Catalytic Activity of Tri(µ-chloro)-BridgedDinuclear Ruthenium Complex Confined in aPolymer Membrane as an Artificial Model of

Photosynthetic Oxygen Evolving Center

Masayuki Yagi,*,† Yukihiro Osawa,† Noriko Sukegawa,† andMasao Kaneko‡

Faculty of Education, Niigata University, 8050 Ikarashi-2,Niigata 950-2181, Japan, and Faculty of Science, Ibaraki

University, 2-1-1 Bunkyo, Mito 310-8512, Japan

Received February 23, 1999. In Final Form: June 22, 1999

Introduction

Water oxidation to evolve dioxygen (O2) catalyzed atthe oxygen evolving center (OEC) in photosynthesis isone of the most important and fundamental chemicalprocesses in nature.1,2 Electrons are provided to the wholephotosynthetic system by this process, which is the originof all the biological activities. It is important to investigatemolecule-based water oxidation catalysts and to establishan aritificial OEC model not only for understanding andsimulating photosynthetic OEC but also for constructingan artificial photosynthesis3-5 which is attracting a greatdeal of interest for the conversion of solar energy intofuels. Although O2 evolution from water is of greatimportance for studying water oxidation catalysis, therehave been few reports on molecule-based water oxidationcatalyst capable of evolving O2 from water.6-9

Activity analyses of molecule-based water oxidationcatalysts are important to (1) evaluate the intrinsicactivities of the catalysts, (2) determine the factors whichaffect the catalysis, and (3) optimize the catalysis con-sidering the factors.10,11 In the present paper, efficientfour-electron water oxidation to evolve O2 using a CeIV

oxidant will be demonstrated using a tri(µ-chloro)-bridgeddinuclear ruthenium ammine complex, [(NH3)3Ru(µ-Cl)3Ru(NH3)3]2+ (abbreviated to Ru(µ-Cl)3Ru), and itscatalytic activity will be analyzed in a homogeneousaqueous solution as well as a heterogeneous Nafionmembrane.

Experimental Section

Ru(µ-Cl)3Ru was prepared and purified according to theprocedure of Mercer et al.12 Nafion membrane incorporating Ru(µ-Cl)3Ru was prepared as described in our earlier papers.10,11 ExcessCeIV oxidant (cerium(IV) diammonium nitrate from Wako PureChemical Ind.) was added quickly to a Ru(µ-Cl)3Ru aqueoussolution or pure water containing the membrane that incorpo-rated Ru(µ-Cl)3Ru in a reaction vessel where the atmospherewas replaced with argon gas. The vessel was kept at 25 °C usinga thermostated water bath. The pH was 0.43 without any bufferunder the entire complex concentration range. The pH changewas less than 0.03 before and after O2 evolution, and the pHchange by H+ released in water oxidation is negligible. Theamount of O2 evolved was obtained by analyzing an aliquot (0.09cm3) of the gas phase (ca. 1 cm3) on a gas chromatograph(Shimadzu GC-8A) equipped with a 5 Å molecular sieve columnusing argon carrier gas (40 cm3 min-1 flow rate) at 50 °C.

Results and Discussion

Dioxygen bubbles were observed when a large excessof a CeIV oxidant was added to an aqueous Ru(µ-Cl)3Rusolution. Since no O2 was evolved in the solution containingthe same amount of CeIV oxidant without Ru(µ-Cl)3Ru,the complex acts as a catalyst for four-electron wateroxidation to evolve O2. In the present system using a CeIV

oxidant (redox potential of CeIV/CeIII 1.53 V vs NHE, pH) 0.43), water oxidation takes place by a four-electronoxidation process to produce O2 rather than a two-stepoxidation process via H2O2 (redox potential of H2O/H2O21.74 V vs NHE, pH ) 0.43) and a four-step mechanismof a one-electron oxidation process via OH‚ (redox potentialof H2O/OH‚ 2.71 V vs NHE, pH ) 0.43).10

The overall catalytic O2 evolution is expressed by theoxidation (eq 1) of the complex by a CeIV oxidant and wateroxidation by the oxidized complex ((complex)ox) (eq 2) whenassuming water oxidation is catalyzed by one complexwhich has a four-electron oxidation ability.

The dependence of initial O2 evolution rate (VO2 (mol s-1))on the CeIV concentration was investigated in an aqueoussolution containing 2.0 × 10-4 mol dm-3 Ru complex. VO2

increased with the CeIV concentration and reached aconstant value at 6.0 × 10-2 mol dm-3 of its concentration(the molar ratio of CeIV to Ru complex is 300). Since wateroxidation by the high oxidation state of the complex (eq2) is the rate-determining step for overall O2 evolutionunder such CeIV concentration conditions, the VO2 can begiven by eq 311

where kO2 (s-1) is the pseudo-first-order rate constant ofwater oxidation in eq 2, c (mol dm-3) is the total complexconcentration, andv (dm3) is the reaction volume. Equation3 means that the VO2 is first order with respect to thecomplex concentration under conditions of the large excess

† Niigata University.‡ Ibaraki University.(1) Ort, D. R.; Yocum, C. F. Oxygenic Photosynthesis; Kluwer:

Dordrecht, 1996.(2) Kaneko, M. Macromolecular Complexes: Dynamic Interaction and

Electronic Processes; Tsuchida, E., Ed.; VCH Publishers, Inc.: NewYork, 1991; p 353.

(3) Graetzel, M.; Kalyanasundaram, K. Curr. Sci. 1994, 66, 706.(4) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 24, 141.(5) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163.(6) Naruta, Y.; Sasayama, M.; Sasaki, T. Angew. Chem., Int. Ed.

Engl. 1994, 33, 1839.(7) (a) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R., Jr.; Geselowitz,

D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc.1985, 107, 3855. (b) Geselowitz, D.; Meyer, T. J. Inorg. Chem. 1990, 29,3894.

(8) Ramaraj, R.; Kira, A.; Kaneko, M. Angew. Chem., Int. Ed. Engl.1986, 25, 1009.

(9) Ramaraj, R.; Kira, A.; Kaneko, M. J. Chem. Soc., Chem. Commun.1987, 227.

(10) Yagi, M.; Nagoshi, K.; Kaneko, M. J. Phys. Chem. B 1997, 101,5143.

(11) Yagi, M.; Tokita, S.; Nagoshi, K.; Ogino, I.; Kaneko, M. J. Chem.Soc., Faraday Trans. 1996, 92, 2457. (12) Mercer, E. E.; Gray, L. W. J. Am. Chem. Soc. 1972, 94, 6426.

4CeIV + complex f 4CeIII + (complex)ox (1)

(complex)ox + 2H2O 98kO2

O2 + 4H+ + complex (2)

VO2) kO2

cv (3)

7406 Langmuir 1999, 15, 7406-7408

10.1021/la9901985 CCC: $18.00 © 1999 American Chemical SocietyPublished on Web 08/05/1999

of CeIV if four-electron water oxidation is catalyzed by one(complex)ox.

VO2 increased with the complex concentration in theaqueous solution, after passing a maximum at 4 × 10-3

mol dm-3, and tended to decrease at high concentrationsas shown in Figure 1. The plots of VO2 vs complexconcentration at low complex concentrations give astraight line (inset of Figure 1) showing that O2 evolutionis first order with respect to the complex concentration.This indicates that water oxidation is catalyzed by oneoxidized complex which possesses a four-electron oxidationability. The kO2 value was estimated as (8.3 ( 0.5) × 10-2

s-1 from the slope of the linear plots according to eq 3. N2was evolved at high complex concentrations in the aqueoussolution. Since no N2 was detected in an aqueous solutioncontaining the same amount of CeIV oxidant without thecomplex, N2 evolution can result only from oxidation ofthe ammine ligands of the complex. The initial N2 evolutionrate (VN2 (mol s-1)) was second order with respect to thecomplex concentration, showing that a bimolecular de-composition of the complex takes place in the solution.The decrease of VO2 at high concentrations is ascribed tobimolecular decomposition.

Figure 2 shows plots of VO2 vs complex concentration inthe Nafion membrane. These plots gave a straight line atlow concentrations (inset of Figure 2) showing that the

complex works as a four-electron oxidation catalyst alsoin the membrane. The kO2 value in the membrane wasobtained from the slope of the straight line as (6.3 ( 0.2)× 10-2 s-1. It should be noted that the VO2 did not decreasemuch even at the high concentration of 2.2 × 10-2 moldm-3. N2 was not observed in the membrane system.

In order to compare the catalytic activities of the complexin the aqueous solution and membrane, an apparent first-order rate constant of O2 evolution (kapp (s-1)) was definedas

where nRu (mol) is the amount of the complex present inthe reaction system. The dependencies of kapp on thecatalyst concentration are shown in Figure 3. In both theaqueous solution and membrane, the kapp decreased withincrease of the complex concentration. However, thedecrease of kapp is much slower in the membrane than inthe aqueous solution, which can be ascribed to stabilizationof the complex against the bimolecular decomposition bythe adsorption of the complex in the membrane.

On the basis of the mechanism that the first-order wateroxidation by the (complex)ox is in competition with thesecond-order bimolecular decomposition of the (complex)ox,eq 5 was derived to analyze the concentration dependencyof kapp

11

where kdeact (dm3 mol-1 s-1) is the second-order rateconstant of the bimolecular decomposition. The plots ofkapp

-1 vs c in both the solution and membrane accordingto eq 5 gave linear relationships as shown in the inset ofFigure 3, consistent with the proposed mechanism in-volving competition between the O2 evolution and thebimolecular decomposition as noted from eq 5. The kO2

and kdeact were obtained from the intercepts and slopes ofthe straight lines, respectively to obtain kO2 ) (5.6 ( 1.7)× 10-2 s-1 and kdeac ) (1.4 ( 1.0) × 102 dm3 mol-1 s-1 inthe solution and kO2 ) (6.3 ( 0.8) × 10-2 s-1 and kdeac )8.4 ( 3.2 dm3 mol-1 s-1 in the membrane.

The kO2 values in solution (5.6×10-2 s-1) and membrane(6.3 × 10-2 s-1) are reasonable when considering thoseobtained from the initial slope in the insets of Figures 1

Figure 1. Plots of initial O2 evolution rates, VO2 (mol s-1), (O),and N2 evolution rate, VN2 (mol s-1) (2), vs complex concentrationin an aqueous solution system. CeIV oxidant, 6.0 × 10-3 mol in5 cm3 water.

Figure 2. Plots of initial O2 evolution rate, VO2 (mol s-1) (O),vs complex concentration in Nafion membrane system. Mem-brane volume, 1.1 × 10-4 dm3; CeIV, 6.0 × 10-3 mol in 5 cm3

water.

Figure 3. Dependencies of apparent catalytic activities (kapp(s-1)) on the complex concentration in the aqueous solution (b)and the membrane systems (O). The solid line is the calculatedcurve based on eq 6. Inset illustrates the plots of kapp

-1 vscomplex concentration in the aqueous solution (b) and themembrane systems (O) according to eq 5.

kapp ) VO2/nRu (4)

kapp-1 ) 1/kO2

+ (kdeact/kO2

2)c (5)

Notes Langmuir, Vol. 15, No. 21, 1999 7407

(8.3 × 10-2 s-1) and 2 (6.3 × 10-2 s-1), respectively. ThekO2 values are close in both the systems, showing that theincorporation of the complex into the membrane does notcause a significant loss of its intrinsic activity. Both thekO2 were larger than those11 of [(NH3)5Ru(µ-O(Ru(NH3)4-(µ-O)Ru(NH3)5]6+ (5.1 × 10-2 s-1 in an aqueous solution,4.5 × 10-2 s-1 in a Nafion membrane)11 which is the mostactive molecule-based water oxidation catalyst studied todate.

The kdeact (8.4 dm3 mol-1 s-1) in the membrane is lowerthan that (1.4 × 102 dm3 mol-1 s-1) in solution by 17 times.In the present system, the cationic complex is electro-statically attached to anionic sulfonate groups on theNafion chain, so that there is a strong restriction for thecomplex to diffuse in the membrane. The bimoleculardecomposition would be suppressed by such restriction ofthe diffusion, resulting in the lower kdeact value in themembrane.

We have already reported the activity analyses ofmolecule-based water oxidation catalysts in a polymermembrane based on intermolecular distance distribu-tion.10,11 When assuming that the complexes existingwithin a critical decomposition distance, rd (nm), undergoa bimolecular decomposition to deactivate, kapp is repre-sented by eq 611

where k (s-1), s (nm), R, and NA (mol-1) are an intrinsic

activity of the catalyst, contact distance between thecatalysts, localization degree (40)11 of the catalysts in themembrane, and Avogadro’s number, respectively. Thevalue for s (0.95 nm) was tentatively taken as a radius ofa sphere which has the same volume as the complexregarded as a cylindrical form calculated from thedistances and angles (Ru-N, 0.210 nm;13 Ru-Cl, 0.238nm;14 N-H, 0.101 nm;13 ∠H-N-Ru, 110°;13 ∠Ru-Cl-Ru, 72.6°)14 as well as the van der Waals radius of hydrogen(0.12 nm).13 Equation 6 was applied to the plots of kapp vsc for the membrane in Figure 3 using a nonlinear least-squares method, and the best fitting was obtained whenk and rd values were (6.7 ( 0.3) × 10-2 s-1 and 1.22 ( 0.03nm, respectively. The k is almost the same as the kO2 (6.3× 10-2 s-1) calculated from kinetic analysis based on thecompetition between water oxidation and bimoleculardecomposition. The rd value (1.22 nm) is reasonable whenconsidering the molecular size (0.95 nm) of the complex,showing that the bimolecular decomposition takes placebetween the complexes in close proximity with each other.It is important to separate the distance between thecomplexes over 1.22 nm in the membrane.

Acknowledgment. The present work was supportedby the Itoh Science Foundation.

LA9901985

(13) CRC Handbook of Chemical and Physics, 73th ed.; Lide, D. R.,Ed.; CRC Press: Boca Raton, FL, 1992.

(14) Bino, A.; Cotton, F. A. J. Am. Chem. Soc. 1980, 102, 608.

kapp ) k exp{-4π(rd3 - s3)RcNA × 10-24/3} (6)

7408 Langmuir, Vol. 15, No. 21, 1999 Notes