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

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Abstract
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.

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