Reaction of Phenyl and Methyl Disulfide with ·Cr(CO)3C5Me5 and HCr(CO)3C5Me5. Metal Radical and Metal Hydride Reactivity at the Sulfur−Sulfur Bond. Different Mechanisms for Oxidative Addition of Alkyl and Aryl Disulfides

Abstract

The enthalpies of reaction of the ·Cr(CO)3C5Me5 and disulfides (RSSR) forming RS−Cr(CO)3C5Me5 have been measured by solution calorimetry, ΔH = −13.3 + 1.5 kcal/mol (R = Ph) and −11.2 + 1.2 kcal/mol (R = Me). These data lead to Cr−SR bond strength estimates of 35 and 43 kcal/mol, respectively. The rates of oxidative addition have been investigated by FTIR spectroscopy. Phenyl disulfide reacts by a second-order mechanism whose rate-determining step is the attack of the chromium radical on the sulfur−sulfur bond, yielding chromium thiolate and a thiyl radical [k(298 K) = 1.3 M-1 s-1, ΔHΦ = +10.2 kcal/mol, ΔSΦ = −24.4 cal/(mol deg)]. Methyl disulfide reacts by a third-order mechanism going through a termolecular transition state in which the stronger sulfur−sulfur bond in the alkyl disulfide is attacked simultaneously by two chromium radicals [k(298 K = 393 M-2 s-1, ΔHΦ = −0.2 kcal/mol, ΔSΦ = −47 cal/(mol deg)]. The rates of reaction of MeSSMe and PhSSPh with the hydride H−Cr(CO)3C5Me5 have also been investig...