Expanded coordination cavity stabilizes a high-spin Fe(III) state in a six-coordinate oxypyriporphyrin with mixed neutral/anionic axial ligation.

Abstract

Oxypyriporphyrin (OxyPyP) possesses an unusually expanded N4 coordination cavity, yet its influence on the electronic structures of coordinated metal ions remains poorly understood. Herein, we investigate a series of OxyPyHEP-Fe(III) complexes with different axial-ligation environments to clarify how the expanded coordination cavity influences their structures and spin states. Among these complexes, the six-coordinate complex [OxyPyHEPFe(III)(1-MeIm)(Cl)] (2), bearing 1-methylimidazole (1-MeIm) and chloride as axial ligands, was isolated and structurally characterized. 2 features a mixed neutral/anionic axial-ligation environment. Single-crystal X-ray diffraction analysis reveals an expanded N4 cavity (8.738 Å2) with elongated Fe-N(1-MeIm) (2.181 Å) and Fe-Cl (2.354 Å) bonds. Magnetic susceptibility measurements (μeff = 5.9μB at 300 K) and EPR spectroscopy (g = 8.15, 3.44, and 1.66) establish a high-spin (HS) Fe(III) (S = 5/2) state despite the six-coordinate geometry. In contrast, the corresponding bis(1-MeIm) complex adopts a low-spin state (μeff = 2.0μB at 300 K), highlighting the critical role of axial ligation in controlling spin-state energetics. The isolation of 2 provides direct structural evidence for the mono(1-MeIm) intermediate proposed from solution titration studies. These findings reveal that expansion of the OxyPyP coordination cavity promotes stabilization of a six-coordinate HS Fe(III) state, providing a structural basis for tuning the electronic and magnetic properties of porphyrinoid iron complexes.