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

• Published in: Chemical science (Cambridge) Vol. 14; no. 3; pp. 573 - 585
• Main Authors: Van den Eynden, Dietger, Pokratath, Rohan, Mathew, Jikson Pulparayil, Goossens, Eline, De Buysser, Klaartje, De Roo, Jonathan
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 18-01-2023
• Subjects: Carboxylic acids; Chain branching; Chemistry; Distribution functions; Esterification; Ethanol; Fatty acids; Infrared analysis; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Ligands; Mass spectrometry; Materials science; Metal clusters; Nanocrystals; NMR; Nuclear magnetic resonance; Oleic acid; Polyoxometallates; Purification; Single crystals; Thermogravimetry; Zirconium oxides
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d2sc05037d

Metal oxo clusters of the type M
6
O
4
(OH)
4
(OOCR)
12
(M = Zr or Hf) are valuable building blocks for materials science. Here, 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). The fatty acid capped oxo clusters have a high solubility but do not crystallize, precluding traditional purification and single-crystal XRD analysis. We thus develop alternative purification strategies and we use X-ray total scattering and Pair Distribution Function (PDF) analysis as our main method to elucidate the structure of the cluster core. We identify the correct structure from a series of possible clusters (
Zr3
,
Zr4
,
Zr6
,
Zr12
,
Zr10
, and
Zr26
). Excellent refinements are only obtained when the ligands are part of the structure model. Further evidence for the cluster composition is provided by nuclear magnetic resonance (NMR), infrared spectroscopy (FTIR), thermogravimetry analysis (TGA), and mass spectrometry (MS). We find that hydrogen bonded carboxylic acid is an intrinsic part of the oxo cluster. Using our analytical tools, we elucidate the conversion from a
Zr6
monomer to a
Zr12
dimer (and
vice versa
), induced by carboxylate ligand exchange. Finally, we compare the catalytic performance of
Zr12
-oleate clusters with oleate capped, 5.5 nm zirconium oxide nanocrystals in the esterification of oleic acid with ethanol. The oxo clusters present a five times higher reaction rate, due to their higher surface area. Since the oxo clusters are the lower limit of downscaling oxide nanocrystals, we present them as appealing catalytic materials, and as atomically precise model systems. In addition, the lessons learned regarding PDF analysis are applicable to other areas of cluster science as well, from semiconductor and metal clusters, to polyoxometalates.
Metal oxo clusters of the type M
6
O
4
(OH)
4
(OOCR)
12
(M = Zr or Hf) are valuable building blocks for materials science.