Synthesis, structure, and spectroscopy of two benzil-based -diimine ligands and their palladium(II) complexes

Two -diimine ligands were prepared in 60-70% yield via p-toluenesulfonic acid-catalyzed condensation reactions from benzil with 4-bromoaniline and with p-anisidine. Palladium(II) complexes were prepared from both ligands in 70-80% yield. X-ray structures were obtained for the ligand prepared from p-anisidine and its palladium(II) complex. A notable feature observed in the former was its unconjugated C-N double bonds, both in the (E)-configuration. The latter structure possessed two molecules of the metal complex in its unit cell, both of which have diimine cores with a degree of conjugation and a nonideal square-planar geometry around palladium caused by the small bite angles (79.61(3) and 79.15(3)degrees) of the diimine ligands. Solution-phase electronic absorption spectra of the ligands in chloroform have two bands from and n transitions at 269-345nm. Absorption spectra of the complexes in chloroform exhibited bands attributed to ligand-centered transitions that were red-shifted as compared to free ligands. Only the spectrum obtained from a chloroform solution of the palladium(II) complex with the diimine ligand prepared from p-anisidine featured a band at approximately 520nm, which was assigned to a combination of d(Pd) and n(Cl) transitions.