Electron Shuttling in High-Valent Heterobimetallic NiFe-Porphyrin Dimers: Stabilization of Ni(III) and Fe-Phenoxyl Radicals

A series of axially phenoxide-bound Ni(II)-Fe(III) heterobimetallic porphyrin dimers, as a synthetic mimic of the diheme enzyme MauG, have been synthesized, which upon 4e- oxidations revealed the oxidized complex involving porphyrin π-cation radicals, an Fe(III)-coordinated phenoxyl radical, and a Ni(III) center. Redox cooperativity and the extensive π-conjugation through the bridge make the oxidized complex behave like a single supramolecular entity having unpaired electrons spread over five magnetic centers. Several isoelectronic states for the oxidized Fe(por) center, i.e., Fe(IV)(por)(PhO•) ↔ Fe(IV)(por•+)(PhO) ↔ Fe(III)(por•+)(PhO•), are possible that can be in equilibrium with low-energy barriers between them. DFT studies demonstrate spin delocalization with a dramatic change in the Fe-OTyr bond upon oxidation, which thereby becomes an active modulator of various electronic states of high-valent intermediates in diheme enzymes, such as MauG and bacterial diheme cytochrome c peroxidase (bCcP), for their function.