Enhanced and Selective Oxygen Reduction by Iron porphyrin with a Biguanidine Residue in the Second Coordination Sphere

Abstract

The role of 2^nd sphere residues in catalytic processes that require multiple-protons and multiple-electrons is a growing area of interest. These 2^nd sphere effects manifest through hydrogen bonding, proton transfer and electrostatic interactions resulting in very selective catalysis. Here a biguanide functional group, which can form very strong hydrogen bonding with several water molecules, is installed in the 2^nd sphere of an iron porphyrin. The resulting porphyrin, Fe-biguanide, is found to reduce O₂ selectively, by 4e^-/4H⁺, to H₂O in both aqueous, where the biguanide is protonated, and organic medium, where it is neutral, with rates much faster than any porphyrin reported till date. The presence of hydrogen bonding with water molecules in the 2^nd sphere is confirmed using several spectroscopic techniques. These results indicate that water molecules, held by 2^nd sphere residues, can accelerate the kinetics of 4e^-/4H⁺ reduction of O₂as well as any other functional group.