Download Synthesis, structure and catalytic activity of NHC-Ag(I)

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Synthesis, structure and catalytic activity of NHC-Ag(I)
carboxylate complexes
Valerie H. L. Wong,[a],[b] Sai V. C. Vummaleti,[c] Luigi Cavallo,[c] Andrew J. P. White,[a] Steven P.
Nolan,[d],[e] and King Kuok (Mimi) Hii*[a]
Abstract: A general synthetic route was used to prepare 15 new Nheterocyclic carbene (NHC) silver(I) complexes bearing anionic
carboxylate ligands, [Ag(NHC)(O2CR)], including a homologous
series of complexes of sterically flexible ITent ligands, permitting a
systematic spectroscopic and theoretical study of the structural and
electronic structures of these compounds. The complexes displayed
significant ligand accelerated effect in the intramolecular cyclisation
of propargylic amides to oxazolidines. The substrate scope is highly
complementary to that achieved previously by NHC-Au and pyridylsilver(I) complexes.
Introduction
Applications of NHC complexes of Group 11 (coinage) metals
are truly diverse, widely exploited for their anti-microbial and
anti-cancer, as well as optical and electronic properties. 1,2 In the
field of catalysis, Cu and Au-NHC complexes have been
extensively investigated. In contrast, while Ag-NHC complexes
are well-known as anti-microbial and anti-cancer agents,3-5 their
catalytic activity has remained largely unreported. This may be
due to the fact that neutral [Ag(NHC)X] have the weakest M-C
bond among the group 11 complexes. 6, 7 Consequently, there is
a tendency for [Ag(NHC)X] to undergo ligand exchange in
solution to form bis-ligated [Ag(NHC)2][X] complexes.8 This
unique property is widely exploited in organometallic synthesis,
where silver-NHC complexes are widely used as carbenetransfer reagents.9 In synthesis, silver-NHC complexes have
been used as a latent catalyst to initiate the polymerization of
lactides10-12 and transesterification reactions.13 In fact, there are
no more than a handful of examples where the metal centre of
Ag-NHC complexes have been implicated as the active site, and
these include carbene insertion of ethyl diazoacetate into C-H
and ketones, 14,15 aerobic oxidation of alcohols,16 O-glycosidation
[a]
[b]
[c]
[d]
[e]
Dr. V. H. L. Wong, Dr. A. J. P. White, Dr. K. K. Hii
Department of Chemistry
Imperial College London
Exhibition Road, South Kensington, London SW7 2AZ, U.K.
E-mail:
Department of Chemistry
National University of Singapore
3 Science Drive, Kent Ridge Crescent, Singapore.
Dr. S. V. C. Vummaleti, Prof. L. Cavallo
KAUST Catalysis Centre, Physical Sciences and Engineering
Division, 4700 King Abdullah University of Science and Technology,
Thuwal 23955-6900, Kingdom of Saudi Arabia.
Prof. S. P. Nolan
Chemistry Department, College of Science, King Saud University,
Riyadh 11451, Saudi Arabia
Department of Inorganic and Physical Chemistry, Universiteit Gent,
Krijgslaan 281 - S3, 9000 Ghent, Belgium.
Supporting information for this article is given via a link at the end of the
document.
of carbohydrates,17 three-component reaction of aldehyde,
amines and alkynes,18 and hydroboraton of alkynes.19 In all
these cases, either [Ag(NHC)X] or [Ag(NHC) 2]X (where X =
halides) can be employed as catalyst precursors. In comparison,
however, the catalytic activity of NHC-Ag-carboxylate complexes
is generally under-developed and poorly understood. As far as
we are aware, there is only one example where a neutral
silver(I) carboxylate complex, [Ag(IPr)(OAc)], was reported to
catalyse the carboxylative cyclisation of allenylmethylamines
with CO2.20
The Hii group at Imperial College London has an ongoing
interest in the catalytic activity of cheaper coinage metal (Cu 21, 22
and Ag23, 24) catalysts for the heterofunctionalisation of
unsaturated carbon-carbon bonds. More recently, this was
extended to the intramolecular addition of N-H and O-H bonds to
alkynes to form substituted oxazolines at ambient conditions
(Scheme 1).16, 25, 26 Catalytic turnovers were found to be very
responsive to the electronic characteristics of pyridine ligands:
Notably, N-H addition was promoted by electron-deficient pyridyl
ligands (eqn 1), while the opposite trend was observed for O-H
addition (eqn 2).
Scheme 1. Ligand-effects in silver-catalysed addition of N-H and O-H to
alkynes.
The catalytic cycles for these processes are envisaged to
compose of three key steps (Scheme 2): (i) activation of the
alkyne moiety by its coordination to the cationic silver (displacing
a labile pyridyl ligand); (ii) C-X bond formation via 5-exo-dig
cyclisation to a putative vinyl-silver intermediate; and (iii)
protodemetallation to release the heterocycle and achieve
catalytic turnover, where the pyridyl ligand may be involved as a
proton shuttle (via pyridinium salts). In the case of N-H addition
(Y = NH, Z = O), the reaction promoted by 4-acetyl pyridine
affording the fastest turnover.16 This suggests that step (iii) is
likely to be turnover-limiting, which can be facilitated by a more
Brønsted acidic pyridinium salt for the protodemetallation step.
Conversely, for the O-H addition (Y = O, Z = NH), the rate of
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turnover is likely to be governed by step (i), as a more electronrich ligand (4-methoxyl pyridine) promotes greater -back
donation of electrons from Ag to the CΞC moiety, lowering the
energy of its LUMO orbital towards nucleophilic attack. These
observations led us to propose that N-heterocyclic carbene
ligands may be better than pyridyl ligands for the O-H addition
reaction: As better  donors, they could facilitate the activation of
the unsaturated carbon-carbon bond (step i), while the
protodemetallation step (step iii) may also be more facile in the
absence of a basic ligand.
(‘tentacular’) ligands,28 these were developed by the Nolan group
as a sterically flexible class of ligands for Pd-catalysed SuzukiMiyaura and Buchwald-Hartwig coupling reactions,28,29 Rucatalysed
olefin
metathesis,30
and
gold-catalysed
31
transformations.
In this study, we will describe the first
preparation, as well as the first catalytic applications of ITent-Agcarboxylate complexes.
Figure 1. Catalyst design: tuneable sites of a [(NHC)Ag(carboxylate)] complex.
Results and Discussion
Synthesis. To date, [Ag(NHC)(O2CR)] complexes are generally
prepared by halide extraction by the addition of excess
Ag(O2CR) to [Ag(NHC)X] (X = Cl, Br, I);20 or the reaction of one
equivalent of the NHCˑHX salt with 2 equivalents of Ag(O 2CR).32
In both cases, these reactions generate an equivalent of an AgX
byproduct in addition to excess Ag(O2CR), which can be
intractable and difficult to remove from the product.
Scheme 2. Key steps of the catalytic cycle.
Initial studies using [Ag(NHC)Cl] (NHC = IPr, SIMes), a
homoleptic [(IPr)2Ag]PF6 complex and a recently synthesized
[Ag(SIPr)(OTf)]27 as catalysts were unsuccessful (Table S6, SI).
With this in mind, we began to explore the activity of
[Ag(NHC)(O2CR)] complexes. It was envisaged that new catalyst
design offers several independent tunable sites for the
optimization of catalyst activity (Fig. 1). Steric and electronic
properties can be adjusted by: (i) the degree of saturation in the
heterocyclic ring, which alters the -donicity of the ligand; (ii)
bulky ‘wingtip’ N-substituents (R1)
prevents catalyst
decomposition; and (iii) the appropriate carboxylate ligand (R2)
to maintain the balance between catalyst stability and reactivity
(carboxylate ligands can dissociate from Ag more readily than
halides).
Herein, we report a general method for the synthesis of a
series of new [Ag(NHC)(carboxylate)] complexes. This allows
spectroscopic analyses of their stereo-electronic properties to be
performed in the solid and solution states. The study was
performed in a systematic way: (i) varying the carboxylate ligand,
including acetate and benzoates; and (ii) introduction of a
homologous series of bulky yet flexible imidazolium NHC
ligands: IPr, IPent, IHept and INon. Known as ‘ITent’
Scheme 3. Preparation of fifteen [(AgNHC)(O2CR)] complexes.
In 2013, three independent research groups of Nolan, 33 Cazin34
and Gimeno35 reported a practical procedure for the synthesis of
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group 11 metals of the type [M(NHC)X] (M = Cu, Ag, Au; X = Cl,
Br, I), whereby a mild base K2CO3 is used to deprotonate the
imidazolium salt in the presence of a suitable metal source (MX).
We have successfully adapted this method to produce a series
of [Ag(NHC)(O2CR2)] complexes (Scheme 3), where R2 = Me or
substituted Ph, a-d and NHC = ‘ITent’ ligands (IPr, IPent, IHep
and INon; 1, 4-6); as well as alkyl-substituted (IAd, 3) and
saturated (SIPr, 2) NHC complexes. The criterion for success is
the use of the NHC·HBF4 salt as the precursor, which allows
deprotonation by the weak base. For the preparation of the
acetate complexes (R2 = Me), the use of 20 equivalents of
K2CO3 is necessary to suppress the formation of the homoleptic
complexes [Ag(NHC)2][X]. This is attributed to the fact that acetic
acid is a weaker Brønsted acid than benzoic acids. Hence, a
larger amount of the carbonate base is required to deprotonate
acetic acid to generate sufficient amount of the counteranion, to
prevent the formation of the homoleptic complex.
A total of fifteen complexes were prepared using this method.
Each complex was isolated, after a single recrystallisation, as
analytically pure, air- and moisture-stable white crystalline solids
in moderate to good yields (41-79%). The first indication of the
relative stability of these compounds could be inferred by their
ESI-MS spectra: Using acetonitrile as the carrier solvent, the
[Ag(NHC)(MeCN)]+ fragment can be detected in the spectra of
IPr, SIPr and IPent complexes (1-2 and 4-6). In contrast, only
the [NHC-H]+ ion was observed in the spectra of 3a and 3b,
suggesting weaker NHC-Ag bonding in these IAd complexes.
Solid-state structures. Single crystals suitable for X-ray
diffraction were obtained for all fifteen complexes by
recrystallisation from CH2Cl2/hexane at 0 ºC; the key structural
parameters are presented in Table 1 (see SI for more extensive
discussions and structure parameters). In general, the
complexes adopt linear arrangements with 1-coordinated
carboxylate ligands, except 2a and 6d, where the anions bind in
an asymmetric, bidentate (2-) manner. Notably, no significant
argentophilic interactions were observed in any of the acquired
crystal structures (with Ag-Ag bond distance <3.44 Å).36 Ag-C
bond lengths are within the expected range for similar
complexes. In most cases, the Ag-C bond lengths are ca. 2.06 Å,
irrespective of the ring saturation, e.g. IPr (1a) vs SIPr (2a), or
the carboxylate ligand (acetate or benzoate), which is somewhat
unexpected. The only exceptions are the longer bond lengths
displayed by the IAd complexes 3a and 3b, which are also less
stable (as suggested earlier by mass spectrometry studies).
The steric environment imposed by the NHC ligands around
the metal centre were calculated using the SambVca web
application,37 which revealed percentage buried volumes (%Vbur)
between 37.0 and 53.6%. Despite having identical wingtip
substituents (2,6-diisopropylphenyl), SIPr complexes (2a-d) are
larger than the corresponding IPr complexes (1a-d). This is
attributed to greater torsion and rigidity in the saturated NHC
backbone, which reinforces the proximity of the aromatic rings to
silver. Within the ITent series, IPent has the largest %Vbur; the
extended alkyl chain in IHept and INon are too distal from the
primary coordination sphere to impose any steric effect.
Within the IPr and SIPr series, changing the carboxylate
ligand did not impose significant changes to %Vbur. However, the
nature of anionic ligand does influence the silver complex
coordinated by the bulkiest ligand IPent. In this case, %V bur
values spanning 5.1 units, from 48.5 in the acetate complex to
53.6 for the chlorobenzoate. This bulky yet flexible property of
the IPent ligand was subsequently found to be important for the
catalytic activity of these silver complexes (vide infra).
Table 1. Structural parameters (solid-state).
Cplx
mode
1a[e]
1-
1b[e]
1-
1c
1d
2a[e]
112-
2b
2c
2d
3a
3b
4a
4b[e]
1111111-
4d
5d
6d
112-
Ag-C/Å[b]
Ag-O/Å[b]
C-Ag-O/º[c]
%Vbur[d]
2.064(4)
2.064(5)
2.071(2)
2.059(2)
2.063(3)
2.059(3)
2.058(7)
2.060(7)
2.058(8)
2.060(7)
2.112(3)
2.113(3)
2.098(2)
2.094(2)
2.114(2)
2.121(2)
2.151(6)/
2.590(7)
2.338(7)/
2.415(7)
2.326(7)/
2.419(8)
2.301(6)/
2.445(6)
2.131(2)
2.101(2)
2.101(2)
2.117(2)
2.123(2)
2.111(2)
2.100(3)
2.089(3)
2.100(2)
2.1222(18)
2.293(2)/
2.408(3)
176.8(1)
177.6(2)
174.85(7)
177.80(7)
175.51(8)
175.26(9)
167.2(3)
161.1(3)
152.6(3)
160.7(3)
41.7,
44.6
43.6,
46.1
41.5
41.5
45.1–
46.3
170.2(1)
171.83(9)
171.85(9)
170.1(1)
175.2(1)
174.5(1)
164.0(1)
166.2(1)
165.7(1)
169.38(8)
158.37(11)
145.88(11)
45.9
43.4
43.4
39.3
37.0
48.5
49.1,
52.1
53.6
52.6
53.2
2.069(3)
2.060(3)
2.059(2)
2.075(4)
2.103(3)
2.067(3)
2.059(3)
2.069(3)
2.064(3)
2.060(2)
2.058(3)
[a] Binding mode of the carboxylate ligand to the metal. [b] Determined by
single crystal X-ray crystallography (SI). [c] Bond angle between NHC and
carboxylate ligands. [d] Calculations were performed using the experimentally
determined bond length for the Ag-C bond, a sphere radius of 3.5 Å, bond radii
scaled by 1.17, and mesh spacing of 0.05. H atoms were excluded.
[e]Asymmetric unit contains more than one independent molecule. The
number of Ag-C values reflects the number of independent structures.
Spectroscopic properties. Key spectroscopic data are
summarized in Table 2. The solid-state IR absorption
frequencies for the carboxylate group’s asymmetric and
symmetric stretches, asym(COO) and sym(COO), were recorded
and analyzed. The observed  values38 (asymsym) for the
NHC complexes are significantly larger than that recorded for
unligated silver carboxylate salts (>200 cm -1), which is
characteristic of monodentate39,40 or asymmetric bidentate41
carboxylate ligands (verified by the X-ray crystal structures, vide
infra). No clear trends between the structures and the COO
absorption bands could be detected, however, indicating that the
coordination mode of the carboxylate is independent of the steric
and electronic properties of the NHC ligand. Next, the solution
state structures were examined by analysing the 13C NMR
resonance signal corresponding to the coordinated C 2 carbenic
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carbon, easily identifiable as two doublets due to coupling to
magnetically active 107Ag and 109Ag isotopes. The observed
1 107/109
J(
Ag-13C) values mirror the gyromagnetic ratio of the two
silver isotopes; suggesting slow or no ligand exchange on the
NMR timescale.8, 42 This also applies to the relatively unstable
IAd complexes 3a and 3b, which decomposed slowly during
acquisition of their NMR spectra.
Table 2. 13C NMR and IR spectroscopic data.
Complex
AgOAc
AgOBz
Ag(OBz-Me)
Ag(OBz-Cl)
[Ag(IPr)(OAc)], 1a
[Ag(IPr)(OBz)], 1b
[Ag(IPr)(OBz-Me)], 1c
[Ag(IPr)(OBz-Cl)], 1d
[Ag(SIPr)(OAc)], 2a
[Ag(SIPr)(OBz), 2b
[Ag(SIPr)(OBz-Me), 2c
[Ag(SIPr)(OBz-Cl), 2d
[Ag(IAd)(OAc)], 3a
[Ag(IAd)(OBz)], 3b
[Ag(IPent)(OAc)], 4a
[Ag(IPent)(OBz)], 4b
[Ag(IPent)(OBz-Cl)], 4d
[Ag(IHep)(OBz-Cl)], 5d
[Ag(INon)(OBz-Cl)], 6d
C data[a]
asym[b]
184.5
(249, 288)
184.4
(251, 290)
184.5
(250, 289)
184.1
(252, 291)
207.9
(234, 271)
207.9
(235, 271)
207.9
(235, 271)
207.6
(236, 273)
173.7
(251, 289)
173.5
(256, 290)
184.3
(250, 288)
184.2
(250, 289)
183.9
(252, 291)
184.1
(251, 290)
184.2
(250, 289)
1509
1513
1510
1509
1380
1379
1379
1371
129
134
131
138
1588
1326
262
1612
1346
266
1603
1359
244
1606
1365
241
1587
1326
261
1614
1347
267
1604
1360
244
1607
1366
241
1595
1372
223
1598
1368
230
1600
1376
224
1607
1365
242
1608
1370
238
1605
1380
225
1600
1372
228
13
sym[b]
[c]
SIPr, in accord with increased deshielding of C 2. This would
appear to contradict an earlier study of electronic and steric
properties of NHC ligands by experimentally-observed IR
stretching frequencies of Ni(CO)3(NHC) complexes, where it was
established that: ‘saturated NHC ligands are not better donors
than their unsaturated analogues’ (in this case, SIPr vs IPr), and:
‘alkyl-substituted NHCs (e.g. IAd) are only marginally more
electron-donating than their aryl-substituted counterparts’.43 In
comparison, the anionic ligand (a-d) has a smaller but
discernable effect on the chemical shift of C2. In all cases,
substituting the anionic ligand with chloro-benzoate led to a
slight shielding of the C2 resonance (ca. 0.3 ppm), supported by
a slight increase in the 1J(AgC) coupling, implying that the
strength of the Ag-C bond is only marginally affected by the
carboxylate ligands.
Computational studies. DFT investigations were performed
to understand the reason behind the observed experimental
trend of the NMR resonance of the C2 atom in the
[Ag(NHC)(OAc)] complexes, using the IP, SIPr and IAd
complexes 1a, 2a and 3a as representative cases. DFTcalculated carbene NMR chemical shift () and shielding () for
the three NHC-Ag(I) complexes are summarised in Table 3.
Overall, an excellent correlation can be obtained between the
experimental and the calculated chemical shifts (Fig. 2). This
validates our subsequent analysis, based on a decomposition of
the DFT isotropic shielding into paramagnetic and diamagnetic
terms,  = p + d. The result indicates that the paramagnetic
shielding term (p) is the dominant contributor for these silver
complexes, ranging over 26 ppm, which is also consistent with
our recent findings for selenoureas and phosphinidine adducts. 44
Once the paramagnetic component is discounted, the
diamagnetic shielding of the carbenic carbon (d) only covers a
narrow range of 1 ppm. This would suggest that the C2 carbenic
carbon in these three complexes is electronically quite similar.
Table 3. Details of experimental and DFT calculated carbene NMR
chemical shielding for three NHC-Ag(I) complexes. The DFT
chemical shift, (), is referenced to TMS.
Complexes
3a
1a
2a
expt.
173.5
184.5
207.9

176.7
186.5
203.7

243.03
244.41
243.88
p
-236.27
-247.44
-264.20
tot
6.76
-3.03
-20.32
[a] Recorded in CDCl3, unless otherwise specified. c of the carbenic carbon in
ppm. Value in parenthesis correspond to 1J(107/109AgC). [b] (COO) absorption
bands (cm-1), recorded as solid samples. [c] asym-sym.
The chemical shifts of C2 for all the homologous series of
imidazol-2-ylidene complexes 1, 3, 5 and 6 are all found at ca.
184 ppm, compared to the SIPr complexes 2a-d, which were
recorded downfield at 207.9 ppm. Conversely, the C2 resonance
of the IAd complexes 4a and 4b was recorded upfield at 173.5
ppm. This observed trend will initially appear to indicate
differences between the electronic properties of these NHC
ligands (saturated vs unsaturated vs alkyl-substituted), with an
increased -donation from CAg in the order: IAd < ITent <
Figure 2. Plot of carbene experimental vs. calculated chemical shifts for the
selected NHC-Ag(I) complexes.
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Table 4. Calculated bond dissociation energies (BDE) and bond lengths of
three NHC-Ag(I) complexes.
Complex
3a
1a
2a
Ag-C (kcal/mol)
68.4
68.0
67.9
Ag-Cl[a] (Å)
2.068
2.048
2.054
Ag-O[a] (Å)
2.092
2.090
2.089
Comparative NHC→Ag binding energies (BDE) are presented
against the calculated Ag-C and Ag-O bond lengths (Table 4),
revealing very similar bond strengths between the three
complexes. Indeed, the NHC→Ag is even marginally stronger
for the IAd complex (3a) than IPr (1a) or SIPr (2a), even though
it has the longer Ag-C bond length. The longer Ag-C bond and
the reduced stability of complex 3a are therefore attributed to
greater kinetic lability associated with its smaller %Vbar volume.
The large paramagnetic shielding (p) observed in the 13C
NMR data deserves further comment. It is known that p results
from transitions of electrons between symmetry-related occupied
and virtual orbitals. For metal-NHC complexes this is related to
transitions between the filled  orbital (the lone pair) and the
empty p orbital of the carbene; the amount of the paramagnetic
shielding is directly related to the energy difference between
these orbitals.45 With this in mind, we focused on the energy
transitions from filled molecular orbitals (MOs) corresponding to
the  NHC-Ag bond, oriented along the x-axis, to empty MOs
corresponding to the π NHC-Ag bond, perpendicular to the xy
plane (Fig. 3). This analysis allowed to us select transitions from
a set of six filled MOs, having significant contribution from the
NHC-Ag -bond, to a set of four empty MOs having significant
contribution from the NHC-Ag π bond (see Fig. S22, SI). An
Illustrative example of the highest occupied and lowest
unoccupied orbitals (HOMO and LUMO) shows maximum
electron density along the IPr-Ag bond in the HOMO, while the
LUMO is mostly localised on the carbene atom (Fig. S23, SI). In
the case of complex 3a, the total contribution of these Ag-NHC
() → Ag-NHC (π*) transitions result in a paramagnetic shielding
of -134.8 ppm, while the same transitions contribute -151.3 and 162.3 ppm for complexes 1a and 2a, respectively, resulting in a
downfield shift of the carbenic signal of 1a and 2a by 16.5 and
27.5 ppm relative to 3a, which are in good agreement with the
experimental evidence.
To understand the origins of such effects, the geometry of
the free NHC’s were optimised under the constraint of C 2v
symmetry, with the NHC ring lying in the xy plane and the
carbene lone pair along the x direction (orientation depicted in
Fig. 3). The HOMO and LUMO energies of the free NHC ligands
are subsequently calculated and tabulated in Table 5. The data
indicates that the energy gap between the HOMO and the
LUMO decreases in the order: IAd > IPr > SIPr. 46 This results in
stronger magnetic coupling between the HOMO and the LUMO
in SIPr, compared to IPr and IAd complexes. In summary, this
analysis suggests that the reduced HOMO-LUMO gap in the
NHCs is at the origin of the increased paramagnetic shielding
(and downfield chemical shift) of the C2 atom from 3a to 1a and
2a.
Table 5. Energy levels of the frontier orbitals and energy gaps for three NHC
ligands.
NHC
IAd
IPr
HOMO E (eV)
-4.642
-5.048
LUMO E (eV)
0.019
-0.468
HOMO-LUMO, E (eV)
4.66
4.58
Scheme 4. Gold-catalysed cyclisation of propargylic amides.
Catalytic studies. To date, cationic gold(I) complexes are the
most effective catalysts for 5-exo-dig cyclisation of propargylic
amides oxazoline derivatives (Scheme 4), 47-49 including
substrates with terminal substituents (R4 ≠ H), under mild
conditions. However, competitive formation of the 6-endo-dig
oxazone product 9 are reported in certain cases.
In our earlier work, we have shown that reaction catalysed
by silver-pyridyl complexes proceeds exclusively to the
oxazoline 8.26 As an initial benchmark, the cyclisation of the
unsubstituted substrate 7a to the oxazoline 8a were monitored
by 1H NMR spectroscopy in the presence of selected NHC-Ag
complexes at ambient temperature (Fig. 3). We were delighted
to observe that the rate of the reaction is faster using the NHC
ligands. Significantly, the turnover is dependent upon both the
NHC and carboxylate ligands – this serves as an interesting
contrast to previous studies using NHC-Au catalysts, where the
nature of the carbene ligand was found to have negligible
effect.50 Control experiments were conducted using the
corresponding silver(I) acetate and benzoate salts – none of
which were active in the absence of NHC ligands (Table S6, SI).
Similarly, perhaps unsurprisingly, the unstable IAd complexes 3a
and 3b were catalytically inactive.
Synergistic effects between the NHC and carboxylate
ligands can be demonstrated by comparing the catalytic activity
of silver(I) complexes of IPr (1a and 1b) with their saturated
congener SIPr (2a and 2b) (Fig. 3A). Overall, the SIPr complex
afforded faster turnover than IPr complexes. However, this effect
is more prominent in the acetate than the benzoate complexes.
The effect of the wingtip N-substituents on catalytic turnover
was investigated by comparing results obtained with the series
of ITent-Ag complexes (Fig. 3B). The [Ag(IPent)(OAc)] (4a) was
found to be much more active than the corresponding IPr or SIPr
complexes (1a and 2a); its catalytic activity can be enhanced
further by using its chlorobenzoate complex 4d. Extending the
N-alkyl chain length, however, did not lead to any improvement:
IHept (5d) and INon (6d) complexes afforded slower reactions;
complete conversions were eventually realised in 8 and 12 h,
respectively (Table S6, SI). This suggests that while the
extended alkyl chain length does not affect the immediate steric
environment (%Vbur), steric interactions in the secondary
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coordination sphere can impede the coordination of the
substrate to the active site, leading to slower turnover.
Figure 3. Relative rates of cyclisation of 7a to 8a, monitored by 1H NMR
spectroscopy. Reaction conditions: 7a (0.4 mmol, 1.0 eq.), [Ag(NHC)(O2CR)]
(0.04 mol, 0.1 eq.), CD2Cl2 (1.0 mL), 23 °C.
Last but not least, the study concluded with a brief exploration of
substrate scope using the most active [Ag(IPent)(4-Cl-Bz)]
complex 4d (Figure 4). In previous work,26 silver-pyridyl catalysts
was found to be effective for the cyclisation of non-terminal
propargylic amides, but was ineffective for reactions with
substrates containing electron-withdrawing amide substituents
(R1). With this in mind, the cyclisation of a number of propargylic
amides was examined using 5 mol% of complex 4d, and the
results were compared to that achieved previously using 10
mol% [Ag(MeO-Py)2]PF6 under identical conditions.
Figure 4. Cyclisation of terminal propargylic amides. Unless otherwise stated,
reactions were conducted using substrate 7a-i (0.4 mmol), 4d (5 mol%) in
CD2Cl2 (1 mL), 23 C, 18 h. Conversions were determined by 1H NMR
spectroscopy (1,3,5-trimethoxybenzene internal standard). Values in
parenthesis were results obtained using 10 mol% [Ag(MeO-Py)2]PF6 in CD2Cl2
(23 °C, 24 h).26 [a] 24 h. [b] 1 mol% catalyst, 0.25 h. [c] 1 h. [d] 5 mol%, 0.5 h.
Initial tests were performed with substrates 7a-g with different
amide substituents (R1). In all but one case (7d→8d), higher and
faster turnovers were achieved using the NHC complex.
Particularly gratifying are the substantial improvements achieved
with substrates 7e-g, where the Ag-pyridyl system was
ineffectual. The cyclisation of substrate 7g to 8g is particularly
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notable. This reaction was only reported once before: Using 5
mol% of [Au(PPh3)]OTs, the reaction proceeded to 87%
conversion in 48 h.49 In comparison, a comparable 92% yield
can be obtained using catalyst 4d in just 18 h under essentially
the same reaction conditions. On the other hand, double
cyclisation of di-propargylic amide substrates 7h and 7i
proceeded in 65% and essentially quantitative conversions to 8h
and 8i, respectively. The lower productivity of the former was
attributed to catalyst inhibition by chelation of the bis-oxazolidine
8h to the silver catalyst. Conversely, the cyclisation of substrate
7j to 8j was achieved in 20 minutes at 1 mol% catalyst loading
(entry 10), facilitated by the presence of gem-dimethyl groups at
the propargylic position (Thorpe-Ingold effect). This also allowed
the cyclisation of the non-terminal alkyne substrate 7k to be
achieved in a good yield. In contrast, the silver(I)-pyridyl catalyst
proved to be superior for the cyclisation of 7l and 7m. These
results demonstrate that the silver-NHC catalyst is rather
sensitive to bulky substituents at the propargylic position. It is
conceivable that catalytic activity may be re-established by
exploring the synergistic effects between the neutral and anionic
ligands, but this is beyond the scope of this study.
Conclusions
A series of fifteen new [Ag(NHC)(carboxylate)] complexes has
been prepared, containing saturated and unsaturated NHC
(including ITent), as well as acetate and benzoate ligands. The
steric and electronic properties of these complexes have been
comprehensively and systematically explored by spectroscopic,
crystallographic and DFT methods. These studies suggest that
the coordination chemistry and stability of these complexes are
dominated largely by steric, rather than electronic effects
imposed by the NHC ligand. The steric environment surrounding
the Ag metal centre is responsive to ligand saturation, wingtip
substituents and, in the case for IPent, the nature of the
carboxylate ligand.
The catalytic activity of the NHC-Ag-carboxylate
complexes was assessed in the cyclisation of propargylic
amides to give oxazolines. In contrast with Au-NHC complexes,
ligand acceleration effect is clearly observed using these Ag
catalysts. Comparisons were also made with previously reported
pyridyl-Ag complexes. In all cases, reaction proceeded under
mild conditions to afford 5-exo-dig oxazoline products. The use
of [Ag(IPent)(OBz-Cl)] 4d overcomes limitations of earlier
catalytic systems, particularly towards electron-deficient amide
substituents. On the other hand, [Ag(4-MeO-Py)2][PF6] is a
better catalyst for substrates containing substituents at the
propargylic and terminal substituents, i.e. two silver catalytic
systems are highly complementary in their reaction scope. The
most significant result of this study is the observation of
synergistic effects between the NHC and carboxylate ligands.
This is interesting, as it can potentially offer independently
tunable sites to optimize catalyst stability and reactivity towards
a certain substrate. It is conceivable that the better catalytic
performance may be achieved if saturated series of ITent
ligands, i.e. SITent, are available. The synthesis of these new
[Ag(NHC)(carboxylate)] complexes, and their applications in
other catalytic reactions, will be explored in future work.
Experimental Section
General procedure for the synthesis of [(Ag(NHC)(O2CR)]
complexes. A mixture of NHCHBF4 (0.2 mmol, 1 eq.) and Ag(O2CR)
(0.24 mmol, 1.2 eq.) in CH2Cl2 (5 or 10 mL) was stirred for 15 min and
K2CO3 (4.0 mmol, 20 eq.) was added. After a prescribed period of time (x
h, see SI), the reaction mixture was filtered through Celite and solvent
was removed in vacuo. The residue was washed with 5 mL of Et2O to
obtain the crude product as a white solid. Recrystallisation from
CH2Cl2/hexane at 0 ºC or -20 ºC afforded the desired product as
colourless crystals. Single-crystal X-ray diffraction data for all 15
complexes have been deposited with the Cambridge Cystallographic
Data Centre (CCDC 1449329 to 1449343).
Computational details. All calculations were performed using density
functional theory (DFT) with the gradient corrections for exchange and
correlation proposed by Becke51 and Perdew52 (BP86), as implemented
in the ADF package,53-55 in combination with a fine integration parameter
(with a numerical integration parameter set to 5). A tripletwo polarization functions on all atoms (TZ2P) was used. Electrons of the
core shells have been treated within the frozen core approximation. 56
Scalar relativistic corrections and spin orbit were included with the
zeroth-order regular approximation (ZORA).57 All geometries were
optimized without any symmetry constraint. These geometries were used
to obtain the carbene NMR chemical shielding properties (including spin
orbit corrections) in the studied NHC-Ag(I) complexes. We included spin
orbit term in the NMR calculations to understand the spin orbit coupling
effects and to estimate the amount of electron transitions from the filled
NHC-Ag (σ) MO to the empty NHC-Ag (π*) MO and their contribution to
the overall calculated paramagnetic shielding.
General procedure for Ag(I)-NHC catalysed conversion of 7 to 8. The
substrate (0.2 mmol) and the 1,3,5-trimethoxybenzene internal standard
(ca. 6.8 mg) were dissolved in CD2Cl2 (0.5 mL) at room temperature,
followed by the catalyst (0.01 or 0.05 equiv.). The solution was
transferred to an NMR tube, and the reaction progress was monitored at
ambient temperature by 1H NMR spectroscopy. The yields and
conversions were calculated by comparison of the integrals of the
product and substrate resonances to that of the internal standard.
Acknowledgements
The authors are grateful to NUS and Imperial College London
for the award of a scholarship (to VHLW). LC and SPN thank
KAUST for support through the CCF project.
Keywords: Silver • N-heterocyclic carbene • Catalysis • Ligand
effects
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