Well-Defined Surface Imido Amido Tantalum(V) Species from Ammonia and Silica-Supported Tantalum Hydrides

Abstract

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·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·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−15NH2] 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 15NH3, 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 TaN, 1.89(1) Å for Ta−O, and 1.98(2) Å for Ta−N.