Thermodynamic and Kinetic Studies of H Atom Transfer from HMo(CO)3(η5-C5H5) to Mo(N[t-Bu]Ar)3 and (PhCN)Mo(N[t-Bu]Ar)3: Direct Insertion of Benzonitrile into the Mo-H Bond of HMo(N[t-Bu]Ar)3 forming (Ph(H)C=N)Mo(N[t-Bu]Ar)3

Synthetic studies are reported that show that the reaction of either H2SnR2 (R = Ph, n-Bu) or HMO(CO)(3)(CP) (1-H, Cp = eta(5)-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 (Delta H = -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 calorimetrically as Delta H = -35.1 +/- 2.1 kcal mol(-1). This value combined with the enthalpy of hydrogenation Of [MO(CO)3(CP)12 (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 nitrile insertion into 2-H would be exothermic by similar to 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 Delta H-double dagger = 11.9 +/- 0.4 kcal mol(-1), Delta S double dagger = -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)/k(D) approximate to 1.6, Delta H-double dagger = 13.1 +/- 0.4 kcal mol(-1) and Delta S-double dagger = 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 1/21(2) showed generation of variable amounts of a complex proposed to be (Ar[t-Bu]N)(3)Mo-Mo(CO)(3)(Cp) (1-2). Complex 1-2 can also be formed in small equilibrium amounts by direct reaction of excess 2 and 1(2). 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 degrees C and 0.26 +/- 0.04 M-1 s(-1) at 17 degrees C. Study of the rate of reaction of 1-D yielded k(H)/k(D) = 1.00 +/- 0.05 consistent with an early transition state in which formation of the adduct (Ar[t-Bu]N)(3)MO center dot center dot center dot HMo(CO)(3)(Cp) is rate limiting.