Pd–C bond formation with the indole ring in palladium complexes of N,N,O-donor ligands. Effect of the nitrogen donor properties

•Some novel phenolate and the indole-binding Pd(II) complexes were characterized.•All the Pd phenolates can convert to the indole-binding species at >60 °C.•Formation properties of Pd-indole complexes depend on the nitrogen donor property.•For steric reasons, introduction of methyl groups into the aromatic nitrogen donor is more effective for the conversion.

The Pd(II) complexes of three new N,N,O-donor ligands with a pendent indole ring and participation of the indole ring in Pd(II) binding have been studied by synthetic, structural, and kinetic methods. The reactions of ligands, 2-[N-2-(1-methylimidazolyl)methyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethyl-3-indole (Hieip), 2-[N-2-(1,4,5-trimethylimidazolyl)methyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethyl-3-indole (HMe3-ieip) and 2-[N-2-(6-methypyridyl)-methyl-N-2-hydroxy-3,5-di(tert-butyl)benzylamino]ethyl-3-indole (HMe-iepp), with [PdCl2(CH3CN)2] at room temperature gave the complexes [Pd(ieip)Cl], [Pd(Me3-ieip)Cl] and [Pd(Me-iepp)Cl], respectively, as reddish-brown crystals. The X-ray crystal structure analyses and 1H NMR spectra revealed that all the complexes have a phenolate coordination in a square-planar geometry and that the pendent indole ring has no characteristic intramolecular interaction with the Pd(II) ion and the coordinated pyridine and imidazole moieties. The distortion of the coordination plane is in the order [Pd(ieip)Cl] < [Pd(Me3-ieip)Cl] < [Pd(Me-iepp)Cl]. Heating a solution of each of the phenolate complexes gave yellow crystals, which were shown to be the indole-C2 binding complex by X-ray analysis. The Pd(II)-O(phenolate) to Pd(II)–C(indole) conversion characteristics of these complexes depends on the nitrogen donor properties of the trans position of the coordinated indole moiety, and the order of the conversion rate of Pd-indole binding complexes is in good correlation with the coordination plane distortion. On the basis of kinetic studies, the influence of the aromatic nitrogen donors to form the indole-C2 binding complexes and the detailed mechanism of the conversion were discussed.

Graphical abstractWe prepared three new N,N,O-donor ligands with a pendent indole ring and their Pd(II) complexes, and characterized the conversion from phenolate to indole-binding species. The conversion properties of these complexes are dependent on the nitrogen donor properties, the yield of the indole-binding species being in the order [Pd(ieip)Cl] > [Pd(Me3-ieip)Cl] > [Pd(Me-iepp)Cl]. On the basis of kinetic studies, the dependence of the tendency to form the indole-C2 binding complexes on the nitrogen-donors and the detailed mechanism of the conversion were discussed.Figure optionsDownload full-size imageDownload as PowerPoint slide