Reduced Nickel Centers Essay

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Reduced nickel centers are important in a number of interesting and useful transformations including catalytic cross coupling of alkyl electrophiles,145,146 evolution of dihydrogen by protonation of nickel hydrides,147 dioxygen activation,148 as well as imparting catalytic activity to several metalloenzymes.149 Our interest in nickel(I) centers derives from our longstanding interest in reductive dehalogenation reactions using transition metal complexes.31,70,76-79,122,150
Nickel-mediated dehalogenation has been observed with factor F430 (Figure 4.1), the nickel-containing prosthetic group associated with methyl-coenzyme M reductase in methanogenic organisms.22,49,151 This cofactor has been shown to dehalogenate a range of substrates in
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Among the macrocyclic nickel complexes, there is a link between cavity size, ligand flexibility, saturation, and the oxidation state of the metal and the ligand under reducing conditions. For example, flexible and saturated tetraaza macrocyclic ligands were found to readily reorganize to stabilize the nickel(I) oxidation state, while analogous complexes with imine functionality underwent ligand reduction.158 Similar trends were observed with various nickel porphyrins,159 chlorin, porphycene, and isobacteriochlorin.160Axial ligation, solvent, and ligand substituents also play a role in the oxidation state of the metal and the ligand.161
Non-macrocyclic reduced nickel complexes have been stabilized by phosphine or sulfur ligand spheres,147,148,162-165 and aromatic amines or imine functionality.145,146,166-170 With the aromatic amine and imine donor ligands, the complexes are thought to be stabilized by the π-accepting capability of the ligands. These features have also led to the formation of ligand-based radicals in a number of macrocyclic and non-macrocyclic nickel complexes due to low lying π* orbitals on the ligand. Specifically bis(pyridine-2,6-diiminenickel,144 α-iminopyridine,171 α-iminoketone,172 and α-diimine173,174 nickel complexes structural, spectroscopic, and electronic features indicated the formation of ligand-based radicals. Crystal structure data shows that the imine bond lengths in reduced and

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