Presentation on theme: "Metal Complexation of Novel Thia- Crown Ether Macrocycles by ESI-MS Sheldon M. Williams, Wendi M. David and Jennifer S. Brodbelt Department of Chemistry."— Presentation transcript:
Metal Complexation of Novel Thia- Crown 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 150 o C. 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 1 S S S S HO OH S S S S 3 2 S S S S 4 S S S S N N H H 5 O S S S 6 7 O O S S S 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.
It was observed that Zn 2+ 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 Cd 2+ 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 Zn 2+ and Cd 2+ affinities. Pb 2+ 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 Hg 2+ selectivities. One host, 8, which contains only three ethylene- heteroatom 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 + ClO 4 ) + 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.
Table 2 Relative Efficiencies of Mercury(II) Extraction by Sulfur Containing Macrocycles
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 HgCl 2 and 7 in the aqueous and organic phase, respectively.
Figure 5 ESI-MS Signal-to-Noise Ratio Versus HgCl 2 Concentration HOST = 7
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.