Carboxylate Coordination Chemistry of a Mononuclear Ni(II) Center in a Hydrophobic or Hydrogen Bond Donor Secondary Environment:Relevance to Acireductone Dioxygenase

摘要

A series of Ni(II) 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(O2C(CH2)2SCH3)]CIO4 centre dot CH2Cl2 (4 centre dot CH2Cl2) and [(6-Ph2TPA)Ni(O2-CCH2SCH3)]CIO4 centre dot 1.5CH2CI2 (5 centre dot 1.5CH2Cl2) revealed that each complex contains a distorted octahedral Ni(II) 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 centre dot 0.5CH2Cl2,and 5 from wet organic solvents yielded a second series of crystalline Ni(II) carboxylate complexes having a coordinated monodentate carboxylate ligand ([(6-Ph2TPA)Ni(H2O)(O2CPh)]ClO4 (6),[(6-Ph2TPA)Ni(H2O)(O2C(CH2)2SCH3)]ClO4 centre dot 0.2CH2Cl2 (7 centre dot 0.2CH2Cl2),[(6-Ph2TPA)Ni(H2O)(O2CCH2SCH3)]ClO4 (8)) which is stabilized by a hydrogen-bonding interaction with a Ni(II)-bound water molecule.In the cationic portions of 7 centre dot 0.2CH2Cl2 and 8,weak CH/pi 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(H2O)(O2CH)]ClO4 (9) was isolated and characterized.The mononuclear Ni(II) carboxylate complexes [(bnpapa)Ni(O2CPh)]ClO4 (10),[(bnpapa)Ni(O2C(CH2)2SCH3)]ClO4 (11),[(bnpapa)Ni(O2CCH2SCH3)]ClO4 (12),and [(bnpapa)Ni(O2CH)]ClO4 (13) were isolated and characterized.Two crystalline solvate forms of 10 (10 centre dot CH3CN and 10 centre dot CH2Cl2) were examined by X-ray crystallography.In both,the distorted octahedral Ni(II) center is ligated by a bidentate benzoate ligand,one Ni(II)-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(II) center in acireductone dioxygenase.