Studies of ligand exchange in N-heterocyclic carbene silver(I) complexes

Article abstract of DOI:10.1021/om300340w

A series of N-heterocyclic carbene (NHC) Ag(I) complexes have been prepared and used to study the dynamics of NHC ligand exchange in these Ag(I) complexes. These studies used solution-state variable-temperature (VT) ¹³C NMR spectroscopy and the temperature-dependent changes in ¹³C- ^107/109Ag coupling to determine activation energies for the ligand exchange process. The effects of concentration, bridging anions, and additives on the exchange process have been studied. The experimental activation energies for the NHC ligand exchange processes of these silver complexes are also compared with DFT calculations. The results are consistent with an associative mechanism for the Ag(I)-NHC exchange process.

Full text of DOI:10.1021/om300340w

Article
pubs.acs.org/Organometallics
Studies of Ligand Exchange in N-Heterocyclic Carbene Silver(I)
Complexes
Haw-Lih Su,† Lisa M. Perez,‡ Sheng-Jui Lee,§ Joseph H. Reibenspies,† Hassan S. Bazzi,∥
́
and David E. Bergbreiter*,†
†Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States
‡Laboratory for Molecular Simulation, Texas A&M University, P.O. Box 30012, College Station, Texas 77843-3255, United States
§Institute of Chemistry, Academia Sinica, Taipei, Taiwan 115, Republic of China
∥Department of Chemistry, Texas A&M University at Qatar, P.O. Box 23874, Doha, Qatar
S
* Supporting Information
ABSTRACT: A series of N-heterocyclic carbene (NHC) Ag(I)
complexes have been prepared and used to study the dynamics of
NHC ligand exchange in these Ag(I) complexes. These studies
used solution-state variable-temperature (VT) 13C NMR spectros-
copy and the temperature-dependent changes in 13C−107/109Ag
coupling to determine activation energies for the ligand exchange
process. The effects of concentration, bridging anions, and additives on the exchange process have been studied. The
experimental activation energies for the NHC ligand exchange processes of these silver complexes are also compared with DFT
calculations. The results are consistent with an associative mechanism for the Ag(I)−NHC exchange process.
have described NHC−Ag complexes which have carbene
carbons that appear as doublets of doublets, as broad doublets,
as broad singlets, or as sharp singlets in 13C NMR
spectroscopy.23−27 Since Ag has two almost equally abundant
INTRODUCTION
■
N-heterocyclic carbenes (NHCs) have been become increas-
ingly important as organocatalysts1,2 and as ligands for
transition metals.3−5 The importance of these species in
organic and biological chemistry was originally recognized in
seminal studies by Breslow in the 1950s,6,7 and their
importance as strong σ-donor and weak π-acceptor ligands in
organometallic chemistry was recognized after key reports by
Arduengo in 19918 and Herrmann in 1995.9 The tunability of
electron density and sterics make NHC ligands useful as
alternatives to phosphines, and many examples of NHC-
complexed transition-metal catalysts have now been de-
scribed.3,5,10,11 This has been especially true for Ru-based
olefin metathesis catalysts12,13 and for Pd cross-coupling
catalysts.3,5,10,11,14 NHC-ligated Ag(I) complexes have been
used both as catalysts15,16 and as intermediates in the synthesis
of other NHC transition metal complexes by transmetala-
tion.17−22 However, while Lin has proposed an associative
mechanism for NHC exchange in Ag(I) halide complexes
(Figure 1),17 relatively little attention has been paid to the
dynamic behavior of NHC ligands on metals.
1
spin /2 nuclei (52% of 107Ag and 48% of 109Ag), the carbene
carbon of a NHC−Ag(I) complex should be a doublet of
doublets if the NHC ligand does not exchange rapidly on the
NMR time scale. If NHC ligand exchange were fast on the
NMR time scale, this carbene carbon peak would be a singlet.
This exchange process could also be temperature dependent, in
which case dynamic behavior could be observed. Indeed, one
report noted that the carbene carbon peak of a bithiophene-
substituted silver NHC complex was a broad singlet at room
temperature and a doublet of doublets at −50 °C.28
VT NMR spectroscopy has been used previously to study
such exchange processes of other ligands on Ag(I).29−37 For
example, the VT 31P NMR spectrum of a P-containing Ag
ligand is a doublet of doublets at low temperature that coalesces
to a broad doublet, then to a broad singlet, and finally to a
singlet when the temperature is increased. Such temperature-
dependent spectra have proven to be useful in studying
mechanistic details of such ligand exchange processes.33−37
Recently, we observed that the carbene carbon signal of (N-
mesityl-N′-methylimidazolylidene)silver iodide is a singlet at
room temperature and a doublet of doublets at −80 °C in 13C
NMR spectroscopy. This suggested that these complexes could
be used to study the dynamics of NHC exchange in (NHC)AgI
exchange processes using line-shape studies of these complexes’
Evidence that ligand exchange in NHC−Ag(I) complexes
can occur at varying rates comes from literature reports that
Figure 1. Proposed mechanism of NHC ligand exchange of NHC−Ag
complexes described in ref 17.
Received: April 25, 2012
© XXXX American Chemical Society
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VT NMR spectra. As shown below, NHC ligands with 13C
labels at the carbene carbon allow these processes to be studied
in detail. The results show that the exchange process is an
associative process and that both counterions and added ligands
significantly affect exchange rates for NHC ligands on Ag(I).
RESULTS AND DISCUSSION
■
Synthesis of NHC−Silver Halide Complexes. The very
low intensity of the 13C signal for the carbene carbon makes
studying NHC−Ag(I) exchange processes problematic. To
address this issue, we used the strategy first reported by
Clyburne to prepare 13C-labeled NHC ligands to facilitate
solution and solid-state NMR spectroscopic studies of (N,N′-
dimesitylimidazolylidene)silver chloride ((IMes)AgCl).38 In
this work, we prepared N-mesityl-N′-methylimidazolium salts
with a 13C-labeled carbon at the 2-position of the imidazolium
ring and used these species to prepare Ag(I) halide complexes
with 13C labels at the carbene carbon. This provided spectra
with sufficient signal to noise to study line broadening in the
VT 13C NMR spectra of these Ag complexes, even when the
NHC−Ag(I) complexes were present at mN concentrations.
Our synthesis of 13C-labeled imidazole 1 and NHC−Ag
halide complexes from 13C-labeled formaldehyde used
Arduengo’s method (Scheme 1).39 Integration of the doublet
Scheme 1. (N-Mesityl-N′-methylimidazolylidene)silver
Halide Complex Syntheses
Figure 2. ORTEP views of X-ray crystal structures of (a) (N-mesityl-
N′-methylimidazolylidene)silver chloride (5) and (b) (N-mesityl-N′-
methylimidazolylidene )silver bromide (6) with thermal ellipsoids at
the 50% probability level. Further details regarding the crystal
structures are available in the Supporting Information.
Table 1. Selected Bond Distances (Å) and Bond Angles
(deg) of Compounds 5 (C26H32Ag2N4Cl2) and 6
(C26H32Ag2N4Br2)
5 (X = Cl)
Bond Lengths
6 (X = Br)
Ag(1)−C(1)
Ag(1)−X(1)
Ag(1)−X(1A)
C(1)−N(1)
C(1)−N(2)
2.074(6)
2.082(10)
2.495(3)
2.928(3)
1.350(12)
1.363(14)
2.3844(14)
2.9017(13)
1.346(6)
1.374(7)
1
due to 13C−1H coupling in the H NMR spectrum of the
Bond Angles
labeled imidazole versus the singlet for the 12C-containing
impurity showed that 1 was 92% enriched with a 13C label. N-
methylation of 1 with methyl halides formed the imidazolium
salts (2−4). While methyl iodide reacted readily, methylations
with methyl bromide or methyl chloride required forcing
conditions and sealed-tube reactions. The most difficult of
these reactions was methylation with methyl chloride, which
had to be carried out at 120 °C in CH3CN in a sealed tube with
an excess of methyl chloride. 13C-enriched NHC−silver halide
complexes were then prepared from these 13C-labeled
imidazolium halide salts using Ag2O (Scheme 1).
The silver complexes 5−7 so formed were characterized by
1H and 13C NMR spectroscopy and by elemental analysis. The
(N-mesityl-N′-methylimidazolylidene)silver chloride (5) and
bromide (6) products were also crystallized from CH3CN/
ether and characterized crystallographically. Their crystal
structures are shown in Figure 2, and selected bond distances
and bond angles are given in Table 1.
C(1)−Ag(1)−X(1)
C(1)−Ag(1)−X(1A)
X(1)-Ag(1)−X(1A)
Ag(1)−X(1A)−Ag(1A)
N(1)−C(1)−N(2)
N(1)−C(1)−Ag(1)
N(2)−C(1)−Ag(1)
163.18(14)
108.38(14)
88.43(4)
91.57(4)
103.2(5)
129.4(4)
127.4(4)
158.42(30)
110.82(31)
90.76(7)
89.27(7)
102.3(9)
128.9(7)
128.4(7)
After repeated attempts, two different crystals were obtained,
but neither had a structure for a neutral (NHC)AgI complex
like that seen for 5 and 6. Elemental analysis of the first type of
crystals had a C, H, and N ratio consistent with an (NHC)AgI
complex containing a 1:1 NHC:AgI ratio. X-ray structural
analysis of these crystals was inconclusive, as attempts to model
the anion failed to produce a sensible model. When the
program PLATON/SQUEEZE was employed to subtract the
electron density from the final refinement of the cation, results
consistent with the structure for the (NHC)2Ag+ cation in
Figure 3 were obtained. Others have noted that complex anions
form during recrystallization of (NHC)AgI complexes. For
Attempts to crystallographically characterize (N-mesityl-N′-
methylimidazolylidene)silver iodide 7 were less successful.
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Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) of Compound 8 (C26H32AgN4I)
Bond Lengths
Ag(1)−C(1)
Ag(1)−I(1)
2.075(11)
3.256(4)
C(1)−N(1)
C(1)−N(2)
1.328(14)
1.3464(15)
Bond Angles
C(1)−Ag(1)−C(1A)
C(1)−Ag(1)−I(1)
N(1)−C(1)−N(2)
175.0(6)
N(1)−C(1)−Ag(1)
N2)-C(1)−Ag(1)
125.3(8)
130.1(9)
92.5(3)
103.7(9)
NMR Studies of NHC−Silver Halide Complexes. A 0.06
N solution of 13C-labeled (NHC)AgI (7) in CD2Cl2 had a
carbene carbon signal that appeared as a broad singlet at 21 °C
in 13C NMR spectroscopy. When the temperature was lowered
to −85 °C, this singlet changed into a doublet of doublets
(Figure 5). VT NMR spectra of solutions of 7 at higher
Figure 3. Ball and stick representation of the bis(N-mesityl-N′-
methylimidazolylidene)silver cation that is thought to be present in 7.
Hydrogen atoms are omitted for clarity. Results from an attempted X-
ray structural determination for 7 suggesting this structure is present
are given in the Supporting Information.
example, Nolan reported a crystal structure for an NHC−Ag
ion pair complex with a complex [Ag4I6]2− anion.26 EDX
(energy dispersive X-ray) analysis of these crystals showed that
the crystals had a 1:1 Ag:I ratio. On the basis of the elemental
analysis and EDX results, the disordered anion in crystals of 7
with the formula (NHC)AgI is presumed to be a silver iodide
−
cluster with a formula equivalent to that of AgI2 , though we
cannot exclude the presence of other more complex anions. A
second crystalline product was also isolated from these
recrystallizations. This product 8 was formed by loss of AgI
and had the formula (NHC)2AgI. This product appeared to
form as a result of loss of AgI due to exposure of 7 to ambient
laboratory light during a recrystallization (eq 1). When 7 was
Figure 5. (a−c) Variable-temperature 13C NMR spectra over a −85 to
+21 °C temperature range for (a) 0.06 N, (b) 0.12 N, and (c) 0.24 N
CD2Cl2 solutions of 13C-enriched (NHC)AgI (7). Coalescence is seen
in all the cases, with the coalescence temperature increasing as the
concentration of 7 decreases. (d) Minor component (7b) clearly
visible in a 0.24 N solution at −85 °C. The concentrations and ratios
of the two components are given on the side.
concentrations of 7 exhibited similar changes, with the
difference that the coalescence temperature was observed to
decrease as the concentration of 7 was increased from 0.06 to
0.12 to 0.24 N.
prepared with exposure to ambient lighting, 8 was the major
1
product. The crystals of 8 so formed were characterized by H
13C NMR spectra of higher concentrations of 7 at −85 °C
showed that solutions of 7 contained more than one
component. This is most apparent in the −85 °C spectrum
of the 0.24 N solution, where two sets of doublets of doublets
are seen, indicating that two kinds of NHC silver species are
and 13C NMR spectroscopy and elemental analysis. The X-ray
structure of this product is shown in Figure 4, and the selected
bond distances and bond angles are given in Table 2.
1
present (Figure 5d). The major component had J(13C−107Ag)
and 1J(13C−109Ag) coupling constants of 241 and 274 Hz, while
the minor component had smaller coupling constants of 179
and 207 Hz. The minor component was also present in more
dilute solutions at a lower relative concentration. The ratio of
the two components was ∼5:1 in a 0.24 N solution of 7 and
was ∼20:1 in a 0.06 N solution of 7. This minor component
had not been noted in our initial studies that did not use a 13C-
enriched version of 7 because of the low intensity of the
unlabeled carbene carbon.
The qualitative change in the ratio of the two components
with changes in the concentration of 7 and the coalescence of
all the peaks seen in the VT NMR spectra suggested that the
two components are in equilibrium with one another. As
discussed below, we believe that the two species in this
Figure 4. ORTEP view of the X-ray crystal structure of bis(N-mesityl-
N′-methylimidazolylidene)silver iodide (8) with thermal ellipsoids at
the 50% probability level. Hydrogen atoms are omitted for clarity.
Further details regarding the crystal structure are available in the
Supporting Information.
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equilibrium mixture are the neutral NHC−silver iodide
complex 7a (the major component) and the bis(N-mesityl-
N′-methylimidazolylidene)silver(I) diiodoargentate ionic spe-
formation of the ion pair. In this work, Nolan assigned the
structure (IMes)AgCl to the major component and the
−
structure (IMes)2Ag+AgCl2 to the minor component. We
cies ((NHC)2Ag+AgI2 ) (7b) (the minor component) (eq 2).
−
prepared solutions of (IMes)AgCl that contained both the
−
neutral (IMes)AgCl and ionic (IMes)2Ag+AgCl2 species and
also observed that the ratio of the neutral and ion pair forms of
this complex changed with changes in solvent polarity. In a ca.
0.4 N CD2Cl2 solution, the ratio of (IMes)AgCl to
(IMes)2Ag+AgCl2 was 3:1 at 23 °C and 4:1 at −80 °C, on
−
1
the basis of H NMR spectroscopic analysis of the signals for
two sets of the four different protons of the neutral and ion
pair. The 13C NMR spectrum at −80 °C showed two doublets
of doublets with 107Ag−13C and 109Ag−13C coupling constants
of 238 and 273 Hz for the major component (the neutral
complex) and 107Ag−13C and 109Ag−13C coupling constants of
180 and 209 Hz for the minor component (the ionic species).
In a 0.1 N DMSO-d6 solution, the ratio of (IMes)AgCl/
VT 13C NMR spectroscopic studies of CD2Cl2 solutions of
silver chloride complex 5 at various concentrations are shown
in Figure 6. The dynamic behavior of 5 is qualitatively similar to
(IMes)2Ag+AgCl2 was ca. 1:1 on the basis of 1H NMR
−
spectroscopic analysis at 23 °C. Nolan reported that the ratio
−
(IMes)AgCl:(IMes)2Ag+AgCl2 was 4:1 in acetonitrile-d3 and
2:1 in DMSO-d6.
The line-shape changes seen in spectra in Figures 5 and 6 are
like those seen for phosphine or alkyl group exchange in other
Ag complexes.29−37 The spectra in Figures 5 and 6 show that
the 13C NMR peaks for 7 at different concentrations coalesce at
different temperatures. For example, the coalescence temper-
ature for 7 was ca. −42 °C for a 0.24 N solution, ca. −25 °C for
a 0.12 N solution, and ca. −5 °C for a 0.06 M solution. This
same trend was also seen in the VT NMR spectroscopic studies
of solutions of (NHC)AgCl (5) and (NHC)AgBr (6). The
coalescence temperature of 5 was ca. 20 °C for a 0.12 N
solution and was greater than room temperature for a 0.06 N
solution (Figure 6). This concentration-dependent increase of
the coalescence temperatures with a decrease of concentration
is consistent with Lin’s proposed mechanism for NHC ligand
exchange via an associative process.
Anion Effects on NHC Ligand Exchange Rates in
NHC−Silver Halide Complexes. Lin’s study of NHC−Ag
complexes17 postulated that halides were bridging anions
connecting two silvers, facilitating NHC exchange (Figure 1).
This postulate is supported by the VT NMR spectra of 5−7,
which show that halide identity has a significant effect on the
coalescence temperature, with the exchange rate variance being
lowest with Cl. Figure 7a shows that when the concentration
was 0.01 or 0.02 N, the coalescence temperatures for complexes
5 and 6 were above and about 20 °C, respectively, suggesting
Figure 6. VT 13C NMR spectra of 13C-enriched (NHC)AgCl (5) at
concentrations of (a) 0.01 N, (b) 0.02 N, (c) 0.06 N, and (d) 0.12 N
in CD2Cl2.
the behavior of 7 seen in Figure 4. For 5, coalescence of the
doublet of doublets at low temperature to form a broad singlet
was only seen for the 0.12 N solution of 5. In more dilute
solutions, and in particular in the 0.02 and 0.01 N solutions of
5, the 22 °C spectra only show broadened doublets. The
spectra of 5, like those of 7, show small amounts of a second
species whose relative ratio increases with concentration. These
two species are presumed to be the neutral (N-mesityl-N′-
methylimidazolylidene)silver chloride complex (NHC)AgCl
(5a) as the major component and the bis(N-mesityl-N′-
methylimidazolylidene)silver(I) dichloroargentate ionic species
((NHC)2Ag+AgCl2 ) (5b) as the minor component (eq 2).
−
Our assignment of ionic structures for 5b and 7b as the
minor peak in the low-temperature spectra above is based on
the assumption that the Ag−C coupling constant is lower for
the ion pair than for the neutral species. This premise is based
on prior reports that have described NHC-ligated Ag(I) with
various anions.17,23,24,40,41 These reports consistently note that
(NHC)2Ag+ complexes have 107Ag−13C coupling constants for
ionic species that are in the range of 168−186 Hz. These
107Ag−13C coupling constants are consistently lower than those
1
of neutral complexes, whose J(107Ag−13C) values are in the
range of 219−253 Hz. Our measured 107Ag−13C and
109Ag−13C coupling constants for the assigned neutral (NHC)-
AgX species and ionic (NHC)2Ag+AgX2 species are in the
−
same range of coupling constant values for these other neutral
and ion pair NHC−Ag complexes in the literature.
These prior reports also suggest that mixtures of neutral and
ionic NHC-ligated silver halide complexes exist in equilibrium
with one another.23,24,40 While the concentration dependence
of the neutral and ion pair structures we noted here has not
been reported, Nolan did observe that the ratio of the two
carbene complexes formed in the synthesis of (IMes)AgCl
varied with solvent polarity.26 Higher solvent polarity favored
Figure 7. 13C NMR CD2Cl2 spectra (20 °C) of (a) 0.01 and 0.02 N of
5 and 6 and (b) 0.12 and 0.06 N of 5 and 7, showing the different
coalescence temperatures with different anions.
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107Ag−13C and 109Ag−13C coupling constants of 178 and 206
Hz seen for 8 were the same as the 107Ag−13C and 109Ag−13C
coupling constants seen for the minor component present in a
0.24 N solution of 7 (i.e., the presumed ionic species). This
may indicate that the same (NHC)2Ag+ cation is present in
both 7b and 8.
that the ligand exchange in 6 is more rapid than in 5. Likewise,
5 and 7 at 0.06 or 0.12 N have coalescence temperatures about
and below 20 °C, respectively, suggesting that NHC ligand
exchange in 7 is more rapid than in 5.
Addition of tetrabutylammonium halides to 0.06 N solutions
of 5 provides further evidence of the effects of halide on the
exchange process. Figure 8 shows that adding (n-Bu)4N+Cl− or
VT 13C NMR studies 8 showed that adding very small
amounts of 7 to solutions of 8 promotes NHC ligand exchange.
When 0.01 equiv of 7 was added to a 0.04 N solution of 8, the
carbene signal in the 13C NMR spectrum became a broad
doublet at room temperature. When 0.03 equiv of 7 was added,
the carbene signal became a broad singlet at room temperature.
This shows that NHC ligand exchange between ionic species
and neutral species is fast on the NMR time scale.
Effects of Added Phosphine Ligands on (NHC)AgI
Solutions. When 0.2, 0.4, 0.9, 1.2, 1.6, and 3.0 equiv of
triphenylphosphine (Ph3P) were added to a 0.12 M solution of
7, 13C NMR spectroscopy showed that the solution of 7
containing primarily the neutral complex 7a was gradually
converted into a complex that had a 13C NMR spectrum
identical with that of 8. As shown in Figure 10, the −85 °C 13
C
Figure 8. VT NMR spectra of a 0.06 N solution of 5 (center) and a
0.06 N solution of 5 with addition of 1 equiv of (n-Bu)4N+Cl− (left) or
0.9 equiv of (n-Bu)4N+Br− (right).
(n-Bu)4N+Br− to a 0.06 N solution of 5 leads to decreased
coalescence temperatures, consistent with a faster NHC
exchange rate. This effect follows the trend noted above, with
bromide anion having a larger effect than chloride anion. Figure
9 shows that increasing the amount of added (n-Bu)4N+Br−
Figure 10. 13C NMR spectra (−85 °C) showing the effect of Ph3P on
the ratio of 7 and 8 when (a) 0.0, (b) 0.2, (c) 0.4, and (d) 1.2 equiv of
Ph3P were added to 0.12 N of 7 in CD2Cl2.
NMR spectra of these experiments show that the ratio of
(NHC)2Ag+ and (NHC)AgI increased when more Ph3P was
added. We suggest that formation of 8 from 7 occurs according
to the process shown in Scheme 2. In this scheme, additional
Scheme 2. Proposed Equilibria Forming Ph3P−AgI
Complexes on Addition of Ph3P to a Solution of 7
Figure 9. VT 13C NMR spectra of (a) a 0.06 N solution of 5 with
addition of (b) 0.3, (c) 0.6, (d) 0.9, and (e) 1.69 equiv of (n-
Bu)4N+Br−.
both decreases the coalescence temperature and affects the
phosphine can coordinate to silver iodide to form complexes of
AgI and Ph3P. Increasing amounts of Ph3P shifted the
equilibrium according to Le Chatelier’s principle. When more
̂
ratio of the neutral (NHC)AgX form and ion pair
(NHC)2Ag+AgX2 . This result is consistent with Nolan’s
−
work,26 as an increase in the amount of (n-Bu)4N+Br− would
increase solution polarity and the relative amount of the ion
pair.
than 0.5 equiv of Ph3P was added, only a trace amount of
(NHC)AgI (7) remained. The Ag species were present as a
combination of phosphine−silver complexes and 8. 31P NMR
spectroscopy confirmed the formation of (triphenylphosphine)
silver iodide complexes in accordance with the equilibria shown
in Scheme 2.
As noted above, we were unable to obtain crystals of (N-
mesityl-N′-methylimidazolylidene)silver(I) iodide (7) but did
obtain a derivative, bis(N-mesityl-N′-methylimidazolylidene)-
silver(I) iodide (8). We briefly examined the dynamic behavior
of the NHC groups in 8. Exchange of the NHC ligands of 8 did
not occur on the NMR time scale. A doublet of doublets was
observed at all temperatures from −80 to +20 °C. The
VT 13C spectroscopy showed that Ph3P addition also affected
the NHC exchange rate. As the amount of added Ph3P
increased, NHC exchange was slowed to the point that
coalescence was not observed below 25 °C. These experiments
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Figure 11. MPW1PW91 results for the ligand exchange process of (N-mesityl-N′-methylimidazolylidene)silver halides.
showed that less than 0.5 equiv of added Ph3P had only a small
effect on the coalescence temperature but that addition of 0.9
equiv or more of Ph3P resulted in higher coalescence
temperatures. The slower NHC exchange rate in the presence
of ca. 1 equiv of Ph3P is understandable in the context of our
other results and Scheme 2 if larger amounts of added Ph3P
shift the equilibria such that the NHC−silver complex left in
solution is 8 ((NHC)2Ag+I−). While we have not characterized
the AgI−triphenylphosphine complexes that form, we noted
several new signals (as doublets of doublets consistent with
Ag−P coupling) in 31P NMR spectra of −85 °C CD2Cl2
solutionsresults that would be consistent with the formation
of a series of different AgI−triphenylphosphine complexes.
DNMR Studies of Exchange Kinetics. The use of NMR
line-shape phenomena to study dynamic processes in spin
systems has a long history both in organometallic chemistry and
in studies of ligand exchange for Ag complexes.33,36,42−48 By
matching simulated spectra to observed spectra, it is possible to
determine kinetic parameters for these exchange processes. In
this work, we matched the observed VT NMR spectra with the
simulated spectra calculated using MEXICO, a computer
program for NMR chemical exchange line-shape calcula-
tion.45,49,50 The calculated rate constants obtained in this way
were then used with the Eyring equation to determine the
activation parameters for the exchange process. Activation free
energies at 20 °C for 5−7 are ca. 12.6, 11.2, and 11.5 kcal/mol,
respectively (details of these calculations are given in the
Supporting Information).
exchange for (NHC)AgX (X = Cl, Br, I) complexes, we carried
out a series of computational studies using density functional
theory51 (MPW1PW91)52 as implemented in Gaussian 09.53
Two mechanisms for NHC exchange were evaluated. First, a
mechanism involving (NHC)AgCl bond dissociation was
considered. It was found that the dissociation energy is ca. 55
kcal/mol in the gas phase and 43 kcal/mol in solution
(CH2Cl2). While these results suggest that the (NHC)AgCl
bond is not a strong bond, the bond dissociation energy is too
large to explain the results of the DNMR spectroscopy studies
described above. Calculations for an associative reaction
pathway shown in Figure 1 for a process in CH2Cl2 solution54
yielded the results shown in Figure 11.
The ΔG⧧ values calculated for this associative reaction path
are 19.5, 17.5, and 12.5 kcal/mol for X = Cl, Br, I, respectively.
While the calculated ΔG⧧ values are somewhat larger than the
experimentally determined values (12.6, 11.2, and 11.5 kcal/
mol), they are consistent with our results.
In our calculation results, a structure with a halide bridging
two silvers is the energy maximum in the reaction pathway,
indicating that the bridging halide plays an important role in
stabilizing the transition state. This transition-state structure
helps explain the experimental results in this report and in the
literature. First, more polarizable halides may serve as better
bridging groups and stabilize this proposed transition state.
Thus, complexes with a bromide or iodide exchange NHC
ligands faster than a complex with a less polarizable chloride
anion. Second, this transition-state structure could also explain
how different N substituents on the NHC ligands affect NHC
exchange rates. In this transition-state structure, one of the N-
Computational Studies of the NHC Exchange Process.
To further understand the mechanism for the NHC ligand
F
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2H), 6.89 (d, J = 7 Hz, 1H), 2.32 (s, 3H) and 1.97 (s, 6H). 13C NMR
(100 MHz, CDCl3, room temperature): large peak at δ 137.3, multiple
small peaks between 140 and 132, 129.0, 120.1, 21.0, and 17.3. The
92% 13C labeling in 1 was determined by integrating the signal for the
doublet due to 13C−1H coupling for the proton signal on C2 of the N-
mesitylimidazole versus the singlet for the 12C-containing N-
mesitylimidazole.
substituent groups (the methyl group in this case) of the
transferring NHC has steric interactions with the N-mesityl
group of the other NHC groups. These steric interactions are
minimized with N-methyl substitutuents. However, when there
are two bulky substitution groups on the nitrogens (e.g., two
mesityl groups in (N,N′-dimesitylimidazolylidene)silver chlor-
ide ((IMes)AgCl)), these same steric interactions would be less
favorable, leading to the observed slower NHC ligand exchange
rate. This explains the lower NHC exchange rate for more
bulky (NHC)AgI halide complexes, where the carbene carbon
signal at 23 °C is a doublet of doublets.26,38
N-Mesityl-N′-methylimidazolium Chloride (2). In a pressure
tube equipped with a magnetic stir bar was added N-mesitylimidazole
(0.192 g, 1.03 mmol), a 1.0 M solution of chloromethane in tert-butyl
methyl ether (5 mL, 5 mmol), and acetonitrile (5 mL). The reaction
mixture was stirred in a sand bath regulated at 120 °C for 4 days. After
the mixture was cooled to room temperature, the solvent was removed
under reduced pressure to yield a white solid. Analysis of this solid
CONCLUSIONS
1
■
with H NMR spectroscopy showed a 75% conversion of the starting
13C-labeled (N-mesityl-N′-methylimidazolylidene)silver halides
have been prepared, and VT 13C NMR spectroscopy has been
used to study the NHC ligand exchange processes for these
complexes. Using 13C-labeled derivatives in this study improved
the signal to noise ratio of the low-intensity carbene carbon
signals and made it possible to obtain spectra at a broader range
of concentrations. The use of these labeled NHC complexes
also facilitated the detection of low concentrations of ion
species that are in equilibrium with neutral NHC silver halide
complexes. The concentration-dependent coalescence temper-
ature suggested that an associative process was involved in the
exchange, and this mechanism is in agreement with DFT
calculations for such an exchange process. Bridging anions are
shown to affect the rate of NHC ligand exchange. Added
phosphine ligands also affect the nature of these spectra and
decrease the NHC ligand exchange rate. These studies provide
a more detailed understanding of the solution behavior of
NHC−silver complexes that underlies the use of these materials
in reactions such as transmetalation and organocatalysis.
material. The desired imidazolium chloride product was separated
from this mixture by first dissolving this solid in 0.2 mL of acetonitrile
and adding this solution to 10 mL of hexanes. Isolation of the white
1
precipitate afforded 2 as a white solid (0.122 g, 50%). H NMR (500
MHz, CDCl3, room temperature): δ 10.63 (s, 1H), 7.57−7.54 (m,
1H), 7.11−7.09 (m, 1H), 6.98 (s, 2H), 4.37 (s, 3H), 2.32 (s, 3H), and
2.05 (s, 6H). 13C NMR (125 MHz, CDCl3, room temperature): δ
141.1, 138.5, 134.2, 130.7, 129.7, 124.4, 122.9, 37.3, 21.1, and 17.6.
Anal. Calcd for C13H17ClN2 (236.74): C, 65.95; H, 7.24; N, 11.83.
Found: C, 65.78; H, 7.26; N, 11.80. In addition to a much more
intense C2 carbon in its 13C NMR spectrum, 13C-labeled 2 had a
1
doublet at δ 10.56 (1J(13C−1H) = 224 Hz) in its H NMR spectrum.
N-Mesityl-N′-methylimidazolium Bromide (3). In a pressure
tube equipped with a magnetic stir bar was added N-mesitylimidazole
(0.183 g, 0.98 mmol), a 2.0 M solution of bromomethane in tert-butyl
methyl ether (2 mL, 4 mmol), and acetonitrile (2.5 mL). The reaction
mixture was stirred in a sand bath regulated at 60 °C for 20 h. After the
mixture was cooled to room temperature, the solvent was removed
under reduced pressure until there was 0.5 mL of the solution. When
the solution was slowly added to 10 mL of hexanes, a white precipitate
formed. Filtration of the mixture afforded a yellowish solid (0.188 g,
1
68%). H NMR (300 MHz, CDCl3, room temperature): δ 10.17−
EXPERIMENTAL SECTION
10.11 (m, 1H), 8.02−7.99 (m, 1H), 7.12−7.09 (m, 1H), 6.92 (s, 2H),
4.29 (s, 3H), 2.27 (s, 3H), and 2.00 (s, 6H). 13C NMR (75 MHz,
CDCl3, room temperature): δ 141.1, 137.9, 134.1, 130.5, 129.7, 124.7,
122.9, 37.2, 21.0, and 17.6. Anal. Calcd for C13H17BrN2 (281.19): C,
55.53; H, 6.09; N, 9.96. Found: C, 55.39; H, 6.11; N, 9.93. In addition
to a much more intense C2 carbon in its 13C NMR spectrum, 13C-
■
General Procedures. All reactions were carried out under an inert
atmosphere unless otherwise noted. All reagents and solvents were
purchased from commercial sources and used without further
purification. 1H NMR spectra were obtained using 300, 400, and
500 MHz spectrometers. 13C NMR spectra were measured on these
spectrometers operating at 75, 100, and 125 MHz. Chemical shifts of
1H and 13C NMR spectra are reported in parts per million (δ) relative
to residual proton resonances in the deuterated solvents (CDCl3 and
CD2Cl2). Coupling constants (J values) are reported in hertz (Hz),
and spin multiplicities are indicated by the following symbols: s
(singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet),
br (broad), and m (multiplet). Temperatures recorded for VT NMR
experiments were calibrated with 100% MeOH. Simulated spectra
were calculated using MEXICO version 3.0.
1
labeled 3 had a doublet at δ 10.43 (1J(13C−1H) = 214 Hz) in its H
NMR spectrum.
N-Mesityl-N′-methylimidazolium Iodide (4). In a 25 mL
round-bottomed flask equipped with a magnetic stir bar and rubber
septum was added 0.107 g of N-mesitylimidazole (0.57 mmol), methyl
iodide (0.12 g, 0.85 mmol), and dichloromethane (2 mL). The
reaction mixture was stirred at room temperature for 18 h. At this
point, the solvent was removed under reduced pressure to yield 0.16 g
(86%) as a yellow oil. 1H NMR (500 MHz, CDCl3, room
temperature): δ 9.85−9.83 (m, 1H), 7.88−7.86 (m, 1H), 7.18−7.16
(m, 1H), 6.96 (s, 2H), 4.32 (s, 3H), 2.30 (s, 3H), and 2.06 (s, 6H).
13C NMR (125 MHz, CDCl3, room temperature): δ 141.3, 137.5,
134.2, 130.4, 129.8, 124.7, 123.1, 37.9, 21.0, and 17.9. Anal. Calcd for
C13H17IN2 (328.19): C, 47.58; H, 5.22; N, 8.54. Found: C, 47.48; H,
5.24; N, 8.56. In addition to a much more intense C2 carbon in its 13C
NMR spectrum, 13C-labeled 4 had a doublet at δ 9.90 (1J(13C−1H) =
13C-Labeled N-Mesitylimidazole (1). In a 25 mL round-
bottomed flask equipped with a magnetic stir bar and rubber septum
was added glacial acetic acid (1.83 g, 30.5 mmol), 20% aqueous 13C-
labeled formaldehyde (1.1 g, 7.11 mmol), and 40% aqueous glyoxal
(1.04 g, 7.17 mmol). The reaction vessel was flushed with dry
nitrogen, and then the flask was placed in an oil bath regulated at 70
°C. A mixture of glacial acetic acid (1.82 g, 30.3 mmol), ammonium
acetate (0.546 g, 7.1 mmol), water (0.16 g), and mesitylamine (0.89 g,
6.6 mmol) was added dropwise over a period of about 20 min with a
glass syringe. The reaction mixture was stirred for 19 h at 70 °C. At
this point, the reaction mixture was cooled to room temperature and
added dropwise into a saturated NaHCO3 aqueous solution (60 mL).
The mixture was stirred for 1 h at room temperature. The precipitate
that formed was isolated by filtration and washed with 20 mL of water
to yield a brown solid, which was purified by column chromatography
with silica gel (hexane then ethyl acetate) to give 0.83 g (67%) of 1 as
1
224 Hz) in its H NMR spectrum.
(N-Mesityl-N′-methylimidazolylidene)silver(I) Chloride (5).
In a 10 mL round-bottomed flask equipped with a magnetic stir bar
and rubber septum was added a solution of N-mesityl-N′-
methylimidazolium chloride (56 mg, 0.24 mmol) in acetonitrile (2
mL) and silver oxide (28 mg, 0.12 mmol). The flask was covered with
aluminum foil, and the reaction mixture was stirred under nitrogen for
18 h. At this point, the stirring was stopped and the reaction mixture
was filtered through Celite. The resulting solid was washed twice with
1 mL of acetonitrile. The acetonitrile solutions were combined, and
the solvent was removed under reduced pressure to yield a white solid
1
a yellow solid. H NMR (400 MHz, CDCl3, room temperature): δ
1
7.46 (d, J(13C−1H) = 200 Hz, 1H), 7.23 (d, J = 7 Hz, 1H), 6.95 (s,
G
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Article
(65 mg, 79%). The product was crystallized from acetonitrile/ether.
1H NMR (400 MHz, CD2Cl2, room temperature): δ 7.22−7.18 (m,
1H), 7.00 (s, 2H), 6.98−6.96 (m, 1H), 3.91 (br, 3H), 2.35 (s, 3H),
and 1.97 (br, 6H). 13C NMR (100 MHz, CD2Cl2, room temperature):
δ 182.1, 140.1, 136.1, 135.5, 129.8, 123.3, 122.8, 39.3, 21.4, and 18.0.
Anal. Calcd for C13H16AgClN2 (343.60): C, 45.44; H, 4.69; N, 8.15.
Found: C, 45.33; H, 4.67; N, 8.12. In addition to a much more intense
C2 carbon in its 13C NMR spectrum, 13C-labeled 5 had a doublet at δ
39.3 (J(13C−13Cmethyl) = 11.2 Hz) in its 13C NMR spectrum.
(N-Mesityl-N′-methylimidazolylidene)silver(I) Bromide (6).
This complex was prepared by following the procedure used for 5
and was isolated as a yellowish solid (111 mg, 95%). The product was
crystallized from acetonitrile/ether. 1H NMR (300 MHz, CD2Cl2,
room temperature): δ 7.33 (d, J = 1.8, 1H), 6.97−6.93 (m, 3H), 3.85
(s, 3H), 2.36 (s, 3H), and 1.85 (s, 6H). 13C NMR (100 MHz, CD2Cl2,
room temperature): δ 182.5, 139.8, 136.0, 135.4, 129.6, 123.3, 123.2,
39.1, 21.4, and 17.9. Anal. Calcd for C13H16AgBrN2 (388.05): C,
40.24; H, 4.16; N, 7.22. Found: C, 40.26; H, 4.15; N, 7.20. In addition
to a much more intense C2 carbon in its 13C NMR spectrum, 13C-
labeled 6 had a doublet at δ 39.2 (J(13C−13Cmethyl) = 11.1 Hz) in its
13C NMR spectrum.
(N-Mesityl-N′-methylimidazolylidene)silver(I) Iodide (7). This
complex was prepared by following the procedure used for 5 and
isolated as a yellowish solid (0.12 g, 90%). 1H NMR (400 MHz,
CD2Cl2, room temperature): δ 7.35−7.33 (m, 1H), 6.93−6.96 (m,
3H), 3.85 (3H), 2.36 (s, 3H), and 1.84 (s, 6H). 13C NMR (100 MHz,
CD2Cl2, room temperature): δ 182.7, 139.7, 135.9, 135.4, 129.6, 123.3,
122.2, 39.1, 21.3, and 17.8. Anal. Calcd for C13H16AgIN2 (435.05): C,
38.89; H, 3.71; N, 6.44. Found: C, 35.93; H, 3.69; N, 6.48. In addition
to a much more intense C2 carbon in its 13C NMR spectrum, 13C-
labeled 7 had a doublet at δ 38.8 (J(13C−13Cmethyl) = 11.2 Hz) in its
13C NMR spectrum. Repeated recrystallizations of 7 yielded a mixture
of crystals of 7b and 8, as described in the text.
dimensional profiling algorithm, and all data were corrected for
Lorentz and polarization factors, as well as for crystal decay effects.
Finally the data were merged and scaled to produce a suitable data set.
The absorption correction program SADABS was employed to correct
the data for absorption effects. Systematic reflection conditions and
statistical tests for the data suggested the space group. A solution was
obtained readily using SHELXTL (SHELXS). All non-hydrogen atoms
were refined with anisotropic thermal parameters. The Hydrogen
atoms bound to carbon were placed in idealized positions (C−H =
0.96 Å, Uiso(H) = 1.2[Uiso(C)]). The structure was refined (weighted
least-squares refinement on F) to convergence.
Energy Dispersive X-ray Analysis (EDX). Elemental analysis of
Ag, I, and Cl was analyzed using a Cameca SX50 electron microprobe
equipped with four wavelength-dispersive spectrometers (WDS). A
WDS spectrometer is a monochromator which consists of a rotatable,
oriented diffraction crystal coupled with a proportional gas-filled
ionization detector in a geometric relationship following Bragg’s law.
WDS was used rather than EDS because of the former’s better
resolution, detection limits, accuracy, and reproducibility. Analyses
were carried out at an accelerating voltage of 15 kV at a beam current
of 10 nA and a beam diameter of 20 μm using PET diffracting crystals.
The sample was prepared by pressing several milligrams of the
powdered material between two highly polished (0.25 μm) stainless
steel cylinders to yield a thin millimeter diameter smooth-surfaced
pellet that was then transferred onto a conductive carbon tape. The
pellet was coated with approximately 150 Å of spec pure carbon to
prevent charge buildup under the electron beam. During analysis, the
sample stage was moved repetitively between 10 adjacent points at 2 s
intervals to obtain more representative sampling of the pellet surface
and to minimize any possible thermal damage to the sample. The
following standards were used: Ag2S for Ag, RbI for I, and NaCl for Cl.
All analyses were processed through the Cameca PAP (Pouchot and
Pichoir) full-quantitative ZAF type matrix correction program. In
order to more properly correct for interelement matrix effects, carbon
was calculated by difference to sum the analyses to 100% by weight.
BSE (backscattered electron) imaging was used to examine the surface
for homogeneity. BSE examination yields an image where the
brightness is directly proportional to the mean atomic number.
Computational Methodology. All calculations were performed
with the Gaussian 09 (G09) suite of programs.53 Full geometry
optimizations were performed using density functional theory
(DFT)51 with the MPW1PW91 (Barone’s modified Perdew−Wang
1991 exchange functional and Perdew and Wang’s 1991 correlation
functional)52 using the SDD (Stuttgart−Dresden) basis set and
effective core potential (ECP)55 for the silver atoms and the 6-
311G(d,p) basis set56 for all other elements. To estimate solvation
effects, the geometry optimizations were performed with the SMD
parametrized54 PCM (polarized continuum model) for CH2Cl2.
Geometry optimizations were performed on 5a−7a and the transition
states to their respective ionic species [NHC2Ag][AgX2] (X = Cl, Br,
I: 5b−7b, respectively), and the theoretical models of free NHC and
free AgCl.
Bis(N-mesityl-N′-methylimidazolylidene)silver(I) Iodide (8).
This complex was prepared using the procedure used to form 7, with
the difference that no effort was made to avoid exposure of the
solution to ambient laboratory lighting. After filtration and
concentration, a yellowish solid containing of a mixture of 8 and a
trace amount of 7 was afforded. Recrystallization of this mixture
afforded 8. 1H NMR (400 MHz, CD2Cl2, room temperature): δ 7.36−
7.34 (m, 1H), 6.93−6.92 (m, 3H), 3.82 (3H), 2.37 (s, 3H), and 1.79
(s, 6H). 13C NMR (100 MHz, CD2Cl2, room temperature): δ 139.7,
135.9, 135.4, 129.6, 123.3, 123.2, 39.0, 21.4, and 17.8. Anal. Calcd for
C26H32AgIN4 (635.33): C, 49.15; H, 5.08; N, 8.82. Found: C, 49.01;
H, 5.10; N, 8.79. At room temperature, the 13C-labeled form of 8 had a
dominant carbene carbon peak at δ 182.1 (1J(13C−107Ag) = 179 Hz
1
and J(13C−109Ag) = 207 Hz) in its 13C NMR spectrum.
Crystal Structure Determination. A Leica MZ 75 microscope
was used to identify a suitable multifaceted crystal with very well-
defined faces from a representative sample of crystals of the same
habit. The crystal mounted on a nylon loop was then placed in a cold
nitrogen stream maintained at 150 K. An Bruker APEXII X-ray (three-
circle) diffractometer was employed for crystal screening, unit cell
determination, and data collection. The goniometer was controlled
using the APEX2 software suite. The sample was optically centered
with the aid of a video camera such that no translations were observed
as the crystal was rotated through all positions. The detector was set at
6.0 cm from the crystal sample. The X-ray radiation employed was
generated from a Mo sealed X-ray tube (λ(Mo Kα) = 0.701 73 Å with
a potential of 40 kV and a current of 40 mA) fitted with a graphite
monochromator in the parallel mode (175 mm collimator with 0.5
mm pinholes). The reflections were used in the autoindexing
procedure to determine the unit cell. A suitable cell was found and
refined by nonlinear least squares and Bravais lattice procedures. The
unit cell was verified by examination of the hkl overlays on several
frames of data. No supercell or erroneous reflections were observed.
After careful examination of the unit cell, a standard data collection
procedure was initiated using ω scans. Integrated intensity information
for each reflection was obtained by reduction of the data frames with
the program APEX2. The integration method employed a three-
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, tables, and CIF files giving spectroscopic data,
NMR line shape analysis, computational details, and crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: bergbreiter@chem.tamu.edu.
Notes
The authors declare no competing financial interest.
H
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Organometallics
Article
(29) Bergbreiter, D. E.; Lynch, T. J.; Shimazu, S. Organometallics
1983, 2, 1354−1359.
ACKNOWLEDGMENTS
■
We gratefully acknowledge support from the Robert A. Welch
Foundation to D.E.B. (A-0639) and the Qatar National
Research Fund to D.E.B. and H.S.B. (Projects 28-6-7-7 and
4-081-1-016). We thank the X-ray crystallography laboratory at
the Department of Chemistry and the Laboratory for Molecular
Simulation (LMS) at Texas A&M University for support and
the simulation programs and supercomputer in the research
laboratory of Dr. Chao-Phing Hsu, Academia Sinica, Taiwan,
for the simulation programs and supercomputer time. Helpful
discussions with Professors Daniel A. Singleton and Michael B.
Hall are also acknowledged.
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dx.doi.org/10.1021/om300340w | Organometallics XXXX, XXX, XXX−XXX