Abstract
Multiply-bonded metal-ligand systems such as akylidyne, amido, and imido groups have long been proposed as key intermediates in heterogeneous catalytic reactions such as olefin metathesis1,2, polymerization1,2, and ammoxidation.1,2,3,4 Despite the importance of these intermediates, the characterization of such species on catalyst surfaces has been hampered by their low concentration and transient nature.1 Recent efforts to characterize similar species to the purposed intermediates has included the production of organometallic species containing these functional groups that are subsequently grafted to silica surfaces by wet impregnation.5 The use of a novel method, such as using derived porous optical glasses containing catalytic sites for studying these species, can potentially better characterize the species. These materials are fabricated by co-condensing metal alkoxides with tetramethoxysilicate to produce isolated metal oxide sites bound into a porous silica matrix and then forming the catalyst. The metal-silica gel catalysts differ from most bulk catalysts in that they are both porous and optically clear, permitting precise electronic and vibrational spectroscopic measurements.
The amido and imido groups been proposed as a key intermediates in several very interesting reactions and the wide availability of small molecule metal-phenylamido and metal-phenylimido complexes makes an ideal prototype system to try and generate and a metal-amido or metal-imido functional group on a supported metal oxide heterogeneous catalyst. This study shows the characterization of the products from the reaction of silica bound chromium(VI) sites with aniline. The resulting chromium-phenylamido species was characterized using UV-vis, IR, Raman, GC, EPR, and SQUID spectroscopy. While the generation of phenylamido groups on chromium and other early transition metals is well established for small molecule analogs, we believe that the research presented here is the first unambiguous generation of such a group on a surface-bound metal.
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