Spin crossover Fe^III complexes with a substituted Hqnal ligand: effects of anions and solvents

A new substituted Hqnal ligand, Hqnal-5-Br_q, and four resulting Fe^III complexes [Fe(qnal-5-Br_q)₂]A·sol (A = NO₃^-, sol = CH₃OH 1; A = ClO₄^-, sol = CH₃OH 2; A = OTf^-, sol = 2CH₃OH·H₂O 3; and A = NTf₂^-, sol = CH₂Cl₂4; Hqnal-5-Br_q = N-(5-bromo-8-quinolinyl)-2-hydroxynaphthaldimine), have been synthesized and characterized. All four complexes, despite having different anions, adopt similar 1D [Fe(qnal-5-Br_q)₂]⁺ cation chains linked by orthogonal π⋯π interactions. These chains are further connected to form 2D and 3D structures by other supramolecular interactions. Complexes 1-3 all exhibit abrupt spin crossover behaviors, with the transition temperatures being 230, 189, and 185 K, respectively, while complex 4 is in a high-spin state. The influence of solvents on spin crossover properties was assessed via magnetic measurements on the desolvated samples. Following desolvation, while complexes 1 and 2 show slight variations in their transition temperatures, complexes 3 and 4 undergo significant changes in their magnetic profiles. Desolvation in complex 3 leads to either a HS state or a very incomplete hysteretic transition with a low transition temperature for different sample batches. In contrast, the gradual desolvation in complex 4 leads to diminished solvent residues and progressive transition from a stable HS state to an SCO-active state.