Study on the Structure of a Ternary Cu(II)-Diamine-Amino Acid Complex Involving Intr a m o lecular Aromatic Ring Stacking I n t e ractions

Abstract

Ternary metal complexes comprising an aromatic amino acid and an aromatic amine such as 1, 10-phenanthroline (phen) and 2, 2′-bipyridine have recently been shown to have the intramolecular aromatic ring stacking in dilute aqueous solution. With a view to understanding the precise binding mode of the stacking interaction, the crystal structure of [Cu(phen)(L-tryptophan)]ClO₄⋅2.5H₂O has been determined by the X-ray method. Crysta ls of the complex are orthorhombic, space group P 2₁2₁2₁ with a=28.362 (28), b=11.063 (5), c=8.139(12) A, U=2554 (4) Å3, and Z=4. The geometry about the copper(II) ion is five-coordinate square-pyramidal, with the two nitrogen atoms of phen and the nitrogen and oxygen atoms of L-tryptophan coordinated at the equatorial position in an apprOximately planar form and one oxygen atom of the carboxyl group of the neighboring complex molecule at the axial position. The most interesting structural feature of the complex is the existence of the intramolecular stacking between the L-tryptophan indole ring and the aromatic rings of phen with the average spacing of 3.51Å from the vacant axial position. To understand the binding mode of the stacking interactions, we calculated the charge densities of a model system, protonated phen-L-tryptophan, by the CNDO/2 MO method. The charge densities obtained from the MO calculation suggest that the intramolecular stacking interactions are dominated by the electrostatic interactions between the two aromatic rings.