852065-50-0Relevant articles and documents
Solution calorimetric and stopped-flow kinetic studies of the reaction of ?Cr(CO)3C5Me5 with PhSe-SePh and PhTe-TePh. Experimental and theoretical estimates of the Se-Se, Te-Te, H-Se, and H-Te bond strengths
McDonough, James E.,Weir, John J.,Carlson, Matthew J.,Hoff, Carl D.,Kryatova, Olga P.,Rybak-Akimova, Elena V.,Clough, Christopher R.,Cummins, Christopher C.
, p. 3127 - 3136 (2005)
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)3C5Me 5 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)3C5Me5or 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)3C5Me 5 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)3C6Me5. 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.
