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 (H(2)bpdc), and a flexible dipyridyl piperazine ligand, 1,4-bis(4-pyridylmethyl)piperazine (bpmp), with versatile conformations, four divalent metal-organic frameworks (MOFs), namely {[Cu-2(bpmp)(2)(bpdc)(2)(H-2-O)(2)]center dot 1.25DMF center dot 1.5H(2)O}(n) (1), {[Zn-2(bpmp)(bpdc)(2)]center dot 2H(2)O}(n) (2), {[Cd(bpmp)(bpdc)(H2O)(2)]center dot 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.