Metal ion-bridged intramolecular stacking interaction between the tryptophyl residue and the aromatic heterocyclic amine within the ternary complex (1,10-phenanthroline)(L-tryptophanato-O,N)copper(II) perchlorate–water (1/2.5): an X-ray study

Abstract

The crystal and molecular structure of [Cu(L-TrpO)(phen)] ClO₄·2.5H₂O has been determined by X-ray diffraction methods. The copper(II) centre is five-co-ordinate. The four equatorial sites are occupied by a bidentate tryptophanato (TrpO) ligand through a carboxylate oxygen [1.93(1)Å] and an amino nitrogen [2.02(1)Å] and a bidentate 1, 10-phenanthroline (phen) ligand [average 2.00(1)Å]. The axial position is occupied by another carboxylate oxygen [2.29(1)Å] of a symmetry-related neighbouring tryptophanato molecule. Thus a one-dimensional polymeric structure is formed with an infinite (–metal–carboxylate–)_n spiral from which tryptophyl indole and phen rings project outwards and stack on each other. An indole–phen π–π interaction in the metal ion-bridged stacked folded molecular unit (average spacing 3.51 Å and a tilt angle between the planes of 5°) reflects the significant deviation of the C^α atom [0.74(2)Å] from the co-ordination plane and a small angle (18°) between the co-ordination and indole planes. The complex further involves metal–indole ring edge-on close contacts [3.22(2) and 3.24(2)Å]. This is the first crystal structure providing direct evidence for the existence of a metal ion-bridged stacking adduct containing an aromatic amino acid residue.