Aqueous Cu(II)–Organic Complexation Studied in Situ Using Soft X‑ray and Vibrational Spectroscopies

Abstract

In situ aqueous solutions containing copper–ligand mixtures were measured at the Cu L-edge using X-ray absorption near edge structure (XANES) and with attenuated total reflectance infrared (ATR-FTIR) spectroscopies. Copper complexation with environmentally relevant ligands such as EDTA, citrate, and malate provided a bridge between spectroscopic studies and general environmental behavior and will allow for future study of complex environmental samples. XANES results show that the lowest unoccupied molecular orbital (LUMO) energy is governed by the ligand field strength and is related to Lewis acid/base properties of the ligand functional groups. Complementary ATR-FTIR studies confirmed the importance of water molecules in the structure of these Cu–ligand complexes and provided in-depth structural analysis to support the XANES data. Copper–malate is shown to have a 5/6-O-ring structure, and Cu–ethylenediaminetetraacetate has pentadentate coordination. Cu L-edge XANES also revealed direct Cu–N coordination in these aqueous solutions with amide functional groups