Strong CO₂ Chemisorption in a Metal-Organic Framework with Proximate Zn-OH Groups

A novel Zn benzotriazolate metal-organic framework (MOF), [Zn₉(OAc)₆(bbtm)₆] (1, bbtm^2- = bis(benzotriazolyl)methanone, OAc^- = acetate), has been synthesized and structurally characterized using micro-crystal electron diffraction. The framework contains 12-connected nonanuclear Zn clusters with Zn-OAc groups separated by short intercluster Zn···Zn distances of 6.06 Å. Postsynthetic OAc^-/OH^- ligand exchange followed by thermal activation generates 1a-OH, which adsorbs CO₂ at very low pressures (1.37 mmol/g at 2.5 mbar) and requires an unusually high desorption temperature (>160 °C). Diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations have been used to interrogate the CO₂ binding mechanism in 1a-OH. The formation of unsymmetric bridging carbonate ligands within the Zn···Zn pockets accompanied by strong hydrogen bonding of the carbonate with a neighboring zinc aqua ligand explains the remarkably strong CO₂ affinity of 1a-OH.