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

Abstract

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)(SiMe2 CH2)}(NR2)2] at the elevated temperatures. BAC then inserts into the U–C bond of [U{N(R)(SiMe2 CH2)}(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)(SiMe2 CH2)}(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.

Ring-Opening in the Actinide Cyclopropyl Complexes [Cp3U(2,2-Diphenylcyclopropyl)]n− (n = 0, 1)

“…However, this proposed intermediate could not be observed. Similarly, the U­(III) bis­(diisopropylamino)­cyclopropenylidene adduct, [(NR 2 ) 3 U­(BAC)] (R = SiMe 3 ), rearranges upon heating to give 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)}], which we hypothesized was also formed via an unobserved carbene intermediate . Finally, reduction of the U­(III) isocyanide, [U­(NR 2 ) 3 (CN-2,6-Me 2 C 6 H 3 ) 2 ], resulted in isocyanide coupling, and not aminocarbyne formation, as originally hoped. , …”

Dimerization and ring-opening in bis(diisopropylamino)cyclopropenylidene (BAC) mediated by [U(NR2)3(CCPh)] (R = SiMe3)

“…In particular, we hypothesized that BAC could insert into the U-C bond of 1, in analogy with the chemistry observed between [U(NR 2 ) 2 (N(SiMe 3 )SiMe 2 CH 2 )] and BAC (Scheme 1). 48 Complex 2 crystallizes as a discrete cation/anion pair in the triclinic space group P1 ˉ, as the pentane solvate, 2•0.5C 5 H 12 (Fig. 2 and S1 †).…”

“…47 We recently reported the reaction of BAC with the U( iv ) metallacycle, [U(NR 2 ) 2 ( N (SiMe 3 )SiMe 2 C H 2 )] (R = SiMe 3 ), which results in ring-opening of the BAC fragment (Scheme 1). 48 We hypothesized that ring-opening occurs via cyclopropenyl intermediate A , which was formed by BAC insertion into the U–C bond. To probe the generality of this transformation, we endeavoured to explore the insertion chemistry of BAC with other U–C bonds.…”

“…In particular, we hypothesized that BAC could insert into the U–C bond of 1 , in analogy with the chemistry observed between [U(NR 2 ) 2 ( N (SiMe 3 )SiMe 2 C H 2 )] and BAC (Scheme 1). 48 Surprisingly, however, addition of 2 equiv. of BAC to 1 in Et 2 O results in formation of [ cyclo -N( i Pr)C(Me) 2 CH(N i Pr 2 )C{CHC 3 (N i Pr 2 ) 2 }][U(NR 2 ) 2 ( N (SiMe 3 )SiMe 2 C H 2 )(CCPh)] ( 2 ), which can be isolated in 38% yield after work-up (Scheme 2).…”