Thermally and Light-Induced Spin-Crossover in Iron(III) Complexes with Benzophenone-Based Saltrien Ligands: Hysteresis, Two-Step Transitions, and the LIESST Effect

Abstract

The synthesis and characterization of four new iron(III) coordination compounds with saltrien-like hexadentate Schiff base ligands L n , prepared by condensation between triethylenetetramine and 2-hydroxy-3,5-dimethylbenzophenone (L1, C1-C3) or 2-hydroxy-5-methoxybenzophenone (L2, C4), are reported. The complexes [Fe(L n )]XmCH3CN (X = SeCN- for C1 and C4, SCN- for C2 and BPh4 - for C3, m = 0 for C3, m = 1 for C1 and C4, m = 2 for C2) were structurally characterized, and their spin-crossover (SCO) was monitored by magnetic measurements, X-ray powder diffraction analysis, and EPR spectroscopy. Intermolecular interactions relevant to SCO were analyzed through Hirshfeld surface maps and QT-AIM calculations. All compounds exhibit SCO above room temperature in their solvated forms, and ab initio calculations were employed to probe their electronic structures. While the computed 2 T 2g-6 A 1g gaps and 10Dq energies are consistent across the whole series, the experimental T 1/2 values do not directly reflect these energy differences. Instead, SCO is predominantly controlled by crystal packing effects, including intermolecular connectivity, internal pressure, lattice rigidity, and solvation. Upon heating, solvent removal in C1 and C2 shifts their SCO to below room-temperature. The desolvated compounds C1d and C2d exhibit sharp SCO with wide hysteresis, while C2d additionally features a second gradual step (C1d: T 1/2 = 82 K/166 K; C2d: T(1)1/2 = 170 K/153 K, T(2)1/2 = 110 K). Furthermore, both compounds are LIESST active upon blue light irradiation (T(LIESST)=57 K for C1d and 36 K for C2d). These results underscore the crucial role of ligand flexibility, solvation, and intermolecular interactions on SCO and highlight the potential of these iron(III) complexes in molecular switching applications.