A new rhodamine based colorimetric 'off-on' fluorescence sensor selective for Pd2+ along with the first bound X-ray crystal structure

Article abstract of DOI:10.1039/c1cc12845k

A new fluorescence rhodamine derivative bearing an 8-aminoquinoline moiety has been designed and synthesized for selective sensing of Pd²⁺ in the presence of other competing metal ions in aqueous media. Pd²⁺ induced spirolactam ring opening of rhodamine is confirmed for the first time by the X-ray crystal structure of the bound Pd²⁺-complex.

Full text of DOI:10.1039/c1cc12845k

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A new rhodamine based colorimetric ‘off–on’ fluorescence sensor selective
for Pd2+ along with the first bound X-ray crystal structurew
Shyamaprosad Goswami,*a Debabrata Sen,a Nirmal Kumar Das,a Hoong-Kun Fun*b and
Ching Kheng Quahb
Received 15th May 2011, Accepted 6th June 2011
DOI: 10.1039/c1cc12845k
A new fluorescence rhodamine derivative bearing an 8-amino-
quinoline moiety has been designed and synthesized for selective
sensing of Pd2+ in the presence of other competing metal ions in
aqueous media. Pd2+ induced spirolactam ring opening of
rhodamine is confirmed for the first time by the X-ray crystal
structure of the bound Pd2+-complex.
nondestructive in nature.9 Being a paramagnetic species, most
of the reported fluorescence sensors of Pd2+ readily undergo
fluorescence quenching.10 Unfortunately, there are only few
reports of fluorescence enhanced probes for Pd2+, which are
either reaction based11a-d or reversible11e–h in nature. In this
regard the rhodamine architecture, provides an ideal mode
for the construction of new ‘off–on’ type fluorescence enhanced
probes for metal ions which is based on spirolactam (colorless,
nonfluorescence) to ring-open amide (pink colored, fluorescence)
equilibrium.12
Development of colorimetric fluorescence receptors for monitoring
heavy and transition metal cations (HTM) of biological impor-
tance has received continuous attention because of their wide-
spread use in agriculture, chemical and industrial processes that
are causing serious threats to living organisms.1
Herein, we report a new rhodamine based colorimetric
‘off-on’ fluorescence probe containing an 8-aminoquinoline
moiety (Pd-Q1) that can selectively detect Pd2+ (0–25 mM)
over other examined metal ions studied in aqueous solution.
Though there is a number of literature reports of rhodamine
based palladium sensors, this is the first report of the X-ray
crystal structure of a rhodamine-Pd2+ complex.
Among the metal ions, palladium is an important platinum-
group element (PGEs) and is extensively being used in numerous
materials such as alloys, jewellery, dental crowns, fuel cells and
catalysts.2 Palladium(II) salts are predominantly used as oxidizing
reagents and pre-catalysts for many cross-coupling reactions.3
A large number of organic reactions, namely Buchwald–Hartwig,
Heck, Sonogashira and Suzuki–Miyaura are facilitated by
palladium catalysis,4 many of which involve carbon–carbon
bond formation and therefore have a significant role in medicinal
chemistry.5 However, the largest use of palladium today is in
catalytic converters of automobiles, that frequently release a
large quantity of palladium to the environment,6 that may
cause a serious health problem.7 Hence, considerable efforts
have been paid for the development of methods for selective
sensing of palladium. Conventional methods for its selective
detection include atomic absorption spectroscopy (AAS),
inductively coupled plasma atomic emission spectroscopy
(ICP-AES), solid-phase microextraction high-performance
liquid chromatography (SPME-HPLC) etc., all of which are
associated with high cost of instrumentation.8 Colorimetric
and fluorimetric methods for the quick detection of Pd2+ are
very popular in recent years because they are inexpensive and
The detailed synthetic methods for the preparation of the
chemosensors are shown in Scheme 1. Initially coupling of
8-aminoquinoline with N-tert-Boc protected b-alanine in the
presence of DCC gives 1 which on de-amidation of the N-tert-Boc
group affords 2. Finally the chemosensors are obtained by
condensation of 2 with rhodamine-6G in presence of Et3N in
ethanolic solution.
The recognition event of Pd-Q1 is based on the Pd2+
induced opening of the spirolactam ring which is associated
with a brilliant pink coloration and remarkable fluorescence
enhancement of the rhodamine dye (Scheme 2).
Interestingly, the introduction of a carboxamido group is of
extra advantage to the deprotonation of the 8-aminoquinoline
moiety after binding with the metal ion. The deprotonation
process strengthens the electron-donating ability from the
nitrogen atom of 8-amino group to the quinoline ring, and
a Department of Chemistry, Bengal Engineering and Science
University, Shibpur, Howrah 711103, West Bengal, India.
E-mail: spgoswamical@yahoo.com; Fax: +91-3326682916
b X-ray Crystallography Unit, School of Physics,
Universiti Sains Malaysia, 11800 USM, Penang, Malaysia.
E-mail: hkfun@usm.my; Fax: +604 6579150
w Electronic supplementary information (ESI) available. CCDC 824877.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c1cc12845k
Scheme 1 Synthesis of Pd-Q1 and Pd-N1.
c
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Chem. Commun., 2011, 47, 9101–9103 9101

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Scheme 2 Proposed Pd2+ sensing process.
Fig. 2 (a) Job plot diagram of Pd-Q1 for Pd2+ determined by UV-vis
method (Xh is the mole fraction of Pd-Q1 and DI indicates the
change of absorbance). (b) UV-vis absorption spectra of Pd-N1
thus electron transfer from the nitrogen atom of the hetero-
cycle to the metal ion, possibly further enhances the charge
transfer (CT) process. The X-ray crystal structure of the
Pd2+-complex also confirms the above and clearly reveals that
the complex prefers to exist in the enol form (Fig. 3). Although the
pink coloration is presumably due to spirolactam ring opening, the
role of the quinoline nitrogen seems to be very crucial. This
reasoning is proved when we compare the naphthalene analogue.
Only a very faint pink coloration and small CHEF effect are
(c = 2.0 Â 10À5 M) in presence of Pd2+
.
The X-ray crystal structure of the Pd2+-complex of Pd-Q1
shows the mode of binding of Pd-Q1 with PdCl2 (Fig. 3). The
complex crystallizes in the monoclinic crystal system in space
group C2/c from hot aqueous methanol (Tables 1 and 2,
ESIw). Pd2+ is coordinated by two nitrogen atoms of the
8-aminoquinoline moiety (Pd–N 2.003(10) and 2.038(11) A)
and one nitrogen atom of the 3-aminopropionamide moiety
(Pd–N 1.993(8) A), along with one chlorine atom (Pd–Cl
2.309(4) A) to furnish a distorted square planar geometry.
No pink coloration is observed initially when Pd2+ (5.0 equiv.)
is added gradually to Pd-N1 (c = 2.0 Â 10À5 M) solution
observed for Pd-N1 in the presence of excess Pd2+
.
The cation binding properties of the chemosensors are
studied by employing the chloride salts of K+, Na+, Mg2+
Mn2+, Ca2+, Zn2+, Cd2+, Hg2+, Pb2+, Ni2+, Co2+, Fe3+
,
,
Cu2+, Pd2+, Pt2+ and Ru3+ ions in EtOH–H2O (1 : 1, v/v,
25 1C) at pH 7.2 (50 mM HEPES buffer). Upon addition of
Pd2+ to a colorless solution of Pd-Q1, a low-energy strong
absorption band centered at 540 nm is observed. The intensity
of the band increases regularly as the amount of Pd2+ is added
progressively (up to 5.0 equiv.) (Fig. 1a) resulting in a brilliant
pink coloration of the solution accompanying the spirolactam
ring opening of the rhodamine dye (Scheme 2). Most importantly,
neither pink coloration nor large CHEF effect is observed
(S14, ESIw) in the presence of excess Pd(PPh3)4, which implies
that Pd-Q1 is inert towards Pd0 (Fig. 1b). This observation
is indeed helpful for the selective detection of Pd2+ in the
presence of Pd0.
(Fig. 2b). Hence, Pd-N1 is quite innocent towards Pd2+
.
However, a very faint pink coloration is noticed (Fig. 1b) only
on prolonged standing when excess Pd2+ (50.0 equiv.) is
added. These results suggest that for Pd-Q1, the aminoquinoline
moiety is effectively adjusted in terms of electron density and
coordinating properties which lead to better selectivity of Pd-Q1
toward Pd2+ over Pd-N1.
The acid–base titration experiment reveals that Pd-Q1 does
not undergo any significant fluorescence enhancement within
the pH range from 5–10 which suggests that the molecule
prefers the spirocyclic form. But in strong acidic conditions
(pH o 3) protonation causes the coloration due to opening of
the spirolactam ring (inset, Fig. 4). Thus Pd-Q1 may be
employed for the detection of Pd2+ in near-neutral pH range.
The fluorescence spectra of Pd-Q1 (ff = 0.021) are recorded
by excitation of the rhodamine fluorophore at 505 nm. Addition
of Pd2+ to the Pd-Q1 solution leads to a large fluorescence
enhancement (ff = 0.125) and the emission intensity increases
very sharply at 562 nm that is accompanied by spirolactam ring
opening of Pd-Q1 (Fig. 4).
The titration experiment fits nicely a 1 : 1 (host : guest)
binding model as suggested by the Job plot diagram (Fig. 2a)
and the association constant determined for the metal complex-
ation process is 4.17 Â 104 MÀ1 (error o 10%). The HRMS
(TOF-MS) spectrum of the Pd2+-complex shows peaks at
m/z 717.1981 and 752.1743 possibly for [Pd-Q1 + Pd2+ +
]
and [Pd-Q1 + Pd2+ + ClÀ]+ ions, respectively, which also
proves a single mononuclear complex between Pd-Q1 and Pd2+
(S8, ESIw). Thus we assume a four-coordinated square planar
Addition of other examined metal ions, even in excess
amount to Pd-Q1, causes no significant absorption and emission
changes. For Pt2+ and Ru3+, a slight increase in emission
intensity is observed. This is possibly because of their similar
chemical properties. Fig. 5(a) shows a comparative view of
emission intensity of the rhodamine fluorophore Pd-Q1 after
adding 5.0 equiv. of each of the guest cations.
geometry for Pd2+
.
Fig. 1 (a) UV-vis absorption titration spectra of Pd-Q1 (c = 2.0 Â
10À5 M) in the presence of Pd2+ (c = 2.0 Â 10À4 M) at pH 7.2.
(b) Color changes in the presence of (i) Pd-Q1 + PdCl2 (5.0 equiv.);
(ii) Pd-Q1 + Pd(OAc)2 (5.0 equiv.); (iii) Pd-Q1 + Pd(PPh3)4
(10.0 equiv.); (iv) Pd-N1 + PdCl2 (50.0 equiv.).
Fig. 3 View of the X-ray crystal structure of the Pd2+-complex of
Pd-Q1 (water and disordered PdCl2 are omitted for clarity).
c
9102 Chem. Commun., 2011, 47, 9101–9103
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indicating that S2À sequesters Pd2+ from the metal–ligand
complex, liberating free Pd-Q1 (S14, ESIw). Thus Pd-Q1 may
be classified as a reversible chemosensor for Pd2+
.
The authors thank CSIR [No. 01(2292)/09/ EMR-II], Govt.
of India for the financial support and Universiti Sains Malaysia
for the Research University Grant (No. 1001/PFIZIK).
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Fig. 4 Fluorescence titration spectra of Pd-Q1 (c = 2.0 Â 10À5 M) in
the presence of Pd2+ (c = 2.0 Â 10À4 M) at pH 7.2. Inset:
Fluorescence response of Pd-Q1 (c = 1.0 Â 10À5 M, lex = 505 nm)
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,
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Zn2+, Cd2+, Hg2+, Pb2+, Ni2+, Co2+, Fe3+, Cu2+, Pt2+
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,
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would have very little influence towards Pd-Q1 and do not
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.
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is observed, the fluorescence enhancement is less compared to
Pd-Q1 during addition of Pd2+ (S13, ESIw). This again proves
the 8-aminoquinoline mediated amplification of the fluorescence
signal of Pd-Q1. Hence, a smaller CHEF effect is observed for
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Fig. 6 Mn+-selectivity profile of Pd-Q1: (grey bars) change of emission
intensity of Pd-Q1 + 6 equiv. Mn+; (black bars) change of emission
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.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9101–9103 9103