Bridged N-Heterocyclic/Mesoionic (NHC/MIC) Heterodicarbenes as Ligands for Transition Metal Complexes

Following a copper catalyzed alkyne azide cycloaddition (CuAAC) and N-alkylation protocols, we report the preparation of a hybrid N-heterocyclic/mesoionic [NHC(H⁺)-MIC(H⁺)][2I]^2- salt (1) in high yields. The treatment of salt 1 with Cu₂O and KI yields a second hybrid NHC/MIC proligand featuring a tetraiodocuprate anion [NHC(H⁺)-MIC(H⁺)][Cu₂I₄]^2- (2). Through selective deprotonation and metalation, both salts 1 and 2 can generate either the chelate heterodicarbene complexes (3) with the rare [NHC·(M)·MIC]⁺[MX₂]^- general formula (M = Pd^II, Rh^I) or NHC-anchored/pendent triazolium species (4) [NHC·(M)-MIC(H⁺)]. If the triazolium moiety of type 4 complexes is deprotonated with KHMDS in the presence of a second metal center, a series of heterobimetallic complexes of the type [NHC·(M)-MIC·(M')] (5) are achieved. Interestingly, the reaction of salt 2 with KHMDS yields the bimetallic copper heterodicarbene (6) which can be a useful transfer reagent for the preparation of type 3 complexes. A variety of synthetic routes for the preparation of complexes 3-5 and their full characterization in solution and in the solid state will be discussed.