Analysis of High and Selective Uptake of CO2 in an Oxamide-Containing {Cu2(OOCR)4}-Based Metal-Organic Framework

The porous framework [Cu-2(H2O)(2)L]center dot 4H(2)O center dot 2DMA (H4L = oxalylbis(azanediyl)diisophthalic acid; DMA = N,N-dimethylacetamide), denoted NOTT-125, is formed by connection of {Cu-2(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) angstrom, beta = 112.655(3)degrees, space group P2(1)/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 {Cu-2(OOCR)(4)} paddlewheels and six linkers to form an ellipsoid-shaped cavity approximately 24.0 angstrom along its long axis and 9.6 angstrom across its central diameter. Cage B consists of six {Cu-2-(OOCR)(4)} units and twelve linkers and has a spherical diameter of 12.7 angstrom 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 N-2. 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.