Tetranuclear Pd<sup>II</sup> Complexes Containing an Adamantane-Based Tetra-MIC Ligand: Ancillary-Ligand-Dependent Catalytic and Redox Activities
Bhaskar Mondal, Rajat Naskar, Jayanta Bag, Somenath Garai, Ramananda Maity
Abstract
The novel tetra-triazolium salt 1, featuring a tetrasubstituted central adamantane backbone, is introduced for the synthesis of tetranuclear Pd II MIC complexes by quadruple C5–H deprotonation and subsequent addition of the corresponding Pd II precursors. The three newly prepared palladium(II) complexes appeared to be stable and have been characterized using standard spectroscopic and HRMS techniques. The molecular structure of the tetranuclear Pd II complex bearing a 4-methoxypyridine ligand has been established by using single-crystal XRD. All three newly prepared tetranuclear Pd II complexes appeared as active precatalysts for the Sonogashira coupling reaction, which occurred in a homogeneous fashion; however, only the palladium(II) complex bearing a combination of MIC and phosphine ligands was found to be active for the intermolecular α-arylation of 1-methyl-2-oxindole, as the complexes bearing a pyridine or a 4-methoxypyridine ligand decomposed to Pd 0 NPs and appeared to be inactive in the α-arylation reaction. The ancillary-ligand-dependent (Py vs PPh 3 ) interplay between homogeneous and heterogeneous catalyses has also been correlated with the catalytic outcomes in α-arylation of 1-methyl-2-oxindole. The electrochemical responses of these complexes have been measured, and the effect of ancillary ligands in both electrochemistry and catalysis has also been examined.