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

Abstract

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.