The silica-supported tantalum hydride, (≡SiO)2Ta(III)H, is transformed under hydrogen by thermal treatment up to 500 °C into a new well-defined surface species, the trisiloxy tantalum (≡SiO)3Ta(III). The Ta LIII-edge EXAFS gives a first coordination sphere of three oxygen atoms at 1.908 Å. The mechanism of formation of (≡SiO)3Ta(III) has been studied by IR: it involves a hydride transfer from tantalum to silicon and a siloxy transfer from silicon to tantalum. This surface reconstruction occurs by opening of the ≡Si-O-Si≡ bridge of silica probably in the vicinity of the tantalum hydride. (≡SiO)3Ta(III) reacts with pyridine and PMes to give 1:1 adducts. It also reacts with ethanol and water by an oxidation process to give molecular hydrogen and the surface compounds (≡SiO)3Ta(V)(OEt)2 and (≡SiO)3Ta(V)-(OH)2. It still reacts with molecular oxygen to give (≡SiO)3Ta(V)(=O) by an unknown mechanism. Although (≡SiO)3Ta(III) exhibits an extreme electron deficiency and redox properties due to its d2 configuration, this new surface tantalum species does not show any longer the catalytic properties of the tantalum hydride.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry