Formation of anionic NHC complexes through the reaction of benzimidazoles with mercury chloride. Subsequent protonation and transmetallation reactions

Anionic nitrogen heterocyclic carbene complexes of C,N-bound mercuramacrocycles {Hg4Cl4(R-Bim)4} (BimH = benzimidazole; R = Ph, Py) were prepared either from the simple reaction of N-substituted benzimidazoles with HgCl2 or from the reaction of N-substituted benzimidazoles with Hg(OAc)2 and NaCl. The former reaction is unique as no additional base is required, although in low reaction yield. Under similar reaction conditions, only N-bound benzimidazole complexes were obtained when HgBr2 or HgI2 instead of HgCl2 was employed. The different reactivity could be due to the hard-soft acid-base property of mercury(II) and halides, that facilitates the cleavage of Hg-Cl bond and promotes the C-metallation of HgCl2 and benzimidazole. Upon protonation of {Hg4Cl4(R-Bim)4} with HBF4⋅OEt2, NH,NR-NHC complexes of [HgCl(R-Bim-H)][BF4] (R = Ph, Py) were obtained. Preliminary study on the transmetallation reaction of {Hg4Cl4(Ph-Bim)4} with PdCl2(CH3CN)2 and LiCl did provide a C,N-bound NHC palladium complex.