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()][X]·(solv) ( = 3-(2-pyridyl)-5-1,2,4-triazole, = Et (), -Pr (), Me (), Bz (), c-Pr () and CHO (), X = ClO or BF, solv = HO, EtOH or MeCO). In the complexes with ethyl substituents [Fe()][ClO] () and [Fe()][BF] (), 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 and LIESST and reverse LIESST in . 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()][BF]·EtOH (), removal of the solvent molecules to form the unsolvated compound [Fe()][BF] () results in more abrupt and complete spin-crossover.