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 (<i>S</i> = 5/2) <b>1</b>, intermediate-spin (<i>S</i> = 3/2) <b>2</b>, and low-spin (<i>S</i> = 1/2) <b>3</b>, generated from a single cofacial <i>cis</i>-Fe(III) porphyrin dimer (<i><b>cis</b></i><b>-Fe</b>_<b>2</b>) 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 (<b>1</b>, <b>2</b>, and <b>3</b>). 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 (<i><b>trans</b></i><b>-Fe</b>_<b>2</b>) yields exclusively the low-spin six-coordinate complex <b>4</b>. 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.