Supramolecular assemblies based on organometallic quinonoid linkers: a new class of coordination networks

Self-assembly of coordination frameworks exhibiting original architectures is an active area of research. Generally, such assemblies are constructed from organic spacers and transition metals of different geometrical structures. Herein, we report a novel class of supramolecular coordination assemblies with organometallic linkers based on metalated quinonoid and thioquinonoid complexes that serve as spacers. The organometallic ligands are stable and have the general formula [Cp*M(eta(4)-benzoquinone)] (o- and p-benzoquinone, Cp*=C(5)Me(5), M=Rh, Ir) and [Cp*Ir(eta(4)-thiobenzoquinone)] (o- and p-thiobenzoquinone). These units bind through both oxygen or sulfur atoms to metal ions of different coordination geometry, such as Cu(I), Ag(I), and Pt(II), to generate supramolecular coordination networks, with the metalated quinonoid or thioquinonoid linkers acting as backbones and the metal centers as nodes. This novel family of supramolecular assemblies exhibits short pi-pi and MM interactions. These results illustrate successfully the role of the organometallic linkers to produce an impressive range of novel supramolecular architectures that hold promise for the development of functional materials.     

Characterization of multimetallic grid-type complexes by electrospray mass spectrometry

The self-assembly of the terdentate ligands 1a-h, based on terpyridine-like binding sites, with octahedrally coordinated metal ions, such as Fe(II), Co(II), Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II), leads to the formation of the supramolecular grid-type complexes 2a-c(M(II)), 3d-g(M(II)) and 4h(M(II)). The structures and compositions of these coordination complexes in solution were deduced from electrospray mass spectrometry (ESMS) measurements. The results agree with the data available from x-ray radiocrystallography in the solid state and/or NMR spectroscopy in solution. ESMS may be applied in cases where other methods are difficult to use or inconclusive. This study stresses the power of ESMS in supramolecular chemistry.     

Binding sites on the outside of metallo-supramolecular architectures; engineering coordination polymers from discrete architectures

Aggregation of metallo-supramolecular architectures through additional coordination is explored by introducing metal-binding units onto the outside of the supramolecular architectures. This is achieved within the framework of our imine-based approach to supramolecular architecture, by replacing the pyridylimine units with pyrazylketimine units. An advantage of the design is that it retains the ease-of-synthesis which characterises our imine-based approach. Silver(I) complexes of three pyrazylketimine ligand systems are described. The complexes demonstrate that introducing pyrazine donor units does indeed allow higher-order assembly of the distinct supramolecular architectures into engineered coordination polymers. Two distinct types of aggregation are observed. In the first, the donors on the outside of one architecture bind to the metals of another to link the units into a polymeric array. In the second type, the donors on the outside of the architectures bind to separate metal centres which are themselves not part of the architectures, and these separate metal centres link the units to form the macromolecular array. The weaker donor nature of the pyrazine nitrogens (compared to pyridine) also introduces an additional element into the design; higher coordination numbers are favoured and this can lead to arrays with higher connectivity than those observed in the discrete pyridylimine architectures.     

First systematic investigation of C-H...Cl hydrogen bonding using inorganic supramolecular synthons: lamellar, stitched stair-case, linked-ladder, and helical structures

Using a group of six neutral M(II)Cl(2)-containing coordination compounds as building blocks, the first systematic investigation of C-H...Cl hydrogen-bonding interactions was performed. Single-crystal X-ray structural analyses of four new compounds (pseudo-tetrahedral Co(II) and Zn(II); distorted trigonal bipyramidal Zn(II)) authenticate the metal coordination geometry. To provide a unified view of the presence of noncovalent interactions in this class of compounds, we have re-examined the packing diagram of two previously reported compounds (a distorted square-pyramidal Cu(II) complex and a trans-octahedral Co(II) complex). The organic ligands of our choice comprise bidentate/tridentate pyrazolylmethylpyridines and an unsymmetrical tridentate pyridylalkylamine. This systematic investigation has allowed us to demonstrate the existence of versatile C-H...Cl(2)M interactions and to report the successful application of such units as inorganic supramolecular synthons. Additional noncovalent interactions such as C-H...O and O-H...Cl hydrogen bonding and pi-pi stacking interactions have also been identified. Formation of novel supramolecular architectures has been revealed: 2D lamellar (p-cyclophane) and 3D lamellar, 3D "stitched staircase" (due to additional hydrogen-bonding interactions by water tetramers, with an average O-O bond length in the tetramer unit of 2.926 A, acting as "molecular clips" between staircases), 3D linked ladder, and single-stranded 1D helix.     

Intra- and intermolecular second-sphere coordination chemistry: formation of capsules,half-capsules,and extended structures with hexaaquo- and hexaamminemetal ions

In the design of novel extended solids, particularly those based on weaker interactions, reliable "synthons" are a valuable commodity. This work concerns the hydrogen-bonded assemblies which result from the second-sphere coordination interactions between a highly preorganized trisulfonate ligand and hexaaquo metal ions. Significantly, supramolecular structural variation, which may be rationalized on the basis of the features of the molecular building blocks, is observed. The results are formation of second-sphere capsules with trivalent ions (Fe(3+), Cr(3+), Al(3+)), and half-capsules with divalent ions (Mg(2+), Zn(2+)). The divalent systems further assemble into extensively hydrogen-bonded hexagonal nets. Effects of geometrical variation of the building blocks are also observed when a Jahn-Teller-distorted divalent ion (Cu(2+)) is substituted for the perfectly octahedral species. The second-sphere effects on the stabilization of the primary coordination sphere are illustrated by TGA experiments. In these assemblies, the potential of a new supramolecular synthon is illustrated, that being the complementary cis-aquo sulfonate interaction. These complexes illustrate the general utility of second-sphere effects, both as an assembly tool and to stabilize metal complexes in the solid state. Finally, as a comparison, a hydrogen-bonded assembly with a hexaammine complex of a trivalent metal (Co(3+)) is presented, which forms an extended network with a completely altered hydrogen bonding array.     

Reversible supramolecular functionalization of surfaces: terpyridine ligands as versatile building blocks for noncovalent architectures

We report on the reversible and selective functionalization of surfaces by utilizing supramolecular building blocks. The reversible formation of terpyridine bis-complexes, based on a terpyridine ligand-functionalized monolayer, is used as a versatile supramolecular binding motif. Thereby, click chemistry was applied to covalently bind an acetylene functionalized Fe(II) bis-complex onto azide-terminated self-assembled monolayers. By decomplexation of the formed supramolecular complex, the ligand modified monolayer could be obtained. These monolayers were subsequently used for additional complexation reactions, resulting in the reversible functionalization of the substrates. The proper choice of the coordinating transition metal ions allows the tuning of the binding strength, as well as the physicochemical properties of the formed complexes and thus an engineering of the surface properties.     

Vom einfachen Komplex zum komplexen Gitter: Metallo‐supramolekulare Chemie

Supramolecular structures and metal‐complexes play a dominant role in the functionality of biomolecules. Taking nature as an example a major goal of metallo‐supramolecular chemistry is the extension of the traditional coordination chemistry towards supramolecular architectures, utilizing complex ligand systems. Herein we describe a wide range of different geometries such as helicates, linear rod‐like polymers, ladders, racks or grids, which are realized by the combination of supramolecular ligands and coordinating metal ions on the basis of self‐assembly and self‐recognition processes. Besides the pure beauty of the structures, the electro‐, photochemical and magnetic properties of the materials might open avenues to applications as smart coatings, catalysts or optical devices.     

Structural modulation of Cd(II) supramolecular frameworks with a versatile 2,4-dipyridyl-type building block and different dicarboxylate ligands

A series of Cd(II) coordination complexes with an elongated 2,4-dipyridyl-type building block trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene (bpe) and different dicarboxylate ligands have been presented. Generally, bpe shows the unidentate coordination mode and serves as the terminal pendant, whereas the dicarboxylate ligands display various binding fashions to interlink the metal centers to form 1-D comb-like chain, ribbon, and fishbone arrays, as well as 2-D 4⁴ and 4.8² layered networks. Notably, the bpe building block is also involved in secondary interactions such as hydrogen bonding and aromatic stacking to construct the resulting supramolecular architectures. Thermal stability of these complexes has been studied by TG-DTA technique.     

Self‐Assembly through Coordination and π‐Stacking: Controlled Switching of Circularly Polarized Luminescence

Multiple noncovalent interactions can drive self‐assembly through different pathways. Here, by coordination‐assisted changes in π‐stacking modes between chromophores in pyrene‐conjugated histidine (PyHis), a self‐assembly system with reversible and inversed switching of supramolecular chirality, as well as circularly polarized luminescence (CPL) is described. It was found that l ‐PyHis self‐assembled into nanofibers showing P‐chirality and right‐handed CPL. Upon Zn^II coordination, the nanofibers changed into nanospheres with M‐chirality, as well as left‐handed CPL. The process is reversible and the M‐chirality can change to P‐chirality by removing the Zn^II ions. Experimental and theoretical models unequivocally revealed that the cooperation of metal coordination and π‐stacking modes are responsible the reversible switching of supramolecular chirality. This work not only provides insight into how multiple noncovalent interactions regulate self‐assembly pathways.     

Hierarchical self-assembly of supramolecular spintronic modules into 1D- and 2D-architectures with emergence of magnetic properties

Hierarchical self-assembly of complex supramolecular architectures allows for the emergence of novel properties at each level of complexity. The reaction of the ligand components A and B with Fe(II) cations generates the [2x2] grid-type functional building modules 1 and 2, presenting spin-transition properties and preorganizing an array of coordination sites that sets the stage for a second assembly step. Indeed, binding of La(III) ions to 1 and of Ag(I) ions to 2 leads to a 1D columnar superstructure 3 and to a wall-like 2D layer 4, respectively, with concomitant modulation of the magnetic properties of 1 and 2. Thus, to each of the two levels of structural complexity generated by the two sequential self-assembly steps corresponds the emergence of novel functional features.     

How to Make Weak Noncovalent Interactions Stronger

By employing noncovalent interactions, chemists have constructed a variety of molecular aggregates with well‐defined structures and fascinating properties. In fabricating stable and large molecular assemblies, noncovalent interactions with high binding strength are needed. This Concept summarizes some strategies to modify and optimize the structures of building blocks for making weak noncovalent interactions stronger. The strategies include: 1) Preorganization of binding sites; 2) spatial confinement effects; 3) multivalent enhancement; 4) synergistic binding with multiple forces. Examples of the fabrication of supramolecular architectures by utilizing these strategies are presented and discussed. Guidance is offered in the construction and fabrication of stable molecular assemblies and supramolecular materials.     

Synthesis, structure, and stereochemistry of trinuclear metal complexes formed from the phosphorus-based achiral tripodal ligand {P(S)[N(Me)N=CHC6H4-o-OH]3} (LH3): luminescent properties of L2Cd3 x 2H2O

Neutral trinuclear metal complexes L2Cd3 x 2H2O, L2Mn3 x MeOH, and L2Zn3 x MeOH were isolated in the reaction between the phosphorus-centered achiral tris(hydrazone) P(S)[N(Me)N=CHC6H(4)-o-OH]3 (LH3) and the corresponding divalent metal ions. The trinuclear complexes contain two equivalent terminal metal ions (M(t)) and a central metal ion (M(c)). The ligand encapsulates M(t) in a facial N3O3 coordination environment. From the coordination sphere of the two terminal metal ions a pair of phenolic oxygen atoms further coordinate to the central metal ion. The coordination requirements of M(c) are completed by the solvents of coordination. The achiral trianionic tripodal ligand (L)3- induces chirality in the metal complexes. This results in a delta (clockwise) or lambda (anticlockwise) configuration for the terminal metal ions. The enantiomeric complexes 2-4 (delta-delta or lambda-lambda) crystallize as racemic compounds. The supramolecular structures of 2-4 reveal chiral recognition in the solid-state; every molecule with the delta-delta configuration interacts stereospecifically, through C-H...S=P bonds, with two lambda-lambda molecules to generate a one-dimensional polymeric chain. Photophysical studies of the diamagnetic trinuclear complexes reveal that the tricadmium complex is luminescent in the solid state as well as in solution. In contrast LH3 and L2Zn3 x MeOH are nonluminescent.     

Solvent influence on the orthogonality of noncovalently functionalized terpolymers

Poly(norbornene) terpolymers containing palladated sulfur‐carbon‐sulfur (SCS) pincer complexes, cyanuric acid, and thymine moieties in their side‐chains were synthesized by ring‐opening metathesis polymerization. Functionalization of the terpolymers was achieved by self‐assembling (i) the Hamilton wedge to the cyanuric acid receptor, (ii) diaminopyridine to the thymine receptor, and (iii) pyridine to the palladated pincer complexes. While all three noncovalent interactions are fully orthogonal to each other in dichloromethane, the employment of a dioxane/chloroform solvent mixture results in the quantitative disassembly of one of the hydrogen bonding recognition units (the Hamilton wedge:cyanuric acid pair) during the metal‐coordination event. This disassembly is completely independent from the diaminopyridine: thymine hydrogen‐bonding pair and allows for the selective removal of one of the side‐chain functionalities. This removal occurs with a switch‐type mechanism: as one functionality is put on (the pyridine), another one (the Hamilton‐wedge receptor) is taken off quantitatively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 1936–1944, 2008     

从超分子聚合物到超分子有机框架:组装溶液相超分子结构的周期性

单体分子在溶液相自发形成周期性的网络结构,是超分子化学和分子自组装研究领域的重大挑战.多头基分子在溶液相通过分子间非共价键作用可以形成超分子聚合物.提高多头基(三头基和四头基)分子骨架的刚性,可以提高结合位点的结构预组织,进而增强分子间相互作用的协同性和多价性特征,提高自组装结构的有序性或周期性.本文综述了多头基分子自组装形成超分子聚合物的一些重要进展,介绍了二维超分子有机框架(一类新的溶液相周期性自组装网络结构研究的最新进展.     

A survey of interactions in crystal structures of pyrazine‐based compounds

The important role of pyrazine (pz) and its derivatives in fields such as biochemistry and pharmacology, as well as in the study of magnetic properties, is surveyed. Recognition of these properties without extensive investigations into their structural properties is not possible. This review summarizes interactions that exist between these organic compounds by themselves in the solid state, as well as those in coordination polymers with metal ions and in polyoxometalate‐based hybrids. Complexes based on pyrazine ligands can generate metal–organic framework (MOF) structures that bind polyoxometalates (POMs) through covalent and noncovalent interactions. Some biological and magnetic properties involving these compounds are considered and the effect of hydrogen bonding on their supramolecular architectures is highlighted.     

Porphyrin framework solids. Synthesis and structure of hybrid coordination polymers of tetra(carboxyphenyl)porphyrins and lanthanide-bridging ions

New types of porphyrin-based framework solids were constructed by reacting meso-tetra(3-carboxyphenyl)porphyrin and meso-tetra(4-carboxyphenyl)metalloporphyrins with common salts of lanthanide metal ions. The large size, high coordination numbers and strong affinity for oxo ligands of the latter, combined with favorable hydrothermal reaction conditions, allowed the formation of open three-dimensional single-framework architectures by coordination polymerization, in which the tetradentate porphyrin units are intercoordinated by multinuclear assemblies of the bridging metal ions. The latter serve as construction pillars of the supramolecular arrays, affording stable structures. Several modes of coordination polymerization were revealed by single-crystal X-ray diffraction. They differ by the spatial functionality of the porphyrin building blocks, the coordination patterns of the lanthanide-carboxylate assemblies, and the topology of the resulting frameworks. The seven new reported structures exhibit periodically spaced 0.4-0.6 nm wide channel voids that perforate the respective crystalline polymeric architectures and are accessible to solvent components. Materials based on the m-carboxyphenyl derivative reveal smaller channels than those based on the p-carboxyphenyl analogues. An additional complex of the former with a smaller third-row transition metal (Co) is characterized by coordination connectivity in two dimensions only. Thermal and powder-diffraction analyses confirm the stability of the lanthanide-TmCPP (TmCPP=tetra(m-carboxyphenyl)porphyrin) frameworks.     

Acyclic oligopyrroles as building blocks of supramolecular assemblies

The supramolecular chemistry of acyclic oligopyrrole derivatives mainly reported by the author’s group in the last four years has been summarized in this review. The author has demonstrated the “first step” to construct the new materials and concepts based on the new molecular systems consisting of pyrrole rings, which form the complexes, assemblies, and organized structures, by using noncovalent interactions such as metal coordination, hydrogen bonding, and π–π interaction. Acyclic π-conjugated oligopyrroles have exhibited not only host–guest binding behaviors in solutions but also the formation of, for example, (i) metal coordination polymers to give emissive colloidal spheres, (ii) solid-state assemblies of acyclic π-conjugated anion receptors and their anion complexes, (iii) anion-responsive supramolecular gels from the receptors with aliphatic chains, and (iv) solvent-assisted organized structures like vesicles derived from amphiphilic anion receptors.     

Formation of 1 D and 3 D coordination polymers in the solid state induced by mechanochemical and annealing treatments: bis(3-cyano-pentane-2,4-dionato) metal complexes

Bis(3-cyano-pentane-2,4-dionato) (CNacac) metal complex, [M(CNacac)(2)], which acts as both a metal-ion-like and a ligand-like building unit, forms supramolecular structures by self-assembly. Co-grinding of the metal acetates of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) with CNacacH formed a CNacac complex in all cases: mononuclear complex was formed in the cases of Mn(II), Cu(II) and Zn(II), whereas polymeric ones were formed in the cases of Fe(II), Co(II) and Ni(II). Subsequent annealing converted the mononuclear complexes of Mn(II), Cu(II) and Zn(II) to their corresponding polymers as a result of dehydration of the mononuclear complexes. The resultant Mn(II), Fe(II), Co(II), Ni(II) and Zn(II) polymeric complexes had a common 3 D structure with high thermal stability. In the case of Cu(II), a 1 D polymer was obtained. The Mn(II), Cu(II) and Zn(II) polymeric complexes returned to their original mononuclear complexes on exposure to water vapour but they reverted to the polymeric complexes by re-annealing. Co-grinding of metal chlorides with CNacacH and annealing of the mononuclear CNacac complexes prepared from solution reactions were also examined for comparison. [Mn(CNacac)(2)(H(2)O)(2)], [M(CNacac)(2)(H(2)O)] (M=Cu(II) and Zn(II)) and [M(CNacac)(2)](infinity) (M=Mn(II), Fe(II) and Zn(II)) are new compounds, which clearly indicated the power of the combined mechanochemical/annealing method for the synthesis of varied metal coordination complexes.     

Crystal assembly based on 3,5-bis(2′-benzimidazole) pyridine and its complexes

Imdazole, pyridine and their derivatives have been considered as excellent ligands in supramolecular self-assembly. In this paper, a ligand molecule 3,5-bis (2′-benzimidazole) pyridine (BBP) was prepared, and two different crystal architectures based on the ligand molecule were self-assembled by diffusion/solvothermal ways. Furthermore, several crystal architectures of several relative complexes were also successfully assembled. These crystal structures were well defined by X-ray diffractions. Structural resolutions indicated that, as building blocks, this bibenzimidazole pyridine molecules exhibited coordination varieties in constructing the crystal architectures based on its related complexes.     

Beyond Molecules: Mesoporous Supramolecular Frameworks Self‐Assembled from Coordination Cages and Inorganic Anions

Biological function arises by the assembly of individual biomolecular modules into large aggregations or highly complex architectures. A similar strategy is adopted in supramolecular chemistry to assemble complex and highly ordered structures with advanced functions from simple components. Here we report a series of diamond‐like supramolecular frameworks featuring mesoporous cavities, which are assembled from metal‐imidazolate coordination cages and various anions. Small components (metal ions, amines, aldehydes, and anions) are assembled into the hierarchical complex structures through multiple interactions including covalent bonds, dative bonds, and weak C? H???X (X=O, F, and π) hydrogen bonds. The mesoporous cavities are large enough to trap organic dye molecules, coordination cages, and vitamin B₁₂. The study is expected to inspire new types of crystalline supramolecular framework materials based on coordination motifs and inorganic ions.