Preparation, reactions, and reaction mechanisms of a family of difunctional complexes of bis(benzylchromium) cations

Article abstract of DOI:10.1021/ic50203a048

Six new dimetallic organochromium cations, each containing two benzylchromium-like units per molecule, were prepared from selected dibromides and chromium(II). These tetrapositive complexes (and certain related monochromium analogues) were isolated and purified by ion-exchange chromatography in aqueous solution. The complexes were characterized by their UV-visible absorption spectra and by the stoichiometry and composition of the products obtained from reactions with chloropentaamminecobalt(III) ion, with sodium carbonate, and with mercury(II) chloride. These complexes react with oxidizing agents such as iron(III) ion, copper(II) ion, and halopentaamminecobalt(III) ion in reactions that are first order in the concentration of organochromium ion, but independent of the identity as well as the concentration of oxidizing agent (except for iron(III) ion which shows, for some complexes, a small contribution of an Fe³⁺-dependent pathway). These data, and comparisons with mononuclear, substituted benzylchromium cations, suggest that the rate-limiting step is unimolecular homolysis of a chromium-carbon bond. The dichromium cations produce only polymer. The monochromium cation [p-CrCH₂C₆H₄CH₂OH]²⁺ also polymerizes upon oxidation and has k_h = 1.66 × 10^-3 s^-1 at 25°C, consistent with other rates and leading to a Hammett substituent constant for p-CH₂OH of σ_p = +0.19. The mechanism of homolysis and of the polymerization initiation step are discussed in light of the kinetic data which give no evidence of a biphasic or stepwise loss of the benzylchromium absorption bands used to monitor the reaction rate.