Water-soluble trialkylphosphine-ruthenium(II) complexes as efficient catalysts for hydrogenation of supercritical carbon dioxide

Article abstract of DOI:10.1246/cl.2001.1016

A water-soluble ruthenium(II) complex bearing tris(hydroxymethyl)phosphine, RuCl₂[PH(CH₂OH)₂]₂-[P(CH ₂OH)₃]₂, is highly effective for the catalytic hydrogenation of supercritical carbon dioxide (scCO₂) under scCO₂-H₂O multi-phasic conditions.

Full text of DOI:10.1246/cl.2001.1016

1016
Chemistry Letters 2001
Water-Soluble Trialkylphosphine-Ruthenium(II) Complexes as Efficient Catalysts for
Hydrogenation of Supercritical Carbon Dioxide
Yoshihito Kayaki, Tomoyuki Suzuki, and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology
and CREST, JST, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552
(Recieved July 11, 2001; CL-010644)
A water-soluble ruthenium(II) complex bearing
tris(hydroxymethyl)phosphine, RuCl2[PH(CH2OH)2]2-
[P(CH2OH)3]2, is highly effective for the catalytic hydrogena-
tion of supercritical carbon dioxide (scCO2) under scCO2–H2O
multi-phasic conditions.
Hydrogenation of CO2 has been extensively investigated as
a potential alternative method for synthesis of formic acid and its
derivatives from toxic CO and alcohol or amine (Scheme 1).1
2,3
Since we have achieved a rapid hydrogenation of scCO2 cat-
alyzed by a CO2-soluble Ru(II) complex with monodentate phos-
phine ligands, [RuCl2{P(CH3)3}4] (1), there have been a number
of reports on further progress in catalyst precursors demonstrated
by Baiker’s group.4 This CO2 hydrogenation was characterized
by an extremely high initial rate of reaction and high product
selectivity as well as operational simplicity due to scCO2 acting
as a reactant and a unique reaction medium. In fact, complex 1
gave DMF with a high turnover number (TON) and in 99%
selectivity albeit around 30% conversion based on the initially
charged dimethylamine after 40 h.5 The coexisting amine plays
an important role for overcoming the thermodynamically
unfavourable addition of H2 to CO2 and its amount was known to
have strong effect on the rate of the CO2 hydrogenation.
However, it should be required for a practical synthesis of DMF
to achieve high amine conversion. These results prompted us to
reinvestigate the catalytic performance of homogeneous and het-
erogeneous catalyst precursors for DMF synthesis via CO2
hydrogenation under supercritical conditions. Now, we will
describe the effect of water on DMF formation via CO2 hydro-
genation and the improvement in the catalyst’s performance by
using water-soluble Ru complexes with OH-substituted phospho-
based on the amine used under the tested conditions (Run 1–3).7
One of the possible reasons for the serious retardation is an
inhibition of the H2O coproduct which is formed in the dehy-
dration of ammonium formate intermediate to DMF.8 We have
previously reported the effect of water on the hydrogenation of
CO2 at 50 °C.5b Although an addition of a small amount of
water (0.1 mmol) accelerated the reaction, the rate of the reac-
tion in the presence of a large amount of water which precipi-
tates to form a liquid phase dropped drastically because of a
phase separation giving a scCO2–liquid two phase system.
Similarly, a significant drop in the rate of DMF synthesis for
the initial 4 h was observed in the presence of a large amount of
water (amine:water = 1:2, the ratio corresponds to the situation
at 67% conversion based on the amine) as shown in Run 5 in
Table 1. Visual inspection through sapphire view windows of a
50-mL high-pressure reactor confirmed that the ammonium car-
bamate (4 mmol) and H2O (8 mmol) were precipitated as a liq-
uid phase under the reaction condition. The liquid phase
remained even in the presence of CO2-soluble DMF (8 mmol).
Since a hydrous phase containing amine and/or ammonium car-
bonates (possibly formed from the reaction of amine, CO2 and
H2O) does not merge with scCO2, a supercritical single-phase
hydrogenation could not be performed leading to a significant
loss in catalytic activity.5,9
rus ligands.
We first examined the reaction using CO2 and H2 in the pres-
ence of ammonium carbamate (8 mmol, [(CH3)2NH2]+
[OCON(CH3)2]–),6 catalyzed by the complex 1 (1.6 mmol,
amine/catalyst = 10000) under H2 (84–86 atm) and CO2
(128–130 atm) at 100 °C for varying the reaction time. Table 1
summarizes the outcome of the reaction with catalyst 1 as a func-
tion of reaction time. It shows that the reaction was strongly
retarded at the later stages, resulting in incomplete conversion
As an approach to suppression of catalytic deactivation
caused by phase separation due to the formation of H2O, we
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1017
examined the reaction catalyzed by a combination of
[RuCl(cod)]n (COD = 1,5-cyclooctadiene) with several water-
soluble phosphine ligands in the place of P(CH3)3 under the
similar conditions. Screening tests revealed as shown in Table
2 that water-soluble trialkylphosphine ligands containing OH
groups10 such as P(CH2OH)3 or P(CH2CH2CH2OH)3 were
highly effective for the present hydrogenation rather than the
well-known water-soluble triarylphosphine ligands, TPPTS and
TPPMS.11 The low activity of the triarylphosphines is consis-
tent with the previously reported results that a conventional
P(C6H5)3-based catalyst RuH2[P(C6H5)3]4 is less active than the
P(CH3)3 complex.5b A ligand exchange reaction of
[RuCl2(cod)]n with 4 mol amount of P(CH2OH)3 in acetone
solution affords RuCl2[PH(CH2OH)2]2[P(CH2OH)3]2 (2), which
has been structurally characterized by Higham et al.,12 and
hence the isolable complex 2 was used as a catalyst precursor in
supercritical fluids at the initial stage of the reaction. The use
of water-soluble trialkylphosphine ligands could overcome the
catalyst deactivation encountered from the phase separation
during the reaction.
References and Notes
1
2
3
a) P. G. Jessop, T. Ikariya, and R. Noyori, Chem. Rev., 95,
259 (1995). b) W. Leitner, Angew. Chem., Int. Ed. Engl.,
34, 2207 (1995).
a) P. G. Jessop, T. Ikariya, and R. Noyori, Nature, 368, 231
(1994). b) P. G. Jessop, T. Ikariya, and R. Noyori, Science,
269, 1065 (1995).
a) P. G. Jessop, T. Ikariya, and R. Noyori, Chem. Rev., 99,
475 (1999). b) “Chemical Synthesis using Supercritical
Fluids,” ed. by P. G. Jessop and W. Leitner, VCH/Wiley,
Weinheim (1999).
4
5
6
a) O. Kröcher, R. A. Köppel, and A. Baiker, Chem.
Commun., 1997, 453. b) O. Kröcher, R. A. Köppel, and A.
Baiker, Chem. Commun., 1996, 1497.
a) P. G. Jessop, Y. Hsiao, T. Ikariya, and R. Noyori, J. Am.
Chem. Soc., 116, 8851 (1994). b) P. G. Jessop, Y. Hsiao, T.
Ikariya, and R. Noyori, J. Am. Chem. Soc., 118, 344 (1996).
The use of liquid dimethylammonium dimethylcarbamate is
experimentally easier and more reproducible than cooled
liquid dimethylamine. Both methods gave the same
results.5)
7
Baiker’s catalyst [RuCl2(dppe)2] (DPPE = 1,2-
bis(diphenylphosphino)ethane) showed a high initial TOF
(turnover frequency: TON h–1) exceeding 360000 h–1 at the
initial 18% conversion based on dimethylamine.4a
However, no remarkable differences in the catalyst’s per-
formances between the P(CH3)3–complex 1 and the DPPE
complex were observed in our experiments.
8
9
CO strongly inhibits the hydrogenation of CO2.5b Complex
1 did not react with DMF at 100 °C to form less reactive
carbonyl complexes.
the following experiments.
In water, direct hydrogention of carbonate to formate is pos-
sible as reported by Joó et al. However, a much higher
TON could be obtained even with the Ru–TPPMS complex-
es under our reaction conditions; F. Joó, G. Laurenczy, L.
Nadasdi, and J. Elek, Chem. Commun., 1999, 971.
The water-soluble complex 2 showed a considerable resist-
ance to catalyst deactivation by H2O as shown in Table 1. In the
case of the addition of twice as much H2O as amine, TONs of
1500 and 4700 for dimethylammonium formate and DMF (Run
9) were obtained with 2, far greater than those with the P(CH3)3
complex 1 (vide supra). The remarkable advantage using the
water-soluble catalyst 2 can be demonstrated by the complete
conversion of the reaction of H2, CO2, and the ammonium carba-
mate into DMF after 48 h (Run 7). These results clearly show
that the catalyst 2 effected the hydrogenation of CO2 in a mixture
of scCO2 and H2O leading to high catalyst performance in terms
of reactivity and catalyst life time. The hydrophilic catalyst 2
also improves the hydrogenation of scCO2 in the presence of
methanol to a mixture of formic acid and methyl formate formed
via esterification of formic acid giving H2O (Scheme 1). The
reaction catalyzed by 2 (methanol/catalyst = 10000) at 100 °C in
the presence of N(C2H5)3 as the base resulted in a higher total
yield (TON = 1400) than that by 1 (TON = 200).
10 a) A. Fukuoka, W. Kosugi, F. Morishita, M. Hirano, L.
McCaffrey, W. Henderson, and S. Komiya, Chem.
Commun., 1999, 489. b) B. Drieben-Hölscher and J.
Heinen, J. Organomet. Chem., 570, 141 (1998). c) P. A. T.
Hoye, P. G. Pringle, M. B. Smith, and K. Worboys, J.
Chem. Soc., Dalton Trans., 1993, 269.
11 F. Gassner and W. Leitner, J. Chem. Soc., Chem. Commun.,
1993, 1465.
12 L. Higham, A. K. Powell, M. K. Whittlesey, S. Wocadlo,
and P. T. Wood, Chem. Commun., 1998, 1107.
13 a) B. M. Bhanage, Y. Ikushima, M. Shirai, and M. Arai,
Chem. Commun., 1999, 1277. b) B. M. Bhanage, Y.
Ikushima, M. Shirai, and M. Arai, Tetrahedron Lett., 40,
6427 (1999). c) G. B. Jacobson, C. T. Lee, Jr., K. P.
Johnston, and W. Tumas, J. Am. Chem. Soc., 121, 11902
(1999). d) Y. Kayaki, Y. Noguchi, and T. Ikariya, Chem.
Commun., 2000, 2245. e) R. J. Bonilla, B. R. James, and P.
G. Jessop, Chem. Commun., 2000, 941. f) R. A. Brown, P.
Pollett, E. McKoon, C. A. Eckert, C. L. Liotta, and P. G.
Jessop, J. Am. Chem. Soc., 123, 1254 (2001). g) F. Liu, M.
B. Abrams, R. T. Baker, and W. Tumas, Chem. Commun.,
2001, 433.
The phase behavior of the reaction mixture is crucial to the
understanding of chemical reactions in supercritical fluids
because the outcome of the reaction can be strongly affected by
whether a particular catalysis system is single or multi-phasic.13
Phase separation often occurs during the progress of the reac-
tion even if all components including the molecular catalysts,
the starting materials, and the cosolvents are all soluble in