Supramolecular Square Research Articles

Titanium‐Based Molecular Squares and Rectangles: Syntheses by Self‐Assembly Reactions of Titanocene Fragments and Aromatic N‐Heterocycles

This paper reports on the potential of titanium compounds as building blocks for supramolecular polygons. Self-assembly reactions of low-valent titanocene units and N-heterocyclic bridging ligands lead to novel titanium-based supramolecular squares. Pyrazine (3), 4,4'-bipyridine (4), and tetrazine (5) were used as bridging ligands, and the acetylene complexes [Cp2Ti{eta2-C2(SiMe3)2}] (1) and [(tBuCp)2Ti{eta2-C2(SiMe3)2}] (2) as sources of titanocene fragments. Molecular rectangles can be synthesized by stepwise reduction of the titanocene dichlorides [Cp(2)TiCl2] and [(tBuCp)2TiCl2] and consecutive coordination of two different bridging ligands. The resulting complexes are the first examples of molecular rectangles containing bent metallocene corner units. Single-crystal X-ray analyses of the tetranuclear compounds revealed the geometric properties of the molecular polygons in the solid state. Comparison of bond lengths and angles in coordinated and free ligands reveals the reduced state of the bridging ligand in the low-valent titanium compounds. The syntheses and properties of these novel, highly air- and moisture-sensitive compounds are discussed.

Supramolecular squares of porphyrazines

The design, self-assembly, and characterization of discrete arrays of porphyrazine (Pz) squares are presented to illustrate the supramolecular chemistry of these macrocycles, and to highlight that these arrays have physical chemical properties that are different from similar arrays of porphyrinic systems.

Walljet Electrochemistry: Quantifying Molecular Transport through Metallopolymeric and Zirconium Phosphonate Assembled Porphyrin Square Thin Films

By employing redox-active probes, condensed-phase molecular transport through nanoporous thin films can often be measured electrochemically. Certain kinds of electrode materials (e.g. conductive glass) are difficult to fabricate as rotatable disks or as ultramicroelectrodes-the configurations most often used for electrochemical permeation measurements. These limitations point to the need for a more materials-general measurement method. Herein, we report the application of walljet electrochemistry to the study of molecular transport through model metallopolymeric films on indium tin oxide electrodes. A quantitative expression is presented that describes the transport-limited current at the walljet electrode in terms of mass transport through solution and permeation through the film phase. A comparison of the film permeabilities for a series of redox probes measured using the walljet electrode and a rotating disk electrode establishes the accuracy of the walljet method, while also demonstrating similar precision for the two methods. We apply this technique to a system consisting of zirconium phosphonate assembled films of a porphyrinic molecular square. Transport through films comprising three or more layers is free from significant contributions from pinhole defects. Surprisingly, transport through films of this kind is 2-3 orders of magnitude slower than through films constructed via interfacial polymerization of nearly identical supramolecular square building blocks (Keefe; et al. Adv. Mater. 2003, 15, 1936). The zirconium phosphate assembled films show good size exclusion behavior. The details of the observed dependence of permeation rates on probe molecule size can be rationalized with a model that assumes that the walls of the squares are slightly tilted from a strictly vertical geometry, consistent with atomic force microscopy measurements, and assumes that the individual wall geometries are locked by rigid interlayer linkages.

Novel Supramolecular Square Grid-Like Structure: Synthesis and Characterization of ((Me 3 Sn) 2 CrO 4 )

Reactions of K2Cr2O7 and K2CrO4 with Me3SnCl yielded [(Me3Sn)2CrO4] (1) and [(Me3Sn)2CrO4⋅(Me3SnOH)] (2), respectively, which were characterized by elemental analysis, 1H-NMR spectroscopy, and an X-ray diffraction study. Compound 1 shows a novel square grid-like structure consisting of CrO42− and Me3Sn+ ions, whereas 2 exhibits a three-dimensional structure composed of CrO42−, Me3Sn+, and [(Me3Sn)2SnOH]+ ions.

Mass spectrometric characterization and gas-phase chemistry of self-assembling supramolecular squares and triangles.

A detailed mass spectrometric characterization of self-assembling polynuclear metal complexes is described. The complexes can only be ionized as intact species under a surprisingly narrow range of conditions by electrospray ionization. Comparison with the results from NMR experiments shows that several solution-phase features of these squares and triangles (such as trends in bond energies, ligand-exchange reactions, or square-triangle equilibria) are qualitatively reflected in the gas-phase data. Consequently, mass spectrometry represents a valuable method for the characterization of these compounds. Nevertheless, the formation of unspecific aggregates during the ionization process occurs and its implications are discussed. Beyond the chemistry in solution, the fragmentation pathways of these complexes in the gas phase have been studied by infrared multiphoton dissociation (IRMPD) experiments. The results of IRMPD studies allow us to draw conclusions with respect to the structure and energetics of fragmentation products. In this tandem MS experiment, reaction pathways can be observed directly which can hardly be analyzed in solution. According to these results, the equilibration of triangles and squares involves the supramolecular analogue of a neighboring-group effect.

Artificial Enzymes Formed through Directed Assembly of Molecular Square Encapsulated Epoxidation Catalysts.

Supramolecular complex formation imparts stability and substrate selectivity to a simple manganese-porphyrin-based epoxidation catalyst. Lewis acid/Lewis base directed assembly was used to encapsulate the epoxidation catalyst within a supramolecular square structure to form an enzyme-like catalyst system. The "molecular square" takes on the functions of the protein superstructure in an enzyme: it spatially protects the catalytic core from decomposition and induces reaction selectivity.

Supramolecular squares with Rh24+ corners

Six molecular squares were obtained by reacting the singly metal–metal-bonded complex [Rh2(cis-DAniF)2(CH3CNeq)4(CH3CNax)2](BF4)2 (DAniF = N,N′-di-p-anisylformamidinate), and (Et4N+)2(Carb2−) where Carb2− represents the dicarboxylate anion bicyclo[1.1.1]pentane-1,3-dicarboxylate, 1; tetrafluoroterephthalate, 2; 1,4-cubanedicarboxylate, 3; terephthalate, 4; fumarate, 5; trans-1,4-cyclohexanedicarboxylate, 6; these squares stack in the crystal forming square channels capable of accommodating solvent molecules and the NMR spectra are consistent with the presence of highly symmetrical structures even in solution.

Simple routes to supramolecular squares with ligand corners:1:1 AgI:pyrimidine cationic tetranuclear assemblies

Simple, tetranuclear cationic supramolecular squares with ligands at the corners can be constructed using the geometry of the pyrimidine ligand and the directional tendencies of AgI coordination; the 1:1 AgI:pyrimidine squares have an open channel structure which facilitates selective ion exchange of pertechnetate over nitrate.