Mononuclear ferrous and ferric complexes

Room temperature spin crossover properties in a series of mixed-anion Fe(NH2trz)3(BF4)2-x(SiF6)x/2 complexes

A series of mixed-anion Fe(NH2trz)3(BF4)2-x(SiF6)x/2 spin crossover complexes is obtained modifying the reaction time but also using an increase amount of tetraethyl orthosilicate as the source for the production and the incorporation of SiF62- competing with the BF42- anions present in the mother solution. The increase of the SiF62- anion inclusion to the detriment of the BF4- counterpart induces a shift of the temperature transition toward high temperatures leading to interesting bistability properties around room temperature with T1/2 spanning from 300 K to 325 K. Moreover, the implementation of a solid-liquid post synthetic modification approach from the Fe(NH2trz)3(BF4)2 parent complex with identical TEOS proportions and under certain experimental conditions lead systematically to the same Fe(NH2trz)3(BF4)1.2(SiF6)0.4 composition. This compound presents an abrupt spin crossover behaviour with a narrow hysteresis loop just above room temperature (320 K), which is stable under thermal cycling and along time with no specific storage conditions. Such crystalline powder sample incorporates homogeneous rod-shaped particles whose formation and physical properties can be followed simultaneously using infra-red spectroscopy, dynamic light scattering (DLS), transmission electronic microscopy (TEM) and optical reflectance. The observation of a stabilized single ca. 800 nm population of mixed-anion particles starting from insoluble various sizes (from nano- to microscale) Fe(NH2trz)3(BF4)2 particles supports the key role of the solvent (water molecules) on the separation, the reactivity and the reorganization of the 1D iron-triazole chains forming the packing of the structure.

Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}- are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]- units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]- and mononuclear [Ag(CN)2]- units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.

Spin crossover in mixed-anion Fe(NH2trz)3(BF4)(SiF6)0.5 crystalline rod-shaped particles: the strength of the solid-liquid post synthetic modification

A pure mixed-anion Fe(NH₂trz)₃(BF₄)(SiF₆)_0.5 spin crossover complex is obtained implementing a solid-liquid post synthetic modification approach from the Fe(NH₂trz)₃(BF₄)₂ parent complex. This method allows obtaining highly crystalline powder samples incorporating homogeneous micrometric (1 μm long) rod-shaped particles. This compound presents an abrupt spin crossover behaviour with a narrow (10 K) hysteresis loop centred just above room temperature (320 K) which makes it very interesting for future integration into devices for various applications.

Triflate vs Acetonitrile: Understanding the Iron(II)-Based Coordination Chemistry of Tri(quinolin-8-yl)amine

In this study, a synthesis route of tri(quinolin-8-yl)amine (L), a recent member of the tetradentate tris(2-pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ⁴ mode, two cis-oriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations─bis(triflato), bis(acetonitrile), and mixed coligand species─could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mössbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time- and temperature-resolved UV/vis spectroscopy, Mössbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between high- and low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)]²⁺ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment─all of the {N₆}-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.

Anion-driven supramolecular modulation of spin-crossover properties in mononuclear iron(III) Schiff-base complexes

A series of Fe(III) complexes [Fe(5-F-sal-N-1,4,7,10)]Y (Y = PF₆^- for 1, Y = ClO₄^- for 2, Y = I^- for 3 and Y = NO₃^- for 4) have been prepared. Single-crystal X-ray crystallographic studies show that complex 1 crystallizes in the orthorhombic Pna2₁ space group and complexes 2-4 have an isomorphous structure and crystallize in the same monoclinic space group, P2₁/n. Complexes 2-4 have two independent molecules (Fe1 and Fe2) in the unit cell. Magnetic susceptibility measurements demonstrated that complexes 1 and 3 showed a gradual one-step SCO behavior (T_1/2 for 1 = 177 K and for 3 = 227 K) without thermal hysteresis. The magnetic behavior of 2 shows an incomplete two-step SCO process at T_1/2 = 114 K and 170 K, respectively, while 4 is in a high-spin state at all measured temperatures. A careful evaluation of the supramolecular structures of these complexes revealed correlation between the supramolecular packing forces and their SCO behavior. The crystal structure of 1 consists of a three-dimensional (3D) extended network constructed from N-H⋯F and C-H⋯F hydrogen bonds, and C-H⋯π and C⋯C short contacts. In compounds 2-4, the crystal packing is governed by C⋯C, C-H⋯π and p-π interactions for the Fe1 centers and by C-H⋯π/O interactions for the Fe2 centers, which form 1D chains. Additional interactions (C-H⋯F and N-H⋯O/I) connect the neighboring chains and planes to form a complex supramolecular network. The anion⋯π interactions in 4 provide a means for preventing SCO occurring at low temperatures. This suggests that the supramolecular connectivity of the anions influences the magnetic properties.

New Spin-Crossover Compounds Containing the [Ni(mnt)] Anion (mnt = Maleonitriledithiolate)

Two novel salts containing the anion [Ni(mnt)2]− (mnt = maleonitriledithiolate) have been synthesized. The counter-ions, [Fe(II)(L1 or L2)2], are cationic complexes where L1 and L2 are methylated derivatives of 2,6-bis(pyazolyl)pyridine or pyrazine, which are similar to ligands found in a series of spin-crossover (SCO) complexes. Both salts are characterized by variable temperature single crystal X-ray diffraction and bulk magnetization measurements. Compound 1, [Fe(II)(L1)2][Ni(mnt)2]2 displays an incomplete and gradual SCO up to 300 K, followed by a more rapid increase in the high-spin fraction between 300 and 350 K. Compound 2, [Fe(II)(L2)2][Ni(mnt)2]2.MeNO2, shows a gradual, but more complete SCO response centered at 250 K. For compound 2, the SCO is confirmed by variable temperature Mössbauer spectroscopy. In both cases, the anionic moieties are isolated from each other and so no electrical conductivity is observed.

A Fluorescence-Detected Coordination-Induced Spin State Switch

The response of the spin state to in situ variation of the coordination number (CISSS) is a promising and viable approach to smart sensor materials, yet it suffers to date from insensitive detection. Herein, we present the synthetic access to a family of planar nickel(II) complexes, whose CISSS is sensitively followed by means of fluorescence detection. For this purpose, nickel(II) complexes with four phenazine-based Schiff base-like ligands were synthesized and characterized through solution-phase spectroscopy (NMR and UV-vis), solid-state structure analysis (single-crystal XRD), and extended theoretical modeling. All of them reveal CISSS in solution through axial ligating a range of N- and O-donors. CISSS correlates nicely with the basicity of the axial ligand and the substitution-dependent acidity of the nickel(II) coordination site. Remarkably, three out of the four nickel(II) complexes are fluorescent in noncoordinating solvents but are fluorescence-silent in the presence of axial ligands such as pyridine. As these complexes are rare examples of fluorescent nickel(II) complexes, the photophysical properties with a coordination number of 4 were studied in detail, including temperature-dependent lifetime and quantum yield determinations. Most importantly, fluorescence quenching upon adding axial ligands allows a "black or white", i.e. digital, sensoring of spin state alternation. Our studies of fluorescence-detected CISSS (FD-CISSS) revealed that absorption-based CISSS and FD-CISSS are super proportional with respect to the pyridine concentration: FD-CISSS features a higher sensitivity. Overall, our findings indicate a favored ligation of these nickel(II) complexes in the excited state in comparison to the ground state.

Interplay of Spin Crossover and Coordination-Induced Spin State Switch for Iron Bis(pyrazolyl)methanes in Solution

Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Mößbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N₅O} to {N₆} and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Mößbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch.

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.

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.

Spin Crossover in 3D Metal Centers Binding Halide-Containing Ligands: Magnetism, Structure and Computational Studies

The capability of a given substance to change its spin state by the action of a stimulus, such as a change in temperature, is by itself a very challenging property. Its interest is increased by the potential applications and the need to find sustainable functional materials. 3D transition metal complexes, mainly with octahedral geometry, display this property when coordinated to particular sets of ligands. The prediction of this behavior has been attempted by many authors. It is, however, made very difficult because spin crossover (SCO), as it is called, occurs most often in the solid state, where besides complexes, counter ions, and solvents are also present in many cases. Intermolecular interactions definitely play a major role in SCO. In this review, we decided to analyze SCO in mono- and binuclear transition metal complexes containing halogens as ligands or as substituents of the ligands. The aim was to try and find trends in the properties which might be correlated to halogen substitution patterns. Besides a revision of the properties, we analyzed structures and other information. We also tried to build a simple model to run Density Functional Theory (DFT) calculations and calculate several parameters hoping to find correlations between calculated indices and SCO data. Although there are many experimental studies and single-crystal X-ray diffraction structures, there are only few examples with the F, Cl, Br and series. When their intermolecular interactions were not very different, T1/2 (temperature with 50% high spin and 50% low spin states) usually increased with the calculated ligand field parameter (Δoct) within a given family. A way to predict SCO remains elusive.

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

Earth-abundant first row transition metal complexes are important for the development of large-scale photocatalytic and solar energy conversion applications. Coordination compounds based on iron are especially interesting, as iron is the most common transition metal element in the Earth’s crust. Unfortunately, iron-polypyridyl and related traditional iron-based complexes generally suffer from poor excited state properties, including short excited-state lifetimes, that make them unsuitable for most light-driven applications. Iron carbene complexes have emerged in the last decade as a new class of coordination compounds with significantly improved photophysical and photochemical properties, that make them attractive candidates for a range of light-driven applications. Specific aspects of the photophysics and photochemistry of these iron carbenes discussed here include long-lived excited state lifetimes of charge transfer excited states, capabilities to act as photosensitizers in solar energy conversion applications like dye-sensitized solar cells, as well as recent demonstrations of promising progress towards driving photoredox and photocatalytic processes. Complementary advances towards photofunctional systems with both Fe(II) complexes featuring metal-to-ligand charge transfer excited states, and Fe(III) complexes displaying ligand-to-metal charge transfer excited states are discussed. Finally, we outline emerging opportunities to utilize the improved photochemical properties of iron carbenes and related complexes for photovoltaic, photoelectrochemical and photocatalytic applications.

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) complexes with N2O2 coordinating Schiff base-like equatorial ligand and 1,2-bis(pyridin-2-ylethynyl)benzene as axial pincer ligand

Three new unique mononuclear iron(II) pincer complexes were synthesized using 1,2-bis(pyridin-2-ylethynyl)benzene as axially coordinating pincer ligand and N₂O₂ coordinating Schiff base-like equatorial ligands. Magnetic susceptibility measurements reveal that all three complexes remain in the high spin state throughout the entire temperature range investigated. Reasons for this are restraining sterical interactions revealed in the single crystal x-ray structure analysis and extended DFT-computational studies of one of the pincer complexes. Those interactions also lead to the formation of unexpected side products during the synthesis such as a complex with two ethanol molecules as axial ligand, whose x-ray structure was determined.

Solvent Effects on the Spin Crossover Properties of Iron(II) Imidazolylimine Complexes

A series of Fe(II) complexes, fac-[Fe(4-ima-Bp)3](Y)2⋅sol (Y = ClO4; sol = 3EtOH 1, 3MeOH 2; Y= BF4; sol = EtOH⋅4H2O 3, 4H2O 4 and 3.5MeCN 5) have been prepared and structurally and magnetically characterized. The low temperature structures of 1, 2 and 5 have been determined by X-ray crystallography with LS Fe(II) centres found in all cases. Extensive C–H···π interactions between the cations form 2D layers, which are linked to one another through N-H···O and O-H···O hydrogen bonds, resulting in high cooperativity. Despite 5 containing MeCN, N-H···O/F hydrogen bonds, and C–H···π and π-π interactions combine to give similar 2D layers. Magnetic measurements reveal moderately abrupt spin crossover for 1-4; becoming more gradual and only 50% complete in 1 due to solvent loss. The MeCN solvate shows more gradual SCO and reinforces how subtle changes in packing can significantly influence SCO behaviour.

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.

Abrupt spin crossover in iron(iii) complexes with aromatic anions

Two iron(iii) complexes, [Fe(qsal-X)₂]OTs·nH₂O, are found to exhibit abrupt spin crossover with the spin transition temperature substituent dependent, and X⋯O halogen bonds linking the spin centres.

Solvatomorphism and anion effects in predominantly low spin iron(iii) Schiff base complexes

The magnetic properties of a series of iron(iii) complexes [Fe(naphEen)₂]halide·sol are reported.