Synthesis, Characterization, and Reactivity of the Highly Unsaturated Silica-Supported Trisiloxy Tantalum: (⋮SiO)3Ta(III)

The silica-supported tantalum hydride, (≡SiO) 2 Ta ( I I I ) H, is transformed under hydrogen by thermal treatment up to 500 °C into a new well-defined surface species, the trisiloxy tantalum (≡SiO) 3 Ta ( I I I ) . The Ta L I I I -edge EXAFS gives a first coordination sphere of three oxygen atoms at 1.908 A. The mechanism of formation of (≡SiO) 3 Ta ( I I I ) 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) 3 Ta ( I I I ) reacts with pyridine and PMe 3 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) 3 Ta ( V ) (OEt) 2 and (≡SiO) 3 Ta ( V ) -(OH) 2 . It still reacts with molecular oxygen to give (≡SiO) 3 Ta ( V ) (=O) by an unknown mechanism. Although (≡SiO) 3 Ta ( I I I ) exhibits an extreme electron deficiency and redox properties due to its d 2 configuration, this new surface tantalum species does not show any longer the catalytic properties of the tantalum hydride.