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

Metal complexes bearing carbene ligands L_nM=CR¹R² are relevant intermediates in catalytic transformations. The stability of isolated carbene complexes decreases for the case of monosubstituted carbenes (R¹ = H). We have discovered that the copper complex [Tp^MsCu(C(H)NEt₂)] (1) readily transfers the aminocarbene :C(H)NEt₂ to other complexes of metals, such as rhodium, iridium, palladium, platinum, copper, or gold, leading to complexes bearing the M–C(H)NEt₂ moiety. Experimental data allow us to propose that this monosubstituted aminocarbene (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 reactivity patterns, such as the generation of the first stable monosubstituted 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 [Tp^MsCu(C(H)NEt₂)] to trans-[PdCl₂(NCMe)(PPh₃)] led us to ascertain that the transmetalation process involves several intermediates in which the Tp^Ms ligand shifts between the κ² and κ³ coordination modes.