Spin Crossover in Iron(II) Coordination Frameworks: Structure and Host-Guest Chemistry

Abstract

Coordination framework synthesis offers a versatile path towards the construction of novel multifunctional materials. The incorporation of metal centres capable of spin-state switching into robust framework hosts allows for the integration of the two properties of spin crossover and nanoporosity. By combining these two functionalities, the genesis of materials that are able to respond in some physical manner (electronic, structural and optical) to their environment can be envisaged, with applications directed towards displays, information storage and sensing devices. To this aim, a range of iron(II) coordination framework materials have been synthesised based on three general structure types: bimetallic Hofmann-type and Hofmann-related systems of the form [FeII(L)M(CN)4]·xGuest (1-5, 12-14; L = bis-unidentate ligand; M = Ni, Pd, Pt) or [FeII(L)(M(CN)4)2]·xGuest (6-8; M = Ag, Au), SCOF-type systems of the form [FeII(L)2(XCN)2]·xGuest (9-11; X = S, Se) and porphyrinic frameworks (15-20). All were structurally characterised by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. A series of bis-unidentate ligands based on an amide/urea functional group motif were incorporated into the materials 1-11. Systematic alteration of the ligand length in the family of Hofmann-type phases 1-5 revealed strong structure-property correlations. The interpenetrated phases 6-8 were each found to undergo thermally-induced spin transitions, with the SCOF-type phases 9-11 remaining in high-spin to 100 K. Guest-dependent spin crossover behaviour was explored in the Hofmann-type phases 12-14 (M = Ni, Pd, Pt), with one material exhibiting unusual temperature-dependent behaviour with two separate regions of bistability. Finally, a series of frameworks incorporating a tetratopic porphyrinic ligand in combination with FeII and the counterions XCN- (X = S, Se, BH3), N(CN)2 and Au(CN)2- were structurally characterised, and comprise the materials 15-20. “The files in Appendix A are in .cif format (Crystallographic Information File) and can be opened with the software Mercury (see website). Mercury can be run on Windows, Linux or Mac OS X.”