Electrocatalytic CO2 Reduction with a Half‐Sandwich Cobalt Catalyst: Selectivity towards CO

We present herein a Cp*Co(III)‐half‐sandwich catalyst system for electrocatalytic CO₂ reduction in aqueous acetonitrile solution. In addition to an electron‐donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton‐responsive pyridyl‐benzimidazole‐based N,N‐bidentate ligand. Owing to the presence of a relatively electron‐rich Co center, the reduced Co(I)‐state was made prone to activate the electrophilic carbon center of CO₂. At the same time, the proton‐responsive benzimidazole scaffold was susceptible to facilitate proton‐transfer during the subsequent reduction of CO₂. The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H⁺ reduced product HCOOH, in contrast to the only known similar half‐sandwich CpCo(III)‐based CO₂‐reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled‐potential electrolysis.     

Combination of Optimization and Metalated‐Ligand Exchange: An Effective Approach to Functionalize UiO‐66(Zr) MOFs for CO2 Separation

The strategy to functionalize water‐stable metal–organic frameworks (MOFs) in order to improve their CO₂ uptake capacities for efficient CO₂ separation remains limited and challenging. We herein present an effective approach to functionalize a prominent water‐stable MOF, UiO‐66(Zr), by a combination of optimization and metalated‐ligand exchange. In particular, by systematic optimization, we have successfully obtained UiO‐66(Zr) of the highest BET surface area reported so far (1730 m² g^?1). Moreover, it shows a hybrid Type I/IV N₂ isotherm at 77 K and a mesopore size of 3.9 nm for the first time. The UiO‐66 MOF underwent a metalated‐ligand‐exchange (MLE) process to yield a series of new UiO‐66‐type MOFs, among which UiO‐66‐(COONa)₂‐EX and UiO‐66‐(COOLi)₄‐EX MOFs have both enhanced CO₂ working capacity and IAST CO₂/N₂ selectivity. Our approach has thus suggested an alternative design to achieve water‐stable MOFs with high crystallinity and gas uptake for efficient CO₂ separation.     

A Planar,Conjugated N4‐Macrocyclic Cobalt Complex for Heterogeneous Electrocatalytic CO2 Reduction with High Activity

Metal complexes have been widely investigated as promising electrocatalysts for CO₂ reduction. Most of the current research efforts focus mainly on ligands based on pyrrole subunits, and the reported activities are still far from satisfactory. A novel planar and conjugated N₄‐macrocyclic cobalt complex (Co(II)CPY) derived from phenanthroline subunits is prepared herein, and it delivers high activity for heterogeneous CO₂ electrocatalysis to CO in aqueous media, and outperforms most of the metal complexes reported so far. At a molar loading of 5.93×10^?8 mol cm^?2, it exhibits a Faradaic efficiency of 96 % and a turnover frequency of 9.59 s^?1 towards CO at ?0.70 V vs. RHE. The unraveling of electronic structural features suggests that a synergistic effect between the ligand and cobalt in Co(II)CPY plays a critical role in boosting its activity. As a result, the free energy difference for the formation of *COOH is lower than that with cobalt porphyrin, thus leading to enhanced CO production.     

Studies on Photocatalytic CO2 Reduction over NH2‐Uio‐66(Zr) and Its Derivatives: Towards a Better Understanding of Photocatalysis on Metal–Organic Frameworks

Metal–organic framework (MOF) NH₂‐Uio‐66(Zr) exhibits photocatalytic activity for CO₂ reduction in the presence of triethanolamine as sacrificial agent under visible‐light irradiation. Photoinduced electron transfer from the excited 2‐aminoterephthalate (ATA) to Zr oxo clusters in NH₂‐Uio‐66(Zr) was for the first time revealed by photoluminescence studies. Generation of Zr^III and its involvement in photocatalytic CO₂ reduction was confirmed by ESR analysis. Moreover, NH₂‐Uio‐66(Zr) with mixed ATA and 2,5‐diaminoterephthalate (DTA) ligands was prepared and shown to exhibit higher performance for photocatalytic CO₂ reduction due to its enhanced light adsorption and increased adsorption of CO₂. This study provides a better understanding of photocatalytic CO₂ reduction over MOF‐based photocatalysts and also demonstrates the great potential of using MOFs as highly stable, molecularly tunable, and recyclable photocatalysts in CO₂ reduction.     

Metal–Organic Framework (MOF) Compounds: Photocatalysts for Redox Reactions and Solar Fuel Production

Metal–organic frameworks (MOFs) are crystalline porous materials formed from bi‐ or multipodal organic linkers and transition‐metal nodes. Some MOFs have high structural stability, combined with large flexibility in design and post‐synthetic modification. MOFs can be photoresponsive through light absorption by the organic linker or the metal oxide nodes. Photoexcitation of the light absorbing units in MOFs often generates a ligand‐to‐metal charge‐separation state that can result in photocatalytic activity. In this Review we discuss the advantages and uniqueness that MOFs offer in photocatalysis. We present the best practices to determine photocatalytic activity in MOFs and for the deposition of co‐catalysts. In particular we give examples showing the photocatalytic activity of MOFs in H₂ evolution, CO₂ reduction, photooxygenation, and photoreduction.     

Judicious Ligand Design in Ruthenium Polypyridyl CO2 Reduction Catalysts to Enhance Reactivity by Steric and Electronic Effects

A series of Ru^II polypyridyl complexes of the structural design [Ru^II(R?tpy)(NN)(CH₃CN)]²⁺ (R?tpy=2,2′:6′,2′′‐terpyridine (R=H) or 4,4′,4′′‐tri‐tert‐butyl‐2,2′:6′,2′′‐terpyridine (R=tBu); NN=2,2′‐bipyridine with methyl substituents in various positions) have been synthesized and analyzed for their ability to function as electrocatalysts for the reduction of CO₂ to CO. Detailed electrochemical analyses establish how substitutions at different ring positions of the bipyridine and terpyridine ligands can have profound electronic and, even more importantly, steric effects that determine the complexes’ reactivities. Whereas electron‐donating groups para to the heteroatoms exhibit the expected electronic effect, with an increase in turnover frequencies at increased overpotential, the introduction of a methyl group at the ortho position of NN imposes drastic steric effects. Two complexes, [Ru^II(tpy)(6‐mbpy)(CH₃CN)]²⁺ (trans‐[ 3 ]²⁺; 6‐mbpy=6‐methyl‐2,2′‐bipyridine) and [Ru^II(tBu?tpy)(6‐mbpy)(CH₃CN)]²⁺ (trans‐[ 4 ]²⁺), in which the methyl group of the 6‐mbpy ligand is trans to the CH₃CN ligand, show electrocatalytic CO₂ reduction at a previously unreactive oxidation state of the complex. This low overpotential pathway follows an ECE mechanism (electron transfer–chemical reaction–electron transfer), and is a direct result of steric interactions that facilitate CH₃CN ligand dissociation, CO₂ coordination, and ultimately catalytic turnover at the first reduction potential of the complexes. All experimental observations are rigorously corroborated by DFT calculations.     

A Planar,Conjugated N4-Macrocyclic Cobalt Complex for Heterogeneous Electrocatalytic CO2 Reduction with High Activity

Metal complexes have been widely investigated as promising electrocatalysts for CO₂ reduction. Most of the current research efforts focus mainly on ligands based on pyrrole subunits, and the reported activities are still far from satisfactory. A novel planar and conjugated N₄-macrocyclic cobalt complex (Co(II)CPY) derived from phenanthroline subunits is prepared herein, and it delivers high activity for heterogeneous CO₂ electrocatalysis to CO in aqueous media, and outperforms most of the metal complexes reported so far. At a molar loading of 5.93×10⁻⁸ mol cm⁻², it exhibits a Faradaic efficiency of 96 % and a turnover frequency of 9.59 s⁻¹ towards CO at −0.70 V vs. RHE. The unraveling of electronic structural features suggests that a synergistic effect between the ligand and cobalt in Co(II)CPY plays a critical role in boosting its activity. As a result, the free energy difference for the formation of *COOH is lower than that with cobalt porphyrin, thus leading to enhanced CO production.     

Investigation of catalytic properties of two new orthopalladated complexes supported on montmorillonite: Synthesis,characterization and application in aerobic oxidation of alcohols

Two new complexes, [Pd(L₁)(C,N)]NO₃ ( 1 ) and [Pd(L₂)(C,N)]NO₃ ( 2 ) (L₁ = 5‐nitro‐1,10‐phenanthroline, L₂ = 4‐methyl‐1,10‐phenanthroline, C,N = benzylamine), have been synthesized and characterized using infrared and NMR spectroscopies and elemental analysis. Montmorillonite (MMT‐K10 clay) was used as a solid support for incorporating the cationic part of complexes 1 and 2 to produce catalysts 1 and 2 , respectively, as heterogeneous catalysts. Catalyst 1 was identified using powder X‐ray diffraction and scanning and transmission electron microscopies, and the content of palladium obtained from inductively coupled plasma analysis. By changing the electron‐donating group on the L₁ ligand with an electron‐withdrawing one, a minor improvement was observed in the catalytic properties. Catalyst 1 showed better efficiency for oxidation of benzyl alcohol in comparison with catalyst 2 , so catalyst 1 was used for the aerobic oxidation of alcohols to corresponding aldehydes or ketones without over‐oxidation (with and without bubbling of air). This catalytic system showed high activity towards alcohols under mild conditions. Finally, the reusability of catalyst 1 was investigated with multiple reuses of the supported catalyst in subsequent alcohol oxidation reactions.     

Cooperative Cluster Metalation and Ligand Migration in Zirconium Metal–Organic Frameworks

Cooperative cluster metalation and ligand migration were performed on a Zr‐MOF, leading to the isolation of unique bimetallic MOFs based on decanuclear Zr₆M₄ (M=Ni, Co) clusters. The M²⁺ reacts with the μ₃‐OH and terminal H₂O ligands on an 8‐connected [Zr₆O₄(OH)₈(H₂O)₄] cluster to form a bimetallic [Zr₆M₄O₈(OH)₈(H₂O)₈] cluster. Along with the metalation of Zr₆ cluster, ligand migration is observed in which a Zr–carboxylate bond dissociates to form a M–carboxylate bond. Single‐crystal to single‐crystal transformation is realized so that snapshots for cooperative cluster metalation and ligand migration processes are captured by successive single‐crystal X‐ray structures. In³⁺ was metalated into the same Zr‐MOF which showed excellent catalytic activity in the acetaldehyde cyclotrimerization reaction. This work not only provides a powerful tool to functionalize Zr‐MOFs with other metals, but also structurally elucidates the formation mechanism of the resulting heterometallic MOFs.     

Isostructural NiII Metal–Organic Frameworks (MOFs) for Efficient Electrocatalysis of Oxygen Evolution Reaction and for Gas Sorption Properties

Design and synthesis of stable, active and cost-effective electrocatalyst for water splitting applications is an emerging area of research, given the depletion of fossil fuels. Herein, two isostructural Ni^II redox-active metal–organic frameworks (MOFs) containing flexible tripodal trispyridyl ligand ( L ) and linear dicarboxylates such as terephthalate (TA) and 2-aminoterphthalate (H₂NTA) are studied for their catalytic activity in oxygen evaluation reaction (OER). The 2D-layered MOFs form 3D hydrogen bonded frameworks containing one-dimensional hydrophilic channels that are filled with water molecules. The electrochemical studies reveal that MOFs display an efficient catalytic activity towards oxygen evolution reaction in alkaline conditions with an overpotential as low as 356 mV. Further, these 2D-MOFs exhibit excellent ability to adsorb water vapor (180–230 cc g⁻¹ at 273 K) and CO₂ (33 cc g⁻¹ at 273 K). The presence of hydrophilic functionality in the frameworks was found to significantly enhance the electrocatalytic activity as well as H₂O sorption.     

Antibacterial,SOD mimic and nuclease activities of copper(II) complexes containing ofloxacin and neutral bidentate ligands

Drug‐based mixed‐ligand copper(II) complexes of type [Cu(OFL)(Aⁿ)Cl]·5H₂O (OFL = ofloxacin, A¹ = pyridine‐2‐carbaldehyde, A² = 2,2′‐bipyridylamine, A³ = thiophene‐2‐carbaldehyde, A⁴ = 2,9‐dimethyl‐1,10‐phenanthroline, A⁵ = 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline, A⁶ = 4,5‐diazafluoren‐9‐one, A⁷ = 1,10‐phenanthroline‐5,6‐dione and A⁸ = 5‐nitro‐1,10‐phenanthroline) were synthesized and characterized. Spectral investigations of complexes revealed square pyramidal geometry. Viscosity measurement and absorption titration were employed to determine the mode of binding of complexes with DNA. DNA cleavage study showed better cleaving ability of the complexes compared with metal salt and standard drug by conversion of a supercoiled form of pUC19 DNA to linear via circular. From the SOD mimic study, concentration of complexes ranging from 0.415 to 1.305 µM is enough to inhibit the reduction rate of NBT by 50% (IC₅₀) in the NADH‐PMS system. Antibacterial activity was assayed against selective Gram‐negative and Gram‐positive microorganisms using the doubling dilution technique. Copyright © 2010 John Wiley & Sons, Ltd.     

Cobalt Phenanthroline–Indole Macrocycles as Highly Active Electrocatalysts for Oxygen Reduction

The replacement of scarce and expensive platinum species poses a challenge in fuel‐cell development. The design and synthesis of a novel type of Co^II–N₄ macrocyclic complex, [CoN₄], based on the phenanthroline–indole macrocyclic ligand (PIM) is reported. This unique ligand allows the formation of mono‐ and dinuclear complexes with defined active sites that facilitate the direct four‐electron reduction of oxygen. Electrochemical measurements revealed that the [CoN₄]/C (20 wt %) catalysts have a high activity and long‐term stability for the oxygen‐reduction reaction (ORR) under alkaline conditions, similar to the Pt/C catalyst. These structurally well‐defined complexes represent a nonprecious alternative to platinum species for future fuel‐cell applications.     

Encapsulation of Silver Nanoparticles in an Amine‐Functionalized Porphyrin Metal–Organic Framework and Its Use as a Heterogeneous Catalyst for CO2 Fixation under Atmospheric Pressure

A new porphyrin‐based compound, [Zn₃(C₄₀H₂₄N₈)(C₂₀H₈N₂O₄)₂(DEF)₂](DEF)₃ ( 1 ; DEF=N,N‐diethylformamide), has been synthesized by employing 5,10,15,20‐tetrakis(4‐pyridyl)porphyrin, 1,2‐diamino‐3,6‐bis(4‐carboxyphenyl)benzene, and Zn²⁺ salt at 100 °C under solvothermal conditions. The structure, as determined by single‐crystal XRD studies, is three‐dimensional with threefold interpenetration. The usefulness of free −NH₂ groups in the ligand was exploited for anchoring silver nanoparticles through a simple solution‐based route. The silver‐loaded sample, Ag@ 1 , was characterized by powder XRD, energy‐dispersive X‐ray spectroscopy, high‐resolution TEM, SEM, X‐ray photoelectron spectroscopy, and inductively coupled plasma MS analysis, which clearly indicated that silver nanoparticles with a size of 3.83 nm were uniformly distributed within the metal–organic framework (MOF). The Ag@ 1 sample was evaluated for possible catalytic activity for the carboxylation of a terminal alkyne by employing CO₂ under atmospheric pressure; this gave excellent results. The Ag@ 1 catalyst was found to be robust, active, and recyclable. The present studies suggest that porphyrin MOFs not only exhibit interesting structures, but also show good heterogeneous catalytic activity towards the fixation of CO₂.     

Post-modification of metal-organic framework for improved CO2 photoreduction efficiency

Utilizing metal-organic frameworks (MOFs) to design photocatalysts for CO₂ reduction catalysts is an excellent idea but currently restricted by the relatively low activity. Enhancing CO₂ affinity and tuning the oxidation state of metal clusters in MOFs might be a solution to improve the catalytic performance. Herein, the Cl-bridge atoms in the metal clusters of a cobalt MOF were easily exchanged with OH^?, which simultaneously oxidized a portion of Co(II) to Co(III) and resulted in a much enhanced photocatalytic activity for CO₂ reduction. In contrast, the original framework does not exhibit such superior activity. Comprehensive characterizations on their physicochemical properties revealed that the introduction of hydroxyl group not only greatly increases the CO₂ affinity but also alters the oxidation state of metal clusters, resulting in significantly improved photocatalytic activities for CO₂ reduction. This work provides important insight into the design of efficient photocatalysts.     

Nanosized Functional MOFs Loading Ag/AgBr with Throughout‐Visible‐Light Absorption for High‐Efficiency Photocatalysis

Porous metal‐organic frameworks (MOFs) loading metal nanoparticles to form a composite photocatalyst demonstrated unique advantages. Modification of the electron donating group on the aromatic linkers of MOFs could increase the absorption range of light, thereby increasing the photocatalytic activity. In this study, we prepared a composite photocatalyst using a stable NH₂‐functionalized MOF (UiO‐66‐NH₂) to load semiconductor Ag/AgBr nanoparticles, and the resultant composites have intense optical absorption throughout visible light range. The greatly enhanced optical absorption and the unique hetero‐junction between Ag/AgBr and UiO‐66‐NH₂ render efficient separation and utilization of photogenerated electron‐hole pairs. Therefore, Ag/AgBr@UiO‐66‐NH₂ showed much more excellent photocatalytic activity, compared with unmodified UiO‐66 loading Ag/AgBr (Ag/AgBr@UiO‐66) and reported AgX@MOF catalysts. Moreover, the composite photocatalysts showed excellent stability during cycling experiment.     

Conversion of carbon dioxide to cyclic carbonates using diimine Ru(II) complexes as catalysts

Cationic diimine Ru(II) complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from CO₂ and liquid epoxides (propylene oxide, epichlorohydrine, 1,2‐epoxybutane and styrene oxide) which served as both reactant and solvent. The reaction rates not only depended on the type of ligand, but also on reaction conditions such as temperature, pressure, base, the epoxide substrates and the use of an additional solvent. Reaction rates in terms of turnover frequencies up to 4050 mol_product mol_cat.^?1 h^?1 at 99% selectivity were achieved by optimizing the diimine ligand as well as the reaction temperature and CO₂ pressure. Consistent with CV measurements, the electron donating group on the p‐position of the aryl ring accelerated the reaction rate. Copyright © 2008 John Wiley & Sons, Ltd.     

Efficient solvent‐free alternating copolymerization of CO2 with 1, 2‐epoxydodecane and terpolymerization with styrene oxide via heterogeneous catalysis

The alternating copolymerization of CO₂ with the terminated epoxides anchoring long alkyl groups is rarely reported because of their low reactivity and polycarbonate selectivity. This work describes a well‐controlled solvent‐free copolymerization of CO₂ with 1, 2‐epoxydodecane (EDD) with a long electron‐donating alkyl group via the catalysis of Zn‐Co(III) double metal cyanide complex catalyst. The productivity of the catalyst was up to 2406 g polymer/g Zn, that is, EDD conversion was 99.2%. The alternating degree of CO₂‐EDD copolymers were more than 99% and had high number‐average molecular weights (M_ns) of >100 kg mol^?1, while only 1.0 wt % 4‐decyl‐1,3‐dioxolan‐2‐one (DC) were detected. Moreover, by introducing styrene oxide (SO) with electron‐withdrawing phenyl group into EDD‐CO₂ copolymerization system, a new random terpolymer with either electron‐withdrawing or electron‐donating side groups was produced with single glass transition temperatures (T_gs) in a wide range from 3 to 56 °C, which might be potentially used as biodegradable elastomers or plastics. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 737–744     

Efficient Visible‐Light‐Driven Carbon Dioxide Reduction by a Single‐Atom Implanted Metal–Organic Framework

Modular optimization of metal–organic frameworks (MOFs) was realized by incorporation of coordinatively unsaturated single atoms in a MOF matrix. The newly developed MOF can selectively capture and photoreduce CO₂ with high efficiency under visible‐light irradiation. Mechanistic investigation reveals that the presence of single Co atoms in the MOF can greatly boost the electron–hole separation efficiency in porphyrin units. Directional migration of photogenerated excitons from porphyrin to catalytic Co centers was witnessed, thereby achieving supply of long‐lived electrons for the reduction of CO₂ molecules adsorbed on Co centers. As a direct result, porphyrin MOF comprising atomically dispersed catalytic centers exhibits significantly enhanced photocatalytic conversion of CO₂, which is equivalent to a 3.13‐fold improvement in CO evolution rate (200.6 μmol g^?1 h^?1) and a 5.93‐fold enhancement in CH₄ generation rate (36.67 μmol g^?1 h^?1) compared to the parent MOF.     

Isostructural Metal–Organic Frameworks Assembled from Functionalized Diisophthalate Ligands through a Ligand‐Truncation Strategy

Four isostructural metal–organic frameworks (MOFs) with various functionalized pore surfaces were synthesized from a series of diisophthalate ligands. These MOFs exhibit a new network topology of {4.6⁴.8}₂{4².6⁴}{6⁴.8²}₂{6⁶}. Hydrogen uptake as high as 2.67 wt % at 77 K/1 bar and CO₂ uptake of 15.4 wt % at 297 K/1 bar have been observed for PCN‐308, which contains ? CF₃ groups. The isostructural series of MOFs also showed reasonable adsorption selectivity of CO₂ over CH₄ and N₂.     

Photophysical Analysis of 1,10‐Phenanthroline‐Embedded Porphyrin Analogues and Their Magnesium(II) Complexes

The synthesis, characterization, photophysical properties, and theoretical analysis of a series of tetraaza porphyrin analogues ( H? Pn : n=1–4) containing a dipyrrin subunit and an embedded 1,10‐phenanthroline subunit are described. The meso‐phenyl‐substituted derivative ( H? P1 ) interacts with a Mg²⁺ salt (e.g., MgCl₂, MgBr₂, MgI₂, Mg(ClO₄)₂, and Mg(OAc)₂) in MeCN solution, thereby giving rise to a cation‐dependent red‐shift in both the absorbance‐ and emission maxima. In this system, as well as in the other H? Pn porphyrin analogues used in this study, the four nitrogen atoms of the ligand interact with the bound magnesium cation to form Mg²⁺–dipyrrin–phenanthroline complexes of the general structure MgX? Pn (X=counteranion). Both single‐crystal X‐ray diffraction analysis of the corresponding zinc‐chloride derivative ( ZnCl? P1 ) and fluorescence spectroscopy of the Mg‐adducts that are formed from various metal salts provide support for the conclusion that, in complexes such as MgCl? P1 , a distorted square‐pyramidal geometry persists about the metal cation wherein a chloride anion acts as an axial counteranion. Several analogues ( H? Pn ) that contain electron‐donating and/or electron‐withdrawing dipyrrin moieties were prepared in an effort to understand the structure–property relationships and the photophysical attributes of these Mg–dipyrrin complexes. Analysis of various MgX? Pn (X=anion) systems revealed significant substitution effects on their chemical, electrochemical, and photophysical properties, as well as on the Mg²⁺‐cation affinities. The fluorescence properties of MgCl? Pn reflected the effect of donor‐excited photoinduced electron transfer (d‐PET) processes from the dipyrrin subunit (as a donor site) to the 1,10‐phenanthroline acceptor subunit. The proposed d‐PET process was analyzed by electron paramagnetic resonance (EPR) spectroscopy and by femtosecond transient absorption (TA) spectroscopy, as well as by theoretical DFT calculations. Taken together, these studies provide support for the suggestion that a radical species is produced as the result of an intramolecular charge‐transfer process, following photoexcitation. These photophysical effects, combined with a mixed dipyrrin–phenanthroline structure that is capable of effective Mg²⁺‐cation complexation, lead us to suggest that porphyrin‐inspired systems, such as H? Pn , have a role to play as magnesium‐cation sensors.