Robust MOFs of "tsg" Topology Based on Trigonal Prismatic Organic and Metal Cluster SBUs: Single Crystal to Single Crystal Postsynthetic Metal Exchange and Selective CO2 Capture.

Abstract

The self-assembly of a rigid and trigonal prismatic triptycene-hexaacid H6 THA with Co(NO3 )2 or Mn(NO3 )2 leads to isostructural metal-organic frameworks (MOFs) that are sustained by 6-connecting metal cluster [M3 (μ3 -O)(COO)6 ] secondary building units (SBUs). The Co- and Mn-MOFs, constructed from organic and metal-cluster building blocks that are both trigonal prismatic, correspond to the heretofore unknown "tsg" topology. Due to the rigidity and concave attributes of H6 THA, the networks in the Co- and Mn-MOFs are highly porous and undergo 3-fold interpenetration. The interpenetration imparts permanent microporosity and high thermal stability to the MOFs to permit postsynthetic metal exchange (PSME) and gas sorption. The PSME occurs in a single crystal to single crystal fashion when the crystals of Co- or Mn-MOFs are immersed in a solution of Cu(NO3 )2 in MeOH/H2 O. Further, the isostructural robust MOFs exhibit significant gas sorption and remarkable selectivity for CO2 over N2 (ca. 100 fold) at ambient conditions. In fact, the postsynthetically-engineered Cu-THA exhibits better CO2 sorption than Co-THA and Mn-THA. A composite of effects that include pore dimensions (ca. 0.7 nm), unsaturated metal centers, and basic environments conferred by the quinoxaline nitrogen atoms appears to be responsible for the observed high CO2 capture and selectivity. The high symmetry and structural attributes of the organic linker seemingly dictate adoption of the trigonal-prismatic metal cluster SBU by the metal ions in the MOFs.