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)center dot 4H(2)O, and triethyl phosphonoacetate. These compounds, Co(H2O)(4)[(UO2)(2)(PO3CH2CO2)(2)(H2O)(2)] (CoUpAA-1), [Co(H2O)(6)][UO2(PO3CH2CO2)](2)center dot 8H(2)O (CoUPAA-2), Co(H2O)(4)UO2(PO3CH2CO2)](2)center dot 4H(2)O (CoUPAA-3), Co(H2O)(4)[(UO2)(6)(PO3CH2CO2)(2)O-2(OH)(3)(H2O)(3)]2 center dot 3H(2)O (CoUPAA-4), and Co-2[(UO2)(6)(PO3CH2CO2)(3)O-3(OH)(H2O)(2)]center dot 16H(2)O (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 A 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 m(2) g(-1) at 273 K, and no uptake for N-2 at 77 K.