Regulating Spin‐Crossover Behavior in Iron(II) Complexes: Control over SCO Temperatures by Varying NCE‒ Co‐ligands

In this research, we introduce new members to the [Fe(bzL)(NCE)2] complex series by incorporating NCS− and NCBH3− as co‐ligands, expanding on our established tetradentate bzL ligand. The synthesis of three isostructural structures has allowed for a systematic tuning of SCO temperatures, with T1/2 values ranging from 85 K for the S‐α complex, featuring approximately 15 K thermal hysteresis, to 194 K for the Se‐α complex, and a stable low‐spin state up to 300 K for the B‐α complex. This work demonstrates the efficacy of modulating SCO properties through variation of the NCE− ancillary ligands. Furthermore, we conducted a comparative analysis of 21 analogs within this family. This analysis aims to elucidate the similarities and differences among the complexes and their correlation with SCO properties, furthering our understanding of the structural determinants that influence SCO behavior.