Dimeric gold bis(carbene) complexes by transmetalation in water

Article abstract of DOI:10.1021/om2009183

Due to its cost, environmental benefits, and safety advantages, water has become more and more important as a solvent for catalytic reactions and constitutes the best environment for biomedical applications. Therefore, watersoluble and water-stable metal complexes containing strong - donor ligands such as N-heterocyclic carbenes (NHCs) are of great interest in modern coordination chemistry. In this paper we present the successful preparation of two new dinuclear gold(I)bis(NHC) complexes in water, by applying the AgNHC transfer route. This green synthetic strategy is valuable for gold(I) compounds involving N-functionalized neutral and dianionic bis(NHC) ligands. These two water-soluble compounds were analyzed by spectroscopic methods and by X-ray diffraction. Furthermore, ab initio and DFT calculations on the corresponding dinuclear gold complexes illustrate the important influence of the electrostatic environment of the dinuclear entity on the aurophilic interactions and help to understand the molecular arrangement presented in this paper.

Full text of DOI:10.1021/om2009183

Article
pubs.acs.org/Organometallics
Dimeric Gold Bis(carbene) Complexes by Transmetalation in Water
Jeremy Cure,† Romuald Poteau,‡ Iann C. Gerber,‡ Heinz Gornitzka,*,† and Catherine Hemmert*,†
́
́
†CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne, F-31077 Toulouse cedex 4, France, and Universite
de Toulouse, UPS, INP, LCC, F-31077 Toulouse, France
‡Universite
́
de Toulouse, INSA, UPS, CNRS; LPCNO (IRSAMC), 135 avenue de Rangueil, F-31077 Toulouse, France
́
S
* Supporting Information
ABSTRACT: Due to its cost, environmental benefits, and
safety advantages, water has become more and more important
as a solvent for catalytic reactions and constitutes the best
environment for biomedical applications. Therefore, water-
soluble and water-stable metal complexes containing strong σ-
donor ligands such as N-heterocyclic carbenes (NHCs) are of
great interest in modern coordination chemistry. In this paper
we present the successful preparation of two new dinuclear
gold(I)−bis(NHC) complexes in water, by applying the Ag−NHC transfer route. This green synthetic strategy is valuable for
gold(I) compounds involving N-functionalized neutral and dianionic bis(NHC) ligands. These two water-soluble compounds
were analyzed by spectroscopic methods and by X-ray diffraction. Furthermore, ab initio and DFT calculations on the
corresponding dinuclear gold complexes illustrate the important influence of the electrostatic environment of the dinuclear entity
on the aurophilic interactions and help to understand the molecular arrangement presented in this paper.
dianionic or neutral bis(NHC) ligand systems by trans-
metalation in water. Moreover, theoretical calculations have
helped us to better understand correlations between gold−gold
interactions12 and ligand arrangement. Owing to the actual
limitation of density functional theory (DFT) methods with
respect to wave function theory (WFT) methods in the
understanding of aurophilicity, a calibration of DFT is provided
for both model dinuclear gold complexes and an Au2···AuH
INTRODUCTION
■
Gold has long played a marginal role in coordination chemistry
involving NHC ligands. In the last few years gold−NHC
complexes have had an increasing impact in all fields of
applications: interesting results have been obtained in catalysis,1
in medicinal chemistry,1a,2 in nanoscience,3 and in the field of
luminescent compounds.1a,4 There are a number of synthetic
ways for the formation of gold−NHC complexes, but two
methods are now mostly applied: (a) direct metalation starting
from imidazolium salts which are transformed into carbenes by
addition of a base followed by a reaction with a gold(I) salt4c,d,5
or (b) the smooth way, by transmetalation via a silver complex
normally carried out in polar solvents as DMSO and DMF or in
dichloromethane.6 The latter method has been reported to be
robust against the presence of water,7 and recently only one
example using water as solvent has been reported.8 Naturally,
water is highly interesting as a cheap and nontoxic solvent.
Furthermore, the development of water-soluble and water-
stable complexes is very important for catalytic reactions in this
medium9 and is essential for biomedical applications.
Recently, we focused our research on the coordinative
behavior of functionalized bis(NHC) ligands.10 In the case of
silver and gold complexes we have studied the influence of the
noncoordinating functionalized side arms on the luminescent
properties and on the metallophilic interactions in dinuclear
complexes.10b Until now, we always used neutral ligands,
forming dicationic dinuclear entities in the case of group 11
metal complexes. Interested in more drastic changes of the
electronic properties of the ligands, we decided to use the
dianionic carboxylate-functionalized bis(NHC) ligands first
reported by Santini and co-workers in 2009.11 Herein we
report the formation of dinuclear gold complexes bearing
small model recently proposed by Pyykko and co-workers.13
̈
RESULTS AND DISCUSSION
■
The first step is the formation of bis(imidazolyl)methane 1 (see
Scheme 1) using modified protocols of Tejeda14 or Strassner.15
The following step, the addition of the functionalized side arms,
has been carried out either as reported by Santini, an N-
alkylation of 1 by 3-bromopropionic acid under reflux during 3
h in toluene, or by heating directly a mixture of 1 and 3-
bromopropionic acid without solvent at 130 °C for 3 h.
Applying the last method allows us to drive the two first
reactions without organic solvents, which constitutes an
interesting feature from an environmental point of view. In
both cases we obtained a 60/40 mixture of di- and monoacid
functionalized bis(imidazolyl)methane salts, which could not be
separated, either by column chromatography or by precipitation
in a methanol/diethyl ether system. Therefore, we changed the
strategy used by Santini and co-workers, and we followed the
synthesis without separation of the two compounds. We
deprotonated the acidic functions by addition of 2 equiv of
Received: September 30, 2011
Published: January 5, 2012
© 2012 American Chemical Society
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the FAB-HRMS spectrum shows the molecular peak [M + H]+
at m/z 1021.1235. For compounds 2−4, absorption, excitation,
and emission spectra have been recorded (see Figure 1 and
Table 1). It is remarkable that all three compounds are
luminescent in water, even at room temperature.
Scheme 1. Synthesis of the Bis(imidazolium carboxylate)
Proligand 2 and the Corresponding Silver(I) Complex 3
a
a
Legend: (i) (1) KOH (4 equiv), TBAB (0.06 equiv), (2) CH2Br2 (1
equiv); (ii) (1) BrCH2CH2COOH (2.2 equiv), 3 h reflux in toluene,
(2) Na2CO3 (2 equiv), 15 min reflux in water or (ii′) (1)
BrCH2CH2COOH (2 equiv), 3 h at 130 °C, (2) Na2CO3 (2 equiv),
30 min reflux in water; (iii) Ag2O (1 equiv), NaCl (1 equiv) in water.
Na2CO3 under reflux in water. The desired product, namely the
carboxylate-functionalized bis(imidazolium) salt 2, could be
obtained by successive precipitations of a methanol solution by
addition of diethyl ether. Some differences concerning the
published spectroscopic data and our data are remarkable. In
the infrared spectrum of neat 2 Santini observed next to COO−
absorptions also COOH ones, attributed to the presence of
both carboxylic and carboxylate species; in our case we observe
Figure 1. Emission spectra (solid lines) and excitation spectra (dashed
lines) of ligand precursor 2 (black), silver(I) complex 3 (red), and
gold(I) complex 4 (blue) measured in water solutions at room
temperature.
Table 1. Emission Data for Ligand Precursor 2, Silver(I)
a
Complex 3, and Gold(I) Complex 4
1
only the COO− stretching bands. In the H NMR experiment
compd
λexc (nm)
λem (nm)
carried out in D2O, we do not observe the imidazolium proton
at 9.18 ppm.11 The 13C NMR spectrum shows clearly the
presence of an imidazolium salt with a resonance located at
161.7 ppm for the C2 carbon atom.
The silver complex 3 was synthesized by the classical way,
adopting nearly the same process as described by Santini. The
upfield shifts of the signals in the 1H NMR spectrum of
complex 3 by comparison to those of ligand 2 and the
observation of the molecular peak [M + H]+ by mass
spectrometry (FAB+) at m/z 841 show clearly the formation
of 3. In the 13C spectrum the resonance corresponding to the
carbene carbon atom C2 could not be observed.
2
3
4
268
340
340
338
422
420
a
Spectra recorded in water solutions at room temperature.
No significant shift differences could be observed between
the silver and the gold complexes, and it has to be noted that
emission around 530 nm, attributed in the literature to
aurophilic interactions,16 is absent. Single crystals, suitable for
an X-ray diffraction study, have been obtained by slow
evaporation of a water solution of 4 (see Figure 2).
As expected, the gold atoms are coordinated in a nearly linear
manner by two carbene carbon atoms coming from two
different bis(NHC) units. The gold···gold distance of 3.589 Å is
at the limit for an aurophilic interaction.17,18 Surprisingly, only
two of the four carboxylate groups of this dinuclear gold units
are involved in the coordination of the sodium cations. The
other two carboxylate anions form hydrogen bonds with water
molecules. Moreover, the sodium atoms form dinuclear units,
in which the Na cations are bridged by two water molecules;
each sodium atom is coordinated by two oxygen atoms from
one carboxylate unit and by one oxygen atom coming from
another carboxylate group, which is bridging two dinuclear
sodium units. This complex arrangement leads to a three-
dimensional coordination polymer in the solid state (see Figure
3).
In order to verify if we can generalize the transmetalation
reactions in water for these kinds of complexes, we synthesized
a dinuclear gold complex bearing a neutral alcohol-functional-
ized bis(NHC) ligand. Two different synthetic strategies have
been employed: (a) direct metalation of the proligand by
AuCl(SMe2) in the presence of sodium acetate as base and (b)
transmetalation of the corresponding silver complex in water
By taking into account the low solubility of 3 in organic
solvents, we decided to carry out the transmetalation reaction
by stirring a mixture of 1 equiv of 3 and 2 equiv of AuCl(SMe2)
in water at room temperature for 12 h (see Scheme 2). The
formed AgCl was filtered off, and the gold complex 4 was
obtained in 84% yield.
Scheme 2. Synthesis of 4 by Transmetalation in Water
1
All spectroscopic data show the formation of 4. In the H
spectrum, the main characteristics are the two methylenic
protons, which become diastereotopic upon complexation by
gold with two distinct doublets located at 6.97 and 6.12 ppm in
D2O.4d,10b The presence of the gold−carbene bond is
unequivocally confirmed by a characteristic singlet resonance
located at 183.5 ppm in the 13C NMR spectrum. Furthermore,
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Scheme 3. Synthesis of the Bis(imidazolium alcohol)
Proligand 6 and the Corresponding Silver(I) Complex 7 and
a
Gold(I) Complexes 8 and 8′
Figure 2. Molecular structure (50% probability level for the thermal
ellipsoids) of the complex 4·2H2O (in the circle). Hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Au−C1 = 2.004(7), Au−C4 = 2.010(7), Au···Au = 3.589; C−
Au−C = 173.0(3).
a
Legend: (i) dibromoethane (2 equiv), 12 h reflux in acetonitrile; (ii)
(1) AuCl(SMe2) (1 equiv), CH3COONa (2.2 equiv), 2 h in DMF at
120 °C, (2) KPF6 (2 equiv), water; (iii) Ag2O (1 equiv), 14 h in
methanol at 50 °C.
Figure 3. Molecular arrangement of 4 in the solid state.
(see Scheme 3). Silver complex 7 was synthesized as described
for other aliphatic bridged systems.10a,b The gold complex
obtained in (a) has been fully characterized and crystallized by
slow evaporation of an acetonitrile solution. All spectroscopic
data are in agreement with the formation of the dinuclear
dimeric gold bis(NHC) complex 8, which has been confirmed
by an X-ray analysis (see Figure 4).
The complex 8′ (8 and 8′ contain the same dication, only the
exogen anions differ), obtained in reaction (b), shows the same
1H and 13C NMR data in D2O as 8, confirming that
transmetalation in water has taken place.
Figure 4. Molecular structure (50% probability level for the thermal
ellipsoids) of complex 8. Hydrogen atoms and noncoordinated anions
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg): Au1−C17 = 2.012, Au1−C1 = 2.0174, Au1···Au2 = 3.3015(2),
Au2−C10 = 2.027(4), Au2−C26 = 2.031(4); C17−Au1−C1 =
175.91(16), C10−Au2−C26 = 179.11(16).
Computational Studies. To help rationalize the impor-
tance of dispersion effects for these kinds of dinuclear gold
complexes containing the bis(NHC) ligands, we carried out a
comparative study using different theoretical approaches. In
order to estimate dispersion effects, a range separated DFT
+WFT approach19 has been used. It intends to properly take
into account the dispersion effects at the origin of the
aurophilic interaction and can be considered as providing
reference data. It has been applied, together with standard DFT
methods, to an unfunctionalized NHC dinuclear gold complex
with (CH2)3 bridges instead of single CH2 bridges. The total
energies reported in Figure 5, as a function of Au···Au bond
lengths, have been calculated thanks to geometry optimizations
performed for frozen Au2 distances. The longer fluxional bridge,
preferred to a single (CH2) functional group, partially prevents
artifactual energy variations due to excessively constrained C−
N bond lengths in a C1 bridge. As expected, DFT curves show
an underestimation of the aurophilic interaction. However, the
B3PW91 functional used in the present paper does not perform
so badly: the optimal Au2 distance is larger than that obtained
with DFT+WFT calculations by ca. 0.3−0.4 Å (3.6 Å for
B3PW91 vs 3.3 Å or 3.2 Å according to the DFT method
considered in DFT+WFT calculations), whereas the well depth
is smaller by 8−10 kcal mol−1. That is, it is slightly higher than
DFT accuracy. Let us now dwell upon the strength on the
aurophilic interaction in these dinuclear gold complexes. There
is a simple correlation between a typical aurophilic interaction
energy and the Au···Au distance:12a De = −(0.0622 × Reb/n)
exp[Re − a)/b], where a and b are obtained from experimental
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Figure 5. Comparison of the energy variation as a function of the
Au···Au bond length for an unfunctionalized NHC dinuclear gold
complex (see inset), calculated by DFT and DFT+WFT methods (all
energies are rescaled with respect to the 5.6 Å energy calculated at the
DFT+WFT level of theory for the isodesmic reaction
(C3N2H3Me)2Au + 2C3H8 → dinuclear gold complex + 4CH4).
Figure 6. Gas-phase optimized geometries for the closed forms A and
the open forms B, with and without sodium cations. Gold−gold
2+
distances in Å: A2−, 3.04; B2−, 3.85; ANa2, 3.70; BNa2, 3.48; BNa4
,
3.47. Two points of view are given for the closed geometries.
data and n is a fitting parameter near Re. This simple equation is
based on the assumed universal gold−gold repulsion A
exp(−BR) which is overcome by a CR−n attraction. According
carboxylate−Na+ electrostatic interactions arise in ANa2,
whereas only two arise in BNa2, yielding a significantly
underestimated stabilization of the open form. As can be seen
in Figure 3, each Na+ cation makes a bridge between two single
units in the solid state, with an optimal Na+−(−OOC)
interaction and a secondary Na+−−O(OC) one, not accounted
for in BNa2. Such an electrostatic effect will be drastically
lowered in strongly polar solvents.20 This is confirmed by PCM
calculations, which show the relative stability of ANa2 in water
by only ΔE = 19 kcal mol−1 with respect to BNa2, without
significant changes in the geometries. The polarizable
continuum surroundings involves a small contraction of the
structure, which manifests itself in the shortening of the (Au2)2+
equilibrium distances by 0.15 and 0.05 Å in A and B,
respectively. The open form should be preferred by complex 4
in water, owing to the ability of the four carboxylate arms to
interact with counterions, whereas they are not available for
additional electrostatic interactions in ANa2. The resulting
complex, BNa42+, is shown in Figure 6.
The gas phase calculated Au−Au bond lengths in BNa2 (3.48
Å) and in BNa42+ (3.47 Å) are 0.2 Å shorter than in ANa2, and
the corresponding distance in the X-ray structure (3.59 Å) is
exactly between these values. DFT AuI−C bond lengths, which
are calculated to be 2.03−2.04 Å in all four neutral or anionic
complexes, compare well with experiment (2.01 Å). A relative
rotation by 30° of the two Au(NHC)2 groups around the Au−
Au axis is obtained in the open form BNa2, in agreement with
symmetry considerations analogous to those described by
Hoffmann and co-workers.21 No ring puckering occurs in the
crystal monomers, probably owing to the various intermolec-
ular stabilizing factors, which can easily counterbalance the
stabilization energy gained by the distortion of the ring. This is
illustrated by the unsubstituted counterpart of the open form,
in which the ring puckering involves only a slight stabilization
by ca. 2 kcal mol−1 with respect to the optimized geometry
forced to remain planar. Although DFT calculations have a
moderate credibility for accurately describing aurophilic
interactions, a Kohn−Sham orbital analysis is however given
in the present paper as an indication of the intrinsic propensity
of the DFT geometry to be distorted. A careful analysis of the
molecular orbitals (MOs) close to the HOMO shows that the
to the parameters given by Pyykko in ref 12a, the difference
̈
between the aurophilic attraction energies at 3.2 and 3.6 Å is 4
kcal mol−1: i.e., half the LDA+WFT value. According to test
calculations reported in the Experimental Section, the so-called
basis set superposition error is small in such DFT+WFT
calculations. The 8−10 kcal mol−1 stabilization energy obtained
with an appropriate treatment of dispersion force, together with
this simple relationship between interaction energy and Au···Au
distances, probably border the actual aurophilic energy, with a
somewhat highest credibility of the DFT+WFT method owing
to the nature of the method and to the calibration calculations
given in the Experimental Section. It is also noteworthy that all
energy curve plots in Figure 5 are very shallow. It is remarkably
illustrated by the energy difference between the equilibrium and
the 3.2 Å geometries obtained at the B3PW91 level of
calculation, which is close to 2 kcal mol−1. Given the energy
differences between all isomers considered in this paper, as well
as steric effects induced by substituents, one can argue that the
B3PW91 functional should do significantly better than being
semiqualitative. This will be confirmed later in the present
paper as well as in deeper ongoing studies by the comparison of
DFT optimized geometries with X-ray structures.
Therefore, in order to better understand the molecular
structure of 4, we performed DFT computational studies based
on the B3PW91 functional. Ignoring the polymolecular
organization and interactions, calculations have been carried
out on the central unit of 4. Concerning the arrangement of the
side arms, two basic situations could be distinguisheda closed
form A, where the side arms come together, and an open form
B, where the carboxylate groups are in opposite directions.
Starting on the central unit without any counterions, the open
form B2− is more stable than the closed form A2− by 39 kcal
mol−1 in the gas phase and by 10 kcal mol−1 in water (see
Figure 6). In order to be closer to the molecular situation found
in the crystalline solid state and to have an overall neutral
system, two sodium cations have been added to the dianionic
model. Calculations show that the closed form ANa2 is favored
in this case in vacuum by ΔrG° = 68 kcal mol−1 toward the
open form BNa2 (Figure 6), in contradiction with the X-ray
structure. Note that, when considering a single unit, four
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was prepared by following a literature procedure.10a All reactions
involving silver compounds were performed with the exclusion of light.
1H (250 MHz) and 13C spectra (63 MHz) were recorded at 298 K on
a Bruker ARX250 spectrometer in D2O, CDCl3, or DMSO-d6 as
solvent (chemical shifts have been calculated by using the field
correction method integrated in the Bruker NMR spectrometer).22
Elemental analyses were carried out by the “Service de Microanalyse
du Laboratoire de Chimie de Coordination” (Toulouse). Mass
spectrometry analyses were performed on a Nermag R1010 (FAB+
in m-nitrobenzyl alcohol (MNBA)) or a Perkin-Elmer SCIEX Api 365
distortion does not involve a significant stabilization of any
MOs, with the exception of that depicted in Figure 7. In the
́
(ES in MeOH) spectrometer, by the “Service de Spectrometrie de
Masse de Chimie UPS-CNRS” (Toulouse). IR spectra were obtained
on a Perkin-Elmer 100 FT-IR spectrometer. UV−vis spectra were
recorded with a Perkin-Elmer Lambda 35 UV−vis spectrophotometer,
with cuvettes of 1 cm path length. Fluorescence spectra were
performed by using a SAFAS FLX-Xenius spectrofluorimeter. Samples
were placed in a four-sided quartz fluorescence cuvette, and data were
recorded in water at room temperature. The concentrations of the
samples used for fluorescence are 8.35 × 10−5 (2), 3.91 × 10−5 (3), and
3.92 × 10−5 mol/L (4), respectively.
Preparation of the Carboxylate-Functionalized Diimidazo-
lium Salt and the Related Silver(I) and Gold(I) Complexes. The
following picture describes the numbering of H (1H NMR) and C
(13C NMR). These notations are used in the following section.
Figure 7. Comparison of the carbene−gold MO involved in the
stabilization of the distorted geometry of the NHC−gold compounds.
planar form this carbene−gold bonding MO is antibonding
between the two gold subunits, whereas it becomes slightly
bonding in the distorted geometry. This is confirmed by the
shortening of the Au−Au bond length (3.61 Å vs 3.47 Å), as
well as by the Au−Au Wiberg bond index, which, albeit very
small in such an aurophilic interaction, is slightly increased in
the distorted structure (0.11 vs 0.09).
Finally, it is interesting to notice that the removal of
counterions has a significant influence on the metallophilic
interaction, with a Au(I)−Au(I) bond length significantly
lowered in A2− (3.04 Å) and increased in B2− (3.85 Å). An
interesting feature, observed on each gold subunit in A2−, is the
rather short distance between one carboxylate group and a gold
atom, in line with the propensity of carboxylate groups to
interact with available cationic functional groups or atoms. This
is evidenced by the AuI−C(OO−) distance of 3.13 Å, shortened
by almost 1 Å with respect to ANa2.
All these calculations clearly show that modification of the
electrostatic interactions with the surroundings of these
dinuclear gold−NHC complexes can drastically influence the
metallophilic interaction.
Synthesis of 1,1′-Methylenebis(1H-imidazole) (1).14,15 A three-
necked flask was charged with a mixture of imidazole (21.78 g, 320
mmol, 2 equiv), tetrabutylammonium bromide (3.088 g, 9.6 mmol,
0.06 equiv), and powdered potassium hydroxide (35.84 g, 640 mmol, 4
equiv), and the mixture was stirred for 1 h at room temperature. To
the resulting liquid was slowly added dibromomethane (11.26 mL, 160
mmol, 1 equiv) over a period of 2 h. After complete addition, a solid
was obtained. Purification of the crude product was performed by
sublimation to yield 1 as a white solid (5 h, 180 °C). Yield: 14.23 g,
60%. Anal. Calcd for C7H8N4: C, 56.74; H, 5.44; N, 37.81. Found: C,
56.50; H, 5.48; N, 37.99. 1H NMR (250 MHz, CDCl3): δ (ppm) 7.67
(s, 2H, H2), 7.14 (s, 2H, H4), 7.01 (s, 2H, H5), 6.02 (s, 2H, H6). 13C
NMR (75 MHz, CDCl3): δ (ppm) 136.6 (2C, C5), 131.2 (2C, C4),
118.2 (2C, C2), 56.3 (2C, C6).
Synthesis of 3,3′-[Methylenebis(1H-imidazol-3-ium-1,3-diyl)-
dipropanoate (2).11 Method A. A mixture of 1 (1.48 g, 10 mmol)
and 3-bromopropionic acid (3.37 g, 22 mmol, 2.2 equiv) was refluxed
in 100 mL of toluene for 3 h. After the mixture was cooled to room
temperature, the volatiles were removed under vacuum to afford a
white-yellow solid. To the resulting product dissolved in 20 mL of
H2O was added Na2CO3 (1.06 g, 20 mmol, 60 mL, 2 equiv), and the
mixture was refluxed at 100 °C for 15 min to afford a white powder.
Purification of the crude product was performed by successive
precipitations of methanolic solutions by addition of diethyl ether at 0
°C to give a white powder. Yield: 848 mg, 29%.
CONCLUSIONS
■
In summary, these results suggest that generally gold−NHC
complexes could be formed in water by starting from water-
soluble silver complexes, which themselves could be synthe-
sized in water as first shown by Youngs.7c The application of
this “organic-solvent-free-synthesis” approach to mononuclear
systems and to transmetalation reactions via metals other than
gold are under intense investigation because water as solvent
represents a series of attractive features. A general study
comparing classical DFT calculations with calculations more
adopted for dispersion effects shows that in our cases of
dinuclear complexes the geometries are strongly influenced by
electrostatic interactions between the surroundings of the metal
core and the metal core itself, stronger than aurophilic
interactions, and for this, the B3PW91 functional represents a
good approach for this kind of problem.
Method B. A mixture of 1 (1.03 g, 6.94 mmol) and 3-
bromopropionic acid (2.123 g, 13.88 mmol, 2 equiv) was heated at
130 °C for 3 h. After the mixture was cooled to room temperature, the
volatiles were removed under vacuum to afford a white-yellow solid. It
was dissolved in 30 mL of H2O, and Na2CO3 was added (1.47 g, 20
mmol, 60 mL, 2 equiv). The mixture was refluxed at 100 °C for 30 min
to afford a white powder. Purification of the crude product was
performed as described above. Yield: 657 mg, 32%. Anal. Calcd for
C13H16N4O4: C, 53.42; H, 5.52; N, 19.17. Found: C, 54.01; H, 5.70;
N, 18.75. 1H NMR (250 MHz, D2O): δ (ppm) 7.71 (s, 2H, H4), 7.61
(s, 2H, H5), 6.64 (s, 2H, H6), 4.42 (t, 4H, H8, 3J = 6.3 Hz), 2.73 (t, 4H,
EXPERIMENTAL SECTION
■
General Synthetic Considerations. Unless otherwise stated, all
reactions were performed in air. CH3CN and DMF were dried over 4
Å molecular sieves. All other reagents were used as received from
commercial suppliers. 1,1′-Methylenebis(1H-imidazole) (1),14,15 3,3′-
[methylenebis(1H-imidazol-3-ium-1,3-diyl)dipropanoate (2), and the
resulting silver complex 3 were prepared by following modified
literature procedures.11 1-(1H-Imidazol-1-yl)-2-methyl-2-propanol (5)
3
H7, J = 6.3 Hz). 13C NMR (75 MHz, D2O): δ (ppm) 177.7 (2C,
COO−), 161.7 (2C, C2), 123.7 (2C, C4), 122.0 (2C, C5), 58.9 (1C,
C6), 47.5 (2C, C7), 37.0 (2C, C8). IR (cm−1): ν 1575, 1390 (COO−).
MS (ES+): m/z 293.1 [M + H]+, 315.0 [M + Na]+, 337.0 [M + 2Na −
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H]+. UV−vis (H2O, room temperature): λmax/nm (ε/103 M−1cm−1)
269 (1.4).
H6), 3.95 (s, 8H, H7), 2.06 (s, 4H, OH), 1.04 (s, 24H, H9). 13C NMR
(63 MHz, DMSO-d6): δ (ppm) 123.8 (2C, C5), 122.0 (2C, C4), 69.1
(2C, C8), 61.8 (2C, C7), 51.2 (2C, C6), 27.6 (4C, C9). MS (FAB+): m/
z 907 [M − Br]+, 827 [M − 2Br − H]+.
Silver(I) Complex 3.11 Under an atmosphere of nitrogen, Ag2O
(0.396 g, 1.71 mmol) was added to a deoxygenated water solution (10
mL) of 2 (0.500 g, 1.71 mmol). The reaction mixture was stirred for
12 h at 50 °C. NaCl (0.110 g, 1.88 mmol, 1.1 equiv) was added at
room temperature, and the reaction mixture was stirred for 15 min.
The resulting suspension was filtered through a pad of Celite, and the
solution was concentrated under vacuum to afford a brown solid. It
was purified by selective precipitation of a methanolic solution by
addition of diethyl ether and dried under vacuum. Yield: 285 mg, 40%.
Anal. Calcd for C26H28N8O8Ag2: C, 39.22; H, 3.54; N, 14.07. Found:
C, 39.30; H, 3.93; N, 14.29. 1H NMR (250 MHz, D2O): δ (ppm) 7.41
(s, 4H, H4), 7.25 (s, 4H, H5), 6.41 (bs, 4H, H6), 4.32 (t, 8H, H8, 3J =
Gold(I) Complex 8. Under an atmosphere of nitrogen, sodium
acetate (92 mg, 1.125 mmol) was added to a mixture of 6 (234 mg, 0.5
mmol) and [Au(SMe)2Cl] (147 mg, 0.5 mmol) in DMF (5 mL) at
100 °C. The mixture was then heated to 120 °C, and this temperature
was maintained for 2 h. After the mixture was cooled to room
temperature, the solvent was concentrated to ca. 1 mL, and addition of
CH2Cl2 (3 mL) and hexane (20 mL) afforded a white solid. After
filtration, the crude product was dissolved in water (5 mL), and
potassium hexafluorophosphate (0.074 g, 1 mmol) was added. A white
hygroscopic solid precipitated. The desired product was obtained after
filtration as a colorless solid. Yield: 235 mg, 73%. Crystals suitable for
X-ray diffraction analysis were grown by slow evaporation of an
acetonitrile solution of 8 at room temperature. Anal. Calcd for
C32H52N8O4P21F12Au2: C, 29.64; H, 4.04; N, 8.64. Found: C, 28.35; H,
3.88; N, 8.12. H NMR (250 MHz, CD3CN): δ (ppm) 7.22 (d, 4H,
H4, 3,4J = 1.7 Hz), 7.11 (d, 4H, H5, 3,4J = 1.7 Hz), 4.82 (s, 8H, H6),
4.11 (s, 8H, H7), 3.41 (s, 4H, OH), 1.16 (s, 24H, H9). 1H NMR (250
MHz, D2O): δ (ppm) 7.20 (bs, 4H, H4), 7.16 (bs, 4H, H5), 4.83 (s,
8H, H6), 4.06 (s, 8H, H7), 1.12 (s, 24H, H9). 13C NMR (63 MHz,
CD3CN): δ (ppm) 184.2 (2C, C2), 123.8 (2C, C4), 121.5 (2C, C5),
69.6 (2C, C8), 61.1 (2C, C7), 50.1 (2C, C6), 26.5 (4C, C9). 31P NMR
(121 MHz, CD3CN): δ (ppm) −144.6 (septuplet, JPF = 708.8 Hz).
MS (FAB+): m/z 1151 [M − PF6]+.
3
6.2 Hz), 2.60 (t, 8H, H7, J = 6.2 Hz). 13C NMR (63 MHz, D2O): δ
(ppm) 178.7 (4C, COO−), 122.6 (4C, C4), 121.2 (4C, C5), 64.5 (2C,
C6), 49.3 (4C, C7), 39.6 (4C, C8). IR (cm−1): ν 1572, 1399 (COO−).
MS (ES+): m/z 841.0 [M + H]+, 863.3 [M + Na]+. UV−vis (H2O,
room temperature): λmax/nm (ε/104 M−1cm−1) 212 (1.9), 400 (0.1).
Gold(I) Complex 4. Under an atmosphere of nitrogen, chloro-
(dimethyl sulfide)gold(I) (Au(SMe2)Cl; 102 mg, 0.35 mmol) was
added to a deoxygenated water solution (20 mL) of silver complex 3
(145 mg, 0.17 mmol). The reaction mixture was stirred for 12 h at
room temperature. The resulting purple solution was filtered through a
pad of Celite in order to remove AgCl salt, and gold nanoparticles and
the solvent was removed under vacuum to afford a white solid. It was
purified by selective precipitation of a methanolic solution by addition
of methyl tert-butyl ether (MTBE) and dried under vacuum. Yield: 148
mg, 84%. Anal. Calcd for C26H28N8O8Au2Na2: C, 30.60; H, 2.77; N,
10.98. Found: C, 29.72; H, 2.96; N, 10.59. 1H NMR (250 MHz,
D2O): δ (ppm) 7.48 (s, 4H, H4), 7.30 (s, 4H, H5), 6.97 (d, 2H, H6, 2J
Gold(I) Complex 8′. Under an atmosphere of nitrogen, Au(SMe2)
Cl (30 mg, 0.10 mmol) was added to a solution of complex 7 (50 mg,
0.051 mmol) in deoxygenated water (3 mL). A white precipitate of
AgCl formed upon addition of gold(I) salt, and the reaction mixture
was stirred at room temperature for 12 h. After filtration through a pad
of Celite, the solvent was removed under vacuum and the desired
2
3
= 14.0 Hz), 6.12 (d, 2H, H6, J = 14.0 Hz), 4.40 (t, 4H, H8, J = 7.0
Hz), 4.37 (t, 4H, H8, J = 6.8 Hz), 2.67 (t, 8H, H7, J = 6.8 Hz). 13C
NMR (63 MHz, D2O): δ (ppm) 183.5 (4C, C2), 178.4 (4C, COO−),
123.1 (4C, C4), 121.1 (4C, C5), 63.1 (2C, C6), 48.9 (4C, C7), 39.4
(4C, C8). IR (cm−1): ν 1570, 1388 (COO−). MS (ES+): m/z 1021.2
[M + H]+, 1043.2 [M + Na]+. MS (ES−): m/z 1019.3 [M − H]−, 997.2
[M − Na]−. HRMS: m/z 1021.1235 [M + H]+. UV−vis (H2O, room
temperature): λmax/nm (ε/104 M−1cm−1) 212 (1.8), 229 (1.9), 241
(2.1), 253 (2.3).
3
3
1
white solid was obtained. Yield: 53 mg, 90%. H NMR (250 MHz,
D2O): δ (ppm) 7.20 (bs, 4H, H4), 7.16 (bs, 4H, H5), 4.83 (s, 8H, H6),
4.06 (s, 8H, H7), 1.12 (s, 24H, H9). 13C NMR (63 MHz, D2O): δ
(ppm) 183.9 (2C, C2), 123.6 (2C, C4), 122.0 (2C, C5), 70.1 (2C, C8),
60.9 (2C, C7), 50.2 (2C, C6), 26.1 (4C, C9).
Density Functional Theory Calculations. All molecular
calculations have been performed using the Gaussian03 suite of
programs.23 Geometry optimizations and subsequent electronic
property calculations have been carried out in the framework of
density functional theory (DFT), using the B3PW91 hybrid
functional.24 A double-ζ basis set augmented by a set of polarization
functions, namely the standard Pople 6-31G(d,p) basis set, has been
employed for H, C, N, and O atoms.25 Relativistic effective core
potentials developed by the Stuttgart group and their associated basis
sets26 have been used for Au,27 Na,28 and Br.29 This basis set was
augmented with a set of d- or f-polarization functions for Au and Br
atoms (Au, ξf = 1.00 ; Br, ξd = 1.55).30 Geometry optimizations were
performed in the gas phase without symmetry constraints. An
unconstrained geometry optimization of A2− and B2− within the 6-
31+G** basis set for main-group elements has confirmed that their
energy difference is moderately dependent on the basis set (ΔE: −41
kcal mol−1 vs −35 kcal mol−1). The nature of each stationary point was
characterized by Hessian calculations, whereas Gibbs free energies G°
at 298 K were calculated by means of the harmonic frequencies, i.e. by
a straightforward application of the statistical thermodynamic
equations.31 Solvation effects waswere evaluated by means of the
PCM formalism,32 with its default implementation33 included in
Gaussian09.34 Geometries were reoptimized, starting from gas-phase
structures. We have also performed natural population analysis35
(NPA) included in the natural bond orbital (NBO36) routines
available in Gaussian03, yielding the so-called “NBO charges”. This is
preferred to Mulliken population analysis for the calculation of atomic
charges. It confirmsif necessarythe Au(I) character in such
dimeric gold bis(carbene) complexes, with a +0.25e charge. It is rather
insensitive to the presence of counterions or to slight geometrical
changes such as planar vs ring puckering.
Preparation of the Alcohol-Functionalized Diimidazolium
Salt and the Related Silver(I) and Gold(I) Complexes. The
picture below describes the numbering of H (1H NMR) and C (13C
NMR). These notations are used in the following section.
3,3′-(1,2-Ethanediyl)bis[1-(2-hydroxy-2-methylpropyl)-1H-imida-
zol-3-ium] Dibromide (6). A solution of 1-(1H-imidazol-1-yl)-2-
methyl-2-propanol (5;10a 1.139 g, 9.64 mmol) and 1,2-dibromoethane
(362 μL, 4.82 mmol) in acetonitrile (10 mL) was stirred at 80 °C for
12 h. After the mixture was cooled to room temperature, the solvents
were evaporated under vacuum, to afford the desired white solid.
Yield: 1.386 g, 61%. Anal. Calcd for C16H28N4O2Br21: C, 41.04; H,
6.03; N, 11.97. Found: C, 41.07; H, 6.15; N, 11.93. H NMR (250
MHz, DMSO-d6): δ (ppm) 9.02 (bs, 2H, H2), 7.71 (bs, 4H, H4, H5),
5.04 (d, 2H, OH), 4.79 (s, 4H, H6), 4.09 (s, 4H, H7), 1.07 (s, 12H,
H9). 13C NMR (75 MHz, DMSO-d6): δ (ppm) 137.6 (2C, C2), 124.8
(2C, C5), 122.1 (2C, C4), 68.6 (2C, C8), 59.4 (2C, C7), 48.7 (2C, C6),
27.0 (4C, C9). MS (FAB+): m/z 387 [M − Br]+, 307 [M − 2Br − H]+.
Silver(I) Complex 7. Under an atmosphere of nitrogen, a Schlenk
was charged with silver(I) oxide (99.6 mg, 0.43 mmol), 6 (200 mg,
0.43 mmol), and dry methanol (5 mL). The resulting mixture was
stirred at 50 °C for 14 h and then filtered through a pad of Celite. The
solvent was evaporated under vacuum, affording the desired white
powder. Yield: 185 mg, 88%. Anal. Calcd for C32H52N8O4Ag2Br2: C,
38.89; H, 5.30; N, 11.34. Found: C, 38.57; H, 5.61; N, 10.96. 1H NMR
(250 MHz, DMSO-d6) δ (ppm) 7.31 (bs, 8H, H4, H5), 4.73 (s, 8H,
624
dx.doi.org/10.1021/om2009183 | Organometallics 2012, 31, 619−626

Organometallics
Article
Dispersion effects have been evaluated thanks to a coupled DFT
+WFT approach19 implemented in MOLPRO.37 The basic idea of this
scheme is to combine the benefits of both quantum chemistry
approaches. DFT local or semilocal approximations are good in the
treatment of short-range exchange-correlation effects, while they
systematically fail to describe long-range correlations. At the same
time, WFT approaches handle long-range correlations but require
high-level methods to treat short-range effects. On the basis of these
statements, we have proposed a combined approach by splitting the
Coulomb potential into two parts. We have applied a modified short-
range version of the LDA or PBE38 functionals, to treat short-range
exchange-correlation effects, while in the long range Hartree−Fock
exchange was employed with a range separation parameter of 0.5 au−1.
As proposed by Goll et al.,39 we have included dispersion effects
through a local long-range correlation calculation of MP2 type. A
Pipek−Mezey localization scheme,40 combined with the Boughton−
Pulay procedure41 with a selection criterion of 0.98, was used due to
large delocalization. To calibrate our DFT+WFT scheme (see also ref
42, we have calculated interaction energies of the Au2−AuH system
and compared it with recent CCSD(T) calculations (Table 2).13 The
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
AUTHOR INFORMATION
Corresponding Author
■
*Fax: (+)33 561553003. E-mail: gornitzka@lcc-toulouse.fr
(H.G.); hemmert@lcc-toulouse.fr (C.H.).
ACKNOWLEDGMENTS
This work was supported by the Centre National de la
Recherche Scientifique (CNRS). We thank the HPCs CALcul
■
en MIdi-Pyren
Informatique National de l’Enseignement Super
CINES, grant pcn6211) for generous allocations of computer
time.
́
ee
́
s (CALMIP, grant p0611) and the Centre
ieur (GENCI-
́
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LDA+WFT
c
LDA+WFT
Comparison is made with the previous work of Pyykko et al.13 The
basis set superposition error has been corrected, unless stated
otherwise. An additional diffuse f function with an exponent of 0.2
has been added. The basis set superposition error has not been
a
̈
b
c
corrected.
agreement is good, with an equilibrium distance of 2.70 Å (2.77 Å)
and a dissociative energy of 17.5 kJ mol−1 (20.5 kJ mol−1). As already
found by Pyykko and co-workers, PBE overbinds the system by
̈
roughly 50%. Interestingly, B3PW91 underbinds it. As shown in Figure
5, in the case of an unfonctionalized NHC dinuclear gold complex,
these functionals underestimate the aurophilic interaction both in
terms of energy and Au2 bond lengths. The energies given in this
figure are rescaled according to a somewhat old-fashioned calculation:
namely, an isodesmic reaction. The Au···Au dissociation limit of the
curve plot in Figure 5 would also involve covalent bond breaking of
the (CH2)3 bridge, so that it is necessary to find a way to rescale the
energies obtained with different methods with respect to the same
energy reference. This is the aim of the proposed isodesmic reaction.
Note that an additional diffuse f function with an exponent of 0.2
does not optimize the aurophilic interaction in the case of B3PW91,
whereas BSSE effects are small in all cases, even in the framework of
DFT+WFT calculations. This result confirms that such effects, usually
small in DFT calculations, are not significantly increased by long-range
WFT calculations.
ASSOCIATED CONTENT
■
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* Supporting Information
Tables, figures, and CIF files giving crystal data for 4 and 8,
NMR spectra, and DFT geometries of the complexes. This
material is available free of charge via the Internet at http://
pubs.acs.org. CCDC-821066 (4) and 821067 (8) also contain
the supplementary crystallographic data for this paper. These
data can also be obtained free of charge from The Cambridge
625
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Organometallics
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