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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

Year: 2025

Type: article

Abstract: 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)-5 R -1,2,4-triazole, R = Et ( L1 ), i -Pr ( L2 ), Me ( L3 ), Bz ( L4 ), c-Pr ( L5 ) and C 5 H 9 O ( L6 ), X = ClO 4 − or BF 4 −, solv = H 2 O, EtOH or Me 2 CO). In the complexes with ethyl substituents [Fe( L1 ) 3 ][ClO 4 ] 2 ( 1[ClO 4 ] 2 ) and [Fe( L1 ) 3 ][BF 4 ] 2 ( 1[BF 4 ] 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[BF 4 ] 2, and LIESST and reverse LIESST in 1[ClO 4 ] 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 ][BF 4 ] 2 ·EtOH ( 2[BF 4 ] 2 ·EtOH ), removal of the solvent molecules to form the unsolvated compound [Fe( L2 ) 3 ][BF 4 ] 2 ( 2[BF 4 ] 2 ) results in more abrupt and complete spin-crossover.

Source: Inorganic Chemistry

DOI: 10.1021/acs.inorgchem.4c04952