Hydrogen‐Bond and Metal–Ligand Coordination Bond Hybrid Supramolecular Capsules: Identification of Hemicapsular Intermediate and Dual Control of Guest Exchange Dynamics

Hybrid supramolecular capsules self‐assemble by simultaneously forming hydrogen and metal–ligand coordination bonds on mixing a C₂‐symmetrical cavitand (calix[4]resorcinarene‐based cavitands with ureide and terminal 4‐pyridyl units) with platinum or palladium complexes ([Pt(OTf)₂] or [Pd(OTf)₂] with chelating bisphosphines) in 1:1 ratio. Hemicapsular assemblies formed in the presence of excess amounts of cavitand relative to the platinum or palladium complexes are identified as intermediates in the above self‐assembly process by 2D‐NOESY spectroscopy. External‐anion‐assisted encapsulation of a neutral guest, 4,4′‐diiodobiphenyl, inside the hybrid supramolecular capsules accompanied conformational changes in the hydrogen‐bonding moieties. The in/out exchange ratio of the encapsulated guest depends on the bite angle of the bisphosphine ligand. Addition of DMSO accelerates guest exchange by weakening the hydrogen bonds in the encapsulation complex. Therefore, variations in the structure of the metal complex and amount of polar solvent exert dual control on the dynamics of the guest exchange.     

Self‐Assembled Cyclic Structures from Copper(II) Peptoids

Metal–ligand coordination is a key interaction in the self‐assembly of both biopolymers and synthetic oligomers. Although the binding of metal ions to synthetic proteins and peptides is known to yield high‐order structures, the self‐assembly of peptidomimetic molecules upon metal binding is still challenging. Herein we explore the self‐assembly of three peptoid trimers bearing a bipyridine ligand at their C‐terminus, a benzyl group at their N‐terminus, and a polar group (N‐ethyl‐R) in the middle position (R=OH, OCH₃, or NH₂) upon Cu²⁺ coordination. X‐ray diffraction analysis revealed unique, highly symmetric, dinuclear cyclic structure or aqua‐bridged dinuclear double‐stranded peptoid helicates, formed by the self‐assembly of two peptoid molecules with two Cu²⁺ ions. Only the macrocycle with the highest number of intermolecular hydrogen bonds is stable in solution, while the other two disassemble to their corresponding monometallic complexes.     

Two‐Dimensional Self‐Assembly of a Porphyrin–Polypyridyl Ruthenium(II) Hybrid on HOPG Surface through Metal–Ligand Interactions

The synthesis and self‐assembly behavior of porphyrin–polypyridyl ruthenium(II) hybrid, which consists of a flexible alkyl chain attached with two conjugated moieties is described. The electronic absorption spectrum and emission spectra show that the [C₈‐TPP‐(ip)Ru(phen)₂](ClO₄)₂, abbreviated as (C₈ip)TPPC has optical properties. Scanning tunneling microscopy (STM) studies found that the π–π interaction and metal–ligand interaction allow (C₈ip)TPPC to form self‐assembled structure and have an edge‐on orientation on the highly oriented pyrolytic graphite (HOPG) surface. The multidentate structure in (C₈ip)TPPC molecules act as linkers between the molecules and form metal–ligand coordination, which forces the assembly process in the direction of stable columnar arrays. In addition, although the sample was stored for two months in ambient conditions, STM experiments showed that the order of (C₈ip)TPPC self‐assembly only slightly decreased which indicates that the self‐assembled monolayer is stable. This work demonstrates that introducing a metal‐ligand in the porphyrin‐polypyridyl compound is a useful strategy to obtain novel surface assemblies.     

Transannular Hydrogen Bonding in Planar‐Chiral [2.2]Paracyclophane‐Bisamides

A series of [2.2]paracyclophane‐bisamide regioisomers and alkylated comparators were designed, synthesized, and characterized in order to better understand the transannular hydrogen bonding of [2.2]paracyclophane‐based molecular recognition units. X‐Ray crystallography shows that transannular hydrogen bonding is maintained in the solid‐state, but no stereospecific self‐recognition is observed. The assignment of both transannularly and intermolecularly hydrogen bonded N?H stretches could be made by infrared spectroscopy, and the effect of transannular hydrogen bonding on amide bond rotation dynamics is observed by ¹H‐NMR in nonpolar solvents. The consequences of transannular hydrogen bonding on the optical properties of [2.2]paracyclophane is observed by comparing alkylated and non‐alkylated pseudo‐ortho 4,12‐[2.2]paracyclophane‐bisamides. Finally, optical resolution of 4‐mono‐[2.2]paracyclophane and pseudo‐ortho 4,12‐[2.2]paracyclophane‐bisamides was achieved through the corresponding sulfinyl diastereoisomers for circular dichroism studies. Transannular hydrogen bonding in [2.2]paracyclophane‐amides allows preorganization for self‐complementary intermolecular assembly, but is weak enough to allow rapid rotation of the amides even in nonpolar solvents.     

Design Rules for Self‐Assembly of 2D Nanocrystal/Metal–Organic Framework Superstructures

We demonstrate the guiding principles behind simple two dimensional self‐assembly of MOF nanoparticles (NPs) and oleic acid capped iron oxide (Fe₃O₄) NCs into a uniform two‐dimensional bi‐layered superstructure. This self‐assembly process can be controlled by the energy of ligand–ligand interactions between surface ligands on Fe₃O₄ NCs and Zr₆O₄(OH)₄(fumarate)₆ MOF NPs. Scanning transmission electron microscopy (TEM)/energy‐dispersive X‐ray spectroscopy and TEM tomography confirm the hierarchical co‐assembly of Fe₃O₄ NCs with MOF NPs as ligand energies are manipulated to promote facile diffusion of the smaller NCs. First‐principles calculations and event‐driven molecular dynamics simulations indicate that the observed patterns are dictated by combination of ligand–surface and ligand–ligand interactions. This study opens a new avenue for design and self‐assembly of MOFs and NCs into high surface area assemblies, mimicking the structure of supported catalyst architectures, and provides a thorough fundamental understanding of the self‐assembly process, which could be a guide for designing functional materials with desired structure.     

Chain center‐functionalized amphiphilic block polymers: Complementary hydrogen bond self‐assembly in aqueous solution

Hydrogen bonding self‐assemblies were formed in an aqueous medium from a pair of an amphiphilic ABA triblock copolymer and a hydrophobic homopolymer, both with a triple hydrogen bonding site that was complementary to each other and precisely placed at the main‐chain center: (PEGMA)_m–(MMA)_n– ADA –(MMA)_n–(PEGMA)_m and (MMA)_p– DAD –(MMA)_p ( A = hydrogen acceptor; D = hydrogen donor; PEGMA: PEG methacrylate; MMA: methyl methacrylate). The polymers were synthesized by the ruthenium‐catalyzed living radial polymerization with bifunctional initiators (Br– ADA –Br and Cl– DAD –Cl) aiming at pinpoint chain center functionalization to give a symmetric segmental sequence; ADA and DAD initiators were derived from 2,6‐diaminopyridine and thymine, respectively. On mixed equimolar in tetrahydrofuran (THF), both polymers spontaneously associated, and the apparently 1:1 assembly further grew into higher aggregate particles on subsequent addition of water. The aggregates in water/THF were relatively stable and uniform in size, which most likely stems from the intermolecular complementary hydrogen bond interaction at polymer chain centers. In sharp contrast, an equimolar mixture of ADA ‐block polymer and DAD ‐free poly(MMA) in water/THF resulted in larger and irregular particles, and thus short‐lived to eventually collapse. These results indicate that, however structurally marginal, precise pinpoint functionalization of macromolecular chains allows stable self‐assemblies via complementary hydrogen bond interaction even in aqueous media. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4498–4504     

Single‐Chain Polymer Self‐Assembly Using Complementary Hydrogen Bonding Units

A triblock copolymer containing the complementary hydrogen bonding recognition pair ureidoguanosine–diaminonaphthyridine (UG–DAN) as pendant functional groups is synthesized using ring‐opening metathesis polymerization (ROMP). The norbornene‐based DAN monomer is shown to allow for a controlled polymerization when polymerized in the presence of a modi­fied‐UG molecule that serves as a protecting group, subsequently allowing for the fabrication of functionalized triblock copolymers. The self‐assembly of the copolymers was characterized using dynamic light scattering and ¹H NMR spectroscopy. It is demonstrated that the polymers self‐assemble via complementary hydrogen bonding motifs even at low dilutions, indicating intramolecular interactions.

     

High Degree of Polymerization in a Fullerene‐Containing Supramolecular Polymer

Supramolecular polymers based on dispersion forces typically show lower molecular weights (MW) than those based on hydrogen bonding or metal–ligand coordination. We present the synthesis and self‐assembling properties of a monomer featuring two complementary units, a C₆₀ derivative and an exTTF‐based macrocycle, that interact mainly through π–π, charge‐transfer, and van der Waals interactions. Thanks to the preorganization in the host part, a remarkable log K_a=5.1±0.5 in CHCl₃ at room temperature is determined for the host–guest couple. In accordance with the large binding constant, the monomer self‐assembles in the gas phase, in solution, and in the solid state to form linear supramolecular polymers with a very high degree of polymerization. A MW above 150 kDa has been found experimentally in solution, while in the solid state the monomer forms extraordinarily long, straight, and uniform fibers with lengths reaching several microns.     

From Nongelating to Gelating: Synthesis and Structural Self‐Assembling Property Relationships of a Homologous Series of Oligo(amide–triazole)s

A homologous series of oligo(amide–triazole)s (OAT) [ OAT‐CO₂H‐2 n and OAT‐COPrg‐(2 n +1) ] with an increasing number of primary amide (CONH) and triazole hydrogen‐bonding functionalities was prepared by an iterative synthetic procedure. It was found that their self‐assembly and thermoreversible gelation strength had a strong correlation to the number of hydrogen‐bonding moieties in the oligomers. There also existed a threshold value of the number of CONH units, above which all the oligomers became organogelators. Hence, oligomers with ≤4 CONH units are devoid of intermolecular hydrogen bonding and also non‐organogelating, whereas those that contain >4 CONH units show intermolecular association and organogelating properties. For the organogelators, the T_gel value increases monotonically with increasing number of CONH units. On the basis of FTIR measurements, both the CONH and triazole C? H groups were involved in the hydrogen‐bonding process. A mixed xerogel that consisted of a 1:1 weight ratio of two oligomers of different lengths ( OAT‐CO₂H‐6 and OAT‐CO₂H‐12 ) was found to show microphase segregation according to differential scanning calorimetry, thus indicating that oligomers that bear a different number of hydrogen‐bonding units exhibited self‐sorting to maximize the extent of intermolecular hydrogen bonding in the xerogel state.     

Mechanochemical Encapsulation of Fullerenes in Peptidic Containers Prepared by Dynamic Chiral Self‐Sorting and Self‐Assembly

Molecular capsules composed of amino acid or peptide derivatives connected to resorcin[4]arene scaffolds through acylhydrazone linkers have been synthesized using dynamic covalent chemistry (DCC) and hydrogen‐bond‐based self‐assembly. The dynamic character of the linkers and the preference of the peptides towards self‐assembly into β‐barrel‐type motifs lead to the spontaneous amplification of formation of homochiral capsules from mixtures of different substrates. The capsules have cavities of around 800 ų and exhibit good kinetic stability. Although they retain their dynamic character, which allows processes such as chiral self‐sorting and chiral self‐assembly to operate with high fidelity, guest complexation is hindered in solution. However, the quantitative complexation of even very large guests, such as fullerene C₆₀ or C₇₀, is possible through the utilization of reversible covalent bonds or the application of mechanochemical methods. The NMR spectra show the influence of the chiral environment on the symmetry of the fullerene molecules, which results in the differentiation of diastereotopic carbon atoms for C₇₀, and the X‐ray structures provide unique information on the modes of peptide–fullerene interactions.     

Hydrogen‐Bond‐Guided Self‐Assembly of Nucleotides on a Receptor‐Array Surface

The hydrogen‐bond‐guided self‐assembly of 5′‐ribonucleotides bearing adenine(A), cytosine (C), uracil (U), or guanine (G) bases from aqueous solution on a lipid‐like surface decorated with synthetic bis(Zn^II–cyclen) (cyclen=1,4,7,10‐tetraazacyclodododecane) metal–complex receptor sites is described. The process was studied by using surface plasmon resonance spectroscopy. The data show that the mechanism of nucleotide binding to the 2D template is influenced by the chemistry of the bases and the pH value of the solution. In a neutral solution of pH 7.5, the process is cooperative and selective with respect to Watson–Crick pairs (A–U and C–G), which form stable double planes in accordance with the Chargaff rule. In a more acidic solution at pH 6.0, the interactions between complementary partners become non‐cooperative and the surface also stabilizes mismatched and wobble pairs due to the pH‐induced changes in the receptor coordination state. The results suggest that hydrogen bonding plays a key role in the self‐assembly of complementary nucleotides at the lipid‐like interface, and the cooperative character of the process stems from the ideal matching of the orientation and chemistry of all the interacting components with respect to each other in neutral solution.     

Rational Design of Nanofibers and Nanorings through Complementary Hydrogen‐Bonding Interactions of Functional π Systems

A simple protocol to create nanofibers and ‐rings through a rational self‐assembly approach is described. Whereas the melamine–oligo(p‐phenylenevinylene) conjugate 1 a self‐aggregates to form ill‐defined nanostructures, conjugate 1 b , which possesses an amide group as an additional interactive site, self‐aggregates to form 1D nanofibers that induce gelation of the solvent. AFM and XRD studies have shown that dimerization through the melamine–melamine hydrogen‐bonding interaction occurs only for 1 b . Upon complexation with 1/3 equivalents of cyanuric acid (CA), conjugate 1 a provides well‐defined, ring‐shaped nanostructures at micromolar concentrations, which open to form fibrous assemblies at submillimolar concentrations and organogels in the millimolar concentration range. Apparently, the enhanced aggregation ability of 1 a by CA is a consequence of columnar organization of the resulting discotic complex 1 a ₃ ? CA. In contrast, coaggregation of 1 b with CA does not provide well‐defined nanostructures, probably due to the interference of complementary hydrogen‐bonding interactions by the amide group.     

The Versatile,Efficient, and Stereoselective Self‐Assembly of Transition‐Metal Helicates by Using Hydrogen‐Bonds

A diverse range of dinuclear double‐stranded helicates in which the ligand strand is built up by using hydrogen‐bonding has been synthesized. The helicates, formulated as [Co₂(L)₂(L‐H)₂X₂], readily self‐assemble from a mixture of a suitable pyridine–alcohol compound (L; for example, 6‐methylpyridine‐2‐methanol, 1 ), and a CoX₂ salt in the presence of base. Nine such helicates have been characterized by X‐ray crystallography. For helicates derived from the same pyridine–alcohol precursor, a remarkable regularity was found for both the molecular structure and the crystal packing arrangements, regardless of the nature of the ancillary ligand (X). A notable exception was observed in the solid‐state structure of [Co₂( 1 )₂( 1 ‐H)₂(NCS)₂] for which intermolecular nonbonded contacts between the sulfur atoms (S???S=3.21 Å) lead to the formation of 1D chains. Helicates derived from (R)‐6‐methylpyridine‐2‐methanol ( 2 ) are soluble in solvents such as CH₃CN and CH₂Cl₂, and their self‐assembly could be monitored in solution by ¹H NMR, UV/Vis, and CD titrations. No intermediate complexes were observed to form in a significant concentration at any point throughout these titrations. The global thermodynamic stability constant of [Co₂( 2 )₂( 2 ‐H)₂(NO₃)₂] was calculated from spectrophotometric data to be logβ=8.9(8). The stereoisomerism of these helicates was studied in some detail and the self‐assembly process was found to be highly stereoselective. The chirality of the ligand precursors can control the absolute configuration of the metal centers and thus the overall helicity of the dinuclear assemblies. Furthermore, the enantiomers of rac‐6‐methylpyridine‐2‐methanol ( 3 ) undergo a self‐recognition process to form exclusively homochiral helicates in which the four pyridine–alcohol units possess the same chirality.     

Photoinduced Triplet–Triplet Energy Transfer in a 2‐Ureido‐4(1H)‐Pyrimidinone‐Bridged,Quadruply Hydrogen‐Bonded Ferrocene–Fullerene Assembly

2‐Ureido‐4(1H)‐pyrimidinone‐bridged ferrocene–fullerene assembly I is designed and synthesized for elaborating the photoinduced electron‐transfer processes in self‐complementary quadruply hydrogen‐bonded modules. Unexpectedly, steady‐state and time‐resolved spectroscopy reveal an inefficient electron‐transfer process from the ferrocene to the singlet or triplet excited state of the fullerene, although the electron‐transfer reactions are thermodynamically feasible. Instead, an effective intra‐assembly triplet–triplet energy‐transfer process is found to be operative in assembly I with a rate constant of 9.2×10⁵ s^?1 and an efficiency of 73 % in CH₂Cl₂ at room temperature.     

catena‐Poly[[[(di‐2‐pyridylamine‐κ2N2,N2′)copper(II)]‐μ‐benzene‐1,3‐dicarboxylato‐κ3O1,O1′:O3] monohydrate], a zigzag coordination polymer with strong π–π interactions

The novel title coordination polymer, {[Cu(C₈H₄O₄)(C₁₀H₉N₃)]·H₂O}_n, synthesized by the slow‐diffusion method, takes the form of one‐dimensional zigzag chains built up of Cu^II cations linked by benzene‐1,3‐dicarboxylate (ipht) anions. An exceptional characteristic of this structure is that it belongs to a small group of metal–organic polymers where ipht is coordinated as a bridging tridentate ligand with monodentate and chelate coordination of individual carboxylate groups. The Cu^II cation has a highly distorted square‐pyramidal geometry formed by three O atoms from two ipht anions and two N atoms from a di‐2‐pyridylamine (dipya) ligand. The zigzag chains, which run along the b axis, further construct a three‐dimensional metal–organic framework via strong face‐to‐face π–π interactions and hydrogen bonds. A solvent water molecule is linked to the different carboxylate groups via hydrogen bonds. Thermogravimetric and differential scanning calorimetric analyses confirm the strong hydrogen bonding.     

Di­aqua­(sulfato‐κO)[2,4,6‐tris(2‐pyridyl)‐1,3,5‐triazine‐κ3N1,N2,N6]zinc(II) dihydrate

In the title compound, [Zn(SO₄)(C₁₈H₁₂N₆)(H₂O)₂]·2H₂O, the metal complex is monomeric, with an octahedral Zn^II centre coordinated by the tridentate ligand 2,4,6‐tris(2‐pyridyl)‐1,3,5‐triazine (tpt), two aqua mol­ecules and a monodentate sulfate ion. A complex hydrogen‐bonding scheme is built up out of the profuse availability of donors and acceptors (O—H⋯O/N and C—H⋯O) which, in addition to π–π interactions between tpt groups, define a three‐dimensional assembly.     

Hot π‐Electron Tunneling of Metal–Insulator–COF Nanostructures for Efficient Hydrogen Production

A metal–insulator–semiconductor (MIS) photosystem based on covalent organic framework (COF) semiconductors was designed for robust and efficient hydrogen evolution under visible‐light irradiation. A maximal H₂ evolution rate of 8.42 mmol h^?1 g^?1 and a turnover frequency of 789.5 h^?1 were achieved by using a MIS photosystem prepared by electrostatic self‐assembly of polyvinylpyrrolidone (PVP) insulator‐capped Pt nanoparticles (NPs) with the hydrophilic imine‐linked TP‐COFs having =C=O?H?N= hydrogen‐bonding groups. The hot π‐electrons in the photoexcited n‐type TP‐COF semiconductors can be efficiently extracted and tunneled to Pt NPs across an ultrathin PVP insulating layer to reduce protons to H₂. Compared to the Schottky‐type counterparts, the COF‐based MIS photosystems give a 32‐fold‐enhanced carrier efficiency, attributed to the combined enhancement of photoexcitation rate, charge separation, and oxidation rate of holes accumulated in the valence band of the TP‐COF semiconductor.     

Alkaline‐Earth Metal Cations as Structure Building Blocks for Molecular Cages with Entrapment and Controlled Release of Quintuple Ionic Aggregates

Currently, main‐group metal cations are totally neglected as the structure‐building blocks for the self‐assembly of supramolecular coordination metallocages due to the lack of directional bonding. However, here we show that a common Arrhenius acid–base neutralization allows the alkaline‐earth metal cations to act as charged binders, easily connecting two or more highly directional anionic transition‐metal‐based metalloligands to coordination polymers. With a metal salt such as K⁺PF₆^? added during the neutralization, the main‐group metal‐connected skeleton can be templated by the largest yet reported ionic‐aggregate anion, K₂(PF₆)₃^?, formed from KPF₆ in solution, into molecular metallocages, encapsulating the ion. Crystal‐structure details, DFT‐calculation results, and controlled‐release behavior support the presence of K₂(PF₆)₃^? as a guest in the cage. Upon removal of PF₆^? ions, the cage stays intact. Other ions like BF₄^? can be put back in.     

Template Synthesis,Metalation, and Self‐Assembly of Protic Gold(I)/(NHC)2 Tectons Driven by Metallophilic Interactions

A new protocol for the synthesis of protic bis(N‐heterocyclic carbene) complexes of Au^I by a stepwise metal‐controlled coupling of isocyanide and propargylamine is described. They are used as tectons for the construction of supramolecular architectures through metalation and self‐assembly. Notably a unique polymeric chain of Cu^I with alternate Au^I/bis(imidazolate) bridging scaffolds and strong unsupported Cu^I–Cu^I interactions has been generated, as well as a 28‐metal‐atoms cluster containing a nanopiece of Cu₂O trapped by peripheral Au^I/bis(imidazolate) moieties.     

From molecular to macroscopic engineering: shaping hydrogen-bonded organic nanomaterials

The self‐assembly and self‐organization behavior of chromophoric acetylenic scaffolds bearing 2,6‐bis(acetylamino)pyridine ( 1 , 2 ) or uracyl‐type ( 3 – 9 ) terminal groups has been investigated by photophysical and microscopic methods. Systematic absorption and luminescence studies show that 1 and 2 , thanks to a combination of solvophilic/solvophobic forces and π–π stacking interactions, undergo self‐organization in apolar solvents (i.e., cyclohexane) and form spherical nanoparticles, as evidenced by wide‐field optical microscopy, TEM, and AFM analysis. For the longer molecular module, 2 , a more uniform size distribution is found (80–200 nm) compared to 1 (20–1000 nm). Temperature scans in the range 283–353 K show that the self‐organized nanoparticles are reversibly formed and destroyed, being stable at lower temperatures. Molecular modules 1 and 2 were then thoroughly mixed with the complementary triply hydrogen‐bonding units 3 – 9 . Depending on the specific geometrical structure of 3 – 9 , different nanostructures are evidenced by microscopic investigations. Combination of modules 1 or 2 with 3 , which bears only one terminal uracyl unit, leads to the formation of vesicular structures; instead, when 1 is combined with bis‐uracyl derivative 4 or 5 , a structural evolution from nanoparticles to nanowires is observed. The length of the wires obtained by mixing 1 and 4 or 1 and 5 can be controlled by addition of 3 , which prompts transformation of the wires into shorter rods. The replacement of linear system 5 with the related angular modules 6 and 7 enables formation of helical nanostructures, unambiguously evidenced by AFM. Finally, thermally induced self‐assembly was studied in parallel with modules 8 and 9 , in which the uracyl recognition sites are protected with tert‐butyloxycarbonyl (BOC) groups. This strategy allows further control of the self‐assembly/self‐organization process by temperature, since the BOC group is completely removed on heating. Microscopy studies show that the BOC‐protected ditopic modules 8 self‐assemble and self‐organize with 1 into ordered linear nanostructures, whereas BOC‐protected tritopic system 9 gives rise to extended domains of circular nano‐objects in combination with 1 .