A DIRECT COMPARISON OF THE RATES OF DEGENERATE TRANSFER OF ELECTRONS, PROTONS, AND HYDROGEN-ATOMS BETWEEN METAL-COMPLEXES

Tp*Mo(CO)3H (Tp* = hydridotris(3,5-dimethylpyrazolyl)borate) crystallized in the monoclinic space group P2(1)/c with a = 8.005(3) angstrom, b = 13.873(5) angstrom, c = 18.874(5) angstrom, beta = 97.73(2)degrees, and Z = 4. Oxidation of Et4N[Tp*Mo(CO)3] with [Cp2Fe]PF6 yielded Tp*Mo(CO)3; this 17-electron radical also crystallized in the monoclinic space group P2(1)/c with a = 7.962(4) angstrom, b = 13.885(6) angstrom, c = 19.048(5) angstrom, beta = 96.86(2)degrees, and Z = 4. The pK(a) of TpMo(CO)3H (Tp = hydridotris(pyrazolyl)borate) in acetonitrile was 11.3(2) and D(Mo-H) measured 63(1) kcal/mol. Equilibration of TpMo(CO)3H with Tp*Mo(CO)3 afforded the corresponding values for Tp*Mo(CO)3H: D(Mo-H) = 60(1) kcal/mol and pK(a) = 10.2. The rates of degenerate electron transfer between nBu4N[Tp*Mo(CO)3] and Tp*Mo(CO)3 in THF were measured by H-1 NMR line broadening: k2(30-degrees-C) = 8.6 x 10(6) M-1 s-1, DELTAH(double dagger) = 3.5(2) kcal/mol, DELTAS(double dagger) = -15.3(6) cal/(mol.K). The rates of degenerate proton transfer between Tp*Mo(CO)3H and nBu4N[Tp*Mo(CO)3] in THF were measured by a selective inversion recovery NMR experiment and by H-1 NMR monitoring of the reaction of deuterium-labeled (Tp*-d4)Mo(CO)3H with nBu4N[Tp*Mo(CO)3]: k2(30-degrees-C) = 3.5 M-1 s-1, DELTAH(double dagger) = 3.6(2) kcal/mol, DELTAS(double dagger) = -45(1)cal/(mol.K). The proton transfer was catalyzed by bromide and chloride ions. The rates of degenerate hydrogen atom transfer were measured by monitoring the reaction of Tp*Mo(CO)3H with Tp*Mo((CO)-C-13)3 in THF at -34-degrees-C by C-13 NMR. The hydrogen atom transfer was catalyzed by both base and acid. The slowest rate constant measured (k2(-34-degrees-C) = 9.0 x 10(-3) M-1 s-1) established an upper limit for the rate of hydrogen atom transfer. In this particular system electron transfer is very fast, proton transfer is slow, and hydrogen atom transfer is slower yet.