Transcript Powerpoint Poster

Metal Complexation of Novel ThiaCrown Ether Macrocycles by ESI-MS
Sheldon M. Williams, Wendi M. David and Jennifer S. Brodbelt
Department of Chemistry and Biochemistry
University of Texas at Austin, Austin, TX 78712
Overview
Purpose:
Evaluate relative heavy metal ion binding affinities of crown-ether
macrocycles with S, O, and N heteroatoms.
Determine avidities for extracting heavy metals from aqueous
solution.
Method:
ThermoFinnigan LCQ Duo
50/50 methanol/ chloroform solutions
Extractions from water to chloroform
Results:
Most thia-crown ether macrocycles tested were found to bind
exclusively to mercury(II) ion in competitive assays with
cadmium(II), lead(II), mercury(II), and zinc(II) chlorides
Extraction studies with chloroform and water revealed that several of
the thia-crown ether macrocycles extracted mercury(II) ion efficiently
and exclusively in the presence of cadmium, lead, mercury, and zinc
ions
Introduction
Novel macrocycles are currently being developed and evaluated for
use as selective, recyclable ligands for extraction of heavy metals from
contaminated water. Fast, efficient feedback about metal selectivities and
avidities will aid the design and development process. Electrospray
ionization mass spectrometry (ESI-MS) shows promise for rapid
screening of binding selectivities in host-guest chemistry [1-5], offering
versatility in a variety of solvent systems and requiring minimal sample
consumption. In the present study, ESI-MS is used to evaluate the metal
binding selectivities of an array of novel caged macrocycles (Figure 1)
for mercury(II), lead(II), cadmium(II), and zinc(II) ions. It is found that
the type of heteroatom (S, O, N), cavity size, and presence of other
substituents influence the metal selectivities.
The desired structure of a heavy metal extraction agent should be
one that minimizes its solubility in an aqueous environment and yet is
able to efficiently extract the desired metal ion from the wastewater. For
this reason, several water-insoluble macrocycles that exhibited superior
affinity for particular heavy metal ions in our initial binding assays have
been tested for their ability to extract mercury(II), lead(II), cadmium(II),
and zinc(II) ions from an aqueous environment into an organic
environment.
Methods
Solutions containing a single host with multiple metals are analyzed
for each thia-crown ether macrocycle in 50/50 methanol/ chloroform.
The concentration of the host and each metal are 2.5 x 10-5 M. Initial
extractions of aqueous heavy metal salts to host in chloroform are
conducted with 0.033 M of each metal salt in aqueous solution and 2 x
10-4 M host in chloroform. Experiments conducted for the purpose of
detecting the extraction of low metal ion concentrations used a 1:1
host:metal chloride molar ratio with metal concentrations varying from 1
x 10-4 M to 1 x 10-5 M. For extraction, 1 ml of organic solution with host
and 1 ml aqueous solution with metal are vortexed for five minutes in a
closed 4 ml vial. The organic phase from the extraction experiments is
then analyzed by ESI-MS. All mass spectrometry experiments are
performed on a ThermoQuest LCQ Duo ESI-MS with a needle voltage
of 5 kV and a heated-capillary temperature of 150oC. A flow rate of 10
l/min was used for all ESI-MS experiments except the low metal ion
concentration experiments where a flow rate of 60 l/min was used.
Figure 1
Thia-crown Ether Macrocycle
Structures
S
S
S
S
S
S
S
S
S
S
OH
HO
S
S
S
S
S
S
S
1
4
3
2
S
S
S
S
S
S
S
6
5
8
S
S
S
S
S
7
S
S
9
S
S
HN
S
S
10
Figure 1, cont.
N
S
S
S
S
Ts N
N
S
S
S
N
12
11
13
O
S
S
S
S
S
O
S
O
14
15
S
Results
Macrocycle/ Heavy Metal Binding Survey in 50/50 Methanol/
Chloroform
As presented in Figure 2, the ESI-mass spectra for the solutions
containing a macrocycle and the metal perchlorates in 50/50 methanol/
chloroform typically consist of signals for complexes of a host and a
doubly-charged metal ion as well as singly charged tertiary complexes
including a single counter-ion. The signal intensities of the metal
complexes in the ESI-mass spectra were used to estimate the relative
binding selectivities and avidities of the hosts. A comparison of the
selectivities of every macrocycle tested is compiled in Table 1.
Figure 2A
ESI-MS of 7 with Cd(ClO4)2, Pb(ClO4)2, Zn(ClO4)2 (1:1:1:1) in
50/50 Methanol/Chloroform
(7+Cd+ClO4)+
(7+Zn+ClO4)+
100
(7+Cd)2+
(7+Zn)2+
0
200
(7+Pb+ClO4)+
(7+Cu)+
(7+Pb)2+
400
600
m/z
800
1000
1200
Figure 2B
ESI-MS of 7 with Cd(ClO4)2, Pb(ClO4)2, Hg(ClO4)2, Zn(ClO4)2,
(1:1:1:1:1) in 50/50 Methanol/Chloroform
100
(7+Hg+ClO4)+
(7+Hg)2+
0
200
400
600
800
m/z
1000
1200
It was observed that Zn2+ complexed favorably with hosts with four
ethylene-heteroatom units in the crown ring, but less than three non-sulfur
heteroatoms (4, 6, 12, 14), whereas Cd2+ complexed favorably to the hosts
with four or more ethylene-heteroatom units with two or more non-sulfur
heteroatoms (5, 7, 9, 10, 11, 13, 14, 15). Macrocycles 2 and 3, which are
hosts with four propyl-sulfur heteroatom units, also showed large Zn2+ and
Cd2+ affinities. Pb2+ complexed favorably to 1 and 7. Mercury(II) complex
was usually the dominant species or only species observed for the solutions
containing this ion, thus suggesting that many of the thio-crown ethers have
great Hg2+ selectivities. One host, 8, which contains only three ethyleneheteroatom units in its ring, appears to prefer binding to residual sodium ion
impurity over the much greater quantity of heavy metal ions in solution,
indicating the cavity to be too small to bind any of the heavy metals
efficiently. These initial findings were used to select candidates for
extraction of metal ions from an aqueous phase into a chloroform organic
phase in order to further evaluate the ability of these hosts as agents for
extracting heavy metals from an aqueous environment. Using these selected
compounds, the extraction process served as a model
for wastewater extraction and to determine the selectivities of hosts in
this process towards heavy metal cations.
Selective Mercury Ion Extraction from Aqueous Solution
Due to the exclusive selectivity most of the thia-crown ether
macrocycles showed towards mercury(II) ion, the eight macrocycles
providing the best combination of clean spectra and high signal-to-noise
ratio for the (Host + Hg + ClO4)+ complex were used for studying
mercury(II) ion extraction. Extractions using mercury(II) perchlorate
were generally unsuccessful, and the observed spectra were similar to
that shown for 7 in Figure 3A. However, 2 appears to have extracted
mercury (II) ion with a perchlorate counter-ion, though rather poorly as
shown in Figure 3B. Extraction of mercury(II) chloride was much more
successful as shown in the spectra of 2 and 7 in Figures 4A and 4B,
respectively, and for all the macrocycles tested in Table 2.
Figure 3
ESI-MS Macrocycle-Containing Chloroform Phase After Extraction of
Aqueous Phase Containing Cd(ClO4)2 :Pb(ClO4)2 :Hg(ClO4)2 :Zn(ClO4)2
(1:1:1:1:1)
100
(7+Hg+Cl)+
Host = 7
(7+Cu)+
0
200
100
400
600
m/z
(2+Cu)+
Host = 2
800
1000
800
1000
(2+Hg+Cl)+
(2+Hg+ClO4)+
0
200
400
600
m/z
Table 2
Relative Efficiencies of Mercury(II) Extraction
by Sulfur Containing Macrocycles
Rank
Host
Identified Peaks
1-best
2
3
4
5
6
7
8-worst
7
15
6
2
3
12
14
5
(L+Hg+Cl)+
(L+Hg+Cl)+
(L+Hg+Cl)+
(L+Hg+Cl)+ (L+Cu)+
(L+Hg+Cl)+ (L+Cu)+
(L+Hg+Cl)+ (L+Cu)+
(L+Hg+Cl)+ (L+Cu)+
Noise
Ionization (Host + Hg + Cl)+
Time (ms) Peak Intensity
0.5
9E7
5
1.8E8
10
1.2E8
10
5E7
150
3E7
300
7E7
300
3E7
-
[Host](o):[metal chlorides](aq) 1:167:167:167 Host:CdCl2:ZnCl2:HgCl2
Flow Rate = 10l/min
Since in almost all cases, the metal ion must transfer from the aqueous
solution into the chloroform to complex with a water-insoluble
macrocycle, the metal ion must be bound to two counter-ions to form a
neutral molecule before transfer can occur. In order for the metal ion
bound to two anions to complex with the macrocycle, one of the anions
must pass through the central cavity of the macrocycle. It is believed that
the effective diameter of the metal-bound perchlorate ion, which is likely
in a tetrahedral geometry, is too large too efficiently pass through the
macrocycles’ cavity. In addition, the perchlorate anion is more
hydrophilic than the chloride ion due to its four oxygen atoms, which
increases the water desolvation energy needed to transfer to the
chloroform from the aqueous phase.
Of the macrocycles tested, 7 gave the best extraction results. Its
superior performance is likely due to a combination of several factors.
Primarily, the four sulfurs appear close to the geometry needed for a
square-planar geometry, with the two chloride counter-ions binding at the
axial positions of a near-ideal octahedral structure. The two additional
oxygens can provide ion-dipole interactions to stabilize the
mercury ion in the cavity. For 15, the two additional oxygens may add more
sites for interaction with the mercury ion, but the added space between the
sulfurs probably interferes with the sulfurs attaining a geometry as favorable
as is achieved for 7. The three sulfurs and one oxygen of 6 likely allow for a
similar square-planar geometry, though the presence of the oxygen, and
perhaps the smaller size, reduce its performance compared to 7 and 15.
Macrocycles 6 and 7 may be superior in mercury extraction performance to
2 and 3 because the propyl units between every sulfur in 2 and 3 may result
in a deviation from the ideal square-planar geometry, similar to the effect
with 15, except that there are only four heteroatom interaction sites in the
macrocycle cavity versus six and eight for 7 and 15, respectively. Although
all the macrocycles tested in the extraction experiments had at least four
heteroatoms, those with less than three sulfurs performed the least well.
As a final experiment, 7 was used to determine the lower limit for
detecting extraction of mercury from water into chloroform. Figure 5
presents a plot of the signal-to-noise ratio for the (macrocycle + Hg + Cl)+
peak with decreasing equimolar concentrations of HgCl2 and 7 in the
aqueous and organic phase, respectively.
Figure 5
ESI-MS Signal-to-Noise Ratio
Versus HgCl2 Concentration
Signal-to-Noise Ratio
1000
HOST = 7
100
10
1
1E-5
3E-5
5E-5
[HgCl2](aq) (M)
1E-4
A lower-limit-of-detection of 1 x 10-5 M (2 ppm Hg) was determined,
though using a higher flow rate (>100 l/min), using a greater, constant
macrocycle concentration in the organic phase (>1 x 10-4 M), and
improving tuning of the ESI-MS interface lenses on the LCQ Duo would
likely improve the detection limit by greater than an order of magnitude.
These probable methods of improving the limit-of-detection were not
examined in the current study due to limited sample quantities, but are
planned when additional 7 becomes available.
Conclusions
Crown ether macrocycles with several sulfur heteroatoms and a ring
composed of at least four ethylene heteroatom units are necessary for
efficient, selective mercury extraction.
Macrocycle cavities with several sulfurs and/or additional nitrogens
and oxygens arranged to bind to mercury with a square-planar
geometry appear the most ideal.
Macrocycles with a pair of sulfurs separated by an ethylene unit on
opposite sides of the cavity with a flexible tether between, and with
additional nucleophilic heteroatoms on the tether appears to create the
most ideal mercury extraction agent of those studied.
The presence of small, low hydrophilicity anions in the aqueous
medium greatly enhances mercury ion extraction for the macrocycles
tested in this study.
Sulfur containing crown ether macrocycles have been shown to have
potential as agents for selectively extracting and detecting aqueous
mercury ion over a large concentration range.
Future Work
Optimize lower-limit-of-detection methodology
Examine selectivity in the presence of other common metal ions (alkali
and alkaline earths)
Molecular modeling and ab initio calculations
Acknowledgements
The laboratory of Alan P. Marchand, Department of Chemistry,
University of North Texas, is gratefully acknowledged for synthesizing
macrocycles 5 through 15.
The National Science Foundation, the Welch Foundation, and the Texas
Advanced Technology Program are gratefully acknowledged.
References
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D.B., Anal. Chem., 2001, 73, 384.
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S., J. Am. Soc. Mass Spectrom., 2000, 11, 884.
3. Kempen, E.C., Brodbelt, J.S., Anal. Chem., 2000, 72, 5411.
4. Blair, S.M., Brodbelt, J.S., Marchand, A.P., Kumar, Kalpenchery, A.,
Chong, H.-S., Anal. Chem., 2000, 72, 2433.
5. Kempen, E.C., Brodbelt, J.S., Bartsch, R.A., Jang, Y., Kim, J.S.,
Anal. Chem., 1999, 71, 5493.