Organometallics 1995, 14, 640-649
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Rate and Mechanism of the Reductions of Iron Pentacarbonyl and Chromium Hexacarbonyl to Their Metalate Complexes Christian Amatore,*?+ Paul J. &sic,$ Steen U. Pedersen,g and Jean-Noel Verpeauxt Dhpartement de Chimie de I%cole Normale SupLrieure, URA CNRS 1679,24 rue Lhomond, F-75231 Paris Cedex 05, France, Central Research and Development Department, E. I. du Pont de Nemours and Company, Experimental Station, E328 Wilmington, Delaware 19898, and Department of Chemistry, Aarhus University, DK-8000 Aarhus, Denmark Received April 11, 1994@ The electrochemical reductions of Fe(C0)5 and Cr(C0)s in THF were shown to proceed by a n ECE mechanism leading to the electrogenerated dianions Fe(C0h2- and cr(c0)s2-, respectively, The initial 19-electron anion radical Fe(C0)5- could not be observed by the fastest direct electrochemical methods but was shown to have a n approximate lifetime of 10 ns. CI-(CO)~-also could not be observed by fast scan cyclic voltammetry, and its lifetime was estimated to lie in the range 50 ps to 10 ns. In the absence of a n electrophile, the electrogenerated dianions further react slowly via a nucleophilic substitution reaction with the parent Fe(CO)5 and Cr(CO)s to yield the dimers Fe2(CO)s2- and Cr2(CO)lo2-,respectively. The corresponding rate constants were estimated a t 120 (Fe(C0h2- Fe(C0)5) and 0.95 M-l s-l (Cr(C0)52- Cr(C0)s). Although these rate constants are rather modest, their magnitudes are sufficient to explain why Fez(CO)s2-and Cr2(CO)lo2-are the major products of the electroreductions of Fe(C0)5 and Cr(CO)6 when the electrolyses are performed under classical conditions (viz., batch electrolyses; telecL 0.5 h). Conversely, when generated by fast exhaustive electrolysis in a percolating flow cell (telecless than a few seconds), Fe(C0h2is the single electrolysis product t h a t remains stable for minutes in the dry electrochemical medium in the absence of Fe(CO15.
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Introduction Two-electron reduction via single electron transfer steps of simple metal carbonyls M(CO), is the preferred route to anionic M(C0),-12- “ate” complexes, which are powerful nucleophiles and reducing reagents widely used in organic and organometallic chemistry.lP2 The electrochemical reduction of metal carbonyls has therefore received particular attention from the early days of organometallic electr~chemistry.~ The first reports concerning iron pentacarbonyl led to the conclusion that the anion radical Fe(C0)4-, formed upon one-electron reduction and CO cleavage, underwent a fast dimerization t o yield the binuclear dianion Fe2(C0)s2- in a oneelectron p r o ~ e s s .Such ~ a scheme implied two consequences: (i) the mononuclear dianion Fe(C0h2- could only be formed upon further reduction of Fe2(CO)s2-, Ecole Normale Superieure. E. I. du Pont de Nemours and Co. 3 Aarhus University. Abstract published in Advance ACS Abstracts, December 15,1994. (1)(a) See: Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Eds.; Pergamon Press: Oxford, 1982. (b) Connelly, J. P.; Geiger, W. E. The electron transfer reactions of mononuclear organotransition metal complexes. In Advances in Organometallic Chemistry; Stone, F . G. A,, West, R., Eds.; Academic Press: Orlando, FL, 1984;Vol. 23. (2) Collman, J. P. Ace. Chem. Res. 1975,8,342. (3) (a) Vlcek, A. A. Nature (London) 1956,1043. (b) Dessy, R. E.; Stary, F. E.; King, R. B.; Waldrop, H. J. A m . Chem. SOC.1966,88, 471. (4)(a) F’ickett, C. J.; Pletcher, D. J . Chem. SOC.,Dalton Trans. 1975, 879. (b) Bond, A. M.; Dawson, P. A,; Peake, B. M.; Robinson, B. H.; Simpson, J . Inorg. Chem. 1977,16,2199.(c) Bezems, G.J.; Rieger, P. H.; Visco, S. J . Chem. SOC.,Chem. Commun. 1981,265. (d) El Mum, N.; Chaloyard, A. Inorg. Chem. 1982,21,2206. +
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i.e., by using a second equivalent of powerful reductant such as sodium amalgam or sodium n a ~ h t h a l e n eand ,~ (ii)an electrochemical preparation of this mononuclear dianion would then prove impossible since Fe2(CO)s2is generally not reducible a t an electrode before the reduction of the supporting electrolyte or the medium. In a more recent study,6 the use of microelectrodes allowed us to determine the standard potential for the couple Fe(CO)d2-/Fe(C0)4-. Its value proved unambiguously that dimerization of the anion radical Fe(C0)4could not compete with its reduction to Fe(C0h2- when the potential was set on the reduction wave of iron pentacarbonyl. In other words, reduction of Fe(C015 had to follow an ECE mechanism (reactions 1-3, Scheme 11, leading directly to tetracarbonyl ferrate Fe(C0)42-.
Scheme 1
+ e == Fe(CO),Fe(CO),- - Fe(CO),- + CO Fe(CO),- + e Fe(C0);Fe(C0);- + Fe(CO), - Fe2(C0)?- + CO Fe(CO),
(1) (2)
(3)
(4)
This result was very recently confirmed by infrared spectroelectr~chemistry.~ However, this scheme implies (5)(a) Strong, H.;Krusic, P. J.; San Filippo, J., Jr. Inorg. Synth. 1986,24,157. (b) Heck, R. F.; Breslow, D. S. J . Am. Chem. SOC.1963, 85, 2779. (6)h a t o r e , C.; Verpeaux, J.-N.; Krusic, P. J. Organometallics 1988, 7, 2426.
0276-733319512314-0640$09.00/0 0 1995 American Chemical Society
Reduction of Fe(C0)S and Cr(C0)s to Metalates a two-electron consumption, in apparent disagreement with the actual overall one-electron consumption and with the formation of Fe2(CO)s2- as the major product observed on the longer time scale of preparative elect r ~ l y s i s .To ~ reconcile these results, a coupling reaction between Fe(C0)42- and unreacted iron pentacarbonyl had to be considered (reaction 4, Scheme 1). The reaction of Fe(C0)42-with Fe(C0)5 (eq 4) is wellknown with sodium as countercation (Collman's reagent) and provides a convenient chemical route to NazFe~(C0)8.~ Its rate constant (sodium salt in acetonitrile at 27 "C) was recently measured by infrared stopped-flow techniquesg and was found to be 4.6 M-l S-1. A major consequence of the direct generation of Fe(C0)42- upon ECE reduction of iron pencarbonyl is the possibility of in situ electrogeneration and alkylation of this highly sensitive reagent by electrolysis of Fe(C0)5 in the presence of an electrophile, such as a n organic halide. Such a scheme appears possible if the alkylation is fast enough t o compete with the coupling reaction (4) leading to Fe2(CO)s2-. While investigating the practical feasibility of the electrogeneration of alkylferrates, we found that alkyl halides that should not have been able to compete with Fe(C0)5,according to the second-order Fe(C0)42- coupling rate constant of the Fe(C0)5 reaction (in the lo6 M-l s-l range) that we reported in our preliminary communication,6 actually led to substantial amounts of alkylation.1° We therefore questioned our previous determination and found that the true value (tetrabutylammonium countercation in THF a t 20 "C) is much smaller and more compatible with the value reported by Atwood et ale9 In this paper, we wish to describe in detail the electrochemical reductions of iron pentacarbonyl and chromium hexacarbonyl using modern electrochemical techniques. We also wish to rectify the erroneous rate constant given in our preliminary communication6for the Fe(C0)5 Fe(C0)42-reaction. This error illustrates the difficulties encountered in the determination of the absolute number of electrons involved in transient electrochemical experiments in the absence of independent knowledge of the pertinent diffusion coefficients (for a discussion of such aspects, see ref 11).
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Results and Discussion Electrochemical Reduction of Fe(C0)s and Electrogeneration of Fe(C0)42-. Some of the following results on the cyclic voltammetry of Fe(C0)5 in THF have already been presented in a preliminary communication.6 Two cyclic voltammograms are shown in Figure 1. At a potential scan rate of 1V s-l, the results are essentially the same as those reported by previous author^:^ an irreversible reduction wave R1 (peak potential, EP = -2.67 V us Ag/Ag+) is followed by two associated anodic peaks 0 2 and 0 3 (Figure 1, left). A (7) Curran, D. J.; Graham, P. B.; Rausch, M. D. Organometallics 1993, 12, 2380. (8)(a) Collman, J. P.; Finke, R. G.; Matlock, P. L.; Wahren, R.; Brauman, J. I. J . Am. Chem. SOC.1976, 98, 4685. (b) Collman, J . P.; Finke, R. G.; Matlock, P. L.; Wahren, R.; Komoto, R. G.; Brauman, J. I. J . Am. Chem. SOC.1978,100,1119. (9) Zhen, Y.; Atwood, J. D. Organometallics 1991, 10, 2778. (10)Amatore, C.; Pedersen, S. U.; Verpeaux, J . N., unpublished results (1993-1994). (11)Amatore, C.; Azzabi, M.; Calas, P.; Jutand, A,; Lefrou, C.; Rollin, Y . J . Electroanal. Chem. 1990, 288, 45.
Organometallics, Vol. 14, No. 2, 1995 641
c
n -1.5
h
1
-2.0
I
1
1
-2.5
1 - 1
1
-3.0 -1.5
-2.0
-2.5
-3.0
Potential, V vs AglAg+ Figure 1. Cyclic voltammetry of 2 mM Fe(CO)Sin THF (containing 0.3 M NBu4BF4) at a gold disk electrode (0.5 mm diameter); scan rates of 1(left) or 20 (right) V s-l at 20 "C. third oxidation wave 0 4 can also be seen at a much more positive potential (-0.09 VI. Wave 0 3 is chemically reversible, as shown by the reduction wave R3 observed upon inversion of the potential scan after wave 0 3 . At faster scan rates &e., 20 V s-l and above), wave 0 2 starts showing chemical reversibility (Figure 1,right), while waves 0 3 and 0 4 become less prominent. Existence and Lifetime of the 19-Electron Fe(CO)s-. The very first step of the reduction process is the electron transfer to Fe(C015 and cleavage. Since no chemical reversibility of wave R1 could be observed even at potential scan rates as high as 300 000 V s-l, the initial 19-electron anion radical Fe(C015- must have a lifetime of less than 5 ps.12 Despite its very short lifetime, this intermediate could be trapped by an efficient hydrogen atom donor such as tributyltin hydride to yield the corresponding formyl complex:13 Fe(CO),-
+ R3SnH
-
[Fe(CO),(CHO)]-
+ R3Sn'
To allow an almost quantitative interception of Fe(C0)5under these conditions, the lifetime of this intermediate must be longer than 10 ns.14 It can be concluded that carbon monoxide loss from Fe(C015- occurs with a rate constant in the range of 2 x lo5 to 1 x IO8 s-l (vide infra). Redox catalysis experiments were undertaken in order to improve this rough estimate. Accordingly, several aromatic hydrocarbons giving rise to reversible (12)(a) For an EC system in cyclic voltammetry, a chemically irreversible voltammogram is observed for a scan rate u provided that the follow-up rate constant k is such that k >> u l A E , with AE = Ep, + Ep, (see Nicholson, R. S.; Shain, I. Anal. Chem. 1964,36,704). Since no reversibility was observed a t room temperature (or even at -40 "C) for u greater than lo5 V s-l, the above inequality implies that k > 2 x lo5 5 - l . (b) For an EC bimolecular process, the same limit is valid, provided that k is replaced by kCo (uiz.,kCO >> VIAE),where CO is the initial concentration of the electroactive species. (13) (a) Narayanan, B. A.; Kochi, J. K. J . Orgunomet. Chem. 1984, 272, C49. (b) Narayanan, B. A,; Kochi, J . K.; Amatore, C. Organometallics 1986, 5 , 926. (14) Under the conditions where the cleavage of CO (first-order rate constant, kl) competes with H atom transfer from Bu&d (secondorder rate constant, kz), the yield of the formyl derivative is given by kdBusSnHY(kz[Bu3SnH]+ k1). Since no decarbonylation product ( e g . ,
