Enhanced and selective oxygen reduction by iron porphyrin with a biguanide residue in the second coordination sphere

The role of 2nd sphere residues in catalytic processes that require multiple protons and multiple electrons is a growing area of interest. These 2nd 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 2nd sphere of an iron porphyrin. The resulting porphyrin, Fe-biguanide, selectively reduces O₂ to H₂O via 4e^-/4H⁺ pathway in both aqueous and organic media, with rates much faster than those reported for any porphyrin to date. In the aqueous medium, the biguanide is protonated, whereas in the organic medium, it exists in neutral form. Several spectroscopic techniques confirmed the presence of hydrogen bonding with water molecules in the 2nd sphere. These results indicate that water molecules, held by 2nd sphere residues, can accelerate the kinetics of 4e^-/4H⁺ reduction of O₂ as well as any other functional group.