Fluorinated Anions Promoted "on Water" Activity of Di- and Tetranuclear Copper(I) Catalysts for Functional Triazole Synthesis

Article abstract of DOI:10.1021/acs.organomet.5b00348

A set of di- and tetra-copper(I) compounds [Cu₂(L¹H)₂][BAr^F]₂ (1) (L¹H = bis(5,7-dimethyl-1,8-naphthyridin-2-yl)amine; BAr^F = [B{C₆H₃(CF₃)₂}₄]) and [Cu₄(L¹)₂(L²)₂][BNB^F]₂ (2) (L² = 5,7-dimethyl-1,8-naphthyridin-2-amine; BNB^F = [NH₂{B(C₆F₅)₃}₂]), stabilized by naphthyridine-based ligands and containing fluorinated anions, is synthesized. Their catalytic utility for copper(I)-catalyzed azide-alkyne coupling (CuAAC) reactions in organic solvents and "on water" is evaluated. The dimer analogue [Cu₂(L¹H)₂][BPh₄]₂ (3) with nonfluorinated anion is synthesized for the purpose of comparison. All three compounds show CuAAC activity in organic solvents, although the performance of 3 is considerably lower. Remarkable rate enhancement is displayed by compounds containing fluorinated anions (1 and 2) under "on water" conditions for the model reaction involving benzyl azide, affording 98% conversions in 15-20 min, where compound 3 gives 76% conversions in 50 min. Kinetic experiments reveal the involvement of two coppers in the cycloaddition process. Employing a host of substrates, the usefulness of fluorinated anions to dramatically improve the catalytic activity of Cu(I) compounds under "on water" conditions is demonstrated. (Figure Presented).

Full text of DOI:10.1021/acs.organomet.5b00348

Article
pubs.acs.org/Organometallics
Fluorinated Anions Promoted “on Water” Activity of Di- and
Tetranuclear Copper(I) Catalysts for Functional Triazole Synthesis
Sayantani Saha, Mandeep Kaur, and Jitendra K. Bera*
Department of Chemistry and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur,
208016, India
S
* Supporting Information
ABSTRACT: A set of di- and tetra-copper(I) compounds [Cu2(L1H)2]-
[BArF]2 (1) (L1H = bis(5,7-dimethyl-1,8-naphthyridin-2-yl)amine; BArF =
[B{C6H3(CF3)2}4]) and [Cu4(L1)2(L2)2][BNBF]2 (2) (L2 = 5,7-dimethyl-
1,8-naphthyridin-2-amine; BNBF = [NH2{B(C6F5)3}2]), stabilized by
naphthyridine-based ligands and containing fluorinated anions, is synthe-
sized. Their catalytic utility for copper(I)-catalyzed azide−alkyne coupling
(CuAAC) reactions in organic solvents and “on water” is evaluated. The
dimer analogue [Cu2(L1H)2][BPh4]2 (3) with nonfluorinated anion is
synthesized for the purpose of comparison. All three compounds show
CuAAC activity in organic solvents, although the performance of 3 is
considerably lower. Remarkable rate enhancement is displayed by
compounds containing fluorinated anions (1 and 2) under “on water”
conditions for the model reaction involving benzyl azide, affording 98%
conversions in 15−20 min, where compound 3 gives 76% conversions in 50 min. Kinetic experiments reveal the involvement of
two coppers in the cycloaddition process. Employing a host of substrates, the usefulness of fluorinated anions to dramatically
improve the catalytic activity of Cu(I) compounds under “on water” conditions is demonstrated.
higher than in neat conditions, prompting Sharpless to ascribe
this as an “on water” effect.8
INTRODUCTION
■
The application of organic solvents for organic reactions stems
from solubility and reactivity considerations. Access of light
organics from petrochemicals accompanied by a rapid growth
of organometallic chemistry fueled the use of organic solvents
as a medium for chemical reactions. Environmental and safety
concerns, however, have forced researchers to develop green
and sustainable methods.1 One of the important elements to
pursue green chemistry is to employ cheap and nonhazardous
solvents. Water is obviously an ideal choice.2 Besides being
cheap and nontoxic, water with its the high heat capacity allows
exothermic reactions to be carried out safely. Further, the
product isolation is simpler. Despite all these benefits, the low
water solubility of organics has prevented widespread use of
water in organic reactions. Designing water-soluble catalysts
and reagents is not a viable solution since it cripples the range,
tolerance, and effectiveness of the reactions.3
There has been continued efforts to use water as solvent for
organic reactions. In 1980, Breslow observed significant rate
acceleration of the Diels−Alder reaction in homogeneous
aqueous solution.4 Since then a number of reports have
appeared where organic reactions are accelerated in water.2b,5
The selectivity too is reported to improve in aqueous medium.6
The most interesting discovery was made by Sharpless in 2005.
Unusual rate acceleration of a [2σ + 2σ + 2π] cycloaddition of
quadricyclane and dimethylazodicarboxylate was observed even
when the reactants are not soluble in water.7 The rate
enhancement in aqueous suspension was found significantly
The Cu(I)-catalyzed azide and alkyne coupling (CuAAC)
reaction is the most convenient method for the synthesis of
functional triazole, which is shown to be an effective linker
between building units of biological relevance and resembles
certain characteristics of a peptide bond.9 The cycloaddition
reaction rate is enhanced and the regioselectivity (1,4-
regioisomer) is improved by Cu(I). The CuAAC reaction
tolerates most functional groups, exhibits wide substrate scope
with respect to both azides and alkynes, and operates in a
variety of solvents and a wide range of pH.10 Different Cu(I)
sources are successfully used in this reaction including Cu(I)
salt (CuI), in situ reduction of Cu(II) (particularly CuSO4 and
the reducing agent ascorbic acid), and comproportionation of
Cu(II) and Cu(0).10b,11 Although ligand-free reactions are fairly
effective, nitrogen donor ligands are recognized to play many
significant roles, for example, stabilizing Cu(I) from undesired
oxidation, acting as a base for alkyne deprotonation, and
generating catalytically active clusters from metal aggregates.12
Kinetic, calorimetric, isotope crossover experiments, and DFT
calculations clearly suggest a second-order rate with respect to
Cu.13 It is now well accepted that the reaction proceeds
through a stepwise mechanism where alkyne and azide bind to
two different but closely spaced Cu ions. The second Cu
Received: April 26, 2015
© XXXX American Chemical Society
A
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activates the azide and further engages in π-interaction with
terminal acetylide bound to the first copper. The overall
activation barrier is computed to be substantially lower than a
single Cu pathway.14
We recently demonstrated that fluorinated anion BArF
[tetrakis(3,5-bis(trifluoromethyl))phenyl borate] (Scheme 1a)
Scheme 1. BArF (a) and BNBF (b) Anions and Ligand L1H
(c) Employed in This Work
Figure 1. Dicationic unit of 1 with selective atom labeling.
Displacement ellipsoids are set at 30% probability, and hydrogen
atoms are omitted for clarity. The prime character (′) indicates atoms
at the symmetrically equivalent positions. Selected bond distances (Å)
and angles (deg): Cu1−Cu1′ 2.4813(13), Cu1−N2 1.982(5), Cu1−
N4 2.034(4), Cu1−N5′ 1.935(4), N5′−Cu1−N2 129.1(2), N5′−
Cu1−N4 139.1(2), N2−Cu1−N4 89.46(19), N5′−Cu1−Cu1
88.56(14), N2−Cu1−Cu1 119.67(13), C11−N3−C10 127.8(5).
induces “on water” activity of the Ag(I) salt for the hydration of
terminal alkynes.15 To widen the scope of this chemistry for
reactions that do not necessarily utilize water as one of the
substrates, metal complexes containing fluorinated anions are
examined for “on water” CuAAC reaction. Since the active
catalysts are known to involve at least two copper ions, a 1,8-
naphthyridine-based ligand, capable of stabilizing bimetallic
systems, is used.16 A discrete dinuclear Cu(I) compound is
synthesized containing nitrogen donor ligands bis(5,7-dimeth-
yl-1,8-naphthyridin-2-yl)amine (L1H) (Scheme 1c) and two
BArF anions (Scheme 1a). Use of the ditris-
(pentafluorophenyl)borane amide (BNBF) (Scheme 1b)
anion resulted in the formation of a tertanuclear compound.
A dinuclear analogue with a nonfluorinated BPh4 anion is also
synthesized for the sake of comparison. Di- and tetranuclear Cu
complexes exhibit higher “on water” activity than in organic
solvents. However, complexes containing fluorinated anions are
evidently more effective than complexes containing non-
fluorinated anions. This work thus demonstrates the utility of
fluorinated anions to dramatically improve “on water” activity
of organometallic catalysts.
the molecule related to the other half by a C2 axis bisecting the
Cu···Cu vector. One naphthyridine unit of each ligand bridges
two copper atoms and additionally chelate one copper utilizing
the proximal nitrogen atom of the second naphthyridine. The
geometry around each copper center is distorted trigonal
planar. The Cu···Cu distance is 2.4813(13) Å. The N−Cu−N
angles imposed by three nitrogens of two ligands at Cu are
89.46(19)°, 129.1(2)°, and 139.1(2)°. The Cu−N bond
lengths vary within a range of 1.935(4)−2.034(4) Å. The
ESI-MS spectrum shows a signal at m/z ratio of 392 (where z =
2) assigned for the dicationic unit [Cu2(L1H)2]2+. The IR
spectrum confirms the absence of triflates for the bulk solid.
A reaction identical to that of 1 but with NaBNBF afforded a
tetranuclear complex, [Cu4(L1)2(L2)2][BNBF]2 (2), where L2 is
5,7-dimethyl-1,8-naphthyridin-2-amine (NP-NH2), a ligand
that is presumably generated by hydrolysis of L1H under
reaction conditions. The X-ray structure of 2 (Figure 2)
confirms a tetranuclear structure spanned by two L1 and two L2
ligands. The deprotonated ligand L1 bridges two independent
dicopper units as shown in Figure 2 utilizing two naphthyridine
nitrogens to bridge one dicopper unit and an amido nitrogen
and one naphthyridine nitrogen to bridge the second pair of
copper. Each L2 bridges two copper atoms through
naphthyridine nitrogens, and the amine unit remains free.
The asymmetric unit contains one-half of the tetramer
connected to the other half by a center of inversion. The
Cu1−Cu2 and Cu2−Cu2′ distances are 2.4559(6) and
2.7064(8) Å, respectively.
RESULTS AND DISCUSSION
■
Synthesis and Structures. Room-temperature reaction of
L1H with one equivalent of [Cu(CH3CN)4][OTf] in
acetonitrile afforded a red solid, which was characterized as
[Cu2(L1H)2][OTf]2 by ESI-MS, IR spectroscopy, and CHN
analysis. A signal at an m/z ratio of 392 (z = 2) is attributed to
the dicationic unit [Cu2(L1H)2]2+ on the basis of the mass and
isotopic distribution pattern. The IR spectrum exhibits an
absorption at ν = 1070 cm−1 for the triflate anion. Poor
solubility in common organic solvents hindered further
characterization.
Use of NaBPh4 under identical conditions resulted in a
dinuclear complex, [Cu2(L1H)2][BPh4]2 (3), in moderate yield
(55%). The relatively lower yield is due to partial anion
exchange. Analytically pure product is isolated by extraction
with dichloromethane, in which the triflate analogue is
insoluble. IR absorption reveals the absence of triflates in the
product. ESI-MS is identical to 1, suggesting a dinuclear
composition (Figure S1).
The solubility was considerably improved by exchanging the
anions. Mixing a solution of [Cu(CH3CN)4][OTf], L1H ligand,
and NaBArF in acetonitrile afforded [Cu2(L1H)2][BArF]2 (1) in
high yield (90%). The molecular structure of 1 revealed a
discrete dicopper(I) complex (Figure 1) where two L1H ligands
span two copper atoms. The asymmetric unit contains half of
B
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azide, and hexyl azide, afforded corresponding functionalized
triazoles (1v, 1w, 1x) in good yields. The catalytic performance
of 1 compares well with other copper(I) catalysts under
homogeneous conditions10a−c,14d with a notable exception of
Straub’s N-heterocyclic carbene based dicopper(I) catalyst,13c
which displays higher activity in dichloromethane.
“On Water” Condition. A dramatic increase in the rate was
noticed when the model reaction was carried out under “on
water” conditions (phenylacetylene/benzyl azide/catalyst = 1/
1/0.01 mmol; 4 mL of water), affording 98% yield within 20
min (Figure S2). This is despite the fact that both substrates
and catalyst 1 are immiscible with water. Changing the solvent
to D2O drastically lowered the yield, providing a maximum
conversion of 50% in 20 min. For a wide range of substrates
studied in this work (aliphatic or aromatic), the CuAAC
reactions proceeded much faster under “on water” conditions
than in THF. Aromatic alkynes with electron-withdrawing
groups such as p-F and o-CHO reacted readily with benzyl
azide “on water” to form the desired products in near-
quantitative yields (98−100%) within 10 min (Table 1, 1b and
1c). Electron-rich aromatic alkynes bearing electron-donating
groups such as 2,5-Me, p-OMe, and p-Me gave comparatively
lower yields (87−92%) in 30 min (Table 1, 1d, 1e, 1f).
Although 1-ethynyl-3,5-bis(trifluoromethyl)benzene (Table 1,
1i) did not react in THF, 99% yield was obtained within 10 min
under “on water” conditions. The ethynyl group on a
heterocyclic ring such as 3-ethynylthiophene and 2-ethynylpyr-
idine “on-water” afforded 92% and 95% yields within 30 min,
respectively (Table 1, 1g and 1h). The bulkier 2-ethynyl-6-
methoxynaphthalene (Table 1, 1j) gave a moderate yield of
72% after 45 min. Aliphatic alkynes (Table 1, 1k−1s and 1u)
are less reactive and took more time to reach completion, a
trend similar to that observed in THF. 1,3-Diethynylbenzene
gave the monoaddition product with one equivalent of benzyl
azide but gave a mixture of mono- and diaddition products in
80:20 ratio with two equivalents of benzyl azide. 1.7-
Dioctadyene gave exclusively the diaddition product (Table 1,
1t and 1u). Tosyl azide, trimethylsilyl azide, and hexyl azide
gave corresponding triazoles (1v, 1w, 1x) in good yields with
phenylacetylene “on water” in a short reaction time. Cyclo-
addition of terminal alkyne, attached to a purine base, gave the
corresponding triazole (92%, 30 min) (Table 1, 1y). A scaled-
up reaction with 5 mmol of benzyl azide and phenylacetylene
each in 10 mL of water did not cause any appreciable loss of
yield and purity of the product (96%, 20 min).
The catalytic efficiencies of all three compounds are
compared both “on water” and in THF for the model reaction
(Figure 3). Consumptions of alkyne were measured in regular
time intervals by GC-MS using dodecane as internal standard.
All three compounds display remarkably enhanced activity “on
water” than in THF. Compounds 1 and 2, containing
fluorinated anions, are highly active “on water” and give near-
quantitative yields in 15−20 min. Compound 3, containing
BPh4, is the least active under both conditions, but affords
product up to 76% in 50 min “on water”. Compound 2 is found
to be slightly better “on water” than 1, but the difference is
more pronounced in THF. The relative performances of 1, 2,
and 3 are compared with a selected set of substrates, and the
general trends are found to be the same as that of the model
reaction (Table 2). The “on water” protocol with Cu(I)
complexes containing fluorinated anions appears highly efficient
compared to other heterogeneous methods and aqueous and
solvent-free CuAAC reactions.19
Figure 2. Dicationic unit of 2 with selective atom labeling.
Displacement ellipsoids are set at 50% probability, and hydrogen
atoms are omitted for clarity. The prime character (′) indicates atoms
at the symmetrically equivalent positions. Selected bond distances (Å)
and angles (deg): Cu1−Cu2 2.4559(6), Cu2−Cu2′ 2.7064(8), Cu1−
N2 1.978(3), Cu1−N5′ 1.996(3), Cu1−N7 2.038(3), Cu2−N3
2.006(3), Cu2−N4′ 2.025(3), Cu2−N6 2.065(3), N2−Cu1−N5′
128.48(12), N2−Cu1−N7 115.82(12), N5′−Cu1−N7 115.65(11),
N2−Cu1−Cu2 87.20(8), N3−Cu2−N4′ 130.05(12), N3−Cu2−N6
115.71(12), N4′−Cu2−N6 113.20(11), N3−Cu2−Cu1 88.37(9).
Although BArF and BPh4 anions afforded dinuclear
complexes 1 and 3, BNBF gave tetranuclear complex 2. This
is most likely due to adventitious moisture, which breaks down
the ligand L1H to generate L2, and subsequent self-assembly of
Cu(I) and the ligands results in 2. The NaBArF and NaBPh4 are
dried well, but NaBNBF is highly hygroscopic and could not be
made completely moisture free. As a result, the ligand is
hydrolyzed, causing formation of the tetranuclear complex.17
Catalytic Studies. In Organic Solvents. To evaluate the
catalytic efficiency of 1 for the azide−alkyne cycloaddition
reaction, initial experiments were carried out using phenyl-
acetylene (1 mmol), benzyl azide (1 mmol), and catalyst 1
(0.01 mmol) in tetrahydrofuran (THF) at room temperature.
Without any additives such as nitrogen bases, full conversion to
1-benzyl-4-phenyl-1H-1,2,3-triazole was observed after 5 h.
Purification of the product by silica gel column chromatography
with petroleum ether/ethyl acetate (3:2 v/v) provided 95%
isolated yield (Table 1, entry 1a). The solvent effect was
examined using model substrates. No significant change was
observed in other organic solvents including dichloromethane
and acetonitrile. In neat conditions only 42% product was
obtained after 5 h.18 Under optimal conditions, the functional
group tolerance and substrate scope were examined in THF.
Aromatic alkynes (1a−f, 1t, Table 1) as well as heteroaromatic
alkynes (1g, 1h, Table 1) afforded excellent yields (85−98%) of
the corresponding triazoles with the exception of 1-ethynyl-3,5-
bis(trifluoromethyl)benzene (entry 1i), which did not react
under identical conditions. In general, aromatic alkynes with
electron-withdrawing groups reacted faster than the same
bearing electron-donating groups. Aromatic alkyne 2-ethynyl-6-
methoxynaphthalene with a bulky substituent (entry 1j) gave a
moderate yield (60%). All aliphatic alkynes (1k−s, 1u, 1y,
Table 1) gave products with moderate to good yields (58−
80%), and a relatively longer reaction time was required in
comparison to aromatic alkynes. Reactions of phenylacetylene
with other organic azides, such as tosyl azide, trimethylsilyl
C
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a b
,
Table 1. Compound 1-Catalyzed [3 + 2] Cycloaddition of Benzyl azide with Various Alkynes
a
b
c
d
e
Azide (1 mmol), alkyne (1 mmol), and catalyst 1 (0.01 mmol) at room temperature. Isolated yields. In THF (4 mL). On water (4 mL). Two
equivalents of azide were used.
Kinetic Studies. The initial rate of reaction was monitored
(up to ∼10−15% conversion) to determine the order with
respect to catalyst 1. Reactions were performed with varying
concentrations of 1 and equimolar amounts of phenylacetylene,
benzyl azide, and dodecane (internal standard). The initial rate
varied linearly with catalyst concentration, and the reaction was
found to be first-order with respect to 1 (Figure S3a). Further,
According to the integrated rate law for the reaction of the type
A + B → C with the restriction [A] = [B] at t = 0, the 1/
[alkyne]t vs time plot fit well to a second-order kinetics (Figure
S4). Both of these experiments thus suggest the involvement of
catalyst 1, azide, and alkyne, one molecule each, in the rate-
determining step. As one molecule of catalyst 1 consists of two
copper centers, it can be argued that the reaction is taking place
on the bimetallic platform. The effect of temperature on the
rate of the reaction of 1 was also examined. The activation
parameters were determined from the plot of ln(k/T) vs 1/T,
which was linear over the temperature range studied (303−333
K) (Figure S5). The estimated entropy of activation (ΔS⧧) is
−42.50 1.89 cal mol−1 K−1, and the enthalpy of activation
equimolar amounts of azide, alkyne, and dodecane (nazide
=
nalkyne = ndodecane) were mixed with 1 mol % of catalyst 1 in 4
mL of THF. Each aliquot of 0.2 mL was taken out at a regular
time interval and immediately quenched by flash column
chromatography, and the amount of unreacted phenylacetylene
was measured using GC-MS against an internal standard.
D
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enhanced “on water” activity, Marcus and Jung proposed a
model where one OH group out of four water molecules
protrudes out at the organic−aqueous interface (Figure S6).8f
The free surface hydroxyl groups favor “docking” of insoluble
organic molecules via a hydrogen-bonding interaction at the
interface as on a catalyst surface. This water arrangement at the
oil−water interface is attributed to a higher reaction rate. It is
argued that catalysts containing fluorinated anions interact with
the surface hydroxyl hydrogen via hydrogen bond interactions20
and thus accelerate the reaction rate compared to homoge-
neous or neat conditions. Such an interaction is absent with
nonfluorinated anions. In accordance with this model, a
vigorous mixing is needed to achieve the highest efficiency of
the catalyst by maximizing the surface area. Indeed, the reaction
is slow when carried out in D2O due to its higher viscosity
affecting efficient mixing. It is our assertion that both enforced
hydrophobic interactions and enhanced surface interactions
contribute to the improved “on water” activity promoted by the
fluorinated anions.
Figure 3. Conversion vs time plots for catalysts 1, 2, and 3 in THF and
“on water” at 30 °C for the model reaction.
(ΔH⧧) is 6.24
0.59 kcal mol−1. A large and negative ΔS⧧
CONCLUSIONS
value is consistent with a concerted cycloaddition in the rate-
limiting step.
■
In conclusion, we report here three active di- and tetranuclear
copper(I) catalysts with fluorinated BArF and BNBF as well as
nonfluorinated BPh4 for CuAAC reactions. Although these
catalysts are effective in organic solvents, the reaction rate was
greatly enhanced under “on water” conditions. More
importantly, compounds with fluorinated anions display clear
superiority “on water” compared to the one with a non-
fluorinated anion. Enforced hydrophobic interactions between
organic components on the water surface and enhanced
interaction of a fluorinated catalyst with a free hydroxyl
group at the oil−water interface are tentatively attributed to
higher activity. Inclusion of fluorinated anions in cationic
complexes to achieve improved catalytic activity “on water” is
an attractive option in terms of simplicity, short reaction time,
easy product isolation, and sustainability. Further studies are
being carried out with catalysts containing fluorinated anions
for a variety of reactions to establish a general trend and
decipher the underlying reasons for enhanced “on water”
activity.
Similar experiments were carried out with catalyst 2. The plot
of initial rate vs [catalyst 2] revealed that the order with respect
to 2 is 1.49, a value in between first-order and second-order.
The 1/[alkyne]t vs time plot is also not linear (Figure S3b). It
appears that two dicopper units in 2 do not act independently,
and reaction on one dicopper impedes the activity of the
second unit.
“On-Water” Effect. The remarkable increase in rate and
yield of the CuAAC reaction under “on water” conditions
deserves an explanation. The Cu(I) catalysts and alkynes are
dissolved in benzyl azide, creating an organic phase. It is
pertinent to mention here that the solubility of compounds 1
and 2 is higher than that of 3 in common organic solvents.
Nonpolar organic molecules when placed in water are brought
together out of mutual repulsions. The high surface tension of
water favors minimal contact between hydrophobic molecules
and polar water, allowing all components to mix intimately,
causing the reaction rate to accelerate. Furthermore, to explain
a,b
Table 2. Studies under “on Water” Condition
a
Azide (1 mmol), alkynes (1 mmol), and catalysts 1 (0.01 mmol), 2 (0.005 mmol), and 3 (0.01 mmol) on water (4 mL) at room temperature.
Isolated yields.
b
E
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C
132H62B4Cu4F60N18: C, 47.51; H, 1.87; N, 7.56. Found: C, 47.47; H,
EXPERIMENTAL SECTION
■
1.84; N, 7.52.
General Procedures. All reactions with metal complexes were
carried out under an atmosphere of purified nitrogen using standard
Schlenk vessel and vacuum line techniques. The crystallized
compounds were powdered, washed several times with dry petroleum
ether, and dried under vacuum for at least 48 h prior to elemental
Synthesis of [Cu2(L1H)2][BPh4]2 (3). To a stirring suspension of
L1H (50 mg, 0.15 mmol) in 15 mL of acetonitrile was added one
equivalent of [Cu(CH3CN)4(OTf)] (57 mg, 0.15 mmol). The
reaction mixture was allowed to stir for 1 h at room temperature
under a N2 atmosphere. Subsequently one equivalent of NaBPh4 (52
mg, 0.15 mmol) was added, and again the reaction mixture was stirred
4 h. The acetonitrile solvent was evaporated under vacuum, and the
resultant solid mass was dissolved in 10 mL of dichloromethane. The
suspension was filtered through a small pad of Celite. The solvent was
evaporated under vacuum, and the resultant solid mass was dissolved
in 1 mL of dichloromethane. A 10 mL amount of petroleum ether was
added to induce precipitation. The solid precipitate was washed and
filtered. This process was repeated three times, and the orange-red
solid product was dried under vacuum. Yield: 118 mg (55%). 1H NMR
(500 MHz, CD3CN, 292 K): δ 8.94 (br, 2H), 6.78−6.79 (m, 12H),
7.25−7.29 (m, 24H), 7.62 (m, 4H), 7.78−7.80 (m, 4H), 7.99−8.01(m,
8H), 3.10 (s, 12H), 2.64 (s, 12H). 13C NMR (125 MHz, CD3CN, 294
K): δ 163.8, 161.1, 1473, 135.9, 134.3, 129.9, 129.4, 128.6, 126.7,
125.6, 122.2, 121.7,115.5, 24.1, 18.2. ESI-MS, m/z: 392
[Cu2(L1H)2]2+. Anal. Calcd for C88H78B2Cu2N10: C, 74.23; H, 5.52;
N, 9.84. Found: C, 74.19; H, 5.48; N, 9.81.
1
analyses. H NMR spectra were obtained on a JEOL JNM-LA 500
1
MHz spectrometer. H NMR chemical shifts were referenced to the
residual hydrogen signal of the deuterated solvents. The chemical shift
is given as dimensionless δ values and is frequency referenced relative
1
to TMS for H and 13C NMR spectroscopy. Elemental analyses were
performed on a Thermoquest EA1110 CHNS/O analyzer. The GC-
MS experiment was performed on an Agilent 7890A GC and 5975C
MS system. Infrared spectra were recorded in the range 4000−400
cm−1 on a Vertex 70 Bruker spectrophotometer on KBr pellets. ESI-
MS were recorded on a Waters Micromass Quattro Micro triple-
quadrupole mass spectrometer.
Materials. Solvents were dried by conventional methods, distilled
under nitrogen, and deoxygenated prior to use. B(C6F5)3, NaNH2, 1-
bromo-3,5-bis(trifluoromethyl)benzene, and all alkynes used in the
catalysis reaction were purchased from Sigma-Aldrich. NaBPh4 was
purchased from Spectrochem Pvt. Ltd. L1 H,21 [Cu-
(CH3CN)4(OTf)],22 NaBArF,23 and Na[NH2{B(C6F5)3}2] were
synthesized according to the literature procedures.
General Procedure for the Catalytic Reaction in THF. Catalyst
(0.01 mmol for 1 and 3, 0.005 mmol for 2), alkyne (1 mmol), and
benzyl azide (1 mmol) were taken in 4 mL of THF. The reaction
mixture was stirred at room temperature inside a closed glass tube, and
the progress of the reaction was monitored by TLC. After the
completion of the reactions the products were purified by
chromatography on a silica gel column using hexane/EtOAc (3:2 v/
Synthesis of [Cu2(L1H)2](OTf)2. To a stirring suspension of L1H
(60 mg, 0.18 mmol) in 15 mL of acetonitrile was added one equivalent
of [Cu(CH3CN)4(OTf)] (69 mg, 0.18 mmol). The reaction mixture
was allowed to stir for 1 h at room temperature under a N2
atmosphere. The solvent was reduced to 1 mL. A 10 mL amount of
diethyl ether was added to induce precipitation. The solid precipitate
was washed and filtered. This process was repeated three times, and
the red-colored solid product was dried under vacuum. Yield: 174 mg
(88%). ESI-MS, m/z: 392 [Cu2(L1H)2]2+. IR (KBr, cm−1): ν(OTf)
1070. Anal. Calcd for C42H38Cu2F6N10S2O6: C, 46.57; H, 3.54; N,
12.94. Found: C, 46.53; H, 3.50; N, 12.91.
v) as eluent. The isolated products were characterized by H and 13C
NMR and ESI-MS spectra.
1
General Procedure for the Catalytic Reaction Using Water
as Solvent. Catalyst (0.01 mmol for 1 and 3, 0.005 mmol for 2),
alkyne (1 mmol), and benzyl azide (1 mmol) were taken in 4 mL of
water. The reactants are insoluble in water. The mixture was stirred
vigorously at room temperature. After completion of the reaction,
water was decanted off and the solid products were dried under
vacuum. The products were further purified by chromatography on a
silica gel column using hexane/EtOAc (3:2 v/v) as eluent. The
isolated products were characterized by 1H and 13C NMR and ESI-MS
spectra. Alternatively, the solid products were washed with hexane,
dried, and recrystallized from methanol. NMR pure products could be
obtained without any appreciable loss of yields.
Synthesis of [Cu2(L1H)2][BArF]2 (1). To a stirring suspension of
L1H (50 mg, 0.15 mmol) in 15 mL of acetonitrile was added one
equivalent of [Cu(CH3CN)4(OTf)] (59 mg, 0.15 mmol). The
reaction mixture was allowed to stir for 1 h at room temperature
under a N2 atmosphere. Subsequently one equivalent of NaBArF (134
mg, 0.15 mmol) was added, and stirring was continued for an
additional 4 h. The deep red reaction mixture was filtered through a
small pad of Celite. The solvent was evaporated under vacuum, and
the resultant solid mass was dissolved in 1 mL of dichloromethane. A
10 mL amount of petroleum ether was added to induce precipitation.
The solid precipitate was washed and filtered. This process was
repeated three times, and the red-colored solid product was dried
under vacuum. X-ray quality crystals were grown by layering
petroleum ether onto a saturated dichloromethane solution of 1
inside an 8 mm o.d. vacuum-sealed glass tube. Yield: 353 mg (90%).
1H NMR (500 MHz, CD3CN, 292 K): δ 11.99 (br, 2H), 7.46−8.21
(m, 36H), 2.51 (s, 12H), 2.44 (s, 12H). 13C NMR (125 MHz,
CD3CN, 294 K): δ 161.9, 134.7, 128.8, 127.8, 125.6, 123.5, 121.3,
117.5, 22.5 17.5. 19F NMR (500 MHz, CD3CN, 292 K): δ −62.37 (s,
48F). ESI-MS, m/z: 392 [Cu2(L1H)2]2+. Anal. Calcd for
C104H62B2Cu2F48N10: C, 49.71; H, 2.49; N, 5.58. Found: C, 49.67;
H, 2.45; N, 5.53.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic table and supporting schemes and figures are
provided. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.organomet.5b00348.
AUTHOR INFORMATION
■
Corresponding Author
Synthesis of [Cu4(L1)2(L2)2][NH2{B(C6F5)3}2]2 (2). The reaction of
L1H (51 mg, 0.15 mmol), [Cu(CH3CN)4(OTf)] (57 mg, 0.15 mmol),
and NaBNBF (164 mg, 0.15 mmol) was carried out by following a
procedure similar to that described for the synthesis of 1. X-ray quality
crystals were grown by layering petroleum ether onto a saturated
dichloromethane solution of 2 inside an 8 mm o.d. vacuum-sealed
*E-mail: jbera@iitk.ac.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
glass tube. Yield: 404 mg (80%). H NMR (500 MHz, CD3CN, 292
This work is financially supported by the Department of
Science and Technology (DST), India, and the Council of
Scientific and Industrial Research (CSIR) of India. J.K.B. thanks
DST for the Swarnajayanti fellowship. S.S. and M.K. thank
CSIR, India, for fellowships.
K): δ 8.09 (d, J = 9 Hz, 6H), 7.14 (d, J = 9 Hz, 6H), 7.22 (s, 6H), 5.67
(br, 4H), 2.55 (s, 12H), 2.40 (s, 12H). 13C NMR (125 MHz, CD3CN,
294 K): δ 148.8, 146.9, 140.2, 138.3, 137.9, 137.8, 135.9, 135.8, 135.6,
134.8, 118.2, 116.9, 29.9, 21.9. 19F NMR (500 MHz, CD3CN, 292 K):
δ 134.38 (s, 12F), 160.38 (s, 6F), 166.32 (s, 12F). Anal. Calcd for
F
DOI: 10.1021/acs.organomet.5b00348
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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Shi, W.; Ma, C.; Chen, B. Green Chem. 2001, 12, 2120. (c) Girard, C.;
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