PALLADIUM(II) COMPLEX-FORMATION BY INDOLE-3-ACETATE - MIXED-LIGAND COMPLEXES INVOLVING A UNIQUE SPIRO-RING FORMED BY CYCLOPALLADATION

Formation of mixed ligand paladium(II) complexes involving indole-3-ac etate (IA) has been studied by synthetic spectroscopic and X-ray cryst allographic methods. Reaction of IA with Na2PdCl4 in methanol gave NaP d(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 cr ystals. Similar reactions carried out in the presence of water gave Pd (IAH(-1)) center dot 1.5H(2)O (1'), which further gave Pd(IAH(-1))(py) center dot 0.5H(2)O (2') by the reaction with py. X-ray crystal struct ure analysis of 2 revealed a unique dimeric structure, where IAH(-1) c oordinates to Pd(II) through the carboxylate oxygen atom and the tetra hedral C3 atom of the indole nucleus, forming a unique spiro-ring. The two complex units are bridged by the indole nitrogens in the 3H-indol e form, and there is a clese contact (2.75 Angstroms) around the nitro gen-C2 bonds of the five-membered rings of the indole nuclei positione d in parallel with each other. IA in the neutral form was liberated up on refluxing 1 in methanol containing 10 percent acetic acid, showing that IAH(-1) and IA are interconvertible under proper conditions. The H-3 and C-13 NMR spectra in CDCl3-CD3OD indicated that the C3 atom of IA in 1 and 2 is tetrahedral. Large shift differences of the of the C2 proton signals were observed between 1 and 1' and between 2 and 2', w hich indicates that 1 and 2 are dimers in solution whereas 1' and 2' a re monomers and that the differences are due to the close contact betw een the two indole rings in 2 as detected in the solid state and proba bly in 1.