FeIII Spin Crossover Complexes with a Dual-Substituted Hqsal Ligand: Anion, Solvent, and Pressure Effects

Abstract

Three Fe^III complexes featuring a new qsal-type ligand with substitutions on both the quinoline and benzene rings, [Fe(5-CH₃-qsal-5-Br_q)₂]A·sol (A = NO₃^–, sol = CH₃OH 1-a and 1-b; and A = OTf^–, sol = 1.3CH₃OH 2, 5-CH₃-qsal-5-Br_q = N-(5-Bromo-8-quinolyl)-5-methyl-salicylaldimine), were synthesized and characterized. Isomers 1-a (kinetically stable) and 1-b (thermodynamically stable) coexist in varying ratios in different batches, influenced by evaporation time. Isomer 1-a and complex 2, both with one unique Fe^III center, exhibit similar Chain-Layer structures, whereas isomer 1-b, with two unique Fe^III centers, shows slight difference. The different stabilities and ratios of the isomers result in different spin crossover performances with varying high spin proportions across different batches of sample 1. Solvent effects on spin crossover properties were examined for all complexes. For 1-a and 1-b, desolvation leads to a transition from spin crossover behavior to a high spin state, whereas in complex 2, it induces a transition from high spin to spin crossover. A solvent exchange from methanol to water at room temperature leads to the hydrated form 1-b(H₂O), which always stays in the high spin state. Progressive desolvation of complex 2 induces a gradual transition from a pure high spin state to an incomplete spin crossover with hysteresis loops, ultimately leading to a nearly complete spin crossover with minimal loops. Pressure effects have also been examined, showing stabilization of the low spin state for both complexes.