Sulfonated water-soluble N-heterocyclic carbene silver(I) complexes: Behavior in aqueous medium and as NHC-transfer agents to platinum(II)

Article abstract of DOI:10.1021/om400228s

This report describes the synthesis of water-soluble silver(I) and platinum(II) complexes bearing sulfonated mono- or dianionic N-heterocyclic carbene ligands. Thus, treatment of the corresponding zwitterionic imidazolium derivative with silver(I) oxide in water afforded the light-sensitive bis(carbene) complexes Ag[Ag(NHC)₂] (2^Ag+), which were transformed into the stable salts Na[Ag(NHC)₂] (2) by addition of sodium chloride. In contrast, the same reaction in dmso afforded mono(carbenes) of general formula Na[AgCl(NHC)] (3). The solvent-dependence of the reaction product can be rationalized on the basis of the equilibrium [AgCl ₂]^- - AgCl + Cl^-. The precipitation of silver chloride is more favored in protic solvents than in aprotic solvents such as dmso, thus explaining the formation of bis(carbenes) in water. The formation of silver chloride may also promote the hydrolysis of silver NHC complexes under some conditions. The water-soluble platinum(II) complexes Na[PtCl ₂(dmso)(NHC)] were synthesized by using either mono(carbene) silver complexes 3 as carbene-transfer agents or by direct metalation of the imidazolium salt with cis-[PtCl₂(dmso)₂] in the presence of NaHCO₃ as base. The (NHC)Pt(II) complexes were tested as catalysts for the hydration of alkynes in the aqueous phase and found to be active in neat water without the need for acidic cocatalysts.

Full text of DOI:10.1021/om400228s

Article
pubs.acs.org/Organometallics
Sulfonated Water-Soluble N‑Heterocyclic Carbene Silver(I)
Complexes: Behavior in Aqueous Medium and as NHC-Transfer
Agents to Platinum(II)
Edwin A. Baquero, Gustavo F. Silbestri, Pilar Gomez-Sal, Juan C. Flores,* and Ernesto de Jesus*
́
́
Departamento de Química Inorgan
́
ica, Campus Universitario, Universidad de Alcala,
́
28871 Alcala
́
de Henares, Madrid, Spain
S
* Supporting Information
ABSTRACT: This report describes the synthesis of water-soluble
silver(I) and platinum(II) complexes bearing sulfonated mono- or
dianionic N-heterocyclic carbene ligands. Thus, treatment of the
corresponding zwitterionic imidazolium derivative with silver(I) oxide
in water afforded the light-sensitive bis(carbene) complexes Ag[Ag-
(NHC)2] (2Ag+), which were transformed into the stable salts
Na[Ag(NHC)2] (2) by addition of sodium chloride. In contrast, the
same reaction in dmso afforded mono(carbenes) of general formula
Na[AgCl(NHC)] (3). The solvent-dependence of the reaction product
can be rationalized on the basis of the equilibrium [AgCl2]− ↔ AgCl +
Cl−. The precipitation of silver chloride is more favored in protic
solvents than in aprotic solvents such as dmso, thus explaining the
formation of bis(carbenes) in water. The formation of silver chloride may also promote the hydrolysis of silver NHC complexes
under some conditions. The water-soluble platinum(II) complexes Na[PtCl2(dmso)(NHC)] were synthesized by using either
mono(carbene) silver complexes 3 as carbene-transfer agents or by direct metalation of the imidazolium salt with cis-
[PtCl2(dmso)2] in the presence of NaHCO3 as base. The (NHC)Pt(II) complexes were tested as catalysts for the hydration of
alkynes in the aqueous phase and found to be active in neat water without the need for acidic cocatalysts.
the synthesis and applications of water-soluble NHC transition-
metal complexes in catalysis have appeared in the last few
months.19
INTRODUCTION
■
In terms of coordination versatility and stability, N-heterocyclic
carbenes (NHCs) have proven to be a superb class of ancillary
ligands in modern day organometallic chemistry.1,2 In
particular, those derived from imidazole are characterized by
their very robust bonds to metals and their strong σ-donor
capabilities,3 forming excellent catalysts for a broad number of
homogeneous processes.4 Applications outside catalysis have
led to recent developments in the fields of medicinal,
luminescent, or functional materials.5 The advantages of using
water-soluble metal complexes for some of these applications
are evident, and a convenient way to render metal NHC
complexes water-soluble involves the attachment of hydrophilic
substituents to the NHC ligand (sulfonates, carbonates,
ammonium groups, sugar moieties, polyethers, etc.).6 Herr-
mann and co-workers published the first examples of such
water-soluble NHC complexes in a patent filed in 1995.7
Platinum(II) complexes containing conventional monoden-
tate20−23 or chelating23,24 NHC ligands have proven to be
useful in a variety of catalytic processes in organic solvents, such
as for the diboration of unsaturated molecules,25 tandem
hydroboration−cross coupling,26 or reductive cyclization of
diines and enines,27 and also as metal-based chemotherapeutic
agents.28 However, platinum complexes containing hydrophilic
NHC ligands were unknown until recently, when we reported
the synthesis of water-soluble (NHC)Pt(0) complexes that
could be used as recoverable catalysts for the hydrosilylation of
alkynes in water at room temperature.29
Although the applications of water-soluble NHC complexes
are progressing rapidly, there is as yet little information
available concerning basic aspects of the chemical reactivity of
these complexes in water, including the limits of the hydrolytic
stability of the metal−NHC bonds. However, water offers
exceptional chemical reactivity due to its unique properties,
such as its ability to solvate salts and polar compounds or its
high dielectric constant. Our aim with this work was to
undertake a study of water-soluble NHC platinum complexes
and their aqueous-phase chemical reactivity. In this first report,
̈
Several years later, Ozdemir and co-workers reported the
synthesis of 2,3-dimethylfuran catalyzed by a water-soluble
NHC ruthenium catalyst.8 In the past few years, new water-
soluble NHC metal complexes have been reported as catalysts
in aqueous-phase processes, including ruthenium complexes in
olefin metathesis,9 allylic alcohol isomerizations,10 or acetophe-
none hydrogenations,11 palladium complexes in cross-coupling
reactions,12,13 gold complexes in alkyne hydrations,14−16
iridium complexes in transfer hydrogenations,17 or copper
complexes in click reactions.18 Two recent reviews concerning
Received: March 18, 2013
© XXXX American Chemical Society
A
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we discuss the synthesis of water-soluble complexes of silver(I)
and platinum(II) with the sulfonated NHC ligands a−e shown
in Figure 1. The platinum(II) complexes have been tested in
similarly obtained starting from the imidazolium 1e.13 As
discussed below, transformation of imidazolium salt 1a into
complex 2a invariably stopped when the molar ratio of both
compounds reached a value of approximately 40−60%. The
role played by the sodium halide is outlined in Scheme 1b.
Imidazolium derivatives 1 are zwitterionic compounds, with the
cationic charge internally balanced by one sulfonate. As such,
their deprotonation with silver oxide results in compounds 2Ag+
,
which contain an argentate anion [Ag(NHC)2]− and a naked
Ag+ counterion. The latter is replaced by Na+ upon addition of
sodium chloride. This pathway has been demonstrated by the
stepwise isolation of compounds 2eAg+ and 2e, whose structures
have been confirmed by X-ray diffraction studies (vide infra).
The sensitivity of the resulting compound to light is
significantly affected by the nature of the cation. Thus, the
darkening of 2eAg+ in the presence of light is appreciable in the
solid state as well as in solution, whereas the sodium salt 2e is
stable for an indeterminate period of time under both
conditions.
Figure 1. Sulfonate NHC ligands used in this work.
Mono(carbene) complexes with sulfonated NHC ligands of
general formula [AgX(NHC)]− were previously unknown. In
the above procedure to synthesize bis(carbene) complexes 2,
the catalytic hydration of alkynes in aqueous phase. Special
emphasis has been paid to understanding the effects of water on
the formation of silver(I) NHC complexes.
NaCl was added after formation of the Ag[Ag(NHC)2] (2Ag+
)
intermediates. It might be reasonable to infer that the
precipitation of silver chloride after the addition of NaCl
could preclude the rearrangement required to afford mono-
(carbene) complexes. However, the fact is that bis(carbene)
complexes were likewise obtained when NaCl was added to the
imidazolium salts 1 in water before silver oxide. Instead, the
solvent was found to be fundamental in determining the final
compound, with the mono(carbene) complexes 3a−e being
obtained in good yields (>80%) when the reactions were
performed in dimethyl sulfoxide instead of water (Scheme 1c).
As would be expected, mono(carbenes) 3b−e evolved into
the corresponding bis(carbenes) 2b−e in water at room
temperature (Scheme 2). At this point it is important to note
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Mono- and Bis-
(carbene) Silver(I) Complexes. The preparation of water-
soluble NHC Pt(II) complexes was initially attempted by way
of silver intermediates. Silver NHC complexes are excellent
agents for the transfer of carbenes to a variety of other metals in
straightforward reactions and, at the same time, are readily
synthesized by treating imidazolium salts with silver(I)
oxide.30−33 To this end, complexes 2b−d of general formula
Na[Ag(NHC)2] (Scheme 1a) were prepared by the procedure
reported in the literature based on the addition of sodium
chloride to a mixture of silver(I) oxide and the corresponding
imidazolium salt 1b−d in water.15,28 The new complex 2e was
Scheme 2. Solvent-Dependent and Reversible
Transformation of Mono- (3) into Bis(carbene) Silver(I)
Complexes (2)
Scheme 1. Formation of Bis- (2) and Mono(carbene) (3)
Complexes of Silver(I) with Sulfonated NHC Ligands
that the conversion of mono- into bis(carbenes) was
accompanied by the precipitation of silver chloride instead of
the usual formation of the ion [AgCl2]− in solution. The
conversion 3 → 2 was found to be reversible, with complexes
2b−e affording mono(carbenes) 3b−e in the presence of AgCl
and NaCl when water was replaced by dmso. The trans-
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formations were fast in both directions at room temperature
except for the complexes with the most sterically demanding
NHC ligand (2e and 3e). These results unequivocally show
that the outcome of the reaction of imidazolium salts 1 with
Ag2O is governed by the (solvent-dependent) thermodynamic
stability of the reaction products.
Imidazolium halides are known to afford NHC-silver
complexes of stoichiometry AgX(NHC) (X = halide), which
adopt neutral mono- (I) or ionic bis(carbene) (II)
architectures30 that are often in equilibrium in solution
(Scheme 3).34,35 The formation of bis(carbene) complexes of
solvated in aprotic than in protic solvents, whereas the opposite
occurs with the [AgX2]− ions.37 The equilibria in Scheme 3 are
therefore expected to be shifted toward the formation of
bis(carbenes) of type III in protic solvents due to the poor
stability of the AgX2 anions in these solvents irrespective of
the factors governing the mono(carbene)−bis(carbene) equi-
librium I ↔ II.
−
To support the above hypothesis, a sample of the
mono(carbene) complex 3e was dissolved in methanol-d4 and
monitored by 1H NMR spectroscopy. As expected, the
transformation of 3e into 2e was accompanied by AgCl
precipitation, although the process was quite slow, taking more
than one day at 90 °C to go to completion. On the other hand,
a look at the literature confirmed that water-soluble NHC silver
complexes obtained by the reaction of imidazolium salts with
silver oxide in the presence of halide anions have always been
obtained in the form of bis(carbene) complexes in
water.14,38−41 It should be noted that Cazin and co-workers
reported the preparation of mono(carbene) silver complexes of
general formula [(NHC)AgCl] in water.42 However, these
reactions involved the formation of complexes containing
hydrophobic NHC ligands (IPr, IMes, or ICy), whose
insolubility in water is most likely responsible for displacing
the equilibria in Scheme 3 toward the mono(carbene) form I.
The hydrolytic stability of the NHC silver complexes merits a
comment. Bis(carbene) complexes 2b−e were found to be
indefinitely stable in air in the solid state, whereas mono-
(carbenes) 3a−e were extensively hydrolyzed under the same
conditions and had to be stored under an inert atmosphere.
The stability in solution is, however, more complex to describe
because it depends on numerous factors, some of which are
discussed below. Bis(carbenes) 2b−e again showed significant
hydrolytic stability in pure water, and their aqueous solutions
Scheme 3. Equilibria Involving Different Structures Found
for Silver(I) NHC Complexes
−
type III, where the counterion is X− instead of AgX2 , is usually
associated with imidazolium salts containing noncoordinating
anions, although it has also been reported with imidazolium
halides.36 Note that the NHC ligands in the chemical formulas
of I−III are considered neutral for generalization purposes. The
fact that the silver complexes reported here are negatively
charged is irrelevant for the following discussion.
It has been noted previously that the neutral mono(carbene)
form I is, in general, favored with regard to the ionic
bis(carbene) form II by bulkier NHCs and less polar solvents.34
Despite this, our understanding of the formation of bis-
(carbenes) in protic solvents is incomplete without taking into
account the equilibrium between II and III. The thermody-
namics of this equilibrium is mainly determined by the stability
of the dihalidoargentate(1−) anions with regard to the
precipitation of silver halide. The constant K for the
equilibrium [AgX2]− ↔ AgX + X− (X− = halide) can be
determined from the overall formation constant of the complex
1
did not show any noticeable degradation (as evidenced by H
NMR spectroscopy) when heated for hours or even days at 90
°C. Complex 2e was particularly stable in water and remained
unaltered for at least 24 h at 100 °C, and two days at 130 °C,
with only 3% hydrolysis. The behavior of complex 2a, in
contrast, is quite remarkable. This complex has previously been
described to be unstable in water.14 Indeed, we have discussed
above that all attempts to synthesize this complex pure failed, as
it was always obtained as an approximately 40−60% mixture
with the corresponding imidazolium salt 1a. In an attempt to
understand this result, two NMR tubes containing a D2O
solution of the mixture of 2a and 1a (previously separated from
the silver oxide/silver chloride precipitate) were prepared. One
of these samples was heated for 24 h at 90 °C and showed no
apparent changes in the composition of the 2a/1a mixture.
This result clearly indicates that 2a is intrinsically stable toward
hydrolysis. In a different experiment, in which sodium chloride
(10 equiv with respect to silver) was added to the second
sample, complex 2a was completely hydrolyzed in less than one
hour at room temperature. Acceleration of the hydrolysis upon
addition of NaCl was also observed for other bis(carbene)
complexes, although the effect was less marked with NHC
ligands that are hydrolyzed more slowly.43 The same effect was
observed when bis(carbenes) were heated in dmso with a small
amount of added water. For instance, the percentage of
hydrolysis of 2c after 15 h at 90 °C rose from 3% to
approximately 30% in the presence of NaCl. Interestingly, the
formation of small amounts of the mono(carbene) 3c was
observed in the last process.
−
AgX2 (β2) and the solubility constant of the silver halide
(Ks).37 A clear picture emerges from the values shown in Table
1. Complexes [AgX2]− are quite stable in dimethyl sulfoxide,
acetonitrile, or dimethylformamide (K ≈ 10−1−10−2), whereas
the precipitation of silver halides is strongly favored in water or
methanol (K ≈ 104−105). This is because the halides are less
Table 1. Constants for Equilibria Involving Silver(I) Ions
and Halides at 25 °C
equilibrium
X
water methanol dmso acetonitrile
dmf
Ag+ + 2 X−
[AgX2],
⇌
Cl
Br
I
−5.4
−7.6
−11.2
−7.9
−10.6
−14.8
−11.7
−11.4
−12.5
−13.4
−13.7
−16.3
−16.6
−17.8
a
−
pβ2
AgX ⇌ Ag+ + Cl
9.8
12.3
16.0
13.1
15.2
18.3
10.4
10.6
11.4
12.9
12.9
14.5
15.0
15.8
a
X−, pKs
Br
I
[AgX2]−
⇌
Cl
Br
I
−4.4
−4.7
−4.8
−5.2
−4.6
−3.5
1.3
0.8
1.1
0.5
0.8
1.8
1.6
2.0
AgX + X−,
b
pK
a
b
Data from ref 37. Determined as pK = −(pβ2 + pKs).
C
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Any analysis of the hydrolytic stability of mono(carbene)
complexes 3 is necessarily complicated by the fact discussed
above, namely, that they evolve to the corresponding
bis(carbenes) in water. Nevertheless, the behavior of complex
3a was unique because it was completely hydrolyzed in water
instead of evolving to the corresponding bis(carbene). This
result does not necessarily mean that 3a is intrinsically more
reactive with water than the other mono(carbenes) but only
that hydrolysis of the NHC−Ag bond occurs faster than the
transfer of NHC ligands between Ag centers in this case.
Another important point with respect to mono(carbenes) is the
reversibility of the hydrolysis observed in the solid state
(Scheme 4a). Thus, when a hydrolyzed solid sample of 3a
narrow range in which carbenic C2 resonances are found
(176.8−182.7 ppm, Table 2) is characteristic of N-alkyl and N-
Table 2. Chemical Shifts and Coupling Constants for the
Carbenic C2 Carbon in Silver NHC Complexes 2 and 3
1
complex
δ (C2, ppm)
1J(109Ag−13C), J(107Ag−13C) in Hz
Bis(carbene) Complexes, Na[Ag(NHC)2] in D2O
178.4 (singlet)
2a
a
2b
179.7 (singlet)
b
2c
180.6 (two doublets)
182.7 (broad doublet)
181.2 (two doublets)
208, 180
193.9
b
2d
2e
217, 188
Mono(carbene) Complexes, Na[AgX(NHC)] in dmso-d6
3a
3b
3c
3d
3e
176.8 (broad singlet)
177.8 (singlet)
not observed
Scheme 4. Hydrolysis of the Silver NHC Complexes
not observed
181.4
268, 232
a
b
From ref 14. From ref 38.
aryl silver imidazolylidene complexes but cannot be used to
differentiate between mono- and bis(carbene) structures.33,45
The coupling constant of the C2 resonance with 107,109Ag is, in
contrast, very informative when the intermolecular exchange of
NHC ligands between silver centers is slow enough to observe
splitting of the signal.32,35 The 1J(13C−Ag) constants of neutral
mono(carbenes) are, in general, 50−60 Hz larger than those of
their corresponding ionic bis(carbenes) (values in the range
275−255 Hz for 109Ag and 235−220 Hz for 107Ag in
mono(carbenes) versus 220−190 Hz for 109Ag and 190−165
Hz for 107Ag in bis(carbenes)).31,33,35,45 The constants
measured for complexes 2e and 3e, which bear the same
NHC ligand, are within the expected ranges for their proposed
structures (Table 2). In contrast, the C2 resonances of the less
encumbered complexes in Table 2 were observed as singlets
(no 1J(13C−Ag) coupling), thereby suggesting that these
complexes undergo intermolecular NHC ligand exchange that
is fast on the NMR time scale. It is important to note that some
of the bis(carbene) complexes listed in Table 2 undergo fast
NHC exchange even in the absence of [AgCl2]− counterions
(or NaCl/AgCl).
The ESI mass spectra of the silver complexes, recorded in
methanol, further supported the proposed structures. Thus, in
the case of the bis(carbene) complexes 2e and 2eAg+, the most
intense peaks correspond to the whole molecular fragment
ionized by the loss of one or more cations (Na+ or Ag+) in
negative mode or by addition of a hydrogen ion, probably from
the solvent, in positive mode. The same anion [M − Na]− was
also observed in the case of the monocarbene complexes 3a−e,
although the most intense peaks were those corresponding to
bis(carbene) fragments, probably due to the rearrangement of
these complexes in protic solvents (Scheme 2). Conversely, the
ESI mass spectra of 3c recorded in acetonitrile showed intense
peaks for the mono(carbene) ions [M − Na]− and [M − Na −
Cl + CN]−. Acetonitrile appears to be the most likely source of
CN− in the latter fragment.46
containing 24% of the NHC ligand in the form of the Ag
complex (and 76% as imidazolium salt) was heated in dmso at
90 °C, the percentage of the mono(carbene) complex increased
to 80% after 15 h. Similar transformations were observed for
other complexes, although they were in general slower as the
steric demand of the NHC ligand increased.
Although the hydrolysis of silver NHC complexes is a
complex phenomenon in which both thermodynamics and
kinetics play an important role, we can nevertheless extract
several simple ideas. First, we should assume that solvation
plays an important role in the thermodynamics of this
hydrolysis to explain the differences found between mono-
(carbenes) in the solid state and in solution. Second, hydrolysis
of the complexes containing two sulfonatopropyl substituents
(2a and 3a) is quite interesting because forward (hydrolysis)
and reverse processes are fast and therefore controlled by the
thermodynamics. Third, the effect of the addition of sodium
chloride on the hydrolysis of bis(carbenes) is likely
thermodynamic in nature and might be motivated by
displacement of the equilibrium shown in Scheme 4b from
the bis(carbene) 2 to the imidazolium salt 1 due to the
favorable formation of silver chloride from silver oxide (K = 1.1
× 102 in water).44 Indeed, the bis(carbene) complex 2a was
obtained spectroscopically pure by reaction of the imidazolium
salt 1a with silver oxide in the presence of an excess of sodium
hydroxide (Scheme 4c).
The new NHC silver complexes 2e, 2eAg+, and 3a−e were
1
characterized by electrospray mass spectrometry and H and
X-ray Crystal Structures of the Silver(I) Complexes 2e,
2eAg+, and 3a. The crystal structures of the NHC silver
complexes 2e, 2eAg+, and 3a have been determined by X-ray
diffraction methods. In the case of complex 2e, X-ray
determinations were performed on two different crystalline
samples, one obtained from dichloromethane/methanol solvent
13C NMR spectroscopy. These complexes were isolated as
spectroscopically pure solids, which afforded inaccurate C, H,
and N elemental analyses due to the presence of solvent
molecules trapped within their structures (see X-ray structures
below) and the likely contamination with inorganic salts. The
D
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mixtures (2e·6MeOH) and the other from acetone/dmso
(2e·5dmso·H2O). The structures of 2e·6MeOH, 2e·5dmso·-
H2O, and 2eAg+ contain similar [Ag(NHC)2]3− moieties packed
in different arrangements together with counterions and solvent
molecules.
Figure 2 shows the molecular structure of the [Ag(NHC)2]3−
unit in 2e·6MeOH together with a selection of distances and
Figure 3. View of the crystal structure of 2eAg+ along the
crystallographic c axis highlighting the enchainment of [Ag(NHC)2]3−
moieties by interaction of the sulfonate groups with the Ag+
counterions, and the position of the Na+ counterions between the
chains.
methanol molecules (mean Na+···O(H)Me distance of 2.34 Å).
The crystal packing is markedly different in the case of
2e·5Me2SO·H2O. In this case, the biscarbene moieties are
organized centrosymmetrically around a cluster of six sodium
ions arranged in a chair-type disposition with intermetallic
distances in the range 3.42−3.93 Å (Figure 4a). This
hexanuclear cluster of sodium ions is supported by coordination
of the oxygen atoms from the eight sulfonate groups, four
molecules of dmso, and two molecules of water, most of which
bridge the edges and the triangular or square-planar faces of the
cluster (Figure 4b).
Figure 5a shows a view of the crystal structure of 3e, whereas
Figure 5b shows the molecular structure of an isolated moiety,
[AgCl(NHC)]2−. The organometallic silver anions and the
sodium counterions are arranged in layers. The coordination
spheres of the Na+ ions are composed by the oxygen atoms of
two (Na(2)) or three (Na(1)) sulfonates from different silver
moieties and one or two dmso molecules in tetrahedral
environments (mean distances Na+···−OSO2R = 2.27 Å,
Na+···OSMe2 = 2.31 Å). The C(1)−Ag−Cl angle (176.6(2)°)
and the Ag−C(1) ((2.063(7) Å) and Ag−Cl (2.298(3) Å)
distances are very similar to those found in the nonsulfonated
analogue [AgCl(IPr)] (175.2°, 2.056 Å, and 2.316 Å,
respectively).34
Synthesis of Platinum(II) NHC Complexes. No reaction
was observed between the silver bis(carbenes) 2b−e and the
precursor cis-[PtCl2(dmso)2] when a mixture of both was
heated in water at 90 °C for 12−14 h (Scheme 5a). Although
the platinum precursor is poorly soluble in water, the same lack
of reactivity was found in mixtures of water and dimethyl
sulfoxide (50%−50%), in which the platinum precursor is
soluble. In contrast, the reaction of one equivalent of the
mono(carbene) silver complexes 3a−d with the above Pt
precursor in dimethyl sulfoxide led to complexes of general
formula Na[PtCl2(dmso)(NHC)] (4a−d) after heating at 80
°C for several hours (Scheme 5b). No transmetalation was
observed with the most sterically hindered silver complex 3e
under the same conditions. These reactions took place with
formation of the cis isomers of 4a−d as the only Pt complexes.
However, hydrolysis of the silver mono(carbenes) with traces
of water, which are difficult to remove from the dmso solvent,
resulted in concomitant formation of variable amounts of the
corresponding imidazolium salt. The lack of an efficient method
for separation of the platinum complexes and the imidazolium
salts prompted us to search for an alternative approach to the
synthesis of complexes 4.
Figure 2. ORTEP diagram (50% probability ellipsoids) of the
organometallic moiety of 2e·6MeOH (H and Na atoms omitted for
clarity). Essentially identical [Ag(NHC)2]3− moieties are present in
the crystal structures of 2e·5Me2SO·H2O and 2eAg+. Selected bond
lengths (Å) and angles (deg) for 2e·6MeOH: Ag−C(1), 2.115(5);
Ag−C(4), 2.108(5); C(1)−Ag−C(4), 178.4(2); N(1)−C(1)−N(2),
103.2(4); N(3)−C(4)−N(4), 103.4(4). Selected bond lengths (Å)
and angles (deg) for 2e·5Me2SO·H2O: Ag−C(1), 2.092(11); Ag−
C(28), 2.099(11); C(1)−Ag−C(28), 178.3(4); N(1)−C(1)−N(2),
102.9(9); N(3)−C(28)−N(4), 103.5(10). Selected bond lengths (Å)
and angles (deg) for 2eAg+: Ag(1)−C(1), 2.111(7); C(1)−Ag(1)−
C(1′), 179.8(4); N(1)−C(1)−N(2), 103.8(6) [both NHCs are
related by a crystallographic 2-fold axis].
angles found in the three structures that contain this unit. The
two carbene ligands are almost linearly coordinated to Ag in all
three structures, with CNHC−Ag-CNHC angles ranging from
178.3(4)° to 179.8(4)°. The NHC rings are twisted with
respect to each other by between 40.2° and 45.5°, thereby
avoiding the steric hindrance that would otherwise arise
between the bulky 2,6-diisopropylphenyl groups. The Ag−
C
NHC distances are in the narrow range 2.092(11)−2.111(5) Å.
These parameters can be compared with those found in
[Ag(IPr)2][SbF6] [180° for the CNHC−Ag−CNHC angle, 37.8°
for the angle between the NHC rings, and 2.13(2) Å for the
Ag−CNHC distances].47
The extended structure of 2eAg+ consists of one-dimensional
chains assembled by bridging Ag+ counterions lying in a
pseudotetrahedral environment between the sulfonate groups
of two adjacent [Ag(NHC)2]3− moieties, with mean short
Ag+···−OSO2R contacts of 2.59 Å (Figure 3). These negatively
charged chains are aligned along the crystallographic b axis and
stacked along the a axis via the sodium cations, which complete
their coordination spheres with two water molecules (mean
Na+···O distance of 2.33 Å for sulfonate and 2.40 Å for water).
The [Ag(NHC)2]3− moieties in 2e·6MeOH are distributed
along the crystallographic a and b axes in a similar manner to
that described for 2eAg+, although they are packed together
exclusively by sodium cations bridging two or three sulfonate
groups (mean Na+···−OSO2R distance of 2.45 Å). The
coordination sphere of these Na+ ions is completed with
E
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Scheme 5. Synthesis of the NHC Platinum(II) Complexes
Na[PtCl2(dmso)(NHC)] (4)
An improved procedure for the synthesis of complexes 4
consisted in the direct reaction of cis-[PtCl2(dmso)2] with
imidazolium compounds 1 in dimethyl sulfoxide at 90 °C in the
presence of sodium hydrogencarbonate as the deprotonating
agent (Scheme 5c). This procedure afforded complexes 4a−d
as analytically pure solids in good to high yields (60−90%),
with carbon dioxide being generated as the only byproduct.
Reaction times ranged from 17 to 25 h, with longer times being
required for the more sterically demanding NHC ligands.
Indeed, pure samples of the complex with the IPr-like NHC
ligand 4e could not be isolated due to the low conversion (65%
after 10 days at 90 °C). A similar result was obtained when
replacing sodium hydrogencarbonate with sodium tert-butoxide
as the base. Despite this, complex 4e could still be characterized
Figure 4. (a) Packing diagram showing the arrangement of
[Ag(NHC)2]3− moieties in 2e·5Me2SO·H2O around a chairlike
hexanuclear cluster of sodium ions. Solvent molecules have been
omitted for clarity. (b) Detail of the hexanuclear cluster of sodium ions
and its coordination sphere formed by oxygen atoms from sulfonate
groups, dmso, and water.
1
by H and 13C NMR spectroscopy and mass spectrometry. A
Figure 5. (a) View of the crystal structure of 3e along the crystallographic a axis highlighting the interactions of Na+ ions with the sulfonate groups of
the [AgCl(NHC)]2− moieties and with the dmso molecules. (b) ORTEP diagram (50% probability ellipsoids) of the organometallic moiety of 3e (H
and Na atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ag−C(1), 2.063(7); Ag−Cl, 2.298(3); C(1)−N(2), 1.354(8);
C(1)−N(1), 1.344(8); C(2)−N(2), 1.359(9); C(3)−N(1), 1.377(9); C(2)−C(3), 1.351(10); C(1)−Ag−Cl, 176.6(2); N(1)−C(1)−N(2),
104.3(5).
F
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similar procedure was used to synthesize the chelate complex 5,
which was isolated analytically pure in 76% yield from the
zwitterionic bisimidazolium salt 1f (Scheme 6).
Scheme 6. Synthesis of the Bis(NHC) Platinum(II) Complex
5
Figure 6. Molecular structure of the organometallic moiety of 4a (50%
probability ellipsoids). Selected bond lengths (Å) and angles (deg):
Pt−C(1), 1.97(2); Pt−Cl(1), 2.359(5); Pt−Cl(2), 2.309(5); Pt−S(3),
2.208(4); C(1)−Pt−Cl(1), 177.6(5); S(3)−Pt−Cl(2), 177.4(2);
C(1)−Pt−S(3), 91.0(5); C(1)−Pt−Cl(2), 88.3(5); Cl(1)−Pt−
Cl(2), 90.4(2); S(3)−Pt−Cl(1), 90.4(2).
The new water-soluble (NHC)Pt(II) complexes were
1
characterized by H, 13C, and 195Pt NMR spectroscopy, mass
spectrometry, and elemental analysis. The ESI-TOF mass
spectra of platinum complexes 4a−e and 5 were recorded in
methanol in negative mode and showed intense peaks
corresponding to the fragments [M − Na]−, with isotopic
distributions matching the calculated patterns. The only clearly
observed 195Pt satellites in their 13C{1H} NMR spectra,
recorded in dmso-d6, were those of the carbenic carbon of
molecules (not shown in Figure 6). Each sodium ion is adjacent
to one sulfonate, with RS(O2)O−···Na+ distances in the range
of 2.8 to 2.9 Å, whereas the nearest oxygen atoms of the other
surrounding sulfonates are at 3.8−4.2 Å. The square-planar
environment of the Pt atom is almost regular, with the metal
lying 0.01 Å out the coordination plane defined by atoms S(3),
C(1), Cl(1), and Cl(2), and the angles between cis bonds
ranging from 88.3° to 91.0°. The dmso is coordinated to the
metal center via the sulfur atom with the sulfur−oxygen moiety
lying parallel to the Pt−C bond and pointing toward the NHC
ligand. This arrangement presumably reduces any steric
interactions between the methyl groups of the dmso and the
heterocyclic ligand. The imidazolium ring is tilted 75.2(4)°
relative to the platinum coordination plane. The Pt−C(1)
distance of 1.97(2) Å is in the range of those reported for
neutral cis-[PtCl2(dmso)(NHC)] complexes (1.88−1.99
Å).21,22,49 The chlorine atoms are mutually arranged in a cis
position, with the Pt−Cl distance of the chloride trans to the
NHC ligand being significantly larger (2.359(5) versus
2.309(5) Å), as expected in light of the high sensitivity of
Pt−Cl distances to the different influences of their trans
ligands.50
Hydration of Terminal Alkynes in Water. The hydration
of alkynes is an important reaction for the synthesis of
aldehydes or ketones. As such, major efforts have been made
over the last few decades to develop new transition-metal
catalysts for this reaction to replace traditional and highly toxic
mercury-based catalysts.51 Important developments in this area
have included the discovery of the first anti-Markovnikov
addition that yielded aldehydes from terminal alkynes, which is
catalyzed by ruthenium(II) complexes,52 or the publication of
gold complexes that are highly active in acidic media.53 In
addition to gold(I) and gold(III), platinum(II) complexes are
among the best metal catalysts for Markovnikov-selective
alkyne hydration. Chatt and Duncanson reported the first
platinum(II)-catalyzed hydration of alkynes using
Na2PtCl4·H2O in ethanolic solution.54 Subsequently, Jennings
and co-workers showed that Zeise’s dimer and Pt(II) halides
were efficient catalysts for the addition of water to inactivated
(electron-rich) terminal and internal alkynes in THF. These
authors noted that the addition of acid was neither necessary
nor advantageous in this case.55 More recently, Atwood and co-
workers used platinum(II) complexes with water-soluble
complex 4b (1J(13CNHC 195Pt) = 1378 Hz). Solvents of
−
relatively high viscosity, such as dimethyl sulfoxide, increase
the contribution of chemical shift anisotropy to the 195Pt
relaxation, thereby often resulting in the broadening or
disappearance of 195Pt satellites.48 Nevertheless, coordination
of the NHC ligands to platinum was supported by the chemical
shifts of the carbenic carbons (144.1−140.8 ppm). The 195Pt
chemical shifts ranged from −3474 to −3529 ppm in
mono(carbenes) 4a−d, whereas the resonance of the bis-
(carbene) 5 was shifted slightly to higher field (−3572 ppm).
The above 13C and 195Pt chemical shifts are in agreement with
those previously reported for cis-[PtCl2(dmso)(NHC)] com-
plexes.21,22,49 The H and 13C{1H} NMR spectra recorded in
1
dimethyl sulfoxide revealed the existence of some asymmetry in
mono(carbenes) 4a−d, as seen by the lack of equivalence
between (a) the two methylene protons α to the imidazole
nitrogen in the sulfonatepropyl chains of 4a−d; (b) the proton
and carbon-13 nuclei in the ortho and meta positions of the
mesityl or 2,6-bis(isopropyl)phenyl groups in 4c−e; and (c)
the two methyl groups of the dmso molecule coordinated to Pt
in the case of complexes with asymmetrically substituted
NHCs. These observations can only be explained by assuming a
cis stereochemistry of the square-planar metal environment,
with a chloride trans to the carbene ligand and with the NHC
ring arranged perpendicularly to the coordination plane
rotating slowly around the Pt−NHC bond on the NMR time
scale.22 The fact that this rotation is faster in water means that
the α-methylene protons of the sulfonatepropyl chains and the
coordinated dmso of 4a,b appear as single resonances in this
solvent. On the other hand, the chelate coordination in
complex 5 is confirmed by the chemical nonequivalence of the
two protons of the methylene bridge in the 1H NMR spectrum,
as would be expected for a boat conformation of the six-
membered metallacycle with a slow boat-to-boat exchange of
conformers on the NMR time scale.
The crystal structure of 4a has been determined by X-ray
diffraction methods. The asymmetric unit contains the
organometallic dianion [PtCl2(dmso)(NHC)]2− (Figure 6),
together with two (disordered) sodium counterions and water
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a c
−
Table 3. Hydration of Phenylacetylene Catalyzed by Complexes 4a−d and 5 in Water
a
b
c
1
Standard reaction conditions: 0.455 mmol of phenylacetylene; 2 mL of H2O. Conversions determined by H NMR spectroscopy. Reaction at
room temperature.
sulfonated phosphines to hydrate alkynols in this solvent.56 To
the best of our knowledge, although NHC-Pt complexes have
not been studied in this reaction, notable decreases in catalyst
loadings were achieved when NHC ligands were involved with
the same conditions (Table 3, entry 19). The reaction also
progressed at room temperature with catalyst 4d, although the
reaction time had to be prolonged to 24 h for completion
(Table 3, entry 12).
gold complexes.57 More recently, Joo
́
and co-workers have
We subsequently studied the hydration of selected terminal
and internal alkynes catalyzed by 4c in neat water at 80 °C
(Table 4). The activity of 4c in the addition of water to para-
substituted phenylacetylenes decreased in the order −H >
−OMe > −CF3 > NO2 (Table 3, entry 10, and Table 4, entries
1−3), thus correlating with the trend of electron-richness of the
alkynes. The heterogeneous nature of the catalytic reaction
cannot, however, be ignored when interpreting these results.
Indeed, the limited reactivity of 3-nitrophenylacetylene may
well be linked to the insolubility of this solid compound in the
reaction medium (all the other phenylacetylenes tested are
liquids under the same conditions). This may well also explain
the lack of reactivity observed for diphenylacetylene.
reported the hydration of alkynes with water-soluble sulfonated
NHC gold(I) complexes in methanol/water mixtures.14,15
The NHC complexes 4a−d and 5 were found to be active
catalysts for the hydration of alkynes in water in the absence of
an acid cocatalyst. The hydration of phenylacetylene was
initially used as a benchmark reaction to compare the different
Pt catalysts and to optimize the reaction conditions (Table 3).
The activity of mono(carbene) complexes 4 was rather similar,
reaching conversions of phenylacetylene to benzophenone of
around 90% at 80 °C in 1 h (Table 3, entries 1, 8, and 15) and
100% in 3−4 h (Table 3, entries 3, 10, and 17) using Pt
loadings of 2.0 mol %. The only exception was complex 4b,
which required 9 h for complete conversion (Table 3, entry 7).
The bis(carbene) complex 5 also showed a similar activity to
that observed for 4a,c,d, completing the reaction in 4 h under
Taking the hydration of phenylacetylene as a reference, the
conversions obtained with 4c can be compared with those
reported for water-soluble NHC gold(I) complexes using the
H
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are very resistant to hydrolysis, even in refluxing water.
However, hydrolysis is favored under some conditions, and
we can especially highlight the fact that the promotion of
hydrolysis by chlorides might explain the problems found in the
literature as regards the synthesis of various NHC silver
complexes in water. Although water is the most natural solvent
for the preparation of water-soluble NHC complexes, the use of
silver complexes for this purpose can prove problematic, as
shown in the synthesis of the water-soluble platinum(II) NHC
complexes 4. Thus, the most suitable procedure for the
synthesis of these complexes was direct metalation of the
corresponding imidazolium salts in dmso, as silver(I) NHC
intermediates failed to transfer their NHC ligands to the Pt
centers in water (but not in dmso). Nevertheless, the transfer of
NHC ligands from Ag to Au, Ru, or Os centers has been
reported to happen in water,14,41,58 and NHC exchange
between Ag centers also occurs rapidly in water on the NMR
time scale in the case of 2a,b. However, steric hindrance
probably makes bis(carbene) complexes kinetically worse
transfer agents and water makes them thermodynamically
more stable. Finally, we have shown that cis-[PtCl2(dmso)-
(NHC)] complexes are active catalysts for the hydration of
alkynes in aqueous media, although their activity is currently
comparable to that reported with platinum(II) complexes of
sulfonated phosphines.
Table 4. Hydration of Selected Alkynes in Water Catalyzed
by Complex 4c
a
b
entry
R1
R2
[Pt] mol % time (h) conversion (%)
1
2
3
4
5
6
7
4-MeO-Ph
4-CF3-Ph
H
2
5
24
24
100
100
2
H
2
3-NO2-Ph
H
2
HO(CH2)3−
HO(CH2)2−
HOCH2−
H
0.5
0.5
0.5
0.5
0.3
0.3
24
24
100
100
CH3
H
c
100 (43 )
c
HOCH2CH2−
H
80 (23 )
a
Standard reaction conditions: 1 mmol of alkyne; 2 mL of H2O.
b
c
1
Conversions determined by H NMR spectroscopy. Ketone yield
1
measured by H NMR spectroscopy using NEt4I as internal standard.
same metal loadings (2.0 mol %). Although a sulfoalkyl-
substituted NHC-Au complex provided conversions of only
17% after 3 h in refluxing water (compared with 100% for 4c in
the same time period at 80 °C, entry 10, Table 3),14 the
reported activities are similar to those observed with 4c with
the use of a bulky IMes-like NHC ligand (90% in 1 h at reflux;
cf. entry 8, Table 3).15 Another reference reaction for 4c is
Atwood’s hydration of water-soluble alkynols with Pt(II)
complexes of sulfonated phosphines.56 Thus, in agreement
with Atwood’s findings, complex 4c directs the hydration of
either 3- or 4-pentynol exclusively to the formation of 5-
hydroxy-2-pentanone, irrespective of the starting alkynol (Table
4, entries 4 and 5). For these reactions, Atwood and co-workers
proposed an anchimeric assistance of the unsaturated bond by
the hydroxyl group of the substrate once the alkyne coordinates
to the Pt center.56 All reactions involving 4c were complete in
only 18 min at 80 °C with Pt loadings as low as 0.5 mol %
(compared with 1 h for the sulfonated phosphine complexes).
In agreement with the mechanism proposed above, the
disfavored participation of the neighboring hydroxyl group
explains the much slower kinetics found for the hydration of
propargyl alcohol and 3-butyn-1-ol under the same conditions
(Table 4, entries 6 and 7). As a consequence, extensive
competitive polymerization was found with the latter alkynes.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed under an
argon atmosphere using standard Schlenk techniques. Unless
otherwise stated, reagents and solvents were used as received from
commercial sources. The complex cis-dichlorobis(dimethyl sulfoxide)-
platinum(II)59 and the imidazolium salts 1a,14 1b,60 1c,d,38 1e,13 and
1f39 were prepared as described in the literature. All solvents were
deoxygenated prior to use. Dimethyl sulfoxide was distilled under
argon over calcium hydride. Deionized water (type II quality) was
1
obtained using a Millipore Elix 10 UV water purification system. H,
13C, 15N, and 195Pt NMR spectra were recorded with a Varian Mercury
300, Unity 300, or Unity 500 Plus spectrometer. Chemical shifts (δ,
parts per million) are quoted relative to SiMe4 (1H, 13C), CH3NO2
(15N), and K2PtCl6 in water (195Pt). They were measured by internal
1
referencing to the 13C or residual H resonances of the deuterated
solvents or by the substitution method in the case of 15N and 195Pt.
Coupling constants (J) are given in hertz. When required, two-
dimensional 1H−13C HSQC and HMBC experiments were carried out
for the unequivocal assignment of 1H and 13C resonances. The
CONCLUSIONS
■
́
Analytical Services of the Universidad de Alcala performed the C, H,
Despite their increasing interest in catalytic or biomedical
applications, the chemistry of water-soluble NHC complexes in
water has barely been studied. One of the aspects considered in
this report is the influence of an aqueous medium on the
preparation and stability of sulfonated NHC silver complexes.
In this respect, we have shown that precipitation of silver
halides makes the difference between protic (such as water and
alcohols) and aprotic solvents. In simple thermodynamic terms,
the sequence of equilibria 2 [AgCl(NHC)] ↔ [Ag(NHC)2]-
[AgCl2] ↔ [Ag(NHC)2]Cl + AgCl is shifted to the right in
protic solvents, partly because hydrogen bonds are involved in
solvation of the chloride anion. This explains why the literature
(and this work) reports the exclusive formation of bis(carbene)
species when water-soluble Ag-NHC complexes are prepared in
this solvent. In addition, we have shown that the sulfonated
mono(carbenes) Na[AgCl(NHC)] (3), which can be prepared
in aprotic solvents such as dmso, evolve reversibly to their
bis(carbene) analogues Na[Ag(NHC)2] (2) when dissolved in
water. The NHC−Ag bonds in these bis(carbene) complexes
and N analyses using a Heraeus CHN-O-Rapid microanalyzer, and the
mass spectra were obtained using an Agilent G3250AA LC/MSD TOF
Multi (MALDI and ESI) mass spectrometer.
Synthesis of the Biscarbene Silver Complexes (2). The silver
NHCs 2b14 and 2c,d38 were prepared according to published
procedures and characterized by comparison of their NMR spectra
with those previously described for all complexes.
Trisodium [1,3-Bis(3-sulfonatepropyl)imidazol-2-ylidene]-
argentate(3−) (2a). A spectroscopically pure sample of 2a was
prepared by heating a mixture of the imidazolium salt 1a (0.1632 g,
0.483 mmol), silver oxide (0.0684 mg, 0.295 mmol), and an excess of
sodium hydroxide (0.0911 g, 2.28 mmol) in water (3 mL) for 36 h at
75 °C. 1H NMR (300 MHz, D2O): δ 7.19 (s, 1H, Imz), 4.12 (t, 3JHH
=
3
6.3, 2H, NCH2), 2.71 (t, JHH = 7.2, 2H, CH2S), 2.13 (m, 2H,
CH2CH2CH2). 13C{1H} NMR (125 MHz, D2O): δ 178.4 (s, Imz C2),
121.3 (s, Imz C4,5), 49.7 (s, NCH2), 47.2 (s, CH2S), 26.4 (s,
CH2CH2CH2). ESI-MS (negative ion, MeOH) m/z: 772.9449 [M −
Na]− (calcd 772.9438) 87%; 750.9629 [M + H − 2Na]− (calcd
750.9619) 85%; 728.9821 [M + 2H − 3Na]− (calcd 728.9799) 100%.
Silver Disodium Bis[1,3-bis(2,6-diisopropyl-4-sulfonatophenyl)-
imidazol-2-ylidene]argentate(3−) (2eAg+). An excess of silver oxide
I
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3
(0.072 g, 0.31 mmol) was added to a solution of the imidazolium salt
1e (0.191 g, 0.335 mmol) in water (7 mL). The mixture was stirred for
4 h at room temperature, then centrifuged and filtered through a plug
of kieselguhr. The resulting solution was evaporated to dryness under
vacuum (4 h, 80 °C, 4 mbar). Compound 2eAg+ was isolated as a light-
2.43 (t, JHH = 7.9, 2H, CH2S), 2.30 (s, 3H, Ar p-Me), 2.11 (m, 2H,
CH2CH2CH2), 1.90 (s, 6H, Ar o-Me). 13C{1H} NMR (75 MHz, dmso-
d6): δ Imz C2 not observed, 138.1 (s, Ar C4), 135.3 (s, Ar C2), 134.0
(s, Ar C1), 128.5 (s, Ar C3), 122.7 (s, Imz C4), 121.9 (s, Imz C5), 49.6
(s, NCH2), 47.5 (s, CH2S), 27.2 (s, CH2CH2CH2), 20.2 (s, Ar p-Me),
16.7 (s, Ar o-Me). ESI-MS (negative ion, MeOH) m/z: 448.9832 [M
− Na]− (calcd 448.9856) 10%; 721.1278 [M − Na − Cl + NHC]−
(calcd 721.1289) 100%. ESI-MS (negative ion, acetonitrile) m/z:
440.0205 [M − Na − Cl + CN]− (calcd 440.0204) 29%; 448.9862 [M
− Na]− (calcd 448.9856) 20%; 721.1293 [M − Na − Cl + NHC]−
(calcd 721.1289) 100%.
1
sensitive white solid (0.415 g, 90%). H NMR (300 MHz, D2O): δ
7.44 (s, 2H, Ar), 7.25 (s, 1H, Imz), 2.13 (m, 2H, CHMe2), 0.86 (d,
3
1
3JHH = 6.8, 6H, CHMe2), 0.58 (d, JHH = 6.8, 6H, CHMe2). H NMR
(300 MHz, dmso-d6): δ 7.75 (s, 1H, Imz), 7.54 (s, 2H, Ar), 2.14 (m,
3
3
2H, CHMe2), 0.99 (d, JHH = 6.8, 6H, CHMe2), 0.69 (d, JHH = 6.8,
6H, CHMe2). 13C{1H} NMR (75 MHz, D2O): δ 181.2 (two d with
1J(13C−109Ag) = 217 and J(13C−107Ag) = 188, respectively, Imz C2),
1
Chlorido[1-(2,6-diisopropylphenyl)-3-(3-sodiumsulfonatepropyl)-
imidazol-2-ylidene]silver(I) (3d). Complex 3d was obtained from 1d
(0.417 g, 1.19 mmol) as a brown solid (0.602 g, 98%) after 20 h of
reaction at 60 °C. 1H NMR (300 MHz, dmso-d6): δ 7.78 (s, 1H, Imz),
7.63 (s, 1H, Imz), 7.50 (t, 3JHH = 7.6, 1H, Ar H4), 7.34 (d, 3JHH = 7.6,
146.8 (s, Ar C3), 144.8 (s, Ar C1), 136.7 (s, Ar C4), 125.1 (d, Imz C4,5,
3J(Ag−13C) = 5.6), 121.7 (s, Ar C2), 28.6 (s, CHMe2), 23.6 (s,
CHMe2), 22.5 (s, CHMe2). ESI-MS (negative ion, MeOH) m/z:
1245.2563 [M − Ag]− (calcd 1245.2563) 36%; 1201.2919 [M − Ag −
2Na + 2H]− (calcd 1201.2929) 100%; 600.1414 [M − Ag − 2Na +
H]2− (calcd 600.1428) 52%.
2H, Ar H3), 4.28 (t, JHH = 6.4, 2H, NCH2), 2.43 (t, JHH = 7.0, 2H,
3
3
CH2S), 2.28 (m, 2H, CHMe2), 2.12 (m, 2H, CH2CH2CH2), 1.14 (d,
3JHH = 6.7, 6H, CHMe2), 1.09 (d, JHH = 6.7, 6H, CHMe2). 13C{1H}
3
Trisodium Bis[1,3-bis(2,6-diisopropyl-4-sulfonatephenyl)-
imidazol-2-ylidene]argentate(3−) (2e). A solution of sodium
chloride (0.0099 g, 0.170 mmol) in water (1 mL) was added dropwise
to a solution of 2eAg+ (0.230 g, 0.170 mmol) in the same solvent (2
mL) in the dark. Formation of a silver chloride precipitate was
observed immediately. Stirring was continued for 30 min at room
temperature; then the solution was centrifuged and filtered through a
plug of kieselguhr. The resulting solution was evaporated to dryness
under vacuum (4 h, 80 °C, 4 mbar). Compound 2e was obtained as a
white solid (0.230 g, 98%). The 1H and 13C NMR data for this
compound are identical to those given above for 2eAg+. ESI-MS
(positive ion, MeOH) m/z: 1269.2538 [M + H]+ (calcd 1269.2539)
10%; 1247.2715 [M − Na + 2H]+ (calcd 1247.2714) 46%; 1225.2858
[M − 2Na + 3H]+ (calcd 1225.2894) 88%; 1203.3044 [M − 3Na +
4H]+ (calcd 1203.3075) 100%. The negative ion ESI mass spectrum is
NMR (75 MHz, dmso-d6): δ Imz C2 not observed, 144.9 (s, Ar C2),
134.6 (s, Ar C1), 129.6 (s, Ar C4), 124.1 (s, Imz C5), 123.4 (s, Ar C3),
121.8 (s, Imz C4), 49.6 (s, NCH2), 47.5 (s, CH2), 27.3 (s,
CH2CH2CH2), 27.2 (s, CHMe2), 23.7 (s, CHMe2), 23.3 (s,
CHMe2). ESI-MS (negative ion, MeOH) m/z: 491.0338 [M − Na]−
(calcd 491.0325) 7.9%; 805.2223 [M − Na − Cl + NHC]− (calcd
805.2228) 100%.
Chlorido[1,3-bis(2,6-diisopropyl-4-sodiumsulfonate)-
phenylimidazol-2-ylidene]silver(I) (3e). Complex 3e was obtained
from 1e (0.682 g, 1.19 mmol) as a white solid (0.701 g, 80%) after 48
1
h of reaction at room temperature. H NMR (300 MHz, dmso-d6): δ
8.01 (s, 1H, Imz), 7.59 (s, 2H, Ar), 2.45 (m, 2H, CHMe2), 1.18 (t, J =
5.7, 12H, CHMe2). 13C{1H} NMR (75 MHz, dmso-d6): δ 181.4 (two
1
1
d with J(13C−109Ag) = 268 and J(13C−107Ag) = 232, respectively,
Imz C2) 149.3 (s, Ar C1), 144.3 (s, Ar C3), 134.1 (s, Ar C4), 124.5 (d,
3J(13C−Ag) = 6.8, Imz C4,5), 120.6 (s, Ar C2), 27.8 (s, CHMe2), 23.7
similar to that found for 2eAg+
.
General Procedure for the Synthesis of Monocarbene
Silver(I) Complexes (3). An excess of silver oxide (0.333 g, 1.44
mmol) was added to a solution of the corresponding imidazolium salt
(1.19 mmol) and sodium chloride (0.070 g, 1.20 mmol) in dimethyl
sulfoxide (5 mL). The mixture was stirred for the time and at the
temperature indicated below for each complex. The precipitate was
then separated by centrifugation and filtration through a plug of
kieselguhr, and the resulting solution evaporated to dryness under
vacuum (6 h, 90 °C, 4 mbar) to give the corresponding NHC silver(I)
complex.
(s, CHMe2), 22.9 (s, CHMe2). ESI-MS (negative ion, MeOH) m/z:
711.0483 [M − Na]− (calcd 711.0495) 1.5%; 653.0820 [M − 2Na −
Cl]− (calcd 653.0915) 6%; 1201.2871 [M − 2Na + NHC + 2H]−
(calcd 1201.2929) 100%.
General Procedure for the Synthesis of Platinum(II)
Complexes 4 and 5. Sodium hydrogencarbonate (0.160 g, 1.91
mmol) was added to a solution of cis-[PtCl2(dmso)2] (0.322 g, 0.764
mmol) and the corresponding imidazolium salt (0.764 mmol) in
dimethyl sulfoxide (5 mL). The mixture was stirred at 90 °C for the
time indicated below, then filtered through a plug of kieselguhr, and
the solvent was partially removed under vacuum to a remaining
volume of 2−3 mL. The platinum complex was then precipitated with
acetone (60 mL), separated by filtration, washed with acetone (3 × 20
mL), and dried under vacuum (2 h, 90 °C, 4 mbar).
Chlorido[1,3-bis(3-sodiumsulfonatepropyl)imidazol-2-ylidene]-
silver(I) (3a). Complex 3a was obtained from 1a (0.398 g, 1.19 mmol)
as an off-white solid (0.547 g, 92%) after 17 h of reaction at 60 °C. 1H
3
NMR (300 MHz, dmso-d6): δ 7.46 (s, 1H, Imz), 4.15 (t, JHH = 6.7,
3
2H, NCH2), 2.38 (t, JHH = 7.0, 2H, CH2S), 2.03 (m, 2H,
CH2CH2CH2). 13C{1H} NMR (75 MHz, dmso-d6): δ 176.8 (br,
Imz C2), 121.4 (s, Imz C4,5), 49.6 (s, NCH2), 47.6 (s, CH2S), 27.1 (s,
CH2CH2CH2). ESI-MS (negative ion, MeOH) m/z: 474.8939 [M −
Na]− (calcd 474.8930) 35%; 750.9629 [M − Na − Cl + NHC + H]−
(calcd 750.9619) 100%.
cis-Dichlorido[1,3-bis(3-sodiumsulfonatepropyl)imidazol-2-
ylidene](dimethyl sulfoxide)platinum(II) (4a). Complex 4a was
obtained from 1a (0.255 g, 0.764 mmol) as a white solid (0.401 g,
75%) after 17 h of reaction at 90 °C. 1H NMR (300 MHz, dmso-d6): δ
7.39 (s, 1H, Imz), 4.51 (m, 1H, NCH2), 4.21 (m, 1H, NCH2), 3.49 (s,
1
3H, Me2SO), 2.49 (m, 2H, CH2S), 2.20 (m, 2H, CH2CH2CH2). H
Chlorido[1-methyl-3-(3-sodiumsulfonatepropyl)imidazol-2-
ylidene]silver(I) (3b). Complex 3b was obtained from 1b (0.243 g,
1.19 mmol) as an off-white solid (0.422 g, 96%) after 17 h of reaction
3
NMR (300 MHz, D2O): δ 7.18 (s, 1H, Imz), 4.37 (t, JHH = 7.3, 2H,
NCH2), 3.42 (s, 3H, Me2SO), 2.84 (m, 2H, CH2S), 2.24 (m, 2H,
CH2CH2CH2). 13C{1H} NMR (75 MHz, dmso-d6): δ 140.4 (s, Imz
C2), 120.9 (s, Imz C4,5), 48.6 (s, NCH2), 47.7 (s, CH2S), 25.6 (s,
CH2CH2CH2). 195Pt NMR (107 MHz, dmso-d6): δ −3518. ESI-MS
(negative ion, MeOH) m/z: 675.9366 [M − Na]− (calcd 675.9355)
73%. Anal. Calcd (%) for C11H20Cl2N2Na2O7PtS3: C, 18.86; H, 2.88;
N, 4.00. Found (%): C, 18.90; H, 3.66; N, 3.84.
1
3
at 60 °C. H NMR (300 MHz, dmso-d6): δ 7.46 (d, JHH = 2.0, 1H,
3
3
Imz), 7.41 (d, JHH = 1.7, 1H, Imz), 4.16 (t, JHH = 6.9, 2H, NCH2),
3
3.76 (s, 3H, NMe), 2.41 (t, JHH = 7.6, 2H, CH2S), 2.03 (m, 2H,
CH2CH2CH2). 13C{1H} NMR (75 MHz, dmso-d6): δ 177.8 (s, Imz
C2), 122.4 (s, Imz C5), 121.4 (s, Imz C4), 49.4 (s, NCH2), 47.6 (s,
CH2S), 37.7 (s, NMe), 27.1 (s, CH2CH2CH2). ESI-MS (negative ion,
MeOH) m/z: 344.9243 [M − Na]− (calcd 344.9230) 29%; 513.0041
[M − Na − Cl + NHC−]− (calcd 513.0037) 100%.
cis-Dichlorido(dimethyl sulfoxide)[1-methyl-3-(3-
sodiumsulfonatepropyl)imidazol-2-ylidene]platinum(II) (4b). Com-
plex 4b was obtained from 1b (0.156 g, 0.764 mmol) as a white solid
Chlorido[1-(3-sodiumsulfonatepropyl)-3-(2,4,6-trimethylphenyl)-
imidazol-2-ylidene]silver(I) (3c). Complex 3c was obtained from 1c
(0.367g, 1.19 mmol) as a brown solid (0.552 g, 98%) after 20 h of
reaction at 60 °C. 1H NMR (300 MHz, dmso-d6): δ 7.74 (s, 1H, Imz),
1
(0.261 g, 60%) after 17 h of reaction at 90 °C. H NMR (500 MHz,
3
3
dmso-d6): δ 7.39 (d, JHH = 2.0, 1H, Imz), 7.35 (d, JHH = 2.0, 1H,
Imz), 4.50 (m, 1H, NCH2), 4.20 (m, 1H, NCH2), 3.85 (s, 3H, NMe),
3
1
7.46 (s, 1H, Imz), 7.05 (s, 2H, Ar), 4.26 (t, JHH = 2.0, 2H, NCH2),
2.50 (m, 2H, CH2S), 2.20 (m, 2H, CH2CH2CH2). H NMR (300
J
dx.doi.org/10.1021/om400228s | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
MHz, D2O): δ 7.11 (d, 3JHH = 1.3, 1H, Imz), 7.05 (d, JHH = 1.6, 1H,
Imz), 4.32 (t, 3JHH = 7.2, 2H, NCH2), 3.78 (s, 3H, NMe), 3.38 (s with
195Pt satellites, 3J(1H−195Pt) = 26.4, 6H, Me2SO), 2.79 (m, 2H,
CH2S), 2.20 (m, 2H, CH2CH2CH2). 13C{1H} NMR (75 MHz, dmso-
6.6, 3H, CHMe2). 13C{1H} NMR (75 MHz, dmso-d6): δ 149.0 (s, Ar
C4), 146.0 (s, Ar C2), 144.9 (s, Ar C2), 144.1 (s, Imz C2), 134.2 (s, Ar
C1), 124.8 (s, Imz C4,5), 120.8 (s, Ar C3), 120.3 (s, Ar C3), 28.1 (s,
CHMe2), 27.6 (s, CHMe2), 25.6 (s, two CHMe2 overlapping), 22.5 (s,
CHMe2), 22.0 (s, CHMe2). ESI-MS (negative ion, MeOH) m/z:
912.0954 [M − Na]− (calcd 912.0920) 1.6%; 444.5531 [M − 2Na]2−
(calcd 444.5516) 100%.
d6): δ 140.8 (s with 195Pt satellites, J(13C−195Pt) = 1378, Imz C2),
1
122.1 (s, Imz C5), 120.9 (s, Imz C4), 48.3 (s, NCH2), 47.6 (s, CH2S),
36.6 (s, NMe), 25.7 (s, CH2CH2CH2). 13C{1H} NMR (75 MHz,
D2O): δ 138.0 (s, Imz C2), 123.6 (s, Imz C5), 121.8 (s, Imz C4), 48.8
(s, NCH2), 47.8 (s, CH2S), 45.4 (s, Me2SO), 44.9 (s, Me2SO), 37.1 (s,
NMe), 25.2 (s, CH2CH2CH2). 15N NMR (50 MHz, dmso-d6): δ
−194.0 (s, NCH2), −206.3 (s, NMe). 195Pt NMR (107 MHz, dmso-
d6): δ −3529. ESI-MS (negative ion, MeOH) m/z: 545.9673 [M −
Na]− (calcd 545.9655) 19%. Anal. Calcd (%) for
C9H17Cl2N2NaO4PtS2: C, 18.95; H, 3.00; N, 4.91. Found (%): C,
18.53; H, 3.64; N, 4.76.
cis-Dichlorido{bis[3-(3-sodiumsulfonatepropyl)imidazol-2-
ylidene]methane}platinum(II) (5). Complex 5 was obtained from 1f
(0.300 g, 0.764 mmol) as a white solid (0.408 g, 76%) after 34 h of
1
reaction at 90 °C. H NMR (500 MHz, dmso-d6): δ 7.51 (d, J = 1.9,
3
2
2H, Imz), 7.34 (d, JHH = 1.9, 1H, Imz), 6.13 (d, JHH = 13.2, 1H,
2
NCH2N), 5.91 (d, JHH = 12.9, 1H, NCH2N), 4.75 (m, 2H, NCH2),
4.18 (m, 2H, NCH2), 2.46 (m, 2H, CH2S), 2.27 (m, 2H, CH2S), 2.06
(m, 4H, CH2CH2CH2). 13C{1H} NMR (75 MHz, dmso-d6): δ 143.6
(s, Imz C2), 121.9 (s, Imz C5), 121.0 (s, Imz C4), 62.3 (s, NCH2N),
48.5 (s, CH2S), 48.5 (s, NCH2), 27.2 (s, CH2CH2CH2). 15N NMR (50
MHz, dmso-d6): δ −190.0 (s, NCH2), −200.0 (s, NCH2N). 195Pt
NMR (107 MHz, dmso-d6): δ −3572. ESI-MS (negative ion, MeOH)
m/z: 677.9590 [M − Na]− (calcd 677.9590) 7.3%; 620.0033 [M −
2Na − Cl]− (calcd 620.0010) 24%. Anal. Calcd (%) for
C13H18Cl2N4Na2O6PtS2: C, 22.23; H, 2.58; N, 7.98. Found (%): C,
22.39; H, 3.09; N, 7.46.
cis-Dichlorido[1-(3-sodiumsulfonatepropyl)-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene](dimethyl sulfoxide)platinum-
(II) (4c). Complex 4c was obtained from 1c (0.236 g, 0.764 mmol)
as a pale yellow solid (0.479 g 93%) after 23 h of reaction at 90 °C.
The solid was isolated by evaporation of the dimethyl sulfoxide
solution to dryness and drying the resulting solid for 24 h at 90 °C and
1
3
4 mbar. H NMR (500 MHz, dmso-d6): δ 7.66 (d, JHH = 2.0, 1H,
Imz), 7.42 (d, 3JHH = 2.0, 1H, Imz), 7.07 (s, 1H, Ar), 7.04 (s, 1H, Ar),
4.71 (m, 1H, NCH2), 4.30 (m, 1H, NCH2), 3.47 (s, 3H, Me2SO), 2.86
(m, 2H, CH2S), 2.81 (s, 3H, Me2SO), 2.33 (m, 2H, CH2CH2CH2),
2.31 (s, 3H, Ar p-Me), 2.18 (s, 3H, Ar o-Me), 2.03 (s, 3H, Ar o-Me).
13C{1H} NMR (125 MHz, dmso-d6): δ 141.7 (s, Imz C2), 138.2 (s, Ar
C4), 135.5 (s, Ar C2), 135.2 (s, Ar C2), 134.1 (s, Ar C1), 128.7 (s, Ar
C3), 128.3 (s, Ar C3), 123.3 (s, Imz C4), 121.4 (s, Imz C5), 49.0 (s,
NCH2), 47.9 (s, CH2S), 45.0 (s, Me2SO), 44.0 (s, Me2SO), 25.7 (s,
CH2CH2CH2), 20.1 (s, Ar p-Me), 18.7 (s, Ar o-Me), 17.7 (s, Ar o-Me).
15N NMR (50 MHz, dmso-d6): δ −190.9 (s, NCH2), −192.0 (s, NAr).
195Pt NMR (107 MHz, dmso-d6): δ −3478. ESI-MS (negative ion,
General Method for the Hydration of Alkynes in Water. The
reaction of phenylacetylene and water is given as an example.
Phenylacetylene (0.05 mL g, 0.455 mmol), the corresponding
platinum complex (9.1 μmol, 2 mol %), and water (2 mL) were
introduced into an ampule tube equipped with a PTFE valve. The
mixture was vigorously stirred at the temperature and for the time
specified in Tables 3 and 4. After cooling to room temperature, sodium
chloride (1 g) was added to the resulting emulsion to facilitate the
separation of layers, and the organics were extracted with diethyl ether
(3 × 15 mL). The combined ethereal layers were dried over MgSO4,
and the solvent was removed under vacuum (25 °C and 500 mbar).
MeOH) m/z: 650.0301 [M − Na]− (calcd 650.0281) 100%. Anal.
Calcd (%) for C17H25Cl2N2NaO4PtS2: C, 30.27; H, 3.74; N, 4.15.
Found (%): C, 30.00; H, 3.92; N, 3.62.
1
Conversions were determined by integration of the H NMR spectra.
X-ray Crystallographic Studies. Suitable single crystals were
obtained by slow diffusion of diethyl ether into a dmso solution
(2eAg+), dichloromethane into a methanol solution (2e·6MeOH),
acetone into a dmso solution (2e·5Me2SO·H2O and 3e), or acetone
into an aqueous solution (4a). A summary of crystal data, data
collection, and refinement parameters for the structural analyses is
given in Table S1 (Supporting Information). Crystals were glued to a
glass fiber using an inert polyfluorinated oil and mounted in the low-
temperature N2 stream (200 K) of a Bruker-Nonius Kappa-CCD
diffractometer equipped with an area detector and an Oxford
Cryostream 700 unit (2e·6MeOH and 2eAg+) or at 100 K
(2e·5Me2SO·H2O) or 296 K (3e and 4a) in a Bruker Kappa Apex
II diffractometer.
c i s - D i c h l o r i d o [ 1 - ( 2 , 6 - d i i s o p r o p y l p h e n y l ) - 3 - ( 3 -
sodiumsulfonatepropyl)imidazol-2-ylidene](dimethyl sulfoxide)-
platinum(II) (4d). Complex 4d was obtained from 1d (0.268 g,
0.764 mmol) as a pale yellow solid (0.492 g, 90%) after 25 h of
reaction at 90 °C. The solid was isolated by evaporation of the dmso
solution to dryness and drying the resulting solid for 24 h at 90 °C and
1
3
4 mbar. H NMR (500 MHz, dmso-d6): δ 7.67 (d, JHH = 2.0, 1H,
Imz), 7.545 (d, 3JHH = 2.0, 1H, Imz), 7.541 (t, 3JHH = 7.8, 1H, Ar H4),
7.40 (d, 3JHH = 7.9, 1H, Ar H3), 7.35 (d, 3JHH = 7.9, 1H, Ar H3), 4.83
(m, 1H, NCH2), 4.28 (m, 1H, NCH2), 3.43 (s, 3H, Me2SO), 3.14 (m,
1H, CHMe2), 2.64 (s, 3H, Me2SO), 2.59 (m, 2H, CH2S), 2.53 (m, 1H,
CHMe2), 2.33 (m, 2H, CH2CH2CH2), 1.29 (d, J = 6.6, 3H, CHMe2),
1.20 (d, J = 6.6, 3H, CHMe2), 1.04 (d, J = 6.6, 3H, CHMe2), 0.88 (d, J
= 6.6, 3H, CHMe2). 13C{1H} NMR (125 MHz, dmso-d6): δ 146.6 (s,
Ar C2), 145.9 (s, Ar C2), 142.7 (s, Imz C2), 133.7 (s, Ar C1), 129.7 (s,
Ar C4), 124.6 (s, Imz C5), 123.6 (s, Ar C3), 123.3 (s, Ar C3), 120.9 (s,
Imz C4), 49.2 (s, NCH2), 47.7 (s, CH2S), 27.4 (s, CHMe2), 27.3 (s,
CHMe2), 26.4 (s, CH2CH2CH2), 25.9 (s, CHMe2), 25.7 (s, CHMe2),
22.2 (s, CHMe2), 22.0 (s, CHMe2). 15N NMR (50 MHz, dmso-d6): δ
−190.5 (s, NCH2), −193.7 (s, NAr). 195Pt NMR (107 MHz, dmso-
d6): δ −3474. ESI-MS (negative ion, MeOH) m/z: 692.0782 [M −
Na]− (calcd 692.0750) 100%. Anal. Calcd (%) for
C20H31Cl2N2NaO4PtS2: C, 33.52; H, 4.36; N, 3.91. Found (%): C,
32.45; H, 4.69; N, 3.80.
Intensities were collected using graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). Data were measured with exposure times of
19 s per frame for 2e·6MeOH (13 sets; 599 frames; phi/omega scans;
1.9° scan-width), 120 s per frame for 2e·5Me2SO·H2O (3 sets; 592
frames; phi/omega scans; 0.5° scan-width), 171 s per frame (4 sets;
288 frames; phi/omega scans; 1.9° scan-width) for 2eAg+, 10 s per
frame for 3e (5 sets; 1662 frames; phi/omega scans; 0.5° scan-width),
and 5 s per frame for 4a (10 sets; 3812 frames; phi/omega scans; 0.5°
scan-width). Raw data were corrected for Lorenz and polarization
effects. The structures were solved by direct methods, completed by
subsequent difference Fourier techniques, and refined by full-matrix
least-squares on F2 (SHELXL-97).61 Anisotropic thermal parameters
were used in the last cycles of refinement for the non-hydrogen atoms.
Absorption correction procedures were carried out using the multiscan
cis-Dichlorido[1,3-bis(2,6-diisopropyl-4-sodiumsulfonatephenyl)-
imidazol-2-ylidene](dimethyl sulfoxide)platinum(II) (4e). Using the
above procedure, only 65% of the imidazolium salt 1e was converted
into 4e after 10 days of reaction at 90 °C (1H NMR analysis). All
attempts to isolate pure samples of complex 4e failed. The
characterization data are nevertheless given. 1H NMR (300 MHz,
SORTAV (semiempirical from equivalent, 2e·6MeOH and 2eAg+ 62 or
)
SADABS programs (2e·5dmso·H2O, 3e, and 4a).63 Hydrogen atoms
were included in the last cycle of refinement from geometrical
calculations and refined using a riding model. All the calculations were
made using the WINGX system.64 In the case of 2e·6MeOH, the
disorder observed for O(21), O(22), and O(23), and for O(56) was
modeled in two positional sets with occupancy factors of 0.54 and
0.46, and 0.81 and 0.19, respectively. In the case of 2eAg+, the disorder
3
dmso-d6): δ 7.81 (s, 1H, Imz), 7.62 (d, JHH = 1.6, 1H, Ar), 7.59 (d,
3JHH = 1.6, 1H, Ar), 3.14 (hept, JHH = 6.6, 1H, CHMe2), 2.97 (hept,
3
3JHH = 6.6, 1H, CHMe2), 1.33 (d, JHH = 6.6, 3H, CHMe2), 1.30 (d,
3
3JHH = 6.6, 3H, CHMe2), 1.05 (d, J = 6.6, 3H, CHMe2), 1.01 (d, 3JHH
=
K
dx.doi.org/10.1021/om400228s | Organometallics XXXX, XXX, XXX−XXX

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observed for O(1), O(2), and O(3) was modeled in two sets with
occupancy factors of 0.85 and 0.15. In this compound, it was not
possible to model some remaining electronic density found in the
difference Fourier map and due to disordered solvent molecules. The
Squeeze procedure65 was used to remove this contribution to the
structure factors.
M. J.; Tessier, C. A.; Cannon, C. L.; Youngs, W. J. Coord. Chem. Rev.
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̈
ASSOCIATED CONTENT
* Supporting Information
■
C. J. Organomet. Chem. 2001, 633, 27−32.
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Crystallographic data and CIF files for compounds 2e·6MeOH,
2e·5Me2SO·H2O, 2eAg+, 3e, and 4a. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: juanc.flores@uah.es (J.C.F.), ernesto.dejesus@uah.es
(E.d.J).
■
(11) Syska, H.; Herrmann, W. A.; Kuhn, F. E. J. Organomet. Chem.
̈
2012, 703, 56−62.
(12) Schonfelder, D.; Nuyken, O.; Weberskirch, R. J. Organomet.
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Notes
Chem. 2005, 690, 4648−4655. Meise, M.; Haag, R. ChemSusChem
2008, 1, 637−642. Roy, S.; Plenio, H. Adv. Synth. Catal. 2010, 352,
1014−1022. Yang, C.-C.; Lin, P.-S.; Liu, F.-C.; Lin, I. J. B.; Lee, G.-H.;
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C.; Poyatos, M.; Peris, E. Organometallics 2011, 30, 684−688. Karimi,
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13, 2071−2077. Luo, F.-T.; Lo, H.-K. J. Organomet. Chem. 2011, 696,
1262−1265.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Spanish Ministerio de
́
Economia y Competitividad (project CTQ2011-24096) and
the Factoria de Crystalizacion
grateful to the Fundacion Carolina for an M.Sc. Fellowship and
to the Universidad de Alcala for a FPI Doctoral Fellowship.
G.F.S. is grateful to the Spanish Ministerio de Educacion for a
́
(CSD2006-00015). E.A.B. is
́
́
(13) Fleckenstein, C.; Roy, S.; Leuthaußer, S.; Plenio, H. Chem.
̈
́
Commun. 2007, 2870−2872.
Postdoctoral Fellowship.
(14) Almas
́
sy, A.; Nagy, C. E.; Ben
́
yei, A. C.; Joo,
́
F. Organometallics
2010, 29, 2484−2490.
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(15) Czegeni, C. E.; Papp, G.; Katho, A.; Joo, F. J. Mol. Catal. A:
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