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The one-step oxidation of benzene to phenol represents a significant and promising advancement in modern industries focused on the production of high-value-added chemical products. Nevertheless, challenges persist in achieving sufficient catalytic selectivity and preventing over-oxidation. Inspired by copper enzymes, we present a nonsymmetric dicopper complex ([CuII2(TPMAN)(μ-OH)(H 2 O)] 3+ , 1) for the selective oxidation of benzene to phenol. Utilizing H 2 O 2 as the oxidant, complex 1 demonstrates remarkable catalytic activity (a TON of 14 000 within 29 hours) and selectivity exceeding 97%, comparable to the finest homogeneous catalyst derived from first-row transition metals. It is noteworthy that the significant substituent effect, alongside a negligible kinetic isotope effect (KIE = 1.05), radical trapping experiments, and an inconsistent standard selectivity test of the ˙OH radicals, all contradict the conventional Fenton mechanism and rebound pathway. Theoretical investigations indicate that the active Cu II (μ-O˙)Cu II -OH species generated through the cleavage of the O-O bond in the Cu II (μ-1,1-OOH)Cu I intermediate facilitates the hydroxylation of benzene via an electrophilic attack mechanism. The nonsymmetric coordination geometry is crucial in activating H 2 O 2 and in the process of O-O bond cleavage.

Water oxidation is a crucial reaction in the artificial photosynthesis system. In the present work, density functional calculations were employed to decipher the mechanism of water oxidation catalyzed by a binuclear cobalt complex, which was disclosed to be a homogeneous water oxidation catalyst in pH=7 phosphate buffer. The calculations showed that the catalytic cycle starts from the Co III,III -OH 2 species. Then, a proton-coupled electron transfer followed by a one-electron transfer process leads to the generation of the formal Co IV,IV -OH intermediate . The subsequent PCET produces the active species, namely the formal Co IV,V =O intermediate (4). The oxidation processes mainly occur on the ligand moiety, including the coordinated water moiety, implying a redox non-innocent behavior. Two cobalt centers keep their oxidation states and provide one catalytic center for water activation during the oxidation process. 4 triggers the O-O bond formation via the water nucleophilic attack pathway, in which the phosphate buffer ion functions as the proton acceptor. The O-O bond formation is the rate-limiting step with a calculated total barrier of 17.7 kcal/mol. The last electron oxidation process coupled with an intramolecular electron transfer results in the generation of O 2 ., (© 2024 Wiley-VCH GmbH.)

How the factors affecting the formation and transformation of the intermediates in Pd(II)-catalyzed aromatic C-H activation: A comprehensive study with the Pd(II)/LA platform. Using the Pd(II)/Lewis acid (LA)-catalyzed C-H activation of electron-rich acetanilides as a platform, the C-H activation intermediates, including the precomplex η 2 -intermediate, the agostic hydrogen intermediate, and the palladacycle compound have been well-characterized. This work presents how the catalyst source, substrate, and solvent affect the formation of the η 2 -intermediate and its equilibrium with the agostic hydrogen intermediate, and the transformation of the agostic hydrogen intermediate to the palladacycle compound through C-H activation. The findings disclosed above are provided as a guideline for the catalyst design of the oxidative olefination of acetanilide with dioxygen, and the catalytic efficiency matched well with the mechanistic findings.

Hydroxyprolines are abundant in nature and widely utilized by many living organisms. Isomerization of trans-4-hydroxy-d-proline (t4D-HP) to generate 2-amino-4-ketopentanoate has been found to need a glycyl radical enzyme HplG, which catalyzes the cleavage of the C-N bond, while dehydration of trans-4-hydroxy-l-proline involves a homologous enzyme of HplG. Herein, molecular dynamics simulations and quantum mechanics/molecular mechanics (QM/MM) calculations are employed to understand the reaction mechanism of HplG. Two possible reaction pathways of HplG have been explored to decipher the origin of its chemoselectivity. The QM/MM calculations reveal that the isomerization proceeds via an initial hydrogen shift from the Cγ site of t4D-HP to a catalytic cysteine radical, followed by cleavage of the Cδ-N bond in t4D-HP to form a radical intermediate that captures a hydrogen atom from the cysteine. Activation of the Cδ-H bond in t4D-HP to bring about dehydration of t4D-HP possesses an extremely high energy barrier, thus rendering the dehydration pathway implausible in HplG. On the basis of the current calculations, conserved residue Glu429 plays a pivotal role in the isomerization pathway: the hydrogen bonding between it and t4D-HP weakens the hydroxyalkyl Cγ-Hγ bond, and it acts as a proton acceptor to trigger the cleavage of the C-N bond in t4D-HP. Our current QM/MM calculations rationalize the origin of the experimentally observed chemoselectivity of HplG and propose an H-bond-assisted bond activation strategy in radical-containing enzymes. These findings have general implications on radical-mediated enzymatic catalysis and expand our understanding of how nature wisely and selectively activates the C-H bond to modulate catalytic selectivity.

The direct double dehydrogenation from primary amines to nitriles without an oxidant or hydrogen acceptor is both intriguing and challenging. In this paper, we describe a non-noble metal catalyst capable of realizing such a transformation with high efficiency. A cobalt-centered N , N- bidentate complex was designed and employed as a metal-ligand cooperative dehydrogenation catalyst. Detailed kinetic studies, control experiments, and DFT calculations revealed the crucial hydride transfer, proton transfer, and hydrogen evolution processes. Finally, a tandem outer-sphere/inner-sphere mechanism was proposed for the dehydrogenation of amines to nitriles through an imine intermediate.

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy 2 PC 6 H 4 )HZnEt ( 3 ) undergoes reversible intramolecular C aryl -H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy 2 PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H 2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D) 2 -Zn upon reaction with D 2 .

In this study, we highlight the impact of catalyst geometry on the formation of O-O bonds in Cu 2 and Fe 2 catalysts. A series of Cu 2 complexes with diverse linkers are designed as electrocatalysts for water oxidation. Interestingly, the catalytic performance of these Cu 2 complexes is enhanced as their molecular skeletons become more rigid, which contrasts with the behavior observed in our previous investigation with Fe 2 analogs. Moreover, mechanistic studies reveal that the reactivity of the bridging O atom results in distinct pathways for O-O bond formation in Cu 2 and Fe 2 catalysts. In Cu 2 systems, the coupling takes place between a terminal Cu III -OH and a bridging μ-O⋅ radical. Whereas in Fe 2 systems, it involves the coupling of two terminal Fe-oxo entities. Furthermore, an in-depth structure-activity analysis uncovers the spatial geometric prerequisites for the coupling of the terminal OH with the bridging μ-O⋅ radical, ultimately leading to the O-O bond formation. Overall, this study emphasizes the critical role of precisely adjusting the spatial geometry of catalysts to align with the O-O bonding pathway., (© 2024 Wiley-VCH GmbH.)

Small molecules with conformationally rigid, three-dimensional geometry are highly desirable in drug development, toward which a direct, simple-to-complexity synthetic logic is still of considerable challenges. Here, we report intermolecular aza-[2 + 2] photocycloaddition (the aza-Paternò-Büchi reaction) of indole that facilely assembles planar building blocks into ladder-shape azetidine-fused indoline pentacycles with contiguous quaternary carbons, divergent head-to-head/head-to-tail regioselectivity, and absolute exo stereoselectivity. These products exhibit marked three-dimensionality, many of which possess 3D score values distributed in the highest 0.5% region with reference to structures from DrugBank database. Mechanistic studies elucidated the origin of the observed regio- and stereoselectivities, which arise from distortion-controlled C-N coupling scenarios. This study expands the synthetic repertoire of energy transfer catalysis for accessing structurally intriguing architectures with high molecular complexity and underexplored topological chemical space., (© 2024. The Author(s).)

Diels-Alder cycloaddition of anthracene with olefin is achieved in a homogeneous solution via energy transfer under visible light. A series of substrates including electroneutral styrene derivatives can be successfully converted into the corresponding cycloadducts in a head-to-head orientation with high to excellent yields. The high ortho-regioselectivity, mild condition, and broad substrate scope enable promising advances in organic transformation.

An unprecedented dearomatization of [2.2]paracyclophane-derived cyclic N -sulfonylimines was conducted through cyclopropanation with sulfur ylides, giving a series of dearomative cyclopropanes with good yields. DFT calculations suggested that the dearomatization was attributed to the relatively weak aromaticity of [2.2]paracyclophane derivatives that resulted from the effect of the unique [2.2]paracyclophane skeleton and the electron-withdrawing N -sulfonyl group. Some downstream elaborations of the products were demonstrated.

The tetradentate PNNP-type Ir III complex Mes-IrPCY2 ([Cl-Ir III -H] + ) is reported to be an efficient catalyst for the reduction of CO 2 to formate with excellent selectivity under visible light irradiation. Density functional calculations have been carried out to elucidate the mechanism and the origin of selectivity in the present work. Calculations suggest that the double-reduced complex 1-H ( 1 [Ir I -H] 0 ) demonstrates higher activity than the single-reduced complex 2-H ( 2 [Ir III (L˙ - )-H] + ), possibly owing to the higher hydride donor ability of the former compared to the latter; thus 1-H functions as the active species in the overall CO 2 reduction reaction. In the HCOO - formation pathway, the hydride of 1-H performs a nucleophilic attack on CO 2 via an outer-sphere fashion to generate species 1-OCHO ( 1 [Ir I -OCHO] 0 ), which then releases HCOO - to produce an Ir I intermediate. A subsequent protonation and chloride coordination of the Ir center leads to the regeneration of catalyst 1 [Cl-Ir III -H] + . For the CO production, a nucleophilic attack on CO 2 takes place by the Ir atom of 1-H via an inner-sphere manner to afford complex O2C-3-H ( 1 [O 2 C-Ir III -H] 0 ), followed by a two-proton-one-electron reduction to furnish the OC-2-H complex ( 2 [OC-Ir III (L˙ - )-H] + ) after liberating a H 2 O. Ultimately, CO is released to form 2-H. The stronger nucleophilicity as well as smaller steric hindrance of the hydride than the Ir atom of the active species 1-H ( 1 [Ir I -H] 0 ) is found to account for the favoring of formate formation over CO formation. Meanwhile, the CO 2 reduction reaction is calculated to be preferred over the hydrogen evolution reaction, and this is consistent with the experimental product distributions.

Exploring bidirectional CO 2 /HCO 2 - catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO 2 /HCO 2 - electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph 2 PC 6 H 4 S - ). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L - ) 2 Fe(III)(μ-HCO 2 )Fe(III)] + , which exhibits remarkable electrocatalytic performance for the HCO 2 - -to-CO 2 transformation with a maximum turnover frequency (TOF max ) ∼10 3 s -1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc +/0 ) and exhibits an impressive FE ∼93 (±3)% for the CO 2 -to-HCO 2 - conversion. Mechanism studies revealed that the HCO 2 - -to-CO 2 electrocatalysis passes through dicationic [(L 2 ) -• Fe(III)(μ-HCO 2 )Fe(III)] 2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO 2 -to-HCO 2 - reduction involves a critical intermediate of [Fe(II)-H] - that was independently synthesized and structurally characterized.

Iron tetraphenylporphyrin complex modified with four trimethylammonium groups (Fe- p -TMA) is found to be capable of catalyzing the eight-electron eight-proton reduction of CO 2 to CH 4 photochemically in acetonitrile. In the present work, density functional theory (DFT) calculations have been performed to investigate the reaction mechanism and to rationalize the product selectivity. Our results revealed that the initial catalyst Fe- p -TMA ([Cl-Fe(III)-LR 4 ] 4+ , where L = tetraphenylporphyrin ligand with a total charge of -2, and R 4 = four trimethylammonium groups with a total charge of +4) undergoes three reduction steps, accompanied by the dissociation of the chloride ion to form [Fe(II)-L ••2- R 4 ] 2+ . [Fe(II)-L ••2- R 4 ] 2+ , bearing a Fe(II) center ferromagnetically coupled with a tetraphenylporphyrin diradical, performs a nucleophilic attack on CO 2 to produce the 1 η-CO 2 adduct [CO 2 •- -Fe(II)-L •- R 4 ] 2+ . Two intermolecular proton transfer steps then take place at the CO 2 moiety of [CO 2 •- -Fe(II)-L •- R 4 ] 2+ , resulting in the cleavage of the C-O bond and the formation of the critical intermediate [Fe(II)-CO] 4+ after releasing a water molecule. Subsequently, [Fe(II)-CO] 4+ accepts three electrons and one proton to generate [CHO-Fe(II)-L •- R 4 ] 2+ , which finally undergoes a successive four-electron-five-proton reduction to produce methane without forming formaldehyde, methanol, or formate. Notably, the redox non-innocent tetraphenylporphyrin ligand was found to play an important role in CO 2 reduction since it could accept and transfer electron(s) during catalysis, thus keeping the ferrous ion at a relatively high oxidation state. Hydrogen evolution reaction via the formation of Fe-hydride ([Fe(II)-H] 3+ ) turns out to endure a higher total barrier than the CO 2 reduction reaction, therefore providing a reasonable explanation for the origin of the product selectivity.

Herein, we disclose the highly enantioselective oxidative cross-coupling of 3-hydroxyindole esters with various nucleophilic partners as catalyzed by copper efflux oxidase. The biocatalytic transformation delivers functionalized 2,2-disubstituted indolin-3-ones with excellent optical purity (90-99 % ee), which exhibited anticancer activity against MCF-7 cell lines, as shown by preliminary biological evaluation. Mechanistic studies and molecular docking results suggest the formation of a phenoxyl radical and enantiocontrol facilitated by a suited enzyme chiral pocket. This study is significant with regard to expanding the catalytic repertoire of natural multicopper oxidases as well as enlarging the synthetic toolbox for sustainable asymmetric oxidative coupling., (© 2023 Wiley-VCH GmbH.)

In canonical organic chemistry textbooks, the widely adopted mechanism for the classic transetherifications between ethers and alcohols starts with the activation of the ether in order to weaken the C-O bond, followed by the nucleophilic attack by the alcohol hydroxy group, resulting in a net C-O/O-H σ-bond metathesis. In this manuscript, our experimental and computational investigation of a Re 2 O 7 mediated ring-closing transetherification challenges the fundamental tenets of the traditional transetherification mechanism. Instead of ether activation, the alternative activation of the hydroxy group followed by nucleophilic attack of ether is realized by commercially available Re 2 O 7 through the formation of perrhenate ester intermediate in hexafluoroisopropanol (HFIP), which results in an unusual C-O/C-O σ-bond metathesis. Due to the preference for the activation of alcohol rather than ether, this intramolecular transetherification reaction is therefore suitable for substrates bearing multiple ether moieties, unparalleled by any previous methods., (© 2023. The Author(s).)

Density functional theory calculations have been utilized to investigate the synergistic effect of palladium(0) and copper(I) catalyzed selective formation of allylated indole from (2-alkynyl)-phenylisocyanate and allyl methyl carbonate. The main competing reaction yielding the N-allyl carbamate was also considered. Calculated results indicate that using the Pd(0)-complex alone, the generation of indole is kinetically much less favored than producing the N-allyl carbamate compound. However, the co-addition of Cu(I) catalyst can considerably decrease the barrier for the intramolecular cyclization step, leading to the formation of the indole product. Analysis of the cyclization process suggests that Cu(I) complex can act as a Lewis acid to activate the linear alkyne group via a π-coordination manner prior to the formation of a 5-membered ring transition state toward indole formation. Altogether, the mechanistic insights revealed in the present study aim at a better understanding of the mechanism and the factors governing the selectivity in synergistic Pd/Cu catalysis., (© 2023 Wiley-VCH GmbH.)

The cytochrome bd oxygen reductase catalyzes the four-electron reduction of dioxygen to two water molecules. The structure of this enzyme reveals three heme molecules in the active site, which differs from that of heme-copper cytochrome c oxidase. The quantum chemical cluster approach was used to uncover the reaction mechanism of this intriguing metalloenzyme. The calculations suggested that a proton-coupled electron transfer reduction occurs first to generate a ferrous heme b 595 . This is followed by the dioxygen binding at the heme d center coupled with an outer-sphere electron transfer from the ferrous heme b 595 to the dioxygen moiety, affording a ferric ion superoxide intermediate. A second proton-coupled electron transfer produces a heme d ferric hydroperoxide, which undergoes efficient O-O bond cleavage facilitated by an outer-sphere electron transfer from the ferrous heme b 595 to the O-O σ* orbital and an inner-sphere proton transfer from the heme d hydroxyl group to the leaving hydroxide. The synergistic benefits of the two types of hemes rationalize the highly efficient oxygen reduction repertoire for the multi-heme-dependent cytochrome bd oxygen reductase family.

Density functional theory (DFT) calculations were conducted to investigate the cobalt porphyrin-catalyzed electro-reduction of CO 2 to CO in an aqueous solution. The results suggest that Co II -porphyrin (Co II -L) undertakes a ligand-based reduction to generate the active species Co II -L⋅ - , where the Co II center antiferromagnetically interacts with the ligand radical anion. Co II -L⋅ - then performs a nucleophilic attack on CO 2 , followed by protonation and a reduction to give Co II -L-COOH. An intermolecular proton transfer leads to the heterolytic cleavage of the C-O bond, producing intermediate Co II -L-CO. Subsequently, CO is released from Co II -L-CO, and Co II -L is regenerated to catalyze the next cycle. The rate-determining step of this CO 2 RR is the nucleophilic attack on CO 2 by Co II -L⋅ - , with a total barrier of 20.7 kcal mol -1 . The competing hydrogen evolution reaction is associated with a higher total barrier. A computational investigation regarding the substituent effects of the catalyst indicates that the CoPor-R3 complex is likely to display the highest activity and selectivity as a molecular catalyst., (© 2023 The Authors. Published by Wiley-VCH GmbH.)

TokK is a B 12 -dependent radical SAM enzyme involved in the biosynthesis of the β-lactam antibiotic asparenomycin A. It can catalyze three methylations on different sp 3 -hybridized carbon positions to introduce an isopropyl side chain at the β-lactam ring of pantetheinylated carbapenem. Herein, we report a quantum chemical study of the reaction mechanism of TokK. A stepwise ''push-pull'' radical relay mechanism is proposed for each methylation: a 5'-deoxyadenosine radical first abstracts a hydrogen atom from the substrate in the active site, then methylcobalamin directionally donates a methyl group to the substrate. More importantly, calculations were able to uncover the origin of observed chemoselectivity and stereoselectivity for the first methylation and regioselectivity for the following two methylations. Further detailed distortion/interaction analysis can help to unravel the main factors controlling the selectivities. Our findings of sequential methylations by TokK could have profound implications for studying other B 12 -dependent radical SAM enzymes., (© 2022 Wiley-VCH GmbH.)

The design and production of active, durable, and nonprecious electrocatalysts toward alkaline hydrogen oxidation and evolution reactions (HOR/HER) are extremely appealing for the implementation of hydrogen economy, but remain challenging. Here, we report a facile electric shock synthesis of an efficient, stable, and inexpensive NiCoCuMoW multi-element alloy on Ni foam (NiCoCuMoW) as a bifunctional electrocatalyst for both HOR and HER. For the HOR, the current density of NiCoCuMoW could reach ∼11.2 mA cm -2 when the overpotential is 100 mV, higher than that for commercial Pt/C (∼7.2 mA cm -2 ) and control alloy samples with less elements, along with superior CO tolerance. Moreover, for the HER, the overpotential at 10 mA cm -2 for NiCoCuMoW is only 21 mV, along with a Tafel slope of low to 63.7 mV dec -1 , rivaling the commercial Pt/C as well (35 mV and 109.7 mV dec -1 ). Density functional theory calculations indicate that alloying Ni, Co, Cu, Mo, and W can tune the electronic structure of individual metals and provide multiple active sites to optimize the hydrogen and hydroxyl intermediates adsorption, collaboratively resulting in enhanced electrocatalytic activity., Competing Interests: The authors declare no conflict of interest., (© 2022 The Authors. Exploration published by Henan University and John Wiley & Sons Australia, Ltd.)

Naturally evolved enzymes, despite their astonishingly large variety and functional diversity, operate predominantly through thermochemical activation. Integrating prominent photocatalysis modes into proteins, such as triplet energy transfer, could create artificial photoenzymes that expand the scope of natural biocatalysis 1-3 . Here, we exploit genetically reprogrammed, chemically evolved photoenzymes embedded with a synthetic triplet photosensitizer that are capable of excited-state enantio-induction 4-6 . Structural optimization through four rounds of directed evolution afforded proficient variants for the enantioselective intramolecular [2+2]-photocycloaddition of indole derivatives with good substrate generality and excellent enantioselectivities (up to 99% enantiomeric excess). A crystal structure of the photoenzyme-substrate complex elucidated the non-covalent interactions that mediate the reaction stereochemistry. This study expands the energy transfer reactivity 7-10 of artificial triplet photoenzymes in a supramolecular protein cavity and unlocks an integrated approach to valuable enantioselective photochemical synthesis that is not accessible with either the synthetic or the biological world alone., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Density functional theory (DFT) calculations have been conducted to investigate the mechanism of cobalt(II) tetraamino phthalocyanine ( CoPc-NH 2 ) catalyzed electro-reduction of CO 2 . Computational results show that the catalytically active species 1 ( 4 [Co II (H 4 L)] 0 ) is formed by a four-electron-four-proton reduction of the initial catalyst CoPc-NH 2 . Complex 1 can attack CO 2 after a one-electron reduction to give a [Co III -CO 2 2- ] - intermediate, followed by a protonation and a one-electron reduction to give intermediate [Co II -COOH] - ( 4 ). Complex 4 is then protonated on its hydroxyl group by a carbonic acid to generate the critical species 6 (Co III -L •- -CO), which can release the carbon monoxide as an intermediate (and also as a product). In parallel, complex 6 can go through a successive four-electron-four-proton reduction to produce the targeted product methanol without forming formaldehyde as an intermediate product. The high-lying π orbital and the low-lying π* orbital of the phthalocyanine endow the redox noninnocent nature of the ligand, which could be a dianion, a radical monoanion, or a radical trianion during the catalysis. The calculated results for the hydrogen evolution reaction indicate a higher energy barrier than the carbon dioxide reduction. This is consistent with the product distribution in the experiments. Additionally, the amino group on the phthalocyanine ligand was found to have a minor effect on the barriers of critical steps, and this accounts for the experimentally observed similar activity for these two catalysts, namely, CoPc-NH 2 and CoPc .

Direct C Ar -F arylation is effective and sustainable for synthesis of polyfluorobiaryls with different degrees of fluorination, which are important motifs in medical and material chemistry. However, with no aid of transition metals, the engagement of C Ar -F bond activation has proved difficult. Herein, an unprecedented transition-metal-free strategy is reported for site-selective C Ar -F arylation of polyfluoroarenes with simple (het)arenes. By merging N , N -bis(2,6-diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide)-catalyzed electrophotocatalytic reduction and anodic nitroxyl radical oxidation in an electrophotocatalytic cell, various polyfluoroaromatics (2F-6F and 8F), especially inactive partially fluorinated aromatics, undergo sacrificial-reagents-free C-F bond arylation with high regioselectivity, and the yields are comparable to those for reported transition-metal catalysis. This atom- and step-economic protocol features a paired electrocatalysis with organic mediators in both cathodic and anodic processes. The broad substrate scope and good functional-group compatibility highlight the merits of this operationally simple strategy. Moreover, the easy gram-scale synthesis and late-stage functionalization collectively advocate for the practical value, which would promote the vigorous development of fluorine chemistry.

The nonheme diiron toluene/o-xylene monooxygenase (ToMO) is the most studied toluene monooxygenase that mediates an aromatic hydroxylation reaction. In this work, QM/MM calculations were performed to understand the reaction mechanism. It is revealed that the μ-η 2 :η 2 peroxodiferric species is the reactive intermediate after the binding of the O 2 molecule to the reduced diferrous center. Subsequently, both a stepwise and a concerted mechanism involving the critical O-O bond cleavage and C-O bond formation were considered. The latter was calculated to be more favorable, suggesting that the formation of a high-valent diferryl Q intermediate is not needed. The isomeric formation of the phenol product was found very facile. The first step was calculated to be rate-limiting, with a barrier of 17.6 kcal/mol for the ortho-hydroxylation., (© 2022 Wiley-VCH GmbH.)

Density functional theory calculations were utilized to elucidate the water oxidation mechanism catalyzed by polyanionic tetramanganese complex a [Mn III 3 Mn IV O 3 (CH 3 COO) 3 (A-α-SiW 9 O 34 )] 6- . Theoretical results indicated that catalytic active species 1 (Mn 4 III,III,IV,IV ) was formed after O 2 formation in the first turnover. From 1, three sequential proton-coupled electron transfer (PCET) oxidations led to the Mn IV -oxyl radical 4 (Mn 4 IV,IV,IV,IV -O⋅). Importantly, 4 had an unusual butterfly-shaped Mn 2 O 2 core for the two substrate-coordinated Mn sites, which facilitated O-O bond formation via direct coupling of the oxyl radical and the adjacent Mn IV -coordinated hydroxide to produce the hydroperoxide intermediate Int1 (Mn 4 III,IV,IV,IV -OOH). This step had an overall energy barrier of 24.9 kcal mol -1 . Subsequent PCET oxidation of Int1 to Int2 (Mn 4 III,IV,IV,IV -O 2 ⋅) enabled the O 2 release in a facile process. Furthermore, apart from the Si-centered complex, computational study suggested that tetramanganese polyoxometalates with Ge, P, and S could also catalyze the water oxidation process, where those bearing P and S likely present higher activities., (© 2022 Wiley-VCH GmbH.)

Reported here is the first catalytic atroposelective electrophilic amination of indoles, which delivers functionalized atropochiral N-sulfonyl-3-arylaminoindoles with excellent optical purity. This reaction was furnished by 1,6-nucleophilic addition to p-quinone diimines. Control experiments suggest an ionic mechanism that differs from the radical addition pathway commonly proposed for 1,6-addition to quinones. The origin of 1,6-addition selectivity was investigated through computational studies. Preliminary studies show that the obtained 3-aminoindoles atropisomers exhibit anticancer activities. This method is valuable with respect to enlarging the toolbox for atropochiral amine derivatives., (© 2022 Wiley-VCH GmbH.)

Mmp10 is a B 12 -dependent SAM radical enzyme that catalyzes C δ -methylation of arginine. The quantum chemical cluster calculations of Mmp10 revealed a "pull-push" radical transfer mechanism in which a 5'-deoxyadenosine radical first abstracts a hydrogen atom from the arginine residue and then methylcobalamin donates a methyl group to the arginine residue. The stereoselectivity and regioselectivity of Mmp10 originated in a specific substrate binding mode enabled by the active site.

Hydroxyprolines are highly abundant in nature as they are components of many structural proteins and osmolytes. Anaerobic degradation of trans -4-hydroxy-l-proline (t4L-HP) was previously found to involve the glycyl radical enzyme (GRE) t4L-HP dehydratase (HypD). Here, we report a pathway for anaerobic hydroxyproline degradation that involves a new GRE, trans -4-hydroxy-d-proline (t4D-HP) C-N-lyase (HplG). In this pathway, cis -4-hydroxy-l-proline (c4L-HP) is first isomerized to t4D-HP, followed by radical-mediated ring opening by HplG to give 2-amino-4-ketopentanoate (AKP), the first example of a ring opening reaction catalyzed by a GRE 1,2-eliminase. Subsequent cleavage by AKP thiolase (OrtAB) yields acetyl-CoA and d-alanine. We report a crystal structure of HplG in complex with t4D-HP at a resolution of 2.7 Å, providing insights into its catalytic mechanism. Different from HypD commonly identified in proline-reducing Clostridia, HplG is present in other types of fermenting bacteria, including propionate-producing bacteria, underscoring the diversity of enzymatic radical chemistry in the anaerobic microbiome.

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph 2 PC 6 H 4 NH)(NH 3 )] + ([ 1 -NH 3 ] + ), which processes electrocatalytic ammonia oxidation to N 2 and H 2 at a mild potential. Through establishing elementary e - /H + conversions with the ferric ammine, a formal Fe(IV)-amido species, [ 1 -NH 2 ] + , together with its conjugated Lewis acid, [ 1 -NH 3 ] 2 + , was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [ 1 -NH 2 ] + induces the reaction of the parent amido unit with NH 3 . The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

Site-selective N-1 and C-3 arylation of indole has been sought after because of the prevalent application of arylindoles and the intricate reactivities associated with the multiple sites of the N-unsubstituted indole. Represented herein is the first regioselective heteroarylation of indole via a radical-radical cross-coupling by visible-light irradiation. Steady and time-resolved spectroscopic and computational studies revealed that the hydrogen-bonding interaction of organic base and its conjugated acid, namely with indole and heteroarylnitrile, determined the reaction pathway, which underwent either proton-coupled electron-transfer or energy-transfer for the subsequent radical-radical cross-coupling, leading to the regioselective formation of C-3 and N-1 heteroarylation of indoles, respectively. The parallel methodologies for regioisomeric N-1 and C-3 heteroaryl indoles with good functional group compatibility could be applied to large-scale synthesis and late-stage derivatization of bioactive compounds under extremely mild reaction conditions., (© 2022 Wiley-VCH GmbH.)

The phosphine-substituted Ru(II) polypyridyl complex, [Ru II -(tpy)(pqn)(MeCN)] 2+ (RuP), was disclosed to be an efficient photocatalyst for the reduction of CO 2 to CO with excellent selectivity. In this work, density functional calculations were performed to elucidate the reaction mechanism and understand the origin of selectivity. The calculations showed that RuP was first excited to the singlet excited state, followed by intersystem crossing to produce a triplet species (3RuIII(L˙-)-S), which was then reduced by the sacrificial electron donor BIH to generate a Ru II (L˙ - ) intermediate. The ligand of Ru II (L˙ - ) was further reduced to produce a Ru II (L 2- ) intermediate. The redox non-innocent nature of the tpy and pqn ligands endows the Ru center with an oxidation state of +2 after two one-electron reductions. Ru II (L 2- ) nucleophilically attacks CO 2 , in which two electrons are delivered from the ligands to CO 2 , affording a Ru II -COOH species after protonation. This is followed by the protonation of the hydroxyl moiety of Ru II -COOH, coupled with the C-O bond cleavage, resulting in the formation of Ru II -CO. Ultimately, CO is dissociated after two one-electron reductions. Protonation of Ru II (L 2- ) to generate a Ru II -hydride, a critical intermediate for the production of formate and H 2 , turns out to be kinetically less favorable, even though it is thermodynamically more favorable. This fact is due to the presence of a Ru 2+ ion in the reduced catalyst, which disfavors its protonation.

A diamido-bridged dicobalt complex supported by a diamidonaphthalene ligand, Cp* 2 Co 2 (μ-1,8-C 10 H 8 (NH) 2 ) ( 1 ), was synthesized, and the reactivity relevant to redox transformations of the Co 2 N 2 core was investigated. It was found that the Co(II)-Co(II) bond allows for protonation by [HPPh 3 ][BF 4 ] resulting in a bridging hydride, [ 1 H] + , with p K a ∼ 7.6 in CH 2 Cl 2 . The diamidonaphthalene ligand can stabilize the binuclear system in the Co(II)Co(III) mixed-valent state ( 1 + ), which is capable of binding CO to afford [ 1 -CO] + . Surprisingly, the mixed-valent complex also activates H 2 O to furnish a Co(III)Co(III) hydroxy complex [ 1 -OH] + accompanied by release of H 2 . The hydroxy ligand in [ 1 -OH] + is exchangeable, as demonstrated by 18 O-labeling experiments on [ 1 -OH] + with H 2 18 O that led to the heavier isotopolog [ 1 - 18 OH] + .

Isethionate sulfite-lyase (IseG) is a recently characterized glycyl radical enzyme (GRE) that catalyzes radical-mediated C-S bond cleavage of isethionate to produce acetaldehyde and sulfite. Herein, we use quantum mechanical/molecular mechanical (QM/MM) calculations to investigate the detailed catalytic reaction mechanism of IseG. Our calculations indicate that a previously proposed direct 1,2-elimination mechanism is disfavored. Instead, we suggest a new 1,2-migration mechanism for this enzymatic reaction: a key stepwise 1,2-SO 3 - radical migration occurs after the catalytically active cysteinyl radical grabs a hydrogen atom from isethionate, followed by hydrogen atom transfer from cysteine to a 1-hydroxylethane-1-sulfonate radical intermediate. Finally, the elimination of sulfite from 1-hydroxylethane-1-sulfonate to result in the final product is likely to occur outside the enzyme. Glu468 in the active site is found to help orient the substrate rather than grabbing a proton from the hydroxyl group of the substrate. Our findings help reveal the mechanisms of radical-mediated C-S bond cleavage of organosulfonates catalyzed by GREs and expand the understanding of radical-based enzymatic catalysis.

Novel cobalt and zinc complexes with the tetradentate ppq (8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2'-yl)quinoline) ligand have been synthesized and fully characterized. Electrochemical measurements have shown that the formal monovalent complex [Co(ppq)(PPh 3 )] + ( 2 ) undergoes two stepwise ligand-based electroreductions in DMF, affording a [Co(ppq)DMF] -1 species. Theoretical calculations have described the electronic structure of [Co(ppq)DMF] -1 as a low-spin Co(II) center coupling with a triple-reduced ppq radical ligand. In the presence of triethylammonium as the proton donor, the cobalt complex efficiently drives electrocatalytic hydrogen evolution with a maximum turnover frequency of thousands per second. A mechanistic investigation proposes an EECC H 2 -evolving pathway, where the second ligand-based redox process (E), generating the [Co(ppq)DMF] -1 intermediate, initiates proton reduction, and the second proton transfer process (C) is the rate-determining step. This work provides a unique example for understanding the role of redox-active ligands in electrocatalytic H 2 evolution by transition metal sites.

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO 2 reduction. Herein, photocatalytic conversion of CO 2 to CO even at low CO 2 concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO 2 binding over proton. The following 2 H + -assisted Fe-CO formation is rate-determining for selective CO 2 -to-CO reduction. A pendant, proton-shuttling α-OH group (2) initiates PCET for predominant H 2 evolution, while an α-OMe group (3) cancels the selectivity control for either CO or H 2 . The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H 2 ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO 2 RR by photocatalysis., (© 2021 Wiley-VCH GmbH.)

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 2 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-N 2 O 2 Cu, 131.6 s -1 ) and binuclear Cu 2 complex (HappCu 2 , 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.

A mononuclear Mn(I) pincer complex [Mn(Ph 2 PCH 2 SiMe 2 ) 2 NH(CO) 2 Br] was disclosed to catalyze the pinacolborane (HBpin)-based CO 2 hydroboration reaction. Density functional calculations were conducted to reveal the reaction mechanism. The calculations showed that the reaction mechanism could be divided into four stages: (1) the addition of HBpin to the unsaturated catalyst C1; (2) the reduction of CO 2 to HCOOBpin; (3) the reduction of HCOOBpin to HCHO; (4) the reduction of HCHO to CH 3 OBpin. The activation of HBpin is the ligand-assisted addition of HBpin to the unsaturated Mn(I)-N complex C1 generated by the elimination of HBr from the Mn(I) pincer catalyst. The sequential substrate reductions share a common mechanism, and every hydroboration commences with the nucleophilic attack of the Mn(I)-H to the electron-deficient carbon centers. The hydride transfer from Mn(I) to HCOOBpin was found to be the rate-limiting step for the whole catalytic reaction, with a total barrier of 27.0 kcal/mol, which fits well with the experimental observations at 90 °C. The reactivity trend of CO 2 , HCOOBpin, HCHO, and CH 3 OBpin was analyzed through both thermodynamic and kinetic analysis, in the following order, namely HCHO>CO 2 >HCOOBpin≫CH 3 OBpin. Importantly, the very high barrier for the reduction of CH 3 OBpin to form CH 4 reconciles with the fact that methane was not observed in this catalytic reaction., (© 2021 Wiley-VCH GmbH.)

Herein, we report the importance of structure regulation on the O-O bond formation process in binuclear iron catalysts. Three complexes, [Fe 2 (μ-O)(OH 2 ) 2 (TPA) 2 ] 4+ (1), [Fe 2 (μ-O)(OH 2 ) 2 (6-HPA)] 4+ (2) and [Fe 2 (μ-O)(OH 2 ) 2 (BPMAN)] 4+ (3), have been designed as electrocatalysts for water oxidation in 0.1 M NaHCO 3 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 Fe IV Fe IV state or via oxo-oxyl coupling pathway at the Fe IV Fe V 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., (© 2021 Wiley-VCH GmbH.)

Chiral photosensitizer-catalyzed stereoselective olefin cyclization has shown its significance in organic synthesis. In this work, we investigated the reaction mechanism, regioselectivity and stereoselectivity of photochemical intramolecular [2 + 2] cycloaddition reaction catalyzed by a chiral thioxanthone molecule using quantum chemical calculations. The reaction proceeded via an energy transfer from the triplet thioxanthone to the substrate, involving stepwise and sequential C-C bond formation. The first C-C bond formation was calculated to be the rate-limiting and selectivity-controlling step. The origin of stereoselectivity was found to be interaction-controlled by distortion/interaction analysis. In addition, the catalyst substituent effects (O vs. S vs. Se) on the stereoselectivity of the photocycloadditions were explored, which provides helpful mechanistic information for the design of related photoinduced reactions.

The energetics for proton reduction in FeFe-hydrogenase has been reinvestigated by theoretical modeling, in light of recent experiments. Two different mechanisms have been considered. In the first one, the bridging hydride position was blocked by the enzyme, which is the mechanism that has been supported by a recent spectroscopic study by Cramer et al. A major difficulty in the present study to agree with experimental energetics was to find the right position for the added proton in the first reduction step. It was eventually found that the best position was as a terminal hydride on the distal iron, which has not been suggested in any of the recent, experimentally based mechanisms. The lowest transition state was surprisingly found to be a bond formation between a proton on a cysteine and the terminal hydride. This type of TS is similar to the one for heterolytic H 2 cleavage in NiFe hydrogenase. The second mechanism investigated here is not supported by the present calculations or the recent experiments by Cramer et al., but was still studied as an interesting comparison. In that mechanism, the formation of the bridging hydride was allowed. The H-H formation barrier is only 3.6 kcal/mol higher than for the first mechanism, but there are severe problems concerning the motion of the protons.

Because of the strong σ-donor and weak π-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO 2 to CO with high activity. However, some NHC-based manganese complexes showed low catalytic activity and required very negative potentials. We report herein that complex fac -[Mn I (bis- Mes NHC)(CO) 3 Br] [ 1 ; bis- Mes NHC = 3,3-bis(2,4,6-trimethylphenyl)-(1,1'-diimidazolin-2,2'-diylidene)methane] could catalyze the electrochemical reduction of CO 2 to CO with high activity (TOF max = 3180 ± 6 s -1 ) at a less negative potential. Due to the introduction of the bulky Mes groups, a one-electron-reduced intermediate {[Mn 0 (bis- Mes NHC)(CO) 3 ] 0 ( 2 • )} was isolated as a packed "dimer" and crystallographically characterized. Stopped-flow Fourier-transform infrared spectroscopy was used to prove the direct reaction between doubly reduced intermediate fac -[Mn(bis- Mes NHC)(CO) 3 ] - and CO 2 ; the tetracarbonyl Mn complex [Mn + (bis- Mes NHC)(CO) 4 ] + ([ 2 -CO] + ) was captured, and its further reduction proposed as the rate-limiting step.

The development of highly active, stable, and inexpensive molecular water oxidation catalysts (WOCs) is important for eventually realizing artificial photosynthesis. The dinuclear earth-abundant molecular WOCs are of great interest as the latent synergy of two adjacent metals of the molecule could enhance the activity of the catalyst for water oxidation, just as the synergistic catalysis effect of metal ions in many metalloenzymes. Herein we report the dinuclear copper complex [L1Cu 2 (μ-OH)](BF 4 ) 3 ( 1 , L1 = N , N '-dimethyl- N , N '-bis{2-[bis(2-pyridinylmethyl)amino]ethyl}ethane-1,2-diamine) with a flexible linker and its mononuclear counterpart [L2Cu(OH 2 )](BF 4 ) 2 ( 2 , L2 = N , N -dimethyl- N ', N '-bis(2-pyridylmethyl)ethane-1,2-diamine) as WOCs. X-ray diffraction analysis showed that in the crystal structure of 1 there is an extraneous water molecule located very close to the bridged O atom, resembling the proposed structure of the transition state for the O-O bond formation. Comparative studies on the performances of 1 and 2 for electrochemical water oxidation at pH 12 manifested that 1 displayed a much higher activity and better stability than that of 2 . The k cat1 of 144 s -1 for 1 is on a par with those of the state-of-the-art earth-abundant molecular WOCs reported to date under similar test conditions. Experimental studies and DFT calculations suggest that the water oxidation catalyzed by 1 proceeds via a unimolecular two-site mechanism with a much lower energy barrier for the O-O bond formation step compared to that for 2 .

Catalytic hydrogenation or transfer hydrogenation of quinolines was thought to be a direct strategy to access dihydroquinolines. However, the challenge is to control the chemoselectivity and regioselectivity. Here we report an efficient partial transfer hydrogenation system operated by a cobalt-amido cooperative catalyst, which converts quinolines to 1,2-dihydroquinolines by the reaction with H 3 N·BH 3 at room temperature. This methodology enables the large scale synthesis of many 1,2-dihydroquinolines with a broad range of functional groups. Mechanistic studies demonstrate that the reduction of quinoline is controlled precisely by cobalt-amido cooperation to operate dihydrogen transfer from H 3 N·BH 3 to the N=C bond of the substrates.

Photosensitization has recently re-emerged owing to the current interest in visible-light catalysis. One of the photoreactions investigated in this context, namely, photo[2+2]cycloaddition of olefins, is established to show high selectivity and wide generality. Here, we describe the results of our studies on selective intermolecular cycloaddition between extended enones (2,4-dien-1-ones and 2-oxo-3-enoates) and olefins under visible-light sensitization. With Ru(bpy) 3 Cl 2 as the triplet energy sensitizer, [2+2] addition of 2,4-dien-1-ones to olefins resulted in the addition to the "ene" part of enones with high efficiency. Generality and functional group tolerance were established by examining a number of enones. 2-Oxo-3-enoates also underwent addition to olefins in the presence of Ru(phen) 3 (PF 6 ) 2 . Both additions were more efficient in the presence of the triplet sensitizer than upon direct irradiation. No Paternò-Büchi product was detected. Density functional theory calculations revealed the origin of high selectivity in the two extended enone systems. Together with spectroscopic studies and control experiments, the cycloaddition has been demonstrated to occur from the excited triplet state of these extended enones, which were generated via the energy transfer process.

Nickel-containing carbon monoxide (CO) dehydrogenase is an enzyme that catalyzes the important reversible carbon dioxide reduction. Several high-resolution structures have been determined at various stages of the reduction, which can be used as good starting points for the present computational study. The cluster model is used in combination with a systematic application of the density functional theory as recently described. The results are in very good agreement with experimental evidence. There are a few important results. To explain why the X-ray structure for the reduced C red1 state has an empty site on nickel, it is here suggested that the cluster has been over-reduced by X-rays and is therefore not the desired reduced state, which instead contains a bound CO on nickel. After an additional reduction, a hydride bound to nickel is suggested to play a role. In order to obtain energetics in agreement with experiments, it is concluded that one sulfide bridge in the Ni-Fe cluster should be protonated. The best test of the accuracy obtained is to compare the computed rate for reduction using -0.6 V with that for oxidation using -0.3 V, where good agreement was obtained. Obtaining a mechanism that is easily reversible is another demanding aspect of the modeling. Nickel oscillates between nickel(II) and nickel(I), while nickel(0) never comes in.

The heterotrinuclear complex A {[Ru II (H 2 O)(tpy)] 2 (μ-[Mn II (H 2 O) 2 (bpp) 2 ])} 4+ [tpy=2,2':6',2''-terpyridine, bpp=3,5-bis(2-pyridyl)pyrazolate] was found to catalyze water oxidation both electrochemically and photochemically with [Ru(bpy) 3 ] 3+ (bpy=2,2'-bipyridine) as the photosensitizer and Na 2 S 2 O 8 as the electron acceptor in neutral phosphate buffer. The mechanism of water oxidation catalyzed by this unprecedented trinuclear complex was studied by density functional calculations. The calculations showed that a series of oxidation and deprotonation events take place from A, leading to the formation of complex 1 (formal oxidation state of Ru1 IV Mn III Ru2 III ), which is the starting species for the catalytic cycle. Three sequential oxidations of 1 result in the generation of the catalytically competing species 4 (formal oxidation state of Ru1 IV Mn V Ru2 IV ), which triggers the O-O bond formation. The direct coupling of two adjacent oxo ligands bound to Ru and Mn leads to the production of a superoxide intermediate Int1. This step was calculated to have a barrier of 7.2 kcal mol -1 at the B3LYP*-D3 level. Subsequent O 2 release from Int1 turns out to be quite facile. Other possible pathways were found to be much less favorable, including water nucleophilic attack, the coupling of an oxo and a hydroxide, and the direct coupling pathway at a lower oxidation state (Ru IV Mn IV Ru IV )., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Metalloenzymes catalyze many different types of biological reactions with high efficiency and remarkable selectivity. The quantum chemical cluster approach and the combined quantum mechanics/molecular mechanics methods have proven very successful in the elucidation of the reaction mechanism and rationalization of selectivities in enzymes. In this review, recent progress in the computational understanding of various selectivities including chemoselectivity, regioselectivity, and stereoselectivity, in metalloenzymes, is discussed.

The conversion of carbon dioxide (CO 2 ) to fuels or value-added chemicals by a photocatalytic system has recently been of growing research interest. One of the challenges is the development of new catalysts with high activity and low cost. Cobalt complexes have long been used as catalysts for the reduction of CO 2 in either electrochemical or photochemical systems. Recently, a series of cis-Co II complexes of tetradentate pyridine-amine ligands (N 4 -ligands) exhibited high activity in the reduction of CO 2 in homogeneous photocatalytic systems. However, only CO was obtained as the reduction product. In this regard, herein, we report a novel cis-Co II complex C1 supported by an N 4 ligand derivatized with TPA (TPA = tris(2-pyridylmethyl)amine). In contrast to the aforementioned Co II catalysts, which contain two halogen atoms at cis-positions, C1 contains one oxygen atom at one cis-coordination site. The structure of C1 was fully characterized by MS, elemental analysis, and single-crystal X-ray diffraction. Experiments on the photocatalytic reduction of CO 2 revealed that C1 is able to convert CO 2 to not only CO but also formate in a homogeneous system containing C1 as a catalyst, Ir(ppy) 3 as a photosensitizer, and triethylamine as an electron donor under visible-light irradiation. The catalytic activity and distribution of reduction products of this system are highly affected by the solvent environment. The presence of water in this system enhances the efficiency of 2H + -to-H 2 and CO 2 -to-formate conversions. Electrochemical and steady-state emission quenching experiments indicate that photoinduced electron transfer from excited Ir(ppy) 3 to C1 is thermodynamically feasible. A photogenerated Co I species is suggested to be the active species involved in the reduction of CO 2 and protons. DFT calculations were performed to elucidate the catalytic pathways of the formation of CO, formate, and H 2 in this system; four pathways, namely, one for the formation of CO, one for the formation of hydrogen, and two for the formation of formate, were suggested. The results revealed that the oxygen atom at the cis-coordination site in C1 plays an important role in stabilizing the transition state during the transformation of CO 2 at the cobalt center.

The identification of an efficient and stable redox mediator is of paramount importance for commercialization of dye-sensitized solar cells (DSCs). Herein, we report a new class of copper complexes containing diamine-dipyridine tetradentate ligands (L1 = N, N'-dibenzyl- N, N'-bis(pyridin-2-ylmethyl)ethylenediamine; L2 = N, N'-dibenzyl- N, N'-bis(6-methylpyridin-2-ylmethyl)ethylenediamine) as redox mediators in DSCs. Devices constructed with [Cu(L2)] 2+/+ redox couple afford an impressive power conversion efficiency (PCE) of 9.2% measured under simulated one sun irradiation (100 mW cm -2 , AM 1.5G), which is among the top efficiencies reported thus far for DSCs with copper complex-based redox mediators. Remarkably, the excellent air, photo, and electrochemical stability of the [Cu(L2)] 2+/+ complexes renders an outstanding long-term stability of the whole DSC device, maintaining ∼90% of the initial efficiency over 500 h under continuous full sun irradiation. This work unfolds a new platform for developing highly efficient and stable redox mediators for large-scale application of DSCs.