A series of entangled MOFs constructed from flexible dipyridyl piperazine and rigid dicarboxylate: interpenetration, self-penetration, and polycatenation

Abstract

The initial evaluation of MOF entanglements as their main limitation is changing to an opportunity since the related benefits are now known. Based on a linear rigid dicarboxylic acid, biphenyl-4,4′-dicarboxylic acid (H₂bpdc), and a flexible dipyridyl piperazine ligand, 1,4-bis(4-pyridylmethyl)piperazine (bpmp), with versatile conformations, four divalent metal–organic frameworks (MOFs), namely {[Cu₂(bpmp)₂(bpdc)₂(H₂O)₂]·1.25DMF·1.5H₂O}_n (1), {[Zn₂(bpmp)(bpdc)₂]·2H₂O}_n (2), {[Cd(bpmp)(bpdc)(H₂O)₂]·2DMA}_n (3), and [Cd(bpmp)_1.5(bpdc)]_n (4), with entangled structures from interpenetration and self-penetration to polycatenation are reported. In all the products, both bpmp and bpdc acted as linkers, connecting the metal centres into different entangled frameworks: 3D inclined polycatenane (1), 4-fold interpenetrated 3D MOF with the pcu topology (2), non-interpenetrated square net 2D MOF with sql topology (3), and self-penetrated 3D MOF (4). The solvent-dependent formations of Cd-MOFs 3 and 4 are discussed in detail as 3 isolated from DMA/water is a simple square net product, while 4 obtained from DMF/water shows a self-penetrating 3D framework in which three square circuits are interlocked with each other to form a cyclic [3]catenane unit via threading of the S-shaped bpmp. These cyclic [3]catenanes were further linked by the V-shaped bpmp, giving rise to a 3D infinite aggregate of cyclic [3]catenanes via the self-penetration, which seems to be unique. Consequently, all the products shared a square net (or square circuit) layer, which was further extended and interlocked in different ways depending on the conformations and roles of the flexible ligands together with the ligand–ligand or ligand–solvent interactions. The photoluminescence properties of the products were compared with those of free ligands and discussed.