Design and Synthesis of Diphosphine Ligands Bearing an Osmium (II

We have designed and prepared novel diphosphine ligands bearing an Os(tpy)22+ moiety as a light-harvesting unit and have found that these diphosphine ...
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Organometallics 2009, 28, 5240–5243 DOI: 10.1021/om900489d

Design and Synthesis of Diphosphine Ligands Bearing an Osmium(II) Bis(terpyridyl) Moiety as a Light-Harvesting Unit: Application to Photocatalytic Production of Dihydrogen Yoshihiro Miyake,† Kazunari Nakajima,† Kouitsu Sasaki,† Ryuichi Saito,† Haruyuki Nakanishi,‡ and Yoshiaki Nishibayashi*,† †

Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan, and ‡Fuel Cell System Development Center, Toyota Motor Corporation, 1200, Mishiku, Susono, Shizuoka 410-1193, Japan Received June 10, 2009

We have designed and prepared novel diphosphine ligands bearing an Os(tpy)22þ moiety as a lightharvesting unit and have found that these diphosphine ligands coordinate to the rhodium atom in a bidentate manner. The rhodium complexes including diphosphine ligands bearing an Os(tpy)22þ moiety have been revealed to work as an effective photocatalyst for the production of dihydrogen. Introduction Considerable attention has been recently paid to the efficient conversion of solar energy to chemical energy because of an interest in environmental issues and the shortage of fossil fuels in the near future. The visible light induced catalytic production of dihydrogen is expected to produce convenient and clean energy from renewable resources.1 Ruthenium(II) tris(2,20 -bipyridyl) complex (Ru(bpy)32þ, bpy = 2,20 -bipyridyl) has so far been extensively studied for its utilization as a photosensitizer for the visible light induced production of dihydrogen2 since the discovery of the photocatalytic system using Ru(bpy)32þ.3 The transformation proceeds via electron transfer from a ruthenium center to a catalytic active center due to their attractive photophysical properties such as absorption of visible light and long excited-state lifetime, which are derived from the metal-to-ligand charge transfer (MLCT) state.4 In addition, several groups have succeeded in the preparation of multinuclear complexes bearing a Ru(bpy)32þ moiety as a *Corresponding author. E-mail: [email protected]. (1) For a review, see: Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107, 4022. (2) For recent examples of the use of Ru(bpy)32þ as a photosensitizer for production of dihydrogen, see: (a) Lei, P.; Hedlund, M.; Lomoth, R.; Rensmo, H.; Johansson, O.; Hammarstr€ om, L. J. Am. Chem. Soc. 2008, 130, 26. (b) Na, Y.; Wang, M.; Pan, J.; Zhang, P.; A˚kermark, B.; Sun, L. Inorg. Chem. 2008, 47, 2805. (3) (a) Lehn, J.-M.; Sauvage, J.-P. Nouv. J. Chim. 1977, 1, 449. (b) Kalyanasundaram, K.; Kiwi, J.; Gr€atzel, M. Helv. Chim. Acta 1978, 61, 2720. (c) Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. Helv. Chim. Acta 1979, 62, 1345. (d) Brown, G. M.; Brungschwig, B. S.; Creutz, C.; Endicott, J. F.; Sutin, N. J. Am. Chem. Soc. 1979, 101, 1298. (4) For reviews, see: (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759. (b) McClenaghan, N. D.; Leydet, Y.; Maubert, B.; Indelli, M. T.; Campagna, S. Coord. Chem. Rev. 2005, 249, 1336 , and references therein . (5) Rau, S.; Walther, D.; Vos, J. G. Dalton Trans. 2007, 915. (6) (a) Gholamkhass, B.; Mametsuka, H.; Koike, K.; Tanabe, T.; Furue, M.; Ishitani, O. Inorg. Chem. 2005, 44, 2326. (b) Sato, S.; Koike, K.; Inoue, H.; Ishitani, O. Photochem. Photobiol. Sci. 2007, 6, 454. (7) (a) Kimura, E.; Bu, X.; Shionoya, M.; Wada, S.; Maruyama, S. Inorg. Chem. 1992, 31, 4542. (b) Kimura, E.; Wada, S.; Shionoya, M.; Okazaki, Y Inorg. Chem. 1994, 33, 770. pubs.acs.org/Organometallics

Published on Web 08/12/2009

light-harvesting unit and their application to photocatalytic reactions.5-14 Recently, Eisenberg’s15 and Bernhard’s groups16 reported that platinum(II) terpyridyl acetylide complexes and iridium(III) bis(2-phenylpyridyl)(2,20 -bipyridyl) complexes work as new efficient photosensitizers for production of dihydrogen, and the development of new candidates has been continuously required. Osmium(II) bis(2,20 :60 ,200 terpyridyl) complex (Os(tpy)22þ), which is a six-coordinate complex containing two terdentate ligands, is also known to show a similar photophysical property to Ru(bpy)32þ.17 (8) (a) Ozawa, H.; Haga, M.; Sakai, K. J. Am. Chem. Soc. 2006, 128, 4926. (b) Ozawa, H.; Yokoyama, Y.; Haga, M.; Sakai, K. Dalton Trans. 2007, 1197. (9) Rau, S.; Sch€afer, B.; Gleich, D.; Anders, E.; Rudolph, M.; Friedrich, M.; G€ orls, H.; Henry, W.; Vos, J. G. Angew. Chem., Int. Ed. 2006, 45, 6215. (10) (a) Elvington, M.; Brown, J.; Arachchige, S. M.; Brewer, K. J. J. Am. Chem. Soc. 2007, 129, 10644. (b) Arachchige, S. M.; Brown, J.; Brewer, K. J. J. Photochem. Photobiol., A 2008, 197, 13. (11) (a) Fihri, A.; Artero, V.; Razavet, M.; Baffert, C.; Leibl, W.; Fontecave, M. Angew. Chem., Int. Ed. 2008, 47, 564. (b) Fihri, A.; Artero, V.; Pereira, A.; Fontecave, M. Dalton Trans. 2008, 5567. (12) Osawa, M.; Hoshino, M.; Wakatsuki, Y. Angew. Chem., Int. Ed. 2001, 40, 3472. (13) (a) Inagaki, A.; Edure, S.; Yatsuda, S.; Akita, M. Chem. Commun. 2005, 5468. (b) Inagaki, A.; Yatsuda, S.; Edure, S.; Suzuki, A.; Takahashi, T.; Akita, M. Inorg. Chem. 2007, 46, 2432. (c) Inagaki, A.; Nakagawa, H.; Akita, M.; Inoue, K.; Sakai, M.; Fujii, M. Dalton Trans. 2008, 6709. (14) A supramolecular complex bearing metalloporphyrin as a lightharvesting unit is known to be applied to photocatalytic production of dihydrogen; see: Li, X.; Wang, M.; Zhang, S.; Pan, J.; Na, Y.; Liu, J.; A˚kermark, B.; Sun, L. J. Phys. Chem. B 2008, 112, 8198. (15) (a) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc. 2006, 128, 7726. (b) Du, P.; Schneider, J.; Li, F.; Zhao, W.; Patel, U.; Castellano, F. N.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130, 5056. (c) Du, P.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130, 12576. (16) (a) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.; Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502. (b) Cline, E. D.; Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47, 10378. (17) For reviews, see: (a) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Cola, L. D.; Flamigni, L. Chem. Rev. 1994, 94, 993. (b) Baranoff, E.; Collin, J.-P.; Flamigni, L.; Sauvage, J.-P. Chem. Soc. Rev. 2004, 33, 147. (c) Medlycott, E. A.; Hanan, G. S. Coord. Chem. Rev. 2006, 250, 1763. r 2009 American Chemical Society

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Scheme 1. Design of Diphosphine Ligands Bearing Os(tpy)22þ as a Light-Harvesting Unit

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Scheme 2. Synthesis of Diphosphine Ligands 1

However, there is no report of utilizing Os(tpy)22þ derivatives as photosensitizers and light-harvesting units for photocatalytic reactions, to the best of our knowledge. We have now succeeded in the design and synthesis of diphosphine ligands (1) bearing an Os(tpy)22þ moiety that can coordinate to transition metals in a bidentate manner and also found that the produced dinuclear complexes can be applicable to photocatalytic production of dihydrogen (Scheme 1). Preliminary results are described here.

Results and Discussion Diphosphine ligands 1 were prepared from osmium(II) bis(5-bromo-2,20 :60 ,200 -terpyridyl) in two steps (Scheme 2). Suzuki-Miyaura coupling reactions of osmium(II) bis(5bromo-2,20 :60 ,200 -terpyridyl) with phenylboronic acid pinacol esters bearing a phosphinothioyl group (3-(R2P(dS)C6H4Bpin)) gave phosphine sulfides 2a and 2b in 93% and 78% yields, respectively. Reduction of 2 with Raney nickel gave the corresponding diphosphines 1a and 1b in 84% and 87% yields, respectively. The coordinating properties of diphosphine ligands 1 were examined by treatment with a rhodium complex (Scheme 3). When the diphosphine ligand 1a was treated with 0.5 equiv of [RhCl(CO)2]2 in acetone-d6 at room temperature, the corresponding dinuclear complex 3a was formed quantitatively as a mixture of two isomers in a ratio of 5:2.18 The 31P{1H} NMR spectrum of the major isomer (δ 29.4 (d, JRh-P=127 Hz), 29.6 (d, JRh-P=128 Hz)) indicated the trans geometry of two phosphorus atoms, while the minor isomer (δ 29.5 (br d, JRh-P = 128 Hz), 46.3 (br d, JRh-P =177 Hz)) was presumed to have a cis geometry. In addition, FAB-MS (1344 [M - 2PF6]) and IR (1974 and 2067 cm-1 (νCO)) spectra also supported the structure of 3a. Similarly, the reaction of 1b with [RhCl(CO)2]2 also led to the quantitative formation of the dinuclear complex 3b.19 These results indicate that diphosphine ligands 1 can smoothly coordinate to rhodium atom in a bidentate manner. The cyclic voltammogram (CV) of 3a showed one reversible oxidation wave assigned to OsII/III at E1/2 = þ0.917 V and two reversible reduction waves assigned to OsII/I and OsI/0 at E1/2 = -1.118 and -1.342 V, which are similar to that of Os(tpy)22þ.17,20 In addition, the oxidation potential of Os(tpy)22þ in the excited state is reported to be -0.83 V.21 At present, we consider that the Os(tpy)22þ unit has an efficient (18) Separately, we confirmed that the immediate evolution of CO from the reaction of the diphosphine ligands 1 with [RhCl(CO)2]2 was detected by GC analysis, and the amount of CO also indicates the efficient formation of dinuclear complexes 3. (19) See Supporting Information for experimental details. (20) Stick Pt as a working electrode and Pt wire as a counter electrode were used in CH3CN containing 0.1 M nBu4NClO4 at room temperature. All potentials were measured against an Ag/Agþ reference electrode and converted to the values vs SCE. (21) Kober, E. M.; Marshall, J, L.; Dressick, W. J.; Sullivan, B. P.; Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755.

Scheme 3. Formation of Dinuclear Complexes Bearing 1

ability to reduce a rhodium species22 in the presence of reductants under photoirradiation.24 Next, we examined the photocatalytic production of dihydrogen in the presence of rhodium complexes and diphosphine ligands 1 (Table 1). When a solution of trifluoromethanesulfonic acid (HOTf) and sodium ascorbate (NaHA) in the presence of a catalytic amount of 3a, prepared from [RhCl(CO)2]2 and 1a in situ, in acetonitrile/water (1:1) was irradiated by visible light (λ > 380 nm) for 18 h under nitrogen atmosphere, the production of dihydrogen was observed with a turnover number (TON) of 36 (Table 1, run 1). Use of RhCl3 3 3H2O in place of [RhCl(CO)2]2 improved the TON (TON = 87) (Table 1, run 2). A prolonged irradiation time further increased it (Figure 1), and the TON achieved was up to 594 for 240 h (Table 1, run 4). Other reductants such as triethanolamine and L-cysteine did not work at all in this catalytic system. A quantum yield of production of dihydrogen in the combination of RhCl3 3 3H2O with 1a was estimated to be 0.7%,19 and pH of the reaction mixture was measured to be 5.2. On the other hand, the ligand 1b showed a slightly lower catalytic activity compared with the ligand 1a (Table 1, run 5). A less amount of dihydrogen was produced in the combination of RhCl3 3 3H2O/dppe (dppe=1,2- bis(diphenylphosphino)ethane) with Os(tpy)22þ (Table 1, run 6). Separately, we confirmed that use of only either RhCl3 3 3H2O/dppe or 1a does not yield any dihydrogen (Table 1, runs 8 and 9). These (22) When we measured the CV of 3a, oxidation waves assigned to rhodium species were not observed. However, the reduction potential of a similar Rh(III) species was reported to be -0.75 V.23 This result indicates that the Rh(III) species generated in situ in the catalytic reaction is sufficient to be reduced by the Os(tpy)22þ moiety. (23) K€ olle, U.; Gr€atzel, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 567. (24) RhCl3Ln complexes are reported to be transformed to Rh(I) species easily under reduction conditions.25

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Table 1. Photocatalytic Production of Dihydrogena

Table 2. Photophysical Properties absorptiona λmax/nm (ε/dm3 3 mol-1 3 cm-1)

run 1 2 3c 4d 5 6 7 8 9

b

[Rh]

[Os]

TON

[RhCl(CO)2]2 RhCl3 3 3H2O RhCl3 3 3H2O RhCl3 3 3H2O RhCl3 3 3H2O RhCl3 3 3H2O/dppe RhCl(CO)(dppe) RhCl3 3 3H2O/dppe

1a 1a 1a 1a 1b [Os(tpy)2][PF6]2

36 87 381 594 52 24 0 0 0

1a

emissiona,b λmax/nm

Φrelc

[Os(tpy)2][PF6]2 476 (18 000), 656 (5600) 715 1.00 1a 485 (12 000), 655 (4300) 726 0.73 d 482 (13 000), 655 (4400) 728 0.52 (0.31)e 3a d RhCl3 3 3H2O/1a 483 (13 000), 655 (4500) 727 0.54 (0.35)e a Measurements were carried out in acetonitrile/water (1:1) at room temperature. b Excitation at 480 nm. c Relative quantum yield versus [Os(tpy)2][PF6]2. d Prepared in situ. e Addition of NaHA (15 000 equiv).

Scheme 4. Plausible Reaction Pathway

a

A solution of HOTf (4.8 mmol) and NaHA (15 mmol) containing rhodium complex (1.0 μmol based on rhodium) and osmium complex (1.0 μmol) in MeCN (10 mL) and H2O (10 mL) was irradiated (λ > 380 nm) for 18 h. b TON (1/2 H2/catalyst) was determined by GC analysis. c For 96 h. d For 240 h.

Figure 1. Time profile of the photocatalytic production of dihydrogen in the presense of RhCl3 3 3H2O and 1a.

results indicate that intramolecular electron transfer from an osmium center to a rhodium center plays a key role in constructing an effective photoinduced reduction system. The photophysical properties of dinuclear complexes are summarized in Table 2. These complexes exhibit two similar visible absorption bands around 480 and 655 nm, which are attributed to 1MLCT and 3MLCT, respectively.17 The emission of these complexes was observed around 720 nm, and their intensity was less than that of 1a. In addition, a remarkable decrease of emission intensities was observed when NaHA was added as a reductant. These results suggest an efficient quenching of the excited state of the Os(tpy)22þ moiety occurs both at the rhodium center oxidatively and by NaHA reductively in this photocatalytic system. Although the detailed reaction mechanism has not yet been clear,26 a plausible reaction pathway is shown in Scheme 4. Quenching of A* at the coordinated rhodium center and by NaHA proceeds smoothly to cause oneelectron reduction of the rhodium center via intramolecular (25) Nishiyama, H.; Kondo, M.; Nakamura, T.; Itoh, K. Organometallics 1991, 10, 500. (26) The result of the mercury test27 suggests the possibility of the formation of colloidal rhodium.2a,15b The detailed investigation to elucidate a catalytic active species is currently in progress. (27) (a) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855. (b) Baba, R.; Nakabayashi, S.; Fujishima, A.; Honda, K. J. Phys. Chem. 1985, 89, 1902.

electron transfer, followed by sequential second electron reduction of the rhodium center to form the reduced rhodium species B. Reaction of B with a proton affords the rhodium hydride species C, which turns into dihydrogen and the rhodium species A by protonolysis. In summary, we have designed and prepared novel diphosphine ligands 1 bearing an Os(tpy)22þ moiety as a lightharvesting unit and found that diphosphine ligands 1 coordinate to the rhodium atom in a bidentate manner. The rhodium complexes bearing diphosphine ligands 1 were revealed to work as an effective photocatalyst for production of dihydrogen. To the best of our knowledge, this is the first successful example of the utilization of the Os(tpy)22þ moiety for photocatalytic reactions. We believe that this finding will open up a new field of the utility of the Os(tpy)22þ moiety as a photosensitizer. Further work is currently in progress to elucidate the reaction mechanism of photoinduced electron transfer and to apply to other photocatalytic reductions.

Experimental Section General Procedures. 1H NMR (270 MHz), 13C NMR (67.8 MHz), and 31P NMR (109 MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in suitable solvent. Elemental analyses were performed at the Microanalytical Center of The University of Tokyo. Mass spectra were measured on a JEOL JMS-700 mass spectrometer and an Applied Biosystem BioSpectrometry Workstation model Voyager-DE STR spectrometer. Absorption and emission spectra were recorded on Shimadzu MultiSpec-1500 and Shimadzu RF-5300PC spectrometers, respectively. All reactions were carried out under a dry nitrogen atmosphere. Photoirradiation was carried out by an Ushio high-pressure mercury lamp USH-250SC (250 W) with a UV cutoff filter (Kenko L-38 filter; transmittance at 380 nm is 50%). Evolved dihydrogen was quantified by gas chromatography using a Shimadzu GC-8A with a TCD detector and a SHINCARBON ST (6 m  3 mm). Cyclic voltammograms were recorded on an ALS/Chi model 610C electrochemical analyzer with platinum working electrode in CH3CN containing 0.1 M n Bu4NClO4 as a supporting electrolyte. All potentials were

Article measured against an Ag/Agþ reference electrode and converted to the values vs SCE. Solvents were dried by the usual methods, then distilled and degassed before use. Preparation of 2. A typical experimental procedure for the preparation of 2a (R = Ph) is described below. In a 50 mL Schlenk flask were placed Pd(OAc)2 (22.5 mg, 0.100 mmol), (2biphenyl)di-tert-butylphosphine (29.8 mg, 0.100 mmol), and KF (174.8 mg, 3.00 mmol) under N2. Anhydrous and degassed DMSO (20 mL) was added, and then the mixture was stirred at room temperature for a while. After the addition of [Os(5Br-tpy)2][PF6]2 (1.105 g, 1.00 mmol) and diphenyl[3-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]phosphine sulfide (1.681 g, 4.00 mmol), the reaction flask was kept at 90 °C for 48 h. The reaction mixture was cooled to room temperature, and then saturated NH4PF6(aq) (60 mL) and water (300 mL) were added. The resulting precipitate was filtered off and washed with water (2 mL  3) and Et2O (1 mL  3). The crude residue was purified by column chromatogaraphy (Al2O3, eluents: dichloromethane to acetonitrile) to give a black solid. After recrystallization from acetone-Et2O, the corresponding bis(phosphine sulfide) 2a was obtained as a black solid (1.418 g, 0.926 mmol, 93% yield). 1H NMR (acetone-d6): δ 9.07 (d, 2H, J=8.1 Hz), 9.04 (d, 2H, J=7.3 Hz), 8.80 (d, 2H, J=8.6 Hz), 8.76 (d, 2H, J=8.0 Hz), 8.15 (dd, 2H, J=8.6 and 2.0 Hz), 8.03 (t, 2H, J=7.6 Hz), 7.92 (td, 2H, J=7.8 and 1.4 Hz), 7.72-7.47 (m, 32H), 7.24 (ddd, 2H, J=7.6, 5.9, and 1.2 Hz). 31P{1H} NMR (acetone-d6): δ 42.5 (s), -144.1 (sep, JP-F=706 Hz). Anal. Calcd for C66H48N6F12P4S2Os: C, 51.76; H, 3.16; N, 5.49. Found: C, 51.53; H, 3.23; N, 5.40. 2b: 78% yield. A black solid. 1H NMR (acetone-d6): δ 9.13 (d, 2H, J=8.2 Hz), 9.12 (d, 2H, J=7.9 Hz), 8.85 (d, 2H, J=8.4 Hz), 8.81 (d, 2H, J=8.4 Hz), 8.22 (dd, 2H, J=8.4 and 2.0 Hz), 8.12 (t, 2H, J=8.0 Hz), 7.98-7.50 (m, 14H), 7.28 (ddd, 2H, J=7.6, 5.8, and 1.5 Hz), 2.66-2.53 (m, 4H), 1.13 (dd, 12H, J = 17.3 and 6.8 Hz), 0.92 (dd, 12H, J = 17.3 and 6.8 Hz). 31P{1H} NMR (acetone-d6): δ 67.6 (s), -144.2 (sep, JP-F=708 Hz). Anal. Calcd for C54H56N6F12P4S2Os: C, 46.48; H, 4.05; N, 6.02. Found: C, 46.27; H, 4.33; N, 5.89. Preparation of 1. A typical experimental procedure for the preparation of 1a (R = Ph) is described below. In a 20 mL Schlenk flask was placed Raney nickel slurry (2.35 g) under N2. After loaded Raney nickel was washed sequnentially with water (10 mL  3), methanol (10 mL  3), and CH3CN (10 mL  3), anhydrous and degassed CH3CN (8 mL) was added. Phosphine sulfide 2a (235.6 mg, 0.154 mmol) was added, and the reaction mixture was stirred at room temperature for 2 h. The resulting solution was filtered through Celite under a nitrogen atmosphere, and the filtrate was concentrated under reduced pressure to give the corresponding diphosphine 1a (188.9 mg, 0.129 mmol, 84% yield) as a black solid. 1H NMR (acetone-d6): δ 9.03 (d, 2H, J=8.6 Hz), 9.00 (d, 2H, J=8.3 Hz), 8.76 (d, 2H, J=8.5 Hz), 8.72 (d, 2H, J = 8.0 Hz), 8.09 (dd, 2H, J=8.5 and 2.2 Hz), 8.00 (t, 2H, J=8.5 Hz), 7.90 (td, 2H, J=8.0 and 1.4 Hz),

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7.59-7.10 (m, 34H). 31P{1H} NMR (acetone-d6): δ -5.3 (s), -144.1 (sep, JP-F=707 Hz). LDI TOF MS: 1178 [M - 2(PF6)]. 1b: 87% yield. A black solid. 1H NMR (CD3CN): δ 8.77 (d, 2H, J=8.3 Hz), 8.74 (d, 2H, J=8.2 Hz), 8.50 (d, 2H, J=3 8.5 Hz), 8.46 (d, 2H, J=7.9 Hz), 8.00 (dd, 2H, J=8.5 and 2.2 Hz), 7.92 (t, 2H, J=8.2 Hz), 7.77 (td, 2H, J=7.7 and 1.4 Hz), 7.49-7.21 (m, 12H), 7.10 (ddd, 2H, J = 7.7, 5.8, and 1.5 Hz), 2.10-1.99 (m, 4H), 1.02 (dd, 12H, J = 15.0 and 6.9 Hz), 0.77 (dd, 12H, J=11.3 and 7.0 Hz). 31P{1H} NMR (CD3CN): δ 10.8 (s), -145.4 (sep, JP-F =706 Hz). LDI TOF MS: 1042 [M - 2(PF6)]. Formation of Rhodium Complexes 3a from Diphosphine Ligands 1a and [RhCl(CO)2]2. In a 20 mL Schlenk flask were placed 1a (8.8 mg, 6.0 μmol) and [RhCl(CO)2]2 (1.2 mg, 3.1 μmol) under N2. After the addition of acetone-d6 (0.7 mL), the resulting mixture was stirred at room temperature for a while. The obtained solution was analyzed by 1H and 31P NMR spectroscopies. After concentrating the solution, the residue was analyzed by mass and IR spectroscopies. 3a: 1H NMR (acetoned6) δ 9.09-8.35 (m, 8H), 8.29-7.18 (m, 40H). 31P{1H} NMR (acetone-d6) trans: δ 29.6 (d, JRh-P=128 Hz), 29.4 (d, JRh-P= 127 Hz), -144.1 (sep, JP-F=707 Hz); cis: δ 46.3 (br d, JRh-P= 177 Hz), 29.5 (br d, JRh-P = 128 Hz). IR (KBr): 1974 and 2067 cm-1 (νCO). FAB-MS: 1344 [M - 2(PF6)]. Photocatalytic Production of Dihydrogen. A typical experimental procedure for the photocatalytic production of dihydrogen using the dinuclear complex 3a is described below. In a 20 mL Schlenk flask was placed 1a (4.4 mg, 3.0 μmol) under N2, and then a 0.5 mM solution of [RhCl(CO)2]2 in CH3CN (3.0 mL, 1.5 μmol) was added and the evolution of CO was immediately observed. The resulting mixture was stirred at room temperature for 30 min, and a 1.0 mM solution of 3a in CH3CN was prepared. To a solution of sodium ascorbate (2.79 g, 15.0 mmol) in CH3CN (9 mL) and water (10 mL) under N2 was added a 1.0 mM solution of 3a in CH3CN (1.0 mL, 1.0 μmol). After the addition of trifluoromethanesulfonic acid (0.42 mL, 4.8 mmol), the reaction mixture was irradiated at λ>380 nm for 18 h. The amount of dihydrogen was determined by GC analysis.

Acknowledgment. The authors are grateful to Prof. Koji Araki at the Institute of Industrial Science, The University of Tokyo, for a valuable suggestion. This work was supported by Grants-in-Aid for Scientific Research for Young Scientists (S) (No. 19675002) and for Scientific Research on Priority Areas (No. 18066003) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Y.M. also thanks TEPCO Research Foundation. Supporting Information Available: Experimental procedures and new compound data. This material is available free of charge via the Internet at http://pubs.acs.org.