The influence of steric effects in substituted 2,2′-bipyridine on the spin state of iron(II) in [FeN 6] 2+ systems

Abstract

Tris(ligand)iron(II) complexes of 6-methyl-2,2′- bipyridine (mbpy) and 2-(pyridin-2-yl)quinoline (pyq) display anomalous magnetic properties which are associated with a temperature-induced singlet ( 1A 1) ⇋ quintet ( 5T 2) transition. Two forms of [Fe(pyq) 3][ClO 4] 2 were obtained: one which is essentially high spin over the range 89–300 K and the other which shows an almost complete, gradual transition in this range. In the three complex salts of [Fe(mbpy) 3] 2+ isolated, the perchlorate, fluoroborate and hexafluorophosphate, appreciable spin- pairing is observed at low temperatures but in no instance is the transition to singlet state species complete at 89 K. Data for solutions of both [Fe (mbpy) 3] [ClO 4] 2 and [Fe(pyq) 3] [BF 4] 2 indicate that negligible spin-pairing is observed down to 210 K. Mössbauer spectral data confirm the change in spin state populations in all solid salts and moreover indicate two sites for the high-spin species at low temperatures. It is believed that these may arise from ordering of anion sites in the lattice. In the room temperature structure of [Fe(mbpy) 3] [ClO 4] 2· 1 2 mbpy disorder of one of the anions as well as the occluded free ligand occurs. In the structure of the complex cation considerable distortion is observed with the average FeN distance being 2.21 Å. The ligand molecules are not planar, the pyridyl rings being twisted 4.3, 16.4 and 17.8° about the inter- ligand bridge. The methyl substituent induces a steric barrier to coordination and also greater inter-ligand repulsions than in complexes of bpy and this is presumably responsible for the accessibility of the quintet state for Fe(II). The steric effects of the fused benzene ring in pyq are predicted to be similar. [Fe(mbpy) 3] [ClO 4] 2· 1 2 mbpy crystallises in space group P2 1/ c ( Z = 4) with cell parameters a = 19.769(10), b = 13.186(4), c = 15.043(8) Å. A total of 2509 reflections with I o > 3σ( I o) were observed and gave a final R = 0.057.