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Modulation of Molybdenum-Centered Redox Potentials and Electron-Transfer Rates by Sulfur versus Oxygen Ligation

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journal contribution
posted on 15.11.2004, 00:00 by Darrell Uhrhammer, Franklin A. Schultz
Temperature-dependent measurements of potential, E°‘, and electron-transfer rate constant, ks,h, are reported for electrochemical reduction (in 0.3 M TBAPF6/CH3CN) of a series of oxomolybdenum(V) complexes, [(Tp*)MoO(X,Y)], where Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E°‘ values shift in the positive direction, and ks,h values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as ΔH = −R ∂[ln(ks,h)]/∂(1/T) and corrected for an outer-shell component by the mean spherical approximation, is ∼10 kJ mol-1 larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of MoV/IV electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo dxy redox orbital.

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