Improving sensitivity of XANES structural fit to the bridged metal–metal coordination

Abstract

Hard X-ray absorption spectroscopy is a valuable in situ probe for non-destructive diagnostics of metal sites. The low-energy interval of a spectrum (XANES) contains information about the metal oxidation state, ligand type, symmetry and distances in the first coordination shell but shows almost no dependency on the bridged metal–metal bond length. The higher-energy interval (EXAFS), on the contrary, is more sensitive to the coordination numbers and can decouple the contribution from distances in different coordination shells. Supervised machine-learning methods can combine information from different intervals of a spectrum; however, computational approaches for the near-edge region of the spectrum and higher energies are different. This work aims to keep all benefits of XANES and extend its sensitivity towards the interatomic distances in the first and second coordination shells. Using a binuclear bridged copper complex as a case study and cross-validation analysis as a quantitative tool it is shown that the first 170 eV above the edge are already sufficient to balance the contributions of Cu–O/N scattering and Cu–Cu scattering. As a more general outcome this work highlights the trivial but often overlooked importance of using `longer' energy intervals of XANES for structural refinement and machine-learning predictions. The first 200 eV above the absorption edge still do not require parametrization of Debye–Waller damping and can be calculated within full multiple scattering or finite difference approximations with only moderately increased computational costs.

In Situ XAS Diagnostics of Reductive Hydroformylation Reaction in Segmented Flow Under Elevated Pressure and Temperature

“…We also increased the sensitivity to Rh−Rh coordination in LCF analysis by converting XANES spectra in the first 250 eV to k 2 •χ(k) form according to the procedure described for bridged metal complexes. 35 Figure 4 shows the results of the linear combination fit analysis. Calculated spectra for DFT-optimized models adequately reproduce the oscillations in the near-edge region of spectra.…”

“…We restricted the fit to only chemically relevant species with minimum energy as obtained from DFT optimization. We also increased the sensitivity to Rh–Rh coordination in LCF analysis by converting XANES spectra in the first 250 eV to k 2 ·χ( k ) form according to the procedure described for bridged metal complexes …”