A highly selective sulfur-free iridium(iii)-complex-based phosphorescent chemidosimeter for detection of mercury(ii) ions

Article abstract of DOI:10.1039/c2dt12120d

A neutral phosphorescent coordination compound bearing a benzimidazole ligand, Ir(pbi)₂(acac) (Hpbi = 1,2-diphenyl-1H-benzo[d]imidazole; Hacac = acetylacetone), is demonstrated to be the first example of a sulfur-free iridium complex for the de

Full text of DOI:10.1039/c2dt12120d

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PAPER
A highly selective sulfur-free iridium(III)-complex-based phosphorescent
chemidosimeter for detection of mercury(II) ions†
Hui Zeng,a Fang Yu,a Jun Dai,a Huiqin Sun,c Zhiyun Lu,*a,b Ming Li,a Qing Jianga and Yan Huang*a
Received 5th November 2011, Accepted 7th February 2012
DOI: 10.1039/c2dt12120d
A neutral phosphorescent coordination compound bearing a benzimidazole ligand, Ir(pbi)2(acac)
(Hpbi = 1,2-diphenyl-1H-benzo[d]imidazole; Hacac = acetylacetone), is demonstrated to be the first
example of a sulfur-free iridium complex for the detection of Hg2+ cations with high selectivity and
sensitivity. Ir(pbi)2(acac) shows a multisignaling response towards mercury(II) ions through UV-vis
absorption, phosphorescence and electrochemistry measurements. Upon addition of Hg2+ ions, solutions
of this complex change from yellow to colorless, which could be observed easily by the naked eye, while
its phosphorescence turns from bright green (λPLmax = 520 nm) into faint skyblue (λPLmax = 476 nm), and
the detection limit is calculated to be 2.4 × 10−7 mol L−1. 1H NMR spectroscopic titration as well as
ESI-MS results indicate that the decomposition of Ir(pbi)2(acac) in the presence of Hg2+ through rupture
of Ir–O bonds is responsible for the significant variations in both optical and electrochemical signals.
fluorescent counterparts, and hence afford advantages like easy
identification of emission spectra from excitation ones, and
Introduction
Mercury is considered to be one of the most hazardous pollu-
tants, for the reason that mercury and its derivatives possess high
affinity to thiol groups in organisms, resulting in the dysfunction
output signals from their fluorescent background in aid of tem-
poral resolution techniques as well.7 In comparison with Pt(II),8
Ru(II)9 and Tb(III)10 complexes, Ir(III)-based coordination com-
of cells which would cause health problems.1 Mercury contami-
nation has pervaded not only in natural water, but also in our
food chain due to bioaccumulation, yet the widely utilized Hg2+-
monitoring methods such as atomic absorption–emission spec-
troscopy and inductively coupled plasma mass spectrometry
demand sophisticated analysis program and sample preparation
along with costly instruments.2 As a result, considerable efforts
have been made to develop more convenient optical (such as
chromogenic3 and fluorogenic4) probes specific for detecting
mercury. Nevertheless, although fluorogenic sensors are regarded
to be quite promising because of their distinct advantages in
high sensitivity and on-line imaging,5 the luminescence quench-
ing caused by heavy metal ions has brought great challenges for
developing fluorescent Hg(II) chemosensors.6 Recently, phos-
phorogenic mercury(II) probes have attracted considerable atten-
tion because of their negligible photoluminescence quenching
toward heavy metal ions due to their triplet luminescence nature.
Moreover, they possess rich photophysical properties such as
larger Stoke’s shifts and longer lifetimes compared to their
pounds11 showing specific response to Hg2+ cations have
aroused more interest due to their relatively high luminescence
efficiency at room temperature and good color diversity, and the
chelate Ir(btp)2(acac) was demonstrated by Li et al. to be the first
example of a neutral iridium complex as a highly selective phos-
phorescent chemosensor for Hg2+ ions.11a Furthermore, as Ir-
(btp)2(acac) bears a benzothiophene moiety in its cyclometallate
ligand, the mercury-probing mechanism for this compound is
tentatively attributed to the specific coordinating interaction
between the sulfur atom and Hg2+ in terms of Pearson’s hard–
soft acid–base theory,12 implying that the presence of sulfur
atoms in the complexes should be an indispensable precondition
for mercury-detection. Based on this mechanism, Li et al. sub-
sequently developed two more iridium chelates11b,11e containing
thiophene or benzothiazole segments as Hg(II) probes, and until
now, all the reported Hg2+-responding neutral Ir(III) complexes
possess thia-heterocycles in their cyclometallate ligands without
exception, and the design of Ir(III) complex-based mercury che-
mosensors is still limited to the sulfur-containing compounds.
Herein, we present our discovery that a sulfur-free benzimida-
zole-ligated Ir(III) complex, bis(1,2-diphenyl-1H-benzimidazo-
lato-N,C2′)iridium(III) (acetylacetonate) [Ir(pbi)2(acac)], also
possesses high specificity and sensitivity towards Hg2+ ions,
suggesting that the step of coordination between Hg2+ ions and
sulfur atoms through lone pair electrons is not absolutely necess-
ary in the probing mechanism. Similar to Ir(btp)2(acac), Ir-
(pbi)2(acac) shows a specific and sensitive multisignaling
aCollege of Chemistry, Sichuan University, Chengdu, 610064,
P. R. China. E-mail: luzhiyun@scu.edu.cn, huangyan@scu.edu.cn;
Fax: +86-28-85410059; Tel: +86-28-85410059
bKey Laboratory of Green Chemistry and Technology, Minister of
Education, Sichuan University, Chengdu, 610064, P. R. China
cAnalytical and Testing Center, Sichuan University, Chengdu, 610064,
P. R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt12120d
optical–electrochemical response toward Hg(II) cations.
A
4878 | Dalton Trans., 2012, 41, 4878–4883
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Scheme 1 Chemical structures of Hpbi and Ir(pbi)2(acac).
possible mechanism for the probing procedure is proposed to be
Hg(II)-induced decomposition of the iridium complex by the
1
rupture of coordinated Ir–O bonds according to H NMR titra-
tion and ESI-MS results. The molecular structures of Hpbi and
Ir(pbi)2(acac) are shown in Scheme 1.
Fig. 1 Changes in the UV-vis absorption spectrum of Ir(pbi)2(acac)
(20 μM) in acetonitrile solution upon titration of Hg2+ (0–24 μM). Inset:
titration curve of Ir(pbi)2(acac) with Hg2+ (absorbance ratio at 411 nm
and 364 nm).
Materials and methods
All the chemicals involved in the synthetic procedure such as
acetylacetone, benzoyl chloride, 2-nitro-N-phenylaniline,
IrCl3·nH2O, anhydrous zinc chloride and iron powder were com-
mercially available and used without further purification. All the
solvents were of analytical grades and freshly distilled prior to
emission measurements, excitation was provided at 384 nm.
Cyclic voltammetry (CV) measurements were carried out on a
PARSTAT 2273 electrochemical workstation at 298 K in an
argon-purged 50 μM anhydrous acetonitrile solution of Ir-
(pbi)2(acac) with tetrabutylammonium perchlorate (0.1 M) as
the supporting electrolyte at a scanning rate of 50 mV s−1. The
CV system was constructed using a platinum plate, a Ag/AgNO3
(0.1 M in acetonitrile) electrode and a platinum wire as the
working electrode, quasi-reference electrode and counter elec-
trode, respectively.
1
use. H NMR spectra were recorded on a Bruker AVANCE-400
spectrometer in CDCl3 and CD3CN using TMS as internal stan-
dard. High resolution MS spectra were obtained from a Q-TOF
Premier ESI mass spectrometer (Micromass, Manchester, UK)
and a micrOTOF-Q II mass spectrometer (Bruker Daltonic Inc.).
The high performance liquid chromatography (HPLC) was
recorded on an Agilent 1100. The FT-IR spectrum was per-
formed on a NEXUS-670 spectrometer with KBr pellets. The
cyclometallate ligand (Hpbi) as well as the objective complex
were prepared with a similar synthetic procedure to that which
we reported previously13 (vide ESI†).
Results and discussion
Electronic absorption properties
Metal cation titration of Ir(pbi)2(acac)
In the UV-vis region at 300–520 nm, Ir(pbi)2(acac) exhibits two
distinguishable absorption bands in acetonitrile solution (shown
in Fig. 1). The intense one at 300–400 nm with a logε of
A acetonitrile stock solution of 1 mM Ir(pbi)2(acac) was pre-
pared, then diluted to 20 μM for spectrophotometric titration
experiments, or 50 μM for electrochemical titration experiments.
2.5 mM stock aqueous solutions of mercury(II) perchlorate trihy-
drate and other metal sulphates (Ag+, Cd2+, Co2+, Cu2+, Mg2+,
Ni2+, Pb2+, Zn2+, Ba2+, Mn2+, K+, Na+, Ca2+, Fe2+, Cr2+) were
prepared freshly before use. All the titration experiments were
carried out in acetonitrile solutions of Ir(pbi)2(acac). Typically,
aliquots of cations were added through a micropipet, and the
UV-vis absorption, photoluminescence (PL) spectra, and cyclic
voltammograms were recorded after 1 min stirring.
1
3.8–4.5 is generally assigned to the spin-allowed (π–π*) tran-
sition due to contributions from the cyclometallate ligands,14
while the weaker low energy feature at 400–520 nm is generally
ascribed to the metal to ligand charge-transfer (1MLCT and
3MLCT) and the spin–orbit coupling enhanced 3(π–π*) tran-
sition.15 Upon addition of Hg2+ cations, the absorbance of the
MLCT transition band at 400–500 nm is weakened gradually;
simultaneously, the absorption intensity of the π–π* transition
band at 310–380 nm increases progressively, resulting in two
isosbestic points at 348 and 317 nm. With the addition of 1 eq.
of Hg2+ ions, the absorption band in the visible region
(400–520 nm) also disappears, leading to a color change from
yellowish green to colorless (vide Fig. 2) with a fairly fast
response within several seconds. Thus Ir(pbi)2(acac) could
serve as a sensitive “naked-eye” indicator for Hg2+. The titration
results of Ir(pbi)2(acac) with Hg2+ according to the variation of
Multisignaling Hg2+ sensing experiments
The UV-vis absorption and PL spectra of 20 μM CH3CN sol-
ution of Ir(pbi)2(acac) in the absence and presence of different
metal cations were recorded from 2.5 mL of 20 μM acetonitrile
solutions in a 1 cm optical path length quartz cuvette, using a
Hitachi U-4100 spectrophotometer and HORIBA Jobin Yvon
Fluoromax-4 spectrophotometer at 298 K respectively. For PL
A
411 nm/A364 nm with the molar fraction of Hg2+ indicate that the
binding of Ir(pbi)2(acac) with Hg2+ shows a 1 : 1 stoichiometry.
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Fig. 2 Photographs of Ir(pbi)2(acac) in acetonitrile solution (500 μM)
in the presence of different metal cations (1.0 eq.): (a) color; (b) phos-
phorescence. 1, blank; 2, Hg(II); 3, Na(I); 4, K(I); 5, Ag(I); 6, Ca(II); 7,
Ba(II); 8, Ni(II); 9, Cu(II); 10, Cd(II); 11, Co(II); 12, Cr(II); 13, Fe(II); 14,
Pb(II); 15, Mn(II); 16, Zn(II); 17, Mg(II).
Fig. 4 Changes in the cyclic voltammogram of Ir(pbi)2(acac)
(50 μM) in acetonitrile solution with various amounts of Hg2+ ions
(0–80 μM). Inset: electrochemical titration of Ir(pbi)2(acac) with Hg2+
(current intensity ratio at 1.02 Vand 0.44 V).
which is consistent with the result determined from the electronic
absorption studies. In air-equilibrated acetonitrile solution, Ir-
(pbi)2(acac) shows a relatively high ΦPL of 0.106 (determined
by using 10 μM deaerated toluene solution of fac-Ir(ppy)3 as a
standard under a λex of 400 nm, ΦPL = 0.4).16 Additionally, a
large extinction coefficient (logε = 3.9) could be found at the
absorption points of 384 nm. Therefore Ir(pbi)2(acac) may
afford widely applied feasibility and an increase in the sensitivity
for detection of mercury(II),10 and the detection limit determined
from the fluorescence titration17 is calculated to be 2.4 × 10−7
M. Moreover, for Ir(pbi)2(acac)/Hg2+(1 eq.) solution, the
addition of the well-known cation chelator, ethylene diamine
tetraacetic acid (EDTA), could not recover the phosphorescence
property of Ir(pbi)2(acac), and the PL emission spectra in the
absence and presence of EDTA are identical (vide Fig. S1 in
ESI†), indicating that the interaction between Ir(pbi)2(acac) and
Hg2+ is irreversible,18 and consequently Ir(pbi)2(acac) should
act as a chemodosimetric probe towards Hg2+.
Fig. 3 Changes in the luminescence spectrum of Ir(pbi)2(acac)
(20 μM) in air-equilibrated acetonitrile solution upon titration of Hg2+
(0–24 μM) (λex = 384 nm). Inset: (a) titration curve of Ir(pbi)2(acac)
with Hg2+ (intensity ratio at 520 nm and 476 nm); (b) Job’s plot (total
concentration of Ir(pbi)2(acac) and Hg2+ ions is 20 μM).
The logε of the bands at 364 nm and 411 nm after addition of
1.0 eq. of Hg2+ are determined to be 4 and 3.5 respectively.
Photoluminescence properties
Electrochemical measurements
The luminescence spectra of Ir(pbi)2(acac) in air-equilibrated
acetonitrile solution in the absence and presence of Hg(II) ions
are shown in Fig. 3. Upon excitation at 384 nm, Ir(pbi)2(acac)
displays green phosphorescence at room temperature with a
λPLmax of 520 nm, which could be assigned to the decay of both
To evaluate whether Ir(pbi)2(acac) may act as an electrochemi-
cal Hg2+ sensor, the cyclic voltammetric behavior of Ir-
(pbi)2(acac) in the presence of mercury(II) is investigated
(depicted in Fig. 4). During the anodic scan, Ir(pbi)2(acac)
shows a reversible one-electron oxidation wave at 0.44 V, which
is mainly attributed to the Ir(III/IV) metal center.19 Upon addition
of Hg2+, the intensity of the oxidation wave at 0.44 V decreases
gradually; simultaneously a more positive and intense reversible
oxidation peak located at 1.02 V emerges. After addition of
1.0 eq. of Hg2+, no further variation of the oxidation waves
could be observed, and the titration experiment result (I1.02
3
3MLCT and ligand-based (π–π*) excited states.13 With addition
of Hg2+, the intensity of the emission band peaked at 520 nm
decreases gradually, while a new emission band emerges with its
λPLmax located at 476 nm. As a result, an isoemissive point at
486 nm could be found, and obvious luminescence variation
from bright green to faint skyblue could be observed (vide
Fig. 2). Both titration experiment results (I520 nm/I476
vs.
V/I0.44
vs. Hg2+) suggests a 1 : 1 binding stoichiometry
nm
Hg2+) and Job’s plot analysis (shown in Fig. 3, inset) suggest a
1 : 1 binding stoichiometry between Ir(pbi)2(acac) and Hg2+,
V
between Ir(pbi)2(acac) and Hg2+, which is in line with the
optical titration results.
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Fig. 5 Absorption response of Ir(pbi)2(acac) (20 μM) in the presence
of various metal cations (1 eq.) in acetonitrile solution. Bars represent
the ratio (A411 nm/A364 nm) of absorbance intensity at 411 and 364 nm
after addition of 1.0 eq. Hg(II) to the above solution. 1, blank; 2, Hg(II);
3, Na(I); 4, Zn(II); 5, Ag(I); 6, Ba(II); 7, Cd(II); 8, Co(II); 9, Cu(II); 10,
K(I); 11, Mg(II); 12, Pb(II); 13, Ni(II); 14, Mn(II); 15, Ca(II); 16, Cr(II);
17, Fe(II).
Fig. 6 Luminescence response of Ir(pbi)2(acac) (20 μM) in the pres-
ence of various metal cations (1.0 eq.) in acetonitrile solution. Bars rep-
resent the ratio (I520 nm/I476 nm) of luminescence intensity at 520 and
476 nm after addition of 1.0 eq. Hg(II) to the above solution. 1, blank; 2,
Hg(II); 3, Na(I); 4, Zn(II); 5, Ag(I); 6, Ba(II); 7, Cd(II); 8, Co(II); 9, Cu(II);
10, K(I); 11, Mg(II); 12, Pb(II); 13, Ni(II); 14, Mn(II); 15, Ca(II); 16,
Cr(II); 17, Fe(II).
Probing mechanism of Ir(pbi)2(acac) towards Hg2+ ions
Selective cation-binding studies
It is widely accepted that the presence of a soft donor like sulfur
atoms in the molecule of chemosensors is generally a favorable
factor for a Hg(II)-induced optical response,20 because as a soft
acid, Hg2+ ions may preferentially interact with S atoms through
coordinational bonding. Accordingly, Li et al. proposed a similar
sensing mechanism for the four phosphorogenic Hg2+-probing
complexes they reported so far, namely, Ir(btp)2(acac),11a Ir-
(thq)2(acac),11b Ir(thq)2(dbm)11d and Ir(bt)2(acac),11e in which
they claim that the coordinational binding of mercury(II) with the
lone-pair electrons of sulfur atoms is the common key procedure.
However, in the case of Ir(pbi)2(acac), there exists no sulfur
atom in the ligands, and not even a heteroatom capable of pro-
viding binding sites with Hg2+ ions after Hpbi has coordinated
with Ir(III) through its N atoms, since the lone-pair electron of
the other nitrogen atom should participate predominantly in the
benzimidazole conjugation system. Thus it is very important to
elucidate the reason why Ir(pbi)2(acac) shows high specificity
toward Hg2+ ions, so that further high performance probing
materials could be constructed.
As high selectivity is a key factor for efficient probes, the cation-
binding capability of Ir(pbi)2(acac) toward different interfering
ions, such as Ag+, Cd2+, Co2+, Cu2+, Mg2+, Ni2+, Pb2+, Zn2+,
Ba2+, Mn2+, K+, Na+, Fe2+, Cr2+ and Ca2+ from main group,
transition or heavy metal elements, are also investigated through
absorption, PL spectrophotometric methods and electrochemical
measurements. As shown in Fig. 2, only the addition of Hg2+
would induce a significant change in color of Ir(pbi)2(acac) sol-
ution, while the existence of an excess of other cations would
lead to little spectral alteration (vide Fig. S2, ESI†), confirming
that Ir(pbi)2(acac) exhibits high selectivity toward Hg2+ ions. In
order to exclude the background interference from the other
coexisting metal ions, competition experiments were carried out
by adding Hg2+ to solutions of Ir(pbi)2(acac) in the presence of
1.0 eq. of Ag+, Cd2+, Co2+, Cu2+, Mg2+, Ni2+, Pb2+, Zn2+, Ba2+,
Mn2+, K+, Na+, Fe2+, Cr2+ or Ca2+. As shown in Fig. 5, the
existence of all these interfering cations leads to hardly any
change in A411 nm/A364
for the Ir(pbi)2(acac) solution.
nm
However, the subsequently added Hg2+ ions result in remarkably
changed absorption spectra, implying Ir(pbi)2(acac) could be
used as a highly selective ratiometric probe for Hg2+ ions.
Similar to the UV-vis absorption results, the PL emission spec-
trum of Ir(pbi)2(acac) shows a specific response toward Hg2+
ions, inducing an obvious alteration in emission color compared
to other metal ions (vide Fig. 2). PL spectra of Ir(pbi)2(acac) in
the presence of different interfering cations (depicted in Fig. 6
and Fig. S3 in ESI†) and competition experiment results confirm
that Ir(pbi)2(acac) shows a high specificity toward Hg2+, and
hence could be used to detect mercury(II) ions in a complicated
matrix. Moreover, the cyclic voltammetric behavior of Ir-
(pbi)2(acac) in solutions of different coexisting cations also
reveals that Ir(pbi)2(acac) is specific towards mercury(II) (vide
Fig. 7 and Fig. S4 in ESI†).
To investigate if the benzimidazole ligand would show high
affinity for Hg2+, and hence result in the formation of a Hpbi–
Hg2+ complex accompanied by the decomposition of Ir-
1
(pbi)2(acac), UV-vis absorption and H NMR spectra of Hpbi
in the absence and existence of Hg2+ were recorded. As depicted
in Fig. 8, Hpbi has an intense absorption band peaked at
291 nm, while the addition of Hg2+ leads to a slight blue-shift of
the absorption peak to 284 nm gradually, suggesting that there
should exist some interaction between Hpbi and Hg2+. To
1
further understand the Hg-probing mechanism, H NMR spectra
of Hpbi, Hacac and Ir(pbi)2(acac) in the absence and presence
of Hg2+ were recorded (vide Fig. 9). Upon addition of Hg2+,
most of the proton signals of Hpbi show a downfield-shift
effect, implying there is some coordinational bonding between
Hpbi and Hg2+. In respect of Hacac, the presence of 1 eq. Hg2+
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Fig. 7 Cyclic voltammogram response of Ir(pbi)2(acac) (50 μM) in
the presence of various metal cations (1.0 eq.) in acetonitrile solution.
Bars represent the ratio (I1.02 V/I0.44 V) of current intensity at 1.02 and
0.44 V after addition of 1.0 eq. Hg(II) to the above solution. 1, blank; 2,
Hg(II); 3, Na(I); 4, Zn(II); 5, Ag(I); 6, Ba(II); 7, Cd(II); 8, Co(II); 9, Cu(II);
10, K(I); 11, Mg(II); 12, Pb(II); 13,Ni(II); 14, Mn(II); 15, Ca(II); 16, Cr(II);
17, Fe(II).
1
Fig. 9 Partial H NMR spectral changes of Ir(pbi)2(acac) in CD3CN
in the presence of Hg(II) (0–1 eq.): (a) 6.0–8.2 ppm; (b) 1.6–5.4 ppm.
complex could be excluded. Furthermore, the addition of
mercury(II) cations induces disappearing of the characteristic
signal at 5.34 ppm that corresponds to the ethylene hydrogen of
acac bonding to the iridium atom, whilst resulting in the release
of free Hacac in both keto and enol form. It is noteworthy that
in the spectrum of Ir(pbi)2(acac) + 0.8 eq. Hg2+, the signal of
the ethylene hydrogen is observed to be 5.61 ppm, which is ana-
logous to that of pure Hacac rather than Hacac + Hg2+, thus the
mechanism of formation of the acac–Hg2+ complex upon intro-
duction of mercury(II) is excluded. Moreover, in the ESI-MS
spectrum of Ir(pbi)2(acac) + 1.0 eq. Hg2+ (vide Fig. S5 and S6,
ESI†), intense signals at m/z = 731.12 and 772.12 attributed to
the species of [Ir(pbi)2]+ and [Ir(pbi)2(CH3CN)]+ respectively
could be observed, suggesting that the Hpbi ligand still binds
with the Ir(III) center atom. As a result, the probing mechanism
of Ir(pbi)2(acac) towards Hg2+ may be assigned to the Hg2+-
induced departure of acac, which is in line with the mechanism
proposed by Li et al. for Ir(thq)2(dbm)11d and Ir(bt)2(acac).11e
However, according to their reports, the binding of sulfur atoms
with mercury(II) ions is involved as the first key step, which
should be excluded in the light of our observations here, and
therefore, we would assign the mechanism merely to the
decomposition of the iridium chelate with the departure of the
relatively weak Ir–O bond.
Fig. 8 Changes in the UV-vis absorption spectrum of Hpbi (20 μM)
in acetonitrile solution with various amounts of Hg2+ ion (0–24 μM).
ions would not only increase the proportion of the enol tauto-
meric form, but also the characteristic signal at 5.61 ppm corre-
sponding to the ethylene hydrogen of Hacac is shifted 0.07 ppm
upfield with respect to Hacac (5.54 ppm), indicating there may
exist weak coordination between Hacac and Hg2+ too. Nonethe-
less, the addition of mercury(II) cations to Ir(pbi)2(acac) induces
1
significant changes in its H NMR spectrum both in the cyclo-
metallate and ancillary ligand moieties (vide Fig. 9). The signal
at 7.59 ppm shifts to 8.04 ppm, while those at 7.35 and
7.16 ppm are 0.16 ppm and 0.09 ppm downfield shifted relative
to those of pristine Ir(pbi)2(acac). As the spectrum of Ir-
(pbi)2(acac) + 1 eq. Hg2+ differs significantly from both those
of Hpbi and Hpbi + 1.0 eq. Hg2+, the mechanism of the
decomposition of Ir(pbi)2(acac) accompanied by the release of
free Hpbi ligand followed by the formation of a pbi–Hg2+
Conclusion
We report herein a new sulfur-free iridium(III)-complex based
multisignaling chemodosimeter for the selective and specific
detection of Hg2+ cations. Upon addition of mercury(II) ions, the
acetonitrile solution of Ir(pbi)2(acac) shows obvious variations
4882 | Dalton Trans., 2012, 41, 4878–4883
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in both optical and electrochemical behavior. The Hg2+-monitor-
ing character of this complex is hardly affected by a large variety
kinds of interfering cations. Moreover, the reason why Ir-
(pbi)2(acac) would show high specificity toward Hg2+ has been
investigated, and a mechanism of decomposition of the iridium
chelate with the departure of the relatively weak Ir–O bond
induced by Hg2+ ions is proposed, without the compulsory
involvement of a pre-coordinational step between the lone-pair
electrons on the ligand and mercury(II) ions. This preliminary
understanding of the mechanism should be helpful for the con-
struction of further high performance Hg2+ probes.
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Acknowledgements
The authors acknowledge the financial support for this work by
the National Natural Science Foundation of China (NSFC)
(Grants No. 20872103, 50803040 and 21072139).
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