a copper(ii) paddle-wheel structure of tranexamic acid: dichloro-tetrakis[μ-4-(ammoniomethyl)cyclohexane-1-carboxylato-o,o′]dicopper(ii) dichloride hexahydrate

Tranexamic acid [systematic name: trans-4-(aminomethyl)cyclohexane-1-carboxylic acid], is an antifibrinolytic amino acid that exists as a zwitterion [trans-4-(ammoniomethyl)cyclohexane-1-carboxylate] in the solid state. Its reaction with copper chloride leads to the formation of a compound with a copper(II) paddle-wheel structure that crystallizes as a hexahydrate, [Cu2Cl2(C8H15NO2)4]2+·2Cl−·6H2O. The asymmetric unit is composed of a copper(II) cation, two zwitterionic tranexamic acid units, a coordinating Cl− anion and a free Cl− anion, together with three water molecules of crystallization. The whole structure is generated by inversion symmetry, with the Cu...Cu axle of the paddle-wheel dication being located about a center of symmetry. The cyclohexane rings of the zwitterionic tranexamic acid units have chair conformations. The carboxylate groups that bridge the two copper(II) cations are inclined to one another by 88.4 (8)°. The copper(II) cation is ligated by four carboxylate O atoms in the equatorial plane and by a Cl− ion in the axial position. Hence, it has a fivefold O4Cl coordination sphere with a perfect square-pyramidal geometry and a τ5 index of zero. In the crystal, the paddle-wheel dications are linked by a series of N—H...Cl hydrogen bonds, involving the coordinating and free Cl− ions, forming a three-dimensional network. This network is strengthened by a series of N—H...Owater, Owater—H...Cl and Owater—H...O hydrogen bonds.