Optical electron-transfer transitions in polynuclear complexes of the

complexes of the type X(NH3)4RuNCRu(bpy)2CNRu(NH3)4Ym+ (X = NH3, py; ... E. Corilo , Marcos N. Eberlin , Guillermo Benítez , Maria E. Vela , Robe...
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Inorg. Chem. 1988, 27, 408-414

408

Contribution from the Dipartimento di Chimica dell'Universit6, Centro di Fotochimica CNR, 44100 Ferrara, Italy, and Dipartimento di Chimica "G. Ciamician" dell'Universit5, Centro di Studio di Elettrochimica Teorica e Preparativa, 40100 Bologna, Italy

Optical Electron-Transfer Transitions in Polynuclear Complexes of the Type X(NH~)~RUNCRU(~~~)~CNRU(NH~)~Y~+ (X = N H , py; Y = N H 3 , py; m = 4-6) Carlo Albert0 Bignozzi,*t Carmen Paradisif Sergio Roffia,t a n d Franco Scandola*+ Received April 7, 1987 The preparation of the binuclear [~~(NH,),RUNCRU(~~~)~CN]'+, [ 3',2], complex and trinuclear [ ~ Y ( N H ~ ) ~ R U N C R U (bpy)2CNRu(NH3)4py]6',[3',2,3'], and [py(NH3)pRuNCRu(bpy)2CNRu(NH,)5]6+, [3',2,3], complexes is described. The chemical or electrochemicalreduction of the Ru(NHJ4py3+and Ru(NH,)?' moieties gives rise to the formation of the semireduced [2',2,3'] and [2',2,3]and fully reduced [2',2], [2',2,2'], and [2',2,2]species. The electrochemicaland spectroscopic properties of these complexes have been studied and compared with those of the [3,2],[2,2],[3,2,3],[3,2,2],and [2,2,2]complexes in which one or two Ru(NH&*'/~' moieties are linked to the central ~is-Ru(bpy)~(CN), unit. The MLCT and IT charge-transfer bands occurring in the UV-vis and near-IR absorption spectra of the complexes have been assigned to transitions between the appropriate and low-energy redox sites. In particular a good correlation is obtained between the redox potentials of the Ru(NH,)~~'/)+ R u ( N H , ) , ~ ~ ~ ' units / ~ + and the energies of two interesting types of electronic transitions: (i) the weak IT absorption due to long-range interaction between the two terminal ruthenium ammine centers of the [2,2,3],[2',2,3],and [2',2,3'] complexes and (ii) the remote MLCT transitions involving the Ru(I1) atom of one ammine moiety and the bpy ligands of the Ru(b~y),(cN)~ unit.

We have recently reported the synthesis of polynuclear comIntroduction plexes containing the c i ~ - R u ( b p y ) ~ ( C N chromophore )~ linked via Following the pioneering work of Taube1*2and Meyer,Is4 a cyano bridges to one and two Ru(NH3)?+l2+ The number of discrete mixed-valence ions based on ruthenium comspectroscopic properties of these molecules were particularly inplexes have been synthe~ized.~Among the various interesting teresting. In fact, several metal-to-ligand charge transfer (MLCT) aspects involved in the chemistry of mixed-valence compounds, and IT absorption bands corresponding to electronic transitions particular attention has been paid to the study of the optical between the various low-energy redox sites were observed, inelectron-transfer transitions between weakly coupled metal centers, bpy MLCT originating in the ruthenium cluding (i) Ru(I1) usually called intervalence-transfer (IT) transitions, which are bpy MLCT oriatom of the R ~ ( b p y ) , ~moiety, + (ii) Ru(I1) related by the Hush theory6 to the kinetic and thermodynamic ginating in the ruthenium atoms of the pentaammine moieties, factors for the corresponding thermal electron-transfer process. Ru(II1) IT between cyano-bridged adjacent ru(iii) Ru(I1) In ligand-bridged mixed-valence ions, the extent of the metRu(II1) IT between remote thenium atoms, and (iv) Ru(I1) al-metal interaction depends mainly on (i) the distance between ruthenium atoms of the pentaammine units. In particular, the the metal centers, (ii) the ability of the bridging ligand in deloend-to-end bands of type iv had previously been observed only calizing the electronic charge, and (iii) the coordination envionce,l4 while the observation of remote MLCT (type ii) was totally ronments of the metal Many examples on the importance new. In order to confirm these observations and to gain further of these three factors have been reported for Ru(II)/Ru(III) insight into the intramolecular charge-transfer transitions of these mixed-valence complexes. Regarding points i and ii, delocalization multisite systems, we have extended the study to analogous syshas been implied for short bridging ligands such as N2: pyrazine tems, based on Ru(bpy),(CN),, in which one or two R u ( N H ~ ) ~ (pz): and cyanogen,1° while 4,4'-bipyridine and trans- 1,2-bis(4pyridy1)ethylene give rise to weakly coupled systems, and no appreciable metal-metal electronic interaction has been observed with 1,2-bis(4-pyridyl)ethane in which two A systems are separated Creutz, C.; Taube, H. J . Am. Chem. SOC.1969,91,3988;1973,95, 1086. by a saturated group." Regarding point iii, a weaker interaction Tom, G.M.; Creutz, C.; Taube, H. J . Am. Chem. SOC.1974,96,7827. between the metal center has been described for the Callahan, R. W.; Brown, G. M.; Meyer, T. J. Znorg. Chem. 1975,14, [(bpy),ClRu(pz)R~C1(bpy)~]~+ ioni2 with respect to the 1443. [(NH3)sRu(pz)Ru(NH3)s]S+ ion9 and assigned to a competitive Powers, M. J.; Callahan, R. W.; Salmon, D. J.; Meyer, T. J. Znorg. back-bonding to the bpy ligands, which decrease the available Chem. 1916,15, 1457. For a review see: (a) Brown, D. B., Ed. Mixed Valence Chemistry; D. electron density for delocalization through the A* orbital of pyReidel: Dordrecht, The Netherlands, 1980; chapters by P. Day, A. razine. Polarizing influence of the pentacyanide environments Ludi, N. S. Hush, T. J. Meyer, and P.N. Schatz. (b) Creutz, C. Prog. complexi3 as was indicated for the (CN)SRu11(pz)Ru11'(NH3)S Znorg. Chem. 1983,30, 1 and references therein. responsible of a low coupling between the Ru"/Ru"' sites. Hush, N. S. Prog. Inorg. Chem. 1967,8,391. Richardson, D. E.;Taube, H.; J . Am. Chem. SOC.1983,105,40. Compared with the number of dinuclear mixed-valence comRichardson, D. E.;Sen, J. P.; Buhr, J. D.; Taube, H. Znorg. Chem. 1982, plexes, a smaller number of trinuclear species have been studied. 21, 3136. In principle this type of molecules could exhibit multisite interFiirholz, U.; Biirgi, H.; Wagner, F. E.; Stebler, A.; Ammeter, J. H.; action, giving rise to optical electron transfer between adjacent Krausz, E.; Clark, R. J. H.; Stead, M. J.; Ludi, A. J . Am. Chem. Soc. 1984,106, 121 and references therein. as well as remote metal centers. In the trans-[(NH,),Ru(pz)Tom, G. M.; Taube, H. J . Am. Chem. SOC.1975,97,5310. R u ( N H ~ ) ~ ( P z ) R u ( N H ~trinuclear ) ~ ] ~ + ion studied by Taube,I4 Callahan, R. W.; Brown, G. M.; Meyer, T. J. Znorg. Chem. 1975,14, an intense intervalence transfer between the terminal ruthenium 1443. atoms was observed. By contrast, both in the case of Callahan, R. W.; Keene, F. R.; Meyer, T. J.; Salmon, D. J. J . Am. [(NH3)sRu(pz)Ru(bpy)2(pz)Ru(NH3) 7+ (bpy = 2,2'-bipyridine) Chem. Soc. 1977,99,1064. Henderson, W. W.;Shepherd, R. E. Znorg. Chem. 1985, 24, 2398. and of the heterotrinuclear ion [(NH,)sRuNCFcCNRu(NH3)S]S+ von Kameke, A.; Tom, G. M.; Taube, H. Znorg. Chem. 1978,17,1790. (Fc = ferrocene) studied by Meyerls and Henryi6 respectively, Powers, M. J.; Callahan, R. W.; Salmon, D. J.; Meyer, T. J. Znorg. no spectroscopic evidence of end-to-end interactions between the Chem. 1976,15,894. ruthenium atoms was obtained. Dowling, N.;Henry, P. M. Znorg. Chem. 1982,21,4088.

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Bignozzi, C. A,; Roffia, S.;Scandola, F. J . A m . Chem. SOC.1985,107, 1644. Roffia, S.; Paradisi, C.; Bignozzi, C. A. J . Electroanal. Chem. Znterfacial Electrochem. 1986,200, 105.

Universiti di Ferrara.

* Universita di Bologna. 0020-1669/88/1327-0408$01.50/0

0 1988 American Chemical Society

Polynuclear Ruthenium Complexes moieties have been substituted by the R u ( N H ~ unit ) ~ (py ~ ~= pyridine). The different redox properties of these two units should give rise to predictable changes in the energy of the MLCT and I T transitions involving these sites. The study of covalently linked supermolecular systems ("diads" or "triads") consisting of an organic photosensitizer unit and various organic donor or acceptor subunits has received relevant attention in recent years and has led to some spectacular results in the context of light-induced charge ~ e p a r a t i o n . l ~ -In ~ ~this respect, systems based on coordination compounds have not received comparable attention. The complexes described in the present study represent a step in this direction. It has been pointed out17,23that the study of MLCT and IT absorption bands that appear in the UV-vis and near-IR spectra may give information about the nature and efficiency of the intramolecular electrontransfer-quenching pathways occurring in these supramolecular systems following photoexcitation. Experimental Section Materials. [Ru(NH,),]CI, (Johnson Matthey) and (NH4)2[RuC16] (Fluka) were used without further purification. Europium(II1) chloride (Schuchardt), ammonium hexafluorophosphate (Merck), and hydrazine hydrate (BDH) were commercial products of reagent grade. Aqueous solutions of Eu2+were prepared by exhaustive reduction of solutions of the corresponding trichlorides with zinc amalgam. Spectrograde organic solvents, deuterium oxide (Fluka) and doubly distilled water were used. The chromatographic separations were made with Sephadex-SPC25-120 strongly acidic cation-exchange resin purchased from Pharmacia. The resin was allowed to swell for at least 3 h in water before packing the column and was cleaned by eluition with 2 M HCI and then with doubly distilled water. Preparations. Mononuclear Ruthenium Compounds. ~ i s - R u ( b p y ) ~ (CN)2.2H20 was prepared according to Demas' procedure.24 [Ru(NH,)SC1]C12and trans- [Ru(NH3),(S02)C1]C1 were prepared by following the procedure outlined by V ~ g t .The ~ ~chloropentaammineruthenium(111) chloride was recrystallized from hot 0.1 M hydrochloride acid, while the two tetraammineruthenium(I1) complexes were used without further purification. trans-[Ru(NH,),(SO,)py]Cl was prepared and purified as indicated by Sutton.26 Anal. Calcd for [Ru(NH,),(SO,)py]CI: C, 15.81; H, 4.51; N , 18.47. Found: C, 15.94; H, 4.52; N , 18.50.

Inorganic Chemistry, Vol. 27, No. 2, I988 409 change chromatography using a 3 X 20 cm column containing the Sephadex-SP-120 resin. After being loaded, the column was first eluted with distilled water in order to remove a small amount of yellow unreacted cis-Ru(bpy),(CN)z. Then, 0.3 M HCI was used to separate a second yellow fraction containing the unreacted tetraammine(pyridine)ruthenium(II) as well as traces of the binuclear Ru/Zn complex. Elution was continued with 0.5 M HCl to collect the green product completely. The green fraction was rotary evaporated to dryness, and the residue was dissolved in a small amount of water. The solid hexafluorophosphate salt was precipitated from this solutionm with a few drops of a saturated aqueous solution of NH4PF6. The solid was redissolved in water, reprecipitated with NH4PF6, filtered, washed with a small amounts of cold water, and dried under vacuum over P205.Anal. Calcd for [~~(NH,),RUNCRU(~~~)~CN] (PF6)&0: C, 27.79; H, 3.02; N, 13.20. Found: C, 27.54; H, 3.03; N, 13.11. [(NH,)sRuNCRu(bpy)2CN](PF6)3?7 This complex has been prepared by a previously reported methodI7 except for the use of a more convenient purification procedure based on ion-exchange chromatography, as described above for the [py(NH3),RuNCRu(bpy),CN](PF& complex.

Trinuclear Ruthenium Compounds. [py(NH,),RuNCRU(~~~)~CNRU(NH,),PY](PF~)~.~' To a solution containing 0.2 g (1.71 X lo4 mol) of the [py(NH3)4NCRu(bpy)2CN](PF6)3*H20 dimer in 50 mL of water

was added 0.068 g (1.79 X lo-, mol) of [ R U ( N H ~ ) ~ ( S O , ) ( ~ ~fol)]C~, lowed by 0.5 mL of 1 M HC1. The green solution was heated up to 40 OC and deaerated under argon for 30 min. Zn amalgam (5 g) was added to the solution, which became brown in color in a few minutes. The solution was kept at 40 OC under argon for 3 h. After removal of the amalgam, 1 mL of 1 M HC1 and a few drops of 35% H 2 0 2were added to the brown solution, which turned almost immediately blue-green in color. The solution was than evaporated under vacuum (at 50 "C) to dryness. The crude product was dissolved in water and purified by ion-exchange chromatography on Sephadex-SP-C25- 120. The column was loaded with the blue-green solution and first eluted with 0.3 M HCI to remove the light yellow unreacted tetraammine(pyridine)ruthenium monomer. Eluition was continued with 0.5 M HC1 to recover the green dimeric reactant, and finally a third blue fraction was collected with 1 M HCI as eluent. The blue solution was evaporated at 50 OC under vacuum to dryness. The product was dissolved in a small amount of water, and few drops of a saturated solution of NH4PF6were added. The product was redissolved in water and reprecipitated as before, washed with a few milliliters of cold water and dried under vacuum. Anal. Calcd C, 20.98; H, Binuclear Ruthenium Compounds. [py(NH3)4RuNCRu(bpy)2CN]- for [~~(NH,),RUNCR~(~~~)~CNR~(NH~)~~~](PF~)~: 2.75; N , 12.23. Found: C, 20.60; H, 2.95; N , 12.10. An alternative (PF6)3.27 A 0.5-g sample (1 X lo-, mol) of R ~ ( b p y ) ~ ( C N ) , - 2 Hin~ 0 procedure for the synthesis of this trimeric complex was also followed, 80 mL of methanol was added to 0.38 g (1 X mol) of [Ru(NH,),starting from the two monomeric reactants ci~-Ru(bpy)~(CN), and (S0,)pylCI in 50 mL of M HCI. The resulting solution was heated [Ru(NH3),(SOp)py]C1. In this case, the reaction was made in the mixed up to 40 OC and deaerated with argon for 30 min. Freshly prepared zinc solvent C H , 0 H / H 2 0 (1:l v/v) with a molar ratio of the reactants of 1:2. amalgam (7 g) was added, and after few minutes the color of the solution The purification of the product was obtained by ion-exchange chromaturned from yellow-orange to brown. The solution was kept at 40 OC tography as reported before with the additional elimination of the ununder argon for 2 h. After removal of the amalgam, few drops of 35% reacted cis-Ru(bpy),(CN), and of the binuclear Ru/Zn complex (see H202were added to the solution, which turned almost immediately green binuclear ruthenium compounds). in color. After the methanol was removed under reduced pressure, the [(NH3)SRuNCRu(hpy)2CNRu(NH,)~](PF6)6.27 As for the dimeric reaction mixture was filtered, leaving a yellow solid on the filter. This pentammine analogue, a literature method was followed for its prepasolid was found to consist of the unreacted c i ~ - R u ( b p y ) ~ ( C plus N ) ~traces ration." Also in this case, the ion-exchange chromatography purification of a binuclear complex containing Zn2+ ions cyano bridged to the Rumethod was adopted. ( b ~ y ) , ~unit. + The solution was rotary evaporated at 70 OC to dryness. [~~(NH,),RUNCR~(~~~)~CNR~(NH,)~](PF~)~.~~ Two equivalent The crude product was dissolved in water and was purified by ion-exroutes of synthesis were followed to obtain this asymmetric trimeric complex: (i) reaction of the dimeric [py(NH3)4RuNCRu(bpy)2CN]2+ (19) Moore, T. A,; Gust, D.; Mathis, P.; Mialocq, J. C.; Chachaty, C.; with the monomeric [ R U ( N H , ) ~ ( H ~ O ) complex ]~+ ion; (ii) reaction of Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A,; Lehman, W. the dimeric [(NH3)5RuNCRu(bpy)zCN]2+with the monomeric [RuR.; Nemeth, G. A,; Moore, A. L. Nature (London) 1984, 307, 630. (NH3)4(H20)py]2+complex ion. The procedure described here refers (20) Seta, P.; Bienvenue, E.; Moore, A. L.; Mathis, P.; Bensasson, R. V.; to reaction i but can be easily applied to reaction ii. [ R U ( N H ~ ) ~ C I ] C I ~ Liddell, P.; Pessiki, P. J.; Joy, A.; Moore, T. A,; Gust, D. Nature (0.050 g, 1.71 X lo4 mol) was added to [py(NH,),RuNCRu(London) 1985, 316, 653. (bpy)2CN](PF6)3.H20(0.2 g, 1.71 X loT4mol) in 50 mL of water fol(21) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J. A m . Chem. SOC.1985, 107, 5562. lowed by 0.5 mL of 1 M HC1. The green solution was heated up to 40 (22) Liddell, P. A.; Barrett, D.; Makings, L. R.; Passiki, P. J.; Gust, D.; OC and deaerated with argon for 30 min. Zn amalgam (5 g) was added Moore, T. A. J . A m . Chem. SOC.1986, 108, 5350. to the solution, which became brown in color in a few minutes. The (23) Scandola, F.; Bignozzi, C. A,; Balzani, V. In Homogeneous and Hetsolution was kept at 40 OC under argon for 3 h. After removal of the erogerneous Photocatalysis; Pellizzetti, E., Serpone, N., Ed.; NATO amalgam, 2 mL of 1 M HC1 were added, and oxygen was vigorously Advanced Study Institute Series 1974; D. Reidel: Dordrecht, The bubbled for 30 min through the solution, which turned slowly from brown Netherlands, 1986; p 29. to blue-green. The separation of the trimeric asymmetric product from (24) Demas, J. N.; Turner, T. F.; Crosby, G. A. Inorg. Chem. 1969,8, 674. the reactants was obtained by following the procedure described for the (25) Vogt, L. H.; Katz, J. L.; Wiberley, S. E. Inorg. Chem. 1965,4, 1157. symmetric [~~(NH,),RUNCRU(~~~)~CNRU(NH~),~~] (PF6)6 complex. (26) Sutton, J. E.; Krentzien, H.; Taube, H. Inorg. Chem. 1982, 21, 2842. (27) Due to the stereochemical stability of the Ru(I1) complexes used in the Anal. Calcd for [py(NH3)4RuNCRu(bpy)zCNRu(NH,),I(PF6)6:C, synthesis, the stereochemistry of the component fragments is assumed 18.32; H, 2.73; N , 12.66. Found: C , 17.95; H, 2.90; N , 12.41. to be maintained in the polynuclear complexes. Therefore, the configApparatus. Absorption spectra in the UV-vis/near-IR regions were uration at the -Ru(bpy)22+ fragment is always cis and that at -Rutaken with a Perkin-Elmer 323 spectrophotometer. (NH3),py2+I3+is always trans, although such labels are omitted in the Checks for emission were performed with a Perkin-Elmer M P F 44 E formulas for the sake of simplicity. Also, in the polynuclear complexes spectrofluorimeter. Attempts to detect transient absorptions were carried the p-CN ligands are assumed to remain C-bonded to the Ru(bpy)?+ unit, as in the starting material. out by using a 30-11s laser flash photolysis apparatus (J&K 2000 Q-

410 Inorganic Chemistry, Vol. 27, No. 2, 1988 switched, frequency-doubled ruby laser, Applied Photophysics detection system). All the electrochemical measurements were made in water that was triply distilled with alkaline permanganate added in the last stage; 0.1 M (C,H,),NBF,([TEA]TBF), Fluka reagent grade, was used as supporting electrolyte. All measurements were taken at 25 f 0.1 OC. The samples of the electrolyzed solutions to be investigated spectroscopically were transferred under argon atmosphere by standard syringe technique from the electrolysis cell to the spectroscopic cell, which was carefully deaerated and fitted with airtight plugs. Polarographic, cyclic voltammetric, and controlled-potential coulometric (CPC) experiments were carried out with an AMEL (Milan). Model 552 potentiostat, an AMEL Model 568 programmable function generator, an AMEL Model 863 X/Y recorder and a Nicolet Model 3091 digital oscilloscope. The minimization of the uncompensate resistance effect in cyclic voltammetry (CV) was achieved with a positive feedback network of the potentiostat. The charge exchanged in CPC experiments was determined with an AMEL Model 721 integrator. A conventional three-electrode cell was used in all experiments. A saturated calomel electrode (SCE), separated from the test solution by a 0.1 M [TEAITFB solution in water sandwiched between two fritted disks, was used as reference electrode, and all potentials are referred to it. A dropping mercury electrode (DME) and a platinum electrode with periodical renewal of the diffusion layer (PRPE) were used as working electrodes in polarographic measurements.28 A hanging mercury drop electrode and a stationary platinum electrode were used in CV.

Bignozzi et al. Table I. Electrochemical Data‘ 4/29

redox couples

V vs. SCE

electrode

[3,31/ [321 [3,21/[2,21 [3/,31/ [3/,21 [3’,21/ ~ 3 1 [3,3,31/[3,2,31 [3,2,31/[3,2,21 [3,2,21/ [2,2,21 [3’,3,31/[3’,2,31 [3’,2,31/ [2’,2,31 [2’,2,31/ [2‘,2,21 [3’,3,3’1/[3’,2,3’1 [3’,2,3’1/[2’,2,3’1 [2’,2,3’1/[2’,2,2’1

+ 1.070b -0.1 + 1,1206

Pt Hg Pt Hg Pt Hg Hg Pt Hg Hg Pt Hg Hg

OOC

+0.110b +1.17OC -0.055‘ -0.125‘ +1.250b +0.160b -0.110b +1.350b +0.183b +0.103b

‘Half-wave Potentials (Ell2)determined in aqueous solutions on 5 X M samples in a supporting electrolyte of 0.1 M [TEAITBF. All of the potentials were determined by cyclic voltammetry and/or polarography. bThis work. cFrom ref 17 and 18.

[3,2,3], [3’,3,3]/3’,2,3], and [3’,3,3’]/[3’,2,3’] couples (Table I), follow the order of the ?r-acceptor capability of the Ru(II1) ammine unit attached to the oxidizable Ru(I1) center. Beside these processes, the complexes present reduction processes at potentials Results near zero, which can be attributed to the Ru(II1) ammine units According to a localized-valence description suggested by the on the basis of the similarity of the redox potentials with those electrochemical and spectroscopic results that will be described of related monomeric The electrode mechanism and by those previously reported on related systems,17J8 the and the determination of the redox potentials of the Ru(II1) following notation for the polynuclear ruthenium complexes will tetraammine and pentaammine units in the trinuclear species will [(NH3)5RuNCRu(bpy)2CN]3+, [3,2]; be used: [(NH~)sR~NCR~(~PY)~CNR~ [3,2,31; ( N H ~ [PY) ~ ~ ~ +be , briefly described. (NH3)4RuNCRu(bpy)zCNI3+, [3’,2]; [ ~ ~ ( N H ~ ) ~ R u N C R U - [3’2,3’]. This complex exhibits an apparently single voltammetric wave with a cathodic peak potential EP,,= 0.085 V, cor( ~ P Y ) z C N R ~ ( N H ~ ) . I P Y[3’,2,3’1; I~+, [PY(NH~),R~NCRUresponding to a process localized on tetraammine(pyridine)ru(bPY)zCNRWH3)5I6+, [3’,2,31. thenium groups. The presence of shoulders on the rising (deChemical Behavior. The substitution of one ammonia ligand scending) part of the cathodic (anodic) peak, the peak to peak of the R u ( N H ~ ) ~ * +moiety / ~ + with pyridine does not introduce separation (1 13 mV), the height of the wave (peak), which is remarkable differences in the solubility and stability of the approximately twice as large as the corresponding wave of [3’,2], polynuclear species derived by the R ~ ( b p y ) , ( c N )unit.’7J8 ~ All the independence of the peak potentials and of the peak current these ions are soluble in water as well as in polar organic solvents function (iP/u1I2) upon sweep rate ( u ) over the entire range exas hexafluorophosphate salts. plored (0.1-200 V s-’), altogether suggest that such a process is Exhaustive reduction of the [3’,2], [3’,2,3’], and [3’,2,3] commade up of two consecutive one-electron reversible processes, with plexes, as well as [3,2] and [3,2,3] complexes, with Zn amalgam, closely spaced standard potentials, according to in argon-purged acidic ( lo-, M HCI) solutions, gives rise to the formation of the corresponding [2’,2], [2’,2,2’], [2’,2,2], [2,2], and [3’,2,3’] e- G [3’,2,2’] (1) [2,2,2]fully reduced species. The same results were obtained by M HCI) solutions of the oxidized reducing deaerated acidic ( [3’,2,2’] + e- s [2’,2,2’] (2) forms with stoichiometric amounts of Eu2+ ions. In order to evaluate the standard potentials Eo1 and Eo2 of the Solutions of the semireduced [2’,2,3’], [2’,2,3], and [2,2,3] two one-electron successive steps, the methods of Richardson and species were either electrochemically generated in water or D 2 0 Taube31 and of Ammar and S a ~ S a nhave t ~ ~ been utilized. In the solutions or chemically obtained by reduction of M HC1 following the standard potentials will be considered equal to solutions of the [3’,2,3’], [3’,2,3], and [3,2,3] complexes with 1 half-wave potentials. According to the first method,31 from the equiv of E d + or by comproportion from equimolar solutions of cathodic and anodic peak potentials, the values of 183 f 3 mV the fully oxidized and reduced complexes. The results obtained and 103 f 3 mV have been obtained for the first and second were independent of the method of prepartion of the semireduced half-wave potentials of the two one-electron successive steps. The complexes. second method32 is based on the convolutive potential sweep In the fully reduced or semireduced state all of the trinuclear voltammetric analysis of the sole cathodic peak. With this procomplexes tend to dissociate slowly in aqueous solution, giving cedure the values E o I = 182 f 3 mV and EoII= 102 f 3 mV R U ( N H ~ ) ~ ( H ~ or O )R~u+( N H ~ ) ~ P Y ( H ~ Oand ) ~ the + , [2’,2] and have been obtained, in very good agreement with the ones pre[2,2] or [3’,2] and [3,2] binuclear species. All the results involving viously determined by using the other method. the [2’,2,2’], [2’,2,2], and [2,2,2] and [2’,2,3’], [2’,2,3], and [2,2,3] The constant K , for the comproportionation equilibrium ions refer to freshly prepared solutions and to time periods (2-3 h) in which the effects of the decomposition reaction were totally negligible. [3’,2,3’] + [2’,2,2’] 2 [3’,2,2’] (3) EdectrochemicalBehavior. Similar to what was found for [3,2,3] determined by means of the equation K, = exp[F(EoI- EoII)/RT] and [3,2],”*18the [3’,2,3’], [3’,2], and [3’,2,3] complexes present was found to be equal to 23 from which it can be deduced that one oxidation wave at rather positive potentials (Table I) and two reduction waves at rather negative potentials (not shown in Table I), which can be indicated as typical of the Ru(bpy),(CN), unit. (29) Matsubara, T.; Ford, P.C. Inorg. Chem. 1976, 15, 1107. The redox potentials of the [3,3]/[3,2], [3’,3]/[3’,2], [3,3,3]/ (30) Diamond, S. E.;Tom, G . M.; Taube, H. J . Am. Chem. SOC.1975, 97,

+

(28) Farnia, G.; Roffia, S. J . Electroanal. Chem. Interfacial Electrochem. 1981, 122, 347.

266 1. (31) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278. (32) Ammar, F.; Saveant, J. M. J . Electroanal. Chem. Interfacial Electrochem. 1973, 47, 215.

Inorganic Chemistry, Vol. 27, No. 2, 1988 411

Polynuclear Ruthenium Complexes

2.5

2.0

1.5

1.0

pm-l

Figure 1. Absorption spectra of [2’,2,2’] (-), [2’,2,3’] (-), and [3’,2,3’] (---) in water.

2.5

2.0

1.5

Figure 2. Absorption spectra of [2’,2,2] (-), [2’,2,3]

1.0 (a*.),

pnl-1

and [3’,2,3]

(---) in water.

the species [3’,2,2’] is present in half-electrolyzed solutions to a 70% extent. [3’,2,3]. In agreement with what is expected on the basis of the inequality of the two lateral Ru(II1) groups, the apparently single wave found at potentials near zero for [3’,2,3’], splits for [3’,2,3] into two waves of equal height, which correspond to two one-electron reversible reductions. The comparison with the potentials of the two redox couples [3’,2,3’]/[2’,2,3’] and [3’,2]/[2’,2] (see Table I) indicates that the more positive reduction process can be attributed to the reduction of the tetraammine(pyridine)ruthenium center according to [3’,2,3]

+ e-

@

[2’,2,3]

(4)

while the more negative one corresponds to the reduction of the Ru(II1) pentaammine unit: [2’,2,3] + e- s [2’,2,2] (5) Since the difference in the potentials of the two sequential electron transfers is high, no convolution technique is required in this case, and the values of the two processes (see Table I) have been calculated as the peak potentials minus 29 mV. The value of the constant K, for the comproportionation equilibrium [3’,2,3]

+ [2’,2,2] & 2 [2’,2,3]

(6)

calculated as previously described for [3’,2,2’], is 30 X lo3, indicating that 99% of [2’,2,3] is present at the equilibrium in a semireduced solution of [3’,2,3] or in an equimolar mixture of [3’,2,3] and [2’,2,2]. UV-Visibile and Near-IR Absorption Spectra. The spectra of aqueous solutions of the trinuclear ions [2’,2,2’], [2’,2,3’], and [3’,2,3’] are reported in Figure 1, and those of [2’,2,2], [2’,2,3], and [3’,2,3] in Figure 2. Considering the equilibrium constants of the comproportionation equilibria shown in eq 3 and 6, the spectrum of the [2’,2,3] ion reported in Figure 2 coincides with that of semireduced [3’,2,3] solutions. The spectrum of [2’,2,3’], which is reported in Figure 1, was obtained from that of semireduced [3’,2,3’] solutions, by considering the equilibrium con-

Figure 3. Schematic representation of the trinuclear ions and the optical electron-transfer processes observable in the UV-vis and N I R spectral regions.

centration of the various species (see previous section) and using the known spectra of the [2’,2,2’] and [3’,2,3’] ions. In agreement with the K , value for the comproportionation equilibrium shown in eq 6, two distinct sets of clean spectral variations were observed during the oxidation of aqueous solutions of the [2’,2,2] ion by air: a first isosbestic point at 1.75 pm-l was maintained until the spectrum of the [2’,2,3] species was reached, and then the spectral variations leading to the [3’,2,3] absorption occurred with two isosbestic points at 1.07 and 1.85 pm-’. The same behavior was observed (in the reverse order) in the electrochemical reduction of [3’,2,3], For a good definition of the low-energy absorption bands in the N I R region, the spectra of [2’,2,3’] and [2’,2,3] ions were recorded in D 2 0 solutions. Photophysical Behavior. Fluid solutions of the [2,2], [3,2], [3,2,3], and [2,2,2] species had not been previously found to exhibit luminescence in the 500-900-nm spectral range or transient absorption signals in laser flash photolysis experiment^.'^ The same behavior was exhibited by the complexes containing the Ru(NH&py unit examined in the present work. Luminescence and absorption laser flash photolysis measurements were also carried out in glassy solvents (CH30H/C2H50H) at 77 K. For all the complexes, the long-lived emission of the R~(bpy)~(cN chromophore )~ was completely quenched by the presence of Ru(NH3);+l3+ and R u ( N H ~ ) ~ ~moieties, ~ ~ + / ~and + no transient phenomena at a time longer than 50 ns were observed. Discussion Absorption Spectra. As in the case of the [2,2,2], [3,2,2], and [3,2,3] ionsI7 the intense absorption bands exhibited by all the trinuclear complexes in the near-UV-vis and near-IR regions can be discussed in terms of electronic transitions between localized ~ i t e s . ~The ~ - ~possible transitions4143 are shown schematically (33) This is true even for transitions involving directly bound metal and ligands. In fact for the d l r * transition of the Ru(bpy),** chromophore, there is now good evidence that the excited state corresponds very closely to a Ru’11(bpy)(bpy’-)2+structure.3w (34) Carlin, C. M.; De Armond, M. K. Chem. Phys. Lett. 1982, 89, 297. (35) Braterman, P. S.;Harriman, A.; Heath, G. A,; Yellowlees, L. J. J . Chem. SOC.,Dalton Trans. 1983, 1801.

412 Inorganic Chemistry, Vol. 27, No. 2, 1988

Bignozzi et al.

Table 11. Energies (pm-I), Half-Widths (pm-’) and Extinction Coefficients (M-lcm-’) of the Charge-Transfer and Intervalence-Transfer Transitions Obtained from a Semiempirical Gaussian Analysis of the Spectra

-

transition d-** Ru, bpy (a) d-a* Rut, d-r* Rut d-x* Rut,

IT Ru, IT Ru,

IT Rut IT Rut, IT Ru

-

+

-

+

[3,21 [2,21 [3’,21 2.48“ 2.42’ 2.50” 0.46 0.50 0.46 6850 5500 6800

py (b) bpy (c)

[3,2,31 2.55“ 0.49 6750

2.00‘ 0.50 1850

bpy (d)

Rut (e)

[2’,21 2.40‘ 0.48 6800 2.4Sb 0.41 8000

[3,2,21 2.51’ 0.50 6200

[2,2,2] 2.50b 0.50 5500

1.92‘ 0.50 1600

2.0OC 0.50 1850

[3’,2,31 2.57” 0.46 6600

2.08‘ 0.46 1800 1.44“ 0.45 3500

1.52“ 0.44 2900

Rut, (f)

1.46” 0.44 3250

1.52b 0.44 2900 1 .36’ 0.44 3000

1.28“ 0.43 3800

Rut (g)

[2’,2,3] 2.52’ 0.46 6200 2.46b 0.41 7600

2.08‘ 0.46 1750 1.47” 0.46 3100

[2’,2,2] 2.45b 0.46 6500 2.46’ 0.41 8000 1.95‘ 0.50 1800 2.09c 0.47 1800

[3’,2,3’] 2.60“ 0.48 6800

1.36“ 0.44 3000

[2’,2,3’] 2.52’ 0.46 6300 2.46b 0.41 7700

[2’,2,2’] 2.45b 0.49 6800 2.45b 0.41 8000

2.07c 0.46 1600

2.08‘ 0.46 1800

1.33” 0.47 2900

0.95d 0.42 350

Rut (h)

1.OOd 0.42 370

Rut, (i)

0.82d 0.42 350

“-dThe band parameters, from top to bottom (i&, Aij,12, emax), have been obtained as explained in ref 44. Ru, = central Ru; Rut = Ru of terminal R U ( N H ~ ) ~ * +unit; / ~ + Rut, = Ru of terminal R u ( N H ~ ) ~ unit. ~ ~ ~ + ~ ~ +

-

I for the various complexes in Figure 3, and can be labeled as I follows: (a) Ru(I1) bpy(d-.n*) MLCT originating in the EX10-3 ruthenium atom of the Ru(bpy)?+ moiety; (b) Ru(I1) py(dlr*) MLCT originating in the ruthenium atom of the R u ( N H ~ ! ~ ~ ~ * + bpy(d-?r*) remote MLCT originating in moiety; (c) Ru(I1) the ruthenium atom of the R U ( N H ~ ) ~ moiety; ’+ (d) Ru(I1) bpy(d-?r*) remote MLCT originting in the ruthenium atom of Ru(II1) IT between the R U ( N H ~ ) moiety; ~ ~ ~ ~(e)+ Ru(I1) ruthenium atoms of adjacent Ru(bpy)?+ and R u ( N H ~ ) ~ ~ + Ru(II1) IT between ruthenium atoms of moieties; (f) Ru(I1) adjacent Ru(bpy)?+ and R U ( N H ~ ) ~ Pmoieties; Y~+ (g) Ru(I1) Ru(II1) remote IT between ruthenium atoms of two terminal pentaammine units; (h) Ru(I1) Ru(II1) remote IT between 2.5 2 . 0 1.5 1.0 pm-1 ruthenium atoms of the terminal tetraammine-pyridine and pentaammine units; (i) Ru(I1) Ru(II1) remote IT between Figure 4. Absorption spectra of [3,2,3] (---I, [3’,2,3] (-.-), and [3’,2,3’] in water. ruthenium atoms of two terminal tetraammine-pyridine units. All of these transitions can be regarded as particular cases of intraI molecular optical electron transfer, although the metal-to-ligand transitions are traditionally called “charge transfer” (MLCT) and the metal-to-metal transitions are traditionally called “intervalence transfer” (IT). The spectra of the trinuclear complexes are shown in Figures 1, 2, and 4. The comparison between spectra of species differing in the oxidation state (Figures 1 and 2) or in the type (Figure 4, and Figure 1 vs Figure 2) of the terminal ruthenium ammine units reveals the presence of several types of transition for each complex.

-

-

-

-

~

-

~

-

-

(e-)

~

~

(36) Bradley, F. G.; Kress, N.; Hornberger, B. A.; Dallinger, R. F.; Woodruff, W. H. J . Am. Chem. SOC.1981, 103, 7441. (37) Forster, M.; Hester, R. E. Chem. Phys. Lett. 1981, 81, 42. (38) Kober, E. M.; Meyer, T. J. Znorg. Chem. 1982, 22, 3967. (39) Smothers, W. K.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 205, 1067. (40) Caspar, J. V.;Westmoreland, T.D.; Allen, G. H.; Bradley, P. G.; Meyer, T. J.; Woodruff, W. H. J . Am. Chem. Soc. 1984,106, 3492. (41) Not all of the possible transitions between localized sites are actually enumerated below, Notably, MLCT and LMCT transitions involving the cyanide ligands and Ru(I1) and Ru(II1) centers, respectively, are not considered. These transitions, in fact, are expected to occur at relatively high energy, based on the spectra of appropriate model cornpounds (Ru(NH )J(CH3CN)2+,42(NH3)5RuNCRu(CN),2-.43Ru(NH3)5(CH3CN)4+$2RU(NH~)~NCRU(CN)~-).” (42) Clarke, R. E.; Ford, P. C. Inorg. Chem. 1970, 9, 227. (43) Vogler, A,; Kisslinger, J. J . Am. Chem. SOC.1982, 104, 231 1.

2.5

2 . 0

1.5

1.0

pm-l

Figure 5. Absorption spectrum of [2’,2,3] in water. The dotted lines represent an approximate Gaussian analysis of the spectrum.

Arguments that can be used to assign the transitions are (i) MLCT transitions disappear by Oxidation Of the Ru(ll) center from which they originate, (ii) IT transitions disappear by oxidizing the Ru(II) center from which they originate or by reducing the Ru(II1) center on which they terminate, and (iii) transitions that originate from the Ru(I1) center (terminate on the Ru(II1) center) of a tetraammine-pyridine unit are displaced to the blue (to the red) by ca. 0.2 eV (Table I) with respect to the corresponding transitions

Polynuclear Ruthenium Complexes

Inorganic Chemistry, Vol. 27, No. 2, I988 413

Scheme I

A

involving the pentammine unit. The types of transitions identified in this way for the various bi- and trinuclear complexes are summarized in Table 11. In all cases, the spectrum can be reproduced with good accuracy as the sum of Gaussian-shaped bands corresponding to the various types of transitions, by using reasonable band parameters (energy, maximum absorptivity, and half-width, also given in Table II).44 An example of spectral analysis is given in Figure 5 for the [2’,2,3] complex. With the addition of a new band, type b, the complexes reported in this work exhibit the same variety of transitions shown by the [2,2,2], [2,2,3], and [3,2,3] systems studied in a previous paper.17 A detailed discussion of these spectra could thus be made along the same lines.17 Importantly, the spectra reported in this work definitely confirm the previous intriguing observation of two types of “remote” optical transitions. A d-?r*, type c transition that involves the ruthenium atom of the terminal pentaammine group and the bipyridine ligand of the central Ru(bpy)?+ unit had been previously identified at 2.1 and 1.92 fim-’, respectively, in the spectra of the [2,2,2] and [2,2,3] ions.17 In addition to IT transitions and ligand-to-ligand charge-transfer tran~itions,4~ this type of “remote” MLCT transition represented a new example of optical charge-transfer transitions involving spatially separated redox sites.17 By comparison of the spectral features of the [2’,2,3] and [2’,2,2] complexes (Figure 2), the absorption band due to the type c transition can readily be identified. As a matter of fact, while the intensity of the 2.5-pm-’ absorption of the [2’,2,3] complex is similar to that of the [2’,2,2] complex, a 50% increase in the intensity a t 2.0 pm-’ with the appearance of a clear shoulder is observed in going from the [2’,2,3] to the [2’,2,2] complex. The analogous d-?r* type d transition should be exhibited by the [2’,2,2’], [2’,2,3’], [2’,2,2] and [2’,2,3] complexes, containing the Ru(NH~)~PY,+ unit. On the basis of the known redox potentials, these transitions are (44) All the bands in the spectra are assumed to have a Gaussian shape, with

the relevant band parameters defined by

As) = fmax exp(-[(e - y,,)/O.O6O1(A~1/,)I2) The band parameters given in Table I1 have been chosen, depending on the particular system, on the basis of (a) direct evidence (when wellresolved bands are present), (b) parameters of the same type of band in suitable model compounds, (c) comparison with systems exhibiting the same type of transition and being related to the one under examination by known differences in the redox potential of the relevant redox sites, and (d) Gaussian fit of residual adsorption. From the values given in Table 11, the actual spectrum of every single species can be reproduced in the spectral range of interest as a sum of individual bands to a precision of f5%. (45) Vogler, A,; Kunkely, H. J . Am. Chem. SOC.1981, 103, 1559 and references therein.

expected to the blue of type c transitions and highly superimposed to type a transitions. Thus this transition does not give rise to definite shoulders, although it contributes to the overall intensity in the region around 2.2 pm-’ (Figures 1, 2, and 5). The most interesting feature of the intervalence properties exhibited by the [2,2,3], [2’,2,3], and [2’,2,3’] complexes is the presence of end-to-end interaction between the terminal ruthenium centers. In a previous study of the [2,2,3] spectrum, a longwavelength tail on the main IT absorption band was assigned to the IT transition between the Ru(I1) and Ru(II1) atoms of the two pentaammine units17 (estimated energy of the corresponding band, 0.95 pm-I).

hv

[2,2,31 (B) [3,2,21 (7) In the same low-energy region, also the [2’,2,3] and [2’,2,3’] complexes show evident band tails very similar to that previously reported for [2,2,3]. These tails are real spectral features that are typical of the semireduced form of the trinuclear complexes. In fact, they are totally absent in the spectra of the fully reduced or oxidized complexes (Figures 2 and 1). These absorption features can be assigned to end-to-end transitions of types h and i. hu

[2’,2,31 [2’,2,3’]

hv

(0

[3’,2,21

(8)

[3’,2,2‘]

(9)

Approximate energies of transitions h and i (Table 11) can be estimated through Gaussian analysis of the spectra of the [2’,2,3] and [2’,2,3’] complexes (see, e.g., Figure 5 ) and appear to be consistent with the energetics of the two systems. An end-to-end IT transition at 0.59 pm-l was observed by Taube in trans-[(NH3)sRu(pz)Ru(NH3),(pz)RU(NH,)5]7+.14The comparison of this energy value with the ones observed in the present work is puzzling.46 In fact, according to Hush’s theory, the reorganizational energy (and the IT energy) should increase as the distance between the redox centers is increased. On the contrary, the end-to-end IT energy is higher for the short cis complexes of this study (estimated end-to-end distance, 7 A) than for the longer trans trimer studied by Taube (estimated end-to-end distance, 14 A).47 This fact may be related to a substantial difference between the two types of complexes that is not considered by the simple Hush model: in these cis complexes the NC-Ru(bpy),-CN “bridge” is bent and the region between the (46) We thank Professor H. Taube for bringing this point to our attention. (47) The distance between the terminal ruthenium atoms has been estimated by using CPK space-filling molecular models.

Inorg. Chem. 1988, 27, 414-417

414

two redox centers is occupied by solvent molecules, whereas in the trans trimer the solvent is kept away from this region by the bulky linear pz-R~(NH,)~-pz“bridge”. It is conceivable that the solvent molecules in this critical intersite region of the cis complexes will experience a substantial reorganization upon electron transfer, a contribution to the overall reorganizational energy that is not present in the case of the trans trimer. Photophysical Behavior. As previously observed for the pentaammine systems [2,2,2], [3,2,2], and [3,2,3],17 the tetraamminepyridine systems investigated in this study ([2’,2,2’], [2’,2,2], [3’,2,3’], [3’,2,3], [2’,2,3’], and [2’,2,3]) do not emit. Although for the oxidized and semioxidized forms perturbation of the d-a* triplet of the Ru(bpy)z chromophore by the paramagnetic Ru(II1) centers could be a possibility, the most likely explanation for the general lack of d-r* emission is radiationless deactivation via low-lying “remote” d-r* (tpye c and d) or IT (type e and f) states. Deactivation schemes for such processes are similar to those previously reported for the pentaammine systems.” An intriguing, new possibility that is specific to the semioxidized mixed pentaammine-tetraammine-pyridine complex [2’,2,3] is depicted in Scheme I. In this scheme, processes 2 + 5 and 1 + 4 represent the possible deactivation pathways of the Ru(bpy)* d-a* excited state via the “remote” d-r* and IT states, respectively. These pathways are common to all the other trinuclear complexes of the series. For this complex, however, an additional possibility is that secondary hole-transfer (process 6) or electron-transfer (process 7) steps convert these intermediate states to the end-to-end IT state. This deactivation scheme is reminiscent of the intramolecular electron transfer sequence involved in photoinduced charge separation in organic molecular triads.l”zz The end-to-end IT state has the reduced and oxidized sites sep-

arated by two bridging groups and a central chromophore, and can thus be considered analogous to the charge-separated state of the organic triads. The end-to-end IT state should be readily observable by spectroscopic means, based on the shift in the visible IT band with respect to the ground state (see, for comparison, the spectra of the [3,2,3] and [3’,2,3’] species). The experimental lack of any observable signal in the laser flash photolysis of the [2’,2,3] must be due either to inefficient population or to the short lifetime of the charge-separated, end-to-end IT state. While the latter explanation may be plausible (calculations using the Hush model and the available spectroscopic information give expected lifetimes for this state in the 0.5-2-11s range), the most likely hypothesis seems to be that of inefficient population of the state. As a matter of fact, if process 4 is compared with process 6, the two processes are virtually identical as far as the electronic factors and the intrinsic barriers are concerned, while the former has a more favorable driving force. As both processes are only moderately exergonic and thus likely to lie in the “normal” free energy region of the Marcus theory,48the former is also expected to be the fastest one. Similar arguments can be made if process 5 and process 7 are compared. Registry No. [2,2], 94499-30-6; [3,2](PF6),, 94499-27-1; [3,3], 11 1557-17-6; [2’,2], 11 1557-19-8; [3’,2](PF,),, 11 1557-12-1; [3’,3], 11 1557-18-7; [2,2,2], 94499-29-3; [3,2,2], 94499-28-2; [3,2,3](PF6)6, 94499-25-9; [2’,2,2], 111557-22-3;[2’,2,3], 11 1557-21-2; [3’,2,3](PF6)6, 111557-16-5; [3’,3,3], 111557-20-1; [2’,2,2’], 111557-25-6; [2’,2,3’], 11 1557-24-5; [3’,2,3’1(PF6)6, 111557-14-3; [3’,3,3’], 11 1557-23-4; trans-[R~(NH~)~(SO~)py]Cl, 63251- 18-3;cis-Ru(bpy),(CN),, 2050636-9; [Ru(NH~)SCI]CI~, 18532-87-1. (48) Marcus, R. A. Annu. Rev. Phys. Chem. 1964, 15 155.

Contribution from the Department of Chemistry, University of Illinois at Chicago, Box 4348, Chicago, Illinois 60680

Magic Numbers in Atom and Electron Counting of Supraclusters Based on Vertex-Sharing Icosahedra Boon K. Teo* and H o n g Zhang Received May 27, 1987

The concept of cluster of clusters (C2) is described. In particular, we consider the series of supraclusters based on vertex sharing of centered icosahedra. General formulas for magic numbers in atom and electron counting are given along with examples and predictions. Possible synthetic approaches are suggested. It is hoped that the present results and the methodology illustrated will provide a rational pathway to large metal clusters of increasing nuclearity formed via vertex, edge, or face sharing of smaller cluster units as building blocks. Introduction High-nuclearity gold phosphine clusters are often based on centered 13-atom icosahedral units (Figure la).l Recently, we reported the structure of a 25-atom cluster containing 13 Au and 12 Ag atoms: viz., [(Ph3P),,Aul3Ag,,Br,]+ (1).2a Its metal framework (Figure l b ) can be considered as two Au-centered Au,Ag6 icosahedra sharing one vertex (nuclearity = 2 X 13 - 1 = 25). We also succeeded in isolating and structurally characterizing a new type of 38-atom cluster, [(R,P),zAulBAgz,CI,,] (1) For reviews of gold phosphine clusters, see, e&: (a) Steggerda, J. J.; Bour, J. J.; van der Velden, J. W. A. R e d . Trav. Chim. Pays-Bas 1982, ZOI, 164. (b) Hall, K. P.; Mingos, D. M. P.Prog. Inorg. Chem. 198s. 237-32s. (c) Puddephatt, R. J. The Chemistry of Gold Elsevier: 1978. (2) (a) Teo, B. K.; et al. to be submitted for publication. (b) Teo, B. K.; Keating, K. J . Am. Chem. SOC. 1984, 106, 2224.

0020-1669/88/1327-0414$01.50/0

(2) (where R = p-MeC6H4).3a This cluster can be visualized as three Au-centered Au7Ag6icosahedra sharing three corners in a triangular arrangement, which gives rise to a hypothetical 36atom cluster (Figure IC); the linkage of two additional exopolyhedral Ag atoms to the top and the bottom Ag, triangles along the threefold axis results in the observed 38-atom cluster (2). The most interesting structural characteristic of these two clusters is that both can be considered as being built up from smaller cluster units by vertex sharing. This opens up new pathways to larger metal clusters via fusion of smaller cluster units as building blocks. We refer to this particular oligomerization (3) (a) Teo, B. K.; Hong, M. C.; Zhang, H.; Huang, D. B., submitted for publication. For a brief account see also: Chem. Eng. News 1987, 65(2), 21. (b) Teo, B. K.; Hong, M. C.; Zhang, H.; Huang, D. B. Angew. Chem., In?. Ed. Engl. 1987, 26, 897.

0 1988 American Chemical Society