Highly active ruthenium complexes with bidentate phosphine ligands for the solvent-free catalytic synthesis of N,N-dimethylformamide and methyl formate

Article abstract of DOI:10.1039/a608150i

Complexes of the type [RuCl₂L₂] [L = Ph₂P(CH₂)_nPPh₂ (n = 1-3), Me₂P(CH₂)₂PMe₂] are shown to be highly active and selective catalysts for the synthesis of formic acid derivatives from carbon dioxide, hydrogen and dimethylamine or methanol-triethylamine, respectively, without any additional solvent, affording at 373 K with [RuCl₂(dppe)₂] an extremely high turnover frequency (TOF) of 360 000 h^-1 in N,N-dimethylformamide synthesis and a TOF of 830 h^-1 in methyl formate synthesis.

Full text of DOI:10.1039/a608150i

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Highly active ruthenium complexes with bidentate phosphine ligands for the
solvent-free catalytic synthesis of N,N-dimethylformamide and methyl formate
Oliver Kro¨cher, Rene´ A. Ko¨ppel and Alfons Baiker*
Department of Chemical Engineering and Industrial Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092
Zu¨rich, Switzerland
Complexes of the type [RuCl2L2] [L = Ph2P(CH2)nPPh2
(n = 1–3), Me2P(CH2)2PMe2] are shown to be highly active
and selective catalysts for the synthesis of formic acid
derivatives from carbon dioxide, hydrogen and dimethyl-
amine or methanol–triethylamine, respectively, without any
additional solvent, affording at 373 K with [RuCl2(dppe)2]
an extremely high turnover frequency (TOF) of 360000 h21
in N,N-dimethylformamide synthesis and a TOF of 830 h21
in methyl formate synthesis.
Although the process reported by Jessop et al. is far more
efficient compared to the liquid-phase synthesis of dmf and
methyl formate, overall reaction rates are still relatively low and
the catalysts are sensitive to air, which impairs technical
utilisation. The aim of our study was to find more active and
stable catalysts for the synthesis of dmf and methyl formate,
obviating these limitations.
Ruthenium complexes of the type [RuCl2L2], with L
representing the chelating ligands dppm = Ph2PCH2PPh2,
dppe = Ph2P(CH2)2PPh2 or dppp = Ph2P(CH2)3PPh2, were
prepared,6 which are easy to characterize, relatively cheap, and
stable to water and air.
Table 1 lists the experimental conditions and the catalytic
results obtained with the various ruthenium complexes for the
synthesis of N,N-dimethylformamide and methyl formate,
respectively. Note that catalytic results presented in Table 1
were confirmed by at least two repetitive experiments. Blank
tests without catalyst performed repeatedly throughout the
study revealed no catalytic activity.
Recently, carbon dioxide has gained renewed interest as an ideal
source of carbon in synthetic chemistry because of its
abundance, low cost and non-toxicity.1,2 Among the possible
valuable chemicals, special attention is given to formic acid and
its derivatives like N,N-dimethylformamide [eqn. (1)] and
methyl formate [eqn. (2)].3,4
CO2 + H2 + Me2NH ? Me2NC(O)H + H2O
CO2 + H2 + MeOH ? HC(O)OMe + H2O
(1)
(2)
The progress made up to 1995 in the homogeneously
catalysed synthesis of N,N-dimethylformamide (dmf) and
methyl formate from carbon dioxide has been reviewed
recently.3,4 Among various transition-metals investigated, ru-
thenium and rhodium complexes were found to exhibit high
activity. Although remarkable efforts have been made to
increase the activity of the catalytic complexes used, the
turnover numbers (TON) and turnover frequencies (TOF)
achieved remained realtively low.3,4
More recently, the use of [RuCl2(PMe3)4] as a catalyst for the
synthesis of formic acid, dmf and methyl formate from carbon
dioxide, hydrogen and dimethylamine or methanol, respec-
tively, has been reported by Jessop et al.3,5 The application of
supercritical carbon dioxide (sc-CO2) as a solvent for hydrogen
and the catalyst enabled the authors to produce dmf with overall
rates of up to 10000 h21 and methyl formate with overall rates
of up to 55 h21. The use of sc-CO2-soluble complexes was
reported to be crucially important for achieving high activities.
The synthesis of dmf proceeded very rapidly at 373 K with up
to 99% yield and always with 100% selectivity to dmf. Analysis
of the gas phase by GC confirmed that no gaseous by-products
(CO, CH4) were formed. A TOF of 360000 h21 could be
reached with [RuCl2(dppe)2], which is 36 times higher than that
with [RuCl2(PMe3)4], the best catalyst reported so far.3,5 In an
experiment using 110 g dimethylamine a maximum TON of
740000 was achieved, which indicates the upper limit of the
catalyst’s lifetime under the conditions applied. Calculated on
the basis of 1 g of ruthenium, 530 kg dmf can be produced
within 2 h. Further improvement in TOF may be possible by
optimizing the reaction parameters. High TOFs could also be
reached with [RuCl2(dppm)2], whereas [RuCl2(dppp)2] ex-
hibited much lower activity (Table 1).
The extremely high catalytic activity achieved with such
trans-[RuCl2L2] complexes was not a priori expected, since the
formation of a monohydride species was reported to be a
prerequisite for catalytic activity for this type of hydrogenation
a
Table 1 Catalytic properties of ruthenium complexes in the production of N,N-dimethylformamide and methyl formate from CO2
Complex (mmol)
Reactant [additive] (mol)
t/h
Product
TONb
TOFc/h21
[RuCl2(PMe3)4]d (6.4)
[RuCl2(dppe)2] (0.58)
[RuCl2(dppe)2] (0.73)
[RuCl2(dppm)2] (1.0)
[RuCl2(dppp)2] (3.1)
[RuCl2(dmpe)2] (8.7)
[RuCl2(PMe3)4]d,e (2.9)
Dimethylamine (3.80)
Dimethylamine (2.44)
Dimethylamine (0.78)
Dimethylamine (0.72)
Dimethylamine (0.72)
Dimethylamine (0.72)
Methanol (0.08)
[triethylamine] (0.030)
Methanol (0.74)
[triethylamine] (0.370)
Methanol (0.74)
70.0
2.05
0.75
0.50
4.05
dmf
dmf
dmf
dmf
dmf
dmf
Methyl formate
420000
6000
740000 360000
200000 265000
95000 190000
10700
30000
3500
2650
2000
55
15.0
64.0
15.5
15.5
[RuCl2(dppe)2] (3.8)
[RuCl2(dppe)2]e (2.3)
Methyl formate
Methyl formate
12900
10100
830
650
[triethylamine] (0.074)
a Reaction conditions: 500 ml stainless-steel autoclave equipped with magnetically coupled stirrer. Analysis: GC equipped with Supelco SPB-1 fused silica
capillary column. p(H2) = 8.5 MPa, p(CO2) = 13.0 MPa, T = 373 K. b TON = mol product/mol complex. c TOF = TON h21
.
d Lit. data3 for the hitherto
best catalyst [RuCl2(PMe3)4]. 300 ml autoclave. e T = 353 K.
Chem. Commun., 1997
453

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catalyst.5,7 No conversion of trans-[RuCl2L2] complexes to the
corresponding monohydrides at normal pressure is described in
the literature.8,9 Despite this, [RuHCl(dppe)2], identified by
NMR, could be found in the reactor after pretreatment of
[RuCl2(dppe)2] with 40 bar of hydrogen and dimethylamine,
confirming monohydrides as key intermediates in the catalytic
cycle. The monitored pressure in the reactor as well as the
catalytic results indicated no further induction period after
addition of carbon dioxide.
Jessop et al. investigated visually the phase behaviour in the
reactor under comparable conditions for different reactants and
additives. They found that a supercritical phase of carbon
dioxide and hydrogen exists besides a liquid phase, highly
swelled by dissolved gases.5 Thus, high concentrations of
carbon dioxide and possibly hydrogen could be assumed in the
liquid-amine phase. Leitner et al. reported high concentrations
of dissolved carbon dioxide in the liquid phase also under
subcritical conditions.10 Due to their insolubility in the sub- or
super-critical carbon dioxide, the complexes trans-[RuCl2L2]
are expected to operate in the liquid phase, which is also
supported by the fact that the catalysts could only be found in
the liquid-product mixture after reaction. Moreover, cleaning of
the lower part of the autoclave was sufficient to exclude any
activity in subsequent blank tests. Variations of the carbon
dioxide partial pressure from super- to sub-critical conditions,
using [RuCl2(dppe)2] as catalyst, resulted in a decrease of
activity from 360000 h21 (8.5 MPa CO2) to 150000 h21 (1.8
MPa CO2), but no drastic activity drop was observed, indicating
supercritical conditions not to be a necessary prerequisite for
high catalytic activity.
h21) than [RuCl2(PMe3)4] (TOF = 55 h21) under comparable
conditions. Increasing the triethylamine concentration and the
temperature resulted in an even higher TOF of 830 h21, which
is 15 times higher than the best previous results.5 Analysis of the
reaction mixture by 1H NMR spectroscopy revealed the
formation of an adduct of formic acid and triethylamine beside
methyl formate. Methyl formate synthesis with our complexes
is favourable compared to that with [RuCl2(PMe3)4], because
higher temperatures can be applied, which is crucial for an
effective esterification of the initially formed formic acid.
In conclusion, we can state that ruthenium complexes with
bidentate phosphine ligands exhibit much higher activity for the
solvent-free synthesis of N,N-dimethylformamide (dmf) and
methyl formate from carbon dioxide than hitherto reported
catalytic systems.3,5 The high concentrations of hydrogen and
carbon dioxide in the liquid dimethylamine or methanol–
triethylamine phase afford high concentrations of all reactants at
the catalytic centres in an ideal reaction design. We have
demonstrated that further improvements in the homogeneous
hydrogenation of carbon dioxide are achievable not only by
developing complexes soluble in the supercritical medium, but
also by the utilisation of better suited ligands for this reaction in
combination with a solvent-free reaction design.
References
1 B. Cornils and W. A. Herrmann, in Applied Homogeneous Catalysis
with Organometallic Compounds, VCH, Weinheim, 1996, pp. 1048–
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2 T. Clifford and K. Bartle, Chem. Ind., 1996, 449.
3 P. G. Jessop, T. Ikariya and R. Noyori, Chem. Rev., 1995, 95, 259.
4 W. Leitner, Angew. Chem., Int. Ed. Engl., 1996, 34, 2207.
5 P. G. Jessop, Y. Hsiao, T. Ikariya and R. Noyori, J. Am. Chem. Soc.,
1996, 118, 344.
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257.
Despite their similar solubility properties, the three related
complexes
[RuCl2(dppm)2],
[RuCl2(dppe)2]
and
[RuCl2(dppp)2] exhibited substantially different activities, with
[RuCl2(dppp)2] being far less active than [RuCl2(dppm)2] and
[RuCl2(dppe)2] (Table 1). In order to compare the catalytic
properties of these complexes, which are insoluble in sc-CO2,
with
a
complex soluble in the supercritical phase,
[RuCl2(dmpe)2] [dmpe = Me2P(CH2)2PMe2] was prepared,11
using bidentate methyl phosphine ligands instead of phenyl
phosphine ligands. Interestingly, [RuCl2(dmpe)2] exhibited
similar low activity to [RuCl2(dppp)2], suggesting a more
pronounced influence of the phosphine ligands than of the
catalysts solubility properties.
11 J. Chatt and R. G. Hayter, J. Chem. Soc., 1961, 896.
[RuCl2(dppe)2] was also tested for methyl formate synthesis
at 353 K and exhibited a 12 times higher activity (TOF = 650
Received, 3rd December 1996; Com. 6/08150I
454
Chem. Commun., 1997