Oxalate radical anion

The oxidation of [Ir(ox)3]3- by cerium(IV) in aqueous acidic sulfate or perchlorate media has given [Ir(ox)3]2. That the product is indeed [Ir(ox)3]2- and not [Irm(ox)2(C207 )]2 finds support from the slow reaction observed in the presence of excess cerium(IV) free or coordinated oxalate radical anion would be expected to react rapidly with CeIv. In solution, [Ir(ox)3]2- undergoes slow pseudo-first-order reaction to yield the highly reactive intermediate [Ir I,(ox)2(C2Oi )]2. ... 

Firstly, the Ru(bpy)3 + is oxidized at the electrode to the Ru(bpy)3 + cation. This species is then capable of oxidizing the oxalate ( 204 ) in the diffusion layer close to the electrode surface to form an oxalate radical anion ( 204 ). This breaks down to form a highly reducing radical anion (C02 , E° = —1.9 V vs. NHE (126)) and carbon dioxide. The reducing intermediate then either reduces the Ru(bpy)3 complex back to the parent complex in an excited state, or reduces Ru(bpy)3 to form Ru(bpy)3+ that reacts with Ru(bpy)3 + to generate the excited state Ru(bpy)3 , which emits light with -620 nm. 

Formation of oxalate radical anion from oxalate dianion)... 

Decomposition of oxalate radical anion into CO2 and CO2 radical anion One-electron oxidation of oxalate radical anion to dioxetanedione which immediately decomposes to 2 CO2... 

The initiating radicals are assumed to be SCN, ONO or N3 free radicals. Tris oxalate-ferrate-amine anion salt complexes have been studied as photoinitiators (A = 436 nm) of acrylamide polymer [48]. In this initiating system it is proposed that the CO2 radical anion found in the primary photolytic process reacts with iodonium salt (usually diphenyl iodonium chloride salt) by an electron transfer mechanism to give photoactive initiating phenyl radicals by the following reaction machanism ... 

Increasing the current density should favour the dimerisation of the radical anion (route 2) while increasing the C02 concentration should favour route 3. If oxalate formation occurs via route 3, then changing the COz concentration and the current density should affect both the CO and C20yields equally whereas if oxalate formation is via route 2, the effect of changing these same two conditions with respect to the CO yield will be opposite to that observed in the yield of oxalate. 

The radical anion COf often features in reactions involving oxalate. The reduction of the rate by Co(III) complexes might be understandable in terms of Sec. 2.2.1(a). With O2, scavenging of COf also occurs. Now another radical OJis formed and a chain reaction is set up, thus modifying the rate law. 

It can be seen from the relative rate constants shown in Sch. 1 that the products formed will depend on the reaction conditions [26]. The production of formate, as shown by the right-hand reaction in Sch. 1, will be enhanced in protic solvents or in more acidic solutions. In water, formic acid is the main product. The production of CO, as shown by the left-hand reaction in Sch. 1, will be enhanced in rapidly stirred solutions in which locally high concentrations of the "C02 radical anion cannot buildup. This will decrease the probability of a bimolecular reaction between 02 radical anions. In quiet solutions and high current densities, the C02 radical anion concentration should be high in the diffusion layer, favoring formation of oxalate. 

As an aside, it should be noted that a large intrinsic ET barrier does not necessarily imply an increase of the bond length upon radical anion formation. A substantial angle deformation also may contribute to the internal intrinsic barrier, AGqj. This was shown in the case of the stepwise dissociative oxidation of oxalate ( O2C-CO2 — 02C-C02 4-e—CO2-F CO2 ), where the data suggest that the oxalate undergoes a substantial increase of... 

In aprotic organic solvents, where there are no protons available to be involved in C02 reduction, the primary products are CO and oxalate, and all reductions must proceed through the C02 radical anion, C02, as discussed previously. The relevant reactions in aprotic media are listed below, as summarized by Halmann and Steinberg [42]. 

Eggins and McNeill compared the solvents of water, dimethylsulfoxide (DMSO), acetonitrile, propylene carbonate, and DMF electrolytes for C02 reduction at glassy carbon, Hg, Pt, Au, and Pb electrodes [78], The main products were CO and oxalate in the organic solvents, while metal electrodes (such as Pt) which absorb C02 showed a higher production for CO. In DMF, containing 0.1 M tetrabutyl ammonium perchlorate and 0.02 M C02 at a Hg electrode, Isse et al. produced oxalate and CO with faradaic efficiencies of 84% and 1.7%, respectively [79], Similarly, Ito et al. examined a survey of metals for C02 reduction in nonaqueous solution, and found that Hg, Tl, and Pb yielded primarily oxalate, while Cu, Zn, In, Sn, and Au gave CO [80, 81]. Kaiser and Heitz examined Hg and steel (Cr/Ni/Mo, 18 10 2%) electrodes to produce oxalate with 61% faradaic efficiency at 6 mA cm-2 [82]. For this, they examined the reduction of C02 at electrodes where C02 and reduction products do not readily adsorb. The production of oxalate was therefore explained by a high concentration of C02 radical anions, COi, close to the surface. Dimerization resulted in oxalate production rather than CO formation. 

When Desilvestro and Pons used in situ IR reflection spectroelectrochemistry to observe the reduction of C02 to oxalate at Pt electrodes in acetonitrile [83], two different forms of oxalate were observed. Similarly, Aylmer-Kelly et al. studied C02 reduction in acetonitrile and propylene carbonate at Pb electrodes [84], by using modulated specular electroreflectance spectroscopy. Subsequently, two radical intermediates were observed which they determined to be the C02 radical anion, C02, and the product of the radical anion and C02, the (C02)2 adduct (see Equations 11.9 and 11.10). Vassiliev et al. also studied the reduction of C02 in... 

Hislop and Bolton (1999) elucidated the complex mechanism of this reduction process. Presumably, it includes a ligand to metal electron transfer in the iron-oxalate complex with formation of an oxalyl radical anion as shown in Eq. 5-13. 

Radical anions of aromatic esters and nitriles lead to catalytic reduction of CO2, yielding exclusively oxalate. However [233], the generation of CO2 radical anion was shown to proceed via a complex process via a S -type reaction, since the oxygen or nitrogen ends of the reduced form of the mediator bind to the carbon of CO2 [233]. 

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