Base-Controlled Directed Synthesis of Metal–Methyleneimidazoline (MIz) and Metal–Mesoionic Carbene (MIC) Compounds

Reactions of a host of metal precursors with pyridyl­(benzamide)-functionalized C²-methyl-protected imidazolium salts [L¹H₂]I and [L²H]I afforded the metal–methyleneimidazoline (MIz) compounds [Ru­(L¹-κC¹)­(p-cymene)]I (1), [Mn­(L¹-κC¹)­(CO)₃] (2), [Ru­(L²-κC¹)­(p-cymene)­Cl]­PF₆ (3), and [Ir­(L²-κC¹)­(Cp*)­Cl]­PF₆ (4) in the presence of different external bases, such as LiHMDS, Na₂CO₃, ^tBuOK, and NaH. However, the use of NaOAc led to the selective formation of the metal–mesoionic carbene (MIC) compounds [Ru­(L²-κC⁵)­(p-cymene)­Cl]­PF₆ (5), [Ir­(L²-κC⁵)­(Cp*)­Cl]­PF₆ (6), [Ir₂(L¹-κC⁵)­(Cp*)₂I]­PF₆ (8), and the ortho-metalated compound [Ir­(L¹)­(Cp*)­I] (7). All compounds have been characterized by spectroscopic techniques and X-ray crystallography. Being more acidic, the C²-methyl is readily deprotonated by the external base to give the metal–MIz products. A metal-bound acetate, in contrast, interacts selectively with the imidazolium C⁵–H and drives the reaction toward the metal–MIC formation. DFT calculations support a concerted metalation–deprotonation pathway for selective C–H activation and metalation.