Thermal spin-crossover with a large hysteresis spanning room temperature in a mononuclear complex.

A large hysteresis centered around room temperature represents one of the mandatory goals of research on functional switchable materials. In the thoroughly studied field of spin-crossover, such behaviour appears very rarely and essentially concerns coordination networks. A new compound showing a large spin-crossover hysteresis spanning room temperature demonstrates in a definitive manner that this goal is achievable in molecular discrete compounds without damaging the single-crystal character.

Structural movies of the gradual spin-crossover in a molecular complex at various physical scales.

The thermally induced Spin-CrossOver (SCO) undergone by the mononuclear iron(ii) complex [Fe(PM-AzA)2(NCS)2] (PM = N-2'-pyridylmethylene, AzA = 4-(phenylazo)aniline) is fully pictured by a quasi-continuous structural determination all along the spin-state modification within the sample. This large scale multi-temperature Single-Crystal X-Ray Diffraction (SCXRD) investigation leads to making structural movies. The latter reveal or confirm some features of the SCO that are subsequently validated by the same systematic investigation performed on a zinc isostructural analogue complex. Notably, the continuous views of the temperature dependencies of the unit-cell parameters, the dilatation tensors, the metal coordination sphere geometry and the intermolecular distances confirm a few of the structure-property relationships already known for SCO materials. In parallel, the examination of the temperature dependencies of the atomic coordinates and the atomic displacement parameters reveals unexpected behaviours in this gradual SCO material such as antagonistic atomic movements due to the single SCO and the pure thermal effects.