Two mixed-valent cerium oxo clusters: synthesis, structure, and self-assembly

Studies on cerium oxo clusters (CeOCs) are not only significant for understanding the redox and hydrolysis behaviors of Ce(III/IV) ions but also crucial for the rational synthesis of novel clusters and nanoceria with specific Ce(III)/Ce(IV) ratios. Here, two sets of reactions were conducted using cerium nitrate and H₂O₂-oxidized cerium nitrate, resulting in the formation of two distinct mixed-valent CeOCs [Ce^III ₄Ce^IV ₁₀O₁₄(OH)₂(PhCO₂)₂₂(DMF)₆] (Ce₁₄) and [Ce^III ₂Ce^IV ₂₂O₂₈(OH)₈(PhCO₂)₃₀(DMF)₄] (Ce_24C). These two clusters exhibit different structures and Ce(III)/Ce(IV) ratios, demonstrating the critical role of cerium oxidation states and the occurrence of redox reactions in cluster formation. Ce₁₄ is the first tetradecanuclear CeOC with a novel structure, whereas Ce_24C differed in its Ce(III)/Ce(IV) ratio, protonation levels of O atoms, and ligands from previously reported 24-nuclear CeOCs. Furthermore, various techniques were employed to investigate the formation process of these two clusters. X-ray photoelectron spectra (XPS) revealed that the white precipitates formed during the preparation of Ce₁₄ contain Ce(III) ions, while the reddish-brown precipitates formed during the preparation of Ce_24C contain a mixture of Ce(III) and Ce(IV) ions. These two precipitations were individually dissolved in N,N-Dimethylformamide (DMF). The evolution of solution color and ultraviolet-visible (UV-Vis) spectra over time revealed the gradual oxidation of partial Ce(III) ions by oxygen in the solution of the white precipitation. As Ce(IV) ions increased in this solution, time-resolved small angle X-ray scattering (SAXS) data demonstrated the self-assembly of the Ce₁₄ clusters after 4 days. In contrast, SAXS data and UV-Vis spectra revealed the rapid assembly of Ce_24C clusters within 2 h due to the initial coexistence of Ce(IV) and Ce(III) ions in the DMF solution of the reddish-brown precipitation. The continued reduction of partial Ce(IV) ions in this solution does not affect Ce_24C clusters' formation and stability. Our studies expand the family of CeOCs and enhance our understanding of the effects of cerium's oxidation states on cluster formation.