A copper-masked monosubstituted carbene as a general transmetalating agent toward stable carbene complexes

Abstract

Metal complexes bearing carbene ligands LnM=CR1R2 are relevant in- termediates in catalytic transformations. The stability of isolated car- bene complexes decreases for the case of monosubstituted carbenes (R1 = H). We have discovered that the copper complex [TpMsCu(C(H) NEt2)] (1) readily transfers the aminocarbene :C(H)NEt2 to other com- plexes of metals, such as rhodium, iridium, palladium, platinum, copper, or gold, leading to complexes bearing the M–C(H)NEt2 moiety. Exper- imental data allow us to propose that this monosubstituted aminocar- bene (MAC) ligand displays a nucleophilicity similar to that of NHC (N-Heterocyclic Carbene) ligands bearing bulky substituents but it has a considerably lower volume. These features allow unprecedented reac- tivity patterns, such as the generation of the first stable monosubsti- tuted carbene adduct of Rh2(OAc)4, a well-known catalyst for carbene transfer reactions. Density functional theory (DFT) studies carried out on the transfer of the MAC ligand from [TpMsCu(C(H)NEt2)] to trans- [PdCl2(NCMe)(PPh3)] led us to ascertain that the transmetalation pro- cess involves several intermediates in which the TpMs ligand shifts be- tween the k2 and k3 coordination modes.