Formation of NH,NR N‐Heterocyclic Carbene Complexes through Mercuration of Mono‐N‐substituted Benzimidazoles

Abstract

A simple two‐step method to prepare NH,NR N‐heterocyclic carbene (NHC) complexes is described herein. In the first step, the reactions of N‐substituted benzimidazoles (R‐BimH) with Hg(OAc)2 in the presence of NaBr rendered 12‐membered mercuramacrocycles [Hg4(R‐Bim)4Br4] [R = Me, pyridyl (Py), Bn]. These complexes consist of four HgII ions bridged by four benzimidazolyl anions through N and C atoms. This reaction presumably proceeds through an initial N‐complexation of the benzimidazole to the HgII ion. Consequently, C2‐H becomes more acidic, and even an acetate anion can promote the deprotonation at C2 to form the C‐metalated products. The second step involves the protonation of these mercuramacrocycles with HBF4·OEt2 to give the protic complexes featuring the NH,NR–NHC ligands of general formula [(R‐BimH)2Hg][BF4]2 (R = Me, Py, Bn).