Evidence of crystal packing effects in stabilizing high or low spin states of iron(II) complexes with functionalized 2,6-bis(pyrazol-1-yl)pyridine ligands

The molecular structures and magnetic properties of homoleptic iron(II) compounds [Fe(bpp-COOMe)2](ClO4)2 (1) and [Fe(bpp-triolH3)2](ClO4)2 (2) have been investigated to ascertain their spin crossover (SCO) behaviour. In these hexacoordinated complexes, the bpp (2,6-bis(pyrazol-1-yl)pyridine) ligands adopt a mer-mer coordination mode and carry COOMe or C(O)NHC(CH2OH)3 para substituents, respectively, on the central pyridyl ring. In spite of the almost equal donor power of the ligands to the iron(II) centre, the two compounds feature different spin state configurations at room temperature. Compound 1 displays a highly-distorted octahedral environment around the iron(II) centre, which adopts a high spin (HS) state at all temperatures, even under an external applied pressure up to 1.0 GPa. By contrast, 2 is characterized by a more regular octahedral coordination around the metal ion and exhibits a low spin (LS) configuration at or below room temperature. However, it shows a thermally-induced SCO behaviour at T > 400 K, along with Light-Induced Excited Spin State Trapping (LIESST) at low temperature, with TLIESST = 38 K. Since DFT (U)M06/6-311+G(d) geometry optimizations in vacuo indicate that both complexes should adopt a HS state and a highly-distorted coordination geometry, the stabilization of a LS configuration in 2 is ultimately ascribed to the effect of intermolecular hydrogen bonds, which align the [Fe(bpp-triolH3)2]2+ cations in 1D chains and impart profound differences in the geometric arrangement of the ligands.