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

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