Entangled metal–organic frameworks modulated by N-donor ligands of different conformations

Based on the aromatic dicarboxylic acid and N-donor ligands with different conformations, four Zn(II) metal–organic frameworks, namely [Zn(mfda)(L¹)] (1), [Zn₂(mfda)₂(L²)]·DMF·H₂O (2), [Zn₂(mfda)₂(L³)(H₂O)]·DMF (3) and [Zn₂(mfda)₂(L⁴)] (4) have been synthesized (mfda = 9,9- dimethylfluorene-2,7-dicarboxylate anion, L¹ = 1,10-phenanthroline, L² = 4,4′-bipyridine, L³ = 2,5-bis(4-pyridyl)-1,3,4-ocadiazole and L⁴ = 1,4-bis(imidazol-1-ylmethyl)benzene). Single-crystal X-ray diffraction has revealed that all compounds exhibit entangled structures. Compound 1 is composed of 1D zigzag chains that are entangled through the π–π stacking interactions to generate a three-fold interpenetrating diamond-like networks. 2 exhibits a two-fold interpenetrating (α-Po) net, which leaves 1D channels with high free volume. In 3, parallel mutual polythreadings of 2D layers are connected by hydrogen bonds into a self-penetrating framework with (4⁴·6¹⁰·7)(4·5·6)(4⁶·5²·6¹⁶·7³·9) topology. For 4, the interpenetrating 2D layers are connected into a self-interpenetrating net with (4⁹·6⁶) topology. The potential of N-donor ligands to produce interesting metal–organic frameworks is investigated. Luminescent studies show that 1–4 exhibit strong blue fluorescent emissions.