Concentration-dependent palladium(II)-indole bond formation in complexes with a 2N-donor ligand containing an indole moiety: Synthesis, characterization, and reaction analysis

The Pd(II) complexes of a 2N-donor ligand containing a pendent indole, 3-(2-pyridylmethylamino)ethylindole (L), were synthesized and characterized. Reaction of the ligand with [PdCl₂(CH₃CN)₂] at room temperature gave [Pd(L)Cl₂] (1) as pale yellow crystals. The X-ray crystal structure analysis and ¹H NMR spectrum of 1 revealed that the complex has a 2N2Cl-donor set 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 moiety. Refluxing a solution of 1 in CH₂Cl₂/DMF for a few hours under basic conditions gave yellow crystals, which were shown to be an indole-C2 binding complex [Pd(L)Cl] (2) by X-ray analysis. Conversion of complex 1 to 2 in DMSO was observed upon dilution of the solution of complex 1. From solution equilibrium and kinetic studies the initial step of the conversion by dilution has been assigned to the replacement of a coordinated Cl⁻ ion with the DMSO molecule. The ligand replacement easily occurred at low concentrations of 1. The complex with a coordinated solvent molecule exhibited a high reactivity and formed a stable Pd-C bond with the indole ring located close to the Pd(II) center. We discussed the concentration dependent formation of the indole-C2 binding complex 2 and its detailed mechanism.