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Valence structures of the diastereomeric complexes meso- andrac-[Ru2(acac)4(l-Q)]n (n = 2-, 1-, 0, 1+, 2+) with the multiple quinonoidbridging ligand Q = 1,2,4,5-tetraimino-3,6-diketocyclohexane†

Doyel Kumbhakar,a Biprajit Sarkar,b Amit Das,a Atanu Kumar Das,b Shaikh M. Mobin,a Jan Fiedler,c

Wolfgang Kaim*b and Goutam Kumar Lahiri*a

Received 6th April 2009, Accepted 15th July 2009First published as an Advance Article on the web 13th August 2009DOI: 10.1039/b906900c

Meso- and rac-configurated diastereoisomers [Ru2(acac)4(m-Q)] have been separated and identified asRuII–Q0 species through a crystal structure analysis of the meso form. The presence of two redox-active{Ru(acac)2} groups (acac- = 2,4-pentanedionate) and quinonoid Q with two equivalent p-conjugateda-diimine chelate sites and one p-quinone function allowed for the full cyclic voltammetric andspectroelectrochemical (UV-vis-NIR, IR, EPR) characterisation of the five accessible states (2-, 1-, 0,1+ and 2+ forms) for both isomers. Oxidation occurs at the metal ions to produce RuIIRuIII

mixed-valent states [Ru2(acac)4(m-Q)]+ (K c ª 104.5) with corresponding EPR features but withoutdetectable intervalence absorption in the near infrared region. IR-spectroelectrochemistry revealsopposite frequency shifts for the n(C=O) and n(NH) stretching vibrations on reduction and oxidation,in agreement with the assumed electronic structure. Reduction leads to strongly stabilised[Ru2(acac)4(m-Q)]- states (K c ª 1011) which show weak NIR shoulders around 1040 nm. The EPRcharacteristics are remarkably different for the two isomeric monoanions, reflecting presumably flexiblegeometry and electronic structure. The observation of broad but detectable EPR resonance at roomtemperature in solution and the g factor anisotropy in the glassy frozen state at 110 K suggest a ratherevenly metal–ligand mixed singly occupied MO. Together with the ZINDO calculations and the partialexperimental results reported previously by Masui et al. (Inorg. Chem., 2000, 39, 141) for[Ru2(bpy)4(m-Q)]n+ (n = 2, 3, 4), the characteristic differences provide an insight into the electronicfeatures such as mixed valency manifestations and the variable extent of mixing of the metal–quinonefrontier orbitals of these systems involving RuII-stabilised Q which is unknown as a free ligand.

Introduction

Apart from their many roles in biochemistry,1 analytical2 andsynthetic chemistry,3 the quinones have also become widely usedas non-innocent ligands in the coordination chemistry of thetransition metals and of main group elements.4 Both o- andp-quinones as well as their monoimino and diimino derivativeshave been employed.5,6 In addition to the characterisation ofnew compounds, recently the focus has been on establishing thevalence and spin situation, especially of the mixed-valent and/orradical odd-electron intermediates,7 and on providing systems forpossible applications in molecular electronics (e.g. through valencetautomerism).8

aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,Mumbai-400076, India. E-mail: lahiri@chem.iitb.ac.inbInstitut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring55, D-70550 Stuttgart, Germany. E-mail: kaim@iac.uni-stuttgart.decJ. Heyrovsky Institute of Physical Chemistry, v.v.i., Academy of Sciences ofthe Czech Republic, Dolejskova 3, CZ-18223 Prague, Czech Republic† Electronic supplementary information (ESI) available: Mass (Fig. S1),IR (Fig. S2) and 1H NMR (Fig. S3) spectra of 1 and 2, selected crystal-lographic parameters (Table S1) and bond distances/angles for 1·2H2O(Table S2). CCDC reference number 726828. For ESI and crystallographicdata in CIF or other electronic format see DOI: 10.1039/b906900c

An unusual multifunctional quinone compound, 1,2,4,5-tetraimino-3,6-diketocyclohexane (Q) which is unknown in itsfree form, has been reported as a coordinated bridging lig-and in the product [Ru2(bpy)4(m-Q)]n (n = 4+, bpy = 2,2¢-bipyridine)9 from the reaction of [Ru(bpy)2(MeOH)2]2+ with1,2,4,5-tetraaminobenzene in the presence of NEt3 and O2. Char-acterisation of the meso diastereoisomer by structure analysis,low-temperature EPR, UV-vis-NIR spectroelectrochemistry in thereduced forms (n = 3+ and 2+) and by ZINDO calculationswas described. Using the redox-active {Ru(acac)2} moiety witha considerably more electron-rich metal4i,4l than in {Ru(bpy)2

2+}we have obtained full access to the more extended redox series[Ru2(acac)4(m-Q)]n (n = 2-, 1-, 0, 1+, 2+) in both separateddiastereoisomeric forms, meso and rac. With a crystal structureanalysis confirming the identity of the materials, the species wereinvestigated using EPR in fluid and frozen solution as well asUV-vis-NIR and IR-spectroelectrochemistry in order to establishunequivocally the nature of the individual redox forms and of thefrontier molecular orbitals involved. Given the complex electrontransfer situation within ruthenium complexes of quinonoidligands in general,4c,d,j,l,m,9 the formation of radical complexesas well as of mixed-valence configurations could be expected.Compound Q, unknown as a free molecule, is particularly attrac-tive as a ligand because it involves two equivalent a-diimine-type

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coordination sites and a p-quinone function in p-conjugation.o-Benzoquinone diimines are particularly strong p-back bondingchelate ligands (especially for ruthenium(II)4l) while p-quinonesoffer rather low-lying p* orbitals for reduction.6a,7c,d Althoughformally related to the hexakis(oxocarbon) system cyclo-[(CO)6] ofoxidised rhodizonate [(CO)6]2-, a well known ligand,10 the presenceof 1,2,4,5-positioned imine sites in conjunction with the affinityof ruthenium for a-diimine chelates raises several questions as tothe valence character of various experimentally accessible redoxstates.

Results and discussion

Synthesis, isomer separation, and identification

The diastereomeric complexes 1 (meso, DK) and 2 (rac, (DD/KK))11 (Scheme 1) have been isolated from the reaction of [RuII-(acac)2(CH3CN)2] with 1,2,4,5-tetraaminobenzene in ethanol inthe presence of NEt3 as a base under aerobic conditions, followedby chromatographic separation using a neutral alumina column.

Scheme 1

The synthesis of the reported analogous bpy complex[Ru2(bpy)4(m-Q)]4+ involves a multi-step procedure involving theinitial reaction under anaerobic conditions, followed by thesubsequent reaction in the presence of an external flow ofO2. Moreover, purification of the initially isolated [Ru2(bpy)4(m-Q)](PF6)4 salt requires repeated reprecipitation by acetone–HClto the corresponding chloride salt.9 In contrast, the synthesis ofthe diastereomeric mixture of 1 and 2 is a single-step reactionunder aerobic conditions, and, more importantly, a simple aluminacolumn not only purifies the mixture of diastereomers but alsofacilitates the isolation of the isomers in their pure forms. It shouldbe noted that the purging of O2 into the refluxing reaction mixturedoes not alter the rate of the reaction or the yield of the products.

The electrically neutral and diamagnetic diastereomeric com-plexes, 1 and 2 exhibit satisfactory microanalytical data (seeExperimental section). The complexes exhibit molecular ion peaksat 764.09 and 764.21 corresponding to 1+ and 2+, respectively(calculated mass, 764.02; Fig. S1, ESI†). The n(C=O) frequenciesof the bridging ligands Q in 1 and 2 lie at 1633 and 1626 cm-1;other bands at lower energies (Fig. S2, ESI†) involve C=NH of Qand C=O of acac-.

The two isolated diastereoisomers (1 and 2) exhibit rathersimilar 1H NMR spectra (Fig. S3, ESI†) suggesting little struc-tural difference. The most pronounced effect, a 0.04 ppm shiftdifference, has been observed for the NH resonances (Fig. S3,ESI†). The spin paired Ru(II) state (t2g

6) and the oxidised quinone

form of Q in 1 and 2 result in one NH(Q), one CH(acac) andtwo CH3(acac) signals in the conventional range of d , 2–13 ppm(see Experimental section, Fig. S3†), corresponding to one fourthof the whole molecule due to internal symmetry. Partial overlapof NMR signals in the obtained mixtures of isomers has beenreported for the tetrakis(bipyridine) compound.9

Differentiation and identification of the meso (1) and rac (2)isomers were possible on the basis of the crystal structure (TableS1, ESI†) of one isomer (Fig. 1). In spite of the relatively poorcrystal quality, the diffraction data confirmed without doubtthat the meso isomer could be crystallised. The approximatestructure data of the Ru(m-Q)Ru core (Fig. 1, Table S2, ESI†)in meso-[Ru2(acac)4(m-Q)] are rather similar to those obtained formeso-[Ru2(bpy)4(m-Q)](ClO4)4 ¥ 4 H2O,9 suggesting the presenceof unreduced Q and, by implication, of ruthenium(II) albeitwith significant p-back bonding interaction. This correspondencedespite the charge difference as caused by the acac- versus bpyco-ligand variation12 signifies an inherent stability of the dinuclear{RuII(m-Q0)RuII} arrangement, as suggested already by the facileformation and by the robustness of both compounds.

Fig. 1 ORTEP diagram of 1. Ellipsoids are drawn at 50% probability. Forclarity one molecule of 1 in the asymmetric unit is shown.

Cyclic and differential pulse voltammetry

Both the meso (1) and rac (2) isomers exhibit rather similarelectrochemical behaviour. Two one-electron oxidation and twoone-electron reduction processes were observed (Fig. 2, Table 1).Significant cathodic shifts of redox potentials have taken place onmoving from [Ru2(bpy)4(m-Q)](ClO4)4

9 to 1 or 2 which are causedby the stronger donor and weaker acceptor effect from acac-

versus bpy. As an important consequence we could investigate theproperties of the oxidised forms [Ru2(acac)4(m-Q)]n (n = 1+, 2+)whereas the corresponding species of the bpy co-ligated complexwere not further studied.9

In addition to the general potential shift, the compropor-tionation constants K c (RT lnK c = nFDE)13 of the odd-electronintermediates (n = 1- and 1+) are noteworthy. While the valuesof K c for the monocationic forms are moderate at about 104.5 andagree with the estimated 105 reported for the bpy system,9 thecomproportionation constants for the monoanions are remark-ably high at about 1011. These values strongly exceed the K c = 106

which can be estimated according to RT lnK c = nFDE13 from the

9646 | Dalton Trans., 2009, 9645–9652 This journal is © The Royal Society of Chemistry 2009

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Table 1 Redox potentialsa ,band comproportionation constants for 1 and 2

E◦298/V (DEp/mV) oxidation E◦

298/V (DEp/mV) reduction

Compound Couple I Couple II K c1c Couple III Couple IV K c2

c

1 (meso) 0.93(116) 0.65(107) 104.7 -0.59(105) -1.24(120) 1011.0

2 (rac) 0.90(108) 0.64(124) 104.4 -0.63(124) -1.29(130) 1011.2

a Potentials E◦298/V (DEp/mV) versus SCE (see Fig. 3). b In CH2Cl2/0.1 mol dm-3 Et4NClO4; scan rate 100 mV s-1. c RT lnK c = nF(DE) for T = 298 K.

Fig. 2 Cyclic voltammograms ( ) and differential pulse voltammograms( ◊ ◊ ◊ ) of (a) 1 and (b) 2 in CH2Cl2/0.1 mol dm-3 [Et4N][ClO4] at 100 mV s-1.

data given by Masui et al.9 An interpretation for this enhancedstabilisation is provided later, together with the discussion of EPRand spectroelectrochemical results.

EPR spectroscopy

The four odd-electron intermediates, [Ru2(acac)4(m-Q)]n (n = 1-and 1+) in their respective meso (1) and rac (2) forms, couldbe studied by X-band EPR spectroscopy following intra muroselectrolysis of the neutral precursors in CH2Cl2/0.1 mol dm-3

Bu4NPF6. Representative spectra are shown in Fig. 3 and 4, Table 2summarises the data.

Fig. 3 EPR spectra of (a) 1- and (b) 1+ in CH2Cl2/0.1 mol dm-3 Bu4NPF6

at 110 K; microwave frequencies of 9.4745 (a) and 9.4729 GHz (b),respectively. Spike at 3400 G (g ª 2.00) is due to an organic decompositionproduct.

Oxidation of both diastereoisomers produces EPR signals onlyat low temperatures in frozen solution with g component splittingsg1 - g3 = Dg ª 0.6 and gav ª 2.19, typical of ruthenium(III) in the low-spin 4d5 configuration.4i The one-electron nature of the process

Table 2 EPR parameters of complex ions [Ru2(acac)4(m-Q)]n from intramuros electrolysis in CH2Cl2/0.1 mol dm-3 Bu4NPF6

n = 1+ n = 1-

1 (meso) 2 (rac) 1 (meso) 2 (rac)

298 Kgiso n.o. n.o. 2.032 2.031DHpp/mT — — 34 10.5

110 Kg1 2.46 2.47 2.190 2.131g2 2.24 2.21 2.043 2.037g3 1.90 1.86 1.860 1.907gav 2.20 2.18 2.031 2.025Dg = g1 - g3 0.56 0.61 0.330 0.223

Fig. 4 EPR spectra of (a) 1- and (b) 2- in CH2Cl2/0.1 mol dm-3 Bu4NPF6

at 298 K; microwave frequencies of 9.4745 GHz (a) and 9.4847 GHz (b),respectively.

suggests the formation of mixed-valent RuIIIRuII species (K c =104.5), although an intervalence absorption could not be detectedin the near IR (cf. below). There is little difference between the meso(1) and rac (2) isomers, and corresponding species [Ru2(bpy)4(m-Q)]5+ were not reported.9

One-electron reduction of [Ru2(acac)4(m-Q)] yields significantlydifferent EPR responses from the two monoanionic diastereoiso-mers. At room temperature in fluid solution rac-[Ru2(acac)4(m-Q)]-

(2-) shows a strong broad signal while the still very much broaderfeature from the meso isomer (1-) is relatively weak (Fig. 4). Thisdifference is observed again in the low-temperature spectra wherethe meso (1) isomer exhibits significantly larger g anisotropy thanthe rac (2) analogue (Fig. 3, Table 2). Obviously, the meso (1)isomer experiences a higher degree of metal contribution to thesingly occupied MO which translates to higher Dg, gav and DHpp

(Table 3) through the high spin–orbit coupling factor of the 4delement ruthenium.6a

Taken together, the EPR results for the [Ru2(acac)4(m-Q)]-

species correspond to a situation with approximately evenly mixed

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Table 3 UV-vis-NIR data for 1n and 2n in various oxidation states fromOTTLE spectroelectrochemistry in CH2Cl2/0.1 mol dm-3 Bu4NPF6

Compound l/nm (e/dm3 mol-1cm-1)

12+ 940 sh, 703 (24 600, ª 4190a), 281 (19 400), 230 (22 000)1+ 667 (22 900, ª 4340a), 423 (7700), 300 sh, 269 (19 900), 230

(20 800)1 544 (41 600, ª 1740a), 429 (9500), 342 (9300), 270 (21 100),

225 (15 500)1- 1035 (2200), 760 sh, 628 (50 100, ª 1900a), 495 (7900), 370

(9900), 272 (30 100), 235 (18 400)12- 695 (25 000, ª 4050a), 645 sh, 429 (8800), 272 (34 300), 235

(19 900)22+ 920 sh, 698 (23 800, ª 4150a), 279 (19 900), 228 (23 200)2+ 673 (22 600, ª 4850a), 422 (7500), 340 sh, 295 sh, 269

(20 600), 228 (22 600)2 544 (55 000, ª 1640a), 429 (12 800), 342 (12 200), 270

(28 900), 227 (22 300)2- 1043 (2300), 775 sh, 628 (53 600, ª 1860a), 495 (9500), 370

(11 900), 273 (34 700), 228 (22 600)22- 705 (33 200, ª 3720a), 635 sh, 450 (11 500), 273 (41 600),

229 (25 200)

a Bandwidth at half height in cm-1.

ligand and metal contributions to the singly occupied MO. Trulyruthenium(II)–radical compounds have typically Dg < 0.01 andgav < 2.005 whereas a mainly ruthenium-centred spin would leadto Dg > 0.2 and gav > 2.10.4i Reduction of the precursor couldlead to RuII(m-Q

∑-)RuII or to RuIII(m-Q2-)RuII (the latter followingintramolecular electron transfer); the RuII(m-Q0)RuI alternative isunlikely although there are mixed-valent intermediates involvingorganometallic ruthenium(I).14

For the related [Ru2(bpy)4(m-Q)]3+ Masui et al.9 have reported a“ligand-centred EPR signal” at g = 1.994, “confirming bridgingligand localisation of the unpaired electron”. In the following,however, they argue that “the broadness and low g value of theEPR signal indicate significant delocalisation of the unpairedelectron over the ruthenium atoms, consistent with ZINDOcalculations” which suggest about 20% metal participation atthe singly occupied MO (SOMO). Apparently, the replacementof bpy by acac- enhances this metal contribution in accordancewith the better stabilisation of ruthenium(III) by the acac- donorligand.15 Nevertheless, a complete spin accommodation by themetal centres does not take place, disfavoured perhaps by theparticular electronic structure of the dianion Q2- (Scheme 2);the unusually high K c values of ca. 1011 for these paramagneticintermediates suggest strong electronic coupling in a mixed-valentdescription or extensive metal–ligand mixing with resulting extrastabilisation.

The pronounced difference in the EPR response of the di-astereoisomeric monoanions is also tentatively attributed to the

particular electronic structure of reduced Q (see Scheme 2). Thissuggests considerable structural flexibility due to the differentcanonical forms of the resonance structures existing in the complexwhich may thus result in differences of forms and energies of theorbitals concerned.

Related unusual effects for potentially quinonoid 1,2,4,5-tetra-modified benzene ligands have been reported for N,N¢,N¢¢,N¢¢¢-tetraorganyl 2,5-diamino-1,4-benzoquinonediimines5c,16 includingazophenine;17,18 the latter showing coordination-induced p-to-o-diiminobenzoquinone restructuring.4k,18

IR-spectroelectrochemistry

The response of isolated N–H and C=O stretching vibra-tions of 1/2 after electron transfer has been monitored byIR-spectroelectrochemistry in CH3CN/0.1 mol dm-3 Bu4NPF6

(Fig. 5). The n(C=O) band at 1630/1637 cm-1 of 1/2 shifts to1669/1671 cm-1 on oxidation and to 1561/1563 cm-1 on reduction.The ca. 50% smaller shift for the former already suggests apredominantly metal-centred process in contrast to the metal–ligand mixed reduction (see discussion above). Unfortunately, theposition of the carbonyl functions at the symmetry axis precludesan investigation of valence (de)localisation by carbonyl vibrationalspectroscopy.19

Fig. 5 IR spectroelectrochemistry of the conversion of 2 → 2+ and 2 →2- in CH3CN/0.1 mol dm-3 Bu4NPF6.

In contrast, the n(NH) bands shift from 3271/3274 cm-1 of 1/2to a lower value of 3247/3248 cm-1 (broad band) on oxidationbut to higher energy (3291/3290 cm-1) on reduction. While thebroadness of the band for the mixed-valent cations may reflectinsufficient valence averaging (class II/III borderline behaviour)on the vibrational time scale,19 the opposite shifts for n(C=O) areseen as a result of the increasing competition between the H and

Scheme 2

9648 | Dalton Trans., 2009, 9645–9652 This journal is © The Royal Society of Chemistry 2009

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Ru electrophiles for the electron density on N on going from thereduced to the oxidised state.

The IR-spectroelectrochemistry in aprotic CH3CN (Fig. 5)confirms that the ligand (Q) based reduction is not associatedwith protonation.

UV-Vis-NIR-spectroelectrochemistry

The accessibility of the two oxidised forms of [Ru2(acac)4(m-Q)]allowed us to study the series of five states, (2-), (1-), 0, (1+), (2+),by spectroelectrochemistry (Fig. 6 and 7, and Table 3).

Fig. 6 OTTLE spectroelectrochemistry for 1n in CH2Cl2/0.1 mol dm-3

Bu4NPF6.

As noted similarly by Masui et al.9 for [Ru2(bpy)4(m-Q)](ClO4)4

and as is well known for RuII complexes of o-diiminoquinones ingeneral,4l the RuII(Q0)RuII precursors meso- or rac-[Ru2(acac)4(m-Q)] are distinguished by intense narrow (Dn1/2 = 1740 cm-1 and

Fig. 7 OTTLE spectroelectrochemistry for 2n in CH2Cl2/0.1 mol dm-3

Bu4NPF6.

1640 cm-1 for 1 and 2, respectively) metal-to-ligand charge transfer(MLCT) bands in the visible, here at lmax = 544 nm. The smallbandwidth indicates that this transition occurs between largelymixed states, most probably involving dp(Ru) and p*(Q) orbitals.On one-electron oxidation, this band is replaced by a broader(Dn1/2 = 4340 cm-1 and 4850 cm-1 for 1 and 2, respectively) butstill intense absorption around 667–673 nm which persists onsecond oxidation and is tentatively assigned to LMCT absorptionsinvolving the acac- ligands and RuIII.7a,d Remarkably, no longer-wavelength bands could be detected for the monocation in the nearinfrared where IVCT transitions are expected for mixed-valentintermediates.20 While RuIIIRuII mixed-valency with very weak oreven without detectable near infrared absorptions is known,21,22

the absence of such a feature is unexpected in the present casewith both moderate comproportionation constants and standardEPR response. The lack of a suitable orbital of the mediator could

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diminish the overlap and thus the IVCT band intensity; like inthe previously observed trinuclear mixed-valent cases withoutNIR absorption and hexaazatrinaphthylene bridges there is aparticular electronic “meta pattern” situation present which doesnot allow for straightforward p-conjugation.21,22 A shift of theIVCT transition into the visible or the mid-IR is unsupported bythe spectroscopic studies and seems unlikely as it would implyunusually strong or very weak metal–metal coupling.

Reduction to the metal–ligand-mixed monoanionic states[Ru2(acac)4(m-Q)]- results in a bathochromic shift of the intensenarrow “MLCT” band to lmax = 628 nm (Dn1/2 = 1900 cm-1

and 1860 cm-1 for 1 and 2, respectively) and in the emergence ofsome weak absorptivity until about 1200 nm in the near infrared.This observation suggests that the monoanions are not classicalmetal-based mixed-valent intermediates but strongly metal–ligandorbital-mixed species, as suggested by EPR. Qualitatively similarresults were reported for the [Ru2(bpy)4(m-Q)]3+ analogue despitethe different EPR response and apparently higher ligand characterof the SOMO.9 The assignments of transitions can be adopted,therefore, from the theoretical considerations of Masui et al.,9

including the two-electron reduced form which shows two majorabsorptions in the visible, attributed to internal transitions of thebridging ligand.

Conclusion

It appears from the studies of both [Ru2(bpy)4(m-Q)]2+/3+/4+ asreported earlier,9 and from the present results on [Ru2(acac)4(m-Q)]n (n = -2, 1-, 0, 1+, 2+) that two ruthenium(II) centrescan stabilise the probably very strong, and in the free form stillunknown, multiple quinonoid p acceptor, 1,2,4,5-tetraimino-3,6-diketocyclohexane Q through back donation, without causinga full reduction to a particular configuration (Scheme 2). Two-step reversible oxidation of diastereomeric 1 and 2 is facilitatedhere relative to the Ru(bpy)2 analogues9 by the electron rich{Ru(acac)2} moieties showing largely metal-centred oxidationwithout revealing a detectable IVCT band in the NIR region forthe moderately coupled (K c ª 104.5) mixed-valent intermediates.Reduction to the strongly stabilised (K c ª 1011) monoanionsleads to intermediates meso- and rac-[Ru2(acac)4(m-Q)]- withdistinctly different metal–ligand mixed electron spin distribution.It is thus believed that back donating but not fully electrontransferring metal complex fragments can also serve to generateother interesting p acceptor configurations which do not have tobe stable in the uncomplexed form.

Experimental

Materials

The precursor complex [Ru(acac)2(CH3CN)2] was preparedaccording to the reported procedure.23 The ligand precur-sor 1,2,4,5-tetraaminobenzene tetrahydrochloride was purchasedfrom Aldrich. Other chemicals and solvents were of reagent gradeand used as received.

Physical measurements

UV-vis-NIR spectroelectrochemical studies were performed inCH2Cl2/0.1 mol dm-3 Bu4NPF6 at 298 K using an optically trans-

parent thin layer electrode (OTTLE) cell24 mounted in the samplecompartment of a J&M TIDAS spectrophotometer. 1H NMRspectra were obtained with a 400 MHz Varian FT spectrometer.The EPR measurements were made in a two-electrode capillarytube25 with a X-band (9.5 GHz) Bruker system ESP300 spectrom-eter using the built-in software. Standard conditions were 0.4 mTmodulation amplitude, 6.34 mW microwave power, and a substrateconcentration of 0.5 mmol dm-3. The microwave frequencies areprovided in the figure captions. Cyclic voltammetric, differentialpulse voltammetric and coulometric measurements were carriedout using a PAR model 273A electrochemistry system. Platinumwire working, auxiliary electrodes and an aqueous saturatedcalomel reference electrode (SCE) were used in a three-electrodeconfiguration. The supporting electrolyte was 0.1 mol dm-3

Et4NClO4 and the solute concentration was ca. 10-3 mol dm-3.The half-wave potential E◦

298 was set equal to 0.5(Epa + Epc),where Epa and Epc are anodic and cathodic cyclic voltammetricpeak potentials, respectively. Elemental analyses were carried outwith a Perkin-Elmer 240C elemental analyser. Electrospray massspectra were recorded on a Micromass Q-ToF mass spectrometer.

Preparation of complexes 1 and 2

[{(acac)2RuII}2(l-Q)] (1 and 2). 0.2 mL (1.44 mmol) of NEt3

was added to a 5 mL ethanolic solution of the ligand 1,2,4,5-tetraaminobenzene tetrahydrochloride (37 mg, 0.13 mmol), lead-ing to the formation of a pink suspension. This was then addeddropwise to a 35 mL ethanolic solution of [Ru(acac)2(CH3CN)2](100 mg, 0.26 mmol). The mixture was heated to reflux for24 h under atmospheric conditions. The initially orange solutiongradually changed to deep violet. The solution mixture was filteredand the filtrate was evaporated to dryness. The residue was purifiedusing a neutral alumina column. Initially a red band of [Ru(acac)3]was eluted with CH2Cl2. Further elution by CH2Cl2 gave a brightpink band of 1, and a bright violet band of 2 was eluted byCH2Cl2 : CH3CN (3 : 2).

1. Yield, 30 mg (30%). Anal. Calc. for C26H32N4O10Ru2: C,40.87; H, 4.22; N, 7.33. Found: C, 40.75; H, 4.17; N, 7.39. ESI MS(in CH3CN): m/z = 764.09 corresponding to [1]+ (calc. molecularweight: 764.02). 1H NMR in CDCl3 (d , ppm): 12.46 (NH), 5.54(CH), 2.40 (CH3), 2.03 (CH3). IR (KBr disk): n(C=O), 1626 cm-1,n(NH), 2924 cm-1.

2. Yield, 35 mg (35%). Anal. Calc. for C26H32N4O10Ru2: C,40.87; H, 4.22; N, 7.33. Found: C, 40.92; H, 4.14; N, 7.29. ESI MS(in CH2Cl2): m/z = 764.21 corresponding to [2]+ (calc. molecularweight: 764.02). 1H NMR in CDCl3 (d , ppm): 12.42 (NH), 5.53(CH), 2.39 (CH3), 2.02 (CH3). IR (KBr disk): n(C=O), 1633 cm-1,n(NH), 2924 cm-1.

Crystal structure determination†

Single crystals of 1·2H2O were grown by slow evaporation of a1 : 1 dichloromethane–ethanol solution at 298 K. Despite severalattempts we failed to get fully satisfactory single crystals of 1.However, the structure of 1 could be partially solved with a veryweakly diffracting crystal. The asymmetric unit consists of twocentro-symmetric molecules and each molecule has one watermolecule. Due to poor quality data the hydrogen atoms associatedwith water molecules could not be generated. X-Ray diffraction

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data were collected using an OXFORD XCALIBUR-S CCDsingle crystal X-ray diffractometer. The structures were solvedand refined by full-matrix least-squares techniques on F 2 usingthe SHELX-97 program.26 The absorption corrections were doneby the multi-scan technique. All data were corrected for Lorentzand polarization effects, and the non-hydrogen atoms except C24,C25 and C26 were refined anisotropically. Hydrogen atoms wereincluded in the refinement process as per the riding model.

Acknowledgements

Financial support received from the Department of Science andTechnology and the Council of Scientific and Industrial Research(New Delhi, India), Mercator Guest Professorship of G. K.L. (DFG-Germany), the DAAD and the DFG (Germany) isgratefully acknowledged.

References

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