INDOLE NITROGEN PALLADIUM(II) BONDING - CHEMICAL AND STRUCTURAL CHARACTERIZATION OF PALLADIUM(II) COMPLEXES OF ALKYLINDOLES AND INTERMEDIACY OF THE H-3 INDOLE RING

The first palladium(II) complexes of alkylindoles (2-methylindole (MI) and 2,5-dimethylindole (DMI)) coordinating through the nitrogen atom in the 3H-indole form have been isolated and characterized by X-ray crystal structure analysis and spectroscopic methods. The complexes [Pd(MI)2Cl2] (1) and [Pd(DMI)2Cl2] (2) were obtained as pale brown crystals from methanol solutions. Both complexes crystallized in the monoclinic space group P21/c with Z = 2. They have a square-planar structure with two indole molecules coordinating to Pd(II) in the trans geometry with two chloride ions occupying the remaining positions. The indole ring is bound to Pd(II) through the sp2 nitrogen atom (N(1)) and is nearly perpendicular to the Pd(II) coordination plane. The N(1)-C(2) bond length (1.305 (12) Å) in 1 is much shorter than that found for 2, 3-diphenylindole (1.392 (2) Å), and the C(2)-C(3) (1.516 (12) Å) and C(3)-C(3a) (1.469 (15) Å) bonds are nearly equal to single bonds, indicating that the indole ring assumes the 3H-indole form. The 1H NMR spectra of 1 in CDCl3 exhibited CH2 signals at ~3.9 ppm but no C(3) and N(1) proton signals observed for MI, which is in accord with the 3H-indole structure. The 13C NMR spectra in solution and in the solid state further confirmed that the 3H-indole structure with the sp3-like C(3) is maintained under both conditions. The results establish the coordination of the indole ring through the “pyridine-like” nitrogen and the intermediacy of the 3H-indole structure in the course of reactions catalyzed, e.g., by metal ions having strong affinity toward nitrogen donors. © 1990, American Chemical Society. All rights reserved.