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The porous framework [Cu2(H2O)2L] ⋅4 H2O ⋅2 DMA (H4L=oxalylbis(azanediyl)diisophthalic acid; DMA= N,N-dimethylacetamide), denoted NOTT-125, is formed by connection of {Cu2(RCOO)4} paddlewheels with the isophthalate linkers in L4−. A single crystal structure determination reveals that NOTT-125 crystallises in monoclinic unit cell with a=27.9161(6), b=18.6627(4) and c=32.3643(8) Å, β=112.655(3)°, space group P21/ c. The structure of this material shows fof topology, which can be viewed as the packing of two types of cages (cage A and cage B) in three-dimensional space. Cage A is constructed from twelve {Cu2(OOCR)4} paddlewheels and six linkers to form an ellipsoid-shaped cavity approximately 24.0 Å along its long axis and 9.6 Å across its central diameter. Cage B consists of six {Cu2(OOCR)4} units and twelve linkers and has a spherical diameter of 12.7 Å taking into account the van der Waals radii of the atoms. NOTT-125 incorporates oxamide functionality within the pore walls, and this, combined with high porosity in desolvated NOTT-125a, is responsible for excellent CO2 uptake (40.1 wt % at 273 K and 1 bar) and selectivity for CO2 over CH4 or N2. Grand canonical Monte Carlo (GCMC) simulations show excellent agreement with the experimental gas isotherm data, and a computational study of the specific interactions and binding energies of both CO2 and CH4 with the linkers in NOTT-125 reveals a set of strong interactions between CO2 and the oxamide motif that are not possible with a single amide. [ABSTRACT FROM AUTHOR]