X-ray and neutron powder diffraction analyses of Gly·MgSO4·5H2O and Gly·MgSO4·3H2O, and their deuterated counterparts

Abstract

We have identified a new compound in the glycine–MgSO4–water ternary system, namely glycine magnesium sulfate trihydrate (or Gly·MgSO4·3H2O) {systematic name:catena-poly[[tetraaquamagnesium(II)]-μ-glycine-κ2O:O′-[diaquabis(sulfato-κO)magnesium(II)]-μ-glycine-κ2O:O′]; [Mg(SO4)(C2D5NO2)(D2O)3]n}, which can be grown from a supersaturated solution at ∼350 K and which may also be formed by heating the previously known glycine magnesium sulfate pentahydrate (or Gly·MgSO4·5H2O) {systematic name: hexaaquamagnesium(II) tetraaquadiglycinemagnesium(II) disulfate; [Mg(D2O)6][Mg(C2D5NO2)2(D2O)4](SO4)2} above ∼330 K in air. X-ray powder diffraction analysis reveals that the trihydrate phase is monoclinic (space groupP21/n), with a unit-cell metric very similar to that of recently identified Gly·CoSO4·3H2O [Tepavitcharovaet al.(2012).J. Mol. Struct.1018, 113–121]. In order to obtain an accurate determination of all structural parameters, including the locations of H atoms, and to better understand the relationship between the pentahydrate and the trihydrate, neutron powder diffraction measurements of both (fully deuterated) phases were carried out at 10 K at the ISIS neutron spallation source, these being complemented with X-ray powder diffraction measurements and Raman spectroscopy. At 10 K, glycine magnesium sulfate pentahydrate, structurally described by the `double' formula [Gly(d5)·MgSO4·5D2O]2, is triclinic (space groupP\overline{1},Z= 1), and glycine magnesium sulfate trihydrate, which may be described by the formula Gly(d5)·MgSO4·3D2O, is monoclinic (space groupP21/n,Z= 4). In the pentahydrate, there are two symmetry-inequivalent MgO6octahedra on sites of \overline 1 symmetry and two SO4tetrahedra with site symmetry 1. The octahedra comprise one [tetraaquadiglcyinemagnesium]2+ion (centred on Mg1) and one [hexaaquamagnesium]2+ion (centred on Mg2), and the glycine zwitterion, NH3+CH2COO−, adopts a monodentate coordination to Mg2. In the trihydrate, there are two pairs of symmetry-inequivalent MgO6octahedra on sites of \overline 1 symmetry and two pairs of SO4tetrahedra with site symmetry 1; the glycine zwitterion adopts a binuclear–bidentate bridging function between Mg1 and Mg2, whilst the Mg2 octahedra form a corner-sharing arrangement with the sulfate tetrahedra. These bridged polyhedra thus constitute infinite polymeric chains extending along thebaxis of the crystal. A range of O—H...O, N—H...O and C—H...O hydrogen bonds, including some three-centred interactions, complete the three-dimensional framework of each crystal.