Two-Step versus One-Step Spin Transitions in Iron(II) 1D Chain Compounds

Spin crossover modulation in a coordination polymer with the redox-active bis-pyridyltetrathiafulvalene (py2TTF) ligand

A one-dimensional Fe^II coordination polymer (CP) has been formed which includes the redox-active ligand bis-pyridyltetrathiafulvalene (py₂TTF) and a Schiff base-like N₂O₂ ligand. This CP is both spin crossover (SCO) and redox-active in the solid-state, and chemical oxidation results in SCO modification.

Iron(II) Spin Crossover Complexes with 4,4'-Dipyridylethyne-Crystal Structures and Spin Crossover with Hysteresis

Three new iron(II) 1D coordination polymers with cooperative spin crossover behavior showing thermal hysteresis loops were synthesized using N₂O₂ Schiff base-like equatorial ligands and 4,4′-dipyridylethyne as a bridging, rigid axial linker. One of those iron(II) 1D coordination polymers showed a 73 K wide hysteresis below room temperature, which, upon solvent loss, decreased to a still remarkable 30 K wide hysteresis. Single crystal X-ray structures of two iron(II) coordination polymers and T-dependent powder XRD patterns are discussed to obtain insight into the structure property relationship of those materials.

Spin-crossover in iron(ii)-Schiff base complexes

A collective overview of iron(ii)-Schiff base complexes, showing abrupt and hysteretic SCO suitable for device applications, and the structure–property relationships governing the SCO of the complexes in the solid-state is presented.

Heteroleptic Fe(II) Complexes with N4S2 Coordination as a Platform for Designing Spin-Crossover Materials

Heteroleptic complexes [Fe(bpte)(bim)]X₂ and [Fe(bpte)(xbim)]X₂ (bpte = S,S'-bis(2-pyridylmethyl)-1,2-thioethane, bim = 2,2'-biimidazole, xbim = 1,1'-(α,α'-o-xylyl)-2,2'-biimidazole, X = ClO₄^-, BF₄^-, OTf^-) were prepared by reacting the corresponding Fe(II) salts with a 1:1 mixture of the ligands. All mononuclear complexes exhibit temperature-induced spin crossover (SCO) with the onset above room temperature. The SCO is rather gradual, due to low cooperativity of interactions between the cationic complexes, as revealed by crystal structure analyses. These complexes expand the range of the recently discovered Fe(II) SCO materials with {N₄S₂} coordination environment.

Synthesis of [Fe(Leq)(Lax)] n coordination polymer nanoparticles using blockcopolymer micelles

Spin-crossover compounds are a class of materials that can change their spin state from high spin (HS) to low spin (LS) by external stimuli such as light, pressure or temperature. Applications demand compounds with defined properties concerning the size and switchability that are maintained when the compound is integrated into composite materials. Here, we report the synthesis of [Fe(L_eq)(L_ax)]_n coordination polymer (CP) nanoparticles using self-assembled polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) micelles as template. Variation of the solvent (THF and toluene) and the rigidity of the axial ligand L_ax (L_ax = 1,2-di(pyridin-4-yl)ethane) (bpea), trans-1,2-di(pyridin-4-yl)ethene (bpee), and 1,2-di(pyridin-4-yl)ethyne) (bpey); L_eq = 1,2-phenylenebis(iminomethylidyne)-bis(2,4-pentanedionato)(2-)) allowed the determination of the preconditions for the selective formation of nanoparticles. A low solubility of the CP in the used solvent and a high stability of the Fe-L bond with regard to ligand exchange are necessary for the formation of composite nanoparticles where the BCP micelle is filled with the CP, as in the case of the [FeL_eq(bpey)] _n @BCP. Otherwise, in the case of more flexible ligands or ligands that lead to high spin complexes, the formation of microcrystals next to the CP-BCP nanoparticles is observed above a certain concentration of [Fe(L_eq)(L_ax)] _n . The core of the nanoparticles is about 45 nm in diameter due to the templating effect of the BCP micelle, independent of the used iron complex and [Fe(L_eq)(L_ax)] _n concentration. The spin-crossover properties of the composite material are similar to those of the bulk for FeL_eq(bpea)] _n @BCP while pronounced differences are observed in the case of [FeL_eq(bpey)] _n @BCP nanoparticles.

Synthesis of [Fe(L)(bipy)]n spin crossover nanoparticles using blockcopolymer micelles

Nowadays there is a high demand for specialized functional materials for specific applications in sensors or biomedicine (e.g. fMRI). For their implementation in devices, nanostructuring and integration in a composite matrix are indispensable. Spin crossover complexes are a highly promising family of switchable materials where the switching process can be triggered by various external stimuli. In this work, the synthesis of nanoparticles of the spin crossover iron(ii) coordination polymer [Fe(L)(bipy)]_n (with L = 1,2-phenylenebis(iminomethylidyne)bis(2,4-pentanedionato)(2-) and bipy = 4,4'-bipyridine) is described using polystyrene-poly-4-vinylprididine blockcopolymer micelles as the template defining the final size of the nanoparticle core. A control of the spin crossover properties can be achieved by precise tuning of the crystallinity of the coordination polymer via successive addition of the starting material Fe(L) and bipy. By this we were able to synthesize nanoparticles with a core size of 49 nm and a thermal hysteresis loop width of 8 K. This is, to the best of our knowledge, a completely new approach for the synthesis of nanoparticles of coordination polymers and should be easily transferable to other coordination polymers and networks. Furthermore, the use of blockcopolymers allows a further functionalization of the obtained nanoparticles by variation of the polymer blocks and an easy deposition of the composite material on surfaces via spin coating.

Dynamic spin interchange in a tridentate Fe(iii) Schiff-base compound

The thermosalient effect is still a rare and poorly understood phenomenon, where crystals suddenly jump, bend, twist or explode upon undergoing a thermally activated phase transition. The synthesis and characterisation of the new spin transition Fe(iii) compound [Fe(5-Br-salEen)2][ClO4] (salEen = N-ethyl-N-(2-aminoethyl)salicylaldiminate) is described and its thermosalient behaviour reported. It is the first example of a thermosalient effect with a spin transition and magnetic, calorimetric, diffraction, microscopy and computational studies are used to characterise these effects. Both thermosalient effect and spin transition occur around 320 K upon heating and are accompanied by an anisotropic unit cell change with conservation of crystal symmetry that causes a large enough stress of the crystal lattice to induce crystal explosion. This stress can ultimately be traced back to a diffusionless and distortive structural perturbation resulting in a coupled spin transition-thermosalient effect.

Iron(ii) spin crossover complexes with diaminonaphthalene-based Schiff base-like ligands: 1D coordination polymers

Iron(ii) spin crossover coordination polymers with diaminonaphthalene derived Schiff base-like ligands were synthesised, and their magnetic and structural properties were analysed. Rigid bridging ligands having up to 40 K wide hysteresis loops were observed.

Influence of conformational changes on spin crossover properties and superstructure formation in 2D coordination polymers [Fe(hbtz)2(RCN)2](ClO4)2

Spin-crossover, governed by conformational changes of nitrile molecule, triggers corrugation of polymeric skeleton and formation of superstructure in [Fe(hbtz)₂(allyl cyanide)₂](ClO₄)₂.

Spin state switching in iron coordination compounds

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligo- and polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.