The triruthenium complex [{(acac)2RuII}3(L)] containing aconjugated diquinoxaline[2,3-a:2�,3�-c]phenazine (L) bridgeand acetylacetonate (acac) as ancillary ligands.Synthesis, spectroelectrochemical and EPR investigation

Srikanta Patra,a Biprajit Sarkar,b Sandeep Ghumaan,a Jan Fiedler,c Wolfgang Kaim*b andGoutam Kumar Lahiri *a

a Department of Chemistry, Indian Institute of Technology – Bombay, Powai, Mumbai–400076,India. E-mail: lahiri@chem.iitb.ac.in

b Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55,D–70550 Stuttgart, Germany. E-mail: kaim@iac.uni-stuttgart.de

c J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic,Dolejskova 3, CZ–18223 Prague, Czech Republic

Received 8th December 2003, Accepted 13th January 2004First published as an Advance Article on the web 30th January 2004

The compound [{(acac)2Ru}3(L)] (1) undergoes three well-separated one-electron oxidation and reduction processes.The EPR results indicate electron removal from the ruthenium() centres on oxidation and the occupation of alargely L-based molecular orbital on reduction. In spite of well-separated (∆E ≥ 340 mV) oxidation no obviousintervalence charge transfer bands were detected in the Vis, NIR or IR regions, suggesting very weak electroniccoupling between the metal centres in the mixed-valent intermediates 1� and 12�. The separated (∆E ≥ 540 mV)stepwise reduction produces weak near-infrared features associated with partially occupied π* orbitals of L, theunusually high g anisotropy in the EPR spectrum of 1� is attributed to the occupation of a degenerate MO by theunpaired electron.

IntroductionThere has been continuous research in the design of poly-nuclear metal complexes incorporating bridging functionalities.1

This is due to their relevance for biological processes,2 molecularelectronics,3 theoretical studies of electron transfer and energytransfer,4 photoactive DNA cleavage for therapeutic pur-poses 5 and non-linear optical behaviour.6 In this context, the1,4,5,8,9,12-hexaazatriphenylene (HAT = L1) bridging ligand 7

which contains a trigonal symmetrical arrangement ofconjugated fused 1,10-phenanthroline-type binding sites, hasbeen used to prepare ruthenium and mixed ruthenium–osmium,8

ruthenium–rhenium,9 ruthenium–rhodium,10 rhodium–iridium,11

chromium,12 cobalt 13 and copper complexes.14

Moreover, the bridging ligand L1 and its derivatives L2–L7

have also been employed to develop e.g. the magnetic molecularsquare [Co(L1)Cl2]4�27H2O,15 the coordination polymer[Ag(L1)ClO4]�2CH3NO2,

16 the enantio- and diastereomericallypure “disk” [{(phen)2Ru}3(L

2)][PF6]6,17 a cylindrical inorganic

cage of nanometric size in copper complexes of L3,18 a cyclichexamer [Co6(L

4)6]4�,19 the anion-trapping host [(Cu-dppe)3-

(L5)6]2�,20 non-linear optical devices based on copper()

complexes of L6,21 or mononuclear and dinuclear PdII/ReI

complexes of L7.22

Although metal complexes encompassing this wide varietyL1–L7 of HAT derivatives have been well explored in recentyears, the HAT derivative diquinoxaline[2,3-a:2�,3�-c]phenazine

(L) was only explored in synthesizing one AgI complex consist-ing of an enantiomorphic pair of interpenetrating nets.23 Thus,as part of our ongoing programme of understanding thespecific role of particular combinations of bridging ligandsand ancillary functions for the intermetallic electronic coupl-ing in a complex matrix, we have intended to scrutinise theefficacy of the hitherto unexplored HAT derivative L in thetriruthenium complex {(acac)2RuII}3(L) = 1 (acac = acetyl-acetonate = pentane-2,4-dionate). It should be noted, that as faras the combination of ruthenium and any HAT derivative isconcerned, only L1/L2 and polypyridine-based 2,2�-bipyridineor 1,10-phenanthroline ancillary functions were considered inthe construction of complex moieties. The present work thusexplores the synthesis, spectroelectrochemical and EPR aspectsof the complex [{(acac)2RuII}3(µ3-η

2:η2:η2-L)] (1).

Results and discussionThe diamagnetic, neutral complex [{(acac)2RuII}3(L)] = 1 wasprepared via the reaction of RuII(acac)2(CH3CN)2 and diquin-oxaline[2,3-a:2�,3�-c]phenazine in a 3 : 1 molar ratio in ethanolfollowed by chromatographic purification using a silica gelcolumn. All attempts to synthesize dinuclear [{(acac)2RuII}2(L)]and mononuclear [{(acac)2RuII}(L)] derivatives using appropri-ate 2 : 1 and 1 : 1 molar ratios of {Ru(acac)2} and L, respect-ively, have failed so far. On every occasion the trinuclear species

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1 was obtained, irrespective of the metal–ligand ratio used. Thecomplex 1 gave satisfactory microanalytical and mass spectraldata (see Fig. 1 and Experimental).

Compound 1 exhibits three successive quasi-reversible stepson oxidation in acetonitrile at E o

1 = 0.34 V (∆Ep = 70 mV)(couple I), E o

2 = 0.68 V (∆Ep =70 mV) (couple II) and E o3 =

1.16 V (∆Ep = 100 mV) (couple III) versus SCE (Fig. 2). The one-electron nature of couple I was confirmed by constant-potentialcoulometry, for couples II and III the same was established bycomparing their differential pulse voltammetric current heightswith that for couple I. The observed steps are tentatively attri-buted to sequential metal-based electron-transfer processes:RuIIIRuIIRuII RuIIRuIIRuII (couple I), RuIIIRuIIIRuII RuIII-RuIIRuII (couple II), and RuIIIRuIIIRuIII RuIIIRuIIIRuII

(couple III). The separation of potentials between the succes-sive couples, 340 mV between couples I and II, and 480 mVbetween couples II and III, correspond to comproportionationconstants of 105.7 and 108.0, respectively [calculated using theequation RT lnKc = nF(∆E ) 24]. These values seem to suggestthat the mixed-valent states RuIIIRuIIRuII (1�) and RuIIIRuIII-

Fig. 1 Electrospray mass spectrum of 1 in CH3CN. Inset shows theisotopic abundance of the molecular ion peak at m/z = 1283.33.

RuII (12�) correspond to moderately to strongly coupled class IIand class III systems, respectively.25 The only other knownrelated triruthenium complexes incorporating a bridging HAT-type ligand, [{(bpy)2RuII}3(L

1)]6� and [(phen)2RuII}3(L1)]6�

(bpy = 2,2�-bipyridine and phen = 1,10-phenanthroline) exhibitKc values of 104.4, 104.2 and 104.5, 104.7 in the respective mixed-valent states.8g,e Thus, a substantial increase in the Kc value hasbeen observed while changing from the combination bpy/phen-L1 to acac-L. The selective introduction of monoanionicσ-donating [acac�] as ancillary function in 1 in place of theneutral and more π-acidic polypyridine ligands lowers not onlythe positive charge of the complex molecule from �6 to 0 butfacilitates also the intermetallic coupling process in the oxidisedmixed-valent states of 1. It should be noted that the enhance-ment of intermetallic electronic coupling in mixed-valent stateson switching from polypyridine to [acac�] ancillary functionshas been recently observed also for diruthenium complexes.26,27

Complex 1 displays three one-electron reduction processes inCH3CN at E o

1 = �0.46 V (∆Ep = 80 mV), E o2 = �1.07 V (∆Ep =

70 mV) and E o3 = �1.61 V (∆Ep = 80 mV) versus SCE. The

separations of 610 and 540 mV correspond to Kc values of 1010.3

and 109.2, respectively. Since neither the [acac�] ligand nor theRuII centres undergo reduction within the potential limit of�2.0 V versus SCE,1d,e,26,27 the reduction processes in 1 are con-sidered to be associated with the conjugated bridging ligand L.The free ligand L displays two irreversible reduction waves withpeak potentials at Epc(1) = �1.05 V and Epc(2) = �1.95 V versusSCE. In case of [{(bpy)2RuII}3(L

1)]6� and [{(phen)2RuII}3-(L1)]6�, the reduction waves for the coordinated HAT ligand(L1) were detected at �0.25, �0.58, �1.07 and �0.30, �0.64,�1.12 V, respectively.1z,8e

UV-Vis-NIR spectroelectrochemical experiments for [1]n�

(n = �3, �2, �1, 0, 1, 2, 3) were performed in acetonitrilesolution at 298 K using an OTTLE cell.28 Spectral data arelisted in Table 1 and the spectra are shown in Figs. 3 and 4. Thepresence of clean isobestic points during each conversion andthe complete electrochemical regeneration of the higher (in caseof reduction) and lower (in case of oxidation) congeners with-out any appreciable degradation established the reversibilityof the redox conversion processes under the conditions forspectroelectrochemistry.

The starting complex 1 exhibits a strong RuII π*(L) MLCTtransition at 622 nm (ε = 31300 dm3 mol�1 cm�1), in addition to

Fig. 2 Cyclic voltammograms (—) and differential pulse voltammo-grams (- - -) of 1 in CH3CN. Scan rate = 50 mV s�1.

Table 1 UV-Vis-NIR data of 1 from spectroelectrochemistry a

Compound λmax/nm (ε/dm3 mol�1 cm�1)

13� 614(10800), 400(sh), 286(42100)12� 796(sh), 648(19000), 476(sh), 378(31700), 287(38700)1� 1075(sh), 652(sh), 596(23500), 348(sh), 278(42700)1 622(31300), 345(sh), 276(51300)1� 1350(sh), 820(sh), 733(21400), 340(sh), 276(58600)12� 1350(sh), 1000(sh), 795(19500), 465(15900), 332(sh),

277(65800)13� 1070(sh), 797(19800), 462(sh), 413(28600), 278(71700)a In CH3CN/0.1M Bu4NPF6.

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intense ligand-based transitions in the UV region (Fig. 3). Onone-electron oxidation to the mixed-valent RuIIIRuIIRuII (1�)state, the MLCT transition is blue-shifted to 596 nm and theintensity of the transition is appreciably diminished (ε = 23500dm3 mol�1 cm�1) due to the decrease in the number of RuII

centres in 1�. On further successive oxidation to the RuIIIRuIII-RuII (12�) and RuIIIRuIIIRuIII (13�) states, the intensity ofthe MLCT transition steadily decreased and one moderatelyintense RuIII-based LMCT transition appeared at 614 nm

Fig. 3 UV-Vis-NIR spectroelectrochemistry of the conversions(a) 1 1�, (b) 1� 12�, (c) 12� 13� in CH3CN/0.1M Bu4NPF6.

Fig. 4 UV-Vis-NIR spectroelectrochemistry of the conversions (a)1 1�, (b) 1� 12�, (c) 12� 13� in CH3CN/0.1M Bu4NPF6.

(ε = 23500 dm3 mol�1 cm�1) for 13�. The mixed-valent states 1�

and 12�, however, did not exhibit clear absorption bands in thenear- and mid-IR regions, save for the weak shoulder at about1075 nm for 1�. Keene and co-workers 8a have also failed toobserve any NIR bands for the mixed-valent RuIIIRuIIRuII andRuIIIRuIIIRuII states of [{(phen)2Ru}3(L

1)]n�, the only othertriruthenium complex incorporating a HAT-type bridgingligand on which the spectroelectrochemistry experiments wereperformed. (The dinuclear [{(phen)2Ru}2(L

1)]5� showed IVCTbands in the expected region.8a) Total valence electron delocal-isation across the entire molecule was considered as the prob-able reason for the absence of IVCT bands in the trinuclearcase.8a Alternative interpretations (which we prefer) invokeweak electronic coupling through a “meta”-bridging aromaticligand; 1,3-substitution patterns are known 29 to diminish themetal–metal interaction in mixed-valent species which may leadto vanishing IVCT band intensities. In addition, the trigonalsymmetry causes orbital degeneracy with e-type MOs beingonly partially occupied for the mixed-valent species. This situ-ation may also be responsible for decreasing oscillator strengthsof the intervalence transitions. Very weak IVCT bands in spiteof large Kc values are not uncommon, they have been reportedfor RuIIRuIII complexes of bis-bidentate ligands.1n,30a

On successive reduction 1 1� 12� 13�, the (RuII L)-based MLCT transition is progressively red-shifted from622 733 795 797 nm, with a drop in intensity (ε =31300 to 19500 dm3 mol�1 cm�1) (Fig. 4). This is probably aconsequence of the sequential addition of electrons into emptyπ* orbitals of the conjugated neutral L.1b,f Moreover, the reduc-tion of L in 1� 13� results in additional low-energy shouldersassociated with intra-ligand transitions. The degenerate frontierorbitals, involving especially the closely spaced low lyingunoccupied a2� and e� MOs of trigonal symmetrical L allow forvarious alternatives in terms of orbital occupancy and electro-nic transitions. In the absence of other π-acidic ligands in 1 itis tempting to assign the low-energy shoulders to internaltransitions associated with the reduced forms of L.1b,f

The in situ generated RuIIIRuIIRuII species 1� shows a slightlyrhombic EPR spectrum in acetonitrile at 4 K (g1 = 2.331, g2 =2.154, g3 = 1.876) (Fig. 5). The g anisotropy g1 � g3 = 0.455 andthe average g factor of <g> = 2.128, derived from <g> = [1/3-(g1

2 � g22 � g3

2)]1/2, indicate a slightly distorted octahedralarrangement around the ruthenium centres in 1� 1c,d,30 and littlecontribution from the ligands. Different situations wereencountered with 1,2-dioxolene ligands and their derivatives.31

The one-electron reduced species 1� exhibits a narrower axialEPR spectrum in acetonitrile at 4 K with g1 = 2.057 and g2 = g3

= 1.924). The relatively 30,32 large g anisotropy g1 � g3 = 0.133and the isotropic g of 1.969, deviating clearly from the free

Fig. 5 EPR spectra of (a) 1� and (b) 1� in CH3CN at 4 K.

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electron value of ge = 2.0023, may imply significant contri-butions from the metals to the singly occupied MO. This wouldbe confirmed by the fact that no EPR signal was observed influid solution. However, we rather attribute the unusually largeg anisotropy for the complex of an anion radical ligand 32 (L��)to the presence of the closely spaced unoccupied orbitals a2�and e� in L. The relatively small distortion possible in the rigidaromatic ligand and the chelate systems causes additionalexcited states to lie very close to the doublet ground state whichwould explain both rapid relaxation and a comparatively largeg anisotropy according to the approximation (1).32

ξ: spin orbit coupling constant; L: angular momentum oper-ator; E0: energy of singly occupied molecular orbital (SOMO).

Continued in situ oxidation or reduction of 1 only causesdiminishing of the EPR signals shown in Fig. 5, no new featureswere detected. No evidence was thus obtained for spin-pairing,triplet or quartet formation in the doubly or triply oxidised orreduced forms.

Experimental

Materials

The starting complex Ru(acac)2(CH3CN)233 and diquinoxaline-

[2,3-a:2�,3�-c]phenazine (L) 34 were prepared according to thereported procedures. Other chemicals and solvents were reagentgrade and used as received. For spectroscopic and electro-chemical studies HPLC grade solvents were used.

Physical measurements

UV-Vis-NIR spectroelectrochemical studies were performed inCH3CN/0.1 M�1 cm�1 Bu4NPF6 at 298 K using an opticallytransparent thin layer electrode (OTTLE) cell 28 mounted in thesample compartment of a Bruins Instruments Omega 10spectrophotometer. FT-IR spectra were taken on a Nicoletspectrophotometer with samples prepared as KBr pellets. Solu-tion electrical conductivity was checked using a Systronic 305conductivity bridge. Magnetic susceptibility was checked with aPAR vibrating sample magnetometer. 1H-NMR spectra wereobtained with a 300 MHz Varian FT spectrometer. The EPRmeasurements were made in a two-electrode capillary tube 35

with a X-band Bruker system ESP300, equipped with a BrukerER035M gaussmeter and a HP 5350B microwave counter.Cyclic voltammetric, differential pulse voltammetric and cou-lometric measurements were carried out using a PAR model273A electrochemistry system. Platinum wire working and aux-iliary electrodes and an aqueous saturated calomel referenceelectrode (SCE) were used in a three-electrode configuration.The supporting electrolyte was [NEt4]ClO4 and the solute con-centration was ∼10�3 M. The half-wave potential E o

298 wasset equal to 0.5(Epa � Epc), where Epa and Epc are anodic andcathodic cyclic voltammetric peak potentials, respectively. Aplatinum wire-gauze working electrode was used in coulo-metric experiments. All experiments were carried out under adinitrogen atmosphere and were uncorrected for junctionpotentials. The elemental analysis was carried out with aPerkin-Elmer 240C elemental analyzer. Electrospray mass spec-trum was recorded on a Micromass O-ToF mass spectrometer.

Preparation of complex [{(acac)2RuII}3(L)] (1)

The starting complex Ru(acac)2(CH3CN)2 (100 mg, 0.26mmol), and the ligand L (33 mg, 0.086 mmol) were added to

(1)

20 cm3 of ethanol, and the mixture was heated to reflux for 12 hunder a dinitrogen atmosphere. The initial orange colour of thesolution gradually changed to blue. The solvent of the reactionmixture was evaporated to dryness under reduced pressure.The solid mass thus obtained was purified by using a silica gelcolumn. Initially, a red compound corresponding to Ru(acac)3

was eluted by CH2Cl2–CH3CN (10 : 1). With CH2Cl2–CH3CN(3 : 1), a blue compound corresponding to 1 was separated lateron. Evaporation of solvent under reduced pressure yieldedcomplex 1.

Anal. calcd. for C54H54N6O12Ru3 (1): C, 50.46; H, 4.24; N,6.54; found: C, 50.10; H, 4.20; N, 6.88%. Yield: 50% (56 mg).Positive ion electrospray mass spectrum of 1 showed themolecular ion peak centred at m/z = 1283.33, corresponding to1� (calculated molecular weight, 1282.27). The 1H-NMR spec-trum of 1 in CDCl3 is also in agreement with the threefoldsymmetry.

Acknowledgements

The work was supported by the Council of Scientificand Industrial Research, New Delhi (India), and DeutscheForschungsgemeinschaft (DFG), the Fonds der ChemischenIndustrie, and the Deutscher Akademischer Austauschdienst(DAAD), Germany.

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