Highly Recoverable Pd(II) Catalysts for the Mizoroki–Heck Reaction Based on N‑Heterocyclic Carbenes and Poly(benzyl ether) Dendrons

Two series of bis­(imidazolylidene)­palladium complexes of general formula [PdBr₂(NHC)₂] have been prepared. The molecular weight of the complexes was enlarged by bonding poly­(benzyl ether) dendrons of increasing size (G0 to G3) at the nitrogen atom of the NHC ligands. Complexes <b>1</b>–<b>4</b> contain monodentate NHC ligands coordinated in a <i>trans</i> mode, whereas complexes <b>9</b>–<b>12</b> contain a chelating ethylene-bridged bis­(NHC) ligand. The complexes were recovered from the product stream of a Mizoroki–Heck reaction by nanofiltration through thermally and chemically stable ceramic membranes. The recoverability is better for larger complexes and chelate ligands. In particular, chelate G3 complex <b>12</b> maintained a constant activity during the 13 recovery cycles performed, affording an accumulated turnover number (TON) of 13 000. On average, around 99.5% of the metal is recovered in each recovery cycle, and contamination by palladium is only 3 mg per kg of product. Several models to explain the efficiency of the recovery are discussed