Unexpected Spin-Crossover and a Low-Pressure Phase Change in an Iron(II)/Dipyrazolylpyridine Complex Exhibiting a High-Spin Jahn–Teller Distortion

A Survey of the Angular Distortion Landscape in the Coordination Geometries of High-Spin Iron(II) 2,6-Bis(pyrazolyl)pyridine Complexes

Reaction of 2,4,6-trifluoropyridine with sodium 3,4-dimethoxybenzenethiolate and 2 equiv of sodium pyrazolate in tetrahydrofuran at room temperature affords 4-(3,4-dimethoxyphenylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L), in 30% yield. The iron(II) complexes [FeL2][BF4]2 (1a) and [FeL2][ClO4]2 (1b) are high-spin with a highly distorted six-coordinate geometry. This structural deviation from ideal D2d symmetry is common in high-spin [Fe(bpp)2]2+ (bpp = di{pyrazol-1-yl}pyridine) derivatives, which are important in spin-crossover materials research. The magnitude of the distortion in 1a and 1b is the largest yet discovered for a mononuclear complex. Gas-phase DFT calculations at the ω-B97X-D/6-311G** level of theory identified four minimum or local minimum structural pathways across the distortion landscape, all of which are observed experimentally in different complexes. Small distortions from D2d symmetry are energetically favorable in complexes with electron-donating ligand substituents, including sulfanyl groups, which also have smaller energy penalties associated with the lowest energy distortion pathway. Natural population analysis showed that these differences reflect greater changes to the Fe-N{pyridyl} σ-bonding as the distortion proceeds, in the presence of more electron-rich pyridyl donors. The results imply that [Fe(bpp)2]2+ derivatives with electron-donating pyridyl substituents are more likely to undergo cooperative spin transitions in the solid state. The high-spin salt [Fe(bpp)2][CF3SO3]2, which also has a strong angular distortion, is also briefly described and included in the analysis.

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.

Defying the inverse energy gap law: a vacuum-evaporable Fe(ii) low-spin complex with a long-lived LIESST state

The novel vacuum-evaporable complex [Fe(pypypyr)₂] (pypypyr = bipyridyl pyrrolide) was synthesised and analysed as bulk material and as a thin film. In both cases, the compound is in its low-spin state up to temperatures of at least 510 K. Thus, it is conventionally considered a pure low-spin compound. According to the inverse energy gap law, the half time of the light-induced excited high-spin state of such compounds at temperatures approaching 0 K is expected to be in the regime of micro- or nanoseconds. In contrast to these expectations, the light-induced high-spin state of the title compound has a half time of several hours. We attribute this behaviour to a large structural difference between the two spin states along with four distinct distortion coordinates associated with the spin transition. This leads to a breakdown of single-mode behaviour and thus drastically decreases the relaxation rate of the metastable high-spin state. These unprecedented properties open up new strategies for the development of compounds showing light-induced excited spin state trapping (LIESST) at high temperatures, potentially around room temperature, which is relevant for applications in molecular spintronics, sensors, displays and the like.

2,6-Bis(pyrazol-1-yl)pyridine (1-bpp) as a General Ligand for the Negishi Alkylation of Alkylpyridinium Salts

A Ni/1-bpp catalyst was demonstrated to be effective in the Negishi alkylation with multiple classes of alkylpyridinium salts, including α-primary and α-secondary. These conditions are also effective for benzylic pyridinium salts, demonstrating the successful Negishi alkylation of benzylic pyridinium salts for the first time. Further, 14 derivatives of 1-bpp were prepared with a variety of steric and electronic properties to study how these changes impact the success of the Negishi alkylation.

Redox and guest tunable spin-crossover properties in a polymeric polyoxometalate

A bifunctionalized polyoxometalate (POM), [V6O19(C16H15N6O)2]2-, which contains a redox active hexavanadate moiety covalently linked to two tridentate 2,6-bis(pyrazol-1-yl)pyridine (1-bpp) ligands, has been prepared and characterized. Reaction of this hybrid molecule with Fe(ii) or Zn(ii) ions produces crystalline neutral 1D networks of formula Fe[V6O19(C16H15N6O)2]·solv (2) and Zn[V6O19(C16H15N6O)2]·solv (3) (solv = solvent molecules). Magnetic properties of 2 show an abrupt spin-crossover (SCO) with the temperature, which can be induced by light irradiation at 10 K (Light-Induced Excited Spin-State Trapping, LIESST effect). Interestingly, this porous and flexible structure enables reversible exchange of solvents in 2, which allows tuning the temperature of the thermal SCO. In 2 and 3, the hexavanadate unit can be reduced by electrochemical or chemical means in a reversible way. Chemical reduction and reoxidation by a postsynthetic method is accompanied by the insertion in the structure of the reductant and oxidant molecules (cobaltocene and tribromide, respectively), which provokes drastic changes in the spin state of Fe(ii). This leads to an elegant switching multifunctional material in which SCO properties of the Fe(ii) complexes coexist with the redox properties of the POM and can be tuned by a variety of stimuli such as temperature, light, solvent exchange or electron transfer. During the reduction process, 3 undergoes a single-crystal-to-single-crystal one-electron reduction, which confirms the presence of cobaltocenium cations by single crystal X-ray diffraction.

Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L¹ ), pyrid-2,6-diyl (L² ), benzene-1,2-dimethylenyl (L³ ) and propane-1,3-diyl (L⁴ ) linkers between the tridentate metal-binding domains. L¹ and L² form [Fe₂ (μ-L)₂ ]X₄ (X^- =BF₄ ^- or ClO₄ ^- ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe₂ (μ-L¹ )₂ ][BF₄ ]₄ exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe₂ (μ-L² )₂ ]⁴⁺ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L¹ -L³ vary with the ligand linker group, by mass spectrometry and ¹ H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe₂ (μ-L¹ )₂ ]⁴⁺ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

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 (ΔT_1/2) 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⋅CH₃NO₂ (X=ClO₄, 1; X=BF₄, 2). Stable spin‐state switching – T_1/2=288 K; ΔT_1/2=62 K – is observed for 1, whereas 2 undergoes above‐room‐temperature lattice‐solvent content‐dependent SCO – T_1/2=331 K; ΔT_1/2=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.

Halogenation affects driving forces, reorganization energies and "rocking" motions in strained [Fe(tpy)2]2+ complexes

Controlling the energetics of spin crossover (SCO) in Fe(II)-polypyridine complexes is critical for designing new multifunctional materials or tuning the excited-state lifetimes of iron-based photosensitizers. It is well established that the Fe-N "breathing" mode is important for intersystem crossing from the singlet to the quintet state, but this does not preclude other, less obvious, structural distortions from affecting SCO. Previous work has shown that halogenation at the 6 and 6'' positions of tpy (tpy = 2,2';6',2''-terpyridine) in [Fe(tpy)₂]²⁺ dramatically increased the lifetime of the excited MLCT state and also had a large impact on the ground state spin-state energetics. To gain insight into the origins of these effects, we used density functional theory calculations to explore how halogenation impacts spin-state energetics and molecular structure in this system. Based on previous work we focused on the ligand "rocking" motion associated with SCO in [Fe(tpy)₂]²⁺ by constructing one-dimensional potential energy surfaces (PESs) along the tpy rocking angle for various spin states. It was found that halogenation has a clear and predictable impact on ligand rocking and spin-state energetics. The rocking is correlated to numerous other geometrical distortions, all of which likely affect the reorganization energies for spin-state changes. We have quantified trends in reorganization energy and also driving force for various spin-state changes and used them to interpret the experimentally measured excited-state lifetimes.

Optimization of crystal packing in semiconducting spin-crossover materials with fractionally charged TCNQ δ- anions (0 < δ < 1)

Co-crystallization of the prominent Fe(ii) spin-crossover (SCO) cation, [Fe(3-bpp)₂]²⁺ (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQ^δ− radical anion has afforded a hybrid complex [Fe(3-bpp)₂](TCNQ)₃·5MeCN (1·5MeCN, where δ = −0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity σ_RT = 3.1 × 10⁻³ S cm⁻¹, and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ⁻ anions, [Fe(3-bpp)₂](TCNQ)₂·3MeCN (2·3MeCN), readily loses the interstitial solvent to afford desolvated complex 2 that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex 2 also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) to obtain [Fe(bzimpy)₂](TCNQ)₆·2Me₂CO (4) and [Fe(bzimpy)₂](TCNQ)₅·5MeCN (5), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with σ_RT = 9.1 × 10⁻² S cm⁻¹ and 1.8 × 10⁻³ S cm⁻¹, respectively.

Co-crystallization of the cationic complex [Fe(3-bpp)2]²⁺ with fractionally charged TCNQ^δ− anions (0 < δ < 1) affords semiconducting spin-crossover (SCO) materials. The abruptness of SCO is strongly dependent on the interstitial solvent content.

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.

The number and shape of lattice solvent molecules controls spin-crossover in an isomorphous series of crystalline solvate salts

Crystals of [FeL2][BF4]2·nMeCN (L = N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide; n = 1 or 2) and [FeL2][ClO4]2·MeCN are isomorphous. When n = 1 the compounds exhibit an abrupt, hysteretic spin-transition below 200 K, but when n = 2 the material remains high-spin on cooling. [FeL2]X2·EtCN (X- = BF4- or ClO4-) are isomorphous with the MeCN solvates and undergo their spin-transition at almost the same temperature. However this now occurs in two-steps via a re-entrant mixed-spin intermediate phase, which correlates with crystallographic ordering of the bent propionitrile molecule.

The effect of tether groups on the spin states of iron(II)/bis[2,6-di(pyrazol-1-yl)pyridine] complexes

The synthesis of six 2,6-di(pyrazol-1-yl)pyridine derivatives bearing dithiolane or carboxylic acid tether groups is described: [2,6-di(pyrazol-1-yl)pyrid-4-yl]methyl (R)-lipoate (L1), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]ethyl (R)-lipoate (L2), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxy]ethyl (R)-lipoate (L3), N-([2,6-di(pyrazol-1-yl)pyrid-4-ylsulfanyl]-2-aminoethyl (R)-lipoamide (L4), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]acetic acid (L5) and 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]propionic acid (L6). The iron(ii) perchlorate complexes of all the new ligands exhibit gradual thermal spin-crossover (SCO) in the solid state above room temperature, except L4 whose complex remains predominantly high-spin. Crystalline [Fe(L6)2][ClO4]2·2MeCN contains three unique cation sites which alternate within hydrogen-bonded chains, and undergo gradual SCO at different temperatures upon warming. The SCO midpoint temperature (T1/2) of the complexes in CD3CN solution ranges between 208-274 K, depending on the functional group linking the tether groups to the pyridyl ring. This could be useful for predicting how these complexes might behave when deposited on gold or silica surfaces.

Sequential single-crystal-to-single-crystal vapochromic inclusion in a nonporous coordination polymer: unravelling dynamic rearrangement for selective pyridine sensing

Unprecedent selective and reversible pyridine solid–vapor sequential inclusion in a nonporous Fe(ii) coordination polymer accompanied by a color change is presented. Although the internal reorganization is significant, the process remains SCSC.

Structural, magnetic and Mössbauer spectroscopic studies of the [Fe(3-bpp)2](CF3COO)2 complex: role of crystal packing leading to an incomplete Fe(ii) high spin ⇋ low spin transition

Mononuclear complex [Fe(3-bpp)₂](CF₃COO)₂ exhibits a thermal (HS + HS) ⇋ (HS + LS) transition at ∼226 K which is not associated with any crystallographic transition.

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.

Unprecedented Reverse Volume Expansion in Spin-Transition Crystals

The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM-pBrA)₂ (NCS)₂ ] exhibits, on the contrary, an increase of the unit-cell volume from HS to LS. This counter-intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single-crystal and powder X-ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light-induced HS state obtained through the Light-Induced Excited Spin-State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials.

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.

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners

Annealing [FeL₂ ][BF₄ ]₂ ⋅2 H₂ O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T_1/2 =205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL₂ ][BF₄ ]₂ ⋅2 H₂ O and [ZnL₂ ][BF₄ ]₂ ⋅2 H₂ O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re-cooling. The low-temperature phase of [CuL₂ ][BF₄ ]₂ involves ordering of its Jahn-Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low-temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high-spin iron compound, before the onset of spin-crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper-doped [Fe_0.97 Cu_0.03 L₂ ][BF₄ ]₂ imply its low-spin phase contains two distinct cation environments in a 2:1 ratio.

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.

Five 2,6-Di(pyrazol-1-yl)pyridine-4-carboxylate Esters, and the Spin States of their Iron(II) Complexes

Two phenyl ester and three benzyl ester derivatives have been synthesized from 2,6-di(pyrazol-1-yl)pyridine-4-carboxylic acid and the appropriate phenyl or benzyl alcohol using N,N’-dicyclohexylcarbodiimide as the coupling reagent. Complexation of the ligands with Fe[BF4]2·6H2O in acetone yielded the corresponding [FeL2][BF4]2 complex salts. Four of the new ligands and four of the complexes have been crystallographically characterised. Particularly noteworthy are two polymorphs of [Fe(L3)2][BF4]2·2MeNO2 (L3 = 3,4-dimethoxyphenyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), one of which is crystallographically characterised as high-spin while the other exhibits the onset of spin-crossover above room temperature. The other complexes are similarly low-spin at low temperature but exhibit gradual spin-crossover on heating, except for an acetone solvate of [Fe(L5)2][BF4]2 (L5 = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at T½ = 273 K with 9 K thermal hysteresis.

Spin-crossover iron(ii) complex showing thermal hysteresis around room temperature with symmetry breaking and an unusually high T(LIESST) of 120 K

The Fe(ii) complex of 4′,4′′ carboxylic acid disubstituted dipyrazolylpyridine shows spin-crossover around room temperature and the LIESST effect with T(LIESST) of 120 K.

Bi-stable spin-crossover characteristics of a highly distorted [Fe(1-BPP-COOC2H5)2](ClO4)2·CH3CN complex

A HS Fe(ii) complex, see figure, with a pronounced angular distortion (ϕ) = 158.83(17)° showed lattice solvent dependent spin-state switching characteristics with a thermal hysteresis width (ΔT) of 101 K.

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.

Abrupt Spin Crossover Behavior in a Linear N1,N2-Triazole Bridged Trinuclear Fe(II) Complex

The synthesis, structures and magnetic properties of a new trinuclear spin crossover complex [FeII3(pyrtrz)6(TsO)6]·10H2O·2CH3OH (C2) and its analogue binuclear [FeII2(pyrtrz)5(SCN)4]·7H2O (C1), are reported here. These two compounds are synthesized based on the pyrrolyl functionalized Schiff base 1,2,4-triazole ligand 4-((1H-pyrrol-2-yl)methylene-amino)-4H-1,2,4-triazole (pyrtrz), which represent rare discrete multi-nuclear species, with µ2-N1,N2-triazole bridges linking the FeII centers. DC magnetic susceptibility measurements revealed an abrupt single-step spin crossover (SCO) behavior for compound 2 on the central FeII site and single-crystal X-ray diffraction (173 K) showed that this compound crystallizes in the monoclinic space group (P21/c), and multiple intramolecular interactions were found responsible for the abrupt transition. Compound 1 is a binuclear complex with thiocyanate as terminal ligands. This compound stays in high spin state over the whole temperature range and displays weak antiferromagnetic exchange coupling.

Cooperativity in Spin Crossover Systems. An Atomistic Perspective on the Devil's Staircase

Cooperativity is key in defining the shape (i.e., gradual, abrupt, or hysteretic) of thermally driven spin transitions in magnetic switches. Despite its importance, there is very little information on its atomistic origin, which hinders the rational design of materials displaying a bistability region (i.e., hysteresis). In this paper, we investigate the spin transition of two solvatomorphs of [Fe(2-pic)₃]Cl₂, an Fe(II)-complex displaying thermal spin crossover (SCO) from a low-spin (LS) to a high-spin (HS) state with either gradual or abrupt two-step character. To do it, we apply a novel computational protocol to study the cooperativity of SCO compounds from DFT calculations, which does not rely on a priori assumptions on the studied system. The distinct shape of the spin transition is successfully captured, and the atomistic origin of cooperativity is traced back to geometrical distortions of the Fe-N₆ core in case of the solvatomorph exhibiting an abrupt transition. According to our calculations, HS and LS molecules contribute differently to cooperativity, which results in a complex energetic evolution of the spin transition that cannot be described by the common Slichter-Drickamer model. The present work opens new avenues for the study of cooperativity of SCO systems having a chemically oriented perspective and demonstrates that quantum chemistry calculations can discriminate the shape of a spin transition, while providing insight into the atomistic underlying factors that contribute to its cooperative behavior.

A Pseudo-Octahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero-Field Single-Molecule Magnet with Easy Axis Anisotropy

The homoleptic mononuclear compound [Co(bpp-COOMe)₂ ](ClO₄ )₂  (1) (bpp-COOMe=methyl 2,6-di(pyrazol-1-yl)pyridine-4-carboxylate) crystallizes in the monoclinic C2/c space group, and the cobalt(II) ion possesses a pseudo-octahedral environment given by the two mer-coordinated tridentate ligands. Direct-current magnetic data, single-crystal torque magnetometry, and EPR measurements disclosed the easy-axis nature of this cobalt(II) complex, which shows single-molecule magnet behavior when a static field is applied in alternating-current susceptibility measurements. Diamagnetic dilution in the zinc(II) analogue [Zn(bpp-COOMe)₂ ](ClO₄ )₂  (2) afforded the derivative [Zn_0.95 Co_0.05 (bpp-COOMe)₂ ](ClO₄ )₂  (3), which exhibits slow relaxation of magnetization even in zero field thanks to the reduction of dipolar interactions. Theoretical calculations confirmed the overall electronic structure and the magnetic scenario of the compound as drawn by experimental data, thus confirming the spin-phonon Raman relaxation mechanism, and a direct quantum tunneling in the ground state as the most plausible relaxation pathway in zero field.

Spin-state dependent conductance switching in single molecule-graphene junctions

Spin-crossover (SCO) molecules are versatile magnetic switches with applications in molecular electronics and spintronics. Downscaling devices to the single-molecule level remains, however, a challenging task since the switching mechanism in bulk is mediated by cooperative intermolecular interactions. Here, we report on electron transport through individual Fe-SCO molecules coupled to few-layer graphene electrodes via π-π stacking. We observe a distinct bistability in the conductance of the molecule and a careful comparison with density functional theory (DFT) calculations allows to associate the bistability with a SCO-induced orbital reconfiguration of the molecule. We find long spin-state lifetimes that are caused by the specific coordination of the magnetic core and the absence of intermolecular interactions according to our calculations. In contrast with bulk samples, the SCO transition is not triggered by temperature but induced by small perturbations in the molecule at any temperature. We propose plausible mechanisms that could trigger the SCO at the single-molecule level.

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.

Evidence of crystal packing effects in stabilizing high or low spin states of iron(ii) complexes with functionalized 2,6-bis(pyrazol-1-yl)pyridine ligands

The molecular structures and magnetic properties of homoleptic iron(ii) compounds [Fe(bpp-COOMe)₂](ClO₄)₂ (1) and [Fe(bpp-triolH₃)₂](ClO₄)₂ (2) have been investigated to ascertain their spin crossover (SCO) behaviour. In these hexacoordinated complexes, the bpp (2,6-bis(pyrazol-1-yl)pyridine) ligands adopt a mer-mer coordination mode and carry COOMe or C(O)NHC(CH₂OH)₃para substituents, respectively, on the central pyridyl ring. In spite of the almost equal donor power of the ligands to the iron(ii) centre, the two compounds feature different spin state configurations at room temperature. Compound 1 displays a highly-distorted octahedral environment around the iron(ii) centre, which adopts a high spin (HS) state at all temperatures, even under an external applied pressure up to 1.0 GPa. By contrast, 2 is characterized by a more regular octahedral coordination around the metal ion and exhibits a low spin (LS) configuration at or below room temperature. However, it shows a thermally-induced SCO behaviour at T > 400 K, along with Light-Induced Excited Spin State Trapping (LIESST) at low temperature, with T_LIESST = 38 K. Since DFT (U)M06/6-311+G(d) geometry optimizations in vacuo indicate that both complexes should adopt a HS state and a highly-distorted coordination geometry, the stabilization of a LS configuration in 2 is ultimately ascribed to the effect of intermolecular hydrogen bonds, which align the [Fe(bpp-triolH₃)₂]²⁺ cations in 1D chains and impart profound differences in the geometric arrangement of the ligands.

Spin-crossover compounds based on iron(ii) complexes of 2,6-bis(pyrazol-1-yl)pyridine (bpp) functionalized with carboxylic acid and ethyl carboxylic acid

New salts of the iron(ii) bppCOOH and bppCOOEt spin crossover complexes have been studied.

Pybox-Iron(II) Spin-Crossover Complexes with Substituent Effects from the 4-Position of the Pyridine Ring (Pybox = 2,6-Bis(oxazolin-2-yl)pyridine)

Spin-crossover (SCO) behavior of a series of [Fe(X-pybox)2](ClO4)2 was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH3O, and CH3S. We confirmed that the mother compound [Fe(H-pybox)2](ClO4)2 underwent SCO above room temperature. After X was introduced, the SCO temperatures (T1/2) were modulated as 310, 230, and 330 K for X = Cl, Ph, and CH3S, respectively. The CH3O derivative possessed the high-spin state down to 2 K. Crystallographic analysis for X = H, Cl, CH3O, and CH3S was successful, being consistent with the results of the magnetic study. Distorted coordination structures stabilize the HS (high-spin) state, and the highest degree of the coordination structure distortion is found in the CH3O derivative. A plot of T1/2 against the Hammett substituent constant σp showed a positive relation. Solution susceptometry was also performed to remove intermolecular interaction and rigid crystal lattice effects, and the T1/2’s were determined as 260, 270, 240, 170, and 210 K for X = H, Cl, Ph, CH3O, and CH3S, respectively, in acetone. The substituent effect on T1/2 became very distinct, and it is clarified that electron-donating groups stabilize the HS state.

Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-triazine Derivatives-The Influence of Ligand Geometry on Metal Ion Spin State

Seven [FeL₂][BF₄]₂ complex salts were prepared, where L is a 6-substituted 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt) derivative. The complexes are all crystallographically high-spin, and exhibit significant distortions from an ideal D_2d-symmetric coordination geometry. In one case, an unusual type of metal ion disorder was observed among a cubic array of ligands in the crystal lattice. The complexes are also high-spin between 3 and 300 K in the solid state and, where measured, between 239 and 333 K in CD₃CN solution. This result is unexpected, since homoleptic iron(II) complexes of related 2,6-di(pyrazol-1-yl)pyridine, 2,6-di(pyrazol-1-yl)pyrazine, and 2,6-di(pyrazol-1-yl)pyrimidine derivatives often exhibit thermal spin-crossover behavior. Gas-phase density functional theory calculations confirm the high-spin form of [Fe(bpt)₂]²⁺ and its derivatives is stabilized relative to iron(II) complexes of the other ligand types. This reflects a weaker Fe/pyrazolyl σ-bonding interaction, which we attribute to a small narrowing of the chelate ligand bite angle associated with the geometry of the 1,3,5-triazinyl ring. Hence, the high-spin state of [Fe(bpt)₂]²⁺ centers does not reflect the electronic properties of its heterocyclic ligand donors but is imposed by the bpt ligand conformation. A high-spin homoleptic iron(III) complex of one of the bpt derivatives was also synthesized.

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives

The following iron(II) complexes of 2,6-bis(oxazolinyl)pyridine (PyBox; L^H ) derivatives are reported: [Fe(L^H )₂ ][ClO₄ ]₂ (1); [Fe((R)-L^Me )₂ ][ClO₄ ]₂ ((R)-2; L^Me =2,6-bis{4-methyloxazolinyl}pyridine); [Fe((R)-L^Ph )₂ ][ClO₄ ]₂ ((R)-3) and [Fe((R)-L^Ph )((S)-L^Ph )][ClO₄ ]₂ ((RS)-3; L^Ph =2,6-bis{4-phenyloxazolinyl}pyridine); and [Fe((R)-L^iPr )₂ ][ClO₄ ]₂ ((R)-4) and [Fe((R)-L^iPr )((S)-L^iPr )][ClO₄ ]₂ ((RS)-4; L^iPr =2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO₂ exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5-300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by ¹ H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD₃ CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Spin state variability in Fe2+ complexes of substituted (2-(pyridin-2-yl)-1,10-phenanthroline) ligands as versatile terpyridine analogues

Fe²⁺ spin crossover complexes [Fe(L)₂]²⁺ (L = substituted (pyridin-2-yl)-1,10-phenanthroline) were prepared and SCO properties were investigated in solution and in the solid state by an experiment and in silico.

A luminescent Pt2Fe spin crossover complex

A heterotrinuclear luminescent [Pt₂Fe] spin crossover (SCO) complex was developed, synthesized, and investigated.

Molecular switching involving metastable states: molecular thermal hysteresis and sensing of environmental changes by chiral helicene oligomeric foldamers

Molecular switching is a phenomenon in which the molecular structure reversibly changes in response to external stimulation. It is crucial in biology and is used in various biological sensing applications and responses. In contrast to the well-studied molecular switching involving two or more thermodynamically stable states, switching involving metastable states exhibits notable non-equilibrium thermodynamic properties. Synthetic chiral helicene oligomeric foldamers that exhibit molecular thermal hysteresis in dilute solution are examples. Molecular switching can be used for sensing environmental changes, including temperature threshold, temperature decrease/increase, rate of temperature decrease, counting the numbers 1 and 2, and concentration increase.