Synthesis and characterization of a new dinuclear platinum(II) alkynyl complex with a ferrocene bridge and its interaction with silver ion

Article abstract of DOI:10.1016/j.jorganchem.2013.02.009

A new dinuclear alkynyl platinum(II) complex with a ferrocene (Fc) bridge (Pt(II)-Fc-Pt(II)), referred as Sensor 1(1), was synthesized and characterized. The photophysical, luminescent and cation-binding properties were studied by UV-Vis spectrophotometric, fluorometric, ¹H NMR titration methods, and the ESI-MS. Sensor 1 showed a significant luminescent quenching in the presence of silver ions (Ag⁺).

Full text of DOI:10.1016/j.jorganchem.2013.02.009

Journal of Organometallic Chemistry 732 (2013) 102e108
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Synthesis and characterization of a new dinuclear platinum(II) alkynyl complex
with a ferrocene bridge and its interaction with silver ion
*
**
Veikko Uahengo, Nana Zhou, Bi Xiong, Ping Cai , Kai Hu, Gongzhen Cheng
College of Chemistry and Molecular Sciences, Wuhan University, Luojiashan Street, Wuhan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new dinuclear alkynyl platinum(II) complex with a ferrocene (Fc) bridge (Pt(II)eFcePt(II)), referred as
Sensor 1(1), was synthesized and characterized. The photophysical, luminescent and cation-binding
properties were studied by UVeVis spectrophotometric, fluorometric, 1H NMR titration methods, and
the ESI-MS. Sensor 1 showed a significant luminescent quenching in the presence of silver ions (Agþ).
Ó 2013 Elsevier B.V. All rights reserved.
Received 24 September 2012
Received in revised form
30 January 2013
Accepted 12 February 2013
Keywords:
Alkynyl platinum(II) complex
Luminescent sensor
Silver ion
Chemosensing
1. Introduction
Therefore, more simple and affordable methods such as lumines-
cent techniques are highly desired, especially by downstream users
The spectroscopic and photophysical behavior of square-planar
platinum(II) alkynyl complexes has been extensively studied,
due to their intriguing spectroscopic and luminescence properties
[1e14]. A series of luminescent platinum(II) terpyridyl complexes
and their spectroscopic and luminescence properties, as well as
their aggregation [1,15e17], biological properties [2] and utilization
in cation sensing have been reported by Prof. V. W.-W. Yam and Dr.
[23e30]. In addition, luminescent probes are suitable for in vivo or
in vitro cellular imaging [31]
Accordingly, silver and its compounds are widely used in elec-
tronic and photographic film production industry as mentioned.
Moreover, silver compounds have been used as clean water disin-
fectants, even though they are more applied in medicinal tools such
as in biofilms, bandages and dental treatments, due to its ionic
form’s antimicrobial activity. Thus, most of the previous silver
related studies were primarily focused on its positive rather than its
negative impacts. However, with the advances in technologies
especially in analytical methods and tools, recent studies have
demonstrated that, in its ionic form silver is the second most toxic
metal of the heavy metals in biosystems, behind mercury only, as it
is consumed through bioaccumulations and natural streams.
Notably, silver contents of environmental samples have been
increasing on daily basis with the increasing use of silver com-
pounds in industry, medicine and technology. The current detec-
K. M.-C. Wong [18]. However, the utilization of the rich p-electron
of alkynyl in such systems in heavy metal ion sensing, such as Agþ,
is very rare in literature. Despite the widespread applications and
the broad prospects of silver in the electronic industry and photo-
graphic and imaging industry, more attention had suddenly been
paid to its negative environmental impacts, especially to organisms
[19e21]. This is due to technological and research advancement,
with new improved analytical tools, gradually able to exploit more
chemical behaviors of ions/elements. Many traditional detection
method approaches, such as atomic absorption spectroscopy, ICP
atomic emission spectroscopy, inductively coupled plasma-mass
spectroscopy and anodic stripping voltammetric methods have
been employed to measure trace amounts of silver ions [22], but
most of them are expensive and time-consuming in practice.
tion methods at
a trace quantity levels are comparatively
unfavorable due to their high costs and time-consuming. Therefore,
designs of chemosensors targeting Ag(I) ions with excellent
sensitivity and selectivity, simple instrumental implementation
and easy operation remain the highest in the priority list [32e38]
Considering the good spectroscopic and luminescence proper-
ties of platinum(II) complexes, in this communication, a new
dinuclear platinum(II) alkynyl complex with a ferrocene bridge was
designed and synthesized as shown in Scheme 1. With the
* Corresponding author. Tel./fax: þ86 27 68752701.
** Corresponding author.
E-mail addresses: caiping@whu.edu.cn, caipingwhu@163.com (P. Cai).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.009

V. Uahengo et al. / Journal of Organometallic Chemistry 732 (2013) 102e108
103
Scheme 1. The synthesis route of the Sensor 1.
exception of being a magnificent building block, ferrocene-bearing
systems have been useful for incorporating redox functions into
supramolecular complexes to bind and allow electrochemical
sensing of ions and neutral molecules by change in the oxidation
potential of Fe(III)/Fe(II) redox couples [39e41]. Additionally, such
assembly is well-known for providing interesting electronic and
optical properties [42e49]. The photophysical properties and ion-
binding properties of Sensor 1 with metal ions such as Naþ, Kþ,
Ca2þ, Ni2þ, Mg2þ, Co2þ, Cu2þ, Zn2þ, Cd2þ, Agþ and Hg2þ ions were
investigated.
0.86 (t, 9H,
J
¼
7.2 Hz). Elemental analysis calcd. (%) for
C68H131ClO3P2Pt: C 63.35, H 10.24; found: C 63.32, H 10.28.
2.3. Synthesis of compound 3
A reaction mixture of 1,10-dicarboxylferrocene (1.37 g, 5 mmol),
oxalyl dichloride (2.2 mL, 25 mmol), and 2 drops of pyridine in
50 mL dry CH2Cl2 was stirred for 4 h at room temperature, then
refluxed overnight under nitrogen. After removing the solvent and
excessive oxalyl dichloride under vacuum, the resulting solid was
redissolved in 50 mL dry CH2Cl2. To
a solution of 4-ethy-
2. Experimental
nylbenzenamine (1.40g, 12 mmol) in 30 mL dry CH2Cl2 with 5 mL
dry triethylamine, was added dropwise to the above solution at
room temperature under nitrogen. After addition, the reaction
mixture was stirred at room temperature for further 4 h, then
washed with water and dried in MgSO4. The product was purified
by column chromatography (silica gel) in ethyl acetate mobile
phase. Red crystal was obtained, 1.17 g, 50%. Elemental analysis
calcd. (%) for C28H18FeO2N2: C 71.51, H 3.86, N 5.96; found: C 71.49,
H 3.88, N 5.94. 1H NMR (in CDCl3): 8.66 (br, 2H, NH), 7.68 (d, 4H,
J ¼ 8.0 Hz, AreH), 7.37 (d, 4H, J ¼ 8.0 Hz, AreH), 4.51 (t, 4H,
J ¼ 1.6 Hz, CpeH), 4.36 (t, 4H, J ¼ 1.6 Hz, CpeH), 4.08 (s, 2H).
2.1. Materials and methods
5-Ethynyl-1,2,3-tris(hexadecyloxy)benzene (Compound 1) was
synthesized according to the literature procedure [50].
All other chemicals were of reagent grade quality obtained from
commercial sources and used without further purification.
Elemental analyses (C, H and N) were carried out on a Perkine
Elmer 240C analytical instrument. Electrospray mass spectra were
carried out on an LCQ system (Finnigan MAT, USA) using methanol
as mobile phase. Electronic absorption spectra were obtained at
room temperature on a Shimadzu 3100 spectrophotometer in
CHCl3:MeOH (9:1) solution. Luminescent spectra were measured
on an Edinburgh Instruments analyzer model FL920 in
CHCl3:MeOH (9:1) solution.
2.4. Synthesis of Sensor 1
A reaction mixture of compound 2 (25 mg, 0.05 mmol), com-
pound 3 (119 mg, 0.1 mmol) with a small amount of CuI as a catalyst
in 10 mL THF and 10 mL diethylamine was stirred at room tem-
perature for 24 h. After evaporating most of the solvent, the
resulting crude was extracted with dichloromethane, the organic
phase was washed with brine and dried with Na2SO4. The product
was purified by column chromatography (silica gel). The impurity
co-product was removed with dichloromethane, the pure product
obtained by dichloromethane with 5% triethylamine. Yellow solid,
75 mg, 50%. Elemental analysis calcd. (%) for C166H278FeO8N2P4Pt2:
C 66.46, H 9.34, N 0.93; found: C 66.46, H 9.33, N 0.92. 1H NMR (in
CDCl3): 8.69 (br, 2H, NH), 7.64 (d, 4H, J ¼ 8.0 Hz, AreH), 7.27 (d, 4H,
J ¼ 8.0 Hz, Ar-H), 6.46 (s, 4H, AreH), 4.59 (t, 4H, J ¼ 1.6 Hz,
Cp-H), 4.43 (t, 4H, J ¼ 1.6 Hz, Cp-H), 3.87e3.93 (m, 12H, OCH2),
2.2. Synthesis of compound 2
[HeC^Ce(C6H2)e(OC16H33)3] (360 mg, 0.44 mmol) was added
to a degassed solution of cis-Pt(PEt3)2Cl2(250 mg, 0.47 mmol) in the
mixed solvents including THF (5 mL), dichloromethane (5 mL) and
diethyamine (5 mL). The reaction mixture was stirred at room
temperature for 24 h under argon. After removing the solvent, the
crude solid was purified by column chromatography on Silica gel/
dichloromethane to give a white solid; 0.41 g, yield 72%. 1H NMR
(600 MHz, CDCl3):
(m, 12H), 1.68e1.78 (m, 6H), 1.38e1.45 (m, 6H), 1.10e1.35 (m, 90H),
d
6.42 (s, 2H), 3.90 (q, 6H, J ¼ 6.4 Hz), 1.95e2.07

104
V. Uahengo et al. / Journal of Organometallic Chemistry 732 (2013) 102e108
2.12e2.20 (m, 24H, PCH2), 1.68e1.77 (m, 12H), 1.40e1.45 (m,
12H),1.17e1.34 (m, 180H), 0.86 (t, 18H, J ¼ 6.4 Hz, CH3).
complexes have indicated that their characteristic low-energy
bands in the absorption spectra are perturbed by complexation [40].
In addition, both the size and chemical properties of Agþ toward
Sensor 1 have played a major role in its selectivity. Meanwhile, the
3. Results and discussion
addition of the other cations, such as Naþ, Kþ, Ca2þ, Ni2þ, Mg2þ
,
Co2þ, Cu2þ, Zn2þ and Cd2þ did not yield any substantial changes,
except the slight response for Hg2þ (Fig. 2) which is perhaps due to
closer chemical links to Agþ.
3.1. UVevis absorption spectra
The UVevis spectrum of Sensor 1 in CHCl3:MeOH (9:1) solution
is characterized by intense high-energy absorption bands at
wavelengths below 350 nm with extinction coefficients in the order
of105 dm3,molꢀ1,cmꢀ1, and a moderate to low intense absorption
bands at 367 and 430 nm respectively (Fig. 1). The high-energy
intense absorption bands are assigned to an admixture of intra-
3.3. Photoluminescent studies
Upon photoexcitation at 395 nm, Sensor 1 displayed intense
emission characterized by several emission peaks (412 nm, 437 nm
and 463 nm) with maximum at 437 nm. Vibronic emission bands
ligand (IL)
), Ar ¼ aromatic [1e14]. The moderate to low-energy absorption
bands at approximately 367e430 nm are assigned as [d (Pt) /
*(eC^CeAr)] metal-to-ligand charge transfer (MLCT) transi-
tions mixed with predominant * intraligand charge transfer
p / p* transitions of the two alkynyl ligands (AreC^Ce
are signals of multiplets originating from different p-electron sys-
tems within the structure of Sensor 1. Fig. 3a shows luminescent
quenching upon the addition of Agþ. The luminescence intensities
of Sensor 1 at all four bands (412 nm, 437 nm and 463 nm) were
linearly reduced (Fig. 3b) with the increasing concentration of Agþ.
The intensity was quenched more than 70% when the concentra-
tion of Agþ reached two equivalents. This may be due to the for-
mation of a ground-state non-luminescent complex of AgeSensor
1, attributed to energy or electron transfer resulting in
luminescence quenching [62].
p
p
p
/ p
(ILCT) transitions [51e57]. In addition, it is well established that
ferrocenyl moieties exhibit two charge transfer bands in the UVevis
region, with a lower energy band assigned to a donor-acceptor
charge transfer transitions and the higher lying absorption as an
intraligand
of ferrocenyl moiety has supposedly overlapped with the intra-
ligand (IL) * transitions of aryl alkynyl, the low lying ab-
p / p* transition [49,54]. While the p / p* transitions
p
/ p
3.4. 1H NMR and ESI-MS studies
sorption band at 430 nm is believed to emanate from the donor-
acceptor charge transfer of the ferrocene moiety with slight MLCT
The 1H NMR experiment was performed to further understand
and comprehend the spectroscopic analysis of Sensor 1 interaction
with Agþ. Sensor 1 was titrated with Agþ under 1H NMR in molar
ratio to verify the binding activity of AgeSensor 1, and their peaks in
CHCl3:MeOH(9:1) aredisplayedinFig. 4. Therespectiveprotonpeaks
are clearly assigned as shown and their changes upon the addition of
Agþ are displayed as well. Upon the addition of Agþ, significant
changes of 1H NMR peaks are observed in theregionsof 2.0e2.7 ppm,
4.6 ppm, 6.4 ppm, 7.0e7.8 ppm and 8.5e9.4 ppm, respectively. The
peak in the region of 8.5e9.4 ppm is initially assigned to eNH proton
of the amino moiety of Sensor 1. After the addition of little amounts
(1e2 equiv.) of Agþ, the eNH band of Sensor 1 broadened and dis-
appeared, which suggested that the first binding activities should
happen between the Agþ and nitrogen atoms, resulting in Age
[d
p(Pt) / p*(eC^CeAr)] characteristics, resulting in a slight blue
shift [51].
3.2. Cation-binding study
The cation-binding studies of square-planar Pt(II) alkynyl com-
plexes is limited in literature, except for alkali and alkaline-earth
metals [58e61]. In this work, cation-binding properties of metal
nitrate salts such as Naþ, Kþ, Ca2þ, Ni2þ, Mg2þ, Co2þ, Cu2þ, Zn2þ
,
Cd2þ, Agþ and Hg2þ in CHCl3:MeOH (9:1) and the luminescent
studies for the binding metal were investigated. Fig. 1 shows the
absorption spectra of Sensor 1 upon the addition of Agþ (aeh)
in molar ratio, where a slight red shift shoulder of 28 nm (367e
395 nm) was observed. Previous studies on ferrocene-based
Sensor
1 complexation. At the same time, changes were
simultaneously observed around 4.62 (Fc) and 7.29 (Ar) ppm,
assigned to ferrocene and aromatic protons (Fig. 4), respectively,
which broadened with the addition of Agþ, the action ascribed to
space effect for being too close in proximity to the bonding field.
Fig. 1. UVevis spectra of Sensor 1 (1.0 ꢁ 10ꢀ5 M) in CDCl3:MeOH (9:1) in the presence
Fig. 2. Absorption changes at 395 nm of the Sensor 1 (10 mM) in the presence of
10 equiv. of different metal ions.
of different amounts of Agþ
.

V. Uahengo et al. / Journal of Organometallic Chemistry 732 (2013) 102e108
105
Fig. 3. (a) Luminescent response of Sensor 1 (10ꢀ5 M) in CHCl3:MeOH (9:1) upon addition of Agþ at lex ¼ 395 nm, and (b) Job’s plot of Sensor 1 and Agþin CHCl3:MeOH (9:1, v/v).
Furthermore, careful observation and analysis suggested that a
two successive step-action was experienced by the sensor as the
amount of Agþ increased. Gradually, as the addition of Agþ
increased to certain amounts (3e5 equiv.), more changes were
experienced at 2.18 (PeCH2), 4.62 (Fc), 7.29 (Ar) and 7.65 (Ar) ppm,
where peaks accordingly shifted to 2.56 (PeCH2, downfield), 5.19
(Fc, downfield), 6.94 (Ar, upfield) and 7.53 (Ar, upfield) ppm,
respectively. And the initially disappeared amino moiety (eNH)
protons were observed again. All of these changes are suggestive
to different binding modes after the addition of more than 2 equiv.
protons shifts at 2.176 (PeCH2) and 7.647 (Ar) ppm, the same
protons experienced secondary changes (upfield or downfield
shifts) which are completely different in nature to the initial effects,
the action attributable to the second mode of interaction. More
evidence to the secondary activities can be pronounced through the
late upfield shift of the protons at 2.176 (PeCH2), which in practice
are distant from the coordination area, thereby resulting in a
slightly tilted structure. The late shifts of the PR3 protons are
strongly evident to
p / p stacking bonding, as they lay in the midst
of the rich
p-electron alkynyl units. However, the stabilities of
Agþ [63,64]. Considering the rich
p-electron in Sensor 1 and the
Sensor 1 and Sensor 1eAgþ have to be determined.
coordination between the Agþ and alkynyl group, the second
changes of proton shifts may be reasonable. This can also explain
why (at first) peaks at 8.657 (NH), 4.622 (Fc) and 7.289 (Ar) ppm
experienced intensity decrease (only) upon gradual addition of
minimum amount Agþ, presumably attributed to being closer to
the coordination (N and O donor atoms) area, while in addition to
In order to further understand Sensor 1 and its interaction with
Agþ, mass spectrometry technology (ESI-MS) was used to provide
more information as shown in Figs. 5 and 6, respectively. The ESI-
MS results of Sensor 1 indicates that the structure is slightly un-
stable due to the mesh of fragments obtained as shown (Fig. 5), this
can be ascribed to its large molecular area and structure. Even
Fig. 4. 1H NMR spectra of Sensor 1 upon addition of Agþ (0e5 equiv.) in CDCl3:CD3OD (9:1).

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V. Uahengo et al. / Journal of Organometallic Chemistry 732 (2013) 102e108
Fig. 5. ESI-MS spectra of Sensor 1 upon addition of Agþ (5 equiv.) in CH3Cl:CH3OH (9:1).
Fig. 6. ESI-MS spectra of Sensor 1 with Agþ (5 equiv.) in CH3Cl:CH3OH (9:1).

V. Uahengo et al. / Journal of Organometallic Chemistry 732 (2013) 102e108
107
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though, it has not been possible to identify all the peaks, it is
obvious most of the detected fragments could very well originate
from Sensor 1. For example, a number of peaks could be identified,
m/z 1270.0 belongs to a platinum(II) fragment (AreC^CePteH2O),
m/z 821.0 could possibly come from the cycloxybenzyl fragment,
while most of the peaks in the range of m/z 1280e1450 results from
the charged ion of Sensor 1([Sensor 1], 1488.95 12þ, 992.6313þ
744.47, 14þ etc.) and fragments of [AreC^CePteFceAreC^CeR1,
R1 ¼ CH3]. There is also strong evidence that the PR3(R ¼ C6H15
,
)
ligands coordinated to Pt(II) might have been removed (in some
cases) in the process, resulting in several possible fragments such as
m/z (1þ) 1430.42, 1388.5, 1370.87,1251.78, etc. In addition, after
losing the cycloxybenzyl fragment (m/z 820), the remaining frag-
ments at m/z 1369.13, 1332.10, 1298.71, 1161.95, 1052.16,968.06
(etc.), as a result of [RPteFcePtR, R ranges from PR3, CH3OH, CH3Cl,
H2O or combination, n ¼ 1 /4], were observed. The principal peak
at m/z 1326.9 is strongly associated to the RePteFcePteR frag-
ments. Additional information on ESI-MS can be found on the
electronic supplementary information (ESI) addendum.
Interestingly, the addition of Agþ [Sensor 1eAgþ] resulted in
fewer peaks (Fig. 6) than Sensor 1 (Fig. 5) over the same range. The
majority of peaks subsequently disappeared in the presence of Agþ,
especially those in the range of m/z 1000e1500, which are associ-
ated with RPteFcePtR, except the peak at m/z 1270 (AreC^CePte
H2O), which was still visible and suggestive that the chemical
bonds between Pt and eFceAreC^Ce were broken. Accordingly,
m/z at 343.1 was assigned to the [AgeC^CeAreFceAreC^Ce
Ag]2þ. The difference between the ESI-MS results of Sensor 1 and
it is the complex (Sensor 1eAgþ) provides another piece of evi-
dence that the presence of Agþ has a significant effect to the Sensor
1. The existing ESI-MS results and the 1H NMR data are all in good
agreement. Therefore it can be concluded that the Sensor 1 can
form an adduct with Agþ in the presence of a small amount of silver
ions. However, in the presence of excessive amount of silver ions,
Sensor 1 may display different behaviors.
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The new dinuclear alkynyl platinum(II) complex was success-
fully synthesized and characterized. Like all other alkynyl plati-
num(II) complexes, it showed excellent photophysical and
luminescent properties. In addition, Sensor 1 has a unique property
of discriminating Agþ through luminescent quenching among other
cations. Only a handful of platinum(II) based alkynyl complexes
have been reported so far capable of cation sensing, which are all
based on alkali and alkaline-earth metal ions. Sensor 1 can be a
good contribution to opening a new dimension toward soft heavy
metal ion sensing.
Acknowledgments
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This work was supported by the National Natural Science
Foundation of China (No. 21101121), the Natural Science Fund
(2010CDB01301) of Hubei Province and Dalian University of Tech-
nology State Key Laboratory of Fine Chemicals Fund (KF0912).
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