Spontaneous resolution of binary copper(II) complexes with racemic dipeptides: Crystal structures of glycyl-l-α-amino-n-butyrato copper(II) monohydrate, glycyl-d-valinato copper(II) hemihydrate, and glycyl-l-valinato copper(II) hemihydrate

Copper(II) complexes with glycyl-dl-α-amino-n-butyric acid (H2gly-dl-but), glycyl-dl-valine (H2gly-dl-val), glycyl-dl-norleucine (H2gly-dl-norleu), glycyl-dl-threonine (H2gly-dl-thr), glycyl-dl-serine (H2gly-dl-ser), glycyl-dl-phenylalanine (H2gly-dl-phe), and glycyl-l-valine (H2gly-l-val), have been prepared and characterized by IR, powder diffuse reflection, CD and ORD spectra, and magnetic susceptibility measurements, and by single-crystal X-ray diffraction. The crystal structures of the copper complex with H2gly-dl-but, the copper complex with H2gly-dl-val, and [Cu(gly-l-val)]n · 0.5nH2O have been determined by a single-crystal X-ray diffraction method. As for the structure of the copper complex with H2gly-dl-but, the configuration around the asymmetric carbon atom is similar to that of [Cu(gly-l-val)]n · 0.5nH2O. Therefore it is concluded that the copper complex with H2gly-dl-but is [Cu(gly-l-but)]n · nH2O. On the contrary, as for the structure of the copper complex with H2gly-dl-val, the configuration around the asymmetric carbon atom is different from that of [Cu(gly-l-val)]n · 0.5nH2O. Therefore it is concluded that the copper complex with H2gly-dl-val is [Cu(gly-d-val)]n · 0.5nH2O. So during the crystallization of the copper(II) complexes with H2gly-dl-but and H2gly-dl-val, spontaneous resolution has been observed; the four complexes have separated as [Cu(gly-d-but)]n · nH2O, [Cu(gly-l-but)]n · nH2O, [Cu(gly-d-val)]n · 0.5nH2O, and [Cu(gly-l-val)]n · 0.5nH2O, respectively. [Cu(gly-l-but)]n · nH2O is orthorhombic with the space group P212121. [Cu(gly-d-val)]n · 0.5nH2O and [Cu(gly-l-val)]n · 0.5nH2O are monoclinic with the space group C2. In these complexes, the copper atom is in a square-pyramidal geometry, ligated by a peptide nitrogen atom, an amino nitrogen atom, a carboxyl oxygen atom, and a carboxyl oxygen atom and a peptide oxygen atom from neighboring molecules. So these complexes consist of a two-dimensional polymer chain bridged by a carboxyl oxygen atom and a peptide oxygen atom from neighboring molecules. The axial oxygen atom is located above the basal plane and the side chain of an amino acid is located below it. These polymer chains consist of only one or the other type of optical isomers; no racemic dipeptides are found. Therefore, spontaneous resolution has been observed in the crystallization of copper(II) complexes with H2gly-dl-but and H2gly-dl-val. The crystal structure of [Cu(gly-d-val)]n · 0.5nH2O agrees almost completely with that of [Cu(gly-l-val)]n · 0.5nH2O, except for the configuration around the asymmetric carbon atom.