Soft X-ray absorption spectroscopy for analysing aluminium complexes in solution

A methodology for solution Al K-edge X-ray absorption spectroscopy (XAS) has been developed to investigate the coordination geometry and electronic structure of aluminium complexes in organic solvents. Using a controllable thin liquid flow cell under a helium atmosphere, Al K-edge XAS spectra of both penta- and hexacoordinate aluminium complexes were successfully obtained. Tris(acetylacetonato)aluminium (Al(acac)₃) exhibited identical spectral features in various solvents, reflecting its rigid and symmetric octahedral structure. In contrast, an Al-salen complex displayed pronounced solvent-dependent spectral variations corresponding to coordination number changes induced by Lewis base coordination. In non-coordinating solvents such as 1,2-dichloroethane, the Al-salen complex retained a pentacoordinate structure, whereas coordination of strongly donating solvents such as DMSO led to the formation of a hexacoordinate species, accompanied by a high-energy shift in the absorption edge. Notably, these structural and electronic changes were detected by XAS, highlighting the superior sensitivity of Al K-edge XAS to local electronic environments. This study demonstrates the feasibility and utility of solution-phase Al K-edge XAS as a powerful spectroscopic probe for elucidating coordination and electronic structures of aluminium complexes.