Metallo-supramolecular modules as a paradigm for materials science

Hydrogen Bonding Directed Self-Assembly of a Binuclear Ag(I) Metallacycle into a 1D Supramolecular Polymer

An Ag(I) metallacycle obtained unexpectedly during the preparation of Pd(II) complexes of the bifunctional ligand 5-([2,2′-bipyridin]-5-yl)pyrimidine-2-amine (L) has been characterized using X-ray structure determination as a binuclear, metallacyclic species [Ag₂L₂](SbF₆)₂, where both the bipyridine and pyrimidine-N donors of L are involved in coordination to the metal. The full coordination environment of the Ag(I) defines a case of highly irregular 4-coordination. In the crystal, the Ag-metallacycles assemble into one-dimensional supramolecular metalladynamers linked together by hydrogen-bonding interactions.

Effect of N-salicylidene hydrazide protonation on the solid state structural diversity of its Cu(ii), Ni(ii) and Zn(ii) complexes

Study of the effect of ligand protonation on the structural diversity of Cu(ii), Ni(ii) and Zn(ii) compounds constructed from a N-salicylidenehydrazide Ligand.

Redox-Responsive Colloidal Particles Based on Coordination Polymers Incorporating Viologen Units

Colloidal particles based on supramolecular polymers are emerging as promising functional materials because of their intrinsic dynamic features and the possibility of stimuli responsivity. In this work, ≈200 nm self-assembled redox-responsive colloidal particles made of 1D-coordination polymers were readily prepared. In these metallopolymers, organic entities made of bis(viologen) groups covalently associated with terpyridine units are spontaneously bridged by Zn²⁺ cations through the formation of coordination bonds. The properties of these particles were analyzed and their redox activities investigated. Upon reduction of the viologen units, the formation of π-dimers between the reduced viologen moieties was demonstrated by spectroscopic experiments. It was shown that intermolecular π-dimers (i.e., between different polymer chains) that do not exist in homogeneous polymer solutions were, nevertheless, formed in the particle's structure because of the effects of confinement. The presence of these π-dimers allows stabilization of the charge in the colloids.

Self-healing, luminescent metallogelation driven by synergistic metallophilic and fluorine-fluorine interactions

Square planar platinum(ii) complexes are attractive building blocks for multifunctional soft materials due to their unique optoelectronic properties. However, for soft materials derived from synthetically simple discrete metal complexes, achieving a combination of optical properties, thermoresponsiveness and excellent mechanical properties is a major challenge. Here, we report the rapid self-recovery of luminescent metallogels derived from platinum(ii) complexes of perfluoroalkyl and alkyl derivatives of terpyridine ligands. Using single crystal X-ray diffraction studies, we show that the presence of synergistic platinum-platinum (PtPt) metallopolymerization and fluorine-fluorine (FF) interactions are the major driving forces in achieving hierarchical superstructures. The resulting bright red gels showed the presence of highly entangled three-dimensional networks and helical nanofibres with both (P and M) handedness. The gels recover up to 87% of their original storage modulus even after several cycles under oscillatory step-strain rheological measurements showing rapid self-healing. The luminescence properties, along with thermo- and mechanoresponsive gelation, provide the potential to utilize synthetically simple discrete complexes in advanced optical materials.

Redox-Triggered Folding of Self-Assembled Coordination Polymers incorporating Viologen Units

We report the study of stimuli-responsive Zn^II and Fe^II coordination polymers (MC3⁴⁺ or MC2⁴⁺ with M=Fe²⁺ or Zn²⁺ ). These soluble metallopolymers were formed spontaneously by reaction of an organic ligand (C3⁴⁺ or C2⁴⁺ ) with one molar equivalent of metal ions. The C3⁴⁺ and C2⁴⁺ ligands incorporate two chelating terpyridine groups bridged by a redox responsive hinge featuring two viologen units (viologen=N,N'-dialkyl-4,4'-bipyridinium) linked either with propyl (C3⁴⁺ ) or ethyl (C2⁴⁺ ) chains. The viologen units in the polymer chains were reduced (1 e^- per viologen group) either by bulk electrolysis or by visible-light irradiation carried out in the presence of a photosensitizer. The 1 e^- reduction of the viologen units in the MC2⁴⁺ polymers induced a slight decrease in the viscosity of the solutions due to a modification of the overall charge carried by the metallopolymers. In strong contrast, reduction of coordination polymers involving propyl linkers (MC3⁴⁺ ) led to a remarkable increase (≈+400 %) in observed viscosity. This reversible effect was attributed to a folding of the polymer chains triggered by π-dimerization of the photo-generated viologen cation radicals.

pH-Induced fragmentation of colloids based on responsive self-assembled copper(ii) metallopolymers

Responsive colloids made from copper(ii) coordination polymers are readily dissolved in acidic medium following a controlled depolymerization of the polymer chains.

Supramolecular reactions of metallo-architectures: Ag2-double-helicate/Zn4-grid, Pb4-grid/Zn4-grid interconversions, and Ag2-double-helicate fusion

Supramolecular reactions are of importance in many fields. We report herein three examples where complexes of hydrazone-based ligands are involved. A Ag2-double-helicate was converted, by treatment with Zn(OTf)2, into a Zn4-grid (exchange of metal ions and change of the nature of the initial complex). A Pb4-grid was converted, upon reaction with ZnCl2 or ZnBr2, into a Zn4-grid (exchange of metal ions, but conservation of the nature of the initial complex). The reverse conversions were also achieved. The fusion of a Ag2-double-helicate with another Ag2-double-helicate was performed (exchange of ligands, but conservation of the nature of the complexes) and resulted in a mixture of three helicates (two homostranded ones and one heterostranded one).

A new high-capacity metal ion-complexing gel containing cyclen ligands

“Hairy” nano-scale objects cross-linked into a high-capacity metal-binding hydrogel.

Generation of a Multicomponent Library of Disulfide Donor-Acceptor Architectures Using Dynamic Combinatorial Chemistry

We describe here the generation of new donor-acceptor disulfide architectures obtained in aqueous solution at physiological pH. The application of a dynamic combinatorial chemistry approach allowed us to generate a large number of new disulfide macrocyclic architectures together with a new type of [2]catenanes consisting of four distinct components. Up to fifteen types of structurally-distinct dynamic architectures have been generated through one-pot disulfide exchange reactions between four thiol-functionalized aqueous components. The distribution of disulfide products formed was found to be strongly dependent on the structural features of the thiol components employed. This work not only constitutes a success in the synthesis of topologically- and morphologically-complex targets, but it may also open new horizons for the use of this methodology in the construction of molecular machines.

Hedgehog-shaped {Mo368} cluster: unique electronic/structural properties, surfactant encapsulation and related self-assembly into vesicles and films

The hedgehog-shaped {Mo368} cluster shows unique electronic (extremely high extinction coefficient) and structural features, especially regarding its size, the high number of delocalized electrons which allows to measure the surface enhanced Raman scattering (SERS) spectrum and the option for coordination chemistry inside the cavity. Its relative instability in aqueous solution can be overcome by embedment in a hydrophobic shell of dimethyldioctadecylammonium cations. The resulting hybrid self-assembles into spherical vesicles in acetone-water mixtures, according to a process directed by hydrophobic-hydrophilic interactions. It also forms rather stable Langmuir monolayers while a second layer evolves under higher surface pressure, in accordance with a rather low alkyl surface density.

Mesophase Transformation in Amphiphilic Hyperbranched Polymers Induced by Transition Metal Ion Complexation. Creating Well-Defined Metallo-Supramolecular Assemblies from "Ill-Defined" Building Blocks

Self-organized metallo-supramolecular heterostructures have potential applications that include molecular electronics, photovoltaics, and magnetic devices, among other examples. The main challenge that scientists typically face when designing advanced supramolecular materials is to achieve structurally defined assemblies by resolving conflicting demands on the topological and/or chemical features of the constituting building blocks. Accordingly, the formation of well-defined metallo-supramolecular arrays using ill-defined, highly polydisperse, self-assemblable starting compounds marks a profound departure from traditional supramolecular paradigms. The present work describes the first observation of spontaneous mesophase transformation of well-defined metallo-supramolecular assemblies in solution as a result of the complexation of transition metal ions into the ionophilic domains of highly branched unimolecular micelles constituted of N-acylated hyperbranched polyethylenimine. Experimental results based on a combination of different synchrotron-based techniques provide unprecedented experimental evidence revealing that ion-induced self-assembly of amphiphilic hyperbranched polymers can be used to achieve highly ordered metallo-supramolecular structures not only in solution but also on solid surfaces. We believe that this emerging conceptual framework can open extremely interesting new synthetic and technological opportunities in the area of self-assembly of well-defined metallo-supramolecular architectures obtained from building blocks with poor structural regularity but easily provided in large quantities by simple and inexpensive preparative chemistries.

Optimized synthesis of a tert-butyl-phenyl-substituted tetrapyridophenazine ligand and its Ru(ii) complexes and determination of dimerization behaviour of the complexes through supramolecular “Fingerhakel”

A high dimerization constant of a ruthenium complex is observed with the aid of ¹H-NMR spectroscopy. The solid state molecular structure indicates that multiple π-interactions are the reason for strong dimerization.

Three-dimensional Fe(II)-based metallo-supramolecular polymers with electrochromic properties of quick switching, large contrast, and high coloration efficiency

A series of Fe(II)-based metallo-supramolecular polymers with three-dimensional (3-D) structures were synthesized by the stepwise complexation of an Fe(II) salt with different ratios of a linear bis(terpyridine) ligand and a branched tris(terpyridine) ligand. Atomic force microscopy images of the polymer films showed a drastic change in the surface morphology upon varying the amount of the branched ligand. The surface of a designed 3-D construction film showed a highly porous structure (pore size: approximately 30-50 nm in diameter), probably due to the formation of a hyperbranched polymer structure. All the 3-D polymers had a blue color based on the metal-to-ligand charge-transfer (MLCT) absorption and exhibited excellent electrochromic properties. The most highly porous 3-D-structured film showed the best electrochromic performance; as compared with a 1-D linear polymer, the switching times were improved 38.7% for the coloring (0.31 → 0.19 s) and 37.9% for the bleaching (0.58 → 0.36 s). The transmittance change (ΔT) increased 21.8% (41.6 → 50.7%). Also, the coloration efficiency (η) was enhanced 45.3% (263.8 → 383.4 cm(2) C(-1)). The redox in the 3-D film was diffusion-controlled, as supported by the linear relationship between the current and square root of the scan rate. It is considered that the porous structure of the 3-D polymer films contributed to smooth ionic transfer during the redox and to the improved electrochromic properties.

Visible light photooxidative performance of a high-nuclearity molecular bismuth vanadium oxide cluster

The visible light photooxidative performance of a new high-nuclearity molecular bismuth vanadium oxide cluster, H3[{Bi(dmso)3}4V13O40], is reported. Photocatalytic activity studies show faster reaction kinetics under anaerobic conditions, suggesting an oxygen-dependent quenching of the photoexcited cluster species. Further mechanistic analysis shows that the reaction proceeds via the intermediate formation of hydroxyl radicals which act as oxidant. Trapping experiments using ethanol as a hydroxyl radical scavenger show significantly decreased photocatalytic substrate oxidation in the presence of EtOH. Photocatalytic performance analyses using monochromatic visible light irradiation show that the quantum efficiency Φ for indigo photooxidation is strongly dependent on the irradiation wavelength, with higher quantum efficiencies being observed at shorter wavelengths (Φ395nm ca. 15%). Recycling tests show that the compound can be employed as homogeneous photooxidation catalyst multiple times without loss of catalytic activity. High turnover numbers (TON ca. 1200) and turnover frequencies up to TOF ca. 3.44 min(-1) are observed, illustrating the practical applicability of the cluster species.

Balance of coordination and hydrophobic interaction in the formation of bilayers in metal-coordinated surfactant mixtures

Metal-ligand coordination and hydrophobic interaction are two significant driving forces in the aggregation of mixtures of M(n+) surfactants and alkyldimethylamine oxide (CnDMAO) in aqueous solutions. The coordinated systems exhibit rich aggregation behavior. This study investigated the effect of M(n+) ions (Zn(2+), Ca(2+), Ba(2+), Al(3+), Fe(3+), La(3+), Eu(3+), and Tb(3+)) and hydrophobic chains (hydrocarbon and fluorocarbon) on the formation of metal-coordinated bilayers. We found that fluorocarbon chains and branched hydrocarbon chains are preferable to the corresponding linear hydrocarbon chains for the formation of an Lα phase. Moreover, Lα phases formed by fluorocarbon chains exhibited higher viscoelasticity than ones formed by the hydrocarbons, and the bilayers formed by branched chains were rather flexible, revealing obvious undulation. The construction of bilayers was also strongly affected by metal ions due to their variable coordination ability with CnDMAO. Our results contribute to the understanding of the formation of metal-coordinated bilayers, which is driven by the interplay of noncovalent forces.

One-Pot Endgroup-Modification of Hydrophobic RAFT Polymers with Cyclodextrin by Thiol-ene Chemistry and the Subsequent Formation of Dynamic Core–Shell Nanoparticles Using Supramolecular Host–Guest Chemistry

Poly(methyl methacrylate) PMMA, synthesized using reversible addition fragmentation chain transfer (RAFT) polymerization, was heated in a solvent at 100°C for 24 h leading to the loss of the RAFT endfunctionality and the complete conversion into a vinyl group. Mono(6-deoxy-6-mercapto)-β-cyclodextrin (β-CD-SH) was subsequently clicked onto the polymer by a thiol-ene reaction leading to PMMA with one β-CD as a terminal group (PMMA70–β-CD). Meanwhile, a RAFT agent with an adamantyl group has been prepared for the polymerization of 2-hydroxyethyl acrylate (HEA) leading to PHEA95–Ada. Two processes were employed to generate core–shell nanoparticles from these two polymers: a one-step approach that employs a solution of both polymers at stoichiometric amounts in DMF, followed by the addition of water, and a two step process that uses PMMA solid particles with surface enriched with β-CD in water, which have a strong tendency to aggregate, followed by the addition of PHEA95–Ada in water. Both pathways led to stable core–shell nanoparticles of ~150 nm in size. Addition of free β-CD competed with the polymer bound β-CD releasing the PHEA hairs from the particle surface. As a result, the PMMA particles started agglomerating resulting in a cloudy solution. A similar effect was observed when heating the solution. Since the equilibrium constant between β-CD and adamantane decreases with increasing temperature, the stabilizing PHEA chains cleaved from the surface and the solution turned cloudy due to the aggregation of the naked PMMA spheres. This process was reversible and with decreasing temperature the core–shell nanoparticles formed again leading to a clear solution.

Insulin analogs for the treatment of diabetes mellitus: therapeutic applications of protein engineering

The engineering of insulin analogs represents a triumph of structure-based protein design. A framework has been provided by structures of insulin hexamers. Containing a zinc-coordinated trimer of dimers, such structures represent a storage form of the active insulin monomer. Initial studies focused on destabilization of subunit interfaces. Because disassembly facilitates capillary absorption, such targeted destabilization enabled development of rapid-acting insulin analogs. Converse efforts were undertaken to stabilize the insulin hexamer and promote higher-order self-assembly within the subcutaneous depot toward the goal of enhanced basal glycemic control with reduced risk of hypoglycemia. Current products either operate through isoelectric precipitation (insulin glargine, the active component of Lantus(®); Sanofi-Aventis) or employ an albumin-binding acyl tether (insulin detemir, the active component of Levemir(®); Novo-Nordisk). To further improve pharmacokinetic properties, modified approaches are presently under investigation. Novel strategies have recently been proposed based on subcutaneous supramolecular assembly coupled to (a) large-scale allosteric reorganization of the insulin hexamer (the TR transition), (b) pH-dependent binding of zinc ions to engineered His-X(3)-His sites at hexamer surfaces, or (c) the long-range vision of glucose-responsive polymers for regulated hormone release. Such designs share with wild-type insulin and current insulin products a susceptibility to degradation above room temperature, and so their delivery, storage, and use require the infrastructure of an affluent society. Given the global dimensions of the therapeutic supply chain, we envisage that concurrent engineering of ultra-stable protein analog formulations would benefit underprivileged patients in the developing world.

From an eight-component self-sorting algorithm to a trisheterometallic scalene triangle

Using motifs from 3-fold completive self-sorting in an eight-component library, we report on the design and fabrication of a fully dynamic trisheterometallic scalene triangle, a demanding supramolecular structure that complements the so far known triangular structures.

Cis-trans switchable metallosupramolecular polymers: computer modeling

Using computer simulations we study linear oligomers end functionalized with ligands that can form trans- or cis-2:1 complexes with metal ions in a salt-screened good solvent. We show that trans-cis isomerization of ligand-metal complexes can significantly increase the average molecular weight as well as trigger formation of reversible metallosupramolecular network based on 3:1 ligand-metal complexes acting as cross-linkers. We predict the conditions under which the most dramatic changes in the properties of metallosupramolecular polymers, such as network formation or increase in elastic plateau modulus of the network, occur upon isomerization.

Metastability and sol phases: two keys for the future of molecular gels?

The future of molecular gels may rely on a deeper theoretical understanding of the gel-to-solid phase-separation process that usually limits the lifetime of molecular gels. Stable sol phases, when available, can be the appropriate vectors to transfer the 1D self-assembled and functional morphologies to solid substrates that may be used for technological devices.

From specular reflectivity to time-resolved grazing incidence X-ray scattering out of the specular plane (GISAXS): equilibrium and nonequilibrium states of organic/inorganic monolayers at liquid surfaces

The grazing incidence diffuse X-ray scattering out of the specular plane technique (GISAXS) is presented, and its capabilities are compared to the more classical X-ray specular reflectivity technique (XSR). Three experimental illustrations are given to prove the efficiency of GISAXS. First, the structure of a DPPC phospholipid monolayer is analyzed. Second, the time-resolved kinetics of lipid desorption from a monolayer interacting with a mineral gel subphase is studied. Third, the structural evolution of biomembranes at extremely low temperatures is illustrated. The GISAXS technique appears to compete efficiently with neutron reflection techniques by taking advantage of remarkably short acquisition times crucial in kinetics experiments.