Effects of water removal on the structure and spin-crossover in an anilato-based compound

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)].2.5H(2)O (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 100K with no hysteresis. The loss of the water molecules results in a phase transition from a P2(1)/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 195/202K with a hysteresis of ca. 7K. 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 T-LIESST of ca. 70K.