Well-defined surface imido amido tantalum(v) species from ammonia and silica-supported tantalum hydrides

The MCM-41 supported hydrides (triple bond]SiO)(2)TaH(3)], 1a, and (triple bond]SiO)(2)TaH(3)], 1b, cleave N-H bonds of ammonia at room temperature to yield the well-defined imido amido surface complexes (triple bond]SiO)(2)Ta(NH)(NH(2))], 2, and 2xNH(3). Additionally, the surface silanes triple bond]Si-H] that exist in close proximity to 1a and 1b also react with ammonia at room temperature to give the surface silylamido Si-NH(2)]. Such reaction is tantalum assisted: surface silanes were synthesized independently and in absence of tantalum by reaction of highly strained silica, SiO(2-1000), with SiH(4) and no reaction with ammonia was observed. Surface-supported complexes 2, 2xNH(3), and triple bond]Si-NH(2)] have been characterized by, inter alia, solid-state NMR, IR, and EXAFS and independent synthesis of triple bond]Si-NH(2)]. The NMR studies on the fully 15N-labeled samples have led to unambiguous discrimination between imido, amido, and amino resonances of 2*, 2*x(15)NH(3), and triple bond]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 NH(3) and 15NH(3), and after H/D exchange has yielded the determination of all the NH(x) 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) Angstrom for Ta=N, 1.89(1) Angstrom for Ta-O, and 1.98(2) Angstrom for Ta-N.