U-C Bond Insertion, Ring-Opening, and C-H Activation in a Uranium Bis(diisopropylamino)cyclopropenylidene (BAC) Adduct

Reaction of [U(NR2)3] (R = SiMe3) with 1 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) in Et2O results in the formation of [(NR2)3U(BAC)] (1), which can be isolated in modest yields. Thermolysis of 1 in C6D6 at 85 °C results in the formation of the ring-opened U(IV) product, [(NR2)2U{N(R)(SiMe2CH═C(NiPr2)C(NiPr2)═CH)}] (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(NR2)3], which converts into [U{N(R)(SiMe2CH2)}(NR2)2] at the elevated temperatures. BAC then inserts into the U-C bond of [U{N(R)(SiMe2CH2)}(NR2)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)(SiMe2CH2)}(NR2)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.