cis-Bis(L-DOPA-κ2 N,O)copper(II) monohydrate: synthesis, crystal structure, and approaches to the analysis of pseudosymmetry

The crystal structure of the cis isomer of cis-bis(L-DOPA-κ2 N,O)copper(II) monohydrate (L-DOPA is 3,4-dihydroxy-L-phenylalanine) (CuLD), [Cu(C9H10NO4)2]·H2O, is a singular example of a structurally characterized, homoleptic, crystalline metal L-DOPA complex. CuLD crystallizes in the space group P21, with Z′ = 2. The two independent molecules are square planar, and are interconnected by a linear hydrogen-bonded chain containing 12 independent hydrogen bonds. The copper ions in both molecules have weak apical intermolecular Cu...O interactions [2.739 (2) and 2.973 (2) Å] with catechol –OH groups. A survey of the Cambridge Structural Database suggested that cis and trans isomers of Cu(NH2–C–CO2)2 amino acid complexes are equally likely to occur. 12 strong O—H...O and N—H...O hydrogen bonds stabilize an unusual linear arrangement of the Cu complexes. The Cu...Cu′ distances along the chain are nearly equal [5.0739 (3) and 5.1107 (3) Å] and the Cu...Cu′...Cu angles are nearly linear [176.75 (1)°]. The MATCH procedure available in the Oxford University Crystals for Windows package was used to carry out a detailed analysis of the relationship between the two independent molecules. MATCH has some particular advantages in studying the details of pseudosymmetry, which include: (i) no atomic-order requirements; (ii) the pseudosymmetry matrix is readily available, which allows quick insight into the symmetry elements involved and their location; and (iii) the differences between molecular centroids, as well as between all atomic positions and torsion angles, are listed. A tutorial presentation is designed to attract new users to the technique. In the present case, a search for a pseudosymmetric relationship between the two independent molecules showed that they are related by a pseudo-42 axis along the crystallographic c direction. A detailed analysis shows that the pseudo-42 symmetry is disrupted by torsions about the CH2—C(ipso) bonds, and that there is no supergroup that can be used to describe the crystal structure.