Spin crossover FeIII complexes with a substituted Hqnal ligand: effects of anions and solvents†

Abstract

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.