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Unusual Stabilization of Ferriheme in Three Different Spin States upon Stepwise Cyanide Coordination in a Single
Subhadip Pramanik1, Nidhi Awasthi1, Sankar Prasad Rath1
1Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India.
Abstract:
Cyanide (CN-) is widely recognized as a potent inhibitor of respiratory oxidases, yet its strong field and π-accepting character also make it a powerful ligand for manipulating electronic states in ferriheme. Herein, we report the first example of a ferric porphyrin framework that stabilizes three distinct spin states of iron: high-spin (S = 5/2) 1, intermediate-spin (S = 3/2) 2, and low-spin (S = 1/2) 3, generated from a single cofacial cis-Fe(III) porphyrin dimer (cis-Fe2) upon stepwise cyanide coordination. Cyanide serves as both a terminal and bridging ligand, giving rise to five-coordinate and six-coordinate Fe(III) species that coexist in solution (1, 2, and 3). Remarkably, the individual spin isomers are unambiguously distinguished by the combined application of paramagnetic 1H NMR, Mössbauer, and EPR spectroscopy, in which each technique provides very characteristic fingerprints at a distinct spectral region for each spin state. The populations of the three spin states can also be tuned by varying the CN- concentration, whereas the analogous trans-dimer (trans-Fe2) yields exclusively the low-spin six-coordinate complex 4. Single-crystal X-ray diffraction of the latter and DFT calculations further substantiate the coordination motifs. These findings establish a unique molecular platform that enables spectroscopically resolvable spin-state diversity in ferriheme, thereby opening new avenues for spin-dependent reactivity and the design of functional materials.
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