Atomistic Origin of Structural, Vibrational, and Electronic Fingerprints in Iminopyridine Iron(II) Dichloride Complexes

Abstract Density functional theory (DFT) investigations were performed on a diverse library of iron(II) dichloride complexes bearing iminopyridine (ImPy) ligands to establish robust structure–property relationships governing their electronic and vibrational behavior. The pronounced structural flexibility of ImPy ligands generates multiple coordination motifs and spin-state manifolds, which frequently complicate experimental characterization and spectroscopic assignment. By systematically correlating geometric descriptors with infrared (IR) and ultraviolet-visible (UV–Vis) signatures, we identify reliable markers of both spin states and local coordination environments across mononuclear and dinuclear complexes. The computational trends show strong agreement with experimental benchmarks, establishing a diagnostic spectroscopic interpretation framework of coordination geometry and spin states in structurally flexible ImPy–Fe complexes, even in the absence of crystallographic data. Ultimately, this study offers a valuable framework for the informed design of next-generation iron (pre)catalysts, helping to tune electronic landscapes and spin-state populations for optimized reactivity.