Two new copper(II) complexes [Cu₂(L)₂(OAc)₂(H₂O)₂] (1) (L = 3-methyl-5-pyridin-2-yl-1,2,4-triazole) and [CuL₂] (2) were prerared and thoroughly studied. The complexes are able to selectively catalyze the oxidation of styrene towards benzaldehyde and of cyclohexane to KA oil. The 2D coordination polymer 1 showed an antiferromagnetic behaviour attributed to the intrachain magnetic coupling.

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.

The title compound, [Cu(HL)₂(H₂O)₂] or [Cu(C₄H₄N₃O₂)₂(H₂O)₂], is a mononuclear octahedral Cu^II complex based on 5-methyl-1H-1,2,4-triazole-3-carboxylic acid (H₂ L). [Cu(HL)₂(H₂O)₂] was synthesized by reaction of H₂ L with copper(II) nitrate hexahydrate (2:1 stoichiometric ratio) in water under ambient conditions to produce clear light-blue crystals. The central Cu atom exhibits an N₂O₄ coordination environment in an elongated octahedral geometry provided by two bidentate HL ⁻ anions in the equatorial plane and two water molecules in the axial positions. Hirshfeld surface analysis revealed that the most important contributions to the surface contacts are from H...O/O...H (33.1%), H...H (29.5%) and H...N/N...H (19.3%) interactions.

The title compound, bis[μ-3-ethyl-5-(pyridin-2-yl)-1H-1,2,4-triazol-1-ido]bis[acetato(dimethylformamide)copper(II)], [Cu₂(C₉H₉N₄)₂(C₂H₃O₂)₂(C₃H₇NO)₂] or [Cu₂(L ^Et)₂(OAc)₂(dmf)₂], is a triazolate complex, which contains two 3-(2-pyridyl)-5-ethyl-triazolates (L ^Et)⁻ in bidentate-bridged coordination modes. Both copper atoms are involved in the formation of a planar six-membered metallocycle Cu–[N—N]₂–Cu. The inversion center of the complex is located at the mid-point of the Cu...Cu vector. Each Cu^II atom has a distorted trigonal–bipyramidal environment formed by the three nitrogen atoms of the deprotonated bridging 3-(2-pyridyl)-5-ethyl-triazolate unit, oxygen atoms of the OAc⁻ group and dmf molecule. In the crystal, C—H...O hydrogen bonds link the molecules into chains running along the c-axis direction.