Bisimino-functionalized dibenzo[a,c]acridines as highly conjugated pincer frameworks for palladium(II): synthesis, characterization and catalytic performance in Heck coupling
A new pair of highly conjugated ligands, 10-[1-(arylimino)ethyl]-14-[(arylimino)methyl]dibenzo[a,c]acridine (aryl = 2,6-Me2Ph L1, 2,6-Et2Ph L2) incorporating both aldimine and ketimine units, have been prepared by a straightforward sequence of organic transformations from phenanthrene-9,10-dione. Cyclopalladation occurs readily at ambient temperature on treating L1 or L2 with PdCl2(NCCH3)2 in aprotic solvents to afford exclusively (NketimineNC)Pd(II) chloride pincer complexes Pd1 or Pd2, respectively. By contrast in methanol, Pd1 or Pd2 are isolated as the minor product with aldehyde-containing Pd3 or Pd4 as the major one, the result of hydrolysis of the pendant aldimine units in Pd1 and Pd2, respectively. All the ligands and palladium complexes have been characterized by FT-IR, 1H and 13C NMR spectroscopy, mass spectrometry and elemental analysis; the molecular structures for L1, Pd1, Pd2, Pd3 and Pd4 are also reported. Using low catalyst loadings (0.0005–0.002 mol%) and elevated temperatures (140–200 °C), Pd1–Pd4 are able to efficiently mediate the coupling of haloarenes with vinyl-containing substrates with turnover numbers as high as 174000; the effects of steric/electronic variation within the substrate and NNC-pincer complex on catalyst performance are examined.