Ru(ii) complexes containing dmso and pyrazolyl ligands as catalysts for nitrile hydration in environmentally friendly media

Article abstract of DOI:10.1039/c3dt51580j

The synthesis of two Ru-dmso complexes containing the ligands 2-(3-pyrazolyl)pyridine (pypz-H), and pyrazole (pz-H), [Ru^IICl ₂(pypz-H)(dmso)₂], (2) and [Ru^IICl ₂(pz-H)(dmso)₃], (3), has been described. Both complexes have been fully characterized in solution through ¹H-NMR and UV-Vis techniques and also in the solid state through monocrystal X-ray diffraction analysis. The redox properties of both complexes have also been studied by means of cyclic voltammetry. Exposure of 2 to visible light in acetonitrile produces a substitution of one dmso ligand by a solvent molecule generating a new complex, [Ru^IICl₂(MeCN)(pypz-H)(dmso)] (4). Also, UV-visible spectroscopy points out that complex 2 presents a thermal and photochemical substitution of dmso ligands in aqueous solution. Finally, the reactivity of complexes 2 and 3 has been tested with regard to the hydration of nitriles using water as a single solvent, displaying good efficiency and selectivity for the corresponding amide derivatives. In general, better performance is achieved with complex 3. Reuse of these catalysts in water and glycerol has been explored for the first time in ruthenium-mediated nitrile hydration catalysis.

Full text of DOI:10.1039/c3dt51580j

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Ru(II) complexes containing dmso and pyrazolyl ligands
as catalysts for nitrile hydration in environmentally
friendly media†
Cite this: Dalton Trans., 2013, 42, 13461
Íngrid Ferrer, Jordi Rich, Xavier Fontrodona, Montserrat Rodríguez* and
Isabel Romero*
The synthesis of two Ru-dmso complexes containing the ligands 2-(3-pyrazolyl)pyridine (pypz-H), and
II
II
pyrazole (pz-H), [Ru Cl
2
(pypz-H)(dmso)
2
], (2) and [Ru Cl
2
(pz-H)(dmso)
3
], (3), has been described. Both
1
complexes have been fully characterized in solution through H-NMR and UV-Vis techniques and also in
the solid state through monocrystal X-ray diffraction analysis. The redox properties of both complexes
have also been studied by means of cyclic voltammetry. Exposure of 2 to visible light in acetonitrile produces
II
a substitution of one dmso ligand by a solvent molecule generating a new complex, [Ru Cl
2
(MeCN)-
(
pypz-H)(dmso)] (4). Also, UV-visible spectroscopy points out that complex 2 presents a thermal and
photochemical substitution of dmso ligands in aqueous solution. Finally, the reactivity of complexes 2
and 3 has been tested with regard to the hydration of nitriles using water as a single solvent, displaying
good efficiency and selectivity for the corresponding amide derivatives. In general, better performance is
achieved with complex 3. Reuse of these catalysts in water and glycerol has been explored for the first
time in ruthenium-mediated nitrile hydration catalysis.
Received 14th June 2013,
Accepted 9th July 2013
DOI: 10.1039/c3dt51580j
www.rsc.org/dalton
On the other hand, catalyst recovery is an important topic
in chemistry and, in this context, solvents are of prime impor-
Introduction
Chemical transformations, as well as other industrially pro- tance. For homogeneous catalysts with high solubility in water,
ductive processes, are experiencing a profound transformation an efficient method for recycling the catalyst can be based on
to meet sustainability criteria, moving from old methods to the higher solubility of the catalyst in the aqueous phase than
3
new ones developed in agreement with green chemistry prin- in the extraction organic solvents. Recently, glycerol appeared
1
4,5
ciples. Substitution of harmful and hazardous chemicals with as a valuable green solvent and has also been described as a
others more compatible with human health and the environ- possible solvent for the immobilization of homogeneous cata-
6
ment is mandatory, and among these the solvent replacement lysts in a similar way.
is especially important since amounts of solvents are usually
much larger than those of reagents and products.
The hydration of nitriles to generate the corresponding
amides is an important transformation from both academic
7
Water has been much under-investigated as a solvent and industrial points of view. Amides not only constitute ver-
8
for chemical transformations basically because of poor solubi- satile building blocks in synthetic organic chemistry, but also
2
lity of organic molecules; however, water is the “ideal solvent”
exhibit a wide range of industrial applications and pharma-
9
being economical, nontoxic, noninflammable and compatible cological interest. This reaction is also of biotechno-
with the environment. Substitution of organic solvents by water logical interest since nitrile hydratases, a family of non-heme
is desirable, but it becomes especially suited for those chemical iron enzymes, are used in the industrial preparation of
1
0
transformations in which water is one of the reagents.
relevant amides, such as acrylamide, nicotinamide and
1
1
5-cyanovaleramide.
Conventionally, amides have been synthesized by the
1
2
13
hydration of nitriles, catalyzed by strong acids and bases.
Departament de Química i Serveis Tècnics de Recerca, Universitat de Girona,
Campus de Montilivi, E-17071 Girona, Spain. E-mail: marisa.romero@udg.edu,
montse.rodriguez@udg.edu
However, under these conditions, various by-products, such as
carboxylic acids, are formed. Moreover, many sensitive func-
tional groups do not endure such harsh conditions, which
consequently decrease the selectivity of the reaction. There-
fore, the development of efficient procedures for the synthesis
†
Electronic supplementary information (ESI) available: Additional structural,
spectroscopic and electrochemical information of the compounds described.
CCDC 933633 and 933634. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt51580j
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 13461–13469 | 13461

View Article Online
Paper
Dalton Transactions
of amides that circumvent the use of stoichiometric reagents
and/or acidic and basic media is highly desirable.
Reaction of equimolecular amounts of [RuCl
and the pypz-H or pz-H ligands in methanol or ethanol at
2 4
(dmso) ], 1,
Extreme acidity and basicity can be avoided by using tran- reflux under nitrogen atmosphere produces the cis-Cl, cis-
sition-metal complexes. Activation of the CN bond occurs dmso complex 2 and the cis-Cl, fac-dmso complex 3, respect-
2
4
through coordination to the metal atom, thus enhancing the ively (compound 3 has been previously synthesized but in
1
4
rate of the hydration step. A variety of transition-metal com- this work, we introduce a new synthetic method together with
1
5
plexes (mainly of groups 8–12) have been investigated, with a complete characterization of the complex).
1
6
Murahashi’s ruthenium dihydride [RuH (PPh ) ], Parkins’s
In the complexes, the nomenclature cis or trans refers to the
2
3 4
1
7
platinum hydride [PtH(PMe
acetonate complex cis-[Ru(acac)
based system [{Rh-(m-OMe)(cod)} ]/PCy (cod = 1,5-cycloocta- meridional disposition of three dmso ligands.
2
OH){(PMe
2
O)
2
H}], the acetyl- relative position of two identical monodentate ligands (Cl or
1
8
2
(PPh
2
py)
2
]
and the Rh(I)- dmso), and the term fac or mer is used to indicate the facial or
2
3
1
9
diene) showing remarkable activities and selectivities under
The substitution of two dmso ligands in 1 by the unsym-
mild conditions though in all cases operating in organic metric ligand pypz-H can potentially lead to six different
media in the presence of only small amounts of water. stereoisomers (including two pairs of enantiomers) for
2
0
Recently, Cadierno and others developed excellent hydration complex 2 which are depicted in Scheme 2a, whereas three
protocols in pure water under neutral conditions using different isomers, including one pair of enantiomers, can be
arene-ruthenium(II) and bis(allyl)-ruthenium(IV) containing formed for complex 3 (Scheme 2b).
P-donor ligands as catalysts. However, to the best of our
It is remarkable that in both cases we have detected a
knowledge, no report of complexes with N- or S-donor ligands single geometrical isomer (the pairs of enantiomers Λ/Δ-2a for
applied to hydration of nitriles can be found in the literature. 2 and Λ/Δ-3a for 3), either when the reflux time is limited to
This prompted us to study the behaviour of two Ru–dmso com- 15′ or when it is extended up to 24 h. This fact can be rational-
pounds as catalysts in the hydration of nitriles in environmentally ized taking into account the following structural and electronic
6
friendly media. These dmso compounds have been described as factors: (a) Ru(II) is a d ion; it forms strong bonds with pyri-
21
potent antitumor compounds or as precursors for the synthesis dylic ligands and they do not exchange in solution with other
22
23c,25
of a large variety of compounds but, although catalytic appli- coordinated ligands;
(b) the existence of a strong hydrogen
cations for Ru–dmso complexes are known, they have never bonding between the oxygen atom of a dmso ligand and the
been used as mediator complexes in nitrile hydration. pyrazolylic hydrogen atom (see below for the X-ray structure for
23
In the present work we describe the preparation and charac- both complexes); (c) the synergistic π-donor and π-acceptor
terization of two Ru-dmso complexes containing the nonsym- effects between the Cl and dmso ligands mutually placed in trans.
metric didentate ligand 2-(3-pyrazolyl)pyridine, pypz-H,
complex 2, or the monodentate pyrazole ligand, pz-H, complex diffraction analysis. Fig. 1 displays the molecular structure of
(ligands are depicted in Scheme 1). We also report the complexes 2 and 3 whereas their main crystallographic data
The crystal structure of complex 2 has been solved by X-ray
3
efficient hydration of different nitrile substrates to their corres-
ponding amides in water media together with some prelimi-
nary studies for the reuse of the catalytic systems.
Results and discussion
Synthesis and crystal structures
The synthetic strategy followed for the preparation of the com-
plexes described in the present paper is outlined in Scheme 1.
Scheme 1 Synthetic strategy and ligands used in this work.
Scheme 2 Possible stereoisomers for 2 and 3.
1
3462 | Dalton Trans., 2013, 42, 13461–13469
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
−
1
9
plex,
20–930 cm range indicates a sulfur-bonded dmso com-
2
1e,26c,e
as confirmed by the X-ray structures obtained.
The one-dimensional (1D) and two-dimensional (2D) NMR
spectra of complexes 2 and 3 were recorded in CD CN or
CD Cl and are presented in the ESI.† The resonances found
3
2
2
for both complexes are consistent with the structures obtained
in the solid state. The complexes exhibit two sets of signals:
one in the aromatic region corresponding to the nitrogen
ligands and the other one in the aliphatic region assigned to
the methyl groups of the bonded dmso ligands.
Complex 2 is an asymmetric molecule, and it generates four
different methyl resonances due to the dmso ligands. The reso-
nances at lower chemical shift (2 and 3.01 ppm) are assigned
to the methyl groups of the axial dmso ligand, due to the an-
isotropic effect of the aromatic ligand over the C11 and
C12 methyl groups. On the other hand, the methyl resonances
of the equatorial dmso ligand are influenced by the deshield-
ing effect of the Cl ligands over C9 and C10 methyl groups. In
the aromatic part, the deshielding effect produced by the
hydrogen bond between the pyrazolic hydrogen H2b and the
oxygen atom of the equatorial dmso ligand allows us to ident-
ify this proton at 13.10 ppm. A similar deshielding effect is
exerted by the equatorial Cl ligand over the alfa pyridyl H1
atom; the rest of the resonances are then easily identified
through the COSY spectrum.
Fig. 1 Ortep plot and labeling schemes for compounds 2 and 3.
and selected bond distances and angles can be found in the
ESI section (Tables S1–S4†). In both cases, the Ru metal
centers adopt an octahedrally distorted type of coordination;
in complex 2 the pypz-H ligand acts in a didentate fashion and
the other coordination sites are occupied by two chloro and
two dmso ligands, which adopt a cis coordination with respect
to each other. All bond distances and angles are within the
2
1e,26
expected values for this type of complexes.
For complex 2 it is interesting to note that the Ru–N1 bond
length (2.123 Å), where the pyridyl N atom is placed trans to
the S atom of dmso, is larger than the distance found for Ru–
N3 (2.031 Å) bond, where the ligand in trans with respect to
the pyrazole ring is a Cl atom. This denotes the stronger trans
influence of the dmso ligand with respect to the chloro ligand,
since a shorter Ru–N bond distance should be expected for the
more π-acceptor pyridyl ring.
The N(1)–Ru(1)–N(3) angle is of 76.92(6)° showing the geo-
metrical restrictions imposed by the didentate pypz-H ligand,
which is considered to define the equatorial plane of the struc-
ture; as a consequence of this, the rest of the equatorial angles
are larger than the 90° expected for an ideal octahedral geome-
try. As mentioned earlier, the complex displays strong hydro-
gen bonding between the oxygen atom of the equatorial dmso
ligand and the hydrogen atom of the pyrazole ring (H(2b)–O(1)
Complex 3 shows three resonances corresponding to each of
the dmso ligands, with two magnetically equivalent methyl
groups per ligand; a NOE effect is observed between the pyrazolic
hydrogen H2d and the C3, C4 methyl groups which allows one to
identify these methyl groups unambiguously; the resonances of
the two remaining dmso ligands are tentatively assigned on the
basis of the deshielding effect produced by the two Cl ligands.
The UV-Vis spectra for both complexes 2 and 3 are dis-
played in the ESI (Fig. S4†). The complexes exhibit ligand
based π–π* bands below 300 nm and relatively intense bands
27
above 300 nm mainly due to dπ–π* MLCT transitions.
The redox properties of the compounds have been deter-
mined by cyclic voltammetry (CV) experiments; complex 2,
containing the didentate pypz-H ligand, exhibits a reversible
monoelectronic Ru(III/II) redox wave at around 0.98 V vs. SSCE
whereas for complex 3, containing the monodentate pz-H
ligand, the Ru(III/II) redox wave is observed at 1 V vs. SSCE. The
higher redox potential observed for 3 arises from the stronger
π-acceptor capacity of dmso with regard to the pyridyl ring
present in 2 instead, though the effect is relatively scarce.
In order to obtain some information about the lability of
the ligands in the synthesized complexes, we have investigated
the photochemical behavior of complex 2. A 0.65 mM solution
of 2 in acetonitrile has been exposed to visible light at room
temperature and its evolution has been monitored through
UV-visible, NMR and cyclic voltammetric experiments. Upon
exposure to light for a few minutes, the color of the solution
changes from light to deep yellow, indicating the occurrence of
=
2.194 Å), then placing the methyl dmso groups above and
below the equatorial plane.
The comparison of complexes 2 and 3 reveals differences in
some structural parameters. For instance, it is interesting to
note that the Ru–Npz bond length (Ru–N1, 2.145 Å) in 3,
where the pyrazolic ring has a dmso ligand in trans, is larger
than the analogous distance for complex 2 (2.031 Å), again
manifesting the strong trans influence of dmso.
Complex 3 also forms an intramolecular hydrogen bond
(H(2d)–O(1) = 1.905 Å) which is stronger than the analogous
bond found in complex 2, due to the lower geometrical restric-
tions of the pz-H monodentate ligand with respect to the
didentate pypz-H ligand.
Spectroscopic properties
The IR spectra for both complexes (see ESI†) show a band light induced processes. In order to follow the process an 80
around 1092 cm− that can be assigned to a νS–O stretching, W lamp was used as the light source to irradiate the complex
and the absence of any significant vibration in the and the spectrophotometric changes were monitored using a
1
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 13461–13469 | 13463

View Article Online
Paper
Dalton Transactions
Fig. 3 Cyclic voltammograms of 2 (1 mM in a 0.1 M TBAH acetonitrile solution)
after irradiation: t = 0, 21, 45, 70 and 85 minutes.
Fig. 2 UV-visible spectroscopy corresponding to the photochemical transform-
ation of 2 into 4.
water as a solvent at 80 °C. The remaining substrate has been
quantified through GC chromatography with biphenyl as the
internal standard and the hydrolysis products have been ana-
lysed by NMR spectroscopy and compared to pure samples of
the corresponding amide and acid derivatives. Conversion and
selectivity values are summarized in Table 1, together with the
conditions used in the catalysis.
Firstly, blank experiments without any catalyst were carried
out by keeping the substrates in water at 80 °C for 20 h. In all
cases, the nitrile was quantitatively recovered except for the ali-
phatic chloronitriles (chloro and dichloroacetonitrile, entries 8
and 9), where a conversion around 40% was achieved in both
cases. However, no traces of the amide product were found
after the blank test for the chloroacetonitrile substrate, in con-
trast to the dichloroacetonitrile, where the amide was quanti-
tatively formed. Thus, the latest substrate was not further
tested in catalytic experiments.
UV-Vis apparatus at 22.0 ± 0.1 °C. The evolution of the UV-Vis
spectra (Fig. 2) shows one isosbestic point at 340 nm confirm-
ing the net conversion to a new compound that presumably
2
corresponds to cis-[RuCl (MeCN)(pypz-H)(dmso)], 4, as
inferred from spectrophotometric, cyclic voltammetric and
NMR experiments (vide infra). In the UV-vis spectra, a shift to
lower energy absorptions is observed for the new complex 4
with regard to the initial complex 2, as expected from the
higher σ-donor and lower π-acceptor capacity of the MeCN
ligand with regard to dmso, which provokes a destabilization
of the dπ (Ru) donor orbital.
1
The changes in the H-NMR spectrum of the aliphatic
region upon 2 → 4 photochemical substitution (Fig. S5†)
clearly show that free dmso (δ 2.6 ppm) is progressively gener-
ated, along with the appearance of new signals which reveal
the presence of two different species during the substitution
process. The four dmso singlets (located at 1.98, 2.92, 3.44 and
As we can observe in Table 1, both complexes were found to
be active towards nitrile hydration, with conversion values
above 80% in some cases. However, the most remarkable
feature is the excellent selectivity observed for the corres-
ponding amides in the vast majority of cases, with only one
exception (entry 4a) where a minor amount (lower than 10%)
of the corresponding acid has been also detected. Regarding
the ether substrate in entry 2, we have observed the hydrolytic
cleavage of the C–O bond in the hydration reaction using 2 as
a catalyst, yielding around 5% of benzonitrile. Among all the
substrates tested, the hydration of acrylonitrile mediated
by complex 3 is particularly interesting (entry 7), where the
industrially relevant acrylamide product is quantitatively
obtained.
3
.48 ppm) gradually decrease and are almost quantitatively
replaced in 40 minutes by the resonances of free dmso and
two new resonances at 3.38 and 3.45 ppm. From the chemical
shifts of the new signals obtained, we can assert that the
released dmso ligand is the one formerly occupying the axial
coordination site. This is consistent with (1) the fact that the
remaining equatorial dmso ligand is stabilized by H-bonding
with the pyrazole ring in cis and (2) the axial dmso ligand is labi-
lized thanks to a kinetic trans effect exerted by the Cl(2) ligand.
The substitution process has also been followed through
cyclic voltammetry (CV) experiments (see Fig. 3). The initial
redox wave at 0.98 V progressively decreases upon irradiation,
in parallel with the appearance of a new reversible wave at 0.64 V.
This value is consistent with the substitution of one dmso by
MeCN since the substitution of an anionic Cl ligand by a
A comparison of the conversion values displayed by the two
catalysts allows concluding that, as a general trend, complex 3
2
8
neutral MeCN would generate much higher redox potentials.
The new wave observed around 1.2 V corresponds to the oxi-
dation of free dmso.
(
with three dmso ligands) displays higher conversion values
for a larger number of substrates, though selectivity is excel-
lent for both catalysts. The relatively higher performance of 3
could be explained by the higher flexibility of a putative inter-
mediate species which would contain only monodentate
Catalytic hydration of nitriles
We have checked our complexes 2 and 3 in the hydration ligands, in parallel with the occurrence of a higher number of
process of different nitriles under neutral conditions using potentially labile sites (presumably those occupied by dmso
1
3464 | Dalton Trans., 2013, 42, 13461–13469
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 1 Ru-catalyzed hydration of nitriles to amides in water using complexes
a
2
and 3 as catalysts
b
Entry
Substrate
Cat
Conv. [%]
Select. [%]
c
1
1
a
b
2
3
86
80
>98
>98
c
d
d
2
2
36
>98
Fig. 4 Evolution of an aqueous solution of complex 2 by warming at 60 °C for
3
3
a
b
2
3
13
45
>98
>98
2
h. Isosbestic points are found at 280 and 302 nm.
4
4
a
b
2
3
62
88
90
>98
Cl substituents have on the aliphatic substrates (entries 8 and
), where the electronic influence is dominated by the electro-
9
5
5
a
b
2
3
37
55
>98
>98
negativity of the Cl substituents, leading in both cases to a
considerable degree of hydrolysis without the need of a
catalyst.
To shed some light on the changes undergone by the cata-
lysts under the experimental conditions of the hydration
6
6
a
b
2
3
26
41
>98
>98
process, a 0.15 mM solution of complex 2 in H
2
O was kept at
7
3
61
>98
60 °C and the evolution was followed by UV-vis spectroscopy.
For the first 2 hours (Fig. 4), isosbestic points were found at
280 and 302 nm, thus indicating the net formation of a
unique complex species presumably by substitution of a dmso
ligand by water (see below). However, keeping the temperature
for longer (up to 4 hours) led to the disappearance of the iso-
8
8
a
b
2
3
e
85
53
>98
>98
9
40
>98
a
Reactions performed at 80 °C using 1 mmol of nitrile in 3 ml of sbestic points, consequently indicating that further ligand sub-
b
water. [Substrate] : [Ru] ratio = 100 : 1. Time: 20 h reaction. Selectivity
=
7
stitutions must be taking place.
c
(amide yield/substrate conversion) × 100. Amide isolated yield [%] =
5 and 71 for catalysts 2 and 3, respectively. 5% of conversion
d
A similar evolution is observed when irradiating an
corresponds to cleavage of the ether group. Selectivity is calculated aqueous solution of 2 at room temperature with visible light
e
with regard to the conversion of the ether substrate. Experiment was
performed without a catalyst.
(
Fig. S7† displays the final spectra obtained after the two pro-
cedures described). However, in the case of photosubstitution
the spectrum corresponding to the initial complex 2 comple-
ligands). Decoordination of dmso is supported by the fact that tely vanishes within 1.5 hours, thus indicating that the photo-
free dmso is found in all cases when analysing the hydrolysis chemical substitution process is faster than the thermal one.
products by NMR spectroscopy.
In both cases, a new MLCT band appears at higher wavelength
The electronic properties of the substrates also influence (around 355 nm), which is consistent with the replacement of
the extent of the hydration reaction, provided that a nucleophi- a dmso ligand by a less π-acceptor aqua ligand.
lic attachment of water (or hydroxo anions) on the nitrile
carbon atom takes place.
Given the efficacy of catalysts 2 and 3 in the hydration reac-
Thus, lower performances are tion, we proceeded to test their reusabilities in water and gly-
1
4a,b
displayed by substrates either linked to aliphatic groups cerol as single solvents. As mentioned previously, glycerol
entries 6a–b) or having para-electron donating groups in the appears as a valuable solvent potentially useful for the immo-
(
aromatic ring (entries 2 and 3a–b). On the other hand, halide- bilization of homogeneous catalysts. In this context we have
substituted benzonitriles (entries 4 and 5) are expected to carried out a preliminary test on the reuse of these catalysts
display better performances thanks to the electron-withdraw- using p-fluorobenzonitrile and benzonitrile itself as substrates
ing character of the halide substituents (inductive effect). (the nitriles chosen are the ones displaying better results
However, in the case of p-chlorobenzonitrile the performance among the ones previously tested for each catalyst), and the
is clearly lowered for both catalysts when compared to benzo- results are shown in Table 2. It is notable that the amide
nitrile, indicating that a deactivating effect, probably caused product (which is evaluated after the last run for each set of
by the resonance delocalization of the Cl lone pairs throughout reuses) is obtained quantitatively in all cases.
the aromatic system, is taking place (the same resonance effect
A first glance at Table 2 allows evidencing that the first run
is expected to be much less significant for the smaller fluoride was slower in glycerol than in water for a given substrate. Both
substituent). This is in contrast with the activating effect that catalysts could be reused for at least a second run and, in the
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 13461–13469 | 13465

View Article Online
Paper
Dalton Transactions
Table 2 Consecutive reuses of catalysts 2 and 3 in the hydration of nitriles to easier replacement of dmso ligands in complex 3 leading to a
a
amides in water and glycerol
higher number of labile coordination sites. More studies related
to the mechanism are being developed in our laboratory.
Entry
1
Substrate
Cat.
Solvent
Run
Conv. [%]
2
2
2
2
2
H O
2
H O
2
H O
2
H O
1
2
3
4
86
35
49
20
Experimental procedures
Materials
2
2
2
2
2
H O
5
1
2
3
2
62
59
1
All reagents used in the present work were obtained from
Aldrich Chemical Co. and were used without further purifi-
cation. Reagent grade organic solvents were obtained from
SDS and high purity de-ionized water was obtained by passing
distilled water through a nano-pure Mili-Q water purification
system. RuCl ·2H O was supplied by Johnson and Matthey Ltd
2
3
Glycerol
Glycerol
Glycerol
3
3
3
3
2
H O
2
H O
2
H O
2
H O
1
2
3
4
88
55
27
5
3
2
and was used as received.
4
3
3
3
3
Glycerol
Glycerol
Glycerol
Glycerol
1
2
3
4
48
56
42
2
Instrumentation and measurements
IR spectra were recorded using an ATR MK-II Golden Gate
Single Reflection. UV-Vis spectroscopy was performed using a
Cary 50 Scan (Varian) UV-Vis spectrophotometer with 1 cm
quartz cells. Cyclic voltammetric (CV) experiments were per-
formed using an IJ-Cambria IH-660 potentiostat using a three
electrode cell. The glassy carbon electrode (3 mm diameter)
from BAS was used as the working electrode, a platinum wire
as the auxiliary and SSCE as the reference electrode. All cyclic
voltammograms presented in this work were recorded under a
nitrogen atmosphere unless explicitly mentioned. The com-
plexes were dissolved in solvents containing the necessary
a
Reactions performed at 80 °C using 1 mmol of nitrile in 3 ml of
water. [Substrate] : [Ru] ratio = 100 : 1. Time: 20 h reaction.
case of aqueous media, the overall turnover number was above
1
75 for both substrates and catalysts. However, the decrease of
activity observed after the first cycle was very pronounced in
water, in contrast to the case of glycerol, where both catalysts
maintain their performance unaltered for a second run. Yet,
the overall TON in glycerol is lower than in water (121 and 146
for entries 2 and 4 of Table 2, respectively). To the best of our
knowledge, this is the first metal-based catalytic system
applied to nitrile hydration catalysis in glycerol media.
Although the performance obtained in glycerol is slightly
lower than in water, it could also be a promising solvent for
this kind of reactions.
+
−
4 6
amount of n-Bu NH PF (TBAH) as the supporting electrolyte
to yield a 0.1 M ionic strength solution. All E1/2 values reported
in this work were estimated from cyclic voltammetric exper-
iments as the average of the oxidative and reductive peak
potentials (Ep,a + Ep,c)/2. Unless explicitly mentioned the con-
1
centration of the complexes was approximately 1 mM. H-NMR
spectroscopy was performed using a Bruker DPX 400 MHz.
Samples were run in CDCl , CD Cl or d -acetonitrile with
3
2
2
3
internal references (residual protons and/or tetramethylsilane).
Elemental analyses were performed using a CHNS-O Elemental
Conclusions
In summary, we have developed two ruthenium dmso com- Analyser EA-1108 from Fisons. Monochromatic irradiations
pounds for the nitrile hydration in water and glycerol media, were carried out using an 80 W lamp source from Phillips on
and they have displayed very good selectivity for the amide pro- complex solutions, typically 1 mM. Gas chromatography exper-
ducts as well as moderate recyclability in solution. These com- iments were performed by capillary GC, using a GC-2010 Gas
pounds constitute the first example of ruthenium dmso Chromatograph from Shimadzu, equipped with an Astec
compounds successfully applied to this type of reaction in CHIRALDEX G-TA column (30 m × 0.25 mm diameter) incor-
environmentally friendly media, and the experiments porating a FID detector. All the product analyses in the cataly-
described in glycerol are the first reported for this type of cata- tic experiments were performed by means of GC using
lytic process in such a solvent.
biphenyl as the internal standard.
A possible mechanism for the nitrile hydration could be
initiated in the case of our catalysts by the substitution of one
Crystallographic data collection and structure determination
or more of the dmso ligands by the solvent (water in our case), Data collection: The measurements were carried out at 300 K
as traces of free dmso are found in the NMR spectra after per- using a BRUKER SMART APEX CCD diffractometer using graph-
forming the catalytic reaction and also from the evidence ite-monochromated Mo K
α
radiation (λ = 0.71073 Å) from an
found in UV-visible experiments. Thus, dmso ligands would X-ray tube. Full-sphere data collection was carried out with ω
II
constitute the labile coordination sites that could allow the and φ scans. Crystals of [Ru Cl
coordination of the corresponding nitrile to the metal center. were grown from CH Cl . Crystals of [Ru Cl
The improved efficiency of 3 versus 2 could arise from the (3), were grown from methanol. The measurements were made
2
(pypz-H)(dmso)
2
] (OH
2
)
1/2, (2),
II
2
2
2
(pz-H)(dmso)
3
],
1
3466 | Dalton Trans., 2013, 42, 13461–13469
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
in the range 1.96–28.30° and 2.55–28.79° for θ respectively. For C, 23.1; H, 3.9; N, 5.9. Calc. for C
9
H
22Cl
2
N
2
O
3
RuS
3
: C, 23.0; H,
1
compound 2 a total of 28 194 reflections were collected of 3.9; N, 6.0. H-NMR (CD Cl , 400 MHz) δ 3.12 (s, 6H, H4, H3),
2
2
which 4562 [R(int) = 0.0207] were unique. For compound 3 a 3.41 (s, 6H, H1, H2), 3.46 (s, 6H, H5, H6), 6.42 (dd, 1H, H8, J8,9
total of 8492 reflections were collected of which 3561 [R(int) = 3.02 Hz; J8,7 = 3.78 Hz), 7.72 (dd, 1H, H7, J7,8 = 3.78 Hz; J7,9
=
=
2
9
0
.0229] were unique. Programs used: data collection, Smart;
0.9 Hz;), 8.48 (dd, 1H, H9, J = 3.02 Hz ; J9,7 = 0.9 Hz), 14.01
9
,8
3
0
31
13
data reduction, Saint+; absorption correction, SADABS.
Structure solution and refinement was done using SHELXTL.
(s, 1H, H2D). C-NMR (CDCl
47.20 (C1, C2), 47.71 (C3, C4), 107.40 (C8), 131.17 (C7), 142.32
The structures were solved by direct methods and refined (C9). For the NMR assignments we use the same labeling
2
, 400 MHz): δ 45.97 (C5, C6),
3
2
2
−1
by full-matrix least-squares methods on F . The non-hydrogen scheme as for the X-ray structures (Fig. 1). IR (vmax, cm ):
atoms were refined anisotropically. The H-atoms were placed 3102, 3004, 2916, 2841, 1531, 1407, 1091, 1012, 934, 771, 674,
in geometrically optimized positions and forced to ride on the 551. E (CH CN) = 1 V vs. SSCE. UV-Vis (CH CN): λmax, nm
1
1
/2
3
3
−
−1
atom to which they are attached, except for N–H and O–H (ε, M cm ) 354 (450).
hydrogen atoms of compound 2 which were located in the
difference Fourier map. The N–H hydrogen was refined freely Catalytic studies
while the O–H bond distance was constrained to 0.85(1) Å.
The ruthenium catalyst (0.01 mmol), water (3 ml) and the
corresponding nitrile (1 mmol) were introduced into a sealed
tube and the reaction mixture was stirred at 80 °C. The
nitrile was extracted with chloroform and quantified by GC,
whereas the identity of the resulting amides was assessed by
Preparations
The [RuCl (dmso)
3
3
2
4
]
complex and the 2-(3-pyrazolyl)pyri-
3
4
dine ligand (pypz-H) were prepared according to literature
procedures. All synthetic manipulations were routinely per-
formed under a nitrogen atmosphere using Schlenk tubes and
vacuum line techniques. Electrochemical experiments were
performed under an N2 atmosphere with degassed solvents.
All spectroscopic, electrochemical and synthetic experiments
were performed in the absence of light unless explicitly
1
H-NMR.
The catalytic reactions with glycerol were carried out under
the same experimental conditions as described for water
except for the extraction step which was done with CH Cl . For
2
2
the recycling process, a new load of the corresponding nitrile
was added to the solution after extraction with chloroform or
CH Cl .
mentioned.
cis,cis-[Ru Cl
2
2
II
2
(pypz-H)(dmso)
sample of pypz-H and 0.15 g (0.31 mmol) of [RuCl
2
], 2. A 0.045 g (0.31 mmol)
4
(dmso) ]
2
were dissolved in 20 ml of ethanol and the resulting solution
refluxed for 2 h. A light orange solid was formed and was fil-
Acknowledgements
tered on a frit, washed with ether and vacuum-dried. Yield: This research has been financed by MICINN of Spain
8 mg (40%). Anal. Found: C, 30.6; H, 3.98; N, 8.62; S, 13.54. (CTQ2010-21532-C02-01). IF and JR thank UdG for a pre-
Calc. for C12 RuS : C, 30.45; H, 4.04; N, 8.87; S, doctoral grant. Johnson & Matthey Ltd are acknowledged for a
, 400 MHz) δ 2.00 (s, 3H, H12), 3.02 (s, RuCl ·nH O loan.
5
H
19Cl
2
N
Cl
2 2
3
O
2
2
1
1
3.54. H-NMR (CD
3
2
3
H, H11), 3.52 (s, 3H, H9), 3.54 (s, 3H, H10), 6.99 (d, 1H, H7,
J7,8 = 2.8 Hz), 7.54 (ddd, 1H, H2, J2,1 = 6.8 Hz; J2,3 = 7.4 Hz; J2,4
=
=
7
1.5 Hz), 7.79 (d, 1H, H8, J8,7 = 2.8 Hz), 7.95 (ddd, 1H, H4, J4,3
7.4 Hz; J4,2 = 1.5 Hz; J4,1 = 0.9 Hz), 8.02(t, 1H, H3, J3,2 = J3,4
.4 Hz; J3,1 = 1.5 Hz), 9.43 (ddd, 1H, H1, J1,2 = 6.8 Hz; J1,3 = 1.5
Notes and references
=
1 (a) P. T. Anastas and J. C. Warner, Green Chemistry: Theory
and Practice, Oxford University Press, Oxford, U.K., 1998;
(b) M. Lancaster, in Handbook of Green Chemistry and Tech-
nology, ed. J. H. Clark and D. J. Macquarrie, Blackwell Pub-
lishing, Abingdon, U.K., 2002; (c) M. Lancaster, Green
Chemistry: An Introductory Text, RSC, London, 2002;
(d) M. Poliakoff, J. M. Fitzpatrick, T. R. Farren and
P. T. Anastas, Science, 2002, 297, 807; (e) P. T. Anastas and
M. Kirchhoff, Acc. Chem. Res., 2002, 35, 686;
(f) R. A. Sheldon, Green Chem., 2005, 7, 267;
(g) R. A. Sheldon, I. Arends and U. Hanefeld, Green Chem-
istry and Catalysis, Wiley-VCH, Weinheim, Germany, 2007.
2 (a) C.-J. Li, Chem. Rev., 2005, 105, 3095; (b) J. H. Clark and
S. J. Taverner, Org. Process Res. Dev., 2007, 11, 149;
(c) C. I. Herrerías, X. Yao, Z. Li and C.-J. Li, Chem. Rev.,
2007, 107, 2546; (d) C.-J. Li and T. H. Chan, Comprehensive
Organic Reactions in Aqueous Media, Wiley-VCH, Weinheim,
Germany, 2007; (e) U. M. Lindström, Organic Reactions in
1
3
Hz; J1,4 = 0.9 Hz), 13.10 (s, 1H, H2B). C-NMR (CDCl
3
): δ
4
1
1
4.19, 45.12 (C11, C12), 45.59, 46.13 (C9, C10), 105.47 (C7),
22.10 (C4), 125.21 (C2), 132.06 (C8), 139.07 (C3), 151.33 (C6),
52.52 (C1, C5). For the NMR assignments we use the same
labeling scheme as for the X-ray structures (Fig. 1). IR (νmax,
−
1
cm ): 3107, 3052, 3025, 1968, 1609, 1438, 1093, 1042, 1010,
67, 781, 681. E1/2(CH CN) = 0.98 V vs. SSCE. UV-Vis (CH CN):
λmax, nm (ε, M cm ) 330 (2030), 400 (1058). ESI-MS (m/z):
9
3
−1
3
−
1
+
4
38 [M − Cl] .
II
[
Ru Cl
2 3
(pz-H)(dmso) ], 3. This complex was prepared
through a modification of the method previously described in
the literature.
2
4
A 0.034 g (0.5 mmol) sample of pz-H and 0.12 g
0.25 mmol) of [RuCl (dmso) ] were dissolved in 10 ml of
(
2
4
methanol and the resulting solution refluxed for 2 h. A light
yellow solid was formed and was filtered on a frit, washed with
methanol and vacuum-dried. Yield: 59 mg (49%). Anal. Found:
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 13461–13469 | 13467

View Article Online
Paper
Water, Blackwell Publishing, Oxford, U.K., 2007;
Dalton Transactions
Acta, 2005, 358, 1; (d) V. Yu. Kukushkin and
A. J. L. Pombeiro, Chem. Rev., 2002, 102, 1771.
(
(
f) A. Chanda and V. V. Fokin, Chem. Rev., 2009, 109, 725;
g) F. M. Kerton, Alternatives Solvents for Green Chemistry, 15 (a) A. W. Parkins, Platinum Met. Rev., 1996, 40, 169;
RSC Publishing, Cambridge, U.K., 2009; (h) R. Skouta,
Green Chem. Lett. Rev., 2009, 2, 121; (i) Handbook of Green
(b) T. J. Ahmed, S. M. M. Knapp and D. R. Tyler, Coord.
Chem. Rev., 2011, 255, 949.
Chemistry, ed. P. T. Anastas and C.-J. Li, Wiley-VCH, Wein- 16 (a) S.-I. Murahashi, S. Sasao, E. Saito and T. Naota, J. Org.
heim, Germany, 2010, Vol. 5.
Chem., 1992, 57, 2521; (b) S.-I. Murahashi, S. Sasao, E. Saito
and T. Naota, Tetrahedron, 1993, 49, 8805;
(c) S.-I. Murahashi and H. Takaya, Acc. Chem. Res., 2000, 33,
3
4
C.-J. Li and L. Chen, Chem. Soc. Rev., 2006, 35, 68–82.
(a) D. Tavor, O. Sheviev, C. Dlugy and A. Wolfson,
Can. J. Chem., 2010, 88, 305; (b) J. Francos and V. Cadierno,
225.
Green Chem., 2010, 12, 1552; (c) S. Balieu, A. El Zein, R. De 17 (a) T. Ghaffar and A. W. Parkins, Tetrahedron Lett., 1995, 36,
Sousa, F. Jérôme, A. Tatibouët, S. Gatard, Y. Pouilloux,
J. Barrault, P. Rollin and S. Bouquillon, Adv. Synth. Catal.,
8657; (b) J. Akisanya, A. W. Parkins and J. W. Steed, Org.
Process Res. Dev., 1998, 2, 274; (c) T. Ghaffar and
A. W. Parkins, J. Mol. Catal. A: Chem., 2000, 160, 249;
(d) X.-B. Jiang, A. J. Minnaard, B. L. Feringa and J. G. de
Vries, J. Org. Chem., 2004, 69, 2327.
2
010, 352, 1826.
(a) B. Lastra-Barreira, J. Díez and P. Crochet, Green Chem.,
009, 11, 1681; (b) B. Lastra-Barreira and P. Crochet, Green
5
6
2
Chem., 2010, 12, 1311.
(a) Y. Gu and F. Jérôme, Green Chem., 2010, 12, 1127;
18 T. Oshiki, H. Yamashita, K. Sawada, M. Utsunomiya,
K. Takahashi and K. Takai, Organometallics, 2005, 24, 6287.
(
b) A. Díaz-Alvarez, J. Francos, B. Lastra-Barreira, P. Crochet 19 A. Goto, K. Endo and S. Saito, Angew. Chem., Int. Ed., 2008,
and V. Cadierno, Chem. Commun., 2011, 47, 6208. 47, 3607.
N. V. Kaminskaia and N. M. Kostic, J. Chem. Soc., Dalton 20 (a) V. Cadierno, J. Francos and J. Gimeno, Chem.–Eur. J.,
7
8
Trans., 1996, 3677.
2008, 14, 6601; (b) V. Cadierno, J. Diez, J. Francos and
J. Gimeno, Chem.–Eur. J., 2010, 16, 9808; (c) A. Cavarzan,
A. Scarso and G. Struckul, Green Chem., 2010, 12, 790;
(d) R. García-Álvarez, J. Díez, P. Crochet and V. Cadierno,
Organometallics, 2010, 29, 3955; (e) R. García-Álvarez,
J. Díez, P. Crochet and V. Cadierno, Organometallics, 2011,
30, 5442; (f) R. García-alvarez, P. Crochet and V. Cadierno,
Green Chem., 2013, 15, 46.
(a) J. Zabicky, The Chemistry of Amides, Wiley-Interscience,
New York, 1970; (b) A. Grenberg, C. M. Breneman and
J. F. Liebman, The amide linkage: Structural significance in
chemistry, biochemistry and materials science, Wiley,
New York, 2000.
9
(a) H. Yamada and M. Kobayashi, Biosci., Biotechnol.,
Biochem., 1996, 60, 1391; (b) A. Bhattacharya, B. P. Scott,
N. Nasser, H. Ao, M. P. Maher, A. E. Dubin, D. M. Swanson, 21 (a) G. Sava, E. Alessio, A. Bergamo and G. Mestroni, Topics
M. P. Shankley, A. D. Wickenden and S. R. Chaplan, J. Phar-
macol. Exp. Ther., 2007, 323, 665.
0 (a) P. Lugo-Mas, A. Dey, L. Xu, S. D. Davin, J. Benedict,
W. Kaminsky, K. O. Hodgson, B. Hedman, E. I. Solomon
and J. A. Kovacs, J. Am. Chem. Soc., 2006, 128, 11211;
in Biological Inorganic Chemistry, Springer-Verlag GmbH &
Co., Berlin, 1999, p. 143; (b) G. Sava, K. Clerici, I. Capozzi,
M. Cocchietto, R. Gagliardi, E. Alessio, G. Mestroni and
A. Perbellini, Anti-Cancer Drugs, 1999, 10, 129; (c) G. Sava,
R. Gagliardi, A. Bergamo, E. Alessio and G. Mestroni, Anti-
cancer Res., 1999, 19, 969; (d) I. Bratsos, D. Urankar,
E. Zangranado, P. Genova-Kalou, J. Kosmrlj, E. Alessio and
I. Turel, Dalton Trans., 2011, 40, 5188; (e) I. Bratsos,
C. Simonin, E. Zangranado, T. Gianferrara, A. Bergamo and
E. Alessio, Dalton Trans., 2011, 40, 9533.
1
1
(b) J. A. Kovacs, Chem. Rev., 2004, 104, 825.
1 (a) H. Yamada and M. Kobayashi, Biosci., Biotechnol.,
Biochem., 1996, 60, 1391; (b) I. Endo, M. Nojiri,
M. Tsujimura, M. Nakasako, S. Nagashima, M. Yohda and
M. Odaka, J. Inorg. Biochem., 2001, 83, 247; (c) S. van Pelt,
F. van Rantwijk and R. A. Sheldon, Focus on Catalysis Appli- 22 (a) I. P. Evans, A. Spencer and G. Wilkinson, J. Chem. Soc.,
cations (supplement to Chimica Oggi/Chemistry Today),
Teknoscienze Srl, Milano, Italy, 2008, vol. 26, p. 2;
Dalton Trans., 1973, 204; (b) E. Alessio, G. Mestroni,
G. Nardin, W. M. Attia, M. Calligaris, G. Sava and S. Zorzet,
Inorg. Chem., 1988, 27, 4099; (c) E. Alessio, Chem. Rev.,
2004, 104(9), 4203; (d) J. Mola, I. Romero, M. Rodríguez,
F. Bozoglian, A. Poater, M. Sola, T. Parella and J. Benet-
Buchholz, Inorg. Chem., 2007, 46, 10707.
(d) S. Sanchez and A. L. Demain, Org. Process Res. Dev.,
2
011, 15, 224; (e) B. Li, J. Su and J. Tao, Org. Process Res.
Dev., 2011, 15, 291.
2 J. N. Moorthy and N. Singhal, J. Org. Chem., 2005, 70, 1926.
1
1
3 (a) N. Kornblum and S. Singaram, J. Org. Chem., 1976, 44, 23 (a) H. L. Kwong, W. S. Lee, T. S. Lai and W. T. Wong, Inorg.
4
1
727; (b) A. R. Katrizky, B. Pilarski and L. Urogdi, Synthesis,
989, 949.
Chem. Commun., 1999, 2, 66; (b) C. Sens, M. Rodríguez,
I. Romero, A. Llobet, T. Parella, B. P. Sullivan and J. Benet-
Buchholz, Inorg. Chem., 2003, 42, 2040; (c) I. Suresh and
S. K. Aruna, Indian J. Chem., Sect. B: Org. Chem. Incl. Med.
Chem., 2003, 42B, 2805; (d) S. Naz, H. Adams and
J. A. Thomas, Inorg. Chim. Acta, 2006, 359, 759.
1
4 (a) N. E. Katz, F. Fagalde, N. D. Lis de Katz, M. G. Mellace,
I. Romero, A. Llobet and J. Benet-Buchholz, Eur. J. Inorg.
Chem., 2005, 3019; (b) J. Mola, D. Pujol, M. Rodríguez,
I. Romero, X. Sala, N. E. Katz, T. Parella, J. Benet-Buchholz,
X. Fontrodona and A. Llobet, Aust. J. Chem., 2009, 62, 1675; 24 M. M. Taqui Khan, N. H. Khan, R. I. Kureshy and
(c) V. Yu. Kukushkin and A. J. L. Pombeiro, Inorg. Chim.
K. Venkatasubramanian, Polyhedron, 1992, 11, 431.
1
3468 | Dalton Trans., 2013, 42, 13461–13469
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
2
5 I. Romero, M. Rodríguez, A. Llobet, M. N. Collomb, 28 A. Gerli, J. Reedijk, M. T. Lakin and A. L. Spek, Inorg.
A. Deronzier, T. Parella and H. Stoeckli-Evans, J. Chem.
Soc., Dalton Trans., 2000, 1689.
6 (a) E. Alessio, E. Iengo, E. Zangrando, S. Geremia,
Chem., 1995, 34, 1836.
29 Bruker Advanced X-ray Solutions. SMART: Version 5.631,
1997–2002.
2
P. A. Marzilli and M. Calligaris, Eur. J. Inorg. Chem., 2000, 30 Bruker Advanced X-ray Solutions. SAINT +, Version 6.36A,
207; (b) M. Calligaris, Coord. Chem. Rev., 2004, 248, 351; 2001.
c) C. Sens, M. Rodríguez, I. Romero, T. Parella, J. Benet- 31 (a) G. M. Sheldrick, Empirical Absorption Correction
2
(
Buchholz and A. Llobet, Inorg. Chem., 2003, 42, 8385;
d) B. Serli, E. Zangrando, T. Gianferrara, C. Scolaro,
Program, Universität Göttingen, 1996; (b) Bruker Advanced
X-ray Solutions. SADABS Version 2.10, 2001.
(
P. J. Dyson, A. Bergamo and E. Alessio, Eur. J. Inorg. Chem., 32 (a) G. M. Sheldrick, Program for Crystal Structure Refinement,
2
005, 3423; (e) J. Mola, M. Romero, M. Rodríguez,
Universität Göttingen, 1997; (b) Bruker Advanced X-ray Solu-
tions. SHELXTL Version 6.14, 2000–2003.
F. Bozoglian, A. Poater, M. Solà, T. Parella, J. Benet-Buch-
holz, X. Fontrodona and A. Llobet, Inorg. Chem., 2007, 46, 33 I. P. Evans, A. Spencer and J. J. Wilkinson, Chem. Soc.,
0707. Dalton Trans., 1973, 2, 204.
7 V. Balzani, A. Juris and M. Veturi, Chem. Rev., 1996, 96, 34 W. Huckel and H. Bretschneider, Chem. Ber., 1937, 9,
59. 2024.
1
2
7
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 13461–13469 | 13469