Experimental and computational insights into the direct cyclopalladation of different unsymmetrical, yet closely related pincer ligands with thione sulfur donors

Three different unsymmetrical, yet closely related pincer ligands are designed to probe the effect of a spectator heteroatom in one of the coordination arms on their direct cyclopalladation both in solution and in the solid phase. The solvent-free methodology is proven to be a viable alternative to the solution-based techniques. [Display omitted] A series of three closely related unsymmetrical pincer ligands featuring thione sulfur donors have been derived from m-thiophosphorylated phenyl isothiocyanate to probe the effect of a spectator heteroatom in the flanking ring on their direct cyclopalladation. The reaction efficiency is shown to drastically increase on passing from a thionated thiazolidinone derivative to its oxazolidinone analog, and further to an imidazolidinone ligand. The investigations by IR and NMR spectroscopy as well as X-ray diffraction confirmed the realization of S,C,S'-type pincer coordination but did not reveal considerable differences in the molecular structures of the Pd(II) complexes obtained; however, the results of DFT calculations provided some valuable insights into the peculiarities of their formation. In addition, the possibility of solid-phase synthesis of the target pincer complexes upon thermally induced CH activation has been demonstrated. This methodology does not afford a significant yield enhancement over the conventional solution-based technique but appears to be highly advantageous in terms of the absence of a solvent at the synthesis step and reaction time.