Synthesis, Characterization, and Reactivity of the Highly Unsaturated Silica-Supported Trisiloxy Tantalum: (⋮SiO)3Ta(III)
journal contributionposted on 24.10.2002, 00:00 by Guillaume Saggio, Aimery de Mallmann, Barry Maunders, Mostafa Taoufik, Jean Thivolle-Cazat, Jean-Marie Basset
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 PMe3 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.