An organopalladium chromogenic chemodosimeter for the selective naked-eye detection of Hg2+ and MeHg+ in water-ethanol 1: 1 mixture

Article abstract of DOI:10.1039/b807670g

An organopalladium chemical dosimeter of Hg²⁺ that methylates Hg²⁺, undergoing a colour change in 1: 1 ethanol-water with submicromolar sensitivity, gives rise to an aqua-palladium complex that is methylated by MeHg⁺ in th

Full text of DOI:10.1039/b807670g

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An organopalladium chromogenic chemodosimeter for the selective
naked-eye detection of Hg2+ and MeHg+ in water–ethanol 1 : 1
mixturew
O. del Campo, A. Carbayo, J. V. Cuevas, A. Munoz, G. Garcıa-Herbosa,*
˜ ´
´
D. Moreno, E. Ballesteros, S. Basurto, T. Gomez and T. Torroba*
Received (in Cambridge, UK) 6th May 2008, Accepted 8th July 2008
First published as an Advance Article on the web 5th August 2008
DOI: 10.1039/b807670g
An organopalladium chemical dosimeter of Hg2+ that methyl-
ates Hg2+, undergoing a colour change in 1 : 1 ethanol–water
with submicromolar sensitivity, gives rise to an aqua–palladium
complex that is methylated by MeHg+ in the presence of a
dithiol compound, undergoing another colour change, thus mak-
ing the system suitable for the naked-eye detection of Hg2+ and
MeHg+, two environmentally important species of Hg2+
.
Environmental mercury contamination is a major concern
because of the huge amount of mercury released to the
environment by human activities (especially coal-fired power
stations, primary metals production and the chlor-alkali in-
dustry) and because of the persistence of mercury in the
environment, not only as the volatile mercury metal but also
as the water soluble mercury[II] (Hg2+) and methylmercury[II]
(MeHg+) cations, and the less common but highly toxic
dimethylmercury (Me2Hg).1 All species of Hg2+ are strongly
interconnected in the environment because of the natural cycle
of mercury metal that keeps Hg2+ and MeHg+ concentra-
tions in natural water resources,1a which need to be monitored
in the contaminated regions and in food products,2 especially
fish,3 produced in these regions. Chemical probes are useful
for fast detection of mercury contamination. Usually, they
work by complexation of Hg2+ by colorimetric4 or fluoro-
genic5 reagents. Complementing these methodologies, chemi-
cal dosimeters act by specific reactions with Hg2+, which
subsequently undergo a color6 or fluorescence7 change. Re-
generative chemodosimeters are less common.8 Neither che-
mical sensors nor dosimeters that extend to MeHg+ and
Me2Hg exist, which is in strong contrast to the enormous
interest that bacterial mercury methylation and demethylation
promotes.9 Palladium[II] complexes are able to transfer alkyl
groups to and from mercury derivatives10 therefore they could
mimic bacterial behaviour, opening the way to biomimetic
selective molecular probes for mercury[II] species. Following
this idea we tested several palladium[II] complexes for their
Scheme 1 Synthesis of complexes 2–3.
ability to interact with mercury[II] species as well as other
metal cations. In this paper we report the first organopalla-
dium regenerative chemodosimeter for the selective naked-eye
detection of Hg2+ and MeHg+ in water–ethanol 1 : 1 mixture.
The dosimeter was synthesized by reaction of ligand 111 and
[PdCl(CH3)(1,5-cyclooctadiene)] (1 equiv.) in CH2Cl2 in the
conditions used for other examples,12 from which
2
(transC–Pd–N isomer, from NOESY experiments, 85%) was
obtained as a yellow, air-stable solid (Scheme 1). Treatment of
2 with NaOMe (1 equiv.) in CH2Cl2 gave 3 (62%) as a red air-
stable solid. Compounds 2–3 were characterized by the usual
spectroscopic and analytical techniques.
A 10ꢀ4 M solution of compound 3 gave a yellow solution in
ethanol–water 1 : 1 that changed to purple in the presence of
one or more equivalents of Hg2+, but was insensitive to the
presence of several equivalents of every one of the rest of the
cations as perchlorate or triflate salts (Fig. 1). In addition, a
solution of 3 in the presence of 1 equiv. of all the rest of the
cations (Ag+, Ni2+, Sn2+, Cd2+, Zn2+, Pb2+, Cu2+, Fe3+
,
Sc3+, Al3+) changed from yellow to purple after addition of 1
equiv. Hg2+ in mixtures of ethanol–water. The absence of
interaction of the Ag+ cation is remarkable.
By quantitative titration of a 10ꢀ4 M solution of 3 in
EtOH–H2O 1 : 1, the original absorption at 455 nm diminished
as Hg2+ was added, and a new absorption at 500 nm,
´
Departamento de Quımica, Facultad de Ciencias, Universidad de
Burgos, 09001 Burgos, Spain. E-mail: ttorroba@ubu.es;
gherbosa@ubu.es; Fax: +34 947258831; Tel: +34 947258088
w Electronic supplementary information (ESI) available: Experimental
procedures, physical and spectral data of new products, colorimetric
studies and calculation of detection limits. CCDC reference number
675181. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b807670g
Fig. 1 Color changes of a 10ꢀ4 M solution of 3 in EtOH–H2O 1 : 1 in
the presence of 1 equiv. of every cation as perchlorate or triflate salts.
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4576 | Chem. Commun., 2008, 4576–4578

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Fig. 2 Titration curves and titration profile of a 10ꢀ4 M solution of 3
in EtOH–H2O 1 : 1 with Hg2+
.
Fig. 4 1H NMR titration of 3 and Hg2+ in CD3OD.
responsible for the colour change, as well as two isosbestic
points, at 480 nm between 0 and 1 equiv. Hg2+, and at 500 nm
between 1 and 2 equiv. Hg2+, appeared (Fig. 2). The titration
profile was fitted to a 1 : 2 model, from which titration
constants were obtained: log K1 = 3.14 ꢂ 0.10 and log K2
= 4.93 ꢂ 0.68. Related constants were obtained from
titrations at fixed pH values (log K1 = 2.66 ꢂ 0.06 and log
K2 = 5.73 ꢂ 0.06 at pH = 7.40; log K1 = 2.39 ꢂ 0.05 and
log K2 = 4.99 ꢂ 0.05 at pH = 4.15; log K1 = 2.39 ꢂ 0.04 and
log K2 = 4.99 ꢂ 0.04 at pH = 8.25). The detection limit of a
10ꢀ5 M solution of 3 in EtOH–H2O 1 : 1 was calculated by the
blank variability method13 and was found to be 3.16 ꢃ 10ꢀ7 M.
To isolate the complex responsible for the detection mechan-
ism, an equimolecular solution of 3 and Hg(ClO4)2 was crystal-
lized by slow diffusion in ether–MeOH for 12 days, from which
red needles were obtained. X-Ray diffraction analysis of the
needles afforded the perchlorate aqua palladium cationic structure
[4(H2O)]+, in which the methyl group has disappeared and
instead a water molecule is bonded to the palladium atom (Fig. 3).
The complex crystallized with two additional water mole-
cules (not shown in Fig. 3) whose hydrogen bonds contributed
to the crystal packing, as it is usual for this kind of com-
plexes.14 1H and 31P NMR spectra confirmed the absence of a
methyl group in the structure, and the UV–visible spectrum of
4 matched exactly to the UV–visible spectrum of a 1 : 1
mixture of 3/Hg(ClO4)2 in EtOH–H2O 1 : 1, confirming that
4 was the species responsible for the Hg2+ detection.
In order to detect the fate of the methyl group, we performed
1H NMR titrations of a 7.73 ꢃ 10ꢀ3 M solution of 3 and Hg2+
in CD3OD (Fig. 4). Under addition of 0.2 to 0.75 equiv. Hg2+
to a solution of 3 in CD3OD the methyl–palladium signal
2
(doublet, d 0.4, J(1H,31P) = 3 Hz) diminished and a Me2Hg
signal [singlet, d 0.15, two satellites, 2J(1H,199Hg) = 105 Hz]15a
appeared in addition to a MeHg+ signal [singlet, d 1, two
satellites, 2J(1H,199Hg) = 269 Hz].15 After addition of 1 equiv.
Hg2+ the signal of Me2Hg practically disappeared and at higher
concentrations of Hg2+ the only methyl signal corresponded to
the MeHg+ signal. Another transient signal was found at d 0.77
in the interval 0.2–1 equiv. Hg2+ which was close to the value
d 0.71 of the Me–Pd group in 2, therefore it should correspond
to a Me–Pd signal of an intermediate complex related to 2.
Therefore, the detection mechanism corresponded to a
mercury[II] methylation as shown in Scheme 2 (left to right).
In order to regenerate the dosimeter 3 we treated an
equimolecular purple solution of 3 and Hg2+ in EtOH–H2O
1 : 1 with 2 equiv. (1 mol = 2 equiv.) of 3,6-dioxa-1,8-
octanedithiol 5, a good complexing reagent for Hg2+, from
which the solution turned yellow. Further addition of 2 equiv.
Hg2+ turned the solution purple again (lmax 510 nm), and this
was repeated consecutively for several times, with the addi-
tional effect of dilution of samples, thus proving that the
dosimeter can be regenerated indefinitely, and this was also
confirmed by UV–visible titration of a 1 : 1 solution of
3/Hg2+, titrated with 5 in EtOH–H2O 1 : 1. 1H NMR titration
experiments in CD3OD confirmed the mechanism.
To test the practical application of the process depicted in
Scheme 2 for the detection of MeHg+, we titrated an
Scheme 2 Methylation/demethylation of Hg2+ by 3.
Chem. Commun., 2008, 4576–4578 | 4577
Fig. 3 X-Ray diffraction structure of [4(H2O)]ClO4.z
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Notes and references
z Crystal data for [4(H2O)]ClO4, C25H24ClNO7.50PPd, M = 631.30,
monoclinic, C2/c,
14.7100(17) A,
a
=
=
18.345(2) A,
b
=
22.576(3) A,
901;
c
V
=
=
.
a
901, 122.192(2)1,
b
=
g
=
5155.7(11) A3, Z = 8, Dcalc = 1.619 g cmꢀ1, m(Mo-Ka) = 0.933 mmꢀ1
Purple prism, (0.30 ꢃ 0.20 ꢃ 0.20) mm3. 24 303 measured reflections,
4531 independent (Rint = 0.0428), 3884 observed (I 4 2s(I)). R1
0.0596, wR2 = 0.1529 (all data). CCDC 675181.
=
Fig. 5 Colour changes of 3 on silica before and after addition of
Hg2+ and then 5.
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each reagent were fast and clearly detected, therefore suitable
for practical applications.
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In conclusion, compound 3 worked as a regenerative che-
mical Hg2+ dosimeter by methylation of Hg2+ and formation
of 4 (characterized as the aqua–palladium perchlorate), under-
going a colour change from yellow to purple in 1 : 1
water–ethanol. MeHg+ produced in the titration was further
titrated with dithiol 5, resulting in a reversed colour change
and formation of the original complex 3. The reported equili-
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eye from the colour change associated with the displacement
of the methyl group form MeHg+ by dithiol 5 with subsequent
methylation of the palladium complex 4 in 1 : 1 water–ethanol,
with formation of complex 3. The system is therefore suitable
for the naked-eye detection of Hg2+ and MeHg+, two environ-
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solutions and constitutes the first example of a new class of
regenerative chemical dosimeters of Hg2+ with sub-micro-
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We gratefully acknowledge financial support from the DGI
of Spain (CTQ2006-15456-C04-04BQU), Junta de Castilla y
Leon, Consejerıa de Educacion y Cultura, y Fondo Social
´ ´ ´
Europeo (BU013A06), European Commission (LIFE05 ENV/
E/000333-Hydrosolar 21), and SCAI-UBU.
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This journal is The Royal Society of Chemistry 2008
4578 | Chem. Commun., 2008, 4576–4578