Preparation of Rhodium(III) Di-NHC Chelate Complexes Featuring Two Different NHC Donors via a Mild NaOAc-Assisted C–H Activation

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 Ag2O followed
by transmetalation to [Rh­(Cp*)­(Cl)2]2 to give
the RhIII-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 C∧C chelate ligand. Reaction
of [2]I or [3]Br with NaOAc and [Rh­(Cp*)­(Cl)2]2 yields the C∧C 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 HBF4·Et2O to give complexes [8]­BF4 and [11]­BF4, 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]­BF4, bearing a unique unsymmetrical (NHC)∧C­(NHC′)
chelate ligand.