Indole rings in palladium(II) complexes. Dual mode of metal binding and aromatic ring stacking causing syn-anti isomerism

In view of the interesting metal-binding properties of lichen substances and the indole ring as a potential ligand, we studied the Pd(II) complexes of indole-containing ligands, N-(indole-3-ethyl)-Δ2,11-enaminousnic acid (IEU) and 4-[2-(indol-3-yl)-ethylamino]pent-3-en-2-one (IEP) obtained by condensation of tryptamine with usnic acid and its model acetylacetone, respectively. Reactions of Pd(II) with IEU and IEP gave isomeric complexes resulting from coordination of the C(3) atom of the indole ring in the 3H-indole form, Pd2(IEUH-2)2 (1 and 2) and Pd2(IEPH-2)2 (3 and 4) (IEUH-2 and IEPH-2 denote doubly deprotonated forms of IEU and IEP, respectively). Complexes 1−4 were determined, from the NMR and MS spectra, to have dimeric structures doubly bridged by the indole rings, which were stacked in different orientations. X-ray crystal structure analysis of 3 and 4 established that they are indole-bridged dimers of the [C,N,O]-donor palladacycles with the Pd(II) centers in a square-planar geometry formed by a...