σ–π Continuum in Indole–Palladium(II) Complexes

The intrinsic features of (hetero‐arene)–metal interactions have been elusive mainly because the systematic structure analysis of non‐anchored hetero‐arene–metal complexes has been hampered by their labile nature. We report successful isolation and systematic structure analysis of a series of non‐anchored indole–palladium(II) complexes. It was revealed that there is a σ–π continuum for the indole–metal interaction, while it has been thought that the dominant coordination mode of indole to a metal center is the Wheland‐intermediate‐type σ‐mode in light of the seemingly strong electron‐donating ability of indole. Several factors which affect the σ‐ or π‐character of indole–metal interactions are discussed.     

Controllable assembly of a three‐dimensional metal–organic supramolecular framework displaying hydrogen‐bonding and π–π stacking interactions

The complex poly[[aqua(μ₂‐phthalato‐κ²O¹:O²){μ₃‐2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetato‐κ⁴N²,N³:O:O′}{μ₂‐2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetato‐κ³N²,N³:O}dizinc(II)] dihydrate], {[Zn₂(C₁₀H₈N₃O₂)₂(C₈H₄O₄)(H₂O)]·2H₂O}_n, has been prepared by solvothermal reaction of 2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetonitrile (PPAN) with zinc(II). Under hydrothermal conditions, PPAN is hydrolyzed to 2‐[3‐(pyridin‐2‐yl)‐1H‐pyrazol‐1‐yl]acetate (PPAA⁻). The structure determination reveals that the complex is a one‐dimensional double chain containing cationic [Zn₄(PPAA)₄]⁴⁺ structural units, which are further extended by bridging phthalate ligands. The one‐dimensional chains are extended into a three‐dimensional supramolecular architecture via hydrogen‐bonding and π–π stacking interactions.     

[2,5‐Bis(2,2′‐bipyridyl‐6‐yl)‐3,4‐diazahexa‐2,4‐diene]dichloridomanganese(II): from a mononuclear compound to a three‐dimensional supramolecular framework through C—H...Cl hydrogen bonds and π–π stacking interactions

The title compound, [MnCl₂(C₂₄H₂₀N₆)], has been synthesized and characterized based on the multifunctional ligand 2,5‐bis(2,2′‐bipyridyl‐6‐yl)‐3,4‐diazahexa‐2,4‐diene (L). The Mn^II centre is five‐coordinate with an approximately square‐pyramidal geometry. The L ligand acts as a tridendate chelating ligand. The mononuclear molecules are bridged into a one‐dimensional chain by two C—H...Cl hydrogen bonds. These chains are assembled into a two‐dimensional layer through π–π stacking interactions between adjacent uncoordinated bipyridyl groups. Furthermore, a three‐dimensional supramolecular framework is attained through π–π stacking interactions between adjacent coordinated bipyridyl groups.     

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.     

Synthesis,Characterization and Crystal Structure of （2,2′—Bipyridine）（μ—maleato）copper（Ⅱ）Dihydrate

The title polymeric complex [Cu(C₄H₂O₄)(C₁₀H₈N₂)]_n·2nH₂O was prepared and its crystal structure was determined by X‐ray diffraction methods. The crystal belongs to space group P2₁/c with cell dimensions of a = 1.0104(1), b = 1.9952(2), c = 0.7357(2) nm, β= 98.38(2)° and Z = 4. The complex forms zig‐zag chains along crystallographic axis c via Cu—O (carboxyl) bond in the apical direction. Each repeated unit consists of a square pyramidal Cu(II) centre with one maleate dianion and one 2,2′‐bipyridine forming a basal plane. Adjacent chains link to each other by H‐bonding between carboxyl groups and crystalline water. The distance of 0.3482 nm between parallel bipyridine rings shows a π‐π stacking interaction. The title complex was also characterized by IR, UV and ESR spectra.     

π‐stacking and C—X...D (X = H,NO2; D = O, π) interactions in the crystal network of both C—H...N and π‐stacked dimers of 1,2‐bis(4‐bromophenyl)‐1H‐benzimidazole and 2‐(4‐bromophenyl)‐1‐(4‐nitrophenyl)‐1H‐benzimidazole

Molecules of 1,2‐bis(4‐bromophenyl)‐1H‐benzimidazole, C₁₉H₁₂Br₂N₂, (I), and 2‐(4‐bromophenyl)‐1‐(4‐nitrophenyl)‐1H‐benzimidazole, C₁₉H₁₂BrN₃O₂, (II), are arranged in dimeric units through C—H...N and parallel‐displaced π‐stacking interactions favoured by the appropriate disposition of N‐ and C‐bonded phenyl rings with respect to the mean benzimidazole plane. The molecular packing of the dimers of (I) and (II) arises by the concurrence of a diverse set of weak intermolecular C—X...D (X = H, NO₂; D = O, π) interactions.     

Two‐Component Self‐Assembly: Hierarchical Formation of pH‐Switchable Supramolecular Networks by π–π Induced Aggregation of Ion Pairs

Two‐component self‐assembly is a promising approach to construct functional nanomaterials. Interaction of a flexible guanidiniocarbonyl pyrrole tetra‐cation ( 1 ) with naphthalene diimide dicarboxylic acid (NDIDC) in aqueous DMSO leads to the formation of supramolecular networks. First, the carboxylate groups of NDIDC bind to the guanidiniocarbonyl pyrrole cations of 1 in a 1:2 stoichiometry. Further π–π induced aggregation then leads to 3D networks, as established by dynamic light scattering studies (DLS), NMR, fluorescence titration, viscosity measurements, AFM, and TEM microscopy. Due to ion pairing, the resulting aggregates can be switched between the monomers and the aggregates reversibly using external stimuli like protonation or deprotonation. At high concentration, a stable colloidal solution is formed, which shows an extensive Tyndall effect. Increasing the concentrations even further leads to formation of a supramolecular gel.     

The tropolone–isobutylamine complex: a hydrogen‐bonded troponoid without dominant π–π interactions

Tropolone long has served as a model system for unraveling the ubiquitous phenomena of proton transfer and hydrogen bonding. This molecule, which juxtaposes ketonic, hydroxylic, and aromatic functionalities in a framework of minimal complexity, also has provided a versatile platform for investigating the synergism among competing intermolecular forces, including those generated by hydrogen bonding and aryl coupling. Small members of the troponoid family typically produce crystals that are stabilized strongly by pervasive π–π, C—H…π, or ion–π interactions. The organic salt (TrOH·iBA) formed by a facile proton‐transfer reaction between tropolone (TrOH) and isobutylamine (iBA), namely isobutylammonium 7‐oxocyclohepta‐1,3,5‐trien‐1‐olate, C₄H₁₂N⁺·C₇H₅O₂⁻, has been investigated by X‐ray crystallography, with complementary quantum‐chemical and statistical‐database analyses serving to elucidate the nature of attendant intermolecular interactions and their synergistic effects upon lattice‐packing phenomena. The crystal structure deduced from low‐temperature diffraction measurements displays extensive hydrogen‐bonding networks, yet shows little evidence of the aryl forces (viz. π–π, C—H…π, and ion–π interactions) that typically dominate this class of compounds. Density functional calculations performed with and without the imposition of periodic boundary conditions (the latter entailing isolated subunits) documented the specificity and directionality of noncovalent interactions occurring between the proton‐donating and proton‐accepting sites of TrOH and iBA, as well as the absence of aromatic coupling mediated by the seven‐membered ring of TrOH. A statistical comparison of the structural parameters extracted for key hydrogen‐bond linkages to those reported for 44 previously known crystals that support similar binding motifs revealed TrOH·iBA to possess the shortest donor–acceptor distances of any troponoid‐based complex, combined with unambiguous signatures of enhanced proton‐delocalization processes that putatively stabilize the corresponding crystalline lattice and facilitate its surprisingly rapid formation under ambient conditions.     

Crystallographic report: Crystal structure of bis[dipyrido[3,2,‐a:2′,3′‐c]phenazine]lead(II)diiodide

The mononuclear lead(II) complex of formula [PbI₂(DPPZ)₂] (DPPZ = dipyrido[3,2,‐a:2′,3′‐c]phenazine) has two‐fold symmetry and features a distorted octahedral geometry for lead defined by an N₄I₂ donor set. Copyright © 2004 John Wiley & Sons, Ltd.     

Complexes of nickel(II) and copper(II) with 1,2,4‐triazole‐3‐carboxylic acid and of cobalt(III) with 3‐amino‐1,2,4‐triazole‐5‐carboxylic acid

Because of their versatile coordination modes and strong coordination ability for metals, triazole ligands can provide a wide range of possibilities for the construction of metal–organic frameworks. Three transition‐metal complexes, namely bis(μ‐1,2,4‐triazol‐4‐ide‐3‐carboxylato)‐κ³N ²,O :N ¹;κ³N ¹:N ²,O‐bis[triamminenickel(II)] tetrahydrate, [Ni₂(C₃HN₃O₂)₂(NH₃)₆]·4H₂O, (I), catena‐poly[[[diamminediaquacopper(II)]‐μ‐1,2,4‐triazol‐4‐ide‐3‐carboxylato‐κ³N ¹:N ⁴,O‐[diamminecopper(II)]‐μ‐1,2,4‐triazol‐4‐ide‐3‐carboxylato‐κ³N ⁴,O :N ¹] dihydrate], {[Cu₂(C₃HN₃O₂)₂(NH₃)₄(H₂O)₂]·2H₂O}_n , (II), (μ‐5‐amino‐1,2,4‐triazol‐1‐ide‐3‐carboxylato‐κ²N ¹:N ²)di‐μ‐hydroxido‐κ⁴O :O‐bis[triamminecobalt(III)] nitrate hydroxide trihydrate, [Co₂(C₃H₂N₄O₂)(OH)₂(NH₃)₆](NO₃)(OH)·3H₂O, (III), with different structural forms have been prepared by the reaction of transition metal salts, i.e. NiCl₂, CuCl₂ and Co(NO₃)₂, with 1,2,4‐triazole‐3‐carboxylic acid or 3‐amino‐1,2,4‐triazole‐5‐carboxylic acid hemihydrate in aqueous ammonia at room temperature. Compound (I) is a dinuclear complex. Extensive O—H…O, O—H…N and N—H…O hydrogen bonds and π–π stacking interactions between the centroids of the triazole rings contribute to the formation of the three‐dimensional supramolecular structure. Compound (II) exhibits a one‐dimensional chain structure, with O—H…O hydrogen bonds and weak O—H…N, N—H…O and C—H…O hydrogen bonds linking anions and lattice water molecules into the three‐dimensional supramolecular structure. Compared with compound (I), compound (III) is a structurally different dinuclear complex. Extensive N—H…O, N—H…N, O—H…N and O—H…O hydrogen bonding occurs in the structure, leading to the formation of the three‐dimensional supramolecular structure.     

A Double Cation–π‐Driven Strategy Enabling Two‐Dimensional Supramolecular Polymers as Efficient Catalyst Carriers

The cation–π interaction is a strong non‐covalent interaction that can be used to prepare high‐strength, stable supramolecular materials. However, because the molecular plane of a cation‐containing group and that of aromatic structure are usually perpendicular when forming a cation–π complex, it is difficult to exploit the cation–π interaction to prepare a 2D self‐assembly in which the molecular plane of all the building blocks are parallel. Herein, a double cation–π‐driven strategy is proposed to overcome this difficulty and have prepared 2D self‐assemblies with long‐range ordered molecular hollow hexagons. The double cation–π interaction makes the 2D self‐assemblies stable. The 2D self‐assemblies are to be an effective carrier that can eliminate metal‐nanoparticle aggregation. Such 2D assembly/palladium nanoparticle hybrids are shown to exhibit recyclability and superior catalytic activity for a model reaction.     

Shifting from Hydrogen Bond Network to π–π Stacking: A Key Mechanism for Reversible Thermochromic Sulfonated Poly(Ether Ether Ketone)

Sulfonated poly(ether ether ketone) (SPEEK) thin film performs reversible thermochromic property by developing the color to be yellowish at the temperature above 190 °C. The detailed analyses based on temperature‐dependent techniques suggest the thermal treatment inducing the shifting of the hydrogen bond network between the sulfonated group and the hydrated water molecules to the π–π stacking among aromatic rings in SPEEK chains. Although it is general that the polymer chain packing is unfavorable at high temperature, the present work shows a good example that when the polymer chains can form specific molecular interaction, such as π–π stacking, even in harsh thermal treatment, a rearrangement will effectively occur, which leads to an external stimuli‐responsive property.

     

Dissecting the concave–convex π‐π interaction in corannulene and sumanene dimers: SAPT(DFT) analysis and performance of DFT dispersion‐corrected methods

The characteristics of the concave–convex π‐π interactions are evaluated in 32 buckybowl dimers formed by corannulene, sumanene, and two substituted sumanenes (with S and CO groups), using symmetry‐adapted perturbation theory [SAPT(DFT)] and density functional theory (DFT). According to our results, the main stabilizing contribution is dispersion, followed by electrostatics. Regarding the ability of DFT methods to reproduce the results obtained with the most expensive and rigorous methods, TPSS‐D seems to be the best option overall, although its results slightly tend to underestimate the interaction energies and to overestimate the equilibrium distances. The other two tested DFT‐D methods, B97‐D2 and B3LYP‐D, supply rather reasonable results as well. M06‐2X, although it is a good option from a geometrical point of view, leads to too weak interactions, with differences with respect to the reference values amounting to about 4 kcal/mol (25% of the total interaction energy). © 2017 Wiley Periodicals, Inc.     

π–π Stacking‐Interlinked Molecular Squares with Hepta‐Coordinated Cadmium(II) Corners, [Cd(DMAP)3(NO2)2]·0.5H2O

The single crystal X‐ray analysis data of the new hepta‐coordinate cadmium(II) complex of N,N‐dimethyl‐N‐(4‐pyridyl)amine (DMPA), [Cd(DMPA)₃(NO₂)₂]·0.5H₂O, shows that the coordination environment around the Cd^II is pentagonal bipyramidal. Furthermore, self‐assembly of this complex as molecular squares that interlink via π–π stacking interactions is observed. This network contains voids that are filled by water molecules.     

Synthesis,crystal structures and luminescence properties of seven mononuclear zinc(II), cadmium(II), cobalt(II) and nickel(II) complexes with 5‐(4‐methylphenyl)‐3‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazole

Due to their versatile coordination modes and metal‐binding conformations, triazolyl ligands can provide a wide range of possibilities for the construction of supramolecular structures. Seven mononuclear transition metal complexes with different structural forms, namely aquabis[3‐(4‐methylphenyl)‐5‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazolato‐κ²N ¹,N ⁵]zinc(II), [Zn(C₁₄H₁₁N₄)₂(H₂O)], (I), bis[5‐(4‐methylphenyl)‐3‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazole‐κ²N ³,N ⁴]bis(nitrato‐κO )zinc(II), [Zn(NO₃)₂(C₁₄H₁₂N₄)₂], (II), bis(methanol‐κO )bis[3‐(4‐methylphenyl)‐5‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazolato‐κ²N ¹,N ⁵]zinc(II), [Zn(C₁₄H₁₁N₄)₂(CH₄O)₂], (III), diiodidobis[5‐(4‐methylphenyl)‐3‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazole‐κ²N ³,N ⁴]cadmium(II), [CdI₂(C₁₄H₁₂N₄)₂], (IV), bis[5‐(4‐methylphenyl)‐3‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazole‐κ²N ³,N ⁴]bis(nitrato‐κO )cadmium(II), [Cd(NO₃)₂(C₁₄H₁₂N₄)₂], (V), aquabis[3‐(4‐methylphenyl)‐5‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazolato‐κ²N ¹,N ⁵]cobalt(II), [Co(C₁₄H₁₁N₄)₂(H₂O)], (VI), and diaquabis[3‐(4‐methylphenyl)‐5‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazolato‐κ²N ¹,N ⁵]nickel(II), [Ni(C₁₄H₁₁N₄)₂(H₂O)₂], (VII), have been prepared by the reaction of transition metal salts (Zn^II, Cd^II, Co^II and Ni^II) with 3‐(4‐methylphenyl)‐5‐(pyridin‐2‐yl)‐1H‐1,2,4‐triazole (pymphtzH) under either ambient or hydrothermal conditions. These compounds have been characterized by elemental analysis, IR spectroscopy and single‐crystal X‐ray diffraction. All the complexes form three‐dimensional supramolecular structures through hydrogen bonds or through π–π stacking interactions between the centroids of the pyridyl or arene rings. The pymphtzH and pymphtz⁻ entities act as bidentate coordinating ligands in each structure. Moreover, all the pyridyl N atoms are coordinated to metal atoms (Zn, Cd, Co or Ni). The N atom in the 4‐position of the triazole group is coordinated to the Zn and Cd atoms in the crystal structures of (II), (IV) and (V), while the N atom in the 1‐position of the triazolate group is coordinated to the Zn, Co and Ni atoms in (I), (III), (VI) and (VII).     

Tuning the π‐π stacking distance and J‐aggregation of DPP‐based conjugated polymer via introducing insulating polymer

The close π–π stacking and the high J‐aggregation during the formation of fibrillar morphology in films of the poly[[2,5‐bis(2‐octyldodecyl)?2,3,5,6‐tetrahydro‐3,6‐dioxopyrrolo[3,4‐c]pyrrole‐1,4‐diyl]‐alt–[[2,2′‐(2,5‐thiophene)bis‐thieno[3,2‐b]thiophen]‐5,5′‐diyl]] (PDPPTT‐T) are demonstrated via blending with polystyrene (PS). The hydrodynamic radius (R_h) of PDPPTT‐T is decreased from 16.7 nm in the neat solution to 12.7 nm in the blend solution at the ratio of 1/20(PDPPTT‐T/PS). This phenomenon suggests that blending PS is beneficial for the disentanglement of PDPPTT‐T. The disentanglement of PDPPTT‐T facilitates the formation of fibrillar morphology. The growth of the fibrils occurs along the molecular backbones and the width of the fibrils is parallel to the π–π stacking direction. The disentanglement of PDPPTT‐T helps the molecules adjust conformation to improve J‐aggregation and decrease the π–π stacking distance. The maximum absorption is red‐shifted from 825 nm to 849 nm and the relative intensity of J‐aggregation (the 0‐0/0‐1 ratio) is increased from 1.19 to 1.60. The π–π stacking distance decreases from 3.57 to 3.52 Å. The charge‐carrier mobility will be improved in the fibrillar morphology with close π–π stacking and high J‐aggregation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 838–847     

Intramolecular π–π Interactions in Flexibly Linked Partially Fluorinated Bisarenes in the Gas Phase

Three compounds with phenyl and pentafluorophenyl rings bridged by (CH₂)₃ and (CH₂)₂SiMe₂ units were synthesized by hydrosilylation and C−C coupling reactions. Their solid‐state structures are dominated by intermolecular π stacking interactions, primarily leading to dimeric or chain‐type aggregates. Analysis of free molecules in the gas phase by electron diffraction revealed the most abundant conformer to be significantly stabilized by intramolecular π–π interactions. For the silicon compounds, structures characterized by σ–π interactions between methyl and pentafluorophenyl groups are second lowest in energy and cannot be excluded completely by the gas electron diffraction experiments. C₆H₅(CH₂)₃C₆F₅, in contrast, is present as a single conformer. The gas‐phase structures served as a reference for the evaluation of a series of (dispersion‐corrected) quantum‐chemical calculations.     

A threefold interpenetrated two‐dimensional zinc(II) supramolecular architecture based on 3‐nitrobenzoic acid and 4,4′‐bipyridine

With regard to crystal engineering, building block or modular assembly methodologies have shown great success in the design and construction of metal–organic coordination polymers. The critical factor for the construction of coordination polymers is the rational choice of the organic building blocks and the metal centre. The reaction of Zn(OAc)₂·2H₂O (OAc is acetate) with 3‐nitrobenzoic acid (HNBA) and 4,4′‐bipyridine (4,4′‐bipy) under hydrothermal conditions produced a two‐dimensional zinc(II) supramolecular architecture, catena‐poly[[bis(3‐nitrobenzoato‐κ²O,O′)zinc(II)]‐μ‐4,4′‐bipyridine‐κ²N:N′], [Zn(C₇H₄NO₄)₂(C₁₀H₈N₂)]_n or [Zn(NBA)₂(4,4′‐bipy)]_n, which was characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis and single‐crystal X‐ray diffraction analysis. The Zn^II ions are connected by the 4,4′‐bipy ligands to form a one‐dimensional zigzag chain and the chains are decorated with anionic NBA ligands which interact further through aromatic π–π stacking interactions, expanding the structure into a threefold interpenetrated two‐dimensional supramolecular architecture. The solid‐state fluorescence analysis indicates a slight blue shift compared with pure 4,4′‐bipyridine and HNBA.