Palladium(II)-N-Heterocyclic Carbene Metallacrown Ether Complexes: Synthesis, Structure, and Catalytic Activity in the Suzuki-Miyaura Reaction

Pd(II)-N-heterocyclic carbene (NHC) metallacrown ether complexes have been synthesized for the first time. Reaction of bis(imidazolium) salts with a long polyether chain with Ag2O afforded silver-NHC complexes [Ag-2(L)Cl-2] 5, which reacted as carbene transfer reagents with palladium salts to give Pd(II)-NHC metallacrown ether complexes 6a,b. The Ag(I)-NHC complex 5b possesses a one-dimensional chain configuration by each Ag(I) center coordinated to one carbene and two bridging chlorides. Each of Pd(II)-NHC metallacrown ether complexes 6a,b contains a 25-membered macrocyclic NHC metallacrown ether formed by a bidentate chelate bis(carbene) ligand with a long flexible linkage and a Pd(II). The coordination geometry on the Pd(II) is approximately square planar. Pd(II)-NHC metallacrown ether complexes 6a and 6b have been shown to be highly effective in the Suzuki-Miyaura reactions of a variety of aryl bromides in neat water without the need of inert gas protection.