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

TpMo(CO)3H (Tp* = hydridotris(3,5-dimethylpyrazolyl)borate) crystalli zed 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[TpMo(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 TpMo(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 el ectron transfer between nBu4N[TpMo(CO)3] and Tp*Mo(CO)3 in THF were m easured 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 TpMo(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 re action of deuterium-labeled (Tp-d4)Mo(CO)3H with nBu4N[Tp*Mo(CO)3]: k 2(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 c atalyzed by bromide and chloride ions. The rates of degenerate hydroge n atom transfer were measured by monitoring the reaction of TpMo(CO)3 H with TpMo((CO)-C-13)3 in THF at -34-degrees-C by C-13 NMR. The hydr ogen atom transfer was catalyzed by both base and acid. The slowest ra te constant measured (k2(-34-degrees-C) = 9.0 x 10(-3) M-1 s-1) establ ished an upper limit for the rate of hydrogen atom transfer. In this p articular system electron transfer is very fast, proton transfer is sl ow, and hydrogen atom transfer is slower yet.