Control of Oxo-Molybdenum Reduction and Ionization Potentials by Dithiolate Donors

Abstract

The compounds (L-N3)MoO(qdt) and (L-N3)MoO(tdt) [(L-N3) = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; tdt = toluene-3,4-dithiolate; qdt = quinoxaline-2,3-dithiolate] have been studied by cyclic voltammetry and photoelectron, magnetic circular dichroism, and electronic absorption spectroscopies, and the experimental data have been interpreted in the context of ab initio molecular orbital calculations on a variety of dithiolate dianion ligands. The PES data reveal very substantial differences between (L-N3)MoO(qdt) and (L-N3)MoO(tdt) in that the first ionization (originating from the Mo dxy orbital) for (L-N3)MoO(qdt) is about 0.8 eV to deeper binding energy than that of (L-N3)MoO(tdt). This stabilizing effect is also reflected in the solution reduction potentials, where (L-N3)MoO(qdt) is approximately 220 mV easier to reduce than (L-N3)MoO(tdt). A direct correlation between the relative donating ability of a given dithiolate ligand and the reduction potential of the (L-N3)MoO(dithiolate) complex has been observed, and a linear relationship exists between the calculated Mulliken charge on the S atoms of the dithiolate dianion and the Mo reduction potential. The study confirms previously communicated work (Helton, M. E.; Kirk, M. L. Inorg. Chem. 1999, 38, 4384-4385) that suggests that anisotropic covalency contributions involving only the out-of-plane S orbitals of the coordinated dithiolate control the Mo reduction potential by modulating the effective nuclear charge of the metal, and this has direct relevance to understanding the mechanism of ferricyanide inhibition in sulfite oxidase. Furthermore, these results indicate that partially oxidized pyranopterins may play a role in facilitating electron and/or atom transfer in certain pyranopterin tungsten enzymes which catalyze formal oxygen atom transfer reactions at considerably lower potentials.

Synthesis, characterization, and spectroscopy of model molybdopterin complexes

“…That the specific substituents on the dithiolene can have an enormous effectalmost 1 V-on the Mo reduction potential has been previously noted [15,47].…”

“…Furthermore, detailed electronic absorption, MCD, and EPR studies have been performed on related oxomolybdenum mono-dithiolene complexes, and the new studies reported herein provide a context in which to interpret the effects of a pterin substituent on the electronic structure of the coordinated dithiolene in the enzymes [15,[48][49][50][51][52][53].…”

Investigation of metal–dithiolate fold angle effects: Implications for molybdenum and tungsten enzymes

“…3) at 7.04 eV is similar in appearance to the low energy band of related complexes having alkoxide ligands (23,52) that has been assigned to ionization arising primarily from the half-filled metal-based orbital (21, 23, 52). The two ionizations between 7.25 eV to 8.00 eV can then be assigned to ionizations associated with symmetric (S ϩ ) and antisymmetric (S Ϫ ) combinations of S orbitals based on previous assignment of analogous complexes (21,22). More detailed assignment of the S ϩ and S Ϫ ionizations is not possible because of the significant overlap of these low-energy features and the beginning of the Tp* ligand based ionizations.…”

“…Subsequent photoelectron spectroscopic studies of other pairs of (Tp*)MoE(dithiolate) complexes (E ϭ O, NO) have shown that the first ionization energies of the members of each pair are within Ϸ0.2 eV of one another. For a series of (Tp*)MoO(dithiolate) complexes with varying peripheral substitutions on the dithiolate ligand, the first ionization energies show a range of Ϸ0.7 eV and correlate with the lowest energy oxidation potential in solution (11,22). Gas-phase photoelectron spectroscopy avoids possible complications of solvent and solid state effects, but a major drawback is that only neutral species are easily studied by this technique, and the (Tp*)MoO(dithiolate) system does not allow access to the important d 0 and d 2 electron configurations of the catalytic cycle.…”

Redox reactions of the pyranopterin system of the molybdenum cofactor

“…Evolutionary conservation of the molybdopterin ligand is strong evidence that it serves an important function to the metal site in catalysis. Features unique to a dithiolene chelate which might be important to the Mo and W cofactors have been identified by a considerable number of coordination chemistry studies comparing dithiolene with other sulfur donor ligands [5][6][7][8][9][10][11][12].…”