Temperature-dependent hysteretic two-step spin crossover in two-dimensional Hofmann-type compounds

Abstract

Hofmann-type spin-crossover (SCO) frameworks attract much attention due to their predictable structures and diverse SCO behaviours. In this paper, we report two isostructural two-dimensional (2D) Hofmann-type SCO frameworks [Fe^II(ppe)₂M^II(CN)₄]·3H₂O (ppe = 1-(2-pyridyl)-2-(4-pyridyl)ethylene and M = Pd for 1 and Pt for 2). X-ray crystallographic studies, magnetic measurements and Mössbauer spectra jointly reveal that both compounds undergo complete hysteretic two-step SCO, i.e. HS^1.0 ⇌ HS^0.6–0.5LS^0.4–0.5 ⇌ LS^1.0 conversion with thermal hysteresis loops of 14 K and 28 K wide for 1 and 13 K and 26 K wide for 2. The dehydrated samples, 1-dehydrated and 2-dehydrated, display different SCO behaviours, unveiling that the SCO behaviours are affected by guest water molecules.