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(<b>L</b>)₃][X]₂·(solv)_<i>n</i> (<b>L</b> = 3-(2-pyridyl)-5<i>R</i>-1,2,4-triazole, <i>R</i> = Et (<b>L1</b>), <i>i</i>-Pr (<b>L2</b>), Me (<b>L3</b>), Bz (<b>L4</b>), c-Pr (<b>L5</b>) and C₅H₉O (<b>L6</b>), X = ClO₄^- or BF₄^-, solv = H₂O, EtOH or Me₂CO). In the complexes with ethyl substituents [Fe(<b>L1</b>)₃][ClO₄]₂ (<b>1[ClO</b>_<b>4</b><b>]</b>_<b>2</b>) and [Fe(<b>L1</b>)₃][BF₄]₂ (<b>1[BF</b>_<b>4</b><b>]</b>_<b>2</b>), 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 <b>1[BF</b>_<b>4</b><b>]</b>_<b>2,</b> and LIESST and reverse LIESST in <b>1[ClO</b>_<b>4</b><b>]</b>_<b>2</b>. 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(<b>L2</b>)₃][BF₄]₂·EtOH (<b>2[BF</b>_<b>4</b><b>]</b>_<b>2</b><b>·EtOH</b>), removal of the solvent molecules to form the unsolvated compound [Fe(<b>L2</b>)₃][BF₄]₂ (<b>2[BF</b>_<b>4</b><b>]</b>_<b>2</b>) results in more abrupt and complete spin-crossover.