11916 J. Phys. Chem. 1995,99, zyxwvut 11916-11922 Phenazine-Photosensitized Reduction of C02 Mediated by a Cobalt-Cyclam Complex through Electron and Hydrogen Transfer Tomoyuki Ogata? Yoshie Yamamoto? Yuji WadaJ Kei Murakoshi? Mitsuhiro Kusaba? Nobuaki Nakashima," Akito Ishida? Setsuo Takamuku? and Shozo Yanagida**t Chemical Process Engineering, Faculty zyxwvut of Engineering, Institute of Laser Engineering, and Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka 565, Japan Received: January 5, 1995; In Final Form: April zyxwvut 1, 1999 Photoreduction of carbon dioxide (C02) to formate (HCOz-) can be achieved by UV-irradiation of the system involving phenazine (Phen) as a photosensitizer, a cobalt complex of cyclam (Co"'L, L zyx = cyclam = 1,4,8,- 1 1-tetraazacyclotetradecane) as an electron mediator, and triethylamine (TEA) zyxw as an electron donor. Reduction products from the system are HC02- and a small quantity of CO and H2. The quantum yield is 0.07 for the formation of HC02- at 1 = 313 nm. Preferential electron transfer from the photoformed radical anion of phenazine (Phen'-) to Co'I'L is confirmed by EPR analysis and the reaction between Phen'- and Co"'L with a second-order rate constant, k = 4.3 zyxwvu x lo9 M-' s-I. The resulting Co"L reacts with phenazinyl radical, giving [CoL(H)I2+ by hydrogen transfer from phenazinyl radical. The effective insertion of a C02 molecule into [CoL(H)I2+yields HC02- selectively. Introduction Reduction of carbon dioxide (C02) is an interesting area from the viewpoint of coping with global environmental problems. Various efforts were conducted toward development of elec- trochemical or photochemical reduction of C02, which requires potentials (C02/C02'-) more negative than -2.0 V vs SCE.'-3 To make this reduction potentially favorable, several complexes were investigated as electrocatalysts. Cobalt(I1) and nickel(I1) macrocycles including Ni11cyclam,4-6phorphyrin~,~ or phtha- locyanine~~~~ were shown to have a catalytic activity for the electroreduction of C02. The cobalt and nickel macrocycles were also used as catalysts in photoassisted electroreduction of C02 at relatively less negative potentials on semiconductor electrodes.'0," Photocatalytic activities of metal complexes in CO2-photoreduction systems were also reported by L e h r ~ , ' ~ - ' ~ Tinneman~,'~ and Calvin et al.I6 With regards to the C02 photoreduction through organic electron transport sensitization, it was found that most condensed aromatic photosensitizers tend to undergo photocarboxylation during the photolysis with C02. In fact, Tazuke et al. reported the carboxylation of phenanthrene, anthracene, or pyrene through their photoformed radical anion^."^'^ The carboxylation of phenylethylenes, biphenyl, or 1-methyl-2-phenylindole was also achieved by photolysis with aromatic amines and CO2.I9 Recently, we found in our studies on photosensitization of p-terphenyl (OPP-3) that cobalt complexes of cyclam or related 14-membered tetraazamacrocycles should mediate electron transfer from the radical anion photoformed through reductive quenching of the singlet state of OPP-3 ('OPP-3*), leading to efficient formation of both CO and HC02- in the photoreduction of COZ.~O CO evolution exceeds HC02- formation in the OPP- 3/Co111L/TEA system. The reaction mechanism in the system was found to involve efficient electron transfer from the radical anion of OPP-3 (OPP-3'-) to a cyclam complex such as Co"'L or CoIIL, yielding the most reduced COIL. It should be noted + Faculty of Engineering. 1 Institute of Laser Engineering. 8 Institute of Scientific and Industrial Research. @ Abstract published in Advance ACS Abstracts, July 15, 1995. in the photosensitization that the cobalt complexes can retard the photodecomposition of OPP-3'- through effective electron mediation. In addition, the resulting Co' complexes have C02 fixing activities, leading to simultaneous reduction to CO or HC02-. CO evolution was considered through the Co'L(C02) complex formation by coordination of the resulting COIL by COZ. The proposed mechanism for HC02- production is shown in Scheme 1. Since OPP-3'- is a powerful reductant (El12 = -2.45 V vs SCE in dimethylamine),2' OPP-3'- can reduce Co"L to COIL (E'" = -1.9 V vs SCE in SCHEME 1 OPP-3 + TEA - OPP-3'- + TEA" OPP-3'- + Co"L - OPP-3 + COIL Co'L + H+ - CoL(H) CoL(H) + CO, -. Co"'L(0,CH) CO"'L(O,CH) + e- - Co"L + HC0,- On the other hand, several photoreaction systems using het- eroaromatic compounds as photocatalysts or photosensitizers were studied as well. For instance, photocarboxylation of acridine,z2 diazariboflavin-photosensitized reduction of bicar- bonate to and photocarboxylation of a-methylstyrene in the presence of phenazine24 were reported. We also found that poly(pyridine-2,5-diyl) (PPy), a nitrogen-containing het- eroaromatic compound, exhibits photocatalysis toward the reduction of water, carbonyl compounds, and alkenes in the presence of TEA.25 In this system, carbonyl compounds whose redox potentials are relatively negative (E1/2 < -1.8 V vs SCE) are reducible and PPy is shown to induce not only electron transfer but also hydride transfer under certain conditions. With these facts in mind, we searched for nitrogen-containing heteroaromatics which may act as a photosensitizers through the characteristic mechanism. In our screening test, phenazine 0022-3654/95/2099-11916$09.00/0 0 1995 American Chemical Society