Reduced Nickel Centers Essay

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 its reduced form and its dehalogenation activity has been modeled experimentally using reduced nickel octaethylisobacteriochlorin anion,123-126,152 variously saturated nickel porphyrins,152,153 and tetraazamacrocyclic nickel complexes.154,155

Figure 4.1. F430

One challenge in understanding the reactivity of F430 and other nickel-containing complexes is the interactions between the nickel atom and the surrounding ligands in the reduced form, specifically, whether the unpaired electron resides on the metal or is delocalized onto the ligand. Reduction of nickel(II) complexes can result in the formation of nickel(I) complexes, nickel(II) complexes with a ligand radical, or a mixed species with both nickel(I) character and ligand radical character. The degree to which the metal and/or ligand is reduced can significantly affect the structure of the reduced complex, its spectroscopic features and its reactivity.
Theoretical studies confirm experimental demonstration that F430, unlike other macrocy...


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...is work, Manuel and Rhode prepared the same complex and came to the same conclusion.137
Nickel(II) precursor complexes: arylaminoiminopyridinenickel (AIPiPrNiCl2) and bis-(arylimino)pyridinenickel (BIPiPrNiCl2)135,136 were synthesized and characterized. These were subsequently reduced using zinc amalgam, characterized, and reacted with benzyl bromide. The differences in the reactivity of the two reduced complexes, AIPiPrNiCl and BIPiPrNiCl, (Figure 4.2) illustrate the surprisingly large impact of a single unit of ligand unsaturation on the reactivity of the complex. These systems not only provide simple models with which to illustrate the fundamental chemistry of reduced nickel complexes, but they enhance understanding of reductive dehalogenation in the biological context that may lead to development of inexpensive and effective catalysts for pollution remediation.

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