Anion-directed self-assembly in coordination networks: architectural control via cooperative noncovalent interactions

The self-assembly of a new flexible tritopic pyrazine-pyridine ligand (pz-3-py) with HgX(2) (X = Cl, Br) was investigated. The results show that coordinated chloride and bromide anions play different roles, and two architecturally different coordination polymers were obtained with the anions used. Where X = Cl, in [Hg(μ(3)-pz-3-py)Cl(2)](n) (1), the 2D network is isolated, while for X = Br, in [Hg(μ-pz-3-py)Br(2)](n) (2), a 1D zigzag chain is constructed. Our results show that noncovalent interactions such as hydrogen bond, halogen···halogen, and halogen···π interactions, when acting cooperatively, are driving forces for the selection of different structures.     

Bridged coordination polymer multilayers with tunable properties

Coordination multilayers consisting of Pd(II) pincer-type complexes and poly(vinyl pyridine) were synthesized and characterized. Film properties were found to be dependent on and could be tuned by varying bath deposition concentrations, polymer molecular weight, and solution additives that compete with binding. Generally, smoother, thinner films were obtained with lower poly(vinyl pyridine) deposition bath concentrations. Likewise, film thickness and roughness could be reduced by employing a higher-molecular-weight poly(vinyl pyridine). Film properties could also be influenced by using acetonitrile as a solution additive, effectively driving the binding equilibrium slightly toward the free species.     

Tuning secondary structure and self-assembly of amphiphilic peptides

Self-assembly is one of nature's mechanisms by which higher order structures are obtained. Two of the main driving forces for self-assembly, hydrophobic interactions and hydrogen bonding, are both present within amphiphilic peptides. Here, it is demonstrated how the intricately interconnected folding and assembly behavior of an N-terminally acylated peptide, with the sequence GANPNAAG, has been tuned by varying its hydrophobic tail and thermal history. The change in interplay between hydrophobic forces and peptide folding allowed the occurrence of different types of aggregation, from soluble peptides with a random coil conformation to aggregated peptides arranged in a beta-sheet assembly, which form helically twisted bilayer ribbons.     

Ring-opening polymerization of gold macrocycles and self-assembly of a coordination polymer through hydrogen-bonding

The equilibrium between digold and tetragold rings and a ring-opened oligomer and polymer is established by NMR and ESI-MS studies in solution and by structure determinations in the solid state; the polymer containing amide-derivatized ligands undergoes self-assembly through hydrogen bonding to give an ordered network.     

Promoting self-assembly of collagen-related peptides into various higher-order structures by metal-histidine coordination

Collagen is an important and widely used biomaterial and therapeutic. The construction of large-scale collagen structures via the self-assembly of small collagen-related peptides has been extensively studied in the past decade. Here, we report a highly effective and simple means to assemble small synthetic collagen-related peptides into various higher-order structures by utilizing metal-histidine coordination. In this work, two short collagen-related peptides in which histidine residues were incorporated as metal binding sites were designed and chemically synthesized: HG(PPG)(9)GH (X9) and HG(PPG)(4)(PHG)(PPG)(4)GH (PHG). Circular dichroism measurements indicated that these two peptides form only marginally stable collagen triple helices but that their stability can be increased upon the addition of metal ions. Dynamic light scattering analyses, turbidity measurements, TEM, and SEM results demonstrated the metal ion-dependent self-assembly of X9 and PHG into supramolecular structures ranging from various nanofibrils to microscale spherical, laminated, and granulated assemblies. The topology and size of these higher-order structures depends both on the metal ion identity and the location of the binding sites. Most intriguingly, the assembled fibrils show similar D-periodicity to that of natural collagen. Our results demonstrate that metal-histidine coordination can serve as an effective force to induce the self-assembly of unstable collagen-related peptides into higher-order structures.     

Urea-bearing copolymers for guest-dependent tunable self-assembly

RAFT polymerization was used to synthesize urea-bearing methyl methacrylate copolymers for binding carboxylate isosteres.     

A lanthanum pyromellitate coordination polymer with three-dimensional structure

A new three-dimensional metal-organic coordination polymer, [La₂(H₂O)₂(H₂BTEC)(BTEC)], 1, was hydrothermally synthesized and characterized by single crystal X-ray diffraction. The three-dimensional framework is built up from La₂O₁₆ dimers connected by carboxylate anions. The polymer exhibits strong photoluminescence at room temperature with the main emission band at 390 nm (λ_ex = 338 nm). Crystal data: triclinic, space group P(−1),a = 6.4486(3),b = 9.4525(5),c = 9.6238(5) ?, α= 88.24(1), β = 74.67(2), γ= 76.76(1)°,V = 550.45(5)     

Synthesis and structure of a stable pentavalent-uranyl coordination polymer

The polymeric complex {[UO2Py5][KI2Py2]}n was isolated by controlled oxidation of uranium tris-iodide in pyridine and structurally characterized using X-ray diffraction. The described synthetic method allows us to isolate a stable derivative of the elusive pentavalent UO2+ species providing a potential starting material for the development of anhydrous UO2+ coordination chemistry.     

From coordination complexes to coordination polymers through self-assembly

Metal ion coordination in metallo-supramolecular assemblies offers the opportunity to fabricate and study devices and materials that are equally important for fundamental research and new technologies. Metal ions embedded in a specific ligand field offer diverse thermodynamic, kinetic, chemical, physical and structural properties that make these systems promising candidates for active components in functional materials. In particular, dynamic coordination polymers offer exciting opportunities to provide materials with responsive properties. In addition, this approach allows to incorporate the well known properties of metal complexes in polymeric architectures. This review highlights the improvements and the possible applications based on metallo-supramolecular systems with an emphasis on materials science. Examples for new materials such as molecular magnets, coordination polymers as carrier package as well as molecular electronics are featured in this article.     

Anion-directed self-assembly of lanthanide-notp compounds and their fluorescence, magnetic, and catalytic properties

Reactions of 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid) [notpH(6), C(9)H(18)N(3)(PO(3)H(2))3] with different lanthanide salts result in four types of Ln-notp compounds: [Ln{C(9)H(20)N(3)(PO(3)H)(2)(PO(3))}(NO(3))(H(2)O)].4H2O (1), [Ln = Eu (1 Eu), Gd (1 Gd), Tb (1 Tb)], [Ln{C(9)H(20)N(3)(PO(3)H)(2)(PO(3))}(H2O)]Cl.3H2O (2) [Ln = Eu (2 Eu), Gd (2 Gd), Tb (2 Tb)], [Ln{C(9)H(20)N(3)(PO(3)H)(2)(PO(3))}(H2O)]ClO4.8H2O, (3) [Ln = Eu (3 Eu), Gd (3 Gd)], and [Ln{C(9)H(20)N(3)(PO(3)H)(2)(PO(3))}(H2O)]ClO4.3H2O (4), [Ln = Gd (4 Gd), Tb (4 Tb)]. Compounds within each type are isostructural. In compounds 1, dimers of {Ln2(notpH4)2(NO3)2(H2O)2} are found, in which the two lanthanide atoms are connected by two pairs of O-P-O and one pair of mu-O bridges. The NO3- ion serves as a bidentate terminal ligand. Compounds 2 contain similar dimeric units of {Ln2(notpH4)2(H2O)2} that are further connected by a pair of O-P-O bridges into an alternating chain. The Cl- ions are involved in the interchain hydrogen-bonding networks. A similar chain structure is also found in compounds 3; in this case, however, the chains are linked by ClO4- counterions through hydrogen-bonding interactions, forming an undulating layer in the (011) plane. These layers are fused through hydrogen-bonding interactions, leading to a three-dimensional supramolecular network with large channels in the [100] direction. Compounds 4 show an interesting brick-wall-like layer structure in which the neighboring lanthanide atoms are connected by a pair of O-P-O bridges. The ClO4- counterions and the lattice water molecules are between the layers. In all compounds the triazamacrocyclic nitrogen atoms are not coordinated to the Ln(III) ions. The anions and the pH are believed to play key roles in directing the formation of a particular structure. The fluorescence spectroscopic properties of the Eu and Tb compounds, magnetic properties of the Gd compounds, and the catalytic properties of 4 Gd were also studied.     

Surface-assisted coordination chemistry and self-assembly

This article discusses different approaches to build up supramolecular nanoarchitectures on surfaces, which were simultaneously investigated by scanning tunneling microscopy (STM) on the single-molecule level. Following this general road map, first, the hydrogen-bonding guided self-assembly of two different, structural-equivalent molecular building blocks, azobenzene dicarboxylic acid and stilbene dicarboxylic acid, was studied. Secondly, the coordination chemistry of the same building blocks, now acting as ligands in metal coordination reactions, towards co-sublimed Fe atoms was studied under near surface-conditions. Extended two-dimensional tetragonal network formation with unusual Fe2L(4/2)-dimers at the crossing points was observed on copper surfaces. Complementary to the first two experiments, a two-step approach based on the solution-based self-assembly of square-like tetranuclear complexes of the M4L4-type with subsequent deposition on graphite surfaces was investigated. One- and two-dimensional arrangements as well as single molecules of the M4L4-complexes could be observed. Moreover, the local electronic properties of a single M4L4-complexes could be probed with submolecular resolution by means of scanning tunnelling spectroscopy (STS).     

Anion-directed self-assembly of a 2,6-bis(2-anilinoethynyl)pyridine bis(amide) scaffold

Bis(sulfonamide) receptors based on the 2,6-bis(2-anilinoethynyl)pyridine scaffold form persistent dimers with water and halides in solution and in the solid-state. The structurally related bis(amide) receptor derived from 3,5-dinitrobenzoyl chloride is a dimer in the solid-state with two HCl molecules directing the self-assembly. The 2+2 dimer, with a twisted ‘S’-shaped backbone, is held together by six hydrogen bonds. Dissolution of the (H2⁺√Cl ^? )₂ adduct in CHCl₃ results, however, in a monomeric structure. DOSY and ¹H NMR experiments were used to identify the dominance of monomer in solution for both 2 and H2⁺√Cl ^? . The ‘OFF–ON’ fluorescence response of 2,6-bis(2-anilinoethynyl)pyridine is retained with amide arms.     

A luminescent coordination polymer based on bisterpyridyl ligand containing o-carborane: two tunable emission modes

We designed a novel ditopic bisterpyridyl ligand containing o-carborane that can construct a coordination polymer by complexation with metal ions. Through the use of Sonogashira-Hagihara coupling, the desired ligand molecule was successfully synthesized. Addition of Zn(II) ions rapidly underwent the generation of a fluorescent coordination polymer, which was confirmed by (1)H NMR, UV-vis, and fluorescent titration experiments. Furthermore, the electronic structure of the bisterpyridyl ligand molecules was drastically changed upon the complexation with metal ions. The obtained coordination polymer showed light blue emission derived from the intraligand charge transfer (ILCT) state, whereas a bare ligand molecule exhibited yellowish-green aggregation-induced emission (AIE) in a poor solvent such as water, because of the variable C-C bond in o-carborane.     

A successful chemical strategy to induce oligothiophene self-assembly into fibers with tunable shape and function

Functional supramolecular architectures for bottom-up organic nano- and microtechnology are a high priority research topic. We discovered a new recognition algorithm, resulting from the combination of thioalkyl substituents and head-to-head regiochemistry of substitution, to induce the spontaneous self-assembly of sulfur overrich octathiophenes into supramolecular crystalline fibers combining high charge mobility and intense fluorescence. The fibers were grown on various types of surfaces either as superhelices or straight rods depending on molecular structure. Helical fibers directly grown on a field effect transistor displayed efficient charge mobility and intrinsic 'memory effect'. Despite the fact that the oligomers did not have chirality centers, one type of hand-helicity was always predominant in helical fibers, due to the interplay of molecular atropisomerism and supramolecular helicity induced by terminal substituents. Finally, we found that the new sulfur overrich oligothiophenes can easily be prepared in high yields through ultrasound and microwave assistance in green conditions.     

Synthesis of polypyrrole intercalated into channels of nanoporous metal-organic coordination polymer

A procedure has been developed for the intercalation of pyrrole into nanocavities of a metal-organic coordination polymer, chromium terephthalate MIL-101, and oxidative polymerization of pyrrole has been accomplished. The new composite nanomaterial thus obtained, polypyrrole@MIL-101, has been characterized by the data of elemental analysis, X-ray powder diffraction, thermal analysis, and scanning electron microscopy. Its specific surface area has been estimated at 1700 m²/g according to Brunauer-Emmett-Teller.     

A new calcium trimellitate coordination polymer with a chain-like structure

The calcium trimellitate, Ca(H₂O)[(O₂C)₂–C₆H₃–CO₂H], was hydrothermally synthesized from a mixture of calcium hydroxide, 1,2,4-benzenetricarboxylic (or trimellitic) acid and water at 180 °C for 24 h (under autogenous pressure). Its crystal structure has been determined by single-crystal X-ray diffraction analysis using synchrotron radiation (station 9.8, SRS Daresbury, UK). It consists of infinite chains of calcium bicapped trigonal prismatic polyhedra connected to each other through the 1,2,4-benzenetricarboxylate ligand. The eight-fold coordinated calcium cation is bonded to one terminal water molecule, two carboxylate groups with a chelating conformation and three carboxylate groups in a monodentating mode. One of the monodentate carboxylate is terminal with the occurrence of protonated C–OH bonding.Triclinic space group P-1 with a = 6.9073(4) Å, b = 6.9917(4) Å, c = 10.3561(6) Å, α = 87.178(1)°, β = 83.233(1)°, γ = 69.576(1)°, V = 465.41(5) Å³.     

First direct assembly of molecular helical complexes into a coordination polymer

Luminescent triple-stranded helicates, formed between Tb(iii) ions and bis-acylpyrazolones, were directly assembled into a 1-D polymeric system.