Nonaqueous formation mechanism of zirconium and hafnium oxo clusters

Metal oxo clusters are interesting catalysts and valuable building blocks for Metal-Organic Frameworks (MOFs) and 3D-printing. Here, we study the nonaqueous formation mechanism of the prototypical M6O4(OH)4(RCOO)12 (M = Zr or Hf) clusters using a suite of in situ and ex situ characterization techniques. In the reaction of metal alkoxide with excess carboxylic acid, an asymmetric trinuclear complex is transiently formed en route to the hexanuclear oxo cluster. The reaction produces ester as by-product and we determine that precisely 4/3 equivalent of ester is required for every Zr atom to reach full yield of the oxo cluster. The esterification can be divided in (i) a fast process, producing 1/3 equivalent of ester and one μ3-O linker, and (ii) a slow process, producing the remaining equivalent of ester to complete the condensation of the oxo cluster. The mechanism is supported by kinetic modeling. These insights are crucial to develop oxo clusters synthesis by design, and even shed light on nanocrystal synthesis from an atomically precise perspective.