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 α-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-Br2)-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−Npyridyl bond trans to the phenolate
oxygen is 0.133 Å 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]·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 (pπ) → iron(III) (dπ*) (λmax ≈
500 nm, ε ≈ 3000 M-1 cm-1) and phenolate (pπ) → iron(III) (dσ*) (λmax ≈ 320 nm, ε ≈ 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.