Outer electron transfer in methylene chloride: concentration, salt, and temperature dependences of the oxidation of trans-ReX2(cis-1,2 bis(diphenylphosphino)ethylene)2 (X = Cl, Br) by the clathrochelate [Co(1,2-cyclohexanedionedioximate)3(BButyl)2]BF4

Abstract

The oxidation of ReC1 2 ( cis -1,2 bis(diphenylphosphino)ethylene) 2 and ReBr 2 ( cis -1,2 bis(diphenylphosphino)ethylene) 2 by the clathrochelate [Co(1,2-cyclohexanedione dioximate) 3 (BButyl) 2 ]BF 4 have been studied by the stopped-flow method as a function of temperature (−85 to −50°C), added Bu 4 NBF 4 (0–0.1 M), and reactant concentration in methylene chloride. The rate constants were fit to a mechanism involving separate paths for the free ion and the ion paired Co(III) oxidant. The result was the enthalpy and entropy of ion pairing for both Bu 4 NBF 4 and the oxidant, and the enthalpy and entropy of activation for both the free ion path and the ion paired path for each reaction. This study makes this one of the best characterized outer-sphere electron transfer reactions occurring in the low dielectric constant solvent methylene chloride. The results will allow comparison of the reactivity of these mononuclear complexes with that of metal-metal bonded dirhenium complexes. Using the temperature dependences of the reduction potentials of the Re and Co complexes as well as decamethylferrocene, and the kinetics of the reduction of the Co(III) complex by decamethylferrocene in methylene chloride, also as a function of added salt, estimates of the high electron self-exchange rate constants of the two Re complexes have been made. They are both very similar and are ca 10 8 M −1 S −1 at 25°C.