Preparation of Rhodium(III) di-NHC chelate complexes featuring two different NHC donors via a mild NaOAc-assisted C-H activation

Abstract

Two molecules of 4,5-dichloroimidazole react with dibromomethane to give the diimidazole 1. Reaction of 1 with iodomethane or 2-fluorobenzyl bromide yields the monoalkylated imidazolium/imidazole salts [2]I and [3]Br, respectively. Salt [2]I reacts with AgO followed by transmetalation to [Rh(Cp∗)(Cl)] to give the Rh-NHC complex [4], bearing an NHC ligand with a pendant imidazole group. The pendant imidazole can be deprotonated using NaOAc to yield complex [5], bearing a doubly C-metalated CC chelate ligand. Reaction of [2]I or [3]Br with NaOAc and [Rh(Cp∗)(Cl)] yields the CC chelate complexes [7] and [9], respectively, in a one-pot reaction. The imine ring nitrogen atom in complexes [7] and [9] can be protonated using HBF·EtO to give complexes [8]BF and [11]BF, each bearing a C(NHC)C(pNHC) chelate ligand (pNHC = protic NH,NR-NHC ligand). Alkylation of the imine ring nitrogen atom in [9] yields complex [10]BF, bearing a unique unsymmetrical (NHC)C(NHC′) chelate ligand.