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

Abstract

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 Cu 2 O and KI yields a second hybrid NHC/MIC proligand featuring a tetraiodocuprate anion [NHC(H + )-MIC(H + )][Cu 2 I 4 ] 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] + [MX 2 ] − 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.