Binding of HgCl2 by tripodal ligands controlled by AgPF 6: Receptors for the PF6- anion

Article abstract of DOI:10.1039/b714654j

Mercuric chloride can readily enter into the biological cycle where it gets converted into other forms of mercury and causes severe neurotoxic, genotoxic and immunotoxic effects; a number of tripodal ligands have been synthesized that selectively bind mercuric chloride and in the presence of AgPF₆, mercury can be removed and supramolecular complexes of the PF₆ ^- anion are formed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b714654j

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www.rsc.org/dalton | Dalton Transactions
Binding of HgCl2 by tripodal ligands controlled by AgPF6: receptors for the
−
PF6 anion†‡
Ashutosh S. Singh and Parimal K. Bharadwaj*
Received 24th September 2007, Accepted 15th November 2007
First published as an Advance Article on the web 27th November 2007
DOI: 10.1039/b714654j
Mercuric chloride can readily enter into the biological cycle
where it gets converted into other forms of mercury and causes
severe neurotoxic, genotoxic and immunotoxic effects; a num-
ber of tripodal ligands have been synthesized that selectively
bind mercuric chloride and in the presence of AgPF6, mercury
Mercury is an extremely toxic, corrosive and environmentally
hazardous element. Some mercury salts (such as HgCl2) are
sufficiently volatile to exist as an atmospheric gas.1 Once released
into the biosphere through natural events or human activities,
−
can be removed and supramolecular complexes of the PF6
anion are formed.
Indian Institute of Technology Kanpur, Department of Chemistry, Kanpur,
208016, India
† CCDC reference numbers 657109–657114. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b714654j
‡ Electronic supplementary information (ESI) available: Experimental
details and crystal data. See DOI: 10.1039/b714654j
Scheme 1
◦
˚
Fig. 1 Structures of the Hg(II) complexes and their selected bond lengths (A) and angles ( ). 1 (L1 + HgCl2): N1–Hg 2.583(6), O1–Hg 2.987(2), Hg–Cl1
2.312(2), Hg–Cl2 2.318(2), Cl1–Hg–Cl2 167.91(8), Cl1–Hg–N1 98.86(15), Cl2–Hg–N1 92.68(15); 2 (L2 + HgCl2): N1–Hg 2.540(8), O2–Hg 2.818(2),
O3–Hg 2.811(1), Hg–Cl1 2.303(3), Hg–Cl2 2.323(3), Cl1–Hg–Cl2 162.22(10), Cl1–Hg–N1 99.1(2), Cl2–Hg–N1 98.4(2); 3 (L3 + HgCl2): N1–Hg 2.486(5),
O1–Hg 2.774(2), O2–Hg 2.839(1), Hg–Cl1 2.329(2), Hg–Cl2 2.315(2), Cl1–Hg–Cl2 153.40(6), Cl1–Hg–N1 99.09(13), Cl2–Hg–N1 107.48(12) and 4
(L4 + HgCl2): N1–Hg 2.487(8), O1–Hg 2.840(2), O4–Hg 2.791(2), Hg–Cl1 2.317(3), Hg–Cl2 2.305(3), Cl1-Hg–Cl2 157.38(10), Cl1–Hg–N1 99.52(19),
Cl2–Hg–N1 102.92(19).
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it readily moves and cycles through the environment.2 Some
microorganisms can produce organic mercury, particularly methyl
mercury, from other forms of mercury that can distribute uni-
formly throughout the human body leading to numerous disorders
of the neurological, psychological, nephrological, immunological,
cardiac and reproductive systems.3 The multiple pathway of
mercury through air, water, food, vaccines, cosmetic products, etc.,
poses a serious concern, because it persists in the environment and
accumulates in the food chain.4–8
The existing methods of removal of mercury compounds
through biodegradation lead to secondary contamination from
by-product formation.12 In the present communication, we report
several tripodal ligands containing O,N-donors (Scheme 1) that
reversibly bind HgCl2 and form the basis for probing its removal.
Mercury(II) chloride exists in its molecular form in water13
and readily reacts with nitrogen donors.14 Its binding ability to
ethereal O is also known.15 Its structural versatility is displayed
by the existence of Hg(II)xCly anions.16 So, tripodal ligands
n−
Removal of mercuric chloride, from electrochemical processes
and crude oil that may contain a variety of mercury compounds,
is a challenging problem as such compounds can damage refinery
equipment as well as pollute the environment.9–11
incorporating N and O donors are chosen. Attachment of electron-
withdrawing groups at the terminals are added to reduce the
binding ability of ethereal O atoms towards transition metal ions.
Besides, tripodal ligands are likely to form complexes that are
thermodynamically stable and kinetically less labile.
The ligands L1–L4§ readily react at room temperature open
to atmosphere, with HgCl2 in MeOH or MeCN, affording the
products in 90–95% yield. All the Hg(II) complexes are air stable
and insoluble in common organic solvents at room temperature
but soluble in solvents like MeOH and MeCN on heating. In each
case, the ligand is bound to HgCl2 through the bridgehead N
and one or two ethereal O atoms in the sidegroups. The terminal
cyano groups remain uncoordinated. With L1, a four-coordinate
Hg(II) complex is formed while the remaining three ligands form
five-coordinate Hg(II) complexes (Fig. 1).¶ All bond distances
and bond angles17 involving the metal ions are similar to known
complexes.16
When a hot methanolic solution of either 2 or 3 is treated with
two equivalents of AgPF6, a white precipitate appears almost
instantaneously. After stirring for 30 min, it is filtered and the
filtrate upon evaporation at RT, affords colorless, block-shaped
crystals of 5 and 6 respectively in ∼85% isolated yield.
In the supramolecular complex 5, the protonated N-bridgehead
of one podand is strongly H-bonded to a CN group of another
˚
podand (N1–H1 · · · N4 2.139 A) forming a chain. Two neigh-
˚
boring chains are linked together through C–H · · · p (2.551 A)
−
interactions forming a cavity in the 2D-sheet (Fig. 2) where a PF6
˚
anion is encapsulated through H-bonding (2.425–2.809 A). The
−
encapsulated PF6 anion of one 2D-sheet is further linked to the
neighboring sheet via H-bonding forming an overall 3D network
(Fig. 3). Metal salts in the presence of moisture, generate protons.17
−
Fig. 2 Encapsulation of PF6 anion inside the cavity in 5.
Fig. 3 Packing of 5 showing the 3D architecture in the solid state.
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−
It is presumed that these protons are responsible for protonating
the bridgehead N atoms of the podands.
The PF6 anion encapsulated within is disordered. Each 1D-
chain is linked with another chain through p–p interactions of
the benzene rings (Fig. 4b), forming an overall 3D-architecture.
The supramolecular complex 6 on the other hand, shows a
structure where two (L3 + H)+ units orient themselves to form an
intermolecular capsule holding a PF6− anion (Fig. 4a) inside. This
anion pulls the three arms through H-bonding interactions with
meta-hydrogen of the nearby benzene ring‡ to form a cone-shaped
architecture. The second PF6− anion links two capsules forming a
1D chain.
Few PF6 encapsulated systems are available in the literature.18
−
Either 5 or 6 dissolved in water in the presence of a few drops of
Et3N, can be extracted with CHCl3. Upon removal of CHCl3 the
white residue is dissolved in methanol and treated with HgCl2 to
afford the corresponding Hg(II) complex. The entire cycle may be
represented as in Fig. 5.
Fig. 5 Schematic representation of removal of HgCl2 by a tripodal
podand (L3).
In conclusion, we are able to show that HgCl2 can be removed
reversibly with tripodal podands in the presence of interfering
metal ions. The present work will provide a new insight for removal
of HgCl2 over existing technology. These podands can be made
easily from commercially available cheap starting compounds.
−
Encapsulation of the PF6 anion in the cyclic process of the
removal of HgCl2, also suggests a new way to make selective and
effective anion receptors.
Notes and references
§ Selected spectroscopic data: L1: Yield 75%; Anal. calcd for C27H24N4O3:
1
C, 71.67; H, 5.35; N, 12.38%. Found: C, 70.47; H, 5.55; N, 12.78%. H-
NMR (CDCl3, 400 MHz) d 3.35 (t, 6H, J = 4.76 Hz), 4.27 (t, 6H, J =
4.76 Hz), 6.91–7.03 (m, 6H), 7.46–7.50 (m, 6H); 13C NMR (CDCl3, 100
MHz) d 55.1 (×3C), 69.8 (×3C), 101.4 (×3C), 112.2 (×3C), 116.7 (×3C),
120.6 (×3C), 133.3 (×3C), 134.4 (×3C), 160.8 (×3C); IR (KBr) cm−1 3111,
3072, 2878, 2222, 1596, 1493, 1449, 1290, 1248, 1166, 1109, 1038, 760. L2:
Yield 60%; Anal. calcd for C27H24N4O3: C, 71.67; H, 5.35; N, 12.38%.
Found: C, 70.47; H, 5.55; N, 12.78%. 1H-NMR (CDCl3, 400 MHz) d 3.15
(t, 6H, J = 5.48), 4.08 (t, 6H, J = 5.48), 7.08 (m, 3H), 7.23 (dd, 3H),
7.32–7.36 (m, 6H); 13C-NMR (CDCl3, 100 MHz) d 54.20 (×3C), 67.25
(×3C), 113.06 (×3C), 117.30 (×3C), 118.53 (×3C), 119.62 (×3C), 124.59
(×3C), 130.33 (×3C), 158.59 (×3C); IR (KBr) cm−1 3056, 2828, 2230,
1724, 1597, 1581, 1484, 1433, 1284, 1148, 1040, 704. L3: Yield 70%; Anal.
calcd for C27H24N4O3: C, 71.67; H, 5.35; N, 12.38%. Found: C, 71.07; H,
5.65; N, 12.08%. 1H NMR (CDCl3, 400 MHz) d 3.15 (t, 6H, J = 5.24 Hz),
4.10 (t, 6H, J = 5.24 Hz), 6.87 (d, 6H, J = 8.8 Hz), 7.53 (d, 6H, J = 8.8
Hz); 13C NMR (CDCl3, 100 MHz) d 54.1 (×3C), 67.2 (×3C), 104.1 (×3C),
115.1 (×6C), 118.9 (×3C), 133.9 (×6C), 161.8 (×3C); ESI-MS [M + 1]+
453. IR (KBr) cm−1 2874, 2224, 1727, 1605, 1510, 1255, 1174, 1043, 834.
L4: Yield 70%; Anal. calcd for C27H24N4O9: C, 56.25; H, 4.72; N, 10.93%.
Found: C, 55.05; H, 5.02; N, 10.83%. 1H-NMR (CDCl3, 400 MHz) d 3.20
(t, 6H, J = 5.48 Hz), 4.16 (t, 6H, J = 5.48 Hz), 6.89 (d, 6H, J = 4.88 Hz),
7.53 (d, 6H, J = 4.88 Hz); 13C-NMR (CDCl3, 100 MHz) d 54.2 (×3C),
67.7 (×3C), 114.3 (×3C), 125.9 (×6C), 141.6 (×3C), 163.5 (×6C); IR
(KBr) cm−1 3113, 3083, 2826, 1596, 1500, 1335, 1259, 1169, 1109, 1033,
848. 1: Yield 92%. Anal. calcd for C27H24N4O3Cl2Hg: C, 44.79; H, 3.34; N,
7.74%. Found: C, 44.97; H, 3.56; N, 7.49%. 2: Yield ∼ 94%. Anal. calcd for
C27H24N4O3Cl2Hg: C, 44.79; H, 3.34; N, 7.74%. Found: C, 44.54; H, 3.46;
35 N, 7.57%. 3: Yield ∼92%. Anal. calcd for C27H24N4O3Cl2Hg: C, 44.79;
H, 3.34; N, 7.74%. Found: C, 44.41; H, 3.39; N, 7.56%. 4: Yield ∼95%.
−
Fig. 4 Crystal structure of 6: (a) encapsulation of PF6 anion inside
the molecular capsule formed by self-assembly of two units of (L1 + H)+
−
and (b) encapsulation of the PF6 anion through anion · · · p interactions
˚
˚
(P–F · · · p 3.055 A.) inside the cavity formed by p · · · p stacking (3.475 A)
of aromatic rings of (L1 + H)+.
740 | Dalton Trans., 2008, 738–741
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Anal. calcd for C24H24N4O9Cl2Hg: C, 36.77; H, 3.09; N, 7.15%. Found: C,
36.21; H, 3.17; N, 7.07%.
¶ Crystal data: 1: C27H24Cl2HgN4O3; Mr = 723.99, monoclinic, space
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◦
˚
group P21/n, a = 7.828(5), b = 24.454(5), c = 14.274(5) A, b = 102.386(5) ,
V = 2669(2) A , Z = 4, qcalc = 1.802 Mg m−3, Mo-Ka radiation (k =
3
˚
˚
0.71069 A), T = 100 K; colorless cube. Of 17 724 reflections measured,
6596 were independent (Rint = 0. 0.0501). The final wR2 was 0.1164 (all
9 M. J. Holmes, S. J. Miller and C. M. Nyberg, Center for Air Toxic Metals
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data). 2: C27H24Cl2HgN4O3; Mr = 723.99, monoclinic, space group P21/◦c,
˚
a = 11.6156(9), b = 16.1637(13), c = 14.9583(12) A, b = 107.491(1) ,
V = 2678.6(4) A , Z = 4, qcalc = 1.795 Mg m−3, Mo-Ka radiation (k =
3
˚
˚
0.71073 A), T = 100 K; colorless cube. Of 17 500 reflections measured,
6631 were independent (Rint = 0.0507). The final wR2 was 0.1546 (all
¯
data). 3: C27H24Cl2HgN4O3; Mr = 723.99, triclinic, space group P1, a =
˚
8.338(5), b = 8.372(5), c = 20.101(5) A, a = 92.608(5), b = 92.608(5),
◦
c = 106.888(5) , V = 1338.7(12) A , Z = 2, qcalc = 1.796 Mg m−3
,
3
˚
˚
Mo-Ka radiation (k = 0.71069 A), T = 100 K; colorless cube. Of 8871
reflections measured, 6254 were independent (Rint = 0.0434). The final wR2
was 0.1127 (all data). 4: C24H24Cl2HgN4O9; Mr = 783.96, triclinic, space
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presented at the AESF/EPA Conference, Orlando, FL, January 28–
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¯
˚
group P1, a = 8.190(5), b◦= 8.396(5), c = 20.920(5) A, a = 80.781(1), b =
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Desalination, 2006, 199, 405.
3
˚
88.390(1), c = 73.641(1) , V = 1362.2(12) A , Z = 2, qcalc = 1.911 Mg
m−3, Mo-Ka radiation (k = 0.71069 A), T = 100 K; yellow needle. Of 9141
13 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, Wiley,
˚
reflections measured, 6441 were independent (Rint = 0.0478). The final
wR2 was 0.1638 (all data). 5: C29H28F6N5O3P; Mr = 639.53, monoclinic,
New York, 5th edn, 1988, p. 611.
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˚
space group C◦2/c, a = 29.780(4), b = 10.3967(11), c = 21.165(3) A,
b = 120.922(4) , V = 5621.6(13) A , Z = 4, qcalc = 1.463 Mg m−3, Mo-
3
˚
˚
Ka radiation (k = 0.71073 A), T = 100 K; colorless needle. Of 18 407
reflections measured, 6929 were independent (Rint = 0.0638). The final
wR2 was 0.1614 (all data). 6: C9H8.33N1.33OP0.33F2; Mr = 208.88, trigonal,
¯
˚
space group R3, a = 13.3330(17), b = 13.3330(17), c = 26.773(4) A, c =
◦
120 , V = 4121.8(10) A , Z = 3, qcalc = 1.493 Mg m−3, Mo-Ka radiation
3
˚
˚
(k = 0.71073 A), T = 100 K; colorless needle. Of 6983 reflections measured,
1564 were independent (Rint = 0.0577). The final wR2 was 0.1548 (all data).
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