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

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)]ClO4·CH2Cl2 (4·CH2Cl2) and [(6-Ph2TPA)Ni(O2CCH2SCH3)]ClO4·1.5CH2Cl2 (5·1.5CH2Cl2) 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, 4·0.5CH2Cl2, 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)]ClO4·0.2CH2Cl2 (7·0.2CH2Cl2), [(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 7·0.2CH2Cl2 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(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·CH3CN and 10·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.