Synthesis of Palladium On Cyclometalated Complexes of 2-β-Disubstituted Ethenyl-8-quinolinols and Steric Interactions of Adjacen t Substituents on Chelate Ring Stability

Abstract

2-Ethenyl-8-quinolinols containing bulky substituents in the 2-position of the ethenyl group have been prepared. From the NMR and X-ray analysis data, O, N and 2-C atoms of the ligand are expected to be in-plane of the tridentate. Cyclometalation of the ligands with Na₂[PdCl₄] in the presence of pyridine in MeOH took place at 25°C to give (ca.60%) the neutral chelate complex possessing a Pd (II)-C σ bond. When the pyridine was exchanged with triphenylphosphine, steric interactions arose throu gh three substituents bonded to α-C, β-C of chelate ring and PPh₃. As revealed from X-ray analysis bis (benzyl ester) groups in Pd complex, the mutual repulsion by adjacent groups causes the dis tinction between disubstituents. The resolution of the diastereomer was thus accomplished by the column chromatography. These indicate that Pd-C bond in cyclometalate ring is stable even under the inherent structural molecular strain.