Cobalt photoreduction

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

Photoreduction of Some Bridged Binuclear Cobalt Ammines 179... 

Photoreduction of Cobalt(III) Complexes Arising from Irradiation of Ion Pairs. 183... 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

Photolysis of cobalt(III) ammines in the solid state and in various organic media have been reported. Irradiation of crystals of potassium cobaltioxalate gives Co(II) and C02. Study of this reaction by ESR techniques has been reported.69 Photoreduction of Co(NH3)5(H20)l3 gives CoI42- as the cobalt-containing product similar results were obtained with the corresponding bromide and chloride. Solid-state photoreduction of Co(en)3Cl3 has also been reported.70... 

Photoreduction of cobalt(III) complexes in nonaqueous solvent systems has been little studied because of the limited solubility of cobalt(III) complexes and their tendency to photooxidize the solvent. Irradiation with 365-mjj. light of cis- or trans-Co(en)2C 2 + and Co(en)2Cl(DMSO)2+ in dimethylsulfoxide (DMSO) leads rapidly to production of a green tetrahedral cobalt(II) product apparently with concurrent solvent oxidation.53,71 Irradiation with 365-mjx light of the molecular Co(acac)3 in benzene rapidly gives a red precipitate which may be the cobalt(II) acetylacetonate.53... 

Several other cobalt(III) oxalate photoreductions have been reported. Qualitative observations of both reduction and racemization... 

We have initiated a study of the photoreduction of a series of paramagnetic peroxy-bridged dicobalt ammines. All of these complexes, which are shown below, contain the (Co-O-O-Co)5 + group, which best present evidence indicates should be represented as (Co -O-O-Com)5+, although ESR studies indicate that the extra electron is to some extent delocalized over the cobalt centers.78,79,82 Our idea was derived from Sykes report in 196363 that solutions of l-p in sunlight in dilute acid were decomposed according to the stoichiometry shown in reaction (29). 

Since chloride and trifluoroacetate ions affect neither the efficiency nor the Co(II)/Co(III) stoichiometry of photoreduction of 1-p or 2-p, kinetic analysis is possible of the reactions of the cobalt(III) intermediate produced in the photoreduction with water or added anions as shown by reaction (34). Calculation of the ratio, R = kx-jkH20, has... 

Cobalt(III) cage complexes can also perform as electron transfer agents in the photoreduction of water.180181 Because of the kinetic inertness of the encapsulated cobalt(II) ion, the cobalt(II)/co-balt(III) redox couple can be repeatedly cycled without decomposition. Thus these complexes are potentially, useful electron transfer agents, e.g, in the photochemical reduction of water, in energy transfer and as relays in photosensitized electron transfer reactions.180,181 The problem of the short excited-state lifetimes of these complexes can be circumvented by the formation of Co(sep)3+ ion pairs, so that the complexes can be used as photosensitizers for cyclic redox processes.182 183... 

A multicomponent positive-imaging process using ammonia release has been described by Ricoh.211 The components of the system are (1) a cobalt(III) hexaammine complex, (2) a quinone photoreductant, (3) a chelating agent such as dimethylglyoxime, (4) a leuco dye (triarylmethane type), (5) a photooxidant (biimidazole) and (6) an organic acid (toluenesulfonic acid). 

The photoreduction of cobalt(III) complexes by these complexes takes place catalytically, in which one-electron oxidized [Ru(( — )-menbpy)3]3+ and [Ru(S( —) PhEtbpy)3]3 + are reduced by ethanol in the solvent [26]. This is because these complexes have more positive reduction potential than that of [Ru(bpy)3]2+. 

The different group also reported that the overall stereoselectivity is influenced by the reverse reaction in the photoreduction of cobalt(III) complexes with A-[Ru(bpy)3]2 + [33]. Kato and collaborators carried out the stereoselective photoreduction of tris(oxalate)cobaltate(III), [Co(ox)3]3-, and tris(acetyl ace-tonato) cobalt(III) with A-[Ru(bpy)3]2 +. ... 

In the quenching reaction of A-[Ru(bpy)3]2+ by [Co(ox)3]3- and Co(acac)3, only the homochiral preference was observed in water, whereas the stereoselectivity of the quenching by [Co(ox)3]3 becomes reverse in 80% methanol-water. These results suggest that the stereoselectivity is determined not only by the photoin-duced electron transfer but also by the different elementary step such as the reverse reaction. The photoreduction of the cobalt(III) complex by the ruthen-ium(II) complex involves various elementary steps, as shown in Scheme 11. Considering this scheme, one can easily understand that the overall photoreduction of the cobalt(III) complexes is determined by not only the quenching process but also the reverse reaction between the reduced Co(II) complexes and the oxidized ruthenium(III) complex. This conclusion is essentially the same as that reported by Ohkubo and his collaborators. 

Since the copper complexes, [Cu(NN)2]+ and [Cu(NN)(PR3)2]+ (NN = 1,10-phenanthroline, 2,2 -bipyridine, and their derivatives) were applied to stoichiometric and catalytic photoreduction of cobalt(III) complexes [8a,b,e,9a,d], one can expect to perform the asymmetric photoreduction system with the similar copper(l) complexes if the optically active center is introduced into the copper(I) complex. To construct such an asymmetric photoreaction system, we need chiral copper(I) complex. Copper complex, however, takes a four-coordinate structure. This means that the molecular asymmetry around the metal center cannot exist in the copper complex, unlike in six-coordinate octahedral ruthenium(II) complexes. Thus we need to synthesize some chiral ligand in the copper complexes. 

---