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

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.