Metal oxo clusters are explored as an atomically defined building unit of nanocrystals where the metal atoms are connected by oxo bridges.[1] Here, we aim to investigate the catalytic activity of zirconium oxo clusters through an esterification reaction. Several zirconium alkoxides, oxides and MOFs have been explored as esterification catalysts. Here, we show that oxo clusters can be considered as an uprising catalyst which at times can also outperform nanocrystals and MOFs that have the same active site.
The higher surface-to-volume ratio of oxo clusters compared to nanocrystals and their homogeneous nature make them an ideal candidate for further catalytic explorations. Looking into the turnover number (TON) per surface metal site reveals that the clusters are also intrinsically more catalytically active. Also, the easily accessible active sites compared to MOFs make them superior. We find that for large substrates, e.g., oleic acid, UiO-66 has negligible activity while clusters provide almost quantitative conversion. For smaller substrates like acetic acid, UiO-66 has an appreciable activity, albeit still lower than the homogeneous clusters.
Focusing on cluster catalysts for large substrates, we obtain high conversion in solvent-free reactions, for sterically hindered alcohols (hexanol, 2-ethylhexanol, benzyl alcohol and neopentyl alcohol). We further expanded our research to hafnium oxo clusters, showing that they behave similarly to zirconium oxo clusters in catalytic reactivity (hexyl oleate ester formation).
The homogeneous cluster catalyst can be recovered and shows identical activity. The structural integrity of the cluster after catalysis is confirmed using Pair Distribution Function analysis (PDF), Nuclear Magnetic Resonance (NMR) spectroscopy and IR spectroscopy.[2] We have dived deep into zirconium-based esterification catalysis and found that when zirconium alkoxides are used as catalysts, the same oxo cluster is retrieved at the end of the reaction. These results thus shed light on the active catalytic species in "homogeneous", zirconium-catalyzed esterification and is also the first step to a more rational design of (enantioselective) esterification catalysts.

[1] Van den Eynden, D.; Pokratath, R.; Mathew, J.P.; Goossens, E.; De Buysser, K.; De Roo, J., Chem. Sci. 2023, 14, 573.
[2] Pulparayil Mathew, J.; Seno, C.; Jaiswal, M.; Simms, C.; Reichholf, N.; Van den Eynden, D.; Parac-Vogt, T. N.; De Roo, J., Small Science 2024, 2400369.