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)(2)bpym(NCS)(2) (c) and Fe(py)(2)phen(NCS)(2) (d), and those presented herein for Fe(Me(2)bpy)(2)(NCS)(2) (1), [Fe(2-pic)(3)]Cl-2.EtOH (2), [Fe(2-pic)(3)]Cl-2.H2O (3) and [Fe(btr)(2)(NCS)(2)].H2O (4) [Me(2)bpy = 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 n(LS) vs. P plots (n(LS) = low-spin fraction) are discussed. Transition pressures, 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 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 P-1/2.