Examining the Scope and Thermodynamics of Assembly in Nesting Complexes Comprising Molecular Baskets and TPA Ligands.

Molecular baskets capture various tris(2-pyridylmethyl)amine ligands, with and without zinc(II) cation, to form nesting complexes. The results of our computational (MD) and experimental (1H NMR/ITC) studies suggest that the assembly is driven by the hydrophobic effect with the charge of complementary molecular components playing an important role in the formation of nesting complexes. In brief, the complexation only takes place when the basket and the ligand carry either oppositely charged or noncharged groups.

Anion-Redox Mechanism of MoO(S2)2(2,2'-bipyridine) for Electrocatalytic Hydrogen Production.

Redox processes of molybdenum-sulfide (Mo-S) compounds are important in the function of materials for various applications from electrocatalysts for the hydrogen evolution reaction (HER) to cathode materials for batteries. Our group has recently described a series of Mo-S molecular HER catalysts based on a MoO(S2)2L2 structural motif. Herein, reductive pathways of MoO(S2)2bpy (Mo-bpy) (bpy = 2,2'-bipyridine) are presented from both experimental and theoretical studies. We tracked chemical reduction of Mo-bpy with UV-vis spectroscopy using sodium napthalenide (NaNpth) as the reducing agent and found that Mo-bpy undergoes anionic persulfide reduction to form the tetragonal Mo(VI) complex [MoOS3]2-. We also identified silver mercury amalgam as an inert working electrode (WE) for spectroectrochemical (SEC) studies. UV-vis spectra in the presence of trifluoroacetic acid with an applied potential confirmed that Mo-bpy maintains its structure during catalytic cycling. Finally, theoretical catalytic reaction pathways were explored, revealing that Mo=O may function as a proton relay. This finding together with the observed anion reduction as the redox center is of broad interest for amorphous Mo-S (a-MoSx) electrocatalytic materials and anion-redox chalcogel battery materials.

Assembly and Folding of Twisted Baskets in Organic Solvents.

A synthetic method for obtaining enantiopure and twisted baskets of type (P)-3 is described. These chiral cavitands were found to fold quinoline gates, at the rim of their twisted platform, in acetonitrile and give molecular capsules that assemble into large unilamellar vesicles. In a less polar dichloromethane, however, cup-shaped (P)-3 packed into vesicles but with the quinoline gates in an unfolded orientation. The ability of twisted baskets to form functional nanostructured materials could be of interest for building stereoselective sensors and catalysts.

Two-Dimensional Supramolecular Polymers Embodying Large Unilamellar Vesicles in Water.

We hereby describe a strategy for obtaining novel topological nanostructures consisting of dual-cavity basket 1, forming a curved monolayer of large unilamellar vesicles in water (CAC < 0.25 µM), and bivalent guests 4/5 populating the cavities of such bolaamphiphilic hosts. On the basis of the results of (1)H NMR spectroscopy, electron microscopy, and dynamic light scattering measurements, we postulated that divalent guest molecules 4/5 cover the curved vesicular surface in a lateral fashion to satisfy the complexation [2 + 2] valency and thereby give stable two-dimensional supramolecular polymers [1⊂4]n and [1⊂5]n. The results of experimental studies are also supported with coarse-grained molecular dynamics simulations and molecular mechanics. Our discovery about the assembly of novel vesicular structures could be of interest for stabilization/functionalization of liposomal surfaces as well as detection of polyvalent molecules and removal of targeted substances from aqueous environments.

Russian Nesting Doll Complexes of Molecular Baskets and Zinc Containing TPA Ligands.

In this study, we examined the structural and electronic complementarities of convex 1-Zn(II), comprising functionalized tris(2-pyridylmethyl)amine (TPA) ligand, and concave baskets 2 and 3, having glycine and (S)-alanine amino acids at the rim. With the assistance of (1)H NMR spectroscopy and mass spectrometry, we found that basket 2 would entrap 1-Zn(II) in water to give equimolar 1-Zn⊂2in complex (K = (2.0 ± 0.2) × 10(3) M(-1)) resembling Russian nesting dolls. Moreover, C3 symmetric and enantiopure basket 3, containing (S)-alanine groups at the rim, was found to transfer its static chirality to entrapped 1-Zn(II) and, via intermolecular ionic contacts, twist the ligand's pyridine rings into a left-handed (M) propeller (circular dichroism spectroscopy). With molecular baskets embodying the second coordination sphere about metal-containing TPAs, the here described findings should be useful for extending the catalytic function and chiral discrimination capability of TPAs.

Tunable Molecular MoS2 Edge-Site Mimics for Catalytic Hydrogen Production.

Molybdenum sulfides represent state-of-the-art, non-platinum electrocatalysts for the hydrogen evolution reaction (HER). According to the Sabatier principle, the hydrogen binding strength to the edge active sites should be neither too strong nor too weak. Therefore, it is of interest to develop a molecular motif that mimics the catalytic sites structurally and possesses tunable electronic properties that influence the hydrogen binding strength. Furthermore, molecular mimics will be important for providing mechanistic insight toward the HER with molybdenum sulfide catalysts. In this work, a modular method to tune the catalytic properties of the S-S bond in MoO(S2)2L2 complexes is described. We studied the homogeneous electrocatalytic hydrogen production performance metrics of three catalysts with different bipyridine substitutions. By varying the electron-donating abilities, we present the first demonstration of using the ligand to tune the catalytic properties of the S-S bond in molecular MoS2 edge-site mimics. This work can shed light on the relationship between the structure and electrocatalytic activity of molecular MoS2 catalysts and thus is of broad importance from catalytic hydrogen production to biological enzyme functions.

Dimeric [Mo2 S12 ](2-) Cluster: A Molecular Analogue of MoS2 Edges for Superior Hydrogen-Evolution Electrocatalysis.

Proton reduction is one of the most fundamental and important reactions in nature. MoS2 edges have been identified as the active sites for hydrogen evolution reaction (HER) electrocatalysis. Designing molecular mimics of MoS2 edge sites is an attractive strategy to understand the underlying catalytic mechanism of different edge sites and improve their activities. Herein we report a dimeric molecular analogue [Mo2 S12 ](2-) , as the smallest unit possessing both the terminal and bridging disulfide ligands. Our electrochemical tests show that [Mo2 S12 ](2-) is a superior heterogeneous HER catalyst under acidic conditions. Computations suggest that the bridging disulfide ligand of [Mo2 S12 ](2-) exhibits a hydrogen adsorption free energy near zero (-0.05 eV). This work helps shed light on the rational design of HER catalysts and biomimetics of hydrogen-evolving enzymes.

Dual-cavity basket promotes encapsulation in water in an allosteric fashion.

We prepared dual-cavity basket 1 to carry six (S)-alanine residues at the entrance of its two juxtaposed cavities (289 Å(3)). With the assistance of (1)H NMR spectroscopy and calorimetry, we found that 1 could trap a single molecule of 4 (K1 = 1.45 ± 0.40 × 10(4) M(-1), ITC), akin in size (241 Å(3)) and polar characteristics to nerve agent VX (289 Å(3)). The results of density functional theory calculations (DFT, M06-2X/6-31G*) and experiments ((1)H NMR spectroscopy) suggest that the negative homotropic allosterism arises from the guest forming C-H···π contacts with all three of the aromatic walls of the occupied basket's cavity. In response, the other cavity increases its size and turns rigid to prevent the formation of the ternary complex. A smaller guest 6 (180 Å(3)), akin in size and polar characteristics to soman (186 Å(3)), was also found to bind to dual-cavity 1, although giving both binary [1⊂6] and ternary [1⊂62] complexes (K1 = 7910 M(-1) and K2 = 2374 M(-1), (1)H NMR spectroscopy). In this case, the computational and experimental ((1)H NMR spectroscopy) results suggest that only two aromatic walls of the occupied basket's cavity form C-H···π contacts with the guest to render the singly occupied host flexible enough to undergo additional structural changes necessary for receiving another guest molecule. The structural adaptivity of dual-cavity baskets of type 1 is unique and important for designing multivalent hosts capable of effectively sequestering targeted guests in an allosteric manner to give stable supramolecular polymers.

Twisted baskets.

A preparative procedure for obtaining a pair of twisted molecular baskets, each comprising a chiral framework with either right ((P)-1syn) or left ((M)-1syn) sense of twist and six ester groups at the rim has been developed and optimized. The racemic (P/M)-1syn can be obtained in three synthetic steps from accessible starting materials. The resolution of (P/M)-1syn is accomplished by its transesterification with (1R,2S,5R)-(-)-menthol in the presence of a Ti(IV) catalyst to give diastereomeric 8(P) and 8(M). It was found that dendritic-like cavitands 8(P) and 8(M), in CD2Cl2, undergo self-inclusion ((1)H NMR spectroscopy) with a menthol moiety occupying the cavity of each host. Importantly, the degree of inclusion of the menthol group was ((1)H NMR spectroscopy) found to be greater in the case of 8(P) than 8(M). Accordingly, it is suggested that different folding characteristic of 8(P) and 8(M) ought to affect the physicochemical characteristics of the hosts to permit their effective separation by column chromatography. The absolute configuration of 8(P)/8(M), encompassing right- and left-handed "cups", was determined with the exciton chirality method and also verified in silico (DFT: B3LYP/TZVP). Finally, the twisted baskets are strongly fluorescent due to three naphthalene chromophores, having a high fluorescence quantum yield within the rigid framework of 8(P)/8(M).

Trapping of organophosphorus chemical nerve agents in water with amino acid functionalized baskets.

We prepared eleven amino-acid functionalized baskets and used (1) H NMR spectroscopy to quantify their affinity for entrapping dimethyl methylphosphonate (DMMP, 118 Å(3) ) in aqueous phosphate buffer at pH=7.0±0.1; note that DMMP guest is akin in size to chemical nerve agent sarin (132 Å(3) ). The binding interaction (Ka ) was found to vary with the size of substituent groups at the basket's rim. In particular, the degree of branching at the first carbon of each substituent had the greatest effect on the host-guest interaction, as described with the Verloop's B1 steric parameter. The branching at the remote carbons, however, did not perturb the encapsulation, which is important for guiding the design of more effective hosts and catalysts in future.