Versatile Catalytic Hydrogenation Using A Simple Tin(IV) Lewis Acid

Despite the rapid development of frustrated Lewis pair (FLP) chemistry over the last ten years, its application in catalytic hydrogenations remains dependent on a narrow family of structurally similar early main‐group Lewis acids (LAs), inevitably placing limitations on reactivity, sensitivity and substrate scope. Herein we describe the FLP‐mediated H₂ activation and catalytic hydrogenation activity of the alternative LA iPr₃SnOTf, which acts as a surrogate for the trialkylstannylium ion iPr₃Sn⁺, and is rapidly and easily prepared from simple, inexpensive starting materials. This highly thermally robust LA is found to be competent in the hydrogenation of a number of different unsaturated functional groups (which is unique to date for main‐group FLP LAs not based on boron), and also displays a remarkable tolerance to moisture.     

Self‐Assembly of Complex DNA Tessellations by Using Low‐Symmetry Multi‐arm DNA Tiles

Modular DNA tile‐based self‐assembly is a versatile way to engineer basic tessellation patterns on the nanometer scale, but it remains challenging to achieve high levels of structural complexity. We introduce a set of general design principles to create intricate DNA tessellations by employing multi‐arm DNA motifs with low symmetry. We achieved two novel Archimedean tiling patterns, (4.8.8) and (3.6.3.6), and one pattern with higher‐order structures beyond the complexity observed in Archimedean tiling. Our success in assembling complicated DNA tessellations demonstrates the broad design space of DNA structural motifs, enriching the toolbox of DNA tile‐based self‐assembly and expanding the complexity boundaries of DNA tile‐based tessellation.     

Competition between Hydrophilic and Argyrophilic Interactions in Surface Enhanced Raman Spectroscopy

The competition for binding and charge‐transfer (CT) from the nitrogen containing heterocycle pyrimidine to either silver or to water in surface enhanced Raman spectroscopy (SERS) is discussed. The correlation between the shifting observed for vibrational normal modes and CT is analyzed both experimentally using Raman spectroscopy and theoretically using electronic structure theory. Discrete features in the Raman spectrum correspond to the binding of either water or silver to each of pyrimidine's nitrogen atoms with comparable frequency shifts. Natural bond orbital (NBO) calculations in each chemical environment reveal that the magnitude of charge transfer from pyrimidine to adjacent silver atoms is only about twice that for water alone. These results suggest that the choice of solvent plays a role in determining the vibrational frequencies of nitrogen containing molecules in SERS experiments.     

Modifications of boronic ester pro-chelators triggered by hydrogen peroxide tune reactivity to inhibit metal-promoted oxidative stress

Several new analogs of salicylaldehyde isonicotinoyl hydrazone (SIH) and salicylaldehyde benzoyl hydrazone (SBH) that contain an aryl boronic ester (BSIH, BSBH) or acid (BASIH) in place of an aryl hydroxide have been synthesized and characterized as masked metal ion chelators. These pro-chelators show negligible interaction with iron(III), although the boronic acid versions exhibit some interaction with copper(II), zinc(II) and nickel(II). Hydrogen peroxide oxidizes the aryl boronate to phenol, thus converting the pro-chelators to tridentate ligands with high affinity metal binding properties. An X-ray crystal structure of a bis-ligated iron(III) complex, [Fe(SBH(m-OMe)(3))(2)]NO(3), confirms the meridonal binding mode of these ligands. Modifications of the aroyl ring of the chelators tune their iron affinity, whereas modifications on the boron-containing ring of the pro-chelators attenuate their reaction rates with hydrogen peroxide. Thus, the methoxy derivative pro-chelator (p-OMe)BASIH reacts with hydrogen peroxide nearly 5 times faster than the chloro derivative (m-Cl)BASIH. Both the rate of pro-chelator to chelator conversion as well as the metal binding affinity of the chelator influence the overall ability of these molecules to inhibit hydroxyl radical formation catalyzed by iron or copper in the presence of hydrogen peroxide and ascorbic acid. This pro-chelator strategy has the potential to improve the efficacy of medicinal chelators for inhibiting metal-promoted oxidative stress.     

Coherent carbon cryogel-ammonia borane nanocomposites for H2 storage

Coherent carbon cryogel-ammonia borane (C-AB) nanocomposites were synthesized, and improved H2 storage properties are reported. Porous carbon cryogels were impregnated with AB in tetrahydrofuran solution at 25 degrees C under argon; 30% of the carbon cryogel pore volume was filled to produce a 24 wt % C-AB nanocomposite. Nitrogen sorption isotherms, X-ray diffraction, differential scanning calorimetry, differential thermal/thermal gravimetric analyses, mass spectrometry, and 11B NMR were used to characterize the coherent C-AB nanocomposites. Findings include a merged two-step hydrogen release reaction with an appreciable reduction in the dehydrogenation temperature to <90 degrees C as well as the suppression of borazine release. The possible nanosize effects on the H2 storage properties are discussed.     

Trapping of a dopaquinone intermediate in the TPQ cofactor biogenesis in a copper-containing amine oxidase from Arthrobacter globiformis

The biogenesis of the topaquinone (TPQ) cofactor of copper amine oxidase (CAO) is self-catalyzed and requires copper and molecular oxygen. A dopaquinone intermediate has been proposed to undergo 1,4-addition of a copper-associated water molecule to form the reduced form of TPQ (TPQ(red)), followed by facile oxidation by O(2) to yield the mature TPQ (TPQ(ox)). In this study, we have incorporated a lysine residue in the active site of Arthrobacter globiformis CAO (AGAO) by site-directed mutagenesis to produce D298K-AGAO. The X-ray crystal structure of D298K-AGAO at 1.7-A resolution revealed that a covalent linkage formed between the epsilon-amino side chain of Lys298 and the C2 position of a dopaquinone derived from Tyr382, a precursor to TPQ(ox). We assigned the species as an iminoquinone tautomer (LTI) of lysine tyrosylquinone (LTQ), the organic cofactor of lysyl oxidase (LOX). The time course of the formation of LTI at pH 6.8 was followed by UV/vis and resonance Raman spectroscopies. In the early phase of the reaction, an LTQ-like intermediate was observed. This intermediate then slowly converted to LTI in an isosbestic manner. Not only is the presence of a dopaquinone intermediate in the TPQ biogenesis confirmed, but it also provides strong support for the proposed intermediacy of a dopaquinone in the biogenesis of LTQ in LOX. Further, this study indicates that the dopaquinone intermediate in AGAO is mobile and can swing from the copper site into the active-site wedge to react with Lys298.     

Linking metal–organic cages pairwise as a design approach for assembling multivariate crystalline materials

Using metal–organic cages (MOCs) as preformed supermolecular building-blocks (SBBs) is a powerful strategy to design functional metal–organic frameworks (MOFs) with control over the pore architecture and connectivity. However, introducing chemical complexity into the network via this route is limited as most methodologies focus on only one type of MOC as the building-block. Herein we present the pairwise linking of MOCs as a design approach to introduce defined chemical complexity into porous materials. Our methodology exploits preferential Rh-aniline coordination and stoichiometric control to rationally link Cu₄L₄ and Rh₄L₄ MOCs into chemically complex, yet extremely well-defined crystalline solids. This strategy is expected to open up significant new possibilities to design bespoke multi-functional materials with atomistic control over the location and ordering of chemical functionalities.

A new strategy to design atomically precise multivariate metal–organic frameworks is presented. This is achieved by linking two preformed metal–organic cages via a precisely tuned Rh–aniline interaction.     

Chemoselective S-benzylation of indoline-2-thiones using benzyl alcohols

Indoline-2-thiones were chemoselectively S-benzylated using a variety of benzyl alcohols by boron trifluoride etherate-catalyzed reactions. The aryl substituent effect on the reactivity of the benzyl alcohols toward S-benzylation is also discussed.     

Replica molding of high-aspect-ratio polymeric nanopillar arrays with high fidelity

Polymeric nanostructures with high aspect ratios, so-called nanopillars, are of interest for a wide range of applications. However, it remains a challenge to fabricate high-density, polymeric nanopillars using soft lithography when the feature size is decreased to hundreds of nanometers and the structures are close to each other. Here, we investigate the fidelity of replica molding technique to fabricate polymer nanopillar arrays with diameters ranging from 300 nm to 1 mum, and we compare the experimental results to the theoretical prediction to understand the nature of the instability of nanopillars. Nanopillars molded from soft materials, poly(dimethylsiloxane) (PDMS), mainly ground collapse due to the adhesive force when the aspect ratio is above 6, whereas those from stiffer materials, polyurethane and epoxy, collapse laterally at a much higher aspect ratio (>/=12), of which the critical value is dependent on the nanopillar's feature size, spacing, height, and shape. Further, we attempt to restore the collapsed high-aspect-ratio nanopillars using supercritical CO(2) drying.