New Amine-Templated Zinc Phosphates with a Temperature-Induced Increase of Structural Dimensionality

Three new amine-templated zinc phosphates, [C₄N₂H₁₄][Zn(HPO₄)₂]·H₂O, AU-I, [C₄N₂H₁₄][Zn₂(H_0.5PO₄)₂(H₂PO₄)], AU-II, and [C₄N₂H₁₄][Zn₅(H₂O)(PO₄)₄], AU-III, are prepared by hydrothermal synthesis using an organic amine, N,N‘-dimethylethylendiamine CH₃NHCH₂CH₂NHCH₃, as structure-directing agent. The three materials are prepared from the same reaction mixture, 1Zn(CH₃CO₂)₂:3.05H₃PO₄:2.25CH₃NHCH₂CH₂NHCH₃:138H₂O (pH = 5.1), AU-I at RT, AU-II at 60 °C, and AU-III at 170 °C. The materials are built from corner-sharing ZnO₄ and PO₄ tetrahedra forming chains, layers, or framework structures for AU-I to III, respectively, and are linked together by hydrogen bonds via the diprotonated amine ions. The complete hydrogen-bond scheme is resolved for these new compounds and reveals some interesting phenomena, for example, a hydrogen shared between two phosphate groups in AU-II, thereby forming H_0.5PO₄ groups. Furthermore, the water molecules are different; that is, in AU−I they act as hydrogen-bond donor and acceptor, whereas they act as ligand in AU-III with coordination to Zn. The structures of the compounds are determined by single-crystal X-ray diffraction analysis. AU-I, [C₄N₂H₁₄][Zn(HPO₄)₂]·H₂O, crystallizes in the triclinic space group P-1, a = 8.215(2), b = 8.810(3), c = 8.861(3) Å, α = 88.001(4)°, β = 89.818(5)°, and γ = 89.773(5)°, Z = 2. AU-II, [C₄N₂H₁₄][Zn₂(H_0.5PO₄)₂(H₂PO₄)], is monoclinic, P2/n, a = 11.7877(4), b = 5.2093(2), c = 12.2031(4) Å, β = 98.198(1)°, Z = 2. AU-III, [C₄N₂H₁₄][Zn₅(H₂O)(PO₄)₄], crystallizes in the orthorhombic space group Pna2₁ with lattice parameters, a = 20.723(2), b = 5.2095(6), c = 17.874(2) Å, Z = 4. The phase stability investigated by systematic hydrothermal synthesis is presented, and the materials are further characterized by ³¹P solid-state MAS NMR, for example, by determination of ³¹P chemical shift anisotropies for AU-III, while the thermal behavior is investigated by thermogravimetry (TG).