| Abstract
| - We report the results of spectroscopic studies on the pyranopterin Mo enzyme model compounds (L-N3)MoO(qdt) and (L-N3)MoO(tdt). Photoelectron spectroscopy shows that the Mo dxy ionization of (L-N3)MoO(qdt) is stabilized by 0.77 eV relative to that of (L-N3)MoO(tdt), and this is reflected in the solution reduction potentials (ΔE1/2 ∼220 mV). The spectroscopic results have been interpreted in the context of a given dithiolate's donor ability toward Mo, resulting in a reduction of the effective nuclear charge (Zeff) on the metal. Finally it was found that a linear relationship exists between the reduction potential of (L-N3)MoO(dithiolate) and the calculated charge localized on the S atoms of the respective dithiolate dianion.
- 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 andphotoelectron, magnetic circular dichroism, and electronic absorption spectroscopies, and the experimental datahave been interpreted in the context of ab initio molecular orbital calculations on a variety of dithiolate dianionligands. The PES data reveal very substantial differences between (L-N3)MoO(qdt) and (L-N3)MoO(tdt) in thatthe first ionization (originating from the Mo dxy orbital) for (L-N3)MoO(qdt) is about 0.8 eV to deeper bindingenergy than that of (L-N3)MoO(tdt). This stabilizing effect is also reflected in the solution reduction potentials,where (L-N3)MoO(qdt) is ∼220 mV easier to reduce than (L-N3)MoO(tdt). A direct correlation between the relativedonating ability of a given dithiolate ligand and the reduction potential of the (L-N3)MoO(dithiolate) complexhas been observed, and a linear relationship exists between the calculated Mulliken charge on the S atoms of thedithiolate 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 contributionsinvolving only the out-of-plane S orbitals of the coordinated dithiolate control the Mo reduction potential bymodulating the effective nuclear charge of the metal, and this has direct relevance to understanding the mechanismof ferricyanide inhibition in sulfite oxidase. Furthermore, these results indicate that partially oxidized pyranopterinsmay play a role in facilitating electron and/or atom transfer in certain pyranopterin tungsten enzymes whichcatalyze formal oxygen atom transfer reactions at considerably lower potentials.
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