Activation of the Hg-C Bond of Methylmercury by [S2]-Donor Ligands

Article abstract of DOI:10.1021/acs.inorgchem.7b01048

Here we report that [S₂]-donor ligands Bmm^OH, Bmm^Me, and Bme^Me bind rapidly and reversibly to the mercury centers of organomercurials, RHgX, and facilitate the cleavage of Hg-C bonds of RHgX to produce stable tetracoordinated Hg(II) complexes and R₂Hg. Significantly, the rate of cleavage of Hg-C bonds depends critically on the X group of RHgX (X = BF₄^-, Cl^-, I^-) and the [S₂]-donor ligands used to induce the Hg-C bonds. For instance, the initial rate of cleavage of the Hg-C bond of MeHgI induced by Bme^Me is almost 2-fold higher than the initial rate obtained by Bmm^OH or Bmm^Me, indicating that the spacer between the two imidazole rings of [S₂]-donor ligands plays a significant role here in the cleavage of Hg-C bonds. Surprisingly, we noticed that the initial rate of cleavage of the Hg-C bond of MeHgI induced by Bme^Me (or Bmm^Me) is almost 10-fold and 100-fold faster than the cleavage of Hg-C bonds of MeHgCl and [MeHg]BF₄ respectively, under identical reaction conditions, suggesting that the Hg-C bond of [MeHg]BF₄ is highly inert at room temperature (21 °C). We also show here that the nature of the final stable cleaved products, i.e. Hg(II) complexes, depends on the X group of RHgX and the [S₂]-donor ligands. For instance, the reaction of Bmm^Me with MeHgCl (1:1 molar ratio) afforded the formation of the 16-membered metallacyclic dinuclear mercury compound (Bmm^Me)₂Hg₂Cl₄, in which the two Cl atoms are located inside the ring, whereas due to the large size of the I atom, a similar reaction with MeHgI yielded polymeric [(Bmm^Me)₂HgI₂]_m·(MeHgI)_n. However, the treatment of Bmm^Me with ionic [RHg]BF₄ led to the formation of the tetrathione-coordinated mononuclear mercury compound [(Bmm^Me)₂Hg](BF₄)₂, where BF₄^- serves as a counteranion.

Full text of DOI:10.1021/acs.inorgchem.7b01048

Article
pubs.acs.org/IC
Activation of the Hg−C Bond of Methylmercury by [S2]‑Donor
Ligands
Ramesh Karri, Mainak Banerjee, Ashish Chalana, Kunal Kumar Jha, and Gouriprasanna Roy*
Department of Chemistry, School of Natural Sciences, Shiv Nadar University, NH91, Dadri, Gautam Buddha Nagar, Uttar Pradesh
201314, India
S
* Supporting Information
ABSTRACT: Here we report that [S2]-donor ligands BmmOH
,
BmmMe, and BmeMe bind rapidly and reversibly to the mercury
centers of organomercurials, RHgX, and facilitate the cleavage of
Hg−C bonds of RHgX to produce stable tetracoordinated Hg(II)
complexes and R2Hg. Significantly, the rate of cleavage of Hg−C
−
bonds depends critically on the X group of RHgX (X = BF4 , Cl−,
I−) and the [S2]-donor ligands used to induce the Hg−C bonds. For
instance, the initial rate of cleavage of the Hg−C bond of MeHgI
induced by BmeMe is almost 2-fold higher than the initial rate
obtained by BmmOH or BmmMe, indicating that the spacer between
the two imidazole rings of [S2]-donor ligands plays a significant role
here in the cleavage of Hg−C bonds. Surprisingly, we noticed that the initial rate of cleavage of the Hg−C bond of MeHgI
induced by BmeMe (or BmmMe) is almost 10-fold and 100-fold faster than the cleavage of Hg−C bonds of MeHgCl and
[MeHg]BF4 respectively, under identical reaction conditions, suggesting that the Hg−C bond of [MeHg]BF4 is highly inert at
room temperature (21 °C). We also show here that the nature of the final stable cleaved products, i.e. Hg(II) complexes,
depends on the X group of RHgX and the [S2]-donor ligands. For instance, the reaction of BmmMe with MeHgCl (1:1 molar
ratio) afforded the formation of the 16-membered metallacyclic dinuclear mercury compound (BmmMe)2Hg2Cl4, in which the
two Cl atoms are located inside the ring, whereas due to the large size of the I atom, a similar reaction with MeHgI yielded
polymeric [(BmmMe)2HgI2]m·(MeHgI)n. However, the treatment of BmmMe with ionic [RHg]BF4 led to the formation of the
tetrathione-coordinated mononuclear mercury compound [(BmmMe)2Hg](BF4)2, where BF4 serves as a counteranion.
−
egg samples of California Black skimmers collected from the
remnant coastal wetlands.10,11 The binding affinity of Hg2+ for
Cl− or I− is very high, and thus, the Hg−Cl or Hg−I bond in
MeHgX may remain intact in aqueous solution in the absence
of a strongly electron donating ligand.8a,12 Thus, a clear
understanding of the role of X (X = Cl−, I−) on the cleavage of
Hg−C bonds of MeHgX is of great importance.
INTRODUCTION
■
Organomercurials (often referred to as RHg+), including
methylmercury (MeHg+), are among the most toxic com-
pounds to humans and animals. MeHg+ exists in the
environment in various chemical forms, including MeHgOH,
MeHgCl, and MeHgI. The toxicity of MeHg+ is mainly
attributed to its lipophilicity and strong binding affinity toward
thiols present in proteins.1−5 On the other hand, due to its
structural similarity with the natural amino acid L-methionine,
the L-cysteine-conjugated MeHg+ (MeHgCys) acts as a
substrate for the membrane transporter L-type large neutral
amino acid transporter 1 (LAT1), which is expressed in many
tissues and actively facilitates the transportation of large neutral
amino acids including L-methionine and MeHgCys in multiple
tissues, including brain.4a,6,7 Thus, the ability of MeHg+ to
interact with L-cysteine is critical for its ability to gain
intracellular access and exert its cytotoxicity. A report suggests
that the toxicological implications associated with the ingestion
of MeHg+ may differ according to its chemical form.8
Murakami and co-workers showed that the pharmacokinetic
properties such as the mechanism of membrane transport and
volume of distribution are quite different between MeHgCl and
MeHgCys.9 Moreover, MeHgI is an another organic pollutant
present in the environment, and it has also been detected in the
On the other hand, several bacteria are resistant to MeHg+
due to the presence of a mer operon that codes for two
enzymes: namely, organomercurial lyase (MerB) and mercurial
reductase (MerA). The enzyme MerB first cleaves the Hg−C
bond of MeHg+ to form Hg(II) and subsequently transfers
Hg(II) to another enzyme MerA, which reduces it to elemental
Hg(0).13−15 The active site of MerB consists of a catalytic triad
of two cysteine residues (Cys) and either an aspartic acid (Asp)
or a serine (Ser) residue.14−17 Although the two Cys residues
are conserved in the active sites of all known variants of MerB
(total 65 complete protein sequences of organomercurial lyases
are identified as of January 2010, UniProtKB database), four
variants of organomercurial lyases (Bacillus megaterium MerB2,
Bacillus subtillis MerB2, Bacillus sp. (RC607) MerB2, and
Received: April 27, 2017
© XXXX American Chemical Society
A
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Clostridium butyricum MerB2) carry a Ser residue instead of Asp
in their catalytic triad. Significantly, recent studies showed that
the presence of a Ser residue in the active site substantially
reduces the catalytic activity as well as the substrate specificity
of these four MerB2 variants.18−20 However, the mechanism by
which these serine-containing organomercurial lyases (MerB2)
cleave the Hg−C bonds of organomercurials has not been
studied yet. In this context, therefore, it is beneficial to develop
chemistry between [S2]-donor ligands with various MeHg+
compounds (such as ionic [MeHg]BF4, MeHgCl, and MeHgI)
which may help in understanding the mechanism of
detoxification of MeHg+ by microorganisms. In this study, we
have employed the three imidazole-based [S2]-donor ligands
BmmOH, BmmMe, and BmeMe with different N substitutions and
spacers between the two imidazole rings (Figure 1), to
Figure 1. Chemical structures of [S2]-donor ligands.
investigate their potential in the cleavage of inert Hg−C
bonds of various organomercurials RHgX (where R = Me, Et,
Ph and X = BF4 , Cl−, I−) at physiological temperature (37
−
°C).
Figure 2. (a) Synthetic route for the cleavage of Hg−C bonds of
[RHg][BF4] by BmmOH. (b) 1H NMR spectra, in DMSO-d6, of
BmmOH, a mixture of BmmOH and [MeHg]BF4, and [(BmmOH)2Hg]-
(BF4)2 (isolated), also showing the formation of Me2Hg (*, DMSO-
d6; #, H2O in DMSO-d6).
RESULTS AND DISCUSSION
■
The [S2]-donor ligands BmmOH, BmmMe, and BmeMe were
synthesized by in situ generation of reactive carbenes from
various bis-imidazolium salts by following the literature
procedures.21,22 Detailed synthetic procedures are mentioned
in the Supporting Information. Although the cleavage of Hg−C
bonds of MeHg+ by the [S3]-donor ligand tris(2-mercapto-1-R-
imidazolyl)hydroborato [TmR],23 the [P3]-donor ligand N-
(CH2CH2PPh2)3,24 and other ligands,25 in the absence or
presence of proton donor (thiol or CF3CO2H), has been
reported previously in the literature, the cleavage of Hg−C
bonds of organomercurials by any [S2]-donor ligands and the
development of coordination chemistry between them have not
been studied earlier. In this study, we use BmmOH, BmmMe, or
BmeMe to understand the cleavage of Hg−C bonds of RHgX.
The treatment of BmmOH with 1 equiv of [RHg]BF4 (R = Ph,
Et, Me) at 21 °C led to the formation of the corresponding 1:1
RHg-conjugated complexes [(BmmOH)HgR]BF4 (Figure 2).
The NMR spectroscopy studies of the reaction solutions of
BmmOH and [RHg]BF4 have provided very useful information
about coordination of S atoms of BmmOH to the mercury
centers of [RHg]BF4. Interestingly, the resulting solution of
BmmOH and [MeHg]BF4 in a 1:1 molar ratio showed only a
spectroscopy suggests that both S atoms of BmmOH are
involved in coordination to the mercury center of [MeHg]BF4
in solution (two chemically equivalent imidazole rings). These
observations suggest that, in solution, all of the BmmOH
molecules are involved in interacting with the mercury center
of [MeHg]BF4 through S atoms and this interaction is rapid, is
faster than the NMR time scale, and is reversible in nature
(fluxional behavior prevents the identification of the excess free
ligands in solution).26 A similar phenomenon was also noticed
at low temperature (−30 °C in CDCl3), indicating that this
exchange process is highly facile even at low temperature.
Accordingly, the 199Hg NMR signal of [MeHg]BF4, in DMSO-
d6, shifted from −1086 to −712 ppm after addition of 1 equiv
of BmmOH, indicating the coordination of S atoms of BmmOH
to the mercury center of [MeHg]BF4 (Figure S46 in the
Supporting Information). The formation of [(BmmOH)HgMe]+
in solution was detected by mass spectrometry (m/z for
[(BmmOH)HgMe]+ 517.0644). However, when the reaction
was continued at high temperature (37 °C) for 7 days, the
cleavage of the Hg−C bond of [(BmmOH)HgMe]+ and
subsequently the formation of the stable tetracoordinated
mononuclear mercury compound [(BmmOH)2Hg](BF4)2 (m/z
for [(BmmOH)2Hg]2+ 401.0538) and volatile dimethylmercury
(Me2Hg) were observed (Figure 3). The 199Hg NMR signals
for [(BmmOH)2Hg]2+ and Me2Hg appeared at −443 and −113
ppm, respectively, in DMSO-d6 (Figure 3c).27
single set of signals of [(BmmOH)HgMe]BF4 in H NMR
1
spectroscopy, as shown in Figure 2b and the Supporting
Information. The signal for the methyl group of −HgMe
appeared at 0.71 ppm in DMSO-d6, which was in a slightly
different position in comparison to that for [MeHg]BF4 alone
(at 0.79 ppm in DMSO-d6) (Figures S20 and S21 in the
Supporting Information). Surprisingly, the addition of more
BmmOH to the above solution ([BmmOH]/[MeHg+] = 2 or
more) resulted in only a single set of signals. In addition, the
observation of two signals for four olefinic hydrogen atoms (or
(imidazole)CH) of two imidazole rings of BmmOH in 1H NMR
Recently we have shown that N-substituted imidazole and
benzimidazole-based thiones and selones having an N−
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RHg-conjugated complexes [(BmmMe)HgR]BF4 (m/z (for M+)
519.0627 when R = Ph; m/z 471.0633 when R = Et; m/z
457.0467 when R = Me) (Figure 4a and Figured S7−S9 in the
Figure 3. HRMS of [(BmmOH)2Hg]2+ (a) and [(BmmOH)HgPh]+ (b)
and 199Hg NMR spectra of [(BmmOH)2Hg](BF4)2, Ph2Hg, and
Me2Hg in DMSO-d6 (c).
CH2CH2OH substituent are remarkably effective in detoxifying
various organomercurials by converting them into less toxic
insoluble HgS(s) and HgSe(s) nanoparticles in the presence of
1 equiv of strong or weak base.28 However, in this case the
addition of 1 equiv of KOH to the reaction mixtures of BmmOH
plus [RHg]BF4 failed to produce HgS(s) nanoparticles at 37
°C. Interestingly, the 1H NMR spectrum of the cleaved product
[(BmmOH)2Hg](BF4)2 showed that the −OH groups of
BmmOH remained free in solution and were not involved in
coordination to the mercury center (Figure 2b). The large
downfield chemical shift of the bridging methylene group (N−
CH2−N) (0.45 ppm) and the olefinic H atoms of imidazole
rings (0.46 ppm) was observed in [(BmmOH)2Hg](BF4)2 in
comparison to those in the free ligand BmmOH. Likewise, the
199Hg NMR of a reaction mixture containing BmmOH and
[PhHg]BF4 in 1:1 molar ratio showed a signal at −1154 ppm
which was shifted almost 225 ppm downfield in comparison to
that observed for free [PhHg]BF4 at −1379 ppm, indicating a
strong coordination between the S atoms of BmmOH to the
mercury center of [PhHg]BF4. However, similar to the case of
BmmOH and [MeHg]BF4, as the reaction between BmmOH and
[PhHg]BF4 was allowed to proceed for 48 h at 37 °C, the
formation of cleaved product [(BmmOH)HgPh]+ and diphe-
nylmercury (Ph2Hg) were observed. The monopositive
[(BmmOH)HgPh]+ (m/z 579.0793) and the dipositive
[(BmmOH)2Hg]2+ (m/z 401.0538) species were detected by
mass spectrometry (Figure 3). Ph2Hg was extracted with a
nonpolar solvent (hexane) and characterized by NMR
spectroscopy. The 199Hg NMR of Ph2Hg (0.1 M) was detected
at −814 ppm in DMSO-d6.29
Figure 4. (a) Synthetic route for the synthesis of [(BmmMe)2Hg]-
1
(BF4)2 and the mass spectrum of [(BmmMe)2Hg]2+. (b) H NMR
spectra, in DMSO-d6, of BmmMe, a mixture of BmmMe and [EtHg]BF4,
and isolated [(BmmMe)2Hg](BF4)2 (*, DMSO-d6; #, H2O in DMSO-
d6).
Supporting Information). The 199Hg NMR signals of
[(BmmMe)HgR]BF4 were observed at −1154 ppm (R = Ph),
−873 ppm (R = Et), and −750 ppm (R = Me), respectively.
However, at high temperature (37 °C), the 1:1 complexes
[(BmmMe)HgR]BF4 gradually degraded to the stable tetra-
coordinated mononuclear Hg(II) complex [(BmmMe)2Hg]-
(BF4)2, where a BF4− ion serves as the counterion (m/z for M2+
341.0377; 199Hg NMR signal appeared at −719 ppm), and
1
R2Hg in solution. H NMR of the isolated cleaved product
[(BmmMe)2Hg](BF4)2 showed large downfield chemical shifts
of the N−CH2−N group (0.36 ppm) and the olefinic hydrogen
atoms of imidazole rings (0.4 ppm) in solution in comparison
to those in free BmmMe molecule (Figure 4b).
The single-crystal X-ray structure analysis of [(BmmMe)2Hg]-
(BF4)2 showed that both S atoms of BmmMe are indeed
coordinated to the same Hg(II) atom in a rather unsymmetrical
manner (Figure 5). The S1 (or S1′) atom of BmmMe is
coordinated very strongly to the Hg(II) atom (dHg−S1 = 2.465
Å, dHg−S1′ = 2.458 Å), whereas the S2 atom (or S2′) of the same
BmmMe molecular unit is coordinated weakly to the same
mercury center (dHg−S2 = 2.651 Å, dHg−S2′ = 2.613 Å). As a
consequence, the C−S bond length between the C2A and S1
Although the coordination complexes of BmmMe and BmeMe
with other metal ions, except mercury, have been reported in
the literature,22,30 cleavage of the Hg−C bonds of organo-
mercurials by BmmMe or BmeMe has not been studied yet.
Rabinovich et al. has reported a coordination complex of
Hg(II) with the [S2]-donor ligand bis(mercaptoimidazolyl)-
borate (BmR)31 but no coordination complex of BmmMe or
BmeMe with mercury has been reported in the literature. To
investigate the effect of N substitution on the cleavage of the
Hg−C bond, we have treated BmmMe with [RHg]BF4 (R = Ph,
Et, Me) under identical reaction conditions to produce the 1:1
atoms of [(BmmMe)2Hg](BF4)2 (dC2A−S1 = 1.714 Å; dC2A′−S1′
=
1.720 Å) is slightly longer than the other C−S bond length of
the same BmmMe molecular unit, i.e. between the C2B and S2
C
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= 2.279−2.628 Å) with the F atoms of BF4− anions in the solid
state.
To understand the effect of imidazole-based [S2]-donor
ligands on RHgCl, we have treated BmmMe with RHgCl (R =
Me, Et, Ph), in a 1:1 molar ratio, which afforded the immediate
formation of the 1:1 adduct (BmmMe)HgRCl (Figure 7a). The
Figure 5. ORTEP diagram of [(BmmMe)2Hg](BF4)2. Counterions are
omitted for clarity (CCDC1534008).
atoms (dC2B−S2 = 1.704 Å; dC2B′−S2′ = 1.705 Å), and significantly
longer than the C−S bond length in the free BmmMe molecule
(dC−S = 1.680 Å). The bond angle among the S1, Hg, and S1′
atoms is wider (S1−Hg−S1′ = 132.65°) in comparison to the
bond angle among S2, Hg, and S2′ atoms (S2−Hg−S2′ =
102.46°), which is quite expected considering the strong
coordination of S1 and S1′ atoms to the Hg(II) atom.
Figure 7. (a) Chemical structures of (BmmMe)HgRCl and (BmmOH)-
Hg2Ph2Cl2. (b) Formation of (BmmMe)2Hg2Cl4 in the reaction of
BmmMe with RHgCl (1:1 molar ratio).
affinity of Cl− to mercury is very high; therefore, it presumably
remains intact with mercury in solution, which is further
supported by the isolation and X-ray structure determination of
(BmmOH)Hg2Ph2Cl2 (CCDC 1534015) from a solution of
BmmOH and PhHgCl, in a 1:2 molar ratio, at low temperature
(Supporting Information). The adducts (BmmMe)HgRCl,
however, were unstable in solution at 37 °C and led to the
facile cleavage of the Hg−C bonds and yielded the formation of
the stable 16-membered neutral metallacycle (BmmMe)2Hg2Cl4
as the end product, in which the two chloride ions are
From X-ray crystal structure analysis and quantum
mechanical calculations (DFT) of [S2]-donor ligands it is
quite evident that the structures of free BmmOH, BmmMe, and
BmeMe primarily exist in zwitterionic form with negative charge
on the S atoms and the delocalized positive charge on the
imidazole ring (Table 2, Figure S5 in the Supporting
Information).30l Moreover, because of the coordination to the
Hg(II) center by p-type sulfur lone pairs, the overall positive
charge on the imidazole rings of [(BmmMe)2Hg](BF4)2 is
expected to increase further in comparison to that observed in
free ligands. Crystal packing arrangements of [(BmmMe)2Hg]-
(BF4)2 in the solid state showed the presence of multiple
nonbonded intermolecular C−H···F hydrogen-bonding inter-
actions (Figure 6). The olefinic H atoms (or (imidazole)CH
atom), the H atoms of the N−Me group, and the H atoms of
the N−CH2−N group are engaged in strong nonbonded
intermolecular C−H···F hydrogen-bonding interactions (dCH···F
1
covalently attached with each mercury center (Figure 7b). H
NMR spectroscopy was used to monitor the cleavage of Hg−C
bonds in the reaction of RHgCl with BmmMe or BmmOH (in a
1:1 molar ratio), as illustrated in Figure 8 and the Supporting
Information.
During the crystallization process of solutions containing
BmmMe and RHgCl ([BmmMe]/[RHgCl] = 1) we have isolated
two different types of crystals; one was the diamond-shaped
crystal (BmmMe)2Hg2Cl4, which was always cocrystallized with
various solvent molecules (CCDC 1534011, (BmmMe)2Hg2Cl4·
2ACN; CCDC 1534012, (BmmMe)2Hg2Cl4·2DMF), and the
other one was needle-shaped crystals of the starting material
BmmMe (Table 3). The X-ray structure determination of the
isolated product (BmmMe)2Hg2Cl4 showed very interesting
structural features (Figure 9). Two sulfur atoms of BmmMe are
coordinated symmetrically and somewhat strongly to the two
different mercury centers, leading to the formation of the 16-
membered metallacycle (BmmMe)2Hg2Cl4. As a result, a
significant amount of increase in C−S bond lengths of
coordinated BmmMe in (BmmMe)2Hg2Cl4 is observed in
comparison to those in free BmmMe. The lengths of Hg−S
and C−S bonds in (BmmMe)2Hg2Cl4 are 2.490 and 1.714 Å,
respectively. Interestingly, (BmmMe)2Hg2Cl4 contains two
completely different types of Hg−Cl bonds (dHg1−Cl1 = 2.521
Å, dHg1−Cl2 = 2.655 Å, dHg2−Cl3 = 2.655 Å, dHg2−Cl4 = 2.521 Å).
Figure 6. Crystal packing diagram of [(BmmMe)2Hg](BF4)2 showing
the nonbonded intermolecular C−H···F hydrogen-bonding interac-
−
tions with BF4 anions (image drawn with Mercury 3.8 software).
D
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[(BmmMe)2Hg](BF4)2 respectively, indicating that the anion
(BF4 or Cl−) has a significant role in deciding the final
−
products in these reactions, we have investigated the effect of
BmmMe on MeHgI. Treatment of MeHgI with BmmMe in a 1:1
molar ratio in dichloromethane afforded a white precipitate of
the 1:1 complex (BmmMe)HgMeI at room temperature, which
was isolated by filtration and characterized thoroughly by
various techniques. However, when a solution of this complex
(BmmMe)HgMeI in methanol was stirred at high temperature
(37 °C), we observed a gradual formation of Me2Hg and the
cleaved product [(BmmMe)HgI2]m·(MeHgI)n, which was
crystallized from methanol (Scheme 1 and Figure 11).
Scheme 1. Synthetic Routes for the Synthesis of
[(BmmMe)HgI2]m·(MeHgI)n and BmeMeHgI2
Figure 8. 1H NMR spectra, in DMSO-d6, of solutions containing
BmmMe and EtHgCl (a), BmmMe and MeHgCl (b), and isolated
(BmmMe)Hg2Cl4 (*, DMSO-d6; #, H2O in DMSO-d6).
Figure 9. ORTEP diagram of (BmmMe)2Hg2Cl4.
The two chlorine atoms Cl1 and Cl4, which are oriented
toward the outside of the 16-membered ring, formed shorter
Hg−Cl bonds (2.521 Å), whereas the other two Cl atoms, Cl2
and Cl3, which are sandwiched between the two imidazole
rings, formed significantly longer Hg−Cl bonds (2.650 Å), as
shown in Figure 10.
Figure 11. 1H NMR spectra in DMSO-d6 showing the cleavage of the
Hg−C bond of MeHgl by BmmMe (a) and BmeMe (b), in a 1:1 molar
ratio, and the formation of Me2Hg (*, DMSO-d6; #, H2O in DMSO-
d6; ≠, mesitylene; [MeHgI] = 0.1 M).
In view of the fact that the reactions of BmmMe with RHgBF4
and RHgCl afforded completely two different types of
coordination compounds, namely (BmmMe)2Hg2Cl4 and
X-ray analysis of colorless needle-shaped crystals showed the
formation of [(BmmMe)HgI2]m·(MeHgI)n, where MeHgI was
cocrystallized with the cleaved product [(BmmMe)HgI2]m (m =
1
n, Figure 12; CCDC 1534014). Thus, H NMR spectroscopic
studies and an X-ray structure analysis have confirmed that the
[S2]-donor ligands can facilitate the cleavage of the Hg−C
bond of MeHgI. X-ray structure analysis of [(BmmMe)HgI2]m·
(MeHgI)n revealed that the two S atoms of BmmMe interact
with the two different mercury centers, resulting in the
formation of a polymeric structure, as illustrated in Figures
12 and 13. As both S atoms of the CS moieties of BmmMe
Figure 10. Molecular structure of (BmmMe)2Hg2Cl4 showing the
orientation of Cl atoms in the molecule.
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(dC2A‑S1 = 1.706 Å; dC2B−S2 = 1.698 Å) from 1.680 Å in the free
BmmMe molecule. The high affinity of iodide ion for Hg(II) has
been confirmed through the presence of two Hg−I bonds in
[(BmmMe)HgI2]m. The two Hg−I bonds in [(BmmMe)HgI2]m·
(MeHgI)n are equal in length with an average bond distance of
2.738 Å, which is slightly longer than the Hg−I bond distance
(2.62 Å) present in MeHgI that is cocrystallized with the
complex. Interestingly, the crystal packing of [(BmmMe)HgI2]m·
(MeHgI)n showed that MeHgI molecules formed a bridge
between the two parallel 1D linear chains of [(BmmMe)HgI2]m.
The iodine atom of MeHgI involves nonbonded intermolecular
C−H···I hydrogen-bonding interactions with the olefinic H
atom of the imidazole ring of one chain. On the other hand, the
H atom of the methyl group of MeHgI forms a nonbonded
interaction with the π electrons of the imidazole ring of the
other 1D linear chain of [(BmmMe)HgI2]m (Figure 13).
Figure 12. ORTEP diagram of [(BmmMe)HgI2]m·(MeHgI)n.
We have studied in detail the cleavage of Hg−C bonds of
MeHgX (X = Cl−, I−) induced by [S2]-donor ligands BmmOH
,
BmmMe, and BmeMe to understand the role of the X group of
MeHgX and the spacer group between the two imidazole rings
of [S2]-donor ligands. The cleavage of Hg−C bonds of MeHgX
1
was monitored by H NMR spectroscopy, and all experiments
were performed at room temperature (21 °C) (for more details
see the Experimental Section); the initial rates are given in
Table 1. We found that the initial rates of degradation of Hg−C
bonds of MeHgX (0.1 M) by BmeMe (1:1 molar ratio) are ∼2-
fold greater than those obtained by BmmOH or BmmMe under
identical reaction conditions. This observation clearly suggests
that the spacer between the two imidazole rings of [S2]-donor
ligands (−CH2− in case of BmmOH and BmmMe vs
−CH2CH2− in case of BmeMe) plays an important role in
the cleavage of the Hg−C bond of MeHgX. Interestingly, we
have observed that the rates of cleavage of the Hg−C bond of
MeHgI induced by [S2]-donor ligands are almost 10-fold faster
than those obtained for MeHgCl under identical reaction
conditions, indicating that the X group of MeHgX plays an
important role in the cleavage of Hg−C bonds of MeHgX
induced by [S2]-donor ligands.
X-ray crystal structure analysis and DFT calculations of [S2]-
donor ligands showed that the negative charge on the S atoms
of BmeMe is comparatively more than the negative charge on
the S atoms of BmmOH or BmmMe (Table 2). This might be the
possible reason for the relatively high reactivity of BmeMe over
BmmOH or BmmMe. On the other hand, due to the strong
interaction between mercury and iodide, the Hg−C bond of
MeHgI (2.121 Å) is relatively longer (weaker) than the Hg−C
bond of MeHgCl (2.101 Å); hence, the cleavage of the Hg−C
bond of MeHgI is expected to occur at a faster rate in
comparison to the cleavage of the Hg−C bond of MeHgCl
(Table S6 in the Supporting Information). However, unlike the
case of MeHgI, the rates of cleavage of the Hg−C bond of ionic
[MeHg]BF4 (the Hg−C bond length of [MeHg]BF4 is 2.086
Å) induced by [S2]-donor ligands were found to be extremely
Figure 13. Packing diagram of [(BmmMe)HgI2]m·(MeHgI)n.
interacted almost equally with the tetracoordinated Hg(II)
atom, the two Hg−S bonds are equal in length (dHg1−S1 = 2.620
Å and dHg1′−S2 = 2.615 Å). Likewise, the corresponding C−S
bonds of coordinated BmmMe are also equally elongated
Table 1. Initial Rate of Degradation of MeHg+ Compounds by [S2]-Donor Ligands
a
initial rate (M h−1
)
ligand
MeHgI
MeHgCl
[MeHg]BF4
BmmMe
BmmOH
BmeMe
(1.3 0.1) × 10−3
(1.26 0.1) × 10−3
(2.76 0.3) × 10−3
(1.75 0.3) × 10−4
(1.70 0.1) × 10−4
(3.64 0.5) × 10−4
(5.1 0.3) × 10−5
(4.9 0.02) × 10−5
(7.6 1.5) × 10−5
a
All experiments were done in an NMR tube at room temperature (21 °C) in DMSO-d6 ([MeHgX] = 0.1 M).
F
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volatile Me2Hg.37 In addition, the formation of Me2Hg from
MeHg+ in the presence of H2S,38 inorganic and organic reduced
sulfur surfaces,39 benzimidazole-based thione28b has been
reported in the literature. Me2Hg is known to decompose to
gaseous CH4 and MeHg+ in acidic medium.37,38 We have also
noticed that the Me2Hg obtained in the reaction was
decomposed slowly into CH4 in acidic medium in the presence
of an [S2]-donor ligand.40
In summary, in the reactions of [S2]-donor ligands with
RHgX, in a 1:1 molar ratio, we have observed the formation of
R2Hg and different types of tetracoordinated Hg(II) complexes.
The R2Hg produced in the above reactions further decomposed
to RH in acidic medium. The possible explanation for the
formation of R2Hg could be due to the exchange of R groups
between two mercury centers in solution in the presence of an
electron-donating ligand (BmmMe, BmmMe, or BmeMe), as
illustrated in Figure 15. The exchange of R groups between two
mercury centers in solution has been reported in the
literature.41 To confirm the role of the [S2]-donor ligand in
the exchange of R groups between two Hg−R groups, we have
treated (BmmMe)HgMeCl with PhHgCl (in 1:1 molar ratio) in
Table 2. C−S Bond Lengths, Bond Orders, and Charge on
the S Atoms of [S2]-Donor Ligands
C−S bond length (Å)
a
a
ligand
exptl
calcd
1.448
charge on S (au)
b
BmmMe
BmmOH
BmeMe
1.682
−0.294
−0.298, −0.304
−0.319
1.686, 1.684
1.444, 1.437
1.434
c
1.682
a
Optimization and NBO analysis were performed at the B3LYP/6-
b
c
311++G(2d,p) level of theory. Obtained from ref 22a. Obtained
from ref 30f.
slow, almost 100-fold slower than the rates of cleavage of the
Hg−C bond of MeHgI induced by [S2]-donor ligands under
identical reaction conditions. In fact, we have observed only
∼15−20% cleavage of Hg−C bonds of MeHgCl and
[MeHg]BF4 after 7 days at room temperature (21 °C).
However, the amounts of cleaved products increased at higher
temperature (37 °C). We do need further study in the future to
identify a better molecule which can facilitate the cleavage of
the Hg−C bond of [MeHg]BF4 at a faster rate at room
temperature. Unlike the crystal structure of [(BmmMe)HgI2]m·
(MeHgI)n, where two S atoms of BmmMe are weakly
coordinated to two different Hg(II) atoms (dHg1−S1 = 2.620
Å and dHg1′−S2 = 2.615 Å), leading to the formation of a
polymeric structure, the single-crystal X-ray structure of
(BmeMe)Hgl2, (CCDC 1544311) showed that both S atoms
of BmeMe are coordinated equally strongly to the same Hg(II)
atom (dHg1−S1 = 2.578 Å) (Figure 14).
Figure 14. ORTEP diagram of (BmeMe)HgI2.
The formation of R2Hg as one of the degradation products in
the reactions of RHg+ with [S2]-donor ligands (BmmOH
,
BmmMe, and BmeMe) is quite interesting here. Notably, under
aerobic conditions a few soil and aquatic microorganisms have
shown the unique ability to metabolize phenylmercuric acetate
(PhHgOAc) and one of the major metabolic products,
identified in this case, was diphenylmercury (Ph2Hg).32
Formation of Ph2Hg was also observed in the reactions of
PhHgOAc with different thiol-containing amino acids and 2,3-
dimercaptopropanol (BALH3).33 On the other hand, several
sulfate-reducing bacteria showed mercury tolerance due to their
ability to convert MeHg+ to insoluble HgS(s) and Me2Hg.34−37
In the culture studies with axenic cultures of D. desulfuricans
strain LS, Baldi and co-workers observed the degradation of
soluble MeHg+ to insoluble black metacinnabar (β-HgS), and
Figure 15. Possible mechanisms for formation of stable tetracoordi-
nated Hg(II) complexes in the reaction of BmmMe with RHgX, where
−
X = BF4 (a), Cl− (b), I− (c).
G
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methanol/dichloromethane (1/1) solvent at 21 °C, which
afforded the unsymmetrical arylalkylmercury compound
PhHgMe along with the other expected products Ph2Hg,
Me2Hg, and [(BmmMe)2Hg2Cl4].
Hg−C bonds of MeHgX induced by [S2]-donor ligands
(BmmOH, BmmMe, and BmeMe) are highly dependent on the
X group present in MeHgX and the spacer between the two
imidazole rings of [S2]-donor ligands. The rates of cleavage of
the Hg−C bond of MeHgI induced by [S2]-donor ligands are
found to be almost 10−100-fold faster than the rates obtained
for MeHgCl and [MeHg]BF4, respectively, under identical
reaction conditions. Moreover, the rate of degradation of
MeHgI by BmeMe is almost 2-fold faster than the rate of
degradation of MeHgI by BmmOH or BmmMe. Interestingly, we
noticed that the nature of the stable tetracoordinated Hg(II)
complexes also depends critically on the X group of RHgX and
the [S2]-donor ligands used for the cleavage of Hg−C bonds.
For example, the reactions of BmmMe with ionic [RHg]BF4
afforded the formation of the mononuclear mercury complex
[(BmmMe)2Hg](BF4)2, where both S atoms of BmmMe are
coordinated to the same Hg(II) atom in an unsymmetrical
The 2J(199Hg−1H) coupling constant values for Me2Hg,
PhHgMe, and (BmmMe)HgMeCl in DMSO-d6 are 105, 113,
and 221 Hz respectively (Figure 16).42 PhHgMe was isolated
−
manner and BF4 serves as a counteranion. However, the
reactions of BmmMe with RHgCl afforded the formation of
(BmmMe)2Hg2Cl4, where the two S atoms of BmmMe are
coordinated to the two different Hg(II) atoms, resulting in the
formation of a stable 16-membered cyclic dinuclear mercury
complex. On the other hand, treatment of BmmMe with MeHgI
afforded a polymeric structure of [(BmmMe)HgI2]m·(MeHgI)n,
in which MeHgI is cocrystallized with the cleaved product.
However, a similar reaction with BmeMe afforded the formation
of the discrete mononuclear mercury compound (BmeMe)HgI2.
A common feature of all three of these structures,
(BmmMe)2Hg2Cl4, [(BmmMe)HgI2]m·(MeHgI)n, and (BmeMe)-
HgI2, is that the halides are covalently attached with the
mercury center. Our finding may help in understanding the
coordination behavior of various organomercurials with [S2]-
donor ligands and the mechanism of cleavage of Hg−C bonds
of organomercurials by many aquatic microorganisms.
1
Figure 16. H (showing only the signal of the Hg−CH3 group) and
199Hg NMR spectra of Me2Hg (a), PhHgMe (b), and (BmmMe)-
HgMeCl (c) showing J(199Hg−1H) coupling constants in DMSO-d6.
2
EXPERIMENTAL SECTION
Caution! Organomercurials are highly toxic to humans, and thus
appropriate safety precautions must be taken in handling these toxic
chemicals.
■
from the above reaction mixture by extracting with nonpolar
solvent and characterized thoroughly (Figure 16 and Figure
S41 in the Supporting Information). However, the treatment of
MeHgCl with PhHgCl in a 1:1 molar ratio in the absence of
any electron-donating ligand BmmMe did not produce
PhHgMe, Ph2Hg, and Me2Hg at room temperature in 2 h
(this reaction was performed in an NMR tube in DMSO-d6).
These observations suggest that in the presence of an electron-
donating [S2]-donor ligand (in 1:1 molar ratio), the Hg−R
bonds in 1:1 RHg-conjugated complexes [(BmmMe)HgR]X (X
General Experimental Considerations. Imidazole, phenyl-
mercury chloride, methylmercury iodide, and ethylmercury chloride
were purchased from Alfa Aesar. Methylmercury chloride and sulfur
powder were purchased from Sigma-Aldrich, and other chemicals were
purchased from local companies. All experiments were carried out
under anhydrous and anaerobic conditions using standard Schlenk
techniques for the synthesis. Electrospray ionization high-resolution
mass spectrometry (ESI-HRMS) was carried out with an Agilent 6540
Q-TOF instrument (Agilent Technologies, USA). 1H (400 MHz), 13
C
−
= BF4 ) and (BmmMe)HgRX (X = Cl−, I−) are activated and,
(100 MHz), and 199Hg (71.6 MHz) NMR spectra were obtained on a
Bruker Advance 400 NMR spectrometer using the solvent as an
therefore, undergo facile exchange between them (with another
molecule of 1:1 RHg-conjugated complex) or with the another
molecule of RHgX present in the solution. This possible
mechanism is also well supported by the isolation of crystals of
free ligand BmmMe from the reaction mixture of BmmMe plus
MeHgCl (1:1 molar ratio) and the isolation of crystals of
[(BmmMe)HgI2]m·(MeHgI)n, from the reaction mixture of
BmmMe plus MeHgI (1:1 molar ratio), where MeHgI is
cocrystallized with the cleaved product [(BmmMe)HgI2]m.
In conclusion, we have demonstrated that the [S2]-donor
ligands facilitate the cleavage of Hg−C bonds of various
internal standard for H and 13C. Chemical shifts (1H, 13C) are cited
1
with respect to tetramethylsilane (TMS). 199Hg NMR spectra are
reported in ppm relative to neat Me2Hg (δ 0 ppm), and HgCl2 (δ
−1501 ppm for 1 M solution in DMSO-d6) was used as an external
standard.43
Synthesis of [(BmmOH)HgMe]BF4. To a solution of MeHgCl (50
mg, 0.2 mmol) in 5 mL of dichloromethane at room temperature (21
°C) was added AgBF4 (38.7 mg, 0.2 mmol). The immediate formation
of a white precipitate of AgCl was observed in the above reaction
mixture. The suspension was stirred for 3 h and filtered into a flask
containing BmmOH (59.7 mg, 0.2 mmol) in 5 mL of methanol. The
resulting solution was stirred for 2 h at 21 °C, and solvent was
removed by evaporation to obtain a solid crude product. The crude
product was further washed with 5 mL of acetonitrile and Et2O (2 × 5
mL) and dried under vacuum to give a white solid of [(BmmOH)-
organomercurials RHgX (R = Me, Et, Ph; X = BF4 , Cl−, I−).
−
The reactions of BmmOH, BmmMe, and BmeMe with RHgX
afforded the formation of tetracoordinated Hg(II) complexes
and R2Hg, which is further degraded to RH in acidic medium.
Our experimental results show that the rates of cleavage of the
1
HgMe]BF4. Yield: 75.5 mg, 63%. H NMR (DMSO-d6): δ 0.71 (s,
H
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3H), 3.65−3.69 (q, J = 4.0 Hz, 4H), 3.98−4.01 (t, J = 4.0 Hz, 4H),
4.95 (t, J = 4.0 Hz, 2H), 6.26 (s, 2H), 7.20−7.21 (d, J = 2.8 Hz, 2H),
7.50−7.50 (d, J = 2.8 Hz, 2H). 13C NMR (DMSO-d6): δ 8.9, 49.9,
56.3, 58.4, 118.1, 119.7, 159.6. 199Hg NMR (DMSO-d6): δ −712 (0.1
M at 300 K). HR-ESIMS (m/z): calcd for C12H19N4O2S2Hg [M]+
517.0648, found 517.0644.
2H), 7.15 (br s, 2H), 7.20−7.41 (m, 5H), 7.43 (br s, 2H). 13C NMR
(DMSO-d6): δ 34.6, 56.1, 117.8, 119.0, 127.8, 128.2, 136.7, 151.6,
161.5. 199Hg NMR (DMSO-d6): δ −1154 (0.1 M at 300 K). HR-
ESIMS (m/z): calcd for C15H17N4S2Hg [M]+ 519.0593, found
519.0627.
Synthesis of [(BmmMe)2Hg](BF4)2. To obtain the cleaved product
[(BmmMe)2Hg](BF4)2, the 1:1 complex [(BmmMe)HgPh]BF4 (100
mg, 0.16 mmol) was stirred in acetonitrile/water (1/1) solution for 48
h at 37 °C. Then the solvents were removed under vacuum and the
residue was washed several times with the nonpolar solvent hexane (4
× 5 mL) to remove Ph2Hg followed by Et2O (2 × 5 mL) to obtain
[(Bmmme)2Hg](BF4)2 as a white solid. Yield: 62.1 mg, 44%. A suitable
single crystal was obtained by slow evaporation of an ACN/DMSO
(2/1) solution.1H NMR (DMSO-d6): δ 3.58 (s, 12H), 6.49 (s, 4H),
7.52 (br s, 4H), 7.58 (br s, 4H). 13C NMR (DMSO-d6): δ 35.6, 57.0,
119.3, 121.8, 154.8. 199Hg NMR (DMSO-d6): δ −719 (0.05 M at 300
K). HR-ESIMS (m/z): calcd for C18H24N8S4Hg [M]2+ 341.0348,
found 341.0377.
Synthesis of [(BmmOH)HgEt]BF4. [(BmmOH)HgEt]BF4 was
synthesized following a procedure similar to that for [(BmmOH)-
HgMe]BF4 by using EtHgCl (50 mg, 0.19 mmol) in place of MeHgCl,
AgBF4 (36.7 mg, 0.19 mmol), and BmmOH (56.6 mg, 0.19 mmol).
1
Yield: 78.8 mg, 68%. H NMR (DMSO-d6): δ 1.24−1.28 (t, J = 8.0
Hz, 3H), 1.57−1.64 (q, J = 8.0 Hz, 2H), 3.68−3.71 (t, J = 7.6 Hz, 4H),
4.09−4.12 (t, J = 7.6 Hz, 4H), 5.07 (s, 2H), 6.38 (s, 2H), 7.36 (br s,
2H), 7.59−7.59 (d, J = 2.4 Hz, 2H). 13C NMR (DMSO-d6): δ 14.1,
24.2, 56.9, 58.6, 118.8, 120.5, 157.1. 199Hg NMR (DMSO-d6): δ −898
(0.1 M at 300 K). HR-ESIMS (m/z): calcd for C13H21N4O2S2Hg [M]+
531.0804, found 531.0798.
Synthesis of [(BmmOH)HgPh]BF4. [(BmmOH)HgPh]BF4 was
synthesized following a procedure similar to that for [(BmmOH)-
HgMe]BF4 by using PhHgCl (50 mg, 0.16 mmol) in place of
MeHgCl, AgBF4 (31.1 mg, 0.16 mmol), and BmmOH (47.9 mg, 0.16
mmol). Yield: 80.3 mg, 76%. 1H NMR (DMSO-d6): δ 3.65 (br s, 4H),
3.99−4.02 (d, J = 8 Hz, 4H), 4.92 (br s, 2H), 6.19 (s, 2H), 7.115−
7.119 (d, J = 1.6 Hz, 2H), 7.16−7.35 (m, 3H), 7.41−7.50 (m, 3H).
13C NMR (DMSO-d6): δ 49.7, 55.8, 58.3, 117.6, 119.0, 127.8, 128.2,
136.8, 151.4, 161.4. 199Hg NMR (DMSO-d6): δ −1154 (0.1 M at 300
K). HR-ESIMS (m/z): calcd for C17H21N4O2S2Hg [M]+ 579.0805,
found 579.0793.
Synthesis of (BmmMe)HgMeCl. To a solution of BmmMe (50 mg,
0.21 mmol) in 10 mL of dichloromethane was added MeHgCl (52.2
mg, 0.21 mmol), and the resulting suspension was stirred for 2 h at 21
°C. The volatile components were removed by evaporation, and the
residue was washed with Et2O (2 × 5 mL) and then dried over high
vacuum pressure to give (BmmMe)HgMeCl as a white powder. Yield:
1
70.1 mg, 68%. H NMR (DMSO-d6): δ 0.74 (s, 1H), 3.47 (s, 6H),
6.15 (s, 2H), 7.13−7.13 (d, J = 4.0 Hz, 2H), 7.41−7.42 (d, J = 4.0 2H).
13C NMR (DMSO-d6): δ 5.9, 34.5, 56.0, 117.7, 118.9, 162.2. 199Hg
NMR (DMSO-d6): δ −837 (0.1 M at 300 K). HR-ESIMS (m/z):
calcd for C10H15N4S2HgCl [M]+ 492.0115, found C10H15N4S2Hg [M
− Cl]+ 457.0467.
Synthesis of [(BmmOH)2Hg](BF4)2. To obtain the cleaved product
[(BmmOH)2Hg](BF4)2, the 1:1 complex [(BmmOH)HgPh]BF4 (100
mg, 0.15 mmol) was stirred in acetonitrile/water (1/1) solution for 48
h at 37 °C. The solvents were evaporated, and the residue was washed
several times with the nonpolar solvent hexane (4 × 5 mL) to remove
Ph2Hg followed by Et2O (2 × 5 mL) to obtain a white solid of
Synthesis of (BmmMe)HgEtCl. (BmmMe)HgEtCl was synthesized
following a procedure similar to that for (BmmMe)HgMeCl, except
EtHgCl (55.1 mg, 0.21 mmol) was added in place of MeHgCl. Yield:
75.3 mg, 71%. 1H NMR (DMSO-d6): δ 1.23−1.27 (t, J = 7.8 Hz, 3H),
1.61−1.67 (q, J = 7.8 Hz, 2H), 3.46 (s, 6H), 6.14 (s, 2H), 7.13−7.13
(d, J = 2.4 Hz, 2H), 7.40−7.41 (d, J = 2.4 Hz, 2H). 13C NMR
(DMSO-d6): δ 13.7, 22.4, 34.5, 56.0, 117.6, 118.8, 162.34. 199Hg NMR
(DMSO-d6): δ −993 (0.1 M at 300 K). HR-ESIMS (m/z): calcd for
C11H17N4S2HgCl [M]+ 506.0271, found C11H17N4S2Hg [M − Cl]+
471.0633.
1
[(BmmOH)2Hg](BF4)2.Yield: 52.6 mg, 36%. H NMR (DMSO-d6): δ
3.70−3.71 (t, J = 5.2 Hz, 8H), 4.13−4.17 (t, J = 5.6 Hz, 8H), 4.94−
4.96 (t, J = 4.8 Hz, 4H), 6.61 (s, 4H), 7.54−7.55 (d, J = 2.4 Hz, 2H),
7.89−7.90 (d, J = 2.4 Hz, 2H). 13C NMR (DMSO-d6): δ 50.7, 57.3,
58.5, 120.1, 122.0, 154.0. 199Hg NMR (DMSO-d6): δ −443 (0.05 M at
300 K). HR-ESIMS (m/z): calcd for C22H32N8O4S4Hg [M]2+
401.0560, found 401.0538.
Synthesis of (BmmMe)HgPhCl. (BmmMe)HgPhCl was synthe-
sized following a procedure similar to that for (BmmMe)HgMeCl,
except PhHgCl (65.1 mg, 0.21 mmol) was added in place of MeHgCl.
Yield: 91.8 mg, 79%.1H NMR (DMSO-d6): δ 3.48 (s, 6H), 6.18 (s,
2H), 7.15−7.16 (d, J = 4.0 Hz, 2H), 7.20−7.35 (m, 3H), 7.42−7.45
(m, 3H). 13C NMR (DMSO-d6): δ 34.5, 56.1, 117.7, 118.9, 127.8,
128.2, 136.7, 151.3, 161.9. 199Hg NMR (DMSO-d6): δ −1160 (0.1 M
at 300 K). HR-ESIMS (m/z): calcd for C15H17N4S2HgCl [M]+
554.0272, found C15H17N4S2Hg [M − Cl]+ 519.0627.
Synthesis of [(BmmMe)HgMe]BF4. To a solution of MeHgCl (50
mg, 0.2 mmol) in 5 mL of dichloromethane at room temperature (21
°C) was added AgBF4 (38.7 mg, 0.2 mmol). The immediate formation
of AgCl precipitate was observed. The suspension was stirred for 3 h
and filtered into a solution of BmmMe (48.1 mg, 0.2 mmol) in 5 mL of
dichloromethane. The resulting solution was stirred for 2 h at 21 °C,
and solvent was removed by evaporation. The residue was washed with
Et2O (3 × 5 mL), and volatile components were removed under
vacuum pressure to afford [(BmmMe)HgMe]BF4 as a white solid.
Yield: 65 mg, 60%. 1H NMR (DMSO-d6): δ 0.74 (s, 3H), 3.56 (s, 6H),
6.41 (s, 2H), 7.46−7.47 (d, J = 4.0 Hz, 2H), 7.68−7.69 (d, J = 4.0 Hz,
2H). 13C NMR (DMSO-d6): δ 8.5, 35.3, 57.4, 119.7, 121.4, 155.1.
199Hg NMR (DMSO-d6): δ −750 (0.1 M at 300 K). HR-ESIMS (m/
z): calcd for C10H15N4S2Hg [M]+ 457.0436, found 457.0467.
Synthesis of [(BmmMe)HgEt]BF4. [(BmmMe)HgEt]BF4 was
synthesized following a procedure similar to that for [(BmmMe)-
HgMe]BF4 by using EtHgCl (50 mg, 0.19 mmol) in place of MeHgCl,
AgBF4 (36.7 mg, 0.19 mmol), and BmmMe (45.6 mg, 0.19 mmol).
Yield: 60 mg, 68%. 1H NMR (DMSO-d6): δ 1.22−1.26 (t, J = 7.6 Hz,
2H), 1.60−1.64 (q, J = 7.6 Hz, 2H), 3.47 (s, 6H), 6.16 (s, 2H), 7.12−
7.13 (d, J = 1.6 Hz, 2H), 7.41−7.410 (d, J = 1.6 Hz, 2H). 13C NMR
(DMSO-d6): δ 13.8, 22.6, 34.5, 56.1, 117.7, 118.9, 162.0. 199Hg NMR
(DMSO-d6): δ −873 (0.1 M at 300 K). HR-ESIMS (m/z): calcd for
C11H17N4S2Hg [M]+ 471.0592, found 471.0633.
Synthesis of (BmmMe)2Hg2Cl4. (BmmMe)2Hg2Cl4 was obtained
from the reaction solution of (BmmMe)HgPhCl (100 mg, 0.18 mmol)
in acetonitrile/water (1/1) after stirring for 48 h at 37 °C. Solvent was
removed under reduced pressure, and the resulting precipitate was
washed several times with hexane (2 × 5 mL) to remove Ph2Hg
followed by Et2O (2 × 5 mL) to obtain the cleaved product
(BmmMe)2Hg2Cl4 as a white solid. Yield: 72.4 mg, 42%. The pure
compound (BmmMe)2Hg2Cl4 was crystallized in various solvents to
obtain suitable single crystals of (BmmMe)2Hg2Cl4, which was always
cocrystallized with the solvent molecules, as confirmed by an X-ray
1
structure determination. H NMR (DMSO-d6): δ 3.62 (s, 12H), 6.57
(s, 2H), 7.53−7.54 (d, J = 4.0 Hz, 2H), 7.85−7.85 (d, J = 4.0 Hz, 2H).
13C NMR (DMSO-d6): δ 35.8, 57.3, 120.3, 122.1, 154.1. 199Hg NMR
(DMSO-d6): δ −886 (0.05 M at 300 K). HR-ESIMS (m/z): calcd for
C18H24N8S4Hg2Cl4 [M + H]+ 1023.95, found [M − HgCl3]+
717.0430, [M − C9H12N4S2HgCl3]+ 476.9911.
Synthesis of [(BmmMe)HgPh]BF4. [(BmmMe)HgPh]BF4 was
synthesized following a procedure similar to that for [(BmmMe)-
HgMe]BF4 by using PhHgCl (50 mg, 0.16 mmol) instead of MeHgCl,
AgBF4 (31.1 mg, 0.16 mmol), and BmmMe (38.4 mg, 0.16 mmol).
Synthesis of (BmmOH)HgMeCl. To a solution of BmmOH (50 mg,
0.16 mmol) in 10 mL of methanol was added MeHgCl (52.2 mg, 0.16
mmol), and the resulting suspension was stirred for 2 h at 21 °C. The
volatile components were removed by evaporation, and the residue
was washed with Et2O (2 × 5 mL) and then dried over high vacuum
1
Yield: 71.6 mg, 74%. H NMR (DMSO-d6): δ 3.48 (s, 6H), 6.18 (s,
I
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pressure to give (BmmOH)HgMeCl as a white solid. Yield: 54.7 mg,
62%. 1H NMR (DMSO-d6): δ 0.74 (s, 3H), 3.63−3.67 (q, J = 4.0 Hz,
4H), 3.99−4.01 (t, J = 4.0 Hz, 4H), 4.90−4.92 (t, J = 4.0 Hz, 2H), 6.18
(s, 2H), 7.09−7.10 (d, J = 2.8 Hz, 2H), 7.43−7.43 (d, J = 2.8 Hz, 2H).
13C NMR (DMSO-d6): δ 5.85, 49.6, 55.7, 58.2, 117.5, 118.9, 162.0.
199Hg NMR (DMSO-d6): −842 (0.1 M at 300 K). HR-ESIMS (m/z):
calcd for C12H19N4O2S2HgCl [M]+ 552.0326, found C12H19N4O2S2Hg
[M − Cl]+ 517.0644.
199Hg NMR (DMSO-d6): δ −1092 (0.1 M at 300 K). HR-ESIMS (m/
z): calcd for C11H17N4S2HgI [M]+ 597.9637, found C11H17N4S2Hg [M
− I]+ 471.0641.
Synthesis of (BmeMe)HgI2. (BmeMe)HgI2 was synthesized
following a procedure similar to that for [(BmmMe)HgI2]m·(MeHgI)n,
except that (BmeMe)HgMeI (25 mg, 0.041 mmol) was added in place
of (BmmMe)HgMeI. Yield: 20.1 mg, 69%. Suitable block-type single
crystals were obtained by slow evaporation of a methanolic solution of
1
(BmeMe)HgI2. H NMR (DMSO-d6): δ 3.59 (s, 6H), 4.81 (s, 2H),
Synthesis of (BmmOH)HgEtCl. (BmmOH)HgEtCl was synthesized
following a procedure similar to that for (BmmOH)HgMeCl, except
EtHgCl (42.4 mg, 0.16 mmol) was added in place of MeHgCl. Yield:
59.7 mg, 66%. 1H NMR (DMSO-d6): δ 1.23−1.25 (t, J = 8.0 Hz, 3H),
1.61−1.67 (q, J = 8.0 Hz, 2H), 3.63−3.71 (q, J = 5.2 Hz, 4H), 3.98−
4.01 (t, J = 5.2 Hz, 4H), 4.91−4.93 (t, J = 5.20 Hz, 2H), 6.18 (s, 2H),
7.09−7.10 (d, J = 2.4 Hz, 2H), 7.43−7.43 (d, J = 2.4 Hz, 2H). 13C
NMR (DMSO-d6): δ 13.8, 22.6, 49.6, 55.7, 58.2, 117.5, 118.9, 162.0.
199Hg NMR (DMSO-d6): δ −1004 (0.1 M at 300 K). HR-ESIMS (m/
7.41−7.41 (d, J = 2.0 Hz, 2H), 7.49−7.50 (d, J = 2.0 Hz, 2H). 13C
NMR (DMSO-d6): δ 35.9, 45.5, 120.5, 121.5, 153.6. 199Hg NMR: not
observed.
Isolation of Ph2Hg. The nonpolar product of Ph2Hg was extracted
from the reaction solution of BmmOH (or BmmMe or BmeMe) with
PhHgCl (1:1 molar ratio) by using hexane. The hexane solution was
concentrated and dried under high vacuum to yield a white solid of
1
Ph2Hg. Yield: ∼22%. H NMR (DMSO-d6): δ 7.13−7.17 (t, J = 7.4
Hz, 2H), 7.32−7.36 (t, J = 7.6 Hz, 4H), 7.51−7.53 (dd, J = 7.8 Hz,
4H). 13C NMR (DMSO-d6): δ 127.1, 127.8, 138.1, 170.9. 199Hg NMR
(DMSO-d6): δ −814 (0.1 M at 300 K).
z): calcd for
C
13H21N4O2S2HgCl [M]+ 566.0483, found
C13H21N4O2S2Hg [M − Cl]+ 531.0798.
Synthesis of (BmmOH)HgPhCl. (BmmOH)HgPhCl was synthe-
sized following a procedure similar to that for (BmmMe)HgMeCl,
except PhHgCl (52.1 mg, 0.16 mmol) was added in place of MeHgCl.
Isolation of PhHgMe. To a solution of (BmmOH)HgMeCl or
(BmmMe)HgMeCl in chloroform/methanol (1:1) was added 1 equiv
of PhHgCl, and the reaction mixture was stirred for 24 h at room
temperature. After 24 h, the reaction mixture was extracted by the
nonpolar solvent hexane. The solvent was concentrated and dried
under high vacuum to yield a white solid of PhHgMe. Yield: ∼14%. 1H
NMR (DMSO-d6): δ 0.36 (s, 3H), 7.09−7.13 (t, J = 7.4 Hz, 2H),
7.29−7.33 (t, J = 7.6 Hz, 4H), 7.41−7.43 (dd, J = 7.8 Hz, 4H). 13C
NMR (DMSO-d6): δ 13.7, 126.8, 127.9, 137.3, 178.1. 199Hg NMR
(DMSO-d6): δ −457 (0.05 M at 300 K).
1
Yield: 70.6 mg, 72%. H NMR (DMSO-d6): δ 3.64−3.67 (t, J = 5.2
Hz, 4H), 3.99−4.01 (t, J = 5.2 Hz, 4H), 4.93 (br s, 2H), 6.20 (s, 2H),
7.11−7.12 (d, J = 1.6 Hz, 2H), 7.19−7.22 (t, J = 7.2 Hz, 1H), 7.26−
7.30 (t, J = 7.2 Hz, 2H), 7.41−7.43 (d, J = 7.2 Hz, 2H), 7.45−7.46 (t, J
= 1.6 Hz, 2H). 13C NMR (DMSO-d6): δ 49.7, 55.8, 58.3, 117.7, 119.1,
127.8, 128.2, 136.8, 151.9, 161.4. 199Hg NMR (DMSO-d6): δ −1156
(0.1 M at 300 K). HR-ESIMS (m/z): calcd for C17H21N4O2S2HgCl
[M]+ 614.0484, found C17H21N4O2S2Hg [M − Cl]+ 579.0793.
Synthesis of [(BmmOH)2Hg2Cl4]. (BmmOH)2Hg2Cl4 was synthe-
sized following a procedure similar to that for (BmmMe)2Hg2Cl4,
except (BmmOH)HgPhCl (100 mg, 0.16 mmol) was added in place of
(BmmMe)HgPhCl. Yield: 100 mg, 54%. 1H NMR (DMSO-d6): δ
3.69−3.72 (t, J = 5.4 Hz, 8H), 4.13−4.17 (br s, 8H), 4.97 (s, 4H), 6.61
(s, 4H), 7.56 (br s, 4H), 7.90−7.91 (d, J = 2.4 Hz, 4H), 13C NMR
(DMSO-d6): δ 51.0, 57.2, 58.5, 120.6, 122.2, 152.3. HR-ESIMS (m/z):
calcd for C22H32N8S4O4Hg2Cl4 [M + H]+: 1143.9541, found [M −
HgCl3]+ 837.0792, [M − C11H16N4S2O2HgCl3]+ 537.0085.
Synthesis of (BmmOH)Hg2Ph2Cl2. To a solution of BmmOH (25
mg, 0.043 mmol) in 5 mL of methanol was added PhHgCl (27 mg,
0.086 mmol), and the reaction mixture was stirred for 4 h at 21 °C.
After completion of the reaction, methanol was removed under
vacuum to obtain (BmmOH)Hg2Ph2Cl2 as a white precipitate. Suitable
single crystals of (BmmOH)Hg2Ph2Cl2 were obtained after slow
evaporation of the solvent mixture ACN/DMSO (2/1). 1H NMR
(DMSO-d6): δ 3.5 (br s, J = 5.2 Hz, 4H), 3.99−4.01 (t, J = 5.2 Hz,
4H), 4.93 (br s, 2H), 6.20 (s, 2H), 7.11−7.12 (d, J = 1.6 Hz, 2H),
7.19−7.22 (t, J = 7.2 Hz, 1H), 7.26−7.30 (t, J = 7.2 Hz, 2H), 7.41−
7.43 (d, J = 7.2 Hz, 2H), 7.45−7.46 (t, J = 1.6 Hz, 2H). 13C NMR
(DMSO-d6): δ 49.7, 55.9, 58.3, 117.7, 119.2, 127.9, 128.2, 136.8,
151.4, 161.2. 199Hg NMR (DMSO-d6): δ −1177 (0.1 M at 300 K).
Procedure for Kinetic Studies. The cleavage of Hg−C bonds of
Synthesis of (BmmMe)HgMeI. To a solution of BmmMe (50 mg,
0.21 mmol) in 10 mL of dichloromethane was added MeHgI (71.2
mg, 0.21 mmol), and the reaction mixture was stirred for 2 h at 21 °C.
The solvent was removed by evaporation, the residue was washed with
Et2O (2 × 5 mL), and volatile components were removed under
vacuum pressure to give (BmmMe)HgMeI as a white solid. Yield: 70.3
mg, 58%. 1H NMR (DMSO-d6): δ 0.89 (s, 1H), 3.47 (s, 6H), 6.14 (s,
2H), 7.13−7.13 (d, J = 2.4 Hz, 2H), 7.40−7.41 (d, J = 2.4 Hz, 2H).
13C NMR (DMSO-d6): δ 14.7, 35.0, 56.5, 118.2, 119.3, 162.9. 199Hg
NMR (DMSO-d6): δ −1133 (0.1 M at 300 K). HR-ESIMS (m/z):
calcd for C10H15N4S2HgI [M]+ 583.9481, found C10H15N4S2Hg [M −
I]+ 457.0467.
−
MeHgX (X = BF4 , Cl−, I−) induced by [S2]-donor ligands was
followed by 1H NMR spectroscopy, and all experiments were
performed in NMR tubes at 21 °C. In general, to a solution of
MeHgX (0.1 M) in DMSO-d6 was added the [S2]-donor ligand
(BmmMe/BmeMe/BmmOH) in a 1:1 molar ratio in an NMR tube, and
this mixture was kept in a shaker for continuous gentle shaking at 21
1
°C. H NMR spectra were recorded at various times. In order to
calculate the rate of demethylation of MeHgX by [S2]-donor ligands,
we used mesitylene (12 mM) as an external standard. Integral values of
the singlet resonance of methyl protons (−CH3) of MeHgX (at
various time points) with respect to the aromatic singlet resonance of
mesitylene (which is considered as 1 and it does not interfere with
other signals) were used for calculating the rate. The initial value at 0
min was considered as 100% of MeHgX (0.1 M).
Synthesis of [(BmmMe)HgI2]m·(MeHgI)n. To obtain the cleavage
product [(BmmMe)HgI2]m·(MeHgI)n, we stirred the 1:1 complex
(BmmMe)HgMeI (25 mg, 0.043 mmol) in 5 mL of methanol for 48 h
at 37 °C. The solvent was removed, and the crude product was washed
several times with acetonitrile followed by Et2O (2 × 5 mL). The
product was dried under reduced pressure for several hours to obtain a
yellow solid of [(BmmMe)HgI2]m·(MeHgI)n. Yield: 12 mg, 42%.
Suitable needle-shaped single crystals were obtained from a methanol
solution of [(BmmMe)HgI2]m·(MeHgI)n. 1H NMR (DMSO-d6): δ
3.55 (s, 6H), 6.54 (s, 2H), 7.47−7.48 (d, J = 4.0 Hz, 2H), 7.91−7.92
(d, J = 4.0 Hz, 2H). 13C NMR (DMSO-d6): δ 35.8, 56.9, 119.8, 121.5,
156.7. 199Hg NMR (DMSO-d6): δ −2126 (0.025 M at 300 K).
Synthesis of (BmeMe)HgMeI. (BmeMe)HgMeI was synthesized
following a procedure similar to that for (BmmMe)HgMeI, except that
BmeMe (53.4 mg, 0.21 mmol) was added in place of BmmMe. Yield:
X-ray Crystal Analysis. Crystal structures of these compounds
were determined by measuring X-ray diffraction data on a D8 Venture
Bruker AXS single-crystal X-ray diffractometer equipped with a CMOS
PHOTON 100 detector having monochromatised microfocus sources
(Mo Kα = 0.71073 Å). Single crystals of [(BmmMe)2Hg](BF4)2
(CCDC 1534008), [(BmmMe)2Hg2Cl4]·ACN (CCDC 1534011),
[(BmmMe)2Hg2Cl4]·DMF (CCDC 1534012), (BmmOH)Hg2Ph2Cl2
(CCDC 1534015), [(BmmMe)HgI2]m·(MeHgI)n (CCDC 1534014),
and BmeMeHgI2 (CCDC 1544311) suitable for X-ray diffraction
studies were obtained from a slow evaporation process using various
solvents.43 All crystal data were collected at room temperature.
Structures were solved using the SHELX program implemented in
1
95.2 mg, 76%. H NMR (DMSO-d6): δ 0.87 (s, 1H), 3.46 (s, 6H),
4.32 (s, 4H), 6.87−6.88 (d, J = 2.4 Hz, 2H), 7.10−7.11 (d, J = 2.4 Hz,
2H). 13C NMR (DMSO-d6): δ 14.0, 34.5, 45.2, 117.4, 118.6, 161.0.
J
DOI: 10.1021/acs.inorgchem.7b01048
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Crystallographic Data and Refinement Parameters for [(BmmMe)2Hg](BF4)2 (I), [(BmmMe)2Hg2Cl4]·2ACN (II),
[(BmmMe)2Hg2Cl4]·2DMF (III), (BmmOH)Hg2Ph2Cl2 (IV), [(BmmMe)HgI2]m·(MeHgI)n (V), and (BmeMe)HgI2 (VI)
I
II
III
IV
V
VI
1544311
CCDC no.
lattice
1534008
triclinic
1534011
1534012
1534015
1534014
monoclinic
C22H30Cl4Hg2N10S4
1105.78
C2/m
monoclinic
C24H38Cl4Hg2N10O2S4
1169.86
C2/c
monoclinic
C23H26Cl2Hg2N4O2S2
926.68
monoclinic
C10H15Hg2I3N4S2
1037.26
P21/c
monoclinic
C10H12.21HgI2N4S2
706.96
Pnma
formula
C18H24B2F8HgN8S4
854.9
formula wt
space group
a/Å
P1
C2/c
̅
11.1499(18)
11.791(2)
13.963(2)
66.115(7)
68.598(7)
64.060(6)
1469.5(4)
2
17.3949(14))
12.0892(14)
10.6711(10)
90
18.3396(12)
12.3326(12)
18.0958(13)
90
18.3515(9)
14.7223(7)
13.7269(6)
90
12.1256(12)
8.3332(8)
21.188(2)
90
9.1474(7)
12.9853(9)
14.4597(10)
90
b/Å
c/Å
α/deg
β/deg
126.202(4)
90
109.396
90
111.942(2)
90
97.800(4)
90
90
γ/deg
V/Å3
90
1810.8(3)
2
3860.5(5)
4
3440.0(3)
4
2121.1(4)
4
1717.5(2)
4
Z
temp/K
300(2)
0.71073
1.932
297(2)
0.71073
2.028
297(2)
0.71073
2.013
298(2)
0.71073
1.789
301(2)
0.71073
3.248
299(2)
0.71073
2.741
radiation (λ)/Å
ρ/g cm−3
μ(Mo Kα)/mm−1
θmax/deg
5.598
9.024
8.476
9.214
19.02
12.791
25.682
16589
26.022
25314
25.679
31550
26.024
69460
25.684
26.462
53466
no. of data collected
no. of data
no. of params
R1 (I > 2σ(I))
wR2 (I > 2σ(I))
R1 (all data)
wR2 (all data)
Rint (all data)
GOF
32252
5790
1809
3803
3287
4263
1694
388
108
213
150
194
93
0.041
0.018
0.018
0.0457
0.034
0.0258
0.0615
0.0277
0.0625
0.0428
1.136
0.099
0.042
0.037
0.1347
0.075
0.052
0.02
0.024
0.0544
0.062
0.105
0.042
0.039
0.1418
0.0862
0.0952
1.026
0.0568
1.017
0.0557
1.037
0.0458
1.035
0.0831
1.064
APEX3.43−48 The non-H atoms were located in successive difference
Fourier syntheses and refined with anisotropic thermal parameters. All
hydrogen atoms were placed at calculated positions and refined using a
riding model with appropriate HFIX commands. The program
Mercury was used for molecular packing analysis.49 The disordered
fluorine atoms (F1, F2, F3, F5, F7, and F8) of [(BmmMe)2Hg](BF4)2
were treated using the PART command with occupancy 36%:64%
(FA:FB). In addition, the anisotropic displacement parameters for
disordered fluorine atoms were fixed using the EADP restraint.50 The
crystal structure of (BmmOH)Hg2Ph2Cl2 has disorder in the solvent
molecules, which was removed by the SQUEEZE option using
PLATON software.51 Crystallographic data and refinement parameters
for I−VI are given in Table 3.
Accession Codes
CCDC 1517829, 1534008, 1534011−1534012, 1534014−
1534015, and 1544311 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Computational Details. All the calculations were carried out
using B3LYP level of theory as implemented in the Gaussian 09
package.52 The 6-311++G(2d,p) basis set was used for all atoms
(except Hg and I), whereas the Stuttgart−Dresden basis set (SDD)
was used for Hg and I atoms with respective relativistic effective core
potentials.53 Frequency calculations of all the optimized structures
were performed to ensure that the optimized structures were the local
energy minima structure without any imaginary frequencies. The NBO
Version 3.1 program implemented in Gaussian 03 was used to perform
NPA.54
*E-mail for G.R.: Gouriprasanna.Roy@snu.edu.in.
ORCID
Gouriprasanna Roy: 0000-0002-0047-3575
Author Contributions
G.R. and R.K. planned the research work, designed experi-
ments, and wrote the manuscript. R.K. carried out most of the
experiments mentioned in this paper. R.K., M.B., K.K.J., and
A.C. solved the crystal structures. G.R. and R.K. analyzed and
interpreted data.
ASSOCIATED CONTENT
* Supporting Information
■
Notes
S
The authors declare no competing financial interest.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01048.
ACKNOWLEDGMENTS
■
Synthesis methods, HR-ESIMS analysis of compounds,
1H, 13C, and 199Hg NMR spectra of [S2-donor ligands
and their complexes with RHgX, DFT calculations, and
optimized geometries and coordinates of the optimized
structures (PDF)
We are grateful to Prof. Rupamanjari Ghosh (VC, SNU) for
continuous support and encouragement. R.K., M.B., K.K.J., and
A.C. thank the SNU for fellowships. We thank the SNU and
SERB (SB/S1/IC-44/2013), government of India, for financial
support.
K
DOI: 10.1021/acs.inorgchem.7b01048
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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