A Phenolate-lnduced Trans Influence:Crystallographic Evidence for Unusual Asymmetric Coordination of an alpha-Diimine in Ternary Complexes of Iron(lll) Possessing Biologically Relevant Hetero-Donor N-Centered Tripodal Ligands

摘要

Three mononuclear ternary complexes of iron(lll) with an alpha-diimine (bipy or phen) and a derivative of N,N-bis(2-hydroxybenzyl)glycinate (L~(3-)) have been synthesized and characterized by magnetic susceptibility measurements,electron paramagnetic resonance (EPR) spectroscopy,vibrational spectroscopy,and electronic absorption spectroscopy.Single-crystal X-ray structure determinations of the pseudo-octahedral complexes [Fe(bipy)L]-MeCN [L = (3,5-Br_2)-L~(3-) or (5,3-CI,Me)-L~(3-)] revealed a considerable and consistent distortion in the coordination of bipy to iron(lll) attributable largely to electronic effects.In both crystal structures,the Fe-N_(pyridyl) bond trans to the phenolate oxygen is 0.133 A longer than the other one positioned trans to the tertiary amine nitrogen,a relatively weaker donor.This coordination behavior of bipy is of structural interest and has not been observed previously for iron(lll).The electronic and EPR spectra of the compounds [Fe(L'-L')L]-MeCN (L'-L' = bipy or phen) are consistent with the spin state of the central metal atom (S = 5/2).The charge-transfer transitions arising from the strong interactions of the phenolate moieties with the ferric ion have been identified as phenolate (p_(pi)) - iron(lll) (d_(pi~*)) (gamma_(max) approx= 500 nm,epsilon approx= 3000 M~(-1) cm~(-1)) and phenolate (p_(pi)) - iron(lll) (d_(sigma~*)) (gamma_(max) approx = 320 nm,epsilon approx= 5200 M~(-1) cm~(-1)).The presence of the phenolate moieties in the quadridentate hetero-donor tripodal ligands,H_3L,lends these iron(lll) ternary complexes the potential to model the specific metal-coordination,metal-substrate interactions,and physicochemical behaviors of several iron-tyrosinate proteins.