Alternating copolymerization of carbon dioxide and propylene oxide under bifunctional cobalt salen complexes: Role of Lewis base substituent covalent bonded on salen ligand

Alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO₂) was realized under mild conditions with a moderate turnover frequency (TOF), employing sole bifunctional cobalt salen complexes containing Lewis acid metal center and covalent bonded Lewis base on the ligand. Variation of the covalent bonded Lewis base substituents on the salen ligands could tailor the catalytic activity with TOF changing from 19.3 to 34.9 h^?1, polymeric/cyclic carbonate selectivity from 95.3 to 72.8%, and the head‐to‐tail structure in the polymer from 72.2 to 86.0%. The IR analysis confirmed that the Lewis base moiety on one molecule could coordinate with cobalt center of adjacent molecule, playing similar role to the Salen metal complex/Lewis base binary catalytic system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 359–365, 2010     

An Anthracene-Based Metal-Organic Framework for Selective Photo-Reduction of Carbon Dioxide to Formic Acid Coupled with Water Oxidation

A Zr-based metal-organic framework has been synthesized and employed as a catalyst for photochemical carbon dioxide reduction coupled with water oxidation. The catalyst shows significant carbon dioxide reduction property with concomitant water oxidation. The catalyst has broad visible light as well as UV light absorption property, which is further confirmed from electronic absorption spectroscopy. Formic acid was the only reduced product from carbon dioxide with a turn-over frequency (TOF) of 0.69 h⁻¹ in addition to oxygen, which was produced with a TOF of 0.54 h⁻¹. No external photosensitizer is used and the ligand itself acts as the light harvester. The efficient and selective photochemical carbon dioxide reduction to formic acid with concomitant water oxidation using Zr-based MOF as catalyst is thus demonstrated here.     

Correlating Metal Redox Potentials to Co(III)K(I) Catalyst Performances in Carbon Dioxide and Propene Oxide Ring Opening Copolymerization

Carbon dioxide copolymerization is a front-runner CO₂ utilization strategy but its viability depends on improving the catalysis. So far, catalyst structure-performance correlations have not been straightforward, limiting the ability to predict how to improve both catalytic activity and selectivity. Here, a simple measure of a catalyst ground-state parameter, metal reduction potential, directly correlates with both polymerization activity and selectivity. It is applied to compare performances of 6 new heterodinuclear Co(III)K(I) catalysts for propene oxide (PO)/CO₂ ring opening copolymerization (ROCOP) producing poly(propene carbonate) (PPC). The best catalyst shows an excellent turnover frequency of 389 h⁻¹ and high PPC selectivity of >99 % (50 °C, 20 bar, 0.025 mol% catalyst). As demonstration of its utility, neither DFT calculations nor ligand Hammett parameter analyses are viable predictors. It is proposed that the cobalt redox potential informs upon the active site electron density with a more electron rich cobalt centre showing better performances. The method may be widely applicable and is recommended to guide future catalyst discovery for other (co)polymerizations and carbon dioxide utilizations.     

Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts for Carbon Dioxide and Cyclohexene Oxide Ring Opening Copolymerizations

A series heterodinuclear catalysts, operating without co-catalyst, show good performances for the ring opening copolymerization (ROCOP) of cyclohexene oxide and carbon dioxide. The complexes feature a macrocyclic ligand designed to coordinate metals such as Zn(II), Mg(II) or Co(III), in a Schiff base ‘pocket’, and Na(I) in a modified crown-ether binding ‘pocket’. The 11 new catalysts are used to explore the influences of the metal combinations and ligand backbones over catalytic activity and selectivity. The highest performance catalyst features the Co(III)Na(I) combination, [N,N′-bis(3,3’-triethylene glycol salicylidene)-1,2-ethylenediamino cobalt(III) di(acetate)]sodium ( 7 ), and it shows both excellent activity and selectivity at 1 bar carbon dioxide pressure (TOF=1590 h⁻¹, >99 % polymer selectivity, 1 : 10: 4000, 100 °C), as well as high activity at higher carbon dioxide pressure (TOF=4343 h⁻¹, 20 bar, 1 : 10 : 25000). Its rate law shows a first order dependence on both catalyst and cyclohexene oxide concentrations and a zeroth order for carbon dioxide pressure, over the range 10–40 bar. These new catalysts eliminate any need for ionic or Lewis base co-catalyst and instead exploit the coordination of earth-abundant and inexpensive Na(I) adjacent to a second metal to deliver efficient catalysis. They highlight the potential for well-designed ancillary ligands and inexpensive Group 1 metals to deliver high performance heterodinuclear catalysts for carbon dioxide copolymerizations and, in future, these catalysts may also show promise in other alternating copolymerization and carbon dioxide utilizations.     

Electrochemical Behavior of Cobalt-Dimethylglyoxime Complex Co(DMG)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)<Subscript>2</Subscript> in the Carbon Dioxide Reduction Process

The electrochemical behavior of a complex of cobalt with dimethylglyoxime Co(DMG)₂(H₂O)₂ is studied by cyclic voltametry. Peaks corresponding to redox transitions Co(III)/Co(II) and Co(II)/Co(I) are observed in the potential region 0.4 to ?1.8 V (Ag/AgCl). The product of reduction of the initial complex interacts with carbon dioxide to form a stable compound, probably an intermediate product of electrocatalytic reduction of CO₂ to CO in the presence of N₄-macrocyclic complexes of cobalt.     

Iron and cobalt salicylaldimine complexes as catalysts for epoxide and carbon dioxide coupling: effects of substituents on catalytic activity

The synthesis and characterization of substituted ONNO-donor salen-type Schiff base complexes of general formula [M^III(L)Cl] (L = Schiff base ligand, M = Fe, Co) is reported. The complexes have been applied as catalysts for the coupling of carbon dioxide and styrene oxide in the presence of tetrabutylammonium bromide as a co-catalyst. The reactions were carried out under relatively low-pressure and solvent-free conditions. The effects of the metal center, ligands, and various substituents on the peripheral sites of the ligand on the coupling reaction were investigated. The catalyst systems were found to be selective for the coupling of CO₂ and styrene oxide, resulting in cyclic styrene carbonate. The cobalt(III) complex with no substituents on the ligand showed higher activity (TON = 1297) than the corresponding iron(III) complex (TON = 814); however, the iron(III)-based catalysts bearing electron-withdrawing substituents on the salen ligands (NEt₃, TON = 1732) showed the highest catalytic activity under similar reaction conditions. The activity of one of the cobalt(III) complexes toward the coupling of 1-butene oxide, cyclohexene oxide and propylene oxide with CO₂ was evaluated, revealing a notable activity for the coupling of 1-butene oxide.     

碱性燃料电池Co-N/C复合催化剂的电化学性能

碳黑经过酸处理后再加入醋酸钴经氨气900 ℃热处理后, 以其制备的气体扩散电极在6 mol•L^―1 KOH溶液中对氧还原反应(ORR)的电催化性能得到大大提高. XRD物相分析表明: 碳粉中加入醋酸钴经氨气热处理生成了氮化钴(Co_5.47N). 通过极化曲线和交流阻抗方法对制备的气体扩散电极在空气中的性能进行了研究. 室温时在－0.2 V (vs. Hg/HgO)电位下, 未经处理的碳电极对氧还原基本没有电流产生; 用酸处理后的碳电极在空气中的电流密度提高到57 mA•cm^―2; 而Co-N/C复合电极在同样条件下电流密度可达170 mA•cm^―2, 交流阻抗显示氮化物的生成减小了氧还原反应的阻抗, 增强了对氧还原反应的电催化作用.     

Electrochemical investigations on pentagonal bipyramidal complexes of manganese(II), iron(II), and cobalt(II) with a heptadentate Schiff-base ligand in non-aqueous solvents

Summary The voltammetric properties of the complexes formed by manganese(II), iron(II), and cobalt(II) ions with a heptadentate Schiff-base ligand have been investigated by cyclic voltammetry and controlled-potential coulometry at mercury and platinum electrodes in acetonitrile and dimethyl sulfoxide solvents.All the species undergo a single one-electron oxidation process leading to the corresponding stable metal(III) complexes which have been isolated and characterized.The cathodic behaviour of manganese(II) and iron(II) derivatives is very similar, in that the less cathodic process occurs at nearly equal potential values, indicating that the ligand moiety is reduced rather than the metal centre. The one-electron reduction process of the cobalt(II) complex leads to the corresponding cobalt(I) derivative, stable in the electrolysis solution.     

A new cobalt complex of 1-decyl-1<Emphasis Type="Italic">H</Emphasis>-benzo[<Emphasis Type="Italic">d</Emphasis>]imidazole: synthesis,characterization and electrocatalysis

The present work reports on the synthesis, characterization and performance of a new cobalt(II) complex, [Co(C₁₀H₂₁-bim)₂(SCN)₂] (bim = benzimidazole) as electrocatalyst for trichloroacetic acid and bromate reduction. Its structure was characterized by X-ray crystallography, IR spectroscopy and elemental analysis. The cobalt atom adopts a distorted tetrahedral geometry by coordinating to four nitrogen atoms from two thiocyanates and two 1-decyl-1H-benzo[d]imidazole ligands. The electrochemical behavior and electrocatalysis of the title complex bulk-modified carbon paste electrode (Co-CPE) have been studied by cyclic voltammetry. The Co-CPE shows good electrocatalytic activities toward the reduction of trichloroacetic acid and bromate. The detection limit and the sensitivity are 0.02 μM, 34.63 μA μM⁻¹ for trichloroacetic acid detection, and 0.03 μM, 78.92 μA μM⁻¹ for bromate detection, respectively. This modified electrode shows good reproducibility, high stability, low detection limit, technical simplicity and possibility of rapid preparation, which is important for practical application.     

Electrocarboxylation of chloroacetonitrile mediated by electrogenerated cobalt(I) phenanthroline

The electrocarboxylation of chloroacetonitrile mediated by [Co(II)(phen)₃]²⁺ has been investigated. Cyclic voltammetry studies of [Co(II)(phen)₃]²⁺ have shown that [Co(I)(phen)₃]⁺, an 18 electron complex, activates chloroacetonitrile by an oxidative addition through the loss of a phenanthroline ligand to give [RCo(III)(phen)₂Cl]⁺. The unstable one-electron-reduced complex underwent Co–C bond cleavage. In carbon dioxide saturated solution, CO₂ insertion proceeds after reduction of the alkylcobalt complex. A catalytic current is observed which corresponds to the electrocarboxylation of chloroacetonitrile into cyanoacetic acid. Electrolyses confirmed the process and gave faradic yield of 62% in cyanoacetic acid at potentials that are about 0.3 V less cathodic than the one required for Ni(salen).     

Study on the polarographic catalytic wave of the system cobalt(II)-dimethylglyoxime

In the cobalt (II)-dimethylglyoxime-NH₃-NH₄Cl (pH 9) system, tne complex Co(II)A₂ exhibits a sensitive polarographic wave. The mechanism of this catalytic wave has been investigated by linear potential sweep voltammetry, cyclic voltammerty and anedic stripping voltammetry. The experimental evidences showed that a zero-valence “active cobalt” or its complex formed during the irreversible reduction of Co(II)A₂, which is adsorbed on the mercury electrode surface, and simultaneously DMG is catalytically reduced by this “active cobalt”. The mechanism of this system with the conflicting explanations of a catalytic hydrogen wave or only adsorptive complex wave is discussed.     

Influence of the electrochemical conversion of [(LH)Mn(II)Cl2] into [(L)Mn(III)Cl]+ on the protonic state of a phenol-containing ligand

A bischloromanganese(II) complex [(LH)MnCl2] (1), where LH is the pentadentate ligand N,N-bis(2-pyridylmethyl)-N'-salicylidene- ethane-1,2-diamine, has been synthesized. Elemental analysis, UV-visible, and cyclic voltammetry experiments showed that the phenol function of the ligand LH remains protonated. Exhaustive electrolysis at 1.0 V vs SCE led to the formation of the Mn(III) derivative [(L)MnCl]+ (3) with the concomitant expulsion of H+ and Cl-. The formation of the Mn(III) species was confirmed by UV-visible spectroscopy and X-ray crystallography. Complex 1 could be regenerated by the reduction of complex 3 in the presence of H+ and Cl-.     

Role of ligand to control the mechanism of nitric oxide reduction of copper(II) complexes and ligand nitrosation

The nitric oxide reactivity of two copper(II) complexes, 1 and 2 with ligands L(1) and L(2), respectively, [L(1) = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, L(2) = 5,5,7-trimethyl-[1,4]-diazepane] have been studied. The copper(II) center in complex 1 was found to be unreactive toward nitric oxide in pure acetonitrile; however, it displayed reduction in methanol solvent in presence of base. The copper(II) center in 2, in acetonitrile solvent, on exposure to nitric oxide has been found to be reduced to copper(I). The same reduction was observed in methanol, also, in case of complex 2. In case of complex 1, presumably, the attack of nitric oxide on the deprotonated amine is the first step, followed by electron transfer to the copper(II) center to afford the reduction. Alternatively, first NO coordination to the Cu(II) followed by NO(+) migration to the secondary amine is the most probable in case of complex 2. The observation of the transient intermediate in UV-visible and FT-IR spectroscopy prior to reduction in case of complex 2 also supports this possibility. In both cases, the reduction resulted into N-nitrosation; in 1, only mononitrosation was observed whereas complex 2 afforded dinitrosation as major product along with a minor amount of mononitrosation. Thus, it is evident from the present study that the macrocyclic ligands prefer the deprotonation pathway leading to mononitrosation; whereas nonmacrocyclic ones prefer the [Cu(II)-NO] intermediate pathway resulting into nitrosation at all the available sites of the ligand as major product.     

Electrochemistry of the bis(1,4,7-triazacyclodecane) cobalt(III) complex and its role in the catalytic reduction of hydrogen

The electrochemistry of the bis(1,4,7-triazacyclodecane) cobalt(III) complex at a mercury electrode, HMDE, in aqueous Britton–Robinson buffer solutions was investigated using cyclic voltammetry, double-potential-step chronoamperometry and chronocoulometry. The cyclic voltammetric data were analyzed by digital simulation to confirm and to measure the heterogeneous and homogeneous parameters for the suggested electrode mechanism. Generally, the complex is electrochemically reduced giving rise to two cyclic voltammetric waves. The first wave is a diffusion-controlled reversible wave. It is assigned to the stable Co(III)/Co(II) redox couple. The second one is found to be irreversible and corresponding to a reduction of Co(II) to Co(I) species. The monovalent cobalt, highly unstable, is rapidly protonated, and then forms cobalt hydride. The hydride decomposes to hydrogen molecules and regenerates Co(II) species following a disproportionation pathway. The overall reduction mechanism is concluded to be an EECC kinetics.     

Electrochemical reduction of carbon dioxide catalyzed by cofacial dinuclear metalloporphyrin

Cofacial dinuclear metalloporphyrins exhibited a catalytic activity for the electrochemical reduction of carbon dioxide. The cofacial dinuclear porphyrin was automatically generated by mixing a cationic cobalt porphyrin (CoTMPyP) and an anionic metalloporphyrin (MTPPS) in solution. The redox system of this complex was examined by electrochemical methods. According to the cyclic voltammogram, the catalytic active species was generated at −1.8V vs. Ag/Ag⁺, which was considered to be a monovalent cobalt porphyrin, Co(I)TMPyP. The catalytic activity of the dinuclear complex was two times greater than that of the mononuclear one because the anionic porphyrin acted as an electron mediator.     

Synthesis and spectroscopic characterization of cobalt porphyrins and their catalytic activity towards dioxygen reduction

The synthesis and spectroscopic characterization of cobalt(Ⅱ) 5-(4-pyridyl)-10,15,20-triphe-nylporphyrin,cobalt(Ⅱ) 5-(4-N-hexadecylpyridiniumyl)-10,15,20-triphenylporphyrin bromide andcobalt(Ⅱ) 5-(2-aminophenyl)-10,15,20-triphenyl-porphyrin are reported.The corresponding copperand vanadyl derivatives ((TriP)Cu,[(hTriP)Cu]~+Br~- and [(hTriP)VO]~+Br~-) were also studied.Eachmetalloporphyrin was characterized by UV-visible,ESR and ~1H NMR spectroscopy.These me-talloporphyrins can be firmly adsorbed on the glassy carbon (GC) surface.The catalytic reduction ofdioxygen at GC electrodes modified by these catalysts was studied by cyclic voltammetry (CV).Thekinetic process of dioxygen reduction at the cobalt porphyrin-modified electrodes was studied with arotating ring disk electrode.     

Electrocatalytic Reduction of Dioxygen at Carbon Paste Electrode Modified with a Novel Cobalt(III) Schiff's Base Complex

A carbon paste modified electrode with a new cobalt(III) Schiff's base complex (CPME) and its application to electrocatalytic activity for dioxygen reduction is developed. The electrochemical behavior and stability of the CPME as well as the two‐electron reduction of O₂ at the electrode were investigated using cyclic voltammetry, chronoamperometry and rotating disk electrode methods. At the CPME, the reduction of dioxygen to hydrogen peroxide occurs at potentials where it is not observed at a bare carbon paste electrode. The CPME exhibited potent and persistent electrocatalysis for O₂ reduction in acetate buffer solutions of pH 4.0 with an overpotential of about 800 mV lower than unmodified CPE and drastic increase in the peak current. The heterogeneous rate constant for the reduction of O₂ at the surface of CPME was determined by hydrodynamic voltammetry using the Koutecky–Levich plot. A possible catalytic mechanism is proposed and discussed.     

Synergetic activation of carbon dioxide molecule using phenylazomethine dendrimers as a catalyst

Phenylazomethine dendrimers bearing a cobalt porphyrin core act as catalysts for CO₂ reduction in the presence of a strong Lewis acid such as lanthanide trifluoromethanesulfonate (Ln(OTf)₃). We investigated the catalytic activity using electrochemical measurements (cyclic voltammetry) on a glassy carbon electrode in a DMF solution. Dissolving CO₂ gas into the solution, the cyclic voltammograms displayed an irreversible increase of the cathodic current. This result suggests the catalytic reduction of CO₂. The redox potential (–1.3 V versus Fc/Fc⁺) at which the catalytic behavior was observed is 1.1 V higher than that catalyzed by cobalt tetraphenylporphyrin (CoTPP). The generation number (n) dependence of the dendrimer catalysts showed the maximum activity at n = 3. A significant decrease of the activity for the largest dendrimer (n = 4) indicates a steric effect, which prevents transmission of the substrate (CO₂ molecule) and electrons to the catalytic center (cobalt porphyrin core). For more efficient catalysis, a novel open-shell dendrimer having a pocket on one side of the molecule was designed and synthesized. Because the accessibility to the core in the opened shell improved, this dendrimer exhibited the highest catalytic activity. These results suggest that tuning of the local domain around the cobalt porphyrin center would lead to a decisive solution for further activation of the CO₂ molecule. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5229–5236, 2006     

Electrochemical derivatization of carbon surface by reduction of diazonium salts in situ generated from nitro precursors in aqueous solutions and electrocatalytic ability of the modified electrode toward hydrogen peroxide

The carbon electrode was covalently modified by electrochemical reduction of nitro precursor in the presence of NaNO₂ in aqueous solutions. The nitro precursor used is p-nitrophenyl phosphate, a well-known chromogenic substrate for the determination of acid and alkaline phosphatases. It is the first method for the covalent modification of carbon surface with a phosphate group. The modified electrode was characterized via cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. It displays good electrocatalytic activity toward hydrogen peroxide reduction.