Water-soluble analogs of [RuCl3(NO)(PPh3) 2] and their catalytic activity in the hydrogenation of carbon dioxide and bicarbonate in aqueous solution

Article abstract of DOI:10.1016/S1381-1169(03)00293-0

The new water-soluble ruthenium-nitrosyl complexes [RuCl ₃(NO)(TPPMS)₂] and [RuCl₃(NO)(TPPTS) ₂] were synthetized and characterized by IR, ¹H and ³¹P NMR spectroscopies. The NO stretching frequencies, v _NO = 1870 cm^-1 (TPPMS) and 1883 cm^-1 (TPPTS) suggest a linear Ru-N-O arrangements. Reactions with OH^- yield the corresponding [RuCl₃(NO₂)(P)₂] derivatives, furthermore, [RuH(NO)(P)₃] is formed with TPPMS or TPPTS, respectively, under 100 bar H₂ pressure. The new complexes are suitable precatalysts for the hydrogenation of carbon dioxide and/or bicarbonate in aqueous solutions up to a tumover frequency of 400 h ^-1 under relatively mild conditions (30 bar H₂, 70 °C).

Full text of DOI:10.1016/S1381-1169(03)00293-0

Journal of Molecular Catalysis A: Chemical 204–205 (2003) 143–148
Water-soluble analogs of [RuCl3(NO)(PPh3)2] and
their catalytic activity in the hydrogenation of
carbon dioxide and bicarbonate in aqueous solution
Ágnes Kathóa,∗, Zsuzsanna Oprea, Gábor Laurenczyb, Ferenc Joóa,c
a
Institute of Physical Chemistry, University of Debrecen, P.O. Box 7, Debrecen-10 H-4010, Hungary
b
Institut de Chimie Moléculaire et Biologique, Ecole Polytechnique Fédérale de Lausanne,
CH-1015 Lausanne, Switzerland
Research Group of Homogeneous Catalysis, Hungarian Academy of Sciences,
c
P.O. Box 7, Debrecen 10 H-4010, Hungary
Received 22 August 2002; received in revised form 15 January 2003; accepted 17 January 2003
Dedicated to Professor Renato Ugo on the occasion of his 65th birthday in recognition of his outstanding contributions
to organometallic chemistry and catalysis
Abstract
The new water-soluble ruthenium-nitrosyl complexes [RuCl3(NO)(TPPMS)2] and [RuCl3(NO)(TPPTS)2] were synthetized
and characterized by IR, 1H and 31P NMR spectroscopies. The NO stretching frequencies, νNO = 1870 cm−1 (TPPMS) and
1883 cm−1 (TPPTS) suggest a linear Ru–N–O arrangements. Reactions with OH− yield the corresponding [RuCl3(NO2)(P)2]
derivatives, furthermore, [RuH(NO)(P)3] is formed with TPPMS or TPPTS, respectively, under 100 bar H2 pressure. The new
complexes are suitable precatalysts for the hydrogenation of carbon dioxide and/or bicarbonate in aqueous solutions up to a
turnover frequency of 400 h−1 under relatively mild conditions (30 bar H2, 70 ◦C).
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Carbon dioxide; Hydrogenation; Nitrosyl complexes; Ruthenium; Water-soluble
1. Introduction
described so far is [Ru(NO)2(TPPTS)2] (TPPTS:
tris(3-sulfonatophenyl)phosphine, trisulfonated triph-
enylphosphine) [2]. The 31P NMR and IR spec-
troscopic characteristics of the TPPTS-containing
derivative matched exactly those of the well char-
acterized [Ru(NO)2(PPh3)2] and therefore a similar
distorted tetrahedral structure with linear Ru–N–O
groups was postulated. One of the most stud-
ied nitrosylphosphine complex of ruthenium is
[RuCl3(NO)(PPh3)2] which—among others—serves
as a useful starting material for the preparation
of various other Ru–NO complexes. For example
the reduction of [RuCl3(NO)(PPh3)2] in refluxing
The important physiological and environmental
role of NO has generated an increased interest towards
the properties and reactions of its metal com-
plexes, especially in the case of the water-soluble
ones [1]. In the early 1970s several nitrosyl com-
plexes of ruthenium, containing also tertiary phos-
phine ligands, were prepared, however, the only
water-soluble ruthenium compound of this type
∗
Corresponding author. Fax: +36-52-512915.
´
E-mail address: katho@tigris.klte.hu (A. Katho´).
1381-1169/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00293-0

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A. Katho´ et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 143–148
144
ethanolic KOH in the presence of excess PPh3
yields [RuH(NO)(PPh3)3], a catalyst of isomer-
ization of olefins and hydrogenation of olefins,
aldehydes and ketones [3–5]. The lack of data on
the properties of analogous water-soluble Ru–NO
compounds promoted us to investigate the prepa-
ration and reactions of [RuCl3(NO)(P)2]-type com-
plexes with the hydrophilic tertiary phosphine lig-
ands TPPTS, (3-sulfonatophenyl)diphenylphosphine,
monosulfonated triphenylphosphine (TPPMS) and
1,3,5-triaza-7-phosphaadamantane (PTA). [{RuCl2
(TPPMS)2}2] [6] and [RuCl2(PTA)4] [7] have al-
ready been studied as catalyst precursors for hydro-
genation of olefins [8], aldehydes [9] and carbon
dioxide [10,11] in homogeneous solutions or in aque-
ous/organic biphasic systems. The fixation of carbon
dioxide is an important chemical and environmen-
tal question, and therefore we wished to assess the
specific influence of the NO-ligand in the catalytic
hydrogenation of CO2 in aqueous systems.
2.1. Synthesis of [RuCl3(NO)(TPPMS)2]
2.1.1. Method I
TPPMS of 400 mg (1 mmol) was dissolved with
magnetic stirring in 15 ml deaerated ethanol in a
Schlenk flask under N2 at room temperature. Then
a solution of 193 mg (0.5 mmol) K2[RuCl5(NO)] in
3 ml water was added and the mixture was refluxed
under N2 for 90 min. After cooling to room temper-
ature the yellow product was filtered, washed with
a small amount of cold ethanol and dried in vacuo.
Yield: 250 mg (52%). For C36H28Cl3NO7P2S2Na2Ru
calculated (found): C (%), 44.76 (44.44); H (%), 2.92
(3.13%); N (%), 1.45 (1.52); Cl (%), 11.01 (11.37).
νNO (KBr) = 1870 cm−1
.
31P NMR: δ 13.6 ppm (s)
(D2O/DMSO, 1/1); 12.9 ppm (s) (CDCl3).
2.1.2. Method II
TPPMS of 2.4 g (6 mmol) was dissolved with mag-
netic stirring in 45 ml deaerated ethanol in a Schlenk
flask under N2 at reflux temperature. A solution of
390 mg (1.5 mmol) RuCl3·3H2O in 5 ml ethanol was
added, followed by 600 mg (3 mmol) of Diazald in
10 ml ethanol. The mixture was refluxed for 30 min
and was worked up as above. Yield: 847 mg (58%).
Found: C (%), 45.05; H (%), 3.22; N (%), 1.50; Cl
(%), 11.10.
2. Experimental
TPPMS [6], TPPTS [12] and PTA [13] were pre-
pared according to the literature. RuCl3·3H2O was a
loan of Johnson Matthey, K2[RuCl5(NO)] was pur-
chased from Alfa. N-Methyl-N-nitroso-p-toluenesul-
fonamide (Diazald) was supplied by Aldrich.
13C-enriched (99%) NaHCO3 and D2O (99.9%) were
obtained from Cambridge Isotope Laboratories. Dou-
bly distilled water was used throughout, and all ma-
nipulations were done in an inert atmosphere. 1H and
31P NMR spectra were recorded on Bruker WP 360
SY and DRX400 equipments in D2O or D2O/H2O
mixtures. Pressurized samples were studied using
a thick-wall glass NMR tube with a Teflon valve
(Aldrich; up to 10 bar) or medium pressure sapphire
NMR tubes (up to 100 bar). Chemical shifts are ref-
erenced to 3-(trimethylsilyl)-1-propanesulfonic acid
Na-salt (TSPSA, Fluka) and 85% H3PO4, respec-
tively. Infrared spectra were taken on a PE Paragon
1000 PC FT-IR spectrophotometer in KBr discs or as
Nujol mulls. For the pH-potentiometric titrations, a
radiometer ABU-90 autoburette was used, equipped
with a combined Ag/AgCl-glass electrode (Radelkis
OP-0808P) calibrated against 0.5 M potassium hydro-
gen phthalate.
2.2. Synthesis of [RuCl3(NO)(TPPTS)2]
TPPTS of 627 mg (1 mmol) was dissolved with
magnetic stirring in 15 ml deaerated ethanol in a
Schlenk flask under N2 at room temperature. Then
a solution of 193 mg (0.5 mmol) K2[RuCl5(NO)] in
3 ml water was added and the mixture was refluxed
under N2 for 90 min. After cooling to room temper-
ature the greenish yellow product was filtered and
washed with a small amount of cold ethanol. Yield:
521 mg (76%). νNO (KBr) = 1883 cm−1
.
31P NMR:
δ 16.0 ppm (s) (D2O).
2.3. Attempted synthesis of [RuCl3(NO)(PTA)2]
PTA of 79 mg (0.5 mmol) was dissolved with mag-
netic stirring in 7.5 ml deaerated ethanol in a Schlenk
flask under N2 at room temperature. Then a solution
of 97 mg (0.25 mmol) K2[RuCl5(NO)] in 1.5 ml water
was added and the mixture was refluxed under N2 for

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145
90 min. After cooling to room temperature the brown
product was filtered and washed with a small amount
of cold ethanol. Yield: 107 mg (78%; for the above
formula). νNO (KBr) = 1854 cm−1. No consistent el-
ementary analysis data could be obtained. The product
is very sparingly soluble in water and other ordinary
solvents and was not investigated further.
Scheme 1. Synthesis of the water-soluble [RuCl3(NO)(TPPMS)2]
and [RuCl3(NO)(TPPTS)2] complexes.
to that of [RuCl3(NO)(PPh3)2], i.e. νNO = 1870 cm−1
(TPPMS), 1883 cm−1 (TPPTS) and 1875 cm−1 (PPh3,
[15]), respectively. The 31P NMR spectra display sin-
glet resonances at δ 13.6 ppm (TPPMS, D2O/DMSO:
1/1) and at 16.0 ppm (TPPTS, D2O). These spectral
features are consistent with an octahedral complex
geometry, having the two phosphine ligands in trans
position in the equatorial plane and the NO and one
of the Cl− ligands in axial positions. The νNO fre-
quencies suggest a linear Ru–N–O arrangements. For
[RuCl3(NO)(PPh3)2] this structure has been unequiv-
ocally established by single crystal X-ray diffraction
[16] and this lends support to the structural conclu-
sions regarding the TPPMS- and TPPTS-containing
complexes, too.
At room temperature, [RuCl3(NO)(TPPMS)2] and
[RuCl3(NO)(TPPTS)2] give stable aqueous solu-
tions, but as expected, show the characteristic re-
action [1] of linear nitrosyl complexes with OH−.
The pH-potentiometric titration of an aqueous solu-
tion of [RuCl3(NO)(TPPMS)2] resulted in a typical
S-shaped pH versus [OH−] curve with an inflexion
point at pH = 8.3. Fig. 1 shows a pH-static titration
2.4. Hydrogenation experiments
The following procedure is typical. A medium
pressure sapphire NMR tube (o.d. 10 mm) was used
as a reactor. In this tube under an N2 atmosphere,
5.41 mg
(0.005 mmol)
[RuCl3(NO)(TPPMS)2],
6.26 mg (0.016 mmol) TPPMS, 91 mg (1.08 mmol)
NaH13CO3 were dissolved in 1.80 ml H2O + 0.20 ml
D2O together with 4.4 mg TSPSA (0.02 mmol). The
reactor was filled with CO2 up to the desired pressure,
then 30 bar H2 was added. The tube was securely
placed on top of a laboratory shaker equipped with a
controllable heating mantle to accommodate the re-
actor, and the reaction mixture was shaken at 70 ◦C.
The reaction was followed by recording the 1H NMR
spectra of the solution in time, and comparing the
integral of the formate C–H signal to that of TSPSA.
In the pH variation studies, the actual pH was set by
choosing the appropriate bicarbonate concentration
and CO2 pressure. The pH of the solution was cal-
culated as pH = pKa1 + ln([HCO3−]/[CO2]), where
pKa1 is the first ionization constant of carbonic acid,
6.35 at 25 ◦C [14]. The actual [HCO3−]/[CO2] ratio
was calculated from the 13C NMR spectra taken at
this temperature.
3. Results and discussion
3.1. Synthesis and properties of the water-soluble
Ru–NO–phosphine complexes
The reaction of K2[RuCl5(NO)] with two equiva-
lents of TPPMS or TPPTS yields well-defined com-
plexes of the formula [RuCl3(NO)(P)2]. [RuCl3(NO)
(TPPMS)2] was also prepared in the direct reaction of
RuCl3·3H2O, TPPMS and N-methyl-N-nitroso-p-tolu-
enesulfonamide (Diazald) with equally good yield and
purity (Scheme 1). These yellow compounds are well
soluble in water. Their IR spectral parameters are close
Fig. 1. Base consumption in the reaction of [RuCl3(NO)(TPPMS)2]
in dilute KOH solution. [Ru] = 4.8 mM, pH = 10.6, [KCl] =
0.2 M, V = 10 ml, T = 35 ◦C.

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A. Katho´ et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 143–148
146
of [RuCl3(NO)(TPPMS)2]. In this experiment, the
complex was dissolved in an aqueous KOH solution
of pH 10.6, and any acid produced in its subsequent
reactions was neutralized by the addition of a KOH
solution of known concentration by an autoburette.
It has been found that two equivalents of KOH per
1 mol of the Ru-complex was consumed, and it is
conceivable therefore that it can be rendered to the nu-
cleophilic addition of OH− on the ruthenium-bound
NO to yield a nitro-derivative (Eq. (1)).
the 1H NMR spectrum at δ = −6.8 ppm with a
2
coupling constant JP–H = 28 Hz. Under simi-
lar conditions, the product of the hydrogenation of
[RuCl3(NO)(TPPTS)2] showed a similar quartet at
δ = −6.9 ppm (2JP–H = 27 Hz). The quartets collapse
to singlets on phosphorus decoupling. Similarly, the
doublet 31P resonances at 62.4 ppm (2JP–H = 27 Hz)
(TPPMS) and at 66.7 ppm (2JP–H = 27 Hz) (TPPTS)
show up as singlets in the 31P{ H} spectra. These data
1
1
are in excellent agreement with the H NMR param-
eters of [RuH(NO)(PPh3)3] (CD2Cl2): δ = −6.6 ppm
[RuCl3(NO)(TPPMS)2] + 2OH−
2
(q, JP–H = 30 Hz) [3,19]. Based on these findings,
→ [RuCl3(NO2)(TPPMS)2] + H2O
(1)
we suggest trigonal bipyramidal structures for the
[RuH(NO)(P)3] complexes (P = TPPMS or TPPTS)
with equatorial phosphines and axial hydride and
nitrosyl ligands; in the solid state a similar, slightly
distorted trigonal bipyramidal coordination geometry
was confirmed in the case of [RuH(NO)(PPh3)3] by
X-ray crystallography [20].
Note, that the reaction is rapid; at room temperature
the half time is about 1 min. Fig. 1 also shows that no
noticeable further reactions happen in the next 10 min.
It can be concluded from these results, that already in
slightly basic solutions (approximately above pH = 8)
the initial nitrosyl-complex is rapidly converted to the
nitro-compound.
3.2. Hydrogenation of carbon dioxide and
bicarbonate in aqueous solution
On prolonged heating at 70 ◦C in aqueous or ethano-
lic solutions [RuCl3(NO)(TPPMS)2] and [RuCl3(NO)
(TPPTS)2] react with an excess of TPPMS and
TPPTS, respectively, yielding strong green solutions
showing a singlet 31P resonance at 15.0 ppm (TPPMS)
and 24.0 ppm (TPPTS). Uncharacterized green im-
purities have been observed in the preparation of
[RuCl3(NO)(PPh3)2] [17,18]. The singlet 31P signal
observed by us is consistent with a trigonal bipyrami-
dal structure and reduction by the added phosphine
can lead to the formation of mer-[RuCl(NO)(P)3].
This compound was not further investigated, although
it may be an intermediate in the formation of the [RuH
(NO)(P)3] complexes as indicated by the rapid colour
change of the solutions from green to yellow under H2
pressure together with the appearance of the charac-
teristic NMR signals of [RuH(NO)(P)3] (vide infra).
In the presence of excess phosphine, the reaction of
[RuCl3(NO)(TPPMS)2] and [RuCl3(NO)(TPPTS)2]
with H2 yields the corresponding hydrido-complexes
(Eq. (2), P = TPPMS or TPPTS).
The hydrogenation of carbon dioxide is of great
interest because of its possible use as a C1 source,
and also due to environmental concerns [21]. Sev-
eral very active homogeneous catalysts are known,
including [RuCl2(PMe3)4] [22], but it was only re-
−
cently, that CO2 and HCO3 were reduced to formic
acid and formate in fully aqueous solutions using
[{RuCl2(TPPMS)2}2] [10] and [RuCl2(PTA)4] [11] as
catalysts. Furthermore, mer-[Mo(CO)H(NO)(PMe3)3]
and mer-[W(CO)H(NO)(PMe3)3] showed rapid inser-
tion of CO2 into the M–H bond giving the correspond-
ing formate-complexes [23,24] and these insertions
were facilitated by the hydridic polarization of the
M–H bond by the NO ligand. Therefore, it seemed
interesting to study the catalytic properties of the
water-soluble [RuH(NO)(P)3] complexes prepared in
situ from [RuCl3(NO)(P)2], H2 and added phosphine,
P = TPPMS or TPPTS.
Both [RuCl3(NO)(TPPMS)2] and [RuCl3(NO)
(TPPTS)2] proved suitable catalyst precursors in the
hydrogenation of carbon dioxide and bicarbonate
(Eq. (3)).
[RuCl3(NO)(P)2] + H2 + P
→ [RuH(NO)(P)3] + H+ + Cl−
(2)
Thus, in phosphate buffer solutions of [RuCl3(NO)
(TPPMS)2] at pH = 6.0 and under 100 bar H2 pres-
sure a quartet hydride resonance was observed in
CO2 + H2 → HCO2H and
HCO3− + H2 → HCO2− + H2O
(3)

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147
ipation of HCO3− rather than hydrated CO2 in the cat-
alytic cycle although we do not have direct evidence
of the intermediates, and at this stage of the investiga-
tions further speculations on the reaction mechanism
would be premature.
Several water-soluble Ru- and Rh-complexes have
been shown earlier to catalyze the isotope exchange
between H2 and D2O, as well as between D2 and H2O
with high efficiency (TOFs up to 1250 h−−1) [25]. In
the aqueous hydrogenation of CO2/HCO3 with the
same catalysts, including, e.g. [{RuCl2(TPPMS)2}2],
−
the formation of large proportions of DCO2 was
indicated by the characteristic splitting of the 13C
−
Fig. 2. Turnover frequency of the hydrogenation of CO2/HCO3
with [RuCl3(NO)P2] catalysts as function of the pH,
1
resonance at 170.7 ppm (t, JC–D = 30.5 Hz). It is
a
P = TPPMS (᭜), TPPTS (᭿). [Ru] = 2.6 mM, [P] = 7.8 mM,
noteworthy, that in the case of the nitrosyl complexes
under study no formate deuteration was observed.
Under strictly identical experimental condi-
tions ([Ru] = 2.6 mM, [TPPMS] or [TPPTS] =
[NaHCO3] = 0.54 M, PCO = 5–58 bar, PH = 30 bar, T = 70 ◦C.
2
2
Under typical experimental conditions ([Ru]
=
7.8 mM, [CO2] = 0.1 M, [NaHCO3] = 0.8 M, PH
2
2–8 mM, [HCO−3 ] = 0.1–0.9 M, PCO = 30 bar, PH
= 30 bar, T = 70 ◦C) the following specific activi-
ties (TOF) were observed: [RuCl3(NO)(TPPMS)2],
102 h−1; [RuCl3(NO)(TPPTS)2], 197 h−1; [{RuCl2
(TPPMS)2}2], 501 h−1. This comparison shows, that
of the three catalysts the NO-complexes are less ac-
tive, although this activity is still fairly high for the
homogeneously catalyzed CO2 hydrogenations [22].
Based on this limited set of data the specific role of
the NO ligand cannot be assessed. It is possible that
2
2
= 30 bar, T = 70 ◦C) the catalytic activity of both
complexes, expressed as mol product (mol Ru h)−1
(turnover frequency, TOF) were in the range of
50–400 h−1. The rate of product formation proved
first order dependent on both the catalyst concentra-
tion and the hydrogen pressure, and increased with
increasing phosphine concentration levelling off at the
[phosphine]/[Ru] ratio of 6. This latter behaviour may
be explained by the formation of the [RuH(NO)(P)3]
complexes as the actual catalytic species. The effect
of pH on the reaction rate is more complex (Fig. 2).
As can be seen with both complexes, the rate shows
a maximum in the function of the pH and drops sharply
above pH = 7, in as much that aqueous NaHCO3 so-
lutions (pH = 8.31 at 1 M) could not be hydrogenated
when no CO2 pressure was applied. This observation
contrasts the case of the [{RuCl2(TPPMS)2}2] [10]
and [RuCl2(PTA)4] [11] catalysts which showed high
activity for the hydrogenation of bicarbonate. It seems
likely, that the deactivation is caused by the formation
of the nitro-complexes [RuCl3(NO2)(TPPMS)2] and
[RuCl3(NO2)(TPPTS)2] rapidly taking place in basic
solutions (vide supra). This sets an upper pH limit of
the use of the nitrosyl complexes for catalytic hydro-
genation in aqueous solutions. On the other side of the
maximum, increasing the pressure of CO2 (with con-
comitant lowering of the pH) leads to gradually lower
rates of hydrogenation. This effect points to the partic-
–
the mechanisms of the HCO3 hydrogenations differ
in the case of the [RuH(NO)(P)3] and [RuH2(P)4]
(P = TPPMS or TPPTS) catalysts, formed from
the nitrosyl complexes and [{RuCl2(TPPMS)2}2],
respectively. Further work to elucidate the reaction
mechanism and to extend the scope of catalysis by the
new water-soluble Ru–NO complexes is in progress.
Acknowledgements
The authors are indebted to Dr. L. Nádasdi and Dr. J.
Kovács for their valuable help with the medium pres-
sure NMR measurements and the pH-potentiometric
titrations, respectively. The financial support by the
Hungarian Ministry of Education (FKFP 0416) and
by the Swiss National Science Foundation (grant
2000-067976.02) are gratefully acknowledged. We
thank Johnson Matthey for a loan of hydrated RuCl3.
Zs.O is grateful for a travel grant from the EPF de

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