Stereochemical effects on the spin-state transition shown by salts of FeL22+ L = 2,6-di(pyrazol-1-yl)pyridine

The syntheses of Fe(L1H)2X2 (L1H = 2,6-di(pyrazol-1-yl)pyridine L1H; X− = BF4−, PF6−) are described. Solvent-free Fe(L1H)2BF42 shows an approximately D2d-symmetric metal centre in the crystal, and undergoes an unusual abrupt spin-state transition centered at 261 K in the solid, or at 248 K in acetone solution. A solvated phase Fe(L1H)2BF42·2.9CH3NO2·0.25H2O can be grown at 240 K, which undergoes an irreversible spin-state transition between 260 and 265 K. In contrast, solid Fe(L1H)2PF62 adopts an unusual C2-symmetric coordination geometry, reflecting a ca. 28° twist of one L1H ligand with respect to each other. This salt is high-spin in the range 10–330 K. DFT calculations have rationalised this unusual structure as a Jahn–Teller distortion of the 5E ground state of the six-coordinate Fe(II) ion. This distortion is favoured by the restricted bite-angle of the L1H ligands.