Access to silver-NHC complexes from soluble silver species in aqueous or ethanolic ammonia

Article abstract of DOI:10.1016/j.jorganchem.2017.04.009

In this communication, we report the use of aqueous ammonia as original conditions for the metalation of imidazol(in)ium ligands. This reaction, performed in homogenous conditions via a soluble silver-ammine complex is a rapid, scalable and often efficient access to silver-NHC complexes. Moreover, modification of the reported reaction conditions allowed the preparation of unprecedented heteroleptic NHC-Ag-phosphine in case of bulky IPr and SIPr ligands.

Full text of DOI:10.1016/j.jorganchem.2017.04.009

Journal of Organometallic Chemistry 840 (2017) 70e74
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Access to silver-NHC complexes from soluble silver species in aqueous
or ethanolic ammonia
a
,
1
,
1
b
a
ꢀ
ꢀ
ꢀ a
ꢀ
Clementine Gibard , Kevin Fauche , Regis Guillot , Laurent Jouffret ,
Mounir Traïkia a, Arnaud Gautier a, Federico Cisnetti a
Universite Clermont Auvergne, CNRS, Sigma Clermont, ICCF, F-63000 Clermont-Ferrand, France
,
*
a
ꢀ
b
ꢀ
ꢀ
^
ꢀ
Institut de Chimie Moleculaire et des Materiaux d’Orsay, Bat. 420, Universite Paris-Sud, UMR CNRS 8182, F-91405 Orsay Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this communication, we report the use of aqueous ammonia as original conditions for the metalation
of imidazol(in)ium ligands. This reaction, performed in homogenous conditions via a soluble silver-
ammine complex is a rapid, scalable and often efficient access to silver-NHC complexes. Moreover,
modification of the reported reaction conditions allowed the preparation of unprecedented heteroleptic
NHC-Ag-phosphine in case of bulky IPr and SIPr ligands.
Received 29 November 2016
Received in revised form
3 March 2017
Accepted 11 April 2017
Available online 13 April 2017
© 2017 Elsevier B.V. All rights reserved.
Keywords:
NHC
Silver
Water
Ammonia
1. Introduction
that [Ag(NHC)2]þ complexes with non-coordinating counter-ions
are difficult to prepare by this method [19]. Moreover, it has been
The preparation of silver(I) complexes of N-heterocyclic car-
benes (NHC) is a very common step to access NHC complexes with
other metallic elements via transmetalation [1,2]. Additionally,
silver(I)-NHC complexes display intrinsic interests in catalysis
either as thermolabile precursors of free carbenes [3] or as active
species containing {NHC-Ag} moieties [4e10]. Moreover, silver-
NHC complexes are bioactive species, displaying antibacterial
properties or cytotoxicity towards cancer cells [11e17]. For all these
purposes, the efficiency of the synthetic conditions for the metal-
NHC complexes is of paramount importance [18].
For the preparation of silver-NHC complexes, the so-called Ag2O
route (usually performed in organic aprotic solvents) may now be
considered as a classical method. Silver(I) oxide acts as base and
metal source [2]. Its advantages include the use of stable reactants,
the lack of special precautions regarding exclusion of moisture and
air, and (in most cases) generally high yields in reasonable reaction
times. Despite being successful in most cases, some limitations may
arise with some problematic systems [2]. It has been highlighted
shown that NHC ligands may yield, either heteroleptic [Ag(NHC)X]
or homoleptic [Ag(NHC)2]þ complexes with a variety of counter-
ions, depending on the experimental conditions both for conven-
tional [20] or functionalized [21,22] NHC ligands. Although the
metalation by Ag2O has been modeled at the molecular level [23],
one of its obvious characteristics is its heterogeneity. While this is
an advantage to separate the excess of silver reactant, the hetero-
geneity of the reaction may also account for reluctant reactions and
for the difficulty to modulate the reaction outcome by variation of
experimental conditions. Thieuleux et al. noted that using
Ag(OC(CF3)3) e a preformed soluble silver metalating agent e may
be an asset, especially if the reaction is to be performed with het-
erogenized azolium salts [24]. Other alternative preparations of
silver-NHC complexes have been reported. Gimeno et al. described
a procedure using AgNO3 as the silver source with a mild base e
K2CO3 e giving only heteroleptic complexes with very high yields
[25]. Cazin et al. reported the synthesis of [AgCl(NHC)] type com-
plexes by refluxing the imidazolium chloride and Ag2O in water for
24 h [26]. Zhu et al. described the efficient (but high temperature)
preparation of silver-NHC complexes in unconventional conditions
(K2CO3/3-chloropyridine) [27]. As an alternative to classical solu-
tion conditions, protocols using ball-milling [19] or microwave
irradiation [28,29] have been recently reported. Overall, alternative
* Corresponding author.
E-mail address: federico.cisnetti@uca.fr (F. Cisnetti).
These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.jorganchem.2017.04.009
0022-328X/© 2017 Elsevier B.V. All rights reserved.

C. Gibard et al. / Journal of Organometallic Chemistry 840 (2017) 70e74
71
methods to prepare silvereNHC complexes remain scarce.
classical Tollens' reagent (reactive dissolution of Ag2O [32] with
excess ammonia in water) and its addition to a solution of the
azolium salt and 2) the addition of ammonia to a suspension of
Ag2O in a solution of the azolium salt. Noteworthy, silver(I)
ammonia complexes e chiefly [Ag(NH3)2]þ e are not oxidizable.
Hence, all syntheses were performed without any experimental
precaution under aerobic conditions, (no improvement of the iso-
lated yields was observed if the reactions were performed under
argon atmosphere). The preparation of heteroleptic complexes
[AgCl(NHC)] (NHC ¼ IPr, SIPr) proceeded using either conditions
(Scheme 1, Table 1). In all cases, silver was introduced in nearly
stoichiometric proportions (1.1 equiv.). With IPr and SIPr, the re-
action was performed in water and ethanol in both conditions
(Table 1, entries 1e6). For conditions 2, longer reaction times were
necessary, as detected visually by the disappearance of black Ag2O
(entries 2,3,5,6). Although the desired [AgCl(NHC)] complexes were
recovered in all cases, the use of water in conditions 2 resulted in
lower yields in some cases, most probably due to solubility issues,
(entries 2 and 5). As unreacted azolium salts were removed
washing the recovered solids, this reduced yield is indicative of
limited conversion. Solubility issues could be mitigated by using
ethanol as solvent (entries 3 and 6) [33]. Finally, to check the
scalability of the present protocol, entries 1 and 6 were repeated at
the 10 mmol scale (~5 g of starting material). The procedure
demonstrated to be easily scalable as the same yields of products
were obtained as in the mmol scale. For IMes and SIMes (entries
7e10), the starting material experienced a full conversion, but
complexes were isolated as mixture of homoleptic [Ag(NH-
C)2](AgCl2) and heteroleptic [AgCl(NHC)] species, with the former
being predominant, whatever the conditions. It is well-known that
silver complexes may undergo ligand redistribution equilibria [34],
and the use of a solvent with a high dielectric constant may favor
the cationic homoleptic complex. The obtention of a mixture of
species was also observed for the metalation of the same azolium
salts with copper in aqueous ammonia [30], although in the latter
case heteroleptic complexes predominated.
We report herein the use of ammonia as modifying reagent in
Ag2O metalations performed in aerobic aqueous or ethanolic con-
ditions. A similar protocol has already proven its interest for the
preparation of copper(I)-NHC complexes [30]. The efficient and
scalable preparation of several widely used metal-NHC complexes
in aqueous aerobic medium is reported. Interestingly, these con-
ditions inspired by classical aqueous coordination chemistry allow
in some cases the determination of the reaction outcome by the
modulation of the experimental conditions. Also, we discovered
that [Ag((S)IPr)(NH3)]þ are stable, soluble species in ethanolic so-
lution and that their reactivity may be harnessed to prepare [Ag((S)
IPr)(phosphine)]þ complexes in very practical one-pot conditions.
2. Experimental
General protocol for the synthesis of Ag-NHC complexes from
azolium chlorides: conditions 1), NHC.HCl (1.0 mmol) was dis-
solved/suspended in 5 mL water. Tollens’ reagent was prepared by
mixing silver(I) oxide (127 mg, 0.548 mmol) and concentrated
ammonia (0.73 mL, 15.3 mol/L, 11 mmol) until complete dissolu-
tion. The reagent was diluted with quantum satis water for a final
volume of 5 mL. An instantaneous precipitation was observed. After
30 min at RT, the white solid was filtered and washed with water.
Conditions 2), NHC.HCl (1.0 mmol) was dissolved/suspended in
10 mL of water or ethanol. Silver(I) oxide (127 mg, 0.549 mmol) and
ammonia (0.65 mL, 15.3 mol/L, 9.9 mmol) were added. After
30e200 min at RT, the white solid was isolated by filtration and
washed with water (IMes, SIMes) or ethanol (IPr, SIPr).
General protocol for the synthesis of Ag-NHC complexes from
azolium hexafluorophosphates: NHC.HPF6 (1.0 mmol) was sus-
pended in 5 mL ethanol. Silver(I) oxide (64.3 mg, 0.277 mmol) and
ammonia (0.65 mL, 15.3 mol/L, 9.9 mmol) were added. The mixture
was stirred at RT during 30 min and filtered. The white solid was
washed with cold ethanol.
General protocol for the synthesis of heteroleptic [Ag((S)IPr)
(phosphine)](PF6) complexes: (S)IPr.HPF6 (1.0 mmol) was sus-
pended in 10 mL absolute ethanol. Silver(I) oxide (116 mg,
0.50 mmol), followed by aqueous ammonia (0.65 mL, 15.3 mol/L,
9.9 mmol) were added and the mixture was treated in an ultra-
sound bath until total dissolution of the product (approximately
5 min). The phosphine (0.95 mmol) was added. The mixture was
treated with ultrasounds during 10 min and stirred at room tem-
perature for a further 30 min. It was filtered and the resulting white
solid was washed with ethanol then with ether.
The reported reaction times were dependent on solubility issues
rather than chemical reactivity. Indeed, a modification of the pro-
tocol in which both partners were soluble before reaction (IPr.HCl
in ethanol and Tollens reagent in water) resulted in reactions pro-
ceeding apparently instantaneously upon mixing both solutions
(Video 1). However, conditions 1 or 2 were more convenient from a
practical point of view [35].
Supplementary video related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2017.04.009.
The propensity of IMes and SIMes to form homoleptic com-
plexes was turned into a facile synthetic procedure (Scheme 2 and
Table 2). Due to the insolubility of azolium salt starting materials in
water (hexafluorophosphate as the counter-ion), ethanol and
conditions 2 were selected. Again, silver(I) was introduced in very
slight excess (0.275 equiv. Ag2O). NMR analysis indicated complete
conversions within 30 min following Ag2O dissolution. At this
point, the complexes could be isolated by filtration as
[Ag(NHC)2](PF6) salts.
3. Results and discussions
As a first set of experiments, we decided to investigate the scope
and limitations of the use of ammonia as basic and complexing
reagent in silver metalation. Hence, we sought to prepare hetero-
leptic [AgCl(NHC)] complexes (NHC ¼ IPr, SIPr, IMes, SIMes) and
homoleptic [Ag(NHC)2]þ complexes (NHC ¼ IMes, SIMes) as PF6
salts.
Initially, by analogy to our previously reported copper metal-
ation protocol (aqueous ammonia and copper(I) chloride in water
as metalation medium) [30], silver chloride was considered as the
metal source to prepare [AgCl(NHC)] complexes. The desired
complexes were indeed obtained in these conditions, albeit in very
limited conversions (~30%) after several hours. This could be
rationalized by considering that solubility of AgCl is ~25 times
lower than the one of CuCl [31]. As the efficient formation of a
soluble metalating agent is crucial in our approach, we decided to
prepare [Ag(NH3)2]þ in solution. This may be achieved using two
protocols, 1) the preparation of a solution corresponding to the
To further exemplify the present method, a new complex
Scheme 1. Preparation of silver-NHC complexes from azolium chlorides. Ar ¼ dipp,
Mes.

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C. Gibard et al. / Journal of Organometallic Chemistry 840 (2017) 70e74
Table 1
Preparation of silver complexes with IPr, SIPr, IMes and SIMes ligands: conditions 1
(preformed Tollens' reagent) or 2 (in situ addition of ammonia).
Entry
Ligand
Cond.
Solvent
Time (min)
Isolated yield
1
2
3
4
5
6
7
8
9
10
IPr
IPr
IPr
SIPr
SIPr
SIPr
IMes
IMes
SIMes
SIMes
1
2
2
1
2
2
1
2
1
2
H2O
H2O
EtOH
H2O
H2O
EtOH
H2O
H2O
H2O
H2O
30
84%a
200
200
30
200
200
30
30
30
30
51%a
82%a
91%a
57%a
92%a
81% (36:64)b
93% (24:76)b
92% (29:71)b
79% (46:54)b
Fig. 1. Thermal ellipsoid plot (50% probability) of the cationic moiety of [Ag(triaz)2]þ
a
100% heteroleptic [AgCl(NHC)] complex.
deduced from X-ray structural data (PFꢀ6 anion and hydrogen atoms omitted for the
b
Obtained as a mixture of heteroleptic and homoleptic complexes.
sake of clarity). Key structural parameter: Ag-C ¼ 2.0872 (11) Å.2
1
of the experiment towards the formation of a homoleptic complex.
This may be connected to our previous hypothesis of the existence
of a similar [Cu(IPr)(NH3)]þ as an intermediate in the synthesis of a
[Cu2(IPr)2 (m
-OH)]þ dinuclear complex [39]. In the case of IPr, we
decided to perform detailed experiments to prove this hypothesis.
First, as the putative [Ag(IPr)(NH3)]þ species is soluble in ethanol,
we precipitated it by addition of diethyl ether and n-pentane in the
medium. We obtained a material containing this species at > 95%
purity (with a trace of homoleptic bis-NHC complex as contami-
nant). The IR spectrum of this solid shows a strong PF6 absorption at
830 cmꢀ1 as well as two bands at high wavenumbers (3394 and
3312 cmꢀ1). The IR data is consistent with a [Ag(IPr)(NH3)](PF6)
composition. Although we were not able to obtain crystals suitable
for X-ray diffraction, the assumption that the compound possesses
a 1:1 metal-NHC ratio is strongly supported spectroscopically by
the 1JAgeC coupling constants deduced from the 13C NMR spectrum
(ESI) [40]. The values (265 Hz and 229 Hz) confirm that only one
NHC ligand is bound to silver. Also, in the 1H NMR spectrum, a
Scheme 2. Preparation of homoleptic Ag-NHC complexes.
Table 2
signal displaying an integral of 3H is observed at
d ~3 ppm (con-
Preparation of homoleptic silver complexes with IMes and SIMes ligand (isolated as
PF6 salts).
centration and temperature sensitive signal) which could be
attributed to the ammonia protons (a similar signal was present for
copper [39]). IR and 1H NMR data are fully consistent with a pre-
vious example of a well-defined ligand-silver-ammine complex
[41].
Entry
Ligand
Cond.
Solvent
Time (min)
Isolated yield
1
2
3
4
IMes
IMes
SIMes
SIMes
1
2
1
2
EtOH
EtOH
EtOH
EtOH
30
30
30
30
97%
97%
88%
97%
To further proof the mononuclear nature of this species, a
mixture corresponding to the outcome of the metalation reaction
of IPr.HPF6 described in Fig. S7 was subjected to a diffusion ordered
2D NMR (DOSY) analysis [42]. This sample contained IPr.Hþ, minor
amounts of [Ag(IPr)2]þand the putative [Ag(IPr)(NH3)]þ species in
an approximate ~1.63:0.12:1 ratio. The diffusion coefficients as
determined from the DOSY NMR data (Fig. S12) were 1,82.
10ꢀ10 m2 sꢀ1, 1,54. 10ꢀ10 m2 sꢀ1 and 1,94. 10ꢀ10 m2 sꢀ1, respectively.
As the measured diffusion coefficient of the latter species is inter-
mediate between the one of the corresponding azolium cation and
homoleptic complex, the formation of higher nuclearity complexes
may be excluded, and the assignment of the observed set of signals
to [Ag(IPr)(NH3)]þ is confirmed.
Interestingly, prolonged heating in refluxing water resulted in
the obtention of a solid consisting essentially in [Ag(IPr)2](PF6) [43]
along with a black insoluble solid (likely Ag2O). The homoleptic
complex could be obtained as monocrystals suitable for X-ray
diffraction. This structure is very similar to the previously reported
SbF6 salt of the same complex [44] (Fig. S39, see ESI for details). In
stark contrast, the same reaction performed with NHC ¼ SIPr
resulted e even in anoxic conditions e in the oxidative degradation
of the NHC as an urea along with the formation of a silver mirror.
Hence, for the obtention of a fairly stable [Ag(NHC)(NH3)]þ species
in solution, the imidazol-2-ylidene core seems to be required.
containing a 1,2,4-triazol-3-ylidene was prepared. The synthesis of
a 1,2,4-triazolium hexafluorophosphate (triaz.HF6) was made easier
in comparison to the method reported by Rovis et al. [36,37] (see
ESI). This starting material was used to prepare a homoleptic
complex in 94% yield (Scheme 2) adapting directly the conditions
described for IMes and SIMes (30 min reaction time). For the sake of
comparison, 1,2,4- triazol-5-ylidene complexes of silver have been
obtained previously using conventional procedures (Ag2O in
dichloromethane) which required lengthy reaction times (12e48 h)
[20,38]. X-ray diffracting crystals of this compound were obtained.
Although the crystals displayed twinning, this was handled as
described in the supporting information and the structure depicted
in Fig. 1 was obtained.
When IPr.HPF6 and SIPr.HPF6 were considered for the prepara-
tion of homoleptic complexes, only unreacted azolium salts were
isolated by filtration. 1H-NMR analysis of a sample of the reaction
medium afforded interesting information. In addition to the signals
corresponding to the azolium salt, another set of signals with
approximately the same integrations was detected (Fig. S7). We
hypothesize that a soluble [Ag(NHC)(NH3)]þ (NHC ¼ IPr, SIPr)
species may be stable in solution and not reactive in the conditions

C. Gibard et al. / Journal of Organometallic Chemistry 840 (2017) 70e74
73
metalation proceeding in aqueous conditions. By conducting the
reaction with a dissolved silver reagent, the reactions were per-
formed akin to classical aqueous coordination chemistry with very
short reaction times and predictable outcomes. In the most favor-
able cases, the crudes proved to be pure NHC complexes, thus
removing the need of purification. This protocol may be helpful for
chemists working in the various areas where silver-NHC complexes
are considered. Moreover, for the bulky imidazol-2-ylidene (S)IPr
ligand, the presence of a relatively stable [Ag ((S)IPr)NH3]þ inter-
mediate could be harnessed to prepare unprecedented [Ag((S)
IPr)(PR3)]þ complexes. Finally, perspectives of this work may arise
in the area of metalation of azolium salts used in (nano)materials
functionalization [24,48,49] where the use of soluble metalating
agents is critical.
Scheme 3. Preparation of heteroleptic NHC-silver-phosphine complexes (R ¼ Cy, Ph, o-
tolyl).
Table 3
Isolated yield of the hexafluorophosphates salts of [Ag(NHC)(Phosphine)]þ
.
NHC
Phosphine
Isolated yield
IPr
SIPr
IPr
SIPr
IPr
SIPr
PPh3
PPh3
PCy3
PCy3
P(o-tol)3
P(o-tol)3
89%
87%
94%
66%
88%
59%
Acknowledgements
ꢀ
The authors thank Melanie Krywalski (undergraduate student)
for experimental support. C.G. and K.F. acknowledge the French
Ministry of National Education, Higher Education and Research for
their PhD fellowships.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2017.04.009.
Abbreviations
dipp
Mes
IPr
2,6-diisopropylphenyl
mesityl
N,N0-Bis(2,6-diisopropylphenyl)imidazol-2-ylidene
N,N0-Bis(2,6-diisopropylphenyl)imidazolin-2-ylidene
N,N0-Bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
N,N0-Bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene
2-Phenyl-6,7-dihydro-5H-pyrrolo[2,1-c][1,2,4]triazol-3-
ylidene
Fig. 2. Thermal ellipsoid plot (50% probability) of the cationic moiety of
[Ag(IPr)(PCy)3]þ deduced from X-ray structural data (PFꢀ6 anion, solvent molecule and
hydrogen atoms omitted for the sake of clarity). Key structural parameters: Ag-
C ¼ 2.119 (8); Ag-P ¼ 2.344 (2) Å; P-Ag-C ¼ 177.34 (19)ꢁ.
SIPr
IMes
SIMes
Triaz
The relative stability of [Ag(NHC)(NH3)]þ prompted us to
investigate whether it could be used to prepare heteroleptic NHC-
Ag-phosphine complexes. Indeed, unlike for their gold and cop-
per congeners [45e47], there is a lack of reliable methods to pre-
pare these potentially useful compounds. To exemplify this
reaction, we prepared six [Ag(NHC)(PR3)]þ complexes (NHC ¼ IPr,
SIPr, R ¼ Cy, Ph, o-tolyl) as hexafluorophosphate salts (Scheme 3). In
optimized conditions, the addition of 0.95 equiv. of the phosphine
ligand to a solution of [Ag(NHC)(NH3)]þ prepared as above afforded
the heteroleptic NHC-Ag-phosphine complexes. These were iso-
lated as hexafluorophosphate salts by simple filtration after 30 min
reaction at room temperature. Isolated yields of the complexes are
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