From Layered Structures to Cubic Frameworks: Expanding the Structural Diversity of Uranyl Carboxyphosphonates via the Incorporation of Cobalt

Five heterobimetallic U(VI)/Co(II) carboxyphosphonates have been synthesized under mild hydrothermal conditions by reacting UO3, Co(CH3CO2)2·4H2O, and triethyl phosphonoacetate. These compounds, Co(H2O)4[(UO2)2(PO3CH2CO2)2(H2O)2] (CoUPAA-1), [Co(H2O)6][UO2(PO3CH2CO2)]2·8H2O (CoUPAA-2), Co(H2O)4[UO2(PO3CH2CO2)]2·4H2O (CoUPAA-3), Co(H2O)4[(UO2)6(PO3CH2CO2)2O2(OH)3(H2O)3]2·3H2O (CoUPAA-4), and Co2[(UO2)6(PO3CH2CO2)3O3(OH)(H2O)2]·16H2O (CoUPAA-5), range from two- to three-dimensional structures. CoUPAA-1 to CoUPAA-3 all possess uranyl carboxyphosphonate layers that are separated by the Co(II) cation with varying degrees of hydration. CoUPAA-4 contains both UO7 pentagonal bipyramids and UO8 hexagonal bipyramids within the uranyl carboxyphosphonate plane. Unlike the first four low-symmetry compounds, CoUPAA-5 is a cubic, three-dimensional network with large cavities approximately 16 Å in diameter that are filled with cocrystallized water molecules. Differential gas absorption measurements performed on CoUPAA-5 displayed a surface area uptake for CO2 of 40 m2 g−1 at 273 K, and no uptake for N2 at 77 K.