Abstract
The kinetics of the oxidative addition of PhSeSePh and PhTeTePh to the stable 17-electron complex •Cr(CO)3C5Me5 have been studied utilizing stopped-flow techniques. The rates of reaction are first-order in each reactant, and the enthalpy of activation decreases in going from Se (ΔH ‡ = 7.0 ± 0.5 kcal/mol, ΔS ‡ = −22 ± 3 eu) to Te (ΔH ‡ = 4.0 ± 0.5 kcal/mol, ΔS ‡ = −26 ± 3 eu). The kinetics of the oxidative addition of PhSeH and •Cr(CO)3C5Me5 show a change in mechanism in going from low (overall third-order) to high (overall second-order) temperatures. The enthalpies of the oxidative addition of PhE−EPh to •Cr(CO)3C5Me5 in toluene solution have been measured and found to be −29.6, −30.8, and −28.9 kcal/mol for S, Se, and Te, respectively. These data are combined with enthalpies of activation from kinetic studies to yield estimates for the solution-phase PhE−EPh bond strengths of 46, 41, and 33 kcal/mol for E = S, Se, and Te, respectively. The corresponding Cr−EPh bond strengths are 38, 36, and 31 kcal/mol. Two methods have been used to determine the enthalpy of hydrogenation of PhSeSePh in toluene on the basis of reactions of HSPh and HSePh with either •Cr(CO)3C5Me5 or 2-pyridine thione. These data lead to a thermochemical estimate of 72 kcal/mol for the PhSe−H bond strength in toluene solution, which is in good agreement with kinetic studies of H atom transfer from HSePh at higher temperatures. The reaction of H−Cr(CO)3C5Me5 with PhSe−SePh is accelerated by the addition of a Cr radical and occurs via a rapid radical chain reaction. In contrast, the reaction of PhTe−TePh and H−Cr(CO)3C5Me5 does not occur at any appreciable rate at room temperature, even in the presence of added Cr radicals. This is in keeping with a low PhTe−H bond strength blocking the chain and implies that H−TePh ≤ 63 kcal/mol. Structural data are reported for PhSe−Cr(CO)3C5Me5 and PhS−Cr(CO)3C5Me5. The two isostructural complexes do not show signs of an increase in steric strain in terms of metal−ligand bonds or angles as the Cr−EPh bond is shortened in going from Se to S. Bond strength estimates of the PhE−H and PhE−EPh derived from density functional theory calculations are in reasonable agreement with experimental data for E = Se but not for E = Te. The nature of the singly occupied molecular orbital of the •EPh radicals is calculated to show increasing localization on the chalcogenide atom in going from S to Se to Te.
