Cis-Bis(l-DOPA-κ2N,O)copper(II) monohydrate: Synthesis, crystal structure, and approaches to the analysis of pseudosymmetry

Abstract

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.