Azo-containing pyridine amide ligand. A six-coordinate nickel(II) complex and its one-electron oxidized species: Structure and properties

A new potentially tridentate ligand HL(11) consisting of 2-pyridinecarboxamide unit and azo functionality has been used, in its deprotonated form, to prepare a nickel(II) complex which has been structurally characterized. The ligand L(11)( ) affords a bis-complex [Ni(II)(L(11))(2)] (1). In 1, the two L(11)( ) ligands bind to the NiII center in a mer configuration. The relative orientations within the pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The Ni(II)N(2)(pyridyl)N'(2)(amide)N"(2)(azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni-N(am) (am = amide) bond lengths are the shortest and the Ni-N(azo) bond lengths are the longest. Complex 1 exhibits a quasi-reversible Ni(III)/Ni(II) redox process. Moreover, the complex displays two ligand-centered (azo group) quasi-reversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically-generated nickel(III) species. To understand the nature of metal-ligand bonding interactions Density Functional Theory (DFT) calculations have been performed on 1 at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results. (C) 2010 Elsevier B.V. All rights reserved.