Electron transfer: classical approaches and new frontiers

Electron transfer, under conditions of weak interaction and a medium acting as a passive thermal bath, is very well understood. When electron transfer is accompanied by transient chemical bonding, such as in interfacial coordination electrochemical mechanisms, strong interaction and molecular selectivity are involved. These mechanisms, which take advantage of "passive self-organization," cannot yet be properly described theoretically, but they show substantial experimental promise for energy conversion and catalysis. The biggest challenge for the future, however, may be dynamic, self-organized electron transfer. As with other energy fluxes, a suitable positive feedback mechanism, through an active molecular environment, can lead to a (transient) decrease of entropy equivalent to an increase of molecular electronic order for the activated complex. A resulting substantial increase in the rate of electron transfer and the possibility of cooperative transfer of several electrons (without intermediates) can be deduced from phenomenological theory. The need to extend our present knowledge may be derived from the observation that chemical syntheses and fuel utilization in industry typically require high temperatures (where catalysis is less relevant), whereas corresponding processes in biological systems are catalyzed at environmental conditions. This article therefore focuses on interfacial or membrane-bound electron transfer and investigates an aspect that nature has developed to a high degree of perfection: self-organization.     

A stable silicon(0) compound with a Si=Si double bond

Dative, or nonoxidative, ligand coordination is common in transition metal complexes; however, this bonding motif is rare in compounds of main group elements in the formal oxidation state of zero. Here, we report that the potassium graphite reduction of the neutral hypervalent silicon-carbene complex L:SiCl4 {where L: is:C[N(2,6-Pri2-C6H3)CH]2 and Pri is isopropyl} produces L:(Cl)Si-Si(Cl):L, a carbene-stabilized bis-silylene, and L:Si=Si:L, a carbene-stabilized diatomic silicon molecule with the Si atoms in the formal oxidation state of zero. The Si-Si bond distance of 2.2294 +/- 0.0011 (standard deviation) angstroms in L:Si=Si:L is consistent with a Si=Si double bond. Complementary computational studies confirm the nature of the bonding in L:(Cl)Si-Si(Cl):L and L:Si=Si:L.     

A stable aminyl radical metal complex

Metal-stabilized phenoxyl radicals appear to be important intermediates in a variety of enzymatic oxidations. We report that transition metal coordination also supports an aminyl radical, resulting in a stable crystalline complex: [Rh(I)(trop2N.)(bipy)]+OTf- (where trop is 5-H-dibenzo[a,d]cycloheptene-5-yl, bipy is 2,2'-bipyridyl, OTf- is trifluorosulfonate). It is accessible under mild conditions by one-electron oxidation of the amide complex [Rh(I)(trop2N)(bipy)], at a potential of -0.55 volt versus ferrocene/ferrocenium. Both electron paramagnetic resonance spectroscopy and density functional theory support 57% localization of the unpaired spin at N. In reactions with H-atom donors, the Rh-coordinated aminyl behaves as a nucleophilic radical.     

Chemistry and chemical intermediates in supersonic free jet expansions

Short-lived intermediates often play key roles in determining the course of chemical reactions. Recently the combination of sophisticated laser techniques and supersonic free jet expansions has offered new insight into the structure and reactivity of such intermediates. Because of their extremely reactive nature the intermediates are produced in situ in the expansion. The free jet expansion provides cooling of the intermediates to very low temperatures, so that even complex organic free radicals and molecular ions can be identified and characterized. Radical-radical reactions and ionic cluster formation likewise proceed in the expansion and can be monitored by laser spectroscopy.     

硒缓解植物重金属胁迫和累积的机制

硒(Se)在提高植物抗逆性、缓解重金属胁迫以及降低植物对重金属吸收方面有着重要的作用。本文综述了Se参与缓解植物重金属胁迫和累积的机制,Se能够缓解重金属的胁迫,主要是因为在植物体内由Se转化而来的相关产物的生理生化作用产生的综合效果。Se是谷胱甘肽过氧化物酶(GSH-Px)的必需组分,GSH-Px利用谷胱甘肽(GSH)将有毒的过氧化物还原成无毒的物质,清除由重金属引起的自由基。Se可以激活植物螯合肽(PC)合成酶及增加PC合成的前体,使植物产生更多的PC,形成更多的重金属-PC配合物。Se还可以与重金属形成大分子的复合物,降低重金属的毒害。Se能够与多种重金属元素产生拮抗效应,降低植物对重金属的吸收。     

汽油电化学催化氧化脱硫机理探索

以乙硫醇、甲硫醚和噻吩为模型硫化物,分析反应前后模型硫化物的变化,探索汽油电化学催化氧化脱硫的机理。推测电化学催化氧化脱硫的机理,在酸性电解体系中为具有催化功能的金属离子Mn+,首先在阳极表面被氧化为金属离子M(n+m)+,后者将硫醇类氧化为二硫化物,将硫醚类氧化为亚砜,将噻吩氧化为噻砜。在碱性电解体系中为阳极表面的OH-离子首先失去电荷生成高活性的氢氧自由基,将硫醇类首先氧化为二硫化物,并进一步氧化为二亚砜;将硫醚类氧化为亚砜或砜,将噻吩类氧化为噻砜类有机硫化物。     

Organometallic chemistry in homogeneous catalysis

Many reactions catalyzed by soluble transition metal compounds proceed by way of organometallic intermediates, even though the original catalyst may be a simple salt. This generality is illustrated for three industrial syntheses of acetic acid that use homogeneous catalysts. Some developments in organometallic chemistry that may extend the utility of homogeneous catalysis are photoactivation of catalysts and the recognition of the importance of metallacyclic intermediates.     

Chemical and spectroscopic evidence for an FeV-oxo complex

Iron(V)-oxo species have been proposed as key reactive intermediates in the catalysis of oxygen-activating enzymes and synthetic catalysts. Here, we report the synthesis of [Fe(TAML)(O)]- in nearly quantitative yield, where TAML is a macrocyclic tetraamide ligand. Mass spectrometry, M?ssbauer, electron paramagnetic resonance, and x-ray absorption spectroscopies, as well as reactivity studies and density functional theory calculations show that this long-lived (hours at -60 degrees C) intermediate is a spin S = 1/2 iron(V)-oxo complex. Iron-TAML systems have proven to be efficient catalysts in the decomposition of numerous pollutants by hydrogen peroxide, and the species we characterized is a likely reactive intermediate in these reactions.     

Studying enzyme mechanism by 13C nuclear magnetic resonance

High-resolution carbon-13 nuclear magnetic resonance (NMR) spectra of enzyme-inhibitor and enzyme-substrate complexes provide detailed structural and stereochemical information on the mechanism of enzyme action. The proteases trypsin and papain are shown to form tetrahedrally coordinated complexes and acyl derivatives with a variety of compounds artificially enriched at the site or sites of interest. These results are compared with the structural information derived from x-ray diffraction. Detailed NMR studies have provided a clearer picture of the ionization state of the residues participating in enzyme-catalyzed processes than other more classical techniques. The dynamics of enzymic catalysis can be observed at sub-zero temperatures by a combination of cryoenzymology and carbon-13 NMR spectroscopy. With these powerful techniques, transient, covalently bound intermediates in enzyme-catalyzed reactions can be detected and their structures rigorously assigned.     

Exciplexes and electron transfer reactions

Electronically excited molecules, being better electron donors and acceptors than their ground states, form charge-transfer complexes (exciplexes) which can lead to radical ions. Exciplex emission is widely used to probe polymers and organized media such as membranes and micelles. Exciplexes are also intermediates in photoreactions that lead to unique products. Photochemical electron-transfer processes, which are the basis of silver halide photography and electrophotography, are involved in many reactions of wide scope. Recent studies have led to the discovery of several electron-transfer photooxygenations with a diversity that will probably rival that of singlet oxygen. Both exciplex emission and photochemical electron transfer play important roles in organic photochemistry.     

Tryptophan-accelerated electron flow through proteins

Energy flow in biological structures often requires submillisecond charge transport over long molecular distances. Kinetics modeling suggests that charge-transfer rates can be greatly enhanced by multistep electron tunneling in which redox-active amino acid side chains act as intermediate donors or acceptors. We report transient optical and infrared spectroscopic experiments that quantify the extent to which an intervening tryptophan residue can facilitate electron transfer between distant metal redox centers in a mutant Pseudomonas aeruginosa azurin. Cu(I) oxidation by a photoexcited Re(I)-diimine at position 124 on a histidine(124)-glycine(123)-tryptophan(122)-methionine(121) beta strand occurs in a few nanoseconds, fully two orders of magnitude faster than documented for single-step electron tunneling at a 19 angstrom donor-acceptor distance.     

吡嗪嘧啶铱(Ⅲ)配合物的合成及其磷光材料性质

该研究采用4-苯基嘧啶(PPY)作为辅助配体合成了新型的配合物(MDPP)2Ir(Cl)PPY(MDPP为5-甲基-2,3-二苯基吡嗪).紫外-可见吸收光谱显示该配合物在475nm处有三重态的金属到配体的电荷跃迁3MLCT吸收峰;该配合物的光致发光光谱中,在558nm处有相应的3MLCT发射峰.该发射峰较用乙酰丙酮(acac)作为辅助配体得到的配合物(MDPP)2Ir(acac)蓝移了22nm,这说明PPY是一类有潜在应用价值的辅助配体,可以用于金属配合物发光颜色的调节.同时,该文还讨论了天然化合物作为有机配体的可行性.     

Activation of alkanes with organotransition metal complexes

Alkanes, although plentiful enough to be considered for use as feedstocks in large-scale chemical processes, are so unreactive that relatively few chemical reagents have been developed to convert them to molecules having useful functional groups. However, a recently synthesized iridium (lr) complex successfully converts alkanes into hydridoalkylmetal complexes (M + R-H --> R-M-H). This is a dihydride having the formula Cp(*)(L)lrH(2), where Cp(*) and L are abbreviations for the ligands (CH(3))(5)C(5) and (CH(3))(3)P, respectively. Irradiation with ultraviolet light causes the dihydride to lose H(2), generating the reactive intermediate Cp(*)lrL. This intermediate reacts rapidly with C-H bonds in every molecule so far tested (including alkanes) and leads to hydridoalkyliridium complexes Cp(*)(L)lr(R)(H). Evidence has been obtained that this C-H insertion, or oxidative addition, reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. Thus the intermediate Cp(*)lrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity that is characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules-alkyl halides-by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp(*)lrL. Oxidative addition of the corresponding rhodium complexes Cp(*)RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable and undergo reductive elimination at -20 degrees C. These and other recent observations provide an incentive for reexamining the factors that have been assumed to control the rate of reaction of transition metal complexes with C-H bonds-notably the need for electron-rich metals and the proximity of reacting centers.     

Synthesis, structure, and reactivity of an iron(V) nitride

Despite being implicated as important intermediates, iron(V) compounds have proven very challenging to isolate and characterize. Here, we report the preparation of the iron(V) nitrido complex, [PhB((t)BuIm)(3)Fe(V)≡N]BAr(F24) (PhB((t)BuIm)(3)(-) = phenyltris(3-tert-butylimidazol-2-ylidene)borato, BAr(F24) = B(3,5-(CF(3))(2)C(6)H(3))(4)(-)), by one electron oxidation of the iron(IV) nitrido precursor. Single-crystal x-ray diffraction of the iron(V) complex reveals a four-coordinate metal ion with a terminal nitrido ligand. M??bauer and electron paramagnetic resonance spectroscopic characterization, supported by electronic structure calculations, provide evidence for a d(3) iron(V) metal center in a low spin (S = 1/2) electron configuration. Low-temperature reaction of the iron(V) nitrido complex with water under reducing conditions leads to high yields of ammonia with concomitant formation of an iron(II) species.     

Supramolecular chemistry: receptors, catalysts, and carriers

Supramolecular chemistry is the study of the structures and functions of the supermolecules that result from binding substrates to molecular receptors. Macropolycyclic receptors and coreceptors have been designed that form cryptate inclusion complexes and display molecular recognition towards spherical, tetrahedral, and linear substrates of various kinds (metal cations, inorganic anions, and organic or biological cations or anions). Anion binding has led to the development of anion coordination chemistry. Metalloreceptors simultaneously bind organic molecules and metal ions; speleands combine polar and nonpolar binding subunits. Receptors bearing reactive functional groups may act as molecular reagents or catalysts, performing a chemical transformation on the bound substrates (by such reactions as hydrogen transfer, ester cleavage, and protoadenosinetriphosphatase and protokinase activities). Receptors fitted with lipophilic groups can operate as molecular carriers, translocating bound species through a membrane; this transport can be coupled to chemical potentials (proton and redox gradients).     

Chemistry of molecular growth processes in flames

Chemical mechanisms of pyrolysis, growth, and oxidation processes in flames have traditionally been inferred from spatial profile measurements of species concentrations. Experimental investigations now include the detection of numerous minor species such as reactive radicals and intermediate hydrocarbons. In assessing a proposed mechanism important new constraints can be established when the detailed species profile data are combined with velocity and temperature measurements and analyzed to determine production and destruction rates for specific molecules. Recent results on hydrocarbon diffusion flames provide new information on the interplay between chemical and transport processes. These measurements have led to direct tests of proposed routes for the formation of aromatic hydrocarbons and the first, small soot particles. The inception chemistry of hydrocarbon growth reactions and initial particle formation is thought to control soot formation, flame radiation and energy transfer, and pollutant emission in combustion environments.     

Molecular dynamics of a cytochrome c-cytochrome b5 electron transfer complex

Cytochrome c and cytochrome b5 form an electrostatically associated electron transfer complex. Computer models of this and related complexes that were generated by docking the x-ray structures of the individual proteins have provided insight into the specificity and mechanism of electron transfer reactions. Previous static modeling studies were extended by molecular dynamics simulations of a cytochrome c-cytochrome b5 intermolecular complex. The simulations indicate that electrostatic interactions at the molecular interface results in a flexible association complex that samples alternative interheme geometries and molecular conformations. Many of these transient geometries appear to be more favorable for electron transfer than those formed in the initial model complex. Of particular interest is a conformational change that occurred in phenylalanine 82 of cytochrome c that allowed the phenyl side chain to bridge the two cytochrome heme groups.     

Solar Energy Conversion by Water Photodissociation: Transition metal complexes can provide low-energy cyclic systems for catalytic photodissociation of water

The basic concepts for direct and catalyzed photodissociation of water have been summarized. Water dissociation in closed-cycle processes based on endothermic photochemical reactions offers a potential solution to the solar energy conversion problem. Transition metal complexes, whose excited state chemistry is extremely rich (23, 24) although mostly unexplored, are, in principle, suitable "catalysts" for cycles of this type. The most interesting cycles are those involving metal hydrido complexes or binuclear complexes in which the two metal atoms are bound into a macrocyclic ligand. Systematic investigations of the photochemistry of transition metal complexes with the aim of designing suitable systems for solar energy conversion have long-range promise and merit further consideration.     

Shape changes of supported Rh nanoparticles during oxidation and reduction cycles

The microscopic insight into how and why catalytically active nanoparticles change their shape during oxidation and reduction reactions is a pivotal challenge in the fundamental understanding of heterogeneous catalysis. We report an oxygen-induced shape transformation of rhodium nanoparticles on magnesium oxide (001) substrates that is lifted upon carbon monoxide exposure at 600 kelvin. A Wulff analysis of high-resolution in situ x-ray diffraction, combined with transmission electron microscopy, shows that this phenomenon is driven by the formation of a oxygen-rhodium-oxygen surface oxide at the rhodium nanofacets. This experimental access into the behavior of such nanoparticles during a catalytic cycle is useful for the development of improved heterogeneous catalysts.