Determination of S−H Bond Strengths in Dimolybdenum Tetrasulfide Complexes

Homolytic solution bond dissociation free energies (SBDFE) for S−H bonds have been determined for soluble dimolybdenum tetrasulfide complexes through thermochemical cycles using electrochemical potentials and pKa values in acetonitrile. In spite of the importance and extensive use of metal sulfide catalysts, these S−H bond strengths are among the first experimentally determined values for metal sulfide systems. For [Cp*Mo(μ-S)(μ-SMe)2(μ-SH)MoCp*]+ (S4Me2H+), [Cp*Mo(μ-S)(μ-SMe)(μ-SH)2MoCp*]+ (S4MeH2+), and [Cp*Mo(μ-S)(μ-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 S4Me2H+ to S4Me2•+, S4MeH2+ to S4MeH•+, and S4H3+ to S4H2•+ were determined to be 60.8 ± 1.0, 61.8 ± 1.6, and 61.9 ± 2.0 kcal/mol, 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. Solid state structures are presented for S4MeH and S4H2.