Ping-Pong at Gold: Proton Jump Between Coordinated Phenyl and η1-Benzene Ligands,A Computational Study

Manik Kumer Ghosh,† Mats Tilset,‡ Ajay Venugopal,† Richard H. Heyn,† and Ole Swang*,†

Department of Hydrocarbon Process Chemistry, SINTEF Materials and Chemistry, P.O. Box 124 Blindern,N-0314 Oslo, Norway and Centre for Theoretical and Computational Chemistry, Department of Chemistry,UniVersity of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway

ReceiVed: May 4, 2010; ReVised Manuscript ReceiVed: June 22, 2010

A DFT computational investigation predicts that the Au(III) complex (bpy)Au(C6H5)2+ reacts with benzeneto furnish square planar (bpy)Au(C6H5)(η1-C6H6)2+. Intramolecular processes that occur within this specieshave been located, and the energetics of all processes have been quantified. The dynamic processes that havebeen identified are (1) benzene ring rotation with respect to Au, (2) direct hydrogen transfer from the benzeneto the phenyl ligand, (3) hydrogen transfer from the ipso to the ortho positions in the coordinated benzeneligand, and (4) hydrogen transfer from the benzenium ligand formed by the ipso/ortho isomerization to thephenyl ligand. Similarities and differences are seen between the behavior of (bpy)Au(C6H5)(η1-C6H6)2+ andpreviously reported isoelectronic Pt(II) complexes. Preliminary experimental results related to this chemistryare reported, and possible consequences for C-H bond activation mediated by gold are discussed.

I. Introduction

Direct catalytic functionalization of methane and othersaturated hydrocarbons is an important problem of modernchemistry.1-3 Since early work by Garnett and Shilov establishedthat aqueous Pt(II) salts are capable of activating C-H bondsof arenes and alkanes,4-6 the quest to understand and utilizedirect C-H activation reactions has been the focus of intenseresearch effort, as evidenced by numerous publications.2,7-10 Themechanism of C-H activation and related reactions at Ptrelevant to the Shilov chemistry has been reviewed,10-12 andliterature results strongly suggest that the reaction often startswith hydrocarbon ligation followed by oxidative addition of aC-H bond at the Pt(II) center. The reaction has been thoroughlyinvestigated by computational methods.13-21

Au(III) is isoelectronic with Pt(II) and possesses the samed8 electronic configuration, and square planar Au(III) andPt(II) chloride complexes have many similar characteristics.22

A notable exception is the inability of Au(III) to undergooxidative addition, as Au(V) is all but unknown, whereasoxidative addition to Pt(II) is a well-known route to Pt(IV)complexes. The activation of C-H bonds by gold compoundsis a poorly explored area of metal complex catalysis. Therehave been only sparse theoretical publications dealing withthe reactivity of gold complexes toward alkanes.23,24 To ourknowledge, alkane or arene functionalization mediated bygold complexes under mild conditions has so far not beendemonstrated experimentally. Selective oxidation of methaneto methanol catalyzed by homogeneous cationic gold has beenobserved,25 but then under rather forcing conditions withfuming sulfuric acid as the solvent and selenic acid as anoxidizing agent, at temperatures near 200 °C.

The mechanisms of activation of aromatic hydrocarbons by(N-N)Pt(II) complexes (where N-N is a neutral diimine ligand)

have been explored in detail by the Tilset26-30 and Bercaw26,31,32

groups. C-H activation at (N-N)Pt(CH3)(L)+ complexes appearsto proceed by initial η2-coordination of the arene at Pt by associativesubstitution of the weakly coordinated ligand L, i.e., the “naked”3-coordinate species (N-N)Pt(CH3)+ appears not to be involved.Once the hydrocarbon is coordinated, C-H activation may occurby oxidative addition to furnish Pt(IV) hydridoaryl complexes.Quite recently, Tilset and co-workers28-30 described the low-temperature protonation chemistry of (N-N)PtPh2 complexes anddemonstrated that Pt(II) (N-N)Pt(C6H5)(η2-C6H6)+ and Pt(IV)(N-N)Pt(C6H5)2(NCMe)H+ complexes can be generated in dichlo-romethane and dichloromethane/acetonitrile mixtures, respectively(Scheme 1). Both species release benzene in the presence ofacetonitrile, the former by an associative mechanism and the latterby a dissociative mechanism. Sun and co-workers investigated thesame systems computationally using density functional theorycalculations21 in an effort to gain insight into and understandingof the factors that affect every step of the C-H activation process.In particular, they proposed that the exchange of a proton betweenthe benzene and the phenyl ligands in (N-N)Pt(C6H5)(η2-C6H6)+,

* To whom correspondence should be addressed. E-mail: Ole.Swang@sintef.no.

† SINTEF Materials and Chemistry.‡ University of Oslo.

SCHEME 1: Relevant Pt Chemistrya

a See text for details.

J. Phys. Chem. A 2010, 114, 8135–8141 8135

10.1021/jp1040508 2010 American Chemical SocietyPublished on Web 07/15/2010

observed by 1H NMR spectroscopy by Tilset and co-workers,28

occurred by an in-plane migration of a proton between the twoligands rather by oxidative addition. This conclusion has recentlybeen supported by experimental evidence.29 Thus, C-H bondcleavage is mediated by the Pt metal center but without theinvolvement of Pt(IV) intermediates.

Considering the background presented above, the notedsimilarities between Pt(II) and Au(III) and the current intensefocus on the application of gold compounds in catalysis thequestion arises: Can appropriately tailored gold complexes, inanalogy with platinum complexes, activate C-H bonds inalkanes or arenes under mild conditions?

In this contribution, we present an extensive theoretical studyof the potential-energy surface of possible reaction mechanismsthat may be involved in a putative benzene C-H cleavageprocess at (N-N)Au(C6H5)(C6H6)2+, which is isoelectronic withthe well-studied (N-N)Pt(C6H5)(C6H6)+ moiety. Modest at-tempts at experimental observation of the theoretically predictedreactions are also described.

II. Computational and Experimental Details

The potential-energy surface (PES) for the reacting systemswas explored using the Gaussian03 program system.33 Theequilibrium and transition-state structures along the reactionpathways were computed using gradient-corrected densityfunctional theory (DFT) with the Becke three-parameter ex-change functional34 and the Lee-Yang-Parr correlation func-tional35 (B3LYP). For gold, a multielectron-adjusted, quasire-lativistic effective core potential (ECP) covering 60 electrons([Kr]4d104f14) and an (8s7p6d)/[6s5p3d]-GTO valence basis set(31111, 22111, 411, 21) were used.36 To further improve thequality of the basis set, f-type polarization functions for goldwere optimized by choosing three exponents in an even-tempered fashion, i.e., separated by factors of �5. CCSD(T)calculations were carried out for the gold atom in its doublet Sground state, and the energy was minimized by varying thef-type exponents, keeping the ratios between them constant.Hence, only one parameter was optimized. Finally, the threeoptimized primitive functions were contracted to two using a(21) scheme, taking the contraction coefficients from the atomiccalculation. Exponents and coefficients are available as Sup-porting Information. All nonmetal atoms (C, H, and N) weredescribed with the Dunning correlation-consistent cc-pVDZ andcc-pVTZ basis sets.37-39 Geometries of all stationary points werecalculated at the B3LYP/cc-pVDZ level. To validate the DFTresults for relative energies of stationary points, single-pointenergy calculations were performed with the cc-pVDZ and cc-pVTZ basis sets in both B3LYP and second order Moller-PlessetPerturbation theory (MP2).40 Hence, reaction and activationenergies for all elementary reaction steps have been predictedby using four model chemistries MCn (n ) 1-4; note that theECP and extended valence basis set described above were usedfor gold in all model chemistries): MC1, B3LYP/cc-pVDZ//B3LYP/cc-pVDZ; MC2, B3LYP/cc-pVTZ//B3LYP/cc-pVDZ;MC3, MP2/cc-pVDZ//B3LYP/cc-pVDZ; MC4, MP2/cc-PVTZ//B3LYP/cc-pVDZ. The model to the left of the double slash isthe one at which the energy was computed, and the model tothe right of the double slash is the one at which the moleculargeometry was optimized. We note that the results vary littleupon increasing basis set size from cc-pVDZ to cc-pVTZ bothfor DFT and MP2 energies, indicating that the chosen basissets are of sufficient quality.

Minimum energy reaction paths were determined by firstoptimizing the geometries of the energy minima and transition

states. To follow the minimum energy path (MEP), also calledintrinsic reaction coordinate (IRC), the Gonzalez-Schlegelsecond-order method41 was used. To characterize all stationarypoints, the Hessian (matrix of energy second derivatives) wascalculated and diagonalized at each stationary point, which alsoyielded zero-point energy (ZPE) corrections.

IRC calculations were performed to examine the reaction pathleading down from a transition structure in both directions onthe PES to ensure that the transition states connected the desiredreactants and products. Further, the normal modes correspondingto the imaginary frequencies of the transition-state structureswere examined with molecular visualization to verify that thenuclear motion tends to deform the transition-state structurealong the pertinent reaction coordinate. To model the Au(III)moiety, all systems have a net charge of +2.

Experimental Procedures. All procedures were conductedusing Schlenk techniques, under argon and with dried anddegassed solvents. [(bpy)AuCl2]NO3 was prepared accordingto a literature procedure.42

Synthesis of [(bpy)AuCl(Ph)]NO3. Solid HgPh2 (0.355 g, 1mmol) was slowly added to a suspension of [(bpy)AuCl2]NO3

(0.486 g, 1.00 mmol) in 25 mL of methanol at 0 °C and stirredfor 2 h at this temperature. The reaction mixture turned violet,and a small quantity of metallic gold was formed. The reactionmixture was not stable beyond 2 h at 0 °C or above. The reactionmixture was filtered at 0 °C to obtain a colorless solution, andthe solvent was removed under vacuum. The residue was washedwith 3 × 10 mL of ice-cold dichloromethane and extracted with2 × 25 mL of ice-cold methanol. Storage of this MeOH solutionat -45 °C for 24 h provided an off-white crystalline precipitateof [(bpy)Au(Ph)Cl]NO3 (0.256 g, 0.50 mmol, 50% yield). 1HNMR (300 MHz, CD3OD) δ 9.27 (d, 1 H, bpy-H1), 8.83 (t, 2H, bpy-H4), 8.55 (q, 2 H, bpy-H3), 8.12 (t, 1 H, bpy-H2), 8.01(d, 1 H, bpy-H1), 7.83 (t, 1 H, bpy-H2), 7.46-7.32 (m, 5 H,C6H5), TOF MS ES: m/z 465.1 [M+], 410.1, 254.0, 157.4.

III. Results and Discussion

The essential reaction that has been subjected to scrutiny involvesthe reaction of (bpy)Au(C6H5)2+ with benzene. As the benzeneapproaches the Au moiety, coordination occurs to give an Au(III)η1-benzene complex (bpy)Au(C6H5)(η1-C6H6)2+. Computationssuggest that this initial product exhibits considerable dynamicbehavior. The dynamic processes include (i) rotation of the benzenering with respect to Au, (ii) endo/exo isomerization of thecoordinated benzene with respect to the Au moiety, and (iii) protonexchange between the benzene and phenyl ligands, which servesto interchange the two ligands. The PES for these processes havebeen explored in detail, starting from the initial interaction of(bpy)Au(C6H5)2+ with benzene. The behavior is quite analogousto the process observed experimentally and investigated compu-tationally for the (diimine)Pt(C6H5)(η2-C6H6)+ complexes but withsome notable differences that will be discussed.

The relative energies of the ground-state and transition-statestructures that have been located are presented in Table 1 atdifferent levels of theory, while all reaction paths that have been

TABLE 1: Relative Energies (kcal/mol) of the Reactants,Transition States, and Products Using Different Levels ofTheory As Described in Section II

model I1 I2 I3, I3′ TS1 TS2 TS3 TS4 TS5 TS6

MC1 0.0 -1.3 4.1 2.8 1.1 5.8 9.4 15.0 15.3MC2 0.0 -1.2 4.6 2.5 1.1 5.3 9.2 14.6 14.8MC3 0.0 -4.4 7.9 2.0 -3.0 4.1 8.9 12.0 13.1MC4 0.0 -4.8 7.5 1.8 -3.6 3.8 7.8 11.1 12.8

8136 J. Phys. Chem. A, Vol. 114, No. 31, 2010 Ghosh et al.

located, with energies of all stationary points, are plotted inFigure 1. The structures of all optimized stationary points arepresented in Chart 1, with the pertinent geometrical parametersprovided in Table 2. Energies given in the following arecomputed using the MC4 model described above, ZPE correc-tions added.

A. π-Benzene Complex Formation and Its DynamicBehavior. When benzene approaches and binds to (bpy)Au-(C6H5)2+, our calculations suggest that two possible π-benzenecomplexes may be formed, which differ in the relative orienta-tion of the phenyl and coordinated benzene groups (Scheme2). I1 and I2 are the two different η1-(C)-coordinated conformerswhere the incoming benzene molecule is facing the bipyridineor phenyl ligands, termed exo and endo conformations, respec-tively (see also Chart 1). Note that in the exo conformation,the proton at the benzene ipso carbon points toward the phenylligand. The bond distances of Au1-N2, Au1-N3, Au1-C4,and Au1-C5 are almost identical for I1 and I2. Conformer I2lies 4.8 kcal/mol lower in energy than I1, presumably for stericreasons, and therefore, the endo coordination mode of benzenerepresents the absolute ground state of the complex. The relativepopulation of the two conformers is expected to be temperaturedependent, and both may be eligible starting points for thermalchemical transformations.

It is interesting that the π-benzene species are calculated tobe η1-(C) coordinated at these dicationic Au(III) species. Theisoelectronic, monocationic (N-N)Pt(CH3)(C6H6)+ complexes,on the other hand, are believed to be η2-(C,C) coordinated. NMRexperiments suggested that these also exist as endo and exoisomers in a dynamic equilibrium,28 a notion that was recentlysupported by DFT calculations; the endo conformation here wasmore stable than the exo one by 2.8 kcal/mol.21

The (bpy)Au(C6H5)2+ moiety in I1 and I2 is able to migratewith respect to the face of the coordinated benzene ring whileretaining its endo and exo coordination modes. Thus, from theexo complex I1 one passes through transition-state structure TS1

in which benzene is η2-bonded to the Au center before arrivalat a new η1-coordination mode at the neighboring C atom; thebarrier height for this ring rotation is a mere 1.8 kcal/mol.Similarly, the benzene ring in the endo complex I2 undergoesan analogous rotation with respect to the Au center throughtransition state TS2, located 1.2 kcal/mol above I2. From thelow calculated activation energies, it may be inferred that theserotation processes will proceed freely even below room tem-perature. In both transition states, the coordinated CdC bondsare oriented almost perpendicularly to the coordination planeof gold, and the gold atom is equidistant to the coordinatingcarbon atoms (C4 and C6) of the benzene. The perpendicularorientation was also calculated for the Pt-arene complexes21

and is of course well established43 for the structure of the anionCl3Pt(C2H4)- of the classic Zeise’s salt and numerous relatedmolecules. These transition-state structures are similar to thosefor the η2-(C,C) benzene complexes reported for Pt but whichrepresent energy minima; η1-(C) structures are transition statesin the platinum case! For comparison, in the Pt system, thebarrier for the ring rotation process connecting η2-benzenespecies was 1.6 kcal/mol for the endo conformer.21 Our failureto computationally observe stable η2-complexes of benzene atAu may be attributed to the instability of Au in coordinationnumbers larger than 4. It is also possible that these dicationicspecies are in need of extra (when compared to analogousmonocationic Pt(II)) charge stabilization in the η1-structure,which has as a resonance structure an Au+-bound benzeniumcation monocation.

Starting from the ground-state endo structure I2, it is alsopossible to initiate a rotation of the η1-coordinated benzenearound the Au-C bond axis; this process serves to transformthe endo structure I2 to the exo one I1. The transition stateTS3 for this process was located at 8.6 kcal/mol above I2. Thus,the Au-(η1-C6H6) system is highly fluxional: Only very lowbarriers have to be traversed in order to convert endo and exocoordination modes as well as for the ring rotation of coordi-nated benzene rings in both conformers. The activation barrierfor the transition from endo to exo in the Pt system was 7.4kcal/mol.44

B. Intramolecular Proton Transfer between CoordinatedBenzene and Phenyl C-H Activation Paths. η2-Arene com-plexes are commonly considered to be key intermediates inaromatic C-H activation reactions,45-54 although exceptionswhere η2-coordination prior to C-H activation is not involvedhave been reported.55,56 As already mentioned, in the presentinvestigation energy minima corresponding to η2-benzenecomplexes could not be located computationally. It will beshown in the following that the η1-benzene Au(III) complexesare key intermediates for intramolecular C-H bond cleavagereactions of coordinated benzene, thus playing a similar role asthe isoelectronic η2-benzene complexes of Pt(II). These η1-benzene complexes may be viewed as analogues of Whelandintermediates in electrophilic aromatic substitution. The C-Hbond cleavage reactions under consideration are intramolecularproton transfers between the benzene and phenyl moieties, aprocess that serves to interchange the two ligands.

Computationally, we find two different reaction paths thatconstitute this type of C-H activation. The exo conformer I1is the starting point for both, as this is the only one in which

Figure 1. Potential-energy surfaces (PES) in terms of MP2 energies(kcal/mol) for the different paths of gold-mediated biphenyl C-Hactivation and π-complex reactions.

Ping-Pong at Gold J. Phys. Chem. A, Vol. 114, No. 31, 2010 8137

the migrating proton of the coordinated benzene has theappropriate orientation for migration to occur to the adjacentphenyl ring. Therefore, there is a need for the absolute ground-state conformer endo I2 to reorient to exo I1 by the rotationprocess via TS3 before the proton can migrate.

In the first reaction path, the ipso C-H bond of benzene isactivated, as this proton is smoothly transferred in the coordina-tion plane of the molecule to the adjacent phenyl group, whichin turn becomes a coordinated benzene molecule (Scheme. 3).The process resembles a ping-pong ball being played back andforth between two rackets. This proton transfer is reminiscentof the experimentally observed proton exchange observed in

(diimine)Pt(C6H5)(η2-C6H6)+ complexes,28-30 except that thestarting (and ending) benzene rings here are η1-coordinated.The migrating proton undergoes an ipso/ipso′ shift relative tothe points of attachment of the phenyl groups at Au. Thetransition state TS4 is perfectly Cs symmetric: The gold atomis equidistant to the nitrogen atoms (N2 and N3) of the bpyligand and to the nearest carbon atoms (C4 and C5) of the twophenyl groups. The migrating proton, H8, is also equidistant tothe donor and acceptor carbon atoms (C4 and C5) of the phenylgroups. The Au1-H8 distance in TS4 is calculated to be 2.09Å. This value may be compared to the sum of the van der Waalsradii, 2.86 Å, the 1.70 Å Au-H bond distance in a binuclear

CHART 1: Optimized Stationary States

8138 J. Phys. Chem. A, Vol. 114, No. 31, 2010 Ghosh et al.

complex reported by Escalle et al.,57 and the 1.706 Å distancereported by Schwerdfeger et al.58 for the linear Au(III) dihydrideAuH2

-. These data suggest that interaction with the Au centermediates the proton transfer in our system. It could be notedthat this mechanism closely resembles the so-called σ-CAMmechanism, as reviewed by Perutz and Sabo-Etienne.59 Thereaction barrier for this path is calculated to be 7.8 kcal/molwith respect to the exo structure I1 that is responsible for themigration; this corresponds to 12.6 kcal/mol with respect to theground-state minimum I2. The corresponding reaction barrierin the Pt systems was 14.7 kcal/mol with respect to the endoground-state geometry and 11.9 kcal/mol with respect to theexo conformer from which the migration occurs.21

In the second reaction path, a proton transfer first occurs fromthe ipso to the ortho positions of the coordinated benzene ligandof exo conformer I1, furnishing the isomer I3 (Scheme. 4).Thereafter, the same proton undergoes transfer to the ortho′

position of the other phenyl group. The starting point of thisreaction path, I1, is connected to the isomer I3 through thetransition state TS5 for the 1,2-shift of H8 from C4 to C6, withan activation barrier of 11.1 kcal/mol. The ortho/ortho′ transferof the proton to the adjacent phenyl ring then occurs via asymmetrical transition state TS6, 5.3 kcal/mol higher in energythan I3, 12.8 kcal/mol higher than the starting exo conformerI1. Proton exchange via the ipso/ipso′ (TS4) and the ortho/ortho′(TS6) pathways therefore differs by 5.0 kcal/mol for the highest-lying barriers. The energy difference is not very large, and thedistribution between the two paths will depend strongly ontemperature. The I3 isomer is 7.5 kcal/mol less stable than I1and is definitely not present to any significant extent atexperimentally relevant temperatures. The structure of I3resembles a Pt-substituted benzenium ion. In the ipso/ortho TS5and ortho/ortho′ TS6 exchange transition states, the migratingproton H8 has no discernible interaction with gold (R(Au-H8) )2.94 Å). Thus, the ipso/ipso′ proton exchange pathway appearsto be favored overall, and this is the one mediated by the Aucenter, at least as far as we solely consider activation energies.The ping-pong process leads to a larger change in the shape ofthe molecule than the ortho-ortho process. Hence, one mightspeculate that solvent interactions could lead to a smallerpreexponential factor for the latter. This suggests interestingvenues for new experimental work; any conclusions here woulddepend on the availability of new experimental data.

The present results clearly suggest that benzene should reactwith (bpy)Au(C6H5)2+ under mild conditions. The reaction pathssummarized in Figure 1 show that the η1-benzene ligand caneasily undergo ring rotation relative to the Au center, passingthrough η2-(C,C)-coordinated transition states with energybarriers of less than 2 kcal/mol. I1 is the key intermediate ofall reaction paths including C-H activation. Benzenium speciesmay form at sufficiently high temperatures; even at subambienttemperatures, our results predict scrambling of all aromaticprotons.

Attempts at experimental observation of the ping-pongphenomenon were unsuccessful, but some results are given inthe Appendix.

IV. Conclusions

We performed quantum chemical simulations by means ofDFT and ab initio calculations of intramolecular reactions inthe complex (bpy)Au(C6H5)(η1-C6H6)2+. The following reactionshave been computationally characterized with respect to reactionand activation energies: (1) benzene ring rotation, (2) directproton transfer from the benzene to phenyl ligand, (3) protontransfer from ipso to ortho positions in the benzene ligand, and(4) proton transfer from the benzenium ligand formed by theipso/ortho isomerization to the phenyl ligand.

The computations provide activation energies that are lowenough to suggest that the processes described should beobservable at subambient temperatures. In an attempt to observe

TABLE 2: Selected Geometrical Parameters of OptimizedStructures of Different Stationary States

parameters I1 I2 I3 I3′ TS1 TS2 TS3 TS4 TS5 TS6

Au1-N2 2.12 2.12 2.14 2.19 2.12 2.11 2.14 2.14 2.12 2.15Au1-N3 2.21 2.21 2.19 2.14 2.22 2.21 2.14 2.14 2.18 2.15Au1-C4 2.24 2.22 2.03 2.05 2.06 2.05 2.10 2.10 2.05 2.04Au1-C5 2.05 2.05 2.05 2.03 2.46 2.42 2.10 2.10 2.05 2.04Au1-C6 2.98 2.82 3.06 2.98 2.46 2.42 3.04 3.04 3.08 3.00Au1-H8 2.51 2.51 3.23 3.23 2.72 2.68 2.09 2.09 2.71 2.94C4-C5 1.44 1.45 1.48 1.40 1.43 1.44 1.42 1.42 1.45 1.44C4-H8 1.10 1.10 2.10 2.81 1.09 1.09 1.40 1.40 1.29 2.05C5-H8 2.22 3.80 1.12 2.10 2.58 3.96 1.40 1.40 2.40 2.05C6-H8 2.15 2.18 2.81 2.75 2.19 2.20 2.27 2.27 1.36 1.45C7-H8 2.81 4.69 2.75 1.12 3.61 5.02 2.27 2.27 2.97 1.45N2-Au1-N3 77.3 77.9 77.2 77.2 77.4 78.2 77.7 77.7 77.5 77.5C4-Au1-C5 83.2 91.1 83.9 83.9 83.2 93.4 77.7 77.7 85.1 84.3

SCHEME 2

SCHEME 3

SCHEME 4

SCHEME 5

Ping-Pong at Gold J. Phys. Chem. A, Vol. 114, No. 31, 2010 8139

these proton transfer mechanisms experimentally, [(bpy)Au-Cl(Ph)]NO3 was synthesized as a potential precursor to I1. Thethermal instability of this compound prevented, however,experimental confirmation of this mechanism. The ping-pongmechanism (reaction 2 above) will be the subject of furtherinvestigations by computational and experimental means.

Acknowledgment. The authors thank the GASSMAKSprogram of the Research Council of Norway for economicsupport through project no. 185513/I30. A generous grant ofcomputing time from the NOTUR programme (http://www.notur.no, account no. NN2174K) is gratefully acknowledged.

Appendix: Experimental Attempts To Observe thePing-Pong Phenomenon

In an attempt to observe the modeled proton “ping-pong”mechanism experimentally, we targeted the synthesis of either[(bpy)Au(Ph)(L)]2+ (L ) weakly coordinating ligand) or [(bpy)-AuPh2]+, each of which could thereafter potentially provide[(bpy)Au(C6H5)(η2-C6H6)]2+ by displacement of L by benzeneor by protonation, respectively. A potential precursor to bothof these targeted compounds is [(bpy)Au(Ph)Cl]+ (1+), whichcan be synthesized from [(bpy)AuCl2]+ (2+) and 1 equiv ofHgPh2 in methanol at 0 °C. The synthetic route to 1+ and theanticipated routes to the precursors of [(bpy)Au(C6H5)(η2-C6H6)]2+ are shown in Scheme 5. The spectroscopic data for[1+]NO3 are consistent with the proposed formulation; inparticular, 1H NMR spectroscopy clearly shows six distinctsignals for the bpy ligand, a result of the asymmetry generatedby substitution of one Cl- ligand with a phenyl group. Analternative synthesis of [1+] starting from in-situ-generated[PhAuCl2]2 and bpy, analogous to the known reaction of[PhAuCl2]2 with monodentate amines,60 only gave minisculequantities of product. Reproduction of the synthesis of [1+]NO3

is not straightforward, since the isolated salt is thermallysensitive, decomposing rapidly to metallic gold at T > 0 °C,while both MeOH and MeCN solutions of [1+]NO3 begin todecompose to metallic Au within 15 min at room temperature.Reaction of [2+] with 2 equiv of HgPh2 shows incompletereaction of the Au starting material, together with the presenceof biphenyl, which probably arises via a reductive eliminationpathway from the presumed product [(bpy)AuPh2]+. Purificationof [1+]NO3 is also complicated by the presence of biphenyl,which suggests that either the monophenylation of [2+] isnonselective (providing some [(bpy)AuPh2]+) or [1+] has somepropensity toward disproportionation. Reaction of impuresamples of [1+]NO3 with 2 equiv of either AgBF4 or AgOTfalso failed to give characterizable products of the dication[(bpy)AuPh(L)]+ (L ) NCMe, BF4

-, or TfO-), even though1H NMR spectroscopy suggested the presence of asymmetryof the coordinated bpy ligand and/or coordinated MeCN in somereactions. Difficulties in removing coproduced Ag+ salts and/or thermal instabilities prevented further isolation of pure Audications.

Supporting Information Available: Cartesian coordinatesfor all stationary states and exponents and coefficients for theF-type polarization functions on gold. This material is availablefree of charge via the Internet at http://pubs.acs.org.

References and Notes

(1) Shilov, A. E. Metal Complexes in Biomimetic Chemical Reactions;CRC Press: Boca Raton, FL, 1997.

(2) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879–2932.

(3) Conley, B. L.; Tenn, W. J., III; Young, K. J. H.; Ganesh, S. K.;Meier, S. K.; Ziatdinov, V. R.; Mironov, O.; Oxgaard, J.; Gonzales, J.;Goddard, W. A., III; Periana, R. A. J. Mol. Catal. A: Chem. 2006, 251,8–23.

(4) Garnett, J. L.; Hodges, R. J. J. Am. Chem. Soc. 1967, 89, 4546–4547.

(5) Gol’dshleger, N. F.; Shteinman, A. A.; Shilov, A. E.; Eskova, V. V.Zh. Fiz. Khim. 1972, 46, 1353–1354.

(6) Gol’dshleger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A.Zh. Fiz. Khim. 1969, 43, 2174–2175.

(7) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H.Acc. Chem. Res. 1995, 28, 154–162.

(8) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437–2450.(9) Jones, W. D. Science 2000, 287, 1942–1943.

(10) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471–2526.(11) Heyduk, A. F.; Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. In

ActiVation and Functionalization of C-H Bonds; Goldberg, K. I., Goldman,A. S., Eds.; American Chemical Society: Washington, D.C., 2004; pp 250-263.

(12) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259–320.(13) Siegbahn, P. E. M.; Crabtree, R. H. J. Am. Chem. Soc. 1996, 118,

4442–4450.(14) Hill, G. S.; Puddephatt, R. J. Organometallics 1998, 17, 1478–

1486.(15) Heiberg, H.; Swang, O.; Ryan, O. B.; Gropen, O. J. Phys. Chem.

A 1999, 103, 10004–10008.(16) Mylvaganam, K.; Bacskay, G. B.; Hush, N. S. J. Am. Chem. Soc.

2000, 122, 2041–2052.(17) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. J. Am. Chem. Soc.

2000, 122, 1456–1465.(18) Bartlett, K. L.; Goldberg, K. I.; Borden, W. T. Organometallics

2001, 20, 2669–2678.(19) Gilbert, T. M.; Hristov, I.; Ziegler, T. Organometallics 2001, 20,

1183–1189.(20) Iron, M. A.; Lo, H. C.; Martin, J. M. L.; Keinan, E. J. Am. Chem.

Soc. 2002, 124, 7041–7054.(21) Li, J.-L.; Geng, C.-Y.; Huang, X.-R.; Zhang, X.; Sun, C.-C.

Organometallics 2007, 26, 2203–2210.(22) Mason, W. R., III; Gray, H. B. J. Am. Chem. Soc. 1968, 90,

5721–9.(23) Pichugina, D. A.; Shestakov, A. F.; Kuz’menko, N. E. Russ. Chem.

Bull. 2006, 55, 195–206.(24) Pichugina, D. A.; Shestakov, A. F. Kinet. Catal. 2007, 48, 305–

315.(25) Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen,

R. J.; Oxgaard, J.; Goddard, W. A., III Angew. Chem., Int. Ed. 2004, 43,4626–4629.

(26) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am.Chem. Soc. 2000, 122, 10846–10855.

(27) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem.Soc. 2001, 123, 6579–6590.

(28) Wik, B. J.; Lersch, M.; Krivokapic, A.; Tilset, M. J. Am. Chem.Soc. 2006, 128, 2682–2696.

(29) Parmene, J.; Ivanovic-Burmazovic, I.; Tilset, M.; van Eldik, R.Inorg. Chem. 2009, 48, 9092–9103.

(30) Parmene, J.; Krivokapic, A.; Tilset, M. Eur. J. Inorg. Chem. 2010,1381-1394.

(31) Procelewska, J.; Zahl, A.; van Eldik, R.; Zhong, H. A.; Labinger,J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 2808–2810.

(32) Zhong, H. A.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.2002, 124, 1378–1399.

(33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.;Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Rev.C.02; Gaussian Inc.: Wallingford, CT, 2004.

(34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.(35) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B: Condens. Matter 1988,

37, 785–9.(36) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor.

Chim. Acta 1990, 77, 123–141.

8140 J. Phys. Chem. A, Vol. 114, No. 31, 2010 Ghosh et al.

(37) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007–23.(38) Kendall, R. A.; Dunning, T. H., Jr.; Harrison, R. J. J. Chem. Phys.

1992, 96, 6796–806.(39) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358–

71.(40) Moller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618–22.(41) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523–7.(42) Ivanov, M. A.; Puzyk, M. V.; Balashev, K. P. Russ. J. Gen. Chem.

2003, 73, 1821–1822.(43) Love, L. A.; Koetzle, T. F.; Williams, G. J. B.; Andrews, L. C.;

Bau, R. Inorg. Chem. 1975, 14, 2653.(44) Cho, S.-H.; Whang, D.; Kim, K. Bull. Korean Chem. Soc. 1991,

12, 107–109.(45) Chin, R. M.; Dong, L.; Duckett, S. B.; Jones, W. D. Organome-

tallics 1992, 11, 871–876.(46) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones,

W. D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685–7695.(47) Cronin, L.; Higgitt, C. L.; Perutz, R. N. Organometallics 2000,

19, 672–683.(48) Iverson, C. N.; Lachicotte, R. J.; Muller, C.; Jones, W. D.

Organometallics 2002, 21, 5320–5333.(49) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G. J. Am.

Chem. Soc. 2003, 125, 1403–1420.

(50) Sweet, J. R.; Graham, W. A. G. J. Am. Chem. Soc. 1983, 105,305–306.

(51) Sweet, J. R.; Graham, W. A. G. Organometallics 1983, 2, 135–140.

(52) Jones, W. D.; Dong, L. J. Am. Chem. Soc. 1989, 111, 8722–8723.(53) Cordone, R.; Taube, H. J. Am. Chem. Soc. 1987, 109, 8101–8102.(54) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1986, 108, 4814–

4819.(55) Vigalok, A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.; Martin,

J. M. L.; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539–12544.(56) Peterson, T. H.; Golden, J. T.; Bergman, R. G. J. Am. Chem. Soc.

2001, 123, 455–462.(57) Escalle, A.; Mora, G.; Gagosz, F.; Mezailles, N.; Le Goff, X. F.;

Jean, Y.; Le Floch, P. Inorg. Chem. 2009, 48, 8415–8422.(58) Schwerdtfeger, P.; Boyd, P. D. W.; Burrell, A. K.; Robinson, W. T.;

Taylor, M. J. Inorg. Chem. 1990, 29, 3593–607.(59) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,

2578–2592.(60) Fuchita, Y.; Utsunomiya, Y.; Yasutake, M. J. Chem. Soc., Dalton

Trans. 2001, 2330–2334.

JP1040508

Ping-Pong at Gold J. Phys. Chem. A, Vol. 114, No. 31, 2010 8141