Inorg. Chem. 1985, 24, 2671-2680 = u(F,2)/2F0) and u(F2) = [u2(r)+ (Ar)2]1/2/Lp, the fudge factor A being assumed equal to 0.03. The final values of the conventional R and R, agreement indices were 0.042 and 0.053, respectively. The final difference-Fourier map showed some residual peaks of up to ca. 1.5 e A-3, close to the rhenium atoms, which prevented a direct location of the hydridic atoms. Fourier maps, computed at decreasing values of (sin B ) / X ) , did not show peaks suitable for these atoms. They were placed in idealized positions but not included in the structure factor calculations. The locations were calculated by determining the intersection point of the two (C0)-Re vectors trans to each hydride. This led to two different situations: the hydrides bridging the long Re-Re edges and almost lying in the Re3 plane (Hy(1) and Hy(2)) gave mean Re-H and Re-H-Re bond parameters of 1.70 A and 140°, while the hydrides bridging the short metal-metal bond (Hy(3) and Hy(4)) gave corresponding mean values of 1.94 A and 92'. We have then moved the hydrides along the lines defined by these calculated points and the centers of the subtended Re-Re edges, ca. 0.30 A outward in the first case (up
2671
to mean Re-H and Re-H-Re parameters of 1.82 A and 122') and slightly inward (ca. 0.12 A) in the second case (up to mean Re-H and Re-H-Re parameters of 1.86 A and 97'). The final bond parameters result quite similar to analogous interactions determined by neutron diffraction studies.3*6a*23 The final positional parameters are reported in Table 11.
Acknowledgment. W e thank the Italian CNR for financial support. Registry No.
1, 64478-51-9; [ReH2(CO),](NEt4), 67047-76-1;
[Re3(cl-H),(CO)l,l(NEt4)2, 60544-31-2; [Re,(cl-H)2(CO)121(NEt4), 96791-00-3; [Re,(p-H),(CO) 12]-.5 1779-06-7; [Re3(fi-H)4(CO)IO] (PPN), 96760-23-5; Re3(g-H)3(CO)12,73463-62-4. Supplementary Material Available: Tables of thermal parameters, calculated positions for the hydrogen atoms, and observed and calculated structure factors (34 pages). Ordering information is given on any current masthead page.
Contribution from the Division of Applied Organic Chemistry, CSIRO, Melbourne, Victoria 3001, Australia, and Research School of Chemistry, The Australian National University, Canberra, ACT 2601, Australia
Energy Transfer vs. Electron Transfer in the Excited-State Quenching of Tris( 2,2'-bipyridine-N,N')ruthenium(II) Complexes by Cobalt (111) Cage Complexes: Applications to the Photoreduction of Water INGE I. CREASER,la LAWRENCE R. GAHAN,l' RODNEY J. GEUE,la ANTON LAUNIKONIS,lb PETER A. LAY,lb JOHN D. LYDON," MARK G. McCARTHY,'~ALBERT W.-H. MAU,*lb ALAN M. SARGESON,*Ia and WOLFGANG H. F. SASSE*Ib Received December 21, 1984
A variety of cobalt cage complexes act as electron-transfer agents (ETA) in the [R~(bpy),]~+/ETA/Pt(PVA)/edta-mediated photoreduction of water (bpy = 2,2'-bipyridine; Pt(PVA) = colloidal platinum dispersion on polyvinyl alcohol; edta = 1,2ethanediamine-N,N,N',N'-tetraacetic acid). At pH 5.0 a very narrow potential range of the Co(III)/Co(II) couple (-4.25 to 5 0.10) photoreduction of water. The most effective cage -0.42 V vs. NHE) is suitable for acceptably efficient (0.05 S @(1/2H2) complex is [Co(CLsar)]'+ (CLsar = 1-chlorosar = 1-chloro-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane). At 4 X M, M, mv2+ is methylviologen (mv2+)and [Co(CLsar)13+ are comparable in their abilities to produce hydrogen, while at 2 X about 3 times as efficient. The narrow bell-shaped potential vs. rate of H2 production curve obtained is due to the lack of thermodynamic driving force for water reduction at more positive potentials and an increased competition of energy-transfer over electron-transfer quenching of [Ru*(bpy),12+ at more negative potentials. With [Ru(4,4'-Me2bpy),12+ as sensitizer, the curve is displaced to more negative potentials in concert with the greater reducing power of its excited state compared with that of [ R ~ * ( b p y ) ~ ] The ~ + . quenching constants observed for the cage complexes reacting with the Ru(I1) excited states are generally lower than those of viologen molecules of the same electrochemical potential. This partially explains the increased concentrations of cage complexes required to obtain maximum rates of H2 production. The sulfur-containing capten cobalt cage complexes (capten = l-methyl-6,13,19-triaza-3,lO,l6-trithiabicyclo[6.6.6]eicosane) and sar cage complexes with potentials 2-0.2 V vs. NHE are very efficient oxidative quenching agents for [Ru*(bpy),12+with quenching rate constants approaching those of mv2+and quantum yields approaching unity. Thus solvent-cage escape before back electron transfer is not considered a limiting factor to oxidative quenching by cage complexes in contrast to the case for viologens. Although the reduction potentials of these complexes are too high (-0 V vs. NHE) at pH 5.0 for reduction of water, at pH -0 quantum yields for H2 production approach unity. Cage complexes based on sar with neutral substituents become more effective quenching agents of the excited states of the ruthenium sensitizers as the Co(III)/Co(II) redox potential is made more positive. Those with potentials more positive than -0.34 V vs. NHE quench [Ru*(bpy),12+ exclusively (295%) by electron transfer; however, their ability to produce hydrogen at pH 5.0 is limited by zero to unfavorable free energy changes. By contrast, electron transfer contributes less than 50% to the quenching of [Ru*(4,4'-Me2bpy)J2+ by cage complexes. The major advantage of the cage complexes is their long-term stability in the photochemical cycles compared to that of viologens.
(1) (a) The Australian National University. (b) CSIRO. (2) Sargeson, A. M. Pure Appl. Chem. 1978,50,905; Chem. Br. 1979,15, 23; ACS Symp. Ser. 1980, No. 119, 115. (3) Harrowfield, J. M.; Herlt, A. J.; Sargeson, A. M. Inorg. Synrh. 1980, 20 115 --,
Creaser, I. I.; Harrowfield, J. M.; Herlt, A. J.; Sargeson, A. M.; Springborg, J.; Geue, R. J.; Snow,M. R.J. Am. Chem. SOC.1977,99, ,3181. Creaser, I. I.; Geue, R. J.; Harrowfield, J. M.; Herlt, A. J.; Sargeson, A. M.; Snow, M. R.; Springborg, J. Ibid. 1982, 104, 6016. . . Geue, R. J.; Hambley, T. W.; Harrowfield, J. M.; Sargeson, A. M.; Snow, M. R. J. Am. Chem. SOC.1984, 106, 5478. (6) Hammersh~i,A.; Sargeson, A. M. Inorg. Chem. 1983, 22, 3554. (7) Gahan, L. R.; Hambley, T. W.; Sargeson, A. M.; Snow, M. R. Inorg. Chem. 1982, 21, 2699.
Geue, R. J.; McCarthy, M. G.; Sargeson, A. M. J . Am. Chem. SOC. 1984, 106, 8282. Geue, R. J.; McCarthy, M. G.; Sargeson, A. M.; Skelton, B. W.; White, A. H. Inorg. Chem. 1985, 24, 1607. Gainsford, G. J.; Geue, R. J.; Sarneson, A. M. J . Chem. Soc.. Chem. Commun. 1982, 233. Petri, W.; Sargeson, A. M.; Snow, M. R., to be submitted for publication. Gahan, L. R.; Sargeson, A. M. Ausr. J. Chem. 1981, 34, 2499. Lay, P. A.; Sargeson, A. M., to be submitted for publication in Inorg. Chem.
Lay, P. A. Ph.D. Thesis, The Australian National University, 1981. Bond, A. M.; Lawrance, G. A.; Lay, P. A,; Sargeson, A. M. Inorg. Chem. 1983,22, 2010.
0020-1669/8S/1324-2671$01.~0/0 0 1985 American Chemical Society
2672 Inorganic Chemistry, Vol. 24, No. 17, 1985
Creaser e t al.
It also facilitates detailed analysis of the thermodynamics and kinetics of their electron-transfer pr~perties.~~~~~.~~''-~~ Some cage complexes have been found to be effective electron-transfer agents (ETA) in the photochemical reduction of water,2628 while energy transfer predominates with othersz6 Therefore, it is clear that cobalt cage complexes possess desirable properties for use as ETA's, including the following: (i) very high stability in the Co(II1) and Co(I1) oxidation states; (ii) photochemical inertness to solar radiation in both oxidation states; (iii) ready control of redox potential of the Co(III)/Co(II) couple in the region useful for Scheme I by changing apical ligand substituents; (iv) the ability to change the rate of electron self-exchange of the Co(III)/Co(II) couple by alteration of the structure of the cage ligand (Table VIII); (v) the ability to control the charge of the complexes by the introduction of charged substituents and hence alter electrostatic work terms; (vi) the ability to alter steric factors that may affect quenching.
Scheme I
-
[RU(bPY)312+ + hv
3t12t
3+/2+
11 3t/2t
[Ru*(bpy)3IZ+
ki
[Ru* (bPY1 3 1z+ [Ru*(bpy),l2+ [Ru*(bpy),lZ'
+ ETA + ETA
+ 2ETA- + 2H'.
kr
-
[ R ~ ( b p y ) ~ ] ~ ETA'
[Ru(bpy)J3+
k3
[ R u ( b ~ y ) ~ ] ~ ETA+
(2)
[Ru(bpy),l2'
+ + ETA*
(2')
+
(3)
[ R ~ ( b p y ) ~ ] ~ ETA +
111
IV 3t/2t
3t/2t
k,, Pt(PVA)
+ edta edta+.
(1)
[ R O P Y1312+
ki
+ H2 [Ru(bpy3)]*++ edta'. ' 2ETA
products
(4) 55)
(6)
VI
V
ETA = Co(II1) cages or viologens These factors are now examined to provide insights into improving hydrogen-producing cycles such as those shown in Scheme I, by the use of the cage complexes. The sar (I)z6and sep (11)2628 cage complexes were shown to be effective ETA's for the production of hydrogen, while the oxosar (111) and capten (IV) cage complexes were n 0 t . ~ ~ 3In~ this ~ paper, we have extended our
work,26 to include detailed studies on the pH dependencies of rates of Hzproduction, quenching rate constants, quantum yields, and redox potentials of these complexes and the related absar (V) and the 1,2-cyclohexanediamine-based(char) (VI) cage c o m p l e ~ e s , ~ ~ in systems using tris(2,2'-bipyridine-N,N')ruthenium(II), [Ru( b p ~ ) ~ ]as ~ +a ,sensitizer in the sacrificial system shown in Scheme I. Equation 2' is an energy-transfer pathway that a t present we cannot distinguish from the back-reaction of Ru"'.ETA- within the solvent cage. Equation 3 is the back-reaction for solventseparated Ru(II1) ETA-. These systems are compared with the ~ e l l - s t u d i e d ~ systems "~~ that utilize viologens as ETA's.
(16) Lawrance, G. A,; Lay, P. A.; Sargeson, A. M., to be submitted for publication in J . Am. Chem. SOC. (17) Lay, P. A.; Sargeson, A. M., to be submitted for publication in J . Am. Chem. SOC. (18) Lay, P. A.; Hupp;J. T.; Sargeson, A. M.; Weaver, M. J., to be submitted for publication in Inorg. Chem. (19) Hupp, J. T.; Lay, P. A,; Liu, H.-Y.; Petri, W.; Sargeson, A. M.; Weaver, M. J. J. Electroanal. Chem. Interfacial Electrochem. 1984, 163, 371. (20) Creaser, I. I.; Sargeson, A. M.; Zanella, A. W. Inorg. Chem. 1983, 22, 4022. (21) Bakac, A.; Espensen, J. H.; Creaser, I. I.; Sargeson, A. M. J. Am. Chem. SOC.1983, 105, 7624. (22) Dubs, R. V.; Gahan, L. R.; Sargeson, A. M. Inorg. Chem. 1983, 22, 2523. (23) Hammershai, A.; Lawrance, G. A,; Sargeson, A. M., to be submitted for publication in Inorg. Chem. (24) Gahan, L. R.; Lawrance, G. A.; Sargeson, A. M. Inorg. Chem. 1984, 23, 4369. ( 2 5 ) Lydon, J. D.; Geue, R. J.; McCarthy, M. G., unpublished results. (26) Lay, P. A.; Mau, A. W.-H.; Sasse, W. H. F.; Creaser, I. I.; Gahan, L. R.; Sargeson, A. M. Inorg. Chem. 1983, 22, 2347. (27) Houlding, V.; Geiger, T.; K611e, V.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1982, 681. (28) Scandola, M. A. R.; Scandola, F.; Indelli, A,; Balzani, V. Inorg. Chim. Acta 1983, 76, L67. (29) ?%e trivial nomenclature used for the cage complexes has k e n described in detail e1se~here.l~ The CM substituent is chloromethyl: sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane; sep = 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane; oxosar = 2-oxo3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane; capten = 1-methyl6,13,19-triaza-3,10,16-trithiabicyclo[6.6.6]eicosane; absar = 3,6,10,13,15,18-hexaazabicyclo[6.6.5]nonadecane; char = 3,10,14,2 1.24.3 1-hexaazapentacyclo[ 10.10.10.04~9.015~20.02S~30]dotriacontane; bpy = 2.2'-bipyridine; 4,4'-Me2bpy = 4,4'-dimethyL2,2'-bipyridine.
+
Moradpour, A.; Amouyal, E.; Keller, P.;Kagan, H. Nouu. J. Chim. 1978, 2, 547. Krasna, A. I. Photochem. Photobiol. 1980, 31, 75. Kiwi, J.; Gratzel, M. J . Am. Chem. SOC.1979, 101, 7214. Kiwi, J.; GrPtzel, M. Nature (London) 1979, 281, 657. Adams, M. W. W.; Rao, K. K.; Hall, D. 0. Photobiochem. Photobiphys. 1979, I , 3 3 .
Gohn, M.; Getoff, N. 2.Naturforsch., A 1979, 34A, 1135. Keller, P.; Moradpour, A,; Amouyal, E.; Kagan, H. B. Nouu. J. Chim. 1980, 4, 377. Keller, P.;Moradpour, A.; Amouyal, E.; Kagan, H. J . Mol. Catal. 1980, 7 , 539. Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Gleria, M. Science (Washington, D.C.)1975, 189, 852. Scbumacher, E. Chimia 1978, 32, 193. Bolton, J. R. Science (Washington, D.C.) 1978, 202, 705. Koryakin, B. V.; Dzhabiev, T. S.; Shilov, A. E. Dokl. Phys. Chem. (Engl. Transl.) 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.
Excited-State Quenching of [Ru(bpy)J*+
Inorganic Chemistry, Vol. 24, No. 17, 1985 2613
Table I. Quenching of [Ru*(bpy),12' by Co(II1) Cage Complexes at 20 OCq EO, V 1W8k electron-transfer agent (NaC1O4)"4 E O , Vasc M-l
l-:
~
[Co(HYMEo~osar-H)1~+ [Co(CMMEabsar)] 3+ [Co(EFMEoxosar-H)lZt ._ [~o(sar)l3+ [Co(AZAMEsar)13' [Co(AMMEsar)] 3+ le13-[Co(diAZAchar)]3+
[Co(C1sar)l3+ [Co(seP)l'+ [Co(MENOsar)] 3t [Co(CLHOsar)] 3t [Co(diCLsar)]3t
[Co(diAMsar)]'* lelp-[Co(diAMchar)]3t
@(Co(II))I
