Axial Ligation Equilibria and Dynamics in a Redox Polyether Hybrid: An Fe Tetraphenylporphyrin Melt

Abstract

Pyridine axial ligation is investigated in a molten Fe tetraphenylporphyrin fashioned by attaching oligomeric (MW 550) ethylene glycol chains at the p-phenyl ring positions ([Fe(T550PP)Cl]). Dissolution of LiClO4 electrolyte renders the highly viscous, room-temperature melt (0.40 M in porphyrin sites) modestly ionically conductive, so that microelectrode voltammetry can be employed to follow axial pyridine ligation in a thin film of Fe porphyrin melt resting on the electrode assembly. Introduction of pyridine at the gas/melt interface produces distinctive changes in the microelectrode voltammetry, from which relative populations of chloride and pyridine-coordinated Fe(III) porphyrin can be measured and equilibrium constants for ligation of the Fe(III) and Fe(II) oxidation states estimated. Removal of the pyridine at the porphyrin melt/gas interface causes dissociation of the ligating pyridine in an overall process that has a time constant on the order of 5−10 min at 25 °C.