Interactions of a β-dipeptide with monovalent metal cations: crystal structures of (anthranoyl)anthranilic acid and its lithium, sodium and thallium salts

Abstract

X-ray crystal structure analyses have been performed on the β-dipeptide (anthranoyl)anthranilic acid [HAnthAnthOH] and its lithium, sodium and thallium salts [HAnthAnthOM] to give a first set of data for this representative model ligand. Crystals of the β-dipeptide are orthorhombic, space group Pca2 1. The unit cell contains two molecules of (anthranoyl)anthranilic acid which form a dimer via hydrogen bonds. The components of the β-dipeptide are rotated into the trans-conformation which allows for internal hydrogen bonds. The p K S value of (anthranoyl)anthranilic acid (9.80 ± 0.14) shows a slight decrease as compared to anthranilic acid; the metal salts can therefore be prepared by direct neutralization of the β-dipeptide with metal hydroxides or carbonates. The alkali compounds crystallize as the trihydrates [HAnthAnthOM(H 2O) 3, M=Li, Na] in the triclinic space group P 1 ̄ . Both metal ions show a clear preference for water molecules over the (anthranoyl)anthranilate anions as ligands in their coordination spheres. As a consequence, the [HAnthAnthO] − anions are only partially involved in metal complexation. The cell plots of both compounds exhibit a stacking with an alternation of oppositely charged layers. The negatively charged layers are composed exclusively of (anthranoyl)anthranilate anions. The thallium compound crystallizes as the hemihydrate [HAnthAnthOTl(H 2O) 0.5] in the monoclinic space group C2/ c. In the dinuclear units, the thallium ions accommodate one nitrogen and four oxygen atoms of the anions in their coordination sphere and in addition entertain weak Tl-arene contacts. In contrast to the alkali compounds, the water molecules are not involved in metal complexation, but contribute to a network of hydrogen bonding.