Oxidation state assignment in cerium nanoclusters: conflicting conclusions from single-crystal X-ray diffraction and spectroscopic data

Abstract

Over the last decade, ceria nanoparticles have received much attention as an alternative to bulk cerium dioxide (CeO2) due to their increased reactivity at lower temperatures. Further interest in Ce based clusters and nanoparticles stems from the nuclear community as cerium has historically been used as a surrogate for plutonium owing to their similar ionic radii, similar coordination environments, and accessible 3+/4+ redox couples. Several cerium clusters have been isolated using organic capping ligands to passivate the surface, thereby halting further oligomeric growth. Clusters capped by inorganic ligands, such as Cl-, are a bit more elusive. Yet our group has recently harnessed nonbonding interactions to isolate several Ce nanoclusters capped by chloride ions and water molecules; however, the valence state assignment of the Ce sites and the 3-O/OH sites on the cluster surface remains unclear. In one such example, we have crystallized a compound whose structure is built from Ce38 clusters. Analysis by single crystal X-ray diffraction and bond valence summation point towards exclusively tetravalent cerium but there is ambiguity in the assignment of 3-O/OH sites and the occupancy of outer sphere cations that charge balance the clusters, both of which have implications on the Ce oxidation state assignment. Additionally, other experimental data including X-ray absorption and X-ray photoelectron spectroscopies suggest the presence of Ce3+ in the cluster; charge compensation of the Ce4+/Ce3+ ratio can be accounted for through the relative ratio of 3-O/OH surface sites. Several plausible formula for the clusters have been determined. Taking into account both the SCXRD and spectroscopic data, “would you publish this”?