A structural, magnetic and Mössbauer spectroscopic study of an unusual angular Jahn–Teller distortion in a series of high-spin iron(ii) complexes

Abstract

Single crystal X-ray structures and susceptibility data are described for six homoleptic iron(II) complex salts, of 2,6-di(pyrazol-1-yl)pyridine or a 3,3"-disubstituted derivative of it. Zero field Mossbauer spectroscopic data for four of the complexes, and one previously reported analogue, are also discussed. Four of these compounds exhibit an unusual angular Jahn-Teller distortion towards C(2) symmetry to differing degrees, while the other two exhibit structures close to the "ideal" D(2d) symmetry for this ligand set. This structural distortion has two components: a twisting of the plane of one ligand relative to the other about the N{pyridine}-Fe-N{pyridine} vector, so that the two ligands are no longer perpendicular; and a rotation of one ligand about the Fe ion, so that the N{pyridine}-Fe-N{pyridine} angle < 180 degrees. Susceptibility data show that all the complexes are fully high-spin between 5 and 300 K, but yield an unusually wide range of zero-field splitting parameters for the different compounds of between 2.6 and 13.4 cm(-1). Magnetostructural correlations suggest that a low value of |D| is diagnostic for a high degree of "rotation" distortion. The Mossbauer spectra imply that an increased quadrupole splitting might also be diagnostic for the presence of the angular distortion.

Iron(II) complexes of 2,6-di(1-alkylpyrazol-3-yl)pyridine derivatives – The influence of distal substituents on the spin state of the iron centre

“…Hence it was unexpected that none of the complexes in this work should exhibit spin-crossover as bulk materials, or in solution. Conversely, however, for those compounds where a direct comparison is available, none of the complexes in this work is isostructural with its corresponding salt from the 2 ] 2+ series [22,37,44]. Therefore, although the differences between [Fe(L R ) 2 ] 2+ and 2 ] 2+ derivatives bearing the same distal 'R' substituents are small at the molecular level (Table 4 and Fig.…”

“…The iron coordination geometry in 1[PF 6 ] 2 is unique in this study, in being significantly distorted from the ideal D 2d symmetry associated with a [Fe(bpp) 2 ] 2+ centre ( Table 2). This angular distortion is is a manifestation of the JahnTeller effect in a high-spin d 6 ion [36][37][38], which is common in high-spin 2 ] 2+ derivatives but has rarely been seen thus far in their 2 ] 2+ analogues [7]. The distortion involves a reduction in the trans-N{pyridyl}-Fe-N{pyridyl} angle () from its ideal value of 180°, and/or a twisting of the two tridentate ligands away from the perpendicular ( < 90º, where  is the dihedral angle between the least squares planes of the two ligands).…”

“…4, bottom). This is unexpected, since the parent complex Notably the iron complex of the corresponding 1-bpp derivative, 2,6-di(3-isopropylpyrazol-1-yl)pyridine, is high-spin under the same conditions which was also attributed to the steric properties of the isopropyl groups in that compound [37,42]. (Fig.…”

“…Such close contacts will prevent spin-crossover on steric grounds, by inhibiting the associated contraction of the Fe-N bonds [42]. In [Fe(L iPr 2 (X -= BF 4 -and PF 6 -) this was to be expected, owing to the steric influence of the L i Pr isopropyl substituents [37,42]. For the other complexes, whose ligand substituents are less bulky, the origin of their high-spin nature is less clear.…”

“…<Insert (2) 150 (2) 150 (2) 150 (2) 150 (2) 150 (2)  calc (g.cm -3 ) [7,35], while  and  are measures of the angular Jahn-Teller distortion sometimes shown by [Fe(bpp) 2 ] 2+ centers in their high-spin state (see the text for details) [36,37]. Typical values of these parameters in [Fe(bpp) 2 ] 2+ -type derivatives are given in ref.…”