Formation of organically surface-modified metal oxo clusters from carboxylic acids and metal alkoxides: a mechanistic study

The reaction of Zr(OR)4 (OR = OnBu, OnPr) with methacrylic acid was monitored by extended X-ray absorption fine structure (EXAFS), 1H and 13C NMR, HPLC, and Raman measurements. The study revealed that the initial dimeric zirconium alkoxides react rapidly with the carboxylic acid and form higher aggregated multinuclear compounds. EXAFS investigations showed that the structures of the resulting aggregates in solution depend on the carboxylic acid to alkoxide ratio. In addition, the reaction rates also differ depending on this ratio; while for a methacrylic acid to zirconium alkoxide ratio of 4∶1 the reaction is slow, metal oxo clusters form rapidly with a 7∶1 ratio. Methacrylic acid ester is simultaneously formed during the reaction, with concomitant production of water, required for the formation and condensation reactions in the cluster preparation.