Low-spin [MII(L)2] and [MIII(L)2]+ (M = Fe and Co) complexes of tridentate azo-containing pyridine/pyrazine amide ligands: structures, properties and redox potential correlations.

Abstract

Using deprotonated forms of tridentate azo-containing pyridine-2-/pyrazine-2-carboxamide 2-[N-(2-phenylazo)carbamoyl]-pyridine/pyrazine, seven bis-ligand complexes of FeII/CoII and FeIII/CoIII 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 MII/IIIN2(pyridine/pyrazine)N'2(amide)N''2(azo) coordination. Complexes of FeII and CoIII are diamagnetic and those of FeIII and CoII are paramagnetic (S = 1/2; room-temperature magnetic data and EPR spectra). Cyclic voltammetry experiments in CH2Cl2 reveal facile metal-centred FeIII/FeII and CoIII/CoII redox responses, and all complexes display quasireversible-to-irreversible ligand(azo)-centred redox processes. The E1/2 values of MIII/MII redox processes for Fe, Co and Ni (reported earlier) complexes of the pyridine amide ligand linearly correlate with those for six-coordinate [MIII(bpy)3]3+/[MII(bpy)3]2+, [MIII(terpy)2]3+/[MII(terpy)2]2+, [MIII(L)]+/[MII(L)]0 or [MIII(L')2]+/[MII(L')2]0 (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.