An efficient ruthenium tris(bipyridine)-based luminescent chemosensor for recognition of Cu(ii) and sulfide anion in water

Article abstract of DOI:10.1039/c3dt51047f

A novel efficient luminescent chemosensor based on a 1,4,7,10- tetraazacyclododecane (cyclen)-tethered Ru(bpy)₃²⁺ derivative (Ru-cyclen) has been synthesized and characterized. It displays an ON-OFF-type luminescence change with excellent selectivity towards Cu(ii) amongst 16 metal ions in 100% aqueous solution. The binding stoichiometry of Ru-cyclen with Cu²⁺ was established by Job plot analysis and mass spectral evidence. Furthermore, the in situ generated Ru-cyclen-Cu ensemble recovered luminescence in the presence of S^2-, indicating an 'OFF-ON'-type sensing process. Similar phenomena were not observed with other common anions and biothiols, making it a high selective sulfide probe. Finally, the sensing mechanism is confirmed to be displacement approach by NMR, mass and emission spectrometry.

Full text of DOI:10.1039/c3dt51047f

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An efficient ruthenium tris(bipyridine)-based
luminescent chemosensor for recognition of Cu(II) and
Cite this: Dalton Trans., 2013, 42, 13509
sulfide anion in water†
Minna Li,* Qichao Liang, Meiqing Zheng, Chenjie Fang, Shiqi Peng and Ming Zhao
A novel efficient luminescent chemosensor based on a 1,4,7,10-tetraazacyclododecane (cyclen)-tethered
2+
Ru(bpy)3 derivative (Ru–cyclen) has been synthesized and characterized. It displays an ON–OFF-type
luminescence change with excellent selectivity towards Cu(II) amongst 16 metal ions in 100% aqueous
solution. The binding stoichiometry of Ru–cyclen with Cu2+ was established by Job plot analysis and mass
spectral evidence. Furthermore, the in situ generated Ru–cyclen–Cu ensemble recovered luminescence in
the presence of S2−, indicating an ‘OFF–ON’-type sensing process. Similar phenomena were not observed
with other common anions and biothiols, making it a high selective sulfide probe. Finally, the sensing
mechanism is confirmed to be displacement approach by NMR, mass and emission spectrometry.
Received 22nd April 2013,
Accepted 29th June 2013
DOI: 10.1039/c3dt51047f
www.rsc.org/dalton
high sensitivity and selectivity. Until now, few selective sensors
for sulfide have been reported18,19 relative to other widely
Introduction
Being a toxic traditional pollutant, the sulfide anion has a investigated anions such as F− and CN−.20–22 Among these
huge environmental impact and is generated not only from sensors, two main types are involved in the pathway of sulfide
industrial process but also biological metabolism.1 Continu- sensing. One is termed a ‘reactive sensor’ based on irreversible
ous exposure to sulfide ions can damage mucous membranes chemical reactions and the other is a ‘competitive sensor’
and cause unconsciousness and respiratory problems.2,3 Once based on metal–anion affinity. Most organic reactions are
protonated, HS– or H2S becomes more toxic. At low concen- time-consuming and reaction conditions are relatively strict,
trations, H2S can induce dizziness and at higher concen- which limits their application. Encouraged by biologically
trations it can result in permanent damage of brain tissues, or compatible conditions, reversible selective sulfide sensors
even suffocation.2–5 The protonated sulfide has also attracted based on copper sulfide affinity are attractive to us,23–26 since
attention due to its role in various physiological processes. For CuS is a well-known stable and low solubility compound with
example, recent studies have identified H2S as the third most a Ksp value of 6.3 × 10−36. A few fluorescence sensors for
biologically active gas following NO and CO.6,7 It is involved in sulfide have been reported,21–26 however, multifunctional
the mediation of neurotransmission,8 inhibition of insulin sig- probes with sufficient selectivity over other biothiols in
naling,9 reduction in blood pressure,10 relaxation of vascular aqueous media are still rare and there is a need for further
smooth muscles, and regulation of inflammation.11 Therefore, investigation.
development of quick and sensitive method for sulfide detec-
tion in aqueous media and in biological systems is of great cyclic compound which has proved to be a versatile metal
importance.
chelator with excellent water-solubility.27,28 Moreover, it has
1,4,7,10-Tetraazacyclododecane (cyclen) is an azamacro-
The most commonly used methods for selective sulfide been successfully applied in biology, recognition and imaging
anion detection, include chemiluminescence,12 inductively fields.29–32 In this work it is employed as an ionophore.
coupled plasma atomic emission spectroscopy,13,14 electro- Additionally, ruthenium(II) tris-bipyridine complexes have
chemical methods,15 ion chromatography16 and titration.17 favorable photochemical and photophysical properties, includ-
Fluorogenic methods in conjunction with suitable probes have ing good photo-stability, long lifetimes, high quantum yields
received great attention because of their simple operation, and long Stokes shifts. These complexes have potential appli-
cations as probes, sensors and imaging agents as different cell
localizations can be obtained by changing the substituents on
School of Chemical Biology and Pharmaceutical Sciences, Capital Medical
the ligands.33–37 Herein, it behaves as chromophore. There-
University, Beijing 100069, China. E-mail: minnali78@126.com
fore, in this paper, a water-soluble ruthenium tris(bipyridine)-
based chemosensor by incorporating the cyclen unit
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3dt51047f
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Fig. 1 Luminescence emission spectra of Ru–cyclen (10 μM) upon addition of
10 equiv. of various metal ions (Li+, Na+, Mg2+, Al3+, Co2+, Ca2+, Mn2+, Fe2+, Ba2+
Cd2+, Fe3+, Hg2+, Ni2+, Pb2+, Zn2+ in 100% aqueous solution.
,
Scheme 1 Synthesis of complex Ru–cyclen (i) (1) SeO2, dioxane, reflux 24 h;
(2) NaBH4, MeOH, rt 1 h; (ii) cis-Ru(bpy)2Cl2, EtOH–H2O (1 : 1), reflux, 4 h; (iii)
SOCl2, reflux, 4 h; (iv) cyclen, Et3N, CH3CN, reflux, 4 h.
of free Ru–cyclen is extremely similar to that of the parent
complex [Ru(bpy)3]2+, consisting of a strong sharp band at
286 nm (π–π* transition of bipyridine), two less intense bands
around 244 nm and 450 nm (MLCT transitions). Additionally,
almost no significant change in the UV-vis absorption spectra
of Ru–cyclen was observed upon addition of various amounts
of Cu2+ ions (Fig. S1, ESI†). Thus, the Cu2+ complexing pro-
perties with Ru–cyclen had to be investigated only by emission
spectra in the following experiments.
(Ru–cyclen, Scheme 1) was designed and synthesized. Conse-
quently, Ru–cyclen exhibits ‘ON–OFF–ON’ mode luminescence
change with alternate addition of Cu2+ and S2− in 100%
aqueous media. Moreover, it has excellent selectivity toward
Cu(II) amongst 16 metal ions and the in situ generated
Ru–cyclen–Cu ensemble is found to be a high sensitive and
selective sulfide probe.
To evaluate the sensing properties of Ru–cyclen, the emis-
sion was studied using various metal ions in 100% water solu-
tion at pH = 7.0 upon excitation at 450 nm (Fig. 1 and Fig. S2,
ESI†). The free Ru–cyclen exhibited a broad emission peak at a
max. of 604 nm. Upon addition of 10 equiv. of Li+, Na+, Mg2+,
Al3+, Co2+, Ca2+, Mn2+, Fe2+, Ba2+, Cd2+, Fe3+, Hg2+, Ni2+, Pb2+,
Zn2+, no marked change in the emission profile was noted.
However, introduction of 1 equiv. of Cu2+ elicited a large
decrease in the emission intensity at 604 nm and the quench-
ing efficiency was nearly 80%. Such good selectivity of
Ru–cyclen toward Cu2+ amongst 16 metal ions may be due
to the good thermodynamical stability38 or particularly large
formation constants of the resulting Cu(II)–cyclen complex
determined by glass electrode potentiometric titration or
spectroscopic titration.39–41
To learn more about the responsive nature of Ru–cyclen to
Cu2+, the complex was titrated with Cu2+ ions in varying con-
centrations in 100% aqueous solution (Fig. 2). The emission
intensity gradually decreased with the addition of increasing
concentrations of Cu2+ ions and became saturated at about
1 equiv. of Cu2+ ions. In good agreement with this finding, the
Job plot and ESI-MS (Fig. S3 and S12, ESI†) also show the for-
mation of a 1 : 1 binding mode between Ru–cyclen and Cu2+.
Based on the 1 : 1 binding mode, the binding constants (K)
with Cu2+ derived from the luminescence titration was calcu-
Result and discussion
Synthesis and characterisation
The synthesis of Ru–cyclen was carried out via a four-step reac-
tion. (Scheme 1) The target complex Ru–cyclen was readily pre-
pared by reaction of an excess of cyclen with chloromethyl
substituted ruthenium tris(bipyridine) (4), which was obtained
by halogenation of the corresponding hydroxylmethyl ana-
logue (3) in the presence of SOCl2. Complex 3 was prepared by
reaction of cis-Ru(bpy)2Cl2 with the appropriate 4-hydroxy-
methyl-4′-methyl-2,2′-bipyridine (2) in a mixture of ethanol
and water under reflux. The key intermediate 2 was syn-
thesized by oxidation of the easily accessible 4,4′-dimethyl-2,2′-
bipyridine (1) using SeO2 in dioxane and subsequent reduction
with NaBH4 in methanol according to the method of Sun et al.
All the ruthenium complexes were purified by crystallization
and well characterized by 1H NMR, 13C NMR and TOF-MS
(Fig. S11, S14–21, ESI†) complex Ru–cyclen showed an ESI+-MS
molecular ion at m/z 384.1453, consistent with the formula
C40H46N10Ru.
Luminescent response of Ru–cyclen to various metal ions
The target complex Ru–cyclen is water soluble, which makes it lated to be 2.36 × 104 with a good correlation coefficient (R2 =
useful for practical applications. The UV-vis absorption spec- 0.997) using the Bensi–Hidebrand plot (Fig. S4, ESI†).25,42 The
trum of Ru–cyclen was measured in 100% aqueous solution at luminescence response of Ru–cyclen toward Cu2+ was influ-
pH = 7.0. As shown in Fig. S1 (ESI†), the absorption spectrum enced by pH and the maximal signal was observed in the
13510 | Dalton Trans., 2013, 42, 13509–13515
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Fig. 2 Changes in the luminescence emission spectrum of Ru–cyclen (10 μM) with increasing concentrations of Cu2+ ions (0–5 equiv.) in 100% aqueous solution
(left). The emission intensity changes at 604 nm of Ru–cyclen (10 μM) with the amount of Cu2+ ions (right).
pH range of 5–11 (Fig. S5, ESI†), indicating that Ru–cyclen can anions in 100% aqueous media. As shown in Fig. 3, the emis-
be used to sense Cu2+ over a wide pH range. From the Cu2+ sion intensity increased with the addition of S2− and the early
concentration-dependent luminescence change, the detection part of sigmoidal titration with sulfide appears saturated at
limit43–45 of Ru–cyclen for the determination of Cu2+ was 15 equivalents of S2−, which may be due to the multiple
calculuated to be 5.4 × 10−6 M (Fig. S6, ESI†). In comparison equilibria between Ru–cyclen, Cu2+ and added S2− 46
or maybe
,
with the limit of 2 × 10−5 M copper in drinking water deter- the formation of CuxSx rings in solution, which are the
mined by EPA (the U.S. Environmental Protection Agency) and building blocks for aqueous CuS molecular clusters, leading to
with such a high selectivity toward Cu2+ rather than other CuS precipitation.47 From the sulfide concentration-dependent
metal ions mentioned above, Ru–cyclen could be used as a luminescence change, the detection limit43–45 of the
highly sensitive and selective luminescent ‘ON–OFF’ probe for Ru–cyclen–Cu ensemble for the determination of S2− was esti-
Cu2+.
mated to be 3.7 × 10−5 M (Fig. S7, ESI†). Furthermore, no
obvious luminescence change was observed in response to
HEPES buffer solution under the same conditions (Fig. S8,
ESI†) however the addition of S2− resulted in a dramatic
Luminescence response of the in situ generated Ru–cyclen–Cu
ensemble to various anions
As described above, when the metal ion of the complex was enhancement of the intensity. Therefore, the in situ generated
replaced by an anion, based on the metal-anion affinity, chelat- Ru–cyclen–Cu ensemble can also be employed as a S2− sensor
ing ligands could release the metal and consequently recog- under physiological conditions.
nize the anions. Accordingly, it is reasonable to hypothesize
To investigate the selectivity of the in situ generated
that in this paper the in situ generated Ru–cyclen–Cu ensemble Ru–cyclen–Cu ensemble, the response of Ru–cyclen–Cu
could be a promising ‘OFF–ON’-type luminescent sensor for towards physiologically and environmentally important anions
the S2− anion, which could bind with Cu2+ to form stable CuS, were evaluated (Fig. 4). It was found that 100 equiv. of
resulting in the release of Ru–cyclen. To test this idea, the common anions (e.g. F−, Cl−, Br−, I−, HCO3−, HSO4−, H2PO4
,
−
in situ generated Ru–cyclen–Cu ensemble was titrated with S2− SO32−, S2O32−, SO42−, CO32−, PO43−, OAC−, ClO4−, NO3−) and
Fig. 3 Changes in the luminescence emission spectrum of Ru–cyclen–Cu system with increasing concentrations of sulfide ions (0–200 μM) in 100% aqueous solu-
tion. [Ru–cyclen–Cu] = 5 μM (left). Emission intensity at 604 nm as a function of S2− concentration (right).
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fluorescence detection of sulfide anions, suggesting that the
Ru–cyclen–Cu ensemble could be useful for selectively sensing
sulfide even involving these relevant anions. The luminescence
response of the Ru–cyclen–Cu ensemble to S2− is related to pH
and the result indicates it can be employed in a wide pH range
(pH = 6–11) (Fig. S9, ESI†).
The proposed sensing mechanism
To determine the sensing mechanism, both NMR and mass
spectrometry were used to study the sensing process. When
addition of 1 equiv. of Cu2+ ions to Ru–cyclen in D2O, the
1H NMR spectrum was very broad due to the complexation of
paramagnetic Cu2+ to Ru–cyclen. Whereas, further addition of
20 equiv. of sulfide anions to the Ru–cyclen–Cu ensemble led
to a well-resolved spectrum, which is almost identical to that
of free Ru–cyclen (Fig. S10, ESI†). Additionally, the ESI-MS
spectrum of free Ru–cyclen showed an intense peak at m/z
384.1453 corresponding to [Ru–cyclen]2+ (Fig. S11, ESI†). After
treatment of Ru–cyclen with Cu2+ in aqueous media, new
peaks at 414.5938 and 433.5816 corresponding to [Ru–cyclen +
Cu2+ − 2H+]2+ and [Ru–cyclen + Cu2+ + Cl− − H+]2+ respectively
appeared and the intense peak at m/z 384.1453 disappeared
(Fig. S12, ESI†). However, further addition of sulfide anions
resulted in the disappearance of the peaks at m/z 414.5938 and
433.5816 and the reappearance of the peak at m/z 384.1369
(Fig. S13, ESI†). Therefore, the studies of NMR, mass
spectrometry, and the emission spectrometry indicate that the
sensor probably functions by a displacement mechanism
(Scheme 2).
Fig. 4 Emission spectra of Ru–cyclen–Cu (3 μM) ensemble in the presence of
various anions and biothiols (20 equiv. for S2−; 100 equiv. for F−, Cl−, Br−, I−,
HCO3−, HSO4−, H2PO4−, SO32−, S2O32−, SO42−, CO32−, PO43−, OAC−, ClO4−, NO3
;
−
1000 equiv. for GSH, DTT, and L-cysteine) in 100% aqueous solution.
Conclusions
In summary, a water-soluble luminescent sensor Ru–cyclen
has been synthesized and characterized, for the sequential
recognition of Cu(II) and sulfide anions based on the displace-
ment approach . Ru–cyclen exhibits high selectivity and sensi-
tivity for Cu2+ in water, which can serve as a luminescent
‘ON–OFF’ mode sensor. Subsequently, the in situ generated
Ru–cyclen–Cu ensemble is a good sensor for S2− over other
common anions and biothiols in the same media and has the
ability to work well over a wide pH range. Thus, it can behave
Fig. 5 Emission intensity changes of Ru–cyclen–Cu(II) (1 μM) with sulfide
anions (20 μM) in the presence of various of anions or biothiols (100 μM) in
water. Key: (a) S2−; (b) S2− + F−; (c) S2− + Cl−; (d) S2− + Br−; (e) S2− + I−; (f ) S2−
+
CO32−; (g) S2− + HCO3−; (h) S2− + H2PO4−; (i) S2− + SO32−; ( j) S2− + S2O32−; (k)
2−
S2− + SO42−; (l) S2− + PO43−; (m) S2− + OAc−; (n) S2− + ClO4−; (o) S2− + HPO4
;
(p) NO3−; (q) S2− + cysteine; (r) S2− + GSH; (s) S2− + DDT.
1000 equiv. of biological biothiols, including reduced gluta-
thione (GSH), dithiothreitol (DTT) and L-cysteine did not
cause any marked luminescence change. In contrast, 20 equiv.
of S2− led to a pronounced enhancement of the emission
intensity. Thus, the in situ generated Ru–cyclen–Cu ensemble
can behave as a high selective luminescent ‘OFF–ON’ sensor
for S2−
.
Additionally, to explore whether the Ru–cyclen–Cu ensem-
ble could maintained its sensing response to S2− under the
potential competition of other relevant anions, it (1 μM) was
treated with sulfide anions (20 μM) in the presence of various
anions or biothiols (100 μM) in water. As shown in Fig. 5, all
Scheme 2 The proposed sensing mechanism of Ru–cyclen for Cu2+ and S2−
the relevant anions tested have virtually no influence on the in H2O.
13512 | Dalton Trans., 2013, 42, 13509–13515
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as a potential ‘OFF–ON’ sensor for S2−. Above all, an ‘ON–OFF– and the mixture was filtered to remove insoluble products.
ON’ mode luminescence recognition system has been The solvent was evaporated under reduced pressure and
constructed and it will be potentially useful in physiological the resulting solid was crystallized with methanol and ether,
and environmental applications for sulfide anion detection.
filtered, and dried in vacuum. The target complex 3
was obtained in a yield of 75% (71 mg). 1H NMR (d6-DMSO)
δ 2.53 (s, 3H), 4.74 (s, 2H), 7.37 (d, 1H, J = 5.0 Hz), 7.46 (d, 1H,
J = 6.5 Hz), 7.51–7.55 (m, 5H), 7.65 (d, 1H, J = 6.0 Hz), 7.74 (t,
4H, J = 6.0 Hz), 8.17 (t, 4H, J = 8.0 Hz), 8.71 (s, 1H), 8.74 (s,
1H), 8.83 (d, 4H, J = 8.5 Hz). 13C NMR (CD3CN) δ 19.98, 61.23,
120.91, 123.98, 124.39, 124.84, 127.27, 128.13, 137.42, 150.36,
150.51, 150.80, 151.30, 151.44, 153.89, 156.25, 156.56, 156.87.
Experimental
General
1H and 13C NMR spectra were recorded on a Bruker Avance II
500 MHz spectrometer in solution (DMSO-d6, D2O or CDCl3)
with tetramethylsilane as the internal standard. Mass spectra
were measured using Bruker microTOF-Q. UV-vis absorption
spectra and photoluminescence spectra were obtained on a
TOF MS ES+ (m/z) 307.0775 [M
−
2PF6−]2+, calculated.
307.0683.
SHIMADZU UV-2550 spectrophotometer and
a HITACHI
Bis(2,2′-bipyridine)-ruthenium-4′-chloromethyl-4-methyl-2,2′-
bipyridine hexafluorophosphate (4)
F-2500 fluorescence spectrometer respectively. All chemicals
were purchased from commercial corporations and used
without further purification except for methanol, which was
distilled from magnesium. cis-[Ru(bpy)2Cl2]2H2O48 was pre-
pared according to the literature procedures.
Bis(2,2′-bipyridine)-ruthenium-4′-hydroxylmethyl-4-methyl-2,2′-
bipyridine hexafluorophosphate (3) (45 mg, 0.05 mmol) was
reflux in SOCl2 (2 mL) for 4 h under argon. The solvent was
removed under reduced pressure and the resulting red-orange
solid was crystallized methanol and ether, filtered, and dried
in vacuum to afford the final product 4 (45 mg, 98%). 1H NMR
(d6-DMSO) δ 2.53 (s, 3H), 4.96 (s, 2H), 7.40 (d, 1H, J = 6.0 Hz),
7.52–7.58 (m, 6H), 7.72 (d, 1H, J = 7.0 Hz), 7.75 (t, 4H, J =
6.0 Hz), 8.17 (t, 4H, J = 6.0 Hz), 8.76 (s, 1H), 8.84 (d, 4H, J =
8.0 Hz), 8.90 (d, 1H, J = 1.5 Hz). 13C NMR (CD3CN) δ 20.51,
43.18, 123.23, 124.08, 125.17, 126.43, 127.36, 128.42, 137.58,
148.31, 150.48, 150.61, 151.40, 151.52, 151.64, 155.89, 156.80,
156.90, 157.36. TOF MS ES+ (m/z) 316.0646 [M − 2PF6−]2+, cal-
culated. 316.0571.
4-Hydroxymethyl-4′-methyl-2,2′-bipyridine (2)
The compound 2 was prepared according to the procedure pre-
viously described by Sun et al.49 with a slight modification. To
a solution of 4,4′-dimethyl-2,2′-bipyridine (0.3 g, 1.65 mmol) in
1,4-dioxane (20 mL), SeO2 (0.3 g, 0.27 mmol) was added, then
the mixture was heated to reflux for 24 h. The reaction mixture
was allowed to cool to room temperature and the black solid
was filtered. The solvent was removed under reduced pressure.
The resulting pink solid was re-dissolved in chloroform, and
the suspension was filtered to remove selenium by-products.
After three successive dissolution and filtration treatments, the
crude product (0.2 g) was obtained. The resulting solid was
suspended in methanol (15 mL) at 0 °C, and sodium boro-
hydride (0.05 g) was added gradually. The mixture was raised
to room temperature and stirred for one hour. Saturated
Na2CO3 solution (1 mL) was added to the mixture and the
crude product was extracted with chloroform (3 × 10 mL). The
combined organic phase was dried over Na2SO4 and the
solvent was evaporated. 4-Hydroxymethyl-4′-methyl-2,2′-bipyri-
dine (0.15 g, 70%) was obtained after purification by silica gel
column chromatography with MeOH–CH2Cl2 as eluent (1 : 10).
All analytical data were in agreement with those reported by
Geren et al.50
Bis(2,2′-bipyridine)-ruthenium-4′-[(1,4,7,10-
tetraazacyclododecane)-1-methyl]-4-methyl-2,2′-bipyridine
hexafluorophosphate (Ru–cyclen)
Bis(2,2′-bipyridine)-ruthenium-4′-chloromethyl-4-methyl-2,2′-
bipyridine hexafluorophosphate (4) (36 mg, 0.04 mmol) was
dissolved in 2 mL of CH3CN. 1,4,7,10-Tetraazacyclododecane
(52 mg, 0.3 mmol) and Et3N (30 mg, 0.3 mmol) were added to
the solution, and the mixture was refluxed for 4 h under
argon. After removal of the solvent by rotary evaporation, the
residue was subjected to add 5 mL of water and additional
32 mg of NH4PF6 to afford a red-orange solid, which was
collected by filtration, washed with water (3 × 10 mL) and
dried. The product was crystallized with methanol and ether
to give 4 (30 mg, 78%). 1H NMR (D2O) δ 2.42 (s, 3H),
Bis(2,2′-bipyridine)-ruthenium-4′-hydroxylmethyl-4-methyl-2,2′-
bipyridine hexafluorophosphate (3)
Bis-2,2′-bipyridyl ruthenium dichloride (55 mg, 0.11 mmol) 2.73–2.95 (br, 16H), 3.88 (s, 2H), 7.13 (d, 1H, J = 5.0 Hz),
and 4-hydroxymethyl-4′-methyl-2,2′-bipyridine (1) (22 mg, 7.23–7.27 (m, 5H), 7.53 (d, 1H, J = 6.0 Hz), 7.69–7.73 (m, 5H),
0.11 mmol) were resolved in the solution of EtOH and H2O 7.92–7.95 (m, 4H), 8.28 (s, 1H), 8.32 (s, 1H), 8.42 (d, 4H, J =
(20 mL, 1 : 1). The mixture was refluxed for 4 h under argon. 8.0 Hz). 13C NMR (CD3CN) δ 20.04, 42.81, 44.42, 45.82,
The solvent was removed by rotary evaporation and the residue 47.62, 58.37, 123.82, 123.92, 123.95, 124.83, 127.24, 128.14,
was dissolved in 5 mL of water, saturated NH4PF6 was added. 137.41, 150.16, 150.49, 151.02, 151.20, 151.34, 151.39, 156.00,
The resulting precipitate was filtered off, washed with water, 156.69, 156.75. TOF MS ES+ (m/z) 384.1470 [M − 2PF6−]2+, cal-
and dried in vacuum to give the red-orange solid. The solid culated. 384.1475; 457.1237 [M − PF6 + H+]2+, calculated.
−
was re-dissolved a minimum volume (2 mL) of methanol 457.1283.
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20 S. Kumar, V. Luxami and A. Kumar, Org. Lett., 2008, 10,
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of Beijing Municipal Commission of Education 23 M. G. Choi, S. Cha, H. Lee, H. L. Jeon and S. K. Chang,
(SQKM201210025006), and the Natural Science Foundation of
Capital Medical University in China (2010ZR06).
Chem. Commun., 2009, 7390.
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