A Comparison of the Stability and Reactivity of Diamido‐ and Diaminocarbene Copper Alkoxide and Hydride Complexes

The mononuclear N‐heterocyclic carbene (NHC) copper alkoxide complexes (6‐NHC)CuOtBu] (6‐NHC=6‐MesDAC ( 1 ), 6‐Mes ( 2 )) have been prepared by addition of the free carbenes to the tetrameric tert‐butoxide precursor Cu(OtBu)]₄, or by protonolysis of (6‐NHC)CuMes] (6‐NHC=6‐MesDAC ( 3 ), 6‐Mes ( 4 )) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2 , the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring‐opening reactions, the latter affording (6‐MesDAC)Cu(OC(O)CMe₂C(O)N(H)Mes)(CNMes)] ( 6 ). The intermediacy of (6‐MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu₂O as an additional product. Attempts to generate an isolable copper hydride complex of the type (6‐MesDAC)CuH] by reaction of 1 with Et₃SiH resulted instead in migratory insertion to generate (6‐MesDAC‐H)Cu(P(p‐tolyl)₃)] ( 9 ) upon trapping by P(p‐tolyl)₃. Migratory insertion was also observed during attempts to prepare (6‐Mes)CuH], with (6‐Mes‐H)Cu(6‐Mes)] ( 10 ) isolated, following a reaction that was significantly slower than in the 6‐MesDAC case. The longer lifetime of (6‐Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi‐reduction of alkynes and hydrosilylation of ketones.