Chemistry of nickel(II) complexes of relevance to acireductone dioxygenase

The work outlined in this dissertation focused on an examination of mononuclear octahedral Ni(II) complexes supported by the tripodal N4-donor ligand (6-Ph2TPA). Several of these complexes are relevant to Ni(II)-containing acireductone dioxygenase (Ni(II)-ARD). This enzyme is the first example of a nickel-containing dioxygenase. The metal center in the resting state of Ni(II)-ARD exhibits octahedral coordination with six O/N-donors, of which 3--4 are histidine residues. This enzyme catalyzes the oxidation of acireductone to propionic acid, CO, and formic acid in a reaction that is a shunt out of the methionine salvage pathway in Klebsiella pneumoniae. It is proposed that Ni(II)-ARD may be involved in the regulation of methionine levels in vivo.;By using a nitrogen-donor chelate ligand, nickel complexes relevant to the resting state of Ni(II)-ARD, as well as complexes that are first generation models for the substrate-bound form of Ni(II)-ARD, have been prepared and fully characterized (X-ray crystallography, FTIR, FAB-MS, EA, and 1H and 2H NMR). The 1H NMR features of each complex are unique and this method has been found to be very useful for monitoring reactions of Ni(II) complexes with model acireductone substrates.;The preparation, characterization, and reactivity of a Ni(II) cis-beta-keto-enolate complex supported by Ph2TPA ligand are presented. A bulky analogue of acireductone, 2-hydroxy-1,3-diphenylpropan-1,3-dione, is used as a reactive model substrate. A monoanionic form of this molecule coordinates to a Ni(II) ion that is also supported by the 6-Ph2TPA ligand. This Ni(II) cis-beta-keto-enolate complex is reactive with dioxygen in the presence of a base to produce a biscarboxylate Ni(II) complex and CO. This is the first example of a reactive model system relevant to the Ni(II)-containing acireductone dioxygenase.;To further explore the oxidation chemistry of acireductone-type model substrates, the reaction of the Ni(II) cis-beta-keto-enolate adduct model complex in the absence of any base is explored. In addition, the reaction of a monoanionic form of the substrate with O2 in the absence of the Ni(II) ion is evaluated. Both of these reactions resulted in the formation of Ni(II)-ARD-type products. Notably, cis -beta-keto-enolate Ni(II) complexes derived from dibenzoylmethane, 2,4-pentanedione, or 1,2-dibenzoylethenol have been synthesized, characterized, and found to be stable toward dioxygen.;Reactions involving the Ni(II) cis-beta-keto-enolate adduct model complex and O2 result in the formation of mononuclear Ni(II) carboxylate complexes of the 6-Ph2TPA ligand. The coordination chemistry of these carboxylate derivatives has been further explored in terms of reactivity in the presence of acid and factors influencing the coordination mode of the carboxylate to the Ni(II) center.