Ions Can Move a Proton‐Coupled Electron‐Transfer Reaction into Tunneling Regime

The effects of charged species on proton‐coupled electron‐transfer (PCET) reaction should be of significance for understanding/application of important chemical and biological PCET systems. Such species can be found in proximity of activated complex in a PCET reaction, although they are not involved in the charge transfer process. Reported here is the first study of the above‐mentioned effects. Here, the effects of Na⁺, K⁺, Li⁺, Ca²⁺, Mg²⁺, and Me₄N⁺ observed in PCET reaction of ascorbate monoanions with hexacyanoferrate(III) ions in H₂O reveal that, in presence of ions, this over‐the‐barrier reaction entered into tunneling regime. The observations are: a) dependence of the rate constant on the cation concentration, where the rate constant is 71 (at I = 0.0023), and 821 (at 0.5M K⁺), 847 (at 1.0M Na⁺), and 438 M ^?1 s^?1 (at 0.011M Ca²⁺); b) changes of kinetic isotope effect (KIE) in the presence of ions, where k_H/k_D=4.6 (at I = 0.0023), and 3.4 (in the presence of 0.5M K⁺), 3.3 (at 1.0M Na⁺), 3.9 (at 0.001M Ca²⁺), and 3.9 (at 0.001M Mg²⁺), respectively; c) the isotope effects on Arrhenius pre‐factor where A_H/A_D=0.97 (0.15) in absence of ions, and 2.29 (0.60) (at 0.5M Na⁺), 1.77 (0.29) (at 1.0M Na⁺), 1.61 (0.25) (at 0.5M K⁺), 0.42 (0.16) (at 0.001M Ca²⁺) and 0.16 (0.19) (at 0.001M Mg²⁺); d) isotope differences in the enthalpies of activation in H₂O and in D₂O, where ΔΔH^?(D,H)=3.9 (0.4) kJ mol^?1 in the absence of cations, 1.3 (0.6) at 0.5M Na⁺, 1.8 (0.4) at 0.5M K⁺, 1.5 (0.4) at 1.0M Na⁺, 5.5 (0.9) (at 0.001M Ca²⁺), and 7.9 (2.8) (at 0.001M Mg²⁺) kJ mol^?1; e) nonlinear proton inventory in reaction. In the H₂O/dioxane 1 : 1, the observed KIE is 7.8 and 4.4 in the absence and in the presence of 0.1M K⁺, respectively, and A_H/A_D=0.14 (0.03). The changes when cations are present in the reaction are explained in terms of termolecular encounter complex consisting of redox partners, and the cation where the cation can be found in a near proximity of the reaction‐activated complex thus influencing the proton/electron double tunneling event in the PCET process. A molecule of H₂O is involved in the transition state. The resulting ‘configuration’ is more ‘rigid’ and more appropriate for efficient tunneling with Na⁺ or K⁺ (extensive tunneling observed), i.e., there is more precise organized H transfer coordinate than in the case of Ca²⁺ and Mg²⁺ (moderate tunneling observed) in the reaction.     

Outer‐Sphere 2 e−/2 H+ Transfer Reactions of Ruthenium(II)‐Amine and Ruthenium(IV)‐Amido Complexes

A diverse set of 2 e⁻/2 H⁺ reactions are described that interconvert [Ru^II(bpy)(en*)₂]²⁺ and [Ru^IV(bpy)(en‐H*)₂]²⁺ (bpy=2,2′‐bipyridine, en*=H₂NCMe₂CMe₂NH₂, en*‐H=H₂NCMe₂CMe₂NH⁻), forming or cleaving different O−H, N−H, S−H, and C−H bonds. The reactions involve quinones, hydrazines, thiols, and 1,3‐cyclohexadiene. These proton‐coupled electron transfer reactions occur without substrate binding to the ruthenium center, but instead with precursor complex formation by hydrogen bonding. The free energies of the reactions vary over more than 90 kcal mol⁻¹, but the rates are more dependent on the type of X−H bond involved than the associated ΔG °. There is a kinetic preference for substrates that have the transferring hydrogen atoms in close proximity, such as ortho ‐tetrachlorobenzoquinone over its para ‐isomer and 1,3‐cyclohexadiene over its 1,4‐isomer, perhaps hinting at the potential for concerted 2 e⁻/2 H⁺ transfers.     

Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time‐Delayed Hydrogen Generation

While natural photosynthesis serves as the model system for efficient charge separation and decoupling of redox reactions, bio‐inspired artificial systems typically lack applicability owing to synthetic challenges and structural complexity. We present herein a simple and inexpensive system that, under solar irradiation, forms highly reductive radicals in the presence of an electron donor, with lifetimes exceeding the diurnal cycle. This radical species is formed within a cyanamide‐functionalized polymeric network of heptazine units and can give off its trapped electrons in the dark to yield H₂, triggered by a co‐catalyst, thus enabling the temporal decoupling of the light and dark reactions of photocatalytic hydrogen production through the radical′s longevity. The system introduced here thus demonstrates a new approach for storing sunlight as long‐lived radicals, and provides the structural basis for designing photocatalysts with long‐lived photo‐induced states.     

Polymeric Design for Electron Transfer in Photoinduced Hydrogen Generation through a Coil–Globule Transition

To realize a renewable energy society, a polymeric system for photoinduced hydrogen generation utilizing a copolymer containing an electron acceptor was designed. In this system, the redox changes of viologen introduced into poly(N‐isopropylacrylamide) cause cyclic conformational changes owing to the shifting of the phase transition temperature (PTT). The polymeric coil–globule transitions with hydrophilic/hydrophobic changes accelerate the electron transfer for hydrogen generation. In particular, hydrogen generation using visible‐light energy with high efficiency is achieved around the PTT. In contrast to conventional solution systems, our polymeric system enables efficient hydrogen generation in a close molecular arrangement without the aggregation of catalytic nanoparticles. The utilization of conformational changes will provide a new strategy for synthesizing artificial photosynthetic hydrogels that split water to generate both hydrogen and oxygen.     

A Ruthenium Complex–Porphyrin–Fullerene‐Linked Molecular Pentad as an Integrative Photosynthetic Model

A ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long‐lived hole–electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited‐state of a zinc porphyrin (¹ZnP*) was quenched by fullerene (C₆₀) to afford a radical ion pair, ^1,3(ZnP^.+‐C₆₀^.−). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (Ru^II) to ZnP^.+ to give the long‐lived charge‐separated state, Ru^III‐ZnP‐C₆₀^.−, with a lifetime of 14 μs. The ZnP worked as a visible‐light‐harvesting antenna, while the C₆₀ acted as an excellent electron acceptor. As a consequence, visible‐light‐driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.     

Proton‐Coupled Electron Transfer of Unsaturated Fatty Acids and Mechanistic Insight into Lipoxygenase

A proton‐coupled electron transfer (PCET) process plays an important role in the initial step of lipoxygenases to produce lipid radicals which can be oxygenated by reaction with O₂ to yield the hydroperoxides stereoselectively. The EPR spectroscopic detection of free lipid radicals and the oxygenated radicals (peroxyl radicals) together with the analysis of the EPR spectra has revealed the origin of the stereo‐ and regiochemistry of the reaction between O₂ and linoleyl (= (2Z)‐10‐carboxy‐1‐[(1Z)‐hept‐1‐enyl]dec‐2‐enyl) radical in lipoxygenases. The direct determination of the absolute rates of H‐atom‐transfer reactions from a series of unsaturated fatty acids to the cumylperoxyl (= (1‐methyl‐1‐phenylethyl)dioxy) radical by use of time‐resolved EPR at low temperatures together with detailed kinetic investigations on both photoinduced and thermal electron‐transfer oxidation of unsaturated fatty acids provides the solid energetic basis for the postulated PCET process in lipoxygenases. A strong interaction between linoleic acid (= (9Z,12Z)‐octadeca‐9,12‐dienoic acid) and the reactive center of the lipoxygenases (Fe^III? OH) is suggested to be involved to make a PCET process to occur efficiently, when an inner‐sphere electron transfer from linoleic acid to the Fe^III state is strongly coupled with the proton transfer to the OH group.     

Peroxyl Radical Reactions in Water Solution: A Gym for Proton‐Coupled Electron‐Transfer Theories

The reactions of alkylperoxyl radicals with phenols have remained difficult to investigate in water. We describe herein a simple and reliable method based on the inhibited autoxidation of water/THF mixtures, which we calibrated against pulse radiolysis. With this method we measured the rate constants k_inh for the reactions of 2‐tetrahydrofuranylperoxyl radicals with reference compounds: urate, ascorbate, ferrocenes, 2,2,5,7,8‐pentamethyl‐6‐chromanol, Trolox, 6‐hydroxy‐2,5,7,8‐tetramethylchroman‐2‐acetic acid, 2,6‐di‐tert‐butyl‐4‐methoxyphenol, 4‐methoxyphenol, catechol and 3,5‐di‐tert‐butylcatechol. The role of pH was investigated: the value of k_inh for Trolox and 4‐methoxyphenol increased 11‐ and 50‐fold from pH 2.1 to 12, respectively, which indicate the occurrence of a SPLET‐like mechanism. H(D) kinetic isotope effects combined with pH and solvent effects suggest that different types of proton‐coupled electron transfer (PCET) mechanisms are involved in water: less electron‐rich phenols react at low pH by concerted electron‐proton transfer (EPT) to the peroxyl radical, whereas more electron‐rich phenols and phenoxide anions react by multi‐site EPT in which water acts as proton relay.     

Poly(diallyldimethylammonium chloride) as an Efficient Phase Transfer Catalyst in Solvent‐Free Reduction of Carbonyl Compounds with NaBH4

Poly(diallyldimethylammonium chloride) resin was used as an efficient polymeric phase transfer catalyst, which catalyzed the chemoselective reduction of aldehydes and ketones by NaBH₄ to give the corresponding alcohols in high yields under solvent-free conditions.     

A Highly Active Iron‐Based Catalyst System for the AGET ATRP of Styrene

The atom transfer radical polymerization of styrene using activators generated by electron transfer (AGET ATRP) has been carried out in bulk in a limited amount of air at 110 °C, using 1,3,5‐(2′‐bromo‐2′‐methylpropionato)benzene (BMPB) as an initiator and FeCl₃ · 6H₂O/tris(3,6‐dioxaheptyl) amine (TDA‐1)/ascorbic acid (VC) as a novel Fe^III‐mediated catalyst system. The results of the polymerizations demonstrate the features of ‘living’/controlled free‐radical polymerization, such as the number‐average molecular weights being close to their corresponding theoretical values and increasing linearly with monomer conversion, and narrow molecular weight distributions ( = 1.18–1.26). The end functionality of the obtained polymers was confirmed by ¹H and ¹³C NMR spectra as well as a chain‐extension reaction.

     

Excited‐State Hydrogen Bonding Strengthening and Weakening with Intramolecular Charge Transfer in Resorufin‐Water Complexes: A TD‐DFT Study

The geometric structures, infrared spectra and hydrogen bond binding energies of the various hydrogen‐bonded Res^?‐water complexes in states S0 and S1 have been calculated using the density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) methods, respectively. Based on the changes of the hydrogen bond lengths and binding energies as well as the spectral shifts of the vibrational mode of the hydroxyl groups, it is demonstrated that hydrogen bonds HB‐II, HB‐III and HB‐IV are strengthened while hydrogen bond HB‐I is weakened in the four singly hydrogen‐bonded Res^?‐Water complexes upon photoexcitation. When the four hydrogen bonds are formed simultaneously between one resorufin anion and four water molecules in the Res^?‐4Water complex, all the hydrogen bonds are weakened in both the ground and excited states compared with those in the corresponding singly hydrogen‐bonded Res^?‐Water complexes. Furthermore, in complex Res^?‐4Water, hydrogen bonds HB‐II and HB‐IV are strengthened while hydrogen bonds HB‐I and HB‐III are weakened after the electronic excitation. The hydrogen bond strengthening and weakening in the various hydrogen‐bonded Res^?‐water complexes should be due to the redistribution of the charges among the four heteroatoms (O_1‐3 and N₁) within the resorufin molecule upon the optical excitation.     

水在超临界二氧化碳中作为直接氢供体：一种经典有效的Zn-H2O-CO2体系化学选择性还原硝基苯类化合物为芳香胺

本文报道了在绿色廉价的还原体系Zn-H2O-CO2中,一系列硝基苯类化合物高选择性,高产率(80-97%)地被还原成为相应芳香胺的过程。该反应体系以H2O作为氢源,以CO2作为反应溶剂,实现了还原反应的绿色化。     

A DFT Quantum‐Chemical Study of the Structure of Precursors and Active Sites of Catalyst Based on 2,6‐Bis(imino)pyridyl Fe(II) Complexes

Summary: A DFT method has been applied for quantum‐chemical calculations of the molecular structure of charge‐neutral complex LFeMe(μMe)₂AlMe₂ which is formed in system LFeMe₂ + AlMe₃ (L = 2,6‐bis(imino)pyridyl). Calculations suggested the formation of highly polarized complex LFeMe(μMe)₂AlMe₂ ( II ) in system LFeMe₂ + AlMe₃, characterized by r(Fe μMe) = 3.70 Å and r(Al μMe) = 2.08 Å and deficient electron density on fragment [LFeMe]^Q (Q = +0.80 e). Polarization of the complex progresses with the bounding of two AlMe₃ molecules (complex LFeMe(μMe)₂AlMe₂ · 2AlMe₃ ( III )) and with replacement of AlMe₃ by MeAlCl₂ (complex LFeMe(μMe)₂AlCl₂ ( IV )). The activation energy of ethylene insertion into the Fe Me bond of these complexes has been calculated. It was found that the heat of π‐complex formation increases with increasing of polarization extent in the order II < III < IV . Activation energy of the insertion of coordinated ethylene into Fe Me bond decreases in the same order: II > III > IV .

Calculated model complex (NH₃)₃FeMe₂; tridentate bis(imino)pyridyl ligand was substituted by three coplanar NH₃ groups.     

A Simple and Efficient Electrochemical Sensor for Electrocatalytic Reduction of Nitrite Based on Poly(4‐aminoacetanilide) Film Using Carbon Paste Electrode

In this study, the electrochemical reduction of nitrite was investigated on poly(4‐aminoacetanilide) (PPAA) forming by cyclic voltammetry at the surface of carbon paste electrode. The electrochemical properties of the modified electrode have been studied by cyclic voltammetry and double potential step chronoamperometry. Results showed that in the optimum condition (pH = 0.00) the reduction of nitrite occurred at a potential about 667 mV more positive than that unmodified carbon paste electrode. This amount of electrocatalytic ability is high compared with other electrocatalysts. Using a chronoamperometric method, the catalytic rate constant (k) was calculated 8.4 × 10⁴ cm³ mol^‐1 s^‐1. Also, the electrocatalytic reduction peak currents was found to be linear with the nitrite concentration in the ranges of 5 × 10^‐4 M to 2.5 × 10^‐2 M and 2 × 10^‐5 M to 7 × 10^‐3 M with detection limits (2σ) were determined as 4.5 × 10^‐4 M and 1 × 10^‐5 M by cyclic voltammetry (CV) and hydrodynamic amperometry methods respectively. Recovery experiments exhibit the satisfactory results.