Sulfur K-edge X-ray absorption spectroscopy and time-dependent density functional theory of arsenic dithiocarbamates

Abstract

S K-edge X-ray absorption spectroscopy(XAS)and time-dependent density functional theory(TDDFT)calculations were performed on a series of As[S2CNR2]3 complexes, where R2 = Et2,(CH2)5 and Ph2, to determine how dithiocarbamate substituents attached to N affect As[S2CNR2]3 electronic structure. Complimentary [PPh4][S2CNR2] salts were also studied to compare dithiocarbamate bonding in the absence of As. The XAS results indicate that changing the orientation of the alkyl substituents from trans to cis(R2 = Et2 vs.(CH2)5)yields subtle variations whereas differences associated with a change from alkyl to aryl are much more pronounced. For example, despite the differences in As 4p mixing, the first features in the S K-edge XAS spectra of [PPh4][S2CNPh2] and As[S2CNPh2]3 were both shifted by 0.3 eV compared to their alkyl-substituted derivatives. DFT calculations revealed that the unique shift observed for [PPh4][S2CNPh2] is due to phenyl-induced splitting of the π* orbitals delocalized over N, C and S. A similar phenomenon accounts for the shift observed for As[S2CNPh2]3, but the presence of two unique S environments(As–S and As⋯S)prevented reliable analysis of As–S covalency from the XAS data. In the absence of experimental values, DFT calculations revealed a decrease in As–S orbital mixing in As[S2CNPh2]3 that stems from a redistribution of electron density to S atoms participating in weaker As⋯S interactions. Simulated spectra obtained from TDDFT calculations reproduce the experimental differences in the S K-edge XAS data, which suggests that the theory is accurately modeling the experimental differences in As–S orbital mixing. The results highlight how S K-edge XAS and DFT can be used cooperatively to understand the electronic structure of low symmetry coordination complexes containing S atoms in different chemical environments. © 2014 The Partner Organisations.