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

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posted on 13.12.2019 by Mandeep Kaur, Kamaless Patra, Noor U Din Reshi, Jitendra K. Bera
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(L1C1)­(p-cymene)]I (1), [Mn­(L1C1)­(CO)3] (2), [Ru­(L2C1)­(p-cymene)­Cl]­PF6 (3), and [Ir­(L2C1)­(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(L2C5)­(p-cymene)­Cl]­PF6 (5), [Ir­(L2C5)­(Cp*)­Cl]­PF6 (6), [Ir2(L1C5)­(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.