Stepwise Spin Transitions of Spin-Crossover Complexes Based in 3-(2-Pyridyl)-1,2,4-triazole Ligands Associated with Symmetry Change in Hydrogen Bonding Interactions

Iron(II) compounds of a family of 3-(2-pyridyl)-1,2,4-triazole ligands with aliphatic substituents of different lengths and branching have been prepared and characterized together with the benzyl derivative with formula [Fe(L)3][X]2·(solv)n (L = 3-(2-pyridyl)-5R-1,2,4-triazole, R = Et (L1), i-Pr (L2), Me (L3), Bz (L4), c-Pr (L5) and C5H9O (L6), X = ClO4- or BF4-, solv = H2O, EtOH or Me2CO). In the complexes with ethyl substituents [Fe(L1)3][ClO4]2 (1[ClO4]2) and [Fe(L1)3][BF4]2 (1[BF4]2), the length and flexibility of the alkyl chain are appropriate to obtain a structure without solvent molecules and strong intermolecular interactions (hydrogen bonds), leading to remarkable spin-crossover properties such as multistep abrupt spin transitions with thermal hysteresis associated with structural phase transitions, light or thermally induced excited spin state trapping effects (LIESST and TIESST, respectively) in 1[BF4]2, and LIESST and reverse LIESST in 1[ClO4]2. The presence of disordered solvent molecules and, subsequently, weaker intermolecular interactions in the other compounds with shorter or bulkier substituents gives rise to gradual and incomplete spin-crossover, as usually found for these types of complexes. In [Fe(L2)3][BF4]2·EtOH (2[BF4]2·EtOH), removal of the solvent molecules to form the unsolvated compound [Fe(L2)3][BF4]2 (2[BF4]2) results in more abrupt and complete spin-crossover.