Two N-heterocyclic carbene silver(I) cyclophanes: Synthesis, structural studies, and recognition for p -phenylenediamine

Article abstract of DOI:10.1021/om1012117

The diimidazolium salt 1,5-bis[1-ethylimidazoliumylethoxy]naphthalene hexafluorophosphate (2) and the dibenzimidazolium salt 1,4-bis[1-(n-butyl) benzimidazoliumylethoxy]benzene hexafluorophosphate (4), as well as their N-heterocyclic carbene silver(I) cyc

Full text of DOI:10.1021/om1012117

ARTICLE
pubs.acs.org/Organometallics
Two N-Heterocyclic Carbene Silver(I) Cyclophanes: Synthesis,
Structural Studies, and Recognition for p-Phenylenediamine
Qing-Xiang Liu,* Zhao-Quan Yao, Xiao-Jun Zhao, Ai-Hui Chen, Xiao-Qiong Yang, Shu-Wen Liu, and
Xiu-Guang Wang
Tianjin Key Laboratory of Structure and Performance for Functional Molecule, College of Chemistry, Tianjin Normal University,
Tianjin 300387, People's Republic of China
S Supporting Information
b
ABSTRACT: The diimidazolium salt 1,5-bis[1-ethylimidazo-
liumylethoxy]naphthalene hexafluorophosphate (2) and the
dibenzimidazolium salt 1,4-bis[1-(n-butyl)benzimidazoliumyl-
ethoxy]benzene hexafluorophosphate (4), as well as their
N-heterocyclic carbene silver(I) cyclophanes [naphthalene-
(OCH2CH2imyEt)2Ag]2(PF6)2 (5) and [benzene(OCH2CH2-
bimynBu)2Ag]2(PF6)2 (6), have been prepared and character-
ized. Each cation in complexes 5 and 6 contains a 30-membered
macrometallacycle. In the crystal packing of 5 and 6, 3D
architectures are formed via intermolecular weak interactions,
including πꢀπ interactions, hydrogen bonds, and CꢀH
π
3 3 3
contacts. The recognition of neutral p-phenylenediamine (PDA) molecules using 5 and 6 as receptors via fluorescent and UV/vis
spectroscopic titrations have been investigated.
’ INTRODUCTION
’ RESULTS AND DISCUSSION
The development of receptors for recognizing cations and
anions has received a significant amount of attention in molecular
recognition study and supramolecular chemistry.1 Additionally,
the design and synthesis of receptors capable of binding neutral
guests are of crucial importance due to their potential applica-
tions in environmental and biological processes.2 However, this
work still remains a great challenge in comparison to the more
extensively studied recognition of cations and anions. One of the
main strategies for achieving strong affinity and selective recog-
nition for neutral species is the design and synthesis of receptors
bearing cavities. In the type of receptor, the acting force between
the host and guest mostly derives from several well-known
intermolecular weak interactions, including van der Waals forces,
Synthesis and General Characterization of Precursors 2
and 4. As shown in reaction 1 in Scheme 1, 1,5-dihydroxy-
naphthalene on etherification with 1,2-dibromoethane followed
by a reaction with 1-ethylimidazole to afford the diimidazolium
salt 1,5-bis[1-ethylimidazoliumylethoxy]naphthalene bromide
and subsequent anion exchange with ammonium hexafluoropho-
sphate in methanol gave 1,5-bis[1-ethylimidazoliumylethoxy]-
naphthalene hexafluorophosphate (2). The dibenzimidazolium
salt 1,4-bis[1-(n-butyl)benzimidazoliumylethoxy]benzene hexa-
fluorophosphate (4) was prepared in a manner analogous to that
of 2 (Scheme 1, reaction 2). Precursors 2 and 4 are stable toward
air and moisture, are soluble in polar organic solvents such as
dichloromethane and acetonitrile, and are scarcely soluble in
1
classical hydrogen bonds (such as NꢀH O and NꢀH
effects of cavity size.3 Some other nonclassical hydrogen bonds
N
3 3 3
3 3 3
benzene, diethyl ether, and petroleum ether. In the H NMR
hydrogen bonds), aromatic πꢀπ interactions, and synergistic
spectra of 2 and 4, the imidazolium (or benzimidazolium) proton
signals (NCHN) appear at δ 9.38 and 9.95 ppm, which are
consistent with the chemical shifts of reported imidazolium (or
benzimidazolium) salts.5
Synthesis and General Characterization of NHC Silver(I)
Cyclophanes 5 and 6. The reaction of precursors 2 and 4 with
silver(I) oxide in acetonitrile afforded the N-heterocyclic carbene
silver(I) cyclophanes 5 and 6, respectively (Scheme 2). Com-
plexes 5 and 6 in the solid state are stable toward air and moisture,
are slightly light sensitive, are soluble in DMSO, and are insoluble
(such as CꢀH X hydrogen bonds, X = O, N, F, Cl, Br, I) have
3 3 3
also been reported in recent years.4 During the course of
searching for receptors bearing cavities, we became interested
in N-heterocyclic carbene metal cyclophanes. In this paper, we
report the synthesis and structures of two new N-heterocyclic
carbene silver(I) cyclophanes: [naphthalene(OCH2CH2imyEt)2-
Ag]2(PF6)2 (5) and [benzene(OCH2CH2bimynBu)2Ag]2(PF6)2
(6) (imy = imidazol-2-ylidene and bimy = benzimidazol-2-
ylidene). In particular, the recognition of p-phenylenediamine
(PDA) using these cyclophanes as receptors was investigated on
the basis of fluorescent and UV/vis spectroscopic titrations.
Received: December 30, 2010
Published: June 27, 2011
r
2011 American Chemical Society
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Scheme 1. Preparation of Precursors 2 and 4
ligands and two silver(I) atoms (Figures 1a and 2a), in which
each silver(I) atom is dicoordinated with two carbene carbon
atoms to adopt an almost linear arrangement (CꢀAgꢀC =
179.6(1)° for 5 and 175.1(1)° for 6), and the AgꢀC distance is in
the range 2.074(5)ꢀ2.093(3) Å. In 5 and 6, two CꢀAgꢀC units
are parallel, and the intramolecular separation AgꢀAg in the
macrometallacycle is 11.680(4) Å for 5 and 9.551(8) Å for 6.
These data show us that the hosts molecules 5 and 6 have large
cavity structures which can contain and have astrong affinity for
p-phenylenediamine. Because the longest distance between two
hydrogen atoms in p-phenylenediamine is 6.74 Å, it is suitable for
the size of these two cavities. The X-ray crystal structure analyses
of 5 and 6 show that inversion centers exist in these complexes.
Two imidazole (or benzimidazole) rings in the NHCꢀAgꢀNHC
units form dihedral angles of 3.0° for 5 (11.8° for 6). Two
imidazole (or benzimidazole) rings in the same ligand form
dihedral angles of 2.9° for 5 (13.3° for 6). The dihedral angles of
imidazole (orbenzimidazole) ringsand naphthalene (or benezene)
rings in the same ligand are 72.3 and 77.7° for 5 (70.3 and 82.3°
for 6). In 5, πꢀπ stacking interactions between two parallel
naphthalene rings are observed with a face-to-face distance of
3.596(7) Å (center-to-center distance 3.634(1) Å).8 In 6, two
benzene rings are also parallel, but no πꢀπ stacking interactions
are observed.
Scheme 2. Preparation of Complexes 5 and 6
in diethyl ether and hydrocarbon solvents. In the 1H NMR spectra
of 5 and 6, the disappearance of the resonances for the
imidazolium (or benzimidazolium) protons (NCHN) shows
the formation of the expected metal carbene complexes, and
the chemical shifts of other hydrogens are similar to those of the
corresponding precursors. In 13C NMR spectra, the signal for the
carbene carbon of 5 appears at 206.2 ppm, which is similar to
signals for known carbene silver(I) complexes.6 The signal of the
carbene carbon in 6 was not observed. Similar results have been
reported for some carbene silver(I) complexes, which may result
from the fluxional behavior of the NHC complexes.7
An interesting feature in the crystal packing of 5 (Figure 1b) is
that the 2D supramolecular layers are formed by CꢀH
O
3 3 3
hydrogen bonds9 and πꢀπ stacking interactions from intermo-
lecular imidazole rings with a face-to-face distance of 3.478(5) Å
(center-to-center distance 4.210(1) Å). In the CꢀH
O
3 3 3
hydrogen bonds, the hydrogen atoms are from CH2 groups of
ether chains (data for the CꢀH O hydrogen bonds are given
3 3 3
in Table 1). In addition, PF6ꢀ groups are packed between suc-
cessive 2D supramolecular layers and hold the layers together via
CꢀH F hydrogen bonds10 (Table 1) to expand 2D layers into
3 3 3
a 3D supramolecular architecture (Figure 1c).
Structures of NHC Silver(I) Cyclophanes 5 and 6. Colorless
crystals of 5 and 6 suitable for X-ray diffraction were obtained by
slow diffusion of diethyl ether into their DMSO and CH3OH/
C2H5OH solutions. In complexes 5 and 6, each cation contains a
30-membered macrometallacycle formed by two bidentate carbene
In the crystal packing of 6, a 2D supramolecular layer is formed
via two types of CꢀH π contacts,11 as shown in Figure 2b. In
3 3 3
the first CꢀH π contacts, the hydrogen atoms are from CH2
3 3 3
groups of ether chains and π systems are from benzimidazole
rings (H π distance 2.673(2) Å and CꢀH π angle
3 3 3
3 3 3
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Figure 1. (a) Perspective view of 5 and anisotropic displacement parameters depicting 30% probability. All hydrogen atoms and the PF6ꢀ anion are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Ag(1)ꢀC(5) = 2.075(5), Ag(1)ꢀC(22A) = 2.074(5); C(5)ꢀAg(1)ꢀC(22A) =
179.6(1), N(1)ꢀC(5)ꢀN(2) = 104.1(4). Symmetry code: (i) 2 ꢀ x, ꢀy, 1 ꢀ z. (b) 2D supramolecular layer of 5 showing πꢀπ interactions and
CꢀH O hydrogen bonds. All hydrogen atoms, except those participating in the hydrogen bonds, and all ethyl groups on nitrogen atoms are omitted
3 3 3
for clarity. (c) 3D supramolecular architecture of 5 with πꢀπ interactions and CꢀH O and CꢀH F hydrogen bonds. All hydrogen atoms, except
3 3 3
3 3 3
those participating in the hydrogen bonds, and irrelevant ethyl groups are omitted for clarity. Symmetry code: (ii) x, 1 + y, z.
128.2(1)°). In the second CꢀH π contacts, the hydrogen
Recognition of p-Phenylenediamine (PDA) Using 5 and 6
3 3 3
as Receptors. In the NHC silver(I) cyclophanes 5 and 6, each
cycle of cation contains several types of binding sites (such as
oxygen atoms, nitrogen atoms, and π systems). In accord with
the sizes of cavities and structural characteristics of 5 and 6, the
selective recognition of some neutral molecules using 5 and 6
as receptors can be measured. During the experiments, we
atoms are from benzimidazole rings and π systems are from
benzene rings (H π distance 2.859(2) Å and CꢀH π angle
3 3 3
3 3 3
ꢀ
149.2(3)°). Similar to the case for 5, PF6 groups in 6 are also
packed between successive 2D supramolecular layers and hold the
layers together via CꢀH F hydrogen bonds (Table 1) to expand
3 3 3
2D layers into a 3D supramolecular architecture (Figure 2c).
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Figure 2. (a) Perspective view of 6 and anisotropic displacement parameters depicting 30% probability. All hydrogen atoms and the
ꢀ
PF6 anion are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ag(1)ꢀC(7) = 2.093(3), Ag(1)ꢀC(28A) = 2.086(3);
C(7)ꢀAg(1)ꢀC(28A) = 175.1(1), N(1)ꢀC(7)ꢀN(2) = 106.2(3). Symmetry code: (i) 2 ꢀ x, ꢀy, ꢀz. (b) 2D supramolecular layer of 6
with CꢀH
π contacts. All hydrogen atoms, except those participating in CꢀH
π contacts, and all butyl groups on nitrogen atoms are
3 3 3
3 3 3
π contacts and CꢀH
omitted for clarity. (c) 3D supramolecular architecture of 6 with CꢀH
F hydrogen bonds. All hydrogen atoms,
3 3 3
3 3 3
except those participating in the hydrogen bonds and CꢀH
code: (iii) 1 ꢀ x, ꢀy, ꢀz.
π contacts, and irrelevant butyl groups were omitted for clarity. Symmetry
3 3 3
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Table 1. H-Bonding Geometry for Complexes 5 and 6
DꢀH A,
3 3 3
DꢀH Aa
DꢀH, Å
H
A, Å
D
A, Å
deg
3 3 3
3 3 3
3 3 3
Complex 5
C(19)ꢀH(19) O(5)i 0.97(5) 2.565(3) 3.629(6)
172.6(3)
142.2(3)
135.3(2)
3 3 3
C(3)ꢀH(3) F(1)ii
0.93(4) 2.537(3) 3.320(5)
0.97(5) 2.520(3) 3.281(6)
3 3 3
C(2)ꢀH(2) F(6)ii
3 3 3
Complex 6
C(3)ꢀH(3) F(5)iii
0.93(4) 2.528(4) 3.457(6)
0.97(4) 2.573(3) 3.536(5)
176.1(3)
171.7(2)
3 3 3
C(8)ꢀH(8) F(6)iii
3 3 3
a Symmetry code: (i) 2ꢀ x, 1 ꢀ y, 2 ꢀ z; (ii) x, 1 + y, z; (iii) 1 ꢀ x, ꢀy, ꢀz.
Figure 4. (a) Emission spectra (λex 287 nm) of 6 (1.0 ꢁ 10ꢀ5 mol/L)
in the presence of p-phenylenediamine in acetonitrile at 25 °C. The
concentrations (10ꢀ5 mol/L) of p-phenylenediamine for curves 1ꢀ20
(from top to bottom) are 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 2.4, 3, 3.5, 4, 5, 6, 7, 8,
10, 12, 15, 18, and 21. Inset: variation of fluorescence quenching F/F0 of
6 with increasing p-phenylenediamine concentration. (b) UV/vis
absorption spectra of receptor 6 (1.0 ꢁ 10ꢀ5 mol/L) in acetonitrile at
25 °C. The concentrations (10ꢀ5 mol/L) of p-phenylenediamine for
curves 1ꢀ10 (from bottom to top) are 0, 0.11, 0.25, 0.43, 0.67, 1, 1.5, 2.4,
4, and 9. Inset: Job plot for the 6/p-phenylenediamine complex in
acetonitrile solution at 204 nm.
in acetonitrile at 25 °C. To further study the recognition of
p-phenylenediamine by 5 or 6, fluorescence titrations were con-
ducted (Figures 3a and 4a). The fluorescence intensities of 5 and
6 (at about λem 330ꢀ365 nm for 5 and λem 320ꢀ345 nm for 6)
decreased markedly with increasing concentration of p-phenyle-
nediamine until the fluorescence intensities did not change. The
quenching behaviors of p-phenylenediamine on the fluorescence
of 5 or 6 were found to follow a conventional SternꢀVolmer
relationship12ꢀ16 (Figures S1a and S2a, -Supporting Information),
which may be attributed to the charge transfer interaction
between aromatic systems in the host and p-phenylenediamine
Figure 3. (a) Emission spectra (λex 294 nm) of 5 (1.0 ꢁ 10ꢀ5 mol/L)
in the presence of p-phenylenediamine (PDA) in acetonitrile at 25 °C.
The concentrations (10ꢀ5 mol/L) of p-phenylenediamine for curves
1ꢀ21 (from top to bottom) are 0, 0.25, 0.67, 1, 1.5, 2, 3, 3.5, 4, 5, 7, 10,
12, 15, 18, 23, 28, 33, 43, 53, and 63. Inset: variation of fluorescence
quenching F/F0 of 5 with increasing p-phenylenediamine concentration.
(b) UV/vis absorption spectra of receptor 5 (1.0 ꢁ 10ꢀ5 mol/L) in
acetonitrile at 25 °C. The concentrations (10ꢀ5 mol/L) of p-phenyle-
nediamine for curves 1ꢀ13 (from bottom to top) are 0, 0.11, 0.13, 0.17,
0.25, 0.33, 0.43, 0.67, 1, 1.5, 2.4, 4, and 9. Inset: Job plot for the
5/p-phenylenediamine complex in acetonitrile solution at 295 nm.
F0=F ¼ 1 + KsCPDA
where F0 and F are the fluorescence intensities of 5 or 6 in the
absence and presence of p-phenylenediamine, respectively, and
CPDA is the concentration of p-phenylenediamine. The equations
reveal that F0/F increases in direct proportion to the increasing
concentration of p-phenylenediamine, and the quenching con-
stant Ks defines the quenching efficiency of p-phenylenediamine.
As shown by the SternꢀVolmer plot in Figures S1a and S2a
(inset), the Ks values of p-phenylenediamine are 0.1 ꢁ 105 Mꢀ1
found that each cycle in 5 and 6 has a strong affinity for the
neutral p-phenylenediamine molecule, and these compounds
exhibit interesting p-phenylenediamine complexation properties
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for 5 and 0.28 ꢁ 105 Mꢀ1 for 6, and the linear ranges are
(0ꢀ1.0) ꢁ 10ꢀ4 M for 5 and (0ꢀ4.0) ꢁ 10ꢀ5 M for 6.
The fluorescence intensity for 5 or 6 at about λem 385 nm
increased gradually with increasing concentration of p-phenyle-
nediamine (Figures 3a and 4a), which may derive from the
fluorescence emission of p-phenylenediamine, because the pure
p-phenylenediamine molecule has fluorescence emission in this
position but cyclophanes 5 and 6 have no fluorescence emission
in this position.
π systems) are included. By using the methods of fluorescent and
UV/vis spectroscopic titrations, the values of Ks based on the
SternꢀVolmer equation and K based on the standard Benesiꢀ
Hildebrand equation are similar to each other, which shows that
the cyclophanes 5 and 6 are efficient receptors for p-phenylene-
diamine molecules in acetonitrile. Further studies on the synthesis
of new organometallic compounds from precursors 2 and 4, as
well as analogous ligands, are underway.
On addition of p-phenylenediamine to the solutions of 5 and 6
in acetonitrile at 25 °C, the UV/vis absorption spectra increased
gradually in intensity (Figures 3b and 4b). The absorption
enhancement of 5 or 6 followed a BenesiꢀHildebrand type
equation17
’ EXPERIMENTAL SECTION
General Procedures. All manipulations were performed using
Schlenk techniques, and solvents were purified by standard procedures.
All the reagents for synthesis and analyses were of analytical grade and
were used without further purification. Melting points were determined
ꢀ
ꢁ
1
with a Boetius Block apparatus. H and 13C{1H} NMR spectra were
A0
εr
1
¼
+ 1
recorded on a Varian Mercury Vx 400 spectrometer at 400 and 100 MHz,
respectively. Chemical shifts, δ, are reported in ppm relative to the
internal standard TMS for both 1H and 13C NMR. J values are given in
Hz. Elemental analyses were measured using a Perkin-Elmer 2400C
Elemental Analyzer. The luminescent spectra were conducted on a Cary
Eclipse fluorescence spectrophotometer. UVꢀvis spectra were obtained
using a JASCO-V570 spectrometer.
A0 ꢀ A
εr ꢀ εc KCPDA
where A0 is the absorption intensity of receptor 5 or 6 in the
absence of p-phenylenediamine, A0 ꢀ A is the discrepancy
of absorption intensity between the absence and presence of
p-phenylenediamine, εr is the molar extinction coefficient of 5 or
6, εc is the molar extinction coefficient of the p-phenylenedia-
mine complex with 5 or 6, and CPDA is the concentration of
p-phenylenediamine. The stability constant K between 5 or 6 and
p-phenylenediamine is given by the ratio of intercept to slope
(Figures S1b and S2b, Supporting Information; K = 0.45 ꢁ 105 Mꢀ1
for 5 and 1.06 ꢁ 105 Mꢀ1 for 6).
Preparation of 1,5-Bis(bromoethoxy)naphthalene (1).
A water solution (30 mL) of NaOH (2.997 g, 74.9 mmol) and
tetrabutylammonium bromide (TBAB; 0.201 g, 0.6 mmol) was added
to a solution of 1,5-dihydroxynaphthalene (2.000 g, 12.5 mmol) in 1,2-
dibromoethane (40 mL) and stirred for 12 h at 80 °C. The brown organic
layer was obtained by separating it from the water layer. 1,2-Dibro-
moethane in the organic layer was removed with a rotary evaporator, and
H2O (80 mL) was added to the residue. Then the solution was extracted
with CHCl3 (3 ꢁ 30 mL), and the extracting solution was dried over
anhydrous MgSO4. After CHCl3 was removed, a pale brown powder of
1,5-bis(bromoethoxy)naphthalene (1) was obtained. Yield: 2.737 g
It is notable that a 1:1 complexation stoichiometry for
p-phenylenediamine complexes with 5 and 6 was established by a
Job plot analysis as shown in the insets to Figures 3b and 4b,18
where the products (χΔA) between molar fractions and the dis-
crepancy of the absorption bands at 295 nm for 5 and at 204 nm
for 6 were plotted against molar fractions (χ) of 5 or 6 under the
conditions of a constant total concentration. As such, the
concentration of p-phenylenediamine complexes with 5 and 6
approached a maximum when the molar fractions of hosts/guest
were about 0.5.
1
(59%). Mp: 140ꢀ142 °C. H NMR (400 MHz, DMSO-d6): δ 3.95
(t, J = 6.5, 4H, CH2), 4.49 (t, J = 6.5, 4H, CH2), 7.03 (d, J = 3.8, 2H, PhH),
7.43 (d, J = 3.8, 2H, Ph H), 7.80 (d, J = 3.8, 2H, Ph H).
Preparation of 1,5-Bis[1-ethylimidazoliumylethoxy]-
naphthalene Hexafluorophosphate (2). A solution of 1-ethyli-
midazole (2.468 g, 25.7 mmol) and 1,5-bis(bromoethoxy)naphthalene
(4.000 g, 10.7 mmol) in THF (150 mL) was stirred for seven days under
refluxing, and a yellow precipitate was formed. The product was filtered
and washed with THF to give a yellow powder of 1,5-bis[1-ethylimida-
zoliumylethoxy]naphthelene bromide. Yield: 4.542 g (75%). M.p.:
166ꢀ168 °C.
In the recognition of p-phenylenediamine molecules using 5
and 6 as receptors via fluorescent and UV/vis methods, the
quenching constant Ks values (0.1 ꢁ 105 Mꢀ1 for 5 and 0.28 ꢁ
105 Mꢀ1 for 6) from the SternꢀVolmer equation (fluorescence
method) and the stability constant K values (0.45 ꢁ 105 Mꢀ1 for
5 and 1.06 ꢁ 105 Mꢀ1 for 6) from the BenesiꢀHildebrand
equation (UV/vis method) are similar to each other.19 To the
best of our knowledge, these are the largest Ks or K values, which
show that the cyclophanes 5 and 6 are efficient macrocyclic
receptors for p-phenylenediamine molecules in acetonitrile.
Notably, Ks or K values in 6 are somewhat larger than those
in 5, which show that the binding capability between 6 and
p-phenylenediamine is slightly stronger than that between 5 and
p-phenylenediamine.
NH4PF6 (1.382 g, 8.5 mmol) was added to the methanol solution of
1,5-bis[1-ethylimidazoliumylethoxy]naphthelene bromide (2.000 g,
3.5 mmol) with stirring, and a yellow precipitate was formed immedi-
ately. The yellow powder was collected by filtration and washed with
small portions of methanol to give 1,5-bis[1-ethylimidazoliumylethoxy]-
naphthelene hexafluorophosphate (2). Yield: 1.810 g (81%). M.p.:
200ꢀ202 °C. Anal. Calcd for C24H30F12N4O2P2: C, 41.38; H, 4.34;
1
N, 8.04%. Found: C, 41.57; H, 4.62; N, 8.47%. H NMR (400 MHZ,
DMSO-d6): δ 1.42 (t, J = 7.2, 6H, CH3), 4.25 (q, J = 7.2, 4H, CH2), 4.47
(t, J = 4.4, 4H, CH2), 4.75 (t, J = 4.4, 4H, CH2), 7.04 (d, J = 8.0, 2H, Ph
H), 7.35 (t, J = 8.0, 2H, Ph H), 7.64 (d, J = 8.0, 2H, Ph H), 7.83 (s, 2H, 4
or 5-imi H), 7.95 (s, 2H, 4 or 5-imi H), 9.38 (s, 2H, 2-imi H) (imi =
imidazole).
In order to understand the interactions of 5 or 6 with
p-phenylenediamine at the molecular level, we attempted to deter-
mine the solid-state structures of p-phenylenediamine complexes
with 5 and 6. Unfortunately, no crystals could be obtained.
Preparation of 1,4-Bis(bromoethoxy)benzene (3). This
compound was prepared in a manner analogous to that of 1,5-
bis(bromoethoxy)naphthalene, only 1,4-dihydroxybenzene was
used instead of 1,5-dihydroxynaphthalene. Yield: 3.216 g (55%),
Mp: 102ꢀ104 °C. 1H NMR (400 MHz, CDCl3): δ 3.60 (t, J = 6.0,
4H, CH2), 4.24 (t, J = 6.0, 4H, CH2), 6.86 (s, 4H, Ph H).
’ CONCLUSIONS
In summary, the two new NHC silver(I) cyclophanes 5 and 6
have been synthesized and characterized. In 5 and 6, each cation
possesses a 30-membered macrometallacycle, in which several
types of binding sites (such as oxygen atoms, nitrogen atoms, and
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Preparation of 1,4-Bis[1-(n-butyl)imidazoliumylethoxy]-
benzene Hexafluorophosphate (4). This compound was prepared
in a manner analogous to that of 2, only 1,4-bis(bromoethoxy)benzene
and 1-(n-butyl)benzimidazole were used instead of 1,5-bis(bromoethoxy)-
naphthalene and 1-ethylimidazole, respectively. Yield: 1.960 g (82%),
Mp: 218ꢀ220 °C. Anal. Calcd for C32H40F12N4O2P2: C, 47.88; H, 5.02;
N, 6.98. Found: C, 47.54; H, 5.47; N, 6.83. 1H NMR (400 MHZ, DMSO-
d6): δ 1.73 (t, J = 5.4, 6H, CH3), 2.15 (m, 4H, CH2), 2.19 (m, 6H, CH2),
4.37 (t, J = 5.6, 4H, CH2), 4.57 (q, J = 5.4, 2H, CH), 4.92 (t, J = 5.6, 4H,
CH2), 6.86 (m, 4H, Ph H), 7.70 (m, 4H, Ph H), 8.17 (m, 4H, Ph H), 9.95
(s, 2H, benzimi H) (benzimi = benzimidazole).
range of 1.8° < θ < 25°. An empirical absorption correction was applied
using the SADABS program.21 The structures were solved by direct
methods, and all non-hydrogen atoms were subjected to anisotropic
refinement by full-matrix least squares on F2 using the SHELXTL
package.22 All hydrogen atoms were generated geometrically (CꢀH
bond lengths fixed at 0.96 Å), assigned appropriated isotropic thermal
parameters, and included in the final calculations. Crystallographic data
are summarized in Table S1 (Supporting Information) for 5 and 6.
’ ASSOCIATED CONTENT
Preparation of the Cyclophane [naphthalene(OCH2CH2-
imyEt)2Ag]2(PF6)2 (5). Silver oxide (0.096 g, 0.4 mmol) was added to
a solution of precursor 2 (0.200 g, 0.3 mmol) in acetonitrile (30 mL),
and the suspension was stirred for 24 h with refluxing. The resulting
solution was filtered and concentrated to 5 mL, and diethyl ether (5 mL)
was added to precipitate a white powder. Isolation by filtration yielded
cyclophane 5. Yield: 0.078 g (33%). Mp: 246ꢀ247 °C. Anal. Calcd for
C48H56Ag2F12N8O4P2: C, 43.85; H, 4.29; N, 8.52. Found: C, 44.13; H,
4.47; N, 8.53. 1H NMR (400 MHz, DMSO-d6): δ 1.40 (t, J = 7.2, 12H,
CH3), 4.25 (q, J = 7.2, 8H, CH2), 4.51 (t, J = 4.4, 8H, CH2), 4.76 (t, 8H,
CH2), 7.73ꢀ7.78 (m, 12H, Ph H), 7.84 (d, 4H, imi H), 7.94 (d, J = 7.8,
4H, imi H). 13C NMR (100 MHz, DMSO-d6): δ 206.2 (Ccarbene), 153.3,
136.8, 126.1, 125.9, 123.4, 122.8, 122.1, 112.1, 107.0, and 106.2 (Ph C or
imi C), 52.1 (OCH2), 47.4 (NCH2), 45.5 (NCH2), 18.6 (CH3).
Preparation of the Cyclophane [benzene(OCH2CH2-
bimynBu)2Ag]2(PF6)2 (6). This complex was prepared in a manner
analogous to that of 6, only 4 was used instead of 2. Yield: 0.170 g (45%).
Mp: 245ꢀ247 °C. Anal. Calcd for C64H76Ag2F12N8O4P2: C, 50.34; H,
5.02; N, 7.34. Found: C, 50.65; H, 5.43; N, 7.74. 1H NMR (400 MHZ,
DMSO-d6): δ 0.76 (t, J = 5.6, 12H, CH3), 1.29 (m, 8H, CH2), 1.84
(m, 8H, CH2), 4.23 (t, J = 4.2, 8H, CH2), 4.58 (t, J = 5.6, 8H, CH2), 4.99
(t, J=4.2, 8H, CH2), 6.40 (s, 8H, Ph H), 7.48 (m, 8H, Ph H), 7.84(d, J=6.4,
4H, Ph H), 8.05 (d, J = 6.4, 4H, Ph H). 13C NMR (100 MHZ, DMSO-d6):
δ 152.0, 149.1, 134.2, 133.8, 124.8, 118.3, 115.6, 113.1, and 112.8 (PhC),
67.9 (OCH2), 49.2 (NCH2), 48.8 (NCH2),32.9(CCH2C), 20.3 (CCH2C),
14.3 (CCH3). The carbene carbon was not observed.
UV Titrations. UV titrations were performed on a JASCO-V570
spectrometer using a 1 cm path length quartz cuvette. Acetonitrile used
in the titrations was freshly distilled over calcium hydride. Titrations
were carried out by placing the receptors (1 ꢁ 10ꢀ5 mol/L) into the
4 mL cuvette and adding increasing amounts of the p-phenylenediamine
(0ꢀ9.0 ꢁ 10ꢀ5 mol/L) using a microsyringe. The absorption spectra
were recorded in the range 200ꢀ400 nm. After each addition, an
equilibration time of 8ꢀ10 min was allowed before the absorption
spectra were recorded. Statistical analysis of the data was carried out
using Origin 8.
Fluorescence Titrations. Fluorescence titrations were performed
on a Cary Eclipse fluorescence spectrophotometer using a 1 cm path
length quartz fluorescence cell. Acetonitrile used in the titrations was
freshly distilled over calcium hydride. Titrations were carried out by
placing the receptors (1 ꢁ 10ꢀ5 mol/L) into the 4 mL cuvette and
adding increasing amounts of p-phenylenediamine (0ꢀ63 ꢁ 10ꢀ5 mol/L
for 5 and 0ꢀ21 ꢁ 10ꢀ5 mol/L for 6) using a microsyringe. The receptor
solution was excited at 294 nm for 5 and 287 nm for 6, and the emission
spectra were recorded in the range 300ꢀ450 nm. After each addition, an
equilibration time of 8ꢀ10 min was allowed before the fluorescence
intensity was recorded. Statistical analysis of the data was carried out
using Origin 8.
S
Supporting Information. Figures giving additional Sternꢀ
b
Volmer and BenesiꢀHildebrand plots and a table and CIF files
giving crystallographic data for 5 and 6. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: qxliu@eyou.com.
’ ACKNOWLEDGMENT
This work was financially supported by the National Natural
Science Foundation of China (No. 20872111) and Tianjin
Natural Science Foundation (No.11JCZDJC22000).
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