X-ray absorption spectroscopy of aqueous aluminum-organic complexes

Aqueous-phase X-ray absorption near-edge structure (XANES) spectra were collected on dissolved Al complexes with organic ligands, including desferrioxamine B, EDTA, acetohydroxamate, malate, oxalate, and salicylate. Spectral interpretations were made using the density functional theory-based modeling package StoBe. The goals of this work were to study the geometric and electronic structural characteristics of these complexes relative to Al(H ₂O)₆³⁺ and to examine the utility of the aqueous Al XANES technique as a tool for probing Al speciation and structure. In the case of EDTA, aqueous Fourier-transform infrared spectroscopy was also used to corroborate the structures of the Al(EDTA)^- and AlOH(EDTA) ^2- complexes. Synthetic XANES spectra calculated with StoBe reproduced the observed spectral differences between Al(H₂O) ₆³⁺, Al(dfoB)⁺, and Al(EDTA)^-. The narrower XANES feature observed for Al(dfoB)⁺ relative to Al(H ₂O)₆³⁺ can be attributed to a weaker splitting of the Al 3p - O 2p interactions in the former, while Al(EDTA)^- exhibits split Al 3p - ligand interactions that likely result from the mixed O/N coordination. In complexes with mixed aqua/organic-oxygen ligation (Al-acetohydroxamate, Al-malate, Al-oxalate, and Al-salicylate), spectra exhibit linear, systematic changes in peak width as a function of H₂O to organic ligand ratio in the Al coordination sphere. These results highlight the sensitivity of the aqueous Al K-edge XANES spectrum to coordination environment and demonstrate its utility as an experimental probe for future studies of Al speciation in complex solutions.