Second sphere control of redox catalysis: Selective reduction of O 2 to O2- or H2O by an iron porphyrin catalyst

Abstract

"Click" reaction has been utilized to synthesize porphyrin ligands possessing distal superstructures functionalized with ferrocenes, carboxylic acid esters, and phenols. Both structural and spectroscopic evidence indicate that hydrogen bonding interaction between the triazole residues resulting from the "click" reaction promotes axial ligand binding into the sterically demanding distal pocket in preference to the open proximal side. An iron porphyrin complex with four ferrocene groups is found to bind O 2 and quantitatively reduce it by one electron to O2- in apolar organic solvents. However the same complex electro-catalytically reduces O2 by four electrons to H2O in aqueous medium under fast, moderate, and slow electron fluxes. This selectivity for O2 reduction is governed by the reduction potential of the electron transfer site (i.e., ferrocene) which in turn is governed by the solvent. This catalyst mimics control of catalysis of an enzyme active site by a second sphere electron transfer residue which is often encountered in naturally occurring metallo-enzymes. © 2013 American Chemical Society.