The first organically templated tetravalent uranium phosphates with dimer-structured topologies

Four new organically templated uranium(IV) phosphates, (C₄H ₁₆N₃)[U₂F₃(PO₄) ₂(HPO₄)] (1), (C₆H₂₁N ₄)[U₂F₄(PO₄)(HPO₄) ₂] (2), (C₄H₁₆N₃)₂[U ₂F₁₀(HPO₄)₂] (3), and (C ₆H₁₆N₂)₂(UF₇)(H ₂PO₄) (4), have been successfully synthesized in pure-phase form under mild solvothermal conditions and characterized by single-crystal X-ray diffraction, magnetic susceptibility, thermogravimetric analysis (TGA), elemental analysis, inductively coupled plasma-atomic emission spectroscopy, and fluorine analysis. They are the first tetravalent uranium phosphates with encapsulated organic templates ever reported. All four materials display unique dimer-structured topologies with various dimensionalities in the inorganic/hydrogen-bonded networks. In this series, U⁴⁺ ions are located in the center of bicapped trigonal prisms of UFO₇, UF ₂O₆, UF₆O₂, or UF₈, from which edge-sharing U₂F_mO_14-m dimers (m = 2 and 4 in 1, m = 4 in 2, m = 10 in 3, and m =14 in 4) are formed. The less-fluorinated U₂F₂O₁₂ and U ₂F₄O₁₀ dimers are connected to eight and six phosphate groups to give two-dimensional layer and one-dimensional chain structures, respectively; the more-fluorinated U₂F₁₀O ₄ and fully fluorinated U₂F₁₄ dimers are respectively attached by two or zero phosphate groups to form clustered structures. TGA results indicated that the layered structure of 1 could sustain heating up to ∼300°C and the thermal stability of the series steadily decreased with decreasing structural dimensionality. The 4+ oxidation state of uranium was confirmed by bond-valence-sum calculations and magnetic susceptibility measurements. In view of radioactive mobility, we discovered that 1 and 2 could be more stable than UO₂ under sufficiently oxidizing conditions. Crystal data: triclinic, space group P1̄, a = 9.1657(4) Å, b = 10.0618(4) Å, c = 10.7584(4) Å, α = 73.668(1)°, β= 65.366(1)°, γ = 65.916(1)°, and Z= 2 for 1; monoclinic, P2₁/c, a = 11.1124(4) Å, b = 19.6830(8) Å, c = 10.1466(4) Å, β= 114.362(1)°, and Z = 4 for 2; monoclinic, space group P2₁/n, a = 18.383(1) Å, b = 7.8448(6) Å, c = 18.442(1) Å, β = 113.967(1)°, and Z = 4 for 3; monoclinic, P ₂1/c, a = 10.2346(9) Å, b = 23.846(2) Å, c = 8.8091(8) Å, β = 93.459(2)°, and Z = 4 for 4.