Nucleophilic Reactivity at a ═CH Arm of a Lutidine-Based CNC/Rh System: Unusual Alkyne and CO 2 Activation.

Reaction of an amido pincer complex [(CNC)*Rh(CO)] ( 1 ) (CNC* is the deprotonated form of CNC) with carbon dioxide gave a neutral complex [(CNC-CO 2 ) Mes *Rh(CO)] ( 2 ), which is the result of a C-C bond-forming reaction between the deprotonated arm of the CNC* ligand and CO 2 . The molecular structure of 2 showed a zwitterionic complex, where the CO 2 moiety is covalently connected to the former ═CH arm of the CNC* pincer ligand. The unusual structure of 1 allowed us to explore the reactivity of the CO 2 moiety with selected primary amines RNH 2 (benzylamine and ammonia), which afforded cationic complexes [(CNC) Mes Rh(CO)][HRNC( O )O] (R = Bz ( 3 ), H ( 4 )). Compounds 3 and 4 are the result of a C-N coupling between the incoming amine and the CO 2 fragment covalently connected to the pincer ligand in 2 , a process that involves protonation of the "CH-CO 2 " fragment in 2 from the respective amines. Once revealed the nucleophilic character of the ═CH fragment in 1 , we explored its reactivity with alkynes, a study that enlightened a novel reactivity trend in alkyne activation. Reaction of 1 with terminal alkynes RC≡CH (R = Ph, 2-py, 4-C 6 H 4 -CF 3 ) yielded neutral complexes [(CNC-CH═CHR) Mes *Rh(CO)] (R = Ph ( 5 ), 2-py ( 6 ), 4-C 6 H 4 -CF 3 ( 7 )) in good yields. Deuterium labeling experiments with PhC≡CD confirmed that complex 5 is the product of a formal insertion of the alkyne into the C(sp 2 )-H bond of the deprotonated arm in 1 . This structural proposal was further confirmed by the X-ray molecular structure of phenyl complex 5 , which showed the alkyne covalently linked to the pincer ligand. Besides, this novel transformation was analyzed by DFT methods and showed a metal-ligand cooperative mechanism, based on the initial electrophilic attack of the alkyne to the ═CH arm of the CNC Mes * ligand (making a new C-C bond) followed by the action of a protic base (HN(SiMe 3 ) 2 ), which is able to perform a proton rearrangement that leads to the final product 5 .