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A mononuclear Ni(ll) complex ([(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]CIO4 (1)), supported by the 6-Ph2TPA chelate ligand (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) and containing a cis-β-keto-enolate ligand having a C2 hydroxyl substituent, undergoes reaction with O2 to produce a Ni(ll) monobenzoate complex ([(6-Ph2TPA)Ni(O2CPh)]ClO4 (3)), CO, benzil (PhC(O)C(O)Ph), benzoic acid, and other minor unidentified phenyl-containing products. Complex 3 has been identified through independent synthesis and was characterized by X-ray crystallography, 1H NMR, FAB-MS, FTIR, and elemental analysis. A series of cis-β-keto-enolate Ni(lI) complexes supported by the 6-Ph2TPA ligand ([(6-Ph2TPA)Ni(PhC(O)CHC(O)Ph)]CIO4 (4), [(6-Ph2TPA)Ni(CH3C(O)CHC(O)CH3)]ClO4 (5), and [(6-Ph2TPA)Ni(PhC(O)CHC(O)C(O)Ph) (6)) have been prepared and characterized. While these complexes exhibit structural and/or spectroscopic similarity to 1, all are unreactive with O2. The results of this study are discussed in terms of relevance to Ni(Il)-containing acireductone dioxygenase enzymes, as well as in the context of recently reported cofactor-free, quercetin, and β-diketone dioxygenases. [ABSTRACT FROM AUTHOR]

A series of Ni(ll) carboxylate complexes, supported by a chelate ligand having either secondary hydrophobic phenyl groups (6-Ph2TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) or hydrogen bond donors (bnpapa, N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been prepared and characterized. X-ray crystallographic studies of [(6-Ph2TPA)Ni(02C(CH2)2SCH3)]CIO4CH2CI2 (4∙CH2CI2) and [(6-Ph2TPA)Ni(O2- CCH2SCH3)]CIO41.5CH2CI2 (5∙1.5CH2CI2) revealed that each complex contains a distorted octahedral Ni(ll) center and a bidentate carboxylate ligand. A previously described benzoate complex ([(6-Ph2TPA)Ni(O2CPh)]ClO4 (3)) has similar structural characteristics. Recrystallization of dry powdered samples of 3, 4∙0.5CH2CI2, and 5 from wet organic solvents yielded a second series of crystalline Ni(ll) carboxylate complexes having a coordinated monodentate carboxylate ligand ([(6-Ph2TPA)Ni(H2O)(O2CPh)]CIO4 (6), [(6-Ph2TPA)Ni(H2O)(O2C(CH2)2SCH3)]ClO4∙0.2CH2Cl2 (7∙0.2CH2CI2), [(6-Ph2TPA)Ni(H2O)(O2CCH2SCH3)]CIO4 (8)) which is stabilized by a hydrogen-bonding interaction with a Ni(ll)-bound water molecule. In the cationic portions of 7∙0.2CH2CI2 and 8, weak CH/π interactions are also present between the methylene units of the carboxylate ligands and the phenyl appendages of the 6-Ph2TPA ligands. A formate complex of the formulation [(6-Ph2TPA)Ni(H20)(O2CH)1CIO4 (9) was isolated and characterized. The mononuclear Ni(l I) carboxylate complexes [(bnpapa)Ni(O2CPh)1CIO4 (10), [(bnpapa)Ni(02C(CH2)2SCH3)]C104 (11), [(bnpapa)Ni(O2CCI-I2SCH3)]CIO4 (12), and [(bnpapa)Ni(O2CH)]CIO4 (13) were isolated and characterized. Two crystalline solvate forms of 10 (10∙CH3CN and 10∙CH2CI2) were examined by X-ray crystallography. In both, the distorted octahedral Ni(ll) center is ligated by a bidentate benzoate ligand, one Ni(ll)-bound oxygen atom of which accepts two hydrogen bonds from the supporting bnpapa chelate ligand. Spectroscopic studies of 10-13 suggest that all contain a bidentate carboxylate ligand, even after exposure to water. The combined results of this work enable the formulation of a proposed pathway for carboxylate product release from the active site Ni(ll) center in acireductone dioxygenase. [ABSTRACT FROM AUTHOR]

Zinc complexes of three new amide-appended ligands have been prepared and isolated. These complexes, [(dpppa)-Zn](CIO4)2 (4(CIO4)2; dpppa = N-((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)-amine), [(bdppa)Zn](CIO4)2 (6(CIO4)2; bdppa = N,N-bis((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl). methyl)amine), and [(epppa)Zn](CIO4)2 (8(CIO4)2; epppa = N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been characterized by X-ray crystallography (4(CIO4)2 and 8(CIO4)2), ¹H and 13C NMR, IR, and elemental analysis. Treatment of 4(CIO4)2 or 8(CIO4)2 with 1 equiv of Me4NOH•5H2O in methanol-acetonitrile (5:3) results in amide methanolysis, as determined by the recovery of primary amine-appended forms of the chelate ligand following removal of the zinc ion. These reactions proceed via the initial formation of a deprotonated amide intermediate ([(dpppa-)Zn]CIO4 (5) and [(epppa-)Zn]CIO4 (9)) which in each case has been isolated and characterized (¹H and 13C NMR, IR, elemental analysis). Treatment of 6(CIO4)2 with Me4NOH•5H2O in methanol-acetonitrile results in the formation of a deprotonated amide complex, [(bdppa-)Zn]CIO4 (7), which was isolated and characterized. This complex does not undergo amide methanolysis after prolonged heating 'in a methanol-acetonitrile mixture. Kinetic studies and construction of Eyring plots for the amide methanolysis reactions of 4(CIO4)2 and 8(CIO4)2 yielded thermodynamic parameters that provide a rationale for the relative rates of the amide methanolysis reactions. Overall, we propose that the mechanistic pathway for these amide methanolysis reactions involves reaction of the deprotonated amide complex with methanol to produce a zinc methoxide species, the reactivity of which depends, at least in part, on the steric hindrance imparted by the supporting chelate ligand. Amide methanolysis involving a zinc complex supported by a N2S2 donor chelate ligand (3(CIO4)2) is more complicated, as in addition to the formation of a deprotonated amide intermediate free chelate ligand is present in the reaction mixture. [ABSTRACT FROM AUTHOR]