The Role of Methyl Substitution in Spin Crossover of Fe(III) Complexes with Pentadentate Schiff Base Ligands

A series of mononuclear complexes, [Fe(L5)(bylim)](BPh4), where L5 represents a pentadentate Schiff base ligand, bylim is 1-benzyl-1-imidazole, and BPh4− is the tetraphenylborate anion, was synthesized. The determined crystal structures indicate the absence of significant cooperative interactions, which influence the properties of the eventual spin transition. Changes in magnetic behavior induced by substitution of the pentadentate ligand were investigated through magnetic susceptibility measurements. It was found that only complexes containing a non-substituted secondary amino group exhibit some form of spin crossover, whereas the majority of those with a methyl substituent remain in the high-spin state across the entire measured temperature range (2–300 K). The changes induced by the substitution of the secondary amine group were further explored through theoretical calculations at DFT and CASSCF/NEVPT2 levels of theory. The topology and energetics of electron density and atomic charges were investigated through QT-AIM calculations.