Aug 28, 2014

Nitrite and Hydroxylamine as Nitrogenase Substrates: Mechanistic Implications for the Pathway of N2 Reduction

Publication Date (Web): August 19, 2014 (Article)
DOI: 10.1021/ja507123d


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Investigations of reduction of nitrite (NO2) to ammonia (NH3) by nitrogenase indicate a limiting stoichiometry, NO2 + 6e + 12ATP + 7H+ → NH3 + 2H2O + 12ADP + 12Pi. Two intermediates freeze-trapped during NO2 turnover by nitrogenase variants and investigated by Q-band ENDOR/ESEEM are identical to states, denoted H and I, formed on the pathway of N2 reduction. The proposed NO2 reduction intermediate hydroxylamine (NH2OH) is a nitrogenase substrate for which the H and I reduction intermediates also can be trapped. Viewing N2 and NO2 reductions in light of their common reduction intermediates and of NO2 reduction by multiheme cytochrome c nitrite reductase (ccNIR) leads us to propose that NO2 reduction by nitrogenase begins with the generation of NO2H bound to a state in which the active-site FeMo-co (M) has accumulated two [e/H+] (E2), stored as a (bridging) hydride and proton. Proton transfer to NO2H and H2O loss leaves M–[NO+]; transfer of the E2 hydride to the [NO+] directly to form HNO bound to FeMo-co is one of two alternative means for avoiding formation of a terminal M–[NO] thermodynamic “sink”. The N2 and NO2 reduction pathways converge upon reduction of NH2NH2 and NH2OH bound states to form state H with [−NH2] bound to M. Final reduction converts H to I, with NH3 bound to M. The results presented here, combined with the parallels with ccNIR, support a N2 fixation mechanism in which liberation of the first NH3 occurs upon delivery of five [e/H+] to N2, but a total of seven [e/H+] to FeMo-co when obligate H2 evolution is considered, and not earlier in the reduction process.

Facile Surface Functionalization of Hydrophobic Magnetic Nanoparticles

Publication Date (Web): August 20, 2014 (Communication)
DOI: 10.1021/ja5060324


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Nonpolar phase synthesized hydrophobic nanocrystals show attractive properties and have demonstrated prominent potential in biomedical applications. However, the preparation of biocompatible nanocrystals is made difficult by the presence of hydrophobic surfactant stabilizer on their surfaces. To address this limitation, we have developed a facile, high efficiency, single-phase and low-cost method to convert hydrophobic magnetic nanoparticles (MNPs) to an aqueous phase using tetrahydrofuran, NaOH and 3,4-dihydroxyhydrocinnamic acid without any complicated organic synthesis. The as-transferred hydrophilic MNPs are water-soluble over a wide pH range (pH = 3–12), and the solubility is pH-controllable. Furthermore, the as-transferred MNPs with carboxylate can be readily adapted with further surface functionalization, varying from small molecule dyes to oligonucleotides and enzymes. Finally, the strategy developed here can easily be extended to other types of hydrophobic nanoparticles to facilitate biomedical applications of nanomaterials.

Direct Comparison of Electrochemical and Spectrochemical Kinetics for Catalytic Oxygen Reduction

Publication Date (Web): August 19, 2014 (Communication)
DOI: 10.1021/ja505667t
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Aug 25, 2014

Design and Synthesis of New Stable Fluorenyl-Based Radicals

Publication Date (Web): August 18, 2014 (Article)
DOI: 10.1021/ja507005c

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Organic neutral radicals have long fascinated chemists with a fundamental understanding of structure–reactivity relationships in organic reactions and with applications as new functional materials. However, the elusive nature of these radicals makes the synthesis, isolation, and characterization very challenging. In this work, the synthesis of three long-lived, fluorenyl-based radicals are reported. The geometry and electronic structures of these radicals were systematically investigated with a combination of various experimental methods, besides density functional theory (DFT) calculations, which include X-ray crystallographic analysis, electron spin resonance (ESR), electron nuclear double resonance (ENDOR), cyclic voltammetry, and UV–vis–NIR measurements. Their half-life periods (τ1/2) in air-saturated solution under ambient conditions were also determined. Surprisingly, all three radicals showed remarkable stabilities: τ1/2 = 7, 3.5, and 43 days.

The Cyanide Ligands of [FeFe] Hydrogenase: Pulse EPR Studies of 13C and 15N-Labeled H-Cluster

Publication Date (Web): August 15, 2014 (Communication)
DOI: 10.1021/ja507046w


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The two cyanide ligands in the assembled cluster of [FeFe] hydrogenase originate from exogenous l-tyrosine. Using selectively labeled tyrosine substrates, the cyanides were isotopically labeled via a recently developed in vitro maturation procedure allowing advanced electron paramagnetic resonance techniques to probe the electronic structure of the catalytic core of the enzyme. The ratio of the isotropic 13C hyperfine interactions for the two CN ligands—a reporter of spin density on their respective coordinating iron ions—collapses from ≈5.8 for the Hox form of hydrogenase to <2 for the CO-inhibited form. Additionally, when the maturation was carried out using [15N]-tyrosine, no features previously ascribed to the nitrogen of the bridging dithiolate ligand were observed suggesting that this bridge is not sourced from tyrosine.

Homogeneous Photochemical Water Oxidation by Biuret-Modified Fe-TAML: Evidence of FeV(O) Intermediate

Publication Date (Web): August 13, 2014 (Article)
DOI: 10.1021/ja503753k
 
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Water splitting, leading to hydrogen and oxygen in a process that mimics natural photosynthesis, is extremely important for devising a sustainable solar energy conversion system. Development of earth-abundant, transition metal-based catalysts that mimic the oxygen-evolving complex of photosystem II, which is involved in oxidation of water to O2 during natural photosynthesis, represents a major challenge. Further, understanding the exact mechanism, including elucidation of the role of active metal-oxo intermediates during water oxidation (WO), is critical to the development of more efficient catalysts. Herein, we report FeIII complexes of biuret-modified tetra-amidomacrocyclic ligands (Fe-TAML; 1a and 1b) that catalyze fast, homogeneous, photochemical WO to give O2, with moderate efficiency (maximum TON = 220, TOF = 0.76 s–1). Previous studies on photochemical WO using iron complexes resulted in demetalation of the iron complexes with concomitant formation of iron oxide nanoparticles (NPs) that were responsible for WO. Herein, we show for the first time that a high valent FeV(O) intermediate species is photochemically generated as the active intermediate for the oxidation of water to O2. To the best of our knowledge, this represents the first example of a molecular iron complex catalyzing photochemical WO through a FeV(O) intermediate.
 

Aug 22, 2014

Direct Spectroscopic Observation of Closed-Shell Singlet, Open-Shell Singlet, and Triplet p-Biphenylyloxenium Ion

Publication Date (Web): August 14, 2014 (Article)
DOI: 10.1021/ja505447q


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The photophysics and photochemistry of p-biphenylyl hydroxylamine hydrochloride was studied using laser flash photolysis ranging from the femtosecond to the microsecond time scale. The singlet excited state of this photoprecursor is formed within 350 fs and partitions into three different transients that are assigned to the p-biphenyloxy radical, the open-shell singlet p-biphenylyloxenium ion, and the triplet p-biphenylyloxenium ion, having lifetimes of 40 μs, 45 ps, and 1.6 ns, respectively, in CH3CN. The open-shell singlet p-biphenylyloxenium ion predominantly undergoes internal conversion to produce the closed-shell singlet p-biphenylyloxenium ion, which has a lifetime of 5–20 ns. The longer-lived radical is unambiguously assigned by nanosecond time-resolved resonance Raman (ns-TR3) spectroscopy, and the assignment of the short-lived singlet and triplet oxenium ion transient absorptions are supported by matching time-dependent density functional theory (TD-DFT) predictions of the absorptions of these species, as well as by product studies that implicate the intermediacy of charged electrophilic intermediates. Product studies from photolysis give p-biphenylol as the major product and a chloride adduct as the major product when NaCl is added as a trap. Thermolysis studies give p-biphenylol as a major product, as well as water, ammonium, and chloro adducts. These studies provide a rare direct look at a discrete oxenium ion intermediate and the first detection of open-shell singlet and triplet configurations of an oxenium ion, as well as providing an intriguing example of the importance of excited state dynamics in governing the electronic state population of reactive intermediates.

Aug 21, 2014

The atomic structure of a 68–gold atom nanoparticle is determined without prior structural knowledge or model fitting.

Abstract

Structure determination of gold nanoparticles (AuNPs) is necessary for understanding their physical and chemical properties, but only one AuNP larger than 1 nanometer in diameter [a 102–gold atom NP (Au102NP)] has been solved to atomic resolution. Whereas the Au102NP structure was determined by x-ray crystallography, other large AuNPs have proved refractory to this approach. Here, we report the structure determination of a Au68NP at atomic resolution by aberration-corrected transmission electron microscopy, performed with the use of a minimal electron dose, an approach that should prove applicable to metal NPs in general. The structure of the Au68NP was supported by small-angle x-ray scattering and by comparison of observed infrared absorption spectra with calculations by density functional theory.

Aug 19, 2014

A crucial step in photosynthesis is becoming clearer [Also see Report by Cox et al.]
Electron paramagnetic resonance spectroscopy reveals that photosynthetic production of O2 proceeds from four MnIV ions. [Also see Perspective by Britt and Oyala]
The photosynthetic protein complex photosystem II oxidizes water to molecular oxygen at an embedded tetramanganese-calcium cluster. Resolving the geometric and electronic structure of this cluster in its highest metastable catalytic state (designated S3) is a prerequisite for understanding the mechanism of O-O bond formation. Here, multifrequency, multidimensional magnetic resonance spectroscopy reveals that all four manganese ions of the catalyst are structurally and electronically similar immediately before the final oxygen evolution step; they all exhibit a 4+ formal oxidation state and octahedral local geometry. Only one structural model derived from quantum chemical modeling is consistent with all magnetic resonance data; its formation requires the binding of an additional water molecule. O-O bond formation would then proceed by the coupling of two proximal manganese-bound oxygens in the transition state of the cofactor.

RNA structures have been designed that self-assemble and are programmable and scalable [Also see Report by Geary et al.]
The size of RNA origami nanostructures has been increased with a distinct assembly pattern. [Also see Perspective by Leontis and Westhof]
Artificial DNA and RNA structures have been used as scaffolds for a variety of nanoscale devices. In comparison to DNA structures, RNA structures have been limited in size, but they also have advantages: RNA can fold during transcription and thus can be genetically encoded and expressed in cells. We introduce an architecture for designing artificial RNA structures that fold from a single strand, in which arrays of antiparallel RNA helices are precisely organized by RNA tertiary motifs and a new type of crossover pattern. We constructed RNA tiles that assemble into hexagonal lattices and demonstrated that lattices can be made by annealing and/or cotranscriptional folding. Tiles can be scaled up to 660 nucleotides in length, reaching a size comparable to that of large natural ribozymes.

Origami techniques are used to develop crawling robots that self-fold from flat-pack designs.
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  • Origami can turn a sheet of paper into complex three-dimensional shapes, and similar folding techniques can produce structures and mechanisms. To demonstrate the application of these techniques to the fabrication of machines, we developed a crawling robot that folds itself. The robot starts as a flat sheet with embedded electronics, and transforms autonomously into a functional machine. To accomplish this, we developed shape-memory composites that fold themselves along embedded hinges. We used these composites to recreate fundamental folded patterns, derived from computational origami, that can be extrapolated to a wide range of geometries and mechanisms. This origami-inspired robot can fold itself in 4 minutes and walk away without human intervention, demonstrating the potential both for complex self-folding machines and autonomous, self-controlled assembly. 
Origami folded sheets can be structurally altered by adding defects to control the mechanical properties. [Also see Perspective by You]
Although broadly admired for its aesthetic qualities, the art of origami is now being recognized also as a framework for mechanical metamaterial design. Working with the Miura-ori tessellation, we find that each unit cell of this crease pattern is mechanically bistable, and by switching between states, the compressive modulus of the overall structure can be rationally and reversibly tuned. By virtue of their interactions, these mechanically stable lattice defects also lead to emergent crystallographic structures such as vacancies, dislocations, and grain boundaries. Each of these structures comes from an arrangement of reversible folds, highlighting a connection between mechanical metamaterials and programmable matter. Given origami’s scale-free geometric character, this framework for metamaterial design can be directly transferred to milli-, micro-, and nanometer-size systems.

Catalytic approach could eliminate CO2 emissions from the key step in making fertilizer.
Synergy at a metal-oxide interface generates highly active catalysts for carbon dioxide hydrogenation to methanol.
The transformation of CO2 into alcohols or other hydrocarbon compounds is challenging because of the difficulties associated with the chemical activation of CO2 by heterogeneous catalysts. Pure metals and bimetallic systems used for this task usually have low catalytic activity. Here we present experimental and theoretical evidence for a completely different type of site for CO2 activation: a copper-ceria interface that is highly efficient for the synthesis of methanol. The combination of metal and oxide sites in the copper-ceria interface affords complementary chemical properties that lead to special reaction pathways for the CO2→CH3OH conversion.
Diamond-based quantum teleportation works every time [Also see Research Article by Pfaff et al.]

Proton-Induced Reactivity of NO from a {CoNO}8 Complex

Publication Date (Web): July 29, 2014 (Communication)
DOI: 10.1021/ja5064444
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Research on the one-electron reduced analogue of NO, namely nitroxyl (HNO/NO), has revealed distinguishing properties regarding its utility as a therapeutic. However, the fleeting nature of HNO requires the design of donor molecules. Metal nitrosyl (MNO) complexes could serve as potential HNO donors. The synthesis, spectroscopic/structural characterization, and HNO donor properties of a {CoNO}8 complex in a pyrrole/imine ligand frame are reported. The {CoNO}8 complex [Co(LN4PhCl)(NO)] (1) does not react with established HNO targets such as FeIII hemes or Ph3P. However, in the presence of stoichiometric H+ 1 behaves as an HNO donor. Complex 1 readily reacts with [Fe(TPP)Cl] or Ph3P to afford the {FeNO}7 porphyrin or Ph3P═O/Ph3P═NH, respectively. In the absence of an HNO target, the {Co(NO)2}10 dinitrosyl (3) is the end product. Complex 1 also reacts with O2 to yield the corresponding CoIII1-ONO2 (2) nitrato analogue. This report is the first to suggest an HNO donor role for {CoNO}8 with biotargets such as FeIII-porphyrins.
 
Publication Date (Web): July 30, 2014 (Article)
DOI: 10.1021/ja506193v
 
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Abstract

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Two derivatives of iron tetraphenylporphyrin bearing prepositioned phenolic functionalities on two of the opposed phenyl groups prove to be remarkable catalysts for the reduction of CO2 to CO when generated electrochemically at the Fe0 oxidation state. In one case, the same substituents are present on the two other phenyls, whereas in the other the two other phenyls are perfluorinated. They are taken as examples of the possible role of pendant acid–base groups in molecular catalysis. The prepositioned phenol groups incorporated into the catalyst molecule induce strong stabilization of the initial Fe0CO2 adduct through H-bonding, confirmed by DFT calculations. This positive factor is partly counterbalanced by the necessity, resulting from the same stabilization, to inject an additional electron to trigger catalysis. Thanks to the preprotonation of the initial Fe0CO2 adduct, the potential required for this second electron transfer is not very distant from the potential at which the adduct is generated by addition of CO2 to the Fe0 complex. The protonation step involves an internal phenolic group and the reprotonation of the phenoxide ion thus generated by added phenol. The prepositioned phenol groups thus play both the role of H-bonding stabilizers and high-concentration proton donors. They play the same role in the second electron transfer step which closes the catalytic loop concertedly with the breaking of one of the two C–O bonds of CO2 and with proton transfer. It is also remarkable that reprotonation by added phenol is concerted with the three other events.
 

Identification of the Valence and Coordination Environment of the Particulate Methane Monooxygenase Copper Centers by Advanced EPR Characterization

Publication Date (Web): July 24, 2014 (Article)
DOI: 10.1021/ja5053126
 
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Particulate methane monooxygenase (pMMO) catalyzes the oxidation of methane to methanol in methanotrophic bacteria. As a copper-containing enzyme, pMMO has been investigated extensively by electron paramagnetic resonance (EPR) spectroscopy, but the presence of multiple copper centers has precluded correlation of EPR signals with the crystallographically identified monocopper and dicopper centers. A soluble recombinant fragment of the pmoB subunit of pMMO, spmoB, like pMMO itself, contains two distinct copper centers and exhibits methane oxidation activity. The spmoB protein, spmoB variants designed to disrupt one or the other or both copper centers, as well as native pMMO have been investigated by EPR, ENDOR, and ESEEM spectroscopies in combination with metal content analysis. The data are remarkably similar for spmoB and pMMO, validating the use of spmoB as a model system. The results indicate that one EPR-active Cu(II) ion is present per pMMO and that it is associated with the active-site dicopper center in the form of a valence localized Cu(I)Cu(II) pair; the Cu(II), however, is scrambled between the two locations within the dicopper site. The monocopper site observed in the crystal structures of pMMO can be assigned as Cu(I). 14N ENDOR and ESEEM data are most consistent with one of these dicopper-site signals involving coordination of the Cu(II) ion by residues His137 and His139, the other with Cu(II) coordinated by His33 and the N-terminal amino group. 1H ENDOR measurements indicate there is no aqua (HxO) ligand bound to the Cu(II), either terminally or as a bridge to Cu(I).
 
Publication Date (Web): July 28, 2014 (Article)
DOI: 10.1021/ja5062196
 
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Addition of stoichiometric quantites of 1,2-diarylhydrazines to the bis(imino)pyridine vanadium dinitrogen complex, [{(iPrBPDI)V(THF)}22-N2)] (iPrBPDI = 2,6-(2,6-iPr2-C6H3N═CPh)2C5H3N), resulted in N–N bond cleavage to yield the corresponding vanadium bis(amido) derivatives, (iPrBPDI)V(NHAr)2 (Ar = Ph, Tol). Spectroscopic, structural, and computational studies support an assignment as vanadium(III) complexes with chelate radical anions, [BPDI]•–. With excess 1,2-diarylhydrazine, formation of the bis(imino)pyridine vanadium imide amide compounds, (iPrBPDI)V(NHAr)NAr, were observed along with the corresponding aryldiazene and aniline. A DFT-computed N–H bond dissociation free energy of 69.2 kcal/mol was obtained for (iPrBPDI)V(NHPh)NPh, and interconversion between this compound and (iPrBPDI)V(NHPh)2 with (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), 1,2-diphenylhydrazine, and xanthene experimentally bracketed this value between 67.1 and 73.3 kcal/mol. For (iPrBPDI)V(NHPh)2, the N–H BDFE was DFT-calculated to be 64.1 kcal/mol, consistent with experimental observations. Catalytic disproportionation of 1,2-diarylhydrazines promoted by (iPrBPDI)V(NHAr)NAr was observed, and crossover experiments established exchange of anilide (but not imido) ligands in the presence of free hydrazine. These studies demonstrate the promising role of redox-active active ligands in promoting N–N bond cleavage with concomitant N–H bond formation and how the electronic properties of the metal–ligand combination influence N–H bond dissocation free energies and related hydrogen atom transfer processes.
 

Aug 18, 2014

Mechanism of Alcohol Oxidation Mediated by Copper(II) and Nitroxyl Radicals

Publication Date (Web): August 4, 2014 (Article)
DOI: 10.1021/ja5070137
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2,2′-Bipyridine-ligated copper complexes, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are highly effective catalysts for aerobic alcohol oxidation. Considerable uncertainty and debate exist over the mechanism of alcohol oxidation mediated by CuII and TEMPO. Here, we report experimental and density functional theory (DFT) computational studies that distinguish among numerous previously proposed mechanistic pathways. Oxidation of various classes of radical-probe substrates shows that long-lived radicals are not formed in the reaction. DFT computational studies support this conclusion. A bimolecular pathway involving hydrogen-atom-transfer from a CuII–alkoxide to a nitroxyl radical is higher in energy than hydrogen transfer from a CuII–alkoxide to a coordinated nitroxyl species. The data presented here reconcile a collection of diverse and seemingly contradictory experimental and computational data reported previously in the literature. The resulting Oppenauer-like reaction pathway further explains experimental trends in the relative reactivity of different classes of alcohols (benzylic versus aliphatic and primary versus secondary), as well as the different reactivity observed between TEMPO and bicyclic nitroxyls, such as ABNO (ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl).
 

Photochemical Tyrosine Oxidation in the Structurally Well-Defined α3Y Protein: Proton-Coupled Electron Transfer and a Long-Lived Tyrosine Radical

Publication Date (Web): August 4, 2014 (Article)
DOI: 10.1021/ja503348d
 
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Tyrosine oxidation–reduction involves proton-coupled electron transfer (PCET) and a reactive radical state. These properties are effectively controlled in enzymes that use tyrosine as a high-potential, one-electron redox cofactor. The α3Y model protein contains Y32, which can be reversibly oxidized and reduced in voltammetry measurements. Structural and kinetic properties of α3Y are presented. A solution NMR structural analysis reveals that Y32 is the most deeply buried residue in α3Y. Time-resolved spectroscopy using a soluble flash-quench generated [Ru(2,2′-bipyridine)3]3+ oxidant provides high-quality Y32–O• absorption spectra. The rate constant of Y32 oxidation (kPCET) is pH dependent: 1.4 × 104 M–1 s–1 (pH 5.5), 1.8 × 105 M–1 s–1 (pH 8.5), 5.4 × 103 M–1 s–1 (pD 5.5), and 4.0 × 104 M–1 s–1 (pD 8.5). kH/kD of Y32 oxidation is 2.5 ± 0.5 and 4.5 ± 0.9 at pH(D) 5.5 and 8.5, respectively. These pH and isotope characteristics suggest a concerted or stepwise, proton-first Y32 oxidation mechanism. The photochemical yield of Y32–O• is 28–58% versus the concentration of [Ru(2,2′-bipyridine)3]3+. Y32–O• decays slowly, t1/2 in the range of 2–10 s, at both pH 5.5 and 8.5, via radical–radical dimerization as shown by second-order kinetics and fluorescence data. The high stability of Y32–O• is discussed relative to the structural properties of the Y32 site. Finally, the static α3Y NMR structure cannot explain (i) how the phenolic proton released upon oxidation is removed or (ii) how two Y32–O• come together to form dityrosine. These observations suggest that the dynamic properties of the protein ensemble may play an essential role in controlling the PCET and radical decay characteristics of α3Y.
 

Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism: A Phase Transition Caused by Deuterium and Electron Transfer in a Hydrogen-Bonded Purely Organic Conductor Crystal

Publication Date (Web): August 15, 2014 (Article)
DOI: 10.1021/ja507132m


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A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O]−1-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1̅). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.

Nitric Oxide Reactivity of [2Fe-2S] Clusters Leading to H2S Generation

Publication Date (Web): August 11, 2014 (Communication)
DOI: 10.1021/ja505415c

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The crosstalk between two biologically important signaling molecules, nitric oxide (NO) and hydrogen sulfide (H2S), proceeds via elusive mechanism(s). Herein we report the formation of H2S by the action of NO on synthetic [2Fe-2S] clusters when the reaction environment is capable of providing a formal H (e/H+). Nitrosylation of (NEt4)2[Fe2S2(SPh)4] (1) in the presence of PhSH or tBu3PhOH results in the formation of (NEt4)[Fe(NO)2(SPh)2] (2) and H2S with the concomitant generation of PhSSPh or tBu3PhO. The amount of H2S generated is dependent on the electronic environment of the [2Fe-2S] cluster as well as the type of H donor. Employment of clusters with electron-donating groups or H donors from thiols leads to a larger amount of H2S evolution. The 1/NO reaction in the presence of PhSH exhibits biphasic decay kinetics with no deuterium kinetic isotope effect upon PhSD substitution. However, the rates of decay increase significantly with the use of 4-MeO-PhSH or 4-Me-PhSH in place of PhSH. These results provide the first chemical evidence to suggest that [Fe-S] clusters are likely to be a site for the crosstalk between NO and H2S in biology.

Jul 29, 2014

Photophysical Properties of cis-Mo2 Quadruply Bonded Complexes and Observation of Photoinduced Electron Transfer to Titanium Dioxide

Publication Date (Web): July 21, 2014 (Article)
DOI: 10.1021/ja504944d
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The compounds cis-Mo2(DAniF)2(L)2 have been prepared, where DAniF = (N,N′)-p-dianisyl formamidinate and L = thienyl-2-carboxylate (Th), 2,2′-bithienyl-5-carboxylate (BTh), and 2,2′:5′,5″-terthienyl-5-carboxylate (TTh). The compounds have been characterized by proton nuclear magnetic resonance (1H NMR), ultraviolet–visible (UV–vis) absorption and emission, differential pulse voltammetry, and time-resolved transient absorption and infrared (IR) spectroscopy. An X-ray crystal structure was obtained for the thienyl complex. The related salt [nBu4N]2[Mo2(DAniF)2(TTh–CO2)2], where TTh–CO2 = 2,2′:5′,2″-terthienyl-5,5″-dicarboxylate, has also been prepared and employed in the attachment of the complex to TiO2 nanoparticles. The latter have been characterized by ground-state Fourier transform infrared spectroscopy (FTIR) and femtosecond time-resolved IR spectroscopy. The time-resolved data provide evidence for sub-picosecond charge injection from the Mo2 center to the semiconducting oxide particle.