From single-site tantalum complexes to nanoparticles of Ta( x )N( y ) and TaO( x )N( y ) supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy

Air-stable catalysts consisting of tantalum nitride nanoparticles represented as a mixture of Ta( x )N( y ) and TaO( x )N( y ) with diameters in the range of 0.5 to 3 nm supported on highly dehydroxylated silica were synthesized from TaMe(5) (Me = methyl) and dimeric Ta(2)(OMe)(10) with guidance by the principles of surface organometallic chemistry (SOMC). Characterization of the supported precursors and the supported nanoparticles formed from them was carried out by IR, NMR, UV-Vis, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopies complemented with XRD and high-resolution TEM, with dynamic nuclear polarization surface enhanced NMR spectroscopy being especially helpful by providing enhanced intensities of the signals of (1)H, (13)C, (29)Si, and (15)N at their natural abundances. The characterization data provide details of the synthesis chemistry, including evidence of (a) O(2) insertion into Ta–CH(3) species on the support and (b) a binuclear to mononuclear transformation of species formed from Ta(2)(OMe)(10) on the support. A catalytic test reaction, cyclooctene epoxidation, was used to probe the supported nanoparticles, with 30% H(2)O(2) serving as the oxidant. The catalysts gave selectivities up to 98% for the epoxide at conversions as high as 99% with a 3.4 wt% loading of Ta present as Ta( x )N( y )/TaO( x )N( y ).