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 CN2O donor set and are anti and syn isomers with respect to the bridging indole rings, respectively. On the
basis of the spectral similarity with 4, 1 and 2 were concluded to be stereoisomers assuming the syn form with the
same donor set. The results demonstrate the metal binding and stacking abilities of the indole ring and the
stereoisomerism from the sp3 C(3) atom formed upon coordination.