[2+2] cycloaddition reactions at terminal imido uranium(IV) complexes to yield isolable cycloadducts

The terminal imido complexes U(NDipp)Cl-2(tppo)(3) (tppo = triphenylphosphine oxide) and U(NDipp)Cl-2 (R(2)bpy)(2) (Dipp = 2,6-Pr-i(2)-C6H3; R(2)bpy = 4,4'-R-2-2,2'-bipyridyl; R = Me, tBu) contain reactive U=N bonds, which undergo [2 + 2] cycloaddition reactions with the N-C multiple bonds of isocyanates and benzonitrile. These low valent imido complexes display a preference for forming cycloaddition products, in contrast to high valent bis(imido)complexes, which undergo imido group exchange when treated with isocyanates. This disparity suggests that the U(IV)=NR linkage, already known to be more ionic than U(VI)=NR bonds, is also weaker than its U(VI) congener. The cycloaddition products that were used in this qualitative bond strength analysis have been characterized by X-ray crystallography and NMR spectroscopy. Most importantly, U(NDipp)Cl-2(tppo)(3) and U(NDipp)Cl-2(R(2)bpy) appear to be excellent synthetic precursors to new and intriguing organometallic uranium complexes. (C) 2014 Elsevier B. V. All rights reserved.