Stoichiometry Controlled Structural Variation in Three-Dimensional Zn(II)-Frameworks: Single-Crystal to Single-Crystal Transmetalation and Selective CO2 Adsorption

Solvothermal reaction of the tripodal linker tris(4'-carboxybiphenyl)amine (H3L) and the colinker 3,5-di(pyridin-4-yl)-4H-1,2,4-triazol-4-amine (dpta) in a different molar ratio produced 2-fold interpenetrated but structurally different three-dimensional Zn(II) frameworks: {[Zn-3(L)(2)-(dpta)(DMF)]center dot 18DMF center dot 3H(2)O}(n) (1) and {[Zn-3(L)(2)(dpta)(DMF)]center dot 14DMF center dot 3H(2)O}(n) (2) (L = L3-, DMF = N,N'-dimethylformamide). Both the structures are built with a common [Zn-3(COO)(6)] secondary building unit. While 1 is a pillared-bilayer framework with (4(3)center dot 6(24)center dot 8)(4(3))(2) topology, the construction of 2 is different due to altered disposition of SBUs and is describable by the Schlafli symbol (4(3)center dot 5(8)center dot 6(8)center dot 7(8)center dot 8)(4(3))(2). The structural variation alters the electronic environment and pore sizes in these frameworks, which allows the activated framework 2' to uptake better N-2 and H-2 gases at 77 K, and CO2 at 273 K, than 1'. Compared to 1', framework 2' gives better selectivity of CO2 adsorption over N-2, and H-2 at 273 K, although the selectivity of CO2 over CH4 is reversed. Both 1 and 2 undergo transmetalation reactions with Cu(II) at room temperature keeping crystalinity intact to generate 1(Cu) and 2(Cu), respectively. The Cu-exchanged frameworks are characterized by single-crystal X-ray structures, while transmetalation kinetics are confirmed by energy-dispersive X-ray spectroscopy. In contrast to the activated Zn(II) frameworks, the activated 1(Cu) and 2(Cu) are unstable and show no gas uptake.