Synthetic studies are reported that show that the reaction of either H 2SnR2 (R = Ph, n-Bu) or HMo(CO)3(Cp) (1-H, Cp = η5-C5H5) with Mo(N[t-Bu]Ar)3 (2, Ar = 3,5-C6H3Me2) produce HMo(N[t-Bu]Ar)3 (2-H). The benzonitrile adduct (PhCN)Mo(N[t-Bu]Ar) 3 (2-NCPh) reacts rapidly with H2SnR2 or 1-H to produce the ketimide complex (Ph(H)C=N)Mo(N[t-Bu]Ar)3 (2-NC(H)Ph). The X-ray crystal structures of both 2-H and 2-NC(H)Ph are reported. The enthalpy of reaction of 1-H and 2 in toluene solution has been measured by solution calorimetry (ΔH = -13.1 ± 0.7 kcal mol-1) and used to estimate the Mo-H bond dissociation enthalpy (BDE) in 2-H as 62 kcal mol-1. The enthalpy of reaction of 1-H and 2-NCPh in toluene solution was determined catorimetrically as ΔH = -35.1 ± 2.1 kcal mol -1. This value combined with the enthalpy of hydrogenation of [Mo(CO)3(Cp)]2 (12) gives an estimated value of 90 kcal mol-1 for the BDE of the ketimide C-H of 2-NC(H)Ph. These data led to the prediction that formation of 2-NC(H)Ph via nitrite insertion into 2-H would be exothermic by ∼36 kcal mol-1, and this reaction was observed experimentally. Stopped flow kinetic studies of the rapid reaction of 1-H with 2-NCPh yielded ΔH‡ = 11.9 ± 0.4 kcal mol-1, ΔS‡ = -2.7 ± 1.2 cal K -1 mol-1. Corresponding studies with DMo(CO) 3(Cp) (1-D) showed a normal kinetic isotope effect with k H/kD ≈ 1.6, ΔH‡ = 13.1 ± 0.4 kcal mol-1 and ΔS‡ = 1.1 ± 1.6 cal K-1 mol-1. Spectroscopic studies of the much slower reaction of 1-H and 2 yielding 2-H and 1/212 showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)3Mo-Mo(CO) 3(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 12. The presence of 1-2 complicates the kinetic picture; however, in the presence of excess 2, the second-order rate constant for H atom transfer from 1-H has been measured: 0.09 ± 0.01 M-1 s-1 at 1.3°C and 0.26 ± 0.04 M-1 s-1 at 17°C. Study of the rate of reaction of 1-D yielded kH/kD = 1.00 ± 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N) 3Mo⋯HMo(CO)3(Cp) is rate limiting.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry