Building block libraries and structural considerations in the self-assembly of polyoxometalate and polyoxothiometalate systems

Inorganic metal-oxide clusters form a class of compounds that are unique in their topological and electronic versatility and are becoming increasingly more important in a variety of applications. Namely, Polyoxometalates (POMs) have shown an unmatched range of physical properties and the ability to form structures that can bridge several length scales. The formation of these molecular clusters is often ambiguous and is governed by self-assembly processes that limit our ability to rationally design such molecules. However, recent years have shown that by considering new building block principles the design and discovery of novel complex clusters is aiding our understanding of this process. Now with current progress in thiometalate chemistry, specifically polyoxothiometalates (POTM), the field of inorganic molecular clusters has further diversified allowing for the targeted development of molecules with specific functionality. This chapter discusses the main differences between POM and POTM systems and how this affects synthetic methodologies and reactivities. We will illustrate how careful structural considerations can lead to the generation of novel building blocks and further deepen our understanding of complex systems

Carbonate-Templated Self-Assembly of an Alkylthiolate-Bridged Cadmium Macrocycle

In the presence of Cd(ClO4)2 and a base, a new mixed N,S-donor alkylthiolate ligand supported both carbonate formation from atmospheric CO2 and the self-assembly of a novel bicapped puckered (CdS)6 molecular wheel. The remarkable stability of the complex was demonstrated by slow intermolecular ligand exchange on the 2J(HH) and J(111/113Cd1H) time scales at elevated temperature. Both CO2 and the base were required to convert amorphous “CdLClO4” precipitated in the absence of air to the carbonate complex. The complex shares structural features with the ζ-carbonic anhydrase class associating cadmium(II) with the biogeochemical cycling of carbon and is the first structurally characterized carbonate complex of any metal involving an alkylthiolate ligand

Control of metallo-supramolecular assemblies via steric, hydrogen bonding and argentophilic interactions; formation of a 3-dimensional polymer of circular helicates

This work shows how multiple non-covalent interactions are employed to control metallosupramolecular architectures and we demonstrate that a ligand, which contains two bidentate domains separated by a ArOH spacer, forms a mesocate when complexed with Ag(I). However, changing this to an ArOCH2CH2Ph spacer unit results in a 1-dimensional helical polymer upon reaction with the same cation. Reaction of Ag(I) with the ArOMe derivative gives a hexanuclear circular helicate which forms inter-assembly Ag⋯Ag interactions resulting in a 3-dimensional honeycomb-like polymer of hexanuclear circular helicates

Two-stage directed self-assembly of a cyclic [3]catenane.

Interlocked molecules possess properties and functions that depend upon their intricate connectivity. In addition to the topologically trivial rotaxanes, whose structures may be captured by a planar graph, the topologically non-trivial knots and catenanes represent some of chemistry's most challenging synthetic targets because of the three-dimensional assembly necessary for their construction. Here we report the synthesis of a cyclic [3]catenane, which consists of three mutually interpenetrating rings, via an unusual synthetic route. Five distinct building blocks self-assemble into a heteroleptic triangular framework composed of two joined Fe(II)3L3 circular helicates. Subcomponent exchange then enables specific points in the framework to be linked together to generate the cyclic [3]catenane product. Our method represents an advance both in the intricacy of the metal-templated self-assembly procedure and in the use of selective imine exchange to generate a topologically complex product.This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC) and a Marie Curie fellowship for J.J.H. (ITN-2010–264645). The authors thank the Diamond Light Source (UK) for synchrotron beamtime on I19 (MT7984 and MT8464).This is the author accepted manuscript. The final version is available from NPG via http://dx.doi.org/10.1038/nchem.220

An Interconverting Family of Coordination Cages and a meso-Helicate; Effects of Temperature, Concentration, and Solvent on the Product Distribution of a Self-Assembly Process

The self-assembly between a water-soluble bis-bidentate ligand L^18w and Co­(II) salts in water affords three high-spin Co­(II) products: a dinuclear <i>meso</i>-helicate [Co₂(L^18w)₃]­X₄; a tetrahedral cage [Co₄(L^18w)₆]­X₈; and a dodecanuclear truncated-tetrahedral cage [Co₁₂(L^18w)₁₈]­X₂₄ (X = BF₄ or ClO₄). All three products were crystallized under different conditions and structurally characterized. In [Co₂(L^18w)₃]­X₄ all three bridging ligands span a pair of metal ions; in the two larger products, there is a metal ion at each vertex of the Co₄ or Co₁₂ polyhedral cage array with a bridging ligand spanning a pair of metal ions along every edge. All three structural types are known: what is unusual here is the presence of all three from the same reaction. The assemblies <b>Co</b>_<b>2</b>, <b>Co</b>_<b>4</b>, and <b>Co</b>_<b>12</b> are in slow equilibrium (hours/days) in aqueous solution, and this can be conveniently monitored by ¹H NMR spectroscopy because (i) the paramagnetism of Co­(II) disperses the signals over a range of ca. 200 ppm and (ii) the different symmetries of the three species give characteristically different numbers of independent ¹H NMR signals, which makes identification easy. From temperature- and concentration-dependent ¹H NMR studies it is clear that increasing temperature and increasing dilution favors fragmentation to give a larger proportion of the smaller assemblies for entropic reasons. High concentrations and low temperature favor the larger assembly despite the unfavorable entropic and electrostatic factors associated with its formation. We suggest that this arises from the hydrophobic effect: reorganization of several smaller complexes into one larger one results in a smaller proportion of the hydrophobic ligand surface being exposed to water, with a larger proportion of the ligand surface protected in the interior of the assembly. In agreement with this, ¹H NMR spectra in a nonaqueous solvent (MeNO₂) show formation of only [Co₂(L^18w)₃]­X₄ because the driving force for reorganization into larger assemblies is now absent. Thus, we can identify the contributions of temperature, concentration, and solvent on the result of the metal/ligand self-assembly process and have determined the speciation behavior of the <b>Co</b>_<b>2</b>/<b>Co</b>_<b>4</b>/<b>Co</b>_<b>12</b> system in aqueous solution