| Abstract: | The compound N,N'-bis(mercaptoethyl)-1,5-diazacyclooctanenickel(II), (bme-daco)Ni[^II] or 1, and its derivatives are used to examine the redox chemistry of Ni in sulfur ligation. Investigations include a comparison of the electrochemical effects of various sulfur ligands on the Ni[^III/II] and Ni[^II/I] couples and mechanistic studies of the oxygenation of Ni(II)-bound thiolates. Cyclic voltammetry was used to obtain Ni[^II/I] and Ni[^II/III] redox potential data for structurally characterized NiN2S2 complexes based on (bme-daco)Ni[^II]. Reactions of 1 with electrophiles yield derivatives in which the sulfur donors are varied from thiolate to thioether, sulfinate and metallated thiolate donors. Crystallography has established that the nickel sites in the complexes are largely square planar with distortions towards tetrahedral from 1° to 18.3°. All complexes show reversible reduction waves in CH3CN, assigned to the Ni[^II/I] couple by EPR of chemically reduced solutions. Irreversible oxidations are observed for all complexes with thiolate donors. Changes in the donor character of the sulfur ligands have a consistent and additive effect on the redox potentials: in CH3CN each methylation of the nickel-bound thiolates stabilizes the Ni[^II/I] couple ca. 700 mV; each oxygenation, ca. 300 mV. Potential measurements in water demonstrate large hydrogen-bonding effects for complexes with thiolate or sulfinate donors. The metallated derivatives display variable stabilization of the Ni[^I] state by electron delocalization. A comparison with octahedral N2S2O2 donor derivatives reveals axial ligation inhibits the Ni[^I] state, causing irreversible reductions. Solutions of 1 react slowly with dioxygen, forming sulfur-oxygenated derivatives. The same products are formed by reaction of 1 with H2O2. The mechanistic pathway for the dioxygen reaction is highly dependent on solvent. In nonprotic solvents two sulfinate products are formed. Isotopic labelling experiments show both result from addition of molecular oxygen. The monosulfinate is formed by a single sulfur-site addition, the bissulfinate is formed by addition across adjacent sulfur sites. Methanol inhibits the dioxygen reaction, whereas in water a monosulfenate product is formed, presumably by O-atom addition. Initial studies of the reactivity of the sulfenates are discussed. |
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