Free energy landscapes for S-H bonds in Cp* ₂Mo ₂S ₄ complexes

An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(μ-S) ₂(μ-SMe)(μ-SH)MoCp * and Cp*Mo(μ-S) ₂(μ-SH) ₂MoCp*. These data include electrochemical potentials, pK _a values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile. Thermochemical data ranged from +0.6 to -2.0 V vs FeCp ₂ ^+/o for electrochemical potentials, 5 to 31 for pK _a values, 43 to 68 kcal/mol for homolytic SBDFEs, and 44 to 84 kcal/mol for hydride donor abilities. The observed values for these thermodynamic parameters are comparable to those of many transition metal hydrides, which is consistent with the many parallels in the chemistry of these two classes of compounds. The extensive set of thermochemical data is presented in free energy landscapes as a useful approach to visualizing and understanding the relative stabilities of all of the species under varying conditions of pH and H ₂ overpressure. In addition to the previously studied homogeneous reactivity and catalysis, Mo ₂S ₄ complexes are also models for heterogeneous molybdenum sulfide catalysts, and therefore, the present results demonstrate the dramatic range of S-H bond strengths available in both homogeneous and heterogeneous reaction pathways.