Transition metal complexes of a tridentate ligand bearing two pendant pyridine bases: The X-ray crystal structure of pentacoordinate copper(II) complex

The synthesis of a tridentate ligand, N,N'-bis(2-pyridinyl)-2,6-pyridinedicarboxamide [H2L] is described together with its manganese(II), cobalt(II), nickel(II), copper(II), zinc(II) and cadmium(II) complexes which were characterized based on elemental analysis, conductivity measurements, spectral, magnetic and thermal studies. The IR spectral studies of all the complexes exhibit a similar feature about the ligating nature of the ligand to the metal ions and revealed that the ligand has coordinated through the nitrogens of the deprotonated amides and the central pyridine. The two pendant pyridine nitrogens in all the complexes are protonated and involved in hydrogen bonding with the oxygens of amide groups. This observation is confirmed by the single-crystal X-ray crystallographic studies of copper(H) complex. The geometry around the copper atom can be viewed as a distorted trigonal bipyramid with tau = 0.74 [structural parameter, tau = (beta - alpha)/60; where alpha and beta are the two basal angles in a five coordinate complex]. The electrochemical study of the copper(II) complex shows single quasi-reversible redox peak [Cu(II) <-> Cu(l)]. The EPR spectrum of copper(H) complex exhibits rhombic pattern [g(1) = 2.0276, g(2) = 2.0926 and g(3) = 2.18]. (c) 2005 Elsevier B.V. All rights reserved.