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

摘要

We have identified a new compound in the glycine-MgSO4-water ternary system, namely glycine magnesium sulfate trihydrate (or GlyMgSO4-3H2O) {systematic name: catewa-poly[[tetraaquamagnesium(II)]-/x-glycine-/c2O:O'-[diaquabis(sulfato-/cO)magnesium(II)]-/x-glycine-/c2O:O']; [Mg(SO4)-(CyDsNC^XD^^],,}, 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(C2DsNO2)2(D2O)4](SO4)2} above ~330 K in air. X-ray powder diffraction analysis reveals that the trihydrate phase is monoclinic (space group P2i/n), with a unit-cell metric very similar to that of recently identified Gly-CoSO4-3H2O [Tepavitcharova et al. (2012). /. 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(6 octahedra on sites of 1 symmetry and two SO4 tetrahedra with site symmetry 1. The octahedra comprise one [tetraaquadiglcyinemagnesium]2+ ion (centred on Mgl) and one [hexa-aquamagnesium]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 MgC>6 octahedra on sites of 1 symmetry and two pairs of SO4 tetrahedra with site symmetry 1; the glycine zwitterion adopts a binuclear-bidentate bridging function between Mgl 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.