Spin-State Modulation in Fe(II)-Phenanthroline Complexes: Enhancement of the Ligand Field by Heterocyclic Fusion

The performance regulation of spin-crossover Fe(II) complexes relies critically on understanding the relationship between ligand field effects and electronic structures. In this work, a series of Fe(II) complexes featuring heterocyclic-functionalized phenanthroline ligands have been systematically investigated using density functional theory calculations. The results demonstrate that the introduction of N-heterocycles effectively enhances the ligand field splitting energy, significantly promoting the transition from the high-spin to the low-spin state. Theoretical modeling and orbital analysis reveal that the electronic effects of N-heterocycles play a decisive role in spin-state modulation, with their influence being substantially stronger than that of purely aromatic ring extensions. This study provides theoretical insights and ligand-engineering strategies for the design of spin-crossover materials based on electronic structure tuning.