Examining Pseudohalide (N₃, NCS, NCO) Coordination in Nonheme Fe(II) and Fe(III) Complexes

Pseudohalide ligands (N₃^-, NCS^-, NCO^-) have emerged as valuable probes for studying nonheme iron halogenase reactivity due to their halogen-like behavior and distinct infrared (IR) spectroscopic signatures. Here, we present a systematic structural and vibrational analysis of a series of Fe(II) and Fe(III) complexes ligated by pseudohalides within a consistent tetradentate N₄O ligand framework (BNPA^Ph₂O^-, BNPA^Ph(p-F)₂O^-, and DPA^Ph₂O^-). Single-crystal X-ray diffraction and IR spectroscopy reveal how variations in oxidation state, hydrogen bonding, trans ligands, and ligand electronics modulate Fe-X (X = N₃, NCS, NCO) bond lengths, angles, and stretching frequencies. While hydrogen bonding induces significant geometric distortions, it has minimal impact on azide vibrational energies. In contrast, subtle electronic modifications and trans ligand effects yield measurable shifts in vibrational frequencies, especially for NCS and NCO. Notably, axial versus equatorial ligand positioning leads to consistent vibrational splitting, enabling confident spectroscopic assignment of pseudohalide orientation. Oxidation state studies reveal a blue shift in ν(N₃) from Fe(II) to Fe(III), while NCS shows a red shift and NCO remains largely unaffected. These findings establish pseudohalides as vibrational reporters of coordination environment in nonheme iron systems and offer a foundation for their use in mechanistic studies of metalloenzymes and bioinspired catalysis.