Thermodynamic, Kinetic, Structural, and Computational Studies of the Ph₃Sn–H, Ph₃Sn–SnPh₃, and Ph₃Sn–Cr(CO)₃C₅Me₅Bond Dissociation Enthalpies

Abstract

The kinetics of the reaction of PhSnH with excess •Cr(CO)CMe = •Cr, producing HCr and PhSn-Cr, was studied in toluene solution under 2-3 atm CO pressure in the temperature range of 17-43.5 °C. It was found to obey the rate equation d[PhSn-Cr]/dt = k[PhSnH][•Cr] and exhibit a normal kinetic isotope effect (k/k = 1.12 ± 0.04). Variable-temperature studies yielded ΔH = 15.7 ± 1.5 kcal/mol and ΔS = -11 ± 5 cal/(mol·K) for the reaction. These data are interpreted in terms of a two-step mechanism involving a thermodynamically uphill hydrogen atom transfer (HAT) producing PhSn• and HCr, followed by rapid trapping of PhSn• by excess •Cr to produce PhSn-Cr. Assuming an overbarrier of 2 ± 1 kcal/mol in the HAT step leads to a derived value of 76.0 ± 3.0 kcal/mol for the PhSn-H bond dissociation enthalpy (BDE) in toluene solution. The reaction enthalpy of PhSnH with excess •Cr was measured by reaction calorimetry in toluene solution, and a value of the Sn-Cr BDE in PhSn-Cr of 50.4 ± 3.5 kcal/mol was derived. Qualitative studies of the reactions of other RSnH compounds with •Cr are described for R = Bu,Bu, and Cy. The dehydrogenation reaction of 2PhSnH → H + PhSnSnPh was found to be rapid and quantitative in the presence of catalytic amounts of the complex Pd(IPr)(P(p-tolyl)). The thermochemistry of this process was also studied in toluene solution using varying amounts of the Pd(0) catalyst. The value of ΔH = -15.8 ± 2.2 kcal/mol yields a value of the Sn-Sn BDE in PhSnSnPh of 63.8 ± 3.7 kcal/mol. Computational studies of the Sn-H, Sn-Sn, and Sn-Cr BDEs are in good agreement with experimental data and provide additional insight into factors controlling reactivity in these systems. The structures of PhSn-Cr and CySn-Cr were determined by X-ray crystallography and are reported. Mechanistic aspects of oxidative addition reactions in this system are discussed.