U–C Bond Insertion, Ring-Opening, and C–H Activation in a Uranium Bis(diisopropylamino)cyclopropenylidene (BAC) Adduct
Thien H. Nguyen, Xiaojuan Yu, Greggory T. Kent, Jochen Autschbach, Trevor W. Hayton
Abstract
Reaction of [U(NR 2 ) 3 ] (R = SiMe 3 ) with 1 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) in Et 2 O results in the formation of [(NR 2 ) 3 U(BAC)] ( 1 ), which can be isolated in modest yields. Thermolysis of 1 in C 6 D 6 at 85 °C results in the formation of the ring-opened U(IV) product, [(NR 2 ) 2 U{ N (R)(SiMe 2 CH═C(N i Pr 2 )C(N i Pr 2 )═ C H)}] ( 2 ), which can be isolated in low yields. Mechanistic studies suggest that the formation of 2 proceeds via dissociation of BAC from 1 to regenerate [U(NR 2 ) 3 ], which converts into [U{ N (R)(SiMe 2 C H 2 )}(NR 2 ) 2 ] at the elevated temperatures. BAC then inserts into the U–C bond of [U{ N (R)(SiMe 2 C H 2 )}(NR 2 ) 2 ] to generate a cyclopropenyl intermediate, which undergoes ring opening and C–H activation to afford the final product, 2 . We hypothesize that the ring-opening generates an unobserved carbene intermediate. Notably, thermolysis of a 1:1 mixture of independently prepared [U{ N (R)(SiMe 2 C H 2 )}(NR 2 ) 2 ] and BAC results in clean formation of 2, providing strong support for the proposed mechanism. The formulations of both 1 and 2 were confirmed by X-ray crystallography. Theoretical calculations indicate that the hypothesized uranium carbene intermediate features strong U–C bonding, potentially with some carbyne character in the electronic structure.