Gold η 2 -Coordination to Unsaturated and Aromatic Hydrocarbons: The Key Step in Gold-Catalyzed Organic Transformations

pubs.acs.org/Organometallics Published on Web 11/19/2009 r 2009 American Chemical Society

2 Organometallics 2010, 29, 2–23

DOI: 10.1021/om900900u

Gold η2-Coordination to Unsaturated and Aromatic Hydrocarbons: The

Key Step in Gold-Catalyzed Organic Transformations

Hubert Schmidbaur* and Annette Schier

Department Chemie, Technische Universit€at M€unchen, Lichtenbergstrasse 4, 85747 Garching, Germany

Received October 14, 2009

For a long time, π-complexes of gold in any of its oxidation states were considered elusive species ofuncertain stoichiometry, low stability, and limited relevance. The recent upsurge of research activities inhomo- and heterogeneous catalysis of reactions by gold and its compounds has now provided detailedand extensive information on almost all molecular prototypes with Au0, Auþ, and Au3þ bound toalkenes, alkynes, and even arenes with low η2-hapticity (or, in special cases, η1 and η3). This extremelyimportant progress in organogold chemistry is reviewed herewith an emphasis on the hard experimentalevidence and the structure and energy characteristics of gold π-coordination in molecular systems.

I. Historical Introduction

Zeise’s salt, K[Cl3PtII(C2H4)], was discovered serendipi-

tously upon triturating chloroplatinic acid and potassiumchloride with ethanol under reflux conditions byW. C. Zeisein 1827.1 Four decades later, K. Birnbaum was able toprepare the same compound by treating a solution of hexa-chloroplatinic acid directly with ethylene.2 From the verybeginning, the stoichiometry of Zeise’s salt not only caused acontroversy with contemporary science protagonists such asJ. v. Liebig3 but also remained a “considerable theoreticalembarrassment for over 100 years”,4 until the donor-accep-tor concept proposed byM. J. S. Dewar, J. Chatt, and othersbecame widely accepted in the 1950s.5-7 Even though thecrystal structure was determined (1), first by X-ray diffrac-tion in 1954,8 1965,9 and 197110 and then by neutrondiffraction in 196911 and 1975,12 the details of bondingcharacteristics associated with the η2-coordination of thePt center to the ethylene molecule and the dynamics of thisstructural unit have posed many problems for experimental-ists and theoreticians. Notwithstanding, Zeise’s salt is con-sidered the simplest case and, thus, a prototype of complexesof transition metals with unsaturated hydrocarbons. It was

also recognized early on that the complexation of an olefin toa metal center such as platinum leads to a significant activa-tion of the CdC multiple bond, which provides a basis formetal catalysis of olefin transformations.

Compounds of mercury, which together with platinum is adirect neighbor of gold in the periodic table, had been knownfor considerable time to catalyze addition reactions of bothalkenes and alkynes, and this catalysis was the basis ofimportant industrial processes.13 Mechanistic studies hadshown in the 1960s that both reactions involvedπ-complexa-tion of the unsaturated hydrocarbon which activated thedouble or triple bond for nucleophilic attack (3, 4).14,15

Several of the proposed π-complexes had been isolated.13

Given this background, it was very surprising that thehalides of platinum’s andmercury’s common neighbor in the

*To whom correspondence should be addressed. E-mail:[email protected].(1) Zeise, W. C. Poggendorf’s Ann. Phys. 1827, 9, 632; Ann. Phys.

Chem. 1831, 21, 497(2) Birnbaum, K. Justus Liebigs Ann. Chem. 1868, 145, 68.(3) Liebig, J. v. Justus Liebigs Ann. Chem. 1834, 9, 1; 1837, 23, 12(4) Greenwood, N. N., Earnshaw, A. Chemistry of the Elements;

Pergamon Press: Oxford, U.K., 1984; p 357.(5) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C79.(6) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2941.(7) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 3937.(8) Wunderlich, J. A.; Mellor, D. P. Acta Crystallogr. 1954, 7, 130.(9) Bokii, G. B.; Kukina, G. A. Zh. Strukt. Khim. 1965, 5, 706.(10) Jarvis, J. A. J.; Kilbourn, B. T.; Owsten, P. G. Acta Crystallogr.

1971, B27, 366.(11) Hamilton,W. C.; Klanderman,K.A.; Spratley, R.Acta Crystal-

logr. 1969, A 25, S 172.(12) Love, R. A.; Koetzle, T. F.; Williams, G. J. B.; Andrews, L. C.;

Bau, R. Inorg. Chem. 1975, 14, 2653.

(13) Coates, G. E., Green, M. L. H., Wade, K. OrganometallicCompounds, 3rd ed.; Methuen: London, 1967; Vol. 1, pp 151, 175.

(14) Chatt, J. Chem. Rev. 1951, 48, 7.(15) Budde, W. L.; Dessy, R. E. Chem. Ind. 1963, 735.

Review Organometallics, Vol. 29, No. 1, 2010 3

periodic table, the common gold halides AuX and AuX3,appeared to show no similar reactions. In solutions ofHAuCl4 in ethanol there was no evidence for the complexAuCl3 3C2H4 (2). In fact, until the mid-1960s, when thechemistry of olefin complexes with many metals and cer-tainly with platinum was in full bloom already, no olefincomplexes of gold in any of its oxidation states were known.Perhaps not astonishing, for decades it was also tacitlyassumed or explicitly stated therefore that gold salts wouldnot be able to act as catalysts for organic transformations ofalkene or alkyne, let alone arene type substrates. This atti-tude, of course, was also supported by the idea of the “noblecharacter” of gold, i.e. its general inertness, in which itsextreme redox potentials are reflected.16,17

Early reviews have noted explicitly the gap in the inventoryof established metal complexes of olefins at gold.18,19 In1964, A. J. Chalk, working at the General Electric CompanyResearch Laboratory in Schenectady, NY, published thefirst report on the synthesis of an analytically well-definedadduct of an olefin to AuCl: (1,5-cyclooctadiene)(AuCl)2.This colorless crystalline compound was obtained by irra-diating a yellow solution of tetrachloroauric acid and theolefin in diethyl ether or 2-propanol with UV light until itwas decolorized completely. The same product was formedfrom AuCl and the olefin in ethanol. Apart from theelemental analysis, the IR absorptions of the complex at1530 and 1520 cm-1 also suggested the presence of an olefincomplex. The same author also prepared what was tenta-tively assumed to be the first olefin complex of gold trichlor-ide, C8H12 3AuCl3, but the evidence for this case was notconclusive.20

This paper by Chalk clearly prompted R. H€uttel atUniversit€at M€unchen (Figure 1) to produce a short preli-minary communication already in 1965 disclosing some ofhis independent work.21 Full details were subsequentlyreported in the early 1970s in a series of papers whichdescribe the first systematic investigations of the reactionsbetween gold salts and olefins. The work with anhydrousAuCl, AuBr, AuCl3, and AuBr3 as the acceptors and simplelinear and cyclic olefins as the donors gave clear evidence forthe long-missed complex formation. However, most of thecompounds obtained were of very limited stability anddecomposed at or below room temperature, except foradducts of strained (cyclic) olefins. It also appeared thatgold(I) halides formed the more stable complexes comparedto gold(III) congeners with the same olefin.22-31

In early work, the complexes were identified mainly byelemental analysis, vibrational spectroscopy, and cryoscopic

or osmometric molecular mass determinations.29 A fewcomplementary experiments were carried out in severallaboratories,32 including those of G. Wittig33 and G. J. M.van der Kerk,34 focusing on complexes with 1,5-cycloocta-diene, cyclooctyne, and styrene, respectively, but this workobviously was soon discontinued. The group of J. K. Kochiturned to coordination compounds of gold(I) salts withpoorly coordinating anions and applied NMR spectroscopyto their systems. Gold(I) fluorosulfate and trifluorometha-nesulfonate (triflate) were thus found to give not only 1:1complexes with olefins, the common stoichiometry for gold-(I) halides, but also complexes with ametal-to-ligand ratio of1:3.35 In this early work, cyclooctenes were among the mostwidely studied ligands.36 The first 197Au M€ossbauer spectraof two AuCl complexes with terminal olefins were deter-mined in the laboratory of R. M€ossbauer at TechnischeUniversit€atM€unchen and confirmed the AuI oxidation statein these compounds.37 This work was followed byM€ossbauer studies at Cambridge in the group of P. G.Maddock.38Alongwithmany simple linear and cyclic olefinsinvestigated as ligands in several laboratories, even exoticunsaturated hydrocarbons such as basketene and snoutenewere probed as substrates for AuCl coordination in someearly work by L. U. Meyer and A. de Meijere.39

Figure 1. Rudolf H€uttel (1912-1993), Professor of OrganicChemistry and Chemical Technology at Ludwig-Maximilians-Universit€at M€unchen (1953-1976), who started his pioneeringwork on olefin complexes of gold halides at LMU in 1961. Thiswork followed his previous successful studies of palladium-catalyzed transformations of olefins. In these earlier investiga-tions he identified the first (allyl)palladium complexes. RudolfH€uttel had extensive experience in chemical technology fromhis work at IG-Farben and at Hoechst AG. Together withH. Wieland, G. Hesse, and H. Behringer he investigated a largevariety of natural products (photograph courtesy of theFakult€atsarchiv, Department f€ur Chemie and Pharmazie,Ludwig Maximilians Universit€at M€unchen).

(16) Armer, B.; Schmidbaur, H.Angew. Chem., Int. Ed. Engl. 1970, 9,101.(17) Schmidbaur, H. Naturw. Rdsch. 1995, 48, 443.(18) Bennett, M. A. Chem. Rev. 1962, 62, 611.(19) Chatt, J.; Guy, R. G. Adv. Inorg. Radiochem. 1962, 4, 77.(20) Chalk, A. J. J. Am. Chem. Soc. 1964, 86, 4733.(21) H€uttel, R.; Dietl, H. Angew. Chem., Int. Ed. Engl. 1965, 4, 438.(22) H€uttel, R.; Reinheimer, H.; Dietl, H. Chem. Ber. 1966, 99, 462.(23) H€uttel, R.; Reinheimer, H. Chem. Ber. 1966, 99, 2778.(24) H€uttel, R.; Reinheimer, H.; Nowak, K.; Dietl, H. Tetrahedron

Lett. 1967, 1019.(25) H€uttel, R.; Reinheimer, H.; Nowak, K. Chem. Ber. 1968, 101,

3761.(26) H€uttel, R.; Tauchner, P.; Forkl, H. Chem. Ber. 1972, 105, 1.(27) H€uttel, R.; Forkl, H. Chem. Ber. 1972, 105, 1664.(28) H€uttel, R.; Forkl, H. Chem. Ber. 1972, 105, 2913.(29) Tauchner, P.; H€uttel, R. Tetrahedron Lett. 1972, 46, 4733.(30) Tauchner, P.; H€uttel, R. Chem. Ber. 1974, 107, 3761.(31) Coutelle, H.; H€uttel, R. J. Organomet. Chem. 1978, 153, 359.

(32) Leedham, T. J.; Powell, D. B.; Scott, J. G. V. Spectrochim. Acta1973, 29A, 559.

(33) Wittig, G.; Fischer, S. Chem. Ber. 1972, 105, 3542.(34) De Graaf, P. W. J.; Boersma, J.; van der Kerk, G. J. M.

J. Organomet. Chem. 1976, 105, 399.(35) Komiya, S.; Kochi, J. K. J. Organomet. Chem. 1977, 135, 65.(36) Couch, D. A.; Robinson, S. D. J. Chem. Soc., Chem. Commun.

1971, 1508.(37) Bartunik, H. D.; Potzel, W.; M€ossbauer, R. L.; Kaindl, G. Z.

Phys. 1970, 240, 1.(38) Jones, P. G.; Maddock, A. G.; Mays, M. J.; Muir, M. M.;

Williams, A. F. J. Chem. Soc., Dalton Trans. 1977, 1434.(39) Meyer, L. U.; de Meijere, A. Tetrahedron Lett. 1976, 497.

4 Organometallics, Vol. 29, No. 1, 2010 Schmidbaur and Schier

Several general reviews of the chemistry of metal π-com-plexes,40,41 as well the first accounts of organogold chemistrypublished in 1964,42 1970,16 and 1976,43 the first monographon The Chemistry of Gold published in 1978,44 and theGmelin Handbook on Organogold Chemistry covering theliterature until 1980,45 all reflected this very limited develop-ment of the coordination chemistry of gold with unsaturatedhydrocarbons in only short chapters with some 15 referenceson the subject. This tertiary literature and the later standardcomprehensive treatises46-48 also show that for two decadesafter the first synthesis of a well-defined compound no crystalstructure was determined of any of the early complexes,leaving many questions open about structure and bonding.Prior to or parallel with the first experimental work,

theoretical approaches to the interaction between ethyl-ene and Auþ using semiempirical treatments were pub-lished,49-51 and in 1979 the group of T. Ziegler52 presentedresults obtained with the Hartree-Fock-Slater transition-state method clearly favoring a symmetrical η2-bonding withC2v symmetry as written also in the conventional descriptionof the anion in Zeise’s salt, with a rather long equilibriumAu-C distance of 2.47 A, well above all experimental dataobtained in later work.Structural information was finally first obtained in 198753

on one of H€uttel’s compounds by the group of J. Str€ahle inT€ubingen in a collaboration with F. Calderazzo’s group inPisa for (cis-cyclooctene)AuCl (5), which features Au-Cdistances of 2.15(2) and 2.21(2) A, while the CdC distanceof 1.38(2) A is significantly longer than in the free olefin ligand(1.332(2) A).54 In the IR spectrum of this first structurallywell-defined complex the ν(CdC) stretching vibration is ob-served at 1525 cm-1, well below the 1648 cm-1 reported forthe free olefin,41 a feature common to the vibrational spectraof all olefin complexes of gold(I) already noted by Chalk,H€uttel, and others in most of the earlier work.20-36 Conver-sely, the absence of a similar shift in the ν(CdC) absorption tosmaller wavenumbers in organogold compounds with“dangling” vinyl or allyl groups (as in Ph3PAuC6H4-2-CHdCH2 or Ph3PAuC6H4-2-CH2CHdCH2 with ν(CdC) 1615and 1638 cm-1, respectively) was taken as a proof for the ab-sence of any (olefin)gold coordination.55 The new structural

results53 have been important benchmark data for subsequenttheoretical investigations of (olefin)AuX complexes. Thedecrease of ν(CdC) of tetracyanoethene from 1562 to1178 cm-1 upon complexation to [MeAu(PMe2Ph)] to givea 1:1 complex of unknown structure has remained a remark-able extreme which may require a specific explanation.56

Another important structural work by the group ofFackler in 199457 provided data for a tetranuclear complexof the formula Au4(dppe)2[S2C2(CN)2]Cl2 (6). In this pro-duct one of the four gold(I) centers is attached to the centralCdC bond of one of the two cis-bis(diphenylphosphino)-ethenes (dppe), is chelated by the 1,2-dicyanoethene-1,2-dithiolate ligand, and thus bears a formal negative charge.The relevant bond lengths are Au-C = 2.11(4) and 2.14(4)A and CdC= 1.38(6) A; the internal reference value for theuncomplexed CdC unit is 1.30(6) A. In the IR spectrum ofthis complex the ν(CdC) stretching vibration is observed at1525 cm-1, well below the 1648 cm-1 reported for the freedppee ligand.

It was not until the 1990s that the structural chemistry of(alkyne)gold compounds was developed, mainly in work onthe oligomeric gold(I) alkynyls (RCtCAu)n by D. M. P.Mingos et al.,58 on the interaction of diacetylene “pincers”with gold salts by H. Lang et al.,59-61 and on the complexa-tion of cyclic functional alkynes by AuCl by R. Schulte andU. Behrens, providing insight into a variety of bondingmodes.62 In these compounds the CtC bond lengths of theligands are generally elongated quite considerably, e.g. from1.194 to 1.259 A, upon complexation of O2S(CH2CMe2)2C2

(40) Quinn,H.W.; Tsai, J.H.Adv. Inorg. Chem.Radiochem. 1969, 12,217.(41) Herberhold, M.Metal π-Complexes; Elsevier: Amsterdam, 1972;

Vol. 2.(42) Nesmeyanov, A. N. Metody Elem. Org. Khim. 1974, 1, 109.(43) Schmidbaur, H. Angew. Chem., Int. Ed. Engl. 1976, 15, 728.(44) Puddephatt, R. J. The Chemistry of Gold; Elsevier: Amsterdam,

1978; p 148 ff.(45) Schmidbaur, H. In Gmelin Handbook of Inorganic Chemistry;

Slawisch, A., Ed.; Springer Verlag: Berlin, 1980; Au-Organic Compounds, p197 ff.(46) Puddephatt, R. J. In Comprehensive Organometallic Chemistry;

Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K.,1982; Vol. 2, p 812 ff.(47) Grohmann, A., Schmidbaur, H. In Comprehensive Organome-

tallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;Pergamon: Oxford, U.K., 1995; Vol. 3, p 1 ff.(48) Schmidbaur, H., Schier, A. In Comprehensive Organometallic

Chemistry III; Crabtree, R. H.,Mingos, D.M. P., Eds.; Elsevier: Amsterdam,2007; Vol. 2 (Meyer, K., Ed.), p 251 ff.(49) Hosoya, H.; Nagakura, S. Bull. Chem. Soc. Jpn. 1964, 37, 249.(50) Sakaki, S. Theor. Chim. Acta 1973, 30, 159.(51) Bach, R. D.; Henneike, H. F. J. Am. Chem. Soc. 1970, 92, 5589.(52) Ziegler, T.; Rauk, A. Inorg. Chem. 1979, 18, 1558.(53) Dell’Amico, D. B.; Calderazzo, F.; Dantona, R.; Str€ahle, J.;

Weiss, H. Organometallics 1987, 6, 1207.(54) Traetteberg, M. Acta Chem. Scand. 1975, B29, 29.(55) Aresta, M.; Vasapollo, G. J. Organomet. Chem. 1973, 50, C51.

(56) Johnson, A.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans.1977, 1384.

(57) Davila, R. M.; Staples, R. J.; Fackler, J. P., Jr. Organometallics1994, 13, 418.

(58) Mingos, D. M. P.; Yau, J.; Menzer, S.; Williams, D. J. Angew.Chem., Int. Ed. Engl. 1995, 34, 1894.

(59) Lang, H.; K€ohler, K.; Zsolnai, L. Chem. Commun. 1996, 2043.(60) K€ohler, K.; Silverio, S. J.; Hyla-Krypsin, I.; Gleiter, R.; Zsolnai,

L.; Driess, A.; Huttner, G.; Lang, H. Organometallics 1997, 16, 4970.(61) Lang, H.; Rheinwald, G. J. Prakt. Chem. 1999, 341, 1.(62) Schulte, P.; Behrens, U. Chem. Commun. 1998, 1633.


to AuCl, and the ν(CtC) stretching vibration is loweredfrom 2188 to 1930/1910 cm-1 (7, 8).

The interactions of cyclopentadienyl anions (cp-) withgold(I) were found very early to be not of the π-type (9b) butlead to conventional species with η1-bonding of σ-type(9a).63 However, compounds such as R3PAu-C5H5 arefluxional in solution, with the gold atom performing CfChopping which is very rapid on the NMR time scale at roomtemperature. Substituted cyclopentadienyls behave simi-larly.64-70 This process had been known from previousstudies of the analogous copper(I) compounds.71 CpAu isan insoluble polymer of unknown structure.63 LAu units arealso σ-bonded (η1) to ferrocene and related compounds.45

It should be noted that for a long time there was noevidence for π-complexation of gold in any of its oxidation

states to neutral arenes such as benzene. The first examplesindeed were reported only very recently (below), whileanalogues with copper72-75 and silver75,76 had been knownfor at least half a century.In the mid-1990s the chemistry of π-complexes of gold

finally underwent an explosive growth when some of thecomplexes were used successfully as catalysts for a rapidlygrowing number of important reactions or transformationsof alkynes and alkenes. Of prime importance are gold-catalyzed addition reactions of alkenes and alkynes, shownin eqs 1 and 2. High yields and excellent selectivity achievedunder mild conditions with only small concentrations ofcatalyst in easily controlled homogeneous media have sincemade gold salts the catalysts of choice formany applications.Prominent processes are the additions of water, alcohols,amines, hydrogen halides, etc. leading to aldehydes, ketones,amines, imines, and alkyl/vinyl halides, respectively, and to aplethora of secondary products. Starting in the beginning ofthe 20th century, some of these additions were catalyzed bymercury(II) salts, which are no longer acceptable undermodern environmental protection laws. The field has mean-while been extended to various types of C-C couplingreactions, as summarized in the tertiary literature.

While the total number of publications in the field of goldcomplexes with η2-bonded hydrocarbons was less than 50 inthe three decades from 1965 to 1995 (above), this number ispresently sometimes the monthly productivity rate. A few keypublications fromacademic77,78 andnotablyalso from industriallaboratories79 about catalysis with both gold(I) and gold(III)compounds have triggered enormous interest in research labora-tories worldwide, and recent reviews of the current progressalready list several hundred experimental contributions.80-88

The most frequently used gold catalysts have become commer-cially available.

(63) H€uttel, R.; Raffay, U.; Reinheimer, H. Angew. Chem., Int. Ed.Engl. 1967, 6, 862.(64) Ortaggi, G. J. Organomet. Chem. 1974, 80, 275.(65) Campbell, C. A.; Green, M. L. H. J. Chem. Soc. A 1971, 3282.(66) Werner, H.; Otto, H.; Ngo-Khac; Burschka, C. J. Organomet.

Chem. 1984, 262, 123.(67) Baukova, T. V.; Slovokhotov, Y. L.; Struchkov, Y. T.

J. Organomet. Chem. 1981, 220, 125.(68) Schumann,H.; G€orlitz, F. H.; Dietrich, A.Chem. Ber. 1989, 122,

1423.(69) Bruce, M. I.; Humphrey, P. A.; Williams, M. L.; Skelton, B. W.;

White, A. H. Aust. J. Chem. 1989, 42, 1847.(70) Bruce, M. I.; Walton, J. K.; Skelton, B. W.; White, A. H.

J. Chem. Soc., Dalton Trans. 1983, 809.

(71) Wilkinson, G.; Piper, T. S. J. Inorg. Nucl. Chem. 1956, 2, 32.(72) Turner, R. W.; Amma, E. L. J. Am. Chem. Soc. 1966, 88, 1877.(73) Dines,M. B.; Bird, P. H. J. Chem. Soc., Chem. Commun. 1973, 12.(74) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A. Inorg.

Chem. 1980, 19, 1191.(75) Schmidbaur, H.; Bublak, W.; Huber, B.; Reber, G.; M€uller, G.

Angew. Chem., Int. Ed. Engl. 1986, 25, 1089.(76) Gmelin Handbook of Inorganic Chemistry, 8th ed.; Springer:

Berlin, 1975; Silver (Part B5: Organosilver Compounds).(77) Ito, Y.; Sawamura,M.;Hayashi, T. J. Am.Chem. Soc. 1986, 108,

6405.(78) Fukuda, Y.; Utimoto, K. J. Org. Chem. 1991, 56, 3729.(79) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed.

Engl. 1998, 37, 1415.(80) Hashmi, A. S. K. Gold Bull. 2004, 37, 51.(81) Arcadi, A.; Giuseppe, S. D. Curr. Org. Chem. 2004, 8, 795.(82) Hashmi, A. S. K.; Hutchings, G. J.Angew. Chem., Int. Ed. 2006, 45,

7896.(83) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180.(84) Hashmi, A. S. K.; Rudolph, M. Chem. Soc. Rev. 2008, 37, 1766.(85) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776.(86) Garcia-Mota, M.; Cabello, N.; Maseras, F.; Echevarren, A. M.;

Perez-Ramirez, J.; Lopez, N. ChemPhysChem 2008, 9, 1624.(87) F€urstner, A.; Davies, P. W. Angew. Chem. 2007, 119, 3478;

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In an attempt to fully understand the mechanism of theunderlying reactions occurring at the gold centers, the atten-tion of many research groups has been focused on thispreviously neglected area of organogold chemistry. In thefollowing, these results are summarized covering the litera-ture up to mid-2009. A review has been published recentlyon “Structurally Characterized Coinage Metal-EthyleneComplexes”, where aspects of the specific interaction in(C2H4)nM

þ units (M = Cu, Ag, Au; n = 1, 3) have beenconsidered.89 Regarding homogeneous catalysis of organicreactions, the literature coverage in the present review is notcomprehensive and only those contributions with a focus onthe nature of the catalyst have been included.η2-Complexation of gold cations to unsaturated organic

ligands has also been recognized as an important tool forcrystal engineering and for building supramolecular struc-tures.61 This includes the formation not only of clusters andmultidimensional arrays but also of unusual catenanes.58,90

Many of these aggregates, in which aurophilic interactionsmay also play a role in a synergism of several weak forces,have interesting photophysical properties.91 The structuralaspects are briefly considered for compounds of low nucle-arity, but bonding of alkenes, alkynes, and arenes to goldclusters, nanoparticles, and surfaces has been excluded.

II. Alkene Complexes of Gold in Its VariousOxidation States

1. (Alkene)gold(0) Complexes. Alkene complexes of goldatoms were first obtained by co-condensation of gold vaporand olefins. In typical experiments, ethene in its variousisotopically labeled forms (1H, 2D, 12C, 13C) and its mixtureswith argon were reacted with atomic gold vapor at 8-10 Kand the products investigated by UV and IR spectroscopy.On the basis of the results, in combination with quantum-chemical calculations, a model structure of a 1:1 complexwithC2v symmetry was proposed (10). The η2-bonded ligandshows ν(CdC) at 1476 cm-1, i.e. very close to that of freeC2H4 (1474 cm-1), and a UV/vis absorption maximum at401 nm assigned to a ligand-to-metal charge transfer. Theproduct therefore has a green color. The adduct alreadystarts to decompose above 20 K to give gold clusters.92,93

In later studies, similar co-condensation experiments in anargon matrix provided evidence for the existence of both1:1 and 2:1 complexes, (C2H4)nAu (n = 1, 2), which werecharacterized also by their ESR spectra. For n = 2, a model

structure with parallel CdCaxes has been proposed (11).94-96

In these theoretical investigations it has been shown that theconclusions drawn from the experimental studies may not beentirely consistent regarding the stoichiometry of the speciespresent in the matrices.

In summary it appears that the bonding of ethene mole-cules to gold atoms in their 5d106s1 state is very weak, withbonding energies close to those of dispersion forces and withthe internal bonding characteristics of the olefin only weaklyaffected by the metal coordination. In brief, (C2H4)nAumolecules may be described as van der Waals adducts.97

Clearly, these results do not allow one to directly concludethat ethene adsorption on electrically neutral gold clusters oron surfaces of bulk gold would also be associated with onlyvery low adsorption energies,98,99 but in principle only a lowaffinity is also to be expected.2. Neutral (Alkene)gold(I) Complexes.As described above,

1:1 complexes of olefins with gold(I) halides, AuX, were thefirstπ-complexes of gold to be discovered and they are still thelargest in number. Even though many members of the serieshave only limited stability, under mild experimental condi-tions the complexes are readily accessible via a number ofconvenient routes. Thus, AuCl andAuBr dissolve in solutionsof the olefin (ene) in a polar solvent to give high yields ofproducts (ene)AuX. For dienes (enene), the stoichiometry isof course altered to (enene)(AuX)2 with anAuXunit attachedto each CdC bond. (CO)AuCl can be used instead of AuCl,because CO is an excellent leaving group, but stronger ligandssuch as RNC, R3P, and R2S cannot be displaced by olefins,which indicates the poor donor power of olefins as ligands forgold(I) centers.

Alternatively, the olefinmay be treatedwithHAuCl4 or itssalts in water, in a mixed polar solvent, or in a two-phasesystem with heating or under UV irradiation. The yields aregenerally lower, owing to a variety of side reactions includinginter alia chlorination of the olefin and disproportionation ofthe gold salt. Examples with mono-, di-, and triolefins invarious stoichiometric ligand-to-metal ratios are summari-zed in Tables 1-3, where references are also given.

In theoretical calculations, an NBO analysis was carriedout for an ethene adduct of AuF, assuming that F- is a pureσ-donor. The equilibrium geometries of the model species[Au(η2-C2H4)]

þ and FAu(η2-C2H4) have Au-C and CdCdistances of 2.227/2.193 and 1.404/1.390 A, respectively. Thelatter is the largest as compared with those of the Cu and Aganalogues, suggesting the strongest interactions.100

Neutral complexes (ene)AuX with X other than halogenappear to have limited stability (and high reactivity). Meta-thesis reactions of (ene)AuCl with silver salts such asCF3SO3Ag lead to a ligand redistribution to give cationiccomplexes with a higher olefin tometal ratio. In the presenceof an excess of ene = trans-cyclooctene, the complex(ene)3AuþO3SCF3

- is formed.35

(89) Rasika Dias, H. V.; Wu, J. Eur. J. Inorg. Chem. 2008, 509.(90) Puddephatt, R. J. Chem. Soc. Rev. 2008, 37, 2012.(91) Yam, V.W.-W.; Cheng, E. C.-C.Chem. Soc. Rev. 2008, 37, 1806.(92) McIntosh, D.; Ozin, G. A. J. Organomet. Chem. 1976, 121, 127.(93) McIntosh,D. F.; Ozin,G.A.;Messmer, R. P. Inorg. Chem. 1980,

19, 3321.(94) Kasai, P. H. J. Am. Chem. Soc. 1983, 105, 1983.

(95) Kasai, P. H. J. Am. Chem. Soc. 1984, 106, 3069.(96) Kasai, P. H. J. Phys. Chem. 1988, 92, 2161.(97) Nicolas, G.; Spiegelmann, F. J. Am. Chem. Soc. 1990, 112, 5410.(98) Bond, G. C.; Sermon, P. A. Gold Bull. 1973, 6, 102.(99) Allison, E. G.; Bond, G. C. Catal. Rev. 1972, 7, 233.(100) Kim, Ch. K.; Lee, K. A.; Kim, Ch. K.; Lee, B.-S.; Lee, H. W.

Chem. Phys. Lett. 2004, 391, 321.(101) Hakansson, M.; Eriksson, H.; Jagner, S. J. Organomet. Chem.

2000, 602, 133.(102) Rasika Dias, H. V.; Wu, J. Angew. Chem., Int. Ed. 2007, 46,

7814.(103) Flores, J. A.; Rasika Dias, H. V. Inorg. Chem. 2008, 47, 4448.


The adducts (ene)AuX are generally colorless solids withmelting and decomposition temperatures below 100 �C. Thecomposition of most compounds has been confirmed byelemental analysis. Molecular mass determinations showedthe complexes to be undissociated monomers in solution.Complete sets of NMR and IR data are available in mostcases.

The crystal and molecular structures of two AuCl adductshave been determined. These are the monomeric (cis-cyclooc-tene)AuCl (5)53 and dimeric (exo-dicyclopentadiene)AuCl(12)101 already described in the classical work by H€uttel.21,22

In the latter case, dimerization occurs through long-rangeaurophilic contacts (3.428 A) which appear to have littleinfluence on the mode of coordination of the metal atoms tothe CdCbonds (Au-C=2.16(1), 2.20(1) A, CdC=1.38(1)A) (12). In CHBr3 and CHCl3 solution, the compound is fullydissociated into themonomers.31 The structure determinationof (norbornene)AuCl remained incomplete.53

Compounds (ene)AuXwith ene= olefin and X representingamultidentate ligandappear tohave improvedstability such thateven ethene complexes have become available and could bestructurally characterized. An early example was J. P. Fackler’scomplex with X = 1,2-dicyanoethene-1,2-dithiolate (6), wherethe gold atom is further η2-coordinated to the CdC bond ofdppee.57 Inmore recent work by the group ofH.V.RasikaDiasthe tris(pyrazolyl)borato anion with three or six electron-with-drawing CF3 substituents was introduced as a ligand to gold.Even though only two of the possible three N donor centers ofthis ligand actually become attached to the metal atom, theresulting AuX unit appears to have a high affinity for olefins,including C2H4 (13).

102 No ethylene is lost from the colorlesssamples under vacuum at room temperature. The compoundsare prepared in good yields by reacting AuCl with the appro-priate Na[Pz3BH] salt under an ethylene atmosphere (Pz= 3,5-(CF3)2C3N2H) or (3-CF3)-5-Ph-C3N2H). In solution, themolecules are fluxional with equilibrated free and coordinatedpyrazolyl donor groups. However, ethylene exchange is slow onthe NMR time scale in solutions in C6D6 or CDCl3, and a sepa-rate 1H resonance is observed if an excess of ethylene is present,with a shift difference of Δδ=1.43 ppm (δ 3.81 vs. 5.24 ppm).TheΔδvalue (59.8ppm) is evengreater for the 13C resonancesofbound and free ethylene (63.7 vs 123.5 ppm). These shifts are thelargest within the series of analogous compounds ofCu,Ag, andAu, suggesting the tightest bonding for gold.102

In the crystals, the Au-C distances in the compounds arein the range 2.093(5)-2.108(6) A, and the CdCbond lengths

Table 1. Neutral Gold(I) Complexes (ene)AuX with Monoolefins

(ene) and X = Cl (unless Stated Otherwise)

ene ref

butene-2 24cis-2,3-dimethylbutene-2 31trans-4,4-dimethylpentene-2 312,4,4-trimethylpentene-2 312,4,4-trimethylpentene-1 31octene-1 23trans-octene-2 23decene-1 23duodecene-1 23quatordecene-1 23sextadecene-1 23, 27, 37octadecene-1 23, 27, 37styrene 34cyclopentene 21, 22cyclohexene 21, 22, 35cycloheptene 21, 22norbornene 53cis-cyclooctene

with X = AuCl (5) 21, 22, 35, 53with X = AuOSO2F/AuOSO2CF3 35

trans-cyclooctenewith X = AuCl 35, 36, 38with X = AuOSO2F/AuOSO2CF3 35

trans-cyclodecene 21, 22basketene 39snoutene 39exo-dicyclopentadiene (12) 1016 5713 10214 10320 12022 12223 12224 126, 12725 128

Table 2. Neutral 1:1 Complexes (enene)AuX with Diolefins

(enene) and X = Cl (unless Stated Otherwise)

enene ref

cycloocta-1,5-dienewith X = AuCl 136with X = AuBr 136

cycloocta-cis-1,trans-5-diene 21, 22exo-dicyclopentadiene (12) 21, 22, 25, 31, 37, 101norbornadiene 21hexamethyl Dewar benzene 26cyclooctatetraene 34

Table 3. Neutral 1:2 Complexes (enene)(AuX)2 with Di- and

Triolefins (enene) and X = Cl (unless Stated Otherwise)

enene ref

hexa-1,4-diene 25hexa-1,5-diene 25norbornadiene 22, 25cyclooctadiene

with X = AuCl 20-22, 32with X = AuBr 32

trans-deca-1,4,9-triene 23cyclododeca-cis-1,cis-5,trans-9-triene 21, 22all-trans-cyclododeca-1,5,9-triene 21, 22


are 1.381 and 1.388 A, respectively, compared to 1.313 Afor free ethylene (all data corrected for libration). Thecoordination geometry may be described as distortedsquare planar (counting all four atoms) or distorted trigo-nal planar (considering C2H4 as an entity).102 These char-acteristics are similar to those of Fackler’s dithiolatecompound.57

In another recent study by Rasika Dias and his collabora-tors, triazapentadienyl anions have been employed as aux-iliary ligandsX in (ene)AuX complexes (14).103 Treatment ofa solution of the corresponding (triazapentadienyl)lithiumreagent with C3F7 substituents at the ring carbon and2,6-dichlorophenyl substituents at the donor nitrogen atomswith AuCl under an atmosphere of ethylene gave a colorlesscrystalline product in good yield. This complex is insensitiveto air and light and again does not lose ethylene undervacuum. In CDCl3 solution the 1H and 13C NMR signalsof the ethylene ligand appear at δ 2.71 and δ 59.1 ppm (Δδ2.69/64.2 ppm upfield from C2H4). In the crystal structureanalysis the Au-C bond lengths were found at 2.089(2) and2.098(2) A and the CdC bond length at 1.405(4) A, a verysignificant lengthening as compared to free ethylene(1.313(1) A).

It should be noted that the orientation of the CdCaxis in the structures of the new complexes with biden-tate donor ligands is always virtually parallel to theN- - -N or S- - -S axes of the coordinated nitrogen or sulfuratoms.57,102,103

3. Cationic (Alkene)gold(I) Complexes. Cations [(alkene)-Au]þ (Chart 1; 15) without an additional neutral oranionic ligand (L/X-) are unknown in the condensedphase: i.e., in solution or as salts. However, they have beenidentified in the gas phase mainly by mass spectrometrytechniques, and they were also the subject of numerousquantum-chemical studies on various levels of sophistica-tion.

Gold cations Auþ generated by laser desorption are read-ily trapped by various organic substrate molecules. Therelative affinity of ethylene for Auþ in these ion-moleculereactions is bracketed by CO and C6H6 in a large series ofpotential ligands investigated in exchange reactions. Thebracketing techniques do not provide absolute values forbond dissociation energies (BDE’s),104,105 and only differ-ences and lower limits of BDE’s have been estimated. For[(C2H4)Au]þ the lower and upper limits were set at 59 and

82 kcal/mol, respectively, while theoretical calculations gave73.1 kcal/mol for an optimized molecular geometry withdistances Au-C and CdC of 2.10 and 1.49 A, respectively,and C2v point group symmetry.106-108 Subsequent DFTstudies afforded data between 64.1 and 69.7 kcal/mol. Morerecent work gave corresponding bond lengths of 2.244 and1.400 A and a BDE of 63.0 kcal/mol.86 Investigation of thephotodissociation spectra of [(C2H4)Au]þ gave an extremeupper limit of 82.3 kcal/mol for the BDE, but calculatedvalues also arrived at more realistic values between 53.3 and55.9 kcal/mol, depending on the basis sets of the calculations.The optimized geometry had Au-C and CdC distances of2.227 and 1.397 A, respectively, with the carbon atomspyramidalized to an angle of 164.7� (with an initial HCCHdihedral angle of 180� as a reference).109 The results of adetailed analysis revealed very significant contributions fromrelativistic effects and favored a metallacyclopropane modelof bonding, very different from the situation with the copperand silver analogues.100,107,109,110

Much more extensive results from both experimental andtheoretical approaches are available for cationic (alkene)-gold(I) complexes of the types [LAu(ene)]þ, [Au(ene)2]

þ and[Au(ene)3]

þ (Chart 1; 16-18), where the auxiliary ligand L ispredominantly a tertiary phosphine, a 2,20-bipyridine, or acarbene.

An early example of the former was obtained byR. Us�on et al. upon treating [(C6F5)2Au(PPh3)]ClO4 or

Chart 1. 15-18

(104) Weil, D. A.; Wilkins, C. L. J. Am. Chem. Soc. 1985, 107, 7316.(105) Chowdhury, A. K.;Wilkins, C. L. J. Am. Chem. Soc. 1987, 109,

5336.

(106) Schr€oder, D.; Hrusak, J.; Hertwig, R. H.; Koch,W.; Schwerdt-feger, P.; Schwarz, H. Organometallics 1995, 14, 312.

(107) Hertwig, R. H.; Koch, W.; Schr€oder, D.; Schwarz, H.; Hrusak,J.; Schwerdtfeger, P. J. Phys. Chem. 1996, 100, 12253.

(108) Schr€oder,D.; Schwarz,H.;Hrusak, J.; Pyykk€o, P. Inorg. Chem.1998, 37, 624.

(109) Stringer, K. L.; Citir, M.; Metz, R. B. J. Phys. Chem. A 2004,108, 6996.

(110) Nechaev, M. S.; Rayon, V. M.; Frenking, G. J. Phys. Chem. A2004, 108, 3134.


[(Ph3P)AuClO4] (prepared in situ) with cycloocta-1,5-diene(COD). The product, [(Ph3P)Au(COD)]þClO4

-, is a 1:1electrolyte in acetone. Its structure is unknown.111

Cationic complexes of the same type have been proposedas intermediates in many reactions where a transformationof an olefin is catalyzed by precursor compounds of the typeR3PAuX, in which X is exchanged by an anion having apoor donor capacity for Auþ and thus is a good leavinggroup. These complexes are generally prepared by react-ing a chloride or bromide complex in situ with the silversalts AgBF4, AgPF6, AgSbF6, AgOTf, AgOC(O)CnF2nþ1,etc. in the presence of the olefin79,112-115 but have beenisolated and fully characterized only in very few cases (seebelow).

Recent NMR investigations have shown that the com-plexes [Ph3PAu(ene)]þBF4

- with ene = substituted styr-ene dissolved in CD2Cl2 are virtually fully dissociated,such that there are no close contacts of the BF4

- counter-ion with the metal center.116 The conclusions have beensupported by theoretical calculations. Notwithstand-ing, in catalytic reactions there may still be an anioneffect, depending on the steric characteristics of theligands.117,118

If the olefin unit is not present in a substrate independentof the [R3PAu]þ fragment, as in the AuCl complex tris-(dibenzo[a,d]cycloheptenyl)phosphine or the acetonitrilecomplex derived therefrom, the geometrical constraintsmay prevent olefin-gold coordination (19).119

In contrast, if the alkene group is tethered to the tertiaryphosphine at a flexible side chain, intermolecular (ene)- - -Aucoordination takes place to give a polycationic chain struc-ture surrounded by SbF6

- counterions (20).120 In a single-

crystal structure analysis theAu-Cbond lengths were foundto be 2.25(1) and 2.34(1) A.

Even if steric conditions rule out efficient π-bonding of anolefinic group attached to the tertiary phosphine ligand,55

there may still be significant activation which allows con-certed reactions, as observed in the bromination of o-vinyl-phenyl- or o-allylphenylphosphine complexes (eq 3; 21).121

Shortly before the literature deadline for this review, areport appeared in which a whole family of surprisinglystable compounds of the formula [(tBu3PAu(ene)]þSbF6

-

has finally been described. While the corresponding com-plexes with Ph3P could not be isolated, introduction of tBu3Pas a ligand gave colorless crystalline productswith isobutene,trans-cyclooctene, norbornene, and norbornadiene whichcould be fully characterized by analytical and structuralmethods (22, 23). These unusual properties have beenascribed to the umbrella-type shielding of the gold atom bythe bulky ligand and themuchmore powerful donor effect ofthe trialkylphosphine as compared to that of triarylpho-sphines.122

In the isobutene complex (23), the gold atom is η2-bondedbut shifted slightly toward the terminal C atom: Au-C1 =2.224(9) A and Au-C2 = 2.350(8) A. Upon complexationtheCdCbond is lengthened to 1.349(14) A, and the central Catom becomes pyramidalized. In the norbornene complex

(111) Us�on, R.; Laguna, A.; Sanjoaquin, J. L. J. Organomet. Chem.1974, 80, 147.(112) Zhang, J.; Yang, C.-G.; He, C. J. Am. Chem. Soc. 2006, 128,

1798.(113) Gris�e, C. M.; Rodrigue, E. M.; Barriault, L. Tetrahedron 2008,

64, 797.(114) Garcia-Mota, M.; Cabello, N.; Maseras, F.; Antonio, M.;

Echevarren, A. M.; Perez-Ramirez, J.; Lopez, N. ChemPhysChem2008, 9, 1624.(115) Herrero-Gomez, E.; Nieto-Oberhuber, C.; Lopez, S.; Benet-

Buchholz, J.; Echevarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 5455.(116) Zuccaccia,D.; Belpassi, L.; Tarantelli, F.;Macchioni,A. J. Am.

Chem. Soc. 2009, 131, 3170.(117) Brouwer, C.; He, C. Angew. Chem., Int. Ed. 2006, 45, 1744.(118) Schelwies, M.; Dempwolff, A. L.; Rominger, F.; Helmchen, G.

Angew. Chem., Int. Ed. 2007, 46, 5598.(119) Fischbach, U.; Ruegger, H.; Gr€utzmacher, H. Eur. J. Inorg.

Chem. 2007, 2654.

(120) Shapiro, N.D.; Toste, F. D.Proc. Natl. Acad. Sci. U.S.A. 2008,105, 2779.

(121) Bennett, M. A.; Hoskins, K.; Kneen, W. R.; Nyholm, R. S.;Hitchcock, P. B.; Mason, R.; Robertson, G. B.; Towl, A. D. C. J. Am.Chem. Soc. 1971, 93, 4591.

(122) Hooper, T.N.; Green,M.;McGrady, J. E.; Patel, J. R.; Russell,C. A. Chem. Commun. 2009, 3877.


(22), the gold atom is symmetrically η2-bonded to theexo face of the alkene with a lengthening of the CdC bondfrom 1.334(1) to 1.366(5) A (Au-C= 2.281(3), 2.299(3) A).DFT calculations on a model system have suggestedligand-to-metal donor and metal-to-ligand back-donationcomponents of the bonding, confirming other relevantresults.52,110,123-125 With norbornadiene both the 1:1 andthe 2:1 complex have been obtained. In the latter, both[(tBu3P)Au]þ groups are at the exo faces with similar struc-tural characteristics but a slight slippage of the metal atomsfrom the CdC bond centroids.

In CDCl3 solution the complexes [(tBu3P)Au(ene)]þ showanexchangeof alkene ligandswhich is rapidon theNMRtimescale, with activation barriers of only ca. 3 kcal/mol, possiblywith an associative mechanism and via [(tBu3P)Au(ene)2]

þ

intermediates. Exchange equilibria have shown that the co-ordination shifts Δδ for the resonances of the olefinic 13Catoms in free and complexed alkene are only small fornorbornene but are as much as 15-20 ppm for isobuteneand, therefore, larger than those for analogous complexeswith a carbene (NHC, below) instead of a phosphine ligand,which may have implications regarding the activation of thesubstrate. Surprisingly, the olefins in 22 or 23 are not replacedby tetrahydrofuran present in the solutions.122

[R3PAu(ene)]þ cations were also observed by 31P NMR

spectroscopy in solution: the resonance at δ 25.8 ppm re-corded for the mixture of Ph3PAuCl with AgOTf in toluene isshifted to δ 27.3 and 29.6 ppm upon addition of cyclohexeneand norbornene, respectively. To give an example for thecatalytic activity, thismixture catalyzes the hydroaminationofthese olefins with p-toluenesulfonamide.123

An entirely new entry into the chemistry of cationic(ene)gold(I) complexes was discovered by the group of M. A.Cinellu.125-127 Dicationic dinuclear gold(III) μ,μ-dioxo com-pounds with 6,60-disubstituted 2,20-bipyridyl ligands (bipyR,R’)were found to be reduced by simple olefins (ene) to givemononuclear gold(I) complexes in which the corresponding[(bipyR.R’)Au]þ units have captured the olefin, affording com-poundsof the general formula [(bipy)Au(ene)]þPF6

- (24) . Thereactions gave stable products with ene = ethene, styrene,4-methoxystyrene, R-methylstyrene, cis-stilbene, norbornene,anddicyclopentadiene in low tomoderate yields.With diolefins(enene) such as 2,5-norbornadiene, 1,5-cyclooctadiene, anddicyclopentadiene, dinuclear compounds were obtained.

All complexes are surprisingly stable, with melting pointsabove 100 �C. In acetone solution they behave as 1:1

electrolytes. The colorless crystals do not lose olefin undervacuum and give intact molecular ions in the cation FABmass spectra. Complete sets of NMR data have been com-piledwhich are in full agreementwith the proposed formulas.No dissociation of olefin is observed, even in good donorsolvents such as CD3CN, but there is rapid exchange withexcess olefin. For unsymmetrical olefins such as styrene,hindered rotation about the Au-ene axis can be observedat low temperature. All 1H and 13C signals of the boundolefins are shifted upfield upon coordination as comparedto those of the free olefins (Δδ ca. 1.5 and 50 ppm, re-spectively).

The crystal structures of two styrene complexes withdifferent substituents on the bipy ligand have been deter-mined (24; R = iPr, 2,6-C6H3Cl2). The gold atoms are in analmost perfectly planar N2AuC2 environment with Au-Cdistances of 2.098(5)/2.105(3) and 2.114(6)/2.118(2) A andCdC distances of 1.488(6)/1.484(4) A.126,127 It thus appearsthat the CdC bonds in these complexes are the longest ofall structurally characterized [(ene)AuL]þ cations and[(ene)AuX] molecules, suggesting particularly strong Auf(ene) π back-donation.110

The cationic (alkene)gold(I) complexes [(ene)AuL]þ havealso been prepared with L representing an N-heterocycliccarbene (NHC) ligand. R. A. Widenhoefer and his colla-borators reacted (NHC)AuCl precursor complexes withequimolar quantities of AgSbF6 in CH2Cl2 in the presenceof an olefin and obtained the corresponding complexes[(NHC)Au(ene)]þSbF6

- in almost quantitative yields (25;R = 2,6-C6H3

iPr2).128 The 1,3-bis(2,6-diisopropylphenyl)-

imidazol-2-ylidene ligand L was combined with the olefinscis-2-butene, isobutene, 2-methylbut-2-ene, 2,3-dimethyl-but-2-ene, 1-hexene, norbornene, styrene, and 4-methylstyr-ene to give air- and heat-stable, colorless crystallinecomplexes. Complexation was established by NMR, MS,and elemental analyses. Three of the complexes (ene =isobutene, norbornene, Me2CdCMe2) have been structu-rally characterized by single-crystal X-ray diffraction. Thegold atom is tricoordinate, with Au-C distances averaging2.23 A and CdC distances 1.35 A. It thus appears from thestructural data that while the CdC bond distances are notmuch affected by complexation, the approach of the metalatoms leads to remarkably short metal-ligand equilibriumdistances, similar to those in the analogues with tertiaryphosphines but shorter compared to complexes with bipyri-dyl ligands (above).

The equilibrium constants measured for the exchange ofthe ligand L0 in the complex [(NHC)Au(L0)]þ with L0 =3,5-bis(trifluoromethyl)phenyl cyanide by the olefins have beencorrelated with the σ-donor capacity of the olefins. The

(123) Zhang, J.; Yang, C.-G.; He, Ch. J. Am. Chem. Soc. 2006, 128,1798.(124) Cedeno, D. L.; Sniatynsky, R. Organometallics 2005, 24, 3882.(125) Cinellu, M. A.; Minghetti, G.; Stoccoro, S.; Zucca, A.;

Manassero, M. Chem. Commun. 2004, 1618.


kinetics of the exchange suggest an associative mechanismwith an activation energy of ca. 16 kcal/mol (for ene = iso-butene): [(NHC)Au(ene)]þ þ ene* f [(NHC)Au(ene*)]þ þene. There are notable differences in the data for complexeswith R3P and NHC ligands which may be useful in futurecatalyst design.

The complexes [(NHC)Au(ene)]þ have been proposed tobe intermediates in transformation reactions of olefins cat-alyzed by precursor compounds (NHC)AuX activated withsilver salts such as AgBF4 etc.

129-131 In these solutions, theanions have little effect on the selectivity of the reactions.116

4. Special Role of Vinylgold(I) Compounds. Attentionmust be paid to the entirely different performance of vinyl-or styrylgold(I) compounds CHR0dCHAuPR3 (Scheme 1;26, R = Ph, etc. and R0 = H, Ph, etc.) as substrates for theattack of [LAu]þ electrophiles. Typically, [(R3P)Au]þ ca-tions (generated in situ) become attached regioselectively tothe ipso carbon atom (27), establishing a geminal structuralunit [CAu2] which appears to be stabilized by aurophilicinteractions.45,74,132 Structure 27 thus is clearly preferredover η2-coordination (28).

A mechanism recently proposed by M. R. Gagn�e and hiscollaborators for the hydroarylation of allenes (Scheme 2;30) includes an intermediate with this type of arrange-ment.133,134 Intermediates 31 and 32 have been isolated andcharacterized by analytical and spectral data and by theirreactivity with tertiary phosphine molecules and halideanions, converting 32 back to the corresponding vinylgoldcompound 31. It is very likely that compounds of the types 31and 32 also appear in other reactions of similar substrates.

Moreover, when any other coinage-metal cations arepresent in the reaction mixtures, which is very common,e.g. in all cases where AgY reagents (with or without a ligandL) are used for the activation of LAuX catalyst precursors(X= Cl, Br; Y= BF4, PF6, SbF6, OTf, NTf2), these metals

may become associated similarly with the gold compoundthrough metallophilic interactions (Scheme 1; 29) and assistin the activation of the alkene. Silver salts can therefore beconsidered efficient cocatalysts in gold-induced additionreactions, not only because they are instrumental in remov-ing the halide anion from the gold center but also because ofsome auxiliary role in activating the olefin.135

5. Cationic Bis(alkene)gold(I) Complexes. Compoundswith cations [(ene)2Au]þX- (16) can be found already amongthe products prepared in the classical studies by H€uttel et al.These compounds were obtained by the reactions of thecorresponding olefins with anhydrous AuCl3 in an organicsolvent or with HAuCl4 or NaAuCl4 in organic/aqueoustwo-phase systems, which proceed with partial reductionAu3þ f Auþ. In this process, chlorinated hydrocarbons areformed, and the chloride anions are complexed by AuCl3 togive the AuCl4

- counterions. The products of the generaltype [(ene)2Au]þAuCl4

- thus obtained are given in Table 4together with pertinent references. The saltlike nature hasbeen confirmed by conductivity and molecular mass deter-minations in solutions in chloroform, acetone, and nitro-methane. The yellow color is evidence for the presence of the

Scheme 1. 26-29 Scheme 2. 30-32

Table 4. Cationic Bis(alkene)gold(I) Complexes [(ene)2Au]þX-

with X- = AuCl4- (unless Stated Otherwise)

ene ref

Monoolefins

ethene (calcd, 16) 1372,4,4-trimethylpentene-1 312,4,4-trimethylpentene-2 31trans-cyclodecene 252,6-dimethylstyrene 31

Diolefins

1,5-cyclooctadienewith X = AuCl4

- 21, 22, 25, 31, 32with X = ClO4

- 136norbornadiene 24, 31dicyclopentadiene 25

(126) Cinellu, M. A.; Minghetti, G.; Cocco, F.; Stoccoro, S.; Zucca,A.; Manassero, M.; Area, M. Dalton Trans. 2006, 5703.(127) Cinellu, M. A.; Minghetti, G.; Stoccoro, S.; Zucca, A.;

Manassero, M. Angew. Chem., Int. Ed. 2005, 44, 6892.(128) Brown, T. J.;Dickens,M.G.;Widenhoefer, R.A. J. Am.Chem.

Soc. 2009, 131, 6350.(129) Fremont, P.; Stevens, E.D.; Frucros,M.R.; Dyas-Requejo,M.

M.; Perz, P. J.; Nolan, S. P. Chem. Commun. 2006, 2045.(130) Corberan, R.; Ramyrez, J.; Poyatos, M.; Peris, E.; Fernandez,

E. Tetrahedron: Asymmetry 2006, 17, 1759.(131) Thang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10, 2079.(132) Grandberg, K. I.; Smyslova, E. I.; Kosina, A. N. Izv. Akad.

Nauk SSSR, Ser. Khim. 1973, 2787; Bull. Acad. Sci. [USSR]Div. Chem.Sci. 1973, 2721.(133) Tarselli, M. A.; Gagn�e, M. R. J. Org. Chem. 2008, 73, 2439.(134) Weber, D.; Gagn�e, M. R. GOLD2009 Conference; Heidelberg

University: Heidelberg, Germany, 2009; Abstracts p 219.(135) Ferrer, C.; Echevarren, A. M. Angew. Chem., Int. Ed. 2006, 45,

1105.


AuCl4- anion. A report describing a compound as a color-

less solid is probably incorrect.32 Where tested, thecompounds were diamagnetic and gave no ESR signal.24

To date, the structures of the cationswith substituted alkenes(Table 4) have not been determined by diffraction methods,and the few IR bands andNMRdata listed in the reports arenot conclusive.32 Only in one case was the anion perchlorateinstead of tetrachloroaurate, but no details are available.136

The molecular orbital characteristics and the structure ofthe cation [(C2H4)2Au]þ have been studied in DFT calcula-tions. Models with D2h and D2d point group symmetry werefound to have the same energy. Depending on the level oftheory, the Au-C and CdC bond lengths are in the ranges2.532-2.270 and 1.343-1.368 A, respectively, and thestabilization energy (Auþ þ 2C2H4 f (C2H4)2Auþ) is ca.113 kcal/mol, the largest in the series of the three coinagemetals Cu-Ag-Au.137

6. Cationic Tris(alkene)gold(I) Complexes. Compoundsof the type [(ene)3Au]þX- (17) are known from the workby the groups of H€uttel24,25,31 (X=AuCl4) andKochi (X=OTf).35 The compounds were largely obtained by treatmentof the olefin with anhydrous AuCl3 in chloroform at lowtemperature. Other examples can also be generated byreacting the olefin with aqueous HAuCl4 (Table 5). Workupof the reaction mixtures gave the corresponding chlorinatedhydrocarbons (e.g. 2,3-dichlorobutane from cis-butene-2).The yellow tetrachloroaurates are thermally very unstableand highly sensitive to moisture. Molecular mass determina-tion suggests dissociation as a 1:1 electrolyte (cryoscopy inCHCl3). The triflate example was obtained from the 1:1complex (ene)AuCl on treatmentwithAgOTf in the presenceof excess alkene.35

The reaction of all-Z-tribenzo[12]annulene with AuCl inCH2Cl2 (33) did not afford the expected cyclic tris(alkene)complexwith the gold atom trapped in the ring center. A [4þ2] cycloaddition occurred instead, probably via an inter-mediate with AuCl η2-bonded to one of the three CdCunits.In contrast, both Cuþ and Agþ gave the expected complexcations with trigonal η2:η2:η2 coordination.138

In very recent work the first salt of the parent cation[(C2H4)3Au]þ (34) was found to be readily accessible viaa simple preparative route.139 Treatment of AuCl withAgSbF6 in dichloromethane under an ethylene atmospheregave good yields of a colorless, air-sensitive crystallineproduct identified as [(C2H4)3Au]þSbF6

-. Previous experi-ments had provided evidence for a less stable yellow salt ofthis cationwithAuCl4

- as the counterion, obtained as one ofthe decomposition products of the reaction of AuCl with

ethylene in dichloromethane. Solutions of the SbF6 salt inCD2Cl2 show the 1H and 13CNMR signals at δ 4.94 and 92.7ppm, respectively, compared to values of 5.40 and 123.4 ppmfor free ethylene. The Raman line for ν(CdC) was observedat 1543 cm-1, which is down byΔν=80 cm-1 from the 1623cm-1 for free C2H4. Both data suggest strong (C2H4)-Aubonding, as was also confirmed by theoretical calculations.

The crystal structure analysis has shown that the cationis almost completely planar regarding its non-hydrogenatoms in what has been called a spoke-wheel arrangement.The average CdC bond length is 1.369 A (corrected forlibration), and the average Au-C distance is 2.265 A. It hasbeen calculated that a cation with the CdC axes of the threeC2H4 ligands rotated by 90� into a structure with these axesparallel to the 3-fold axis of the cation has an energyþ19 kcal/mol higher than that for the observed all-planararrangement. The copper(I) and silver(I) complexes areisostructural, but the C2H4-Au bonding appears to havethe highest covalent character of the metallacyclopropanetype.139a,b It is worth noting that the cations [(ene)3Au]þ nowwell established have their counterparts in platinum chem-istry with [(ene)3Pt

0] as the classical isoelectronic analo-gue.13,41 Salts of the corresponding silver cation [(C2H4)3-Ag]þ have recently also been investigated.140

Details on (Alkene)gold(I) Coordination. From the aboveoverview of the results on (alkene)gold(I) complexes itappears that mainly nine different types of coordinationmotifs have to be taken into account (Chart 2), six of whichhave been experimentally realized.

Asmight be anticipated, the strongest electrophilic affinityis found in the 1:1 complexes with the low-coordinateacceptors [(L)Au]þ (A) and AuX (B) (Tables 1-3 andcomplexes 5, 12, 20, 22, 23, and 25) . Perhapsmost surprising,however, is the fact that strong interactions are also found in

Table 5. Cationic Tris(alkene)gold(I) Complexes [(ene)3Au]þX-

with X- = AuCl4- (unless Stated Otherwise)

ene ref

ethene (17, 34) 139trans-cycloctene, with X- = OSO2CF3

- 35cis-butene-2 24trans-butene-2 24trans-hexene-2 24trans-4,4-dimethylpentene-2 31dicyclopentadiene 24, 25norbornene 25norbornadiene 24, 25, 31styrene 31

(136) Roulet, R.; Favez, R. Chimia 1975, 29, 346.(137) Tai, H.-C.; Krossing, I.; Seth, M.; Deubel, D. V. Organome-

tallics 2004, 23, 2343.(138) Yoshida, T.; Kuwatani, Y.; Hara, K.; Yoshida,M.;Matsuyama,

H.; Iyoda, M.; Nagase, Sh. Tetrahedron Lett. 2001, 42, 53.

(139) (a) RasikaDias, H. V.; Fianchini,M.; Cundari, T. R.; Campana,C. F. Angew. Chem., Int. Ed. 2008, 47, 556. (b) Fianchini, M.; Huixiong, D.;Rasika Dias, H. V. Chem. Commun. 2009, 6373.

(140) Reisinger, A.; Trapp, N.; Knapp, C.; Himmel, D.; Breher, F.;R€uegger, H.; Krossing, I. Chem. Eur. J. 2009, 15, 9505.


the type C with two donors L, provided that the two donorcenters are fixed in a rigid chelating unit L∩L (such as 2,20-bipyridyl or a bis(pyrazolyl)borate group; 24 and 13). Thesituation is similar for typeD if the two anionic donor centersare part of a rigid chelating unitX∩X (such as in 1,2-dithiolateor 1,3,5-triazapentadienide anions;6 and14). So far there is nowell-established example for the type E, for which, however,many candidates with anions [L∩X]- can be designed. Thisopportunity should be considered in future work.

It thus seems obvious that the key prerequisite for aneffective interaction with the incoming olefin is the inabilityof the units [L-Au-L]þ, (L)AuX, and [X-Au-X]- withstrong donors L and X to adopt a linear arrangement, whichis the most stable and, therefore, most common and mostunreactive coordination mode for these species. Therefore,there is of course no evidence that complexes such as[(R3P)Au(PR3)]

þ, (RNC)AuCl, [Cl-Au-Cl]-, and [NC-Au-CN]- have the slightest affinity for olefins.

Cations of type F with a 1:2 stoichiometry are recognizedas a special case of type A with L= alkene (Table 4 and 16).Cases of types G and H have not been isolated but areproposed as transition states or intermediates in the alkeneexchange reactions occurring in solution.122,128 In contrast,cations of type I, which are recognized as a special case oftype H with L = alkene, have been experimentally estab-lished for cases where no other donors are present ascompetitors for a coordination site at the gold(I) center(Table 5 and 34).

No species with four-coordinate gold(I) centers have to beconsidered at this time. It should be remembered that even inthe case of the tris(pyrazolyl)borate complex 13, where threestrong donor sites are offered in a sterically favorablearrangement, the third donor function of the ligand is notaccommodated at the metal atom.7. Attempted Synthesis of (Alkene)gold(III) Complexes.

Various alkenes have been reacted with AuCl3, AuBr3,HAuCl4, or NaAuCl4 in methanol or chloroform, or in theabsence of a solvent, but no complexes of the componentscould be identified in the reaction mixtures or isolated

therefrom.141-144 Reduction to gold(I) complexes or to goldmetal occurred in all cases (above).45-48

The complex (C2H4)AuCl3 (2) has been considered inrecent DFT calculations. The optimized model structurehas C2v symmetry with the ethylene CdC axis perpendicularto the AuCl3 plane and Au-C and CdC distances of 2.365and 1.377 A, respectively. The interaction energy for(C2H4)-AuCl3 is negative (-7.91 kcal/mol), while for thecomplex with C2H2 it is positive (þ1.51 kcal/mol). The datafor the propylene complex are similar.86

III. Alkyne Complexes of Gold in Its VariousOxidation States

1. (Alkyne)gold(0) Complexes. Gold vapor was co-con-densed with acetylene or acetylene/argon145 mixtures in away similar to that for ethylene.92,93 Formation of the adduct(C2H2)Au0 has been proposed on the basis of vibrational andUV spectroscopy, but the stoichiometry is not unambiguous.

The ESR analysis of the products of a co-condensation ofacetylene (in its various isotopic enrichments of 1H, 2D, 12C,and 13C) with gold vapor in an adamantane matrix at 77 Khas indicated that a vinylgold radical [H2C=CAu]• is thedominant species. With PhCtCH the isomeric radicals cis-and trans-[AuCHdCPh]• were obtained (in cyclohexanematrix).146

A CF3CtCCF3 matrix was also exposed to gold vapor at-196 �C. A darkly colored product was obtained whichdecomposed at room temperature to give metallic gold andhexakis(trifluoromethyl)benzene, the trimerization productof the alkyne. The low-temperature IR spectrum showed aband at 1725 cm-1, which suggests π-complexation of thealkyne in the primary adduct, but no other analytical dataare available.147

2. Neutral (Alkyne)gold(I) Complexes. Complexes of thetype (yne)nAuXwith n=1, 2 and yne and X representing analkyne and an anionic ligand, respectively, were first pre-pared by the group of H€uttel in the 1970s. Generally, theadducts are of very limited stability, and only few of thecompounds have been well characterized. Table 6 sum-marizes the results of this earlier work. (MeCtCMe)AuCl,as an example, was obtained from the components inCH2Cl2

Chart 2. A-I

(141) Brain, F. H.; Gibson, C. S.; Jarvis, J. A.-J.; Philips, R. F.;Powell, H. M.; Tyabji, A. J. Chem. Soc. 1952, 3686.(142) Monaghan, P.K.; Puddephatt,R. J. Inorg. Chim.Acta 1975, 15,

231.(143) Norman, R. O. C.; Parr,W. J. E.; Thomas, C. B. J. Chem. Soc.,

Perkin Trans. 1 1976, 811.(144) Norman, R. O. C.; Parr,W. J. E.; Thomas, C. B. J. Chem. Soc.,

Perkin Trans. 1 1976, 1983.

(145) Kasai, P. H. J. Am. Chem. Soc. 1983, 105, 6704.(146) Chenier, J. H. B.; Howard, J. A.; Mile, B.; Sutcliffe, R. J. Am.

Chem. Soc. 1983, 105, 788.(147) Klabunde, K. J.; Groshens, I.; Brezinski, M.; Kennelly, W.

J. Am. Chem. Soc. 1978, 100, 4437.


at low temperature in high yield and characterized by its 1HNMRand 197AuM€ossbauer spectra. The singlet signal in theformer differed byΔδ 0.56 ppm from that of the free alkyne,and the doublet in the latter (IS 0.79; QS 6.38 (mm s-1))confirmed the presence of gold(I). In solution, the compoundundergoes rapid ligand exchange with an excess of the sameor another alkyne or with pyridine. Decomposition is rapidabove 0 �C both in solution and in the solid state.27

Compounds prepared with strained cyclic alkynes showhigher stability, as already observed by G. Wittig andS. Fischer.33 Thus, cyclooctyne gave a stable 2:1 complexwithAuBrwhich decomposes above 135 �C. Its IR spectrumshowsν(CtC) at 2035 cm-1, and its 1HNMR spectrum exhibits theexpected resonances, but the results of molecular mass deter-minations have suggested partial dissociation in solution.

Work by P. Schulte and U. Behrens has provided detailedinformation on related cycloheptyne complexes (7, 8) pre-pared from (tht)AuCl and the alkynes (tht = tetrahydro-thiophene). According to crystal structure determinations, inboth cases the gold atom is η2-bonded symmetrically to theCtCunit withAu-Cbond lengths in the range 2.050-2.100A and CtC bond lengths of 1.259(12) and 1.244(11) A,respectively. Intermolecular interactions lead to a dimer ora polymer (7, 8). The ν(CtC) vibrations are shifted by about250 cm-1 to lower wavenumbers, suggesting strong (CtC)-Au interactions.62

This work was complemented by the structural character-ization of H€uttel’s (3-hexyne)AuCl by the group of H. V.Rasika Dias (35a). This compound features monomers withAu-C distances of 2.152(4) and 2.172(5) A and a CtCdistance of 1.224(6) A, which in the crystal are aggregated to1D chains via aurophilic bonding. Solutions in CD2Cl2 showthe 1H and 13C signals shifted to lower field by only small

margins upon complexation, which is in marked contrastto the changes observed in the complexation of alkenes(extremely large upfield shifts). Structure, bonding, and chemi-cal shifts of the complex have also been calculated in quantum-chemical studies, and the results are in goodagreementwith theexperimental data. The calculated enthalpy of formation was37.51 kcal/mol. The bonding model calls for strong alkynefmetal σ-bonding and less metalfalkyne π-back-donation.148a

Also very recently, the 1:1 complex (η2) of cyclododecynewithAuCl (35b) was prepared and structurally characterized bythe group of F€urstner.149 The CtC bond length of the freehydrocarbon (1.196(4) A) is elongated to 1.224(5) A in thiscomplex, and theCtC-Cangles are reduced from175.9(9)� to165(1)� (average). The molecules are aggregated to dimers viaweak aurophilic contacts. Upon complexation, the chemicalshift of the alkyne carbon atoms is only slightly shifted (from81.7 to85.9ppm inCD2Cl2). It shouldbenoted that crystals andsolutions of the complex are only stable when kept below 0 �C.

In another very recent study, (alkyne)gold(I) complexeswith an anionic triazapentadienide ligand have been pre-pared and structurally characterized (36; R = 2,6-C6H3-iPr2).

148b The Au-C distances are 2.069(4) and 2.071(4) A,and the CtC distance is 1.233(7) A, only marginally longerthan in free alkynes (e.g. 1.2022(15) A for tBuCtCtBu).150

The IR/Raman band/line for ν(CtC) appears at 1920 cm-1,down very significantly from 2296 cm-1 for free 3-hexyne.151

Together with a strong bending of the CtC-C angles from180� to 155.0(4)�, these data suggest a very strong (alkyne);Au interaction, the strongest in the coinage metal series. Thedata have been reproduced by DFT calculations.149

In preceding theoretical work, the structure and energeticsof the model adduct (C2H2)AuCl have been investigated.The interaction energy for (RC2H)-AuCl (37; R = H, Me)was found to amount to -34.1 kcal/mol (as compared to-38.5 kcal/mol for (C2H4)-AuCl) for an equilibrium geo-metry with Au-C and CtC distances of 2.22 and 1.235 A,respectively (2.218 and 1.382 A for (C2H4)AuCl), forη2-complexation withC2v point group symmetry. The corre-sponding data for propyne, which is bonded asymmetri-cally, are -36.2 kcal/mol and 2.184/2.296 and 1.238 A.These results would suggest that alkenes are activated morethan alkynes by an AuX catalyst. However, in NMR testexperiments with selected enynes it has been demonstratedthat the relative reaction rates of individual steps and hence

Table 6. Neutral and Cationic (Alkyne)gold Complexes

complex ref

(MeCtCMe)AuCl 27(EtCtCEt)AuCl (35a) 27, 148(PhCtCPh)AuCl 27[(PhCtCPh)Au]AuCl4 27, 28[(MeCtCMe)2Au]AuCl4 27(cyclooctyne)2AuBr 33(MeCtCMe)Au2Cl6 28(MeCtCMe)2Au2Cl6 28(CF3CtCCF3)nAum 147(C2H2)Au 145, 1467 628 6235b 14936 14837 86[(C2H2)Au]þ 86, 106, 110, 15238 15339 12040a 15540b 14940c 149

(148) (a) Wu, J.; Kroll, P.; Rasika Dias, H. V. Inorg. Chem. 2009, 48,423. (b) Rasika Dias, H. V.; Flores, J. A.; Wu, J.; Kroll, P. J. Am. Chem. Soc.2009, 131, 11249.

(149) Fl€ugge, S.; Anoop, A.; Goddard, R.; Thiel, W.; F€urstner, A.Chem. Eur. J. 2009, 15, 8558.

(150) Boese, R., Blaser, D., Latz, R., Baumen, A. Acta Crystallogr.1999, C55, IUC9900016.

(151) Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L. Inorg.Chem. 2006, 45, 5742.


the selectivity of the overall reactionmay not be governed bythe thermodynamic preference in the coordination steps.86

3. Cationic (Alkyne)gold(I) Complexes with and without

Coligands. Cations [(alkyne)Au]þ without a coligand at thegold center have only been observed in the gas phase by massspectrometry techniques. They were also the subject of quan-tum-chemical calculations. DFT studies of the species[(C2H2)Au]þ gave an optimized geometry with C2v pointgroup symmetry, Au-C and CtC bond lengths of 2.222and 1.240 A, respectively, and a (C2H2)-Auþ interactionenergy of-54.1 kcal/mol. For propyne, for which the bindingshows the expected asymmetry, the data are similar: 2.073/2.551 and 1.238 A and -36.2 kcal/mol.86 These results are inagreement with those of previous theoretical investiga-tions.106,110 In another study, the reactions of (C2H2)Auþ

and (C2H2)AuPMe3þ with methanol molecules were followed

in amass spectrometer. In the gas phase no reactions occurred.This result has shown that the gold(I)-catalyzed addition ofMeOH to C2H2 is not successful in bimolecular collisions butrequires the environment of the condensed phase. This may betrue in particular for the hydrogen migration. DFT calcula-tions gave a binding energy (η2-C2H2)-Auþ of 53 kcal/mol.152

Compounds with the cationic units [(alkyne)AuL]þ and[(alkyne)2Au]þ are known only in small numbers. R. H€utteland H. Forkl obtained a product from 2-butyne and AuCl3in CH2Cl2, to which they assigned the formula [(MeCtCMe)2Au]þAuCl4

- . The lemon yellow crystals decomposeat-15 �C. The M€ossbauer spectrum shows two quadrupoledoublets characteristic of Auþ and Au3þ. With excess2-hexyne, there is rapid ligand exchange in solution accord-ing to NMR experiments.27 An ionic formula, [(PhCtCPh)Au]þAuCl4

-, was also tentatively ascribed to a productobtained from this complex on reaction with excess tolane,but no details are available.27

π-Complexation of a gold(I) center has been proposed for acompoundwith a tertiary phosphinine ligand bearing (phenyl-alkynyl)dimethylsilyl groups in the 2,6-positions (38). A 31PNMR signal at δ 227.0 ppm has been assigned to this cation inCH2Cl2 solutionwithGaCl4

- as a counterion.Owing to its lowstability, the systemwas not characterized any further. Specificactivation of the CtC bonds was not observed.153

Side-on complexation of linearly two-coordinated gold(I)units to CtC bonds as additional ligands appears to be

generally of very low energy. Thus, oligomerization of thecomplexes (dppb)Au2(CtCPh)2 and (tppb)Au4(CtCPh)4occurs only through Au- - -Au and not through η2-CtC- - -Au contacts (dppb = 1,4-bis(diphenylphosphino)benzene;tppb=1,2,4,5-tetrakis(diphenylphosphino)benzene). It thusappears;other effects ignored;that even weak aurophilicbonding (with bond energies between 5 and 10 kcal/mol) issuperior to this side-on coordination.154

In the seminal work by J. H. Teles et al., cations of the type[(R3P)Au(alkyne)]þ were proposed as the key intermediatesin the catalysis of the addition of alcohols to alkynes by goldcompounds.79 In an NMR investigation of the addition ofmethanol to 3-hexyne inCD2Cl2 catalyzed by Ph3PAuOTf atlow temperature, the 31P resonance at δ 28 ppm has beenassigned to the complex [(Ph3P)Au(EtCtCEt)]þOSO2CF3

-.This signal persisted up to a temperature of 0 �C. Thecomplex was not isolated, but the influence of the nature ofthe tertiary phosphine and of the substituents of the alkyneon the stability and reactivity of the corresponding com-plexes was studied both experimentally and in ab initiocalculations onmodel systems. Carbene complexes were alsoincluded in preliminary tests.79

The first stable complex with a tertiary phosphine group Las the coligand for an [(alkyne)Au]þ unit was obtained byShapiro and Toste. For the gold(I) center a PPh2Ph

0 ligandwas chosen in which a triisopropylsilyl (TIPS)-capped alky-nyl groupwas tethered to one of the phenyl groups (Ph0) via aflexible hydrocarbon chain. Upon removal of the Cl- anionfrom the AuCl complex of this ligand by AgSbF6 theresulting cation undergoes a head-to-tail dimerization togive a dinuclear dication (39). The alkynyl groups areη2-bonded to the metal centers with Au-C and CtC dis-tances of 2.197(5)/2.270(5) and 1.221(8) A according to anX-ray crystal structure determination. The ν(CtC) vibrationof the complex was found at 2053 cm-1 (reduced from 2171cm-1 for the free ligand). In theoretical calculations these datahave been approached very closely using smaller modelsystems. Calculated natural bond order orbital interaction

(152) Roithova, J.; Hrusak, J.; Schr€oder, D.; Schwarz, H. Inorg.Chim. Acta 2005, 358, 4287.

(153) M�ezailles, N.; Ricard, L.; Mathey, F.; Le Floch, P. Eur. J.Inorg. Chem. 1999, 2233.

(154) Yam, V. W.-W.; Choi, S. W.-K.; Cheung, K.-K. Organometal-lics 1996, 15, 1734.

(155) Akana, J. A.; Bhattacharyya, K. X.; M€uller, P.; Sadighi, J.J. Am. Chem. Soc. 2007, 129, 7736.


energies indicate both strong πfAu and Mfπ* interactions,exceeding those in the Cu and Ag analogues.120

Amononuclear (alkyne)gold(I) complexwithanNHCas thecoligand was reported by the group of J. Sadighi. The reactionof the (SIPr)AuCl precursor complex, where SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, was reactedwith AgBF4 in the presence of 3-hexyne in CH2Cl2 to givethe compound [(SIPr)Au(EtCtCEt)]þBF4

- (40a; R = SIPr)which is stable as a solid at room temperature and moderatelystable inCH2Cl2 solution. It is an efficient novel catalyst for thehydrofluorination of alkynes using NEt3H

þF-.155

A similar complex containing cyclodocecyne instead of 3-hexyne (40b) was recently prepared by the F€urstner group(bond length/angle, chemical shift: d(CtC) = 1.206(11) A;CtC-C = 168(3)� (average); δ(13C) 88.5 ppm (CD2-Cl2)).

148a In the same study, the NHC derived from1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-1-ium chloride via the silver salt method was used as a ligand(40c: d(CtC) = 1.218(16) A, CtC-C = 160(4)� (average),δ(13C) 86.6 ppm (CD2Cl2)).

Ever since the appearance of the publication by J. H. Teleset al.,79 cationic complexes of the type [(L)Au(alkyne)]þwithL representing mainly a tertiary phosphine or an NHC havebeen considered the key intermediates in most gold-cata-lyzed additions of nucleophiles to alkynes. The literature isvast and rapidly increasing. In recent work, complexes withboth tertiary phosphines and NHC’s are equally important.The former in most cases are prepared in situ from(R3P)AuCl or (R3P)AuMe precursors which are convertedinto active species by treatment with a silver salt AgY (Y =OTf, BF4, PF6, AsF6, SbF6, etc.) or with a mineral acid HY,respectively.113 More recently, chiral Brønsted acids havealso been employed to achieve stereoselectivity of the reac-tions.156 In order to achieve similar goals, the nature of thecarbene ligands has been greatly modified, but simple

standardNHC’s employed also in catalysis withmetals otherthan gold are still most widely used.157,158

Details on (Alkyne)gold(I) Coordination. From the aboveoverview of the preparative results on simple (alkyne)gold(I)complexes it appears that so far only a few coordinationmotifs have been realized (Chart 3; the labels correspond tothose chosen for the alkene analogues in Chart 2, andreferences are given in Table 6)

While typesA0 (see 35and 37),B0 (see 39and 40),C0 (see 36),and D0 (see 7) need no further comment, type E0 (representedby compound 8) is remarkable, since it has no counterpart yetin the scheme of alkene analogues. 1:2 complexes are rare foralkynes, and only some ofH€uttel’s andWittig’s products, andMathey’s compound 38, can serve as examples, none of whichhave been fully characterized. Finally, 1:3 complexes haveonly been encountered with metalated alkynes (i.e. metalalkynyls, metal acetylides), as shown below.3. π-Complexation of Gold(I) at Metal Alkynyl Units

MCtCR. It has been observed in the very early work inorganogold chemistry,45-47 and later by G. E. Coates and C.Parkin,159 that compounds of the net formula [RCtCAu]form either insoluble polymers or soluble oligomers, but thestructural motifs of these aggregates were not really clear.Work by D. M. P. Mingos et al. has shown that tBuCtCAudoes not form a cyclic tetramer, as previously proposed, but isactually a dodecanuclear catena compound with two inter-penetrating hexanuclear rings (41).58 Each hexanuclear ringcan be formulated as composed of a set of two [Au(CtC-tBu)2]

- anions, two Auþ cations, and two tBuCtCAu mole-cules. The alkynyl groups of each of the two anions which arealready terminally σ-bonded (η1) to a gold atom become alsoattached η2 to a gold cation, while the other anion has its twoalkynyl groups attached to the gold atoms of the two neutralcomponents. Each ring thus has two- (σ,σ), three- (σ,π), andfour-coordinate (π,π) gold atoms. The catena aggregate 41 isthe ideal reference compound for σ- and π-complexation ofgold(I) to alkynyls. The organization of the catena structure isfurther supported by inter-ring aurophilic bonding betweenthe four three-coordinate gold atoms. There are also threeinequivalent CtC bonds, which is reflected by three ν(CtC)bands at 2002, 1983, and 1964 cm-1. Some fluxionality insolution is indicatedby the appearanceof two1H resonances inthe intensity ratio 2:1 in C6D6 at room temperature.58

It should be noted that there is also a large family of catenacompounds with cyclic components containing LAuCtCR

(156) Han, Z.-Y.; Xiao, H.; Chen, X.-H.; Gong, L.-Z. J. Am. Chem.Soc. 2009, 131, 9182and references therein.

(157) Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.;Bertrand, G. J. Organomet. Chem. 2008, 693, 1674.

(158) Lavallo, V.; Frey, G. D.; Donnadieu, B.; Soleilhavoup, M.;Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 5224.

(159) Coates, G. E.; Parkin, C. J. Chem. Soc. 1962, 3220.


units (L = tertiary phosphine), but these feature noη2(CtC)- - -Au π-interactions. Contacts between the hetero-cycles is based only on aurophilic interactions, which alsoassist in the transformation of open-chain, monocyclic, andcatena species.48,160-162

Side-on η2η2-coordination of AuX units to bis-(alkynyl)metal complexes has been extensively studied bythe group of H. Lang as summarized in a review article.61 Avariety of compounds with the structural unit [M](CtCR)2can act as “pincers” or “π-tweezers” with two -CtC-“tongues” to chelate an incoming coinage metal atom (42).In the majority of cases investigated, [M] is a titanocene unit,but other connectors can also be employed.

The coinage metal entities grabbed by the pincers rangefrom simple Cuþ and Agþ compounds to AuX units withX=Me (42a), CtC-tBu (42b), CtCSiMe3 (42c), and C6H2-2,4,6-(CF3)3 (42d). Compounds of this type were first ob-tained by the reaction of the tweezer complex Cp2Ti-(CtCSiMe3)2 with (Me2S)AuCl, which afforded [Cp2Ti-(CtCSiMe3)2(AuCtCSiMe3)] along with other productsgenerated in a ligand scrambling process (Cp = Me3Si-C5H4). A clean, high-yield reaction was observed betweenthe same tweezer163 and (Me2S)AuCtCSiMe3, which gaveMe2S as the only substitution byproduct. The synthesis ofthe other examples follows the same protocol.61

The crystal structures of the compounds 42b-d featureAu-C distances to the alkyne carbon atoms in the rangefrom 2.217 to 2.270 A and CtC bond lengths in the rangebetween 1.22(1) and 1.26(1) A, a lengthening of ca. 0.02 Afrom the values of 1.203(6) and 1.214(6) A in the free tweezer.The bite angletC-Ti-Ct of the tweezer is narrowed from102.8(2)� to an average of only 95.8� in the complexes,indicating that the alkynyl groups are actually drawn tothe Au center. In the IR spectra of the complexes, the

ν(CtC) vibrations are lowered to the range 1830-1896 cm-1

from 2012 cm-1 in the parent compound. As an addi-tional internal reference, the vibration of the alkynylgroups σ-bonded to gold in 42b,c appear as high as 2054 and2069 cm-1. In the 13CNMR spectra, the signals of the alkynylcarbon atoms are shifted downfield (R) or slightly upfield (β)upon complexation.59,163,164

Inquantum-chemical (EHandDFT) studies the bonding inthe coinage metal complexes of the type 42 has been investi-gated by different groups.60,165,166 Ti-Au interactions havebeen discussed on the basis of the short Ti- - -Au distance of2.995 A observed in the crystals.60 For the molecular skeletalgeometry of the model (C5H5)2Ti(CtCH)2 3AuR (R = Me,Ph, vinyl) the experimental values were used.60 The geometrywas optimized for the model Cl2Ti(CtCH)2 3AuMe, yield-ing a calculated CtC bond length of 1.249 A.166 It has beenpointed out that this considerable lengthening must notnecessarily mean stronger Au-(CtC) bonding as comparedto, for example, theCuandAganalogues, as the natures of theinteractions may differ considerably. The theoretically pre-dicted dissociation energy required for a separation of AuMefrom the ligand is in fact onlyDc = 17.3 kcal/mol, lower thanfor the Cu and Ag analogues (39.2 and 21.2 kcal/mol,respectively). As expected, the π-coordination in the com-plexes leads to bonds with highly polar character.60,166 Ther-mal decomposition of the compounds [(Cp)2Ti(CtCR)2] 3AuR0 gives gold metal and R0

2 (as the product of C-Ccoupling).61 The activation of the CtC bonds in the com-plexes indicated by bond lengthening and by spectral data hasnot yet been exploited for further transformations.

Attempted insertion of anAuþ cation between the two cis-alkynyl ligands of a square-planar bis(pyrazole)bis(phenyl-ethynyl)platinum(II) complex using [Ph3PNPPh3]

þ[Au-(acac)2]

- as the reagent gave a novel hexanuclear mixed-metal aggregate (acac = acetylacetonate).167 The four goldatoms of the Au4Pt2 unit are in three different environmentsin the ratio 2:1:1. Two of the gold atoms are σ-bonded to apyrazole N atom and π-bonded (η2) to an alkynyl group in aroughly trigonal planar array C2AuN, a third one isσ,σ-bonded to two pyrazole nitrogen atoms, and the fourthone is π,π-bonded (η2:η2) to two CtC bonds. In the latter,the two alkynes are not parallel but form an angle of ca. 50�(43). The Au-C distances are in the narrow range 2.174-(6)-2.225(7) A, indicating that the gold atoms are almostsymmetrically bonded to the CtC bonds in all cases. Theν(CtC) vibration is observed at 1948 cm-1, as compared to2115 and 2127 cm-1 for the parent platinum compound,

Chart 3. A0-F0

(160) McArdle, C. P.; Jennings,M. C.; Vittal, J. J.; Puddephatt, R. J.Chem. Eur. J. 2001, 7, 3572.(161) Puddephatt, R. J. Coord. Chem. Rev. 2001, 216, 313.(162) Mohr, F.; Eisler, D. J.; McArdle, C. P.; Christopher, P.; Atieh,

K.; Jennings, M. C.; Puddephatt, R. J. J. Organomet. Chem. 2003, 670,27.(163) Lang, H.; Seyferth, D. Z. Naturforsch. 1990, 45b, 212.

(164) Lang, H.; Herres, M.; Zsolnai, I. Organometallics 1993, 12,5008.

(165) Janssen,M.D.; K€ohler, K.;Herres,M.;Dedieu, A.; Smeets,W.J. J.; Grove, D. M.; Lang, H.; van Koten, G. J. Am. Chem. Soc. 1996,118, 4817.

(166) Kovacs, A.; Frenking, G. Organometallics 1999, 18, 887.(167) Fornies, J.; Fuertes, S.; Martin, A.; Sicilia, V.; Lalinde, E.;

Moreno, M. T. Chem. Eur. J. 2006, 12, 8253.


suggesting a considerable lowering of the bond order upon Aucoordination. According to NMR and IR studies, the structureof the complex is preserved in solution. The results of this workhave thus provided extremely valuable information on the dif-ferentmodes ofπ-coordination of gold(I) cations to alkynyls.167

Side-on η2-coordination of coinage metal cations Mþ

(M=Cu, Ag) to alkynylgold(I) compounds has been ob-served in 1,10-bis[diphenylphosphine(2-methylbut-1-en-3-ynyl)gold)]ferrocene, which can act as a “pincer-type” ligandwith two -CtC- “tongues” (44). So far, no example withM=Au has been prepared.168

However, related work with gold(I) alkynylcalix[4]crown-6 complexes was successful and provided the first exampleswhere two gold(I) cations become imbedded (and η2:η2-bonded) between two parallel CtCAuCtC units tetheredto the calixarene caps (45).169 In the treatment of 2mol of thecalixarene precursorwith two terminal-CtCHgroupswith4 equiv of (tht)AuCl andEt3N inCH2Cl2 the four gold atomsform not only two conventional -CtCAuCtC- units butalso two η2η2-Au bridges between these two almost parallelunits. This arrangement is further supported by aurophilicbonding between the four gold atoms, which are in arhomboidal arrangement (Au- - -Au distances 3.1344(8)and 3.2048(8) A). The side-on Au-C π-contacts (2 � η2)are in the range 2.150(9)-2.359(8) A and cause a lengtheningof the CtC bonds to 1.204(11) and 1.215(11) A. It isremarkable that the atoms of both CtC groups and theirconnecting gold atom form a unique planar C4Au unit. The

compounds with two different bridging units in the crownether part, which appear to increase the rigidity of thecalixarene, show strong UV absorption in CHCl3 solutionand intense emission in the visible region in solution as wellas in the solid state at 77K (glassmatrix or solid) and at roomtemperature (solution and solid).169 The aurophilic bondingbetween AuCtCPh functions clearly causes remarkablephotophysical effects.170,171

The complexes of the type 45 belong to the small group ofstructurally characterized compounds with Auþ cationsη2-bonded to bis(alkynyl)gold(I) anions. This type of coor-dination is much more common for the congeners Cuþ andAgþ. There is a rich literature on polynuclear aggregatesbuilt from these cations and [RCtCAuCtCR]- anions,published mainly by the group of O. M. Abu-Salah. Typicalexamples are shown in 46 and 47.172-180

(168) Yam, V. W.-W.; Cheung, K.-L.; Cheng, E. C.-C.; Zhu, N.;Cheumg, K.-K. Dalton Trans. 2003, 1830.(169) Yip, S.-K.; Cheng, E. C.-C.; Yam,V.W.-W.Angew.Chem., Int.

Ed. 2004, 43, 4954.

(170) Wong,K.M.-C.;Hui, C.-K.;Yu,K.-L.; Yam,V.W.-W.Coord.Chem. Rev. 2002, 229, 123.

(171) Yam, V. W.-W.; Yu, K.-L.; Womg, K. M.-C.; Cheung, K.-K.Organometallics 2001, 20, 721.

(172) Abu-Salah,O.M.; Al-Ohaly,A.-R.A.; Knobler, C. B. J. Chem.Soc., Chem. Commun. 1985, 1502.

(173) Abu-Salah, O. M.; Knobler, C. B. J. Organomet. Chem. 1986,302, C10.

(174) Abu-Salah, O. M.; Al-Ohaly, A. R.-A. J. Chem. Soc., DaltonTrans. 1988, 2297.

(175) Abu-Salah, O. M. J. Organomet. Chem. 1990, 387, 123.(176) Abu-Salah, O. M.; Al-Ohaly, A.-R. A.; Mutter, Z. F.

J. Organomet. Chem. 1990, 389, 427.(177) Abu-Salah, O. M.; Al-Ohaly, A.-R. A.; Mutter, Z. F.

J. Organomet. Chem. 1990, 391, 267.(178) Nast, R.; Schneller, P.; Hengefeld J. Organomet. Chem. 1981,

214, 273.(179) Abu-Salah, O. M.; Al-Ohaly, A.-R. A.; Al-Showiman, S. S.;

Al-Najjar, I. M. Transition Met. Chem. 1985, 10, 207.(180) Schuster, O.; Monkowius, U.; Schmidbaur, H.; Ray, R. Sh.;

Kr€uger, S.; R€osch, N. Organometallics 2006, 25, 1004.


Two main types of mixed-coinage-metal alkynyls havebeen presented The first is found in tetranuclear aggregatesof the formula [(R3P)Ag]2[Au(CtCR0)2]2 with a rhomboidalAg2Au2 core and both silver atoms σ-bonded to the tertiaryphosphine and π-bonded (η2) to two CtC bonds (46).173,175

In the example with R=R0 =Ph the CtC bond lengths arein the narrow range 1.203-1.236 A, and ν(CtC) appears at2075 cm-1 (25-30 cm-1 lower than in [PhCtCAuCtCPh]-

associated with “innocent” cations).178,179

Pentanuclear aggregates in this family have the formula Mþ-[Au3Ag2(CtCR)6]

- with M representing a noncoordinatingcounterion such as NEt4

þ etc. (47). In the complex anion, twoAgþ cations are imbedded between three parallel oriented linear[RCtCAuCtCR] units, where they become η2:η2:η2-coordi-nated to three CtC groups. The five metal atoms form a highlysymmetrical trigonal bipyramid (point group D3h) with shortAg- - -Au and Au- - -Au contacts clearly supported by metallo-philic bonding. The CtC distances are in the range 1.20-(2)-1.25(2) A.180 The analogous Cuþ complex with a Cu2Au3core172,174 shows the IR absorption for ν(CtC) at 2075 cm-1.

Silver(I) π-coordination to gold(I) alkynyls is also present intricationic complexes of the type [Au5Ag8(μ-dppm)4(CtCR)7-(1,2,3-C6R3)]

3þ, where dppm = Ph2PCH2PPh2 and R is asubstituent. The 1,2,3-trisubstituted arene unit was formed in acyclotrimerization of alkynyls induced by the metal atoms andpromoted by photochemical assistance. The silver atoms ex-hibit various mixed hapticities regarding the alkynyl groupsand extensive metallophilic bonding. Apart frommetallophilicinteractions, the gold atoms are only σ-bonded to alkynyl andaryl groups.181 In the preparation of these compounds from thecomponents [(dppm)2Au2]

2þ(SbF6-)2 and [AgCtCR]n in

CH2Cl2 green intermediates were observed which were lateridentified as clusters [Au6Ag13(dppm)3(CtCR)14]

5þ(SbF6-)5.

Each of the gold atoms is the center of a linear [RCtCAuCtCR]- anion, and six of these anions are in a roughlyparallel orientation grouped with D3h symmetry around acentered, trigonal-prismatic cluster of seven silver atoms andsurrounded by three dppm-linked pairs of silver atoms in theperiphery.Except for the central atom,all silver atomsentertainη2-contacts with CtC groups of the anions.182

Even larger aggregates of this type were recently preparedby I. O. Koshevoy et al. employing 1,4-diphosphinobenzeneor 4,40-diphosphinobiphenyl or -terphenyl ligands for themetal alkynyls (Cu, Ag, Au).183,184

4. (Alkyne)gold(III) Complexes. As already mentionedabove, early experiments with the simple alkynes RCtCRand gold(III) halides have led to partial reduction of gold(III)to gold(I) and yielded the corresponding cationic gold(I)complexes with AuX4

- counterions.27,28,136 There is renewedinterest in the complexation of alkynes by gold(III) com-pounds such as AuCl3, AuBr3, and HAuCl4, since it wasfound that several transformations of alkynes can be cata-lyzed by these reagents. To date, however, no complex inter-mediate, e.g. of the type (RCtCR)AuCl3 (2) could beisolated. Pertinent work byY. Fukuda andK.Utimoto aboutthe addition of methanol or water to alkynes catalyzed byaqueous Na[AuCl4]

185 was recently extended by the group ofM. Laguna by using organogold(III) complexes of the type[RAuX3]

- and [R2AuX2]- with suitable quaternary counter-

ions and R = aryl, pentahalophenyl and X = Cl, Br. Theseanions could be converted into compounds [RAuX2] and[R2AuX] using silver salts. In reaction mixtures of PhCtCHwith the latter, an 1H NMR signal at δ 3.76 ppm (in toluene,Δδ 1.34 ppm versus δ 2.42 for free PhCtCH) was tentativelyassigned to the species (C6F5)2AuCl[PhCtCH], but nofurther evidence for this intermediate is available.186

For a series of C-C coupling reactions and annulationsinvolving propargyl esters and related substrates, dichlo-rogold(III) 2-picolinate has been employed. In the proposedmechanism, gold(III) carbenoid intermediates may be morerelevant than any potentially preceding (alkyne)gold(III)complexes.187,188 Gold(III) complexes may also be the cata-lytically active species in cycloisomerizations of allenynes,because there is evidence for disproportionation of gold(I)catalysts which gives rise to precipitation of gold colloids.189

These and other examples suggest that the role of gold(III)catalysts in the reactions and transformations of both alkenesand alkynes is far less clear than that of gold(I) catalysts.

IV. (Arene)gold(I) Complexes

For a long time there was no direct evidence for π-complexa-tion of neutral aromatic hydrocarbons by gold(I) compoundsin the condensed phase.16,43-48 In contrast, there was a growingliterature on the corresponding silver(I) complexes, showing thatsilver salts form a variety of stable crystalline π-complexes withbenzene and related compounds in which the Agþ cation isattached to the arene predominantly in the η2 or η3 mode.76

1. (Arene)gold(I) Complexes. In the gas phase, cationic spe-cies of the type (arene)Auþ and (arene)2Au

þ havebeenobservedbymass spectrometry techniques, and their structures have beenoptimized by quantum-chemical calculations. The affinity of

(181) Wei, Q.-H.; Zhang, L.-Y.; Yin, G.-Q.; Shi, L.-X.; Chen, Z.-N.J. Am. Chem. Soc. 2004, 126, 9940.(182) Wei, Q.-H.; Zhang, L.-Y.; Yin, G.-Q.; Shi, L.-X.; Chen, Z.-N.

Organometallics 2005, 24, 3818.

(183) Koshevoy, I. O.; Koskinen, L.; Haukka, M.; Tunik, S. P.;Serdobintsev, P. Y.; Melnikov, A. S.; Pakkanen, T. A. Angew. Chem.,Int. Ed. 2008, 47, 3942.

(184) Koshevoy, I. O.; Karttunen, A. J.; Tunik, S. P.; Haukka, M.;Selivanov, S. I.; Melnikov, A. S.; Serdobintsev, P. Y.; Khodorkovskiy,M. A.; Pakkanen, T. A. Inorg. Chem. 2008, 47, 9478.

(185) Fukuda, Y.; Utimoto, K. J. Org. Chem. 1991, 56, 3729.(186) Casado, R.; Contel, M.; Laguna, M.; Romero, P.; Sanz, S.

J. Am. Chem. Soc. 2003, 125, 11925.(187) Shapiro, N. D.; Shi, Y.; Toste, F. D. J. Am. Chem. Soc. 2009, 131,

11654.(188) Shapiro, N. D.; Toste, F. D. J. Am. Chem. Soc. 2008, 130, 9244.(189) Lemiere, G.; Gandon, V.; Agenet, N.; Goddard, J.-P.; de

Kozak, A.; Aubert, C.; Fensterbank, L.; Malacria, M. Angew. Chem.,Int. Ed. 2006, 45, 7596.


Auþ for benzene is similar to that for ethylene, as shown bybracketing experiments and as calculated in the theoreti-cal studies (bond dissociation energies are 73 kcal/mol forC2H4- - -Au

þ and ca. 70 kcal/mol for C6H6- - -Auþ).106 The

calculated model structures (C6H6)Auþ and (C6H6)2Au

þ haveCs and C2h point group symmetry, respectively, with the metalatom η1-bonded to a ring carbon atom. For C6F6, coordinationto the fluorine atoms is of comparable energy.104-106,108,190-193

It should be remembered at this point that earlyworkby theschool of Nesmeyanov has shown that arylgold(I) complexesArAuL with Ar= Ph, Tol, etc. and L=R3P (Scheme 3; 48),on treatment with acid HY in the molar ratio 2:1, afford saltswith novel dinuclear cations [Ar(AuL)2]

þY- (49).194 Clearly,the [ArAu]þ unit generated upon dearylation of one of thecomplexes becomes attached to the second molecule at theipso carbon atom to form an aryl-bridged moiety.195 Noaddition as proposed in formula 50 has been observed. Thiscourse of the reaction has later been confirmed in other caseswith different groups Ar, and it has also been found forcyclopentadienyl compounds such as ferrocene (51).45,193

Several structures have later been determined,196-198 andthese experimental results taken together with those oftheoretical studies have suggested that aurophilic interac-

tions between the two gold atoms are responsible for theunexpected regioselectivity of the reactions: Clearly, attach-ment of the [LAu]þ electrophile at any of the ring carbonatoms (η1, η2, or other) other than the ipso carbon atom isinferior in energy.194 The results thus suggest that aurophilicbonding provides significant assistance to the ipso coordina-tion in the particular case of arylgold(I) complexes. The samephenomenon was observed with heterocyclic analogs, as e.g.furans and thiophenes (Scheme 4; 52a, E = O, S).196,199

Again [(R3P)Au]þ electrophiles become attached selectivelyat the ipso carbon atomandno η2-bonding to aCdCbondofthe heterocycle is formed (52b). Note that even the potentialO or S donor functions remain untouched (52c).

Recent preparative work with specially designed ligands hasfinally provided the long-sought stable arene complexes whichcould be subjected to detailed structure determinations. In thefirst group of compounds, weak side-on attachment of an areneto gold complexeswith the typical linear two-coordination of thegold atom has been observed. The group of Z.-Z. Zhang syn-thesized an anthracene-based ditertiary phosphine ligand inwhich the two diphenylphosphino groups are tethered to the9,10-positionsvian-propylaminomethyl spacers.Reactionof thisligandwith (Me2S)AuCl andAgClO4 inCH2Cl2 gave highyieldsof a cyclic compound with the Au atom positioned above thecenter of the anthracene unit (53). TheAu atom is in an approxi-mately linear coordination of the two P atoms (P-Au-P =172�) and is located 3.00 A above the ring with Au-C distancesin the narrow range 3.118-3.246 A. It is of course questionable ifthis positioning is due to any significant π-bonding of the metalatom to the ring system or is simply imposed by the ligandgeometry. (The corresponding silver compound has a similarmolecular geometry,while the copper analogue shows signsof anattraction to the ring in that the P-Cu-P axis is strongly bent to154�.) However, both quantum-chemical calculations and astudy of the photophysical properties (UV-vis absorption andemission) indicate significant arene-metal CT interactions.200

A more convincing example has been provided by usingthe related monodentate ligand shown as a component forcations 54. Crystal structure determinations have shownthat the neutral AuCl complex of this ligand has a confor-mation in which the gold atom has no contact with atoms ofthe anthracenyl group. In contrast, in the correspondingcations with acetonitrile or pyridine as auxiliary ligandsthe structures change consistently into a conformationbringing the gold atom above the central ring. The attach-ment is reduced, however, from an η6 (53) to an η2 contact(54; L=MeCN, Pyr) with distances Au-C2= 2.958 A andAu-C3 = 3.097 A (for the MeCN complex) and Au-C2=3.02 A and Au-C3 = 3.163 A (for the Pyr complex). Both

Scheme 3. 48-50

(190) Dargel, T. K.; Tertwig, R. H.; Koch, W. Mol. Phys. 1999, 96,583.(191) Schr€oder, D.; Brown, R.; Schwerdtfeger, P.; Schwarz, H. Int. J.

Mass Spectrom. 2000, 132, 73.(192) Diefenbach, M., Schwarz, H. In Electronic Encyclopedia of

Computational Chemistry; Schleyer, P. v. R., Ed.; Wiley: Chichester, U.K., 2004; pp 1-21.(193) Schr€oder, D.; Diefenbach, M.; Schwarz, H.; Schier, A.;

Schmidbaur, H. In Relativistic Effects in Heavy-Element Chemistryand Physics; Hess, B. A., Ed.; Wiley: Chichester, U.K., 2002; p 245.(194) Nesmeyanov, A. N.; Perevalova, E. G.; Grandberg, K. I.;

Lemenovskii, D. A.; Baukova, T. V.; Afanassova, O. B. J. Organomet.Chem. 1974, 65, 131.(195) Schmidbaur, H.; Inoguchi, Y. Chem. Ber. 1980, 113, 1646.(196) Review: Schmidbaur, H.; Porter, K. A. In Carbocation Chem-

istry; Olah, G. A., Prakash, G. K. S., Eds.; Wiley-Interscience: Hoboken, NJ,2004; p 291.(197) Us�on, R.; Laguna, A.; Fern�andez, E. J.; Mendia, A. S.; Jones,

P. G. J. Organomet. Chem. 1988, 350, 129.(198) Baukova,T.V.;Kuz’mina,L.G.;Oleinikova,N.A.; Lemenovskii,

D. A.; Blumenfeld, A. L. J. Organomet. Chem. 1997, 530, 27.

(199) Porter, K. A.; Schier, A.; Schmidbaur, H. Organometallics2003, 22, 4922.

(200) Xu, F.-B.; Li, Q.-S.; Wu, L.-Z.; Leng, X.-B.; Li, Z.-C.; Zeng,X.-S.; Chow, Y. L.; Zhang, Z.-Z. Organometallics 2003, 22, 633.


complexes show linearity of the P-Au-N units (179.6and 177.1�, respectively).201 The difference in hapticitybetween the cations shown in 53 (η6 with ligand con-straints) and 54 (η2 in the absence of ligand constraints)indicates that the latter is energetically preferred as alsoproposed in quantum-chemical calculation for (arene)-Agþ/Auþ complexes and amply observed for the silvercomplexes.76,202-206

Compounds of a similar type with sterically crowdedbiphenylylphosphine ligands were prepared by the group ofA. M. Echevarren (55).207 In this model system already theneutral AuCl complex molecule showed a conformation inthe solid state with the gold atom placed above the terminalphenyl group of the biphenylyl substituent. Not unexpect-edly then, this conformation was also confirmed for thecationic complex with an acetonitrile ligand (56). Thecontact (arene)- - -Au is shorter in the MeCN-complexedcation as compared to that in the neutral AuCl complex, in

which the terminal phenyl group appears to be bentaway from the metal atom, ruling out any stronger inter-actions. The Au-C distances therefore are in the range3.16-3.40 A for the cation and 3.15-3.83 A for the AuClcomplex.

It ismost interesting in the present context that the authors

finally observed that recrystallization of theMeCN complex

from toluene or p-xylene gave the corresponding 1:1 arene

complexes, in which the arenes are approximately η2-bondedto the gold atom (57 and 58, forR=Cy, tBu, respectively). It

is immediately obvious that in the products the bonding of

the gold atom to the toluene and p-xylene ligands is much

stronger with Au-C distances in the narrow range between

2.299(7) and 2.423(5) A. Fully consistent X-ray data are

available for four examples of this type, and only in one case

is a slight slippage of a toluene ring indicative of η1/η3-instead of η2-bonding. In this case, one of the Au-C

distances is as short as 2.263(5) A, while the other two are

still 2.535(6) and 2.689(6) A. The Au atom clearly follows a

roughly perpendicular approach to the toluene or p-xylene

molecular plane, but there is no clear preference regarding

the positioning relative to the methyl substituents of the

substrates. The Au-C distances to carbon atoms of the ter-

minal ring of the biphenylyl group are in the range 3.03-3.64

A and provide internal reference data for a comparison of the

two types of Au- - -(arene) interactions in these cations.

From this comparison it appears that the “perpendicular”

η2-bonding is representing the most effective type of inter-

action between [LAu]þ and (arene) components. In CD2Cl2solutions, there is fluxionality probably based on rapid

Scheme 4. 52a-c

(201) Li,Q.-S.;Wan,C.-Q.; Zou,R.-Y.;Xu,F.-B.; Song,H.-B.;Wan,X.-J.; Zhang, Z.-Z. Inorg. Chem. 2006, 45, 1888.(202) Munakata,M.;Wu, L. P.; Ning, G. L.Coord. Chem. Rev. 2000,

198, 171.(203) Lindeman, S. V.; Rathore, R.; Kochi, J. K. Inorg. Chem. 2000,

39, 5707.(204) Ogawa, K.; Kitagawa, T.; Ishida, S.; Komatsu, K. Organome-

tallics 2005, 24, 4842.(205) Schmidbaur, H.; Bublak, W.; Huber, B.; Reber, G.; M€uller, G.

Angew. Chem., Int. Ed. Engl. 1986, 25, 1089.(206) Schmidbaur, H.; Bublak, W.; Haenel, M. W.; Huber, B.;

M€uller, G. Z. Naturforsch. 1988, 43b, 702.(207) Herrero-Gomez, E.; Nieto-Oberhuber, C.; Lopez, S.; Benet-

Buchholz, J.; Echevarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 5455.


ligand exchange according to the results of variable-tem-perature 31P NMR studies.207

Similar results have finally been obtained with (carbene)-gold(I)(arene) complexes. The group of G. Bertrand was ableto isolate compounds of the type shown in 59 and to determinethe structure of a representative example.157,158,208 In theirwork a 1:1 complex of AuClwith a sterically demanding cyclic(alkyl)(amino)carbene ligand (CAAC) was obtained from(Me2S)AuCl and the CAAC in excellent yield and convertedinto the cationic toluene complex on reaction with the power-ful halophile [Et3Si(Tol)]

þ[B(C6F5)4]- with Tol = toluene.

The cation of the salt [(CAAC)Au(Tol)]þ[B(C6F5)4]- has the

gold atom quasi-η2-bonded to C2 and C3 of the toluenemolecule in much the same perpendicular approach as ob-served by the Echevarren group.207,208 The Au-C distancesare also very short at 2.320(3) and 2.234(3) A. This bondingobviously causes little perturbation of the aromatic ring, asexhibited by only slightly alternating ring C-C distances. Thecompound is perfectly stable both in the solid state and insolution, but the NMR data suggest that the hapticity of thetoluene molecule is fluxional at room temperature.208 Thecomplexes are highly active catalysts for various organictransformations of unsaturated compounds.158,208,209 A cata-lyst can also be prepared in situ by reacting (CAAC)AuClwithK[B(C6F5)4] in benzene.206 The reaction with phenylethyne inthe presence of an amine base gives the σ-alkynyl (CAAC)-AuCtCPh, probably via a π-bonded η2-intermediate aftersubstitution of the toluene ligand.208

Details of (Arene)gold(I) Coordination. Only a small num-ber of coordination motifs have been established in recentstudies (Chart 4; the labels correspond to those used inCharts 2 and 3). Only examples for type A00 with L = R3P,carbene (see 57-59) show short η2 contacts of the gold atomsand should be assigned significant (arene)-Au bonding. Incontrast, in examples of the types C00 and E00 (see 53-56) the

Au atom is at a rather long equilibrium distance, suggest-ing very weak interactions. Moreover, the L-Au-L andL-Au-X angles of these species deviate only slightly or notat all from linearity, which confirms the idea of very weakbonding, if any. In agreement with the results of theoreticalcalculations, (arene)-Au interactions are to be considered asthe weakest and least effective as compared to (alkene)-Auand (alkyne)-Au interactions.2. (Arene)gold(III) Complexes. Although many attempts

have been made to prepare complexes of aromatic hydro-carbons with gold(III) halides and other gold(III) com-pounds,45 to date no aggregates of these components couldbe prepared and characterized. Treatment of aromatichydrocarbons with anhydrous AuCl3 in a solvent leads to“auration” of the substrate: i.e. to an electrophilic substitu-tion with elimination of HCl, giving arylgold(III) com-pounds with σ-bonded aryl groups. No intermediates couldbe observed.210

Numerous organic transformations of aromatic com-pounds have recently been catalyzed successfully by additionnot only of suitable gold(I) complexes but also of simplegold(III) compounds. It is unknown if species with (arene)-gold(III) π-bonding play a role in the underlying mechan-isms. In many cases reduction of gold(III) to gold(I) occurs,and it is difficult to assign the catalytic action to a specificintermediate. This is generally true for gold(III) catalysis ofreactions of unsaturated hydrocarbons82-84,211-214 but maybe entirely different for complexes with robust gold(III)complexes using chelating ligands.215

V. Summary and Conclusions

Recent work in gold catalysis of chemical reactions hasbeen extremely successful. This is particularly true for trans-formations of unsaturated hydrocarbons and their deriva-tives with homogeneous gold catalysts. It has long beensuggested that π-complexation of CdC or CtC bonds atAuþ and Au3þ centers with an incomplete residual ligandsphere L is the crucial step of the activation of the substrates.However, only few species of this type could be directlyobserved or isolated for further investigation.Dedicated studies with new ligand systems L representing

in particular a tertiary phosphine, a carbene, a bipyridine, ora triazadienyl have finally given access to all fundamental

Chart 4. A00-F0 0

(208) Lavallo, V.; Frey, G. D.; Kousar, S.; Donnadieu, B.; Bertrand,G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 13569.(209) Zeng, X.; Frey, G. D.; Kousar, S.; Bertrand, G. Chem. Eur. J.

2009, 15, 3056.

(210) Kharasch,M. S.; Isbell, H. S. J. Am. Chem. Soc. 1931, 53, 3053.(211) Corma, A.; Gonzales-Arellano, C.; Iglesias, M.; Sanchez, F.

Angew. Chem., Int. Ed. 2007, 46, 7820.(212) Bardaji,M.; Laguna, A.; Jones, P. G.Organometallics 2001, 20,

3906.(213) Fukuda, Y.; Utimoto, K. Synthesis 1991, 975.(214) Lo, V. K.-Y.; Liu, Y.; Wong, M.-K.; Che, C.-M. Org. Lett.

2006, 8, 1529.(215) Soro, B.; Stoccoro, S.;Minghetti, G.; Zucca,A.; Cinellu,M.A.;

Gladiali, S.; Sansoni, M.; Manassero, M. Organometallics 2005, 24, 53.


types of neutral and cationic species and allowed a detailedinvestigation of the various modes of interaction whichinduce addition reactions and associated secondary trans-formations. Very consistently, upon complexation the CdCor CtC bonds of the substrates are lengthened significantly,and the configuration of the carbon atoms is changed fromtrigonal planar and linear toward pyramidal and bent,respectively. On the basis of structural criteria (Table 7),the interaction is strongest with alkynes, followed by alkenesand arenes. With alkynes, the critical Au-C distances arenear 2.10 A associated with lengthened CtC distances of ca.1.23 A. For alkenes, the approach is more variable withAu-Cdistances between 2.20 and 2.30 A and consequently alarger spread of CdC distances of ca. 1.40( 0.05 A. Finally,for arenes the Au-C distances are well beyond 2.30 A anddepend on the different hapticities (η1-η3), associated withintra-ring C-C distances at ca. 1.425 A. Not surprisingly,therefore, the deviation from linearity of the alkynes is moredramatic than that from planarity of the alkenes, whichsuggests that a description of the former as metallacyclopro-penes is more justified than that of the latter as metallacy-clopropanes (Chart 5; 60, 61).A comparison of the spectroscopic data leads to similar

conclusions: the stretching vibrations ν(CdC) and ν(CtC)are shifted considerably to longer wavelengths. The chemicalshifts δ(13C) and δ(1H) of the atoms concerned undergo largechanges, and the coupling constants 1J(CH) are reducedfrom the standard values for the free alkenes and alkynescorrelated with a reduction in s character of the orbitalsinvolved (sp2 f sp3, sp f sp2). No experimental data areavailable on the binding energies, but the results of theore-tical calculations for ethyne or propyne model cations

indicate that interaction energies of (alkene)- - -Auþ aresmaller or at best comparable to those of (alkyne)- - -Auþ

units (35-45 and 45-55 kcal/mol, respectively).Exchange of bound and free olefin in solution is facile; it

follows an associative mechanism and is associated with lowactivation energies (10-15 kcal/mol or even lower).Most of these data are not very different from those of

π-complexes of the neighboring elements of gold in theperiodic table (Pd, Pt, Ag), showing that gold is not reallyexceptional regarding its affinity toward unsaturated hydro-carbons. Nevertheless, it shows distinctly modified featuresof reactivity, mainly due to differences in coordinationnumbers and oxidation states, which inter alia have theirorigin in particularly strong relativistic effects. In its stan-dard environment, gold(I) has the lowest possible coordina-tion number which gives substrates ready access to the metalcenter. Typically, the specific electrophilic nature of LAuþ

units is governed by only one ligand L and thus can easily betuned by a suitable combination of steric and donor/acceptorproperties and by the choice of the counterion and thesolvent.The new wealth of information will be very important for

future efforts to design active and selective gold catalysts. Itshould be remembered that the development of this field ofgold catalysis was driven inter alia mainly by the renuncia-tion of classical mercury catalysis of addition reactions ofalkynes and by hopes of being able to cut down on the use ofplatinum, which, after all, is significantly more expensivethan gold. The lower coordination number of low-valentgold as compared to low-valent platinum or palladium andmost other metals offers greater opportunities for variationsin the ligand environment, leading to reaction selectivity.With a deeper knowledge of the fundamentals of goldπ-complexation gained in the last two decades, the prospectsof gold catalysis have become even brighter.

Acknowledgment. The work for this review was gene-rously supported by the Fonds der Chemischen Industrie,Frankfurt, Germany.

Table 7. Interatomic Distances (A) in the Hapticity Region of

(Alkene)-, (Alkyne)-, and (Arene)gold(I) Complexes

compd Au-C1 Au-C2 C1-C2 ref

5 2.15(2) 2.21(2) 1.38(2) 536 2.11(4) 2.14(4) 1.38(6) 5712 2.16(1) 2.20(1) 1.38(1) 10113a 2.096(6) 2.108(6) 1.381(10) 10213b 2.093(5) 2.096(5) 1.388(9)14 2.089(2) 2.098(2) 1.405(4) 10322 2.281(3) 2.299(3) 1.366(5) 12223 2.224(9) 2.350(8) 1.349(4) 12224a 2.098(5) 2.105(3) 1.488(6) 12624b 2.114(6) 2.118(2) 1.484(4) 12725a 2.199 2.285 1.331 12825b 2.239 2.230 1.34625c 2.224 2.248 1.37434 2.271(5) 2.267(4) 1.371(7) 139

2.267(4) 2.263(4) 1.351(7)2.269(4) 2.272(4) 1.369(7)

7 2.075(9) 2.055(9) 1.259(12) 628 2.050(7) 2.100(8) 1.244(11) 6235a 2.152(4) 2.172(5) 1.224(6) 14836 2.069(4) 2.071(4) 1.233(7) 14839 2.197(5) 2.270(5) 1.221(8) 12057 2.299(5) 2.423(5) 204

2.263(5) 2.535(6)/2.689(6)58 2.338(7) 2.341(7) 204

2.300(4) 2.354(4)2.308(4) 2.370(4)

59 2.320(3) 2.234(3) 1.425(5) 205

Chart 5. 60, 61

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Gold η 2 -Coordination to Unsaturated and Aromatic Hydrocarbons: The Key Step in Gold-Catalyzed Organic Transformations