Analysis of high and selective uptake of CO₂ in an Oxamide-containing {Cu₂(OOCR)₄}-based metal-organic framework

The porous framework [Cu₂(H₂O)₂L] η4H₂Oη2DMA (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 L^4-. 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 (cageA and cageB) in three-dimensional space. CageA is constructed from twelve {Cu₂(OOCR)₄} paddlewheels and six linkers to form an ellipsoid-shaped cavity approximately 24.0Å along its long axis and 9.6Å across its central diameter. CageB consists of six {Cu₂(OOCR)₄} units and twelve linkers and has a spherical diameter of 12.7Å 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 (40.1wt% at 273K and 1bar) 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.