Cobalt systems, photoreduction

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. 

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... 

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). 

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. 

Since the cobalt(II) complex is inert, its decomposition (114) even in acidic medium k = 1.2 x 10-2 mol T s ) is slow compared with the electron-transfer rate ()e = 5.0 mohU s" ). Therefore, process (113) dominates, and no photoeffect has been observed. The inertness of the [Co(sep)] + cation in photochemical processes is caused by complete encapsulation of the cobalt ion. In the system with the nonmacrocyclic hexamine [Co(NHs)6] trication, photoreduction was found to occur in relatively high quantum yield ( = 0.16). In this case, the photocomposition rate of [Co (NH3)5(NH3)+] + cation formed on irradiation k > 10 s >) is higher than that of electron transfer k = 10-5 mol-T s-i). 

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). ... 

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