Synthesis and characterization of 1D iron(ii) spin crossover coordination polymers with hysteresis

FeII spin crossover complexes containing N4O2 donor ligands

Spin crossover (SCO) is one of the most studied magnetic bistable phenomena because of its application in the field of multifunctional magnetic materials. Fe^II complexes in a N₆ coordination environment have been the most well-studied in terms of their SCO behaviour. Other coordination environments, notably the N₄O₂ coordination environment, has also been quite effective in inducing SCO behaviour in the corresponding Fe^II complexes. This review deals with such systems. The three ligand families that are discussed are: Jager type ligands, hydrazone based ligands and tridentate ligands having salicylaldehyde derivatives. These ligands allow the assembly of both mononuclear and multinuclear complexes that exhibit cooperative spin transitions. Also, Fe^II complexes obtained from some of these ligands are multifunctional and exhibit a coupling of optical and magnetic properties. Most of the Fe^II complexes obtained from these families of ligands are charge neutral which allows easy surface deposition. Further, modulation of these ligand families allows a fine tuning of the ligand field strength which results in varying SCO behavior. In addition some of the Fe^II complexes derived from these ligands exhibit a light induced excited spin state trapping (LIESST) effect. All of the above aspects are reviewed in this review.

Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [Fe^II(L^npdtz)₂(NCX)₂] (X = S; Se), with L^npdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Bifunctional Fe(II) spin crossover-complexes based on ω-(1H-tetrazol-1-yl) carboxylic acids

To increase the supramolecular cooperativity in Fe(ii) spin crossover materials based on N1-substituted tetrazoles, a series of ω-(1H-tetrazol-1-yl) carboxylic acids with chain-lengths of C2-C4 were synthesized. Structural characterization confirmed the formation of a strong hydrogen-bond network, responsible for enhanced cooperativity in the materials and thus largely complete spin-state transitions for the ligands with chain lenghts of C2 and C4. To complement the structural and magnetic investigation, electronic spectroscopy was used to investigate the spin-state transition. An initial attempt to utilize the bifunctional coordination ability of the ω-(1H-tetrazol-1-yl) carboxylic acids for preparation of mixed-metallic 3d-4f coordination polymers resulted in a novel one-dimensional gadolinium-oxo chain system with the ω-(1H-tetrazol-1-yl) carboxylic acid acting as μ2-η2:η1 chelating-bridging ligand.

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.

First Cobalt(II) Spin Crossover Compound with N4S2-Donorset

Herein we report the synthesis and characterization of a novel bis-tridentate 1,3,4-thiadiazole ligand (L = 2,5-bis[(2-pyridylmethyl)thio]methyl-1,3,4-thiadiazole). Two new mononuclear complexes of the type [M^II(L)₂](ClO₄)₂ (with M = Fe^II (C1) and Co^II (C2)) have been synthesized, containing the new ligand (L). In both complexes the metal centers are coordinated by an N₄S₂-donorset and each of the two ligands is donating to the metal ion with just one of the tridentate pockets. The iron(II) complex (C1) is in the low spin [LS] state below room temperature and shows an increase in the magnetic moment only above 300 K. In contrast, the cobalt(II) complex (C2) shows a gradual spin crossover (SCO) with T_1/2 = 175 K. To our knowledge, this is the first cobalt(II) SCO complex with an N₄S₂-coordination.

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.

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.

A mononuclear iron(ii) complex: cooperativity, kinetics and activation energy of the solvent-dependent spin transition

A mononuclear iron(ii) pyrimidine-based complex shows remarkable spin crossover properties and unprecedented thermal robustness.

Iron(ii) spin crossover complexes with diaminonaphthalene-based Schiff base-like ligands: mononuclear complexes

Iron(ii) spin crossover (SCO) complexes with diaminonaphthalene derived Schiff base-like ligand were synthesised, and their magnetic and structural properties analysed.