Stereochemistry of model compounds for pyridoxal-catalysed reactions. Crystal structures of the hydrated complexes bis(pyridoxylidene-DL-valinato)nickel(II) and bis(pyridoxylidene-L-valinato)zinc(II)

Abstract

Crystals of bis(pyridoxylidene-DL-valinato)nickel(II), (I), are orthorhombic, space group Pccn, with unit-cell parameters a= 17·69, b= 19·43, c= 20·83 (all ±0·01)Å, Z= 8. The structure has been solved by the heavy-atom method and refined to R 0·17 for 4784 independent reflections. Crystals of bis(pyridoxylidene-L-valinato)zinc(II), (II), are tetragonal, space group P4₂2₁2, with Z= 8, in a unit cell of dimensions a=b= 18·59 ± 0·02, c= 21·29 ± 0·04 Å. (II) Is strictly isomorphous with the related Co^II complex. A trial model for (II) was deduced from the structure of the nickel complex and refined to R 0·16 for 1568 independent reflections. Both (I) and (II) consist of discrete molecules with octahedral metal ions bonded to two terdentate Schiff bases, and are characterized by the presence of polar cavities in which partially disordered water molecules are located. The ligand is not planar, and the distortion is mostly due to a rotation of the carboxy-group. The stereospecificity of formation of crystalline complexes is discussed.