OBSERVATION OF THE LONGEST FE-N(PYRIDINE) BOND IN AN FEIIN6 CHROMOPHORE - CRYSTAL-STRUCTURE AND H-1 NUCLEAR-MAGNETIC-RESONANCE STUDIES OF [FEL22][CLO4]2 [L2 = 2-(3,5-DIMETHYLPYRAZOL-1-YLMETHYL)-6-(PYRAZOL-1-YLMETHYL)PYRIDINE]

The X-ray structural and H-1 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 [Fe(L)2(2)][CIO4]2 Crystallizes as yellow prisms in the monoclinic space group P2(1)/n with a = 14.122(3), b = 13.612(3), c = 18.393(4) angstrom, beta = 99.11(3)-degrees, 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 angstrom 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 repulsions and this is presumably responsible for the accessibility of the quintet state for Fe(II). The H-1 NMR spectral measurements in CD3CN reveal that its solid-state structure is retained in solution.