The MCM-41 supported hydrides [(≡SiO)2TaH], 1a, and [(≡=SiO)2TaH3], 1b, cleave N-H bonds of ammonia at room temperature to yield the well-defined imido amido surface complexes [(≡=SiO)2Ta-(NH)(NH2)], 2, and 2·NH 3. Additionally, the surface silanes [≡Si-H] that exist in close proximity to 1a and 1b also react with ammonia at room temperature to give the surface silylamido [≡Si-NH2]. Such reaction is tantalum assisted: surface silanes were synthesized independently and in absence of tantalum by reaction of highly strained silica, SiO2-1000, with SiH4 and no reaction with ammonia was observed. Surface-supported complexes 2, 2·NH3, and [≡Si-NH2] have been characterized by, inter alia, solid-state NMR, IR, and EXAFS and independent synthesis of [≡Si-NH2]. The NMR studies on the fully 15N-labeled samples have led to unambiguous discrimination between imido, amido, and amino resonances of 2*, 2*·15NH3, and [≡Si-15NH 2] through the combination of solid-state magic angle spinning (MAS), heteronuclear correlation (HETCOR), 2D proton double-quantum (DQ) single-quantum (SQ) correlation, and 2D proton triple-quantum (TQ) single-quantum (SQ) correlation spectra. The in situ IR monitoring of the reaction of 1a and 1b with regular NH3 and 15NH 3, and after H/D exchange has yielded the determination of all the NHx vibration and deformation modes, with their respective H/D and 14N/15N isotopic shifts. EXAFS study yielded the bond distances in 2 of 1.79(2) Å for Ta=N, 1.89(1) Å for Ta-O, and 1.98(2) Å for Ta-N.
ASJC Scopus subject areas
- Colloid and Surface Chemistry