Iron(ii) complexes with a terpyridine embrace packing motif show remarkably consistent cooperative spin-transitions

Activating a high-spin iron(II) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral-trigonal prismatic distortion pathway. In contrast, [Ni(bpp)2][ClO4]2 (monoclinic, P21) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp)2]2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ ≪ Ni2+ ≪ Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1-x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 250 ± 1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100% completeness in 1a and 2a; that is, the 26% Ni doping in 2a is enough to convert high-spin [Fe(bpp)2][ClO4]2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [FexNi1-x(bpp)2][BF4]2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P21 phase in this system is 0.7 ± 0.1.

Iron(II) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L1), treatment of α,α'-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L2) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L3). Solvated crystals of [Fe(L1)2][BF4]2 (1[BF4]2) and [Fe(L2)2][BF4]2 (2[BF4]2) are mostly high-spin, although one solvate of 1[BF4]2 undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF4]2 which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L3 complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)2L3]Br·0.5H2O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L4)2][BF4]2 (L4 = 2,6-di{quinolin-2-yl}pyridine; 4[BF4]2) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF4]2 and 2[BF4]2 undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules.

Synthesis and Magnetic Studies of Two Neutral, Bis-Ligand Fe(II) Complexes Containing Carbazole-Bis(tetrazole) Ligands

Previously reported carbazole-bis(tetrazole) (CzT^R) ligands (where R = iPr and CH₂-2,4,6-C₆H₂Me₃) were used to synthesize air-stable, six-coordinate, octahedral bis-ligand Fe(II) complexes (CzT^R)₂Fe. The synthesis and characterization of these complexes using ¹H nuclear magnetic resonance (NMR), X-ray crystallography, Mössbauer spectroscopy, and density functional theory (DFT) calculations are reported. Analysis of the magnetic properties revealed that the isopropyl derivative displays thermally induced spin crossover (SCO) over a temperature range of 150-350 K. This transition appears as an abrupt two-step transition in the solid state but simplifies to a smooth one-step transition in solution. The two-step transition in the solid state has been postulated to be due to lattice and solvation effects. In contrast, the slightly bulkier substituted CH₂-2,4,6-C₆H₂Me₃ (CH₂Mes) Fe complex displays dramatically different magnetic behavior with no SCO and magnetic data suggesting low-spin Fe(II) with a possible TIP contribution. DFT calculations support the postulate that the change in magnetic behavior is primarily due to the nature of the ligand substituents.

Sterically crowded di-indazolyl-pyridines: Iron(II) complexation studies

4-(2,6-Di(2H-indazol-2-yl)pyridin-4-yl)benzoic acid (1) and 10-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)anthracene-9-carboxylic acid (2) were required for adsorption studies on Ag(111), with a view to subsequent iron(II) complexation and formation of well-ordered spin-responsive self-assembled monolayers. While the generation of these compounds has remained elusive, several intermediates and by-products were obtained, potentially useful as dipyrazolylpyridine-related derivatives and for metal ion coordination. 3,5-Dichloro-2,6-diindazolylpyridine-4-amine, which forms as a mixture of regioisomers, was synthesised, the mixture separated, and the components characterised (3,5-dichloro-2,6-di(2H-indazol-2-yl)pyridin-4-amine; 3,5-dichloro-2-(1H-indazol-1-yl)-6-(2H-indazol-2-yl)pyridin-4-amine; 3,5-dichloro-2,6-di(1H-indazol-1-yl)pyridin-4-amine). Their iron(II) complexes have been prepared and fully characterised, including single crystal X-ray structure determination. The complexes are instructive examples of the influence of ligand design (“steric jamming”) on the spin-crossover (SCO) activity of FeII centres. Bulky substitution, which entails twisted ligand conformation, increases intramolecular crowding. This prevents contraction of the metal coordination sphere, which would be a prerequisite for thermally inducible SCO. Mössbauer spectroscopy has revealed that the complexes remain predominantly high-spin (HS) between 20 and 200 K, and that a mixture of conformational HS isomers is present in the microcrystalline solid.

Solvatomorphs of Iron(II) Complex with N,N'-Disubstituted 2,6-Bis(pyrazol-3-yl)pyridine with a Temperature-Induced Spin Transition in Solution

Abstract

The reaction of the tridentate ligand 4-(2,6-bis(5-tert-butyl-1-(2,6-dichlorophenyl)-1H-pyrazol-3-yl)pyridin-4-yl)benzonitrile (L) with iron(II) salt gave the complex [Fe(L)2](BF4)2, which was isolated in a pure state and characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction as two crystal polymorphs differing in the nature of the solvent molecule in the crystal (solvatomorphs I and II). According to the results of X-ray diffraction study (CCDC nos. 2104367 (I), 2104368 (II)), the iron(II) ion in these compounds occurs in different spin states and does not undergo a temperature-induced spin transition, which was previously observed for this complex in solution. The details of supramolecular organization of two solvatomorphs that prevent this transition were studied using the Hirshfeld surface analysis.

Spin-Crossover in Supramolecular Iron(II)-2,6-bis(1H-Pyrazol-1-yl)pyridine Complexes: Toward Spin-State Switchable Single-Molecule Junctions

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2'-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.

Iron(II) Complexes of 4-(Alkyldisulfanyl)-2,6-di(pyrazolyl)pyridine Derivatives. Correlation of Spin-Crossover Cooperativity with Molecular Structure Following Single-Crystal-to-Single-Crystal Desolvation

The complex salts [Fe(L 1)2]X2 (1X 2 ; L 1 = 4-(isopropyldisulfanyl)-2,6-bis(pyrazolyl)pyridine; X- = BF4 -, ClO4 -) form solvated crystals from common organic solvents. Crystals of 1X 2 ·Me2CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X 2 ·Me2CO cocrystallizes with other, less cooperative acetone solvates, which all transform into the same solvent-free materials 1X 2 ·sf upon exposure to air, or mild heating. Conversion of 1X 2 ·Me2CO to 1X 2 ·sf proceeds in a single-crystal to single-crystal fashion. 1X 2 ·sf are not isomorphous with the acetone solvates, and exhibit abrupt spin transitions at low temperature with hysteresis loops of 30-38 K (X- = BF4 -) and 10-20 K (X- = ClO4 -), depending on the measurement method. Interestingly, the desolvation has an opposite effect on the SCO temperature and hysteresis in the two salts. The hysteretic spin transitions in 1X 2 ·Me2CO and 1X 2 ·sf do not involve a crystallographic phase change but are accompanied by a significant rearrangement of the metal coordination sphere. Other solvates 1X 2 ·MeNO2, 1X 2 ·MeCN, and 1X 2 ·H2O are mostly isomorphous with each other and show more gradual spin-crossover equilibria near room temperature. All three of these lattice types have similar unit cell dimensions and contain cations associated into chains through pairwise, intermolecular S···π interactions. Polycrystalline [Fe(L 2)2][BF4]2·MeNO2 (2[BF 4 ] 2 ·MeNO2; L 2 = 4-(methyldisulfanyl)-2,6-bis(pyrazolyl)pyridine) shows an abrupt spin transition just above room temperature, with an unsymmetrical and structured hysteresis loop, whose main features are reversible upon repeated thermal scanning.

Structural Insights into Hysteretic Spin-Crossover in a Set of Iron(II)-2,6-bis(1H-Pyrazol-1-yl)Pyridine) Complexes

Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2 ) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes - [Fe(bpp-COOEt)2 ](X)2 ⋅CH3 NO2 (X=ClO4 , 1; X=BF4 , 2). Stable spin-state switching - T1/2 =288 K; ΔT1/2 =62 K - is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO - T1/2 =331 K; ΔT1/2 =43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations - from the Jahn-Teller-distorted HS state to the less distorted LS state - and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements.

Exceptionally high temperature spin crossover in amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complex revealed by variable temperature Raman spectroscopy and single crystal X-ray diffraction

The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH₂) is described. The complex salts [Fe(bppCONH₂)₂](BF₄)₂ and [Fe(bppCONH₂)₂](ClO₄)₂ were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH₂)₂](BF₄)₂ indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ at 445 K with a 14 K hysteresis. For the BF₄^- salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF₄ salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences in the inter-molecular hydrogen bonding in the low spin and high spin states.

Spin-Crossover in Iron(II) Complexes of N,N′-Disubstituted 2,6-Bis(Pyrazol-3-yl)Pyridines: An Effect of a Distal Substituent in the 2,6-Dibromophenyl Group

A series of new bis(pyrazol-3-yl)pyridines (LR) N,N′-disubstituted by 4-functionalized 2,6-dibromophenyl groups have been synthesized to study the effect of a distal substituent on the spin-crossover (SCO) behaviour of the iron(II) complexes [Fe(LR)2](ClO4)2 by variable-temperature magnetometry, NMR spectroscopy, and X-ray diffraction. The SCO-assisting tendency of the substituents with different electronic and steric properties (i.e., the bromine atom and the methyl group) in the para-position of the 2,6-dibromophenyl group is discussed. Together with earlier reported SCO-active iron(II) complexes with N,N′-disubstituted bis(pyrazol-3-yl)pyridines, these new complexes open the way for this family of SCO compounds to emerge as an effective ‘tool’ in revealing structure–function relations, a prerequisite for successful molecular design of switchable materials for future breakthrough applications in sensing, switching, and memory devices.

Interplay between spin crossover and proton migration along short strong hydrogen bonds

The iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T 1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T 2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T 1(LIESST) = 68 K and T 2(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.

Bistable spin-crossover in a new series of [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine; R = CN) complexes

Spin-crossover (SCO) active transition metal complexes are a class of switchable molecular materials. Such complexes undergo hysteretic high-spin (HS) to low-spin (LS) transition, and vice versa, rendering them suitable for the development of molecule-based switching and memory elements. Therefore, the search for SCO complexes undergoing abrupt and hysteretic SCO, that is, bistable SCO, is actively carried out by the molecular magnetism community. In this study, we report the bistable SCO characteristics associated with a new series of iron(ii) complexes-[Fe(BPP-CN)2](X)2, X = BF4 (1a-d) or ClO4 (2)-belonging to the [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine) family of complexes. Among the complexes, the lattice solvent-free complex 2 showed a stable and complete SCO (T1/2 = 241 K) with a thermal hysteresis width (ΔT) of 28 K-the widest ΔT reported so far for a [Fe(BPP-R)2](X)2 family of complexes, showing abrupt SCO. The reproducible and bistable SCO shown by the relatively simple [Fe(BPP-CN)2](X)2 series of molecular complexes is encouraging to pursue [Fe(BPP-R)2]2+ systems for the realization of technologically relevant SCO complexes.

Spin State Behavior of A Spin-Crossover Iron(II) Complex with N,N′-Disubstituted 2,6-bis(pyrazol-3-yl)pyridine: A Combined Study by X-ray Diffraction and NMR Spectroscopy

A series of three different solvatomorphs of a new iron(II) complex with N,N′-disubstituted 2,6-bis(pyrazol-3-yl)pyridine, including those with the same lattice solvent, has been identified by X-ray diffraction under the same crystallization conditions with the metal ion trapped in the different spin states. A thermally induced switching between them, however, occurs in a solution, as unambiguously confirmed by the Evans technique and an analysis of paramagnetic chemical shifts, both based on variable-temperature NMR spectroscopy. The observed stabilization of the high-spin state by an electron-donating substituent contributes to the controversial results for the iron(II) complexes of 2,6-bis(pyrazol-3-yl)pyridines, preventing ‘molecular’ design of their spin-crossover activity; the synthesized complex being only the fourth of the spin-crossover (SCO)-active kind with an N,N′-disubstituted ligand.

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.

Selective signalling of alcohols by a molecular lattice and mechanism of single-crystal-to-single-crystal transformations

A molecular material undergoes spin-switching as it exchanges MeOH, EtOH or ⁿPrOH with acetone from the lattice. The subsequent thermal single-crystal-to-single-crystal desorption of ⁿPrOH is followed by single crystal X-ray diffraction snapshots.

Relationship between the Molecular Structure and Switching Temperature in a Library of Spin-Crossover Molecular Materials

Structure-function relationships relating the spin-crossover (SCO) midpoint temperature (T_1/2) in the solid state are surveyed for 43 members of the iron(II) dipyrazolylpyridine family of SCO compounds. The difference between T_1/2 in the solid state and in solution [ΔT(latt)] is proposed as a measure of the lattice contribution to the transition temperature. Negative linear correlations between the SCO temperature and the magnitude of the rearrangement of the coordination sphere during SCO are evident among isostructural or near-isostructural subsets of compounds; that is, a larger change in the molecular structure during SCO stabilizes the high-spin state of a material. Improved correlations are often obtained when ΔT(latt), rather than the raw T_1/2 value, is considered as the measure of the SCO temperature. Different lattice types show different tendencies to stabilize the high-spin or low-spin state of the molecules they contain, which correlates with the structural changes that most influence ΔT(latt) in each case. These relationships are mostly unaffected by the SCO cooperativity in the compounds or by the involvement of any crystallographic phase changes. One or two materials within each subset are outliers in some or all of these correlations, however, which, in some cases, can be attributed to small differences in their ligand geometry or unusual phase behavior during SCO. A reinvestigation of the structural chemistry of [Fe(3-bpp)₂][NCS]₂·nH₂O [3-bpp = bis(1H-pyrazol-3-yl)pyridine; n = 0 or 2], undertaken as part of this study, is also presented.

Iron(III) Azadiphenolate Compounds in a New Family of Spin Crossover Iron(II)–Iron(III) Mixed-Valent Complexes

A new family of mixed valent, double salt spin crossover compounds containing anionic FeIII and cationic FeII compounds i.e., [FeII{(pz)3CH}2][FeIII(azp)2]2·2H2O (4), [FeII(TPPZ)2][FeIII(azp)2]2]·H2O (5) and [FeII(TPPZ)2][FeIII(azp)2]2]·H2O·3MeCN (6) (where (pz)3CH = tris-pyrazolylmethane, TPPZ = 2,3,5,6, tetrapyridylpyrazine and azp2− = azadiphenolato) has been synthesized and characterised. This is the first time that the rare anionic spin crossover species, [FeIII(azp)2]−, has been used as an anionic component in double salts complexes. Single crystal structures and magnetic studies showed that compound 6 exhibits a spin transition relating to one of the FeIII centres of the constituent FeII and FeIII sites. Crystal structures of the anionic and cationic precursor complexes were also analysed and compared to the double salt products thus providing a clearer picture for future crystal design in double spin crossover materials. We discuss the effects that the solvent and counterion had on the crystal packing and spin crossover properties.

Deciphering crystal packing effects in the spin crossover of six [FeII(2-pic)3]Cl2 solvatomorphs

Six isostructural solvatomorphs of the same Fe(ii) complex have been reported to display completely different SCO transitions due to tiny differences in their crystal environments. In this paper, we unravel the reasons.

Spin crossover in hydrogen-bonded frameworks of FeII complexes with organodisulfonate anions

Four spin crossover Fe^II complexes of hydrogen-bonded frameworks were constructed from the charge-assisted hydrogen bonds between the Fe^II complexes and organodisulfonate anions.

Above room temperature spin crossover in thioamide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complexes

This work describes the synthesis of two novel functionalised 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands, namely 2,6-bis(pyrazol-1-yl)pyridine-4-carbothioamide (bppCSNH2) and N-methyl-2,6-bis(pyrazol-1-yl)pyridine-4-carbothioamide (bppCSNHMe). The corresponding solvated or non-solvated Fe(ii) salts, [Fe(bppCSNH2)2]X2 and [Fe(bppCSNHMe)2]X2 (X = BF4- or ClO4-) were synthesised and their properties measured by SQUID magnetometry, Evans NMR, differential scanning calorimetry and single crystal X-ray diffraction. In the solid state [Fe(bppCSNH2)2]2+ salts persist in the low spin state below 350 K. The structure of [Fe(bppCSNH2)2](BF4)2·2MeNO2 shows a network of intermolecular interactions responsible for the low spin state stabilisation, relative to the prototypical [Fe(bpp)2]2+ spin crossover (SCO) salts. By contrast the complexes of bppCSNHMe both display abrupt SCO above 300 K. [Fe(bppCSNHMe)2](BF4)2·MeNO2 requires solvent loss before SCO can be observed centred at 332 K. The non-solvated [Fe(bppCSNHMe)2](ClO4)2 shows SCO centred at 325 K. Analysis of solvated and non-solvated crystal structures suggests that cooperativity is facilitated by thioamide-group interactions with neighbouring pyrazolyl and pyridyl moieties.

Intramolecular Spin State Locking in Iron(II) 2,6-Di(pyrazol-3-yl)pyridine Complexes by Phenyl Groups: An Experimental Study

Here we report a series of 1-phenyl-5-substituted 2,6-di(pyrazol-3-yl)pyridine complexes with iron(II) ion found in a high spin state in solids (according to magnetochemistry) and in solution (according to NMR spectroscopy), providing experimental evidence for it being an intramolecular effect induced by the phenyl groups. According to X-ray diffraction, the high spin locking of the metal ion is a result of its highly distorted coordination environment (with a very low ‘twist’ angle atypical of 2,6-di(pyrazol-3-yl)pyridine complexes), which remains this way in complexes with different substituents and counterions, in a diamagnetic zinc(II) analogue and in their solutions. Three possible reasons behind it, including additional coordination with the phenyl group, energy penalty incurred by its rotation or intramolecular stacking interactions, are addressed experimentally.

An Incomplete Spin Transition Associated with a Z'=1→Z'=24 Crystallographic Symmetry Breaking

Crystalline [FeL2 ][BF4 ]2 ⋅Me2 CO (L=N-[2,6-di{pyrazol-1-yl}pyrid-4-yl]acetamide) is high-spin at room temperature, and undergoes an abrupt, hysteretic spin-crossover at T1/2 =137 K (ΔT1/2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P21 /c, Z=4) to phase 2 (P21 , Z=48). The cations associate into chains in the crystal through weak intermolecular π⋅⋅⋅π interactions. Phase 2 contains a mixture of high-spin and low-spin molecules, which are grouped into triads along these chains. The perchlorate salt [FeL2 ][ClO4 ]2 ⋅Me2 CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P21 /c, Z=8) without a change in spin state.

Spacer type mediated tunable spin crossover (SCO) characteristics of pyrene decorated 2,6-bis(pyrazol-1-yl)pyridine (bpp) based Fe(ii) molecular spintronic modules

Two isomer-like, pyrene-decorated Fe(ii) spin crossover (SCO) complexes, [Fe(L1)2](BF4)2 and [Fe(L2)2](BF4)2, exhibit a dramatic 140 K (ΔT_1/2) variation in their switching temperatures.

The role of symmetry breaking in the structural trapping of light-induced excited spin states

Six isostructural compounds show differing agreement with the correlation T(LIESST) = T₀ − 0.3T_1/2 (red line; LIESST = Light-Induced Excited Spin-State Trapping). This reflects crystallographic symmetry breaking under LIESST conditions, exhibited by one of the compounds but not by two others.

Iron(II) complexes of tridentate indazolylpyridine ligands: enhanced spin-crossover hysteresis and ligand-based fluorescence

Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)2](2+), [Fe(1,2-bip)2](2+), and [Fe(1-ipp)2](2+) are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)2](2+) and [Fe(2-ipp)2](2+) are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)2](2+) exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)2][BF4]2·2MeNO2 adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T1/2 = 266 K, ΔT = 16-20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers.

Abrupt two-step and symmetry breaking spin crossover in an iron(iii) complex: an exceptionally wide [LS–HS] plateau

A rare iron(iii) symmetry breaking spin crossover compound with an extremely wide [LS–HS] plateau is reported.