Synthesis, Characterization, and Properties of Organometallic Molecular Cylinders Bearing Bulky Imidazo[1,5-a]pyridine-Based N-Heterocyclic Carbene Ligands

Article abstract of DOI:10.1002/chem.201806204

The metal-controlled self-assembly of organometallic molecular cylinders from a series of imidazo[1,5-a]pyridine-based tris-NHC ligands is described in this report. The imidazo[1,5-a]pyridinium salts H₃-L(PF₆)₃ (L=4 a–4 c)

Full text of DOI:10.1002/chem.201806204

A Journal of
Accepted Article
Title: Synthesis, Characterization and Properties of Organometallic
Molecular Cylinders Bearing Bulky Imidazo[1,5-a]pyridine-Based
N-Heterocyclic Carbene Ligands
Authors: Ya-Wen Zhang, Rajorshi Das, Yang Li, Yao-Yu Wang, and
Ying-Feng Han
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To be cited as: Chem. Eur. J. 10.1002/chem.201806204
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Synthesis, Characterization and Properties of Organometallic
Molecular Cylinders Bearing Bulky Imidazo[1,5-a]pyridine-Based
N-Heterocyclic Carbene Ligands
Ya-Wen Zhang,[a] Rajorshi Das,[a] Yang Li,[a] Yao-Yu Wang,[a] and Ying-Feng Han*[a,b]
Abstract: The metal-controlled self-assembly of organometallic
molecular cylinders from a series of imidazo[1,5-a]pyridine-based
tris-NHC ligands has been described. The imidazo[1,5-a]pyridinium
salts H3-L(PF6)3 (L = 4a−4c) react with 1.5 equiv of Ag2O to yield the
trinuclear AgI hexacarbene cages [Ag3(L)2](PF6)3 (L = 4a−4c), where
three AgI ions are sandwiched between the two tricarbene ligands.
The silver(I) complexes [Ag3(L)2](PF6)3 (L = 4a−4c) undergo facile
transmetalation reaction in presence of 3 equiv of AuCl(THT) leading
to the formation of trinuclear AuI cylinder-like cages [Ag3(L)2](PF6)3
(L = 4a−4c) without destruction of the metallosupramolecular
structure. The new hexacarbene assemblies feature a significantly
metal ions. For example, a benzene-bridged 1,4-bisimidazolium
salt was used for the preparation of molecular rectangle of type
A (Figure 1),[6b] whereas, the molecular cylinder of type B has
been built from a disk-shaped 1,3,5-trisimidazolium salt.[9a,c] In
similar fashion, an analogous hexaimidazolium benzene salt
allows the preparation of a coordination cage C featuring six AgI
metal ions sandwiched between the two hexacarbene ligands.[9d]
The linear coordination arrangement of AgI also plays an
important role to hold the two transoid ligands in a face-to-face
manner leading to a pre-defined ‘cylindrical’ structure. Thus, the
silver(I)-mediated self-assembly strategy using a Ag2O method
offers an efficient synthetic pathway for the exclusive
preparation of three-dimensional molecular assemblies with
interstitial cavities. In principle, such cavities may be useful for
the recognition of small molecules or for the development of new
chemical entities, which is only possible by performing reaction
large-sized
cavity
which
can
easily
accommodate
a
dimethylsulphoxide molecule as a molecular guest. This study for
the first time constitutes a unique example of ‘host-guest’ system
using an organometallic cylinder-like cage derived exclusively from
poly-NHC ligands.
in the confined space of a molecular nanocage.[11]
.
Introduction
Over the last couple of decades, N-heterocyclic carbenes
(NHCs) have evolved from niche compounds to one of the
ubiquitous ligands in chemistry.[1] They have been extensively
employed in catalysis,[2] in material science[3] and in biologically-
active metal complexes.[4] Recently, the application of NHCs has
been extended to supramolecular chemistry, where the poly-
NHC ligands are used as essential building blocks for the
construction of metallosupramolecular assemblies.[1a,h,5]
Since the emergence of the first poly-NHC ligand, the
synthesis of two- and three-dimensional supramolecular
architectures with different shapes and sizes has become a
focus of interest. Today, a variety of structures such as
molecular rectangles,[6] squares,[7] metallacycles,[8] cylinder-type
cages[9] and tweezers[10] derived from polycarbene ligands are
known. The overall structure of these metallo-supramolecular
assemblies is strictly dependent on the structural design of the
polycarbene ligands and the coordination geometry of selected
Figure 1. Previously reported two- and three-dimensional molecular
[a]
Y.-W. Zhang, Dr. R. Das, Y. Li, Prof. Dr. Y.-Y. Wang, Prof. Dr. Y.-F.
Han
assemblies derived from poly-NHC ligands.
Key Laboratory of Synthetic and Natural Functional Molecule
Chemistry, College of Chemistry and Materials Science
Northwest University
Xi’an 710127, P. R. China
E-mail: yfhan@nwu.edu.cn
The supramolecular assemblies of type B and C feature a
rather small cavity for the encapsulation of molecular guests.
Attempt for further enhancement of the cavity size with a 1,3,5-
triphenylbenzene linker although yielded a relatively larger cavity,
however, the π···π interactions between the two central
phenylene rings caused by the flexible aromatic backbone again
reduces the possibility for molecular recognition.[9e] We therefore
became interested in synthesizing a new set of tris-NHC ligands
[b]
Prof. Dr. Y.-F. Han
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter
Chinese Academy of Sciences
Fuzhou 350002, P.R. China
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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featuring imidazo[1,5-a]pyridine scaffold, which may enable the
cage with a significantly large-sized hollow cavity.
and K2CO3. Subsequently, the treatment of 2a (or 2b) with 2,4,6-
trimethylaniline (or 2,6-diisopropylaniline) in presence of catalytic
amount of glacial acetic acid afforded the imine products 3a, 3b
or 3c as yellow micro-crystalline powder. Finally, the reaction of
3a (3b or 3c) with paraformaldehyde in 4M HCl/1,4-dioxane
solution followed by the anion exchange with NH4PF6 in
methanol gave trisimidazo[1,5-a]pyridinium hexafluorophosphate
salts H3-4a(PF6)3, H3-4b(PF6)3 or H3-4c(PF6)3 in good yields of
80%, 79% and 77%, respectively, as off-white solids. All the
hexafluorophosphate salts are freely soluble in polar organic
solvents such as acetonitrile, dimethylformamide and
dimethylsulfoxide.
The imidazo[1,5-a]pyridinium salts have recently attracted
attention due to their potential application as N-heterocyclic
carbene precursors with tunable steric and electronic
properties.[12] Utilizing such behavior, here we describe the
synthesis of bulky trisimidazo[1,5-a]pyridinium salts H3-L(PF6)3
(L = 4a−4c) (Scheme 1) and their subsequent use in generating
trinuclear AgI and AuI hexacarbene coordination cages. Not only
the cages feature a significantly large-sized cavity, but also a
larger M···M separation compared to B and C type cylindrical
complexes, which is beneficial for the inclusion of guest
molecules. In addition, the imidazo[1,5-a]pyridinium salts are
rarely used in organometallic discrete NHC‒M complexes,[12]
and the metallosupramolecular assemblies derived from
imidazo[1,5-a]pyridinium salts have been unprecedented.
The formation of H3-4a(PF6)3, H3-4b(PF6)3 and H3-4c(PF6)3 was
confirmed by NMR spectroscopy (1H, 13C{1H}, 2D NMR) and
high-resolution
electrospray
ionization(HR-ESI)
mass
spectrometry. In the 1H NMR spectra, the characteristic
imidazolium C2−H resonances were observed as singlet at δ =
10.07 ppm for H3-4a(PF6)3, 9.96 ppm for H3-4b(PF6)3 and 10.06
ppm for H3-4c(PF6)3 in DMSO-d6. The 13C{1H} NMR spectra
showed the resonances for the N-CH-N carbon atoms at δ =
128.0 ppm (H3-4a(PF6)3), 127.9 ppm (H3-4b(PF6)3) and 128.3
ppm (H3-4c(PF6)3), in the same deuteriated solvent.
Results and Discussion
The imidazo[1,5-a]pyridine-based N-heterocyclic carbene
precursors H3-L(PF6)3 (L = 4a−4c) were synthesized by a three-
step procedure as depicted in Scheme 1. First,
pyridinecarboxaldehydes 2a and 2b were prepared by a cross-
coupling reaction using 3 equiv of 5-bromo-2-formylpyridine with
the boronate esters 1a and 1b, respectively, using of Pd(PPh3)4
The composition of pyridinium salts H3-4a(PF6)3, H3-4b(PF6)3
and H3-4c(PF6)3 was further confirmed by the HR-ESI mass
spectrometry, as the most intense peaks (positive ion mode) at
Scheme 1. Synthesis of the imidazo[1,5-a]pyridine-based N-heterocyclic carbene precursors H3-L(PF6)3 (L = 4a−4c).
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m/z = 464.1969 (calcd for [H3-4a(PF6)]2+ 464.1903), m/z =
337.1746 (calcd for [H3-4b]3+ 337.1699) and m/z = 379.1975
(calcd for [H3-4c]3+ 379.2169) were found for each pyridinium
salt with the identical isotopic distribution.
The reaction of 2 equiv of H3-L(PF6)3 (L = 4a−4c) with 3 equiv
of Ag2O in acetonitrile under exclusion of light yielded the
trinuclear AgI hexacarbene complexes [Ag3(L)2](PF6)3 (L =
4a−4c) in 90%, 73% and 68% yields, respectively (Scheme 2).
The trinuclear complexes were fully characterized by NMR
spectroscopy and HR-ESI mass spectrometry. Upon formation
of [Ag3(L)2](PF6)3 (L = 4a−4c), the characteristic resonances for
the C2−H protons of imidazolium salts are no longer observed in
1
the H NMR spectra. Accordingly, the CNHC resonances at δ =
174.3 ppm ([Ag3(4a)2](PF6)3), 171.5 ppm ([Ag3(4b)2](PF6)3) and
175.6 ppm ([Ag3(4c)2](PF6)3) in DMSO-d6 were obtained in the
13C{1H} spectra. These values are consistent with the previously
reported values for AgI(NHC)2 complexes.[8,9a-f] The HR-ESI
mass spectra (positive ions) feature the highest intense peak at
m/z = 628.5062 (calcd for [Ag3(4a)2]3+ 628.5005), at m/z =
780.5625 (calcd for [Ag3(4b)2]3+ 780.5651) and at m/z =
864.6720 (calcd for [Ag3(4c)2]3+ 864.6591), respectively.
Figure 2. Molecular structure (top view) of the trication [Ag3(4a)2]3+ in
[Ag3(4a)2](PF6)3 (hydrogen atoms omitted for clarity). Selected bond distances
(Å) and bond angles (deg): range Ag···Ag, 9.751−10.101; Ag−CNHC
,
2.054(7)−2.089(7); range CNHC−Ag−CNHC, 175.7(3)−177.0(2); range N−CNHC−N,
101.8(6)−104.2(7).
Single crystals of [Ag3(4a)2](PF6)3∙(DMSO)5 were obtained by
slow
diffusion
of
diethyl
ether
into
a
saturated
acetonitrile/dimethylsulfoxide solution of [Ag3(4a)2](PF6)3 at
ambient temperature. The X-ray diffraction study revealed the
expected cylinder-like complex cation [Ag3(4a)2]3+ as depicted in
Figure 2. Three silver(I) ions are sandwiched between the two
tris-NHC ligands obtained from the corresponding imidazo[1,5-
a]pyridinium salt H3-4a(PF6)3. The metric parameters found in
the complex cation [Ag3(4a)2]3+ [Ag–CNHC bond lengths,
2.054(7)−2.089(7);
CNHC–Ag–CNHC
bond
angles,
175.7(3)−177.0(2) and N−CNHC−N angles, 101.8(6)−104.2(7)] fall
in the range reported previously for related silver(I) polycarbene
assemblies.[8,9] The three silver(I) ions are situated in an
essentially planar triangle featuring almost equidistant Ag···Ag
separations around 1 nm.
Scheme 2. Preparation of trinuclear silver(I) hexacarbene complexes and their
transmetalation reactions to trinuclear gold(I) hexacarbene complexes.
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Since the silver(I)-NHC complexes are excellent carbene
transfer agents, we tried to substitute the AgI ions in
[Ag3(L)2](PF6)3 (L = 4a−4c) by AuI ions. Stirring an acetonitrile
solution of [Ag3(4a)2](PF6)3 with 3 equiv of [AuCl(THT)] for 16 h
at ambient temperature gave exclusively trinuclear gold(I)
hexacarbene complex [Au3(4a)2](PF6)3 as beige solid in an
excellent yield of 73% (Scheme 2). Similarly, the complexes
[Au3(4b)2](PF6)3 and [Au3(4b)2](PF6)3 have also been
synthesized in 65% and 67% yields, respectively.
The formation of transmetalated gold(I) complexes
[Au3(L)2](PF6)3 (L = 4a−4c) was verified by NMR spectroscopy
and HR-ESI mass spectrometry. In the 13C NMR spectra, CNHC
resonances were found at δ = 176.1 ppm ([Au3(4a)2](PF6)3),
162.6 ppm ([Au3(4b)2](PF6)3) and 163.5 ppm ([Au3(4c)2](PF6)3) in
DMSO-d6. The HR-ESI mass spectra (positive ion mode)
showed the most intense peaks at m/z = 717.5714 (calcd for
[Au3(4a)2]3+ 717.5631), m/z = 869.6435 (calcd for [Au3(4b)2]3+
869.6257) and m/z = 953.7269 (calcd for [Au3(4c)2]3+ 953.7196),
respectively.
An X-ray diffraction study with the single crystals of
composition [Au3(4a)2](PF6)3 obtained by the slow diffusion of
diethyl ether into a saturated solution of [Au3(4a)2](PF6)3, unveils
the retention of three-dimentional cylinder-like structure for the
complex cation [Au3(4a)2]3+ (Figure 3). The bond parameters of
[Au3(4a)2]3+ are expected[8a,9c] and found to be almost identical to
that of [Ag3(4a)2]3+ due to the essentially isostructural geometry.
The major differences between the two cations are found in the
slightly shorter Au−CNHC bond lengths [1.986(11)−2.001(11) Å]
compared to Ag−CNHC bond lengths [2.054(7)−2.089(7)], which
is also in good agreement with the previous reports.[8a,9a,c]
Figure 3. Molecular structure (top view) of the trication of [Au3(4a)2]3+ in
[Au3(4a)2](PF6)3 (hydrogen atoms omitted for clarity). Selected bond distances
(Å) and bond angles (deg): range Au−Au, 9.868−9.969; Au−CNHC
,
1.986(11)−2.001(11); range CNHC−Au−CNHC
,
177.5(4)−178.5(4); range
N−CNHC−N, 102.0(9)−103.9(10).
In accord with the previous observation for complex cation
[Ag3(4a)2]3+, the M···M separations, in particular, Au···Au
separations in the cation [Au3(4a)2]3+ are wide enough and the
distances measure around 1 nm. In addition, the central phenyl
rings are oriented in a coplanar fashion and located in 7 Å apart
Figure 4. Comparison of the separations between the two central phenyl rings and Ag···Ag atoms in the previously reported hexacarbene complex cations
of type B[9a,c] (Figure 1) with the molecular structures of cations [Ag3(4a)2]3+ and [Au3(4a)2]3+ (Atom colors are identical to those of Figure 2 and Figure 3).
The red arrows indicate the distances between the central phenyl rings.
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from each other. These values are significantly larger than the
values observed in the equivalent ‘cylinder-like’ hexacarbene
complexes of type B, where the Ph···Ph and Ag···Ag
separations are only 5 Å and 6 Å, respectively (Figure 4).[9a,c]
The elongated distances between the central phenyl rings of
complex cations [Ag3(4a)2]3+ and [Au3(4a)2]3+ can be attributed by
the introduction of imidazo[1,5-a]pyridine moieties into 1,3,5-
trisubstituted benzene skeleton. After complex formation, the
inherent molecular structure of imidazo[1,5-a]pyridine leads to
an increase in the vertical distance between the carbene carbon
atom and the extended imaginary-plane drawn from the central
phenyl ring, which eventually enhances the separation between
the two central phenyl rings in the hexacarbene cages. As a
around λem = 400 nm (Figure 6, also see Supporting Information).
However, the emission intensity is suppressed upon complex
formation. We take this observation as a result of the presence
of silver(I) or gold(I) centers, which may cause emission
quenching due to their heavy atom effects.[14] Under the similar
conditions, silver(I) complexes [Ag3(L)2](PF6)3 (L = 4a−4c)
exhibited weak emission, whereas, almost no emission was
observed for the heavier gold(I) complexes [Au3(L)2](PF6)3 (L =
4a−4c), even though both contain the same number of metal
centers, which further supports the heavy-atom effects.
Interestingly, after removal of the silver(I) metal ions from
[Ag3(L)2](PF6)3 (L = 4a−4c) by a well-established NH4Cl/NH4PF6
protocol[8] (Scheme 2), the high fluorescence emission of
imidazo[1,5-a]pyridine-based NHC precursors H3-L(PF6)3 (L =
4a−4c) can be restored. Potentially, this phenomenon could
operate in a turn-on/off fashion for visualization and/or imaging
with high spatial resolution.
consequence of the enlarged cavity,
a
co-crystallized
dimethylsulfoxide molecule can easily fit into the hollow space of
the cation [Ag3(4a)2]3+ as depicted in Figure 5.
Conclusions
We have demonstrated a facile synthetic strategy for the
preparation of trisimidazo[1,5-a]pyridine-based NHC ligand
precursors, which we have used to access the cylinder-like
trinuclear silver(I) and gold(I) hexacarbene complexes via metal-
controlled
self-assembly.
The
newly
synthesized
metallosupramolecular assemblies feature
a
larger M···M
distance (1 nm) as well as wide enough two central phenyl rings
(7 Å), elaborating the utility of the introduction of trisimidazo[1,5-
a]pyridine scaffolds into the 1,3,5-trisubstituted benzene core.
As a consequence of the significantly enhanced volume of cavity,
a dimethylsufoxide molecule can easily sit inside, which allows a
rich follow-up chemistry. Currently, efforts are being made in our
laboratories to develop molecular recognition and selected
catalytic applications using these imidazo[1,5-a]pyridine-based
N-Heterocyclic carbene complexes.
Figure 5. Orientation of the dimethylsulfoxide molecule (space-filling model)
trapped inside the hollow cavity of cylindrical complex cation [Ag3(4a)2]3+
.
Hexafluorophophate anions are omitted for clarity.
H -4a(PF )
3
6 3
Ag (4a) (PF )
6
3
2
3
Au (4a) (PF )
6
3
2
3
Experimental Section
General Procedures. All manipulations were performed under nitrogen
atmosphere using standard Schlenk techniques. Glassware was dried
overnight in an oven at 110 °C before use. Solvents were freshly distilled
by standard procedures prior to use. 1H, 13C{1H} and 2D NMR spectra
were recorded on Bruker AVANCE III 400 or JEOL JNM-ECZ600R
spectrometers. Chemical shifts (δ) are expressed in ppm downfield from
tetramethylsilane using the residual protonated solvent as an internal
standard. Mass spectra were obtained with a Bruker microTOF-Q II mass
spectrometer (Bruker Daltonics Corp., USA) in the electrospray ionization
(ESI) mode. 1a, 1b and 2a were synthesized according to published
procedures.[15] Unless otherwise stated, all reagents were purchased
from commercial sources and used without further purification. Due to the
fluorine content in the counteranions, satisfactory microanalytical data
were difficult to obtain.[16] A complete set of NMR spectra (1H, 13C{1H},
1H-1H COSY, 1H-13C HSQC and 1H-13C HMBC) and ESI-MS spectra are
provided instead.
350
400
450
500
550
Wavelength (nm)
Figure 6. Emission spectra of H3-4a(PF6)3 (Black), [Ag3(4a)2](PF6)3 (Red) and
−
[Au3(4a)2](PF6)3 (Blue) in CH3CN (PF6 salt, T = 298K, λex = 300 nm, c = 10.0
μM).
Since the imidazo[1,5-a]pyridinium ions are good
fluorophores,[13] we next investigated the luminescence
properties of these salts and their silver(I) and gold(I) complexes.
We have found that these polyheterocyclic hexafluorophosphate
salts H3-L(PF6)3 (L = 4a−4c) display strong emission centered at
Synthesis of 2b. A mixture of 1,3,5-tris(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)benzene (0.500 g, 0.77 mmol), 5-bromo-2-
pyridinecarboxaldehyde (0.612 g, 3.29 mmol), K2CO3 (1.6 g, 11.58 mmol),
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Pd(PPh3)4 (0.300 mg, 0.26 mmol) in anhydrous DMF (100 mL) was
degassed and stirred under N2 atmosphere at 90 ºC for 24 h. The solvent
was removed under reduced pressure and the solid residue triturated
with water, collected by filtration and washed with water (3×15 mL),
diethyl ether (2×10 mL) and hexane (2× 10 mL). The dried solid was
washed with a small amount of cold CH2Cl2 and collected again. The
resulting product was re-dissolved in hot CHCl3, the insoluble materials
filtered off and Et2O added to the filtrate to precipitate out pure product
2b as a pale yellow solid. Yield: 0.287 g (0.462 mmol 60 %). 1H NMR
(400 MHz, CDCl3): δ = 10.15 (s, 3H), 9.10 (d, J = 2.2 Hz, 3H), 8.15 (dd, J
= 7.9, 2.2 Hz, 3H), 8.09 (d, J = 7.9 Hz, 3H), 7.93 (s, 3H), 7.90 (d, J = 7.8
Hz, 6H), 7.81 (d, J = 7.8 Hz, 6H). 13C{1H} NMR (100 MHz, CDCl3): δ
193.1, 151.8, 148.7, 141.9, 141.7, 140.2, 136.1, 135.2, 128.4, 128.2,
125.7, 122.1. HRMS (ESI, positive ions): m/z 311.6100 (calcd for [M +
was added and stirred for 2 hours at ambient temperature to precipitate
compound H3-4a(PF6)3 as an off-white solid. Yield: 0.260 g (0.213 mmol
80 %). 1H NMR (400 MHz, DMSO-d6): δ = 10.07 (s, 3H, H15), 9.26 (s, 3H,
H14), 8.52 (s, 3H, H7), 8.37 (s, 3H, H13), 8.21 (d, J = 9.8 Hz, 3H, H9),
8.07 (d, J = 9.8 Hz, 3H, H10), 7.24 (s, 6H, H3), 2.39 (s, 9H, H1), 2.07 (s,
18H, H5). 13C{1H} NMR (100 MHz, DMSO-d6): δ 140.7 (C2), 137.2 (C12),
134.3 (C4), 131.7 (C6), 129.6 (C11), 129.3 (C3), 129.2 (C8), 128.0 (C15),
126.4 (C13), 125.9 (C10), 122.4 (C14), 118.9 (C9), 115.3 (C7), 20.6 (C1),
16.9 (C5). HRMS (ESI, positive ions): m/z 1073.3454 (calcd for [H3-
4a(PF6)2]+ 1073.3453); m/z 464.1969 (calcd for [H3-4a(PF6)]2+ 464.1903).
Synthesis of H3-4b(PF6)3. Compound H3-4b(PF6)3 was synthesized as
described for the synthesis of compound H3-4a(PF6)3 from compound 3b
(0.200 g, 0.206 mmol), Paraformaldehyde (0.069 g, 0.773 mmol) and 4M
HCl in 1,4-dioxane (0.203 ml, 0.618 mmol). Yield: 0.236 g (0.163 mmol,
79%). 1H NMR (400 MHz, DMSO-d6): δ = 9.96 (s, 3H, H15), 9.06 (s, 3H,
H14), 8.46 (s, 3H, H7), 8.21 (d, J = 7.9 Hz, 6H, H16), 8.18 (s, 3H, H19),
8.11 (d, J = 9.7 Hz, 3H, H9), 8.00 (d, J = 7.8 Hz, 6H, H13), 7.88 (d, J =
9.8 Hz, 3H, H10), 7.23 (s, 6H, H3), 2.39 (s, 9H, H1), 2.07 (s, 18H, H5).
13C{1H} NMR (100 MHz, DMSO-d6): δ 140.9 (C2), 140.6 (br, C17, C18),
134.4 (C12), 134.3 (C4), 131.7 (C6), 129.7 (C11), 129.3 (C3), 129.1 (C8),
128.3 (C16), 127.9 (C15), 127.5 (C13), 125.7 (C10), 124.9 (C19), 121.3
(C14), 118.7 (C9), 115.0 (C7), 20.6 (C1), 16.9 (C5). HRMS (ESI, positive
ions): m/z 578.2318 (calcd for [H3-4b(PF6)]2+ 578.2373); m/z 337.1746
(calcd for [H3-4b]3+ 337.1699).
2H]2+
311.6099).
Elemental
analysis
calcd
(%)
for
C42H27N3O3·CH2Cl2·H2O: C, 71.3; H, 4.3; N, 5.8 found C, 71.3; H, 4.1; N,
5.7.
Synthesis of 3a. 1,3,5-tris(4’,4’’,4’’’-formylpyridine)benzene (2a) (0.300g,
0.763mmol) was dissolved in 30 mL trichloromethane and 10 mL ethanol,
then, 2,4,6-Trimethylaniline (0.402 mL, 2.86 mmol) and acetic acid glacial
(0.014 mL, 0.251 mmol) was added. The mixture was refluxed for 16
hours. The solvent was removed in vacuo and the residue was washed
with methanol yielding ayellow micro-crystalline powder. Yield: 0.470 g
(0.631 mmol, 83 %). 1H NMR (400 MHz, CDCl3): δ = 9.07 (d, J = 2.3 Hz,
3H), 8.46 (d, J = 8.1 Hz, 3H), 8.44 (s, 3H), 8.17 (dd, J = 8.2, 2.3 Hz, 3H),
7.95 (s, 3H), 6.93 (s, 6H), 2.31 (s, 9H), 2.19 (s, 18H). 13C{1H} NMR (100
MHz, CDCl3): δ 163.0, 154.2, 148.2, 147.9, 139.7, 137.3, 135.4, 133.7,
129.0, 127.0, 126.3, 121.5, 20.9, 18.4. HRMS (ESI, positive ions): m/z
373.2017 (calcd for [M + 2H]2+ 373.2043). Elemental analysis calcd (%)
for C51H48N6·CH2Cl2: C, 75.3; H, 6.1; N, 10.1 found C, 75.9; H, 6.5; N,
10.0.
Synthesis of H3-4c(PF6)3. Compound H3-4c(PF6)3 was synthesized as
described for the synthesis of compound H3-4a(PF6)3 from compound 3c
(0.200 g, 0.182 mmol), Paraformaldehyde (0.061 g, 0.682 mmol) and 4M
HCl in 1,4-dioxane (0.184 ml, 0.546 mmol). Yield: 0.203 g (0.140 mmol
77%). 1H NMR (400 MHz, DMSO-d6): δ = 10.06 (s, 3H, H15), 9.06 (s, 3H,
H14), 8.59 (s, 3H, H7), 8.21 (d, J = 8.2 Hz, 6H, H16), 8.19 (s, 3H, H19),
8.12 (d, J = 9.9 Hz, 3H, H9), 8.02 (d, J = 7.9 Hz, 6H, H13), 7.91 (d, J =
9.7 Hz, 3H, H10), 7.70 (t, J = 7.8 Hz, 3H, H5), 7.53 (d, J = 7.8 Hz, 6H,
H4), 2.32 – 2.21 (m, 6H, H2), 1.17 (t, J = 6.7 Hz, 36H, H1, H20).13C{1H}
NMR (100 MHz, DMSO-d6): δ 145.1 (C3), 140.9 (C18), 140.6 (C17),
134.4 (C12), 131.7 (C5), 131.0 (C6), 129.8 (C11), 129.1 (C8), 128.3
(C15), 128.3 (C16), 127.5 (C13), 126.1 (C10), 124.9 (C19), 124.4 (C4),
121.5 (C14), 118.7 (C9), 116.1 (C7), 27.9 (C2), 24.1 (C1/C20), 23.9
(C1/C20). HRMS (ESI, positive ions): m/z 641.2818 (calcd for [H3-
4c(PF6)]2+ 641.3077); m/z 379.1975 (calcd for [H3-4c]3+ 379.2169).
Synthesis of 3b. Compound 3b was synthesized as described for the
preparation of 3a from compound 2b (0.300g, 0.483mmol), an excess of
2,4,6-Trimethylaniline (0.254 mL, 1.81mmol) and acetic acid glacial
(0.010 mL, 0.161 mmol). Yield: 0.418 g (0.430 mmol, 89%). 1H NMR (400
MHz, CDCl3): δ 9.03 (d, J = 1.7 Hz, 3H), 8.41 (s, 3H), 8.38 (d, J = 0.8 Hz,
3H), 8.12 (dd, J = 8.1, 2.3 Hz, 3H), 7.94 (s, 3H), 7.90 (d, J = 8.4 Hz, 6H),
7.86 – 7.78 (m, 6H), 6.93 (s, 6H), 2.31 (s, 9H), 2.18 (s, 18H). 13C{1H}
NMR (100 MHz, CDCl3): δ 163.1, 153.6, 148.0, 141.9, 141.2, 137.5,
136.7, 134.9, 133.6, 129.0, 128.2, 127.9, 127.0, 125.4, 121.4, 20.9, 18.4.
HRMS (ESI, positive ions): m/z 487.2496 (calcd for [M + 2H]2+ 487.2512).
Synthesis of [Ag3(4a)2](PF6)3. A sample of H3-4a(PF6)3 (0.050 g, 0.041
mmol) was dissolved in acetonitrile (20 mL), and to this solution was
added Ag2O (0.357 g, 0.154 mmol). The resulting suspension was
heated to 55 °C for 24 hours under exclusion of light. After the reaction
mixture was cooled to ambient temperature, the obtained suspension
was filtered through a pad of Celite to give a clear solution. The filtrate
was concentrated to 3 mL, and addition of 30 mL of diethyl ether resulted
in the precipitation of a gray solid. The precipitate was filtered off,
washed with diethyl ether, and dried in vacuo. Yield: 0.043 g (0.019 mmol,
90%). 1H NMR (400 MHz, DMSO-d6): δ = 8.65 (s, 3H, H14), 7.98 (s, 3H,
H7), 7.88 (s, 3H, H13), 7.80 (d, J = 9.5 Hz, 3H, H9), 7.36 (d, J = 9.5 Hz,
3H, H10), 7.06 (d, J = 12.0 Hz, 6H, br H3, H16), 2.47 (s, 9H, H1), 1.83 (s,
9H, H5), 1.48 (s, 9H, H17). 13C{1H} NMR (100 MHz, DMSO-d6): δ =
174.3 (dd, J (C-Ag107) = 184.2 Hz, J (C-Ag109) = 217.3 Hz, C15), 138.9
(C12), 138.8 (C2), 135.8 (C6), 134.1 (C4/C18), 133.5 (C18/C4), 129.6
(C8), 129.2 (C3/C16), 128.6 (C16/C3), 128.3 (C11), 127.0 (C13), 126.9
(C14), 124.8 (C10), 118.7 (C9), 118.0 (CH3CN), 114.2 (C7), 20.7 (C1),
17.3 (C5), 16.5 (C17). HRMS (ESI, positive ions): m/z 1015.2471 (calcd
for [[Ag3(4a)2](PF6)]2+ 1015.2332); m/z 628.5062 (calcd for [Ag3(4a)2]3+
628.5005).
Synthesis of 3c. Compound 3c was synthesized as described for the
preparation of 3a from compound 2b (0.300g, 0.483mmol), an excess of
2,6-diisopropylaniline (0.342 mL, 1.81mmol) and acetic acid glacial
(0.010 mL, 0.161 mmol). Yield: 0.403 g (0.367 mmol, 76%). 1H NMR (400
MHz, CDCl3): δ 9.06 (s, 3H), 8.42 (d, J = 4.8 Hz, 3H), 8.40 (s, 3H), 8.15
(dd, J = 8.2, 2.3 Hz, 3H), 7.97 (s, 3H), 7.93 (d, J = 8.3 Hz, 6H), 7.84 (d, J
= 8.4 Hz, 6H), 7.21 (d, J = 6.5 Hz, 6H), 7.19 – 7.13 (m, 3H), 3.05 (h, J =
6.9 Hz, 6H), 1.22 (d, J = 6.9 Hz, 36H). 13C{1H} NMR (100 MHz, CDCl3): δ
162.7, 153.4, 148.6, 148.2, 142.0, 141.3, 137.8, 137.4, 136.8, 135.1,
128.3, 127.9, 125.5, 124.7, 123.2, 121.6, 28.2, 23.6. HRMS (ESI,
positive ions): m/z 550.3222 (calcd for [M + 2H]2+ 550.3217). Elemental
analysis calcd (%) for C78H78N6·CH2Cl2·H2O: C, 78.9; H, 6.9; N, 7.0 found
C, 78.8; H, 7.1; N, 6.8.
Synthesis of H3-4a(PF6)3. The reaction was carried out under an
atmosphere of dry nitrogen: Paraformaldehyde (0.091 g, 1.01 mmol) was
dissolved in 30 ml hot toluene and Compound 3a ( 0.200 g, 0.268 mmol)
was added. 4M HCl in 1,4-dioxane (0.685 ml, 0.804 mmol) was added
dropwise, whereby a light yellow precipitate formed immediately. After
stirring for 18 h at 43 °C, the precipitate was filtered and washed twice
with 2 ml diethyl ether. To remove non-reacted paraformaldehyde the
residue was dissolved in methanole and filtered. A solution of NH4PF6
Synthesis of [Ag3(4b)2](PF6)3. Complex [Ag3(4b)2](PF6)3 was
synthesized as described for the synthesis of complex [Ag3(4a)2](PF6)3
from compound H3-4b(PF6)3 (0.050 g, 0.035 mmol) and Ag2O (0.304 g,
This article is protected by copyright. All rights reserved.

10.1002/chem.201806204
Chemistry - A European Journal
FULL PAPER
0.131 mmol). Yield: 0.035 g (0.013 mmol, 73%). 1H NMR (400 MHz,
DMSO-d6) δ = 9.39 (s, 3H), 8.11–8.05 (m, 6H), 8.04 (s, 3H), 7.87 (d, J =
9.2 Hz, 3H), 7.68 (d, J = 9.9 Hz, 3H), 7.57 (d, J = 4.5 Hz, 6H), 7.50 (s,
3H), 7.03 (s, 6H), 2.48 (s, 9H), 1.87 (s, 18H). 13C NMR (100 MHz,
DMSO-d6) δ = 171.5, 139.6, 138.8, 135.9, 135.2, 133.8, 133.6, 133.4,
129.8, 128.9, 127.5, 127.1, 126.3, 123.6, 118.4, 118.1, 114.1, 22.5, 20.8,
17.1. HRMS (ESI, positive ions): m/z 780.5625 (calcd for [Ag3(4b)2]3+
780.5651).
vacuo. Yield: 0.049 g (0.015 mmol, 67%). 1H NMR (400 MHz, DMSO-d6)
δ = 8.83 (s, 3H), 8.18 (s, 3H), 8.13 – 8.09 (m, 6H), 7.99 (d, J = 8.2 Hz,
6H), 7.87 (s, 3H), 7.84 (s, 3H), 7.60 (d, J = 8.2 Hz, 6H), 7.44 (d, J = 7.8
Hz, 6H), 2.19 (d, J = 7.1 Hz, 6H), 1.23 (d, J = 6.7 Hz, 18H), 1.13 (d, J =
6.8 Hz, 18H). 13C NMR (100 MHz, DMSO-d6) δ = 163.5, 145.1, 141.1,
140.0, 135.2, 134.6, 130.7, 129.0, 128.1, 127.9, 127.5, 124.8, 124.1,
123.4, 118.7, 115.3, 30.5, 28.0, 24.9, 24.0. HRMS (ESI, positive ions):
m/z 953.7269 (calcd for [Au3(4c)2]3+ 953.7196).
Synthesis of [Ag3(4c)2](PF6)3. Complex [Ag3(4c)2](PF6)3 was
synthesized as described for the synthesis of complex [Ag3(4a)2](PF6)3
from compound H3-4c(PF6)3 (0.050 g, 0.032 mmol) and Ag2O (0.278 g,
0.120 mmol). Yield: 0.033 g (0.011 mmol, 68%). 1H NMR (400 MHz,
DMSO-d6) δ = 9.33 (s, 3H, H14), 8.20 (s, 3H, H7), 8.07 (d, J = 8.1 Hz, 6H,
H13), 7.91 (d, J = 9.5 Hz, 3H, H9), 7.70 (d, J = 9.7 Hz, 3H, H10), 7.57 (t,
J = 7.8 Hz, 3H, H5), 7.48 (d, J = 7.9 Hz, 6H, H16), 7.41 (s, 3H, H19),
7.35 (d, J = 7.8 Hz, 3H, H4), 7.17 (d, J = 7.9 Hz, 3H, H4), 2.13 (dd, J =
15.9, 7.2 Hz, 6H, H2), 1.26–0.49 (m, 36H, br H1, H20). 13C NMR (100
MHz, DMSO-d6) δ = 175.6 (C15), 144.4 (C3), 139.5 (C17), 135.2 (C12),
135.0 (C6), 130.2 (C5), 129.5 (C8), 127.3 (C13), 127.2 (C16), 127.0
(C18), 126.5 (C14), 124.1 (C11), 124.1 (C10), 123.7 (C4), 123.2 (C19),
118.4 (C9), 115.9 (C7), 27.8 (C2), 24.3 (C1/C20), 24.2 (C1/C20). HRMS
(ESI, positive ions): m/z 1369.4821 (calcd for [[Ag3(4c)2](PF6)]2+
1369.4710); m/z 864.6720 (calcd for [Ag3(4c)2]3+ 864.6591).
Acknowledgements
The authors gratefully acknowledge financial support from the
NSFC (grant nos. 21722105, 21531007), the National Youth
Top-notch Talent Support Program of China, the Key Science
and Technology Innovation Team of Shaanxi Province, and the
FM & EM International Joint Laboratory of Northwest University.
Conflict of interest
The authors declare no conflict of interest.
Synthesis of [Au3(4a)2](PF6)3. To a solution of [Ag3(4a)2](PF6)3 (0.050 g,
0.022 mmol) in acetonitrile (15 mL) was added solid [AuCl(THT)] (0.023 g,
0.073 mmol). Immediately after the addition the mixture turned from light
reddish to black. The reaction mixture was stirred at ambient temperature
for 16 h and was then slowly filtered through a pad of Celite until a clear
filtrate was obtained. The filtrate was concentrated to 3 mL, and addition
of 30 mL of diethyl ether resulted in the precipitation of a purple solid,
which was collected by filtration, washed with diethyl ether, and dried in
vacuo. Yield: 0.042 g (0.016 mmol, 73%). 1H NMR (400 MHz, DMSO-d6)
δ = 8.61 (s, 3H, H14), 8.02 (s, 3H, H7), 7.94 (s, 3H, H13), 7.84 (d, J = 9.5
Hz, 3H, H9), 7.35 (d, J = 9.3 Hz, 3H, H10), 7.06 (d, J = 10.7 Hz, 6H,
H3/H16), 2.47 (s, 9H, H1), 1.82 (s, 9H, H5), 1.43 (s, 9H, H17). 13C NMR
(100 MHz, DMSO-d6) δ = 176.2, 139.2, 138.5, 135.5, 135.0, 134.1, 134.1,
133.7, 129.3, 129.1, 128.9, 128.6, 125.4, 118.6, 114.5, 20.8, 17.3, 16.5.
HRMS (ESI, positive ions): m/z 1148.8428 (calcd for [[Au3(4a)2](PF6)]2+
1148.8270); m/z 717.5714 (calcd for [Au3(4a)2]3+ 717.5631).
Keywords: self-assembly • N-heterocyclic carbene •
transmetalation • host-guest systems • organometallic chemistry
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Synthesis of [Au3(4b)2](PF6)3. To a solution of [Ag3(4b)2](PF6)3 (0.061 g,
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10.1002/chem.201806204
Chemistry - A European Journal
FULL PAPER
Table of Contents
FULL PAPER
Well done imidazo[1,5-a]pyridine! A
set of novel organometallic molecular
cylinders comprising
Ya-Wen Zhang, Rajorshi Das, Yao-Yu
Wang, and Ying-Feng Han*
Page No. – Page No.
imidazo[1,5a]pyridine-based tris-NHC
ligands has been described. The
incorporation of imidazo[1,5-a]pyridine
scaffolds into the poly-NHC ligands
enables the enhancement of cavity-
size of the newly synthesized
Synthesis, Characterization and
Properties of Organometallic
Molecular Cylinders Bearing Bulky
Imidazo[1,5-a]pyridine-Based N-
Heterocyclic Carbene Ligands
metallacages, which are potentially
capable for molecular recognition.
This article is protected by copyright. All rights reserved.