Influence of Strong pi-Acceptor Ligands on Cr-K-edge X-Ray Absorption Spectral Signatures and Consequences on Interpretation of Surface Sites in the Phillips Catalyst

X-ray absorption spectroscopy (XAS) is one of the most powerful characterization techniques, that has been intensively employed to study the Phillips polymerization catalyst (CrO3/SiO2). While Cr K-edge XAS signatures are used to evaluate the nature of surface (active) sites, they are highly sensitive to oxidation state, geometry and types of ligands, making interpretation challenging. In the specific case of CrO3/SiO2, CO has been particularly used both as a reductant to generate the expected low valent Cr sites and a probe to understand surface Cr sites. Considering the electronic properties of CO, a strong s-donor and pi-acceptor ligand, one may wonder the impact of the coordination of CO on Cr on its XAS signature. We herein built a molecular low-valent Cr library bearing isocyanide ligands, which mimic CO as its isoelectronic counterpart, as a model of low-valent Cr sites interacting with p-acceptor ligand. Cr K-edge XAS augmented with DFT calculations elucidated the profound effect of isocyanide ligand on both XANES and EXAFS regions giving a rise to characteristic features as well as the significant stabilization of low-spin Cr(II/III) species, which potentially alter the ease of interpretation of XAS spectra. Taking the herein demonstrated effect of p-acceptor ligand into account, experimental Cr K-edge spectra of CO-reduced Phillips catalyst at different temperatures were reproduced show-casing the effect of CO on Cr sites.