Collapse or Capture? Guest-Induced Response of Two Structurally Distinct Pillared-MOFs upon exposure to Pyridines and Quinolines

Abstract

Two differently entangled Zn-containing pillared Metal-Organic Frameworks (MOFs) have been used for the inclusion of N-containing heterocycles, such as quinolines and pyridines, through single-crystal-to-single-crystal transformations. These two MOFs have similar chemical compositions, each consisting of a dicarboxylic linker (4,4′-biphenyldicarboxylic acid or 2,6-naphthalenedicarboxylic acid) and the same bis-amide-bis-pyridine pillar. Either the flexible and interpenetrated MOF PUM168 and the rigid and polycatenated PUM210 exhibit good uptake propensity towards quinoline, although the structural modelling of the whole included guest molecules was more successful only for PUM168. For PUM210 only one molecule of quinoline coordinated to a metal center was modeled, the remaining being randomly distributed along the channels. Both PUM168 and PUM210 crystals rapidly degrade once put in contact with liquid pyridines. The decomposition products of PUM210 in pyridine were structurally characterized, giving insights on the degradation pathway. This involves the replacement of the bis-amide-bis-pyridine pillar of the MOF by pyridine with formation of a new homoleptic 1D-coordination polymer in which Zn ions are bound to naphthalenedicarboxylate dianions and pyridine molecules. Finally, uptake of the chelating 8-hydroxy-quinoline by PUM168 led to the protonolysis of the dicarboxylate linker and extraction of Zn ions from the framework, with formation of the bis-chelate complex Zn(8-hydroxyquinolinate)2.