How Counterions Affect the Solution Structure of Polyoxoaurates: Insights from UV/Vis Spectral Simulations and Electrospray Mass Spectrometry

Abstract

Abstract The solution structure of the two presently known polyoxoaurates, [Au 4 As 4 O 20 ] 8– (Au 4 As 4 ) and [Au 4 Se 4 O 16 ] 4– (Au 4 Se 4 ), was elucidated by a combination of time‐dependent density‐functional‐theory (TD‐DFT) calculations, experimental UV/Vis spectroscopy, and electrospray ionization mass spectrometry (ESI‐MS). The TD‐DFT‐predicted electronic spectra for molecular models including different numbers of counterions, along with the main Au 4 As 4 and Au 4 Se 4 structural motifs, were analyzed and compared with the experimental UV/Vis spectra of the two polyanions. The performed analysis revealed important structural differences between the arsenate and the selenite derivatives in aqueous solution. For the selenite oxoaurate, the bare Au 4 Se 4 polyanion itself shows very good agreement between predicted and experimental excitation energies. In contrast, the solution state of the arsenate oxoaurate appears to be best described by a dimeric assembly of the type [Na 5 Au 8 As 8 O 40 ] 11– (Na 5 Au 8 As 8 ) with a stoichiometry and structure very similar to those reported for the solid state. Indirect experimental evidence that Na 5 Au 8 As 8 is the dominant species in aqueous solution is provided by ESI‐MS measurements. Our work demonstrates that the combination of experimental UV/Vis spectroscopy and TD‐DFT calculations complemented with electrospray mass spectrometry may serve as an alternative strategy for rationalizing the solution chemistry of polyoxometalates, which cannot be studied by conventionally used spectroscopic techniques, such as NMR or EPR spectroscopy.