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 UO 3, Co(CH 3 CO 2 ) 2 ·4H 2 O, and triethyl phosphonoacetate. These compounds, Co(H 2 O) 4 [(UO 2 ) 2 (PO 3 CH 2 CO 2 ) 2 (H 2 O) 2 ] (CoUPAA-1), [Co(H 2 O) 6 ][UO 2 (PO 3 CH 2 CO 2 )] 2 ·8H 2 O (CoUPAA-2), Co(H 2 O) 4 [UO 2 (PO 3 CH 2 CO 2 )] 2 ·4H 2 O (CoUPAA-3), Co(H 2 O) 4 [(UO 2 ) 6 (PO 3 CH 2 CO 2 ) 2 O 2 (OH) 3 (H 2 O) 3 ] 2 ·3H 2 O (CoUPAA-4), and Co 2 [(UO 2 ) 6 (PO 3 CH 2 CO 2 ) 3 O 3 (OH)(H 2 O) 2 ]·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 UO 7 pentagonal bipyramids and UO 8 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 CO 2 of 40 m 2 g −1 at 273 K, and no uptake for N 2 at 77 K.