Redox chemistry of nickel thiolate complexes

Abstract

A series of aminedithiolate ligands and corresponding nickel complexes have been synthesized. The dimeric complexes form with nickel (II). The square planar nickel site is composed of one nitrogen, one terminal thiolate and two bridging thiolates. A derivative where nitrogen has been substituted with a thioether has also been synthesized. The nitrogen-containing derivatives exhibit one-electron oxidations. The first oxidation process is reversible for the N-thioether and irreversible for the N-alkyl dimers. The species produced at this oxidation exhibits a rhombic epr spectrum and is thermally unstable. The spectra are remarkably similar to epr spectra found in the enzyme-hydrogenase. The decay process of the oxidized product was monitored by epr and is of first order. Isotopic substitution with \sp{61}Ni (I = 3/2) gives no resolved hyperfine, only broadening of the epr features. EPR simulations of the spectra show that $\u3c$30% of the spin density resides on the nickel centers. This is in good agreement with similar calculations on hydrogenase grown in bacteria in the presence of \sp{61}Ni ($\u3c$30%). The second oxidation of the nitrogen-containing dimers is at least a two-electron process. Coulometric analysis at the first oxidation for the all sulfur-derivative reveals similar behavior. An irreversible reduction at low potentials appears only after the two-electron oxidation process has occurred. The reduction is also at least a two-electron process. The simulations and ENDOR studies of the electrochemically generated cation radical dimers suggest that the product of one-electron oxidation is a delocalized radical that is not centered on nickel. Mechanisms accounting for the thermal instability of the one-electron oxidation products are proposed. Electrochemical and coulometric analysis on the dimers and an analogous disulfide complex formed from the dimers reveal that upon the two-electron oxidation, sulfur based redox chemistry is occurring