Synthesis, Magnetic Properties and X-ray Structure Analysis of a 1-D Chain Iron(II) Spin Crossover Complex with wide 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.

Realizing shape and size control for the synthesis of coordination polymer nanoparticles templated by diblock copolymer micelles

The combination of polymers with nanoparticles offers the possibility to obtain customizable composite materials with additional properties such as sensing or bistability provided by a switchable spin crossover (SCO) core. For all applications, a precise control over size and shape of the nanomaterial is highly important as it will significantly influence its final properties. By confined synthesis of iron(II) SCO coordination polymers within the P4VP cores of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) micelles in THF we are able to control the size and also the shape of the resulting SCO nanocomposite particles by the composition of the PS-b-P4VP diblock copolymers (dBCPs) and the amount of complex employed. For the nanocomposite samples with the highest P4VP content, a morphological transition from spherical nanoparticles to worm-like structures was observed with increasing coordination polymer content, which can be explained with the impact of complex coordination on the self-assembly of the dBCP. Furthermore, the SCO nanocomposites showed transition temperatures of T_1/2 = 217 K, up to 27 K wide hysteresis loops and a decrease of the residual high-spin fraction down to γ_HS = 14% in the worm-like structures, as determined by magnetic susceptibility measurements and Mössbauer spectroscopy. Thus, SCO properties close or even better (hysteresis) to those of the bulk material can be obtained and furthermore tuned through size and shape control realized by tailoring the block length ratio of the PS-b-P4VP dBCPs.

Evidence for surface effects on the intermolecular interactions in Fe(II) spin crossover coordination polymers

From X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS), it is evident that the spin state transition behavior of Fe(II) spin crossover coordination polymer crystallites at the surface differs from the bulk. A comparison of four different coordination polymers reveals that the observed surface properties may differ from bulk for a variety of reasons. There are Fe(II) spin crossover coordination polymers with either almost complete switching of the spin state at the surface or no switching at all. Oxidation, differences in surface packing, and changes in coordination could all contribute to making the surface very different from the bulk. Some Fe(II) spin crossover coordination polymers may be sufficiently photoactive so that X-ray spectroscopies cannot discern the spin state transition.

Confined Crystallization of Spin-Crossover Nanoparticles in Block-Copolymer Micelles

Nanoparticles of the spin-crossover coordination polymer [FeL(bipy)]n were synthesized by confined crystallization within the core of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer micelles. The 4VP units in the micellar core act as coordination sites for the Fe complex. In the bulk material, the spin-crossover nanoparticles in the core are well isolated from each other allowing thermal treatment without disintegration of their structure. During annealing above the glass transition temperature of the PS block, the transition temperature is shifted gradually to higher temperatures from the as-synthesized product (T1/2 ↓=163 K and T1/2 ↑=170 K) to the annealed product (T1/2 ↓=203 K and T1/2 ↑=217 K) along with an increase in hysteresis width from 6 K to 14 K. Thus, the spin-crossover properties can be shifted towards the properties of the related bulk material. The stability of the nanocomposite allows further processing, such as electrospinning from solution.

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.

Crystal structure and Hirshfeld surface analysis of two (E)-N'-(para-substituted benzyl-idene) 4-chloro-benzene-sulfono-hydrazides

Two (E)-N'-(p-substituted benzyl-idene)-4-chloro-benzene-sulfono-hydrazides, namely, (E)-4-chloro-N'-(4-chloro-benzyl-idene)benzene-sulfono-hydrazide, C13H10Cl2N2O2S, (I), and (E)-4-chloro-N'-(4-nitro-benzyl-idene)benzene-sulfono-hydrazide, C13H10ClN3O4S, (II), have been synthesized, characterized and their crystal structures studied to explore the effect of the nature of substituents on the structural parameters. Compound (II) crystallized with two independent mol-ecules [(IIA) and IIB)] in the asymmetric unit. In both compounds, the configuration around the C=N bond is E. The mol-ecules are twisted at the S atom with C-S-N-N torsion angles of -62.4 (2)° in (I), and -46.8 (2)° and 56.8 (2)° in the mol-ecules A and B of (II). The 4-chloro-phenyl-sulfonyl and 4-substituted benzyl-idene rings form dihedral angles of 81.0 (1)° in (I), 75.9 (1)° in (IIA) and 73.4 (1)° in (IIB). In the crystal of (I), mol-ecules are linked via pairs of N-H⋯O hydrogen bonds, forming inversion dimers with an R 2 2(8) ring motif. The dimers are linked by C-Cl⋯π inter-actions, forming a three-dimensional structure. In the crystal of (II), mol-ecules are linked by C-H⋯π inter-actions and N-H⋯O hydrogen bonds, forming -A-B-A-B- chains along the c-axis direction. The chains are linked via C-H⋯O and C-H⋯π inter-actions, forming layers parallel to the bc plane. Two-dimensional fingerprint plots show that the most significant contacts contributing to the Hirshfeld surface for (I) are H⋯H contacts (26.6%), followed by Cl⋯H/H⋯Cl (21.3%), O⋯H/H⋯O (15.5%) and Cl⋯C/C⋯Cl (10.7%), while for (II) the O⋯H/H⋯O contacts are dominant, with a contribution of 34.8%, followed by H⋯H (15.2%), C⋯H/H⋯C (14.0%) and Cl⋯H/H⋯Cl (10.0%) contacts.

Spin-Crossover Iron(II) Coordination Polymer with Fluorescent Properties: Correlation between Emission Properties and Spin State

A spin-crossover coordination polymer [Fe(L1)(bipy)]_n (where L = a N₂O₂^2- coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4'-bipyridine) was synthesized and exhibits a 48 K wide thermal hysteresis above room temperature (T_1/2↑ = 371 K and T_1/2↓ = 323 K) that is stable for several cycles. The spin transition was characterized using magnetic measurements, Mössbauer spectroscopy, and DSC measurements. T-dependent X-ray powder diffraction reveals a structural phase transition coupled with the spin transition phenomenon. The dimeric excerpt {(μ-bipy)[FeL1(MeOH)]₂}·2MeOH of the coordination polymer chain crystallizes in the triclinic space group P1̅ and reveals that the packing of the molecules in the crystal is dominated by hydrogen bonds. Investigation of the emission properties of the complexes with regard to temperature shows that the spin crossover can be tracked by monitoring the emission spectra, since the emission color changes from greenish to a yellow color upon the low spin-to-high spin transition.

Synthesis of Coordination Polymer Nanoparticles using Self-Assembled Block Copolymers as Template

Nowadays there is a high demand in specialized functional materials, for example, for applications as sensors in biomedicine. For the realization of such applications, nanostructures and the integration in a composite matrix are indispensable. Coordination polymers and networks, for example, with spin crossover properties, are a highly promising family of switchable materials in which the switching process can be triggered by various external stimuli. An overview over different strategies for the synthesis of nanoparticles of such systems is given. A special focus is set on the use of block copolymer micelles as templates for the synthesis of nanocomposites. The block copolymer defines the final size and shape of the nanoparticle core. Additionally it allows a further functionalization of the obtained nanoparticles by variation of the polymer blocks and an easy deposition of the composite material on surfaces.

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.

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 new polymorph of 1-({[1,3-dihy-droxy-2-(hy-droxy-meth-yl)propan-2-yl]iminio}meth-yl)naphthalen-2-olate

The title compound, C₁₅H₁₇NO₄, containing two mol­ecules in the asymmetric unit is a polymorph of the crystal structure published by Martínez et al. [(2011). Eur. J. Org. Chem. pp. 3137-3145] which at 120 K is monoclinic with one mol­ecule in the asymmetric unit. Both mol­ecules in the title compound are in the trans form. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds connect mol­ecules, forming a two-dimensional network parallel to (001).

Crystal structure of (Z)-4-[1-(4-acetyl-anilino)ethyl-idene]-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one

In the solid state, the title compound, C20H19N3O2, adopts the keto-amine tautomeric form, with the H atom attached to the N atom, which participates in an intra-molecular N-H⋯O hydrogen bond with an S(6) ring motif. The dihedral angles between the pyrazole ring and the phenyl and benzene rings are 3.69 (10) and 46.47 (9)°, respectively. In the crystal, mol-ecules are linked by weak C-H⋯O hydrogen bonds, generating C(16) chains propagating in [301]. Weak aromatic π-π stacking inter-actions [centroid-centroid distances = 3.6123 (10) and 3.6665 (10) Å] link the chains into a three-dimensional network.

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.

(E)-3-(2-Hy-droxy-5-methyl-phenyl-imino)-indolin-2-one

In the title compound, C₁₅H₁₂N₂O₂, the dihedral angle between the two benzene rings is 83.55 (11)° In the crystal, the molecules are linked by O—H⋯O and N—H⋯O hydrogen bonds.

2-{[4-(Diethylamino)phenyl]iminomethyl}-4,6-diiodophenol

In the title compound, C₁₇H₁₈I₂N₂O, the dihedral angle between the aromatic rings is 5.4 (1)°. An intra­molecular O—H⋯N hydrogen bond generates an S(6) ring motif. The crystal packing is stabilized by C—H⋯π and π–π inter­actions [centroid–centroid distance = 3.697 (1) Å].

N-(4-Chlorobenzylidene)-1-naphthylamine

The title compound, C₁₇H₁₂ClN, represents a trans isomer with respect to the C=N bond; the dihedral angle between the planes of the naphthyl and benzene groups is 66.53 (5)°.

2-Bromo-4-chloro-6-{(E)-[4-(diethyl-amino)-phen-yl]imino-meth-yl}phenol

In the title compound, C(17)H(18)BrClN(2)O, the dihedral angle between the aromatic rings is 3.0 (1)°. The methyl-ethanamine group assumes an extended conformation. An intra-molecular O-H⋯N hydrogen bond generates an S(6) ring motif. The crystal packing is stabilized by C-H⋯π and π-π [centroid-centroid distances = 3.691 (1) and 3.632 (1) Å] inter-actions.

(E)-2-(4-Diethyl-amino-2-hydroxy-benzyl-idene-amino)benzonitrile

The mol­ecule of the title compound, C₁₈H₁₉N₃O, displays a trans configuration with respect to the C=N double bond. The dihedral angle between the planes of the two benzene rings is 2.62 (11)°. A strong intra­molecular O—H⋯N hydrogen bond stabilizes the mol­ecular conformation.

(E)-2-[(5-Bromo-2-hydroxy-benzyl-idene)amino]benzonitrile

In the mol­ecule of the title compound, C₁₄H₉BrN₂O, the dihedral angle between the aromatic rings is 1.09 (4)°. Intra­molecular O—H⋯N hydrogen bonding results in the formation of a planar (r.m.s. deviation = 0.0140 Å) six-membered ring. In the crystal structure, inter­molecular C—H⋯N inter­actions link the mol­ecules into chains.

(E)-3-(2-Hydroxy-4-methoxybenzylideneamino)benzonitrile

In the mol­ecule of the title compound, C₁₅H₁₂N₂O₂, the aromatic rings are oriented at a dihedral angle of 28.11 (3)°. Intra­molecular O—H⋯N hydrogen bonding results in the formation of a planar six-membered ring, which is nearly coplanar with the adjacent ring at a dihedral angle of 1.53 (3)°. In the crystal structure, π–π contacts between the benzene rings [centroid–centroid distance = 3.841 (1) Å] may stabilize the structure.

(Z)-2-[(2-Hydr-oxy-1-naphth-yl)methyl-eneamino]benzonitrile

The title compound, C₁₈H₁₂N₂O, crystallizes in a phenol–imine tautomeric form with a Z conformation for the imine functionality. The dihedral angle between the aromatic rings is 8.98 (9)°. A strong intra­molecular O—H⋯N hydrogen-bond inter­action between the hydroxyl group and imine N atom occurs.

(Z)-1-[(3-Cyano-phen-yl)iminiometh-yl]-2-naphtholate

The title compound, C₁₈H₁₂N₂O, crystallizes in a zwitterionic form. The dihedral angle between the planes of the benzene ring and naphthalene ring system is 13.95 (5)°. An intra­molecular N—H⋯O inter­action results in the formation of a planar six-membered ring, which is oriented at dihedral angles of 13.50 (4) and 4.49 (4)° with respect to the benzene ring and naphthalene ring system, respectively. In the crystal structure, inter­molecular C—H⋯O and C—H⋯N inter­actions link the mol­ecules into a two-dimensional network. π–π contacts between the naphthalene systems [centroid–centroid distance = 3.974 (1) Å] may further stabilize the structure.