Cobalt macrocycle

Ukrainczyk, L., Chibwe, M., Pinnavaia, T.J. Boyd, S. A. (1995). Reductive dechlorination of carbon tetrachloride in water catalyzed by mineral-supported biomimetic cobalt macrocycles. Environmental Science Technology, 29, 439-45. 

Gou ec, R, Savy, M., and Riga, J., Oxygen reduction in acidic media catalyzed by pyrolyzed cobalt macrocycles dispersed on an active carbon the importance of the content of oxygen surface groups on the evolution of the chelate structure during the heat treatment, Electrochim. Acta, 43, 743, 1998. 

Photochemical CO2 reduction to CO (and formate in some cases) has been reported in a catalytic system using Ru(bpy)3 + as the sensitizer, nickel or cobalt macrocycles as the electron relay catalyst, and ascorbate as a sacrificial reductive quencher [9, 15, 16]. These systems also produce H2 via water reduction. Although Ni(cyclam) + is an efficient and selective catalyst for electrochemical CO2 reduc-... 

A redox potential threshold for CO2 binding to the cobalt macrocycles seems to occur at —1.2 V vs. SCE [19]. The CO2 binding constants for a series of Co(I) macrocycles show a strong correlation with the Co / potential. This trend is consistent with charge transfer from the electron-rich cobalt center to the bound CO2. The CO2 binding rate constants in MeCN-MeOH seem also to correlate with the Co / potential in MeCN 1.1 x 10 M s for Co OMD+, 1.7 x 10 M s for Co HMD+ and 3.7 x 10 M s for Co DMD+. 

The dependence of the decay rate of TP on [CO2], measured for solutions containing CO2 with no cobalt macrocycle is not linear in CO2 concentration [28]. A rate constant of <10 s is estimated for the TP -C02 reaction. This sluggish rate constant is consistent with the large reorganization of the C02/C02 couple and modest driving force for the reaction (0.5 V). Under photocatalytic conditions (continuous photolysis) the TP reacts much faster with the cobalt complex than with CO2 and >90 % the photochemically generated reducing equivalents are captured by the cobalt macrocycle. 

PHOTOCHEMICAL CO2 REDUCTION WITH COBALT MACROCYCLES MECHANISTIC AND KINETIC STUDIES... 

Our previous studies indicated that cobalt macrocycles mediate the photoreduction of CO2 to CO with / -terphenyl (TP) as a photosensitizer and a tertiary amine as a sacrificial electron donor in a 5 1 acetonitrile/methanol mixture [22]. The system enhances the activity of the TP by suppressing the formation of dihydroterphenyl derivatives and produces CO and formate efficiently with only small amounts of H2. The total quantum yield of CO and formate is 25% at 313 nm in the presence of triethanolamine (TEOA) and Co(cyclam). ... 

A second, and more chemical, verification is due to Finke et al.,21 who also invented the descriptive phrase persistent radical effect and gave a prototype example to the extreme. The thermal reversible 1,3-benzyl migration in a coenzyme B12 model complex leads to the equilibrium of Scheme 9. Earlier work had shown that the reaction involves freely diffusing benzyl and persistent cobalt macrocycle radicals, but the expected self-termination product bibenzyl of benzyl was missing. Extending the detection limits, the authors found traces of bibenzyl and deduced a selectivity for the formation of the cross-products to the self-termination products of 100 000 1 or 99.999%. Kinetic modeling further showed that over a time of 1000 years only 0.18% of bibenzyl would be formed, and this stresses the long-time duration of the phenomenon. 

For the purposes of this review, it is important to recognize the dual nature of cobalt macrocycle complexes. Both species, LCoinH and LCo1, can add to double and triple bonds forming alkyl and alkenyl cobalt chelates but the products are different. The Co111 hydride reaction occurs in a Markovnikov addition while LCo1 provide anti-Markovnikov products. It is believed that eqs 21 and 22 explain the difference. 

B. D. Curtis, C. J. Dubois, D. L. Electrochemical Reduction of CO2 Catalyzed by a Dinuclear Palladium Cooperativity OrganometalUcs 1995, 14, 4937. (k) Ogata, T. Yanagida, S. Brunschwig, B. S. Fujita, E. Mechanistic and Kinetic-Studies of Cobalt Macrocycles in a Photochemical CO2 Reduction System - Evidence of C0-CO2 Adducts as Intermediates /. Am. Chem. Soc. 1995,117, 6708. (1) Arana, C. Keshavarz, M. Potts, K. T. Abruna, H. D. Electrocatalytic Reduction of CO2 and 0-2 with Electropolymerized Films of Vinyl-Terpyridine Complexes of Fe, Ni and Co Inorg. Chim. Acta 1994, 225, 285. 

According to the scheme above, the total driving force of the reaction will be given by the applied potential and also by the M(III)/M(II) formal potential of the catalyst, so the catalytic activity could be correlated with the formal potential of the catalyst. Many authors have discussed this issue and it is yet not clear what sort of correlation should be expected. Reduction should occur at the potential of reduction of the M(III)02 adduct and not at the potential of the M(III)/(II) couple. The latter should only be observed if the reaction were outer sphere. For the particular case of iron phthalocyanines and other iron macrocyclics, O2 reduction starts at potentials very close to the Fe(III)/(II) couple ". In contrast, for cobalt macrocyclics reduction of O2 begins at potentials much more negative than those corresponding to the Co(III)/(II) couple . Several authors have reported correlations between activity (measured as potential at constant current) and the M(III)/(II) formal potential and volcano-shaped curves have been obtained -see for... 

Contamin, O., C. Debiemme-Chouvy, M. Savy, and G. Scarbeck (2000). O2 electroreduction catalysis Effect of sulfur addition on some cobalt macrocycles. J. New Mat. Electrochem. Systems 3, 61-1 A. 

Hu X, Brunschwig BS, Peters JC (2007) Electrocatalytic hydrogen evolution at low overpotentials by cobalt macrocyclic glyoxime and tetraimine complexes. J Am Chem Soc 129(29) 8988-8998. doi 10.1021/ja067876b... 

Fujita E, Creutz C, Sutin N, Brunschwig BS (1993) Carbon dioxide activation by cobalt macrocycles evidence of hydrogen bonding between bound CO2 and the macrocycle in solution, friorg Chem 32(12) 2657-2662. doi 10.1021Ac00064a015... 

Gouerec P, Savy M. Oxygen reduction electrocatalysis ageing of pyrolyzed cobalt macrocycles dispersed on an active carbon. Electrochim Acta 1999 44(15) 2653-61. 

Table 1 Reactions of cobalt ) macrocycle complexes with H2O2, 7=0.1M, [H+] = 0.05-0.1M ...

Catalytic Chain Transfer. A highly useful variant of chain transfer was discovered in the 1970-1980s in the Soviet Union (216). A number of reviews have been published in recent years (217-221) on this synthetic method which has acquired the nomenclature of either catalytic chain transfer (CCT) or special chain transfer (SCT). The most commonly adopted catalysts are based on low spin cobalt macrocycles, although other metal-containing complexes have also been suggested in the patent and scientific literature. Some typical catalyst structures are shown as 12 and 13. 

ELECTROCHEMICAL STUDIES ON ACTIVATION OF DIOXY6EN AT COBALT MACROCYCLES DERIVED FROM COBALOXIME... 

Electrocatalytic ORR carries out in three pathways the 1-electron transfer pathway, producing superoxide ion the 2-electron transfer pathway, producing hydrogen peroxide and the 4-electron transfer pathway, producing water. In a non-aqueous aprotic solvent system, a room-temperature ionic liquid system, and on specific transition-metal, macrocyclic-compounds-coated graphite electrodes in alkaline solutions, 1-electron reduction can be observed. Carbon materials, quinone and derivatives, mono-nuclear cobalt macrocyclic compounds, and some chalcogenides can only catalyze 2-electron ORR. Noble metal, noble metal alloy materials, iron-macrocyclic complexes, di-nuclear cobalt macrocyclic complexes, some chalcogenides, and transition-metal carbide-promoted Pt catalysts can catalyze 4-electron reduction. 

A spectroscopic study on prim,rac-CoL(C02) (L = 5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazacyclotetradeca-4,ll-diene), a cobalt-macrocycle CO2 complex in which the CO2 ligand occupies the axial metal coordination site on the primary macrocycle face, has... 

Ogata T, Yanagida S, Brunshwig BS, Fujita E (1995) Mechanistic and kinetic studies of cobalt macrocycles in a photochtaiucal CO2 reduction system evidence of C0-CO2 adducts as intermediates. J Am Chem Soc 117 6708-6716... 

Other systems for electrochemical CO2 reduction utilize transition metal complexes of nitrogen-containing (nickel and cobalt) macrocycles (including porphyrins and phthalocyanines) and (ruthenium, cobalt, and rhenium) complexes of 2,2 -bipyridine. ... 

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