Well-defined surface imido amido tantalum(V) species from ammonia and silica-supported tantalum hydrides
The MCM-41 supported hydrides [( SiO)(2)TaH], 1a, and [( SiO)(2)TaH3], 1b, cleave N-H bonds of ammonia at room temperature to yield the well-defined imido amido surface complexes [( SiO)(2)Ta-(NH)(NH2)], 2, and 2 center dot NH3. 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 center dot 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 N-15-labeled samples have led to unambiguous discrimination between imido, amido, and amino resonances of 2*, 2*, (NH3)-N-15, and [ Si-(NH2)-N-15] 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 (NH3)-N-15, and after H/D exchange has yielded the determination of all the NH x vibration and deformation modes, with their respective H/D and N-14/N-15 isotopic shifts. EXAFS study yielded the bond distances in 2 of 1.79(2) angstrom for Ta=N, 1.89(1) angstrom for Ta-O, and 1.98(2) angstrom for Ta-N.