C─C Bond Formation via Reductive Elimination at Rare Earth Centers

Reductive elimination is one of the most important elementary reactions employed for constructing various chemical bonds in transition metal complex-mediated transformations. However, due to the lack of two-electron transfer, rare earth (RE) centers have been considered incapable of promoting reductive elimination, thus limiting the reactivity of RE complexes and their applications in synthetic chemistry. In the presented study, we demonstrated the reactions of β-diketiminate-supported dialkyl RE(III) complexes (L^Nacnac-κ²N,N')RE(III)(CH₂SiMe₃)₂(THF) (1-RE, RE = Y, Er, Yb, and Lu; L^Nacnac = {[(2,4,6-Me₃C₆H₂)NCMe]₂CH}^-) with azobenzene at ambient conditions, resulting in the formations of (L^Nacnac-κ²N,N')RE(III)(η²-cis-PhNNPh^-•)₂(THF) (2-RE) and 2-[(trimethylsilyl)methyl]azobenzene (I) via a reductive elimination-powered pathway. The key intermediate (L^Nacnac-κ²N,N')Lu(III)(PhNNC₆H₅-κ² N,C)(CH₂SiMe₃) (3-Lu), which is formed via ortho-H deprotonation in azobenzene by the (trimethylsilyl)methyl group in 1-Lu, was isolated and structurally identified. The computational studies indicate that the subsequent η²-coordination of a cis-azobenzene to the RE(III) centers in this intermediate triggers the reductive elimination between the (trimethylsilyl)methyl group and the neighboring PhNNC₆H₅-κ²N,C group, consequently enabling the C─C cross-coupling that affords product I. In this elementary process, the two N═N π*-orbitals mainly act as the energetically accessible acceptors for the two electrons being transferred, thereby facilitating the reductive elimination by avoiding the thermodynamically unfavorable 2e-reductions of RE centers.