Organometallics 1996, 14,2501-2505
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Electrochemical Behavior and Electron-Transfer Chain (ETC) Reactions of &RUq(CO)12 Domenico Osella,* Carlo Nervi, and Mauro Ravera Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universith di Torino, Via P. Giuria 7,10125 Torino, Italy
J a n Fiedler The J. Heyrovsky Institute of Physical Chemistry, The Academy of Sciences of the Czech Republic, Dolejskova 3, 182 23 Prague 8, Czech Republic
Vladimir V . Strelets Russian Academy of Sciences, Institute of Chemical Physics in Chernogolovka, Moscow Region, Chernogolovka 142432,Russian Federation Received November 15, 1994@ The electrochemical reduction of I-&R~q(C0)12 in tetrahydrofuran (THJ?) is studied by means of dc and ac polarography, cyclic voltammetry, and FT-IR spectroscopy following electrolysis in a optically transparent thin-layer electrochemical (OTTLE)cell. The reduction of the title complex generates a transient radical anion, [H4R~(C0)12]-, which produces the stable anion [H3R~q(C0)121-on a longer time scale. In the presence of triphenylphosphine, nucleophilic substitutions readily take place by electrochemical initiation and produce monoand bisubstituted derivatives in yields depending on PPh3 concentration. The termination side chain reaction is the loss of a hydrido ligand, to give the above-mentioned anion, which is inert to substitution even in the presence of a large excess of Lewis base.
Introduction Ru3(COh and H4RudCO)lZ are known to be excellent substrates for radical-anion-initiated reactions, in which nucleophilic substitutions occur very easi1y.l When a catalytic amount of sodium benzophenone ketyl (BPK) is added to a THF solution of phosphine ligand and Ru~(C0)12or H4Rua(C0)12,the substitution of the Lewis bases for CO is fast, selective, and efficientel In contrast, thermally activated nucleophilic substitutions take place producing mixtures of derivatives, with Ru3(CO)gL3 being the major product in the former case.2 The excellent results obtained by employing chemical reducing agents for both Ru3(C0)12 and H~RU~(CO)E should imply the existence of reasonably stable radical anions as intermediates of electron-transfer chain (ETC) reactions. Surprisingly, chemical and electrochemical initiation of Lewis base substitution on Ru~(C0)12did not give equivalent results.3 The redox behavior of the clusters M3(C0)12 (M = Fe, Ru, Os) has been extensively studied and this apparent conflict tentatively ex~ l a i n e d .To ~ our knowledge, no data on the electrochemical behavior of H4Ru4(C0)12 have been reported, and furthermore, in a recent review the tetrahydro @Abstractpublished in Advance ACS Abstracts, April 1, 1995. (1)(a)Bruce, M. I.; Kehoe, D. C.; Matisons, J. G.; Nicholson, B. K.; Rieger, P. H.; Williams, M. L. J. Chem. SOC.,Chem. Commun. 1982, 442.(b) Bruce, M. I.; Matison, J. G.; Nicholson, B. K.; Williams, M. L. J. Organomet. Chem. 1982,236,C57. ( c ) Bruce, M. I.; Matison, J. G.; Nicholson, B. K. J. Organomet. Chem. 1983,247,321. (2) Bruce, M. I. In Comprehensive Organometallic Chemistry, Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982. (3)(a) Cyr, J. C.; DeGray, J. A,; Gosser, D. K.; Lee, E. S.; Rieger, P. H. Organometallics 1986,4,950.(b) Downard A. J.; Robinson, B. H.; Simpson J. J. Organomet. Chem. 1987,320,363. ( c ) Cyr,J. E.; Rieger, P. H. Organometallics 1991,10,2153.(d) Osella D.; Hanzlik, J. Znorg. Chim. Acta 1993,213,311.
cluster is reported t o be redox i n a ~ t i v e ! Since ~ ETC reactions are strictly correlated with electrochemical properties, electrochemical and spectroelectrochemical methods are employed here in order t o elucidate this aspect of H4Ru(C0)12 reactivity.
Results and Discussion Electrochemical Behavior of R R U ~ C O (1). )~~ The dc polarographic response of a THF solution of 1 (Figure 1) exhibits a reduction wave (A)at E1dA) = -1.58 V us the ferrocenelferrocenium (Fc/Fc+)couple. The slope of the logarithmic plot analysis is 60 mV, near ~ the value expected for a l e Nernstian p r o ~ e s s .However, comparison with the l e oxidation wave of Fc (added in equimolar concentration as an internal standard)6 gives a limiting current ratio, il(A)lil(Fc),corrected for the different diffusion coefficients, equal t o 1.6. The diffusion coefficients of 1 and Fc have been estimated from their molecular sizes by using the Stokes-Einstein equation.? The molecular radii have been calculated from the crystallographic volumes of the molecule^^^^ assuming a spherical shape. A dc polarographic limiting current ratio higher than that expected for a l e process indicates that chemical complications (4)Drake, S.R. Polyhedron 1990,9, 455. (5)(a)Bard, A. J.; Faulkner, L. L. Electrochemical Methods; Wiley: New York, 1980.(b) Brown, E.R. Sandifer, J. R. In Physical Methods ofchemistry; Rossiter, B. W., Hamilton, G. F., Eds.; Wiley: New York, 1986;Vol. 11, Chapter IV. (6)Adams, R. N.; Electrochemistry at Solid Electrodes; Marcel Dekker: New York, 1969;p 214. (7)Bockris, J. O'M.; Reddy, A. K. M. Modern Electrochemistry; Plenum: New York, 1970;Chapter IV,p 380. (8)Seiler, P.; Dunitz, J. D. Acta Crystallogr. 1979,B35,2020. (9)Wilson, R. D.; Wu, S. M.; Love, R. A,; Bau, RInorg. Chem. 1978, 17,1271.
0276-733319512314-2501$09.0010 0 1995 American Chemical Society
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12l2-(by loss of hydrogen),which in turn can be reduced a t a more cathodic potential (wave B). On (13)Inkrott, K. E.;Shore, S. G . Inorg. Chem. 1979, 18,2817.
the contrary, several slow reactions are able to generate the stable anion [H3Ru4(C0)121-, further reducible a t E1/2(D) potential. Electron-Transfer Chain (ETC) Reaction of EF~Ru~(CO)I~ with PPb. Figure 7 shows the polarograms obtained when a THF solution of 1 (curve 1)is added with an equimolar amount of PPhs (curve 2) and then gradually electrolyzed a t a mercury-pool electrode, Eappl= -1.55 V (curves 3 and 4). The original polarographic wave A decreases and after consumption of 0.25 F/mol (curve 4) totally disappears. Two well-resolved waves are observed at El/z(E) = -1.73 and El/z(F)= -1.98 V, respectively, in addition t o wave D due to reduction of [H3Ru(C0)12]-. The new waves are unambiguously assigned to reduction of H4Ru4(CO)llPPh3 and H4Ru(CO)lo(PPh3)2,respectively, on the basis of polarographic results on authentic samples,2recorded in the same experimental conditions. The composition of the solution after 0.25 F/mol electrolysis can be estimated from the heights of the polarographic waves in the likely hypothesis that the diffusion coefficients of the products are the same: H ~ R u ~ ( C O ) I I(60%), PP~~
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Figure 7. Polarograms of a THF solution of 1 (1x M).Curve 1 corresponds to the polarogram of the initial solution, curve 2 corresponds to the polarogram obtained after addition of PPh3 (molar ratio l:l), and curves 3 and 4 correspond to polarograms obtained for the abovementioned mixture during exhaustive electrolysis at a mercury-poolelectrode, E,,] = -1.55 V, when 0.10 and 0.25 F/mol are consumed, respectively.
H ~ R u ~ ( C O ) ~ O (20%), ( P P ~ [H3Ru(C0)121~)~ (20%). The electrolyzed residue, extracted with hot cyclohexane, gives TLC and IR results qualitatively and quantitatively consistent with the electrochemical ones, as far as the neutral substituted products are concerned. An analogous experiment (Eappl= -1.55 V) with 5-fold excess of PPh3 leads, after 0.25 F/mol, to an electrolyzed solution containing H4Ru4(CO)llPPh3 (20%), H4Ru(C0)10(PPh3)2(70%),and [H3Ru(C0)121- (10%). In both experiments, the current yield of substitution is more than loo%, as expected for an ETC process. It is noteworthy t o recall that the electrochemical initiation must be always performed a t a potential corresponding to the foot of the polarographic wave A in order to produce the reactive radical anion 1-. When more cathodic potentials are applied, the efficiency of the process strongly decreases since an enhanced amount
of the stable anion [H~RU~(CO)~Z]is obtained, which proved to be totally inert toward PPh3 substitution. This makes the electrochemical activation poorly selective, since one can pilot the reaction only by means of the phosphinelsubstrate ratio and not by the applied potential. Interestingly, Bruce in his comprehensive worklCon BPK-initiated nucleophilic substitutions proposed that the minor efficiency found for H4Ru(CO)l2 with respect to Ru~(CO)~Z would be ascribed t o a chain-termination process, the likely loss of hydride ligands. Indeed, BPK initiation was not able to produce tri- and tetrasubstituted derivatives with PPh3; however, trisubstituted derivatives could have been obtained with ph0sphites.l' Multiscan CV responses a t a HMDE electrode of a THF solution of 1 are in agreement with the polarographic results: the original reduction peak A decreases in height when PPh3 is added, and two new peaks appear in correspondence of the reduction of H4Ru(CO)11(PPhd(Ep(E) = -1.76 V) and &Rw(CO)IO(PP~~)Z (Ep(F) = -1.99 V). A very weak peak is further observed only when a 10 times excess of PPh3 is employed: it corresponds to H4Ru4(CO)g(PPh3)3reduction as verified on an authentic sample (Ep= -2.31 V, 0.2V s - ~ ) As . ~ usual in ETC reactions,14the increases of scan rate (which shortens the time scale available for the chemical reaction) decreases the height of peaks due to PPh3-substituted derivatives. Experimental Section H4RudC0)12 (1) was synthesized from Ru3(CO)lZ according to the literature method.16 Triphenylphosphine was used as purchased from Aldrich. THF was distilled from sodium benzophenone ketyl just before use. Tetrabutylammonium (14)Kochi, J. K. J. Organomet. Chem. 1986,300, 139. (15)Knox, S.A. R.; Koepke, J. W.; Andrews, M. A.; Kaesz, H. D. J. Am. Chem. SOC.1975,97,3942.
Organometallics, hexafluorophosphate (Aldrich) was recrystallized three times from 95% ethanol and dried in a vacuum oven at 110 "C overnight. Electrochemistry was performed with an EG&G PAR 273 electrochemical analyzer connected t o a n interfaced personal computer and to an EG&G PAR Model 5210 lock-in amplifier (for ac measurements). A standard three-electrode cell was designed to allow the tip of the reference electrode to closely approach the working electrode. The pseudo-reference electrode was a silver wire dipped in a 0.1 M solution of [BudNI[PFs] in THF and separated from the cell solution by a Vycor frit. At the end of each experiment the potential of the ferrocene(O/l+) couple was measured, to which all data are referred. The working electrode for CV was a HMDE (Metrohm Model 6.0335); for polarography a dropping mercury electrode (DME) with flow rate of 1.22 mg s-l at a reservoir height of 0.5 m was employed. Drop time was controlled by an electromechanical hammer. The auxiliary electrode was a platinum wire sealed in glass. Positive feedback iR compensation was applied routinely. All measurements were carried out under Ar in anhydrous deoxygenated THF; solutions were 5x M with respect t o the compounds under study and 1 x 10-1 M with respect t o the supporting electrolyte, [Bu~NI-
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[PFs]. The temperature of the solution was kept constantly at 25 f 1 "C, by circulation of a thermostated water-ethanol mixture through the double wall of the cell. Spectroelectrochemistry was performed in an optically transparent thin-layer electrochemical (OTTLE) cell assembled as previously described.16 The corresponding IR spectra were measured on a Philips 9800 FT-IR spectrometer.
Acknowledgments. We thank the Minister0 dell'Universita e della Ricerca Scientifica e Tecnologica (MURST, Rome) for a fellowship (to J.F.) and the Council of National Research (CNR, Rome) for financial support. Johnson Matthey Ltd. is acknowledged for a generous loan of RuCl3, and Mr. P. A. Loveday (University Chemical Laboratory, Cambridge, U.K.) for highpressure synthesis of Ru3(CO)12. We are indebted to Professor M. I. Bruce (University of Adelaide) for his interest in this work. OM940870N (16)Krejcik, M.; Danek, M.; Hartl, F.J.Electroanal. Chem. 1991, 317, 179.
