Synthesis of multi-imidazolium salt ligands containing calixarene fragments and their N-heterocyclic carbene Ag(I) macrocyclic complexes

Article abstract of DOI:10.1016/j.poly.2014.09.043

The unsymmetric bis-imidazolium salts ligands 4a and b were prepared by stepwise reactions of 2,6-bis(bromomethylene)-4-(t-butyl)anisole (1) with two different N-substituted imidazoles. Reaction of the bis-imidazolium ligands with Ag₂O in acetonitrile gave the dinuclear bis(N-heterocyclic carbene) complexes 5a and b. The molecular structure of complex 5a was determined by single-crystal X-ray diffraction analysis. In complex 5a and b, the two C_carbene-Ag-C_carbene axes are in a crossed conformation, two O atoms from phenoxyl groups weakly coordinate to different Ag ions and the two p-t-butylphenoxyl groups adopted a staggered conformation. To regulate the molecular shape of the N-heterocyclic carbene Ag(I) macrocyclic complexes, two tetradentate imidazolium ligands (9a and b) were prepared. The bimetallic NHC-Ag(I) complexes 10a and b were obtained by the reaction of 9a and b and Ag₂O in acetonitrile. Constrained by the chain (the dioxoethylene or dioxomethylenebenzene chain in 10a or 10b, respectively) at the O-position of the phenoxyl group, the two p-methylphenoxyl groups in complexes 10a and b might be predicted to have a cone conformation. The ligands and the corresponding NHC metal complexes have been characterized by elemental analysis, ¹H NMR and ¹³C NMR, HRMS spectra, as well as TGA and DTA spectra being determined for complexes 10a and b. The TGA curves show that these NHC-Ag complexes have good thermal stability.

Full text of DOI:10.1016/j.poly.2014.09.043

Accepted Manuscript
Synthesis of multi-imidazolium salt ligands containing calixarene fragments and
their N-heterocyclic carbene Ag(I) macrocyclic complexes
Lin Guo, Xi-Zhen Song, Cai-Xia Lin, Qing-Shan Li, Chuan Liu, Wen-Hu Wang,
Feng-Bo Xu
PII:
S0277-5387(14)00665-2
DOI:
http://dx.doi.org/10.1016/j.poly.2014.09.043
POLY 11016
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
28 April 2014
30 September 2014
Please cite this article as: L. Guo, X-Z. Song, C-X. Lin, Q-S. Li, C. Liu, W-H. Wang, F-B. Xu, Synthesis of multi-
imidazolium salt ligands containing calixarene fragments and their N-heterocyclic carbene Ag(I) macrocyclic
complexes, Polyhedron (2014), doi: http://dx.doi.org/10.1016/j.poly.2014.09.043
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Synthesis of multi-imidazolium salt ligands containing
calixarene fragments and their N-heterocyclic carbene Ag(I)
macrocyclic complexes
Lin Guo, Xi-Zhen Song, Cai-Xia Lin, Qing-Shan Li*, Chuan Liu, Wen-Hu Wang,
Feng-Bo Xu*
State Key Laboratory of Elemento-Organic Chemistry, Department of Chemistry,
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Nankai University, Tianjin 300071, P. R. China
Abstract
The unsymmetric bis-imidazolium salts ligands 4a-b were prepared by stepwise
reactions of 2,6-bis(bromomethylene)-4-(t-butyl)anisole (1) with two different
N-substituted imidazoles. Reaction of the bis-imidazolium ligands with Ag2O in
acetonitrile gave the dinuclear bis(N-heterocyclic carbene) complexes 5a-b. The
molecular structure of complex 5a was determined by single-crystal X-ray diffraction
analysis. In complex 5ab, the two Ccarbene-Ag-Ccarbene axes are in a crossed
conformation, two O atoms from phenoxyl groups weakly coordinate to different Ag
ions and the two p-t-butylphenoxyl groups adopted a staggered conformation. To
regulate the molecular shape of the N-heterocyclic carbene Ag(I) macrocyclic
complexes, two tetradentate imidazolium ligands (9a-b) were prepared. The
bimetallic NHC-Ag(I) complexes 10a-b were obtained by the reaction of 9a-b and
Ag2O in acetonitrile. Constrained by the chain (the dioxoethylene or
dioxomethylenebenzene chain in 10a or 10b, respectively) at the O-position of the
phenoxyl group, the two p-methylphenoxyl groups in complexes 10a−b might be
predicted to have a cone conformation. The ligands and the corresponding NHC metal
1
complexes have been characterized by elemental analysis, H NMR and 13C NMR,
* Corresponding author. Tel. & Fax: +86-22-23503710.
Email address: qli@nankai.edu.cn (Q. S. Li).
1

HRMS spectra, as well as TGA and DTA spectra being determined for complexes
10a-b. The TGA curves show that these NHC-Ag complexes have good thermal
stability.
Keywords: N-heterocyclic carbene, Macrocyclic complexes, Silver complexes,
Molecular structure.
1. Introduction
Metal-organic framework (MOF) functional material molecules with special
structural shapes, such as a molecular structure with a cage-, cave- or bowl shape,
have been studied extensively in recent years [1-4]. Usually these complexes are
synthesized through the coordination of N-, O-, S- or P-ligands to some transition
metal ions. Diphospine ligands showed good coordinative ability with some soft metal
ions, such as low valent metal ions and late transition metal ions. Compared to
trivalent phosphine ligands, NHCs (N-heterocyclic carbenes) are weak π-acceptors
and strong σ-donors and they can form strong M−C bond with transition metal ions
[5-6]. Because of the high stability of the M−C bond in NHC-metal complexes, some
MOFs complexes constructed with NHC-metal bonding can applied as a type of good
host in molecular recognition to some special organic neutral molecules and anions,
or as efficient catalysts to some coupling or polymerization reactions [7-8]. Due to
their readily accessibility and structural diversity, imidazolium salts provide various
donor sources for the preparation of NHC-metal complexes. This type of ligand along
with metal ions readily forms cyclic NHC-metal complexes, which may have
potential application in organic reactions and in molecular recognition. An efficient
way to prepare the NHC metal complexes is using the deprotonation of imidazolium
in the presence of a strong inorganic base, and the continuous reaction with the metal
ions. Another method is from the reaction of the imidazolium with the metal oxide
directly [9-10].
As is known, calixarenes are a class of important and versatile host molecules
because of their unique conformational and special cavity structures [11-13]. Some
2

attempts have been made to prepare metal NHC-calixarene complexes by attaching
NHC-metal substituents at the upper/lower rims of the calixarene [14-16]. In recent
years we have been engaged in the synthesis of macrocyclic bis-/multi-N-heterocyclic
carbene metal complexes containing calixarene fragments. Since these complexes are
assembled via M−C bonds of high stability and calixarene fragments, they could be
predicted to have the property of both metallacycles and calixarenes. Herein, two
unsymmetric bis-imidazolium precursors (4a-b) linked by calixarene fragments were
designed and used to react with Ag2O directly to prepare the dinuclear NHC−Ag(I)
macrocyclic complexes 5a-b. The structural analysis showed that complex 5a has an
essential metallamacrocyclic structure with a staggered conformation of the two
p-t-butylphenoxyl groups. For comparison, two tetra-imidazolium ligands (9a-b) were
designed. The tetradentate ligands reacted with Ag2O and the metallamacrocyclic
complexes (10a-b) were prepared. We hoped the complexes 10a-b would have a
bowl-like structure in molecular shape, since the dioxoethylene or
p-dioxomethlylenebenzene linker can act as the bottom of the bowl and strain the
lower rim of the two p-methylphenoxyl groups in the metallamacrocycles.
2. Experimental
2.1. General
The solvents were purified by standard methods. The bridged
bis-p-methylphenol tetra-bromomethylene derivatives 8a and 8b (Scheme 2) were
prepared from p-methylphenol according to the literature methods [17-18]. 1H and 13C
NMR spectra were recorded on a Bruker AV400 spectrometer. The HRMS spectra
were obtained on an Agilent 6520 Q−TOF spectrometer. Elemental analyses were
measured using a Vario EL CUBE elemental analyzer. Thermogravimetric analysis
(TGA) was performed using a TG209 instrument at a heating rate of 10 °C min-1
under high purity N2 with a flow rate of 40 mL min-1. The single crystal X-ray
diffraction intensity data was collected on a Rigaku Saturn724 CCD diffractometer.
The melting points were determined with an X−4 digital display micromelting point
apparatus.
3

2.2. Preparation of bis-imidazolium ligands 4a and 4b
2.2.1. Preparation of the mono-imidazolium bromide 2
To a solution of 2,6-bis(bromomethyl)-4-t-butylanisole (1) (3.53 g, 0.01 mol) in
30 ml dioxane, a solution of N-anthracenomethylene-imidazole (2.61 g, 0.01 mol) in
dioxane was added dropwise. The mixture was refluxed for 7 hours. After cooling, the
mixture was filtered and the filtrate was evacuated to remove the solvent. The
resulting yellow oil was subjected to flash column chromatography using an eluent of
ethyl acetate/methanol (2:1) to give compound 2 (Rf 0.3 in 2:1 ethyl acetate/methanol)
1
as a clear yellow solid. Yield: 3.26 g (53%). M.p. 133-135 °C. H NMR (400 MHz,
DMSO-d6) δ: 9.13 (s, 1H, NCHN of Im), 8.86 (s, 1H, An−H), 8.50 (d, J = 7.9 Hz, 2H,
An−H), 8.23 (d, J = 7.3 Hz, 2H, An−H), 7.73-7.61 (m, 5H), 7.48 (s, 1H), 7.06 (s, 1H),
6.57 (s, 2H), 5.37 (s, 2H), 4.66 (s, 2H), 3.69 (s, 3H, OCH3), 1.14 (s, 9H, t−Bu). 13C
NMR (100 MHz, DMSO-d6) δ: 153.95 (C−OCH3 of Ph), 147.25 (C−t−Bu of Ph),
136.03 (NCHN of Im), 131.07, 130.99, 130.60, 130.12, 129.37, 127.86, 127.77,
127.16, 125.56, 123.59, 123.36, 122.82, 61.69 (OCH3), 47.85 (Im−CH2−Ph), 45.00
(Br−CH2−Ph), 34.10 (Im−CH2−Ph), 30.90 (t−Bu), 28.95 (CH3 of t−Bu). HRMS,
[M−Br−]+ m/z: 527.1698, Found: 527.1696. Anal. Calcd for C31H32Br2N2O: C, 61.20;
H, 5.30; N, 4.60, Found: C, 60.98; H, 5.47; N, 4.48%.
2.2.2. Preparation of bis-imidazolium ligands 4a-b
A 1,4-dioxane (40 ml) solution of bromide 2 (1.52 g, 2.5 mmol) and N-methyl
imidazole (0.21 g, 2.5 mmol) was refluxed for 12 hours. After cooling, the
bis-imidazolium bromide (3a) was precipitated at the bottom of the flask as a
pale-yellow solid. The upper solvent was poured off, then the bromide was dissolved
in methanol (35 ml). NH4PF6 (0.41 g, 2.5 mmol) was added and the mixture was
stirred for 3 hours to precipitate another pale-yellow solid. The solid was collected,
washed with Et2O and dried in a vacuum to give the NHC precursor 4a. Yield: 1.68 g
1
(82%). M.p.: 256-257 °C. H NMR (400 MHz, DMSO-d6) δ: 9.09 (s, 1H, NCHN of
Im), 9.02 (s, 1H, NCHN of Im), 8.86 (s, 1H, An−H), 8.46 (d, 2H, J = 8.6 Hz, An−H),
4

8.23 (d, 2H, J = 8.1 Hz, An−H), 7.74 (s, 1H, Im−H), 7.69-7.59 (m, 6H, Im−H and
An−H), 7.39 (d, 1H, J = 2.0 Hz, C6H2), 6.98 (d, 1H, J = 2.0 Hz, C6H2), 6.53 (s, 2H,
An−CH2−Im), 5.35 (d, 4H, J = 3.6 Hz, Im−CH2−Ph), 3.83 (s, 3H, Im−CH3), 3.58 (s,
13
3H, OCH3), 1.10 (s, 9H, t−Bu). C NMR (100 MHz, DMSO−d6) δ: 154.0 (C−OCH3
of Ph), 147.8 (C−t–Bu of Ph), 136.7 (NCHN of Im), 136.1 (NCHN of Im), 131.1,
130.6, 130.2, 129.5, 128.3, 128.1, 127.8, 127.7, 127.2, 125.6, 123.8, 123.5, 123.2,
−
−
123.1, 122.9, 122.4, 61.8 (OCH3), 47.6 (Im CH2−Ph), 45.1 (Im CH2−Ph), 35.9
(Im−CH3), 34.2 (t−Bu), 30.9 (CH3 of t−Bu). ESI-MS, Calcd for C35H38F6N4OP+
− +
([M−PF6 ] ) m/z: 675.2687, Found: 675.3. Anal. Calcd for C35H38F12N4OP2: C, 51.23;
H, 4.67; N, 6.83, Found: C, 51.04; H, 4.78; N, 6.53%.
Ligand 4b was prepared using the same procedure as that for 4a, except using
N-benzylimidazole instead of N-methyl imidazole. Yield: 1.63 g (73%). M.p.:
1
139-140 °C. H NMR (400 MHz, DMSO-d6) δ: 9.35 (s, 1H, NCHN of Im), 9.04 (s,
1H, NCHN of Im), 8.86 (s, 1H, An−H), 8.46 (d, J = 8.7 Hz, 2H, An−H), 8.23 (d, J =
8.1 Hz, 2H, An−H), 7.81 (s, 1H, Im−H), 7.73 (s, 1H, Im−H), 7.67 (s, 2H, Im−H),
7.65-7.59 (m, 4H, An−H), 7.42-7.36 (m, 5H, Ph−H), 7.25 (d, J = 2.1 Hz, 1H, C6H2),
6.99 (d, J = 2.0 Hz, 1H, C6H2), 6.53 (s, 2H, An−CH2−Im), 5.43 (s, 2H, Im−CH2−Ph),
5.40 (s, 2H, Im−CH2−Ph), 5.34 (s, 2H, Im−CH2−Ph), 3.57 (s, 3H, OCH3), 1.08 (s, 9H,
13
t−Bu). C NMR (100 MHz, DMSO-d6) δ: 153.76 (C−OCH3 of Ph), 147.63 (C−t−Bu
of Ph), 136.47 (NCHN of Im), 136.04 (NCHN of Im), 134.93, 131.08, 130.58, 130.15,
129.41, 128.98, 128.76, 128.15, 128.04, 127.79, 127.67, 127.05, 125.59, 123.50,
123.27, 123.08, 123.00, 122.84, 122.79, 61.78 (OCH3), 45.02 (Im−CH2−Ph), 47.55
(Im−CH2−Ph), 47.77 (Im−CH2−Ph), 51.94 (Im−CH2−An), 34.14 (t−Bu), 30.81 (CH3
- +
of t−Bu). HRMS, Calcd. for C41H42F6N4OP+ ([M−PF6 ] ) m/z: 751.3000, Found:
751.2986. Anal. Calcd. for C41H42F12N4OP2: C, 54.92; H, 4.72; N, 6.25, Found: C,
54.66; H, 4.98; N, 6.11%.
2.3. Preparation of the N-heterocyclic carbene Ag(I) macrocyclic complexes 5a-b
To a solution of 4a (0.33 g, 0.4 mmol) in acetonitrile, Ag2O (0.09 g, 0.4 mmol)
was added and the mixture was stirred at 50 °C for 12 hours with the exclusion of
5

light. After cooling to room temperature, the mixture was filtered to remove the
suspension. The volatiles were evaporated in vacuo at room temperature. Pure
complex 5a was afforded by recrystallization from a mixture of acetonitrile and
diethyl ether. Yield: 0.48 g (77%). M.p.: 244-245 °C. 1H NMR (400 MHz, DMSO-d6)
δ: 8.40 (s, 2H, An−H), 8.33 (d, 4H, J = 8.4 Hz, An−H), 8.01 (s, 2H, Im−H), 7.86 (d,
4H, J = 8.4 Hz, An−H), 7.71 (s, 2H, Im−H), 7.59-7.47 (m, 4H, An−H), 7.41-7.32 (m,
4H, An−H), 7.25 (d, 2H, J = 1.7 Hz, C6H2), 7.23 (s, 2H, Im−H), 6.73 (s, 2H, Im−H),
6.35 (s, 4H, Im−CH2−An), 6.01 (d, 2H, J = 1.7 Hz, C6H2), 5.14 (s, 4H, Ph−CH2−Im),
4.01 (s, 2H, Ph−CH2−Im), 2.94 (s, 6H, Im−Me), 1.86 (s, 3H, OMe), 0.89 (s, 18H,
t−Bu). 13C NMR (100 MHz, DMSO-d6) δ: 181.8 (Ccarbene), 178.7 (Ccarbene), 152.2
(C−OMe of Ph), 146.4 (C−t−Bu of Ph), 131.4, 131.1, 130.7, 128.8, 128.6, 127.2,
126.7, 126.2, 125.1, 123.7, 123.2, 123.2, 122.8, 122.8, 121.4, 121.3, 119.9 (Im), 119.8
(Im), 60.6 (OMe), 48.7 (Im−CH2−Ph), 48.3 (Im−CH2−Ph), 46.0 (Im−CH2−Ph), 36.6
−
(Im−CH3), 33.8 (t−Bu), 30.8 (CH3 of t−Bu). ESI−MS, [M−2PF6 ]/2 m/z: 635.2 (for
–
107Ag), 637.2 (for 109Ag). HRMS, Calcd for C35H37AgN4O+ ([(M–2PF6 )/2)]+) m/z:
– +
636.1939, Found: 636.2018. Calcd for C70H72Ag2F6N8O2P+ ([M–PF6 ] ) m/z:
1417.3519, Found: 1417.3481.
Complex 5b was prepared using the same procedure as that for complex 5a, but
from the reaction of ligand 4b with Ag2O. Yield: 0.34 g (54%). M.p.: 180-181 °C.
1H NMR (400 MHz, DMSO-d6) δ: 8.69 (d, J = 15.4 Hz, 2H, An−H), 8.40 (dd, J =
18.4, 8.7 Hz, 4H, An−H), 8.11-8.01 (m, 4H, An−H), 7.93 (s, 1H, C6H2), 7.83 (s, 1H,
C6H2), 7.51 (m, 14H, An−H, Ph−H), 7.22 (s, 6H, An−H, Ph−H), 6.82 (s, 1H, Im−H),
6.56 (s, 1H, Im−H), 6.39 (s, 2H, Im−H), 6.30 (s, 2H, Im−H), 6.24 (s, 1H, Im−H), 6.16
(s, 2H, Im−CH2−Ph), 6.08 (s, 1H, Im−H), 5.38 (s, 2H, Im−CH2−Ph), 5.04 (s, 2H,
Im−CH2−Ph), 4.87 (s, 2H, Im−CH2−Ph), 4.49 (s, 2H, Im−CH2−An), 4.33 (s, 2H,
Im−CH2−An), 4.09 (s, 2H, Im−CH2−Ph), 2.86 (s, 3H, OCH3), 1.86 (s, 3H, OCH3),
0.68 (s, 18H, t−Bu). 13C NMR (100 MHz, DMSO-d6) δ: 180.43 (Ccarbene), 177.96
(Ccarbene), 151.98 (C−OCH3 of Ph), 150.89 (C−OCH3 of Ph), 146.45 (C−t−Bu of Ph),
136.89, 135.53, 131.17, 131.04, 130.59, 130.36, 130.11, 129.77, 129.07, 128.81,
128.64, 128.61, 128.16, 127.92, 127.80, 127.22, 126.32, 125.25, 123.65, 123.24,
6

122.98, 122.69, 122.45, 121.96, 121.78, 121.54, 121.36, 60.35 (OCH3), 59.82 (OCH3),
57.64 (Im−CH2−Ph), 57.45 (Im−CH2−Ph), 53.91 (Im−CH2−Ph), 49.98 (Im−CH2−Ph),
48.84 (Im−CH2−Ph), 48.50 (Im−CH2−Ph), 46.22 (Im−CH2−An), 45.86 (Im−CH2−An),
33.67 (t−Bu), 33.57 (t−Bu), 30.49 (CH3 of t−Bu). HRMS, Calcd for C41H40AgN4O+
–
([(M–2PF6 )/2)]+) m/z: 712.2252, Found: 712.2251. Anal. Calcd for
C82H16Ag2F12N8O10P2 (5b·8H2O): C, 52.97; H, 5.20; N, 6.03, Found: C, 52.96; H,
5.14; N, 6.09%.
2.4. Preparation of tetra-imidazolium ligands 9a and 9b
A
1,4-dioxane (30 ml) solution of the bridged p-methylphenol
tetra-bromomethylene derivative 8a (1.54 g, 2.5 mmol) and N-methylimidazole (0.82
g, 10 mmol) was refluxed for 12 hours to give the tetra-imidazolium bromide as a
white solid. The bromide was dissolved in methanol (35 ml) and NH4PF6 (0.41 g, 2.5
mmol) was added. The mixture was stirred for 3 hours to precipitate a white solid.
The solid was collected, washed with Et2O and dried in a vacuum to give the
precursor 9a. Yield: 2.26 g (75%). M.p.: 234-235 °C. 1H NMR (400 MHz, DMSO-d6)
δ: 9.18 (s, 4H, NCHN of Im), 7.74 (s, 4H, Im−H), 7.71 (s, 4H, Im−H), 7.09 (s, 4H,
C6H2), 5.50 (s, 8H, Im−CH2−Ph), 4.32 (s, 4H, O−CH2), 3.87 (s, 12H, Im−CH3), 2.26
(s, 6H, Ph−CH3). 13C NMR (100 MHz, DMSO−d6) δ: 151.89 (Cipso of Ph), 136.90
(NCHN of Im), 135.15, 130.53, 128.49, 124.00, 122.58, 73.54 (O−CH2−Ph), 47.05
(Im−CH2−Ph), 35.95 (Im−CH3), 20.33 (Ph−CH3). HRMS, Calcd for
+
− +
C36H46F18N8O2P3 ([M−PF6 ] ) m/z: 1057.2669, Found: 1057.2627. Anal. Calcd. for
C36H46F24N8O2P4: C, 35.95; H, 3.86; N, 9.32, Found: C, 36.07; H, 3.69; N, 9.24 %.
Precursor 9b was prepared using the same method as above mentioned, except
that compound 8b was used as the start material. Yield: 1.85 g (65%). M.p.: 219-220
°C. 1H NMR (400 MHz, DMSO-d6) δ: 9.07 (s, 4H, NCHN of Im), 7.72 (s, 4H, Ph−H),
7.64 (s, 4H, Im−H), 7.62 (s, 4H, Im−H), 7.07 (s, 4H, C6H2), 5.46 (s, 8H, Im−CH2−Ph),
13
5.00 (s, 4H, O−CH2−Ph), 3.85 (s, 12H, Im−CH3), 2.26 (s, 6H, Ph−CH3). C NMR
(100 MHz, DMSO-d6) δ: 152.47 (C−OCH2 of Ph), 137.36 (NCHN of Im), 137.01,
135.53, 131.01, 129.09, 128.95, 124.52, 123.02, 76.20 (O−CH2−Ph), 47.79
7

+
(Im−CH2−Ph), 36.41 (Im−CH3), 20.84 (CH3). HRMS, Calcd for C42H50F18N8O2P3
− +
([M−PF6 ] ) m/z: 1133.2982, Found: 1133.2955. Anal. Calcd for C42H50F24N8O2P4: C,
39.45; H, 3.94; N, 8.76, Found: C, 39.51; H, 3.79; N, 8.83%.
2.5. Preparation of the N-heterocyclic carbene Ag(I) macrocyclic complexes 10a-b
A sample of precursor 9a (0.50 g, 0.4 mmol) was dissolved in 30 mL of
acetonitrile and silver oxide (0.09 g, 0.4 mmol) was added to the solution. The
mixture was stirred at 50 °C for 8 hours, with the exclusion of light. After cooling to
room temperature, the suspension was filtered off through Celite and the filtrate was
collected. The volatiles were evaporated in vacuo at room temperature. Pure complex
10a was afforded by recrystallization from a mixture of acetonitrile and diethyl ether.
1
Yield: 0.29 g (65%). M.p.: 245-246 °C. H NMR (400 MHz, DMSO-d6) δ: 7.46-7.38
(m, 12H, 4Im−H, 2C6H2), 5.32 (s, 8H, Im−CH2−Ph), 4.27 (s, 4H, O−CH2), 3.70 (s,
12H, Im−CH3), 2.32 (s, 6H, Ph−CH3). 13C NMR (100 MHz, DMSO-d6) δ: 182.06
(Ccarbene), 152.90 (Cipso of Ph), 134.42, 132.57, 130.11, 122.90, 121.42, 73.22
(O−CH2−Ph), 48.96 (Im−CH2−Ph), 38.16 (Im−CH3), 20.35 (Ph−CH3). HRMS, Calcd
–
for C18H21AgN4O+ ([(M–2PF6 )/2)]+) m/z: 417.0765, Found: 417.0763. Anal. Calcd
for C36H44Ag2F12N8O3P2 (10a·H2O): C, 37.85; H, 3.88; N, 9.81, Found: C, 37.62; H,
3.88; N, 9.51%.
Complex 10b was prepared using the same procedure as described above, but
1
with the precursor 9b and Ag2O. Yield: 0.34 g (70%). M.p.: 229-231 °C. H NMR
(400 MHz, DMSO-d6) δ: 7.49 (s, 4H, Im−H), 7.45 (s, 4H, Im−H), 7.34 (s, 4H, C6H2),
6.54 (s, 4H, C6H4), 5.20 (s, 8H, Im−CH2−Ph), 4.75 (s, 4H, O−CH2−Ph), 3.83 (s, 12H,
Im−CH3), 2.03 (s, 6H, Ph−CH3). 13C NMR (100 MHz, DMSO-d6) δ: 181.06 (Ccarbene),
150.70 (C−OCH2 of Ph), 136.32, 134.33, 130.33, 128.09, 127.84, 123.50, 122.17,
75.65 (O−CH2−Ph), 48.53 (Im−CH2−Ph), 38.05 (Im−CH3), 20.48 (CH3). HRMS:
− +
Calcd for C42H46Ag2F6N8O2P+ ([M−PF6 ] ) m/z: 1055.1484, Found: 1055.1448.
–
Cacld for C21H23AgN4O+ ([(M–2PF6 )/2)]+) m/z: 455.0921, Found: 455.0926. Anal
calcd for C42H54Ag2F12N8O6P2 (10b·4H2O): C, 39.64; H, 4.28; N, 8.81, Found: C,
39.90; H, 4.10; N, 8.62%.
8

2.6. X−ray crystallography.
A single crystal of complex 5a suitable for X-ray diffraction analysis was grown
by slow diffusion of diethyl ether into its acetonitrile solution at room temperature.
Diffraction intensity data of 5a were collected at 113(2) K on a Rigaku Saturn724
CCD diffractometer using Mo-Kα radiation (0.71073 Å). Absorption corrections were
performed using CrystalClear software. All structures were solved using direct
methods, which yielded the positions of all non-hydrogen atoms. These were refined
first isotropically and then anisotropically. For the structure of 5a, each asymmetric
unit contains two molecules of CH3CN as co-crystallized solvent. One molecule of
CH3CN is disordered and the atoms of the CH3CN (C71, C72 and N9) are assigned
-
with a partial site occupancy of 0.61. Two PF6 anions are disordered and the affected
atoms (F1−F6 and F7−F12) are assigned with partial site occupancies of 0.57 and
0.53, respectively. All of the disordered parts were restrained using DFIX, ISOR and
SIMU instructions to make the displacement parameters more reasonable. All the
hydrogen atoms of the ligand were placed in calculated positions with fixed isotropic
thermal parameters and included in the structure factor calculations in the final stage
of full-matrix least-squares refinement. All the calculations were performed using the
SHELXTL system of computer programs [19]. Crystallographic data of complex 5a
are summarized in Table 1.
9

Table 1. Summary of the crystallographic data for complex 5a.
5a·2CH3CN
998978
C74H78Ag2F12N10O2P2
1645.14
CCDC
Chemical Formula
Formula weight
Crystal system
Space group
a (Å)
Triclinic 1
P−1
14.9069(19)
15.0888(17)
18.682(3)
b (Å)
c (Å)
79.078(7)
α (deg)
β (deg)
82.424(7)
63.050(5)
γ (deg)
Volume (Å3)
3672.8(9)
2
Z
Dcalcd (g/cm3)
1.488
Abs coeff (mm–1)
F (000)
0.660
1868
Crystal size (mm)
0.18 × 0.16 × 0.14
1.53, 25.02
o
θmin, θmax
/
Reflections collected/unique
R(int)
31411 /12911
0.0371
Max. & min. transmission
0.9133, 0.8905
12911 / 521 / 1068
Data/restraints/ parameters
Goodness−of−fit on F2 a
0.938
Final R indices b [ I >2s( I)]
0.0301, 0.0726
R1, wR2
0.0359, 0.0750
R & Rw (all data) R1, wR2
Largest diff. Peak & hole (e/Å3)
0.608 and −0.595
2
a Goof = [∑w(Fo – Fc2)2/(n − p)]1/2, where n is the number of reflections and p is the number of
b
2
2
∑(||
∑|
parameters refined. R1 = Fo| – |Fc||)/ Fo|; wR2 = 1/[σ (Fo ) + (0.0691P) + 1.4100P] where P =
2
(Fo + 2Fc2)/3.
10

3. Results and discussion
3.1. Preparation of the macrocyclic NHC-Ag complexes 5a−b
The reaction of 2,6-dibromomethyl-4-t-butylanisole (1) with 1 equiv. of
(9-anthracenylmethylene)imidazole gives the mono-imidazolium salt (2). After
separation, compound 2 continued to react with another N-substituted imidazole,
giving the unsymmetric bisimidazolium dibromide 3a-b as an oil. Without separation,
anion exchange reactions of 3a-b with NH4PF6 were conducted in methanol and the
NHC precursors 4a-b were obtained conveniently in powder form [20] (Scheme 1).
N
N
N
N
R
Br
N
Br
Br
Br
N
dioxane
dioxane
OCH3
OCH3
1
2
2Br
2PF6
NH4PF6
N
N
N
N
N
N
N
N
R
R
methanol
OCH3
OCH3
4a, R = Me
4b, R = benzyl
3a, R = Me
3b, R = benzyl
2PF6
N
N
N
R
N
O
O
Ag2O
CH3CN
Ag
Ag
R
N
N
N
N
5a, R = Me
5b, R = benzyl
Scheme 1
11

In the NMR spectra of the unsymmetric bis-imidazolium precursors 4a-b, the
characteristic signal for 2−H of the imidazole ring appeared in the region 9.02−9.35
ppm, which disappeared in the spectra of the Ag complexes 5a-b, indicating the
formation of NHC-Ag macrocyclic complexes from the reaction of the imidazolium
salts with Ag2O. Accordingly, the 2−C signal of the imidazolium ligand for 5a-b
shifted down to the lower field region of 177.96-181.82 ppm compared to the
corresponding signals (136.04-136.72 ppm) in 4a-b. It should be noted that for either
2-H of the imidazole in the precursors 4a-b or 2-C of the imidazole in the macrocyclic
complexes 5-b, the NMR spectra showed two isolated signals due to the
characteristics of the unsymmetric bis-imidazolium ligands. For example, the 2-H
signals for the unsymmetric ligand 4a appeared at 9.02 and 9.09 ppm, but for the
symmetric ligand 9a this signal appeared at 9.18 ppm (see Supplementary materials,
Table S1). Similarly, the 2-C signals for complex 5a appeared at 178.73 and 181.82
ppm, but for complex 10a this signal appeared at 182.06 ppm. The HRMS spectra of
the metal complexes 5a-b showed characteristic signals for [(M−2PF6)/2]+ at
636.2018 and 712.2251, respectively, which are consistent with their calculated
theoretical values.
3.2. Single crystal determination of the macrocyclic complex 5a.
To confirm the molecular structure of the prepared dinuclear macrocyclic
complexes, a single crystal of 5a was grown and its molecular structure was
determined by X-ray diffraction analysis. The structural descriptions of complex 5a
are summarized in Figure 1 and Table 2. As can be seen in Figure 1, a [2+2]
macrocyclic structure of the NHC-Ag complex 5a is formed by the coordination of
two unsymmetric ligands with two Ag ions in a head-to-tail fashion. The two
p-t-butylphenoxyl groups adopt a staggered conformation. No large cavity can be
observed in the solid structure of 5a. The two NHC rings in the ligands generally
adopt a cis-conformation, corresponding to the backbone p-t-butyl-anisole plane. The
Ag−C(carbene) bond distances vary in the range 2.082 to 2.093 Å. The two Ag atoms
12

in the complex have a nearly linear coordination geometry, with C(69)−Ag(1)−C(18)
and C(34)−Ag(2)−C(53) bond angles of 174.39° and 176.15°, respectively, which are
similar to those reported for Ag(I)-NHC complexes [21].
(a)
(b)
Figure 1 (a) Molecular structure of complex 5a (30% probability ellipsoids),
13

hydrogen atoms, CH3CN and anions have been omitted for clarity. Selected bond
lengths (Å) and angles (o): Ag(1)−C(69) = 2.082(3), Ag(1)−C(18) = 2.095(2),
Ag(2)−C(34) = 2.086(3), Ag(2)−C(53) = 2.093(3), C(69)−Ag(1)−C(18) = 176.15(10),
C(34)−Ag(2)−C(53) = 174.39(9). (b) The supramolecular chain of complex 5a via
intermolecular π−π interactions and C−H···F hydrogen bonds. Irrelevant hydrogen
atoms have been omitted for clarity.
It should be noted that the two C(69)−Ag(1)−C(18) and C(34)−Ag(2)−C(53)
axes in 5a cross each other with a dihedral angle of 69.3°, and the Ag-Ag separation is
5.697 Å. However, in previously reported complexes [21] prepared from symmetric
bis-imidazolium ligands and silver oxide by the same method, the two
NHC−Ag(I)−NHC axes were in mutually parallel positions and the Ag-Ag
separations were longer (in the range 6.792 to 16.986 Å). The reason causing the
crossed arrangement of the two NHC−Ag(I)−NHC axes is contributed to the weak
coordination interaction of the methoxy groups to the Ag(I) ions in 5a (Ag(1)···O(2) =
3.104(1) Å and Ag(2)···O(1) = 2.871(1) Å), which also directs the twisted staggered
conformation of complex 5a compared to the rectangular shape of similar complexes
reported by us [21].
Table 2 Structural description of complex 5a, according to the X-ray structure
analysis.
Space intramolecular Φ1/o
Φ2/o
Φ3/o
Φ4/o
Φ5/o
group
Ag···Ag/Å
P−1
5.697(1)
82.0(1) 69.3(1)
89.1(1)
3.6
72.0(1) 88.8(1)
85.0(1) 75.8(1)
5a
Φ1 are the dihedral angles between the imidazolium ring and its N−attached anthracenyl ring in
the molecule; Φ2 is the torsion angle between two C−Ag−C links; Φ3 is the dihedral angle
between the two anthracenyl rings in the molecule; Φ4 are the dihedral angles between the two
NHC rings within a ligand; Φ5 are the dihedral angles between the pair of NHC rings in one
NHC−Ag−NHC unit.
As can be seen from Table 2, the dihedral angles between the imidazole rings
and their attached anthracenyl rings are 82.0 and 89.1o, respectively. In 5a, the two
14

anthracenyl ring weakly π−interact to the imidazolium ring of the other ligand, with
dihedral angles of 11.2 and 19.3o respectively. The dihedral angles between the pair of
NHC rings in one NHC−Ag−NHC unit are 88.8 and 75.8o.
Our previous investigations from single-crystal determinations of these
complexes showed that group 11 metal cations−π−arene interactions exist extensively
in these metal-organic ligand framework complexes [22-23]. In complex 5a, the two
anthracenyl rings in the side chain of the imidazolium adopted a η3-coordination
fashion, attaching to the two central Ag(I) atoms. The nearest separation of
Ag−C(arene) for the two Ag ions is 3.000 and 3.046 Å (Figure 1). Detailed Ag−arene
weak interaction information is listed in Table 3.
Table 3 Structural features of 5a with Ag(I)−arene π interactions and Ag(I)−O weak
interactions.
type a
d1 /Åb
d2 /Å c
d3 /Å d
d4 /Å e
Ag1
Ag2
η3
3.158
2.886
Ag(1)−C(1) = 3.000(3)
Ag(1)−C(2) = 3.206(3)
Ag(1)−C(14) = 3.221(3)
Ag(1)−O(2) = 3.104(1)
η3
3.341
3.0607
Ag(2)−C(36) = 3.046(3) Ag(2)−O(1) = 2.871(1)
Ag(2)−C(37) = 3.237(3)
Ag(2)−C(49) = 3.453(3)
a
b
The coordination mode between the silver atom and arene π–electron system. d1 are the
c
distances of the silver atoms to the centers of the nearest six-membered interacting arene rings.
d2 are the perpendicular distances of the silver atoms to the nearest six-membered interacting
arene rings. d d3 are the distances of the silver atoms to the weak interacting C atoms of the arene
interaction. e d4 are the distances of the silver atoms to the weak interacting O atoms of the
methoxy group.
As depicted in Figure 1b, intermolecular π−π stacking is found between the two
anthracenyl groups in two close molecules. The dihedral angle between the interacting
anthracenyl rings is 3.6 Å. The perpendicular distance between the two anthracenyl
rings is 3.0381 Å. The distance between the two centroids of the two interacting
15

anthracenyl rings is 3.561 Å. Also, weak intermolecular interactions involved C−H···F
hydrogen bonds exist extensively (see Supplementary materials, Table S1). The π−π
interactions and the C−H···F hydrogen bonds direct the molecular stacking into a
polymeric chain.
There are many intramolecular C−H⋅⋅⋅π interactions that exist in the molecular
structure of 5a in the solid state, which contribute to the twisted staggered
conformation of the macrocyclic complex 5a. Also, the intermolecular C−H⋅⋅⋅π
interactions are found in the stacking of the molecular structure (see supplementary
materials, Figure S1 and Table S3).
3.3. Bimetallic NHC−Ag macrocyclic complexes 10a-b
To regulate the molecular shape of these macrocyclic NHC-metal complexes,
two bis-imidazolium salts were designed to linked together through bridging (−
CH2CH2− or p−CH2C6H4CH2−) on the O-position of two p-methylphenoxyl groups,
forming the tetra-dentate imidazolium ligands 9a-b. Ligands 9a-b reacted with Ag2O
in acetonitrile in the dark and the bimetallic NHC−Ag(I) complexes 10a-b were
obtained. Constrained by the middle bridge chain, the two p-methylphenoxyl groups
in complexes 10a-b should have a cone conformation, rather than the staggered
conformation in 5a. If the middle bridge in complexes 10a-b was regarded as the
bottom in the cave structure of the NHC-Ag complexes, the whole molecule might be
predicted to have
a
bowl-like shape. The preparative route of the
lower-rim-constrained macrocyclic NHC-Ag complexes is shown in Scheme 2.
16

HO
OH
Br
Br
X
O
O
O
O
Br
Br
PBr3
X
X
HO
OH
dioxane
K2CO3, acetone
OH
HO
OH
Br
Br
7a, X = CH2-CH2
7b, X = p-CH2-C6H4-CH2
8a, X = CH2-CH2
8b, X = p-CH2-C6H4-CH2
6
4PF6
2PF6
N
N
N
N
N
N
N
N
N
O
O
O
, dioxane
Ag2O
(1)
N
Ag
X
Ag
X
CH3CN
(2)
NH4PF6, methanol
O
N
N
N
N
N
N
N
N
9a, X = CH2-CH2
9b, X = p-CH2-C6H4-CH2
10a, X = CH2-CH2
10b, X = p-CH2-C6H4-CH2
Scheme 2
In the NMR spectra of the tetra-imidazolium salts ligands 9a-b, the signals for
the positive 2−H proton of the imidazolium rings were found at 9.07 and 9.18 ppm,
respectively, and the signal for 2−C appeared at 136.90 and 137.36 ppm, respectively
(see Supplementary materials, Table S1), which are consistent with those for the NHC
precursors reported previously [21]. In the HRMS spectra of 9a-b, the characteristic
MS peaks for 9a and 9b were found at m/z = 1057.2627 (calcd: 1057.2669) and
1133.2955 (calcd: 1133.2982), respectively, which correspond to the ions of
− +
[M−PF6 ] . After metallation of the imidazolium salts with Ag2O, the complexes
10a-b were isolated as brown powders. In the NMR spectra of 10a-b, the proton
signal for 2−H in the ligand disappeared and the 2−C signals of the imidazolium ring
in complexes 10a and 10b downshifted to 181.06 and 182.06 ppm, respectively,
showing the formation of the NHC−Ag complexes. The HRMS spectra confirmed the
molecular constitution of the macrocyclic complexes 10a-b. The characteristic peaks
17

for 10a and 10b were found at m/z = 417.0763 (calcd: 417.0765 for [(M−2PF6)/2]+)
and 455.0926 (calcd: 455.0921 for [(M−2PF6)/2]+), respectively.
Many attempts to grow single crystals of complexes 10a-b have not been
successful. To study the molecular shape of the prepared NHC-Ag complexes, 2D
NMR experiments of the complexes were carried out (see Supplementary materials,
Figures S2 and S3). In the ROESY NMR spectra of 10a, the peaks at 5.33 and 4.27
ppm were assigned to the H-H ROESY signal of (phenoxyl)-CH2-(imidazolium) with
–CH2CH2-. It clearly showed that the correlated two groups -CH2- and –CH2CH2-
were at the bottom position of the molecule. Thus, the molecule of 10a might have a
bowl shaped structure. However, In the NOESY NMR spectra of 10b, the
corresponding correlation signal has not been observed. The other COSY signals
showed the molecular structure of 10b might be a twisted conformation, but detailed
structural information is unknown as yet.
The NHC-Ag complexes are stable in the solid state and in solution. The
thermogravimetric (TG) analyses of complexes 10a-b (see Supplementary materials,
Figure S4) were representatively performed to investigate the stability of these
macrocyclic complexes. The TG curve (Figure 2) of complex 10a showed an initial
loss of 1.6% of the crystallized solvent before decomposition of the complex at about
268.1 °C, implying the high stability of the dinuclear Ag(I)−NHC macrocycles
[24-25]. In the following step, two significant weight losses of ca.7.7% and 32.8%
occurred in the regions 268.1-292.7 and 292.7-450 °C, respectively. In the final step, a
slow decomposition process of the residue was observed with a loss of 8% in the
region 450-800 °C.
18

268.1 oC,
98.4%
100
90
80
70
60
50
292.7oC,
90.7%
800OC,
49.2%
450oC,
57.9%
0
100 200 300 400 500 600 700 800
o
Temperature/ C
Figure 2. The TG analysis of complex 10a
4. Conclusion
A series of bis- and tetra-imidazolium precursors of N-heterocyclic carbene
ligands containing calixarene fragments were synthesized and were used to prepare
NHC−Ag macrocyclic complexes. The structure of the Ag complex 5a was
determined by X-ray single crystal diffraction analysis. The molecular structure
determination unambiguously showed a macrocyclic bimetallic metal complex
structure, in which the two p-t-butylphenoxyl groups adopt a staggered conformation.
To regulate the molecular shape of the NHC-Ag complexes, 10a-b were synthesized
through the reaction of the tetra-imidazolium salts 9a-b with Ag2O. 2D NMR studies
showed that 10a might have a bowl shaped molecular structure. But for 10b, detailed
structural information has not been obtained from the NOESY NMR spectrum.
Further studies on these macrocyclic compounds will focus on the synthesis of NHC
metal complexes with a cavity structure and the inclusion of some guest molecules
within the cavity complexes.
Acknowledgments
This work is supported by National Natural Science Foundation of China (No.
20772060 and No. 21172114).
19

Appendix A. Supplementary data
CCDC 998978 contains the supplementary crystallographic data for complex 5a.
The
data
can
be
obtained
free
or
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
from
the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary materials
associated with this article containing the weak interaction analysis of 5a, the 2D
NMR spectra and TG curves of 10a-b can be found in a supplementary file.
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21

Short abstract and figure for the contents page
Two unsymmetrical bis-imidazolium ligands, two symmetric tetra-imidazolium
ligands and their dinuclear NHC-Ag macrocyclic complexes have been prepared and
characterized. The X-ray diffraction analysis showed that the molecule 5a adopts a
twisted macrocyclic structure, in which the two Ccarbene-Ag-Ccarbene axials are crossed
and the two p-t-butylphenoxyl groups are in a staggered conformation.
2PF6
N
N
N
N
R
O
O
Ag
Ag
Br
Br
R
OCH3
N
N
N
N
Calixarene fragment
NHC-Ag macrocyclic complexes
22