Unusual Stabilization of Ferriheme in Three Different Spin States upon Stepwise Cyanide Coordination in a Single Molecular Framework

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.