UNEXPECTED DISPROPORTIONATION MECHANISM FOR PROTON-TRANSFER REACTIONS BETWEEN 17-ELECTRON METAL HYDRIDE CATION RADICALS AND NEUTRAL 18-ELECTRON METAL-HYDRIDES

The hydrides CpRu(PR3)2H ((PR3)2 = (PPh3)2, 1; dppm, 2; dppe, 3; and dppp, 4) undergo one-electron oxidations at -0.1 to -0.3 V vs Cp2Fe/Cp2Fe+. The oxidations of 1 and 4 are nearly chemically reversible in acetonitrile; in THF, the oxidation of 3 also exhibits partial chemical reversibility. Chemical oxidation of 1-4 in acetonitrile yields 1:1 mixtures of CpRu(PR3)2(NCMe)+ and the dihydride or dihydrogen complexes CpRu(PR3)2H2+, indicative of proton transfer from the hydride cation radicals to the neutral hydrides. A detailed kinetic and mechanistic investigation of the reactions of 1.+ and 4.+ was performed by derivative cyclic voltammetry. Kinetic data are in accord with a disproportionation mechanism. The disproportionation is facilitated by acetonitrile coordination at the 17-electron cation radicals and is followed by proton transfer from highly acidic dications, rather than from the less acidic cation radicals, of the substrates. By the use of a thermochemical cycle that combines the electrode potentials for 2 and 3 and the pK(a) data for the corresponding dihydrogen complexes, it is found that H-2 is activated by ca. 100 kJ/mol toward homolysis when coordinated at the Ru centers. The structure of 1 was determined by X-ray crystallography. Crystal data for 1: triclinic space group, P1BAR, Z = 2, a = 9.736(1) angstrom, b = 10.117(1) angstrom, c = 17.659(3) angstrom, alpha = 90.91(1)degrees, beta = 105.93(1)degrees, gamma = 99.19(1)degrees, R = 0.041, R(w) = 0.047.