Catalytic\nBenefits of an Ethynylphenyl-Based Pd<sup>II</sup> Bis(NHC) over a\nPhenyl-Based Pd<sup>II</sup> Bis(NHC): Insights\ninto the Structural Aspects and Donor Strength Probing

The amount of cooperation between the metal centers by\ntailored\nligand design is vital to achieve the multimetallic catalytic benefits.\nHerein, a bimetallic Pd<sup>II</sup> NHC/pyridine complex is synthesized\nutilizing a novel extended <i>C</i><sub>2</sub>-symmetric\nbis-NHC ligand featuring 1,4-disubstituted ethynylbenzene as a central\nligand platform. The complex was further employed for the preparation\nof a dinuclear complex possessing a combination of NHC and PPh<sub>3</sub> ligands. Dinuclear complexes have also been synthesized using\na 1,4-di(NHC)-substituted parent benzene ring platform with no acetylene\nlinker units. The donor strengths of both bis-NHC ligands were compared\nwith the help of <sup>13</sup>C and <sup>31</sup>P chemical shift\nvalues as probes. All of the bimetallic complexes were tested as precatalysts\nin the intermolecular α-arylation of 1-methyl-2-oxindole and\nSonogashira coupling reactions. The dinuclear Pd<sup>II</sup> complexes\nwith the ethynylbenzene-bridged bis-NHC ligand showed impressive catalytic\noutcomes that outweigh the catalytic outcomes obtained with dinuclear\nbis-NHC complexes possessing no such acetylene linkers. The corresponding\nmononuclear Pd<sup>II</sup> complexes were synthesized for comparison\npurposes utilizing both types of central ligand platforms. The dinuclear\ncomplexes appeared more active than the corresponding mononuclear\ncomplexes in catalysis. Furthermore, higher cooperative index values\nwere obtained in catalysis using the dinuclear Pd<sup>II</sup> complexes\nwith an acetylene-bridged bis-NHC ligand compared to those with no\nsuch acetylene linkers.