Thermodynamic, Kinetic, Structural, and Computational Studies of the Ph3Sn-H, Ph3Sn-SnPh3, and Ph3Sn-Cr(CO)3C5Me5 Bond Dissociation Enthalpies

Xiaochen Cai; Subhojit Majumdar; George C Fortman; Anjaneyulu Koppaka; Leonardo Serafim; Burjor Captain; Manuel Temprado; Carl D Hoff

doi:10.1021/acs.inorgchem.6b01978

Thermodynamic, Kinetic, Structural, and Computational Studies of the Ph₃Sn-H, Ph₃Sn-SnPh₃, and Ph₃Sn-Cr(CO)₃C₅Me₅ Bond Dissociation Enthalpies

Inorg Chem. 2016 Oct 17;55(20):10751-10766. doi: 10.1021/acs.inorgchem.6b01978. Epub 2016 Oct 5.

Authors

Xiaochen Cai¹, Subhojit Majumdar¹, George C Fortman¹, Anjaneyulu Koppaka¹, Leonardo Serafim¹, Burjor Captain¹, Manuel Temprado², Carl D Hoff¹

Affiliations

¹ Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
² Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Universidad de Alcalá , Ctra. Madrid-Barcelona Km. 33,600, Madrid 28871, Spain.

PMID: 27704803
DOI: 10.1021/acs.inorgchem.6b01978

Abstract

The kinetics of the reaction of Ph₃SnH with excess •Cr(CO)₃C₅Me₅ = •Cr, producing HCr and Ph₃Sn-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[Ph₃Sn-Cr]/dt = k[Ph₃SnH][•Cr] and exhibit a normal kinetic isotope effect (k_H/k_D = 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 Ph₃Sn• and HCr, followed by rapid trapping of Ph₃Sn• by excess •Cr to produce Ph₃Sn-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 Ph₃Sn-H bond dissociation enthalpy (BDE) in toluene solution. The reaction enthalpy of Ph₃SnH with excess •Cr was measured by reaction calorimetry in toluene solution, and a value of the Sn-Cr BDE in Ph₃Sn-Cr of 50.4 ± 3.5 kcal/mol was derived. Qualitative studies of the reactions of other R₃SnH compounds with •Cr are described for R = ⁿBu, ^tBu, and Cy. The dehydrogenation reaction of 2Ph₃SnH → H₂ + Ph₃SnSnPh₃ was found to be rapid and quantitative in the presence of catalytic amounts of the complex Pd(IPr)(P(p-tolyl)₃). 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 Ph₃SnSnPh₃ 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 Ph₃Sn-Cr and Cy₃Sn-Cr were determined by X-ray crystallography and are reported. Mechanistic aspects of oxidative addition reactions in this system are discussed.