Palladium(II) complex formation by indole-3-acetate. Mixed ligand complexes involving a unique spiro-ring formed by cyclopalladation

Abstract

Formation of mixed ligand palladium(II) complexes involving indole-3-acetate (IA) has been studied by synthetic, spectroscopic and X-ray crystallographic methods. Reaction of IA with Na 2PdCl 4 in methanol gave NaPd(IAH −1)Cl ( 1) (IAH −1=IA deprotonated from the indole ring), which reacted with pyridine (py) to give [Pd(IAH −1)(py)] ( 2) as orange crystals. Similar reactions carried out in the presence of water gave Pd(IAH −1)·1.5H 2O ( 1′), which further gave Pd(IAH −1)(py)·0.5H 2O ( 2′) by the reaction with py. X-ray crystal structure analysis of 2 revealed a unique dimeric structure, where IAH −1 coordinates to Pd(II) through the carboxylate oxygen atom and the tetrahedral C3 atom of the indole nucleus, forming a unique spiro-ring. The two complex units are bridged by the indole nitrogens in the 3 H-indole form, and there is a close contact (2.75 Å) around the nitrogen-C2 bonds of the five-membered rings of the indole nuclei positioned in parallel with each other. IA in the neutral form was liberated upon refluxing 1 in methanol containing 10% acetic acid, showing that IAH −1 and IA are interconvertible under proper conditions. The 1H and 13C NMR spectra in CDCl 3CD 3OD indicated that the C3 atom of IA in 1 and 2 is tetrahedral. Large shift differences of the C2 proton signals were observed between 1 and 1′ and between 2 and 2′, which indicates that 1 and 2 are dimers in solution whereas 1′ and 2′ are monomers and that the differences are due to the close contact between the two indole rings in 2 as detected in the solid state and probably in 1.