Water-soluble chiral coordination polymers of Li+, Na+, K+, and Ba2+ with an anionic iron(iii) complex of a l-threonine derivative and a significant red shift of visible spectra with Al3+ salt

Abstract

Four salts of an anionic iron(III) bis-complex, [Fe(L^L-thr)₂]¹⁻, were synthesized from water or methanol. H₂L^L-thr is a tridentate ligand derived from the L-threonine amino acid, and the cations used are Li⁺ (1), Na⁺ (2), K⁺ (3), and Ba²⁺ (4). Single-crystal X-ray diffraction showed that all the complexes are coordination polymers of different dimensionalities. The iron(III) complex binds to cations through its coordinated phenolate and non-coordinated carboxylate oxygen atoms. While Li⁺ forms a linear chain, all others have a pair of bridged cations intervening the iron(III) complexes. The 3D network of Ba²⁺ salt has a sizeable solvent-accessible space occupied by aquated chloride ions. The differences in circular dichroism (CD) spectra and significantly lower conductance values in water and methanol support partial retention of the polymeric nature in methanol. The visible spectra of 4 in methanol or water showed an ∼10 nm shift of the charge transfer bands from 3. However, the addition of Al³⁺ salt to 2 showed a significant colour shift. Further investigation confirmed that the colour shift is due to partial protonation of the complex with protons generated from salt hydrolysis. Most reports on visual aluminium detection consider aluminium's binding as the shift's source. The present results show that protonation due to hydrolysis of aluminium salt can skew the observation.