title: Cyclopalladation of the indole ring in palladium(II) complexes of 2N1O-donor ligands and its dependence on the O-donor properties.
abstract: Synthetic, structural, spectroscopic, and kinetic studies have been carried out on the Pd(II) complexes of new 2N1O-donor ligands containing a pendent indole, 3-(N-2-pyridylmethyl-N-2-hydroxy-5-methoxybenzylamino)ethylindole (HMeO-iepp), 3-(N-2-pyridylmethyl-N-2-hydroxy-5-nitrobenzylamino)ethylindole (HNO2-iepp), and (N-2-pyridylmethyl-3-indolylethylamino)acetic acid (Hiepc) (H denotes a dissociable proton). [Pd(MeO-iepp)Cl] (2), [Pd(NO2-iepp)Cl] (3), and [Pd(iepc)Cl] (4) were prepared and revealed by X-ray analysis to have a pyridine nitrogen, an amine nitrogen, a phenolate or carboxylate oxygen, and a chloride ion in the coordination plane. UV absorption and 1H NMR spectral changes indicated that all the complexes could be converted to the indole-binding complexes where the O donor was replaced by the indole C2 atom by cyclopalladation in DMSO or DMF in the temperature range of 40-60 degrees C. Formation of the indole-binding complex species obeyed the first-order kinetics, from which the activation parameters were estimated. The formation rate was dependent on the properties of the O-donor group, a lower pKa value of its conjugate acid causing faster conversion to the indole-binding species in the order 2 (methoxyphenolate) < 3 (nitrophenolate) < 4 (carboxylate). On the other hand, the ratio of the indole-binding complex to the O-donor complex as a result of the conversion was greater for the complexes with a higher pKa value of the ligand OH group, the order being 2 > 3 > 4.
authors.affiliation: Institute for Materials Chemistry and Engineering, Kyushu University, Higashi-ku, Fukuoka 812-8581, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Shimazaki
authors.forename: Minoru
authors.initials: M
authors.lastname: Tashiro
authors.forename: Takeshi
authors.initials: T
authors.lastname: Motoyama
authors.forename: Satoshi
authors.initials: S
authors.lastname: Iwatsuki
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Yajima
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Nakabayashi
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Naruta
authors.forename: Osamu
authors.initials: O
authors.lastname: Yamauchi
mesh_terms: D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D009589:Nitrogen Oxides / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D010165:Palladium / Q000737:chemistry*
chemical_list: D009589:Nitrogen Oxides; D010165:Palladium; D010100:Oxygen