The crucial role played by a crystallization water molecule in the spin crossover (SCO) temperature and its hysteresis is described and discussed in compound [NBu4][Fe(bpp)2][Cr(C6O4Br2)3]⋅2.5H2O (1), where bpp = 2,6-bis(pyrazol-3-yl)pyridine and (C6O4Br2)2− = dianion of the 3,6-dibromo-2,5-dihydroxy-1,4-benzoquinone. The compound has isolated [Fe(bpp)2]2+ cations surrounded by chiral [Cr(C6O4Br2)3]3− anions, NBu4+ cations, and a water molecule H-bonded to one of the non-coordinated N–H groups of one bpp ligand. This complex shows a gradual almost complete two-step spin transition centered at ca. 180 and 100 K with no hysteresis. The loss of the water molecules results in a phase transition from a P21/n phase with only one independent [Fe(bpp)2]2+ cation to a chiral Pn phase with two independent [Fe(bpp)2]2+ cations. Besides, there is an increase in the SCO temperature to 195/202 K with a hysteresis of ca. 7 K. In the dehydrated phase, only one of the two independent [Fe(bpp)2]2+ cations shows the SCO, whereas the second one remains in a high spin configuration at any temperature. In addition, compound 1 exhibits the LIESST (light-induced excited spin-state trapping) effect with a TLIESST of ca. 70 K.
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