Different Spin-State Behaviors in Isostructural Solvates of a Molecular Iron(II) Complex

The complex [FeL2][BF4]2 (1; L=4-(isopropylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) forms solvate crystals 1 solv (solv=MeNO2, MeCN, EtCN, or Me2CO). Most of these materials lose their solvent sluggishly on heating. However, heating 1 MeNO2 at 450K, or storing 1 EtCN under ambient conditions, leads to single-crystal to single-crystal exchange of the organic solvent for atmospheric moisture, forming 1 H2O. Solvent-free 1 (1 sf) can be generated in situ by annealing 1 H2O at 370K in the diffractometer or magnetometer. The different forms of 1 are isostructural (P21/c, Z=4) and most of them exhibit spin-crossover (SCO) at 141≤T1/2≤212K, depending on their solvent content. The exception is the EtCN solvate, whose pristine crystals remain high-spin between 3-300K. The cooperativity of the spin-transitions depends on the solvent, ranging from gradual and incomplete when solv=acetone to abrupt with 17K hysteresis when solv=MeCN. Our previously proposed relationship between molecular structure and SCO explains some of these observations, but there is no single structural feature that correlates with SCO in all the 1 solv materials. However, changes to the unit cell dimensions during SCO differ significantly between the solvates, and correlate with the SCO cooperativity. In particular, the percentage change in unit cell volume during SCO for the most cooperative material, 1 MeCN, is 10 times smaller than for the other 1 solv crystals.