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

Abstract

The first chiral bis(pyridine) N–C(H)–N pincer ligand, (5 S,7 S)-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 K 2[PtCl 4] 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 Na 2[PdCl 4] 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.