Organonitrile ligated silver complexes with perfluorinated weakly coordinating anions and their catalytic application for coupling reactions

Article abstract of DOI:10.1039/b414060e

Homogeneous catalytic processes mediated by silver(I) complexes are relatively rare. This work describes the synthesis and characterization of acetonitrile ligated silver salts with three weakly coordinating anions [B(C₆F₅)₄]^- [B{C₆H ₃(CF₃)₂}₄]^- and [(C ₆F₅)₃B-C₃H₄N ₂-B(C₆F₅)₃]^-. The silver cation is coordinated either by four or by two acetonitrile ligands. All examined Ag(I) complexes show catalytic activity in coupling reactions of terminal alkynes with aldehydes and amines. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b414060e

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P A P E R
Organonitrile ligated silver complexes with perfluorinated weakly
coordinating anions and their catalytic application for coupling
reactions
Yanmei Zhang,a Ana M. Santos,a Eberhardt Herdtweck,a Janos Minkbc and Fritz E. Ku
¨
hn*a
a Anorganisch-chemisches Institut der Technischen Universitat Munchen, LichtenbergstraXe 4,
D-85747 Garching bei Munchen, Germany. E-mail: fritz.kuehn@ch.tum.de
¨
¨
¨
b Molecular Spectroscopy Department, Chemical Research Centre of the Hungarian
Academy of Sciences, PO Box 17, H-1525 Budapest, Hungary
c Analytical Chemistry Research Group of the Hungarian Academy of Sciences,
´ ´
Veszprem University PO Box 158, H-8201 Veszprem, Hungary
Received (in Durham, UK) 13th September 2004, Accepted 17th December 2004
First published as an Advance Article on the web 17th January 2005
Homogeneous catalytic processes mediated by silver(I) complexes are relatively rare. This work describes the
synthesis and characterization of acetonitrile ligated silver salts with three weakly coordinating anions
[B(C6F5)4]ꢀ, [B{C6H3(CF3)2}4]ꢀ and [(C6F5)3B–C3H4N2–B(C6F5)3]ꢀ. The silver cation is coordinated either
by four or by two acetonitrile ligands. All examined Ag(I) complexes show catalytic activity in coupling
reactions of terminal alkynes with aldehydes and amines.
bromobenzene with n-butyllithium in diethyl ether at ꢀ78 1C;
Introduction
after reacting for ca. 30 min BCl3 was added. The reaction
mixture was then brought to room temperature and the
potassium salts were obtained by adding potassium chloride.
The potassium was subsequently exchanged by silver through
the addition of silver nitrate. K[(C6F5)3B–C3H4N2–B(C6F5)3]
was synthesized according to a literature procedure18 and
converted into the corresponding silver salt by exchange with
silver nitrate.
Coordination chemistry has made extensive use of non-coor-
dinating and weakly coordinating anions. Some of the weakꢀly
coordinating, fluorine-containing anions such as BF4ꢀ, PF6
,
SO3CF3ꢀ, B(C6F5)4ꢀ, OTeF5ꢀ, etc. play a significant role in
chemistry.1 Their large size frequently aids the stabilization of
complex cations,2 and their weak coordination mode allows
the introduction of other (somewhat less) weakly coordinating
ligands3 or the creation of vacant coordination sites and
virtually ‘‘naked’’ metal atoms.4
Ag[B(C6F5)4] and Ag[(C6F5)3B–C3H4N2–B(C6F5)3] were re-
crystallized in acetonitrile at ꢀ35 1C yielding crystalline, tetra-
hedrally coordinated [Ag(NCCH3)4][B(C6F5)4] (1) and
[Ag(NCCH3)4][(C6F5)3B–C3H4N2–B(C6F5)3] (3) complexes
(Chart 1). The X-ray crystal structure of complex 1 has been
reported previously.19 The silver salt of B{C6H3(CF3)2}4] is
more difficult to crystallize. The recrystallization of it in
In spite of the fact that heterogeneous silver-catalyzed olefin
epoxidation is one of the most important and thoroughly
investigated industrial processes,5 homogeneous catalytic pro-
cesses mediated by silver(I) are relatively rare. Reports on
silver(I)-catalyzed processes include carbon–hydrogen inser-
tion,6 carbon–halogen bond activation,7 aziridination,8 silacy-
clopropanation,9
cyclopropanation11 and propargylic amine synthesis.12
ketone-hydroxylation
reactions,10
Silver ions with weakly coordinating anions are normally
used as metathesis reagents and starting materials for the syn-
thesis of asymmetric catalysts13 and supramolecules.14 A recent
investigation showed [Ag(PPh3)]1[CB11H6Br6]ꢀ to be air and
moisture stable as well as to be the best catalyst for a series of
hetero Diels–Alder reactions.15 Recently Li et al.16 as well as
other groups17 have described the direct addition of terminal
alkynes to aldehydes and amines to afford propargyl alcohols
and propargyl amines. In this paper, we report the synthesis and
characterization of nitrile ligated silver salts with three types of
weakly coordinating anions and their catalytic activity towards
the coupling reaction of terminal alkynes with aldehydes and
amines. The applied weakly coordinated anions are [B(C6F5)4]ꢀ,
[B{C6H3(CF3)2}4]ꢀ and [(C6F5)3B–C3H4N2–B(C6F5)3]ꢀ.
Results and discussion
Synthesis and characterization
Ag[B(C6F5)4] and Ag[B{C6H3(CF3)2}4] were prepared by react-
ing pentafluorobenzene bromide and 3,5-bis(trifluoromethyl)-
Chart 1 Complexes 1–3.
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
366
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 6 6 – 3 7 0

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Table 1 Selected bond lengths (A) and angles (deg) for complex 2
Ag(1)–N(1)
Ag(2)–N(2)
2.097(2)
2.066(4)
N(1)–Ag(1)–N(1_a)
N(2)–Ag(2)–N(2_b)
Ag(1)–N(1)–C(1)
Ag(2)–N(2)–C(3)
180
180
171.9(3)
177.2(3)
Complexes 1–3 were additionally characterized by elemental
1
analysis, IR and H NMR spectroscopy.
The shifts corresponding to the nCN adsorption (CN asym-
metric stretching vibration) in the IR spectra are changed from
2266.5 cmꢀ1 for gaseous CH3CN to 2286 cmꢀ1, 2296 cmꢀ1 and
2285 cmꢀ1 for complexes 1, 2 and 3, respectively. While the
n
CN adsorptions in the tetrahedrally coordinated compounds 1
and 3 are identical within the error range, the same band for
compound 2 is observed 10 cmꢀ1 higher. The CN stretching
frequency shifts for all examined complexes, leading to a
considerable increase of the CN stretching force constants of
compounds 1 and 3 to ca. 19 N cmꢀ1 as compared to those of
the free acetonitrile of 17.5 N cmꢀ1. In the case of complex 2
the force constant increases even more. These observations are
in good accord with what has been reported previously for
related Cu(I) complexes.20
Fig. 1 ORTEP style plot of the cationic part of compound 2 in the
solid state. Both molecules, A and B, are located on a centre of
symmetry. Thermal ellipsoids are drawn at a 50% probability level.
acetonitrile did not result in the isolation of the expected
tetrahedrally coordinated Ag(I) complex. Only an oily product
could be obtained that was, however, successfully recrystallized
from
a
dichloromethane–n-hexane solution, yielding
[Ag(NCCH3)2][B{C6H3(CF3)2}4] (2), bearing only two acetoni-
trile ligands according to elemental analysis.
1
The H NMR shifts corresponding to the methyl group of
acetonitrile (measured in methylene chloride) were observed
2.07 ppm, 2.03 ppm and 1.87 ppm for complexes 1, 2 and 3,
respectively, slightly shifted to low field in comparison to
CH3CN (1.83). In this case, however, no clear difference in
the chemical shift of the CH3 protons of the linearly coordi-
nated compound 2 in comparison to the tetrahedrally coordi-
nated compounds 1 and 3 can be observed.
The molecular structure of compound 2 (Fig. 1) was deter-
mined by single-crystal X-ray crystallography.w The key bond
distances and angles are listed in Table 1. The silver atoms
appear linearly coordinated. The bond distances for Ag–N in
complex 2 are ca. 20 pm shorter than those of complex 1 where
Ag is tetrahedrally surrounded by acetonitrile ligands (Table
2). In the case of closely related Cu(I) complexes similar
observations were made.20 The linearly coordinated
[Cu(NCCH3)2]1 compounds display significantly shorter
M–N bonds than observed in tetrahedrally coordinated
[Cu(NCCH3)4]1, where the Cu–N bonds are ca. 0.15 A longer
Thermogravimetry/mass spectrometry
Complex 1 shows its first decomposition onset at 80 1C, being a
mass loss of ca. 10%. Since an analogous decomposition step
can not be found in acetonitrile free K[B(C6F5)4], it must
correspond to the loss of two acetonitrile ligands, amounting
to 8.62% of the total mass of compound 1. This interpretation
is further supported by mass spectroscopy (MS) of the frag-
ments observed during the decomposition process (TG-MS). In
the first decomposition steps no fluorine or CF containing
fragments but extensive amounts of CN and CH3 containing
fragments are detected. The second decomposition step starts
at 130 1C and corresponds to a mass loss of ca. 60% of the
original mass. Since K[B(C6F5)4] shows a mass loss of 50%
starting at approximately the same temperature it can be
argued that this decomposition step accounts for anion frag-
mentation as well as the loss of two additional acetonitrile
molecules (8.62%). In the second step fragments of both the
nitrile ligands and CF containing moieties are found by MS
spectroscopy. The last decomposition step at 270 1C associated
with ca. 10% mass loss in the case of complex 1 also occurs in
K[B(C6F5)4] at the same temperature and the same, fluorine
containing fragments are found and therefore can be assigned
to further anion degradation. Compound 2 shows a higher
thermal stability than compound 1 (decomposition starting at
140 1C) and decomposes in only one step. Since only two
acetonitrile molecules are bonded to the metal, one can argue,
in good agreement with the X-ray results, that these two
ꢀ
than in the linear Cu compound.20 Interestingly, the B(C6F5)4
counter ion, allowing a linear coordination of the Cu atom by
NCCH3 molecules, does not permit such a coordination in the
Ag(I) case as seen by the tetrahedral coordination of complex 1.
While in the case of [Cu(NCCH3)2][B(C6F5)4] a weak fluorine–
Cu interaction can be found (F–Cu ¼ 271 pm),20 the distance
between Ag and F is 313 pm in the case of compound 2 and
therefore nearly identical with the sum of the van der Waals
radii (319 pm) of Ag and F. In the Cu–F case the sum of the
van der Waals radii amounts to 287 pm. In the case of the Cu
complexes the bond length difference between linear and
tetrahedral Cu–N bonds is ca. 15 pm. We were not able to
isolate an ‘‘intermediate’’ compound, where the Ag(I) ion is
coordinated by three acetonitrile ligands. The same inability
has been reported for the closely related Cu compounds.20
The results presented here demonstrate the ease of acetoni-
trile ligand removal once again (as had been observed already
for related Ag(I) complexes21), by showing the structures of an
acetonitrile rich and an acetonitrile poor compound. Most
interestingly, in both cases of compound 1 and of compound
2, no interaction with the counter ion is observed, which
contrasts the linearly coordinated Cu case.
w C36H18AgBF24N2, Mr ¼ 1053.20, data collected at 123 K, colorless
ꢀ
fragment (0.38 ꢁ 0.43 ꢁ 0.81 mm), triclinic, P1 (No. 2), a ¼ 12.6083(1),
b ¼ 12.8156(1), c ¼ 13.7369(1) A, a ¼ 75.0063(4), b ¼ 76.7008(4), g ¼
64.5828(4)1, V ¼ 1918.45(3) A3, Dc ¼ 1.823 cmꢀ3, Z ¼ 2, m ¼ 0.674
Table 2 Selected bond lengths (A) and angles (deg) for complex 1
mmꢀ1, reflections collected 39 206, independent reflections 7026 (Rint
¼
Ag–N(1)
Ag–N(2)
Ag–N(3)
Ag–N(4)
2.281(3)
2.265(3)
2.279(3)
2.298(3)
N(1)–Ag–N(2)
N(1)–Ag–N(3)
N(1)–Ag–N(4)
N(2)–Ag–N(3)
N(2)–Ag–N(4)
N(3)–Ag–N(4)
103.43(12)
125.53(12)
98.77(12)
107.29(12)
124.86(12)
99.00(12)
0.042), observed reflections (I 4 2s(I)) ¼ 5921, data/restraints/para-
meters 7026/0/678, R1 (observed/all data) 0.0369/0.0467, wR2 (ob-
served/all data) 0.0886/0.0940, GOF (observed/all data) 1.022/1.022.
CCDC reference number 258315. See http://www.rsc.org/suppdata/nj/
b4/b414060e/ for crystallographic data in .cif or other electronic
format.
N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 6 6 – 3 7 0
367

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remaining nitriles are more strongly bound than the nitrile
ligands in compound 1 and that their displacement occurs
concomitantly with the degradation and loss of the anion. The
mass loss is finished at 270 1C with a residual value of 12%.
Compound 2 contains 10.24% Ag, which contributes, also
according to EA of the residue, the bulk of the remaining
material.
Astonishingly, compound 3 displays the highest thermal
stability of the Ag and K complexes examined here. The
presence of the acetonitrile stabilized silver cation seemingly
even increases the stability of the complex in comparison to the
precursor complex K[(C6F5)3B–C3H4N2–B(C6F5)3]. This latter
complex displays its first decomposition step at ca. 120 1C. In
the case of compound 3 two decomposition steps can be
observed with onsets at 200 1C and 340 1C and associated with
mass losses of 35 and 30%, respectively, which can be assigned
to anion decomposition and to the loss of the acetonitriles
(12% of the original mass). The C6F5 groups contribute 74%
to the total mass of compound 3 and can therefore be only
partially lost during these decomposition steps. However, the
loss of the fluorine containing fragments takes place in both
decomposition steps according to MS. K[(C6F5)3B–C3H4N2–
B(C6F5)3] shows three decomposition steps with onsets at ca.
120 1C, 300 1C and 480 1C. All three steps are associated with
the loss of F containing fragments, according to MS.
Chart 2 Reaction products from Table 3 (entries 1–18).
compound 2 in entry 15 is due to higher workup losses during
the isolation of the product. Lowering the catalyst charge from
3 to 0.3% does not decrease the activity significantly (entries 13
and 14), which makes this catalyst comparable to the best
catalysts described in the literature.12
Catalysis of coupling reactions with complexes 1–3
Complexes 1–3 were found to be effective catalysts for the
three-component coupling reaction of phenylacetylene, alde-
hydes and amines to generate propargylamines in toluene [eqn.
(1)]. The catalytic reactions were performed without using any
co-catalysts or additives. Phenylacetylene was coupled with
several aldehydes and dialkylamines, the results being summar-
ized in Table 3 and Chart 2. Both aromatic and aliphatic
aldehydes were found to undergo addition reactions to afford
the corresponding three-component propargylic amines effec-
tively. The coupling reaction was highly affected by the nature
of aldehydes, for instance the use of 2-pyridine aldehyde led to
the lowest conversion obtained in this work. Complexes 1–3
show a similar catalytic behavior as can be seen by comparing
entries 13, 15, 18 and 10, 16, 17. The lower yield obtained for
ð1Þ
Previous experiments have shown that metallic silver does
not show any catalytic activity,12 rendering therefore unlikely
the possibility of a reduction of the applied complexes to Ag(0)
during the catalytic cycle. The mechanism most likely involves
the substitution of the H of the alkyne by an Ag(I) species,
probably involving the displacement of one or more acetoni-
trile molecules in solution. This silver acetylide intermediate
can then react with the iminium ion resulting from the reaction
of aldehydes with secondary amines, regenerating the cata-
lyst.16 Experiments to clarify the nature of the catalytically
active species are currently under way in our laboratories.
Table 3 Coupling of aldehydes, phenylacetylene and amines catalyzed
by the silver(I) complexes (1–3) in toluene
Entrya
Aldehyde
Amine
Yield(%)b
Conclusion
1
PhCHO
Piperidine
Piperidine
96
92
87
96
98
80
91
90
95
85
41
98
93
94c
82
95
91
90
2
Ph(CH2)2CHO
p-MePhCHO
o-MePhCHO
c-C6H11CHO
p-MeOPhCHO
p-CF3PhCHO
m-MePhCHO
p-MePhCHO
o-MePhCHO
2-PyCHO
Complexes 1–3 are easily accessible and can be obtained in
high yields. The X-ray crystal structure of compound 2 (bear-
ing only two acetonitrile ligands) was determined and its
features compared with the X-ray crystal structure of com-
pound 1 (having four, more weakly coordinated acetonitrile
ligands), which was determined recently and with closely
related Cu(I) complexes. Compounds 1–3 have proven to be
effective catalysts for the coupling reaction of phenylalkyne,
aldehydes, and amines in toluene at 75 1C without using
co-catalysts or additives. Yields up to 98% can be achieved.
3
Piperidine
Piperidine
4
5
Piperidine
Piperidine
6
7
Piperidine
Piperidine
8
9
Pyrrolidine
4-Me-piperidine
Pyrrolidine
Pyrrolidine
4-Me-piperidine
4-Me-piperidine
4-Me-piperidine
Pyrrolidine
Pyrrolidine
4-Me-piperidine
10
11
12
13
14
15
16
17
18
a
o-MePhCHO
p-MePhCHO
p-MePhCHO
p-MePhCHO
o-MePhCHO
o-MePhCHO
p-MePhCHO
Experimental
All preparations and manipulations were carried out under
argon atmosphere using standard Schlenk techniques. Methyl-
ene chloride was distilled over calcium hydride, n-hexane
distilled over Na–benzophenone and kept over 4 A molecular
sieves, acetonitrile dried over calcium hydride and kept over 3
A molecular sieves. Unless otherwise stated, all the chemicals
were used as received from Aldrich. The mid-IR spectra were
Entries 1–14 were catalyzed by 1. Entries 15–16 were catalyzed by 2.
b
Entries 17–18 were catalyzed by 3. Yields were determined based on
c
aldehyde. Using 0.3% of 1.
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N e w J . C h e m . , 2 0 0 5 , 2 9 , 3 6 6 – 3 7 0

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recorded with a Digilab (Bio-Rad) FTS-60A interferometer
using HgCdTe detector, the far-IR spectra were measured with
a Digilab (Bio-Rad) FTS-40 interferometer using a wire mesh
beam splitter, high pressure mercury source and a deutero-
triglycinesulfate detector. Raman spectra were recorded with a
dedicated Digilab (Bio-Rad) or a Nicolet FT-Raman 950
X-Ray single crystal structure determinations
Preliminary examination and data collection were carried out
on a KappaCCD device (NONIUS MACH3) with an Oxford
Cryosystems cooling device at the window of a rotating anode
(NONIUS FR591) with graphite monochromated Mo–Ka
radiation (l ¼ 0.710 73 A). Data collection was performed
using the Collect Software.23 The detector to crystal distance
was 40 mm. A correction for absorption effects and/or decay
was applied during the scaling procedure.24 The structures
were solved by a combination of direct methods25 and differ-
ence-Fourier syntheses.26 All non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms
were found and refined with individual isotropic displacement
parameters. Full-matrix least-squares refinements were carried
1
spectrometers. H NMR measurements were performed on a
Bruker AVANCE-DPX-400 spectrometer. Elemental analysis
were measured at the Mikroanalytisches Labor of The TU
Munchen (M. Barth). The potassium precursors of compounds
¨
1–3 were prepared according to literature procedures.18,22
[Ag(NCCH3)4][B(C6F5)4](1)
2
2 2
out by minimizing Sw(Fo ꢀ Fc ) and converged with a
maximum shift/error o0.001. The final difference-Fourier
maps show no striking features. A disorder of two CF3 groups
could be resolved clearly. The asymmetric unit cell contains
two crystallographically independent molecules A and B. Both
are located on a centre of symmetry. CCDC reference number
258315. See http://www.rsc.org/suppdata/nj/b4/b414060e/ for
crystallographic data in .cif or other electronic format.
A dry acetonitrile (10.0 ml) solution of silver nitrate (0.204 g,
1.2 mmol) was added to a (15.0 ml) acetonitrile solution of
K[B(C6F5)4]21 (0.861 g, 1.2 mmol). A white precipitate of
KNO3 began to form immediately. The reaction mixture was
stirred for about 10 min and diethyl ether was added to
accelerate precipitation. After the reaction the solution phase
was removed by filtration. The filtrate was concentrated in
vacuo to dryness (without heating). The solid was redissolved
in dichloromethane while minimizing light exposure and the
remaining KNO3 was removed by filtration. The filtrate was
brought to dryness and the solid was redissolved in acetonitrile.
The solution was reduced to 2.0 ml under oil pump vacuum
and kept at ꢀ35 1C. The desired product was obtained as a
white crystalline solid. Yield 0.936 g (82%). Calcd. for
C32H12AgBF20N4: C 40.41; H 1.27; N 5.89. Found: C 40.52;
H 1.30; N 5.82%. Selected IR (KBr, cmꢀ1): nCN, 2266, 2286.
1H NMR (400 MHz, CDCl3, rt, d(ppm)) 2.07 (CH3, s, 12H).
Typical procedure for coupling reactions of aldehydes,
phenylacetylene and amines
(3.0 mmol, 306.0 mg) phenylalkyne, (2.2 mmol, 187.0 mg)
piperidine, (2.0 mmol, 224.0 mg) and (0.06 mmol, 54.0 mg)
[Ag(NCCH3)4][B(C6F5)4] were heated in 1.0 ml toluene at
about 75 1C. The product was purified by chromatography
with ethyl acetate and hexane, being the yields given isolated
yields. The identity of the product was proven by GC-MS and
1H NMR measurements.
[Ag(NCCH3)2][B{C6H3(CF3)2}4] (2)
Acknowledgements
Silver nitrate (0.52 g, 3.06 mmol) was dissolved in 10 ml dry
The DFG (Deutsche Forschungsgemeinschaft) and the FCI
(Fonds der Chemischen Industrie) are acknowledged for finan-
cial support. AMS thanks the Alexander von Humboldt
Foundation for a postdoctoral research grant. FEK and JM
are grateful to the DAAD for financial support.
acetonitrile and the solution added to
a solution of
K[B{C6H3(CF3)2}4]21 (2.76 g, 3.06 mmol) in acetonitrile. A
white precipitate of KNO3 formed immediately. The reaction
mixture was stirred for another 10 min and diethyl ether was
added to complete precipitation. After filtration, the resulting
solution was brought to dryness by oil pump vacuum and the
resulting solid redissolved in dichloromethane in darkness. The
solution was layered with hexane at ꢀ35 1C and the desired
product was obtained as a white crystalline solid. Yield 2.32 g
(72%). Calcd. for C36H18AgBF24N2: C 41.06; H 1.75; N 2.66.
Found: C 40.99; H 1.71; N 2.65%. Selected IR (KBr, cmꢀ1):
nCN, 2296. 1H NMR (400 MHz, CDCl3 rt, d(ppm)): 2.03
(CH3, s, 12H), 7.65–7.78 (C6H3, s, 12H).
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18
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mixture was stirred for another 10 min and diethyl ether was
added to complete precipitation. After filtration, the resulting
solution was brought to dryness by oil pump vacuum and the
resulting solid redissolved in dichloromethane in darkness The
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again evaporated to dryness, redissolved in acetonitrile, va-
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41.67; H 1.23; N 6.64%. Selected IR (KBr, cmꢀ1) nCN,
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12H), 6.74 (C3H3, s, 3H) 7.44 (CH, s, 1H).
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