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

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 group P2₁/n), with a unit‐cell metric very similar to that of recently identified Gly·CoSO₄·3H₂O [Tepavitcharova et 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 group P, Z = 1), and glycine magnesium sulfate trihydrate, which may be described by the formula Gly(d₅)·MgSO₄·3D₂O, is monoclinic (space group P2₁/n, Z = 4). In the pentahydrate, there are two symmetry‐inequivalent MgO₆ octahedra on sites of symmetry and two SO₄ tetrahedra 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 MgO₆ octahedra on sites of symmetry and two pairs of SO₄ tetrahedra 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 the b axis 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.