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

## TL;DR

This paper studies new iron complexes that change their spin state with temperature and light, showing potential for molecular switches.

## Contribution

The study introduces new iron(III) complexes with unique spin-crossover behaviors, including hysteresis and two-step transitions.

## Key findings

- Desolvated compounds exhibit sharp spin-crossover with wide hysteresis.
- Compound C2d shows a second gradual spin-crossover step.
- Both compounds respond to blue light with 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
)]­X·mCH3CN (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.