AgI-directed triple-stranded helicates with meta-ethynylpyridine ligands

Article abstract of DOI:10.1002/ejic.201402345

A series of triple-stranded complexes, [1a₃Ag]BF₄, [1b₃Ag]BF₄, [2a₃Ag₂](BF ₄)₂, [2b₃Ag₂](BF₄) ₂, and [3₃Ag₃

Full text of DOI:10.1002/ejic.201402345

FULL PAPER
DOI:10.1002/ejic.201402345
AgI-Directed Triple-Stranded Helicates with
meta-Ethynylpyridine Ligands
Qiaolian Li,[a] Fu Huang,[b] Yaxun Fan,[b] Yilin Wang,[b]
Jianfeng Li,[a] Yujian He,*[a] and Hua Jiang*[b,c]
Keywords: Helical structures / Self-assembly / Structure elucidation / Ligand design / Silver
A
series of triple-stranded complexes, [1a3Ag]BF4,
solid state for 1b and 2b. A combination of NMR spec-
troscopy, HRMS (ESI), and isothermal titration microcalorim-
etry studies further confirmed that strands 1, 2, and 3 are also
able to form triple-stranded helical structures in the presence
of AgI ions in solution.
[1b3Ag]BF4, [2a3Ag2](BF4)2, [2b3Ag2](BF4)2, and [33Ag3]-
(BF4)3, have been obtained through the self-assembly of AgI
ions with p-phenylene ethynylpyridine ligands. X-ray crys-
tallographic studies unambiguously demonstrated a AgI-di-
rected monodentate triple-stranded helical structure in the
nitrogen donors (8-hydroxyquinoline derivatives),[4a,4b] as
well as sulfur or phosphorus donors (benzene-o-dithiolato
group).[6] These ligands selectively coordinate with diverse
metal ions to form a variety of novel triple-stranded hel-
icates. The preference of the oligooxygen donors to coordi-
nate some alkali metal ions[7] (e.g., Li+, Na+, K+) has led to
efficacious templated synthesis of triple-stranded structures.
For instance, N-oxide binaphthol derived ligands wrap
around three independent Na+ or Li+ ions to form
trinuclear triple-stranded helicates.[7a] Additionally,
recent studies demonstrated that transition-metal ions
Introduction
Helical structures are ubiquitous in biological molecules
such as protein, DNA, and collagens.[1] In the past few dec-
ades, a great deal of effort has been devoted to the design
and synthesis of artificial molecules that are able to mimic
biomacromolecules both structurally and functionally. The
research has led to a good level of understanding of the
relationship between structures and functions as well as the
self-assembly processes involved in biomacromolecule for-
mation, and the work ultimately led to the discovery of fol-
damers.[2] Helicates, which are an intensively studied class
of foldamers, are oligonuclear metal complexes in which
two or more linear organic oligodonor ligands wrap around
a series of metal ions in a helical fashion.[3] Because signifi-
cant attention has been paid to the self-assembly of polynu-
clear helicates in recent years, huge progress has been made
in this area.[4–11] Among these helicates, ligands that are
composed of strands are classified according to their coor-
dination centers, for example, oligonitrogen donors (oligo-
pyridine, imine, five-membered azaheterocycles),[4a] oli-
gooxygen donors (catecholato group),[5] mixed oxygen/
[Co(II/III),[8a,8c,8d] GaIII [8c]
,
InIII [8c]
,
Fe(II/III),[8d–8h]
and
Cu(I/II),[8h,8i,10] ZnII,[8b,8j,8k] MnII,[8k,8l] AgI[9i,11]
]
LnIII[8b,9] (Ln = La, Eu, Ce, Tb, Lu) also play a very impor-
tant role in the formation of triple-stranded helicates. The
metallo centers in the helicates impart a cationic charge,
which is expected to enhance the potential use of such tri-
ple-stranded helicates in chemical and biochemical do-
mains, particularly, in DNA labeling and recognition.[8d,9d]
It is noteworthy that almost all of the ligands in the triple-
stranded helicates are bidentate, tridentate, or even multi-
dentate.[7–9] In contrast, there are few reports on metal-di-
rected monodentate triple-stranded helicates,[10,11] in par-
ticular, AgI-mediated monodentate triple-stranded hel-
icates.[11] The group of Piguet reported an infinite, linear,
triple-stranded helicate polymer in the solid state, which
consisted of lanthanide helicates and AgI ions. However,
AgI ions served as a bridge connecting two adjacent lantha-
nide helicates so as to form the polymers.[9i] The silver ion
was, in fact, a cofactor for the formation of the triple helic-
ate polymers but was not a crucial factor for the formation
of individual triple helicates in Piguet’s system. In this pa-
per, we designed and synthesized a novel monodentate tri-
ple-stranded helicate in which AgI acts as a crucial directing
agent.
[a] School of Chemistry and Chemical Engineering, University of
Chinese Academy of Sciences,
Beijing 100049, P. R. China
E-mail: heyujian@ucas.ac.cn
http://chem.ucas.ac.cn
[b] Beijing National Laboratory for Molecular Sciences, Institute
of Chemistry, Chinese Academy of Science,
Beijing 100190, P. R. China
E-mail: hjiang@iccas.ac.cn
[c] College of Chemistry, Beijing Normal University,
Beijing 100875, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402345.
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It is well established that pyridine is a suitable donor due we concluded that strands with three or less pyridine rings
to the strength of the metal–nitrogen coordination bond in gave rise to effective and stable metal complexes, i.e., triple-
some supramolecular architectures such as cages[12] and fol- stranded helicates. In contrast, strands with four or more
damers.[4,10,13–16a] A survey of CCDC data shows that AgI pyridines resulted in the formation of insoluble complexes.
and pyridine are able to form many complexes with dif- In addition, we introduced n-butyl groups at both ends of
ferent stoichiometries up to 1:4, which inspired us to design the ligands to enhance the solubility of the ligand in organic
AgI-mediated monodentate triple-stranded helicates by solvents.
carefully controlling the stoichiometry of AgI and pyridine.
In this work, we report a series of strands consisting of p-
phenylene ethynylpyridine ligands that possess a AgI-coor-
dination site at each pyridine unit (Figure 1 and
Scheme S1). We hypothesized that novel monodentate tri-
ple-stranded helicates could be achieved due to the steric
bulk of the 2,6-diethynyl-pyridine ligand. A methylene
group was inserted in two p-phenylene ethynylpyridine li-
gands as a hinge in the long strands to relax the rigidity of
the long triple-stranded helicates. After screening a variety
of arrays of the benzene and pyridine rings in the strand,
Results and Discussion
Synthesis and Characterization of the Ligands
The synthetic procedures for ligands 1–3 are shown in
Scheme 1. Starting from 4,4Ј-methylenedianiline, the diiod-
ide derivative bis(4-iodophenyl)methane (4) was synthesized
by diazo reaction.[21] The Sonogashira coupling reaction of
4 with trimethylsilylacetylene in the presence of diisopropyl-
amine, catalyzed by tetrakis(triphenylphosphine)palladium
and cuprous iodide, gave 5 in 83% yield, which was sub-
sequently ethynylated by tetrabutylammonium fluoride in
dichloromethane to give diacetylene 6 in 95% yield. Com-
mercially available 2,6-dibromopyridine was subjected to
Sonogashira reaction with 2.5 or 0.2 equiv. of (trimethyl-
silyl)acetylene (TMSA) to yield 7 and 9, respectively, which
was followed by treatment with tetrabutylammonium
fluoride to generate diacetylene
8 (92% yield) and
monoacetylene 10 (88% yield), respectively. The monomeric
strand of 1a and 1b were synthesized by the coupling reac-
tion of diacetylene 8 with 1-butyl-4-iodobenzene and iodo-
Figure 1. Structures of ligands 1–3.
Scheme 1. Synthetic route to ligands 1–3.
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benzene, in yields of 90 and 81%, respectively. Monoacet-
ylene 10 was allowed to react with 1-butyl-4-iodobenzene
and iodobenzene, producing 11a and 11b in 86 and 84%
yield, respectively. The Sonogashira coupling of bromides
11a–b with diacetylene 6 afforded the dimeric strand of 2a
(80% yield) and 2b (91% yield). Sonogashira reaction and
ethynylation converted bromide 11a into acetylide 13. The
reaction of diacetylene 8 with an excess of diiodide deriva-
tive 4 gave 14 in 72% yield. Finally, the trimeric strand of
3 was obtained by the Sonogashira coupling reaction of di-
iodide 14 with acetylide 13. All intermediates of the mono-
mers, dimers and trimer were purified by column
1
chromatography and characterized on the basis of H and
Figure 2. X-ray crystal structure of AgI complex of 1b. Upper: stick
diagrams; lower: space-filling diagrams; left: side view; right: top
view. In both diagrams, alternate ligands are colored separately, C,
H: red, blue and green for different strands; N: pink; Ag: light blue,
counterions have been omitted for clarity.
13C NMR spectroscopy, mass spectra, and melting point
measurements. Furthermore, 1D-NOEDIF, 2D 1H-1H
COSY and ROESY spectra were recorded to identify the
signals of the protons of 1–3 and the protons of the corre-
sponding AgI complexes (see Figure S2–S9 in the Support-
ing Information).
[2b3Ag2](BF4)2 adopted a head-to-head triple-stranded di-
nuclear helical structure with a C3-symmetric axis passing
through the AgI ions, as shown in Figure 3. The three-
stranded helicate crystallized as a racemate in the trigonal
Solid-State Characterization of 1b and 2b
¯
system with the space group R3 (Table S1). Three strands
Various attempts to obtain crystals of the AgI complexes
of 1a and 2a that were suitable for X-ray diffraction analysis
were unsuccessful, probably due to the disorder of the butyl
moiety. Fortunately, crystals of complexes of 1b and 2b
without a butyl moiety were obtained for X-ray diffraction
analysis.
of 2b, each containing two coordinated sites, wrap around
two AgI ions with Ag1–Ag2 distance of 14.882 Å between
the two coordination sites, and each three pyridyl groups
bridge one AgI atom in a monodentate model. The three
nitrogen atoms and AgI ion exhibit a trigonal planar geom-
etry, in which the Ag–N distance is in the range of 2.242(3)
to 2.247(3) Å and the corresponding chelate angles are
approximately 119.68(15) to 119.99(3)°. The dihedral angle
between adjacent methylene-bridged benzene rings C and
D is about 101.9°. Notably, there are three C–H groups par-
ticipating in the C–H···F type hydrogen bonds with one
Crystals of the AgI complex of 1b were achieved by slow
evaporation from an ethanol/H2O solution. The head-to-
head three-stranded helicate crystallized as a racemate in
¯
the trigonal system with the space group R3c, and both left
and right helicates are present in the solid state, as shown
in Figure 2 and Table S1. The AgI ion is coordinated by
three pyridyl groups from three ligands, in which each pyr-
idyl moiety acts as a monodentate ligand. Each three-
stranded helicate has a C3 axis. The three nitrogen atoms
and the AgI ion display a trigonal planar geometry, which
is uncommon in comparison to the reported AgI-pyridine
complexes,[14,15] with a Ag–N distance of 2.248(6) Å and a
–
counterion BF4 (C···F distance 3.333 Å, and C–H···F
angle 130.88°). The distance between C5 and the corre-
sponding phenyl ring A in the adjacent ligand is about
3.884 Å, with a dihedral angle between the two phenyl rings
of about 116.02°, indicating a slight C–H···π interaction
–
chelate angle of approximately 119.72(3)°. BF4 is bound
by six C–H groups from three different ligands through six
hydrogen bonds.[8g] Clearly, all six C–H···F bonds point to
the same F atom (F2), which sits at the C3 axis (C···F dis-
tance 3.370–3.418 Å, average 3.394 Å and C–H···F angles
123.07–126.07°, average 124.57°). It is worth mentioning
that one of the terminal benzene rings (labeled as C) inter-
acts with one of the pyridine rings (labeled as B) (Fig-
ure S18) at the adjacent ligands in a nearly parallel orienta-
tion, with a dihedral angle about 161.02° and a interplanar
distance of approximately 4.599 Å (centroid to centroid),
suggesting the existence of weak π···π interactions.[17a,17b]
These C–H···F bonds and π···π interactions may also play
important roles in the stabilization of the helicate.
Figure 3. X-ray crystal structures of the AgI complex of 2b. Upper:
stick diagrams; lower: space-filling diagrams; left: side view; right:
top view. In both diagrams, alternate ligands are coloured sepa-
rately, C, H: red, blue and green for different strands; N: pink; Ag:
Crystals of the AgI complex of 2b were obtained by slow
evaporation from a mixed solvent of CHCl3 and ethanol.
The X-ray crystallographic analysis explicitly indicated that light blue, counterions have been omitted for clarity.
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(Figure S19).[17b,17c] Similar C–H···π interactions were also
observed between other carbon atoms and the correspond-
ing aromatic rings (e.g., C17···benzene ring C, distance
3.818 Å, C1···pyridyl ring B, distance 3.945 Å, C39···pyridyl
ring E, distance 3.761 Å). All of these C–H···F bonds and
slight C–H···π interactions may play a significant role in
stabilizing the helicate.
Having found that triple-stranded helicates were present
in the solid state, the coordination behavior of these ligands
1
in solution was then investigated on the basis of H NMR
spectra, HRMS (ESI) and isothermal titration microcalo-
rimetry (ITC) measurements.
NMR Titration Studies
1H NMR titration experiments with monomeric strand
1a and AgI ions were performed in [D6]acetone at room
temperature (Figure 4, a). The spectra showed a simple
1
Figure 4. Partial H NMR spectra of 1a, 2a and 3 and the corre-
sponding AgI complex at 298 K: (a) 1a (upper) and 1a/Ag (1:2)
pattern containing one set of signals, which suggested a ra- (lower) in [D6]acetone, 400 MHz; (b) 2a (upper) and 2a/Ag (1:2)
(lower) in CDCl3/[D6]acetone (3:2, v/v), 400 MHz; (c) 3 (upper)
pid equilibrium was established on the NMR timescale, pre-
and 3/Ag (1:2) (lower) in CDCl3/[D6]acetone (9:1, v/v), 500 MHz.
sumably due to labile AgI ions involved in the coordination
[1a] = [2a] = [3] = 2.0 mm; * represent solvent peaks.
process.[16b] No further significant changes occurred when
the number of equivalents of AgI ions was increased above
1.5, whereupon the coordination and dissociation of AgI
to high field (Figure 4, b). These upfield shifts originate
complex achieved dynamic equilibrium (Figure S10). Upon
from the shielding effect of the corresponding phenyl rings
increasing the amount of AgI ions, the chemical shifts of
of the counterpart strands in the triple helicates.[16a]
the pyridyl moiety displayed a clear downfield shift because
The titration of trimeric strand 3 showed a set of rela-
coordination occurred between the pyridyl moiety and the
tively complex 1H NMR spectra in the absence of AgI ions,
AgI ions.[13b] The pyridyl proton peaks Ha and Hb at δ =
due to the presence of a variety of aromatic rings. However,
7.87 and 7.59 ppm moved to δ = 8.17 and 7.88 ppm, respec-
tively. The changes of chemical shifts were up to Δδ = 0.30
and 0.29 ppm for Ha and Hb, respectively. In contrast, the
chemical shifts of the aromatic protons Hc and Hd on the
upon the addition of 1 equiv. of AgI ions into a solution of
1
trimeric strand 3, the H NMR spectrum became simple
and clear, indicating the formation of the AgI complex of 3
(Figure 4, c and Figure S12). The most evident changes
were observed in the signals of the pyridyl proton. In detail,
terminal benzene rings displayed little variation (δ =
7.46 ppm, Δδ = –0.10 ppm and δ = 7.24 ppm, Δδ =
the peaks HaЈ at δ = 8.19 ppm and HiЈ at δ = 8.15 ppm
–0.08 ppm, respectively).
showed significant downfield shifts of Δδ = 0.55 and
0.51 ppm, which exhibited two sets of triplet peaks in the
complex but an apparent single triplet in the free strand.
Similar changes were observed for HbЈ, HhЈ and HjЈ with
Δδ = 0.48, 0.48, and 0.36 ppm, respectively. Simultaneously,
the signals split into two sets of doublet peaks at δ = 7.89
and 7.77 ppm in the complex. Other aromatic protons expe-
rienced diverse upfield shifts, as expected. For example, the
1H NMR titration studies of 2a and 3 were performed in
the mixed solvent of CDCl3 and [D6]acetone to improve
their solubility (Figure S11–S12). Upon addition of AgI
ions into a solution of dimeric strand 2a, the chemical shifts
of all aromatic protons experienced similar variations to
that of 1a (Figure 4, b), suggesting that coordination took
place between pyridine units and AgI ions. The changes in
the chemical shifts were saturated upon addition of
aromatic protons HkЈ at δ = 7.23 ppm and HlЈ at δ =
7.00 ppm shifted to high field with Δδ = –0.23 and
–0.12 ppm, respectively. The aromatic protons HcЈ, HdЈ,
1.2 equiv. of AgI ions (Figure S11). In detail, the Ha* peak
of the pyridine units at δ = 8.25 ppm displayed a remark-
able downfield shift with Δδ = 0.48 ppm. As for Hb* and
HfЈ, and HgЈ also experienced moderate upfield shifts (Δδ
Hc*, its chemical shift appeared as a doublet at δ =
from –0.22 to –0.28 ppm).
7.56 ppm in the free strand. The titrations of AgI ions in-
duced the signals of Hb* and Hc* to split into two sets of
doublet peaks at δ = 7.96 and 7.91 ppm with significant
downfield shifts of Δδ = 0.40 and 0.35 ppm, respectively. In
HRMS (ESI) Analysis
addition, the other protons Hd*, He*, Hf*, Hg* of the
phenyl rings (δHd* = 7.10 ppm, Δδ = –0.13 ppm; δHe*
To determine the stoichiometry of the complexes in solu-
tion, HRMS (ESI) were recorded for the AgI complexes of
=
7.32 ppm, Δδ = –0.20 ppm; δHf* = 7.34 ppm, Δδ = 1a and 2a (Figure 5, Figure S13 and S14 in the Supporting
–0.18 ppm; δHg* = 6.99 ppm, Δδ = –0.28 ppm) and Hh* of Information). The mass spectrometric studies with 1a
the methylene linker (δ = 3.96 ppm, Δδ = –0.10 ppm) shifted showed a major peak at m/z 1282.59 that was assigned to
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the [1a3Ag]+ complex (Figure 5, a). Two other peaks at m/z respectively, were also observed under the same experimen-
783.47 and 891.36, assigned to [1a2H]+ and [1a2Ag]+, tal conditions (Figure S13). The coexistence of [1a2H]+ and
[1a2Ag]+ species, together with the [1a3Ag]+ complex, indi-
cated that a dynamic equilibrium involving a series of solu-
tion species occurred, as observed in other reported sys-
tems.[16b,18] However, the observation of a major peak at
m/z 1282.59 for [1a3Ag]+ illustrated a 3:1 stoichiometry for
the 1a/AgI complex was the dominant coordination mode
in solution.[18a]
Similar behavior was observed for the AgI complex of
2a. Here, the base peak at m/z 1131.91 was assigned to the
expected twofold charged 3:2 complex [2a3Ag2]2+, which
showed that AgI ions indeed formed dinuclear triple-
stranded complex with 2a in solution (Figure 5, b). Other
signals at m/z 790.24 and 1473.58 were observed with low
intensity for two bimetallic complexes with two and four
strands, respectively [(2a2Ag2)2+ and (2a4Ag2)2+, Fig-
ure S14]. These results revealed that the 3:2 binding
stoichiometry for the complex formed by 2a and AgI ions
was also the dominant coordination mode in solution.[18b]
For the AgI complex of 3, no featured peaks were detected,
presumably because of the poor solubility of the complex
in polar solvents such as acetonitrile and methanol used in
the ESI experiments.
Isothermal Titration Calorimetry (ITC) Experiments
ITC is a powerful tool for investigating the thermody-
namics of small molecule/metal coordination systems. The
exothermic or endothermic heat that is evolved during a
Figure 5. Positive HRMS (ESI) of the AgI complexes for 1a and
reaction can be directly measured by ITC to determine a
2a. (a) The AgI complex of 1a: (upper) showing the experimented
binding equilibrium constant (β) and an enthalpy change
signal of [1a3Ag]+ at m/z 1282.59; (lower) The calculated signal of
(ΔH0) for the reaction. Entropy changes (TΔS0) can also be
[1a3Ag]+ (b) The AgI complex of 2a: (upper) showing the experi-
calculated (Figure S15–S17).[19,20] Another benefit of ITC is
that the binding stoichiometry of certain complexation
mented signal of [2a3Ag2]2+ at m/z 1131.91; (lower) The calculated
signal of [2a3Ag2]2+
.
Figure 6. ITC profile for the titration of ligands 1a, 2a, and 3 with AgBF4 at 298 K. (Upper) Raw ITC data; each peak corresponds to
the thermal power evolved from the solution of ligand on addition of aliquots of AgBF4 solution. (Lower) Normalized titration curve;
the red solid line represents the best-fit curve obtained from the assumed stoichiometry.
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reactions were carried out under an atmosphere of nitrogen with
freshly distilled solvents. Generally, three vacuum/purge cycles were
applied to the reaction tube to remove air and refilled with nitro-
gen. Tetrahydrofuran (THF) and diisopropylamine (iPr2NH) were
freshly distilled before use from sodium/benzophenone and CaH2,
respectively.
events can be easily determined from the inflection point in
the titration curve plotting heat versus the molar ratio of
metal ion to ligand.[20a]
ITC experiments with 1a and AgI ions was performed in
ethanol, and ITC experiments of 2a and 3 were performed
in a mixed solvent of chloroform and ethanol to improve
their solubility (Figure 6); the data are listed in Table 1. The
molar ratios of AgI ions to 1a, 2a and 3 were 0.34Ϯ0.001,
0.70Ϯ0.04, and 0.83Ϯ0.001, respectively, corresponding to
stoichiometries of 1:3, 2:3, and 3:3 for AgI complexes of 1a,
2a and 3. These results were consistent with the observa-
tions discussed above. Considering that the solvent effect
on coordination to metal ions is indistinct, no attempt was
made to interpret these thermodynamic parameters. How-
ever, the molar ratios abstracted from ITC experiments
could represent the stoichiometries of the complexes in
solution.
Single-crystal X-ray data for the helicate were collected with a Ri-
gaku ST Saturn724+ diffractometer. NMR spectra were recorded
with a 400 MHz (100 MHz for 13C) Bruker AV, a 500 MHz
(125 MHz for 13C) Bruker AV III, or a 600 MHz Bruker AV spec-
trometer at 25 °C using residual protonated solvent signals as in-
ternal standard {1H: δ (CDCl3) = 7.26 ppm, δ ([D6]acetone) =
2.05 ppm, and 13C: δ (CDCl3) = 77.16 ppm}. Mass spectrometry
was performed with a Shimadzu LC-MS (ESI-MS) or a Bruker-
Autoflex III (MALDI-TOF MS). The metal ion complexes were
measured with a Bruker-Apex IVFTMS (ESI-HRMS) and a
Bruker-Solar 1X (ESI-HRMS). ITC experiments were conducted
with a TAM2277–201 microcalorimetric system (Thermometric
AB, Järfälla, Sweden). Melting points were measured with a Tech
x-4 micro melting point apparatus.
Table 1. Thermodynamic parameters for ligands 1–3 binding to
AgBF4 at 298 K.
Synthetic Procedures for Compounds 1–14
n[a]
β[19]
ΔH0
TΔS0
Bis{4-[(trimethylsilyl)ethynyl]phenyl}methane (5): To a mixture of 4
(2.24 g, 5.3 mmol), Pd(PPh3)4 (0.051 g, 0.27 mmol), CuI (0.312 g,
0.27 mmol), and diisopropylamine (2 mL) in THF (20 mL) was
added trimethylsilylacetylene (TMSA; 1.3 g, 13.25 mmol). The re-
action mixture was stirred for 12 h at room temperature then the
resulting mixture was filtered. The filtrate was poured into aqueous
NH4Cl, and extracted with dichloromethane. The extract was
washed with brine, dried with MgSO4, and filtered. The filtrate was
concentrated by rotary evaporator and the residue was purified by
silica gel (neutral) column chromatography (hexane/CH2Cl2, 30:1)
to give 5 (1.58 g, 83%) as a white solid.[22]
[kJmol–1]
[kJmol–1]
1a 0.34Ϯ0.001 1.1ϫ103
n.d.[b]
n.d.
–1.02
–51.53
2a 0.70Ϯ0.04 (1.56Ϯ0.26)ϫ106 –36.34Ϯ0.44
3
0.83Ϯ0.001 (2.90Ϯ0.18)ϫ106 –88.39Ϯ0.28
[a] n (molar ratio = [Metal]/[Ligand]) ratio at which an inflection
point was observed on the titration curve. [b] n.d.: not determined.
In the ITC experiment of 1a, we could not obtained a complete
titration curve that contained two plateaus as 2a and 3, because
of the relatively low binding equilibrium constant β, therefore, the
corresponding ΔH0 and TΔS0 could not be obtained precisely.
Bis(4-ethynylphenyl)methane (6): Compound 5 (1.26 g, 3.5 mmol)
was dissolved in dichloromethane (100 mL), and tetrabutylammo-
nium fluoride (TBAF; 2.65 g, 8.4 mmol) was added. The resulting
mixture was stirred at room temperature for 0.5 h then poured into
water. After extraction with dichloromethane, the organic layer was
washed with brine, dried with MgSO4, and filtered. The solvent
was evaporated and the residue was purified by silica gel (neutral)
column chromatography (hexane/CH2Cl2, 25:1) to give 6 (0.72 g,
95% yield) as a pale-yellow solid.[22]
Conclusions
We have successfully designed and prepared a series of
AgI-directed monodentate triple-stranded helicates con-
sisting of p-phenylene ethynylpyridine ligands that possess
AgI coordination sites at each pyridine unit. The X-ray sin-
gle-crystal studies unambiguously established the triple-
stranded helical structure of [1b3Ag]BF4 and [2b3Ag2]-
2,6-Bis[(trimethylsilyl)ethynyl]pyridine (7): Prepared as reported in
the literature with modifications.[23] To a mixture of 2,6-dibromo-
pyridine (2.37 g, 10 mmol), Pd(PPh3)4 (0.58 g, 0.5 mmol), CuI
(0.095 g, 0.5 mmol), and diisopropylamine (5 mL) in THF (20 mL),
was added TMSA (2.5 g, 25 mmol). The reaction mixture was
stirred for 12 h at room temperature and the resulting mixture was
filtered. The filtrate was poured into aqueous NH4Cl and extracted
with dichloromethane. The organic layer was then washed with
brine, dried with MgSO4, and filtered. The filtrate was evaporated
in vacuo and the residue was purified by silica gel (neutral) column
chromatography (hexane/CH2Cl2, 85:15) to give 7 (2.4 g, 90%
yield) as a white solid.
1
(BF4)2 in the solid state. H NMR spectroscopic analysis
confirmed the complexes of strands 1a, 2a, and 3 with AgI
binding in solution. HRMS (ESI) and ITC studies showed
that the stoichiometries of the triple helix formation were
1a/AgI = 3:1, 2a/AgI = 3:2, and 3/AgI = 1:1, which are sig-
nificantly different from the previous report that the pyr-
idine-containing ligands and AgI ions formed double-
stranded helicates.[15] Further investigations to establish
whether the ligands can coordinate other transition metal
cations, especially copper(I) and palladium(II), are in pro-
gress in our laboratory.
2,6-Diethynylpyridine (8): To a solution of 7 (2.4 g, 9 mmol) in
dichloromethane (100 mL) was added TBAF (6.82 g, 21.6 mmol).
The resulting mixture was stirred at room temperature for 0.5 h
then poured into water. After extraction with dichloromethane, the
organic layer was washed with brine, dried with MgSO4 and fil-
tered. The solvent was evaporated and the residue was purified by
silica gel (neutral) column chromatography (hexane/CH2Cl2, 50:50)
to give 8 (1.05 g, 92% yield) as a white solid.[23]
Experimental Section
General Methods: All starting materials were purchased from com-
mercial suppliers and used without further purification unless
otherwise noted. Compound 4 was synthesized according to the
previously reported methods.[21] All of the Sonogashira coupling
Eur. J. Inorg. Chem. 2014, 3235–3244
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2-Bromo-6-ethynylpyridine (10): Prepared as reported in the litera-
ture with modifications.[15] To a mixture of 2,6-dibromopyridine
(11.86 g, 50.1 mmol), Pd(PPh3)4 (0.58 g, 0.5 mmol), CuI (0.1 g,
0.5 mmol), and diisopropylamine (10 mL) in THF (40 mL), was
added TMSA (0.98 g, 10.02 mmol) in THF (100 mL) dropwise over
5 h. The reaction mixture was stirred overnight at room tempera-
ture, then the resulting mixture was filtered. The filtrate was poured
into aqueous NH4Cl, and extracted with dichloromethane. The or-
ganic layer was then washed with brine, dried with MgSO4 and
filtered through a thin pad of silica gel. The filtrate was evaporated
in vacuo to give a mixture of 2,6-dibromopyridine, 9, and a small
amount of 7. To the residue was added dichloromethane (200 mL)
and TBAF (23.8 g, 75 mmol) and the resulting mixture was stirred
at room temperature for 0.5 h, and poured into water. After extrac-
tion with dichloromethane, the organic layer was washed with
brine, dried with MgSO4 and filtered. The solvent was evaporated,
and the residue was purified by silica gel (neutral) column
chromatography (hexane/CH2Cl2, 70:30) to give recovered 2,6-di-
bromopyridine and 10 (1.59 g, 88% yield) as a white solid.
7.17 (d, J = 8.0 Hz, 2 H), 2.62 (t, J = 7.7 Hz, 2 H), 1.61 (dt, J =
15.5, 7.6 Hz, 2 H), 1.35 (dq, J = 14.7, 7.3 Hz, 2 H), 0.93 (t, J =
7.3 Hz, 3 H), 0.27 (s, 9 H) ppm. 13C NMR (100 MHz, CDCl3): δ
= 144.43, 144.04, 143.46, 136.35, 132.11, 128.59, 126.52, 126.27,
119.32, 103.35, 95.36, 90.13, 87.88, 35.73, 33.38, 22.38, 13.99,
–0.22 ppm. MS (ESI): m/z calcd. for C22H25NSi [M + H+] 332.18;
found 332.2.
2-[(4-Butylphenyl)ethynyl]-6-ethynylpyridine (13): Compound 12
(0.7 g, 2.1 mmol) was dissolved in dichloromethane (100 mL), and
TBAF (0.8 g, 2.52 mmol) was added. The resulting mixture was
stirred for 0.5 h at room temperature then poured into water. After
extraction with dichloromethane, the organic layer was washed
with brine, dried with MgSO4 and filtered. The solvent was evapo-
rated, and the residue was purified by silica gel (neutral) column
chromatography (hexane/CH2Cl2, 40:60) to give 13 (0.48 g, 89%
1
yield) as a pale-yellow solid, m.p. 59–61 °C. H NMR (400 MHz,
CDCl3): δ = 7.65 (t, J = 7.8 Hz, 1 H), 7.50 (d, J = 7.8 Hz, 2 H),
7.48 (d, J = 7.7 Hz, 1 H), 7.41 (d, J = 7.7 Hz, 1 H), 7.17 (d, J =
8.0 Hz, 2 H), 3.16 (s, 1 H), 2.62 (t, J = 7.7 Hz, 2 H), 1.60 (dt, J =
15.3, 7.5 Hz, 2 H), 1.35 (dq, J = 14.6, 7.2 Hz, 2 H), 0.93 (t, J =
7.3 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 144.59,
144.29, 142.80, 136.53, 132.20, 128.67, 126.93, 126.34, 119.31,
90.36, 87.77, 82.52, 77.65, 35.80, 33.44, 22.45, 14.04 ppm. MS
(ESI): m/z calcd. for C19H17N [M + H+] 260.14; found 260.1.
2-Bromo-6-[(4-butylphenyl)ethynyl]pyridine (11a): To a mixture of
1-butyl-4-iodobenzene (1.88 g, 7.2 mmol), Pd(PPh3)4 (0.347 g,
0.3 mmol), CuI (0.057 g, 0.3 mmol), and diisopropylamine (5 mL)
in THF (20 mL) was added a solution of 10 (1.1 g, 6 mmol) in
THF (10 mL). The reaction mixture was stirred overnight at room
temperature, and the resulting mixture was filtered. The filtrate was
poured into aqueous NH4Cl, and extracted with dichloromethane.
The organic layer was washed with brine, dried with MgSO4, and
filtered. The filtrate was evaporated in vacuo and the residue was
purified by silica gel (neutral) column chromatography (hexane/
CH2Cl2, 80:20) to give 11a (1.6 g, 86% yield) as a white solid, m.p.
73–75 °C. 1H NMR (400 MHz, CDCl3): δ = 7.56–7.40 (m, 5 H),
7.18 (d, J = 8.0 Hz, 2 H), 2.63 (t, J = 7.7 Hz, 2 H), 1.60 (dt, J =
15.3, 7.5 Hz, 2 H), 1.36 (dq, J = 14.8, 7.4 Hz, 2 H), 0.93 (t, J =
7.3 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 144.49,
143.90, 141.53, 138.24, 131.90, 128.44, 127.02, 125.84, 118.71,
91.13, 86.98, 35.51, 33.13, 22.19, 13.82 ppm. MS (ESI): m/z calcd.
for C17H16BrN [M + H+] 314.05; found 314.0.
2,6-Bis{[4-(4-iodobenzyl)phenyl]ethynyl}pyridine (14): To a mixture
of Pd(PPh3)4 (0.24 g, 0.2 mmol), CuI (0.04 g, 0.2 mmol), diiso-
propylamine (10 mL), THF (20 mL), and an excess of 4 (4.2 g,
10 mmol), was added a solution of 8 (0.26 g, 2 mmol) in THF
(100 mL) dropwise over 6 h. The reaction mixture was stirred and
heated overnight at 60 °C. After cooling to room temperature, the
resulting mixture was poured into aqueous NH4Cl, and extracted
with dichloromethane. The organic layer was then washed with
brine, dried with MgSO4 and filtered. The filtrate was evaporated
in vacuo and the residue was purified by silica gel (neutral) column
chromatography (hexane/CH2Cl2, 55:45) to give 14 (1.02 g, 72%
yield) as a pale-yellow solid, m.p. 217–219 °C. 1H NMR (400 MHz,
CDCl3): δ = 7.66 (t, J = 8.0 Hz, 1 H), 7.62 (d, J = 8.2 Hz, 4 H),
7.52 (d, J = 8.1 Hz, 4 H), 7.45 (d, J = 7.8 Hz, 2 H), 7.15 (d, J =
7.9 Hz, 4 H), 6.93 (d, J = 8.0 Hz, 4 H), 3.93 (s, 4 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 143.99, 141.84, 140.15, 137.79,
136.52, 132.50, 131.17, 129.09, 126.28, 120.26, 91.71, 89.88, 88.34,
41.49 ppm. MS (MALDI-TOF): m/z calcd. for C35H23I2N [M +
H+] 711.99; found 712.1.
2-Bromo-6-(phenylethynyl)pyridine (11b): The synthetic procedure
for 11a was generally followed to synthesize 11b, in which reaction
the reagent 1-butyl-4-iodobenzene was substituted by iodobenzene.
The reaction mixture was purified by silica gel (neutral) column
chromatography (hexane/CH2Cl2, 90:10) to give 11b (0.6 g, 84%
yield) as a white solid, m.p. 80–82 °C. 1H NMR (400 MHz,
CDCl3): δ = 7.58 (t, J = 7.8 Hz, 1 H), 7.59 (d, J = 7.7 Hz, 1 H),
7.53 (d, J = 7.7 Hz, 1 H), 7.46 (ddd, J = 13.8, 7.6, 1.0 Hz, 2 H),
7.41–7.34 (m, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 143.99,
141.84, 138.42, 132.19, 129.42, 128.52, 127.46, 126.12, 121.83,
90.91, 87.54 ppm. MS (ESI): m/z calcd. for C13H8BrN [M + H+]
257.98; found 257.9.
Monomer 1a: To a mixture of 1-butyl-4-iodobenzene (1.25 g,
4.8 mmol), Pd(PPh3)4 (0.116 g, 0.1 mmol), CuI (0.02 g, 0.1 mmol),
and diisopropylamine (2 mL) in THF (20 mL) was added a solu-
tion of 8 (0.25 g, 2 mmol) in THF (10 mL). The reaction mixture
was stirred overnight at room temperature then the resulting mix-
ture was filtered. The filtrate was poured into aqueous NH4Cl, and
extracted with dichloromethane. The organic layer was washed with
brine, dried with MgSO4, and filtered. The filtrate was evaporated
in vacuo, and the residue was purified by silica gel (neutral) column
chromatography (hexane/CH2Cl2, 75:25) to give 1a (0.7 g, 90%
yield) as a white solid, m.p. 143–145 °C. 1H NMR (400 MHz,
CDCl3): δ = 7.66 (t, J = 7.8 Hz, 1 H), 7.51 (d, J = 8.1 Hz, 4 H),
7.45 (d, J = 7.8 Hz, 2 H), 7.17 (d, J = 8.1 Hz, 4 H), 2.63 (t, J =
7.7 Hz, 4 H), 1.60 (dt, J = 15.4, 7.6 Hz, 4 H), 1.36 (dq, J = 14.7,
7.3 Hz, 4 H), 0.93 (t, J = 7.3 Hz, 6 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 144.40, 144.06, 136.39, 132.12, 128.59, 126.06, 119.37,
90.06, 87.98, 35.74, 33.38, 22.39, 13.99 ppm. MS (ESI): m/z calcd.
for C29H29N [M + H+] 392.23; found 392.3.
2-[(4-Butylphenyl)ethynyl]-6-[(trimethylsilyl)ethynyl]pyridine
(12):
To mixture of 11a (0.81 g, 2.6 mmol), Pd(PPh3)4 (0.15 g,
a
0.13 mmol), CuI (0.025 g, 0.13 mmol), and diisopropylamine
(5 mL) in THF (20 mL) was added TMSA (0.31 g, 3.12 mmol). The
reaction mixture was stirred for 12 h at room temperature, and the
resulting mixture was filtered. The filtrate was poured into aqueous
NH4Cl and extracted with dichloromethane. The organic layer was
washed with brine, dried with MgSO4, and filtered. The filtrate
was evaporated in vacuo and the residue was purified by silica gel
(neutral) column chromatography (hexane/CH2Cl2, 85:15) to give
12 (0.7 g, 82% yield) as a white solid, m.p. 92–94 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.62 (t, J = 7.8 Hz, 1 H), 7.49 (d, J =
Monomer 1b: The synthetic procedure for 1a was generally followed
8.0 Hz, 2 H), 7.44 (d, J = 7.7 Hz, 1 H), 7.39 (d, J = 7.7 Hz, 1 H), to synthesize 1b, in which reaction the reagent 1-butyl-4-iodo-
Eur. J. Inorg. Chem. 2014, 3235–3244
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benzene was substituted by iodobenzene. The reaction mixture was
Single Crystals for X-ray Diffraction
purified by silica gel (neutral) column chromatography (hexane/
[1b3Ag]BF4: AgBF4 (5.0 mg, 0.026 mmol) and 1b (19.56 mg,
0.07 mmol) were dissolved in ethanol/H2O (5 mL, 9:1, v/v) and
stirred for 2 h at 50 °C. The resultant colorless solution was allowed
to evaporate in the dark at room temperature for 3 weeks to give
colorless polyhedrons of the AgI complex of 1b.
CH2Cl2, 80:20) to give 1b (0.5 g, 81% yield) as a white solid, m.p.
1
137–139 °C. H NMR (400 MHz, CDCl3): δ = 7.69 (t, J = 7.8 Hz,
1 H), 7.61 (dd, J = 6.5, 3.2 Hz, 4 H), 7.48 (d, J = 7.8 Hz, 2 H),
7.41–7.34 (m, 6 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 143.94,
136.53, 132.21, 129.19, 128.50, 126.33, 122.24, 89.76, 88.42 ppm.
MS (ESI): m/z calcd. for C21H13N [M + H+] 280.10; found 280.1.
[2b3Ag2](BF4)2: To a solution of 2b (17.1 mg, 0.03 mmol) in CHCl3
(5 mL) was added a solution of AgBF4 (4.28 mg, 0.022 mmol) in
ethanol (2 mL). After stirring for 2 h at 50 °C, a clear colorless
solution was obtained. Colorless block-like crystals of the AgI com-
plex of 2b were obtained by slow evaporation in darkness at room
temperature for 4 weeks.
Dimer 2a: To a mixture of 11a (0.75 g, 2.4 mmol), Pd(PPh3)4
(0.06 g, 0.05 mmol), CuI (0.01 g, 0.05 mmol), and diisopropylamine
(2 mL) in THF (20 mL) was added a solution of 6 (0.22 g, 1 mmol)
in THF (10 mL). The reaction mixture was stirred overnight at
room temperature, and the resulting mixture was filtered. The fil-
trate was poured into aqueous NH4Cl, and extracted with dichloro-
methane. The organic layer was washed with brine, dried with
MgSO4, and filtered. The filtrate was evaporated in vacuo, and the
residue was purified by silica gel (neutral) column chromatography
(hexane/CH2Cl2, 30:70) to give 2a (0.54 g, 80% yield) as a white
1H MNR Titration Experiments of Ligands 1a, 2a, and 3: Ligands
were dissolved in different solvents due to their distinct solubility
[1a in [D6]acetone; 2a in CDCl3/[D6]acetone (3:2, v/v); 3 in CDCl3/
[D6]acetone (9:1, v/v)]. The stock AgBF4 solutions (50.0 mm) were
prepared accordingly in [D6]acetone, CDCl3/[D6]acetone (3:2, v/v)
and CDCl3/[D6]acetone (9:1, v/v), respectively, which were titrated
into the solution of ligands (2.0 mm) in each portion of 2 μL
(0.1 equiv.) using a microsyringe (25 μL). The 1H MNR spectra was
measured to observe the change 10 min after injection of AgBF4
solution. The titration was finished when no further significant
change was observed.
1
solid, m.p. 264–266 °C. H NMR (400 MHz, CDCl3): δ = 7.67 (t,
J = 7.8 Hz, 2 H), 7.53 (dd, J = 12.0, 8.1 Hz, 8 H), 7.45 (d, J =
7.8 Hz, 4 H), 7.18 (dd, J = 8.1, 3.6 Hz, 8 H), 4.02 (s, 2 H), 2.63 (t,
J = 7.7 Hz, 4 H),1.60 (dt, J = 15.3, 7.5 Hz, 4 H), 1.36 (dq, J =
14.7, 7.3 Hz, 4 H), 0.93 (t, J = 7.3 Hz, 6 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 144.50, 144.18, 143.99, 141.77, 136.47,
132.50, 132.19, 129.20, 128.66, 126.23, 126.17, 120.32, 119.40,
90.19, 89.71, 88.42, 87.98, 41.96, 35.80, 33.45, 22.45, 14.05 ppm.
MS (MALDI-TOF): m/z calcd. for C51H42N2 [M + H+] 683.33;
found 683.6.
HRMS (ESI) of AgI Complexes of 1a and 2a
AgI Complex of 1a: AgBF4 (11.68 mg, 0.06 mmol) and 1a
(11.74 mg, 0.03 mmol) were dissolved in acetone (5 mL) and stirred
for 2 h at 50 °C. After cooling to room temperature, the clear color-
less solution was obtained for HRMS (ESI) measurement.
Dimer 2b: The synthetic procedure used for 2a was generally fol-
lowed to synthesize 2b, in which reaction the reagent 11a was sub-
stituted by 11b. The reaction mixture was purified by silica gel (neu-
tral) column chromatography (hexane/CHCl3, 40:60) to give 2b
(0.52 g, 91% yield) as a white solid, m.p. 247–249 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.67 (t, J = 7.8 Hz, 2 H), 7.61 (dd, J = 6.5,
3.1 Hz, 4 H), 7.54 (d, J = 8.1 Hz, 4 H), 7.47 (d, J = 8.1 Hz, 4 H),
7.42–7.32 (m, 6 H), 7.19 (d, J = 8.1 Hz, 4 H), 4.02 (s, 2 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 144.06, 143.98, 141.79, 136.51,
132.50, 132.25, 129.20, 128.52, 126.34, 126.28, 122.31, 120.29,
89.80, 89.77, 88.48, 88.38, 77.36, 41.96 ppm. MS (MALDI-TOF):
m/z calcd. for C43H26N2 [M + H+] 571.21; found 571.1.
AgI Complex of 2a: To the solution of 2a (13.65 mg, 0.02 mmol) in
CHCl3 (2 mL) was added
a solution of AgBF4 (7.79 mg,
0.04 mmol) in acetone (5 mL), and the mixture was stirred for 2 h
at 50 °C. The solvent was evaporated, and the resulting residue was
dissolved in acetone (5 mL) for HRMS (ESI) measurement.
Isothermal Titration Microcalorimetry (ITC): The calorimetric
measurements were made by using a stainless steel sample cell
(1 mL) at 25.00Ϯ0.01 °C. The sample cell of the microcalorimeter
was initially loaded with 600 μL solvent or ligand solution [2.0 mm
of 1a in ethanol; 1.0 mm of 2a in CHCl3/ethanol (3:2, v/v); 1.0 mm
of 3 in CHCl3/ethanol (9:1, v/v)]. Concentrated metal solution
[8.5 mm in ethanol, 2.5 mm in CHCl3/ethanol (3:2, v/v), 2.5 mm in
CHCl3/ethanol (9:1, v/v) for the corresponding titration of 1a, 2a
and 3] was injected consecutively into the stirred sample cell in
portions of 5, 10, and 5 μL, respectively, using a 500 μL Hamilton
syringe controlled by a Thermometric 612 Lund pump until the
desired concentration range had been covered. During the whole
titration process, the system was stirred at 60 rpm with a gold pro-
peller, and the interval between two injections was sufficiently long
for the signal to return to the baseline. The enthalpy (ΔH0) was
obtained by integrating the areas of the peaks in the plot of thermal
power against time. The reproducibility of experiments was within
Ϯ4%.
Trimer 3: To a mixture of 14 (0.36 g, 0.5 mmol), Pd(PPh3)4 (0.12 g,
0.1 mmol), CuI (0.02 g, 0.1 mmol), and diisopropylamine (5 mL) in
THF (20 mL) was added a solution of 13 (0.39 g, 1.5 mmol) in
THF (10 mL). The reaction mixture was stirred and heated for 12 h
at 60 °C. After cooling to room temperature, the resulting mixture
was poured into aqueous NH4Cl, and extracted with dichlorometh-
ane. The organic layer was washed with brine, dried with MgSO4
and filtered. The filtrate was evaporated in vacuo, and the residue
was purified by silica gel (neutral) column chromatography (hex-
ane/CHCl3, 10:90). The final product 3 was isolated as a white solid
(0.29 g, 59% yield) by washing with CH2Cl2 to remove unreacted
1
starting materials and soluble impurities, m.p.Ͼ 300 °C. H NMR
(500 MHz, CDCl3): δ = 7.64 (t, J = 7.8 Hz, 3 H), 7.54 (d, J =
8.0 Hz, 8 H), 7.51 (d, J = 8.0 Hz, 4 H), 7.45–7.43 (m, 6 H), 7.19–
7.16 (m, 12 H), 4.02 (s, 4 H), 2.63 (t, J = 7.7 Hz, 4 H), 1.62 (dt, J
= 15.2, 7.6 Hz, 4 H), 1.37 (dq, J = 14.6, 7.3 Hz, 4 H), 0.94 (t, J =
7.3 Hz, 6 H) ppm. 13C NMR (125 MHz, CDCl3, 318 K): δ =
144.52, 144.36, 144.20, 144.16, 141.83, 141.80, 136.40, 132.56,
132.25, 129.23, 128.68, 126.27, 126.22, 126.16, 120.50, 120.47,
119.57, 90.27, 89.84, 89.78, 88.55, 88.51, 88.09, 42.04, 35.83, 33.43,
22.47, 14.00 ppm. MS (MALDI-TOF): m/z calcd. for C73H55N3 [M
+ H+] 974.44; found 974.4.
CCDC-974845 (for [1b3Ag]BF4) and -974846 (for [2b3Ag2](BF4)2)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): 1D-NOEDIF spectra, 2D NMR spectra, 1H NMR titration
experiments, HRMS (ESI), ITC measurements, crystal structure
1
determination, H and 13C NMR spectra.
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Acknowledgments
The authors thank the National Natural Science Foundation of
China (NSFC) (grant numbers 21125205 and 21332008) for finan-
cial support.
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