Catalysis of redox isomerization of allylic alcohols by [RuClCp(mPTA)2](OSO2CF3)2 and [RuCp(mPTA)2(OH2-κO)](OSO2CF3)3·(H2O)(C4H10O)0.5. Unusual influence of the pH and interaction of phosphate with catalyst on the reaction rate

Article abstract of DOI:10.1016/j.molcata.2010.05.003

In aqueous solutions at 80 °C, [RuClCp(mPTA)₂](OSO₂CF₃)₂ (1) and [RuCp(mPTA)₂(OH₂-κO)](OSO₂CF₃)₃·(H₂O)(C₄H₁₀O)_0.5 (3) (mPTA: N-methyl-PTA) were found effective catalysts of the redox isomerization of alk-1-en-3-ols to the corresponding ketones, characterized by initial turnover frequencies (TOF) of 162 h^-1 (3) and 9.6 h^-1 (1) in 1-octen-3-ol isomerization. A sharp maximum of the reaction rate as a function of pH was observed with 1 in phosphate buffer solutions. Kinetic and ³¹P NMR measurements revealed for the first time in aqueous organometallic catalysis that component(s) of phosphate buffer (most probably HPO₄^2-) strongly interact(s) with the catalyst complexes and this interaction leads to a dramatic loss of the catalytic activity.

Full text of DOI:10.1016/j.molcata.2010.05.003

Journal of Molecular Catalysis A: Chemical 326 (2010) 15–20
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Editor’s Choice paper
Catalysis of redox isomerization of allylic alcohols by
[RuClCp(mPTA)2](OSO2CF3)2 and
[RuCp(mPTA)2(OH2-␬O)](OSO2CF3)3·(H2O)(C4H10O)0.5. Unusual influence
of the pH and interaction of phosphate with catalyst on the reaction rate
Beatríz Gonzáleza, Pablo Lorenzo-Luisa,∗, Manuel Serrano-Ruizb, Éva Pappc, Marianna Feketec,
c
c
Klára Csépke , Katalin Osz , Ágnes Kathóc,d, Ferenc Joóc,d,∗∗, Antonio Romerosab,∗ ∗ ∗
˝
a Departamento de Química Inorgánica, Facultad de Química, Universidad de La Laguna, La Laguna, Tenerife, Spain
b Área de Química Inorgánica, Facultad de Ciencias, Universidad de Almería, 04120 Almería, Spain
c Institute of Physical Chemistry, University of Debrecen, P.O. Box 7, H-4010 Debrecen, Hungary
d Research Group of Homogeneous Catalysis, Hungarian Academy of Sciences, P.O. Box 7, H-4010 Debrecen, Hungary
a r t i c l e i n f o
a b s t r a c t
In aqueous solutions at 80 ◦C, [RuClCp(mPTA)2](OSO2CF3)2 (1) and [RuCp(mPTA)2(OH2-
␬O)](OSO2CF3)3·(H2O)(C4H10O)0.5 (3) (mPTA: N-methyl-PTA) were found effective catalysts of the
redox isomerization of alk-1-en-3-ols to the corresponding ketones, characterized by initial turnover
frequencies (TOF) of 162 h−1 (3) and 9.6 h−1 (1) in 1-octen-3-ol isomerization. A sharp maximum of the
reaction rate as a function of pH was observed with 1 in phosphate buffer solutions. Kinetic and 31P
NMR measurements revealed for the first time in aqueous organometallic catalysis that component(s)
of phosphate buffer (most probably HPO42−) strongly interact(s) with the catalyst complexes and this
interaction leads to a dramatic loss of the catalytic activity.
Article history:
Received 7 March 2010
Received in revised form 9 May 2010
Accepted 11 May 2010
Available online 20 May 2010
Keywords:
Allylic alcohols
Redox isomerization
Phosphate coordination
Ruthenium
© 2010 Elsevier B.V. All rights reserved.
Methyl-PTA
1. Introduction
phosphaadamantane) [9,10] and its various derivatives have often
been used for the synthesis of water-soluble organometallic com-
Water is an environment-friendly solvent for organic reac-
tions and for that reason it has attracted increasing interest
recently both from industrial and academic viewpoints [1–4].
The most common and efficient catalysts in aqueous media are
transition metal complexes containing water-soluble phosphines,
such as the monosulfonated and trisulfonated triphenylphos-
phines, usually as their sodium salts (mtppms-Na [5,6] and
mtppts-Na3 [7,8], respectively). In addition to sulfonated ter-
tiary phosphines, the water-soluble aliphatic caged phosphine PTA
(1,3,5-triaza-7-phosphatricycle[3.3.1.13,7]decaneor1,3,5-triaza-7-
plexes. A wide range of water-soluble homogeneous catalysts
containing rhodium, ruthenium, palladium, iridium, and other
metal ions has been reported [1–14].
Catalytic isomerization of allylic alcohols (e.g. Scheme 1) is an
attractive strategy for the synthesis of the corresponding ketones
(and aldehydes) [15–17]. Such internal redox reactions (transpo-
sitions) show 100% atom economy and therefore much effort has
been spent on the study of these processes following the seminal
works of Blum and co-workers [18] and Trost and Kulawiec [19].
In most previous investigations, isomerization processes have
been studied in organic solvents, such as THF [20,21] and alka-
nes [22]. Very efficient catalyst systems were developed based
on ruthenium complexes [23–29] with a turnover frequency
(TOF) as high as 62500 h−1 [TOF = (mol converted substrate)(mol
catalyst)−1h−1] [26]. Several of these catalysts are catalytically
active in aqueous or aqueous-organic biphasic systems, too
[24–29]. In addition to tertiary phosphine-containing catalysts, a
few complexes with N-heterocyclic carbene ligands were also used
as catalysts of redox isomerization of allylic alcohols in aqueous
media [30,31].
∗
Corresponding author. Tel.: +34 922316502x5423; fax: +34 922318461.
Corresponding author at: Institute of Physical Chemistry, University of Debre-
∗∗
cen, 1, Egyetem tér, P.O. Box 7, H-4010 Debrecen, Hungary.
Tel.: +36 52 512900x22382; fax: +36 52 512915.
∗ ∗ ∗
Corresponding author. Tel: +34 950015305; fax: +34 950015008.
E-mail addresses: plorenzo@ull.es (P. Lorenzo-Luis), fjoo@delfin.unideb.hu
(F. Joó), romerosa@ual.es (A. Romerosa).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.003

16
B. González et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 15–20
Table 1
Isomerization of various allylic alcohols catalyzed by [RuClCp(mPTA)2](OSO2CF3)2
(1).
Substrate
Conversion (%)
TOF (h−1
)
Oct-1-en-3-ol
Hept-1-en-3-ol
Hex-1-en-3-ol
Pent-1-en-3-ol
But-1-en-3-ol
2-Methylprop-2-en-1-ol
Prop-2-en-1-ol
50
28
20
20
17
27
0
18.7
10.0
7.1
7.1
6.1
9.6
0
Scheme 1. Isomerization of alk-1-ene-3-ols.
Ruthenium(II) complexes of PTA and its derivatives have been
widely used recently as water-soluble catalysts of hydrogenation
[32], hydroformylation [33], and hydrogen transfer [34] reactions
as well as promising compounds of anticancer activity [35,36].
Some of us have reported earlier the synthesis of [RuClCp
(mPTA)2](OSO2CF3)2 (1) and its reactions in aqueous solutions to
yield – inter alia – [RuCp(OH-␬O)(mPTA)2](OSO2CF3)2·(C4H10O)
(2) and [RuCp(mPTA)2(OH2-␬O)](OSO2CF3)3·(H2O)(C4H10O)0.5 (3)
where mPTA: N-methyl-PTA [37,38]. Here we report the results
of the investigation of 1 and 3 as catalysts (precursors) in the
redox isomerization of enols in aqueous solutions or in aqueous-
organic biphasic systems. A large effect of the solution pH on the
reaction rates was observed together with a remarkable influ-
ence of phosphate buffer on catalysis; the general consequences
of such interactions in aqueous organometallic catalysis is also
discussed.
Conditions: 1 mmol substrate, 12 mg catalyst (0.014 mmol), 3 mL 0.1 M phosphate
buffer, pH 4.75, 2 h reaction time, 80 ◦C.
stirring. After the required time the resulting mixture was cooled to
room temperature and extracted by CHCl3, filtered through MgSO4
and analyzed by gas chromatography. The identity of the products
was established by comparison with commercially available pure
samples. The conversion of allyl alcohol was determined by 1H NMR
spectroscopy.
2.3. Reaction of 3 in H2O with NaCl at 25 and 80 ◦C
In a 5 mm NMR tube containing 0.5 mL of H2O was dissolved 3
(10 mg, 0.012 mmol) and 5 equivalents of NaCl (3.5 mg, 0.6 mmol).
2. Experimental
1
The 31P{ H} NMR at room temperature showed the exclusive pres-
ence of 1 (ı = −10.77 ppm) which remained the unique detected
2.1. Materials and equipment
compound at 80 ◦C.
[RuClCp(mPTA)2](OSO2CF3)2
(1),
[RuCp(mPTA)2(OH2-
2.4. Study of the behaviour of 3 in H2O at 25 and 80 ◦C
␬O)](OSO2CF3)3·(H2O)(C4H10O)0.5 (3) [37], PTA [39] and mPTA
(triflate salt) [40] were synthesized according to published proce-
dures. All other reagents were obtained from Sigma–Aldrich-Fluka
and Lancaster and used without further purification.
Compound 3 (10 mg, 0.012 mmol) was dissolved in 0.5 mL of
1
H2O in a 5 mm NMR tube. The 31P{ H} NMR spectra evolved from
−10.61 ppm at room temperature to a broad signal at −10.50 ppm
which returned to the original signal when temperature was
reduced to 25 ◦C.
All reactions and manipulations were routinely performed
under an atmosphere of dry nitrogen or argon using vacuum-line
and standard Schlenk techniques. Solvents were dried and deoxy-
genated under nitrogen/vacuum before use. Doubly distilled water
was used throughout. Gas chromatographic measurements were
made on a Hewlett-Packard HP 5890 Series II equipment using a
Varian CP-Wax 52 CB, 30 m, 0.32 mm, 0.25 ␮m (CP884) column and
FID and on a Varian (Walnut Creek, CA, USA) CP-3380 GC gas chro-
matograph equipped with a 1177 split/splitless injector (250 ◦C),
Factor Four VF23 capillary column and flame ionization detector
(280 ◦C). The elution was performed in split mode (1/20 split ratio),
isothermal at 80 ◦C and nitrogen as carrier gas at 1 mL/min. NMR
spectra were recorded on a Bruker AVANCE DRX300 spectrometer
operating at ca. 300 MHz (1H) and ca. 75.4 MHz (13C), respectively.
Peak positions are reported relative to tetramethylsilane calibrated
against the residual solvent resonance (1H) or the deuterated sol-
2.5. Study of the behaviour of 3 in 0.1 M phosphate buffer at pH of
4.75 (25, 80 ◦C)
Into a 5 mm NMR tube was introduced 3 (10 mg, 0.012 mmol)
and 0.5 mL of a 0.1 M phosphate buffer of pH 4.75. The 31P{ H}
1
NMR spectrum obtained after 5 min at room temperature showed
signals at 8.64 ppm (2.50%; broad), 0.66 ppm (60.93%; broad; phos-
phate), −5.32 ppm (0.91%; broad), −8.95 ppm (3.77%; broad) and
−10.58 ppm (31.90%) which did not change during 2 h. At 80 ◦C
the observed signals were at 9.13 ppm (3.52%; broad), 1.23 ppm
(72.91%; broad; phosphate), −3.63 ppm (0.76%; broad), −8.52 ppm
(4.22%; broad) and −10.45 ppm (18.59%), and were not significantly
changed after 2 h at this temperature. The sample was let to cool
and the spectrum recorded newly at room temperature showed the
same signal pattern what was previously observed at this temper-
ature.
1
vent multiplet (13C). 31P{ H} NMR spectra were recorded on the
same instrument operating at ca. 121 MHz. Chemical shifts were
measured relative to external 85% H3PO4 with downfield values
taken as positive.
2.2. General procedure for catalytic isomerization of allylic
alcohols
3. Results and discussion
We have found that on the catalytic action of
[RuClCp(mPTA)2](OSO2CF3)2 (1) at 80 ◦C several enols with
the notable exception of allylic alcohol were selectively isomerized
to the corresponding ketones with turnover frequencies in the
range of 6.1–18.7 h−1 (Table 1).
The catalytic properties of 1 were studied in detail in the reaction
of 1-octen-3-ol, as a representative example of the redox isomer-
ization of allylic alcohols. Some of the reactions were carried out
in the mixture of neat oct-1-en-3-ol with the aqueous solvent and
the progress of the reaction was determined upon termination of
1.74 mL of deoxygenated solvent (water, Na-phosphate or cit-
rate buffer of appropriate pH) was introduced into a Schlenk tube.
After closing, the air was removed from the tube by several evac-
uation/refill (Ar) cycles. Then the catalyst precursor (0.1–4 mol%),
and the allylic alcohol (1 mmol) were added under an argon atmo-
sphere. In some experiments 3 mL of the aqueous phase and 1.5 mL
of toluene as the organic phase were used. The tube was placed into
a water bath the temperature of which was controlled by a Julabo
F25 circulator. The reaction was started by starting the magnetic

B. González et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 15–20
17
Fig. 1. Isomerization of oct-1-en-3-ol catalyzed by 1 as a function of time. Con-
ditions: 12 mg catalyst (0.014 mmol), 160 ␮L oct-1-en-3-ol (1 mmol), 3 mL 0.1 M
phosphate buffer, pH 4.75, 1.5 mL toluene, 80 ◦C.
Fig. 3. Isomerization of oct-1-en-3-ol catalyzed by 1 as a function of pH. Conditions:
12 mg catalyst (0.014 mmol), 160 ␮L oct-1-en-3-ol (1 mmol), 3 mL 0.1 M phosphate
buffer, 2 h reaction time, 80 ◦C.
the process by extraction with chloroform followed by GC analy-
sis. Other reactions were run in biphasic mixtures of the aqueous
buffer and toluene; by sampling the organic phase we could conve-
niently follow these reactions. However, since in these cases where
the substrate was applied in dilute toluene solutions in the bipha-
sic reaction mixtures, the reactions proceeded less rapidly than
without toluene in the organic phase.
The reaction shows no induction period and at low conversions
the yield of 3-octanone is a linear function of time (Fig. 1). Increasing
the amount of the substrate in the organic phase results in lowering
of the overall conversion, nevertheless the yield of 3-octanone pro-
duced in 2 h reactions varies according to a saturation curve upon
the increase of substrate concentration (Fig. 2). The reaction rate
(2 h conversions) shows a linear dependence on the catalyst con-
centration (see Supplementary Material). The effect of temperature
on the conversion in the 40–80 ◦C range is surprisingly linear (see
Supplementary Material) what may be the result of the different
temperature sensitivity of the various chemical and physical pro-
cesses (including e.g. mass transfer between phases) and of that of
the isomerization reaction itself. It is emphasized that these mea-
surements were done with 0.1 M phosphate buffer of pH 4.75 as
the aqueous phase.
most of the further studies the pH of the reaction mixture was
adjusted to 4.75.
These observations are consistent with an ␩3-oxo-allyl mecha-
nism of the isomerization of enols to ketones [17]. According to this
mechanism the substrate enol coordinates as a bidentate enolate
with a concomitant deprotonation, while the isomerized product is
liberated by protonation of the ␩3-oxo-allyl intermediate. In prin-
ciple, the balance of these two processes may lead to a maximum
in the reaction rate at a particular pH.
In addition to the direct role of proton in the reaction mech-
anism of a catalytic transformation, the reaction rate can also be
influenced by the pH through its effect on the composition or
structure of the catalyst. In the 2.8–11.0 range, pH-potentiometric
titration of 1 did not reveal a proton consuming or releasing pro-
cess involving the complex. (This also means that the coordinated
mPTA ligand is not protonated at pH ≥ 2.8 so this process cannot be
involved in the pH dependence of the rate of catalytic redox isomer-
1
ization of oct-1-en-3-ol.) UV–vis spectrophotometric and 31P{ H}
NMR measurements indicated reversible reactions of 1 in strongly
basic solutions (between pH 11 and 13); unfortunately these were
not definitive with respect to the composition of the new species.
Besides, the changes in such strongly alkaline solutions do not
have relevance for the explanation of the rate changes around
pH 5.
A striking feature of the reaction is that its rate shows a pro-
nounced maximum as a function of the pH of the aqueous phase
(Fig. 3). Under the conditions of Fig. 3, maximum conversion (50%)
was observed at pH 4.75, and both in more acidic and more basic
solutions the catalytic activity of 1 dropped sharply. Therefore in
It is known from our previous studies [37,38] that the
chloride ligand in [RuClCp(mPTA)2](OSO2CF3)2 (1) is replaced
by hydroxyde in strongly alkaline aqueous solutions yield-
ing [RuCp(OH-␬O)(mPTA)2](OSO2CF3)2·(C4H10O) (2). However,
dissociation of chloride was not observed in water at room temper-
ature, and the corresponding aqua-complex [RuCp(mPTA)2(OH2-
␬O)](OSO2CF3)3·(H2O)(C4H10O)0.5 (3) could be prepared only via
chloride abstraction by AgOTf (OTf: OSO2CF3). Complex 3 is stable
in aqueous solutions and can be deprotonated by NaOH to yield 2. In
contrast to 1, the closely related complex [RuClCp(PTA)2] is exten-
sively dissociated (Keq = 6.18 × 10−4) to yield [RuCp(PTA)2(H2O)]+
in aqueous solutions already at room temperature [41]. The
less closer analog [Ru(␩6-p-cymene)Cl2(PTA)] (p-cymene: 4-
isopropyltoluene) readily looses both of its Cl− ligands upon
dissolution in water at 25 ◦C [42]. The lack of chloride dissocia-
tion from 1 at room temperature may be the consequence of the
fact that the resulting species 3 is a tripositive ion. Nevertheless we
reasoned that at the reaction temperature of oct-1-en-3-ol isomer-
ization (80 ◦C), complex 3 may be present in the reaction mixture
in substantial quantities and therefore we studied the redox iso-
merization of oct-1-en-3-ol with 3, too, as catalyst. Furthermore,
Fig. 2. Isomerization of oct-1-en-3-ol catalyzed by 1 as a function of amount of
substrate. Conditions: 12 mg catalyst (0.014 mmol), 3 mL 0.1 M phosphate buffer, pH
4.75, 2 h reaction time, 80 ◦C.

18
B. González et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 15–20
Fig. 4. Isomerization of oct-1-en-3-ol catalyzed by 3 as a function of time. Condi-
tions: 5.2 mg catalyst (0.005 mmol), 80 ␮L oct-1-en-3-ol (0.5 mmol), 1.74 mL water,
80 ◦C.
Fig. 5. Isomerization of oct-1-en-3-ol catalyzed by 3 in phosphate buffer as a func-
tion of pH. Conditions: 5.2 mg catalyst (0.005 mmol), 80 ␮L oct-1-en-3-ol (0.5 mmol),
1.74 mL 0.1 M phosphate buffer, 1 h reaction time, 80 ◦C.
1
31P{ H} NMR spectra of 1 and 3 were recorded as a function of
interaction of citrate and 1 cannot be excluded, this finding allows
the conclusion that it is the interaction of 1 and phosphate what is
manifested in the maximum curve dependence of the conversions
on pH in the range of 4.0–6.0.
temperature both in the absence and in the presence of phosphate.
The compound 3-octanone was efficiently obtained in a bipha-
sic mixture of water and neat oct-1-en-3-ol catalyzed by 3 (Fig. 4).
In fact, the reactions catalyzed by 3 in water were much faster than
with 1 in phosphate buffer as catalyst. At 80 ◦C, the initial TOF (cal-
culated from conversions at 15 min) was as high as 162 h−1, and
TOF-s calculated from conversions belonging to 1 h reaction times
were in the range of 85–100 h−1 in the case of higher substrate
amounts (in the range of [catalyst]/[substrate] ratio 100–320).
Under the same conditions, hept-1-en-3-ol and pent-1-en-3-ol
were similarly converted to the corresponding ketones with high
efficiency (TOFs at 1 h: 99 and 89 h−1, respectively; Table S1 in Sup-
plementary Material). Large substrate loadings were also possible
with 3 albeit requiring longer reaction times for high conversion;
with a substrate/catalyst ratio of 1000, 93% conversion was reached
in 20 h (Table S2 in Supplementary Material). It is interesting to note
that 3 could be used in the presence of air, as well, with no appar-
ent drop in catalytic activity. This is an important practical aspect,
however, it was not investigated in detail.
Similar to the case of 1, the rate was found to be a linear func-
tion of temperature in the range of 50–80 ◦C (see Supplementary
Material). However, the reaction rate decreased monotonously
upon raising the pH from 3.0 to 8.2 by using Na-phosphate solu-
tions to set the desired pH (Fig. 5) which contrasts the findings with
1 as shown earlier in Fig. 3. The presence of phosphate had a detri-
mental effect on the reaction rate: under comparable conditions
(pH 6.5), instead of 90% conversion obtained using water as solvent
only 12% conversion to 3-octanone was determined using the same
catalyst, 3, but in phosphate buffer.
1
Dissolved in H2O at 25 ◦C, 1 displayed a single sharp 31P{ H}
NMR signal at −10.77 ppm. However, increasing the temperature
to 80 ◦C led to the shift of this resonance to −10.33 ppm, and to
the appearance of several other weak signals of which the ones at
−9.93 and −9.62 ppm were clearly discernible. When the solution
was cooled back to 25 ◦C, all changes reverted and only the singlet
at −10.77 ppm was observed indicating a complete reversibil-
ity of the process. These changes can be attributed to chloride
dissociation from 1, since in the presence of chloride the origi-
nal singlet resonance at −10.77 ppm remained unchanged. When
sufficient amounts of Na2HPO4 and NaH2PO4 were added to set
the pH at 5.0, a strong singlet was seen at room temperature at
−10.77 ppm (referring to 1 still being the major species, present
in approximately 65% at this temperature), accompanied by weak
singlets at −10.61 ppm (sharp) and at −9.74 ppm (broad). Most
strikingly, upon heating the solution to 80 ◦C, only a sharp sin-
glet was observed at −10.58 ppm with no additional weak signals.
The finding that 3 is a much better catalyst than 1 for the
redox isomerization of oct-1-en-3-ol is in agreement with the
generally assumed need for a free coordination site to allow the
interaction between the substrate and catalyst. In the case of 3, an
O-coordinated ligand (H2O) has to be replaced by the ␬O-substrate
(assuming an ␩3-oxo-allyl mechanism) and this step may take place
with a low activation barrier. In addition, the observations strongly
refer to an interaction of the catalyst with component(s) of the
phosphate buffer. Therefore, we determined the effect of the pH
on the rate of oct-1-en-3-ol isomerizations with catalyst 1 using
citrate buffer instead of phosphate. In the pH range 3.7–5.7, where
a sharp maximum of the rate was observed with phosphate, the use
of citrate buffer as the aqueous phase led to monotonously decreas-
ing conversions of low values (5–25%, Fig. 6). Although a specific
Fig. 6. Isomerization of oct-1-en-3-ol catalyzed by 1 in citrate buffer as a function
of pH. Conditions: 12 mg catalyst (0.014 mmol), 160 ␮L oct-1-en-3-ol (1 mmol), 3 mL
0.1 M citrate buffer, 2 h reaction time, 80 ◦C.

B. González et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 15–20
19
According to these data, the molecular state of the Ru(II)-complex
4. Conclusion
in phosphate buffer is clearly different from that in water both at
25 and 80 ◦C.
Both [RuClCp(mPTA)2](OSO2CF3)2 (1) and [RuCp(mPTA)2(OH2-
␬O)](OSO2CF3)3·(H2O)(C4H10O)0.5 (3) actively catalyzed the redox
isomerization of alk-1-en-3-ols to the corresponding ketones in
aqueous solutions or in aqueous-organic biphases at 80 ◦C with 3
being considerably more active than 1. The catalytic activity of 1
dissolved in phosphate buffer showed a pronounced maximum at
around pH 4.75. In contrast, the catalytic activity of 3 at this pH
was largely diminished by phosphate buffer (compared to water
as solvent) and decreased further monotonously with increasing
pH. Based on kinetic and NMR measurements, it is concluded that
Similar to 1, 3 was also found to be stable in aqueous solu-
tions both at room temperature and at 80 ◦C (see Section 2). On the
addition of phosphate buffer (pH 4.75), the singlet at −10.61 ppm
1
(
31P{ H} NMR) observed at room temperature for 3 was shifted
to −10.58 ppm, and weak singlets at 8.64 ppm, −5.32 ppm, and
−8.95 ppm were also displayed. These signals showed small and
reversible shifts upon heating the sample to 80 ◦C and then cool-
ing it back to room temperature (see Section 2 and Supplementary
Material).
The interaction of phosphate buffer with the analogous [Ru(␩6-
p-cymene)Cl2(PTA)] has been noted by Dyson et al. The UV–vis
spectrum of this compound was essentially the same in water and
in phosphate buffer of pH 2 (showing the presence of [Ru(␩6-
p-cymene)Cl(H2O)(PTA)]+), while at pH 7 it was different and
coincided with that recorded at pH 12; that was considered
as “indicating that the phosphate interacts with the complex”
[36,42].
components of phosphate buffer (first of all H2PO4 and HPO4
)
−
2−
strongly interact with the dipositive and tripositive complex ions
of 1 and 3. Similar interactions were already noticed in bioinor-
ganic/bioorganometallic chemistry, however, those are reported
here for the first time for aqueous organometallic catalysis. The
findings stress that although the use of buffers is essential in aque-
ous organometallic catalysis, and Na-phosphate buffers have been
used frequently, their innocence cannot be taken for granted with
all catalysts and must be scrutinized in all mechanistic investiga-
tions.
In aqueous solutions the pKa values of phosphoric acid are 2.14,
7.20, and 12.34 [43,44], therefore in the pH range of 4.50–5−.00,
where the maximum rate of isomerization is observed, H2PO4 is
the dominant species with a mole fraction >99%. However, the mole
Acknowledgement
2−
fraction of HPO4
increases exponentially above pH 5. Its con-
centration in 0.1 M phosphate buffer solutions at pH 6.00 reaches
5.93 mM which is in the range of the usual catalyst concentrations
(2.7–4.7 mM) in this study. Strong interaction of the dinegative
Funding provided by Junta de Andalucía through PAI (research
teams FQM-317) and Excellence Projects FQM-03092, MCYT
(Spain) by project CTQ2006-06552/BQU and the COST Action
CM0802 (WG2, WG3, WG4), the Hungarian-Spanish Intergovern-
mental Collaboration in Science and Technology (TéT E-10/2005;
HH2005-0001) and the Hungarian National Research and Tech-
nology Office - National Research Fund (NKTH-OTKA K 68482
and OTKA IN 78040). We acknowledge Dr. José Elías of Analyti-
cal Department of ULL for the gas chromatographic measurements
facilities. Beatriz González thanks the Gobierno Autónomo de
Canarias for a predoctoral fellowship, and Éva Papp is grateful to the
Hungarian Ministry of Education for a Békesy György postdoctoral
scolarship.
2−
HPO4 with the di- or tripositive ions of 1 or 3 may lead to poi-
soning of the catalyst.
The interaction of 1 or 3 with HPO42− can explain the sharp drop
of catalytic activity between pH 5 and 7. Note that albeit the concen-
−
tration of H2PO4 is rapidly decreasing in this pH range, its actual
concentration in the 0.1 M phosphate buffer (61.3 mM at pH 7.00)
is still much higher than that of the catalyst. Consequently, if there
−
was any interaction between the catalyst and H2PO4 (it should
be beneficial, anyway, since the rate maximum is observed at a pH
−
where the concentration of H2PO4 is at its maximum) it would
probably not contribute to the overall change in the rate of iso-
merization. Deprotonation of [RuCp(mPTA)2(H2O-␬O)]3+ requires
addition of NaOH [37] so it is unlikely to proceed in such mildly
acidic or neutral solutions and therefore it is unlikely to contribute
to the changes in the reaction rate.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.05.003.
There are not many catalysts of redox isomerization in
aqueous systems that can be compared to 1. The complex
Na2[RuClCp(mtppms)2] [45] is an analogous water-soluble half-
sandwich complex of ruthenium. This compound showed excellent
catalytic activity (up to TOF = 2200 h−1) in the isomerization of oct-
1-en-3-ol at 80 ◦C using 0.1 M phosphate buffer [27]. However, in
acidic solutions, the reaction rate was only slightly effected by
changes in the pH: the catalytic activity (TOF) of this catalyst dimin-
ished only slightly from 2200 h−1 (pH 2.2) to 2000 h−1 (pH 5.0) and
only in more basic solutions could be a sharp drop of the catalytic
activity observed (TOF 690 h−1 at pH 7.0).
Na-phosphate buffers are widely used as solvents in aque-
ous organometallic catalysis [1–4], however, so far no specific
salt effect of phosphate on the catalytic activity or selectivity
has been described in the literature. Conversely, in bioinor-
ganic/bioorganometallic chemistry interactions of phosphate
buffer with aquated Cisplatin [46] and [Ru(␩6-p-cymene)Cl2(PTA)]
[36,40] were clearly recognized, although the structures of the
several resulting phosphate-containing species could not be fully
determined. Our results unambiguously show that the species
present in varying concentrations in phosphate buffers at various
pH may interact with the catalytically active metal complexes and
can strongly influence the catalytic properties of the latter.
References
[1] F. Joó, Aqueous Organometallic Catalysis, Kluwer, Dordrecht, 2001.
[2] B. Cornils, W.A. Herrmann (Eds.), Aqueous-Phase Organometallic Catalysis, 2nd
ed., Wiley-VCH, Weinheim, 2004.
[3] D.J. Adams, P.J. Dyson, S.J. Tavener, Chemistry in Alternative Reaction Media,
Wiley, Chichester, 2004.
[4] C.J. Li, Chem. Rev. 105 (2005) 3095–3165.
[5] S. Ahrland, J. Chatt, N.R. Davies, A.A. Williams, J. Chem. Soc. (1958) 276–288.
[6] F. Joó, J. Kovács, Á. Kathó, A.Cs. Bényei, T. Decuir, D.J. Darensbourg, Inorg. Synth.
32 (1998) 1–8.
[7] E. Kuntz, CHEMTECH 17 (1987) 570–575.
[8] W.A. Herrmann, C.W. Kohlpaintner, Inorg. Synth. 32 (1998) 8–25.
[9] D.J. Darensbourg, F. Joo, M. Kannisto, A. Katho, J.H. Reibenspies, Organometallics
11 (1992) 1990–1993.
[10] A.D. Phillips, L. Gonsalvi, A. Romerosa, F. Vizza, M. Peruzzini, Coord. Chem. Rev.
248 (2004) 955–993.
[11] B. Cornils, W.A. Herrmann, I.T. Horváth, W. Leitner, S. Mecking, H. Olivier-
Bourbigou, D. Vogt (Eds.), Multiphase Homogeneous Catalysis, Wiley-VCH,
Weinheim, 2005.
[12] F. Joó, Á. Kathó, in: J.G. de Vries, C.J. Elsevier (Eds.), Handbook of Homogeneous
Hydrogenation, Wiley-VCH, Weinheim, 2007, pp. 1327–1359.
[13] K.H. Shaughnessy, Chem. Rev. 109 (2009) 643–710.
[14] M. Ito, S. Kitahara, T. Ikariya, J. Am. Chem. Soc. 127 (2005) 6172–6173.
[15] R.C. van der Drift, E. Bouwman, E. Drent, J. Organomet. Chem. 650 (2002) 1–24.
[16] R. Uma, C. Crévisy, R. Grée, Chem. Rev. 103 (2003) 27–52.
[17] V. Cadierno, P. Crochet, J. Gimeno, Synlett (2008) 1105–1124.
[18] A. Zoran, Y. Sasson, J. Blum, J. Org. Chem. 46 (1981) 255–260.

20
B. González et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 15–20
[19] B.M. Trost, R.J. Kulawiec, J. Am. Chem. Soc. 115 (1993) 2027–2036.
[20] J.-E. Bäckvall, U. Andreasson, Tetrahedron Lett. 34 (1993) 5459–5462.
[21] P. Crochet, M.A. Fernández-Zúmel, J. Gimeno, M. Scheele, Organometallics 25
(2006) 4846–4849.
[22] B. Martín-Matute, K. Bogár, M. Edin, F.B. Kaynak, J.-E. Bäckvall, Chem. Eur. J. 11
(2005) 5832–5842.
[35] C.S. Allardyce, P.J. Dyson, D.J. Ellis, S.L. Heath, Chem. Commun. (2001)
1396–1397.
[36] C. Scolaro, A. Bergamo, L. Brescacin, R. Delfino, M. Cocchietto, G. Laurenczy, T.J.
Geldbach, G. Sava, J. Paul, Dyson, J. Med. Chem. 48 (2005) 4161–4171.
[37] B. González, P. Lorenzo-Luis, P. Gili, A. Romerosa, M. Serrano-Ruiz, J. Organomet.
Chem. 694 (2009) 2029–2036.
[23] C. Slugovc, E. Rüba, R. Schmid, K. Kirchner, Organometallics 18 (1999)
4230–4233.
[38] E.M. Pen˜a-Méndez, B. Gonzalez, P. Lorenzo, A. Romerosa, J. Havel, Rapid Com-
mun. Mass Spectrom. 23 (2009) 3831–3836.
[24] V. Cadierno, J. Francos, J. Gimeno, N. Nebra, Chem. Commun. (2007) 2536–2538.
[25] V. Cadierno, S.E. García-Garrido, J. Gimeno, Chem. Commun. (2004) 232–233.
[26] V. Cadierno, S.E. Garcia-Garrido, J. Gimeno, A. Varela-Álvarez, J.A. Sordo, J. Am.
Chem. Soc. 128 (2006) 1360–1370.
[39] D.J. Daigle, Inorg. Synth 32 (1998) 40–45.
[40] A. Romerosa, T. Campos-Malpartida, C. Lidrissi, M. Saoud, M. Serrano-Ruiz,
M. Peruzzini, J.A. Garrido-Cárdenas, F. García-Maroto, Inorg. Chem. 45 (2006)
1289–1298.
[27] T. Campos-Malpartida, M. Fekete, F. Joó, Á. Kathó, A. Romerosa, M. Saoud, W.
Wojtków, J. Organomet. Chem. 693 (2008) 468–474.
[28] V. Cadierno, P. Crochet, J. Francos, S.E. García-Garrido, J. Gimeno, N. Nebra,
Green Chem. 11 (2009) 1992–2000.
[29] B. Lastra-Barreira, J. Díez, P. Crochet, Green Chem. 11 (2009) 1681–1686.
[30] P. Csabai, F. Joó, Organometallics 23 (2004) 5640–5643.
[31] M. Fekete, F. Joó, Catal. Commun. 7 (2006) 783–786.
[41] C.A. Mebi, R.P. Nair, B.J. Frost, Organometallics 26 (2007) 429–438.
[42] C. Scolaro, C.G. Hartinger, C.S. Allardyce, B.K. Keppler, P.J. Dyson, J. Inorg.
Biochem. 102 (2008) 1743–1748.
[43] K.J. Powell, P.L. Brown, R.H. Byrne, T. Gajda, G. Hefter, S. Sjöberg, H. Wanner,
Pure Appl. Chem. 77 (2005) 739–800.
[44] R.N. Goldberg, N. Kishore, R.M. Lennen, J. Phys. Chem. Ref. Data 31 (2002)
232–371.
[32] S.S. Bosquain, A. Dorcier, P.J. Dyson, M. Erlandsson, L. Gonsalvi, G. Laurenczy,
M. Peruzzini, Appl. Organomet. Chem. 21 (2007) 947–951.
[33] P. Smolen´ ski, F.P. Pruchnik, Z. Ciunik, T. Lis, Inorg. Chem. 42 (2003) 3318–3322.
[34] D.J. Darensbourg, F. Joó, M. Kannisto, Á. Kathó, J.H. Reibenspies, D.J. Daigle,
Inorg. Chem. 33 (1994) 200–208.
[45] A. Romerosa, M. Saoud, T. Campos-Malpartida, C. Lidrissi, M. Serrano-Ruiz, M.
Peruzzini, J.A. Garrido-Cárdenas, F. García-Maroto, Eur. J. Inorg. Chem. (2007)
2803–2812.
[46] M.S. Davies, S.J. Berners-Price, T.W. Hambley, Inorg. Chem. 39 (2000)
5603–5613.