Spin crossover active iron(II) complexes of selected pyrazole-pyridine/pyrazine ligands

Unravelling Kinetics of Intramolecular NdIII → FeII Energy Transfer in Spin Crossover Single Molecules: Dotting the i's and Crossing the t's

Compared with the ripple of visible EuIII-based emission intensity induced by appended [FeIIN6] spin crossover (SCO) units, as detected in the triple-stranded [EuFe(L1)3]5+ helicate, the lanthanide-based luminescent detection of FeII spin-state equilibria could be improved significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing EuIII with NdIII in [NdFe(L1)3]5+ (i) maintains the favorable SCO properties in acetonitrile [critical temperature T1/2 = 322(2) K], (ii) saturates nonradiative vibrational relaxation processes in the 233-333 K range, and (iii) boosts the crucial intramolecular NdIII → FeII energy transfer rate processes, which are sensitive to the spin state of the FeII metallic center. Consequently, the steady-state NIR Nd(4F3/2 → 4IJ) emission of the luminophore is amplified and linearly correlated with the low-spin-[FeIIN6] and high-spin-[FeIIN6] mole fractions controlled by the SCO equilibrium. This paves the way for a straightforward and direct NIR luminescent reading/sensing of the FeII spin state in single molecules.

New cyanido-bridged iron(II) spin crossover coordination polymers with an unusual ladder-like topology: an alternative to Hofmann clathrates

Two new cyanido-bridged {FeIIMII} double chains were obtained by reacting cyanido anions [M(CN)4]2- with complex cations [FeII(tptz)]2+ (preformed in situ by mixing a hydrated tetrafluoroborate salt of iron(II) and a tptz ligand, tptz = 2,4,6-tri(2-pyridyl)-1,3,5-triazine) having the general formula [FeII(tptz)MII(CN)4]·2H2O·CH3CN, where M = Pd (1) or Pt (2). Additionally, two molecular complexes formulated as [FeII(tptz)2][MII(CN)4]·4.25H2O, where M = Pd (3) or Pt (4), were subsequently obtained from the same reaction, as secondary products. Single crystal X-ray analysis revealed that 1 and 2 are isostructural and crystallize in the P-1 triclinic space group. Their structure consists of a double-chain with a ladder-like topology, in which cyanido-based [M(CN)4]2- metalloligands coordinate, through three CN- ligands and three [FeII(tptz)]2+ complex cations. Compounds 3 and 4 are also isostructural and crystallize in the P1̄ triclinic space group, and the X-ray structural data show the formation of [FeII(tptz)2]2+ and [MII(CN)4]2- ionic units interconnected through H-bonds and π⋯π stacking supramolecular interactions. The static DC magnetic measurements recorded in the temperature range of 2-300 K showed that 1 and 2 exhibit incomplete spin transition on cooling, which is also confirmed by single crystal XRD analysis and Mössbauer spectroscopy. Compounds 3 and 4 are diamagnetic, most likely due to the encapsulation of Fe(II) in a tight pocket formed by two tptz ligands that preserve the low-spin state in the temperature range of 2-400 K.

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

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.

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.

Spontaneous Chiral Resolution of a MnIII Spin-Crossover Complex with High Temperature 80 K Hysteresis

Non-centrosymmetric spin-switchable systems are of interest for their prospective applications as magnetically active non-linear optical materials and in multiferroic devices. Chiral resolution of simple spin-crossover chelate complexes into the Δ and Λ forms offers a facile route to homochiral magnetic switches, which could be easily enantiomerically enriched. Here, we report the spontaneous resolution of a new hysteretic spin-crossover complex, [MnIII (sal2 323)]SCN ⋅ EtOH (1), into Δ and Λ forms, without the use of chiral reagents, where sal2 323 is a Schiff base resulting from condensation of 1,2-bis(3-aminopropylamino)ethane with 2-hydroxybenzaldehyde. The enantiopurity of the Δ and Λ isomers was confirmed by single crystal X-ray diffraction and circular dichroism. Quantum chemistry calculations were used to investigate the electronic structure. The opening of a wide 80 K thermal hysteresis window at high temperature highlights the potential for good magneto-optical function at ambient temperature for materials of this type.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

Binuclear spin-crossover [Fe(bt)(NCS)2]2(bpm) complex: A study using first principles calculations

The spin-crossover [Fe(bt)(NCS)₂]₂(bpm) complex is studied using spin-polarized density functional theory within the generalized gradient approximation, the Hubbard U and the weak van der Waals interactions in conjunction with the projector augmented wave method in its molecular and periodic arrangements. It is shown that the considered complex has three magnetic configurations [high spin state (HS)-HS, HS-low spin state (LS), and LS-LS] corresponding to those observed experimentally after two transition temperatures T_c ⁽¹⁾ of 163 K and T_c ⁽²⁾ of 197 K. For the HS-HS magnetic state, we found that the two Fe centers are antiferromagnetically coupled for both molecular and periodic structures in good agreement with the experimental observations. Our results show that the computed total energy difference between the magnetic state configurations of the considered Fe₂ complex is significantly smaller compared to those reported in the literature for other mono- or binuclear compounds.

Iron(II) Complexes of 2,6-Di[4-(ethylcarboxy)pyrazol-1-yl]pyridine with Reversible Guest-Modulated Spin-Crossover Behavior

Three solvatomorphs of the iron(II) complex of 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (bpCOOEt₂p) of formulas [Fe(bpCOOEt₂p)₂](ClO₄)₂·1.5MeNO₂ (1), [Fe(bpCOOEt₂p)₂](ClO₄)₂·MeNO₂ (2), and [Fe(bpCOOEt₂p)₂](ClO₄)₂·2MeNO₂ (3) have been prepared and characterized. They show interesting spin-crossover (SCO) properties ranging from partial to complete thermal spin transitions and a light-induced excited spin-state trapping (LIESST) effect. In solvatomorph 2, a robust structure is formed with channels that enable the entrance or removal of solvent molecules by vapor diffusion without losing the crystallinity. Thus, solvent-exchanged samples [Fe(bpCOOEt₂p)₂](ClO₄)₂·MeNO₂ (2·MeNO ₂ ), [Fe(bpCOOEt₂p)₂](ClO₄)₂·MeCN (2·MeCN), [Fe(bpCOOEt₂p)₂](ClO₄)₂·0.5Me₂CO (2·Me ₂ CO), and [Fe(bpCOOEt₂p)₂](ClO₄)₂·MeCOOH (2·MeCOOH) were prepared by vapor diffusion of the solvents in a crystal of the compound previously heated to 400 K in a single-crystal to single-crystal (SCSC) fashion. Interestingly, this causes a change of spin state with a stabilization of the low-spin state in 2·Me ₂ CO and the high-spin state in 2·MeCN. Therefore, the SCO properties of 2 can be tuned in a reversible way by exposure to different solvents.

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.

Ag2O-Induced Regioselective Huisgen Cycloaddition for the Synthesis of Fully Substituted Pyrazoles as Potential Anticancer Agents

Efficient regioselective synthesis of novel fully substituted pyrazoles has been achieved through Huisgen cycloaddition reaction of δ-acetoxy allenoates with hydrazonoyl chlorides by the addition of Ag₂O. The present approach offers the advantages of simpleness, high efficiency, mild conditions, wide substrate scope, and good-to-excellent regioselectivities. The strategy could be performed on a large-scale pattern to allow access to structurally versatile pyrazoles, of which a key intermediate of lonazolac (303), a nonsteroidal anti-inflammatory drug, could be synthesized efficiently. Moreover, several pyrazoles show obvious growth-inhibitory activity of Huh-7 cells, expected as potential anticancer agents.

Push and Pull Effect of Methoxy and Nitro Groups Modifies the Spin-State Switching Temperature in Fe(III) Complexes

We have explored the impact of electron-donating (methoxy) and electron-withdrawing (nitro) substituents on SalEen ligand based spin crossover (SCO) behavior of Fe(III) complexes. Thus, 3-X-substituted SalEen ligands were employed to prepare [Fe(3-X-SalEen)₂]·NCSe, where X = OMe (1), H (2), and NO₂ (3) (3-X-SalEen is the condensation product of 3-substituted salicylaldehyde and N-ethylethylenediamine). The characteristic spin transition temperature (T _1/2) is shown to shift to a lower temperature when an electron-donating substituent (OMe) is used and to a higher temperature when an electron-withdrawing substituent (NO₂) is used. We used experimental and theoretical methods to determine the reasons for this behavior. The solid-state magnetic data revealed the transition temperatures for complexes 1, 2, and 3 to be 219, 251, and 366 K, respectively. The solution-state magnetic data also support this trend in T _1/2 values. UV-vis spectra analysis indicates that there is greater delocalization in the π-manifold of the ligand when the nitro group is the substituent. Theoretical studies through density functional theory methods suggest the methoxy substituent decreases the energy gap between the t_2g and e_g orbitals (explaining the lower T _1/2 value), while the nitro substituent increases the energy gap between the t_2g and e_g orbitals and thus increases the T _1/2 value.

Subcomponent self-assembly of circular helical Dy6(L)6 and bipyramid Dy12(L)8 architectures directed via second-order template effects

In situ metal-templated (hydrazone) condensation also called subcomponent self-assembly of 4,6-dihydrazino-pyrimidine, o-vanillin and dysprosium ions resulted in the formation of discrete hexa- or dodecanuclear metallosupramolecular Dy6(L)6 or Dy12(L)8 aggregates resulting from second-order template effects of the base and the lanthanide counterions used in these processes. XRD analysis revealed unique circular helical or tetragonal bipyramid architectures in which the bis(hydrazone) ligand L adopts different conformations and shows remarkable differences in its mode of metal coordination. While a molecule of trimethylamine acts as a secondary template that fills the void of the Dy6(L)6 assembly, sodium ions take on this role for the formation of heterobimetallic Dy12(L)8 by occupying vacant coordination sites, thus demonstrating that these processes can be steered in different directions upon subtle changes of reaction conditions. Furthermore, Dy6(L)6 shows an interesting spin-relaxation energy barrier of 435 K, which is amongst the largest values within multinuclear lanthanide single-molecular magnets.

Environmental effects on the singlet fission phenomenon: a model Hamiltonian-based study

In the screening of compounds for singlet fission, the relative energies of the constitutive units are decisive to fulfil the thermodynamic rules. From a model Hamiltonian constructed on the local spin states of an active chromophore and its environment, it is suggested that embedding greatly influences the energy differences of the active monomer spin states. Even in the absence of charge transfer, the field generated by a singlet environment produces an increase of the [E(S₁) - E(S₀)]/[E(T₁) - E(S₀)] critical ratio by up to 6% as compared to the one of a free chromophore. Besides, variations are observed when the intimate electronic structure of the singlet environment is modified. This propensity towards singlet fission is even more pronounced (10%) when the environment is switched to the triplet state. Finally, the embedding is likely to reverse the spin state ordering in the limit of vanishing atomic orbital overlaps. Despite its simplicity, the model stresses the importance of the environment spin nature in the quest for singlet fission candidates, and more generally in spectroscopy analysis.

Trend in light-induced excited-state spin trapping in Fe(II)-based spin crossover systems

A computational study of the light-induced excited spin-state trapping (LIESST) in a number of Fe(II) spin crossover complexes, coordinated by monodentate, bidentate and multidentate ligands is carried out, with the goal to uncover the trend in the low temperature relaxation rate. A nine order of magnitude change in low temperature relaxation rate is observed among the complexes. The trend is rationalized in terms of the change in metal-ligand covalency, numerically estimated by the crystal orbital Hamiltonian population, thus influencing the back donation or delocalization of the electrons from the low-lying Fe(II)-centered molecular orbital to the empty low-lying ligand-centered π* antibonding molecular orbitals.

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.

High spin iron(ii) complexes based on imidazolyl- and 1,2,3-triazolyl-thione ligands and NCE (E = S, Se or BH3) co-ligands: effect of the S-functional group on the structural and magnetic properties

Differing magnetic behaviour in complexes of the type [FeII(N2S2)(NCE)2]: thione-based complexes are stabilised in the HS-state, whilst a previously reported picolylbenzylthioether–iron(ii) complex shows a hysteretic spin crossover behaviour.

Heteroleptic iron(ii) complexes of chiral 2,6-bis(oxazolin-2-yl)-pyridine (PyBox) and 2,6-bis(thiazolin-2-yl)pyridine ligands – the interplay of two different ligands on the metal ion spin sate

The spin-crossover properties of [Fe(LR)L][ClO4]2 (LR = a chiral PyBox {L1R} or ThioPyBox {L2R} derivative) show subtle differences depending on the tridentate ‘L’ co-ligand.

Accurate calculation of spin-state energy gaps in Fe(III) spin-crossover systems using density functional methods

Fe(III) complexes are receiving ever-increasing attention as spin crossover (SCO) systems because they are usually air stable, as opposed to Fe(II) complexes, which are prone to oxidation. Here, we present the first systematic study exclusively devoted to assess the accuracy of several exchange-correlation functionals when it comes to predicting the energy gap between the high-spin (S = 5/2) and the low-spin (S = 1/2) states of Fe(III) complexes. Using a dataset of 24 different Fe(III) hexacoordinated complexes, it is demonstrated that the B3LYP* functional is an excellent choice not only for predicting spin-state energy gaps for Fe(III) complexes undergoing spin-transitions but also for discriminating Fe(III) complexes that are either low- or high-spin in the whole range of temperatures. Our benchmark study has led to the identification of a very versatile Fe(III) compound whose SCO properties can be engineered upon changing a single axial ligand. Overall, this work demonstrates that B3LYP* is a reliable functional for screening new spin-crossover systems with tailored properties.

Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands

Iron(II) complex salts of 2,6-di(1,2,3-triazol-1-yl)pyridine (L¹) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L¹)₄(H₂O)₂][ClO₄]₂ and [Fe(NCS)₂(L¹)₂(H₂O)₂]·L¹, containing L¹ bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 4⁴ grid structures through O-H···N hydrogen bonding; a solvate of a related 4⁴ coordination framework, catena-[Cu(μ-L¹)₂(H₂O)₂][BF₄]₂, is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L²) and 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L³) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L³)₂][ClO₄]₂ are low-spin and diamagnetic in the solid state and in solution, while [Fe(L²)₂][ClO₄]₂ and [Co(L³)₂][BF₄]₂ are fully high-spin. Treatment of L³ with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L⁴) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L⁵). While salts of [Fe(L⁵)₂]²⁺ are low-spin in the solid state, [Fe(L⁴)₂][ClO₄]₂·H₂O is high-spin, and [Fe(L⁴)₂][ClO₄]₂·3MeNO₂ exhibits a hysteretic spin transition to 50% completeness at T_1/2 = 128 K (ΔT_1/2 = 6 K). This transition proceeds via a symmetry-breaking phase transition to an unusual low-temperature phase containing three unique cation sites with high-spin, low-spin, and 1:1 mixed-spin populations. The unusual distribution of the spin states in the low-temperature phase reflects "spin-state frustration" of the mixed-spin cation site by an equal number of high-spin and low-spin nearest neighbors. Gas-phase density functional theory calculations reproduce the spin-state preferences of these and some related complexes. These highlight the interplay between the σ-basicity and π-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Brønsted basicities of L¹-L³ are very sensitive to the linkage isomerism of their triazolyl donors, which explains why their iron complex spin states show more variation than the better-known iron(II)/2,6-dipyrazolylpyridine system.

Elastic Modeling of Two-Step Transitions in Sterically Frustrated 1D Binuclear Spin-Crossover Chains

Among the large family of spin-crossover materials, binuclear systems play an important role due to their specific molecular configurations, allowing the presence of multi-step transitions and elastic frustration. Although this issue benefited from a significant number of spin-based theories, there is almost no elastic description of the spin transition phenomenon in binuclear systems. To overcome this deficiency, in this work we develop the first elastic modeling of thermal properties of binuclear spin-crossover solids. At this end, we investigated a finite spin-crossover open chain constituted of elastically coupled binuclear (A = B) blocks, ⋯A=B−A=B−A=B⋯, in which the considered equivalent A and B sites may occupy two configurations, namely low-spin (LS) and high-spin (HS) states. The sites of the binuclear unit interact via an intramolecular spring and couple to the neighboring binuclear units via other springs. The model also includes the change of length inside and between the binuclear units subsequent to the spin state changes. When injecting an elastic frustration inside the binuclear unit in the LS state, competing interactions between the intra- and the inter-binuclear couplings emerge. The latter shows that according to the intra- and inter-binuclear elastic constants and the strength of the frustration, multi-step transitions are derived, for which a specific self-organization of type (HS = HS)-(LS-LS)-(HS = HS)⋯ is revealed and discussed. Finally, we have also studied the relaxation of the metastable photoinduced HS states at low temperature, in which two relaxation regimes with transient self-organized states were identified when monitoring the elastic frustration rate or the ratio of intra- and intermolecular elastic interactions. These behaviors are reminiscent of the thermal dependence of the order parameters of the system. The present model opens several possibilities of extensions of elastic frustrations acting in polynuclear spin-crossover systems, which may lead to other types of spin-state self-organizations and relaxation dynamics.

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.

The dynamic interplay between intramolecular and intermolecular interactions in mononuclear manganese(III) SCO complexes

The effect of counter anions on thermally induced manganese(iii)-based SCO within the [Mn(5-F-sal-N-1,5,8,12)]Y family has been investigated. All the complexes are crystallized without any lattice solvents. Crystal packing and intermolecular forces influence the spin-state stabilization and spin-transition profiles. Magnetic measurements indicate that salts with octahedral anions, [Mn(5-F-sal-N-1,5,8,12)]PF6 (1), [Mn(5-F-sal-N-1,5,8,12)]AsF6 (2) and [Mn(5-F-sal-N-1,5,8,12)]SbF6 (3), show HS electronic configurations between 2 and 300 K, and there exist π-π stackings between the phenyl groups from the neighboring [Mn(5-F-sal-N-1,5,8,12)]+ cations. As for the tetrahedral anions, complex [Mn(5-F-sal-N-1,5,8,12)]BF4 (4) exhibits a gradual and incomplete spin conversion. Complex [Mn(5-F-sal-N-1,5,8,12)] ClO4 (5) shows a nearly complete SCO with T1/2 = 100 K. The remaining salts with spherical anions form Nam-HX (X = Cl, Br, I) hydrogen bonds between Mn(iii) cations and counterions. Complexes [Mn(5-F-sal-N-1,5,8,12)]Cl (6) and [Mn(5-F-sal-N-1,5,8,12)] Cl0.28Br0.72 (7) feature gradual SCO behaviors with T1/2 = 220 K and 235 K, respectively. Complex [Mn(5-F-sal-N-1,5,8,12)]I (8) exhibits a more gradual spin conversion and is far from a complete HS state with a χMT value of 1.89 cm3 mol-1 K at 400 K.

Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+ complexes: what for and who cares about it?

The [FeN₆] chromophores found in [Fe(didentate)₃]²⁺ complexes, where didentate is a non-symmetrical 2-(6-membered-heterocyclic ring)-benzimidazole ligand (Lk), exist as mixtures of two geometrical mer (C₁-symmetry) and fac (C₃-symmetry) isomers. Specific alkyl-substituted six-membered heterocyclic rings connected to the benzimidazole unit (pyridines in ligands L1-L3, pyrazines in L4-L5 and pyrimidines in L6-L7) control the ligand field strength and the electron delocalization so that [Fe^II(Lk)₃]²⁺ display tunable thermally-induced spin transitions in solution. Thermodynamic, spectroscopic (UV-Vis, NMR) and magnetic studies in solution demonstrate that [Fe(L6)₃]²⁺ (L6 = 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d]imidazole) exhibits a close to room temperature spin transition (T_1/2 = 273(3) K) combined with a high stability formation constant ( in acetonitrile), which makes this complex suitable for the potential modulation of lanthanide-based luminescence in polymetallic helicates. A novel method is proposed for assigning specific thermodynamic spin crossover parameters to fac-[Fe(L6)₃]²⁺ and mer-[Fe(L6)₃]²⁺ isomers in solution. The observed difference relies mainly on the entropic content ΔS-ΔS = 11(1) J mol^-1 K^-1, which favors the spin transition for the meridional isomer. Intermolecular interactions occurring in the crystalline state largely overcome minor thermodynamic trends operating in diluted solutions and a single configurational isomer is usually observed in the solid state. Among the thirteen solved crystal structures 1-13 containing the [M(Lk)₃]²⁺ cations (M = Fe, Ni, Zn, Lk = L6-L7), pure meridional isomers are observed six times, pure facial isomers also six times and a mixture (44% mer and 56% fac) is detected only once. Solid-state magnetic data recorded for the Fe^II complexes show the operation of slightly cooperative spin transitions in 7 (fac-[Fe(L6)₃]²⁺) and 12 (mer-[Fe(L7)₃]²⁺). For the meridional isomer in 6, a two-step spin state transition curve, associated with two phase transitions, is detected.

Direct Access for the Regio- and Stereoselective Synthesis of N-Alkenylpyrazoles and Chromenopyrazoles

A highly regio- and stereoselective method was developed for the preparation of N-alkenylpyrazoles and chromenopyrazoles by the reaction of N-tosylhydrazones and salicyl N-tosylhydrazones with alkynes under neat conditions in the presence of La(OTf)₃. The present study was found to be efficient and convenient for direct access to N-alkenylpyrazoles and chromenopyrazoles through C-C, C-N, and C-O bond forming reactions. Structure assignment of N-alkenylpyrazole compound 5c was confirmed by X-ray analysis.

Structural Transformations and Spin-Crossover in [FeL2 ]2+ Salts (L=4-{tert-Butylsulfanyl}-2,6-di{pyrazol-1-yl}pyridine): The Influence of Bulky Ligand Substituents

4-(tert-Butylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L) was obtained in low yield from a one-pot reaction of 2,4,6-trifluoropyridine with 2-methylpropane-2-thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [FeL₂ ][BF₄ ]₂ ⋅solv (1[BF₄ ]₂ ⋅solv) and [FeL₂ ][ClO₄ ]₂ ⋅solv (1[ClO₄ ]₂ ⋅solv; solv=MeNO₂ , MeCN or Me₂ CO) exhibit a variety of structures and spin-state behaviors including thermal spin-crossover (SCO). Solvent loss on heating 1[BF₄ ]₂ ⋅x MeNO₂ (x≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a "reverse-SCO" step in its warming cycle at a scan rate of 5 K min^-1 . The reverse-SCO is not observed in a slower 1 K min^-1 measurement, however, confirming its kinetic nature. The final product [FeL₂ ][BF₄ ]₂ ⋅0.75 MeNO₂ was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF₄ ]₂ ⋅y Me₂ CO (y≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low-spin 1[ClO₄ ]₂ ⋅0.5 Me₂ CO is not isostructural with its BF₄ ^- congener, and undergoes single-crystal-to-single-crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P 1 ‾ space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low-temperature phases when the materials are fully low-spin.

Spin crossover phenomena in long chain alkylated complexes

This review presents a discussion of soft metal complexes with a focus on spin crossover behaviours that are associated with structural phase transition, including liquid crystal LC transition.

Hexakis-adducts of [60]fullerene as molecular scaffolds of polynuclear spin-crossover molecules

A family of hexakis-substituted [60]fullerene adducts endowed with the well-known tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand for spin-crossover (SCO) systems has been designed and synthesized. It has been experimentally and theoretically demonstrated that these molecular scaffolds are able to form polynuclear SCO complexes in solution. UV-vis and fluorescence spectroscopy studies have allowed monitoring of the formation of up to six Fe(ii)–bpp SCO complexes. In addition, DFT calculations have been performed to model the different complexation environments and simulate their electronic properties. The complexes retain SCO properties in the solid state exhibiting both thermal- and photoinduced spin transitions, as confirmed by temperature-dependent magnetic susceptibility and Raman spectroscopy measurements. The synthesis of these complexes demonstrates that [60]fullerene hexakis-adducts are excellent and versatile platforms to develop polynuclear SCO systems in which a fullerene core is surrounded by a SCO molecular shell.

Polynuclear spin-crossover molecules showing both thermal and photoinduced spin transitions have been prepared using a [60]fullerene hexakis-adduct endowed with Fe(ii) complexes of tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand.

Allosteric Spin Crossover Induced by Ligand-Based Molecular Alloying

The spin crossover (SCO) phenomenon represents a source of multistability at the molecular level, and dilution into a nonactive host was originally key to understand its cooperative nature and the parameters governing it in the solid state. Here, we devise a molecular alloying approach in which all components are SCO-active, but with significantly different characteristic temperatures. Thus, the molecular material [Fe(Mebpp)₂](ClO₄)₂ (2) has been doped with increasing amounts of the ligand Me2bpp (Mebpp and Me2bpp = methyl- and bis-methyl-substituted bis-pyrazolylpyridine ligands), yielding molecular alloys with the formula [Fe(Mebpp)_2-2x(Me2bpp)_2x](ClO₄)₂ (4x; 0.05 < x < 0.5). The effect of the composition on the SCO process is studied through single-crystal X-ray diffraction (SCXRD), magnetometry, and differential scanning calorimetry (DSC). While the attenuation of intermolecular interactions is shown to have a strong effect on the SCO cooperativity, the spin conversion was found to occur at intermediate temperatures and in one sole step for all components of the alloys, thus unveiling an unprecedented allosteric SCO process. This effect provides in turn a means of tuning the SCO temperature within a range of 42 K.

Recent progress in chemosensors based on pyrazole derivatives

Colorimetric and fluorescent probes based on small organic molecules have become important tools in modern biology because they provide dynamic information concerning the localization and quantity of the molecules and ions of interest without the need for genetic engineering of the sample. In the past five years, these probes for ions and molecules have attracted great attention because of their biological, environmental and industrial significance combined with the simplicity and high sensitivity of absorption and fluorescence techniques. Moreover, pyrazole derivatives display a number of remarkable photophysical properties and wide synthetic versatility superior to those of other broadly used scaffolds. This review provides an overview of the recent (2016-2020) findings on chemosensors containing pyrazole derivatives (pyrazoles, pyrazolines and fused pyrazoles). The discussion focuses on the design and physicochemical properties of chemosensors in order to realize their full potential for practical applications in environmental and biological monitoring (sensing of metal ions, anions, explosives, and biomolecules). We also present our conclusions and outlook for the future.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors.

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.

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4-L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C₃-symmetrical dinuclear triple-stranded helical [LnM(Lk)₃]⁵⁺ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)₃]⁵⁺ (strictly high-spin), [LnFe(L5)₃]⁵⁺ (spin-crossover (SCO) around room temperature), and [LnFe(L6)₃]⁵⁺ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)₃]⁵⁺ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

A fluorescent Cu(ii) complex as a dual functional sensor for selective and sensitive detection of acetone and Cd(ii) pollutants

In this study, Cu²⁺ perchlorate complexes of 3,5-dimetylpyrazole(dmpy), namely [Cu(dmpy)₃(H₂O)₂](H₂O)(ClO₄)₂ (1), were prepared and characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared (IR) spectroscopy, elemental analysis, mass spectrometry, and ultraviolet visible (UV-Vis) spectroscopy. It was determined that Cu²⁺ coordinated to the nitrogen atoms of three dmpy molecules as well as to the oxygen atoms of two H₂O molecules in a square pyramidal geometry. To balance the charge, two uncoordinated perchlorate anions and one H₂O molecule were also present in the lattice. The resulting Cu²⁺ complex was involved in extensive hydrogen bonding, which stabilized the structure. The fluorescence properties of complex 1 were studied in acetonitrile at room temperature upon the addition of different organic solvents. It was confirmed that the complex exhibited remarkable acetone selectivity via a fluorescence quenching mechanism. At low concentrations, the fluorescence intensities were nearly completely quenched via a turn-off mode. Moreover, complex 1 displayed exceptional selectivity, fast detection time, and high sensitivity for Cd²⁺ in aqueous solutions via a fluorescence enhancement mechanism (turn-on mode). Hence, the prepared complex showed potential for application in Cd²⁺ detection. Notably, in an aqueous solution, complex 1 exhibited highly selective fluorescence enhancement effects (turn-on mode) with respect to Cd²⁺ and was not affected by the interference of other metal ions.

The prepared complex 1 exhibited highly sensitive and selective fluorescence quenching (turn-off) effect in the presence of acetone as well as fluorescence enhancement (turn-on) effect toward Cd²⁺ ions in aqueous solutions.

Heteroleptic iron(ii) complexes with naphthoquinone-type ligands

A series of heteroleptic mononuclear iron(ii) complexes with naphthoquinone-type ligands were synthesized, and their structures, magnetic behavior and spin states were investigated.