title: 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 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 (kH/kD= 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+ Ph3SnSnPh3was 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 Ph3SnSnPh3of 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.
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: Xiaochen
authors.initials: X
authors.lastname: Cai
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: Subhojit
authors.initials: S
authors.lastname: Majumdar
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: George C
authors.initials: GC
authors.lastname: Fortman
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: Anjaneyulu
authors.initials: A
authors.lastname: Koppaka
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Serafim
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: Burjor
authors.initials: B
authors.lastname: Captain
authors.affiliation: Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering, Universidad de Alcalá , Ctra. Madrid-Barcelona Km. 33,600, Madrid 28871, Spain.
authors.forename: Manuel
authors.initials: M
authors.lastname: Temprado
authors.affiliation: Department of Chemistry, University of Miami , 1301 Memorial Drive, Coral Gables, Florida 33124, United States.
authors.forename: Carl D
authors.initials: CD
authors.lastname: Hoff