Di- and trivalent iron complexes with redox-active 1-(2-pyridylazo)-2-phenanthrol (papl)

The synthesis and study of redox-active ligand complexes has received much recent attention. We have recently explored the coordination chemistry of arylazo redox-active ligand 1-(2-pyridylazo)-2-phenanthrol (papl) and herein expand the series of complexes to the Group 8 transition metal ion, iron. We report the preparation and structure of a divalent iron complex with the papl anion, Fe(papl)₂, which is diamagnetic and features ligand centered redox activity and intense visible electronic transitions. Three polymorphs of the Fe(papl)₂complex are reported, including one that exhibits a structural phase transition at low temperature. Oxidation of Fe(papl)₂with molecular iodine has produced the low-spin trivalent complex [Fe³⁺(papl)₂]⁺as an ionic complex with the I₃anion. Density functional theory (dft) calculations and data from variable temperature Mössbauer spectroscopy experiments are provided to shed light on the electronic structures of these iron complexes.