Structural effects on electron transfer behavior of molybdenum and tungsten complexes

The object of this work is to investigate effects of structural changes on electron transfer behavior of Mo and W complexes. Three aspects will be studied: (1) the ability of sulfur vs. oxygen to influence the potential of Mo-centered redox processes, (2) the ability of sulfur vs. oxygen to enhance the rates of Mo-centered electron transfer reactions, and (3) the kinetics and thermodynamics of redox reactions in which two electrons are transferred at one time in conjunction with metal-metal bond cleavage. In part one comparative electrochemical studies are conducted on several six-coordinate MoO4+ complexes that contain a tridentate N,O,O′ or N,O,S-donor Schiff base ligand. The complexes exhibit a reversible Mo(VI/V) reduction, the potential of which shifts in the positive direction as O′ is replaced by S in the Schiff base ligand. The direction of the shift is consistent with greater stabilization of the lower oxidation state by S donor ligands. In part two the temperature dependence to the heterogeneous electron transfer rate constant, ks,h, is investigated for a family of HB(Me2pz)3MoO(X-Y) complexes where X-Y is a bidentate aromatic (XC6H5Y2− ) or aliphatic ligand (XCH2CH2Y2− ), and X and Y are systematically varied between O and S. These compounds show a large increase in ks,h upon replacing O with S in the bidentate ligand. Activation parameters determined from the temperature dependence of ks,h are used to assess contributions from inner-shell reorganization and/or electron tunneling to the rate differences. Part three examines the thermodynamics and kinetics of two-electron transfer in ligand-bridged binuclear M2(μ-PPh2)2(CO)8 (M = Mo, W) complexes. These compounds undergo concerted two-electron transfer accompanied by cleavage of a metal-metal bond. The second reduction is the rate-limiting step in the two-electron transfer and is nearly three times slower for the W compound. Thus, metal-metal bond cleavage makes an important contribution to the activation barrier for two-electron transfer. The difference in rates results from the greater strength for W-W versus Mo-Mo bonds