Tuning the supramolecular isomerism of MOF-74 by controlling the synthesis conditions

Supramolecular isomerism of metal-organic frameworks (MOFs) is known for several MOF structures, having direct implications on the properties of these materials. Although the synthesis of MOF isomers is mainly serendipitous in nature, achieving controlled formation of a target framework is highly relevant for practical applications. This work discusses the influence of additives and synthesis conditions on the formation of porous isomers containing Zn²⁺ as nodes and 2,5-dihydroxy-1,4-benzenedicarboxylate (dobdc(^4-)) as a linker. Using solvent mixtures containing strongly coordinated molecules, e.g. N,N'-dimethylformamide (DMF) and N-methylpyrrolidone (NMP), facilitates the formation of porous structures of type [Zn-₂(dobdc)(S)_x]·yS (S = DMF, NMP) which are built from dinuclear Zn₂(O)₂(CO₂)₃ secondary building units (SBUs) consisting of two different edge-sharing polyhedra with the Zn²⁺ ions in a unsaturated coordinative environment. In the presence of water, the Zn²⁺ dimers are converted to one-dimensional infinite Zn²⁺ chains, in which the number of Zn²⁺-linker bonds increases, therefore giving a hydrolytically more stable coordination environment. The full characterization of the isomers as well as their conversion to the most stable isomer is presented.