Structural effects on electron transfer behavior of molybdenum and tungsten complexes

Darrell Uhrhammer, Purdue University

Abstract

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.

Degree

Ph.D.

Advisors

Schultz, Purdue University.

Subject Area

Chemistry

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