Oct 29, 2014

Light-Induced Proton-Coupled Electron Transfer Inside a Nanocage

Publication Date (Web): October 21, 2014 (Communication)
DOI: 10.1021/ja509761a
 
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Triggering proton-coupled electron-transfer (PCET) reactions with light in a nanoconfined host environment would bring about temporal control on the reactive pathways via kinetic stabilization of intermediates. Using a water-soluble octahedral Pd6L4 molecular cage as a host, we show that optical pumping of host–guest charge transfer (CT) states lead to generation of kinetically stable phenoxyl radical of the incarcerated 4-hydroxy-diphenylamine (1-OH). Femtosecond broadband transient absorption studies reveal that CT excitation initiates the proton movement from the 1-OH radical cation to a solvent water molecule in ∼890 fs, faster than the time scale for bulk solvation. Our work illustrates that optical host–guest CT excitations can drive solvent-coupled ultrafast PCET reactions inside nanocages and if optimally tuned should provide a novel paradigm for visible-light photocatalysis.
 

Selective Electrocatalytic Oxidation of a Re–Methyl Complex to Methanol by a Surface-Bound RuII Polypyridyl Catalyst

Publication Date (Web): October 17, 2014 (Communication)
DOI: 10.1021/ja507979c
 
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The complex [Ru(Mebimpy)(4,4′-((HO)2OPCH2)2bpy)(OH2)]2+ surface bound to tin-doped indium oxide mesoporous nanoparticle film electrodes (nanoITO-RuII(OH2)2+) is an electrocatalyst for the selective oxidation of methylrhenium trioxide (MTO) to methanol in acidic aqueous solution. Oxidative activation of the catalyst to nanoITO-RuIV(OH)3+ induces oxidation of MTO. The reaction is first order in MTO with rate saturation observed at [MTO] > 12 mM with a limiting rate constant of k = 25 s–1. Methanol is formed selectively in 87% Faradaic yield in controlled potential electrolyses at 1.3 V vs NHE. At higher potentials, oxidation of MTO by nanoITO-RuV(O)3+ leads to multiple electrolysis products. The results of an electrochemical kinetics study point to a mechanism in which surface oxidation to nanoITO-RuIV(OH)3+ is followed by direct insertion into the rhenium–methyl bond of MTO with a detectable intermediate.

Oct 28, 2014

Publication Date (Web): October 23, 2014 (Communication)
DOI: 10.1021/ja5092672


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An anionic indium porphyrin framework (UNLPF-10) consisting of rare Williams β-tetrakaidecahedral cages was constructed using an octatopic ligand linked with 4-connected [In(COO)4] SBUs. Remarkably, the extent of indium metalation of porphyrin macrocycles in UNLPF-10 can be facilely tuned in situ depending on the M/L ratio during synthesis, resulting in a controllable framework charge density and photocatalytic activity toward the selective oxygenation of sulfides.

Oct 27, 2014

Mechanistic Insight into the Formation of Cationic Naked Nanocrystals Generated under Equilibrium Control

Publication Date (Web): October 10, 2014 (Article)
DOI: 10.1021/ja508675t


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Cationic naked nanocrystals (NCs) are useful building units for assembling hierarchical mesostructured materials. Until now, their preparation required strongly electrophilic reagents that irreversibly sever bonds between native organic ligands and the NC surface. Colloidal instabilities can occur during ligand stripping if exposed metal cations desorb from the surface. We hypothesized that cation desorption could be avoided were we able to stabilize the surface during ligand stripping via ion pairing. We were successful in this regard by carrying out ligand stripping under equilibrium control with Lewis acid–base adducts of BF3. To better understand the microscopic processes involved, we studied the reaction pathway in detail using in situ NMR experiments and electrospray ionization mass spectrometry. As predicted, we found that cationic NC surfaces are transiently stabilized post-stripping by physisorbed anionic species that arise from the reaction of BF3 with native ligands. This stabilization allows polar dispersants to reach the NC surface before cation desorption can occur. The mechanistic insights gained in this work provide a much-needed framework for understanding the interplay between NC surface chemistry and colloidal stability. These insights enabled the preparation of stable naked NC inks of desorption-susceptible NC compositions such as PbSe, which were easily assembled into new mesostructured films and polymer-nanocrystal composites with wide-ranging technological applications.

Oct 24, 2014

Room-temperature enantioselective C–H iodination via kinetic resolution

  • Ling Chu,
  • Kai-Jiong Xiao,
  • and Jin-Quan Yu
Science 24 October 2014: 451-455.
Palladium catalysis produces benzylamine derivatives of interest in medicinal chemistry.

 

Abstract

Asymmetric carbon-hydrogen (C–H) activation reactions often rely on desymmetrization of prochiral C–H bonds on the same achiral molecule, using a chiral catalyst. Here, we report a kinetic resolution via palladium-catalyzed enantioselective C–H iodination in which one of the enantiomers of a racemic benzylic amine substrates undergoes faster aryl C–H insertion with the chiral catalysts than the other. The resulting enantioenriched C–H functionalization products would not be accessible through desymmetrization of prochiral C–H bonds. The exceedingly high relative rate ratio (kfast/kslow up to 244), coupled with the subsequent iodination of the remaining enantiomerically enriched starting material using a chiral ligand with the opposite configuration, enables conversion of both substrate enantiomers into enantiomerically pure iodinated products.
Simultaneous measurements of structural and optical properties are used to study optically excited vanadium dioxide.
The complex interplay among several active degrees of freedom (charge, lattice, orbital, and spin) is thought to determine the electronic properties of many oxides. We report on combined ultrafast electron diffraction and infrared transmissivity experiments in which we directly monitored and separated the lattice and charge density reorganizations that are associated with the optically induced semiconductor-metal transition in vanadium dioxide (VO2). By photoexciting the monoclinic semiconducting phase, we were able to induce a transition to a metastable state that retained the periodic lattice distortion characteristic of the semiconductor but also acquired metal-like mid-infrared optical properties. Our results demonstrate that ultrafast electron diffraction is capable of following details of both lattice and electronic structural dynamics on the ultrafast time scale.

Oct 22, 2014

Chemistry prize winners pushed microscopes past supposed limit.
For more than 100 years, microscopists thought they would never get a clear close-up look at living organisms in visible light micrographs. The stumbling block was the so-called Abbe diffraction limit, a supposed physical law that stated optical images could never reach a resolution finer than half a wavelength of light. But beginning in the late 1990s, Eric Betzig of the Howard Hughes Medical Institute, Stefan Hell of the Max Planck Institute for Biophysical Chemistry, and William Moerner of Stanford University found a way to blast through that limit by using fluorescence to coax objects to reveal details through their own light. The techniques netted them this year's Nobel Prize in chemistry. 
Syntheses of two natural products reveal a surprising divergence in the plausible biosynthetic precursors of their class.
Cycloaddition is an essential tool in chemical synthesis. Instead of using light or heat as a driving force, marine sponges promote cycloaddition with a more versatile but poorly understood mechanism in producing pyrrole–imidazole alkaloids sceptrin, massadine, and ageliferin. Through de novo synthesis of sceptrin and massadine, we show that sponges may use single-electron oxidation as a central mechanism to promote three different types of cycloaddition. Additionally, we provide surprising evidence that, in contrast to previous reports, sceptrin, massadine, and ageliferin have mismatched chirality. Therefore, massadine cannot be an oxidative rearrangement product of sceptrin or ageliferin, as is commonly believed. Taken together, our results demonstrate unconventional chemical approaches to achieving cycloaddition reactions in synthesis and uncover enantiodivergence as a new biosynthetic paradigm for natural products. 

Intramolecular Iron-Mediated C–H Bond Heterolysis with an Assist of Pendant Base in a [FeFe]-Hydrogenase Model

Publication Date (Web): September 22, 2014 (Article)
DOI: 10.1021/ja5078014
 
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Although many metalloenzymes containing iron play a prominent role in biological C–H activation processes, to date iron-mediated C(sp3)–H heterolysis has not been reported for synthetic models of Fe/S-metalloenzymes. In contrast, ample precedent has established that nature’s design for reversible hydrogen activation by the diiron hydrogenase ([FeFe]-H2ase) active site involves multiple irons, sulfur bridges, a redox switch, and a pendant amine base, in an intricate arrangement to perform H–H heterolytic cleavage. In response to whether this strategy might be extended to C–H activation, we report that a [FeFe]-H2ase model demonstrates iron-mediated intramolecular C–H heterolytic cleavage via an agostic C–H interaction, with proton removal by a nearby pendant amine, affording FeII–[tiebar above startFe′II–CH–tiebar above endS] three-membered-ring products, which can be reduced back to 1 by Cp2Co in the presence of HBF4. The function of the pendant base as a proton shuttle was confirmed by the crystal structures of the N-protonated intermediate and the final deprotonated product in comparison with that of a similar but pendant-amine-free complex that does not show evidence of C–H activation. The mechanism of the process was backed up by DFT calculations.
 
Publication Date (Web): September 30, 2014 (Article)
DOI: 10.1021/ja507665a
 
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We prepared bifunctional MgII porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO2. The activities of 1d, 1h, and 1i, which have Br, Cl, and I counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1jm, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized MgII porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h–1). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO2 pressures in the 1–6 MPa range, suggesting that CO2 was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO2, probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO2. Experiments with 18O-labeled CO2 and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br on the less hindered side of the epoxide to generate an oxyanion, which underwent CO2 insertion to afford a CO2 adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the MgII and Br and a conformational change (induced-fit) of the quaternary ammonium cation.

Oct 21, 2014

Investigations on the Role of Proton-Coupled Electron Transfer in Hydrogen Activation by [FeFe]-Hydrogenase

Publication Date (Web): October 6, 2014 (Article)
DOI: 10.1021/ja508629m
 
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Proton-coupled electron transfer (PCET) is a fundamental process at the core of oxidation–reduction reactions for energy conversion. The [FeFe]-hydrogenases catalyze the reversible activation of molecular H2 through a unique metallocofactor, the H-cluster, which is finely tuned by the surrounding protein environment to undergo fast PCET transitions. The correlation of electronic and structural transitions at the H-cluster with proton-transfer (PT) steps has not been well-resolved experimentally. Here, we explore how modification of the conserved PT network via a Cys → Ser substitution at position 169 proximal to the H-cluster of Chlamydomonas reinhardtii [FeFe]-hydrogenase (CrHydA1) affects the H-cluster using electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. Despite a substantial decrease in catalytic activity, the EPR and FTIR spectra reveal different H-cluster catalytic states under reducing and oxidizing conditions. Under H2 or sodium dithionite reductive treatments, the EPR spectra show signals that are consistent with a reduced [4Fe-4S]H+ subcluster. The FTIR spectra showed upshifts of νCO modes to energies that are consistent with an increase in oxidation state of the [2Fe]H subcluster, which was corroborated by DFT analysis. In contrast to the case for wild-type CrHydA1, spectra associated with Hred and Hsred states are less populated in the Cys → Ser variant, demonstrating that the exchange of −SH with −OH alters how the H-cluster equilibrates among different reduced states of the catalytic cycle under steady-state conditions.

Oct 6, 2014

Hot molecules—off the beaten path

  • Arthur G. Suits and
  • David H. Parker
Science 3 October 2014: 30-31.
Highly excited carbon dioxide dissociates unexpectedly to form molecular oxygen [Also see Report by Lu et al.]
Absorption of high-energy ultraviolet light can break apart CO2 into C and O2 . [Also see Perspective by Suits and Parker]
Photodissociation of carbon dioxide (CO2) has long been assumed to proceed exclusively to carbon monoxide (CO) and oxygen atom (O) primary products. However, recent theoretical calculations suggested that an exit channel to produce C + O2 should also be energetically accessible. Here we report the direct experimental evidence for the C + O2 channel in CO2 photodissociation near the energetic threshold of the C(3P) + O2(X3Σg) channel with a yield of 5 ± 2% using vacuum ultraviolet laser pump-probe spectroscopy and velocity-map imaging detection of the C(3PJ) product between 101.5 and 107.2 nanometers. Our results may have implications for nonbiological oxygen production in CO2-heavy atmospheres.

Colloidal nanoparticle biosensors have received intense scientific attention and offer promising applications in both research and medicine. We review the state of the art in nanoparticle development, surface chemistry, and biosensing mechanisms, discussing how a range of technologies are contributing toward commercial and clinical translation. Recent examples of success include the ultrasensitive detection of cancer biomarkers in human serum and in vivo sensing of methyl mercury. We identify five key materials challenges, including the development of robust mass-scale nanoparticle synthesis methods, and five broader challenges, including the use of simulations and bioinformatics-driven experimental approaches for predictive modeling of biosensor performance. The resultant generation of nanoparticle biosensors will form the basis of high-performance analytical assays, effective multiplexed intracellular sensors, and sophisticated in vivo probes.

Spin-Selective Charge Recombination in Complexes of CdS Quantum Dots and Organic Hole Acceptors

Publication Date (Web): September 17, 2014 (Article)
DOI: 10.1021/ja507301d

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This paper describes the mechanisms of charge recombination on both the nanosecond and microsecond time scales in a donor–acceptor system comprising thiol-modified bis(diarylamino)4,4′-biphenyl (TPD) molecules attached to a CdS quantum dot (QD) via the thiolate linker. Transient absorption measurements, in conjunction with EPR and magnetic field effect studies, demonstrate that recombination on the nanosecond time scale is mediated by radical pair intersystem crossing (RP-ISC), as evidenced by the observation of a spin correlated radical ion pair, the formation of the localized 3*TPD state upon charge recombination, and the sensitivity of the yield of 3*TPD to an applied magnetic field. These experiments show that the radical spins of the donor–acceptor system have weak magnetic exchange coupling (|2J| < 10 mT) and that the electron donated to the QD is trapped in a surface state rather than delocalized within the QD lattice. The microsecond-time scale recombination is probably gated by diffusion of the trapped electron among QD surface states. This study demonstrates that magneto-optical studies are useful for characterizing the charge-separated states of molecule–QD hybrid systems, despite the heterogeneity in the donor–acceptor geometry and the chemical environment of the radical spins that is inherent to these systems.
Publication Date (Web): September 22, 2014 (Article)
DOI: 10.1021/ja5059444

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Oxidation of small organic molecules in a fuel cell is a viable method for energy production. However, the key issue is the development of suitable catalysts that exhibit high efficiencies and remain stable during operation. Here, we demonstrate that amine-modified ZnO nanorods on which ultrathin Au nanowires are grown act as an excellent catalyst for the oxidation of ethanol. We show that the modification of the ZnO nanorods with oleylamine not only modifies the electronic structure favorably but also serves to anchor the Au nanowires on the nanorods. The adsorption of OH species on the Au nanowires that is essential for ethanol oxidation is facilitated at much lower potentials as compared to bare Au nanowires leading to high activity. While ZnO shows negligible electrocatalytic activity under normal conditions, there is significant enhancement in the activity under light irradiation. We demonstrate a synergistic enhancement in the photoelectrocatalytic activity of the ZnO/Au nanowire hybrid and provide mechanistic explanation for this enhancement based on both electronic as well as geometric effects. The principles developed are applicable for tuning the properties of other metal/semiconductor hybrids with potentially interesting applications beyond the fuel cell application demonstrated here.