Designing structural motifs for clickamers: exploiting the 1,2,3-triazole moiety to generate conformationally restricted molecular architectures.

Noncovalent interactions, especially hydrogen-bonding interactions as well as electrostatic forces, confined within one macromolecule are the key to designing foldamers that adopt well-defined conformations in solution. In the context of significant recent activities in the area of triazole-connected foldamers, so-called clickamers, we present a fundamental study that compares various model compounds that bear adjacent N-, O-, or F-heteroatom substituents. The interplay of attractive and repulsive interactions leads to rotational constraints around the single bonds attached to both the 1- and 4-positions of the 1,2,3-triazole moiety and should therefore be able to induce well-defined conformational preferences in higher oligomers and polymers, that is, foldamers. Various compounds were synthesized and characterized with regard to their preferred conformations in all three aggregation states--that is, in the gas phase, in solution as well as in the solid state--by employing DFT calculations, NMR spectroscopic experiments, and X-ray crystallography, respectively. On the basis of the thus-obtained general understanding of the conformational behavior of the individual connection motifs, heterostructures were prepared from different motifs without affecting their distinct folding characteristics. Therefore, this work provides a kind of foldamer construction kit, which should enable the design of various clickamers with specific shape and incorporated functionality.

Complexes of click-derived bistriazolylpyridines: remarkable electronic influence of remote substituents on thermodynamic stability as well as electronic and magnetic properties.

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) ligands with substitution patterns ranging from strongly electron-donating to strongly electron-accepting groups, readily prepared by means of Cu-catalyzed 1,3-dipolar cycloaddition (the "click" reaction), were investigated with regard to their complexation behavior, and the properties of the resulting transition-metal compounds were compared. Metal-btp complexes of 1:1 stoichiometry, that is, [Ru(btp)Cl(2)(dmso)] and [Zn(btp)Br(2)], could be isolated and were crystallographically characterized: they display octahedral and trigonal-bipyramidal coordination geometries, respectively, and exhibit high aggregation tendencies due to efficient pi-pi stacking leading to low solubilities. Metal-btp complexes of 1:2 stoichiometry, that is, [Fe(btp)(2)](2+) and [Ru(btp)(2)](2+), could also be synthesized and their metal centers show the expected octahedral coordination spheres. The iron compounds exhibit quite a complex magnetic behavior in the solid state including spin crossover near room temperature, and hysteresis and locking into high-spin states on tempering at 400 K, depending on the substituents on the btp ligands. Cyclic voltammetry studies of [Ru(btp)(2)](2+) reveal strong modulation of the oxidation potentials by more than 0.6 V and a clear linear correlation to the Hammett constant (sigma(para)) of the substituent at the pyridine core. Isothermal titration calorimetry was used to measure the thermodynamics of the Fe(II)-btp complexation process and enabled accurate determination of the complexation enthalpies, which display a linear relationship with the sigma(para) values for the terminal phenyl substituents. Detailed NMR spectroscopic studies finally revealed that in the case of Fe(II) complexation, dynamics are rapid for all investigated btp derivatives in acetonitrile, while replacing Fe(II) by Ru(II) or changing the solvent to dichloromethane effectively slows down ligand exchange. The results nicely demonstrate the utility of substituent parameters, originally developed for linear free-energy relationships to explain reactivity in organic reactions, in coordination chemistry, and to illustrate the potential to custom-design btp ligands and complexes thereof with predictable properties. The fast equilibration of the [Fe(btp)(2)](2+) complexes together with their tunable stability and interesting magnetic properties should enable the design of dynamic metallosupramolecular materials with advantageous properties.

Modulating large-area self-assembly at the solid-liquid interface by pH-mediated conformational switching.

Dynamic surfaces: The conformational transition of 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridine (BTP) derivatives, triggered by a change in pH, has been observed with a sub-nm resolution by STM at the solid-liquid interface. Upon addition of trifluoroacetic acid two different BTP molecules, each forming a highly ordered physisorbed monolayer, underwent significant conformational changes from their "rosette" to their "tetragon" forms, as reflected in dramatically altered 2D self-assembly over large areas extending over hundreds of nanometers (see graphic).

Multifunctional "clickates" as versatile extended heteroaromatic building blocks: efficient synthesis via click chemistry, conformational preferences, and metal coordination.

Click chemistry has been utilized to access 2,6-bis(1-aryl-1,2,3-triazol-4-yl)pyridines (BTPs) as versatile extended heteroaromatic building blocks for their exploitation in supramolecular chemistry, in particular foldamer and ligand design. In addition to their high-yielding synthesis using Cu(I)-catalyzed Huisgen-type 1,3-dipolar cycloaddition reactions the formed triazole moieties constitute an integral part of the BTP framework and encode both its pronounced conformational preferences as well as its chelating ability. A diverse set of symmetrical and non-symmetrical BTPs carrying electron-donating and -withdrawing substituents at both terminal aryl and the central pyridine moieties has efficiently been synthesized and could furthermore readily be postfunctionalized with amphiphilic side chains and porphyrin chromophores. In both solution and solid state, the BTP scaffold adopts a highly conserved horseshoe-like anti-anti conformation. Upon protonation or metal coordination, the BTP scaffold switches to the chelating syn-syn conformation. Iron and europium complexes have been prepared, successfully characterized by single-crystal X-ray diffraction analysis, and investigated with regard to their spin state and luminescent properties. The extended heteroaromatic BTP scaffold should prove useful for the design of responsive foldamer backbones and the preparation of new magnetic and emissive materials.