Low-spin [M(L)] and [M(L)] (M = Fe and Co) complexes of tridentate azo-containing pyridine/pyrazine amide ligands: structures, properties and redox potential correlations.

Using deprotonated forms of tridentate azo-containing pyridine-2-/pyrazine-2-carboxamide 2-[N-(2-phenylazo)carbamoyl]-pyridine/pyrazine, seven bis-ligand complexes of Fe/Co and Fe/Co have been synthesized. Molecular structures of six of them reveal that these six-coordinate complexes utilize all available donor sites of the ligands and assume MN(pyridine/pyrazine)N'(amide)N''(azo) coordination. Complexes of Fe and Co are diamagnetic and those of Fe and Co are paramagnetic (S = 1/2; room-temperature magnetic data and EPR spectra). Cyclic voltammetry experiments in CHCl reveal facile metal-centred Fe/Fe and Co/Co redox responses, and all complexes display quasireversible-to-irreversible ligand(azo)-centred redox processes. The E values of M/M redox processes for Fe, Co and Ni (reported earlier) complexes of the pyridine amide ligand linearly correlate with those for six-coordinate [M(bpy)]/[M(bpy)], [M(terpy)]/[M(terpy)], [M(L)]/[M(L)] or [M(L')]/[M(L')] (bpy = 2,2'-bipyridine, terpy = 2,2':6',2''-terpyridine, hexadentate L(2-) = 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane and tridentate L'(-) = {2-[2-(arylimino)phenylazo]-pyridine}) couples. Density functional theory (DFT) at the B3LYP level and time-dependent (TD)-DFT calculations rationalize the electronic structure of the present complexes and throw light on the origin of observed electronic transitions.