Observation of the longst Fe-N(pyridine) bond in an Fe^IIN₆ chromophore. Crystal structure and ¹H nuclear magnetic resonance studies of [FeL²₂][ClO₄]₂[L²= 2-(3,5-dimethylpyrazol-1-ylmethyl)-6-(pyrazol-1-ylmethyl)pyridine]

Abstract

The X-ray structural and ¹H NMR spectral properties of a bis(ligand) iron(II) complex of a tridentate pyridylpyrazole ligand L²[L²= 2-(3,5-dimethylpyrazol-1-ylmethyl)-6-(pyrazol-1-ylmethyl)pyridine] have been studied. The complex [FeL²₂][ClO₄]₂ crystallizes as yellow prisms in the monoclinic space group P2₁/_n with a= 14.122(3), b= 13.612(3), c= 18.393(4)Å, β= 99.11(3)°, Z= 4, R= 0.0518 and R'= 0.0634 for 6464 observed reflections. The Fe atom is bound to six nitrogen atoms from two tridentate (N₃) heterocyclic ligands. Considerable distortion is observed with the average Fe-N(pyridine) and Fe-N(pyrazole) distances being 2.274 and 2.173 Å respectively. These bond lengths are markedly longer than corresponding lengths for complexes of less sterically demanding ligands. As expected, the ligand molecules are not planar, the pyridyl and pyrazole rings being twisted relative to each other. The 3-methyl substituent induces a steric barrier to co-ordination and also greater inter-ligand repulsion and this is presumably responsible for the accessibility of the quintet state for Fe^II. The ¹H NMR spectral measurements in CD₃CN reveal that its solid-state structure is retained in solution.