PNNL

Abstract

Homolytic solution bond dissociation free energies (sBDFE’s) for S-H bonds have been determined for soluble dimolybdenum tetrasulfide complexes through thermochemical cycles using electrochemical potentials and pKa values, all measured in acetonitrile. For [Cp*Mo(? S)(? SMe)2(? SH)MoCp*]+ (S4Me2H+), [Cp*Mo(? S)(? SMe)(? SH)2MoCp*]+ (S4MeH2+), and [Cp*Mo(? S)(? SH)3MoCp*]+ (S4H3+), the pKa values were determined to be 5.6 ? 0.4, 5.3 ? 0.3, and 4.9 ? 0.3, respectively. The E1/2 values for S4Me2•+/o, S4MeH•+/o, and S4H2•+/o were measured to be -0.02 ? 0.02, +0.04 ? 0.05, and +0.07 ? 0.07 V vs FeCp2+/o, respectively. Using these experimental values, the homolytic S-H sBDFE for each S4RR’H+ to S4RR’•+ reaction was determined to be 60.8 ? 1.0, 61.8 ? 1.6, and 61.9 ? 2.0 kcal/mol for S4Me2H+, S4MeH2+, and S4H3+ respectively. These sBDFE values can be used to estimate gas phase bond dissociation enthalpies of 65.6, 66.6, and 66.7 kcal/mol, respectively. The gas phase bond dissociation enthalpy for the analogous complex, Cp2Mo2S4Me2H+ was determined to be 64.9 kcal/mol by electronic structure calculations. Solid state structures are presented for S4MeH and S4H2. The Pacific Northwest National Laboratory is operated y Battelle for the U.S. Department of Energy.