Extreme π-Loading as a Design Element for Accessing Imido Ligand Reactivity. A CCC-NHC Pincer Tantalum Bis(imido) Complex: Synthesis, Characterization, and Catalytic Oxidative Amination of Alkenes

Abstract

A rare Ta bis(imido) complex, which has unique reactivity, was prepared by manipulating the coordination sphere of a CCC-NHC pincer Ta complex. The reaction of lithium tert-butylamide with complex 1 yielded (1,3-bis(3′-butylimidazol-2′-yl-1′-idene)-2-phenylene)bis(tert-butylimido)tantalum(V) (2) as a lithium iodide bridged dimer, as determined by the X-ray structure. Complex 2 catalytically cyclized α,ω-aminoalkenes to effect an oxidative amination of alkenes (dehydrogenation by C–H activation) and produced a cyclic imine, an equivalent of reduced substrate, and varying proportions of hydroamination. Various additives and concentration impact the catalytic results. Computational and experimental observations have led to an initial mechanistic hypothesis. Based upon it, precatalyst 2 appears to be the first example of a bifunctional catalyst (MH-NHR) that is highly selective for nonpolar C═C bonds in preference to polar C═X bonds for outer-sphere hydrogenation.