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.