Thermodynamic and Kinetic Aspects of Substituent Effects on the Redox Properties of the Carbonyl-Bridged Dirhodium Complexes (η-C5H5)2Rh2(μ-CO)(μ-t-BuC2-t-Bu) and (η-C5H5)2Rh2(μ-CO)(μ-CF3C2CF3)

J. W. Bixler, A. M. Bond, R. S. Dickson

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The electrochemical reduction and oxidation of the dirhodium(I) carbonyl bridged alkyne complexes (η-C5H5)2Rh2(μ-CO)(μ-t-BuC2-t-Bu) and (η-CF3C2CF3) have been investigated in detail at both platinum and mercury electrodes in dichloromethane and acetonitrile. Remarkable thermodynamic and kinetic differences exist between the t-BuC2-t-Bu and CF3C2CF3 analogues. The oxidation of (η-C5H5)2Rh2(μ-CO)(μ-t-BuC2-t-Bu) in dichloromethane produces a very stable cation, [(η-C5H5)2Rh2(μ-CO)(μ-t-BuC2-t-Bu)]+. This complex can be generated by controlled potential electrolysis and has been examined by ESR and other techniques. The corresponding dication and monoanion are less stable on the synthetic time scale although voltammetrically they can be detected as part of the four membered redox series In contrast to data in the noncoordinating solvent dichloromethane, a considerable degree of chemical irreversibility is observed, even on the voltammetric time scale, when oxidation is achieved in acetonitrile. This is attributed to the reactions [(η-C6H5)2Rh2(μ-CO)(μ-t-BuC2-t-Bu]+/2++ CH3CN [(η-C5H5)2Rh2(Co)(Ch3Cn) (μ- T-Buc2-t-Bu) ]+/2+. These oxidized acetonitrile derivatives readily undergo further oxidation and have no inherent stability. The substitution reaction (η-C5H5)2Rh2(μ-CO)(μ-RC2R) + CH3CN ⇋ (η-C5H5)2Rh2(CO) (CH3CN) (μ-RC2R) actually occurs for the neutral complex when R = CF3 and is formally an oxidative addition since a change in bonding mode of the alkyne accompanies the reaction. In contrast, no reaction with acetonitrile occurs for the neutral complex when R is the bulky t-Bu group. Substantial activation toward substitution after oxidation is therefore noted. Redox data at mercury electrodes are identical with that at platinum for the t-BuC2-t-Bu species. However, formation of a mercury derivative is indicated in the electrochemistry of (η-C5H5)2Rh2(μ-CO)(μ-CF3C2CF3) which is again consistent with the higher reactivity of the hexafluorobut-2-yne complex. In addition to a marked decrease in kinetic stability (increase in reactivity) achieved in replacement of the t-BuC2-t-Bu groups by the less bulky and more electron-withdrawing CF3C2CF3 alkyne, marked thermodynamic changes (E1/2 ≈ E° values) are also found. The CF3C2CF3 complex is harder to oxidize and easier to reduce than the t-BuC2-t-Bu derivative. However, despite the fact that in the thermodynamic redox sense, [(η-C5H6)2Rh2(μ-CO)(μ-RC2R2)]-is more stable when R = CF3 than when R = t-Bu, the t-Bu derivative remains the more kinetically stable complex.

Original languageEnglish
Pages (from-to)1435-1441
Number of pages7
Issue number7
Publication statusPublished - 1 Jan 1986

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