Spin crossover in homo- and heteroligand iron(II) complexes with tris(pyrazol-1-yl)methane derivatives

The studies concerning coordination compounds of various salts of iron(II) with tris(pyrazol-1-yl)methane derivatives (HC(pz)₃) are discussed. The results of a number of studies on the synthesis and investigation of the homo- and heteroligand iron(II) complexes with tris(3,5- dimethylpyrazol-1-yl)methane (HC(3,5-Me₂pz)₃) are considered. The study of the temperature dependence μ_eff (T) showed that the spin crossover (SCO) ¹A₁?⁵T₂ observed in a series of the compounds discussed is accompanied by thermochromism (color change pink (purple) ? colorless). Specific features of the SCO and their dependence on the outer-sphere anion in the iron(II) complexes are discussed. The data of the recently published work devoted to the synthesis of the iron(II) complexes with three N-substituted HC(pz)₃ derivatives (general formula ^xL, where x = H, CH₂C₆H₅ (Bn) and p-SO₃C₆H₄CH₃ (Ts)) are considered.     

Iron(II) closo-borate complexes with 1,2,4-triazole derivatives: Spin crossover in the iron(II) closo-borate complexes with tris(pyrazol-1-yl)methane

Methods of synthesis of iron(II) complexes containing cluster closo-borate anions—[Fe(Htrz)₃]B₁₀Cl₁₀ (I) (HTrz is 1,2,4-triazole), [Fe(NH₂Trz)₃]B₁₀Cl₁₀ · 2H₂O (II) (NH₂Trz is 4-amino-1,2,4-triazole), [Fe{HC(pz)₃}₂]B₁₀Cl₁₀ (III), [Fe{HC(pz)₃}₂]B₁₀H₁₀ (IV), and [Fe{HC(pz)₃}₂]B₁₂H₁₂ · 2H₂O (V) (HC(pz)₃ is tris(pyrazol-1-yl)methane)—have been developed. The compounds have been studied by the static magnetic susceptibility method (78–500 K) and electronic, IR, and EXAFS spectroscopy. Complexes I and II in the temperature range under consideration remain in the high-spin state. Low-spin complex III shows incomplete spin crossover and decomposes on heating above 440 K. Complexes IV and V are characterized by reversible spin crossover ¹ A ₁ ? ⁵ T ₂ accompanied by thermochromism (the pink ? white color change). The crossover temperature (T _c) for IV and V is 375 and 405 K, respectively.     

Spin-crossover in complexes of iron(II) carboranes with tris(pyrazol-1-yl)methane

Synthesis procedures for coordination compounds of iron(II) 1,5,6,10-tetra(R)-7,8-dicarba-nido-undecaborates(-1) (carboranes) with tris(pyrazol-1-yl)methane (HC(pz)₃) of the composition [Fe{HC(pz)₃}₂]A₂·nH₂O (A = (7,8-C₂B₉H₁₂)^? (I), (1,5,6,10-Br₄-7,8-C₂B₉H₈)^? (II), (1,5,6,10-I₄-7,8-C₂B₉H₈)^? (III), n = 0–2) are developed. The compounds are studied by static magnetic susceptibility in the temperature range of 160–500 K, electron (diffuse reflectance spectra), IR, and Mössbauer spectroscopy methods. It is shown that the complexes have high-temperature spin-crossover ¹ A ₁ ? ⁵ T ₂. Transition temperatures (T _c) for I–III are 370 K, 380 K, and 400 K respectively. Spin-crossover is accompanied by thermochromism (color change: pink ? white).     

Spin crossover in the coordination compounds of iron(II) with tris(3,5-dimethylpyrazol-1-yl)methane

Iron(II) complexes with tris(3,5-dimethylpyrazol-1-yl)methane {HC(3,5-Me₂Pz)₃} of the composition [Fe{HC(3,5-Me₂Pz)₃}₂]Am · nH₂O (A = Cl^? (I), ClO ₄ ^? (II), SO ₄ ^2? (III), CF₃SO ₃ ^? (IV), m = 1, 2, n = 0.1) are synthesized. The compounds are studied by static magnetic susceptibility, IR and diffuse reflectance spectroscopy, and X-ray structure analysis. The crystal structures of two polymorphous modifications of the [Fe{HC(3,5-Me₂Pz)₃}₂](ClO₄)₂ (IIa and IIb) and [Fe{HC(3,5-Me₂Pz)₃}₂](CF₃SO₃)₂ (IV) complexes are determined. The temperature dependence ??_eff(T) shows that the spin crossover ¹ A ₁ ai ⁵ T ₂ is observed in the polycrystalline phase of complex I and in one of the single-crystal phases of complex II (IIa) and is accompanied by thermochromism (the change of the dark pink color ai to white).     

High-temperature spin transition in the iron(II) trifluoromethylsulfonate,perrhenate, and tetraphenylborate complexes with tris(pyrazol-1-yl)methane

New coordination compounds of iron(II) trifluoromethylsulfonate, perrhenate, and tetraphenylborate with tris(pyrazol-1-yl)methane (HC(Pz)₃) of the composition [Fe(HC(Pz)₃)₂]A₂ (A = CF₃SO₃ ⁻ (I), ReO₄ ⁻ (II), and B(C₆H₅)₄ ⁻ (III)) were synthesized and studied by the method of static magnetic susceptibility and IR and electronic spectroscopies. The crystal and molecular structures of compounds I and II were determined by X-ray diffraction analysis. The magnetochemical study of complexes I—III in the interval from 275 to 500 K showed that they possessed the high-temperature spin transition ¹ A ₁ ⇄ ⁵ T ₂ accompanied by thermochromism.     

Spin-crossover in coordination compounds of iron(II) with tris(pyrazol-1-yl)methane and cluster anions

Synthesis procedures for new coordination compounds of iron(II) with tris(pyrazol-1-yl)methane (HC(pz)₃), containing cluster anions in the outer sphere, of the composition [Fe{HC(pz)₃}₂][Mo₆Cl₁₄]?2H₂O (I), [Fe{HC(pz)₃}₂][Mo₆Br₁₄]?H₂O (II), and [Fe{HC(pz)₃}₂]₂[Re₆S₈(CN)₆]?2H₂O (III) are developed. The compounds are studied by static magnetic susceptibility, electronic, IR, and Mössbauer spectroscopic methods. The magnetochemical study shows that in the polycrystalline phases of all compounds the spincrossover ¹ А ₁ ? ⁵ Т ₂ is observed which is accompanied by thermochromism.     

Mono- and heteroligand iron(II) complexes with tris(3,5-dimethylpyrazol-1-yl)methane

Coordination compounds of iron(II) thiocyanate with tris(3,5-dimethylpyrazol-1-yl)methane (HC(3,5-Me₂Pz)₃), [Fe(HC(3,5-Me₂Pz)₃)₂](NCS)₂] (I) and [Fe(HC(3,5-Me₂Pz)₃)(Рhz)(NCS)₂] · H₂O (II), where Рhz is phthalazine, are synthesized. The complexes are studied by X-ray diffraction analysis, diffuse reflectance and IR spectroscopy, and static magnetic susceptibility measurements. The single crystals are obtained, and the molecular and crystal structures of complex II and compounds [Fe(HC(3,5-Me₂Pz)₃)(3,5-Me₂Pz)(NCS)₂] · С₂H₅OH (III), where 3,5-Me₂Pz is 3,5-dimethylpyrazole, and [Fe(HC(3,5-Me₂Pz)₃)₂][Fe(HC(3,5-Me₂Pz)₃)(NCS)₃]₂ (IV) are determined (CIF files CCDC 1415452 (II), 1415453 (III), and 1415454 (IV)). The study of the temperature dependence μ_eff(Т) in a range of 2–300 K shows exchange interactions of the antiferromagnetic character between the iron(II) ions in complexes I and II.     

Spin crossover in di-, tri- and tetranuclear, mixed-ligand tris(pyrazolyl)methane iron(II) complexes

A series of polynuclear mixed-ligand tris(pyrazolyl)methane iron(II) complexes displaying high temperature spin crossover behaviour has been synthesised. These complexes are of the type [(Fe((3,5-Me(2)pz)(3)CH))(n)(μ-L)](BF(4))(2n), where μ-L is one of five bridging ligands X(CH(2)OCH(2)C(pz)(3))(n), (X = the central linking moiety, pz = pyrazolyl ring and n = 2 (ditopic), 3 (tritopic) or 4 (tetratopic)). Throughout the series the terminal tris(3,5-dimethylpyrazolyl)methane co-ligand (3,5-Me(2)pz)(3)CH and the BF(4)(-) counter anion were kept constant while variations in the central linking moiety have produced three dinuclear complexes and a trinuclear and tetranuclear complex, all isolated as solvates. The three dinuclear complexes are a 1,4-xylene-bridged complex 1·2DME, a 2,6-naphthalene-bridged complex 2·2.5MeCN.2DME and a 1,4-butene-bridged complex 3·2DME. The trinuclear complex 4·solvent (solvent undefined) has a 1,3,5-mesitylene core and the tetranuclear complex, 5·8MeCN.2(t)BuOMe, has a 1,2,4,5-tetramethylbenzene core (DME = dimethoxyethane, (t)BuOMe = tertiarybutyl-methylether). The trinuclear cluster has a "3-up" cup shape with the cups arranging themselves in pairs to form capsules that contain anion guests. All the solvated compounds have been structurally characterised and both the solvated and desolvated versions have had their magnetic and thermal properties thoroughly investigated by variable temperature magnetic susceptibility, differential scanning calorimetric and M?ssbauer spectral methods. They all display typical low spin iron(II) magnetic behaviour at room temperature and all undergo a spin state transition to high spin iron(II) above room temperature. In particular, complex 1·2DME shows an abrupt spin transition which shifts, upon desolvation, to a lower value of T(1/2) and in addition displays a small thermal hysteresis.     

Thermodynamic properties of complex compounds of iron(II) nitrate with tris(3,5-dimethylpyrazol-1-yl)methane

The temperature dependence of the heat capacity of a complex compound of iron(II) nitrate with tris(3,5-dimethylpyrazol-1-yl)methane is studied by adiabatic calorimetry in the range of 100–300 K. A specific heat anomaly is found and localized in the temperature range corresponding to the sharp spin transition ¹ A ₁ ? ⁵ T ₂ with hysteresis on the temperature dependence of the magnetic susceptibility. The effects of cooperative interaction are revealed on the basis of thermodynamic and magnetochemical data, using two widely used models of spin transition.     

Synthesis of organometallic hydrotris(pyrazol-1-yl)boratoruthenium(II) complexes

The reactions of K[HB(pz)₃] (pz = pyrazol-1-yl) with the coordinatively unsaturated σ-vinyl complexes [Ru(CRCHR)Cl(CO)(PPh₃)₂] (R = H, Me, C₆H₅) proceed with loss of a chloride and a phosphine ligand to provide the compounds [Ru(CRCHR)(CO)(PPh₃){HB(pz)₃}] in high yield. Similar treatment of the complex [Ru(C₆H₄Me-4)Cl(CO)(PPh₃)₂] leads to the related σ-aryl derivative [Ru(C₆H₄Me-4)(CO)(PPh₃){HB(pz)₃}] whilst the complex [RuClH(CO)(PPh₃)₃] treated successively with diphenylbutadiyne and K[HB(pz)₃] provides the unusual derivative [Ru{C(CCPh)CHPh}(CO)(PPh₃){HB(pz)₃}].     

Structural diversity in iron(II) complexes of 2,6-di(pyrazol-1-yl)pyridine and 2,6-di(3-methylpyrazol-1-yl)pyridine

The syntheses, magnetochemistry and crystallography of [Fe(L1)2]I0.5[I3]1.5 (1), [Fe(L1)2][Co(C2B9H11)2]2 (2) and [Fe(L2)2][SbF6]2 (3) (L1 = 2,6-di(pyrazol-1-yl)pyridine; L2 = 2,6-di(3-methylpyrazol-1-yl)pyridine) are described. Compounds 1 and 3 are high-spin between 5-300 K. For 1, this reflects a novel variation of an angular Jahn-Teller distortion at the iron centre, which traps the molecule in its high-spin state. No such distortion is present in 3; rather, the high-spin nature of this compound may reflect ligand conformational strain caused by an intermolecular steric contact in the crystal lattice. Compound 2 exhibits a gradual high --> low spin transition upon cooling with T(1/2) = 318 +/- 3 K, that is only 50% complete. This reflects the presence of two distinct, equally populated iron environments in the solid. One of these unique iron centres adopts the same angular structural distortion shown by 1 and so is trapped in its high-spin state, while the other, which undergoes the spin-crossover, has a more regular coordination geometry. In contrast with 3, the solvated salts [Fe(L2)2][BF4]2 x 4 CH3CN and [Fe(L2)2][ClO4]2 x (CH3)2CO both undergo gradual thermal spin-transitions centred at 175 +/- 3 K.     

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

This review begins with a brief introduction to pyrazole and to spin crossover. The focus then moves to a detailed consideration of the synthesis and magnetic properties of structurally characterized iron(II) spin crossover (SCO) active complexes of pyrazole- and pyrazolate-based ligands that also contain at least one pyridine or pyrazine unit within the ligand motif. The syntheses and crystallization methods reported in the original publications are emphasized in this review. The reason for this is that these factors often affect the exact nature of the final product, including the amount and nature of the crystallization solvent molecules present and/or what polymorph is obtained, and hence they can impact strongly on the SCO properties of the resulting materials, as can be seen in this review.     

Spin crossover in a four-coordinate iron(II) complex

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mo?ssbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.     

Spin crossover in tetranuclear cyanide-bridged iron(II) square complexes: a theoretical study

Spin crossover in a series of six cyanide-bridged iron(II) tetranuclear square complexes was analyzed using density functional theory (DFT) methods. As the spin crossover between the low-spin (LS) and high-spin (HS) states can occur only for two of four iron ions, we characterized energetically and structurally the [LS-LS], [HS-LS], and [HS-HS] spin-state isomers. For all studied complexes, the energy of the mixed [HS-LS] spin state does not deviate essentially from the halfway point between the energies of homogeneous spin states, thereby satisfying the conditions for an one-step transition between the [LS-LS] and [HS-HS]. This fact reflects the weak elastic coupling between the environments of transiting centers. The two-step spin transition observed in one complex can appear only due to the crystal packing effects. We also evaluated the strength of exchange coupling between the paramagnetic ions in the [HS-HS] state.     

Photochemical study of tris(benzotriazol-1-yl)methane

Photodecomposition of tris(benzotrizol-1-yl)methane (1) in benzene gives [1-benzotryazol-1-yl-methylidene]-biphenyl-2-ylamine (2) resulting from the loss of the benzotriazolyl radical and nitrogen followed by addition of benzene. Elimination of the second benzotriazolyl radical from 2 provides the biphenyl-2-ylmethyleneamine radical, which affords phenantridine (3) after ring closure. In contrast, the photolysis of 1 in methanol gives a high yield of benzotriazole (4).     

Copper(II) complexes of bis(pyrazol-1-yl)methane - Synthesis, spectroscopic characterization, X-ray structure and DFT calculations

Two novel copper(II) complexes incorporating bis(pyrazol-1-yl)methane ligand (bpzm) have been synthesized. The compounds [CuCl(bpzm)₂(H₂O)]Cl·H₂O (1) and [Cu(N₃)₂(bpzm)]_n (2) have been studied by IR, UV-Vis spectroscopy and X-ray crystallography. The experimental studies on the compounds 1 and 2 have been accompanied computationally by the density functional theory (DFT) calculations.     

Spin transition in iron(III) and iron(II) complexes

The main stages of the studies on the spin transitions in iron(III) and iron(II) complexes are considered. The types of the spin transitions and the factors responsible for the latter are reported. The problems arising during experiments in this field are discussed.     

Structure-function correlations in Iron(II) tris(pyrazolyl)borate spin-state crossover complexes

Iron(II) poly(pyrazolyl)borate complexes have been investigated to determine the impact of substituent effects, intramolecular ligand distortions, and intermolecular supramolecular structures on the spin-state crossover (SCO) behavior. The molecular structure of Fe[HB(3,4,5-Me3pz)3]2 (pz = pyrazolyl ring), a complex known to remain high spin when the temperature is lowered, reveals that this complex has an intramolecular ring-twist distortion that is not observed in analogous complexes that do exhibit a SCO at low temperatures, thus indicating that this distortion greatly influences the properties of these complexes. The structure of Fe[B(3-(cy)Prpz)4]2.(CH3OH) ((cy)Pr = cyclopropyl ring) at 294 K has two independent molecules in the unit cell, both of which are high spin; only one of these high-spin iron(II) sites, the site with the lesser ring-twist distortion, is observed to be low-spin iron(II) in the 90 K structure. A careful evaluation of the supramolecular structures of these complexes and several similar complexes reported previously revealed no strong correlation between the supramolecular packing forces and their SCO behavior. Magnetic and M?ssbauer spectral measurements on Fe[B(3-(cy)Prpz)4]2 and Fe[HB(3-(cy)Prpz)3]2 indicate that both complexes exhibit a partial SCO from fully high-spin iron(II) at higher temperatures, respectively, to a 50:50 high-spin/low-spin mixture of iron(II) below 100 K. These results may be understood, in the former case, by the differences in ring-twisting and, in the latter case, by a phase transition; in all complexes in which a phase transition is observed, this change dominates the SCO behavior. A comparison of the M?ssbauer spectral properties of these two complexes and of Fe[HB(3-Mepz)3]2 with that of other complexes reveals correlations between the M?ssbauer-effect isomer shift and the average Fe-N bond distance and between the quadrupole splitting and the average FeN-NB intraligand dihedral torsion angles and the distortion of the average N-Fe-N intraligand bond angles.