A strikingly fast route to methylmercury acetylides as a new opportunity for monomethylmercury detection

Article abstract of DOI:10.1016/j.jorganchem.2004.10.004

Methylmercury σ-complexation to 1-alkynes is exploited in a new practical and sensitive quantitation of monomethylmercury in water and in biological tissues; indeed, methylmercury halides are detected at the nanomolar level by 10-(3-trimethylsilyl-2-propynyl)-9-(10H)-acridinone, in dichloromethane and in the presence of Bu₄NF·3H₂O.

Full text of DOI:10.1016/j.jorganchem.2004.10.004

Journal of Organometallic Chemistry 690 (2005) 588–593
www.elsevier.com/locate/jorganchem
A strikingly fast route to methylmercury acetylides as a
new opportunity for monomethylmercury detection
a,
b
b
a,
*
Marco Lombardo *, Ivano Vassura , Daniele Fabbri , Claudio Trombini
a
`
Dipartimento di Chimica ‘‘G. Ciamician’’, Universita di Bologna,via Selmi 2, 40126 Bologna, Italy
`
Laboratorio di Chimica, C.I.R.S.A.,Universita di Bologna, via S. Alberto 163, 48100 Ravenna, Italy
b
Received 16 July 2004; accepted 6 October 2004
Abstract
Methylmercury r-complexation to 1-alkynes is exploited in a new practical and sensitive quantitation of monomethylmercury in
water and in biological tissues; indeed, methylmercury halides are detected at the nanomolar level by 10-(3-trimethylsilyl-2-propy-
nyl)-9-(10H)-acridinone, in dichloromethane and in the presence of Bu4NF Æ 3H2O.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Monomethylmercury; Acetylides; Trimethylsilylalkynes; HPLC; Fluorimetric detection
1. Introduction
conditions, to give mercury acetylides D (path #2) [4].
The latter process is closely related to the key step of
the Sonogashira coupling reaction, where a 1-alkyne,
treated with a Cu(I) salt and a tertiary amine, is con-
verted into a Cu(I) acetylide [5].
Alkynyl ligands behave as good r-donors and weak
p-acceptors towards the group 11 and 12 metals [1], as
discussed in a few reviews on the chemistry of metal–
alkynyl complexes [2]. In particular, the interaction of
acetylene with HgCl2 in the gas phase leading to p-com-
plexes of HgCl2 with one or two molecules of acetylene,
has a zero energy barrier [3].
Scheme 1 summarizes the possible interactions of
Hg++ with a terminal alkyne in aqueous media. Vinyl
cation A or p-complex B present two possible reacting
centers: (i) the positively charged carbon which reacts
with nucleophiles, as happens with water under acidic
conditions, affording C (path #1) which eventually leads
to a carbonyl compound (anti-Markovnikov hydration
of alkynes), (ii) the vinylic proton, whose acidity is mag-
nified by the positive charge on the b-carbon, which re-
acts with bases, as happens with water under alkaline
Alkyne–mercury p-coordination is at the basis of the
mercury-catalysed electrophilic additions to alkynes.
Structures A and/or B are trapped by various nucleo-
philes to give vinyl halides, vinyl esters, ethers and so
on. The mercury-catalysed hydration of acetylene to give
acetaldehyde is an example of an old industrial process
based on this chemistry, as demonstrated by a group of
patents dating back to the beginning of the last century
[6]. Nowadays, acetaldehyde manufacture is no more
based on acetylene but on ethylene, a much cheaper
raw material, through the Wacker process [7]. However,
acetaldehyde plants using acetylene as the feedstock have
operated throughout the world from the 1930s to the
1980s, and often left behind environmental injuries in
terms of land or water bodies pollution by mercury.
The most dramatic example was offered by the outbreak
of methylmercury poisoning in Japan, known as the
Minamata disease [8]; further heavy environmental
*
Corresponding authors. Tel.: +39 51 2099513; fax: +39 51
2099456.
E-mail addresses: marco.lombardo@unibo.it (M. Lombardo),
claudio.trombini@unibo.it (C. Trombini).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.10.004

M. Lombardo et al. / Journal of Organometallic Chemistry 690 (2005) 588–593
589
methylmercury acetylides [19], in analogy to the known
chemistry of Hg(II) which affords diacetylides under
the same reaction conditions [4]. In addition, we previ-
ously demonstrated that the reaction of Hg(II) and
MMHg with phenylacetylene in water is not affected
by the contemporary presence of Cu(II), Zn(II), Cd(II)
and Pb(II) in concentrations 104 higher than mercury
[19b]. On the way to develop a synthesis of methylmer-
cury acetylides in organic solvents, and particularly in
DCM, one of the best solvents for MMHg, we ob-
served that no reaction occurs in this solvent between
CH3HgBr or CH3HgCl and phenylacetylene in the
presence either of inorganic heterogeneous bases (alka-
line carbonates) or of tertiary amines. However, when
phenylacetylene was replaced by trimethylsilyl phenyl-
acetylene, a very fast reaction occurred in the presence
of TBAF. Indeed, using a 5 · 10À6 M standard solu-
tion of CH3HgBr in DCM, an excess of trimethylsilyl
phenyl acetylene (50 equiv.) and TBAF (50 equiv.),
after 20 min at 20 ꢁC conversion of MMHg into meth-
ylmercury acetylide was virtually complete, as deter-
mined by HPLC/UV (Detection Limit = 500 pg as
Hg injected). In a similar way, when an excess (50
equiv.) of fluorescent 10-(3-trimethylsilyl-2-propynyl)-
9-(10H)-acridinone (2) was added to CH3HgBr
(1 · 10À8 M in DCM) and TBAF (50 equiv.) in
DCM, formation of 10-(3-methylmercury-2-propynyl)-
9-(10H)-acridinone 3 was observed in >85% yield after
20 min at 20 ꢁC; 40–50% of 2 was protodesylilated to
1 (Scheme 2).
Scheme 1.
impacts by former acetaldehyde manufacture have been
recently reported in Italy [9], in China [10] and in Kaza-
kistan [11]. Thus, acetylene chemistry has been co-
responsible of the overall anthropogenic contribution
to the mercury budget of the biosphere.
Combining the fast biogeochemical cycling of mer-
cury [12] in the environment and the toxicity associated
to the neurotoxic bioaccumulative monomethylmercury
(MMHg) derivatives [13], it is apparent why so powerful
efforts have been directed to the study of the environ-
mental chemistry of mercury in general, and in particu-
lar to the development of sensors [14] and labels [15], as
testified by the number of papers on Hg(II) signaling
that have recently appeared in the literature [16]. Con-
versely, no MMHg sensors have been developed so far
at the best of our knowledge, thus a simple methodology
for the recognition and detection MMHg derivatives of
general formula CH3HgL, where L is an inorganic or or-
ganic [17] ligand, is highly desirable [18].
We now show that alkyne–mercury coordination
chemistry may be also exploited in an environmental
profitable way for MMHg detection. To this purpose,
the development of new analytical protocols for the fast
and cheap control of priority pollutants represents a
main goal of analytical green chemistry.
To sum up these preliminary results, we have at dis-
posal a MMHg receptor in the form of a silylated alkyne
which interacts with the analyte in DCM in a fast and
efficient way, and the opportunity to exploit the sensitiv-
ity of fluorimetric detection. The analytical protocol en-
sures MMHg recognition up to 6 · 10À9 M scale and
with a Detection Limit of 5 pg, as Hg injected. Further-
more, 1-alkynes as Hg(II) and CH3Hg+ receptors benefit
for a substantial lack of interference from other ions;
only Cu(I) [20a–c] and Ag(I) [21a–c] could in principle
interfere in triple bond complexation, but they are not
Here, we propose to exploit the alkyne–mercury p-co-
ordination chemistry (Scheme 1, path #2), to detect
MMHg at the nanomolar level, by exploiting a stric-
kingly fast reaction of CH3HgBr with 1-trimethylsilyl
alkynes in the presence of tetrabutylammonium fluoride
(TBAF) in dichloromethane (DCM). In particular, a flu-
orescent 1-trimethylsilyl alkyne was selected in order to
profit from the high sensitivity of fluorimetric detection.
2. Results and discussion
The reaction of MMHg with 1-alkynes in alkaline
aq. conditions is known to afford the corresponding
Scheme 2.

590
M. Lombardo et al. / Journal of Organometallic Chemistry 690 (2005) 588–593
Table 1
Analysis of standard solutions of CH3HgBr in DCM
Run
CH3HgBr (lg/L)a
[2] and [TBAF] (equiv.)
t (min)
3b lg/L CLc
Yield (%)
1
2
3
4
5
6
7
a
40
30
20
10
7.5
5
50
50
20
20
20
20
35
35
35
33.2 0.5
24.2 0.4
16.5 0.5
8.5 0.7
6.7 0.7
4.6 0.8
1.15 0.2
83
81
83
85
88
92
95
50
100
110
150
800
1.2
Expressed as mercury.
Determined by interpolation of chromatographic peak areas using a calibration curve.
Confidence limits determined by 95% confidence bands of calibration curve.
b
c
extractable in DCM as MMHg is, unless suitable organ-
ic ligands are added, for example tertiary phosphates for
Cu(I) [22] or dicyclohexano-18-crown-6 for Ag(I) [23].
To develop a complete analytical flow chart, we had
first to develop an efficient sample treatment and, sec-
ond, to ensure lack of interference by other matrix com-
ponents. Thus, MMHg is extracted from an aqueous
sample with DCM and the organic phase analysed
according to a very simple analytical protocol (see Sec-
tion 4). Table 1 collects a series of results obtained with
standard solutions of CH3HgBr in DCM.
To confirm the performance of this analytical proce-
dure, we compared the results of MMHg determinations
via 2b with MMHg analyses using GC-ECD [24d–e].
Two different specimens of tissues of a tuna fish were
used (Table 3, runs 1,2), as well as a sample of clams
(run 3) harvested in an intertidal lagoon in Northern
Adriatic Sea, which was thoroughly examined by us in
the last decade for the heavily mercury contamination
of its sedimentary compartment [9].
In all the three specimens a good agreement between
the two analytical procedures is apparent.
DCM is also recognized as one of the best solvents
for the extraction of MMHg from biological tissues
after leaching the sample with an aq. solution of KBr/
H2SO4/CuSO4 [22].
3. Conclusions
To optimize the overall methodology for biological
samples, a certified reference material (CRM 464-55)
consisting of tuna fish muscle was tested in a series of
analyses (see Section 4). After leaching the sample with
a KBr/H2SO4/CuSO4 aq. solution, MMHg was ex-
tracted as CH3HgBr in DCM; derivatisation was carried
out by means of an excess of 10-(3-trimethylsilyl-2-
propynyl)-9-(10H)-acridinone 2 and TBAF in DCM.
In five replicates carried out on CRM 464-55 (certified
MMHg content = 5.12 lg/g, expressed as Hg), mean
derivatization yield was 88 5% with respect to MMHg
extracted (see Table 2).
The organometallic chemistry of mercury and its
coordination with alkynes is proposed here as a new tool
to detect MMHg; in particular, the use of a fluorescent
trimethylsilylalkyne joins the binding ability of 1-alky-
nes for MMHg to the advantages of fluorimetric analy-
sis in terms of sensitivity and reduction of matrix
interference. As the result, this new mercuration reac-
tion seems promisingly exploitable in MMHg recogni-
tion and quantitation in biological samples, in a
simple analytical protocol using common commercial
instrumentation. This original route to MMHg determi-
nation competes in terms of efficiency with the routinely
Table 2
Analysis of MMHg in the certified material CRM 464-55 (tuna fish muscle, certified MMHg content = 5.12 lg/g, expressed as Hg)
Run
CRM 464-55
Extracted Hgb lg/g (%)
MMHg foundc lg/g CLd
Yield (%)e
Sample weight (g)
Total Hg (lg)a
1
2
3
4
5
a
0.2200
0.2176
0.2065
0.2003
0.0637
1.13
1.11
1.06
1.03
0.326
4.95 (97)
4.40 (86)
4.50 (88)
4.61 (90)
4.71 (92)
4.2 0.1
4.0 0.1
4.2 0.1
3.8 0.2
4.1 0.6
85
91
93
80
87
THg, absolute total mercury content.
Total mercury extracted per gram of sample, determined by standard CVAFS (SD 0.05).
b
c
Expressed as Hg and determined by interpolation of chromatographic peak areas using calibration curve.
Confidence limits determined by 95% confidence bands of calibration curve.
Yield obtained, as percent recovery with respect to mercury extracted.
d
e

M. Lombardo et al. / Journal of Organometallic Chemistry 690 (2005) 588–593
591
Table 3
Analysis of MMHg in tissues
Run
Sample
THga (lg/g)
MMHg found lg/g SDb (this method)
MMHg found lg/g SDc (GC-ECD)
1
2
3
a
Tuna muscle
Tuna gill
Clam muscle
3.30
2.25
0.55
2.9 0.1
2.0 0.1
0.40 0.04
3.0 0.1
2.1 0.1
0.44 0.03
Total mercury, determined by standard CVAFS.
Expressed as mercury and determined by interpolation of chromatographic peak areas using calibration curve (n = 10).
n = 3 [16].
b
c
used GC-ECD techniques, as apparent from Table 3,
and in terms of practicality with the widely used ethyla-
tion-based protocols which require sample pre-
treatment with sodium tetraethyl borate under strictly
controlled conditions, followed by GC-pyrolysis-
CVAFS [25]. Finally, the combined use of an alkyne
as the derivatising agent of MMHgBr joined to an
extraction step from acidic aqueous solutions into
DCM, makes this protocol practically unaffected by
other potential interfering species such as Ag(I)Br and
Cu(I)Br.
(pH 6.88), the aqueous layer is extracted with THF and
the combined organic layers are dried (Na2SO4) and
evaporated at reduced pressure. The desired silylated al-
kyne is obtained in 70% yield (0.32 g, 1.1 mmol) as an oil
after purification by flash-chromatography on silica
1
(cyclohexane:ethyl acetate 80:20). H NMR (300 MHz,
CDCl3) d: 0.16 (s, 9H), 5.06 (s, 2H), 7.35 (dt, J = 0.8/
8.0 Hz, 2H), 7.63 (d, J = 8.5 Hz, 2H), 7.79 (ddd,
J = 1.7/7.0/8.5 Hz, 2H), 8.57 (dd, J = 1.7/8.0 Hz, 2H);
13C NMR (75 MHz, CDCl3) d: 0.30, 37.9, 91.0, 98.5,
114.8, 121.5, 122.6, 127.6, 133.9, 141.7, 178.1; GC–MS
(70 eV) m/z (%): 73 (12), 83 (38), 111 (12), 137 (17),
166 (32), 252 (16), 290 (24), 305 (98), 194 (100).
4. Experimental
4.3. 10-(3-Methylmercury-2-propynyl)-9(10H)-acridinone
(3)
4.1. 10-(2-Propynyl)-9(10H)-acridinone (1)
NaH (0.18g, 7.5 mmol) is added to a solution of
9(10H)-acridinone (0.98 g, 5 mmol) in 25 mL of dimeth-
ylformamide (DMF). The solution was stirred for 30
min at 50 ꢁC and allowed to cool to room temperature.
Propargyl bromide (0.65 mL, 80% solution in toluene, 6
mmol) was added and the solution was stirred at 50 ꢁC
for 6 h. The reaction mixture was quenched with water
(80 mL) and the solid product was recrystallized from
hot ethanol to obtain 0.69 g of pure 1 (2.95 mmol,
59%). m.p. = 212–215 ꢁC (ethanol); 1H NMR (300
MHz, CDCl3) d: 2.43 (t, J = 2.5 Hz, 1H), 5.22 (d,
J = 2.5 Hz, 2H), 7.33 (d, J = 7.9 Hz, 2H), 7.57 (d,
J = 8.5 Hz, 2H), 7.77 (dt, J = 1.7/8.0 Hz, 2H), 8.55
(dd, J = 1.7/8.0 Hz, 2H); 13C NMR (75 MHz, CDCl3)
d: 36.8, 73.9, 77.2, 114.5, 121.8, 122.7, 127.9, 134.1,
141.7, 178.1; GC–MS (70 eV) m/z (%): 69 (13), 75 (16),
102 (14), 140 (58), 166 (81), 194 (82), 204 (65), 232
(43), 233 (100).
CH3HgCl (0.04 g, 0.164 mmol) is added to a solution
of 10-(3-trimethylsilyl-2-propynyl)-9(10H)-acridinone 2
(0.05 g, 0.164 mmol) in 2 mL of CH2Cl2. The solution
is stirred for 5 min at room temperature, tetrabutylam-
monium fluoride (0.052 g, 0.164 mmol) is added and
the reaction mixture is stirred for additional 15 min at
room temperature. The organic phase is evaporated to
dryness and the product is purified by flash-chromatog-
raphy on silica (cyclohexane:ethyl acetate 80:20). The
solid obtained is recrystallized by hot ethanol to yield
0.028 g (0.062 mmol, 38%) of pure acetylide 3. m.p. =
dec. at 155–160 ꢁC (ethanol); 1H NMR (300 MHz,
CDCl3, T = 50 ꢁC) d: 0.63 (d, J relative to 199
Hg = 147.7 Hz, 3H), 0.63 (s, 3H), 5.0 (s, 2H), 7.27–
7.38 (m, 2H), 7.64–7.83 (m, 4H), 8.57 (dd, J = 1.6/8.0
Hz, 2H); 13C NMR (75 MHz, CDCl3, T = 50 ꢁC) d:
6.7, 37.6, 77.2, 98.8, 114.8, 121.6, 122.9, 127.9, 133.9,
141.9, 178.2; GC–MS (70 eV) m/z (%): 140 (26), 166
(52), 194 (60), 204 (50), 217 (6), 232 (100), 255 (5), 449
([M+] relative to 202 Hg, 23).
4.2. 10-(3-Trimethylsilyl-2-propynyl)-9(10H)-acridinone
(2)
4.4. Analytical tools and procedures
BuLi (0.72 mL, 2.5 M solution in hexanes, 1.8 mmol)
is added at À78 ꢁC to a solution of 1 (0.35 g, 1.5 mmol)
in 15 mL of THF and the reaction mixture is stirred for
1 h at À78 ꢁC. Trimethylsilylchloride (0.24 mL, 1.8
mmol) is added and the solution is stirred for 3 h at
À78 ꢁC. The reaction is quenched with phosphate buffer
Analytical HPLC was performed on a Perkin–Elmer
binary pump LC 250 instrument connected to a Perkin–
Elmer LC 240 fluorescence detector using a reversed-
phase column (Supelcosil LC-18, 25 cm · 4.6 mm, 5
lm film-thick). For the analysis of 10-(3-trimethylsilyl-

592
M. Lombardo et al. / Journal of Organometallic Chemistry 690 (2005) 588–593
2-propynyl)-9(10H)-acridinone (tR = 7.34 min, kex = 251
nm, kem = 418 nm, /em = 0.50 in H2O/ CH3CN = 2/8 v/v)
the elution program was: 10 min isocratic CH3CN/
H2O = 55/45 v/v, gradient ramp up to 100% CH3CN
in 10 min, 5 min isocratic 100% CH3CN, flow = 1 mL/
min. Calibration curve for standard CH3HgBr solutions
is linear in the range examined (5–50 lg/L of CH3HgBr
expressed as Hg, r = 0.9994). Cold Vapor Atomic Fluo-
rescence spectroscopy (CVAFS) was performed using a
Tekran Mercury Detector 2500 instrument connected
to a Hewlett Packard integrator HP 3395 and using ar-
at 20 ꢁC and 0.205 mL of a 4 · 10À4 M solution of
TBAF in DCM (8.25 · 10À5 mmol, 110 equiv.) are
added. After additional stirring for 15 min at 20 ꢁC,
the solvent is slowly evaporated to a volume of about
5 mL. The mixture is stirred again for 15 min at 20
ꢁC, the solvent is slowly evaporated (T = 35 ꢁC, P = 28
mmHg), the residue is dissolved into the desired volume
of CH3CN and mercury acetylide 3 is directly analyzed
by HPLC with fluorimetric detection.
4.7. Typical experimental procedure for MMHg analysis
in tissues (Table 2, Run 1)
1
gon as carrier gas. H and 13C NMR spectra were re-
corded using a Varian Inova 300 MHz spectrometer,
using tetramethylsilane (TMS) as internal standard;
chemical shifts are reported in ppm (d) downfield from
TMS. GC–MS analyses (70 eV) were performed with a
Hewlett–Packard 5890 Series II instrument connected
to a Hewlett–Packard 5971 quadrupole mass detector.
Stock solutions of CH3HgBr in DCM (100 mg LÀ1 as
Hg) are prepared by weighting 0.0295 g of CH3HgBr
and diluting to volume with DCM in a 200 mL volumet-
ric flask and are stored at À20 ꢁC. Diluted solutions are
freshly prepared just before use. Stock solutions of silyl-
ated alkyne (122 mg LÀ1, 4 · 10À4 M) are prepared by
dissolving 12.2 mg in DCM in a 100 mL volumetric flask
and diluting to volume with DCM. Stock solutions of
TBAF (105 mg LÀ1, 4 · 10À4 M) are prepared by dis-
solving 10.5 mg of TBAF in DCM in a 100-mL volumet-
ric flask and diluting to volume with DCM.
The lyophilized and homogenized sample (0.1–0.2 g)
is poured into a centrifuge teflon tube and washed with
acetone (2 · 3 mL) and toluene (1 · 3 mL). The sample
is then centrifuged at 3000 rpm for 5 min, the organic
layer is removed and the sample is dried using a gentle
nitrogen flow. A mixture consisting of KBr (1.5 M in
5% H2SO4, 5 mL), CuSO4 (1 M in H2O, 1 mL) and
DCM (5 mL) is added to the residue, the tube is tightly
closed and the resulting mixture is vigorously shaken for
20 min, then centrifuged at 3000 rpm for 20 min. The or-
ganic layer is removed and the extraction is repeated
with a second aliquot of DCM (5 mL). A first 0.5 mL
aliquot of the combined organic layers is analyzed by
CVAFS in order to check the extraction efficiency and
to evaluate the total mercury content; a second 2 mL ali-
quot is further on purified from co-extracted polar or-
ganic components by silica gel (5 g) chromatography
with DCM (25 mL). Mercury recovery in the chromato-
graphic step was confirmed to be quantitative by
CVAFS. A solution of 10-(3-trimethylsilyl-2-propy-
nyl)-9(10H)-acridinone 2 in DCM (4 · 10À4 M, 0.5
mL) is added to the DCM solution of MMHg, the mix-
ture is stirred for 5 min at 20 ꢁC, then a solution of
TBAF in DCM (4 · 10À4 M, 0.5 mL) is added. After
additional stirring for 15 min at 20 ꢁC, the solvent is
slowly evaporated (T = 35 ꢁC, P = 28 mmHg), the resi-
due is dissolved into the desired volume of CH3CN
and mercury acetylide 3 is directly analyzed by HPLC
with fluorimetric detection.
4.5. Typical experimental procedure for CH3HgBr analysis
in standard solutions with Hg content higher than 10 lg/L
(Table 1, Run 1)
A
4 · 10À4-M solution of 10-(3-trimethylsilyl-2-
propynyl)-9(10H)-acridinone 2 in DCM (0.125 mL,
5 · 10À5 mmol, 50 equiv.) is added to 5 mL of a
2 · 10À7 M solution of MMHg in DCM (40 lg/L as
Hg, 1 · 10À6 mmol). The mixture is stirred for 5 min
at 20 ꢁC and 0.125 mL of a 4 · 10À4 M solution of
TBAF in DCM (5 · 10À5 mmol, 50 equiv.) are added.
After additional stirring for 15 min at 20 ꢁC, the solvent
is slowly evaporated (T = 35 ꢁC, P = 28 mmHg), the res-
idue is dissolved into the desired volume of CH3CN and
mercury acetylide 3 is directly analyzed by HPLC with
fluorimetric detection.
Acknowledgement
This work was supported by the University of Bolo-
gna (C.I.R.S.A.).
4.6. Typical experimental procedure for CH3HgBr anal-
ysis in standard solutions with Hg contents lower than 10
lg/L (Table 1, Run 5)
A 4 · 10À4 M solution of 10-(3-trimethylsilyl-2-
propynyl)-9(10H)-acridinone 2 in DCM (0.205 mL,
8.25 · 10À5 mmol, 110 equiv.) is added to 20 mL of a
3.7 · 10À8 M solution of MMHg in DCM (7.5 lg/L as
Hg, 7.5 · 10À7 mmol). The mixture is stirred for 5 min
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