Exploiting the use of hydrogen bonding and metal-coordination in the self-assembly of photoreactive multicomponent networks

Multivalent hydrogen-bonded architectures directed by self-complementarity between [Cu(2,2'-biimidazole)] and malonate building blocks

The synthesis and structural characterization of four novel supramolecular hydrogen-bonded arrangements based on self-assembly from molecular `[Cu(2,2'-biimidazole)]' modules and malonate anions are presented, namely, tetrakis(2,2'-biimidazole)di-μ-chlorido-dimalonatotricopper(II) pentahydrate, [Cu3(C3H2O4)2Cl2(C6H6N4)4]·5H2O or [Cu(H2biim)2(μ-Cl)Cu0.5(mal)]2·5H2O, aqua(2,2'-biimidazole)malonatocopper(II) dihydrate, [Cu(C3H2O4)(C6H6N4)(H2O)]·2H2O or [Cu(H2biim)(mal)(H2O)]·2H2O, bis[aquabis(2,2'-biimidazole)copper(II)] dimalonatodiperchloratocopper(II) 2.2-hydrate, [Cu(C6H6N4)2(H2O)]2[Cu(C3H2O4)(ClO4)2]·2.2H2O or [Cu(H2biim)2(H2O)]2[Cu(mal)2(ClO4)2]·2.2H2O, and bis(2,2'-biimidazole)copper(II) bis[bis(2,2'-biimidazole)(2-carboxyacetato)malonatocopper(II)] tridecahydrate, [Cu(C6H6N4)2][Cu(C3H2O4)(C3H3O4)(C6H6N4)2]·13H2O or [Cu(H2biim)2][Cu(H2biim)2(Hmal)(mal)]2·13H2O. These assemblies are characterized by self-complementary donor-acceptor molecular interactions, demonstrating a recurrent and distinctive pattern of hydrogen-bonding preferences among the carboxylate, carboxylic acid and N-H groups of the coordinated 2,2'-biimidazole and malonate ligands. Additionally, coordination of the carboxylate group with the metallic centre helps sustain remarkable supramolecular assemblies, such as layers, helices, double helix columns or 3D channeled architectures, including mixed-metal complexes, into a single structure.

Halogen-Bond Mediated [2+2] Photodimerizations: À la Carte Access to Unsymmetrical Cyclobutanes in the Solid State

The ditopic halogen-bond (X-bond) donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene (1,2-, 1,3-, and 1,4-di-I-tFb, respectively) form binary cocrystals with the unsymmetrical ditopic X-bond acceptor trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (2,4-bpe). The components of each cocrystal (1,2-di-I-tFb)·(2,4-bpe), (1,3-di-I-tFb)·(2,4-bpe), and (1,4-di-I-tFb)·(2,4-bpe) assemble via N···I X-bonds. For (1,2-di-I-tFb)·(2,4-bpe) and (1,3-di-I-tFb)·(2,4-bpe), the X-bond donor supports the C=C bonds of 2,4-bpe to undergo a topochemical [2+2] photodimerization in the solid state: UV-irradiation of each solid resulted in stereospecific, regiospecific, and quantitative photodimerization of 2,4-bpe to the corresponding head-to-tail (ht) or head-to-head (hh) cyclobutane photoproduct, respectively.

Regioselective Photochemical Cycloaddition Reactions of Diolefinic Ligands in Coordination Polymers

The pure diolefinic ligand 1,4-bis(pyridin-4-yl)-1,3-butadiene (bpbde) is photostable in the crystalline state. With the assistance of coordination-driven metal-organic assemblies, the photoreactivity of this diolefinic ligand can be significantly enhanced. A hydrothermal reaction of bpbde with Cd(NO₃ )₂ ⋅4 H₂ O and the auxiliary ligand adipic acid resulted in the formation of a two-dimensional photoreactive coordination polymer (CP), [Cd(adipate)(bpbde)]_n (1). When the aliphatic carboxylic acid was replaced by pimelic acid, another photoreactive CP [Cd(pimelate)(bpbde)]_n (2) with a three-dimensional framework was obtained. With irradiation of 365 nm UV light, the bpbde ligands in crystalline 1 and 2 underwent a regioselective photochemical [2+2] cycloaddition reaction and converted to 3,4,7,8-tetra(pyridin-4-yl)tricyclo[4.2.0.0^2,5 ]octane (tptco) and 1,3-bis(pyridin-4-yl)-2,4-bis(2-(pyridin-4-yl)vinyl)cyclobutane (bpbpvcb), respectively. The results provide an interesting insight into the rational design of highly regio- or stereoselective photocatalytic reactions for the formation of special organic molecules.

1,2,4,5-Benzenetetracarboxylic acid: a versatile hydrogen bonding template for controlling the regioselective topochemical synthesis of head-to-tail photodimers from stilbazole derivatives

The crystal engineering of hydrogen bonded organic assemblies based on 1,2,4,5-benzenetetracarboxylic acid (H4bta) and stilbazole derivatives (1-10) is exploited to provide regio-controlled [2 + 2] photocycloadditions in the solid state. Single crystal X-ray diffraction analyses have revealed that all the arrays are built-up from the self-assembly of the (H2bta)2- dianion with two stilbazolium cations via O-HO- and N+-HO- charge-assisted H-bonding synthons: (4-Hstilbazolium+)2(H2bta2-). The dianion displays an interesting diversity of H-bonding motifs. Such structural flexibility allowed us to obtain four structure-types defined by the preferential formation of intramolecular or intermolecular hydrogen bonds between carboxylate-carboxylic groups. In these ionic assemblies two predominant structural H-bonding patterns were observed. The first pattern is characterised by the formation of intramolecular H-bonds in the dianion, leading to discrete assemblies based on ternary arrays. The second hydrogen pattern consists of 2-D hydrogen networks built-up from the self-assembly of anions via intermolecular H-bonds that are linked to the cations. Two additional examples, in which the dianion is self-assembled in two types of ribbons, were also observed. Another supramolecular feature predominant in all these arrays is the stacking of the cations in a head-to-tail fashion, which is controlled via cation-π interactions. These arrays are photoactive in the solid state upon UV-irradiation leading to the regioselective synthesis of rctt-cyclobutane head-to-tail-isomers in high to quantitative yield. In this work, the template tolerance either to steric or electronic effects by changing the number or positions of the supramolecular interactions exerted by distinctive functional groups was also explored. In addition, assemblies bearing 2-chloro (7 and 8) and 3-chloro-4-stilbazole (1 and 9) crystallize in two different crystalline forms, leading to novel examples of supramolecular isomers with similar solid state reactivity.

Photochemical reactions of metal complexes in the solid state

This perspective focusses on the solid-state reactivity and structural transformation driven by photochemical methods in discrete metal complexes, organometallic compounds, metallo-macrocycles and cages. Changes in the metal-metal bond distances, racemization of chiral centres, fusion of cages, formation of coordination polymers, expected [2 + 2] and [4 + 4] cycloaddition products, unusual phenyl-olefin dimerization, and linkage isomerization of -SO₂, -NO & -NO₂ ligands cause the structural transformations. Of these, [2 + 2] photo-cycloaddition reactions have been widely studied and the photoreactions are made possible by various supramolecular interactions including hydrogen bonds, metallophilic, ππ and C-Hπ interactions, ligand design and metallic clips to bring the reactive functional groups closely into correct orientation close to the transition state. These photoreactions are often accompanied by crystal bending, mechanical motion, and changes in the magnetic and photoluminescence properties. In several cases, the single crystals have been preserved at the end of the reactions, which are known as single-crystal-to-single-crystal (SCSC) reactions.

Template-stereocontrolled [2 + 2] photoreactions directed by surface recognition on hydrophilic functionalized carbon materials

Supramolecular assistance either in solution or in the solid state to the regioselective synthesis of single photodimers from [2 + 2] photoreactions surface-directed by multivalent H-bonding exo-templates based on hydrophilic carbon nanomaterials is shown.

Solid state [2 + 2] photocycloaddition for constructing dimers of N,N′-diacyl-1,4-dihydropyrazines based on thiourea-induced assembly

Unprecedented photoadducts were obtained through solid state [2 + 2] photocycloaddition of 1,4-dihydropyrazines, assembled using thiourea as a template.

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.

Single-crystal-to-single-crystal conversions of two metal-mediated photoreactive coordination polymers based on stereoselective [2 + 2] photocycloaddition reactions

Two stereoselective [2 + 2] photocycloaddition reactions occur in two metal-mediated coordination polymers via single-crystal-to-single-crystal transformations.

UV-induced single-crystal-to-single-crystal conversion from a coordination ladder to a two-dimensional network through an intermolecular carbon–carbon coupling reaction

UV-induced single-crystal-to-single-crystal transformation from a coordination ladder to a two-dimensional network accompanied by photocontrolled fluorescence.

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.

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.

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.