Carbon dioxide, rhenium complex

B2FgN4WCgHi2, Tungsten(2+), tetrakis-(acetonitrile)dinitrosyl-, cis-, bis[tetrafluoroborate(l —)], 28 66 02B2FgN4WCgHi2, Tungsten(II), tetrakis-(acetonitrile)dinitrosyl-, cis-, bis[tetra-fluoroborate(l —)], 26 133 O2C, Carbon dioxide, rhenium complex, 26 111... 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

Scheme 144 Catalytic cycle of cathodic carbon dioxide reduction with rhenium complexes to carbon monoxide.

Low-valent rhenium complexes are effective in the catalytic reduction of carbon dioxide. The conversion can be accomplished photolytically or electrochemically and is of interest with regard to fuel production and greenhouse gas remediation [9]. Electrocatalytic reduction of CO2 to CO is initiated by the reduction of fac-Re(bpy)(CO)3Cl or a related complex and can be accomplished in homogeneous solution [54, 55] or on a polymer-modified electrode surface [56]. Catalytic current... 

Abstract This review describes recent advances in the photochemistry of complexes of the element rhenium. It covers not only fundamental chemistry but the recent applications of these complexes to supramolecular chemistry, carbon dioxide reduction, bioinorganic chemistry, sensors, and light-emitting devices. 

Selective carbon dioxide reduction to CO has been accomplished in a non-aqueous medium that includes tricarbonyl (2,2 -bipyridinium) rhenium , /ac-Re(bpy) (CO)3X (X=Cl, Br) as light-active component and homogeneous catalyst for C02 reduction [183-185]. In dimethylformamide solutions that include TEOA as sacrificial electron donor, photosensitized reduction of C02 to CO proceeds with a quantum efficiency of

Mechanistic investigations have revealed that reductive ET quenching of the rhenium complex (Eq. (54)) yields the catalytic intermediate active in deoxygenation of C02. It has been suggested that carbon... 

CO2 reduction at metallic electrodes is generally poorly selective [151]. Monoelec -tronic reduction of carbon dioxide may occur at a platinum cathode in non-aqueous solvents, but at very negative potentials. Catalytic activation of CO2 has been described (e.g. at a cathode modified by a rhenium complex in a hydroorganic solvent) the observed conversions did correspond to the formation of CO and formic acid. In organic synthesis, CO2 was mainly used as an electrophile (toward electrogenerated anions from jt -acceptors or electrogenerated nucleophiles when adequate transition metals ions were present in situ) for the purpose of carboxylation. 

Carbon dioxide electroreduction can be carried out on modified polypyrrole electrodes [81,82,180]. The active electrodes are obtained by electropolymerizalion of Re(L)(CO)3Cl complex, where L is a pyrrole-substituted ligand of various length. The active species is the immobilized rhenium complex. The main reaction product in acetonitrile is carbon monoxide, and preparative-scale electrolysis gave efficiencies as high as 90%. 

Direct excitation of the transition metal complex active in COj reduction was demonstrated in a photosystem composed of tricarbonyl(2,2 -bipyridin-ium)rhenium(I), /ac-Re(bpy)(CO)3X (X = C1, Br) as a light-active compound and a homogeneous catalyst [140-142]. Photosensitized reduction of carbon dioxide to CO proceeds in nonaqueous solutions (i.e., dimethylform-amide) including the rhenium(I) complex and TEOA as the sacrificial electron donor. The quantum efficiency for CO formation in the system corresponds to

Mechanistic studies show that the primary step in... 

The resulting reduced photoproduct acts as the catalytic intermediate for COj reduction (Figure 33). It has been suggested that carbon monoxide ligand dissociation, followed by formation of a rhenium-formate intermediate, leads to cyclic reduction of carbon dioxide to CO. Interestingly, a rhenium(I)-carboxylate complex, /ac-Re(0—C(=0)—H)(bpy)(CO)3, has been isolated as a by-product of the photosystem. The latter complex is inactive in COj reduction to CO, and hence a rhenium-formate intermediate formed by COj insertion into the hydride bond was suggested as the active intermediate for CO formation (Figure 33). 

PHOTOCATALYSIS REDUCTION OF CARBON DIOXIDE AND WATER-GAS-SHIFT REACTION PHOTOCATALYZED BY 2 -BIPYRIDINE OR 1,10-PHENANTHROLINE COBALT(H), RUTHENIUM(H), RHENIUM(I) AND IRIDIUM(ni) COMPLEXES ... 

Water and carbon dioxide do not absorb light above 200 nm and their monoelectronic reduction requires an energy too high to be performed by classical transition metal complexes. It is therefore necessary to use a photosensitizer and organometallic complexes which are able to transfer more than one electron (e.g. cobalt(I), ruthenium(O), or rhenium(-I) or iridium(I) complexes). In principle, these species could be oxidized to a higher oxidation state, by reaction with water or carbon dioxide. However this poses certain problems (i) the compatibility of redox potentials between the photosensitizer and the catalyst (ii) finding mediators and... 

Thus, oxidation of the rhenium complex, [Re(OD)3Cp ], with four equivalents of dimethyldioxirane (determined by spectroscopic titration) in anhydrous acetone solution yields IReC Cp ]. Both K and GC analyses of the liberated gas indicated it to be a mixture of carbon monoxide and carbon dioxide. The first step of the reaction is likely to be the loss of a single carbonyl group by oxidative elimination, Eq. 8.58 ... 

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