Synthesis, Characterization, and Application of Pd(II)-NNN Pincer Complexes in Direct C-H Arylation of Heteroarenes with Aryl Bromides.

Abstract

We report the synthesis, characterization, and catalytic activity of two new palladium-(II) complexes featuring benzo-[<i>d</i>]-oxazol-2-yl-based pincer ligands, namely, [Pd-(NNN^mor)-Cl] (<b>C1</b>) [Pd-(NNN^pic)-Cl] (<b>C2</b>), and (where NNN^mor = <i>N</i>-(2-(benzo-[<i>d</i>]-oxazol-2-yl)-phenyl)-2-morpholinoacetamide) NNN^pic = <i>N</i>-(2-(benzo-[<i>d</i>]-oxazol-2-yl)-phenyl) picolinamide. Structures of the ligands and the complexes were confirmed by FTIR, NMR and high-resolution mass spectrometry (HRMS). Further, the molecular structure of <b>C2</b> in the solid state was confirmed by single-crystal X-ray diffraction. The complex showed pincer coordination mode with a distorted square planar geometry. The catalytic activity of these Pd-(II) complexes in the direct arylation of indazoles and other heteroarenes with aryl bromides has been explored, with complex <b>C2</b> exhibiting better reactivity compared to complex <b>C1</b>. Mechanistic investigations revealed that the catalytic pathway involves a Pd^II-Pd^IV-Pd^II cycle. The results are supported by DFT calculations.