Chiral NCN Pincer Pt(II) and Pd(II) Complexes with 1,3-Bis(2′-imidazolinyl)benzene: Synthesis via Direct Metalation, Characterization, and Catalytic Activity in the Friedel-Crafts Alkylation Reaction

The synthesis and characterization of a series of platinum(II) and palladium(II) pincer complexes with chiral NCN ligands based on 1,3-bis(2'-imidazolinyl)benzene are reported. The new NCN ligand precursors 3a-m were easily prepared from inexpensive, readily available isophthalyl chloride or 5-substituted isophthalyl chloride and chiral amino alcohols in two steps. Direct C2 metalation of 3 with K2PtCl4 provided convenient access to the corresponding platinum pincer complexes 4 in 51-84% isolated yields. On the basis of the same methodology, seven palladium pincer complexes 5 were synthesized in 12-54% yields by heating 3 with Pd(OAc)(2) in refluxing HOAc under a nitrogen atmosphere, followed by the addition of LiCl. All the ligand precursors and pincer complexes are air- and moisture-stable both in the solid state and in solution. They were fully characterized by HRMS, H-1 NMR, C-13 NMR, DEPT, HSQC, HMBC, IR, and ESI-MS. The molecular structures of platinum complexes 4c and 4i and palladium complex 5b have been determined by X-ray single-crystal diffraction. Treatment of Pt complex 4l with AgOTf or AgSbF6 in wet acetone gave effective catalysts with moderate stereoselectivity for the enantioselective Friedel-Crafts alkylation reaction between indole and trans-beta-nitrostyrene.