Volume 22 Number 17
Inorganic Chemistry
August 17, 1983
0 Copyright 1983 b y the American Chemical Society
Communications Cobalt Cage Complexes as Electron-Transfer Agents in the Photoreduction of Water
Sir: We wish to communicate the properties of cobalt(II1) cage complexes as electron-transfer agents (ETA) in the Ru(2,2'-bipyridi11e)~~+( R ~ ( b p y ) , ~ + )Pt, catalyst, ethylenediaminetetraacetic acid (edta) mediated photoreduction of water. This study identifies some complexes where the electrontransfer quenching is effective and others where energy-transfer quenching intervenes. The effective complexes are not quite as efficient as methylviologen for the production of hydrogen, but they have turnover numbers at least two orders of magnitude greater. The studies also point to a critical region of redox potential for the electron-transfer agent to achieve rapid H 2 production (i.e. -0.3 V). One of the most widely studied model systems for the photoreduction of consists of the excited state of * R ~ ( b p y ) , ~ as + , a sensitizer, methylviologen (1 ,l'-dimethyl4,4'-bipyridinium, mv2+) as an electron-transfer agent, ethylenediaminetetraacetic acid as an electron donor, and platinum supported on polyvinyl alcohol (pva) as the electronBalzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Gleria, M. Science (Washington, D.C.) 1975, 189, 852. Schumacher, E. Chimia 1978, 32, 193. Bolton, J. R. Science (Washington, D.C.) 1978, 202, 705. Koriakin, B. V.; Dzhabiev, T. S.; Shilov, A. E. Dokl. Akad. Nauk SSSR 1977, 233, 620. Kirch, M.; Lehn, J. M.; Sauvage, J. P. Helu. Chim. Acta 1979,62, 1345. DeLaive, P. J.; Sullivan, B. P.; Meyer, T. J.; Whitten, D. G. J . Am. Chem. SOC.1979,101, 4007. Okura, I.; Kim-Thuan, N. Chem. Lett. 1980, 1511. Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helu. Chim. Acta 1978, 61, 2720. Harriman, A.; Porter, G.; Richoux, M.-C. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1939. Krasna, A. I. Photochem. Photobiol. 1980, 31, 75. Adams, M. W. W.; Rao, K. K.; Hall, D. 0. Photobiochem. Photobiophys. 1979, 1, 33. Moradpour, A,; Amouyal, E.; Keller, P.; Kagan, H. Nouu. J . Chim. 1978, 2, 547. Kiwi, J.; Gratzel, M. Nature (London) 1979, 281, 657. Kiwi, J.; Gratzel, M. J. Am. Chem. SOC.1979, 101, 7214. Keller, P.; Moradpur, A. J . Am. Chem. SOC.1980, 102, 7193. Keller, P.; Moradpour, A.; Amouyal, E.; Kagan, H.J. Mol. Catal. 1980, 7, 539. Keller, P.; Moradpur, A.; Amouyal, E.; Kagan, H. B. Nouu. J. Chim. 1980, 4, 377. Amouyal, E.; Grand, D.; Moradpour, A,; Keller, P., to be submitted for publication. Frank, A. J.; Stevenson, K. L. J . Chem. Soc., Chem. Commun. 1981, 593. Miller, D.; McLendon, G. Inorg. Chem. 1981, 20, 950. Johansen, 0.;Launikonis, A.; Lcder, J. W.; Mau, A. W.-H., Sasse, W. H. F.; Swift, J. D.; Wells, D. Aust. J. Chem. 1981, 34, 981. Johansen, 0.;Launikonis, A.; M e r , J. W.; Mau, A. W.-H.; Sasse, W. H. F.; Swift, J. D.; Wells, D. Aust. J. Chem. 1981, 34, 2347.
0020-1669/83/1322-2347$01.50/0
transfer catalyst. Detailed studies have established that the initial rates of formation and yields of hydrogen are determined by reactions 1-5.8J2J9-2'
ki
* R ~ ( b p y ) ~ ~ +R ~ ( b p y ) , ~ +
+ ETA 2R ~ ( b p y ) ~ ,++ ETARu(bpy),,+ + ETA- 2 R ~ ( b p y ) , ~++ ETA R ~ ( b p y ) ~ ,++ edta R ~ ( b p y ) , ~++ edta'. k ETA- + H+ & W P v a ETA + '/2H2
*Ru(bpy);+
-+ ki
(1) (2) (3) (4)
However, continuing irradiation leads to decreased evolution of hydrogen, which can cease even before the concentration of electron donor has become the limiting factor.15-18 This decline is largely caused by reaction 6, which competes with
the formation of hydrogen and causes failure of the system by depleting the electron-transfer agente2' Besides reaction 6, mv2+and its analogues are also degraded by a photochemical pathway involving * R ~ ( b p y ) , ~and + mv+Sz2 Colloidal platinum is widely used as a catalyst for the production of Hz from mv+. (reaction 5). Other metal catalysts are found to be less efficient. The aim of the current study is to investigate whether Co(II1) cage complexes are efficient reagents to replace mvz+ in the above scheme. The cage complexes used are derived from the ligands
HAH PnNT N W
N-N Y
U
N
H
sepZ3
azacaptenz4
0 1983 American Chemical Society
OXOSX-H*~
2348 Inorganic Chemistry, Vol. 22, No. 17, I983 Table I. Hydrogen Productiona
electron-transfer agenta
E", V
CoIII(meoxosar-H) '+ CO~'~(COOC,H,meoxosar-H)2' C~(sar)~+ Co(NH,me~ai)~+ Co(C1sar) Co(sep)3t Co(azacapten) '+ methylviologen( 2+)
-0.59
-0.5
re1 rate of H, production
reacn mixture yield of H, extra(2 h polated irr ad), turnover &mol/mL no."
not detected not detected
nil nil
4 11 45 39 not detected
0.9 2.4 8.2 7.1 nil 19.6
Communications 2 V has been observedz9for the Co(II1) complexes. Oxidative quenching *Ru(bpy),*+
+ Co(II1) 2Ru(bpy)?+ + Co(1I)
and quenching by energy transfer * R ~ ( b p y ) , ~++Co(II1)
-% R u ( b ~ y ) , ~++ *Co(III)
(2')
fast
Co(II1) (8)
remain to be considered. The absorption spectra of saturated amine Co(II1) complexes indicate that their low-lying triplet states (,TI, ,T2),0 are below that of Ru(bpy)?. In order to >5O0Ob see the relative efficiency of these two quenching paths (re>5O0Ob action 2' and reaction 8), we have determined the quantum -0.05 yield of the formation of Co(I1) (@co(II)) by continuous irra-0.44 100 55 diation of R ~ ( b p y ) , ~at+ 460 nm. Since Co(I1) has no apa Conditions: solution containing electron transfer agent, preciable absorption near the 470-490-nm region,23probing 2X M; edta, 2 X lo-' M;Pt/pva M; Ru(bpy),'+, 6 x the absorbance decrease of Co(II1) (e490 = 608 M-' cm-l for with 5 X lo-, M Pt with acetate buffer at pH 5 (0.15 M total Co(azacapten),+, E490 = 200 M-' cm-' for CoThis is a conservative estimate since the acetate); 25 'C.*' (COOCzH5meoxosar-H)Z+, and €470 = 100 M-' cm-' for all [Ru(bpy),]?+ turnover number is -10 000 under t h e conditions used. The HPLC estimation of the cage complex at the end of other Co complexes) gives @co(lI) as listed in Table 11. The the reaction indicated no measurable (
