V. A. Varnek

Spin-crossover and thermochromism in complexes of iron(II) iodide and thiocyanate with 4-amino-1,2,4-triazole

Abstract New complexes of iron(II) iodide and thiocyanate with 4-amino-1,2,4-triazole were synthesized. The complexes were characterized by magnetic measurements, reflection, Mossbauer and IR spectra. The coordination of iron(II) is discussed on the basis of these data. The compounds posses reversible thermochromism (colour change from white to pink) due to the spin transition 5 T 2 ⇌ 1 A 1 .

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).

Reactivity of pyrrhotite (Fe9S10) surfaces: Spectroscopic studies

Synthetic hexagonal pyrrhotite (Fe9S10) etched in hydrochloric acid solution and then dried in air has been studied using ex situ XPS, X-ray fluorescence, Mossbauer, solid-state NMR and EPR spectroscopies. The metal-deficient non-equilibrium, up to several micrometres thick, layer (NL) formed on pyrrhotite under non-oxidative conditions has been found to be composed predominantly of low-spin Fe2+, nearly equal quantities of di- and polysulfide sulphur (probably, chains of 3–5 atoms) and no or low oxygen. When pyrrhotite with the NL is kept in air, singlet ferrous iron converts into high-spin Fe2+ and Fe3+, oxygen is incorporated into the layer and the surface enrichment in sulfur over iron decreases. A Mossbauer signal with an isomer shift of 0.36 mm s−1 and negligible quadruple splitting has been detected for the etched sample, desiccation in air gives rise to a quadruple split of up to 0.65 mm s−1 and a minor decrease in the isomer shift. The application of variable X-ray tube accelerating voltage has made it possible to obtain depth-resolved Fe-Lα,β spectra of the NL and to find several alteration zones which include different forms of iron. Slow oxidative dissolution of the material in 1 M HCl+0.01 M FeCl3 electrolyte produces only a thin NL with mostly O-bonded Fe3+ and polysulfide prevailing over mono- and disulfide species. Subsequent air-drying of this sample results in an increase in the concentrations of oxygen, S-bonded Fe, and mono- and disulfide species, along with S0 formation. No unpaired electron spins have been registered in any of these NL.

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.

Complexes Fe(HTrz)3B10H10 · H2O and Fe(NH2Trz)3B10H10 · H2O (HTrz = 1,2,4-triazole and NH2Trz = 4-amino-1,2,4-triazole). The spin transition 1A1 ⇆ 5T2 in Fe(HTrz)3B10H10 · H2O

Methods for the synthesis of iron(II) complexes with 1,2,4-triazole (HTrz) and 4-amino-1,2,4-triazole (NH2Trz) containing the decahydro-closo-decaborate ion [B10H10]2-were developed. The empirical formulas of the complexes were Fe(HTrz)3B10H10 · H2O (I) and Fe(NH2Trz)3B10H10 · H2O (II). The complexes were examined by static magnetic susceptibility measurements (2–300 K) and Moessbauer, IR, and electronic spectroscopy. Complex I exhibits the reversible spin transition 1A1 ⇄ 5T2 and pink ⇄ white thermochromism. The temperatures of the forward and reverse transitions in complex I were 246 and 233 K, respectively. Complex II remained in the high-spin state over the whole temperature range. The sharp decrease in its effective magnetic moment at T < 78 K was attributed to antiferromagnetic exchange interactions between Fe2+ ions.

Iron(II) Complexes with 4-R-1,2,4-Triazoles (R = Ethyl, Propyl) Exhibiting 1A1 ⇄ 5T2 Spin Transition

Iron(II) complexes with 1,2,4-triazoles of composition FeL3A2 · nH2O were synthesized, where L is 4-ethyl- or 4-propyl-1,2,4-triazole (Ettrz, Prtrz, respectively) and A = NO–3, ClO–4, Br–; n = 0.5, 1, 2. Magnetochemical studies showed that all these compounds exhibit 1A1 ⇄ 5T2 spin transition (ST) which is accompanied by thermochromism (a reversible pink ⇄ white change of color). The ST pattern, i.e., the temperature of a direct (Ts↑) and reverse (Ts↓) transition, and its contrast substantially depend on the nature of both the ligand and anion and on the availability of water molecules in the complex structure. The highest ST temperatures were observed for Fe(Ettrz)3Br2 · 2H2O: Ts↑ = 327 K, Ts↓ = 314 K.

Iron(II) Complexes with 4-R-1,2,4-Triazoles (R = Ethyl, Propyl, Isopropyl): Synthesis and Properties

Fe(II) complexes with 4-R-1,2,4-triazoles of compositions FeL3A2 · nH2O (where L is 4-propyl-1,2,4-triazol; A is Br- (n = 4), CF3SO3- (n = 5)), and FeL2A2 · nH2O (where L are 4-ethyl-, 4-propyl-, 4-isopropyl-1,2,4-triazoles (Ettrz, Prtrz, Iprtrz, respectively), A = NCS-, NO3- (n = 0-2)) were synthesized. Magnetochemical studies revealed that FeL3A2 · nH2O complexes exhibit reversible spin transition (ST) 1A1 ⇄ 5T2 that is accompanied by thermochromism (a reversible change of color rose ⇄ white). The temperatures of direct (Ts↑) and reverse (Ts↓) spin transitions are, respectively, 252 and 247 K for Fe(Рrtrz)3Br2 · 4H2O and 207 and 202 K for Fe(Рrtrz)3(CF3SO3)2 · 5H2O. Dehydration of the complexes is attended by significant changes in the type and temperatures of ST.

Preparation of supported iron-containing catalysts from a FeSo4 solution: The effect of the support

The iron compounds resulting from the impregnation of the most common supports—alumina and silica gel-with an aqueous FeSO4 · 7H2O solution and subsequent heat treatment in air are identified by thermal analysis and Mössbauer spectroscopy. The state of the active component of the iron-containing catalysts depends strongly on the nature of the support.

Mössbauer study of the effect of iron substitution by nickel on the1A1 5T2 spin transition in Fe(4- amino- 1,2,4- triazole)3(NO3)2

Solid phases of FexNi1- x- (ATr)3(NO3)2(0.2) ≤x ≤1, having a polymer chain structure, were studied by Mössbauer spectroscopy at 295 and 78 K The dependences of the fraction of the high- spin form and of the area and parameters of the spectra on the relative content of nickel in the samples were measured. As the iron atoms are substituted by the nickel atoms, the fraction of the high- spin form, the ionicity of Fe- N chemical bonds, and the degree of distortion of the octahedral environment of iron atoms in the low- spin form increase. A model explaining these effects is suggested. When the iron atoms are substituted by the nickel atoms, an effect of “softening“ of the vibrational spectrum of the crystal lattice for the low- spin form of the complex takes place.

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).