Stepwise Host–Guest [2 + 2] Photoreaction in a Hydrogen-Bonded One-Dimensional Coordination Polymer to a Two-Dimensional Layered Structure

Two hydrogen-bonded interdigitated one-dimensional (1D) coordination polymers (CPs) were synthesized from Cd­(II) nitrate and 1,4-benzenedicarboxylate (bdc) with two 4-styrylpyridine (4spy) derivatives. In the solid state structure of 1D CP containing 2′-fluorostyrylpyridine (2F-4spy), a guest 2F-4spy was sandwiched between the terminal 2F-4spy ligands and the CC bonds of all the 2F-4spy are well-aligned in <i>host–guest-host–host-guest–host</i>- sequence infinitely. From time-dependent photoreaction studies, it has been found that the [2 + 2] photocycloaddition occurs between <i>host–guest</i> molecules first followed by <i>host–host</i> pairs in a two-step process, which resulted in the quantitative conversion of CC bond pairs to cyclobutane rings. It is a rare quantitative photoreaction between the <i>host</i> and <i>guest</i> molecules. Further, solvothermal crystallization of the final photoproduct furnished an interesting three-dimensional CP with <b>bcg</b> topology. Interestingly, 2NO₂-4spy also furnished a similar hydrogen-bonded 1D CP with parallel orientation of olefin bonds. Although the olefin bonds satisfy the Schmidt’s criteria for photoreactivity, it was found to be photoinert under UV light. This has been attributed to the steric hindrance caused by NO₂ substituent or the electron withdrawing effect

Influence of Fluorine Substitution on the Unusual Solid-State [2 + 2] Photo-Cycloaddition Reaction between an Olefin and an Aromatic Ring

Solid-state [2 + 2] photo-cycloaddition reactions observed so far were exclusively between a pair of olefin bonds. Usually when the phenyl–olefin bonds have been closely aligned, they were found to be either photoinert or sliding of molecules takes place for [2 + 2] cycloaddition reaction between olefins in the solid state, although intramolecular phenyl–olefin reactions are well-known in solution. In the crystal structure of [Zn₂(ptol)₄­(4spy)₂] (ptol = <i>para</i>-toluate), the neighboring 4-styrylpyridine (4spy) ligands are organized in a head-to-tail manner. On one side of the complex in the crystal structure, the olefin bonds in the 4spy pairs are perfectly aligned to undergo cycloaddition reaction, but on the other side, the olefin bond pairs are slightly offset and found to be photoinert at 223 K forming only a dimer in single crystals. The sliding of 4spy groups has been restricted by the steric hindrance of the adjacent methyl group of the ptol ligands. A similar packing of 2-fluoro-4′-styrylpyridine (2F-4spy) pairs was found in [Zn₂(ptol)₄­(2F-4spy)₂]. Again, normal cycloaddition reaction occurs on one side of the 2F-4spy ligand pairs, whereas the second offset 2F-4spy ligand pairs undergo a rare [2 + 2] cycloaddition reaction between the fluorophenyl group and olefin bond resulting in the formation of a one-dimensional coordination polymers containing a bicyclic product in a quantitative yield. The bicyclic ring in the photoproduct can be thermally cleaved back to olefin and phenyl groups. These observations have been confirmed by single-crystal X-ray crystallography, ¹H NMR, and ¹⁹F NMR studies. Density functional theory calculations were performed to elucidate the nature of the interactions between the fluorophenyl and olefin groups. The greater reduction of aromaticity of 2F-4spy in the excited singlet state compared to the 4spy system may explain the observed reactivity difference between the two systems. The improved reactivity in 2F-4spy may also be attributed to the fact that the olefin–phenyl distance is shorter in 2F-4spy than in 4spy (3.63 versus 3.69 Å). This solid state phenyl–olefin photodimerization helps to pave the way for making new bicyclic derivatives

Photosalient Behavior of Photoreactive Crystals

Being documented with only about a dozen serendipitous observations, the photosalient effect, where crystals leap when exposed to light, is considered a very rare phenomenon. Here, with a set of structurally related materials that undergo [2 + 2] photocycloaddition we present evidence that this effect is more common than it has been realized in the past, and we seek to establish correlations with the kinematics and the crystal structure toward rational design of photosalient materials. To that end, nine photoreactive complexes AgL₂X₂ (L = 4-styrylpyridine, 2′-fluoro-4-styrylpyridine, and 3′-fluoro-4-styrylpyridine, X = BF₄^–, ClO₄^– and NO₃^–) were prepared. The [AgL₂]⁺ cations in these structures pack by both head-to-head and head-to-tail alignment of the styrylpyridine ligands. Crystals of six out of the nine complexes were photosalient and popped, hopped, and/or leaped when exposed to UV light. It is concluded that the occurrence of the photosalient effect is determined not only by the nature of the ligand but also by the crystal packing which directs the magnitude, direction, and rate of volume expansion during the photoreaction

Fabrication of Gold/Titania Photocatalyst for CO₂ Reduction Based on Pyrolytic Conversion of the Metal–Organic Framework NH₂‑MIL-125(Ti) Loaded with Gold Nanoparticles

Titania exhibits unique photophysical and -chemical properties and can be used for potential applications in the field of photocatalysis. The control of TiO₂ in terms of phase, shape, morphology, and especially nanoscale synthesis of TiO₂ particles still remains a challenge. Ti-containing metal–organic frameworks (MOFs), such as MIL-125, can be used as sacrificial precursors to obtain TiO₂ materials with diverse phase compositions, morphologies, sizes, and surface areas. MIL-125 is composed of Ti/O clusters as the secondary building units (SBUs) bridged by 1,4-benzenedicarboxylate (bdc). In this study, preformed and surfactant-stabilized gold nanoparticles (GNPs) were deposited onto the surface of amino functionalized NH₂-MIL-125 during solvothermal synthesis. Targeted gold/titania nanocomposites, GNP/TiO₂, were fabricated through the pyrolysis of GNP/NH₂-MIL-125 nanocrystals. The modification of TiO₂ with GNPs significantly increased the photocatalytic activity of the MOF derived TiO₂ material for the reduction of CO₂ to CH₄ as compared to TiO₂ reference samples such as P-25 and AUROlite (Au/TiO₂). The new materials GNP/TiO₂ and TiO₂ derived by the MOF precursor route were thoroughly characterized by PXRD, FTIR and Raman, TEM, and N₂ adsorption studies

Solvent-Induced Structural Dynamics in Noninterpenetrating Porous Coordination Polymeric Networks

Three novel soft porous coordination polymer (PCP) or metal–organic framework (MOF) compounds have been synthesized with a new rigid ligand <i>N</i>-(4-pyridyl)-1,4,5,8-naphathalenetetracarboxymonoimide (PNMI) by partial hydrolysis of <i>N,N′</i>-di-(4-pyridyl)-1,4,5,8-naphthalenete-tracarboxydiimide (DPNI) during solvothermal reactions with Zn­(II), Cd­(II), and Mn­(II) salts, and they are [Zn­(PNMI)]·2DMA (<b>1</b>·2DMA, <b>1a</b>), [Cd­(PNMI)]·0.5DMA·5H₂O (<b>2</b>·0.5DMA·5H₂O), and [Mn­(PNMI)]·0.75DMF (<b>3</b>·0.75DMF). The structure of <b>1</b> is based on paddle-wheel secondary building unit (SBU) with a 3,6-connected <b>rtl</b> net topology, whereas <b>2</b> and <b>3</b> are isotypical but the M­(O₂C–C)₂ fragments aggregate in one-dimension and the overall connectivity is the same <b>rtl</b> net topology. All these three MOFs have one-dimensional rhombic channels filled with guest molecules. The guest molecules in <b>1a</b> can be exchanged with EtOH in a single-crystal to single-crystal (SCSC) manner to <b>1</b>·1.25EtOH·0.375H₂O (<b>1b</b>). Further, the guest molecules in <b>1b</b> can be replaced with ethylene glycol, triethylene glycol and allyl alcohol without destroying its single crystal nature. These guest exchanges are accompanied by reduction in volume of the unit cell up to 16%, as well as the void volume up to 33.1%. Similarly, triethylene glycol (TEGly) selectively exchanges EtOH in a mixture of the above solvents, which might be the result of correct fit of the hydrogen-bonded TEGly dimer in the channel of <b>1</b>. While activated <b>1</b> and <b>3</b> exhibit no uptake of N₂ and H₂ at 1 bar and 77 K and very low uptake of CO₂ gas at 1 bar and 196 K, activated <b>2</b> shows selective CO₂ uptake, 278 cm²·g^–1, over N₂ and H₂ at 1 bar and 196 K, which corresponds to 5.87 molecules of CO₂ per formula unit of <b>2</b>

Influence of Angular Dicarboxylate Ligand on the Structures of Single and Double Pillared-Layer Coordination Polymers of Co(II)

Two pillared-layer coordination polymeric compounds, namely, [Co₂(bpeb)₂(obc)₂]·DMF·5H₂O (<b>1</b>) and [Co₂(bpeb)­(obc)₂]·2DMF·H₂O (<b>2</b>), have been synthesized using a long dipyridyl spacer ligand (1,4-bis­[2-(4-pyridyl)­ethenyl]­benzene, bpeb), 4,4′-oxybisbenzoate (obc), and Co­(NO₃)₂·6H₂O under solvothermal conditions, using appropriate ratios of Co­(II) to bpeb. In compound <b>1</b>, the double pillared-layer structure has a dimeric repeating unit and exhibits <b>rob</b> topology, while single pillared-layer compound <b>2</b> has an unusual building block which is a linkage isomer of the well-known paddle-wheel structure and has a <b>pcu</b> topology. Twofold and threefold interpenetrations are observed in <b>1</b> and <b>2</b>, respectively. The variable-temperature magnetic properties of <b>1</b> and <b>2</b> were also investigated. The double pillared-layer structure of <b>1</b> exhibits antiferromagnetic behavior while a relatively rare ferromagnetism has been observed for the single pillared-layer structure of <b>2</b>

Influence of Angular Dicarboxylate Ligand on the Structures of Single and Double Pillared-Layer Coordination Polymers of Co(II)

Two pillared-layer coordination polymeric compounds, namely, [Co₂(bpeb)₂(obc)₂]·DMF·5H₂O (<b>1</b>) and [Co₂(bpeb)­(obc)₂]·2DMF·H₂O (<b>2</b>), have been synthesized using a long dipyridyl spacer ligand (1,4-bis­[2-(4-pyridyl)­ethenyl]­benzene, bpeb), 4,4′-oxybisbenzoate (obc), and Co­(NO₃)₂·6H₂O under solvothermal conditions, using appropriate ratios of Co­(II) to bpeb. In compound <b>1</b>, the double pillared-layer structure has a dimeric repeating unit and exhibits <b>rob</b> topology, while single pillared-layer compound <b>2</b> has an unusual building block which is a linkage isomer of the well-known paddle-wheel structure and has a <b>pcu</b> topology. Twofold and threefold interpenetrations are observed in <b>1</b> and <b>2</b>, respectively. The variable-temperature magnetic properties of <b>1</b> and <b>2</b> were also investigated. The double pillared-layer structure of <b>1</b> exhibits antiferromagnetic behavior while a relatively rare ferromagnetism has been observed for the single pillared-layer structure of <b>2</b>

Regulating photosalient behavior in dynamic metal-organic crystals

Dynamic photoactuating crystals have become a sensation due to their potential applications in developing smart medical devices, molecular machines, artificial muscles, flexible electronics actuators, probes and microrobots. Here we report the synthesis of two iso-structural metal-organic crystals, [Zn(4-ohbz)2(4-nvp)2] (1) and [Cd(4-ohbz)2(4-nvp)2] (2) {H4-ohbz = 4-hydroxy benzoic acid; 4-nvp = 4-(1-naphthylvinyl)pyridine} which undergo topochemical [2 + 2] cycloaddition under UV irradiation as well as sunlight to generate a dimerized product of discrete metal-complex [Zn(4-ohbz)2(rctt-4-pncb)] {rctt-4-pncb = 1,3-bis(4′-pyridyl)-2,4-bis(naphthyl)cyclobutane} (1′) and one-dimensional coordination polymer (1D CP) [Cd(4-ohbz)2(rctt-4-pncb)] (2′) respectively, in a single-crystal-to-single-crystal (SCSC) process. The Zn-based compound demonstrates photosalient behaviour, wherein crystals show jumping, splitting, rolling, and swelling upon UV irradiation. However, the Cd-based crystals do not show such behaviour maintaining the initial supramolecular packing and space group. Thus the photomechanical behaviour can be induced by choosing a suitable metal ion. The above findings are thoroughly validated by quantitative density functional theory (DFT) calculations which show that the Zn-based crystal shifts towards an orthorhombic structure to resolve the anisotropic UV-induced mechanical strain. Furthermore, the mechano-structure-property relationship has been established by complimentary nanoindentation measurements, which are in-line with the DFT-predicted single crystal values.</p

Influence of Angular Dicarboxylate Ligand on the Structures of Single and Double Pillared-Layer Coordination Polymers of Co(II)

Two pillared-layer coordination polymeric compounds, namely, [Co₂(bpeb)₂(obc)₂]·DMF·5H₂O (<b>1</b>) and [Co₂(bpeb)­(obc)₂]·2DMF·H₂O (<b>2</b>), have been synthesized using a long dipyridyl spacer ligand (1,4-bis­[2-(4-pyridyl)­ethenyl]­benzene, bpeb), 4,4′-oxybisbenzoate (obc), and Co­(NO₃)₂·6H₂O under solvothermal conditions, using appropriate ratios of Co­(II) to bpeb. In compound <b>1</b>, the double pillared-layer structure has a dimeric repeating unit and exhibits <b>rob</b> topology, while single pillared-layer compound <b>2</b> has an unusual building block which is a linkage isomer of the well-known paddle-wheel structure and has a <b>pcu</b> topology. Twofold and threefold interpenetrations are observed in <b>1</b> and <b>2</b>, respectively. The variable-temperature magnetic properties of <b>1</b> and <b>2</b> were also investigated. The double pillared-layer structure of <b>1</b> exhibits antiferromagnetic behavior while a relatively rare ferromagnetism has been observed for the single pillared-layer structure of <b>2</b>