Two ligand-functionalized Pb(ii) metal–organic frameworks: structures and catalytic performances

Abstract

A microporous Pb(II) metal–organic framework (MOF) [PbL₂]·2DMF·6H₂O (1) has been assembled from a N-oxide and amide doubly functionalized ligand HL (= N-(4-carboxyphenyl)isonicotinamide 1-oxide). Complex 1 features a three-dimensional (3D) framework possessing one-dimensional (1D) rhombic channels with dimensions of 13 × 13 Ų. The 3D framework is built up from 1D PbO₂ chains that link ligands in parallel fashion to construct single-wall channels. When recrystallizing 1 in a DMSO–DMF mixture (3 : 5 v/v), a new coordination polymer, [PbL₂]·DMF·2H₂O (2), was obtained. Complex 2 is also a 3D framework containing 1D rectangular channels, but the channel dimensions become reduced in size to 13 × 8 Ų due to reorganization of the Pb(II) coordination environment. The PbO₂ chains in 2 are reformed to link ligands in a double-wall fashion, significantly reducing the channel size. Even though, the guest exchange study indicates that the DMF molecules in 2 could be replaced with benzene molecules when immersing in benzene solvent, showing single-crystal-to-single-crystal (SC–SC) guest exchange in the solid state and leading to a daughter crystal [PbL₂]·0.5C₆H₆·2H₂O (2′). Desolvated 1 and 2 display preferential adsorption behaviors of water vapour over CO₂ due to the hydrophilic nature of the channels and the strong host–guest interactions. Catalytic tests indicate that desolvated 1 and 2 have size-selective catalytic activity towards the Knoevenagel condensation reaction.