Fig. 2 (A) Time-dependent TONs to provide HCOOH in H2O for the aqueous hydrogenation of CO2 catalysed by 1b(SO4) at H2 (5.5 MPa) and CO2
(2.5 MPa) at 40 uC. (B) Temperature-dependence of TONs for the aqueous hydrogenation of CO2 catalysed by 1b(SO4) at H2 (5.5 MPa) and CO2 (2.5 MPa)
after 12 h. (C) Dependence of the TONs for the aqueous hydrogenation of CO2 on the pressure of H2 at 1.5 (dark grey circles) and 2.5 (light grey circles)
MPa CO2 catalysed by 1b(SO4) at 40 uC after 12 h.
the reaction of the aqua complex 1 with H2 at pH 2.5–5.0.6 It is
known that the H2O ligand accelerates the heterolytic H2-activation
in polar solvents to release H3O1.7,8 The hydride species 2 reacts
with CO2 to afford the formate complex 3b (Scheme 1). The
formation of 3b was confirmed by 1H NMR and electrospray
ionization mass spectrometry.{
The catalytic conditions were optimised for the reaction time
(Fig. 2A), reaction temperature (Fig. 2B), and pressures of H2 and
CO2 (Fig. 2C). The time dependence of TON to give HCOOH for
the 1b(SO4)-catalysed aqueous hydrogenation with H2 (5.5 MPa)
and CO2 (2.5 MPa) at 40 uC is depicted in Fig. 2A. The TON
increases with reaction time to reach an equilibrium value in 55 h.
The TON in 12 h increases with increasing temperature to reach a
maximum value at 40 uC and then decreases with further increase
in temperature (Fig. 2B). The backward reaction in Scheme 1 at a
higher temperature, which results in a decrease in TON, was
examined by the reaction of 1b with 10 equivalent of HCOOH in
H2O at pH 2.4 at 60 uC. After 1 h, disappearance of HCOOH (w
90%) and evolutions of H2 and CO2 were confirmed by 1H NMR
and GC. On the other hand, the TON in 12 h at 40 uC increases
linearly with increasing H2 pressure and the slope becomes larger at
a higher CO2 pressure (Fig. 2C).
(pH 5.0) in a pressure vessel (25 mL). The temperature was raised to 40 uC,
and then the solution was pressurized with CO2 (2.5 MPa) and H2
(5.5 MPa) for 70 h. After it was returned to atmospheric pressure, the
solution was quickly cooled down to ambient temperature (pH 2.5). The
yield of formic acid was determined by 1H NMR measurement of
the resulting solution with TSP in D2O as the reference and the internal
standard {TSP ~ 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt}.
1 (a) F. Joo´, in Catalysis by Metal Complexes, Vol. 23, Aqueous
Organometallic Catalysis, Kluwer Academic Publishers, Dordrecht,
The Netherlands, 2001, pp. 113–122; (b) W. Leitner, E. Dinjus and
F. Gaßner, in Aqueous-Phase Organometallic Catalysis, Concepts and
Applications, ed. B. Cornils and W. A. Herrmann, Wiley-VCH,
Weinheim, Germany, 1998, pp. 486–498; (c) A. Behr, Carbon Dioxide
Activation by Metal Complexes, VCH, Weinheim, Germany, 1988;
(d) P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1995, 95, 259;
(e) W. Leitner, Angew. Chem., Int. Ed. Engl., 1995, 34, 2207.
2 (a) Handbook of Chemistry and Physics, 83rd edn., ed. D. R. Lide, CRC
Press, Boca Raton, FL, 2002; (b) F. A. Cotton, G. Wilkinson,
C. A. Murillo and M. Bochmann, in Advanced Inorganic Chemistry, 6th
edn., Wiley-Interscience, New York, 1999, pp. 226–227.
3 (a) M. M. Taqui Khan, S. B. Halligudi and S. Shukla, J. Mol. Catal.,
1989, 57, 47; (b) F. Gassner and W. Leitner, J. Chem. Soc., Chem.
Commun., 1993, 1465; (c) F. Joo´, G. Laurenczy, L. Na´dasdi and J. Elek,
Chem. Commun., 1999, 971; (d) G. Laurenczy, F. Joo´ and L. Na´dasdi,
Inorg. Chem., 2000, 39, 5083; (e) F. Joo´, G. Laurenczy, P. Kara´dy,
J. Elek, L. Na´dasdi and R. Roulet, Appl. Organomet. Chem., 2000, 14,
857; (f) J. Elek, L. Na´dasdi, G. Papp, G. Laurenczy and F. Joo´, Appl.
Catal., A, 2003, 255, 59; (g) A. Katho´, Z. Opre, G. Laurenczy and F. Joo´,
J. Mol. Catal. A: Chem., 2003, 204–205, 143; (h) Y. Himeda,
N. Onozawa-Komatsuzaki, H. Sugihara, H. Arakawa and K. Kasuga,
Organometallics, 2004, 23, 1480; (i) H. Horva´th, G. Laurenczy and
A. Katho´, J. Organomet. Chem., 2004, 689, 1036.
In conclusion, the aqueous hydrogenation of CO2 into HCOOH
under acidic conditions has been made possible by using the water-
soluble aqua complexes under the optimised catalytic conditions.
Financial support of this research by the Ministry of Education,
Science, Sports, and Culture, Japan Society for the Promotion of
Science, Grant-in-Aid for Scientific Research (15350033, 16205020,
and 16655022) is gratefully acknowledged.
Notes and references
4 In ref. 3a and 3c, the catalytic hydrogenations of CO2 without base have
been reported, although the pH values of the solutions were not
described.
5 (a) H. Hayashi, S. Ogo, T. Abura and S. Fukuzumi, J. Am. Chem. Soc.,
2003, 125, 14266; (b) S. Ogo, K. Uehara, T. Abura, Y. Watanabe and
S. Fukuzumi, Organometallics, 2004, 23, 3047; (c) S. Ogo, T. Abura and
Y. Watanabe, Organometallics, 2002, 21, 2964.
6 The hydride species 2a has been spectroscopically detected under the
stoichiometric conditions (see ref. 5a). The hydride species 2a and 2b,
however, were not detected in the reaction of the aqua complexes with
H2 in H2O, probably because of the instability under the acidic
conditions.
7 S. Ogo, H. Nakai and Y. Watanabe, J. Am. Chem. Soc., 2002, 124, 597.
8 (a) P. J. Brothers, Prog. Inorg. Chem., 1981, 28, 1; (b) B. R. James and
M. T. Ashby, in Inorganic Reactions and Methods, Vol. 16, Reactions
Catalyzed by Inorganic Compounds, ed. A. D. Norman, VCH Publishers,
New York, USA, 1991, pp. 71–77.
{ Selected data for 1b(SO4): Yield 98%. 1H NMR (300 MHz, in D2O): d ~
2.12 (s, 18H, g6-C6Me6), 4.08 (s, 6H, OMe), 7.42 (dd, J ~ 6.61, 2.57 Hz,
2H, bpy), 7.86 (d, J ~ 2.57 Hz, 2H, bpy), 8.91 (d, J ~ 6.61 Hz, 2H, bpy).
Selected data for 3b(PF6): 1H NMR (300 MHz, in DMSO-d6): d ~ 2.04 (s,
18H, g6-C6Me6), 4.04 (s, 6H, OMe), 7.36 (dd, J ~ 6.42, 2.75 Hz, 2H, bpy),
7.65 (s, 1H, OCHO), 8.17 (d, J ~ 2.75 Hz, 2H, bpy), 8.92 (d, J ~ 6.41 Hz,
2H, bpy). ESI-MS (in MeOH): m/z 525.2 {[3b]1, relative intensity (I) ~
66% in the m/z range 200–1000}.
{ Crystal data for 1b(PF6)2?2H2O: C24H36N2O5P2F12Ru, M ~ 823.56,
˚
monoclinic, a ~ 15.511(5), b ~ 12.878(4), c ~ 16.840(5) A, b ~
3
˚
106.376(3)u, V ~ 3227(1) A , T ~ 173 K, space group P21/a (No. 14), Z ~
4, m (MoKa) ~ 6.91 cm21, 25926 reflections measured, 7212 unique
(Rint ~ 0.035), final R1 [I w 2s(I)] (wR2) ~ 0.050 (0.147) parameters.
crystallographic data in .cif or other electronic format.
§ General procedure: 20.0 mmol of 1 was dissolved in 20 mL of water
C h e m . C o m m u n . , 2 0 0 4 , 2 7 1 4 – 2 7 1 5
2 7 1 5