The first pincer-aryl [M–(NCN)] complexes {M = Pd(II); Pt(II)} with chiral pyridine donors: Synthesis and catalytic activity in C–C bond formation

The first chiral bis(pyridine) N–C(H)–N pincer ligand, (5S,7S)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanquinolin-2-yl)benzene (HL) has been synthesized and characterized by a thorough 1H NMR analysis. Reaction of HL with K2[PtCl4] in acetic acid gives [Pt(L)Cl] (1), where L acts as a tridentate N–C–N pincer ligand. The analogous palladium(II) derivatives [Pd(L)Cl] (2), and [Pd(L)(OAc)] (3), were first prepared through a transmetalation reaction between Pd(OAc)2 and the organomercury compound [Hg(L)Cl] (4). The structures of compounds 1 (Pt) and 2 (Pd), as determined by X-ray diffraction, are reported and compared. Compound 2 can also be obtained from Na2[PdCl4] and HL in refluxing acetic acid, i.e., under the same conditions used for compound 1. Apparently, this is the first palladium pincer derivative of a 1,3-bis(pyridyl)benzene ligand synthesized by direct C–H activation.The neutral complexes 1–3 are catalysts of modest activity, but devoid of enantioselectivity in the Heck reaction between iodobenzene and methyl acrylate and in the aldol condensation of benzaldehyde with methyl isocyanoacetate.

Graphical abstractThe direct synthesis of NCN pincer platinum(II) and palladium(II) complexes derived from the new chiral C2-symmetric ligand (5S,7S)-1,3-bis(6,6-dimethyl-5,6,7,8-tetrahydro-5,7-methanequinolin-2-yl) benzene (HL) is reported. The structures of compounds 1 [Pt(L)Cl] and 2 [Pd(L)Cl], as determined by X-ray diffraction, are described and compared.The catalytic activity of the complexes in some C–C bond forming reactions has been investigated as well.Figure optionsDownload full-size imageDownload as PowerPoint slide