Reduction of CO

The initiating radical is derived from the monomer by addition of the H2O molecule with a reduction of Co " to Co ". (reaction Scheme [29])... 

This lactamization process can be promoted by enzymes such as pancreatic porcine lipase. Reduction of co-azido carboxylic acids leads to macrocyclic lactams. Although treatment of carboxylic acids with amines does not directly give amides, the reaction can be made to proceed in good yield at room temperature or... 

When Na-Hg is used as a reducing agent, care must be taken to insure that no undesired Hg is incorporated into the product e.g., reduction of [Co(Cl)np]BPh4 in THF with Na-Hg followed by BuOH gave the red-brown dimeric npCoHgHgConp in 65% yield , and the reaction of Hg with [NaMnfCOj] and tetraphenylporphy-rin-SnClj in THF gave a porphyrin derivative (II) with a Sn — Mn—Hg—Mn chain. ... 

The above proposed process can be expected to easily put into practice as ammonia is abuandant as the main feedstock for fertilizer. Nevertheless, there is also a problem that Co(NH3)6 is apt to be oxidized to Co(NH3)6 which is unable to form the peroxo binuclear complex and ineffective to O2 solubility enhancement, thus reaction (4) is inhibited. But Co will be relatively stable, and Co may be reduced to Co " by H2O [12]. As a result, a regenration method has also been proposed by using the activated carbon as the catalyst[7], in which Co(NH3)6 dissociation into Co " and NH3 occurs on the activated carbon surface followed by reduction of Co with H2O into Co, O2 and H. ... 

The surface analyses of the Co/MgO catalyst for the steam reforming of naphthalene as a model compound of biomass tar were performed by TEM-EDS and XPS measurements. From TEM-EDS analysis, it was found that Co was supported on MgO not as particles but covering its surface in the case of 12 wt.% Co/MgO calcined at 873 K followed by reduction. XPS analysis results showed the existence of cobalt oxide on reduced catalyst, indicating that the reduction of Co/MgO by H2 was incomplete. In the steam reforming of naphthalene, film-like carbon and pyrolytic carbon were found to be deposited on the surface of catalyst by means of TPO and TEM-EDS analyses. 

It is of incidental interest that a little work has been done on dicobalt systems. Doyle and Sykes have made a study of the reduction of decammine-//-amidodi-cobalt(Ill), (NH3)5Co NH2-Co(NH3)5 , by V(II). Since the rate is independent of hydrogen-ion concentration the mechanism cannot involve an amide bridge and must be outer-sphere, as it is in the case of the reduction of Co(NH3)6 by V(1I) . Both the binuclear complex and Co(NH3)6 are inert to substitution but the former is capable of functioning as a two-equivalent oxidant. Thus the two likely mechanisms are... 

Halide effects have been reported for the Cr reduction of Co(NH3)5L" complexes (where L = NH3, acetato or fumarato, FuH ) . Chloride exerts a stronger catalytic effect than bromide in all cases the second-order rate coefficient takes the form where k is pH-independent for L = NH3... 

The reduction of Co(lll) by Fe(II) in perchloric acid solution proceeds at a rate which is just accessible to conventional spectrophotometric measurements. At 2 °C in 1 M acid with [Co(IlI)] = [Fe(II)] 5 x 10 M the half-life is of the order of 4 sec. Kinetic data were obtained by sampling the reactant solution for unreacted Fe(Il) at various times. To achieve this, aliquots of the reaction mixture were run into a quenching solution made up of ammoniacal 2,2 -bipyridine, and the absorbance of the Fe(bipy)3 complex measured at 522 m/i. Absorbancies of Fe(III) and Co(lll) hydroxides and Co(bipy)3 are negligible at this wavelength. With the reactant concentrations equal, plots of l/[Fe(Il)] versus time are accurately linear (over a sixty-fold range of concentrations), showing the reaction to be second order, viz. 

The reduction of Co(III) by Ag(I) in perchlorate solutions has been studied by Sutcliffe et al. Since the initial product of reaction is the very reactive Ag(Il) species, all solutions were subject to preliminary ozonolysis to remove traces of reducible impurities. The final products of reaction are Co(II) and Ag(l). Kinetic data were obtained spectrophotometrically by following the disappearance of Co(III) at 605 m/i, a small correction being applied for the absorbance of Co(ll). With Ag(I) in excess, the disappearance of Co(III) is second order, i.e., plots of the reciprocal of the corrected absorbance versus time are linear. The rate is directly proportional to the concentration of Ag(I), and inversely proportional to the square of the concentration of Co(II). These results can be understood in terms of the mechanism... 

Addition of a Co(III) sulphate solution in sulphuric acid to an olefin dissolved in acetic acid results in reduction of Co(III) at a rate commensurate with that observed for aqueous solution and with identical kinetics " Prior treatment of the Co(III) solution with acetic acid, however, causes the rate of reduction of Co(III) to become almost independent of olefin concentration. Evidently a Co(III)-acetate complex is formed in the mixture of acids which oxidises only after a rate-determining dissociation. However, this complex cannot be formed instantly, and uncomplexed Co(III) can attack olefins in acetic acid in a manner similar to that in water. 

Yeom and Frei [96] showed that irradiation at 266 nm of TS-1 loaded with CO and CH3OH gas at 173 K gave methyl formate as the main product. The photoreaction was monitored in situ by FT-IR spectroscopy and was attributed to reduction of CO at LMCT-excited framework Ti centers (see Sect. 3.2) under concurrent oxidation of methanol. Infrared product analysis based on experiments with isotopically labeled molecules revealed that carbon monoxide is incorporated into the ester as a carbonyl moiety. The authors proposed that CO is photoreduced by transient Ti + to HCO radical in the primary redox step. This finding opens up the possibility for synthetic chemistry of carbon monoxide in transition metal materials by photoactivation of framework metal centers. 

Other important alternate electrochemical methods under study for pCO rely on measuring current associated with the direct reduction of CO. The electrochemistry of COj in both aqueous and non-aqueous media has been documented for some time 27-29) interferences from more easily reduced species such as O2 as well as many commonly used inhalation anesthetics have made the direct amperometric approach difficult to implement. One recently described attempt to circumvent some of these interference problems employs a two cathode configuration in which one electrode is used to scrub the sample of O by exhaustive reduction prior to COj amperometry at the second electrode. The response time and sensitivity of the approach may prove to be adequate for blood ps applications, but the issue of interfering anesthetics must be addressed more thorou ly in order to make the technique a truly viable alternative to the presently used indirect potentiometric electrode. 

Kim et al. (19) also observed that the ee of recovered epichlorohydrin was reduced to 17% in the second hydrolysis reaction with Jacobsen s Co-OAc salen catalyst, if the catalyst was not regenerated with acetic acid in air. Although they attributed the loss of enantioselectivity to the reduction of Co(lll) to Co(ll) salen complex after the HKR reaction, no spectroscopic evidence was provided. Therefore, we probed the catalyst by UV-Vis and XANES spectroscopy before and after the HKR reaction. 

Uribe et al.117 examined the reduction of CO in liquid NH3-0.1 M KI at -50°C, using various working electrodes such as Pt, Ni, C, and Hg. The reaction of CO with electrogenerated solvated electrons produced dimeric species, which precipitated as K2C202. Electrochemical reduction of CO in an aqueous solution at porous gas-diffusion and wet-proof electrodes of Co, Ni, and Fe was carried out,178 and Cj to C3 hydrocarbons and ethylene were reported to be the products. 

Similar results have been obtained by Chisholm for CC-coupling reactions of related tungsten complexes [9], In all cases, however, the addition of stoichiometric amounts of a reducing agent (e.g. Mg or Zn) proved to be necessary for the reduction of CO [1],... 

A prerequisite for CO activation in 6 is an O-attack of the silane at the coordinated carbonyl ligand. This activation step induces the reduction of CO by formal electron transfer Mn—>C and yields a highly reactive 17e intermediate siloxycarbyne complex which dimerizes to give 8. 

Methanol Oxidation at a Smooth Polished Polycrystalline Platinum Electrode d 3.3—The Electrochemical Reduction of CO[sub(2)]... 

Fig. 11. Sampled-current voltammograms recorded point-by-point at a Pt-RDE for the reduction of Co(II) in the 60.0 m/o AlCh-EtMelmCl melt. The Co(II) concentrations were ( ) 5.00, ( ) 10.0, (A) 25.0, and ( ) 50.0 mmol L-1. Also shown is a voltammogram recorded in pure melt before the addition of Co(II) ( ). The angular velocity of the electrode was 104.7 rad s-1. Adapted from Mitchell et al. [44] by permission of The Electrochemical Society.

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