The First Organically Templated Tetravalent Uranium Phosphates with Dimer-Structured Topologies

Four new organically templated uranium(IV) phosphates, (C<sub>4</sub>H<sub>16</sub>N<sub>3</sub>)[U<sub>2</sub>F<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>(HPO<sub>4</sub>)] (<b>1</b>), (C<sub>6</sub>H<sub>21</sub>N<sub>4</sub>)[U<sub>2</sub>F<sub>4</sub>(PO<sub>4</sub>)(HPO<sub>4</sub>)<sub>2</sub>] (<b>2</b>), (C<sub>4</sub>H<sub>16</sub>N<sub>3</sub>)<sub>2</sub>[U<sub>2</sub>F<sub>10</sub>(HPO<sub>4</sub>)<sub>2</sub>] (<b>3</b>), and (C<sub>6</sub>H<sub>16</sub>N<sub>2</sub>)<sub>2</sub>(UF<sub>7</sub>)(H<sub>2</sub>PO<sub>4</sub>) (<b>4</b>), 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, U<sup>4+</sup> ions are located in the center of bicapped trigonal prisms of UFO<sub>7</sub>, UF<sub>2</sub>O<sub>6</sub>, UF<sub>6</sub>O<sub>2</sub>, or UF<sub>8</sub>, from which edge-sharing U<sub>2</sub>F<sub><i>m</i></sub>O<sub>14−<i>m</i></sub> dimers (<i>m</i> = 2 and 4 in <b>1</b>, <i>m</i> = 4 in <b>2</b>, <i>m</i> = 10 in <b>3</b>, and <i>m</i> =14 in <b>4</b>) are formed. The less-fluorinated U<sub>2</sub>F<sub>2</sub>O<sub>12</sub> and U<sub>2</sub>F<sub>4</sub>O<sub>10</sub> dimers are connected to eight and six phosphate groups to give two-dimensional layer and one-dimensional chain structures, respectively; the more-fluorinated U<sub>2</sub>F<sub>10</sub>O<sub>4</sub> and fully fluorinated U<sub>2</sub>F<sub>14</sub> dimers are respectively attached by two or zero phosphate groups to form clustered structures. TGA results indicated that the layered structure of <b>1</b> 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 <b>1</b> and <b>2</b> could be more stable than UO<sub>2</sub> under sufficiently oxidizing conditions. Crystal data: triclinic, space group <i>P</i>1̅, <i>a</i> = 9.1657(4) Å, <i>b</i> = 10.0618(4) Å, <i>c</i> = 10.7584(4) Å, α = 73.668(1)°, β = 65.366(1)°, γ = 65.916(1)°, and <i>Z</i> = 2 for <b>1</b>; monoclinic, <i>P</i>2<sub>1</sub>/<i>c</i>, <i>a</i> = 11.1124(4) Å, <i>b</i> = 19.6830(8) Å, <i>c</i> = 10.1466(4) Å, β = 114.362(1)°, and <i>Z</i> = 4 for <b>2</b>; monoclinic, space group <i>P</i>2<sub>1</sub>/<i>n</i>, <i>a</i> = 18.383(1) Å, <i>b</i> = 7.8448(6) Å, <i>c</i> = 18.442(1) Å, β = 113.967(1)°, and <i>Z</i> = 4 for <b>3</b>; monoclinic, <i>P</i>2<sub>1</sub>/<i>c</i>, <i>a</i> = 10.2346(9) Å, <i>b</i> = 23.846(2) Å, <i>c</i> = 8.8091(8) Å, β = 93.459(2)°, and <i>Z</i> = 4 for <b>4</b>.