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

摘要

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)_2 bpym(NCS)_2 (c) and Fe(py)_2phen(NCS)_2 (d), and those presented herein for Fe(Me_2bpy)_2(NCS)_2 (1), [F3e(2-pic)_3]Cl_3 center dot EtOH (2), [Fe(2-pic)_3]Cl_2 center dot H_2O (3) and [Fe(btr)_2(NCS)_2] center dot H_2O (4) [Me_2bpy=4,4 min-dimethyl-2,2 min-bipyridine, 2-pic=2-(aminomethyl)pyridine, btr=4,4 min-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 n_LS vs. P plots (n_LS=low-spin fraction are discussed. transition presures, P_(1/2), 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, p_(1/2) is found to vary quasi-linearly with the transition temperature at atmospheric pressure, T_(1/2), 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 P_(1/2) 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 conpound 4, pressure is shown to first induce a structural transition to a new high-spin-to-low-spin transformation of this phase, which results in a comparatively high value of P_(1/2).