Base Catalysis of Sodium Salts of [Ta6−xNbxO19]8− Mixed-Oxide Clusters

The solid base catalysis of sodium salts of Lindqvist-type metal oxide clusters was investigated using a Knoevenagel condensation reaction. We successfully synthesized the sodium salts of Ta and Nb mixed-oxide clusters Na8−nHn[(Ta6−xNbx)O19]·15H2O (Na-Ta6−xNbx, n = 0, 1, x = 0–6) and found them to exhibit activity for proton abstraction from nitrile substrates with a pKa value of 23.8, which is comparable to that of the conventional solid base MgO. The Ta-rich Na-Ta6 and Na-Ta4Nb2 exhibited high activity among Ta and Nb mixed-oxide clusters. Synchrotron X-ray diffraction (SXRD) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and X-ray absorption spectroscopy (XAS) revealed the structure of Na-Ta6−xNbx: (1) The crystal structure changed from Na7H[M6O19]·15H2O to Na8[M6O19]·15H2O (M = Ta or Nb) by the anisotropic expansion of the unit cell with an increase in Ta content; (2) Highly symmetrical Lindqvist [Ta6−xNbxO19]8− was generated in Na-Ta4Nb2 and Na-Ta6 because of the symmetrical association of Na+ ions with [Ta6−xNbxO19]8− in the structure. DFT calculation revealed that the Lindqvist structures with high symmetry have large NBO charges on surface oxygen species, which are strongly related to base catalytic activity, whereas the composition hardly affects the NBO charges. The above results showed that the Brønsted base catalysis was sensitive to the symmetry of the Lindqvist [Ta6−xNbxO19]8− structure. These findings contribute to the design of solid base catalysts composed of anionic metal oxide clusters with alkaline-metal cations.