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

Abstract

A novel Zn benzotriazolate metal–organic framework (MOF), [Zn9(OAc)6(bbtm)6] (1, bbtm2– = 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.