Stereochemical effects on the spin-state transition shown by salts of [FeL2](2+) [L=2,6-di(pyrazol-1-yl)pyridine]

The syntheses of [Fe((LH)-H-1)(2)]X-2 ((LH)-H-1 = 2,6-di(pyrazol-1-yl) pyridine [(LH)-H-1]; X- = BF4-, PF6-) are described. Solvent-free [Fe((LH)-H-1)(2)][BF4](2) shows an approximately D-2d-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((LH)-H-1)(2)][BF4](2).2.9CH(3)NO(2).0.25H(2)O can be grown at 240 K, which undergoes an irreversible spin-state transition between 260 and 265 K. In contrast, solid [Fe((LH)-H-1)(2)][PF6](2) adopts an unusual C-2-symmetric coordination geometry, reflecting a ca. 28degrees twist of one (LH)-H-1 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 E-5 ground state of the six-coordinate Fe(II) ion. This distortion is favoured by the restricted bite-angle of the (LH)-H-1 ligands.