Syntheses and structures of ammonium transition-metal dialuminium tris(phosphate) dihydrates (NH4)MAl4(PO4)3·2H2O (M = Mn and Ni)

The aluminophosphate frameworks of the title compounds, (NH4)MAl2(PO4)3·2H2O (M = Mn and Ni), consist of a three-dimensional array of vertex-sharing tetrahedral PO4 and trigonal–bipyramidal AlO5 moieties, which delineate [001] twelve-membered channels in which the ammonium NH4 + and transition-metal cations (M = Mn2+ and Ni2+) reside as charge compensators for the anionic aluminophosphate framework.


Structural commentary
The aluminophosphate framework of the title compounds with the chemical formula (NH 4 )MAl 2 (PO 4 ) 3 Á2H 2 O (M = Mn and Ni) is composed of [PO 4 ] tetrahedra and [AlO 5 ] trigonalbipyramids. Fig. 1(a) shows the [Al 2 (PO 4 ) 3 ] 1 layers, which are built up from four-and eight-membered rings connected via Al-O-P bonds. These layers stack along the a-axis direction, with the [P2O 4 ] tetrahedra (atom P2 lies on a crystallographic twofold axis) bridging between them, leading to the formation of a three-dimensional network encapsulating twelvemembered channels propagating in the [001] direction. The ammonium and transition-metal cations are respectively located in and on these channels, compensating the negative charge of the aluminophosphate framework [ Fig. 1 (Panz et al., 1998;Meyer & Haushalter 1994;Kiriukhina et al., 2020;Medina et al. 2004) showed similar geometrical features with longer axial Al-O bonds distances.
The transition-metal cations, which lie on crystallographic twofold axes, are octahedrally coordinated by two oxygen atoms of water molecules and four oxygen atoms of the framework (Fig. 2). The mean M-O bond distances for the Mn and Ni compounds are 2.186 Å and 2.079 Å , respectively, which are consistent with the ionic radii of VI Mn 2+ (0.83 Å ) and VI Ni 2+ (0.69 Å ; Shannon 1976). The MO 6 octahedron shares an edge O4Á Á ÁO4 with the adjacent [P2O 4 ] tetrahedron. The length of the shared-edge O4Á Á ÁO4 is the shortest among the twelve edges of octahedrally coordinated transition-metal cations in accordance with the P 5+ -M 2+ cation repulsion (Pauling, 1929(Pauling, , 1960. The positions of the hydrogen atoms in the water molecule, H71 and H72, could be determined by analysing the residual peaks in the difference-Fourier maps. The oxygen atom O7 of the water molecule is coordinated to the transition-metal ions, and hydrogen atoms of H71 and H72 form O-HÁ Á ÁO hydrogen bonds with the oxygen atoms O1 and O3 of the [Al 2 (PO 4 ) 3 ] 1 layer, respectively (Tables 2 and 3). Thus, the H71Á Á ÁO1 and H72Á Á ÁO3 hydrogen bonds contribute to the accumulation of the layers. Symmetry codes: (i) Àx, y, Àz + 1 2 ; (ii) Àx + 1 2 , y À 1 2 , Àz + 1 2 ; (iv) x, Ày + 1, z À 1 2 ; (vi) Àx + 1 2 , y + 1 2 , Àz + 1.  As for the hydrogen-bonding interactions of the ammonium cation (N atom site symmetry 2) within the title compounds, not all the H atoms could be definitively located from difference maps but some structural information could be obtained from the observed distances N1Á Á ÁO5 = 3.085 (5) and 3.103 (4) Å and N1Á Á ÁO6 = 2.906 (4) and 2.862 (3) Å for (NH 4 )MAl 2 (PO 4 ) 3 Á2H 2 O (M = Mn and Ni), respectively. The longer N1Á Á ÁO5 distance and the large isotropic atomic displacement parameters, U iso , of the N1 atom clearly indicate the relatively weaker hydrogen bonding for the presumed N1-HÁ Á ÁO5 cases. This structural feature did not allow us to definitively located the positions of hydrogen atoms within the N1-HÁ Á ÁO5 cases. Nevertheless, some of the hydrogen-atom positions around the ammonium cations could be located in the difference-Fourier maps and coordinates are (0.5382, 0.3998, 0.2391) and (0.5357, 0.4204, 0.2296) for (NH 4 )MAl 2 -(PO 4 ) 3 Á2H 2 O (M = Mn and Ni), respectively. These possible hydrogen-atom positions correspond to those for the N1-HÁ Á ÁO6 cases. Weak hydrogen bonds between NH 4 + and the framework suggests that NH 4 + and a monovalent cation (e.g., alkali cation or H 3 O + ) are exchangeable akin to zeolitic cations in this unique framework structure (Meyer & Haushalter 1994;Kiriukhina et al., 2020). The chemical formula for the group of compounds reported in this study can be denoted by A + M 2+ Al 2 (PO 4 ) 3 Á2H 2 O (A = monovalent cation, M = divalent transition-metal cation).

Synthesis and crystallization
Single crystals of (NH 4 )MAl 2 (PO 4 ) 3 Á2H 2 O (M = Mn and Ni) were obtained as by-products of the laumontite-type zeolite imidazole-templated hydrothermal technique. The precursor solution was prepared by dissolving the chemical agents of imidazole, aluminium-isopropoxide and H 3 PO 4 (85% solution): the transition-metal component (Ni or Mn) was added to the solution. For the insertion of nickel in the system, (CH 3 COO) 2 NiÁ4H 2 O was used and for corresponding manganese analogue (CH 3 COO) 2 MnÁ4H 2 O was added to the as-prepared precursor solution. In each case, the resultant gel mixture was then sealed in a Teflon-lined tube and heated at 453 K for three days.

Refinement details
The crystal data, data collection methods, and structure refinement details are summarized in Table 4. The positions of the hydrogen atoms bonded to O7 were estimated using the residual peaks in the difference Fourier maps and refined using a riding model. The U iso parameters for hydrogen atoms were fixed at 1.5 Â the U iso of O7.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.