Catalytic Benefits of an Ethynylphenyl-Based PdII Bis(NHC) over a Phenyl-Based PdII Bis(NHC): Insights into the Structural Aspects and Donor Strength Probing.

The amount of cooperation between the metal centers by tailored ligand design is vital to achieve the multimetallic catalytic benefits. Herein, a bimetallic PdII NHC/pyridine complex is synthesized utilizing a novel extended C2-symmetric bis-NHC ligand featuring 1,4-disubstituted ethynylbenzene as a central ligand platform. The complex was further employed for the preparation of a dinuclear complex possessing a combination of NHC and PPh3 ligands. Dinuclear complexes have also been synthesized using a 1,4-di(NHC)-substituted parent benzene ring platform with no acetylene linker units. The donor strengths of both bis-NHC ligands were compared with the help of 13C and 31P chemical shift values as probes. All of the bimetallic complexes were tested as precatalysts in the intermolecular α-arylation of 1-methyl-2-oxindole and Sonogashira coupling reactions. The dinuclear PdII complexes with the ethynylbenzene-bridged bis-NHC ligand showed impressive catalytic outcomes that outweigh the catalytic outcomes obtained with dinuclear bis-NHC complexes possessing no such acetylene linkers. The corresponding mononuclear PdII complexes were synthesized for comparison purposes utilizing both types of central ligand platforms. The dinuclear complexes appeared more active than the corresponding mononuclear complexes in catalysis. Furthermore, higher cooperative index values were obtained in catalysis using the dinuclear PdII complexes with an acetylene-bridged bis-NHC ligand compared to those with no such acetylene linkers.