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 = Et2vs. (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.

Solution and Solid‐State Ligand K‐Edge XAS Studies of PdCl2 Diphosphine Complexes with Phenyl and Cyclohexyl Substituents

“…To evaluate the possible changes in Pd–P covalency in solution and the solid‐state, we performed dispersion‐corrected DFT and TDDFT calculations on 1 – 3 with a polarizable continuum model to simulate the CH 2 Cl 2 solution. The B3LYP functional was again selected because it has been shown to be remarkably accurate in the simulation of experimental XAS spectra via time‐dependent density functional theory (TDDFT) , , , , , , . For consistency, TDDFT calculations for 1 – 3 from our previous study were recalculated with dispersion corrections for comparison to the dispersion‐corrected solution calculations presented here.…”

“…The oscillator strengths of the calculated transitions were multiplied by a factor of 300 to bring them on scale with the experimental data. Energy shifts of +49.6 eV (P K‐edge; left) and +64.8 eV (Cl K‐edge; right) were applied to the calculated spectra so the relative peak positions could be compared, as described previously , , …”

“…All XAS data were collected at the Stanford Synchrotron Radiation Lightsource (SSRL). Solid samples of 4 – 6 were prepared as described previously in an N 2 ‐filled glovebox and transferred to the BL in a sealed vessel , . Cl K‐edge XAS data were collected on Beamline (BL) 4–3, which is equipped with a 20‐pole, 2.0 Tesla wiggler and double crystal monochromator cooled with liquid nitrogen.…”

“…Quantifying M–P covalency in solution is necessary when considering how the XAS results correlate to Pd II complexes and precatalysts that may be found in solution at ambient or elevated temperatures in catalytic reactions. Exchanging phenyl substituents for alkyl is important because it has been shown by us that changes in phenyl group orientation can influence ligand K‐edge XAS intensities in aryl‐substituted dithiophosphinates and dithiocarbamates , . Our goal was to determine if similar effects were responsible for the observed increase in Pd–P and Ni–P covalency in Pd(dppe)Cl 2 and Ni(dppe)Cl 2 .…”

Ligand K-edge XAS studies of arsenic-sulfur and metal-phosphorus bonds, and synthetic explorations of 1,8,10,9-triazaboradecalin and related complexes

“…The results revealed that P-M-P angle (also called the bite angle) is not sufficient to account for variations in M-P covalency in diphosphine complexes, which is notable because it is commonly used as a parameter for evaluating structure/function correlations in diphosphine catalysts. [54][55][56][57][58][59][60] Trans influence contributions to M-P covalency were investigated by collecting P K-edge XAS data on trans-Pd(PPh3)2Cl2 (18) for comparison to the cis diphosphine (11), Ni(dppe)Cl2 (12), and Ni(dppp)Cl2 (13). Right -P K-edge XAS spectra of Pd(dppm)Cl2 (14), Pd(dppe)Cl2 (15),Pd(dppp)Cl2 (16), and Pd(dppb)Cl2 (17).…”