Jacqueline Zarembowitch

High-spin α low-spin transition in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

Abstract The FeII ion in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O) has distorted tetragonal symmetry with two trans-oriented NCS− ligands. It shows a very abrupt high-spin α low-spin transition at 123.5 K on cooling and 144.5 K on warming. The compound is rather stable in vacuo at room temperature; however, samples which have passed the spin transition once lose their water of hydration above 240 K in vacuo. The dehydrated substance does not show a spin transition; it is high spin in the whole temperature range. Mossbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed. The spin transition has been followed by EPR measurements with the aid of traces of Cu2+ ions which could be substituted for Fe2+ in the tetragonal structure. In the high-spin phase the EPR signal is very broad and featureless; in the diamagnetic low-spin phase it is very sharp and resolved in hyperfine and superhyperfine structures. This unusual method to follow the spin transition was shown to be quite generally applicable. In the crystal structure of Fe(NCS)2(C4H4N6)2(H2O) the distances FENCS are 2.125(3) A, and FeN(ligand) 2.180(3) and 2.188(2) A. The water molecule is connected to the non-coordinating ligand N-atom by hydrogen bonding.

Spin transition in a Langmuir-Blodgett film

Fe(X2Yphen)2(NCS)2(X2Yphen is a trisubstituted 1,10-phenanthroline with X = C17H35 and Y = C18H37O) incorporated in a Langmuir–Blodgett film shows a spin transition which has been studied by i.r. spectrometry.

Pressure-induced spin-state crossovers at room temperature in iron(II) complexes: comparative analysis; a XANES investigation of some new transitions

An attempt to rationalize the behaviour of iron(II) spin-crossover compounds under hydrostatic pressure at room temperature from a comparative analysis of data is reported. The selected pressure-induced spin transitions are those previously described for Fe(phen)2(NCS)2 [polymorphs I (a) and II (b)], Fe(py)2bpym(NCS)2 (c) and Fe(py)2phen(NCS)2 (d), and those presented herein for Fe(Me2bpy)2(NCS)2 (1), [Fe(2-pic)3]Cl2·EtOH (2), [Fe(2-pic)3]Cl2·H2O (3) and [Fe(btr)2(NCS)2]·H2O (4) [Me2bpy = 4,4′-dimethyl-2,2′-bipyridine, 2-pic = 2-(aminomethyl)pyridine, btr = 4,4′-bis-1,2,4-triazole]. All these transitions have been investigated by XANES spectrometry. In this paper, the spectra of compounds 1–4 and the resulting nLSvs. P plots (nLS = low-spin fraction) are discussed. Transition pressures, P1/2, are found to be 0.30 (1), 1.32 (2), 0.60 (3) and 2.04 (4) GPa. For compounds 1, a, b, c, with closely related formulas, P1/2 is found to vary quasi-linearly with the transition temperature at atmospheric pressure, T1/2, and deviations from linearity are discussed in terms of the variations in entropy and lattice volume associated with the spin change. For compounds 2, 3 and d, the values of P1/2 markedly depart from the above trend. This is ascribed either to the change in ligand-field strength resulting from the effects of pressure on the hydrogen-bonding network (2, 3) or the occurrence of a structural phase transition simultaneously with the spin conversion (d). In the polymeric two-dimensional compound 4, pressure is shown to first induce a structural transition to a new high-spin phase, then to produce a very gradual high-spin-to-low-spin transformation of this phase, which results in a comparatively high value of P1/2.