Catalytic Benefits of an Ethynylphenyl-Based Pd^II Bis(NHC) over a Phenyl-Based Pd^II 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 Pd^II NHC/pyridine complex is synthesized utilizing a novel extended <i>C</i>₂-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 Pd^II 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 Pd^II 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 Pd^II complexes with an acetylene-bridged bis-NHC ligand compared to those with no such acetylene linkers.