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 pK a 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*]+ (S 4 Me 2 H + ), [Cp*Mo(μ-S)(μ-SMe)(μ-SH)2MoCp*]+ (S 4 MeH 2 + ), and [Cp*Mo(μ-S)(μ-SH)3MoCp*]+ (S 4 H 3 + ), the pK a values were determined to be 5.6 ± 0.4, 5.3 ± 0.3, and 4.9 ± 0.3, respectively. The E 1/2 values for S 4 Me 2 •+/o , S 4 MeH •+/o , and S 4 H 2 •+/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 S 4 Me 2 H + to S 4 Me 2 •+, S 4 MeH 2 + to S 4 MeH •+, and S 4 H 3 + to S 4 H 2 •+ 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 S 4 MeH and S 4 H 2 .