Rhenium-mediated photochemical carbon dioxide reduction in compressed carbon dioxide

Article abstract of DOI:10.1246/cl.2000.522

Rhenium-bipyridine complexes soluble in compressed CO₂ were prepared and employed in the first example of photochemical reduction of CO₂ mediated by metal complexes in compressed CO₂, without use of an organic liquid solve

Full text of DOI:10.1246/cl.2000.522

522
Chemistry Letters 2000
Rhenium-Mediated Photochemical Carbon Dioxide Reduction in Compressed Carbon Dioxide
Hisao Hori,* Kazuhide Koike, Koji Takeuchi, and Yoshiyuki Sasaki
National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569
(Received February 14, 2000; CL-000147 )
Rhenium-bipyridine complexes soluble in compressed CO2
were prepared and employed in the first example of photochem-
ical reduction of CO2 mediated by metal complexes in com-
pressed CO2 , without use of an organic liquid solvent.
hexyl groups because hexane has properties similar to CO2, e.g.
similar low polarity, comparable density and a weak solvating
ability. Although the complex [fac-Re(bpy)(CO)3P(OC6H13)3]+
SbF6 (2+SbF6 ) showed a somewhat higher solubility,6 the
absorption at visible to near-ultraviolet region was still negligi-
ble [Figure 1(b)]. Recently, it has been reported that the fluo-
rine substitution on organometallic catalysts enhances their sol-
ubility in compressed CO2.7 We therefore changed the counter
-
−
Compressed (liquid or supercritical) CO2 has been recently
recognized as an innovative and ecologically benign reaction
medium for chemical synthesis, especially metal-catalyzed
processes.1 Compared to conventional organic solvents, com-
pressed CO2 has many advantages such as nonflammability, non-
toxicity, and ease of separation of products or catalysts from the
reaction mixture. However, few reports have focused on photo-
chemical reactions using metal complexes in compressed CO2.2
Furthermore, there have been no reports on photochemical CO2
reduction mediated by metal complexes in compressed CO2.3
The reason for the few examples of photochemical reactions
mediated by metal complexes in compressed CO2 could be the
poor solubility of ionic metal complexes. In the present work,
we prepared rhenium-bipyridine complexes soluble in com-
pressed CO2 and succeeded in causing a photochemical CO2
reduction.
-
anion SbF6 to tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
(BArF−).8 The complex 1+BArF− gave rise to higher solubility,
estimated to be 7.2 × 10-6 mol dm-3 [Figure 1(c)].9 Finally, the
substitution of both the phosphite ligand and the counter anion
created a much CO2-soluble complex 2+BArF−,10 having a
maximum solubility of 2.5 × 10-5 mol dm-3 [Figure 1(d)]. The
UV-VIS spectrum of 2+BArF− clearly shows the triplet metal-
to-ligand charge transfer (3MLCT) absorption band around 340
nm, similar to other rhenium-bipyridine complexes in conven-
tional organic solvents.4b-d,11 It also shows corresponding emis-
sion at maximum wavelength of 520 nm with lifetime of 395 ns
in liquid CO2 (T = 26 °C, P = 7.6 MPa, ρ = 0.73 g cm-3). The
photochemical CO2 reduction system using rhenium complexes
3
requires an amine such as Et3N to quench the MLCT state.4
Rhenium-bipyridine complexes have been known as photo-
catalysts for CO2-to-CO reduction by visible-light irradiation in
conventional organic solvents with amine.4 Among them, phos-
phite-containing complexes such as [fac-Re(bpy)(CO)3P(OR)3]+
X- (bpy = 2,2'-bipyridine, R = C2H5 or i-C3H7, X = SbF6 or PF6)
show high catalytic activities.4c,d However, when [fac-
The addition of Et3N increased the maximum solubility of
2+BArF− in liquid CO2 to 4.4 × 10-5 mol dm-3 (T = 28 °C, P =
7.0 MPa, ρ = 0.70 g cm-3, [Et3N] = 0.42 mol dm-3, molar ratio
of CO2 / Et3N = 36 / 1).
Figure 2 shows the UV-VIS spectral changes of the liquid
CO2 containing 2+BArF−and Et3N under 365 nm irradiation.12
After irradiation started, absorption maxima around 400 and
−
Re(bpy)(CO)3P(Oi-C3H7)3]+SbF6- (1+SbF6 ) was introduced into
liquid CO2, it was found to be insoluble, which is reflected by
almost no absorption in the UV-VIS spectrum of saturated solu-
tion [Figure 1(a)].5 We changed isopropyl groups of 1+SbF6− to
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
523
500 nm appeared. This feature is very similar to that observed
in the organic solvent-amine mixtures of other rhenium-bipyri-
dine complexes under irradiation,4c,d and the new absorption
bands that grow during irradiation are attributed to the one-elec-
tron reduced complex [fac-Re(bpy•−)(CO)3P(OC6H13)3]. The
one-electron reduced rhenium complexes are recognized as key
precursors for the photochemical CO2 reduction in conventional
organic solvent.4,11 Therefore this accumulation suggests the
ability of 2+BArF−to cause photochemical CO2 reduction in liq-
uid CO2.
Anorg. Allg. Chem., 624, 602 (1998).
3
4
An example of heterogeneous photochemical CO2 reduc-
tion using TiO2 in supercritical CO2 was reported, see T.
Mizuno, H. Tsutsumi, K. Ohta, A. Saji, and H. Noda,
Chem. Lett., 1994, 1533.
a) J. Hawecker, J.- M. Lehn, and R. Ziessel, Helv. Chim.
Acta, 69, 1990 (1986). b) O. Ishitani, M. W. George, T.
Ibusuki, F. P. A. Johnson, K. Koike, K. Nozaki, C. Pac, J.
J. Turner, and J. R. Westwell, Inorg. Chem., 33, 4712
(1994). c) H. Hori, F. P. A. Johnson, K. Koike, O. Ishitani,
and T. Ibusuki, J. Photochem. Photobiol. A: Chem., 96,
171 (1996). d) K. Koike, H. Hori, M. Ishizuka, J. R.
Westwell, K. Takeuchi, T. Ibusuki, K. Enjoji, H. Konno,
K, Sakamoto, and O. Ishitani, Organometallics, 16, 5724
(1997).
5
UV-VIS spectra were collected in a high-pressure cell
equipped with sapphire windows (cell volume = 17 cm3,
optical path length = 4 cm). This cell was the same as that
used for the photochemical experiments. Using a pump,
the cell was filled with a weighed amount of CO2 (11.4 –
12.3 g) to adjust the desired pressure and density.
−
6
7
The preparation method of 2+SbF6 was similar to that of
−
1+SbF6 [H. Hori, K. Koike, M. Ishizuka, K. Takeuchi, T.
Ibusuki, and O. Ishitani, J. Organomet. Chem., 530, 169
(1997)], except that P(OC6H13)3 was used as a reactant and
CH2Cl2 / pentane were used as recrystallization solvents.
a) M. J. Burk, S. Feng, M. F. Gross, and W. Tumas, J. Am.
Chem. Soc., 117, 8277 (1995). b) S. Kainz, D. Koch, W.
Baumann, and W. Leitner, Angew. Chem., Int. Ed. Engl.,
36, 1628 (1997). c) H. Hori, C. Six, and W. Leitner,
Macromolecules, 32, 3178 (1999).
8
9
The anion substitution was carried out by mixing the
MeOH solution of SbF6 salt and saturated MeOH solution
-
of NaBArF·2H2O, followed by crystallization with water.
The saturation concentrations were estimated from the
Lambert-Beer equation by using the extinction coefficients
measured at low complex concentrations, wherein the com-
plex is completely soluble in liquid CO2. The extinction
coefficients in liquid CO2 were similar to those in CH2Cl2,
as shown in the Note 10.
Figure 3 shows the dependence of CO formation on irradi-
ation time using 2+BArF− and Et3N in liquid CO2.13 As expect-
ed, CO was formed and the final amount was 1.61 × 10-6 mol
(turnover number = 2.2). In the control experiment with argon-
saturated DMF, no CO was formed. This indicates that the ori-
gin of CO is CO2, rather than the decomposition of 2+BArF−.
On the other hand, when insoluble 1+SbF6− was used under the
same reaction conditions, no CO was formed even after a 19-h
irradiation. Therefore, it is obvious that the complex must be
soluble in liquid CO2 to cause the CO formation.
10 Analytical data of 2+BArF−: Found: C, 46.50; H, 3.47; N,
1.41%. Calcd for C63H59O6N2PBF24Re C, 46.59; H, 3.66;
N, 1.72%. UV-VIS: λmax (liquid CO2 at T = 26 °C, P = 7.9
MPa, ρ = 0.75 g cm-3) / nm (ε / dm3mol−1cm−1) 305
(12700), 314 (13900), 340 (6700); λmax (CH2Cl2) / nm (ε /
dm3mol−1cm−1) 307 (11700), 316 (13500), 347 (4600).
11 C. Kutal, M. A. Weber, G. Ferraudi, and D. Geiger,
Organometallics, 4, 2161 (1985); K. Kalyanasundaram, J.
Chem. Soc., Faraday Trans. 2, 82, 2401 (1986).
In summary, we have demonstrated that the CO2-soluble
rhenium complex 2+BArF− causes photochemical CO2 reduc-
tion in compressed CO2.
References and Notes
1
“Chemical Synthesis Using Supercritical Fluids,” ed by P.
Jessop and W. Leitner, Wiley-VCH, Weinheim (1999).
X.- Z. Sun, M. W. George, S. G. Kazarian, S. M.
Nikiforov, and M. Poliakoff, J. Am. Chem. Soc., 118,
10525 (1996); U. Kreher, S. Schebesta, and D. Walther, Z.
12 A high pressure Hg lamp (500 W) with a band pass filter
(365 nm) was used to produce the monochromatic light.
13 Amount of CO was measured by a gas chromatograph with
a methanizer (Ni catalyst, 400 °C) and a flame ionization
detector.
2