Spin-state versatility in Fe II4L6 supramolecular cages with a pyridyl-hydrazone ligand scaffold modulated by solvents and counter anions

Abstract

Discrete spin crossover (SCO) tetranuclear cages are a unique class of materials that have potential use in next-generation molecular recognition and sensing. In this work, two new edge-bridged SCO Fe^II₄L₆ (L = 2,7-bis(((E)-pyridin-2-ylmethylene)amino)benzo[lmn] [3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) supramolecular cages with different counter anions: ClO₄⁻ (2) and CF₃SO₃⁻ (3) were constructed via subcomponent self-assembly to investigate both solvent and anion influences on their magnetic properties and compare them to cage 1 with a BF₄⁻ anion. Pyridyl-hydrazone bidentate ligand scaffolds were employed to replace the ‘classical’ imidazole/thiazolyl-imine coordination units to induce SCO behaviour in these cages. 2 and 3 were structurally characterized by single-crystal X-ray diffraction analysis and electrospray ionization time-of-flight mass spectrometry. Magnetic susceptibilities of 1–3 and 1–3·desolvated indicate that the solvents’ presence is in favor of the low-spin (LS) state. While different counter anions in 1–3·desolvated affect the spin-state configurations of the four Fe^II metal centers. According to the ⁵⁷Fe Mössbauer spectral analysis, the spin-state distributions in 1–3 at 80 K are [2 high-spin (HS)–2LS], [1HS–3LS] and [2HS–2LS], respectively and density functional theory calculations were employed to investigate the reasons. These findings provide insights to regulate the spin-state versatility of SCO Fe^II cage systems in the solid state.