Metal-organic frameworks containing a tetrapyridylcyclobutane ligand derived from isomerization reaction

Photo- and Stress-Induced Bending of (E)-1,2-Bis(pyridinium-4-yl)ethene Dinitrate Crystals

We report on the elastic and photodynamic properties of (E)-1,2-bis(pyridinium-4-yl)ethene dinitrate [H2Ebpe](NO3)2, whose needle-like crystals can be reversibly deformed by applying external mechanical stress. The macro-scale mechanical properties of [H2Ebpe](NO3)2 crystals were quantified by a three-point bending test, which gave a stress-strain curve with an elastic modulus of 1.18 GPa, and its values are lower than those of other flexible elastic organic crystals. It can also be reversibly bent through the [2+2] cycloaddition reaction of the olefin moiety, depending on the direction of UV irradiation.

Sustainable Zn2+ removal using highly efficient, novel, and cost-effective chitosan-magnetic biochar composite

This study focused on the development of a sustainable and low-cost adsorbent derived from the chitosan-biochar composite for the removal of Zn2+ from an aqueous solution. Biochar was prepared from cotton stalk residue by pyrolysis at 600 °C for 2 h, modified with FeCl3, and composed with chitosan in various ratios (1:3, 1:1, 3:1), leading to the formation of an efficient, thermally stable, and rich with functional groups chitosan-biochar composite denoted as CHB-Fe-CS. Functional groups (hydroxyl, carboxyl, and amine) were identified as key contributors to the adsorption mechanism. Langmuir isotherm (R2 = 0.99) and Pseudo-Second order (R2 = 0.99) were best fitted models with the experimental results indicating chemisorption-driven monolayer adsorption. The results revealed CHB-Fe-CS (3:1) composite obtained the highest adsorption capacity of 117.50 mg/g for Zn2+ under optimal conditions viz., 180 min batch time, 500 mg/l metal concentration, 4 g/l adsorbent dosage, 40 °C solution temperature, and 5.0 pH. Regeneration of the used adsorbent was performed using 0.2 mol/l HCl and obtained desorption efficiency of 67.48% and 51.48% after the 4th and 8th cycles. The adsorption mechanisms were dominated by ion exchange, surface complexation, and electrostatic attraction compared to intra-particle diffusion and physisorption. The CHB-Fe-CS demonstrated an economical, environment friendly, and good performing adsorbent for water decontamination.

Isolation of elusive cyclobutane ligands via template assisted photochemical [2+2] cycloaddition reaction and their utility in engineering crystalline solids

Solid-state photochemical [2+2] cycloaddition reactionis one of the promising routes to isolate highly strained cyclobutane derivatives whichmay otherwise be very difficult to obtain via conventional synthetic methodology. Various methods have...

Square network based upon the molecular salt of the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane and the sulfate anion

The formation and crystal structure of a hydrated molecular salt that results in a square network is reported. The crystalline solid is based upon the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (4H-rtct-TPCB)⁴⁺ along with two sulfate anions (SO₄^2-) and eight waters of hydration, namely, 4,4',4'',4'''-(cyclobutane-1,2,3,4-tetrayl)tetrapyridinium bis(sulfate) octahydrate, C₂₄H₂₄N₄⁴⁺·2SO₄^2-·8H₂O. The fully protonated photoproduct acts as a four-connecting node within the square network by engaging in four charge-assisted N⁺-H...O hydrogen bonds to the sulfate anion. The observed hydrogen-bonding pattern in this square network is akin to T-silica, which is a metastable form of SiO₂. The included water molecules and sulfate anions engage in numerous O-H...O hydrogen bonds to form various hydrogen-bonded ring structures.

Construction of 2D Interdigitated Polyrotaxane Layers and their Transformation to a 3D Polyrotaxane by a Photocycloaddition Reaction between Wheels

Following the pioneering work of Sauvage and Stoddart on rotaxanes, construction of higher dimensional polyrotaxanes in metal-organic frameworks (MOFs) via a modified protocol is challenging. We present the formation of a two-dimensional (2D) polyrotaxane and its conversion to a three-dimensional (3D) polyrotaxane MOF via a photoreaction between interdigitated "olefin wheels". For this purpose, a 2-fold entangled 2D MOF [Pb₂(bpp)(sdc)₂] (1), showing a 2D + 2D → 2D polyrotaxane motif, has been synthesized from the solvothermal reaction of lead(II) nitrate, 3,3'-stilbenedicarboxylic acid (H₂sdc) containing an olefin group, and 1,4-bis(4-pyridyl)piperazine (bpp). The single-crystal X-ray diffraction analysis of 1 revealed that the adjacent entangled 2D layers are interdigitated, with the separation of 3.72 Å between C═C bond pairs in adjacent layers satisfying Schmidt's criteria for the occurrence of a [2 + 2] photocycloaddition reaction. Irradiation of the single crystals of 1 under UV light resulted in formation of a 3D polyrotaxane, [Pb₂(bpp)(rctt-tccb)]_n (2), due to a [2 + 2] photocycloaddition reaction between two wheels via a single-crystal to single-crystal transformation. The photocycloaddition and partial thermal cleavage reaction between 1 and 2 were confirmed by ¹H NMR and powder X-ray diffraction (PXRD) in solution and the solid state, respectively. The present approach could contribute to the understanding of the construction of higher dimensional polyrotaxanes which are not accessible by the traditional routes.

Design of Green-Emitting Salts from Substituted Pyridines: Understanding the Solid-State Photodimerization of trans-1,2-bis(4-pyridyl)ethylene

Polymorphic salts of trans-1,2-bis(4-pyridyl)ethylene (bpe), 2[bpeH₂ ] ⋅ (SO₄ )(2HSO₄ ) (1) and [bpeH₂ ] ⋅ 2HSO₄ (2) have been synthesized and their structures determined by X-ray crystallography. The Schmidt postulate predicts that neither of the salts will give rise to photodimerization so they can both potentially be applied as green light emitters. Despite the predictions, 1 undergoes a stereospecific solid-state photodimerization reaction with 100 % yield. This is due to UV induced combination of sliding and pedal-like movement of the pyridyl ring system that influences the alignment of C=C bonds. The sliding motion is restricted in 2. Consequently, the green emission from 1 is completely quenched after photodimerization. It is evident that counter ions play a dominant role in dis- and enabling photodimerization; their degree of protonization and lattice placement are important solvent controlled design parameters towards crystal structures that can act as future light emitters.

New node on the block for organic solid-state chemists: rtct-tetrakis(pyridin-4-yl)cyclobutane

We now know the symmetry and unit-cell parameters of the functionalized cyclo­butane rtct -TPCB , as well as a multicomponent solid containing it. What other networks and topologies are possible?

Molecular docking, spectroscopic, and quantum chemical studies on aromatic heterocycle tetrakis(4-pyridyl)cyclobutane regioisomers: potential membrane-permeable inhibitors

Aromatic heterocyclic compounds are commonly used in pharmaceutical and agrochemical products. The usage has been also extended in the production of corrosion inhibitors and electronic, opto-electronic, and NLO devices. The four regioisomers of the aromatic heterocyclic molecule, tetrakis(4-pyridyl)cyclobutane (tpcb), were optimized, and their chemical, electronic, and biological properties were examined. The rctt and rtct isomers were isolated and further characterized by the infrared and Raman spectroscopic techniques, and their vibrational modes with attention on important functional groups were discussed. The theoretical vibrational data obtained by DFT/B3LYP/6-311++G(d,p) were in very good agreement with the experimental ones. The occupancy, bond polarization, hybridization of the N-C bond, bonding characteristics between the pyridyl carbon and cyclobutane moiety, and distribution of electron density for the four tpcb isomers have been explored through NBO analysis. The inversion barrier of the cyclobutane moiety was obtained based on the optimized structures of the planar and puckered configurations of the isomers. The rccc isomer exhibited the least electrophilic character compared to the other tpcb isomers. The chemical significance of tpcb has been further explained by the wave function analyses. Molecular docking results revealed that the tpcb ligand would possess a potential anti-viral and anti-bacterial activity via a membrane-permeable mechanism.

Honeycomb molecular network based upon a hydrate of 4,6-dichlororesorcinol and the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane

The formation of a self-interpenetrated honeycomb molecular network based upon 4,6-dichlororesorcinol (4,6-diCl res), a water molecule, and the photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (rtct-TPCB) is reported. Interestingly, only three of the four pyridine rings on the central cyclobutane ring are found to engage in O-H...N hydrogen bonds with either the 4,6-diCl res or an included water molecule, resulting in a three-connected net. Notably, the solid (4,6-diCl res)·(rtct-TPCB)·(H₂O), C₆H₄Cl₂O₂·C₂₄H₂₀N₄·H₂O, contains channels that run along the crystallographic b axis, which are found to be interpenetrated. Although rtct-TPCB has been employed as a bridging ligand in the formation of numerous metal-organic materials, surprisingly neither the single-component X-ray structure nor any multi-component molecular solids based upon this stereoisomer have been reported previously. Lastly, the single-crystal X-ray structure of the photoproduct rtct-TPCB is also reported.

Thermal Elimination of Ethylene from Cyclobutyl Groups Characterized by X-ray Crystallography in a Metal-Organic Framework Matrix

Methods to synthesize and characterize aromatic molecules with vinyl substituents are sought after yet limited in the literature. Here, we introduce cyclobutyl groups into a metal-organic framework (MOF) matrix that are poised to produce ethylene upon heating. The expulsion of ethylene produces vinyl groups on an aromatic core, which are isolated by the crystalline matrix of the framework. This enables full characterization of the thermolysis by single-crystal X-ray diffraction. Further, we modify the vinyl groups by a bromine addition reaction. Importantly, the two transformations happen in a single-crystal-to-single-crystal manner without changing the overall network structure of the parent framework. New insights into the structural and synthetic chemistry of this important class of compound are generated. Installing reactive vinyl tags in materials by the high temperature thermolysis of cyclobutyl groups is a powerful strategy for altering their physicochemical characteristics.

Reversible photoswitching of the DNA-binding properties of styrylquinolizinium derivatives through photochromic [2 + 2] cycloaddition and cycloreversion

It was demonstrated that styrylquinolizinium derivatives may be applied as photoswitchable DNA ligands. At lower ligand:DNA ratios (≤1.5), these compounds bind to duplex DNA by intercalation, with binding constants ranging from K b = 4.1 × 104 M to 2.6 × 105 M (four examples), as shown by photometric and fluorimetric titrations as well as by CD and LD spectroscopic analyses. Upon irradiation at 450 nm, the methoxy-substituted styrylquinolizinium derivatives form the corresponding syn head-to-tail cyclobutanes in a selective [2 + 2] photocycloaddition, as revealed by X-ray diffraction analysis of the reaction products. These photodimers bind to DNA only weakly by outside-edge association, but they release the intercalating monomers upon irradiation at 315 nm in the presence of DNA. As a result, it is possible to switch between these two ligands and likewise between two different binding modes by irradiation with different excitation wavelengths.

A diamondoid net sustained by halogen bonds: employing a cyclobutane to generate a tetrahedral architecture

A purely organic dia net sustained by halogen bonds that contains nodes comprised of rctt-tetrakis (4-pyridyl) cyclobutane, obtained from a solid-state photoreaction, is reported.

Isomerism in double-pillared-layer coordination polymers - structures and photoreactivity

The existence of isomerism in coordination polymeric structures offers opportunities to understand structure-function relationships. Herein the serendipitous isolation is reported of two isomeric double-pillared-layer coordination polymeric structures arising from two different types of carboxyl-ate bonding of benzene-1,4-di-carboxyl-ate ligands to zinc(II), which constitutes a new type of structural isomerism. The different bonding modes not only alter the shape and size of the pores, but also the nature of interpenetration and photoreactivity. Although two trans,cis,trans-bpeb ligands with conjugated olefin bonds are aligned in close proximity in both of the structures, one isomer undergoes a double [2 + 2] cyclo-addition reaction and the second isomer only offers an incomplete single cyclo-addition product. This work demonstrates how small changes in the structural connectivity can have an impact on the overall structural, physical and chemical properties of such materials.

Liquid/vapor-induced reversible dynamic structural transformation of a three-dimensional Cu-based MOF to a one-dimensional MOF showing gate adsorption

A new 3D metal–organic framework (MOF) is reversibly transformed to a 1D chain MOF showing selective adsorption properties.

Single-crystal-to-single-crystal transformation of a coordination polymer from 2D to 3D by [2 + 2] photodimerization assisted by a coexisting flexible ligand

A 2D interdigitated [Ni₂(adipate)₂(spy)₄(H₂O)₂] was transformed to a 3D coordination polymer through [2 + 2] photodimerization with conformational changes of adipate ligands.

Synthon trends according to acid strength and geometry in salts of N-heterocyclic bases

The hierarchy and robustness of homosynthons and heterosynthons formed by N-heterocyclic bases were assessed experimentally in salts of aminopyrazine (ampyz) andtrans-1,2-bis(4-pyridyl)ethane (BPE) with common strong, moderate and weak acids, and theoretically at the M06-2X/6-31+G** level of theory.

Monitoring photo-induced transformations in crystals of 2,6-difluorocinnamic acid under ambient conditions

Several conditions need to be fulfilled for a photochemical reaction to proceed in crystals. Some of these conditions, for example, geometrical conditions, depend on the particular type of photochemical reaction, but the rest are common for all reactions. The mutual directionality of two neighbouring molecules determines the kind of product obtained. The influence of temperature on the probability of a photochemical reaction occurring varies for different types of photochemical reaction and different compounds. High pressure imposed on crystals also has a big influence on the free space and the reaction cavity. The wavelength of the applied UV light is another factor which can initiate a reaction and sometimes determine the structure of a product. It is possible, to a certain degree, to control the packing of molecules in stacks by using fluoro substituents on benzene rings. The crystal and molecular structure of 2,6-difluorocinnamic acid [systematic name: 3-(2,6-difluorophenyl)prop-2-enoic acid], C9H6F2O2, (I), was determined and analysed in terms of a photochemical [2 + 2] dimerization. The molecules are arranged in stacks along the a axis and the values of the intermolecular geometrical parameters indicate that they may undergo this photochemical reaction. The reaction was carried out in situ and the changes of the unit-cell parameters during crystal irradiation by a UV beam were monitored. The values of the unit-cell parameters change in a different manner, viz. cell length a after an initial increase starts to decrease, b after a decrease starts to increase, c increases and the unit-cell volume V after a certain increase starts to decrease. The structure of a partially reacted crystal, i.e. containing both the reactant and the product, namely 2,6-difluorocinnamic acid-3,4-bis(2,6-difluorophenyl)cyclobutane-1,2-dicarboxylic acid (0.858/0.071), 0.858C9H6F2O2·0.071C18H12F4O4, obtained in situ, is also presented. The powder of compound (I) was irradiated with UV light and afterwards crystallized [as 3,4-bis(2,6-difluorophenyl)cyclobutane-1,2-dicarboxylic acid toluene hemisolvate, C18H12F4O4·0.5C7H8] in a space group different from that of the crystal containing the in-situ dimer.

The synthesis of flexible tetrapyridylethanes from pyridylpyrylium dications

Novel tetrapyridylethanes and bis-phenylpyridylmethylenepyrans as potential ligands for coordination chemistry.

Structurally Flexible C₃-Symmetric Receptors for Molecular Recognition and Their Self-Assembly Properties

The bioinspired design and synthesis of building blocks and their assemblies by the supramolecular approach has ever fascinated scientists to utilize such artificial systems for numerous purposes. Flexibility is a basic feature of natural systems. However, in artificial systems this is difficult to control, especially if there is no preorganization of the component(s) of a system. We have designed and synthesized a series of C3 -symmetric N-bridged flexible receptors and successfully utilized them to selectively entrap the notorious and toxic nitrate anion in aqueous medium. This was the first report of highest binding affinity for the nitrate anion in aqueous medium. An impressive self-sorting phenomenon of reversibly formed hydrogen-bonded capsules, which self-assembled from flexible tripodal receptors having branches of similar size and bearing the same amide functionality, has been disclosed. Encapsulated nitrate anion has been further utilized for the photochemical [2+2] cycloaddition reaction for the synthesis of strained four-membered ring structures through dynamic self-assembly. In this Personal Account, we summarize these results showing the utility of naturally inspired flexibility in artificial systems.

Study of temperature and ligand flexibility effects on coordination polymer formation from cyclobutanetetracarboxylic acid

Three novel coordination polymers (CPs) from rctt-H4Cbtc and 4,4’-bpe with Zn and Ni metal nodes have been synthesized. The CP derivates of Zn are an unusual example for the preparation of metastable phases involving temperature dependent and concomitant transformation.

Oxygen-initiated stereoselective thermal isomerisation of a cyclobutane derivative in the solid state

Solid-state [2+2] photochemical cycloaddition reactions have been extensively studied after the classical work of Schmidt in the 1960s. Of these, trans-1,2-bis(4'-pyridyl)ethylene (bpe) is one of the well-studied alkenes to synthesize tetrakis(4-pyridyl)cyclobutane (tpcb). However, almost all the solid-state [2+2] cycloaddition reactions of bpe yielded, almost exclusively, one of the four possible isomers, namely, the rctt-tpcb (r=regio c=cis and t=trans). Here we describe a stereoselective synthesis of the tetrahedrally disposed rtct-tpcb by the solid-state thermal isomerization of the rctt-isomer in atmospheric air. We propose that this isomerization occurs through a topochemical unimolecular mechanism by a radical chain pathway, initiated by molecular oxygen. This is supported by the nature of products formed in air and nitrogen, detection of a radical in ESR spectral studies, ESI-MS crossover experiments, VT PXRD studies along with QM, MD and docking calculations. The formation of a unique isomer by thermal isomerization may be a general phenomenon to quantitatively synthesize other useful stereoisomers from the existing isomers of cyclobutane derivatives.

Role of anions in the synthesis of cyclobutane derivatives via [2 + 2] cycloaddition reaction in the solid state and their isomerization in solution

trans-3-(4'-Pyridyl)acrylic acid (4-PA) is inert to photodimerization reaction both in solution and solid state. It is made photoreactive by forming salts with various acids. The anions of these salts play a key role in directing the packing of 4-PAH(+) in the solid state. The anions CF(3)CO(2)(-), Cl(-), ClO(4)(-), and BF(4)(-) direct the parallel alignments of 4-PAH(+) in head-to-tail (HT) fashion and lead to the formation of HT-photodimer. On the other hand, bivalent anion SO(4)(2-) directs parallel alignment of 4-PAH(+) in head-to-head (HH) fashion and lead to the formation of HH-photodimer. The details of the anion-controlled stereoselective syntheses of these two cyclobutane derivatives are presented. Interestingly, both cyclobutane compounds undergo isomerization from rctt-form to rctc-form in solution catalyzed by acid.

Crystal engineering rescues a solution organic synthesis in a cocrystallization that confirms the configuration of a molecular ladder

Treatment of an achiral molecular ladder of C(2h) symmetry composed of five edge-sharing cyclobutane rings, or a [5]-ladderane, with acid results in cis- to trans-isomerization of end pyridyl groups. Solution NMR spectroscopy and quantum chemical calculations support the isomerization to generate two diastereomers. The NMR data, however, could not lead to unambiguous configurational assignments of the two isomers. Single-crystal X-ray diffraction was employed to determine each configuration. One isomer readily crystallized as a pure form and X-ray diffraction revealed the molecule as being achiral based on C(i) symmetry. The second isomer resisted crystallization under a variety of conditions. Consequently, a strategy based on a cocrystallization was developed to generate single crystals of the second isomer. Cocrystallization of the isomer with a carboxylic acid readily afforded single crystals that confirmed a chiral ladderane based on C(2) symmetry. The chiral ladderane and acid self-assembled to generate a five-component hydrogen-bonded complex that packs to form large solvent-filled homochiral channels of nanometer-scale dimensions. Whereas cocrystallizations are frequently applied to structure determinations of proteins, our study represents the first application of a cocrystallization to confirm the relative configuration of a small-molecule diastereomer generated in a solution-phase organic synthesis.