Free Energy Landscapes for S−H Bonds in Cp*₂Mo₂S₄ Complexes

Abstract

An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(mu-S)(2)(mu-SMe)(mu-SH)MoCp* and Cp*Mo(mu-S)(2)(mu-SH)(2)MoCp*. 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(2)S(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.

Proton management as a design principle for hydrogenase-inspired catalysts

“…Robust artificial molecular electrocatalysts for hydrogen production based on inexpensive and abundant metals that can rival the performance of hydrogenase enzymes are crucial for the practical production of renewable fuels. 27,28,[31][32][33][34] Gas-phase density functional theory (DFT) calculations were carried out with the Gaussian 03 program package 35 on the proposed sequence of intermediates and transition states in the catalytic mechanism using a simplified model of the Ni(PNP) 2 2+ complex, Ni(PNP 0 ) 2 2+ (PNP 0 ¼ H 2 PCH 2 NHCH 2 PH 2 ), as shown in Fig. This reasoning led to the preparation of a hydrogenase-inspired electrocatalyst with a single Ni(II) metal center and two PNP (PNP ¼ Et 2 PCH 2 NMeCH 2 PEt 2 ) ligands.…”

Reducing CO2 to HCO2- at Mild Potentials: Lessons from Formate Dehydrogenase

“…Dimolybdenum complexes store multiple hydride equivalents through metal reduction and accumulation of protons onto bridging sulfur ligands. 80 Attempts at isolating the metal hydride [Rh(Cp*)(bpy)H] + , the putative intermediate in transfer hydrogenation 81 and artificial 1,4-NADH regeneration cycles, [82][83][84] unexpectedly resulted in [Rh(Cp*H)(bpy)] + , where the proton resides on the Cp* ring. 85,86 Combined experimental and computational studies by Nocera and Hammes-Schiffer indicate H2 evolution in several redox active transition metal complexes with pendant acid functionalities likely proceed via metal-ligand cooperation instead of a metal hydride intermediate.…”

“…The examples of orthogonal hydride transfer in synthetic systems provide models for designing new catalysts. In some cases, a ligand bound directly to the metal such as terminal or bridging O/S groups 80,[89][90][91][92][93][94][95] or Cp ring 85,86 can serve as a proton reservoir. In this motif, it is more difficult to independently tune the pKa with the redox properties, as the former would be expected to have some correlation with the latter.…”

A Modular Approach to the Development of Molecular Electrocatalysts for H2Oxidation and Production Based on Inexpensive Metals

“…This quantitative model of how the free energy surfaces and reaction profi les of catalytic cycles respond to changes in reaction conditions and to catalyst structure is extremely useful in catalyst design for this class of nickel complexes [13,14] . These free energy landscapes can also be usefully applied to the development of molybdenum sulfur complexes such as the one shown by structure 2 [40] .…”