Ligand exchange and stereodynamics in the cationic palladium(LL)–π-allyl complex of 2,2′-bis(pyridyl)-1,1′-binaphthalene, a novel atropisomeric N,N-ligand

Abstract

Racemic 2,2′-bis(pyridin-2-yl)-1,1′-binaphthalene (±)-6 was prepared in four synthetic steps from 2-naphthol. Resolution of the novel atropisomeric ligand 6 was effected by repeated preparation, recrystallisation and then liberation of the bis-tartrate salt (D and L). This affords both enantiomers of 6 (>96% enantiomeric excess, ee) whose absolute configurations were assigned by CD. The ligand 6 is stable towards racemisation (ΔG^‡_rac > 167 kJ mol⁻¹) and the structure was confirmed by single crystal X-ray diffraction of the complex [ZnCl₂(6)]. The structure and stereodynamics of the complex [Pd(η³-C₃H₅)(6)][OTf] (OTf = O₃SCF₃) which comprises a C₂-symmetric ligand bound via Pd to a non-C₂-symmetric allyl fragment was studied in detail in solution by 1-D and 2-D ¹H NMR and in the solid state by single crystal X-ray diffraction. Apparent stereodynamic processes of the allyl ligand, as detected in solution by NMR, are shown to arise from ligand 6 stereodynamics and exchange. The ligand 6 represents the first compound/building block for a series of enantiomerically pure N,N-ligands that may be of utility in supramolecular co-ordination chemistry and in asymmetric catalysis.