Coordination chemistry with pyridine/pyrazine amide ligands. Some noteworthy results

Pyridine-2-carboxamide and pyridine-2,6-dicarboxamide-based chelating ligands form a variety of coordination complexes with a number of metal ions, providing varying coordination geometry and nuclearity. Recent years have seen considerable interest in the designing of this class of ligands and to study their structural properties to serve specific stereochemical requirement of a particular metal-binding site. Notably, this class of ligands has been extensively utilized by Mascharak and co-workers to provide low-molecular-weight representations of metallo-proteins/enzymes such as bleomycins, nitrile hydratase. Moreover, the transition metal complexes of this class of ligands are being used as various exogenous nitric oxide (NO) donors. Using over 60 this class of chelating carboxamide ligands, the stereochemical properties of over 150 discrete coordination complexes, studied by single-crystal X-ray crystallography have been analyzed. Various bonding modes fora given chelating ligand are involved, and are reviewed with reference to ligand structure and the resulting coordination complexes. It is shown that the complexes synthesized have served to address notable issues such as effect of ligand structure/donor atom on metal-centered redox potentials, change of spin-state of iron(III), ligand-radical coordinated metal-complexes, interesting chemical reactivity studies, and catalytic potential. The ligands are introduced systematically as a function of their denticity, making easy access to information on specific type of ligands and coordination complexes thereof. X-ray crystallographically determined bond lengths of various donor atoms/groups are collected in a table, thus providing an accessible source for reference purposes. Source material for the review amounts to about 90 references. (C) 2012 Elsevier B.V. All rights reserved.