A sequential method to prepare polymorphs and solvatomorphs of [Fe(1,3-bpp)2](ClO4)2⋅nH2O (n=0, 1, 2) with varying spin-crossover behaviour

Two polymorphs of the spin crossover (SCO) compound [Fe(1,3-bpp)2](ClO4)2 (1 and 2; 1,3-bpp=2-(pyrazol-1-yl)-6-(pyrazol-3-yl)pyridine) were prepared using a novel, stepwise procedure. Crystals of 1 deposit from dry solvents, while 2 is obtained from a solid-state procedure, by sequentially removing lattice H2O molecules from the solvatomorph [Fe(1,3-bpp)2](ClO4)2⋅2 H2O (2⋅2 H2O), using single-crystal-to-single-crystal (SCSC) transformations. Hydrate 2⋅2 H2O is obtained through the same reaction as 1, now with 2.5 % of water added. Compounds 2 and 2⋅2 H2O are unstable in the atmosphere and absorb or lose one equivalent of water, respectively, to both yield the stable solvatomorph [Fe(1,3-bpp)2](ClO4)2⋅H2O (2⋅H2O), also following SCSC processes. The four derivatives have been characterised by single-crystal X-ray diffraction (SCXRD). Furthermore, the homogeneity of the various compounds as well as their SCSC interconversions have been confirmed by powder X-ray diffraction (PXRD). Polymorphs 1 and 2 exhibit abrupt SCO behaviour near room temperature with T1/2↑=279/316 K and T1/2↓=276/314 K (near 40 K of shift) and different cooperativity.