The impact of N,N′-ditopic ligand length and geometry on the structures of zinc-based mixed-linker metal–organic frameworks

Abstract

Combining Zn(NO₃)₂·6H₂O with a series of dicarboxylic acids in the presence of the N,N′-ditopic ligand di(4-pyridyl)-1H-pyrazole (Hdpp) results in a series of mixed-linker metal–organic frameworks (MOFs) that have been crystallographically characterised. The reaction with 1,4-benzenedicarboxylic acid (H₂bdc) gives [Zn₂(bdc)₂(Hdpp)₂]·2DMF 1, which shows Zn₂(μ-carboxylate)₂(carboxylate)₂ secondary building units (SBUs) linked by bdc ligands into sheets, and these are pillared by the Hdpp linkers into a doubly-interpenetrated three-dimensional network. The reaction with 1,4-naphthalene dicarboxylic acid (H₂ndc-1,4) gives two products: [Zn₂(1,4-ndc)₂(Hdpp)]·4DMF 2a forms a three-dimensional network in which sheets, formed from Zn₂(carboxylate)₄ ‘paddle-wheel’ SBUs being linked by 1,4-ndc, are connected together by Hdpp pillars, whereas [Zn(1,4-ndc)(Hdpp)]·DMF 2b forms a fourfold interpenetrated structure based on diamondoid networks with single zinc centres as nodes. The reaction with 1,3-benzenedicarboxylic acid (H₂mbdc) produces [Zn(mbdc)(Hdpp)]·DMF 3, which forms a two-dimensional network with (4,4) topology in which ZnO₂N₂ nodes are interlinked by mbdc and Hdpp linkers. The reaction with 5-methyl-1,3-benzenedicarboxylic acid (H₂mbdc-Me) also forms a two-dimensional network structure, [Zn₂(mbdc-Me)₂(Hdpp)₂]·DMF 4, albeit wherein dicarboxylates bridge between zinc-dicarboxylate tapes, themselves formed by interlinking of Zn₂(μ-carboxylate)₂(carboxylate)₂ SBUs similar to those in 1. Finally, the reaction with 2,6-naphthalene dicarboxylic acid (H₂ndc-2,6) yields two crystalline species, both having the formula [Zn₂(2,6-ndc)₂(Hdpp)]·DMF 5a/5b and possessing infinite zinc-carboxylate chain motifs interlinked by both naphthalene rings and Hdpp linkers into a three-dimensional framework. In compounds 1, 2b, 3 and 4, the pyrazole NH groups are involved in hydrogen bonding that serves to link either interpenetrated networks or neighbouring sheets together. However, in 2a and 5a/5b the NH groups project into the pores of the framework enabling interactions with guest molecules.