Selective Formation of Small and Large Coordination Cages and Their Catalytic Differences.

Self-assembly of CuX2 (X- = BF4-, ClO4-, and CF3SO3-) with a new tridentate 5,5',5″-(((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris(oxy))triisoquinoline (L) gives rise to single-crystal pairs consisting of small and large cages, [X@Cu2X2L4]X and [Cu6X12L8], respectively, via selection of solvents. In particular, the large cage is transformed into a small cage in acetonitrile above 50 °C. A significant difference in heterogeneous catechol oxidation catalysis between the small and large cages is observed. Such notable catechol-oxidation-catalytic effects can be explained by maintenance of the Cu···Cu distance at the catalytic center. This research is a direct systematic example of both cage-size control via solvent selection and the significance of the Cu···Cu distance in catechol oxidation catalysis with copper (Cu).

Crystals of Ni6L12 Ellipsoidal Tubes as Single-Crystal-to-Single-Crystal Adsorption Matrix: Penetrative Study of Self-Assembled Crystals vs Guest-Exchanged Crystals.

Informative similarities/differences between self-assembled and single-crystal-to-single-crystal (SCSC) guest-exchanged crystals based on both the molecular structure and adsorption nature are observed. The self-assembly of Ni(ClO4)2 with a dicyclopentyldi(pyridine-3-yl)silane bidentate ligand (L) in a mixture of toluene and acetonitrile gives rise to purple crystals consisting of double-stranded ellipsoidal tubes, [Ni6(ClO4)4(CH3CN)8L12]·8ClO4·4CH3CN·5C7H8. The coordinated acetonitriles as well as the solvates are removed at 170 °C to transform the purple crystals into blue crystals of [Ni(ClO4)2L2]n that return to the original crystals in the mixture of toluene and acetonitrile. Further, the toluene and acetonitrile solvates of the original crystals are replaced by o-, m-, and p-xylene isomers within 5 min in a SCSC manner. In the present study, SCSC xylene-exchanged crystals were compared with crystals obtained from direct self-assembly in a mixture of each xylene isomer and acetonitrile.

Pair of (Hg2II)3L2 Chiral Cages and Successive Transformation into (HgII)3L2 Chiral Cages: Chiral DOPA Recognition via Chiral Cages.

Self-assembly of Hg(ClO4)2 with a pair of C3-symmetric chiral ligands, (1S,1'S,1″S,2R,2'R,2″R)-(benzenetricarbonyltris(azanediyl))tris(2,3-dihydro-1H-indene-2,1-diyl)trinicotinate (s,r-L) and (1R,1'R,1″R,2S,2'S,2″S)-(benzenetricarbonyltris(azanediyl))tris(2,3-dihydro-1H-indene-2,1-diyl)trinicotinate (r,s-L), produces a pair of chiral cages C4H8O2@[(Hg2II)3(ClO4)6(s,r-L)2(H2O)7](C4H8O2)7 and C4H8O2@[(Hg2II)3(ClO4)6(r,s-L)2(H2O)7](C4H8O2)7, respectively, via straightforward formation of the reduced Hg2II species with an inner cavity in which a single dioxane molecule is nestled. The pair of chiral cages are transformed into their downsized pair of cages, [Hg3II(ClO4)6(s,r-L)2] and [Hg3II(ClO4)6(r,s-L)2], respectively, in the presence of hydrochloric acid. The original chiral cages are more effective than the corresponding downsized cages for enantiorecognition of chiral 3,4-dihydroxyphenylalanine (DOPA) via the shifts of electrochemical oxidation potentials observed by linear sweep voltammetry (LSV) technique. Furthermore, the photoluminescence (PL) spectral shifts show that the downsized chiral cages significantly recognize chiral DOPA.

Chiral Pd6L8 Nanocube Pairs: Recognition of Chiral Amino Acids via Electrochemistry.

The self-assembly of PdX2 (X- = ClO4- and PF6-) with C3-symmetric l- and d-L [L = (2S,2'S,2″S)- and (2R,2'R,2″R)-[benzenetricarbonyltris(azanediyl)]tris(3-phenylpropane-2,1-diyl)triisonicotinate] produces the chiral nanocube pair [Pd6(l-L)8](X)12 and [Pd6(d-L)8](X)12 (X- = ClO4- and PF6-, respectively) with an inner cavity of 12.3 × 12.3 × 12.3 Å3. These chiral nanocubes are effective for the enantiorecognition of various chiral amino acids via the square-wave-voltammetry technique. In the present study, the site of enantiorecognition was confirmed by density functional theory calculated interactions between each nanocube and the chiral amino acids, and the calculated interactions were coincident with the shifts of the electrochemical oxidation potentials.

Recent Advances in Various Metal-Organic Channels for Photochemistry beyond Confined Spaces.

Tailor-made molecular channel architectures are a hot issue in the fields of nanotechnology, molecular sieves, ion sensors, recognition, confined space reactors, and fluidic transport systems. Carbon nanotubes have been a particular focus, though they cannot easily be synthesized to predefined structures and sizes. Rational design and construction of molecular channel structures based on coordination chemistry has been recognized as a useful approach. Metal-organic channel (MOC) structures can be generated by utilizing, at least in principle, molecular self-assembly of metal ions as angular units with designed ligands as spacers. Recent developments in molecular channel chemistry include exciting advances in photochemical applications and supramolecular material functionality, in addition to general applications such as transport, diffusion and exchange, separation, gas storage, catalysis, and simple encapsulation. In order to carry out the diverse channel functionalities, a large number of studies have been conducted on the synthesis of robust and stable 3D coordination polymers, which show permanent porosity without any guest molecules within the channels, in that the robustness of the channel structures after removal of the solvate/guest molecules is of interest because the structural integrity of the extended structures must be maintained during the reactions. These compounds can be regarded as analogous to zeolites. This Account highlights advances in the construction, from metal cations and multidentate pyridyl ligands, of various MOCs and useful molecular materials as photoreaction platforms. We begin by discussing the fact that detailed proof-of-concept construction of various systematic MOC structures has been introduced mainly in terms of the metal ions as angular components and the pyridyl ligands as spacers. This approach leads to structural complexity of assembled MOC motifs such as metallamacrocycles, helical and cylindrical coordination polymers, vertical arrays of 1D coordination polymers, interweaving and eclipsed stacking of 2D coordination polymers, and typical 3D coordination polymers. Notwithstanding the diversity of their skeletal structures, confined spaces of the channels are suitable for the study of photochemical performances including radical trapping, photocyclopropanation, dye inclusion and energy transfer, and guest-to-host structural transmission. The key aspect of their utility is not only the preparation of photoresistant MOCs but also channel tuning for inclusion of photoactive guests. Guest molecules, which are compatible with the size, shape, and polarity of the channels, can be incorporated into the crystals, instead of simple organic solvents, thus giving rise to host-guest complexes in the solid state. Such MOC studies could facilitate the development of chemical sensors, new photocatalytic systems, and useful molecular photochemical reactors.

Construction of Hetero-Four-Layered Tripalladium(II) Cyclophanes by Transannular π⋠⋠⋠π Interactions.

A synthetic strategy for the generation of new molecular species utilizing a provision of nature is presented. Nano-dimensional (23(2)×21(1)×16(1) ų) hetero-four-layered trimetallacyclophanes were constructed by proof-of-concept experiments that utilize a suitable combination of π⋅⋅⋅π interactions between the central aromatic rings, tailor-made short/long spacer tridentate donors, and the combined helicity. The behavior of the unprecedented four-layered metallacyclophane system offers a landmark in the development of new molecular systems.

Organization and Energy Transfer of Fused Aromatic Hydrocarbon Guests within Anion-Confining Nanochannel MOFs.

The self-assembly of Zn(II) ions with 1,3,5-tris(isonicotinoyloxyethyl)cyanurate produces new topological (4(2)⋅12(4))3(4(3))4 2D metal-organic frameworks (MOFs) with anion-confining cages. The eclipsed assembly of each 2D MOF by π-π stacking of cyanurate moieties (3.352(5) Å) forms 3D MOFs consisting of nanochannels (10.5 Å). Two of the three anions are confined in each peanut-type cage, resulting in hydrophobicity of the nanochannels. The hydrophobic nanochannel effectively adsorbs a wide range of fused aromatic hydrocarbons (FAHs) as monomers or dimers, rendering it potentially highly useful as an energy-transfer material.

Coordinating nature of M6L12 double-stranded macrocycles: co-ligand competition of perchlorate, water, and acetonitrile depending on metal(II) ions.

Self-assembly of M(ClO4)2 (M(II) = Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)) with dicyclopentyldi(pyridine-3-yl)silane (L) as a donor in a mixture of acetonitrile and toluene produces crystals consisting of M6L12 double-stranded macrocycles. The geometry around the M(II) cations is a typical octahedral arrangement, but the metallamacrocycles' outer axial coordination environment is sensitive to the M(II) cations. The conformation of the unique metallamacrocycles is informatively dependent on the nature of the coordination around the M(II) cations via subtle co-ligand competition among perchlorate anions, water, and acetonitrile. Both the coordinated acetonitriles and the solvate molecules of the crystals are removed at 170 °C, thereby transforming the crystals into new crystals that return to their original form in the mixture of toluene and acetonitrile. Catalytic oxidation of 3,5-di-tert-butylcatechol using [Cu6(ClO4)8(CH3CN)4L12]4ClO4·5C7H8 is much faster than those using the transformed product, [Cu(ClO4)2L2], and a simple mixture of Cu(ClO4)2 + L.

Self-assembly of Ni(II) with a chiral ligand pair vs. mixture of the chiral ligand pair: structural features and recognition ability of Ni2L4 cages.

The self-assembly of NiCl2 with a chiral bidentate ligand pair, (1R,2S)-(+)- and (1S,2R)-(-)-1-(nicotinamido)-2,3-dihydro-1H-inden-2-yl nicotinate (r,s-L and s,r-L) in a mixture of ethanol and dioxane, gives rise to stable crystals consisting of [2Cl@Ni2Cl2(s,r-L)4(H2O)2]·4C4H8O2·EtOH and [2Cl@Ni2Cl2(r,s-L)4(H2O)2]·4C4H8O2·EtOH chiral cages, respectively, with two encapsulated chloride anions in the cavities. The most interesting feature is that the self-assembly of NiCl2 with the mixture of r,s-L and s,r-L (1 : 1-1 : 4) produces crystals of thermodynamically stable achiral cages, [2Cl·2H2O@Ni2Cl2(s,r-L)2(r,s-L)2(H2O)2]·7C4H8O2, in the molar ratio range. Furthermore, the [2Cl@Ni2Cl2(s,r-L)4(H2O)2]·4C4H8O2·EtOH and [2Cl@Ni2Cl2(r,s-L)4(H2O)2]·4C4H8O2·EtOH chiral crystals can recognize the pairs of L-,D-tryptophan and L-,D-cysteine via cyclic voltammetry (CV) signals, in contrast to the [2Cl·2H2O@Ni2Cl2(s,r-L)2(r,s-L)2(H2O)2]·7C4H8O2 achiral crystal.

M(II) effect on encapsulation of guests into a series of M3L2 chiral cages: enantio-recognition.

Self-assembly of M(ClO4)2 (M2+ = Ni2+, Cu2+, and Zn2+) with (1S,1'S,1''S,2R,2'R,2''R)-(benzenetricarbonyltris(azanediyl))tris(2,3-dihydro-1H-indene-2,1-diyl) trinicotinate (s,r-L) and the corresponding enantiomer (r,s-L) as a pair of chiral tridentate donors gives rise to the chiral cage pairs [M3(s,r- and r,s-L)2](ClO4)6. For the two pairs of [(Me2CO)(H2O)@M3(r,-s and s,r-L)2](ClO4)6 (M2+ = Ni2+ and Zn2+), the inner cavity is occupied by both an acetone and a single water molecule, whereas for the copper(II) pair of [Me2CO@Cu3(r,s- and s,r-L)2](ClO4)6 under the same conditions, the cavity is filled by only one acetone molecule. Thus, the encapsulation of guest molecules into the cages during self-assembly shows significant metal(II) ion effects. These chiral cages are effective for the enantio-recognition of chiral (S)-2-butanol and (R)-2-butanol via the shifts of the electrochemical oxidation potentials obtained by the linear sweep voltammetry (LSV) technique, density functional theory (DFT) calculations, and the chiral 2-butanol adsorption in the single-crystal-to-single-crystal (SCSC) mode.

Formation Process of SiF6@Cu2L4 Chiral Cage Pairs in a Glass Vessel: Catechol Oxidation Catalysis and Chiral Recognition.

Self-assembly of CuX2 (X- = BF4-, PF6-, and SbF6-) with a pair of chiral bidentate ligands, (1R,2S)-(+)- and (1S,2R)-(-)-1-(nicotinamido)-2,3-dihydro-1H-inden-2-yl-nicotinate (r,s-L or s,r-L), in a mixture solvent including ethanol in a glass vessel gives rise to SiF62--encapsulated Cu2L4 chiral cage products. The SiF62- anion from the reaction of X- with SiO2 of the glass-vessel surface acts as a cage template or cage bridge. One of the products, [SiF6@Cu2(SiF6)(s,r-L)4]·3CHCl3·4EtOH, is one of the most effective heterogeneous catalysts for the oxidation of 3,5-di-tert-butylcatechol. Furthermore, an l-DOPA/d-DOPA pair is recognizable by the cyclic voltammetry (CV) signals of its combination with chiral cages [SiF6@Cu2(BF4)2(s,r- or r,s-L)4]·4CHCl3·2EtOH pair and [SiF6@Cu2(SiF6)(s,r- or r,s-L)4]·3CHCl3·4EtOH pair.

Structural properties of [Cu(II)3L6] cages: bridged polyatomic anion effects on unprecedented efficiency of heterogeneous catechol oxidation.

Self-assembly of CuX2 (X- = BF4-, ClO4-, PF6-, and SbF6-) with a bidentate ethylmethylbis(3-pyridine)silane ligand (L) in the presence of additional polyatomic anions (X' = SiF62- and PF6-) gives rise to single crystals consisting of the X'@[Cu(II)3L6] cage motif. These cages exist as discrete or anion-bridged 3D networks depending on outside anions. The anion-bridged 3D networks interpenetrate in a four-fold fashion, and show, to our best knowledge, the most effective heterogeneous catalysis in 3,5-di-tert-butylcatechol oxidation reaction within 20 min at room temperature. Surprisingly, the heterogeneous catalysis is more effective than its corresponding homogeneous catalysis. Such notable catalytic effects can be explained by the maintenance of 3D inter-cage Cu⋯Cu distance as a catalytic center.

Insight into systematic formation of hexafluorosilicate during crystallization via self-assembly in a glass vessel.

Formation of the unexpected hexafluorosilicate (SiF6 2-) anion during crystallization via self-assembly in glassware is scrutinized. Self-assembly of M(BF4)2 (M2+ = Cu2+ and Zn2+) with tridentate N-donors (L) in a mixture solvent including methanol in a glass vessel gives rise to an SiF6 2--encapsulated Cu3L4 double-decker cage and a Zn2L4 cage, respectively. Induced reaction of CuX2 (X- = PF6 - and SbF6 -) instead of Cu(BF4)2, with the tridentate ligands, produces the same species. The formation time of SiF6 2- is in the order of anions BF4 - < PF6 - < SbF6 - under the given reaction conditions. The SiF6 2- anion, acting as a cage template or cage-to-cage bridge, seems to be formed from the reaction of polyatomic anions containing fluoride with the SiO2 of the surface of the glass vessel.

Dual Molecular Design toward a Lysosome-Tagged AIEgen and Heavy-Atom-Free Photosensitizers for Hypoxic Cancer Photodynamic Therapy.

To date, a large number of photosensitizers (PS) have introduced heavy atoms to improve the ISC process and 1O2 generation. However, they often show low efficiency in hypoxic conditions, aggregate states, and turn-off PDT in the dark. Besides that, the toxicity of heavy metals is also concerned. Therefore, we developed lysosome-targeted heavy-metal-free PS (3S and 4S) based on thionated naphthalimide for hypoxic cancer photodynamic therapy (PDT), not only under white light but also in the dark via thermal-induced 1O2 generation. AIEgen (3O and 4O) were prepared for studying the PDT action of PSs (3S and 4S) in lysosome and aggregate state. We also examined the photophysical properties of AIEgen (3O and 4O) and PS (3S and 4S) by UV-vis absorption, fluorescent emission spectra, and theoretical calculations.

Pair of chiral 2D silver(I) enantiomers: chiral recognition of L- and D-histidine via differential pulse voltammetry.

Self-assembly of AgPF6 with a pair of chiral tridentate ligands (1S,1'S,1''S,2R,2'R,2''R) and (1R,1'R,1''R,2S,2'S,2''S)-(benzenetricarbonyltris(azanediyl))tris(2,3-dihydro-1H-indene-2,1-diyl)triisonicotinate (s,r-L) and (r,s-L) in a mixture of methanol and dioxane yields 2D sheets consisting of [Ag(s,r-L)](PF6)·3C4H8O2·0.5H2O and [Ag(r,s-L)](PF6)·3C4H8O2·0.5H2O, respectively. The differential pulse voltammetric (DPV) technique using the pair of chiral 2D-sheet enantiomers was employed for chiral discrimination of amino acid enantiomers, and was found to be an effective tool for enantio-recognition of L- and D-histidines. Both the size and the binding site of amino acids were strongly dependent on electrochemical enantio-recognition via the chiral 2D sheets.

Motion of an isolated water molecule within a flexible coordination cage: structural properties and catalytic effects of ionic palladium(II) complexes.

The unique cage complexes [[(Me(4)en)Pd](3)(L)(2)](X)(6) (L = 1,3,5-tris(isonicotinoyloxyethyl)cyanurate; X(-) = BF(4)(-), ClO(4)(-)) were constructed. A single water molecule in a skeletal cage was reversibly associated and dissociated via a combination of the adequate space, polar environment, and conformational flexibility of the cage. In Suzuki-Miyaura C-C cross-coupling reactions, the cage complex showed significant catalytic activity along with the effects of the isolated single water molecule.

Interconversion between a Pd3L2 trigonal prism and a Pd6L8 cube via anion exchange: binding affinity of monoatomic vs. polyatomic anions.

Systematic interconversion between trigonal prisms [Pd3X6L2] (X- = Cl-, Br-, and I-) and cubic cages [Pd6L8]12+(X-)12 (X- = BF4- and CF3SO3-) via anion exchange was established. Self-assembly of K2PdX4 (X- = Cl- and Br-) with a C3-symmetric tridentate 1,3,5-tris(2-isonicotinamidephenoxy)benzene (L) produces a trigonal prism, [Pd3X6L2]. Further photoreaction of the [Pd3X6L2] (X- = Cl- and Br-) with CH2I2 gives rise to a halide-exchanged species, [Pd3I6L2]. In contrast, anion exchange of [Pd3X6L2] (X- = Cl-, Br-, and I-) with BF4- yields cubic-shaped cages, [Pd6L8]12+(BF4-)12, with an inner cavity of 15.9 × 15.9 × 15.9 Å3. Successive anion exchange of [Pd6L8]12+(BF4-)12 with CF3SO3- gives rises to anion-exchanged [Pd6L8]12+(CF3SO3-)12 and vice versa without cage destruction. Thus, the cage system is specifically sensitive to anions, enabling cage formation to recognize the binding affinity and size of various anions.

A cyclic manipulation of cage isomers via anion exchange and thermal isomerism.

A cyclic manipulation of peanut cage isomers has been achieved via anion exchange and unusual cage isomerism.

Trimetallic coordination cage formation for nitrate encapsulation: transformation of kinetic products into thermodynamic products.

A procedure for the formation of a nitrate-encapsulating tripalladium(II) cage via self-assembly of Pd(NO3)2 with 1,3-bis(dimethyl(pyridin-4-yl)silyl)propane (L) was developed. The self-assembly reaction initially produces spiro-type macrocycles, PdL2, and finally results in transformation into a nitrate-encapsulated cage, [(NO3)@Pd3L6], in the mother liquor. The reaction of PdX2 (X- = BF4-, ClO4-, PF6-, and CF3SO3- instead of NO3-) with L gives rise to a spiro species, PdL2, as the final product, and anion exchange of the spiro products, [PdL2](X)2, with NO3- produces the tripalladium cage [(NO3)@Pd3L6].

Straightforward formation of dianionic acetonylate: self-assembly of mercury(II) with pyridyl donor ligands in acetone.

Self-assembly of Hg(ClO4)2 with a new bidentate ligand (L) in acetone at room temperature produces single crystals consisting of unusual discrete tetranuclear complexes [Hg4(ClO4)4(CH2COCH2)2L2]·CH3COCH3via straightforward formation of dianionic acetonylate -CH2COCH2- in a quantitative yield.