L. G. Lavrenova

1A1 ⇄ 5T2 Spin Transition in New Thermochromic Iron(II) Complexes with 1,2,4-Triazole and 4-Amino-1,2,4-Triazole

New complexes of iron(II) chloride and bromide with 1,2,4-triazole (Htrz) and 4-amino-1,2,4-tri-azole (NH2trz) of composition Fe(Htrz)3Cl2 · 1.5H2O, Fe(NH2trz)3Cl2 · 2H2O, Fe(Htrz)3Br2 · 2H2O, and the Fe(NH2trz)3SO4 · H2O complex were synthesized and studied by magnetochemical, electronic, IR and Mössbauer spectroscopy methods. Magnetochemical studies showed that these complexes exhibit 1A1 ⇄ 5T2 spin transition accompanied by thermochromism (a reversible pink ⇄ white change of color).

Influence of magnetic dilution on the spin transition in the complex of iron(II) nitrate with 4-amino-1,2,4-triazole

Influence of magnetic dilution with ZnII ions on the spin transition in the iron nitrate complex of 4-amino-1,2,4-triazole (AT) was studied by magnetochemistry, Mössbauer spectroscopy, and IR spectroscopy. In studies of the properties of solid phases of FexZn1−x(AT)3(NO3)2 (0.01≤x≤0.8), it was demonstrated that magnetic dilution results in a lowered spin transition temperature and an increased share of the high-spin form of the iron(II) complex.

The Conducting Spin-Crossover Compound Combining Fe(II) Cation Complex with TCNQ in a Fractional Reduction State

The radical anion salt [Fe{HC(pz)3}2](TCNQ)3 demonstrates conductivity and spin-crossover (SCO) transition associated with Fe(II) complex cation subsystem. It was synthesized and structurally characterized at temperatures 100, 300, 400, and 450 K. The compound demonstrates unusual for 7,7,8,8,-tetracyanoquinodimethane (TCNQ)-based salts quasi-two-dimensional conductivity. Pronounced changes of the in-plane direct-current resistivity and intensity of the electron paramagnetic resonance (EPR) signal, originated from TCNQ subsystem, precede the SCO transition at the midpoint T* = 445 K. The boltzmannian growth of the total magnetic response and structural changes in the vicinity of T* uniquely show that half [Fe{HC(pz)3}2] cations exist in high-spin state. Robust broadening of the EPR signal triggered by the SCO transition is interpreted in terms of cross relaxation between the TCNQ and Fe(II) spin subsystems.

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 1A1 ⇄ 5T2 accompanied by thermochromism.

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 1A1 ⇔ 5T2. Transition temperatures (Tc) for I–III are 370 K, 380 K, and 400 K respectively. Spin-crossover is accompanied by thermochromism (color change: pink ⇔ white).

Magnetic properties of new complexes of 3d metal chlorides with 3-amino-4-ethoxycarbonylpyrazole

New complexes of Co(II), Ni(II), and Cu(II) chlorides with 3-amino-4-ethoxycarbonylpyrazole (L) of the composition ML2Cl2 are synthesized. The compounds are studied by powder X-ray diffraction, electronic and IR spectroscopy, static magnetic susceptibility (temperature range 2–300 K). It is found that in the ML2Cl2 complexes (M = Co(II), Ni(II), Cu(II)) ferromagnetic exchange interactions between the unpaired electrons of metal ions occur. In [CoL2Cl2] and [NiL2Cl2] compounds a transition to the magnetically ordered state (Tc ≊ 10–12 K) is observed.

Cobalt(II), nickel(II), and copper(II) bromide complexes with 3-amino-4-ethoxycarbonylpyrazole: Syntheses, structures, and magnetic properties

New complexes of cobalt(II), nickel(II), and copper(II) bromides with 3-amino-4-ethoxycarbo- nylpyrazole (L) of the composition ML4Br2 · 2H2O (M = Co(II) and Ni(II)) and CuL2Br2 are synthesized. The compounds are studied by X-ray diffraction analysis (CIF files CCDC nos. 1006284 (I), 1006286 (II), and 1006285 (III)), IR spectroscopy, and static magnetic susceptibility. In the complexes, the metal ions are in the distorted octahedral environment and the ligand is coordinated to the metal by the N(2) atom of the pyrazole cycle according to the monodentate mode. The Co(II) and Ni(II) complexes are mononuclear, whereas CuL2Br2 is polynuclear due to the bridging function of the bromide ion. Complex CuL2Br2 possesses the thermochromic properties: the transition of the color is brown (at room temperature) ⇔ light brown (at the temperature of liquid nitrogen).

Synthesis and study of cobalt(II), nickel(II), and copper(II) complexes with 4-(4-hydroxyphenyl)-1,2,4-triazole

New complexes of Co(II), Ni(II), and Cu(II) nitrates, chlorides, and perchlorates with 4-(4-hydroxyphenyl)-1,2,4-triazole (L) were obtained and examined by single-crystal X-ray diffraction, X-ray powder diffraction, and electronic absorption and IR spectroscopy. The cations of all the complexes have linear trinuclear structures. Ligand L is coordinated to the metal ions in a bidentate bridging fashion through the N(1) and N(2) atoms of the heterocycle. The coordination polyhedron of the metal atoms is a distorted octahedron. The molecular and crystal structures of the complexes [Co3L6(H2O)6](ClO4)6 · 3C2H5OH · 3.75H2O and [M3L6(H2O)6](ClO4)6 · 6H2O (M = Cu2+ and Ni2+) were determined.

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 μeff(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.

Syntheses, structures, and properties of the Co(II), Ni(II), and Cu(II) complexes with 5-carboxy- and 5-methoxycarbonylpyrazoles

New cobalt(II), nickel(II), and copper(II) complexes based on 5-methoxycarbonyl-3-me-thylpyrazole (MePzCOOMe), [Co(MePzCOOMe)2(H2O)2](NO3)2 (I), [Ni(MePzCOOMe)2(H2O)2] (NO3)2 (II), and [Cu(MePzCOO)2(H2O)] · 3H2O (III), were synthesized. The compounds were studied by X-ray diffraction analysis, IR spectroscopy, and static magnetic susceptibility. The molecular and crystal structures of complexes I and III were determined by X-ray structure analysis.