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The safety of geotechnical engineering in cold region depends on the stability of supporting structure. Wet shotcrete technology (WST) has the advantages of less dust on the working surface, low rebound and high strength of shotcrete layer, so it has been gradually applied in roadway support of broken surrounding rock. In the cold environment, the decrease of the curing temperature of WST will have an impact on the development of shotcrete strength, but the influence of this temperature difference has not been fully considered in the current engineering practice. Aiming at the phenomenon of small thickness, low strength, and easy cracking of shotcrete in the application of WST in a mine in alpine environment, the temperature effect study on the strength evolution of WST was carried out. The research puts forward the optimization measures for WST support in the alpine environment, which effectively improves the structural strength of the shotcrete, the support repair rate is greatly reduced, and the support efficiency is significantly improved, it also provides reference for WST construction under similar engineering conditions.

Water oxidation is the key half reaction to achieve full splitting of water to hydrogen and oxygen. Herein, a binuclear complex, [(L4−)CoIII2(OH)]ClO4, was reported as a stable and efficient homogenous catalyst for electrocatalytic water oxidation in 0.1 M phosphate buffer (pH 7.0). Cyclic voltammetry experiments indicated that the catalytic process proceed via “one-site catalysis with two-sites oxidation” mechanism in which both two metal sites store the required oxidation equivalents for water oxidation and O–O bond formation occurs by single-site water nucleophilic attack (WNA).

Solar-powered water splitting is a dream reaction for constructing an artificial photosynthetic system for producing solar fuels. Natural photosystem II is a prototype template for research on artificial solar energy conversion by oxidizing water into molecular oxygen and supplying four electrons for fuel production. Although a range of synthetic molecular water oxidation catalysts have been developed, the understanding of O-O bond formation in this multielectron and multiproton catalytic process is limited, and thus water oxidation is still a big challenge. Herein, we report a trinuclear copper cluster that displays outstanding reactivity toward catalytic water oxidation inspired by multicopper oxidases (MCOs), which provides efficient catalytic four-electron reduction of O2 to water. This synthetic mimic exhibits a turnover frequency of 20000 s-1 in sodium bicarbonate solution, which is about 150 and 15 times higher than that of the mononuclear Cu catalyst (F-N2O2Cu, 131.6 s-1) and binuclear Cu2 complex (HappCu2, 1375 s-1), respectively. This work shows that the cooperation between multiple metals is an effective strategy to regulate the formation of O-O bond in water oxidation catalysis.

Water oxidation is the bottleneck of artificial photosynthesis. Since the first ruthenium-based molecular water oxidation catalyst, the blue dimer, was reported by Meyer's group in 1982, catalysts based on transition metals have been widely employed to explore the mechanism of water oxidation. Because the oxidation of water requires harsh oxidative conditions, the stability of transition complexes under the relevant catalytic conditions has always been a challenge. In this work, we report the redox properties of a CuIII complex (TAML-CuIII) with a redox-active macrocyclic ligand (TAML) and its reactivity toward catalytic water oxidation. TAML-CuIII displayed a completely different electrochemical behavior from that of the TAML-CoIII complex previously reported by our group. TAML-CuIII can only be oxidized by one-electron oxidation of the ligand to form TAML•+-CuIII and cannot achieve water activation through the ligand-centered proton-coupled electron transfer that takes place in the case of TAML-CoIII. The generated TAML•+-CuIII intermediate can undergo further oxidation and ligand hydrolysis with the assistance of borate anions, triggering the formation of a heterogeneous B/CuOx nanocatalyst. Therefore, the choice of the buffer solution has a significant influence on the electrochemical behavior and stability of molecular water oxidation catalysts.

Molecular catalysis of water oxidation has been intensively investigated, but its mechanism is still not yet fully understood. This study aims at capturing and identifying key short-lived intermediates directly during the water oxidation catalyzed by a cobalt-tetraamido macrocyclic ligand complex using a newly developed an in situ electrochemical mass spectrometry (EC-MS) method. Two key ligand-centered-oxidation intermediates, [(L

The proton-coupled electron transfer (PCET) reaction has drawn extensive attention for its widespread occurrence in chemistry, biology, and materials science. The mechanistic studies via model systems such as tyrosine and phenol oxidation have gradually deepened the understanding of PCET reactions, which was widely accepted and applied to bond activation and transformation. However, direct PCET activation of nonpolar bonds such as the C-H bond, O

Direct activation of O–H bond in water via hydrogen atom transfer (HAT) reactions is a challenge because of its high bond dissociation energy. In this work, a strong hydrogen atom abstracting reage...

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. RuV =O is considered a key intermediate to initiate the O-O bond formation through either a water nucleophilic attack (WNA) pathway or a bimolecular coupling (I2M) pathway. Herein, we report a Ru-based catalyst that displays water oxidation reactivity with RuIV =(O) with the help of a redox-active ligand at pH 7.0. The results of electrochemical studies and DFT calculations disclose that ligand oxidation could significantly improve the reactivity of RuIV =O toward water oxidation. Under these conditions, sustained water oxidation catalysis occurs at reasonable rates with low overpotential (ca. 183 mV).

Five molecular complexes with different non-noble metal centers were synthesized. The Co-based complex displays the highest photocatalytic performance for CO2 to CO conversion in aqueous media. It achieves high activity (TON = 41 017 and TOF = 3.80 s−1) and selectivity (87%) for the production of CO.

Solar-powered water splitting is a dream reaction for constructing an artificial photosynthetic system for producing solar fuels. Natural photosystem II is a prototype template for research on artificial solar energy conversion by oxidizing water into molecular oxygen and supplying four electrons for fuel production. Although a range of synthetic molecular water oxidation catalysts have been developed, the understanding of O-O bond formation in this multielectron and multiproton catalytic process is limited, and thus water oxidation is still a big challenge. Herein, we report a trinuclear copper cluster that displays outstanding reactivity toward catalytic water oxidation inspired by multicopper oxidases (MCOs), which provides efficient catalytic four-electron reduction of O

Photothermal therapy at the NIR-II biowindow (1000-1350 nm) is drawing increasing interest because of its large penetration depth and maximum permissible exposure. Now, the supramolecular radical dimer, fabricated by N,N'-dimethylated dipyridinium thiazolo[5,4-d]thiazole radical cation (MPT.+ ) and cucurbit[8]uril (CB[8]), achieves strong absorption at NIR-II biowindow. The supramolecular radical dimer (2MPT.+ -CB[8]) showed highly efficient photothermal conversion and improved stability, thus contributing to the strong inhibition on HegG2 cancer cell under 1064 nm irradiation even penetrating through chicken breast tissue. This work provides a novel approach to construct NIR-II chromophore by tailor-made assembly of organic radicals. It is anticipated that this study provides a new strategy to achieve NIR-II photothermal therapy and holds promises in luminescence materials, optoelectronic materials, and also biosensing.

Herein, we report the importance of structure regulation on the O-O bond formation process in binuclear iron catalysts. Three complexes, [Fe2 (μ-O)(OH2 )2 (TPA)2 ]4+ (1), [Fe2 (μ-O)(OH2 )2 (6-HPA)]4+ (2) and [Fe2 (μ-O)(OH2 )2 (BPMAN)]4+ (3), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO3 solution (pH 8.4). We found that 1 and 2 are molecular catalysts and that O-O bond formation proceeds via oxo-oxo coupling rather than by the water nucleophilic attack (WNA) pathway. In contrast, complex 3 displays negligible catalytic activity. DFT calculations suggested that the anti to syn isomerization of the two high-valent Fe=O moieties in these catalysts takes place via the axial rotation of one Fe=O unit around the Fe-O-Fe center. This is followed by the O-O bond formation via an oxo-oxo coupling pathway at the FeIV FeIV state or via oxo-oxyl coupling pathway at the FeIV FeV state. Importantly, the rigid BPMAN ligand in complex 3 limits the anti to syn isomerization and axial rotation of the Fe=O moiety, which accounts for the negligible catalytic activity.

Oxidation reactions are fundamental transformations in organic synthesis and chemical industry. With oxygen or air as terminal oxidant, aerobic oxidation catalysis provides the most sustainable and economic oxidation processes. Most aerobic oxidation catalysis employs redox metal as its active center. While nature provides non-redox metal strategy as in pyrroloquinoline quinone (PQQ)-dependent methanol dehydrogenases (MDH), such an effective chemical version is unknown. Inspired by the recently discovered rare earth metal-dependent enzyme Ln-MDH, here we show that an open-shell semi-quinone anionic radical species in complexing with lanthanum could serve as a very efficient aerobic oxidation catalyst under ambient conditions. In this catalyst, the lanthanum metal serves only as a Lewis acid promoter and the redox process occurs exclusively on the semiquinone ligand. The catalysis is initiated by 1e--reduction of lanthanum-activated ortho-quinone to a semiquinone-lanthanum complex La(SQ-.)2, which undergoes a coupled O-H/C-H dehydrogenation for aerobic oxidation of alcohols with up to 330 h-1 TOF. This study suggests a possible functional mode of semiquinone radicals, widely observed with quinoproteins in Nature. Moreover, this unique reductive activation strategy as well as the resulted radical anion as redox ligand creates a new turning point in the development of efficient aerobic oxidation catalysis.

Five molecular complexes with different non-noble metal centers were synthesized. The Co-based complex displays the highest photocatalytic performance for CO

The study of catalytic water splitting is one of the most active areas of research across many sub-disciplines of chemistry. To understand the mechanistic details and design artificial molecular catalysts for both water reduction (Hydrogen Evolution Reaction, HER) and water oxidation (Oxygen Evolution Reaction, OER) continue to be a challenge for the development of artificial photosynthetic system. This chapter will focus on the summarization of recent development in the rapidly growing field of artificial molecular catalysts with pincer ligand for both HER and OER.

An unsymmetrical di-copper complex, ([Cu2(TPMAN)(μ-OH)(H2O)]3+, was prepared and used for electrocatalytic water oxidation in neutral conditions. This complex is a stable and efficient homogeneous catalyst during the electrocatalytic oxygen evolution process ( kcat = 0.78 s-1) with 780 mV onset overpotential in 0.1 M phosphate buffer (pH 7.0). The water oxidation mechanism of the unsymmetrical catalyst [Cu2(TPMAN)(μ-OH)]3+ exhibits different behaviors than that of [Cu2(BPMAN)(μ-OH)]3+, such as two redox steps with different pH dependences, a significant kinetic isotope effect, and buffer concentration dependence. All these changes were ascribed to the open site on the Cu center that is formed by removal of the hemilabile pyridyl site, which acts as an intramolecular proton acceptor to assist the O-O bond formation step.

Visible-light-mediated C(sp3)-H activation has recently emerged as a powerful tool for functionalization of C H bonds. In this work, we reported that the triplet state of Lumiflavin is a good candidate to directly abstract hydrogen atom from C(sp3)-H substrates. Following photoexcitation by visible light (440 nm), the organic molecule Lumiflavin (LF) can form the excited-state 3*LF, which is competent to oxidize a range of C H substrates. Product analysis and kinetic data showed that these reactions occurred via rate determining hydrogen-atom-transfer (HAT), with a linear correlation for log(k) vs BDE (C H) and KIE = 3.2 for 9, 10-dihydroantracene (DHA).

Water oxidation is the key step in both natural and artificial photosynthesis to capture solar energy for fuel production. The design of highly efficient and stable molecular catalysts for water oxidation based on nonprecious metals is still a great challenge. In this article, the electrocatalytic oxidation of water by Na[(L4-)CoIII], where L is a substituted tetraamido macrocyclic ligand, was investigated in aqueous solution (pH 7.0). We found that Na[(L4-)CoIII] is a stable and efficient homogeneous catalyst for electrocatalytic water oxidation with 380 mV onset overpotential in 0.1 M phosphate buffer (pH 7.0). Both ligand- and metal-centered redox features are involved in the catalytic cycle. In this cycle, Na[(L4-)CoIII] was first oxidized to [(L2-)CoIIIOH] via a ligand-centered proton-coupled electron transfer process in the presence of water. After further losing an electron and a proton, the resting state, [(L2-)CoIIIOH], was converted to [(L2-)CoIV═O]. Density functional theory (DFT) calculations at the B3LYP-D3(BJ)/6-311++G(2df,2p)//B3LYP/6-31+G(d,p) level of theory confirmed the proposed catalytic cycle. According to both experimental and DFT results, phosphate-assisted water nucleophilic attack to [(L2-)CoIV═O] played a key role in O-O bond formation.

The performance of water oxidation catalysis by a Cu-based polypyridyl complex, [CuII(TPA)(OH2)]2+ (1H; TPA = tris-(pyridylmethyl)amine), has been investigated in neutral aqueous solution by electrochemical methods. Compared with our previously reported binuclear catalyst, [(BPMAN)(CuII)2(μ-OH)]3+ (2; BPMAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine), mononuclear catalyst 1 has a higher overpotential and lower catalytic activity toward water oxidation under the same conditions. Experimental results revealed that the O-O bond formation occurred via a water nucleophilic attack mechanism in which formal CuIV(O) is proposed as a key intermediate for the mononuclear catalyst 1H. In contrast, for the binuclear catalyst, O-O bond formation was facilitated by bimetallic cooperation between the two CuIII centers.

Water oxidation (2 H2O → 4H+ + 4 e- + O2) is a key step in utilization of solar energy for producing hydrogen of artificial photosynthetic system. Inspired by the oxygen evolution complex (OEC) in PSII, molecular catalysts are extensively studied and the progress both in biomimetic studies and the mechanistic understanding is truly remarkable. While research efforts in the earlier years focused on the Ru-, Mn- and Ir-based complexes, the focus has gradually changed to non-precious catalysts based on earth-abundant metals (Fe, Co, Ni, Cu). These studies have deepened our understanding on the mechanism of water oxidation, including both proton-coupled electron transfer and O-O bond formation. Herein, we briefly review the research progress surrounding the construction of the OEC-like catalytic systems as well as bioinspired multi-nuclear catalysts for water oxidation.

A chiral ion-pair photoredox organocatalyst was reported to facilitate visible-light-meditated asymmetric anti-Markovnikov hydroetherification of alkenols with high reactivity and moderate enantioselectivity.

The reactivity, and even reaction pathway, of excited states can be tuned by proton-coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Bronsted acid (3*BP-COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (3*BP). 3*BP-COOH could remove an electron from benzene at the rate of 8.0×105 m−1 s−1, in contrast to the reactivity of 3*BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Bronsted acid, which enables the reductive quenching of 3*BP by concerted electron–proton transfer.

Water splitting is one of the most promising solutions for storing solar energy in a chemical bond. Water oxidation is still the bottleneck step because of its inherent difficulty and the limited understanding of the O-O bond formation mechanism. Molecular catalysts provide a platform for understanding this process in depth and have received wide attention since the first Ru-based catalyst was reported in 1982. Ru

Here, we report the proton-coupled electron transfer reaction between TEMPO-H and Wurster's blue salt (TMPDA˙+). TMPDA˙+ could abstract a hydrogen atom from 2,2,6,6-tetramethyl-1-hydroxy-piperidine (TEMPO-H) via concerted electron-proton transfer and displayed a large KIE (7.05 ± 0.07 at 298 K). This work shows that a nitrogen-centered radical cation (R3N˙+) such as TMPDA˙+ is more than a weak single-electron oxidant, and can also be an effective PCET reagent for oxidative bond activation.

Simply mixing a Cu(II) salt and 1,2-ethylenediamine (en) affords precursors for both heterogeneous or homogeneous water oxidation catalysis, depending on pH. In phosphate buffer at pH 12, the Cu(II) en complex formed in solution is decomposed to give a phosphate-incorporated CuO/Cu(OH)2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm2 was obtained at an overpotential of 540 mV, a significant enhancement compared to other Cu-based surface catalysts. The results of electrolysis studies suggest that the solution en complex decomposes by en oxidation to glyoxal, following Cu(II) oxidation to Cu(III). At pH 8, the catalysis shifts from heterogeneous to homogeneous with a single-site mechanism for Cu(II)/en complexes in solution. A further decrease in pH to 7 leads to electrode passivation via the formation of a Cu(II) phosphate film during electrolyses. As the pH is decreased, en, with pKb ≈ 6.7, becomes less coordinating and the precipitation of the Cu(II) film inhibits w...

Water oxidation is the key step in both natural and artificial photosynthesis to capture solar energy for fuel production. The design of highly efficient and stable molecular catalysts for water oxidation based on nonprecious metals is still a great challenge. In this article, the electrocatalytic oxidation of water by Na[(L

A visible-light-mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosin Y as photocatalyst in hexafluoro-2-propanol. The catalyst system is also suitable for 1,3-dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.

Electrocatalytic water oxidation using the oxidatively robust 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine ligand (BPMAN)-based dinuclear copper(II) complex, [Cu2(BPMAN)(μ-OH)](3+), has been investigated. This catalyst exhibits high reactivity and stability towards water oxidation in neutral aqueous solutions. DFT calculations suggest that the O-O bond formation takes place by an intramolecular direct coupling mechanism rather than by a nucleophilic attack of water on the high-oxidation-state Cu(IV)=O moiety.

The complex Cu(II)(Py3P) (1) is an electrocatalyst for water oxidation to dioxygen in H2PO4(-)/HPO4(2-) buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84% Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single-site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom-proton transfer in a rate-limiting O-O bond-forming step with HPO4(2-) as the acceptor base or by concerted electron-proton transfer with electron transfer to the electrode and proton transfer to the HPO4(2-) base.

The development of an earth-abundant, first-row water oxidation catalyst that operates at neutral pH and low overpotential remains a fundamental chemical challenge. Herein, we report the first nickel-based robust homogeneous water oxidation catalyst, which can electrocatalyze water oxidation at neutral pH and low overpotential in phosphate buffer. The results of DFT calculations verify that the O-O bond formation in catalytic water oxidation prefers a HO-OH coupling mechanism from a cis-isomer of the catalyst.

Oxidation of the NADH analogue, N-benzyl-1,4-dihydronicotinamide (BNAH), by the 1e(-) acceptor, [Os(dmb)3](3+), and 2e(-)/2H(+) acceptor, benzoquinone (Q), has been investigated in aqueous solutions over extended pH and buffer concentration ranges by application of a double-mixing stopped-flow technique in order to explore the redox pathways available to this important redox cofactor. Our results indicate that oxidation by quinone is dominated by hydride transfer, and a pathway appears with added acids involving concerted hydride-proton transfer (HPT) in which synchronous transfer of hydride to one O-atom at Q and proton transfer to the second occurs driven by the formation of the stable H2Q product. Oxidation by [Os(dmb)3](3+) occurs by outer-sphere electron transfer including a pathway involving ion-pair preassociation of HPO4(2-) with the complex that may also involve a concerted proton transfer.

The six-coordinate Fe(III)-aqua complex [Fe(III)(dpaq)(H2O)](2+) (1, dpaq is 2-[bis(pyridine-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido) is an electrocatalyst for water oxidation in propylene carbonate-water mixtures. An electrochemical kinetics study has revealed that water oxidation occurs by oxidation to Fe(V)(O)(2+) followed by a reaction first order in catalyst and added water, respectively, with ko = 0.035(4) M(-1) s(-1) by the single-site mechanism found previously for Ru and Ir water oxidation catalysts. Sustained water oxidation catalysis occurs at a high surface area electrode to give O2 through at least 29 turnovers over an 15 h electrolysis period with a 45% Faradaic yield and no observable decomposition of the catalyst.

A self-assembly-formed triglycylglycine macrocyclic ligand (TGG(4-)) complex of Cu(II), [(TGG(4-))Cu(II)-OH(2)](2-), efficiently catalyzes water oxidation in a phosphate buffer at pH 11 at room temperature by a well-defined mechanism. In the mechanism, initial oxidation to Cu(III) is followed by further oxidation to a formal "Cu(IV)" with formation of a peroxide intermediate, which undergoes further oxidation to release oxygen and close the catalytic cycle. The catalyst exhibits high stability and activity toward water oxidation under these conditions with a high turnover frequency of 33 s(-1).

Benzoquinone/hydroquinone redox interconversion by the reversible Os(dmb)(3)(3+/2+) couple over an extended pH range with added acids and bases has revealed the existence of seven discrete pathways. Application of spectrophotometric monitoring with stopped-flow mixing has been used to explore the role of PCET. The results have revealed a role for phosphoric acid and acetate as proton donor and acceptor in the concerted electron-proton transfer reduction of benzoquinone and oxidation of hydroquinone, respectively.

A visible-light-mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosin Y as photocatalyst in hexafluoro-2-propanol. The catalyst system is also suitable for 1,3-dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.

Photoinitiated proton-coupled electron transfer (PCET) kinetics has been investigated in a series of four modified tyrosines linked to a ruthenium photosensitizer in acetonitrile, with each tyrosine bearing an internal hydrogen bond to a covalently linked pyridine or benzimidazole base. After correcting for differences in driving force, it is found that the intrinsic PCET rate constant still varies by 2 orders of magnitude. The differences in rates, as well as the magnitude of the kinetic isotope effect (KIE = kH/kD), both generally correlate with DFT calculated proton donor-acceptor distances. An Arrhenius analysis of temperature dependent data shows that the difference in reactivity arises primarily from differences in activation energies. We use this kinetic data to evaluate a commonly employed theoretical model for proton tunneling which includes a harmonic distribution of proton donor-acceptor distances due to vibrational motions of the molecule. Applying this model to the experimental data yields the conclusion that donor-acceptor compression is more facile in the compounds with shorter PT distance; however, this is contrary to independent calculations for the same compounds. This discrepancy is likely because the assumption in the model of Morse-shaped proton potential energy surfaces is inappropriate for (strongly) hydrogen-bonded systems. These results question the general applicability of this model. The results also suggest that a correlation of rate vs proton tunneling distance for the series of compounds is complicated by a concomitant variation of other relevant parameters.

Electrocatalytic reduction of carbon dioxide to carbon monoxide and of water/weak acids to hydrogen by a class of polypyridyl ruthenium complexes both occur by well-defined mechanisms involving common intermediates. Based on available mechanistic insight, conditions have been found for competitive electrocatalytic reduction of CO2-H2O-H2PO4(-) mixtures by a single metal complex catalyst to give syngas products with the H2 : CO ratio controllable by controlling the composition of the solutions.

The enthalpy changes of 47 five-membered heterocyclic compounds (ZH) [33 substituted 2,3-dihydro-2-phenylbenzo[d]imidazoles (1H-5H), 9 substituted 2,3-dihydro-2-phenylbenzo[d]thiazoles (6H), and 5 substituted 2,3-dihydro-2-phenylbenzo[d]oxazoles (7H)] as a class of very important organic hydride donors to release hydride anion were determined by using titration calorimetry. The result shows that the enthalpy change scale of the 47 ZH in acetonitrile ranges from 49.0 to 93.4 kcal/mol. Such a long energy scale evidently shows that the 47 ZH can construct a large and useful library of organic hydride donors, which can provide various organic hydride donors that the hydride-releasing enthalpies are known. The enthalpy changes of the 47 ZH to release hydrogen atom and the 47 ZH+* to release proton and hydrogen atom were also evaluated by using relative thermodynamic cycles according to Hess' law. The results show: (1) the enthalpy change scale of the 47 ZH to release hydrogen atom covers a range from 71.8 to 91.4 kcal/mol, indicating that the 47 ZH all should be weak hydrogen atom donors. (2) The enthalpy change scales of the 47 ZH+* to release proton and to release hydrogen atom range from 17.5 to 25.7 and from 27.2 to 52.4 kcal/mol, respectively, implying that the proton-donating abilities of ZH+* are generally quite larger than the corresponding hydrogen atom-donating abilities. The standard redox potentials of the 47 ZH and the 47 corresponding salts (Z+) were measured by using cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV), the results display that the standard oxidation potential scale of ZH ranges from -0.254 to -0.002 V for 1H-5H and from 0.310 to 0.638 V for 6H-7H, implying that 1H-5H should be strong one-electron reducing agents and 6H-7H should be weak one-electron reducing agents; the standard reduction potential scale of Z+ ranges from -1.832 to -2.200 V for 1+-5+ and from -1.052 to -1.483V for 6+-7+, meaning that 1+-5+ belong to very weak one-electron oxidation agents. The energies of the intramolecular hydrogen bond in 3H, 3H+*, and 3* with a hydroxyl group at ortho-position on the 2-phenyl ring were estimated by using experimental method, the results disclose that the hydrogen bond energy is 3.2, 2.8-3.0, and 3.9-4.0 kcal/mol for 3H, 3H+*, and 3* in acetonitrile, respectively, which is favorable for hydrogen atom transfer but unfavorable for hydride transfer from 3H. The relative effective charges on the active center in ZH, ZH+*, Z*, and Z+, which is an efficient measurement of electrophilicity or nucleophilicity as well as dimerizing ability of a chemical species, were estimated by using experimental method; the results indicate that 1*-5* belong to electron-sufficient carbon-radicals, 6*-7* belong to electron-deficient carbon radicals, they are all difficult to dimerize, and that 1+-5+ belong to weak electrophilic agents, 6+-7+ belong to strong electrophilic agents. All these information disclosed in this work could not only supply a gap of the chemical thermodynamics of the five-membered heterocyclic compounds as organic hydride donors, but also strongly promote the fast development of the chemistry and applications of the five-membered heterocyclic organic hydrides.

Water oxidation is the key step in natural and artificial photosynthesis for solar-energy conversion. As this process is thermodynamically unfavorable and is challenging from a kinetic point of view, the development of highly efficient catalysts with low energy cost is a subject of fundamental significance. Herein, we report on iron-based films as highly efficient water-oxidation catalysts. The films can be quickly deposited onto electrodes from Fe(II) ions in acetate buffer at pH 7.0 by simple cyclic voltammetry. The extremely low iron loading on the electrodes is critical for improved atom efficiency for catalysis. Our results showed that this film could catalyze water oxidation in neutral phosphate solution with a turnover frequency (TOF) of 756 h(-1) at an applied overpotential of 530 mV. The significance of this approach includes the use of earth-abundant iron, the fast and simple method for catalyst preparation, the low catalyst loading, and the large TOF for O2 evolution in neutral aqueous media.

Proton-coupled electron transfer (PCET) from tyrosine (TyrOH) to a covalently linked [Ru(bpy)(3)](2+) photosensitizer in aqueous media has been systematically reinvestigated by laser flash-quench kinetics as a model system for PCET in radical enzymes and in photochemical energy conversion. Previous kinetic studies on Ru-TyrOH molecules (Sjödin et al. J. Am. Chem. Soc. 2000, 122, 3932; Irebo et al. J. Am. Chem. Soc. 2007, 129, 15462) have established two mechanisms. Concerted electron-proton (CEP) transfer has been observed when pHpK(a)(TyrOH), which is pH-dependent but not first-order in [OH(-)] and not dependent on the buffer concentration when it is sufficiently low (less than ca. 5 mM). In addition, the pH-independent rate constant for electron transfer from tyrosine phenolate (TyrO(-)) was reported at pH10. Here we compare the PCET rates and kinetic isotope effects (k(H)/k(D)) of four Ru-TyrOH molecules with varying Ru(III/II) oxidant strengths over a pH range of 1-12.5. On the basis of these data, two additional mechanistic regimes were observed and identified through analysis of kinetic competition and kinetic isotope effects (KIE): (i) a mechanism dominating at low pH assigned to a stepwise electron-first PCET and (ii) a stepwise proton-first PCET with OH(-) as proton acceptor that dominates around pH = 10. The effect of solution pH and electrochemical potential of the Ru(III/II) oxidant on the competition between the different mechanisms is discussed. The systems investigated may serve as models for the mechanistic diversity of PCET reactions in general with water (H(2)O, OH(-)) as primary proton acceptor.