Getting suitable crystals for single-crystal X-ray crystallographic analysis still remains an art. Obtaining single crystals of metal-organic frameworks (MOFs) containing organic polymers poses even greater challenges. Here we demonstrate the formation of a syndiotactic organic polymer ligand inside a MOF by quantitative [2+2] photopolymerization reaction in a single-crystal-to-single-crystal manner. The spacer ligands with trans,trans,trans-conformation in the pillared-layer MOF with guest water molecules in the channels, undergo pedal motion to trans,cis,trans-conformation prior to [2+2] photo-cycloaddition reaction and yield single crystals of MOF containing two-dimensional coordination polymers fused with the organic polymer ligands. We also show that the organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. These MOFs also show interesting photoluminescent properties and sensing of small organic molecules.
The bpeb ligands aligned in a slip-stacked manner in a two-fold interpenetrated non-porous metal-organic framework (MOF) [Zn2(bpeb)(bdc)(fa)2] undergo [2+2] cycloaddition reaction in a single-crystal to single-crystal manner to a non-interpenetrated 3D structure with a new topology comprising an organic polymer ligand and a 2D coordination polymer.
The degree of interpenetration is known to influence the gas sorption, catalytic, magnetic and nonlinear optical properties, chirality, and sensing of various molecules but not the solid-state [2 + 2] photocycloaddition reaction. In our previous studies of a solvothermal reaction using dimethylacetamide (DMA) as one of the solvents, a photoreactive 6-fold interpenetrated metal−organic framework with dia topology, [Zn(bpeb)(bdc)] (1) [bpeb = 1,4-bis[2-(4′-pyridyl)ethenyl]benzene; bdc = 1,4-benzenecarboxylate], was isolated. Because of the slip-stacked alignment of a dipyridyl ligand with two conjugated olefin bonds, the [2 + 2] cycloaddition reaction occurs under UV light leading to the formation of an organic polymer ligand fused with a coordination polymer, 2. On the contrary, under similar conditions when diethylformamide was used instead of DMA, a 5-fold interpenetrated structure, 3, with the same dia topology was obtained in this work. This has been found to be photostable as also predicted from the analysis of the solid state structure. All the solids show interesting photoluminescence properties, and the emissions were preferentially quenched by curcumin to make these materials as potentially useful sensing agents.
Synthesis of an NO2S3-macrocycle (L) incorporating a pyridine subunit and its anion and/or mole ratio-dependent coordination modes in the formations of mercury(II) complexes is reported. When the mercury(II) salts with different anions (ClO4(-) or Br(-)) were reacted with L, the Hg(ClO4)2 afforded a typical endocyclic complex [HgL](ClO4)2 (1). Meanwhile, the HgBr2 gave an exocyclic complex [HgLBr2] (2) in which the metal ion exists outside the macrocyclic cavity. The observed anion effect on the coordination modes can be explained by the anion coordination ability toward the metal cation. In the mole ratio variation experiments, notably, the use of 1.5 equiv or above of HgBr2 in the same reaction condition gave a unique endo/exocyclic dumbbell-type complex 3, [Hg4L2Br6][Hg2Br6]. However, the formation of the endocyclic Hg(ClO4)2 complex 1 shows no mole ratio dependency. To monitor the observed mole ratio-dependent exocoordination products as well as their reactivities and reversibility, systematic powder X-ray diffraction (PXRD) analysis was also applied. From single crystal X-ray and PXRD analyses, it was found that endocyclic complex 1 is not reactive, but complexes 2 and 3 are reactive and show the reversibility between them in the presence of the corresponding reactants.
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