Imidazolate Bridges Enable Spin Communication in Diiron(III) Porphyrin Dimers: A Bioinspired Model of the Diheme
Subhadip Pramanik1, Rupesh Kumar Tiwari2, Sarnali Sanfui1
1Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur208016, India.
Abstract:
Inspired by the long-range electron-transfer pathways in diheme enzymes such as MauG and bacterial diheme cytochrome c peroxidases, where an intervening tryptophan residue mediates electronic communication between spatially separated heme centers, we report a rare class of imidazolate-bridged Fe(III) porphyrin dimers that emulate this biological architecture. The redox-active imidazolate bridge directly couples two iron porphyrin centers, enabling efficient electronic communication. To probe the role of structural flexibility, dimers incorporating rigid cis-ethene and flexible diethyl pyrrole linkers were systematically investigated. Stepwise chemical oxidation modulates the electronic structure and magnetic coupling through the bridge. Paramagnetic 1H NMR spectroscopy reveals oxidation-dependent shifts of the imidazolate proton resonances, providing direct evidence for spin delocalization onto the bridging ligand. Magnetic studies show a nonmonotonic dependence of the JFe-Fe exchange interaction on the oxidation state, weakening after one-electron oxidation but strengthening upon further oxidation. Complementary structural, spectroscopic, and DFT analyses attribute this behavior to subtle changes in Fe-Nμim and Fe-Np bond lengths, axial ligand orientation, and spin density redistribution, which collectively regulate orbital overlap between the iron centers. Overall, this work establishes a structure-property relationship linking the bridge geometry, redox state, and magnetic exchange, providing a molecular design strategy for tuning electronic communication in bioinspired diheme architectures.
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