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.

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.

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.

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(S)L structural motif. Herein, reductive pathways of MoO(S)bpy (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 [MoOS]. 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-MoS) electrocatalytic materials and anion-redox chalcogel battery materials.

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.