Photochemical Modification of Carbene-Based Metallacycles: A Facile Route to Polycarbene Complexes and Their Derivatives

Article abstract of DOI:10.1021/acs.organomet.5b00977

The modification of dinuclear silver metallcycles via photochemical [2 + 2] cycloaddition is described. Reaction of benzimidazole with trans-4,4′-dibromostilbene gives trans-4,4′-bis(1H-benzo[d]imidazolyl)stilbene (L), which after subsequent dialkylation

Full text of DOI:10.1021/acs.organomet.5b00977

Article
pubs.acs.org/Organometallics
Photochemical Modification of Carbene-Based Metallacycles: A
Facile Route to Polycarbene Complexes and Their Derivatives
Chun-Xiang Wang, Yang Gao, Yu-Xin Deng, Yue-Jian Lin, Ying-Feng Han,* and Guo-Xin Jin*
Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, 220 Handan Road,
200433 Shanghai, People’s Republic of China
S
* Supporting Information
ABSTRACT: The modification of dinuclear silver metallcycles via
photochemical [2 + 2] cycloaddition is described. Reaction of
benzimidazole with trans-4,4′-dibromostilbene gives trans-4,4′-bis(1H-
benzo[d]imidazolyl)stilbene (L), which after subsequent dialkylation with
alkyl bromides and anion exchange yields the corresponding dibenzimi-
dazolium salts H2-1(PF6)2 and H2-2(PF6)2. The dibenzimidazolium salts
react with Ag2O to give the dinuclear silver(I) tetracarbene metallacycles
Ag2(1)2(PF6)2 and Ag2(2)2(PF6)2 in high yield. Irradiation (UV light, λ
365 nm) of Ag2(1)2(PF6)2 and Ag2(2)2(PF6)2 in [D6]DMSO resulted in
rapid conversion into the corresponding dinuclear cyclobutane-carbene
complexes Ag2(3)(PF6)2 and Ag2(4)(PF6)2 quantitatively. The cyclo-
butane-bridged polycarbene precursors were isolated in good yields as their tetrabenzimidazolium salts.
postassembly modification and functionalization of target
complexes in the solid state and in solution.10−12
INTRODUCTION
■
In recent decades, metal−ligand-directed assemblies have
played a prominent role in the design and construction of
functional supramolecular architectures with a wide range of
properties.1 In contrast to the intense interest focused on the
construction of typical Werner-type complexes, our knowledge
of organometallic molecular assemblies (i.e., those containing
metal−carbon bonds) is still somewhat underdeveloped.2 N-
heterocyclic carbene (NHC) ligands can form highly stable M−
CNHC bonds due to their strong electron-donor abilities to
metal ions.3 Metal−carbene complexes, formed from NHC
ligands and metals, are an exciting class of structures with utility
in catalysis,4 medicine,5 and functional materials.6 Recently,
intense research efforts have been directed toward the
development of discrete organometallic assemblies based on
poly-NHC ligands.3,7 An efficient method to prepare such
assemblies is silver(I)-mediated self-assembly. Such silver(I)−
dicarbene complexes are usually obtained easily by reacting
Ag2O with the corresponding bis-imidazolium salts. Dinuclear
silver(I)−NHC metallacycles have been reported with a variety
of bridging motifs.7,8 This method can also be used for the
synthesis of three-dimensional metal−carbene assemblies.9
However, to the best of our knowledge, the functionalization
of organometallic assemblies containing poly-NHC ligands has
only been presented recently.9 In previous work, we and others
focused on template-designed photodimerization of olefins.10
We found that a number of metallacycles can be used as
organometallic templates to direct photochemical [2 + 2]
cycloaddition reactions.11 In general, [2 + 2] photodimerization
occurs when the two olefinic groups lie approximately parallel
and are separated by less than 4.2 Å. Photochemical [2 + 2]
cycloaddition reactions have thereby provided an approach to
Over the last two years, we have developed a photochemical
modification method for dinuclear AgI and AuI molecular
rectangles featuring olefin-bridged dicarbene ligands.13 The
metallacycles [M2(dicarbene)2]2+ (M = Ag, Au) are good
candidates for the photodimerization of olefinic double bonds
within molecular rectangles. Following this strategy, a series of
cyclobutane-based tetracarbene complexes were synthesized. In
addition, a new method to polycarbene precursors has been
developed by removal of metal centers. Building upon the
successful modification of imidazolium-derived metallaycles by
photochemical reaction, the photochemical modification of
related benzimidazolium-derived metallacycles is an attractive
goal in order to expand the possibilities for assembly and
postassembly modification of new macromolecular carbene
assemblies. In addition, cyclobutanes, the structural unit formed
by photochemical [2 + 2] olefin dimerization, have attracted
much attraction, in part because they are important in biology
and biotechnology applications.14
Here we describe the preparation of dinuclear silver(I)
dicarbene complexes from benzimidazolium salts featuring
internal olefin groups. These metallacycles can be effectively
modified by photochemical [2 + 2] cycloaddition reactions.
The isolation of the cyclobutane-bridged tetracarbene pre-
cursors was subsequently realized by liberation of silver metals.
Received: November 30, 2015
Published: December 15, 2015
© 2015 American Chemical Society
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DOI: 10.1021/acs.organomet.5b00977
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electrospray ionization mass spectroscopic (ESI-MS) studies
further supported the formation of discrete supramolecular
rectangles. For example, the ESI-MS spectrum for
Ag2(1)2(PF6)2 showed peaks at m/z 576.1379, corresponding
to [Ag2(1)2]2+ (calcd for [Ag2(1)2]2+ m/z 576.1362), and m/z
1297.2386, attributed to [Ag2(1)2(PF6)]+ (calcd for
[Ag2(1)2(PF6)]+ m/z 1297.2371). The ESI mass spectrum
(positive ions) confirmed the formation of [Ag2(2)2](PF6)2 by
exhibiting an intense peak at m/z 632.2006 (calcd for
[Ag2(2)2]2+ m/z 632.1989).
RESULTS AND DISCUSSION
■
trans-4,4′-Dibromostilbene was synthesized from 4-bromoben-
zaldehyde and 1-bromo-4-(bromomethyl)benzene according to
a reported protocol.15 As shown in Scheme 1, the
Scheme 1. Synthesis of the Ligand Precursors
Photolysis of the metallacyclic rectangles Ag2(1)2(PF6)2 and
Ag2(2)2(PF6)2 was investigated (Scheme 3). The photo-
Scheme 3. Photochemical Reactions of Metallacycles and the
Formation of the Polybenzimidazolium Salts
dibenzoimidazolium salts H2-1(PF6)2 and H2-2(PF6)2 were
prepared by a three-step procedure in good yields starting from
trans-4,4′-dibromostilbene and benzimidazole. The ligand (E)-
1,2-bis(4-(1H-benzo[d]imidazol-1-yl)phenyl)ethene (L) was
synthesized by the solid-state reaction of trans-4,4′-dibromos-
tilbene and 1H-benzo[d]imidazole in the presence of CuSO4·
5H2O and K2CO3.
The alkylation of L with ethyl bromide or n-butyl bromide
gave the corresponding dibenzimidazolium salt H2-1(Br)2 or
H2-2(Br)2. The bromide compounds H2-1(Br)2 and H2-2(Br)2
could be converted into H2-1(PF6)2 and H2-2(PF6)2 by anion
exchange with ammonium hexafluorophosphate in methanol.
The 1H NMR spectra of H2-1(Br)2 and H2-2(Br)2 in
[D6]DMSO show characteristic resonances for the benzimida-
zolium C2−H protons at δ 10.28 and 10.30 ppm, respectively.
The corresponding signals were observed upfield at δ 9.25 and
9.26 ppm for H2-1(PF6)2 and H2-2(PF6)2, which are consistent
with previous observations upon anion exchange from bromide
to hexafluorophosphate salts.13 The typical olefinic resonance is
observed at δ 7.67 (H2-1(PF6)2) or 7.53 ppm (H2-2(PF6)2).
The reaction of H2-1(PF6)2 or H2-2(PF6)2 with Ag2O, as
shown in Scheme 2, afforded the desired disilver(I)
tetracarbene complex Ag2(1)2(PF6)2 or Ag2(2)2(PF6)2 in high
yield. Each reaction was complete after 20 h at 55 °C in
acetonitrile. Successful formation of the carbene complexes was
confirmed by 1H and 13C{1H} NMR spectroscopy. Positive-ion
chemical reaction was carried out at room temperature in a
Pyrex tube with a 400 W high-pressure mercury lamp as a light
source. Irradiation (Hg lamp, 365 nm) of Ag2(1)2(PF6)2 in
degassed [D6]DMSO resulted in rapid and practically complete
conversion into the corresponding rctt-tetracarbene-substituted
cyclobutane-bridged dinuclear silver complex Ag2(3)(PF6)2. As
judged by 1H NMR, the [2 + 2] cycloaddition was complete in
Scheme 2. Synthesis of Dinuclear Tetracarbene
Metallacycles
1
30 min. Although the H NMR resonances of Ag2(1)2(PF6)2
are slightly broadened, the corresponding peaks were found to
be sharp after photoreaction. The 1H NMR spectrum of
Ag2(3)(PF6)2 displayed a sharp singlet at 4.98 ppm
corresponding to a single cyclobutane environment. The
13C{1H} NMR spectrum of Ag2(3)(PF6)2 features a signal for
the carbon nuclei of the cyclobutane ring at 44.39 ppm.
Similarly, UV irradiation of Ag2(2)2(PF6)2 afforded the desired
dimer product Ag2(4)(PF6)2 in nearly quantitative conversion.
As can be seen from Figure 1, the spectrum of the
photoreaction product showed the disappearance of the olefin
proton signal at 7.51 ppm and the appearance of the
cyclobutane proton signal at 4.99 ppm (Figure 1b,c).
UV−vis measurements also clearly indicated that photo-
dimerization had occurred (Figures S1 and S2 in the
Supporting Information). In each case, the spectrum showed
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The addition of ammonium chloride successfully resulted in
removal of the silver cations by the precipitate of AgCl from the
solution (Scheme 3). The formed chloride salts were then
converted to the hexafluorophosphate salts H4-3(PF6)4 and H4-
4(PF6)4 by reactions with NH4PF6 in MeOH. In all cases, the
only product observed was the stereospecific tetrabenzimida-
zolium salt. Compounds H4-3(PF6)4 and H4-4(PF6)4 exhibit
typical NMR spectra for highly symmetrical structures in
solution. All of the proton and carbon signals were fully
assigned by 2D NMR measurements (H−H COSY, HSQC,
and HMBC). For example, in H4-4(PF6)4, two sharp singlets
were observed at 9.11 and 4.99 ppm in a 1:1 ratio as expected,
identical with the signals for the C2−H benzimidazolium and
cyclobutane ring protons, respectively. ESI-MS data provided
further evidence for the formation of the tetrabenzimidazolium
salt. For example, the ESI mass spectrum for H4-4(PF6)4
showed isotopically resolved peaks at m/z 671.2748 and
1487.5113, due to [H4-4(PF6)2]2+ and [H4-4(PF6)3]+,
respectively.
Single crystals of the tetrabenzimidazolium hexafluorophos-
phate salts were obtained by slow diffusion of diethyl ether into
saturated acetonitrile solutions of H4-3(PF6)4 at ambient
temperature. The X-ray crystal structure analysis of H4-
3(PF6)4 shows the presence of cyclobutylene-bridged tetra-
benzimidazolium ligands (Figure 3). The C−C single bonds of
the bridging four-membered carbocycle amount to 1.559(4) Å
(C14−C15) and 1.572(4) Å (C14−C15A).
Figure 1. Partial 1H NMR spectra (400 Hz, 300 K) of (a) H2-2(PF6)2
in CD3CN, (b) before and (c) after UV irradiation of Ag2(2)2(PF6)2 in
[D6]DMSO, and (d) H4-4(PF6)4 in CD3CN.
the disappearance of the absorption band around 320 nm after
the photochemical reaction, which is consistent with the
cycloaddition of two olefinic groups. Further fluorescence
experiments also supported this conclusion (Figures S3 and S4
in the Supporting Information).
Although silver(I)−carbene complexes are often slightly
sensitive to light, NMR spectroscopy indicated that no
byproducts were formed during these processes. Attempts to
investigate the photodimerization in the solid state failed
because of the decomposition of silver−carbene complexes
under UV irradiation.13a
Single-crystal X-ray structural analysis revealed the expected
molecular structure containing a cyclobutane ring (Figure 2).16
Metric parameters such as M−CNHC bond lengths (2.080(7)−
2.099(8) Å) and CNHC−M−CNHC angles (177.1(3) and
177.3(3)°) in the [Ag2(4)]2+ cation are in the ranges previously
observed for linearly coordinated bis(NHC) silver complexes.
Figure 3. Crystallographically derived molecular structure of the cation
[H4-3]4+ in H4-3(PF6)4 crystals (hydrogen atoms omitted for clarity).
Selected bond lengths (Å) and angles (deg): N(1)−C(1) 1.322(4),
N(2)−C(1) 1.335(4), C(14)−C(15) 1.559(4), C(14)−C(15A)
1.572(4); N(1)−C(1)−N(2) 110.3(3), N(3)−C(22)−N(4)
110.3(3), C(15)−C(14)−C(15A) 90.0(2), C(14)−C(15)−C(14A)
90.0(2).
In conclusion, we have demonstrated the synthesis of two
new dibenzimidazolium salts containing internal olefinic
groups. Reactions of the obtained dibenzimidazolium salts
with Ag2O yield the corresponding rectangular dinuclear
disilver(I) carbene complexes. Irradiation of Ag2(1)2(PF6)2 or
Ag2(2)2(PF6)2 in solution resulted in rapid conversion into the
corresponding dinuclear rctt-cyclobutane−carbene complex
Ag2(3)(PF6)2 or Ag2(4)(PF6)2. After removal of the metal
centers, the cyclobutane-bridged polycarbene precursors were
isolated as their tetrabenzimidazolium salts. This method
demonstrates the feasibility of utilizing photochemical [2 +
2] cycloaddition reactions at active olefinic groups within
metal−carbene metallacycles. This work presented here also
provides a new entry to the synthesis of potentially very useful
Figure 2. Crystallographically derived molecular structure of the
dication [Ag2(4)]2+ (hydrogen atoms omitted for clarity). Selected
bond lengths (Å) and angles (deg): Ag(1)−C(37) 2.080(7), Ag(1)−
C(1) 2.093(6), Ag(2)−C(58) 2.098(8), Ag(2)−C(22) 2.099(8),
N(1)−C(1) 1.353(8), N(2)−C(1) 1.351(8), N(3)−C(22)
1.353(10), N(4)−C(22) 1.346(9), N(5)−C(37) 1.371(9), N(6)−
C(37) 1.386(10), N(7)−C(58) 1.370(10), N(8)−C(58) 1.349(9),
C(14)−C(15) 1.541(10), C(14)−C(50) 1.574(10), C(15)−C(51)
1.586(10), C(50)−C(51) 1.551(11); C(37)−Ag(1)−C(1) 177.1(3),
C(58)−Ag(2)−C(22) 177.3(3), C(15)−C(14)−C(50) 89.1(5),
C(14)−C(15)−C(51) 87.8(5), C(51)−C(50)−C(14) 87.9(5),
C(50)−C(51)−C(15) 88.3(6).
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1
starting materials. Yield: 0.62 g (0.90 mmol, 90%). H NMR (400
polycarbene precursors. The application of the new cyclo-
butane-bridged tetrabenzimidazolium salts is the subject of
current research in our laboratory.
MHz, DMSO-d6): δ 10.30 (s, 2H, NCHN), 8.26 (d, 3J = 8.1 Hz, 2H),
8.05 (d, J = 8.5 Hz, 4H), 7.92 (m, 6H), 7.73−7.82 (m, 4H), 7.66 (s,
3
2H, CHCH), 4.61 (t, 4H, CH2CH2CH2CH3), 2.01 (m, 4H,
CH2CH2CH2CH3), 1.46 (m, 4H, CH2CH2CH2CH3), 0.98 ppm (t,
6H, CH2CH2CH2CH3). 13C{1H} NMR (100 MHz, DMSO-d6): δ
142.4, 138.7, 132.4, 131.3, 131.0, 129.2, 128.3, 127.4, 127.0,125.6,
114.1, 113.6, 46.8 (CH2CH2CH2CH3), 30.5 (CH2CH2CH2CH3), 19.2
(CH2CH2CH2CH3), 13.4 ppm (CH2CH2CH2CH3). Anal. Calcd for
C36H38Br2N4 (684.15): C, 62.98; H, 5.58; N, 8.16. Found: C, 62.51;
H, 5.86; N, 7.92.
EXPERIMENTAL SECTION
■
Description of Synthetic Procedures. All manipulations were
performed under an argon atmosphere using standard Schlenk
techniques. Glassware was oven-dried at 130 °C prior to use. Solvents
were freshly distilled by standard procedures prior to use. 1H, 13C{1H},
and 2D NMR spectra were recorded on Bruker AVANCE I 400
spectrometers. Chemical shifts (δ) are expressed in ppm downfield
from tetramethylsilane using the residual protonated solvent as an
internal standard. Coupling constants are expressed in hertz. Mass
spectra were obtained with MicroTof (Bruker Daltonics, Bremen,
Germany) spectrometers. UV−vis spectra were obtained using an
Agilent 8453 spectrophotometer. Fluorescence emission spectra were
obtained using a Cary Eclipse spectrofluorophotometer (Varian).
Elemental analyses were performed on an Elementar Vario EL III
analyzer. trans-4,4′-Dibromostilbene17 was synthesized according to
reported procedures. All other chemicals were purchased from
commercial sources and used as received.
Synthesis of H2-2(PF6)2. The bromide salt from the previous
reaction was converted to H2-2(PF6)2 by adding a solution of NH4PF6
(0.36 g, 2.2 mmol) in methanol (8 mL) to a methanolic solution of
H2-2(Br)2 (0.69 g, 1.0 mmol in 10 mL of methanol). The white
hexafluorophosphate salt H2-2(PF6)2 precipitated immediately. The
precipitated solid was collected by filtration, washed with small
portions of cold methanol and diethyl ether, and dried under vacuum.
1
Yield: 0.65 g (0.80 mmol, 80%). H NMR (400 MHz, CD3CN): δ
9.26 (s, 2H, NCHN), 7.96−8.02(m, 6H), 7.73−7.84 (m, 10H), 7.53
(s, 2H, CHCH), 4.55 (t, 4H, CH2CH2CH2CH3), 2.05 (m, 4H,
CH2CH2CH2CH3), 1.52 (m, 4H, CH2CH2CH2CH3), 1.02 ppm (t,
6H, CH2CH2CH2CH3). 13C{1H} NMR (100 MHz, DMSO-d6): δ
141.6, 140.4, 133.5, 132.8, 132.7, 130.4, 129.5), 128.8,128.5, 126.7,
114.8, 48.5 (CH2CH2CH2CH3), 31.6 (CH2CH2CH2CH3), 20.4
(CH2CH2CH2CH3), 13.8 ppm (CH2CH2CH2CH3). Anal. Calcd for
C36H38F12N4P2 (816.24): C, 52.95; H, 4.69; N, 6.86. Found: C, 53.14;
H, 4.62; N, 6.39.
Synthesis of [Ag2(1)2](PF6)2. A sample of H2-1(PF6)2 (76 mg, 0.1
mmol) was dissolved in 10 mL of CH3CN, and to this solution was
added Ag2O (26 mg, 0.11 mmol). The resulting suspension was heated
to 55 °C for 20 h under exclusion of light. After it was cooled to
ambient temperature, the obtained suspension was filtered slowly
through Celite to give a clear solution. The filtrate was concentrated to
3 mL, and diethyl ether (20 mL) was added. This led to the
precipitation of a white solid. The solid was collected by filtration,
washed with diethyl ether, and dried under vacuum. Yield: 64 mg
(0.045 mmol, 89%). 1H NMR (400 MHz, DMSO-d6): δ 8.03 (m, 8H),
7.79−7.83 (m, 8H), 7.48−7.62 (m, 20H), 4.77 (m, 8H, CH2), 1.66
ppm (t, 12H, CH3). ESI-MS (positive ions) for [Ag2(1)2](PF6)2
(C64H56Ag2F12N8P2): m/z 576.1379 (calcd for [Ag2(1)2]2+ 576.1362),
1297.2386 (calcd for [Ag2(1)2(PF6)]+ 1297.2371). Anal. Calcd for
C64H56Ag2F12N8P2·2CH3CN: C, 53.56; H, 4.10; N, 9.19. Found: C,
53.41; H, 3.96; N, 9.27.
Synthesis of [Ag2(3)](PF6)2 (Photochemistry in Solution). A
solution of [Ag2(1)2](PF6)2 (20 mg, 0.014 mmol) in DMSO-d6 (0.6
mL) or CD3CN (0.6 mL) in an NMR tube was irradiated with a
Philips mercury high-pressure lamp (125 W) at ambient temperature
for 30 min. The conversion to [Ag2(3)](PF6)21was quantitative. The
solids were obtained by removal of CD3CN. H NMR (400 MHz,
DMSO-d6): δ 8.02 (d, 3J = 8.20 Hz, 4H), 7.43−7.58 (m, 28H), 4.98 (s,
4H, Hcycobutane), 4.74 (m, 8H, CH2), 1.66 ppm (t, 12H, CH3). 13C{1H}
NMR (100 MHz, DMSO-d6): δ 188.8 (Ccarbene), 141.2, 135.8, 133.9,
132.8, 125.9, 124.9, 124.6, 112.3, 111.9, 44.4 (Ccycobutane), 44.1 (CH2),
16.13 ppm (CH3). ESI-MS (positive ions) for [Ag2(3)](PF6)2
(C64H56Ag2F12N8P2): m/z = 576.1381 (calcd for [Ag2(3)]2+
576.1362), 1297.2373 (calcd for [Ag2(3) (PF6)]+ 1297.2371). Anal.
Calcd for C64H56Ag2F12N8P2: C, 53.28; H, 3.91; N, 7.77. Found: C,
53.07; H, 3.98; N, 7.82.
Synthesis of [Ag2(2)2](PF6)2. A sample of H2-2(PF6)2 (82 mg, 0.1
mmol) was dissolved in 10 mL of CH3CN, and to this solution was
added Ag2O (0.026 g, 0.11 mmol). The resulting suspension was
heated to 55 °C for 20 h with the exclusion of light. After it was cooled
to ambient temperature, the obtained suspension was filtered slowly
through Celite to give a clear solution. The filtrate was concentrated to
3 mL, and diethyl ether (20 mL) was added. This led to the
precipitation of a white solid. The solid was collected by filtration,
washed with diethyl ether, and dried under vacuum. Yield: 71 mg
Synthesis of (E)-1,2-Bis(4-(1H-benzo[d]imidazol-1-yl)-
phenyl)ethene (L). Samples of trans-4,4′-dibromostilbene (0.34 g,
1.0 mmol), benzoimidazole (0.47 g, 4.0 mmol), K2CO3 (0.55 g, 4.0
mmol) and CuSO4·5H2O (0.02 g, 0.08 mmol) were mixed in a 50 mL
round-bottom flask, and this mixture was heated under an argon
atmosphere for 24 h to 180 °C. The reaction mixture was cooled to
ambient temperature and washed three times with hot water. The solid
residue was extracted with dichloromethane (120 mL). The solution
was brought to dryness and then was washed with methanol. The solid
was dried under vacuum to give a colorless product. Yield: 0.35 g (0.85
mmol, 85%). 1H NMR (400 MHz, DMSO-d6) δ 8.61 (s, 2H, NCHN),
7.91 (d, 4H, J = 8.32 Hz, Ar−H), 7.80 (d, 2H, J = 7.68 Hz, Ar−H),
7.74 (d, 4H, J = 8.32 Hz, Ar−H), 7.69 (d, 2H, J = 7.68 Hz, Ar−H),
7.50 (s, 2H, CHCH), 7.35 (m, 4H, Ar−H). 13C NMR (101 MHz,
DMSO-d6): δ 143.86, 143.20, 136.32, 135.20, 132.98, 128.23, 128.06,
123.82, 123.50, 122.49, 119.97, 110.77 ppm. Anal. Calcd for C28H20N4
(412.17): C, 81.53; H, 4.89; N, 13.58. Found: C, 81.46; H, 4.95; N,
13.24.
Synthesis of H2-1(Br)2. A Schlenk flask was charged with trans-
4,4′-bis(1-benzoimidazolyl)stilbene (0.41 g, 1.0 mmol) and an excess
of ethyl bromide (0.44 g, 4.0 mmol). To this mixture was added DMF
(5 mL), and the reaction mixture was heated to 110 °C for 24 h.
During this time a white compound precipitated, which was filtered
off, washed with diethyl ether, and dried under vacuum to give H2-
1(Br)2 as a white solid. Yield: 0.60 g (0.95 mmol, 95%). 1H NMR (400
MHz, DMSO-d6): δ 10.28 (s, 2H, NCHN), 8.24 (d, 3J = 7.9 Hz, 2H),
3
8.05 (d, 4H, J = 8.4 H), 7.92 (m, 6H), 7.74−7.82 (m, 4H), 7.67 (s,
2H, CHCH), 4.65 (q, 4H, CH2), 1.65 ppm (t, 6H, CH3). 13C{1H}
NMR (100 MHz, DMSO-d6): δ 142.3, 138.7, 132.5, 131.0 (br, two
carbon signals), 129.2, 128.3, 127.4, 126.9, 125.5, 114.0, 113.6, 42.5
(CH2), 14.0 ppm (CH3). Anal. Calcd for C32H30Br2N4 (628.08): C,
60.97; H, 4.80; N, 8.89. Found: C, 60.79; H, 4.63; N, 8.74.
Synthesis of H2-1(PF6)2. H2-1(Br)2 was converted to H2-1(PF6)2
by adding a solution of NH4PF6 (0.36 g, 2.2 mmol) in methanol (8
mL) to a methanolic solution of H2-1(Br)2 (0.63 g, 1.0 mmol in 50 mL
of methanol). The white hexafluorophosphate salt H2-1(PF6)2
precipitated immediately. The precipitated solid was collected by
filtration, washed with small portions of cold methanol and diethyl
1
ether, and dried under vacuum. Yield: 0.65 g (0.85 mmol, 85%). H
NMR (400 MHz, CD3CN): δ 9.25 (s, 2H, NCHN), 7.96−8.02 (m,
6H), 7.74−7.84 (m, 10H), 7.53 (s, 2H, CHCH), 4.60 (q, 4H, CH2),
1.69 ppm (t, 6H, CH3). 13C{1H} NMR (100 MHz, CD3CN): δ 141.4,
140.4, 133.5, 132.8, 130.4, 129.5, 128.8, 128.4, 126.6, 118.3, 114.7,
114.7, 44.1 (CH2), 14.4 ppm (CH3). Anal. Calcd for C32H30F12N4P2·
H2O: C, 49.37; H, 4.14; N, 7.20. Found: C, 49.24; H, 3.97; N, 7.35.
Synthesis of H2-2(Br)2. By a procedure similar to that for the
synthesis of H2-1(Br)2, H2-2(Br)2 was obtained as a white solid from
trans-4,4′-bis(1-benzoimidazolyl)stilbene and n-butyl bromide as
1
3
(0.045 mmol, 91%). H NMR (400 MHz, DMSO-d6): δ 8.04 (d, J =
8.20 Hz, 4H), 7.80 (m, 16H), 7.51−7.61 (m, 16H), 4.75 (t, 8H,
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Article
CH2CH2CH2CH3), 2.05 (m, 8H, CH2CH2CH2CH3), 1.51 (m, 8H,
CH2CH2CH2CH3), 1.00 ppm (t, 12H, CH2CH2CH2CH3). 13C{1H}
NMR (100 MHz, DMSO-d6): δ 188.9 (Ccarbene), 137.4, 136.58, 133.5,
133.0, 128.8, 128.1, 125.8, 125.0, 124.9, 112.6, 112.4, 112.3, 49.2
(CH2 CH2 CH2 CH3 ), 32. 6 (CH2 CH2 CH2 CH3 ), 19. 9
(CH2CH2CH2CH3), 13.8 (CH2CH2CH2CH3). ESI-MS (positive
ions) for [Ag2(2)2](PF6)2 (C72H72Ag2F12N8P2): m/z 632.2006
(calcd for [Ag2(2)2]2+ 632.1989). Anal. Calcd for C72H72Ag2F12N8P2
(1552.33): C, 55.61; H, 4.67; N, 7.21. Found: C, 55.33; H, 4.71; N,
7.15.
monochromated Mo Kα radiation (λ = 0.71073 A).18 Diffraction
measurement of H4-3(PF6)4 was carried out at T = 293(2) K on an
Agilent SuperNova AtlasS2 diffractometer equipped with Cu X-ray
source (Cu Kα 1.54184 Å). Diffraction data were collected over the
full sphere and were corrected for absorption. Structure solutions were
found with the SHELXS-97 package using direct methods and were
refined with SHELXL-97 against |F2| values of all data using first
isotropic and later anisotropic thermal parameters (for exceptions see a
description of the individual molecular structures).19 Hydrogen atoms
were added to the structure models in calculated positions. For
crystallographic data, see the Supporting Information.
Synthesis of [Ag2(4)](PF6)2 (Photochemistry in Solution). A
solution of [Ag2(2)2](PF6)2 (20 mg, 0.013 mmol) in DMSO-d6 (0.6
mL) or CD3CN (0.6 mL) in an NMR tube was irradiated with a
Philips mercury high-pressure lamp (125 W) at ambient temperature
for 30 min. The conversion to [Ag2(4)](PF6)21was quantitative. The
solids were obtained by removal of CD3CN. H NMR (400 MHz,
DMSO-d6): δ 8.01 (d, 4H), 7.32−7.65 (m, 28H), 4.99 (s, 4H,
Hcyclobutane), 4.70 (t, 8H, CH2CH2CH2CH3), 2.03 (m, 8H,
CH2CH2CH2CH3), 1.50 (m, 8H, CH2CH2CH2CH3), 1.01 ppm (t,
12H, CH2CH2CH2CH3). 13C{1H} NMR (100 MHz, DMSO-d6): δ
189.3 (Ccarbene), 141.2, 135.5, 134.0, 133.1, 131.6, 127.7, 125.9, 124.9,
124.6, 112.5, 111.9, 49.0 (CH2CH2CH2CH3), 44.3 (Ccycobutane), 32.6
(CH2CH2CH2CH3), 19.9 (CH2CH2CH2CH3), 13.7 ppm
(CH2CH2CH2CH3). ESI-MS (positive ions) for [Ag2(4)](PF6)2
(C72H72Ag2F12N8P2): m/z 632.2002 (calcd for [Ag2(4)]2+ 632.1989).
Synthesis of Ligand H4-3(PF6)4. A sample of [Ag2(3)](PF6)2 (72
mg, 0.05 mmol) was dissolved in a mixture of MeOH (5 mL) and
DMSO (1 mL). To this solution was added NH4Cl (11 mg, 0.2
mmol). White solid AgCl precipitated immediately. The resulting
suspension was filtered through Celite to give a clear solution. The
solvent was removed to give a white solid. The white solid was
dissolved in MeOH (5 mL), and a solution of NH4PF6 (36 mg, 0.22
mmol) in methanol (3 mL) was added. The mixture was stirred at
ambient temperature for 2 h. After this period a white solid
precipitated, which was isolated by filtration, washed with diethyl
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00977.
■
S
UV−vis, fluorescence, and NMR (1H, 13C{1H} and 2D)
spectra (PDF)
Crystallographic data for Ag2(4)(PF6)2 (CIF)
Crystallographic data for H4-3(PF6)4 (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail for Y.-F.H.: yfhan1980@fudan.edu.cn.
*E-mail for G.-X.J.: gxjin@fudan.edu.cn.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
This paper is dedicated to Prof. F Ekkehardt Hahn on the
occasion of his 60th birthday. This work was supported by
NSFC (21371036, 21374019), the National Basic Research
Program of China (2015CB856600), and the Shanghai Pujiang
Program (14PJD006).
ether, and dried under vacuum. Yield: 55 mg (0.036 mmol, 72%). H
NMR (400 MHz, CD3CN): δ 9.11 (s, 4H), 7.97 (d, 4H), 7.67−7.72
(m, 12H), 7.54−7.59 (m, 16H), 4.99 (s, 4H, Hcyclobutane), 4.56 (d, 8H),
1.65 ppm (t, 12H). 13C{1H} NMR (100 MHz, CD3CN): δ 143.75,
141.3, 132.8, 132.3, 131.3, 128.7, 128.4, 125.9, 114.7, 114.3, 47.2, 44.0
(CH2), 14.4 ppm (CH3). Anal. Calcd for C64H60F24N8P4 (1520.35):
C, 50.54; H, 3.98; N, 7.37. Found: C, 49.97; H, 4.12; N, 7.12. Anal.
Calcd for C72H72Ag2F12N8P2 (1552.33): C, 55.61; H, 4.67; N, 7.21.
Found: C, 55.27; H, 4.52; N, 7.01.
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1
ether, and dried under vacuum. Yield: 46 mg (0.035 mmol, 70%). H
NMR (400 MHz, CD3CN): δ 9.13 (s, 4H, NCHN), 7.67 (d, 4H),
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for [H4-4(PF6)2]2+ 671.2733), 1487.5138 (calcd for [H4-4(PF6)3]+
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4.69; N, 6.86. Found: C, 52.46; H, 4.74; N, 6.93.
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diffractometer equipped with a rotation anode using graphite-
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