C-N Bond Coupling Reactions of Ammonia with Acetone Promoted by Iridium and Rhodium Complexes: Experimental and DFT Studies
Treatment of acetone solutions of the known chlorido-bridged complexes [{M(m-Cl)(cod)}2] (M = Ir, Rh; cod = 1,5-cyclooctadiene) under an ammonia atmosphere afforded the cationic complexes [M(cod)(kN,kN-NH2-C(CH3)2-CH2-C(CH3)=NH)]Cl (M = Ir (3), Rh (4)). The molecular structures of 3 and 4 showed the formation of six-membered metallacycles due to the presence of a 4-imino-2-methylpentan-2-amino kN,kN-chelated ligand. Alternatively, the cations [M(cod)(NCCH3)2]BF4 (M = Ir, Rh) reacted with gaseous ammonia at atmospheric pressure affording bis ammine complexes [M(cod)(NH3)2]BF4 (M = Ir (5), Rh (6)), which were found to react with acetone forming cations [M(cod)(kN,kN-NH2-C(CH3)2-CH2-C(CH3)=NH)]BF4 (M = Ir (7), Rh (8)). DFT studies reveal that the transformation of 6 - 8 is mediated by NH3 molecules acting as an external base. The reaction is triggered by deprotonation of an ammonia ligand forming a metal-amido intermediate which further transforms to an acetimino ligand via aldol condensation. The terminal methyl group of one acetimino ligand is deprotonated by NH3 yielding an enamine ligand which can react with the imine ligand via concerted nucleophilic addition affording the metallacycle which is stabilized by protonation.