REACTION OF ORGANIC DISULFIDES WITH COBALT-CENTERED METAL RADICALS - USE OF THE E-BASED AND C-BASED DUAL-PARAMETER SUBSTITUENT MODEL AND QUANTITATIVE SOLVENT EFFECT ANALYSES TO COMPARE OUTER-SPHERE AND INNER-SPHERE ELECTRON-TRANSFER PROCESSES

Treatment of isolable, paramagnetic Cp2Ta(mu-CH2)(2)CoCp (1, Cp = eta( 5)-C5H5) With aromatic disulfides RSSR affords diamagnetic monothiolat e complexes Cp2Ta(mu-CH2)(2)Co(SR)Cp in quantitative yield. The struct ure of Cp2Ta(mu-CH2)(2)Co(SC6H3(CH3)(2))Cp (2a) has been characterized by X-ray crystallography. The mechanism of this disulfide S-S bond cl eavage reaction has been probed using kinetic, substituent effect, and solvent effect techniques. A Hammett sigma/rho substituent constant a nalysis yields a nonlinear correlation. In contrast, application of th e E- and C-based dual-parameter substituent model gives a good fit of the data and demonstrates that the reaction is sensitive to both the e lectrostatic and the covalent characteristics of the entering disulfid e. All substituents examined (excluding p-(trifluoromethyl)phenyl) exh ibited a solvent effect consistent with a modest amount of charge sepa ration in the transition state. The reaction of 1 with the strongly el ectron withdrawing substituted bis[p-(trifluoromethyl)phenyl] disulfid e is much more sensitive to solvent polarity changes. As a comparison system expected to proceed via a limiting outer-sphere electron-transf er mechanism, the reaction of aromatic disulfides with the 19-electron complex cobaltocene (CoCp2, 3), which leads to salts [CoCp2][SR] (4), was investigated using methods parallel to those employed in the stud y of Ta/Co complex 1. In the cobaltocene system, the kinetic, solvent effect, and substituent effect data showed that the reaction is especi ally sensitive to the electrostatic character of the disulfide and rel atively insensitive to its covalent character and steric bulk. The com parison of cobaltocene with Ta/Co complex 1 shows that the latter reac ts with most diaryl disulfides (perhaps with the exception of p-CF3 co mpound 2f, which may be a borderline case) via a transition state with substantially more covalent character, acid correspondingly less char ge separation, than that involved in the outer-sphere reactions of Cp2 Co.