Quantitative photochemical formation of [Ru(tpy)(bpy)H]+

Article abstract of DOI:10.1021/ic901080r

Quantitative photochemical production of [Ru(tpy)(bpy)H]⁺ (Ru-H⁺) was achieved by irradiation of [Ru(tpy)(bpy)(DMF)] ²⁺ (Ru-DMF^2-; DMF = N,N-dimethylformamide) in a tetrahydroturan (THF) solution containing exce

Full text of DOI:10.1021/ic901080r

10138 Inorg. Chem. 2009, 48, 10138–10145
DOI: 10.1021/ic901080r
Quantitative Photochemical Formation of [Ru(tpy)(bpy)H]+
Yasuo Matsubara,† Hideo Konno,‡,§ Atsuo Kobayashi,‡,§ and Osamu Ishitani*,†
†Department of Chemistry, Tokyo Institute of Technology, O-okayama 2-12-1, E1-9, Meguro-ku, Tokyo
152-8551, Japan, and ‡Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo,
Saitama 338-8570, Japan. § Present address (H.K.): Research Institute for Innovation in Sustainable Chemistry,
National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8569,
Japan; (A.K.) DIC Corporation, Oaza Komuro, Ina-machi, Kitaadachi-gun, Saitama 362-8577, Japan.
Received June 4, 2009
Quantitative photochemical production of [Ru(tpy)(bpy)H]+ (Ru-H+) was achieved by irradiation of [Ru(tpy)-
(bpy)(DMF)]2+ (Ru-DMF2+; DMF = N,N-dimethylformamide) in a tetrahydrofuran (THF) solution containing excess
triethylamine (NEt3). The mechanism of the Ru-H+ formation was investigated in detail. A photochemical ligand
substitution reaction of Ru-DMF2+ in THF proceeded to give [Ru(tpy)(bpy)(THF)]2+ (Ru-THF2+) with a quantum yield
of (7.6 ( 0.7) ꢀ 10-2. In the presence of NEt3, a similar photochemical ligand substitution reaction also proceeded
quickly, but the products were an equilibrium mixture of Ru-THF2+ and [Ru(tpy)(bpy)(NEt3)]2+ (Ru-NEt32+) with a
considerable amount of Ru-H+ even in the first stage of the photochemical reaction. The equilibrium constant between
Ru-THF2+ and Ru-NEt32+ was determined as 6.9 ( 2.1. Irradiation to Ru-NEt32+ gave Ru-H+ with a quantum yield of
(9.1 ( 0.5) ꢀ 10-3. An important intermediate, Ru-NEt32+, was isolated, and its properties were investigated in detail.
Introduction
good electron acceptors and radical coupling often
competes with the addition of a proton and another
electron to the one-electron-reduced species as an inter-
mediate.4
Construction of photochemical multielectron reduction
systems has been one of the most significant barriers to
developing artificial photosynthesis that can convert solar
energy to chemical energy. Typical examples of this process
include photocatalytic hydrogen evolution from water1 and
the reduction of CO2,2 which require two (or more) electrons
in order to form products such as H2 and CO. Therefore, a
photosensitizer that initiates photochemical single-electron
transfer should combine with a catalyst which converts
single-electron transfer to a multielectron process. However,
such photocatalytic systems initiated by photochemical elec-
tron transfer cannot fully reduce organic substrates such
asolefins,3a carbonylcompounds,3b and the coenzyme NAD-
(P)+,3c which require two-electron reduction (i.e., hydroge-
nation or hydride reduction), since these substrates are also
There have been only a few photocatalytic systems that
mediate only hydride transfer. One such system, a combina-
tion of [Ru(tpy)(bpy)(py)]2+ (tpy = 2,20;60,200-terpyridine;
bpy=2,20-bipyridine; py=pyridine) as a photocatalyst and
triethylamine (NEt3) as a reductant, can selectively catalyze
the hydride reduction of a NAD(P)+ model compound to the
corresponding 1,4-dihydroform.5 Although this type of
photocatalysis is unique because no intermolecular photo-
electron transfer process is included in the reaction, both
the quantum yield (Φ) and the turnover number (TN)
for formation of the 1,4-dihydroform are low (Φ < 10-8
,
TN ∼ 3). We have previously reported that photochemical
formation of the hydrido complex [Ru(tpy)(bpy)H]+ should
be one of the key processes of photocatalytic hydride trans-
fer,5 but the detail of this step is still unclear because only
3.5% of [Ru(tpy)(bpy)H]+, based on the starting complex,
accumulated in the reaction during steady-state irradiation.
Although some intermediates such as a NEt3-coordinated
complex have been proposed, they have not been isolated, and
*To whom correspondence should be addressed. E-mail: ishitani@chem.
titech.ac.jp.
(1) (a) Amouyal, E. In Homogeneous Photocatalysis; VCH: Weinheim,
Germany, 1997; Vol. 2, pp 263-307. (b) Esswein, A. J.; Nocera, D. G. Chem.
Rev. 2007, 107, 4022–4047.
(2) (a) Arakawa, H..; et al. Chem. Rev. 2001, 101, 953–996. (b) Fujita, E.;
DuBois, D. L. Carbon Dioxide Fixation, BNL-67078, Brookhaven National
Lab.: Upton, NY, 2000; pp 1-34. (c) Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O.
J. Am. Chem. Soc. 2008, 130, 2023–2031.
(3) (a) Pac, C.; Ihama, M.; Yasuda, M.; Miyauchi, Y.; Sakurai, H. J. Am.
Chem. Soc. 1981, 103, 6495–6497. (b) Ishitani, O.; Yanagida, S.; Takamuku, S.;
Pac, C. J. Org. Chem. 1987, 52, 2790–2796. (c) Pac, C.; Kaseda, S.; Ishii, K.;
Yanagida, S.; Ishitani, O. In Photochemical Processes in Organized Molecular
Systems; North-Holland: Amsterdam, 1991; pp 177-186.
(4) In the case of NAD(P)þ model compounds: (a) Eisner, U.; Kuthan, J.
Chem. Rev. 1972, 72, 1–42. (b) Carelli, V.; Liberatore, F.; Casino, A.; Di Rienzo,
B.; Tortorella, S.; Scipione, L. New J. Chem. 1996, 20, 125–130. (c) Kano, K.;
Matsuo, T. Tetrahedron Lett. 1975, 1389–1392.
(5) Ishitani, O.; Inoue, N.; Koike, K.; Ibusuki, T. J. Chem. Soc., Chem.
Commun. 1994, 367–368.
r
pubs.acs.org/IC
Published on Web 10/07/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10139
consequently their properties are not well-understood
(eq 1).
and then n-pentane was added into the solution to afford a
brown solid. The solid was collected by filtration and washed
with n-pentane. Yield: 55.5 mg, 80%. Anal. Calcd for C58H31-
BF27N5O3SRu: C, 46.35%; H, 2.08%; N, 4.66%. Found: C,
46.36%; H, 1.78%; N, 4.78%. ESI MS (m/z): 640.0, [M]+.
Calcd: 640.02. 1H NMR data are shown in the Supporting
Information.
hv
s
2 þ
2 þ
½RuðtpyÞðbpyÞðpyÞꢁ
½RuðtpyÞðbpyÞðNEt Þꢁ
3
NEt3
hv
s
½RuðtpyÞðbpyÞHꢁþ
ð1Þ
0
[Ru(tpy)(bpy)(DMF)](BAr4 )2. A 2 mL DMF solution con-
0
Here, we report an improvement of both the efficiency and
yield of the photochemical formation of [Ru(tpy)(bpy)H]+
by several orders of magnitude. This optimization relied on
detailed mechanistic studies, including the isolation of a
critical intermediate.
taining [Ru(t0 py)(bpy)(CF3SO3)](BAr4 ) (46.6 mg, 31.0 μmol)
and NaBAr4 2H2O (29.5 mg, 32.0 μmol) was stirred for 5 min,
3
and then cold water was added. The brown precipitate was
collected by filtration, washed with cold water, and dried under
a vacuum. The product was dissolved in a minimum amount of
diethylether under a nitrogen atmosphere and precipitated a
second time by the addition of n-pentane. Yield: 63.1 mg, 89%.
Anal. Calcd for C92H50B2F48N6ORu: C, 48.25%; H, 2.20%; N,
3.67%. Found: C, 48.36%; H, 2.12%; N, 3.53%. ESI MS (m/z):
281.8, [M]2+. Calcd: 282.06. UV-vis absorption (DMF and
THF) λmax, nm (ε, 104 M-1 cm-1): 485 (1.0) and 481 (1.0). 1H
NMR data are shown in the Supporting Information.
Experimental Section
1
General Procedures. H NMR spectra were recorded on a
JEOL Lambda 400 NMR spectrometer (400 MHz) at 25 °C.
UV-vis spectra were recorded using a JASCO V-565 spectro-
meter or MCPD-2000 (Otsuka Electronic Co.). The reduced
products of the NAD(P)+ model compounds were analyzed
using a HPLC system with a Nomura ODS-UG-5 column, a
Shimazu ST-50 pump, a Shimazu UV-50 detector (wavelength:
320 nm for 1,4-BCF3H), and a Rheodyne 7125 injector. A mixed
solution of MeOH/KH2PO4-NaOH buffer (0.05 M, 4:1 v/v)
was used as the eluent. Electrospray ionization mass spectra
were obtained with a Shimazu LCMS-2010A system with
HPLC-grade methanol as the mobile phase. Diethylamine was
analyzed using a Shimazu GC-17A gas chromatograph and a
flame ionization detector (GC-FID) with an InertCap for
amines capillary column (GL Sciences Inc.). Wolfram Mathe-
matica 6.0 software was used for global fitting.
11
0
[Ru(tpy)(bpy)(NEt3)](BAr4 )2 (NEt3)0.4
atmosphere, [Ru(tpy)(bpy)(CF3SO3)](BAr4 ) (73.3 mg, 48.8
.
0Under a nitrogen
3
0
μmol) was dissolved in CH2Cl2 (10 mL), and then NaBAr4
(43.3 mg, 48.9 μmol) was added to it. The solution was stirred for
30 min, followed by filtration to remove precipitated NaCF3-
SO3. To avoid the reaction of the dichloromethane with NEt3,
diethylether (1 mL) was added to the red filtrate, followed by the
addition of n-pentane to produce a brown precipitate. The
precipitate was washed three times with n-pentane, dissolved
in a minimum amount of diethylether, and then precipitated
again by the addition of n-pentane. The precipitated solid was
washed three times with n-pentane and dissolved in a minimum
amount of diethylether again. NEt3 (1 mL) was then added to it,
and the solution was evaporated slowly under reduced pressure
until a solid began to precipitate. The resulting purple solids
were washed with n-pentane. Even after the solids were dried
under a vacuum for 6 h, 1H NMR spectrum of the dry CD2Cl2
solution, which dissolved the solids, indicated that 0.4 equiv of
NEt3 was contained as a solvent of crystallization. Yield: 80.1
mg, 70%. Anal. Calcd for C97.4H64B2F48N6.4Ru: C, 49.60%; H,
2.74%; N, 3.80%. Found: C, 49.21%; H, 3.12%; N, 3.75%. 1H
NMR (δ, 396 MHz) spectrum was measured in dry CD2Cl2
containing the ruthenium complex (12.1 mM): 9.63 (br, 1H, bpy-
6), 8.43 (d, 1H, J = 8.1 Hz, bpy-3), 8.29 (d, 2H, J = 7.7 Hz, tpy-
30), 8.19 (ddd, 1H, J = 8.1, 7.5, 1.5 Hz, bpy-4), 8.18 (d, 2H, J =
8.3 Hz, tpy-3), 8.09 (d, 1H, J = 8.2 Hz, bpy-30), 7.97 (t, 1H, J =
7.7 Hz, tpy-40), 7.90 (dd, 1H, J = 7.5, 5.8 Hz, bpy-5), 70.80 (ddd,
2H, J = 8.3, 7.8, 0.8 Hz, tpy-4), 7.72 (m, 16H, BAr4 -o), 7.56
Materials. Dimethylformamide (DMF) was dried over 4A
molecular sieves and distilled at reduced pressure before use.
Tetrahydrofuran (THF) was distilled from Na/benzophenone
under a nitrogen atmosphere prior to use. Dichloromethane and
triethylamine were dried over calcium hydride and distilled
under an argon atmosphere. Dichloromethane-d2 was dried
over calcium hydride, distilled using trap-to-trap techniques,
and stored over activated 4A molecular sieves under a nitrogen
atmosphere. Other chemicals obtained from commercial
sources were used without purification. RuCl3 3H2O was
3
0
kindly supplied by KojimaChemical Co. NaBAr4 2H2O (Ar0 =
3
3,5-bis(trifluoromethyl)phenyl) was purchased from ABCR
GmbH and Company or prepared by the reported procedure.6
0
Anhydrous NaBAr4 was obtained by the removal of water
as the dichloromethane azeotrope prior to use. [Ru(tpy)-
(bpy)(py)](PF6)2,7 [Ru(tpy)(bpy)H](PF6),8 [Ru(tpy)(bpy)Cl]-
(PF6),9 hexafluorophosphate salts of the 1-benzyl-3-trifluoro-
methylpyridinium cation (BCF3+),10 and the corresponding
1,4-dihydroforms (BCF3H)10 were prepared according to re-
0
(1H, bpy-40), 7.55 (2H, tpy-6), 7.54 (m, 8H, BAr4 -p), 7.22 (dd,
2H, J = 7.8, 6.3 Hz, tpy-5), 6.97 (d, 1H, J = 6.0 Hz, bpy-60), 6.83
(ddd, 1H, J = 7.7, 6.0, 1.2 Hz, bpy-50), 2.24 (q, 8.4H, J = 7.2
Hz, -CH2-), 0.78 (t, 12.6H, J = 7.2 Hz, -CH3).
0
ported methods. [Ru(tpy)(bpy)Cl](BAr4 ) was prepared by an-
0
ion exchange of [Ru(tpy)(bpy)Cl](PF6) with NaBAr4 in a
dichloromethane solution.
Photochemical Formation of the Hydrido Complex. Under a
nitrogen atmosphere, a DMF or THF solution (4 mL) contain-
0
0
[Ru(tpy)(bpy)(CF3SO3)](BAr4 ).0A dichloromethane solution
ing [Ru(tpy)(bpy)(DMF)](BAr4 )2 (0.05 mM) and NEt3 (0 -
containing [Ru(tpy)(bpy)Cl](BAr4 ) (60.0 mg, 43.2 μmol) and
AgCF3SO3 (11.9 mg, 46.4 μmol) was refluxed for 24 h under an
argon atmosphere. The precipitated white solid (AgCl) was
removed by filtration. The filtrate was partially evaporated,
4 M) was placed into a quartz cuvette (d = 1 cm) and bubbled
with argon for 15 min, and then the cuvette was sealed with a
rubber septum (Aldrich Z553921). The sample solution was kept
at 25 ( 1 °C using a temperature control unit (TAITEC
LabBath LB-21 JR) and irradiated at 436 nm using a 500 W
high-pressure Hg lamp (Eikosha Co.) combined with a band-
pass filter (436 ( 2 nm, Asahi Spectra Co.). The incident light
intensity into the solution was 0.23 ( 0.02 μeinstein s-1, which
was determined using a K3Fe(C2O4)3 actinometer.12 Formation
(6) Reger, D. L.; Wright, T. D.; Little, C. A.; Lamba, J. J. S.; Smith, M. D.
Inorg. Chem. 2001, 40, 3810–3814.
(7) Hecker, C. R.; Fanwick, P. E.; McMillin, D. R. Inorg. Chem. 1991, 30,
659–666.
(8) Konno, H.; Kobayashi, A.; Sakamoto, K.; Fagalde, F.; Katz, N. E.;
Saitoh, H.; Ishitani, O. Inorg. Chim. Acta 2000, 299, 155–163.
(9) Rasmussen, S. C.; Ronco, S. E.; Mlsna, D. A.; Billadeau, M. A.;
Pennington, W. T.; Kolis, J. W.; Petersen, J. D. Inorg. Chem. 1995, 34, 821–
829.
(10) Konno, H.; Sakamoto, K.; Ishitani, O. Angew. Chem., Int. Ed. 2000,
39, 4061–4063.
(11) (a) Hevia, E.; Perez, J.; Riera, V.; Miguel, D.; Kassel, S.; Rheingold,
A. Inorg. Chem. 2002, 41, 4673–4679. (b) Huhmann-Vincent, J.; Scott, B. L.;
Kubas, G. J. J. Am. Chem. Soc. 1998, 120, 6808–6809.
(12) Kurien, K. C. J. Chem. Soc. B 1971, 2081–2082.

10140 Inorganic Chemistry, Vol. 48, No. 21, 2009
Matsubara et al.
of the hydrido complex [Ru(tpy)(bpy)H]+ (Ru-H+) was ana-
lyzed by three methods as follows: (a) HPLC analysis of BCF3H
formed by the addition of BCF3+PF6- (about 0.1 mM) to the
to the ruthenium complex proceeds photochemically in
THF (eq 2).
irradiated solution, (b) determination using the differential
+
spectrum between, before and after the addition of BCF3
,
and (c) 1H NMR measurement of the irradiated THF-d8 solu-
tion containing Ru-DMF2+ (10 mM) and NEt3 (2 M; Figure S6,
Supporting Information). The differential molar extinction
coefficient (Δε) at 560 nm was (9.5 ( 1.3) ꢀ 103 M-l cm-1 using
the reaction of [Ru(tpy)(bpy)H](PF6) with BCF3+PF6-. Esti-
mation of the quantum yield is mentioned below. Gas samples
were taken using a gastight syringe. The diethylamine was
analyzed using GC-FID as described above.
Determination of Equilibrium Constants. [Ru(tpy)-
0
(bpy)(NEt3)](BAr4 )2 (NEt3)0.4 was dissolved in CD2Cl2 (0.55
3
mL) and transferred to a NMR tube under a nitrogen atmo-
sphere, and then the 1H NMR spectrum was measured. After the
measurement, various volumes of NEt3 or THF were intro-
duced into the NMR tube, and the 1H NMR spectrum was
recorded again. The behavior of the chemical shift of the
methylene and methyl protons in NEt3 allowed for calculation
of the equilibrium constant K1 between [Ru(tpy)(bpy)(NEt3)]2+
(Ru-NEt32+) and [Ru(tpy)(bpy)(CD2Cl2)]2+ (Ru-CD2Cl22+).
The concentration of [Ru(tpy)(bpy)(THF)]2+ (Ru-THF2+) was
directly determined using the proton peaks attributed to the THF
ligand.
Figure 2 illustrates the electronic spectral change of the
THF solution containing Ru-DMF2+ and NEt3 during
irradiation. Although no isosbestic point was observed in
the first stage of the photoreaction (0-3 min, shown in
Figure 2a), the intensity of the absorption band with a
maximum at 485 nm decreased, and new bands appeared
at 385 and 544 nm with isosbestic points at 344, 424, and
500 nm after 3 min of irradiation (Figure 2b). The final
spectrum obtained by irradiation for 120 min was almost
identical to the spectrum of a THF solution containing
Ru-H+ (Figure S1, Supporting Information).
Results
Photochemical Formation of the Hydrido Complex. In a
typical run, a THF solution (4 mL) containing
[Ru(tpy)(bpy)(DMF)]2+ (Ru-DMF2+; 0.20 μmol) and
NEt3 (2 M) was irradiated using 436 nm monochromatic
light for 2 h. The UV-vis absorption spectrum of the
irradiated solution (Figure 1a) was quite similar to that of
[Ru(tpy)(bpy)H]+ (Ru-H+),8 as shown in Figure 1b.
After the irradiation, the 1-benzyl-3-trifluoromethyl-
pyridinium cation (BCF3+, 0.5 μmol) was added to the
reaction solution as hydride acceptor, giving 0.2 μmol of
the corresponding hydride reduction product 1-benzyl-
3-trifluoromethyl-1,4-dihydropyridine (1,4-BCF3H). The
differential electronic spectrum of the irradiated solution
Photochemical Ligand Substitution of Ru-DMF2+. In
the absence of NEt3, irradiation of a THF solution
containing Ru-DMF2+ caused a rapid blue-shift of the
metal-to-ligand charge-transfer absorption band from
481 to 469 nm with isosbestic points at 453 and 393 nm
(Figure 3a) within 3 min, which is attributable to photo-
chemical substitution of the DMF ligand with a THF
molecule to give [Ru(tpy)(bpy)(THF)]2+ (Ru-THF2+
,
eq 3), but further irradiation did not cause any change
of the solution.
hv
2 þ s
2 þ
½RuðtpyÞðbpyÞðDMFÞꢁ
½RuðtpyÞðbpyÞðTHFÞꢁ
þ DMF ð3Þ
+
before and after the addition of BCF3 (inset in
in THF
Figure 1a) was almost identical to that obtained by the
addition of BCF3+ to a THF solution containing Ru-H+
and NEt3 (inset Figure 1b). Comparison between
these two differential spectra also indicates that Ru-H+
was quantitatively formed by the photochemical reac-
tion of Ru-DMF2+ with NEt3.13 A small amount of
water was added to the irradiated solution and then
analyzed by GC-FID. A stoichiometric equivalent
(with respect to Ru-H+) of diethylamine, which
should be produced via hydrolysis of the two-electron
oxidation product of NEt3, Et2N+dCHCH3,14 was
formed during the photoreaction. These results clearly
indicate that quantitative hydride transfer from NEt3
When the irradiated solution was kept in the dark for
7 h, the UV-vis absorption spectrum of the solution
returned to that observed before irradiation (Figure 3b).
Because the reformation process of Ru-DMF2+ from
Ru-THF2+ was much slower than the photochemical
ligand substitution of Ru-DMF2+ with THF and the
concentration of THF (12.3 M) was much higher than
that of DMF (0.051 mM), irradiation of Ru-DMF2+ in
THF should cause almost a quantitative conversion to
Ru-THF2+. As such, the molar extinction coefficient of
Ru-THF2+ at 477 nm was estimated as (9.0 ( 0.2) ꢀ 103
M-1 cm-1. The quantum yield could be determined to be
(7.6 ( 0.7) ꢀ 10-2 by fitting the absorption change
at 477 nm caused by a growth of peaks attributable to
Ru-THF2+ and a corresponding decrease of those from
Ru-DMF2+ (ε477=(9.4 ( 0.2) ꢀ 103 M-1 cm-1, Support-
ing Information).
(13) The differential molar extinction coefficient at 560 nm is (9.5 ( 1.3) ꢀ
103 M-l cm-1, which was obtained using data shown in the inset in Figure 1b.
The 1H NMR spectrum of a THF-d8 solution containing Ru-DMF2þ and
NEt3 after irradiation also provided clear evidence for the photochemical
formation of Ru-Hþ, that is, observation of the hydride ligand peak at
-14.51 ppm (Figure S6, Supporting Information).
(14) (a) Castedo, L.; Riguera, R.; Vazquez, M. P. J. Chem. Soc., Chem.
Commun. 1983, 301–302. (b) Cohen, S. G.; Stein, N. M. J. Am. Chem. Soc.
1971, 93, 6542–6551. (c) Russell, C. D. Anal. Chem. 1963, 35, 1291–1292. (d)
Smith, P. J.; Mann, C. K. J. Org. Chem. 1969, 34, 1821–1826.
Synthesis and Identification of [Ru(tpy)(bpy)(NEt3)]2+
.
The ether complex [Ru(tpy)(bpy)(OEt2)]2+ was chosen as
a precursor and was reacted with NEt3 in a dry diethyl
ether solution at room temperature. Evaporation of the

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10141
Figure 1. (a) UV-vis absorption spectra of the irradiated THF solutions containing Ru-DMF2+ (0.051 mM) and NEt3 (2 M) for 2 h before (a1) and after
(a2) addition of BCF3+ (0.125 mM). (b) UV-vis absorption spectra of the THF solution containing Ru-H+ (0.093 mM) and NEt3 (2 M) before (b1) and
after (b2) addition of BCF3+ (0.2 mM). Each inset shows differential absorption spectra, that is, a1 - a2 and b1 - b2, respectively.
Figure 2. UV-vis absorption spectral changes of a THF solution containing Ru-DMF2+ (0.051 mM) and NEt3 (2 M) duringirradiation at 436 nm (a) for
0-3 min recorded at intervals of 0.5 min and (b) for 3-120 min, recorded at intervals of 3 min.
Figure 3. UV-visabsorption spectralchangesofa THF solution(4 mL) containingRu-DMF2+ (0.051mM) during irradiationusing436 nmlight (0.23(
0.02 μeinstein s-1), recorded up to 3 min at intervals of 10 s (a) and recorded after stopping irradiation at intervals of 30 min (b).
solvent and residual NEt3, with care taken to exclude
dark red solid [Ru(tpy)(bpy)-
EquilibriumbetweenRu-NEt32+ andRu-THF2+.Figure5
shows H NMR spectra of a dry CD2Cl2 solution con-
1
moisture, gave
a
0
(NEt3)](BAr4 )2 with about half of an equivalent of
NEt3 as the solvent of crystallization, the existence of
which was clearly shown by both elemental analysis and
1H NMR. Figure 4 illustrates the 1H NMR spectrum of
the Ru-NEt32+ dissolved in dry CD2Cl2. The quartet and
triplet peaks attributed to the ethyl groups of the NEt3
ligand were shifted downfield by 0.23 ppm and 0.20 ppm,
respectively, compared with free NEt3. While the proton
peaks at the five and six positions of the bpy ligand were
broadened, the other aromatic proton peaks were not.
taining Ru-NEt3 (BAr4)2 0.4NEt3 (1.8 mM) before
and after the addition of NEt3 (20 mM). It is noteworthy
3
2+
that the concentration of Ru-NEt3
in Figure 5a
(1.8 mM) was much lower than that in Figure 4 (12.1
mM). Although in all of the cases only one set of proton
peaks corresponding to the ethyl groups was observed,
their chemical shifts varied depending on the concentrations
of NEt3 and the added complex, as shown in Figure
S3 (Supporting Information). These results clearly indi-
cate that there is an equilibrium between Ru-NEt32+ and

10142 Inorganic Chemistry, Vol. 48, No. 21, 2009
Matsubara et al.
Figure 4. 1H NMR spectra of Ru-NEt3 (12.1 mM; black) and free
2+
NEt3 (red) measured in dry CD2Cl2 at 25 °C.
Figure 6. 1H NMR spectra ofa CD2Cl2 solution containing Ru-NEt3
2+
(1.8 mM) before (a) and after the addition of THF (b, 16 mM; c, 30 mM).
The peaks marked with / are the spinning side bands. The peaks marked
with † are attributed to Et3N+-CD2Cl. The peaks marked with ‡ are
attributed to contaminating n-pentane.
downfield in the presence of a higher concentration of
THF. Two sets of protons attributed to THF were ob-
served in the presence of an excess amount of THF, and
this ratio did not change even after the sample was kept in
the dark for 1 h. The protons attributed to the THF
ligand (3.68 and 1.81 ppm) were shifted downfield
compared with those of free THF (2.88 and 1.64 ppm).
These results indicate that ligand substitution of Ru-
also proceeds with a THF molecule, giving
Figure 5. 1H NMR spectra of a CD2Cl2 solution containing the Ru-
NEt32+ (1.8 mM) (a) before addition of NEt3 and (b) after addition of
NEt3 (20 mM). 1H NMR spectrum of CD2Cl2 containing only NEt3 (c).
The peak marked with † is attributed to Et3N+-CD2Cl, which is
produced via a reaction of NEt3 with CD2Cl2 (see ref 15).
2+
NEt3
[Ru(tpy)(bpy)(THF)]2+ (Ru-THF2+), and that an equi-
librium was attained among Ru-NEt32+, Ru-THF2+, and
Ru-CD2Cl2 as shown in Scheme 1. Calculation of the
2+
equilibrium constant K2 was obtained as (1.4 ( 0.4) ꢀ 103
a solvent complex, probably [Ru(tpy)(bpy)(CD2Cl2)]2+
(Ru-CD2Cl22+) in this case (eq 4). The equilibrium con-
stant K1 was obtained as (9.8 ( 1.2) ꢀ 103 (Supporting
Information).
(Supporting Information). Therefore, the equilibrium con-
stant between Ru-NEt3
and Ru-THF2+ (K) can be
2+
determined to be 6.9 ( 2.1 using the relationship
K = K1/K2.
Determination of the Molar Extinction Coefficients of
Ru-NEt32+. Although the molar extinction coefficient of
Ru-NEt32+ in THF is necessary for determining the true
(not apparent) value of the quantum yield of Ru-H+
formation, it cannot be directly determined using absorp-
tion spectra because of its instability. Therefore, we
attempted to obtain the quantum yield using the follow-
ing method. As shown in Figure 2b, the isosbestic points
were continuously observed in the electronic spectra of
the THF solution containing Ru-DMF2+ (0.051 mM)
The addition of THF to a CD2Cl2 solution containing
Ru-NEt3(BAr4)2 0.4NEt3 affected the H NMR chemi-
cal shifts attributed to the bpy and tpy ligands and NEt3
(Figure 6). For example, the ethyl protons were shifted
1
3

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10143
Scheme 1. Equilibrium among Ru-NEt32+, Ru-THF2+, and Ru-
CD2Cl2
2+
and NEt3 (2 M) during irradiation with 436 nm light after
3 min of preirradiation. This indicates that three com-
plexes, Ru-THF2+, Ru-NEt32+, which are in equilibri-
um, and the product Ru-H+, share the irradiated
photons during this period (eq 5: cRu-H[t] and c0 are
concentrations of Ru-H+ at time t and the total concen-
tration of the three complexes, respectively, and R is the
ratio of Ru-NEt3 to the total of Ru-THF2+ and Ru-
2+
Figure 7. Total absorbance attributedtoRu-THF2+ andRu-NEt32+ at
436 nm in the presence of various concentrations of NEt3. The curve was
fit using eq 6.
NEt32+).
sK
hv
2 þ
½tꢁÞ s
Ru-THF2 þ
Ru-NEt
Ru-Hþ
ð5Þ
F
s
½tꢁÞ R
3
ð1 -RÞðc0 -cRu-H
cRu-H½tꢁ
Rðc0 -cRu-H
where cRu-THF[t], cRu-NEt3[t], and ceq[t] denote the concen-
and their total
2+
trations of Ru-THF2+ and Ru-NEt3
In this case, the absorbance of the reaction solution at
436 nm after irradiation for t sec, measured with a 1 cm
cell, is expressed by eq 6 because of the rapid interconver-
concentration (cRu-THF[t] + cRu-NEt3[t]), respectively, at
time t. Because the reaction solution was well-stirred during
irradiation, the concentration of each complex should be
homogeneous. Therefore, the rate of formation of Ru-H+
is described by eq 8:16
2+
sion between Ru-THF2+ and Ru-NEt3
:
Abs½tꢁ ¼ fεRu-THFð1 -RÞ
þ εRu-NEt3Rgðc0 -cRu-H½tꢁÞ
þ εRu-HcRu-H½tꢁ
n
o
dcRu-H½tꢁ
dt
I0
V
eqceq½tꢁ þεRu-HcRu-H½tꢁÞ
ð6Þ
¼ Φapp
1 -10-ðε
where εRu-THF and εRu-NEt3 denote the molar absorption
coefficients of Ru-THF2+ and Ru-NEt32+, respectively.
The molar extinction coefficient of Ru-H+ (εRu-H) is (4.0
( 0.1) ꢀ 103 M-1 cm-1, and cRu-H and εRu-THF (ε = (6.5
( 0.2) ꢀ 103 M-1 cm-1) could be obtained by the
procedures described above. The best-fit curve shown in
Figure 7 was obtained using eq 6 with εRu-NEt3 = (4.4 (
0.5) ꢀ 103 M-1 cm-1 and R = 0.11, 0.20, 0.34, 0.46, and
0.54 for each concentration of NEt3 (0.5, 1, 2, 3, and 4 M),
respectively. We can also estimate the equilibrium con-
stant between Ru-THF2+ and Ru-NEt32+ to be K = 3.8
( 1.5 by this method. This value is reasonably consistent
with that obtained using NMR techniques (6.9 ( 2.1).
Determination of Quantum Yields. As described
above, Ru-H+ was produced quantitatively from
the equilibrium mixture of Ru-THF2+ and Ru-
εeqceq½tꢁ
ꢀ
ð8Þ
εeqceq½tꢁ þ εRu-HcRu-H½tꢁ
where Φapp is the apparent quantum yield of the formation
of Ru-H+ from the equilibrium mixture. I0 represents
photon flux, V is the volume of the solution, and the
light-path length of the cuvette was 1 cm. The term in the
curlybracketsrepresentsthe ratio of light absorbed byall of
the complexes to the total light flux, and the last term
represents the ratio of light absorbed by Ru-THF2+ and
Ru-NEt32+ to that by all of the complexes in the solution.
The global-fitting method for determining Φapp using eqs 6
and 8 was applied to the absorption change during irradia-
tion between 350 and 650 nm. As an example, Figure 8
shows the absorption changes at 436 and 550 nm during
irradiation and the best-fit curves in the case, where a THF
solution containing Ru-DMF2+ (0.051 mM) and NEt3 (2
M) was used. Using this fit, the apparent quantum yield of
the Ru-H+ formation (Φapp) was (3.4 ( 0.2) ꢀ 10-3. The
apparent quantum yield Φapp was strongly dependent on
the concentration of NEt3, as shown in Figure S5
(Supporting Information).
NEt32+. Because interconversion between Ru-THF2+
2+
and Ru-NEt3
is rapid enough to reach equilibrium
during irradiation, the molar absorption coefficient
of the equilibrium mixture εeq can be defined by
eq 7:
ε
Ru-THFcRu-THF½tꢁ þ εRu-NEt3cRuNEt3 ¼ εeqceq½tꢁ
ð7Þ
Discussion
(15) The chloride complex [Ru(tpy)(bpy)Cl]þ formed by the slow reaction
of Ru-CD2Cl22þ with NEt3 accompanied by the formation of Et3Nþ-CD2Cl
(see: Wulff, C. A.; Wright, D. A. J. Org. Chem. 1970, 35, 4252 and Huhmann-
Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chem. 1999, 38, 115). This reaction
was slowed drastically in the presence of NEt3, THF, or both additives. Therefore,
for the calculation of the equilibrium constants, the amount of [Ru(tpy)(bpy)Cl]þ
produced just before the addition of NEt3 and THF was subtracted from the total
amount of the added complex. The 13C NMR technique is not suitable for
determining the structure of the Ru-CD2Cl22þ because of too rapid an exchange
between CD2Cl2 and NEt3 as a ligand.
Irradiation of a THF solution containing Ru-DMF2+ and
an excess of NEt3 quantitatively produced Ru-H+ and
NEt2H as the two-electron oxidation product of NEt3
(eq 2). Figure 9 shows a dependence of the yield of Ru-H+
on the irradiation time in photochemical reactions with NEt3
(16) (a) Bunce, N. J. J. Photochemistry 1987, 38, 99–108. (b) Tonne, J.;
Prinzbach, H.; Michl, J. J. Photochem. Photobiol. Sci. 2002, 1, 105–110.

10144 Inorganic Chemistry, Vol. 48, No. 21, 2009
Matsubara et al.
Figure 8. UV-vis absorption changes of a THF solution containing
Ru-DMF2+ (0.051 mM) and NEt3 (2 M) during irradiation with 436 nm
light for 180-1200 s recorded with 5 s intervals, detected at 436 nm (blue
circle) and 550 nm (red circle). The fitting curves were obtainedusingeqs 6
and 8.
Figure 10. Dependence of the quantum yield of the formation of Ru-
to that by the
2+
H+ on the ratio of photons absorbed by Ru-NEt3
equilibrium mixture of Ru-THF2+ and Ru-NEt32+ at 436 nm.
2+
-
0
lated Ru-NEt3 as a BAr4 salt (Ar0 = 3,5-bis(trifluoro-
methyl)phenyl). As shown in Figure 4, there are no peaks
1
corresponding to free NEt3 in the H NMR spectrum of
Ru-NEt322++. This strongly suggests that the NEt3 ligand in
Ru-NEt3
is quickly exchanged with free NEt3, with a
pedigree as a solvent of crystallization, on the NMR time
scale. The chemical shift and shape of the 1H NMR peaks
were strongly dependent on the concentrations of both Ru-
2+
NEt3 and NEt3: (1) A lower concentration of the com-
plex without the addition of NEt3, where the concentration
of free NEt3 should also be low, caused broader peaks of
the NEt3 and the bpy and tpy ligands (Figure 4 and 5a). (2)
The addition of NEt3 caused peak sharpening, and both
the ethylene and methylene protons shifted downfield
(Figure 5b). These results also indicate a quick exchange
Figure 9. Formation yield of the Ru-H+ versus irradiation time for the
photochemical reactions of Ru-DMF2+ in THF (red), Ru-DMF2+ in
DMF (black), and Ru-py2þ in DMF (blue). The solutions containing the
complex (0.05 mM) and NEt3 (2.0 M) were irradiated using 436 nm light
(0.23 ( 0.02 μeinstein s-1) under an Ar atmosphere.
of the NEt3 ligand with free NEt3 in solution. The equi-
2+
librium constant (K1 in eq 4) between Ru-NEt3
and
the solvento complex [Ru(tpy)(bpy)(CD2Cl2)]2+ (Ru-
CD2Cl22+) was calculated to be (9.8 ( 1.2) ꢀ 103 by use
of the dependence of chemical shifts of the ethyl protons on
the concentration of NEt3 in a CD2Cl2 solution (Figure S3,
Supporting Information). Two pairs of proton peaks
attributable to THF were observed through the addition
(2 M) under three different conditions: using Ru-DMF2+ in a
THF solution, Ru-DMF2+ in DMF, and [Ru(tpy)(bpy)-
(py)]2+ (Ru-py2þ) in DMF. This indicates that the photoche-
mical formation of Ru-H+ is strongly dependent on both the
monodentate ligand in the starting complex as well as the
solvent. It should be noted that the apparent quantum yield of
Ru-H+ formation using Ru-DMF2+ in THF increased by
about 5 orders of magnitude over that using Ru-py2+ in DMF,5
which are similar conditions to those reported previously. The
formation yield of Ru-H+ was also improved from 3.5%
(Ru-py2+ in DMF) to 100% (Ru-DMF2+ in THF), depending
on the Ru complex added. One of the reasons for this drastic
improvement should be the high efficiency and chemoselectivity
of the photochemical ligand substitution reaction of
Ru-DMF2+ (Φ = (7.6 ( 0.7) ꢀ 10-2 in eq 3). Although
similar photochemical ligand substitution of Ru-py2+ was
observed, its quantum yield was much lower (Φ < 10-5),
and no isosbestic point was observed in the spectral change
during irradiation.
2+
of THF to a CD2Cl2 solution containing Ru-NEt3 (1.8
mM), as shown in Figure 6. The concentration of THF
affected the chemical shifts of proton peaks corresponding
2+
to NEt3. This indicates that Ru-NEt32+, Ru-CD2Cl2
,
and Ru-THF2+ are in equilibrium with each other in
2+
solution. The equilibrium constant between Ru-NEt3
and Ru-THF2+ (K) was calculated as 6.9 ( 2.1.
With this data in hand, we can consider the reaction
mechanism of the photochemical formation of Ru-H+.
Irradiation of Ru-DMF2+ in a THF solution containing
NEt3 leads to a loss of the DMF ligand and a coordination of
THF or NEt3 to give Ru-THF2+ and Ru-NEt32+ in a ratio
that is dependent on the concentration of NEt3 in the solution
(Scheme 2).
Because the photochemical ligand substitution was com-
pleted in the fast stage of the photochemical reaction (within
3 min of irradiation with a light intensity of 0.23 ( 0.02
μeinstein s-1), this process should not affect the efficiency of
Althoughthe triethylamine complex Ru-NEt32+ should be
an important intermediate in the photochemical formation of
Ru-H+, it had not been isolated because of its instability.
Therefore, its properties and reactivity had not been
previously studied. We successfully synthesized and iso-
the following process. The ratio of Ru-THF2+ to Ru-NEt3
2+
in solution is about 4:6 in the presence of 2 M NEt3.
2+
Irradiation of Ru-NEt3 in the solution containing NEt3

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10145
Scheme 2. Photochemical Ligand Substitution of Ru-DMF2+ in
THF Containing NEt3
of Ru-NEt32+ but not Ru-THF2+
.
ε
Ru-NEt3cRu-NEt3
Φapp
¼
ΦRu-H
ð9Þ
ε
Ru-THFcRu-THF þ εRu-NEt3cRu-NEt3
Conclusion
Quantitative photochemical production of Ru-H+ can be
achieved via the irradiation of Ru-DMF2+ in a THF solution
containing excess NEt3. The chemical and quantum yields of
Ru-H+ using the system reported here were about 30 and 105
times higher, respectively, compared with the previously
reported system. Investigations of the properties of Ru-
DMF2+ and Ru-NEt32+ clearly indicate that the formation
of Ru-H+ proceeds via the excitation of Ru-NEt32+, and
both the higher formation and quantum yield are caused by
an improvement of the photochemical ligand substitution of
Ru-DMF2+ and thus the higher steady-state concentration
of Ru-NEt32+ in THF solution.
(2 M) caused the formation of Ru-H+ with an apparent
quantum yield (Φapp) of (3.4 ( 0.2) ꢀ 10-3. The apparent
quantum yield was strongly dependent on the concentration
of NEt3, as shown in Figure S5 (Supporting Information). It
2+
has been previously reported that the irradiation of Ru-NEt3
causes the formation of Ru-H+.5 An intermolecular photo-
chemical reaction of Ru-THF2+ with free NEt3 might be
omitted because of the short lifetime of the excited state of
Ru-THF2+, which does not emit at room temperature in
THF solution. Therefore, if the formation of Ru-H+ is
caused only by irradiation to Ru-NEt32+, the true (not
apparent) value of the quantum yield for the photochemical
formation of Ru-H+ from Ru-NEt3
(ΦRu-H) can be
2+
Acknowledgment. This work was supported by a Grant-
in Aid for Scientific Research on Priority Areas (417)
from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT) of the Japanese Government.
calculated as about (9.1 ( 0.5) ꢀ 10-3 using eq 9, with
an equilibrium constant K = 6.9 ( 2.1 and the molar
extinction coefficients of the complexes at the irradia-
tion wavelength, that is, 436 nm (εRu-THF = (6.5 ( 0.2) ꢀ
103 M-1 cm-1 for Ru-THF2+ and εRu-NEt3 = (4.2 ( 0.9) ꢀ
103 M-1 cm-1 for Ru-NEt32+). A good linear relation-
ship (Figure 10) also strongly supports the hypothesis
that the formation of Ru-H+ occurs via the excitation
Supporting Information Available: Full ref 2a, 1H NMR data
of [Ru(tpy)(bpy)(CF3SO3)]2þ and Ru-DMF2þ, and Figures
S1-S6. This material is available free of charge via the Internet
at http://pubs.acs.org.