Control of oxo-molybdenum reduction and ionization potentials by dithiolate donors

The compounds (L-N-3)MoO(qdt) and (L-N-3)MoO(tdt) [(L-N-3) = hydrotris(3,5- dimethyl-1-pyrazolyl)borate; tdt = toluene-3,4-dithiolate; qdt = quinoxalin e-2,3-dithiolate] have been studied by cyclic voltammetry and photoelectron , magnetic circular dichroism, and electronic absorption spectroscopies, an d the experimental data have been interpreted in the context of ab initio m olecular orbital calculations on a variety of dithiolate dianion ligands. T he PES data reveal very substantial differences between (L-N-3)MoO(qdt) and (L-N-3)MoO(tdt) in that the first ionization (originating from the Mo d(xy ) orbital) for (L-N-3)MoO(qdt) is about 0.8 eV to deeper binding energy tha n that of (L-N-3)MoO(tdt). This stabilizing effect is also reflected in the solution reduction potentials, where (L-N-3)MoO(qdt) is similar to 220 mV easier to reduce than (L-N-3)MoO(tdt). A direct correlation between the rel ative donating ability of a given dithiolate ligand and the reduction poten tial of the (L-N-3)MoO(dithiolate) complex has been observed, and a linear relationship exists between the calculated Mulliken charge on the S atoms o f the dithiolate dianion and the Mo reduction potential. The study confirms previously communicated work (Helton, M. E.; Kirk, M. L. Inorg. Chern. 199 9, 38, 4384-4385) that suggests that anisotropic covalency contributions in volving only the out-of-plane S orbitals of the coordinated dithiolate cont rol the Mo reduction potential by modulating the effective nuclear charge o f the metal, and this has direct relevance to understanding the mechanism o f ferricyanide inhibition in sulfite oxidase. Furthermore, these results in dicate that partially oxidized pyranopterins may play a role in facilitatin g electron and/or atom transfer in certain pyranopterin tungsten enzymes wh ich catalyze formal oxygen atom transfer reactions at considerably lower po tentials.