Thermal Cleavage of Cyclobutane Rings in Photodimerized Coordination-Polymeric Sheets

Self-Catalysis Transformation of Metal-Organic Coordination Polymers

Designing a multidimensional transformation of metal-organic coordination polymers (MOPs) is highly attractive yet very challenging. Herein, by combining the dynamicity of the coordination bond with the controllability of the chemical reaction, the concept of self-catalysis transformation of MOPs is first proposed. It uses the metal in MOPs as the catalyst to catalyze the chemical reaction of the ligand in the frameworks, simultaneously changing the coordination environment of the metal and the structure of the ligand, resulting in the controllable multidimensional transformation in the morphology and structure of MOPs. The self-catalysis transformation of MOPs can be triggered by heat or light, and crystals with various morphologies and structures can be obtained. Significantly, because the self-catalysis reaction is constraint in the framework, the products at different transformation processes are relatively stable. Monitoring and characterizing the transformation of MOPs give evidences for the exploration of the self-catalysis reaction, and a plausible transformation mechanism is proposed and proved. It can be foreseen that this novel self-catalysis transformation strategy might open up a new direction for the diverse development of MOPs and provide a powerful tool for the study of organic reaction mechanism.

Coordination Polymer-Mediated Molecular Surgery for Precise Interconversion of Dicyclobutane Compounds

A Cd(II)-based coordination polymer {[Cd₂(5-F-1,3-bpeb)₂(FBA)₄]·H₂O}_n (CP₁) was obtained from Cd(II) salt, 5-fluoro-1,3-bis[2-(4-pyridyl)ethenyl]benzene (5-F-1,3-bpeb), and p-fluorobenzoic acid (HFBA). Within the one-dimensional chain structure of CP₁, a pair of 5-F-1,3-bpeb was arranged in a face-to-face style. Upon UV irradiation and heat treatment, multiple cyclobutane isomers, including specific monocyclobutanes (1 with an endo-cyclobutane ring in CP₁-1 and 1' with an exo-cyclobutane ring in CP₁-1') and dicyclobutanes (endo,endo-dicyclobutane 2α in CP₁-2α, exo,endo-dicyclobutane 2β in CP₁-2β, and exo,exo-dicyclobutane 2γ in CP₁-2γ) were stereoselectively produced. These isomers could be interconverted inside the CP via cutting/coupling specific bonds, which may be regarded as a type of molecular surgery. The precision of cutting/coupling relied on the thermal stability of the cyclobutanes and the alignment of the reactive alkene centers. The conversion processes were tracked through nuclear magnetic resonance, in situ powder X-ray diffraction, and IR spectroscopy. This approach can be considered as skeletal editing to construct complex organic compounds directly from one precursor.

Photoreactive Crystals Exhibiting [2 + 2] Photocycloaddition Reaction and Dynamic Effects

ConspectusConducting a reaction in the solid state eliminates the usage of solvents. If such reactions are conducted in a single-crystal to single-crystal (SCSC) fashion, then structural characterization by single-crystal X-ray crystallography (SCXRD) techniques provides unequivocal structural details. Although topochemical principles govern, getting single crystals at the end of a SCSC reaction purely depends on the experimental skills of the researchers. SCSC reactions are common among solid-state [2 + 2] cycloaddition reactions (hereafter "photoreaction") after the classical work of Schmidt and co-workers in 1960s. Synthons and tectons in the crystal engineering box can be exploited to bring the functional groups into the required alignment and packing to achieve the desired chemical reactivities and physical properties, respectively. Bringing a pair of alkenes closer together in the organic molecules provides an effective starting point to achieve the goal of crystal engineering.Further, understanding and controlling photoreactivity in the solid state provide a gateway to designing new advanced materials, for example, making cycloreversible optical storage materials, photosalient and photomechanical materials, highly crystalline or even single-crystalline organic polymers, covalent organic framework structures, and organic polymers incorporated inside metal-organic frameworks (MOFs). Photoreactions often proceed in a SCSC manner due to the limited movements of the closely disposed reactive functional groups in the crystals. Thus, these photoreactions yield not only quantitative photoproducts but also regio- and stereospecificity, which are otherwise inaccessible by solution syntheses.The traditional definition of crystals being hard, rigid, and brittle is no longer valid ever since the mechanically responsive crystals were discovered. These dynamic crystals undergo various movements like curling, jumping, hopping, popping, splitting, and wiggling, when exposed to light (called "photosalient effect") or heat (called "thermosalient" effect). These crystals generate new methods of transforming light and heat energy into mechanical work. Recently, photosalient behavior during the [2 + 2] cycloaddition reaction under UV light has been frequently observed. With the emergence of the field of "crystal adaptronics", dynamic photoreactive crystals have emerged as smart actuating materials.This Account aims to provide an overview of the development in this area, since it has garnered much attention among solid state chemists. While presenting selected examples of important strategies, we try to illustrate the intentions and concepts behind the methods developed, which will help in a rational approach for the fabrication of advanced solid state materials. Apart from topochemical transformations, the important roles played by weak interactions, guest solvents, and mechanical grinding have been highlighted in several classes of compounds to show structural transformations that defy the expected outcomes. Overall, the progress of [2 + 2] cycloaddition reaction in solid state materials has been discussed from UV induced structural transformations to the development of smart actuating materials.

Controllable photomechanical bending of metal-organic rotaxane crystals facilitated by regioselective confined-space photodimerization

Molecular machines based on mechanically-interlocked molecules (MIMs) such as (pseudo) rotaxanes or catenates are known for their molecular-level dynamics, but promoting macro-mechanical response of these molecular machines or related materials is still challenging. Herein, by employing macrocyclic cucurbit[8]uril (CB[8])-based pseudorotaxane with a pair of styrene-derived photoactive guest molecules as linking structs of uranyl node, we describe a metal-organic rotaxane compound, U-CB[8]-MPyVB, that is capable of delivering controllable macroscopic mechanical responses. Under light irradiation, the ladder-shape structural unit of metal-organic rotaxane chain in U-CB[8]-MPyVB undergoes a regioselective solid-state [2 + 2] photodimerization, and facilitates a photo-triggered single-crystal-to-single-crystal (SCSC) transformation, which even induces macroscopic photomechanical bending of individual rod-like bulk crystals. The fabrication of rotaxane-based crystalline materials with both photoresponsive microscopic and macroscopic dynamic behaviors in solid state can be promising photoactuator devices, and will have implications in emerging fields such as optomechanical microdevices and smart microrobotics.

Coupling Postsynthetic High-Temperature Oxidative Thermolysis and Thermal Rearrangements in Isoreticular Zinc MOFs

Herein, we report coupling in situ high temperature postsynthetic modifications (PSMs) in metal-organic frameworks (MOFs). Thermo-reactive propargyloxy-functionalized zinc IRMOFs (isoreticular metal-organic frameworks) prepared from 2-(prop-2-yn-1-yloxy)-[1,1'-biphenyl]-4,4'-dicarboxylic acid (H₂bpdcOCH₂CCH) were investigated for their high-temperature postsynthetic rearrangement (PSR) chemistry to heterocyclic chromenes and benzofurans and then coupled to solid-gas reactions with molecular oxygen. The selectivity for the initial molecular rearrangements was found to be inverted in the porous MOF environment compared to conventional melt reactions of the ester compound Me₂bpdcOCH₂CCH and proceeded far more easily than the solid-state transformation from H₂bpdcOCH₂CCH, showing the potential of MOFs to give rise to different chemistry. The major oxidative process was thermolysis of the chromene ring with a minor pathway of allylic-type oxidation to give heterocyclic chromenone functionality. The sequence was also successful on a series of two-component multivariate IRMOF frameworks prepared from thermo-reactive H₂bpdcOCH₂CCH and thermo-resistant H₂bpdcOMe linkers, demonstrating that these reactions can be used with known crystal engineering strategies. All transformations were fully compatible with the requirements to maintain MOF crystallinity and porosity as evidenced by surface area analysis and X-ray powder diffraction measurements. This work contributes to establishing the feasibility of high-temperature solid-gas manifolds for MOF PSM.

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.

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.

Transformation between 2D and 3D Covalent Organic Frameworks via Reversible [2 + 2] Cycloaddition

We report the first transformation between crystalline vinylene-linked two-dimensional (2D) polymers and crystalline cyclobutane-linked three-dimensional (3D) polymers. Specifically, absorption-edge irradiation of the 2D poly(arylenevinylene) covalent organic frameworks (COFs) results in topological [2 + 2] cycloaddition cross-linking of the π-stacked layers in 3D COFs. The reaction is reversible, and heating to 200 °C leads to a cycloreversion while retaining the COF crystallinity. The resulting difference in connectivity is manifested in the change of mechanical and electronic properties, including exfoliation, blue-shifted UV-vis absorption, altered luminescence, modified band structure, and different acid-doping behavior. The Li-impregnated 2D and 3D COFs show a significant room-temperature ion conductivity of 1.8 × 10^-4 S/cm and 3.5 × 10^-5 S/cm, respectively. Even higher room-temperature proton conductivity of 1.7 × 10^-2 S/cm and 2.2 × 10^-3 S/cm was found for H₂SO₄-treated 2D and 3D COFs, respectively.

On the planarity of the cyclobutane ring in the crystal of dimethyl 2,4-bis(3,4-dimethoxyphenyl)cyclobutane-1,3-dicarboxylate: a natural bond orbital and Hirshfeld surface analysis study

The α-form of this 1,3-di-(substituted)-2,4-bis-(substituted)-cyclobutane derivative displays a planar ring in the gas phase and solution. The intermolecular interactions in the crystal have been determined.

A reversible photochemical solid-state transformation in an interpenetrated 3D metal-organic framework with mechanical softness

We synthesized a two-fold interpenetrated 3D MOF with two crystallographically distinct C[double bond, length as m-dash]C bonds, which undergoes [2+2] photo-cycloaddition and thermal reversible reaction, in a single-crystal-to-single-crystal (SCSC) manner. The softer nature and comparable mechanical properties of the crystals of the parent and cyclized MOFs revealed by nanoindentation allowed rationalizing their structural softness and SCSC transformation behaviour.

Spin-crossover modulation via single-crystal to single-crystal photochemical [2 + 2] reaction in Hofmann-type frameworks

This study reports the first modulation of spin-crossover (SCO) behavior via a photochemical [2 + 2] cycloaddition reaction. Here we construct two no-solvent Fe(ii)-Ag(i) bimetallic Hofmann-type frameworks, [Fe(4-spy)2{Ag(CN)2}2] (1) and [Fe(2,4-bpe)2{Ag(CN)2}2] (2) (4-spy = 4-styrylpyridine, 2,4-bpe = trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene). For 1, the dimerization of 4-spy results in a single-crystal to single-crystal (SCSC) transformation from 2D interdigitated layers to a 3D interpenetrated structure. Additionally, a 3D → 3D structural transformation accompanied with Ag(i)-N bond breaking is achieved via the photochemical cycloaddition reaction of 2,4-bpe in 2. More importantly, both the spin transition temperatures and the SCO character are effectively modulated; thus, this approach provides a new strategy for constructing photo-responsive SCO magnetic materials.

Scalable preparation and property investigation of a cis-cyclobutane-1,2-dicarboxylic acid from β-trans-cinnamic acid

Scalable synthesis of β-truxinic acid (CBDA-4) was accomplished by capturing and photodimerizing a metastable crystalline solid of trans-cinnamic acid. This synthetic approach builds a foundation for investigating the properties and applications of the useful diacid. The X-ray crystal structure of CBDA-4 was determined for the first time. The cyclobutane ring in CBDA-4 was cleaved upon heating, making it a promising building block for thermally recyclable/degradable materials.

Liquid-free single-crystal to single-crystal transformations in coordination polymers

Single-crystal to single-crystal (SCSC) transformations not only can create new materials, but also provide an opportunity to explore the process of forming a chemical bond. SCSC transformations discussed in this paper are confined to transformationsviaan absolutely liquid-free mode and involve the breakage and formation of new chemical bonds.

Interpenetrated Double Pillared-Layer CoII MOFs with pcu Topology

Three double pillared-layer CoII metal–organic frameworks (MOFs) with a pcu topology of a long, conformationally flexible, dipyridyl spacer ligand, 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb), and aromatic dicarboxylates (1,4-benzenedicarboxylate (bdc), 2,6-naphthalenedicarboxylate (ndc), and biphenyl-4,4′-dicarboxylate (bpdc)) have been synthesised and structurally characterised by X-ray crystallography. The MOFs are denoted as [Co2(bpeb)2(bdc)2]·DMF·3H2O (1), [Co2(bpeb)2(ndc)2]·1.75DMF·3.75H2O (2), and [Co2(bpeb)2(bpdc)2]·3.5DMF·4H2O (3). In the dinuclear repeating unit, four carboxylates are bonded to two CoII atoms forming a (4,4) layer structure. The axial positions are occupied by bpeb ligands. Of these, 1 and 2 have 2-fold interpenetration, whereas 3 displays 3-fold interpenetration. The two bpeb space ligands in 1 have trans,trans,trans and trans,cis,trans conformations. In contrast, the bpeb ligands in 2 and 3 have a trans,cis,trans conformation. Although the olefin groups in two adjacent bpeb ligands, as the double pillars in 2 and 3, satisfy the conditions for photo-dimerisation to occur, they are photo-inactive. The conformational changes of bpeb, bonding modes of the dicarboxylates, and pore sizes in these double pillared-layer compounds have been discussed.

In Situ Raman Monitoring of Silver(I)-Aided Laser-Driven Cleavage Reaction of Cyclobutane

The cyclobutane cleavage reaction is an important process and has received continuous interest. Herein, we demonstrate the visible laser-driven cleavage reaction of cyclobutane in crystal form by using in situ Raman spectroscopy. Silver(I) coordination-induced strain and thermal effects from the laser irradiation are the two main driving forces for the cleavage of cyclobutane crystals. This work may open up a new avenue for studying cyclobutane cleavage reactions, as compared to the conventional routes using ex situ techniques.

Controlled formation of chiral networks and their reversible chiroptical switching behaviour by UV/microwave irradiation

By using chiral diamines a chiral 3D coordination polymer is assembled to show reversible chiroptical switching behavior by UV/microwave irradiation.

Solid-state polymerisation via [2+2] cycloaddition reaction involving coordination polymers

Recent advancements in the construction of cyclobutane polymersviaphoto-dimerisation reaction in the monocrystalline solids of metal complexes, coordination polymers and metal–organic framework structures are reviewed.

Metal nanoparticle catalyzed cyclobutane cleavage reaction

The pyridine substituted cyclobutane cleavage reaction can be catalyzed directly by metallic silver/gold nanoparticles.

A novel example of double reactivity by either photochemical [2+2] or thermal additions of an ionic organic supramolecular assembly

An example of double reactivity in the solid state was achieved from an ionic array directed by charge-assisted hydrogen bonds.

A Zn(ii) coordination polymer and its photocycloaddition product: syntheses, structures, selective luminescence sensing of iron(iii) ions and selective absorption of dyes

Nanospheres of a Zn(ii) complex are used to selectively sense iron(iii) ions while its photocycloaddition product can selectively absorb dyes.

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.

Photoreactivity of polymorphs of a ladder polymer with criss-cross and parallel orientations of C=C bonds

Two polymorphic forms of a ladder coordination polymer [Zn2(μ-O2C-p-Tol)2(O2C-p-Tol)2(bpe)2] have parallel and criss-cross alignments of the C=C bond pairs. Both polymorphs undergo 100% [2+2] cycloaddition reaction but the one with a parallel orientation of C=C bonds (thermodynamic product) retains its single crystallinity after the reaction.

Diethyl 2,2′-(ethane-1,2-diyldioxy)dibenzoate

The molecular title compound, C20H22O6, was obtained by the reaction of ethyl 2-hydroxybenzoate with 1,2-dichloroethane. The molecule lies on a twofold rotation axis which passes through the middle of the central ethylene bridge. This group exhibits agaucheconformation with the corresponding O—C—C—O torsion angle being 73.2 (2)°. The C atoms of the carboxyl group, the aryl and the O—CH2group are coplanar, with an r.m.s. deviation of 0.01 Å. The two aryl rings form a dihedral angle of 67.94 (4)°. The ester ethyl group is disordered over two sets of sites with an occupancy ratio of 0.59 (2):0.41 (2). The crystal packing is dominated by van der Waals forces.

Post‐synthetic Modification of MOFs

Post‐synthetic modification is increasingly recognised as an important and versatile tool in the preparation of functionalised metal organic frameworks (MOFs). The process involves one or more reactions on a pre‐formed MOF, and it can be used to prepare MOFs that are not accessible by direct combination of metal and linker. This review explores the methods and strategies that have been developed for post‐synthetically modifying MOFs, concentrating on four classes of reaction: covalent transformations of the linker, coordination of a metal centre to a linker, modification of the inorganic part of the MOF and exchange of counter‐ions. Examples of the use of the modified MOFs are given, with a focus on their utility in catalysis.

A reagentless thermal post-synthetic rearrangement of an allyloxy-tagged metal-organic framework

Direct heating of a metal-organic framework provides a simple, controllable way of effecting a covalent post-synthetic modification. Herein we report that an allyloxy-tagged zinc metal-organic framework undergoes a thermally-promoted aromatic Claisen rearrangement through which the framework connectivity and porosity are maintained.