Phosphorus-based ligand effects on the structure and radical scavenging ability of molecular nanoparticles of CeO₂

Abstract

Two new Ce^IV/O^2- clusters, (pyH)₈[Ce₁₀O₄(OH)₄(O₃PPh)₁₂(NO₃)₁₂] (1) and [Ce₆O₄(OH)₄(O₂PPh₂)₄(O₂C^<i>t</i>Bu)₈] (2), have been prepared that contain P-based ligands for the first time. They were obtained from the reaction of (NH₄)₂[Ce(NO₃)₆], PhPO₃H₂ or Ph₂PO₂H, and ^<i>t</i>BuCO₂H in a 2 : 1 : 2 molar ratio in pyridine/MeOH (10 : 1 mL). Both compounds contain a {Ce₆O₄(OH)₄} face-capped octahedral core, with 1 containing an additional four Ce^IV on the outside to give a supertetrahedral Ce₁₀ topology; the {Ce₆O₈} unit is the smallest recognizable fragment of the fluorite structure of CeO₂. The HO˙ radical scavenging activities of 1 and 2 were measured by UV/vis spectral monitoring of methylene blue oxidation by HO˙ radicals in the presence and absence of the Ce/O clusters, and the results compared with those for larger Ce₂₄ and Ce₃₈ molecular nanoparticles of CeO₂ prepared in previous work. 1 and 2 are both very poor HO˙ radical scavengers compared with Ce₂₄ and Ce₃₈, a result that is consistent with reports in the literature that PO₄^3- ions inhibit the radical scavenging ability of traditional CeO₂ nanoparticles and putatively assigned to PO₄^3- binding to the surface.