A phenolate-induced trans influence:: Crystallographic evidence for unusual asymmetric coordination of an α-diimine in ternary complexes of iron(III) possessing biologically relevant hetero-donor N-centered tripodal ligands

Three mononuclear ternary complexes of iron(III) with an a-diimine (bipy or phen) and a derivative of N,N-bis(2-hydroxybenzyl)glycinate (L3-) 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)-L3- or (5,3-Cl,Me)-L3-] revealed a considerable and consistent distortion in the coordination of bipy to iron(III) attributable largely to electronic effects. In both crystal structures, the Fe-N-pyridyl bond trans to the phenolate oxygen is 0.133 angstrom 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(III), The electronic and EPR spectra of the compounds [Fe(L '-L ')L]center dot 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(III) (d(pi*)) (lambda(max) approximate to 500 nm, is an element of approximate to 3000 M-1 cm(-1)) and phenolate (p pi) -> iron(III) (d(sigma*)) (lambda(max) approximate to 320 nm, is an element of approximate to 5200 M-1 cm(-1)). The presence of the phenolate moieties in the quadridentate hetero-donor tripodal ligands, H3L, lends these iron(III) ternary complexes the potential to model the specific metal-coordination, metal-substrate interactions, and physicochemical behaviors of several iron-tyrosinate proteins.