Structural evidence for the intramolecular charge-transfer interaction involving an indole ring in ternary copper(II) complexes with L-tryptophan and aromatic diamines

With a view to understanding the precise binding mode and strength of the stacking interaction in the ternary copper(II) complexes comprising an aromatic diamine such as 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen) and an aromatic amino acid such as L-phenylalanine, L-tyrosine and L-tryptophan (L-trp), the crystal structure of [Cu(bpy)(L-trp)]ClO₄ and the circular dichroism (CD) and absorption spectra of [Cu(bpy)(L-trp)]ClO₄ and [Cu(phen)(L-trp)]ClO₄ have been investigated. The complex [Cu(bpy)(L-trp)]ClO₄ crystallizes in the monoclinic space group, P2₁, with two molecules in a unit cell of dimensions a=13.022(1), b=7.753(1), c=10.533(1) Å, and β=91.18(1)°. The Cu(II) ion is five-coordinate square-pyramidal, with the two nitrogen atoms of bpy and the nitrogen and oxygen atoms of the amino acid coordinated at the equatorial positions in a slightly distorted square-planar form and the carboxylate oxygen atom of the neighboring molecule at the axial position. The most interesting structural feature of the complex is the existence of the intramolecular stacking interaction between the aromatic rings of L-trp and bpy with the average spacing of 3.67 Å from the vacant axial position. The CD spectra in the d-d region for [Cu(bpy)(L-trp)ClO₄ and [Cu(phen)(L-trp)]ClO₄ in aqueous solution showed a large negative peak at 587 and 598 nm, respectively, and the magnitudes were greatly reduced in dioxane-water, which indicates that the aromatic ring stacking interaction is weakened in a hydrophobic environment. The absorption bands due to the charge transfer (CT) interaction between the indole ring and the aromatic diamine have been observed in the difference spectra in the near ultraviolet region. The strength of the stacking interactions has been demonstrated by the CT band intensity and the distance between the stacked rings to be in the order [Cu(phen)(L-trp)]ClO₄>[Cu(bpy)(L-trp)]ClO₄ both in solution and in the solid stare.