The ON–OFF switching of thermal spin crossover by interstitial solvent exchange in a layered Re^V–CN–Fe^II coordination framework

We present an advanced material exhibiting a spin crossover (SCO) effect generated by the combined application of two external stimuli, temperature and chemical treatment related to the crystallization solvent exchange. The reported material is based on a bimetallic {[Fe^II(4-Brphpy)₄]₃[Re^V(CN)₈]₂}⋅7MeOH [1, 4-Brphpy = 4-(4-bromophenyl)pyridine] coordination network built of cyanido-bridged layers and interstitial methanol molecules. A weakly bonded solvent can be exchanged into the water, which results in a hydrated phase, {[Fe^II(4-Brphpy)₄]₃[Re^V(CN)₈]₂}⋅5H₂O (1-hyd). The difference in solvent content between 1 and 1-hyd is sufficient to induce a remarkable change in spin transition properties. Despite the {N₆} coordination environment around Fe(II), 1 reveals a stable high-spin state in the whole 2–300 K temperature range, which was assigned to the stiffening of the framework due to a series of non-covalent interactions involving solvent and 4-Brphpy ligands. Upon the exchange of solvent to water, the framework of 1-hyd becomes more flexible producing a thermally induced SCO effect, which occurs in two distinguishable steps in the broad 70–250 K range. The 1 to 1-hyd transformation can be reversed by immersion in the original solvent, thus, the chemical ON–OFF switching of a thermal SCO effect was achieved. This work shows that solvent exchange processes within bimetallic layered Fe^II–[Re^V(CN)₈]^3– networks give an efficient route for reversible chemical modulation of thermally induced SCO properties.