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 (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-Br₂)-L^3- or (5,3-Cl,Me)-L^3-] 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 Irans 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*) (λ_max ≈ 500 nm, ε ≈ 3000 M^-1 cm^-1) and phenolate (pπ) → iron(III) (dσ*) (λ_max ≈ 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.