Iron(II) Spin Crossover Complexes: a multifaceted approach to rational design, tailor cooperativity, enhance complexity and advanced multifunctionality

Molecular bistability is a key-ingredient for the development of novel molecular materials tailored for technological applicability. One of the most illustrious examples of bistability in coordination chemistry is the spin-crossover (SCO) effect, featuring for 3d4-3d7 metal ions a reversible transition between different electronic states triggered by external stimuli as temperature, pressure, light, etc. The change of the metal centre’s spin-state affects key-properties of the material including magnetic moment, colour, dielectric constant, lattice extension. These attributes make SCO compounds highly desirable for applications in sensors, data storage, and smart materials. Of the utmost importance for any possible application is an abrupt and preferably hysteretic spin transition at or above room temperature, for which a high degree of cooperativity between molecules is required.Modern SCO research focuses mainly on two aspects: the first is a deeper understanding of the SCO phenomenon through rational design of SCO materials to achieve precise control over the SCO phenomenon and exploit the underlying magnetic behavior, but in the last three decades a new trend emerged, namely the synthesis of multifunctional SCO materials bearing an additional property which synergistically depends on the spin transition. In this work, both abovementioned aspects were tackled.In the context of rational design, a decoupling of electronic and steric effects was attempted, by preparing [Fe(PrIm)6]2+ (PrIm = 1-propyl-1H-imidazole) and comparing it with [Fe(3tz)6]2+ (3tz=1-propyl-1H-tetrazole), one of the first and probably the most studied and comprehensively characterized SCO-compound. The study also shed light on the versatility of PrIm as ligand for Fe(II) complexes. Multifunctionality was first approached by synthesizing and characterizing a series of ω-(1H-tetrazol-1-yl) carboxylic acids bifunctional ligands and their Fe(II) complexes aiming to obtain systems showing a strong, extended H-bond network for enhanced cooperativity, ergo improve the quality of the spin transition. In a second step, the bidentate nature of the ligands was exploited for the synthesis of mixed-metallic 3d-4f coordination polymers.Lastly, the coupling of SCO with host-guest chemistry for chemo-sensing application was investigated, by taking a new approach to produce extended Hofmann-type SCO-PCPs. Applying a novel synthetic procedure, an enlargement of the classic structure [Fe(pz)][M(CN)4] (M=Ni, Pd, Pt) based on cyanometallate linkers was performed, using tetrakis-cyanoacetylides linkers to obtain PCPs having the general formula [Fe(pz)][M(C3N)4]. They feature five-fold larger cavities and a drastic increase of the porosity. Most importantly, all the desired properties (hysteretic SCO, guest-dependent spin transition, bidirectional chemoswitching and memory effect) were retained.