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Spin Crossover in New Iron(II) Coordination Compounds with Tris(pyrazol-1-yl)Methane

Olga G. Shakirova
1,2,* and
Ludmila G. Lavrenova
Department of Chemistry and Chemical Technologies, Faculty of Machinery and Chemical Technologies, Federal State Budget Institution of Higher Education, Komsomolsk-na-Amure State University, Lenin Ave. 27, 681013 Komsomolsk-on-Amur, Russia
Department of Chemistry of Coordination, Cluster and Supramolecular Compounds, Laboratory of Synthesis of Complex Compounds, Federal State Budget Institution for Science, Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Acad. Lavrentyev Ave. 3, 630090 Novosibirsk, Russia
Department of Analytical Chemistry, Faculty of Natural Sciences, Novosibirsk State University, Pirogov Str. 2, 630090 Novosibirsk, Russia
Author to whom correspondence should be addressed.
Crystals 2020, 10(9), 843;
Submission received: 27 August 2020 / Revised: 17 September 2020 / Accepted: 19 September 2020 / Published: 22 September 2020
(This article belongs to the Special Issue Coordination Polymers)


We review here new advances in the synthesis and investigation of iron(II) coordination compounds with tris(pyrazol-1-yl)methane and its derivatives as ligands. The complexes demonstrate thermally induced spin crossover accompanied by thermochromism. Factors that influence the nature and temperature of the spin crossover are discussed.

Graphical Abstract

1. Introduction

Spin crossover (SCO) is the change in spin multiplicity under the influence of external conditions, representing one of the most interesting phenomena in the field of coordination chemistry of Cr2+, Mn2+/3+, Fe2+/3+, and Co2+/3+ [1,2,3]. The most common and widely used external factor that causes a spin crossover is temperature (referred to as TIESST—thermally induced excited spin state trapping). However, there are examples of other external stimuli capable of switching the spin state of SCO-active complexes, such as pressure [4], an external magnetic field [5], light of a certain wavelength (LIESST—light-induced excited spin state trapping) [6], and light-controlled ligand isomerization (LDLISC—ligand-driven light-induced spin change) [7] and solvation/desolvation [8].
The interest of our investigation predominantly focuses on six-coordinate iron(II) complexes with the following change of electron configurations and ligand field states: A 1   1 ( t 2 g 6 e g 0 = l o w   s p i n ,   L S ) T 2   5 ( t 2 g 4 e g 2 = h i g h   s p i n ,   H S ) . The most widely studied systems in which the SCO phenomenon is observed are hexacoordinated iron (II) complexes with the coordination core FeN6 [9,10,11,12]. In many of these complexes, the six donor nitrogen atoms surrounding the iron ion belong to different heterocycles, imines, and/or amines. Nevertheless, it was shown that other donor atoms can also be used to obtain compounds with SCO (moreover, not only octahedral, but also other coordination numbers) [13]. Six donor atoms can belong to both six molecules (monodentate ligands), or three (bidentate ligands), two (tridentate/tripodal), and even one (hexadentate ligand). Note that SCO is not always (and not only) realized in complexes with the FeN6 node. The current level of development of the synthetic direction of coordination chemistry convincingly proves the importance of many factors but does not provide a 100% guarantee of the successful production of a material with the desired properties. Therefore, the authors of research works pay great attention to the methods and conditions of synthesis, isolation, storage, and identification. The chemical, physical, and in particular, magnetic properties of iron (II) complexes are different and unique.
Minor tweaks in spin state energetics between different compounds are measurable via the SCO equilibrium midpoint temperature (T½). Another SCO temperature (Tc) is obtained as the top of the derivative of magnetic susceptibility with respect to temperature ∂χ/∂T. The SCO temperature during heating (Tc ↑) and cooling (Tc↓) can differ significantly, their difference is called the width of the hysteresis loop: ΔTc = Tc↑ − Tc↓.
The synthesis of new thermosensitive magnetically active compounds—affordable, environmentally friendly, changing the spin state at different temperatures—is an important task for a chemist. Moreover, thermally induced SCO is provided by a change in the magnetic moment and color (oftentimes, but not always) [14]. These properties determine whether these compounds can be used as materials for information recording and storage systems, displays [15,16], electroluminescent devices [17], and temperature-sensitive contrast agents in magnetic resonance imaging [18]. These compounds could be used as thermochromic indicators and labels in engineering processes and in the security industry [3,19].
This review is a logical extension of our paper [20] and is devoted to the latest studies of spin crossover in Fe(II) complexes with tris(pyrazol-1-yl)methane (HC(pz)3) and its derivatives (Figure 1). We will give a comparative characteristic with bis(B-scorpionate) iron(II) complexes, but we will not touch on the third–fifth generation of scorpionates, which are well described in [21,22]. The steric aspects of ligand design for the control of the metal ion spin state are clear enough. Bulky substituents in the vicinity of the ligand donor atoms that disfavor contraction of the metal–ligand bonds will stabilize the HS state of a complex. A steric bulk at the periphery of the ligand sphere can promote the LS state if it inhibits the expansion of the metal coordination sphere, or the HS state if it leads to distortion of the ligand–metal dative interaction. These effects can be reliably understood or predicted in single compounds using molecular modeling [23].

2. Discussion

Tris(pyrazol-1-yl)methane (HC(pz)3), tris(pyrazol-1-yl)borate (HB(pz)3), and their derivatives are isotypic ligands that show promising prospects for the SCO. The most important feature of these compounds is the presence in the three pyrazole cycles of N atoms capable of coordination to a metal leading to the formation of an octahedral complex with the coordination core FeN6 in the case of the coordination of two ligands. For iron(II) compounds, this means that there is the possibility of SCO. It was shown that these ligands are coordinated to the metal mainly in a tridentate mode (symmetry group C3) [24,25,26]. In the case of (HB(pz)3), a neutral complex is formed, [Fe(HB(pz)3)2] [27]. Any modification of the ligand leads to a change in the inner sphere of the complex and its properties [28,29]. In the case of (HC(pz)3), cationic complexes are formed, [Fe(HC(pz)3)2]2+. Hence, the researcher can modify the ligand and replace the anion to study the effect on the properties of not only the internal sphere of the complex, but also the external one. In addition, for both types of complexes, the influence of the crystallization solvent must be taken into account [30,31,32,33].
The synthesis of several iron(II) compounds with unsubstituted (HB(pz)3) and HC(pz)3, such as [Fe(HB(pz)3)2] and [Fe(HC(pz)3)2]A2 (A = Br, ClO4, BF4), was described lightly in the last century; specifically, [Fe(HB(pz)3)2], which is low-spin up to 280 K and then a gradual change to 4.91 μB at 461 K, typical of the high-spin state; it also undergoes a spin state crossover to the high-spin state upon heating to 420 K and at high pressure [27]. Similarly, variable-temperature magnetic susceptibility measurement in the solid state of the complexes [Fe(HC(pz)3)2](ClO4)2 and [Fe(HC(pz)3)2](BF4)2 suggest the existence of single-quintet spin crossover with the single isomer largely favored at room temperature, a spin crossover to the high-spin state undergoes upon heating [34,35,36], while the complex [Fe(HC(pz)3)2]Br2 was recognized as diamagnetic [25], which was questioned by us.
In the paper [37], the crystal structures of α and β polymorphs of [Fe(HC(pz)3)2](NO3)2 were reported. The density of the β form is considerably greater than that of the α. Some considerations are changes in the angular parameters N–Fe–N, and associated variations in the angles exocyclic to the various five- and six-membered rings. Unfortunately, the authors were not able to obtain enough of the α isomer to measure its magnetic properties for comparison. The β polymorph showed a slight temperature dependence of μeff over the range 4–350 K, with μeff being essentially constant at 1.05 μB between 50 and 270 K and then increasing gradually to 1.26 μB at 350 K. We continued their work and showed with the help of DSC (Figure 2) that the main changes are observed at higher temperatures: Tonset↑ = 427.9 K, Tonset↓ = 415.3 K, ∆Htr = 13.9 ± 0.3 kJ/mol, ∆Str = ∆H/T1/2 = 32.3 ± 0.8 J.mol−1.K−1.
The spin crossover observed upon cooling and at high pressure in the iron(II) complexes formed with various methyl derivatives of mentioned ligands span a variety of different behaviors. [Fe(HB(3,5-(CH3)2pz)3)2], which is high-spin at 295 K, undergoes a spin state crossover to the low-spin state both upon cooling below 195 K and at high pressure. Tris(3,5-dimethylpyrazole-1-yl)methanide (C(3,5-(CH3)2pz)3) obtained by deprotonation of tris(3,5-dimethylpyrazole-1-yl)methane was also used as a ligand for iron (II). However, for the [Fe(C(3,5-(CH3)2pz)3)2] complex, there is a sharp SCO with a small hysteresis with T1/2 = 267 K [38]. Exposed carbanion centers that are located outwards are available for further reactions.
In contrast, [Fe(HB(3,4,5-(CH3)3pz)3)2] remains high-spin between 1.9 and 295 K but is gradually converted to the low-spin state with increasing pressure. In a parallel fashion, [Fe(HC(3,5-(CH3)2pz)3)2]I2 is high-spin at 295 K. The sharp and complete transitions with a hysteresis in the μeff(Т) curves are observed in the single-crystal phase of [Fe(HC(3,5-(CH3)2pz)3)2]I2 [35,39] and in the polycrystalline phase of [Fe(HC(3,5-(CH3)2pz)3)2](NO3)2 [40]. In [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2, which is high-spin at 295 K, a phase transition exhibits upon cooling below 206 K in which only one-half of the iron(II) is converted to the low-spin state; the remaining one-half of the iron(II) remains high-spin upon cooling to 4.2 K [41]. The sharp but incomplete transition is observed also in complex [Fe(HC(3,5-(CH3)2pz)3)2](ClO4)2 [42]. In [Fe(HC(3,5-(CH3)2pz)3)2]I2 and [Fe(HC(3,5-(CH3)2pz)3)2](ClO4)2, spin crossovers are observed only in single crystals, while the polycrystalline phases exhibit no spin crossover. Note that two polymorphous modifications were observed for single crystals in the latter case. The spin crossover is observed in one modification isostructural to [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2, while no spin crossover is observed in the second modification [42].
By now, we synthesized the complexes [Fe(HC(pz)3)2]A2·nH2O, here A= Cl, Вr, I, NO3, CF3SO3, C10H7SO3, C12H25SO4, 7,8-С2B9H12, С2B9H8Br4, С2B9H8I4, C2N3, C8H5O4, ReO4, [B(C6H5)4] , [Ni(dmit)2], TCNQ; [Fe(HC(pz)3)2]A·nH2O, here A= SO42−, SiF62−, [Mo6Br14]2−, [Mo6Cl14]2−, [W6I14]2−, [W6Br14]2−, [W6Cl14]2−, B10H102−, B10Cl102−, B12H122−, [Eu(dipic)2(Hdipic)]2−, [Fe(NO)(CN)5]2−; n = 0–7; and [Fe(HC(pz)3)2]2[Re6S8(CN)6]·2H2O [43,44,45,46,47,48,49]. Complexes were obtained by an exchange reaction between water–alcohol solutions of FeSO4 (С ≈ 0.05 mol/L) and HC(pz)3 with an aqueous solution of salt of the respective anion with a different ratio of iron:ligand:anion, which is usually 1:2-6:3-6. Most of the synthesized iron(II) complexes with HC(pz)3 are stable on storing in the air at room temperature for a long time and non-hygroscopic.
These compounds were studied by static magnetic susceptibility, diffuse reflectance, IR spectroscopy, and powder and single-crystal X-ray diffraction (XRD). We determined the molecular and crystal structure parameters for most of the isolated compounds. According to the XRD data, the complexes are mononuclear and have an octahedral structure of the coordination polyhedron, with FeN6 being the coordination core. The Fe–N bond lengths in the complex cations are in the range 1.96–1.98 Å, which corresponds to the LS form of an iron(II) complex. In the crystals, the complex cations exhibit small distortion of the octahedral polyhedron. The values of the deviation of the coordination polyhedron from the ideal octahedron are calculated and shown in the Table 1, the parameters F and Σ are introduced by the authors [50,51]. For example, Table 2 demonstrates changes in geometric characteristics of the iron(II) environment in complex [Fe(HC(pz)3)2]SiF6 at heating. From these data, you can see that the spin crossover is accompanied by a significant change in all crystallographic parameters: Δd(Fe–N) = 0.1391 Å (7.0%); Δa = 0.0536 Å (0.4%); Δc = 0.3639 Å (2.7%); ΔV = 69.8 Å3 (3.6%); Δρ = −0.06938 g/cm3 (3.4%).
In cases where good single crystals were not obtained for X-ray diffraction analysis, the geometry of high- and low-spin complexes with various anions of the outer sphere was studied by EXAFS spectroscopy, which is suitable for studying the geometry of the local environment of atoms and ions both in crystalline solids and in systems without long-range ordering. Experimental EXAFS spectra [46] show the first maximum corresponding to the electron wave scattering on nitrogen atoms of the first coordination sphere iron ion, and the second maximum, which corresponds to scattering on second-order nitrogen atoms coordination sphere. Upon the transition of the complexes from the LS to the HS state, the Fourier function maxima are shifted toward larger scattering, and their intensities become considerably weaker. The Fourier functions for the samples before and after heating are almost identical, which points to the reversibility of SCO. The interatomic distances obtained by simulation of EXAFS spectra are summarized in Table 3.
The IR spectra analysis of these compounds in the range of metal–ligand stretching vibrations (200–400 cm−1) and the comparison with the literature data [52] show that at room temperature some complexes contain iron(II) not only in LS (FeLS) but also HS (FeHS) states. Composite bands within the range 348–382 cm−1 can be assigned to stretching ν(FeНС–N) vibrations, and the bands at 265–298 cm−1 can be assigned to ν(FeВС–N).
IR spectroscopy data easily reveal that the composition of complexes contain crystal water molecules (the range 3400–3600 cm−1). An analysis of the position of absorption bands in the range 1400–1600 cm−1 is most informative for the conclusion about the ways of coordination of heterocyclic ligands. In the spectrum of HC(pz)3 in the range 1428–1566 cm−1, there are bands corresponding to stretching–bending vibrations of pyrazole rings, which are sensitive to coordination. In the spectra of complexes, these bands are observed in the range 1440–1515 cm−1. Hence, in comparison with the spectrum of HC(pz)3, they are noticeably shifted, which indicates the coordination of nitrogen atoms of the pyrazole ring to the iron(II) ion. Comparing the IR spectra of powder samples with the spectra of crystal complexes of the same composition, we can conclude that in all mentioned compounds HC(pz)3 is a tridentate cyclic ligand coordinated to iron(II) ions by N(2) nitrogen atoms of pyrazole rings.
In [53], the influence of temperature on the character of IR spectra was studied. When the samples are heated (Figure 3), there is a sharp increase in the band intensity at 850 cm−1 and a decrease in the band intensity at 865 cm−1. The intensity of the band increases at 1000 cm−1, the intensity of the bands at 1450 and 1410 cm−1 decreases, and the band 1515 cm−1 increases moderately. Almost all bands undergo a 5–10 cm−1 shift to the low frequency region when the samples are heated. All these trends are reproduced in B3LYP calculations for an isolated [Fe(HC(pz)3)2]2+ cation. (Some computational details for the B3LYP calculations are in the Supplementary Materials.)
The electronic spectra (diffuse reflectance spectra, DRS) of the transition in the complexes at room temperature exhibit a single band in the range 480–570 nm that can be attributed to the d–d transition 1A11T1 in a strong octahedral ligand field. The positions of these bands are consistent with our previous data for LS iron(II) complexes with 1,2,4-triazoles (Htrz), which have a FeN6 core and possess SCO [54].
Using the DRS data (ν, cm−1) obtained at room temperature, we estimated the strength of the ligand field (parameter ΔLS, cm−1) of tris(pyrazol-1-yl)methane in complexes. The following approximations valid for octahedral nitrogen-containing complexes of iron(II) were applied: ν ( A 1   1 T 1   1 )   =   Δ LS     C   +   86 · B 2 / Δ LS   ; with regard to that, Bfree ion = 917 cm−1 [55]; В ≈ (0.7 ÷ 0.8)Bfree ion ≈ 690 cm−1, and С = 4.41В ≈ 3040 cm−1 [56]. The obtained ν values are in the range 17,540–20,830 cm−1; the calculated ΔLS values changes within 18,500–22,000 cm−1. It is seen that the anion nature and size appreciably affect the ligand field strength. The condition 19,000 cm−1 ≤ ΔLS ≤ 23,000 сm−1 [55] that causes the appearance of SCO at heating, is fulfilled (it is satisfied within a 3% relative error).
In addition, the complexes with cluster anions that have a second broad absorption band are observed in the near IR range. Bands with the maxima at 835–860 nm can be assigned to the d–d-transition 5Т25Е in a weakly distorted octahedral ligand field. The position of these bands is typical of the spectra of HS octahedral complexes of iron(II) with nitrogen-containing ligands [20]. From these data, it is possible to easily calculate the values ν(5Т25Е) = ΔHS = 11,630–11,980 cm−1, where ΔHS is the splitting parameter in the crystalline field of HS Fe(II) complexes. The splitting parameters indicate that in these complexes SCO is theoretically possible at cooling because 11,000 cm−1 ≤ ΔHS ≤ 12,500 cm−1 [57].
Let us present our calculations showing that tris(pyrazol-1-yl)methanes are ligands of a stronger field compared to 1,2,4-triazoles (Table 4).
The magnetochemical investigation of the series of complexes demonstrated that they all exhibit reversible SCO 1A15T2, most often with small hysteresis on the μeff(T) curves (Figure 4).
In all cases, the transition is accompanied by a thermochromic effect. The complex [Fe(HC(pz)3)2)](CF3SO3)2 possessed a different behavior for three measurements, which were revolved. The first measurement demonstrated fairly complete SCO in the μeff(T) curve with a large hysteresis loop. The temperatures of the direct (Tc↑) and reverse (Tc↓) transitions are 375 and 300 K, respectively. In the second and third cycles of measurements, an abrupt transition was observed with Tc↑ and Tc↓ values of 310 and 302 K, respectively. It can be seen that the width of the hysteresis loop is greatly reduced. It is possible to assume that this effect is caused by reaching the equilibrium state in the crystals. This effect was previously found for complexes of iron(II) with 1,2,4-triazoles as ligands [58].
For a correct comparison of the magnetochemical data, complexes were analyzed after dehydration. Below, you can see a series of Tc↑ (K) dependences on the outer-sphere anion:
[W6Br14]2− (240*) < [W6I14]2− (248*) < [Mo6Br14]2− (270) < [Mo6Cl14]2− (271) < [W6Cl14]2− (287*) < SO3CF3 (310**) < SiF62− (335) < [Re6S8(CN)6]4− (383) < I (400) ≈ SO42− (400) ≈ [B(C6H5)4] (400) < ReO4 (403) ≈ PF6 (403) < Cl (410) < [Eu(dipic)2(Hdipic)]2− (415) < C12H25SO4 (420) ≈ NO3 (420) < C10H7SO3 (440) < Вr (445) ≈ TCNQ (445)
* for a two-stage transition, the value of the first stage is specified; ** for the second/third cycles of measurements.
This series shows no relationship between the anion size and the increase in Tc. In addition, it does not confirm the pattern found earlier for Fe(II) complexes with 1,2,4-triazoles of the composition FeL3A2, where А = Cl, Br, I, in which in the series of anions of one nature, Tc decreases with an increase in the size of the outer-sphere anion [59].
According to the data for boron-containing complexes, it is possible to derive a series of Tc↑ (K) dependences on the outer-sphere cluster anion:
7,8-С2B9H12 (370) < 1,5,6,10-Br4-7,8-С2B9H8 (380) < 1,5,6,10-I4-7,8-С2B9H8 (400);
B10H102− (375) < B12H122− (405).
The given series shows that with an increase in sizes of both closo-borate and carborane (due to the introduction of the substituent) Tc increases. The opposite trend was observed for SCO coordination polymers, namely, when the anion volume increases, the temperature decreases [60].
It must be noted that the data on the DRS wavelength maxima (λmax, nm) for anions (even triple row for anhydrous powdered samples 1,5,6,10-I4-7,8-С2B9H8 (505) < C12H25SO4 (519) < B10H102− (520)) allow us to make sure that there is no direct connection between Tc and λmax.
Our work [61] was targeted at preparing and characterizing iron(II) complexes with HC(3,5-(CH3)2pz)3 that contain two-charge cluster anions, decahydro-closo-decaborate (B10H10)2− and dodecahydro-closo-dodecaborate (B12H12)2−. μeff(T) studies show low-temperature incomplete spin crossover (only half of the iron(II) ions have experienced high-to-low spin transition) in [Fe(HC(3,5-(CH3)2pz)3)2]B10H10·H2O complex and antiferromagnetic exchange coupling among iron(II) unpaired electrons at temperatures below 90 K in [Fe(HC(3,5-(CH3)2pz)3)2]B12H12·H2O complex (Figure 5), in contrast to [Fe(HC(pz)3)2]B12H12 [46], which has complete and sharp SCO (Figure 4d). Hence, the introduction of alkyl substituents led not only to a decrease in the temperature of the spin transition. In some observed cases, the transition did not occur due to steric difficulties.
A comparison of the properties of the [Fe(HC(pz)3)2][Eu(dipic)2(Hdipic)] complex [62] with data for compounds of this class shows that the introduction of a complex anion of “gigantic” size into the crystal lattice did not lead to a significant change in the Tc and thermal stability of the compound, and there were no noticeable changes in the nature of the μeff(T) dependence curve in the SCO region (Figure 4c). A slight difference in the magnetic behavior of the complex is observed in the low temperature region, where exchange interactions between metal ions of an antiferromagnetic nature are manifested.
It is of a considerable interest to use the octahedral cation complexes of Fe(II), showing reversible SCO between HS and LS states of the metal ion, as a magnetic subsystem in combination with an anion-conducting subsystem. The conducting sublattice of such hybrid materials were represented by the radical anion subsystem based on [Ni(dmit)2]•δ− complexes (dmit = 4,5-dithiolato-1,3-dithiole-2-thione; 0 < δ < 1) and 7,7,8,8,-tetracyanoquinodimethane ((TCNQ)•δ−,0< δ < 1) in a fractional oxidation or reduction state, respectively (Figure 6). Note, the radical anion salt [Fe(HC(pz)3)2](TCNQ)3 demonstrates unusual for 7,7,8,8,-tetracyanoquinodimethane (TCNQ)-based salts quasi-two-dimensional conductivity, which is interpreted in terms of cross relaxation between the TCNQ and Fe(II) spin subsystems [63]. In the [Fe(HC(pz)3)2][Ni(dmit)2]2·2(CH3)2CO complex, radical anions can both dimerize and then break down again into monomers; they can become structurally nonequivalent; they can be shielded by solvent molecules, and they can create stacking layers for electron transport, similarly to [64]. In turn, cations can be in LS and/or HS form, affecting the electrical conductivity of the sample. Hence, the electrical conductivity of the crystals at 300 K is 2·10-6−1·cm−1 for [Fe(HC(pz)3)2][Ni(dmit)2]2·2(CH3)2CO and 1.5·10−2−1·cm−1 for [Fe(HC(pz)3)2](TCNQ)3, which is millions of times higher than typical values for the SCO-active complexes (for example, 2·10−14−1·cm−1 for Fe(Htrz)2(trz)BF4).
The mononuclear iron(II) complex [Fe(HC(pz)3)2](tcp)2 containing the mono-negative tetracyanopyrrolide (tcp) as a counter-anion was characterized by X-ray diffraction and investigated in the solid state by magnetic susceptibility measurements and differential scanning calorimetry [65]. Structural studies at 123 K reveal Fe–N distances typical for a low-spin iron(II) range between 1.964(4) and 1.979(4) Å. The complex undergoes a gradual, incomplete spin crossover in the examined temperature range 1.9–400 K. The maximal experimental effective moment μeff equals 3.29 μB at 400 K, which is close to T1/2. The authors note, the obtained results are closely related to the tetrafluoroborate salt, where the rapid increase of the effective moment starts at 320 K; the latter rises to 3.24 μB at 400 K and reaches full saturation of HS states at 470 K based on Mössbauer spectroscopy studies [36]. The compound shows a distinct thermochromism, associated with the spin transition. The color of the sample changes from pink at 77 K to dark red–violet at 425 K.
For some compounds, substantial residual paramagnetism is observed in the μeff(T) dependence curves at 200 K (see Figure 4f). We estimated the contribution of temperature-independent paramagnetism (TIP) by formulas: μeff(TIP) = (8χTIPT)1/2, χTIP = 4/ΔLS [66]. At Т = 200 K μeff(TIP) must not exceed 0.6 μB. For the calculation we assume that μeff(HS) ≈ 5.1 μB for the class of compounds studied [67]. Then the fractions of the HS form of complexes [Fe(HC(pz)3)2](1,5,6,10-Br4-7,8-С2B9H8)2 and [Fe(HC(pz)3)2](1,5,6,10-I4-7,8-С2B9H8)2 were calculated by the formula α(HS) =100·(μeff2 − μeff(TIP)2)/μeff(HS)2, are ~10% at 200 K; hence, each tenth of the iron(II) atoms does not pass into the LS state. A possible reason for the high value of α(HS) can be the existence of structurally non-equivalent forms of the complexes, one of which does not participate in the SCO process. For the complex [Fe(HC(pz)3)2](7,8-С2B9H12)2 (with the unsubstituted anion), the fraction of the residual HS form calculated similarly is below 0.5%.
Complexes of iron(II) thiocyanate with this class of ligands differ significantly their structures. The typical complex [Fe(HC(3,5-(CH3)2pz)3)2](NCS)2 [68] does not change its HS state with the temperature decreasing to 40 K; then the decreasing μeff indicates exchange interactions of the antiferromagnetic character. For complexes [Fe(HC(pz)3)2][Fe(HC(pz)3)(NCS)3]NCS·2H2O [67] and [Fe(HC(3,5-(CH3)2pz)3)2][Fe(HC(3,5-(CH3)2pz)3)(NCS)3]2 [68] (in both cation and complex anion), tris(pyrazol-1-yl)methanes coordinate via the tridentate cyclic mode. In addition, the anions have the octahedral coordination core FeN6 due to three N atoms of three isocyanate groups (Figure 7). The SCO is exhibited only by the complex with cation Fe[(HC(pz)3)2]2+ (Tc = 380 K) (Figure 8), whereas other particles remain in the HS state over the entire temperature range.
Mössbauer spectra of complexes are poorly resolved lines with parameters (chemical shift δ and quadruple splitting ε) shown in Table 5 and Figure 9. The δ values are in the region of shifts characteristic of LS Fe(II) complexes, lower than those in previously studied Fe(II) complexes with 1,2,4-triazoles (0.416—0.427) [69], which gives evidence that chemical Fe—N bonds in new complexes are more covalent. This fact, in turn, confirms the conclusion drawn in [20] that tris(pyrazol-1-yl)methane is a stronger field ligand in comparison with 1,2,4-triazole and 4-amino-1,2,4-triazole (NH2trz). This makes it possible to understand the main reason why SCO temperatures are higher in iron(II) complexes with HC(pz)3 than those in the complexes with 1,2,4-triazoles [70]. At the same time, there is no direct proportionality between Тс and ΔLS. Small quadruple splittings indicate a weak distortion of the octahedral environment of iron atoms in it (as in Fe(II) complexes with 1,2,4-triazoles).
The measurements at 78 K show that the probability of the Mössbauer effect (f′) in them approximately three times decreases with an increase in the temperature to 295 K, which is appreciably stronger than that for Fe(II) complexes with 1,2,4-triazoles for which f′ lowers no more than 1.7 times. This fact can be related to the island structure of complexes and a weaker cation–anion interaction in comparison with the Fe(II) complexes with 1,2,4-triazoles having a polymeric chain structure. The consequence of this is the weakening of cooperative interactions in the lattices of such complexes, which decreases the 1А15Т2 SCO abruptness.
Then we reported the synthesis and investigations of the complexes of Fe[(HC(pz)3)2]2+ with functionalized calix[4]arenes [44,71] These complexes were obtained from solution by an exchange reaction between aqueous solutions of the complex Fe[{HC(pz)3}2]SiF6 (or sulfate, or nitrate) and the sodium salt of a p-sulfonatothiacalix[4]arene (TC[4]AS4−), p-sulfonatocalix[4]arene (C[4]AS4−/5−). Note that these were the first examples in which macrocyclic anions were used in the synthesis of compounds with SCO. For [Fe(HC(pz)3)2]2(TC[4]AS), it was demonstrated by 1H NMR spectroscopy that the Fe[(HC(pz)3)2]2+ cation is partially immersed in the cyclophane cavity of TCAS4−. This complex exhibits high temperature spin crossover, Tc = 450 K, without hysteresis in the μeff(T) dependence, which is accompanied by the thermochromism. The hydrated structures [Fe(HC(pz)3)2]5(C[4]AS)2.nH2O (n = 16, 62) and [Fe(HC(pz)3)2]2(C[4]AS).24H2O have been analyzed by X-ray diffraction (XRD) and revealed the outer sphere inclusion of [Fe(HC(pz)3)2]2+ into the cyclophane cavity in each case (Figure 10). The magnetochemical investigation of μeff(Т) within the temperature range of 78–500 K has shown that the complex [Fe(HC(pz)3)2]5(C[4]AS)2 (i.e., dehydrated) exhibits a high-temperature 1А15Т2 spin crossover (SCO), which is accompanied by thermochromism. The SCO temperature values Tc↑ and Tc↓ are equal to 390 K and 380 K, respectively (Figure 11). The formation of inclusion complex of iron(II) tris(pyrazol-1-yl)methane complex with p-sulfonatocalix[4]arene leads to thermal stabilization and decreasing of SCO temperature. That allowed us to reach more preferred SCO characteristics (completeness, sharpness, room temperature range) for the compound, which can be used as a promising material for molecular sensors.
The heteroligand iron(II) complexes, which contain in the internal sphere other nitrogen-containing heterocycles (pyridine (Py), 4,4′-bipyridyl (Bipy), imidazole (im), NCS, NCSe [72], tetrazole (Tz), pyrazole (Pz), 3,5-dimethylpyrazole (3,5-(CH3)2pz), 3-amino-4-ethoxycarbonylpyrazole (Ametcpz), and phthalazine (Phz) [73]), along with tris(pyrazol-1-yl) methanes, were synthesized and studied. Specific features of the SCO and their dependence on the ligand complexes are discussed. The study of the temperature dependence μeff(T) within the temperature range 2–350 K showed the gradual and incomplete spin crossover 1A15T2 accompanied by thermochromism (color change “pink ↔ colorless”) in complexes [Fe(HC(3,5-(CH3)2pz)3)(Tz)3](NO3)2, [Fe(HC(3,5-(CH3)2pz)3)(Tz)3](ClO4)2, and [Fe(HC(3,5-(CH3)2pz)3)(Aecpz)(NCS)2] only. The compounds with py, bipy, im, NCS, and NCSe are all high-spin between 5 and 300 K, despite mostly having regular coordination geometries and tripodal ligand conformations that should favor spin-crossover. Doubtlessly, the ligand field is too weak to make the low-spin forms of the compounds accessible.

3. Conclusions

An analysis of the data, being the subject of this review, demonstrates that, there is an essentially marked temperature dependence of the SCO (including the abruptness of the transition, the presence or absence of hysteresis) on the ligand structure and its composition, the nature of the outer-sphere anion, and also on the presence of solvent molecules. This pronounced dependence of Tc can be explained by the ability of anions and the lattice solvent molecules to form a hydrogen network with the coordinated ligands. It is typical both for polynuclear complexes with 1,2,4-triazoles and island complexes with B/C-scorpionates. Effective intermolecular interactions can be created also through π stacking. The effect of an outer-sphere anion is also manifested by the creation of an additional electrostatic field by the anion (ionic atmosphere), which results in additional splitting of the terms of the HS and LS electronic configurations of the central atom and causes SCO. Hence, the prediction of Tc on the nature of the ligand and the anion can be used with a number of restrictions because the SCO-phenomenon depends on several factors, which are working synchronously.
For complexes with tris(pyrazol-1-yl)methane, tris(pyrazol-1-yl)borate, and its derivatives, one can observe either abrupt SCO or gradual transitions, sometimes with small hysteresis. At the same time, the Fe(II) complexes with tris(pyrazol-1-yl)methane have remarkably higher values of Tc than the complexes with 1,2,4-triazoles and B-scorpionates. The substituent introduction in the ligand composition reduces the field strength and Tc of complexes. The introduction of an amino group affects Tc to a much lesser extent than the introduction of alkyl substituents. Thus, the strength of the ligand field is the main determining factor for the spin-crossover temperature, but the nature of the transition (abruptness and hysteresis) is most significantly affected by the structure of the complex. It is necessary to remember that SCO is not only a two-electron but also a phase transition, the main driving factor of which is a change in entropy. According to the calculations performed, the oscillatory component makes the most significant contribution to the entropy change at SCO.
The spin-crossover for complexes with Rtrz is sharper in comparison with complexes with HC(pz)3. In most cases, there is a hysteresis on the curves of the dependence μeff(T). This is due to a high degree of cooperative effect in complexes with Rtrz, which have a polynuclear structure. In some cases, in the low-temperature region, the μeff(T) curve during cooling passes significantly higher than the curve during heating, which is explained by kinetic difficulties in the process of transition HS→LS in the low-temperature region. It is interesting to note that there is no direct relationship between the sharpness of the transition and its Tc; moreover, the Tc can both increase and decrease during complexes dehydration.
The analysis of the results accumulated for Fe(II) complexes with B- and C-scorpionates shows that a promising class of compounds with SCO systems, despite the significant array of data obtained, deserves further research.

Supplementary Materials

The following are available online at, Table S1: Scaling factors for the force fields of the compounds studied.

Author Contributions

Writing review and editing, O.G.S.; supervision, L.G.L. All authors have read and agreed to the published version of the manuscript.


This research was funded by RUSSIAN SCIENCE FOUNDATION, grant number 20-63-46026.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Schematic structure of C-scorpionate tris(pyrazol-1-yl)methanes RC(R′-pz)3 (pz = pyrazolyl; R = H or substituent at the methine carbon; R′ = H or any substituent at the pz ring) and comparison with a scorpion; and the structure of the complex cation [Fe(HC(pz)3}2]2+.
Figure 1. Schematic structure of C-scorpionate tris(pyrazol-1-yl)methanes RC(R′-pz)3 (pz = pyrazolyl; R = H or substituent at the methine carbon; R′ = H or any substituent at the pz ring) and comparison with a scorpion; and the structure of the complex cation [Fe(HC(pz)3}2]2+.
Crystals 10 00843 g001
Figure 2. Termal analysis (DSC) of [Fe(HC(pz)3)2](NO3)2.
Figure 2. Termal analysis (DSC) of [Fe(HC(pz)3)2](NO3)2.
Crystals 10 00843 g002
Figure 3. Influence of the metal spin state on the IR spectra of Fe(II) complexes with tris(pyrazol-1-yl)methane: (a)—B3LYP [Fe(HC(pz)3)2]2+; (b)—[Fe(HC(pz)3)2]I2; (c)—[Fe(HC(pz)3)2]SiF6; (d)—[Fe(HC(pz)3)2](ClO4)2; the blue line indicates LS state; the red line shows HS state.
Figure 3. Influence of the metal spin state on the IR spectra of Fe(II) complexes with tris(pyrazol-1-yl)methane: (a)—B3LYP [Fe(HC(pz)3)2]2+; (b)—[Fe(HC(pz)3)2]I2; (c)—[Fe(HC(pz)3)2]SiF6; (d)—[Fe(HC(pz)3)2](ClO4)2; the blue line indicates LS state; the red line shows HS state.
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Figure 4. An example of μeff(T) dependences: (a)—spin crossover (SCO) with hysteresis; (b)—SCO without hysteresis; (c)—incomplete SCO with exchange interactions between metal ions of an antiferromagnetic nature; (d)—complete and sharp SCO; (e)—two stage SCO; (f)—incomplete and continuous SCO.
Figure 4. An example of μeff(T) dependences: (a)—spin crossover (SCO) with hysteresis; (b)—SCO without hysteresis; (c)—incomplete SCO with exchange interactions between metal ions of an antiferromagnetic nature; (d)—complete and sharp SCO; (e)—two stage SCO; (f)—incomplete and continuous SCO.
Crystals 10 00843 g004aCrystals 10 00843 g004b
Figure 5. μeff(T) plot: (a)—incomplete SCO with exchange interactions between metal ions of an antiferromagnetic nature; (b)—only exchange interactions between metal ions of an antiferromagnetic nature.
Figure 5. μeff(T) plot: (a)—incomplete SCO with exchange interactions between metal ions of an antiferromagnetic nature; (b)—only exchange interactions between metal ions of an antiferromagnetic nature.
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Figure 6. Fragment of the crystal packing with showing alternate cationic and anionic layers: (a)—Coordination interactions Ni⋯S in the anionic dimers and interlayer C−H⋯S contacts (2.81–2.96 Å) between cations and anions; (b)—Stacking interactions in the anionic columns and interlayer C−H⋯N contacts (2.41–2.70 Å) between cations and anions (conducting TCNQ plane (a,b)).
Figure 6. Fragment of the crystal packing with showing alternate cationic and anionic layers: (a)—Coordination interactions Ni⋯S in the anionic dimers and interlayer C−H⋯S contacts (2.81–2.96 Å) between cations and anions; (b)—Stacking interactions in the anionic columns and interlayer C−H⋯N contacts (2.41–2.70 Å) between cations and anions (conducting TCNQ plane (a,b)).
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Figure 7. Unit of [Fe(HC(pz)3)2][Fe(HC(pz)3)(NCS)3]NCS·2H2O.
Figure 7. Unit of [Fe(HC(pz)3)2][Fe(HC(pz)3)(NCS)3]NCS·2H2O.
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Figure 8. μeff(Т) curve of [Fe(HC(pz)3)2][Fe(HC(pz)3)(NCS)3]NCS.
Figure 8. μeff(Т) curve of [Fe(HC(pz)3)2][Fe(HC(pz)3)(NCS)3]NCS.
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Figure 9. Mössbauer spectra of [Fe(HC(pz)3)2]SiF6 at 295 K (left) and at 473 K (right).
Figure 9. Mössbauer spectra of [Fe(HC(pz)3)2]SiF6 at 295 K (left) and at 473 K (right).
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Figure 10. Fragments of crystal structures illustrated the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of p-sulfonatocalix[4]arene (crystallization water is not shown for clarity).
Figure 10. Fragments of crystal structures illustrated the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of p-sulfonatocalix[4]arene (crystallization water is not shown for clarity).
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Figure 11. The μeff(T) plots for [Fe(HC(pz)3)2]5(C[4]AS)2, [Fe(HC(pz)3)2]2(TC[4]AS), and their comparison with [Fe(HC(pz)3)2](ns)2 (where ns = 2-naphthalenesulfonate C10H7SO3).
Figure 11. The μeff(T) plots for [Fe(HC(pz)3)2]5(C[4]AS)2, [Fe(HC(pz)3)2]2(TC[4]AS), and their comparison with [Fe(HC(pz)3)2](ns)2 (where ns = 2-naphthalenesulfonate C10H7SO3).
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Table 1. Basic geometric characteristics of the iron(II) environment in complexes at T = 150 K.
Table 1. Basic geometric characteristics of the iron(II) environment in complexes at T = 150 K.
Complexd(Fe–N)average Σ = i = 1 12 | α i c i s 90 | ,   ° F = 1 24 i = 1 24 | α i t r i 60 | ,   °
Table 2. Basic geometric characteristics of the iron(II) environment in complex [Fe(HC(pz)3)2]SiF6.
Table 2. Basic geometric characteristics of the iron(II) environment in complex [Fe(HC(pz)3)2]SiF6.
Т, K300400
Colour of crystal Crystals 10 00843 i001 Crystals 10 00843 i002
d(Fe–N), Å1.98252.1216
F, °1.9683.865
Σ, °31.59663.912
Spin stateLSHS
a = b, Å12.9805(5)13.0341(16)
c, Å13.4503(4)13.8143(14)
V, Å3V = 1962.66(12)2032.46(41)
ρ, g/cm32.012691.94358
Table 3. Modeling of experimental Fe K-edge EXAFS spectra. Fe–N1 and Fe–N2 are average distances to the nitrogen atoms in the environment of the iron atom.
Table 3. Modeling of experimental Fe K-edge EXAFS spectra. Fe–N1 and Fe–N2 are average distances to the nitrogen atoms in the environment of the iron atom.
Complex300 K (LS)
before Heating
475 K (HS)300 K (LS)
after Cooling
R, Å ± 0.01
R, Å ± 0.01
R, Å ± 0.01
R, Å ± 0.01
R, Å ± 0.01
R, Å ± 0.01
Table 4. Splitting parameters for HS- and LS-complexes of iron(II) with 1,2,4-triazoles and tris(pyrazol-1-yl)methanes.
Table 4. Splitting parameters for HS- and LS-complexes of iron(II) with 1,2,4-triazoles and tris(pyrazol-1-yl)methanes.
ComplexSpin StateΔE, cm−1В, cm−1C, cm−1 10 D q H S , cm−1 10 D q L S ,   cm 1
(21,120–21,230 [57])
(20,450 [57])
[Fe(HC(pz)3)2]A2HS18,690–20,8306903040 19,650–22,010
Table 5. Basic Mössbauer parameters of the iron(II) complexes.
Table 5. Basic Mössbauer parameters of the iron(II) complexes.
ComplexТ, Kδ, mm/secε, mm/sec
for comparison
δ ±0.003±0.01

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Shakirova, O.G.; Lavrenova, L.G. Spin Crossover in New Iron(II) Coordination Compounds with Tris(pyrazol-1-yl)Methane. Crystals 2020, 10, 843.

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Shakirova OG, Lavrenova LG. Spin Crossover in New Iron(II) Coordination Compounds with Tris(pyrazol-1-yl)Methane. Crystals. 2020; 10(9):843.

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Shakirova, Olga G., and Ludmila G. Lavrenova. 2020. "Spin Crossover in New Iron(II) Coordination Compounds with Tris(pyrazol-1-yl)Methane" Crystals 10, no. 9: 843.

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