Ternary Metal(II) Complexes with Tyrosine-Containing Dipeptides. Structures of Copper(II) and Palladium(II) Complexes Involving L-Tyrosylglycine and Stabilization of Copper(II) Complexes Due to Intramolecular Aromatic Ring Stacking

The structures and stabilities of metal(II) complexes of tyrosine- (tyr-) containing dipeptides (L), L-tyr·X [X = glycine (gly), L-/D-alanine, -tyr, -tryptophan, and -phenylalanine] and diamines [DA = ethylenediamine (en), 2,2^′-bipyridine (bpy), and 1,10-phenanthroline (phen)] have been studied by crystallographic, spectroscopic, and Potentiometric methods. The absorption spectra of the 1:1:1 Cu(DA)(L) systems exhibited a single d-d peak at 610–640 nm (pH 6–7) and at 620–640 nm (pH ∼9) with an additional peak at ∼850 nm indicating the formation of a five-coordinate complex. The circular dichroism (CD) spectra showed magnitude anomaly resulting from conformational changes. The stability constants β_pqrs of the ternary complexes Cu_p(DA)_q(L)_rH_s have been determined by Potentiometric titrations at 25 °C and I = 0.1M (KNO₃). The complexes with DA = bpy or phen are stabilized relative to Cu(en)(glycylglycine) by the stacking interaction between the side-chain aromatic ring of L and DA. Two complexes with L = l-tyr·gly, [Pd(bpy)(L-tyr·gly)]·3H₂O (1) and [Cu(phen)(L-tyr·gly)]·3H₂O (2), were isolated as crystals, and the structures were determined by the X-ray diffraction method. Complex 1 crystallizes in the triclinic space group, P1, with one molecule in a unit cell of dimensions a = 10.856(2), b = 8.114(1), c = 7.704(1) Å; α = 81.58(1); β = 112.89(1); and γ = 117.48(1)°. The Pd(II) ion is in a four-coordinate square-planar geometry with the two nitrogens of bpy and two nitrogens of L-tyr·gly. The phenol ring of L-tyr·gly is situated above the coordination plane and stacked with bpy with the average spacing of 3.28 Å. Complex 2 crystallizes in the orthorhombic space group, P2₁2₁2₁, with four molecules in a unit cell of dimensions a = 10.765(2), b = 22.074(3), and c = 10.078(2) A. The Cu(II) ion has a five-coordinate square-pyramidal geometry; the two nitrogens and one oxygen of L-tyr·gly and one of the two nitrogens of phen occupy the equatorial positions in a slightly distorted square plane, and the other nitrogen of phen is coordinated at an axial position. Intramolecular aromatic ring stacking has been detected between the phenol ring of L-tyr·gly and the aromatic ring of phen perpendicular to the Cu(II) coordination plane, the average spacing between the rings being 3.61 Å. The results confirm the stabilization of Cu(DA)(L) (DA = bpy or phen) evaluated from log β_pqrs values and suggest that the conformation of side chain aromatic rings and coordination structures can be regulated by intramolecular stacking.