Can the clusters Zr6O4(OH)4(OOCR)12 and [Zr6O4(OH)4(OOCR)12]2 be converted into each other?

Upon reaction of zirconium alkoxides, Zr(OR)4, with carboxylic acids, the clusters Zr6O4(OH)4(OOCR)12 (Zr6) or [Zr6O4(OH)4(OOCR)12]2 (Zr12) were obtained, depending on the employed carboxylic acid. The structures of Zr12 clusters with acetate, propionate, vinyl acetate or 3,3′-dimethylacrylate ligands were determined by single-crystal X-ray diffraction, as well as that of methacrylate/acetate and methacrylate/propionate mixed-ligand clusters. The structure of the hafnium cluster [Hf6O4(OH)4(acetate)12]2 is also reported for comparison. The Zr12 clusters are structurally related to the Zr6 clusters since they are composed of two Zr6 sub-units which are bridged by four carboxylate ligands. In each compound, carboxylic acid molecules interact with the cluster by hydrogen bonding to part of the μ3-OH ligands. The clusters are highly dynamic in solution, due to site exchange of the carboxylate ligands; low-temperature NMR spectra of the clusters correspond to the solid-state structures. The Zr6 and Zr12 clusters can be clearly distinguished by their solution NMR spectra. Reactions between selected Zr12 clusters and carboxylic acids led to the conclusion, that only part of the ligands is accessible for ligand exchange reactions. In contrast, all ligands were exchanged when the Zr6 cluster Zr6O4(OH)4(methacrylate)12 was treated with an excess of propionic acid, and Zr6O4(OH)4(propionate)12 was formed. However, the Zr12 cluster [Zr6O4(OH)4(propionate)12]2 was obtained from the reaction of Zr(OBu)4 with propionic acid. The Zr6 and Zr12 clusters thus do not interconvert at the given reaction conditions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)