A Zn benzotriazolate metal−organic framework (MOF), [Zn(ZnO 2 CCH 3 ) 4 (bibta) 3 ] (1, bibta 2− = 5,5′-bibenzotriazolate), has been subjected to a mild CH 3 CO 2 − /HCO 3 − ligand exchange procedure followed by thermal activation to generate nucleophilic Zn−OH groups that resemble the active site of α-carbonic anhydrase. The postsynthetically modified MOF, [Zn-(ZnOH) 4 (bibta) 3 ] (2*), exhibits excellent performance for trace CO 2 capture and can be regenerated at mild temperatures. IR spectroscopic data and density functional theory (DFT) calculations reveal that intercluster hydrogen bonding interactions augment a Zn−OH/Zn− O 2 COH fixation mechanism.

Heterobimetallic analogues of CFA-1 [Zn 1+z M 4-z X 4 (bibta) 3 , bibta 2− = 5,5′-bibenzotriazolate, M = Co (z = 0), Ni (z = 1), Cu (z = 2.3), X = Cl − , Br − , CH 3 CO 2 − ] have been prepared via postsynthetic cation exchange. Subsequent postsynthetic X − /HCO 3 − ligand exchange followed by thermal activation generates nucleophilic M−OH groups at the Kuratowski-type metal nodes of the heterobimetallic metal-organic frameworks (MOFs). While the Cu-exchanged MOF suffered from degradation as a result of the postsynthetic modifications, the Co and Ni analogues (Co−OH and Ni−OH) proved to be stable to activation, and room-temperature isotherm measurements show steep CO 2 uptake at pressures compatible with direct air capture and other trace CO 2 removal applications. Ni−OH exhibits a greater low-pressure CO 2 capacity and higher isosteric heat of adsorption than Co−OH and the all-Zn MOF, Zn−OH. In situ diffuse reflectance infrared (IR) spectroscopy experiments indicate that Co−OH and Ni−OH adsorb CO 2 via a M−OH → M−O 2 COH chemisorption mechanism aided by intercluster hydrogen-bonding interactions. However, CO 2 adsorption in Ni−OH gives rise to spectroscopic features that are not observed for Co−OH and Zn−OH and can be attributed to Ni-bicarbonate groups that do not engage in intercluster hydrogen bonding. Density functional theory (DFT) calculations performed on model clusters support the experimentally observed trend in CO 2 affinity.

A series of benzotriazolate MOFs containing nucleophilic transition metal hydroxide (M–OH) groups has been synthesized to compare the effects of framework structure, metal composition, and method of postsynthetic ligand exchange (PSLE) on CO2 adsorption. Analogues of MFU-4 (1a/b-OH, [Zn5(OH)4(bbta)3], bbta2– = benzo-1,2,4,5-bistriazolate) and MFU-4l (2a/b-OH, [Zn5(OH)4(btdd)3], btdd2– = bis­(1,2,3-triazolo)­dibenzodioxin) were prepared by direct Cl–/OH– ligand exchange (a) or Cl–/HCO3 – ligand exchange followed by thermal activation (b). A Ni/Zn heterobimetallic analogue of MFU-4l (2a/b-NiOH) was also synthesized to investigate the effect of metal identity. The products have been characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). All of the M–OH functionalized MOFs show steep CO2 adsorption at low partial pressures. However, materials synthesized using the direct Cl–/OH– ligand exchange method show greater low-pressure CO2 uptake than those prepared by Cl–/HCO3 – PSLE. Notably, the small pore size in 1a/b-OH not only promotes stronger framework–CO2 interactions and higher CO2 uptake than 2a/b-OH but also results in slow adsorption kinetics. The Ni/Zn heterobimetallic analogue 2a-NiOH exhibits the greatest low-pressure CO2 capacity (1.70 mmol g–1 at 2.6 mbar) among the series. In situ DRIFTS studies reveal that both 2a-OH and 2a-NiOH contain weak Zn–OH binding sites that readily desorb CO2 at room temperature. However, 2a-NiOH also contains strong Ni–OH binding sites that are spectroscopically distinct and only desorb CO2 upon heating.

Postsynthetic modification methods have emerged as indispensable tools for tuning the properties and reactivity of metal−organic frameworks (MOFs). In particular, postsynthetic Xtype ligand exchange (PXLE) at metal building units has gained increasing attention as a means of immobilizing guest species, modulating the reactivity of framework metal ions, and introducing new functional groups. The reaction of a Zn−OH functionalized analogue of CFA-1 (1-OH, Zn(ZnOH) 4 (bibta) 3 , where bibta 2− = 5,5′-bibenzotriazolate) with organic substrates containing mildly acidic E−H groups (E = C, O, N) results in the formation of Zn−E species and water as a byproduct. This Brønsted acid−base PXLE reaction is compatible with substrates with pK a (DMSO) values as high as 30 and offers a rapid and convenient means of introducing new functional groups at Kuratwoski-type metal nodes. Gas adsorption and diffuse reflectance infrared Fourier transform spectroscopy experiments reveal that the anilide-exchanged MOFs 1-NHPh 0.9 and 1-NHPh 2.5 exhibit enhanced low-pressure CO 2 adsorption compared to 1-OH as a result of a Zn−NHPh + CO 2 ⇌ Zn−O 2 CNHPh chemisorption mechanism. The MFU-4l analogue 2-NHPh ([Zn 5 (OH) 2.1 (NHPh) 1.9 (btdd) 3 ], where btdd 2− = bis(1,2,3-triazolo)dibenzodioxin), shows a similar improvement in CO 2 adsorption in comparison to the parent MOF containing only Zn−OH groups.