Fatty acid capped, metal oxo clusters as smallest conceivable nanocrystal prototypes

Metal oxo clusters of the type \ce{M6O4(OH)4(RCOO)12} (M = Zr of Hf) are valuable building blocks for material science. Here, we develop them as smallest conceivable nanocrystal prototypes. We synthesize a series of zirconium and hafnium oxo clusters with ligands that are typically used to stabilize oxide nanocrystals (fatty acids with long and/or branched chains). In contrast to previously reported discrete oxo clusters with short/rigid carboxylates (e.g., acetate, benzoate), the fatty acid capped oxo clusters have a high solubility but do not crystallize, precluding traditional purification and single-crystal XRD analysis of clusters. We thus develop alternative purification strategies and structural analysis tools. We use X-ray total scattering and Pair Distribution Function (PDF) analysis as our main tool to elucidate the structure of the cluster core. In contrast to traditional PDF analysis of larger clusters and nanocrystals, we show that the structure models need to include the carboxylate binding groups to obtain excellent refinements. Our methodology is able to pick up the correct structure from a series of possible structure models (\textbf{Zr4}, \textbf{Zr6}, \textbf{Zr12}). Further supporting evidence for the cluster composition (including their ligand shell) is provided by nuclear magnetic resonance (NMR), infrared spectroscopy (FTIR), thermogravimetry analysis (TGA) and mass spectrometry (MS). We find that the ligands have multiple binding modes and that hydrogen bonded carboxylic acid is an intrinsic part of the oxo cluster. Using our analytical tools, we elucidate the conversion from \textbf{Zr6} monomer to \textbf{Zr12} dimer (and vice versa), induced by carboxylate ligand exchange. Finally, we compared the catalytic performance of \textbf{Zr12}-oleate clusters with oleate capped, 3-5 nm zirconium oxide nanocrystals in the esterification of oleic acid with ethanol. The oxo clusters are much more catalytically active, due to their higher surface area. Since the oxo clusters are the limit of downscaling oxide nanocrystals, we thus propose them here as appealing catalytic materials, or at least as atomically precise model systems. In addition, our analytical (PDF) methodology is generally applicable and expected to find use in other areas of clusters as well, and will be especially valuable for clusters with weakly scattering core atoms.