Design and synthesis of diphosphine ligands bearing an osmium(II) bis(terpyridyl) moiety as a light-harvesting unit: Application to photocatalytic production of dihydrogen

Article abstract of DOI:10.1021/om900489d

We have designed and prepared novel diphosphine ligands bearing an Os(tpy)₂²⁺ moiety as a lightharvesting unit and have found that these diphosphine ligands coordinate to the rhodium atom in a bidentate manner. The rhodium complexes

Full text of DOI:10.1021/om900489d

5240 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 light-
harvesting unit and have found that these diphosphine ligands coordinate to the rhodium atom in a
2þ
bidentate manner. The rhodium complexes including diphosphine ligands bearing an Os(tpy)2
moiety have been revealed to work as an effective photocatalyst for the production of dihydrogen.
Introduction
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
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 renew-
able resources.1 Ruthenium(II) tris(2,20-bipyridyl) complex
2þ
(Ru(bpy)3 , bpy = 2,20-bipyridyl) has so far been exten-
complex containing two terdentate ligands, is also known to
2þ 17
sively studied for its utilization as a photosensitizer for the
visible light induced production of dihydrogen2 since the
show a similar photophysical property to Ru(bpy)3
.
2þ 3
discovery of the photocatalytic system using Ru(bpy)3
.
(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.
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
€
(9) Rau, S.; Schafer, B.; Gleich, D.; Anders, E.; Rudolph, M.;
€
Friedrich, M.; Gorls, 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.
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Fontecave, M. Angew. Chem., Int. Ed. 2008, 47, 564. (b) Fihri, A.; Artero,
V.; Pereira, A.; Fontecave, M. Dalton Trans. 2008, 5567.
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2001, 40, 3472.
(13) (a) Inagaki, A.; Edure, S.; Yatsuda, S.; Akita, M. Chem. Com-
mun. 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.
2þ
multinuclear complexes bearing a Ru(bpy)3 moiety as a
*Corresponding author. E-mail: ynishiba@sogo.t.u-tokyo.ac.jp.
(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.; Hammarstrom, L. J. Am. Chem. Soc. 2008,
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130, 26. (b) Na, Y.; Wang, M.; Pan, J.; Zhang, P.; Akermark, B.; Sun, L.
Inorg. Chem. 2008, 47, 2805.
(3) (a) Lehn, J.-M.; Sauvage, J.-P. Nouv. J. Chim. 1977, 1, 449. (b)
(14) A supramolecular complex bearing metalloporphyrin as a light-
harvesting unit is known to be applied to photocatalytic production of
dihydrogen; see: Li, X.; Wang, M.; Zhang, S.; Pan, J.; Na, Y.; Liu, J.;
€
Kalyanasundaram, K.; Kiwi, J.; Gratzel, 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.; Cam-
pagna, 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.
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Akermark, 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.
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H.; Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502. (b) Cline, E. D.;
Adamson, S. E.; Bernhard, S. Inorg. Chem. 2008, 47, 10378.
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C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigelletti, F.; Cola, L. D.;
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r
pubs.acs.org/Organometallics
Published on Web 08/12/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 17, 2009 5241
Scheme 1. Design of Diphosphine Ligands Bearing Os(tpy)22þ as
a Light-Harvesting Unit
Scheme 2. Synthesis of Diphosphine Ligands 1
2þ
However, there is no report of utilizing Os(tpy)2 deriva-
tives 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 dipho-
2þ
sphine ligands (1) bearing an Os(tpy)2 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.
Scheme 3. Formation of Dinuclear Complexes Bearing 1
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(5-
bromo-2,20:60,200-terpyridyl) with phenylboronic acid pina-
col 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
ability to reduce a rhodium species22 in the presence of
reductants under photoirradiation.24
Next, we examined the photocatalytic production of dihy-
drogen in the presence of rhodium complexes and dipho-
sphine ligands 1 (Table 1). When a solution of trifluoro-
methanesulfonic 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 3H2O in place of [RhCl(CO)2]2 im-
3
proved 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
E1/2 = -1.118 and -1.342 V, which are similar to that of
2þ 17,20
In addition, the oxidation potential of Os-
Os(tpy)2
.
(tpy)22þ in the excited state is reported to be -0.83 V.21 At
of RhCl3 3H2O/dppe (dppe=1,2- bis(diphenylphosphino)-
3
present, we consider that the Os(tpy)22þ unit has an efficient
2þ
ethane) with Os(tpy)2 (Table 1, run 6). Separately, we
confirmed that use of only either RhCl3 3H2O/dppe or 1a
does not yield any dihydrogen (Table 1, runs 8 and 9). These
3
(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 tempera-
ture. All potentials were measured against an Ag/Agþ reference elec-
trode and converted to the values vs SCE.
(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) Kolle, U.; Gratzel, M. Angew. Chem., Int. Ed. Engl. 1987, 26,
567.
(24) RhCl3Ln complexes are reported to be transformed to Rh(I)
(21) Kober, E. M.; Marshall, J, L.; Dressick, W. J.; Sullivan, B. P.;
Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755.
species easily under reduction conditions.25

5242 Organometallics, Vol. 28, No. 17, 2009
Miyake et al.
Table 1. Photocatalytic Production of Dihydrogena
Table 2. Photophysical Properties
absorptiona
emissiona,b
λ
max/nm
(ε/dm3 mol-1 cm-1
c
)
λmax/nm
Φrel
3
3
run
[Rh]
[Os]
TONb
[Os(tpy)2][PF6]2
1a
476 (18 000), 656 (5600) 715
485 (12 000), 655 (4300) 726
482 (13 000), 655 (4400) 728
1.00
0.73
1
2
[RhCl(CO)2]2
RhCl3 3H2O
RhCl3 3H2O
RhCl3 3H2O
RhCl3 3H2O
RhCl3 3H2O/dppe
RhCl(CO)(dppe)
RhCl3 3H2O/dppe
1a
1a
1a
1a
1b
36
87
381
594
52
24
0
3ad
0.52 (0.31)e
0.54 (0.35)e
RhCl3 3H2O/1ad 483 (13 000), 655 (4500) 727
3
3c
4d
5
3
3
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).
3
3
6
7
8
9
[Os(tpy)2][PF6]2
3
0
0
3
Scheme 4. Plausible Reaction Pathway
1a
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.
electron transfer, followed by sequential second electron
reduction of the rhodium center to form the reduced rho-
dium 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 dipho-
2þ
sphine ligands 1 bearing an Os(tpy)2 moiety as a light-
harvesting unit and found that diphosphine ligands 1 co-
ordinate 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.
Figure 1. Time profile of the photocatalytic production of
dihydrogen in the presense of RhCl3 3H2O and 1a.
3
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 emis-
sion of these complexes was observed around 720 nm, and
their intensity was less than that of 1a. In addition, a remark-
able decrease of emission intensities was observed when
NaHA was added as a reductant. These results suggest an
Experimental Section
1
General Procedures. H 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 BioS-
pectrometry Workstation model Voyager-DE STR spectro-
meter. Absorption and emission spectra were recorded on
Shimadzu MultiSpec-1500 and Shimadzu RF-5300PC spectro-
meters, 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 chromatog-
raphy 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
nBu4NClO4 as a supporting electrolyte. All potentials were
2þ
efficient quenching of the excited state of the Os(tpy)2
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 one-
electron reduction of the rhodium center via intramolecular
(25) Nishiyama, H.; Kondo, M.; Nakamura, T.; Itoh, K. Organome-
tallics 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.

Article
Organometallics, Vol. 28, No. 17, 2009 5243
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.
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,
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), (2-
biphenyl)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(5-
Br-tpy)2][PF6]2 (1.105 g, 1.00 mmol) and diphenyl[3-(4,4,5,5-
tetramethyl-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: dichlor-
omethane to acetonitrile) to give a black solid. After recrystalli-
zation from acetone-Et2O, the corresponding bis(phosphine
sulfide) 2a was obtained as a black solid (1.418 g, 0.926 mmol,
J
P-F=706 Hz). LDI TOF MS: 1042 [M - 2(PF6)].
Formation of Rhodium Complexes 3a from Diphosphine Li-
gands 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 (acetone-
d6) 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
=
=
1
93% yield). H NMR (acetone-d6): δ 9.07 (d, 2H, J=8.1 Hz),
177 Hz), 29.5 (br d, JRh-P = 128 Hz). IR (KBr): 1974 and
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 C66H48-
N6F12P4S2Os: C, 51.76; H, 3.16; N, 5.49. Found: C, 51.53; H,
3.23; N, 5.40.
2067 cm-1 (νCO). FAB-MS: 1344 [M - 2(PF6)].
Photocatalytic Production of Dihydrogen. A typical experi-
mental procedure for the photocatalytic production of dihydro-
gen 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 deter-
mined by GC analysis.
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 atmo-
sphere, 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),
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.