Synthesis of a Series of Highly Multidentate Podand Ligands as

Chapter 8

Downloaded by CORNELL UNIV on October 17, 2016 | http://pubs.acs.org Publication Date (Web): December 1, 2015 | doi: 10.1021/bk-2015-1210.ch008

Synthesis of a Series of Highly Multidentate Podand Ligands as Possible Water Remediation Agents Candice Kashat, Jenna Payne, Jennifer Roehl, Ashley G. Zerweck, and Mark A. Benvenuto* Department of Chemistry & Biochemistry, University of Detroit Mercy, 4001 W. McNichols Road., Detroit, Michigan 48221-3038 *E-mail: [email protected].

A series of multidentate podand ligands, all incorporating terminal biphenyl moieties, have been synthesized and characterized. These ligands have been examined in terms of their ability to coordinate various metal cations solvated in water and precipitate them from solution, in order to determine their ability to act as water remediators. The ligands were combined with copper as a divalent cation and cobalt as a trivalent cation. In all cases except that of the shortest chain ligand, solid precipitates formed upon mixing.

Introduction The desalination of saline waters, remediation of polluted waters, and extraction of inorganic ions from water is an established field (1–11) but continues to be an important field of study, since humans almost always live near a source or multiple sources of water. Of late, water sources in many areas of the world are being stressed by the presence of ever-increasing populations and by the waste products in water produced by their presence. Additionally, valuable and scarce elements may be recoverable from various waste water streams as important co-products of some industrial processes. For example, gold and silver are produced industrially using large, aqueous-based processes, and the waste streams from this production may contain other cations that are economically feasible to extract (12). Ethylenediaminetetraacetic acid (EDTA) has proven to be an excellent, widely used, relatively inexpensive water © 2015 American Chemical Society Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


remediator (13), but other materials have shown potential as well. Indeed, the field of multidentate ligands in coordination chemistry has a long history going back at least to the production of salen (14). More recently, a large number of multidentate ligands have been produced, although for many reasons besides water remediation (15–21). While effective water remediators such as EDTA exist, a series of complexing agents could be produced that contain Lewis base atoms capable of binding with various cations equal in their ability to remove such ions from water to that of established materials, but that would also contain highly hydrophobic moieties. The purpose of the aromatic terminal portions – the biphenyl terminal groups – would be to maximize the ability of the ligands to separate any resulting complexes from water because of end-group hydrophobicity. Five multidentate podand molecules, from the simplest, ethylenediamine, to the longest and highest in terms of denticity, pentaethylenehexaamine, have been utilized to produce a series of novel ligands, all incorporating biphenyl moieties at the terminal, primary amines.

Results and Discussion Synthesis and Characterization of Ligands The starting materials ethylenediamine, diethylenetriamine, triethylenetetraamine, tetraethylenepentaamine, and pentaethylenehexamine, were all purchased commercially and utilized without further purification. Stoichiometric combinations in a two-to-one ratio of aldehyde to the five amines in ethanolic solution or monoglyme solution produced the five ligands in quantitative yield at room temperature (25 °C) with stirring in the cases of Ligands 1 and 2, and at reflux temperature over 16h for Ligands 3 – 5. Rotary evaporation and subsequent drying on a Schlenk line at 0.5 torr resulted in isolable products that could be unambiguously determined via 1H NMR to be ligands 1 through 5. Figure 1 shows the general synthetic scheme.

Figure 1. Syntheses of Ligands 1 – 5. 176 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


The reaction scheme shown in Figure 1 is that for ligands 1 through 5, which were produced from the following starting amines respectively from shortest to longest: ethylenediamine (less formally, N2), diethylenetriamine (N3), triethylenetetraamine (N4), tetramethylenepentaamine (N5), and pentaethylenehexaamine (N6). 4,4’-Biphenylaldehyde was the common aldehyde starting material for all ligands. Isolation of each ligand via rotary evaporation produced yellow materials. In the case of the N2 starting material, the product, Ligand 1, was a yellow, crystalline solid. Ligand 2, produced from the N3 amine, was a solid, but a glassy, noncrystalline yellow solid material. Ligands 3 – 5, produced from N4, N5, N6 and spermine, were all viscous yellow oils at room temperature after removal of solvent via vacuum. Proton-NMR of the products all showed a diagnostic singlet indicative of the formation of an imine, between 8.4 and 8.5 ppm, while at the same time showing no singlet corresponding to the starting aldehyde, which would occur at 10.1 ppm if unreacted starting material was still present in any of the products. Although the terminal amines had been reacted with an aromatic aldehyde, there is no overlap of the imine singlet with the aromatic signals of the starting biphenylaldehyde, as the signals from the aldehyde all occur as doublets and a triplet, with some overlap, between 7.4 and 8.1 ppm. An examination of the 1H NMR aliphatic regions of the ligands showed progressively more complexity in the signals of the spectra when a comparison is made beginning with Ligand 1, which shows a singlet representing four hydrogen atoms, to Ligand 5, which showed multiple overlapping signals from 2.1 – 4.2 ppm, arising from twenty hydrogen atoms. The aromatic region did not show any significant differences in signal multiplicity, since the addition of the two biphenyl groups, one at each amine terminus, was the same in all six ligands. Evaluation of Ligand Complexing Abilities After characterization, solutions of the ligands were made by adding each ligand to 25 mL of toluene. They appear yellow without any cloudiness or opacity and solvate into the aromatic solution in no more than one minute. Aqueous solutions of metal salts, again in 25 mL volumes, were made by adding a metal salt to distilled water, the ratio of ligand to metal salt being 2:1 in the cases of Ligands 1 and 2, and 1:1 for Ligands 3 – 5. Copper(II) nitrate was used in these metal – ligand complex experiments because metal nitrate salts are typically quite soluble in water. The toluene solution in each experiment was added carefully to the aqueous solution, so that minimal mixing would occur at contact. In the case of Ligand 1 and copper(II) nitrate, the biphasic solution did not appear to undergo visible mixing. Even after the solution was shaken and stirred, a yellow toluene-based phase and a blue aqueous phase were present. Contrary to this, the mixing of a toluene solution of Ligand 2 and copper(II) nitrate produced a blue, cloudy precipitate immediately. The precipitate was sufficiently dense that it sank to the bottom of the aqueous portion of the biphase in less than one minute. The results of similar mixing of Ligand 3, or Ligand 4, and of Ligand 5 with solutions of 177 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


copper(II) nitrate was the same as that for Ligand 2. In each of these cases, a blue precipitate formed immediately, and separated from the aqueous portion of the biphasic mixture. An effective means of determining how completely the ligands were taken into the aqueous phase and the precipitate was to separate each biphase and rotary evaporate the toluene portion to dryness. This was done for the reactions run with ligands 2 – 5, since their precipitation was visually apparent. In each case, no organic residue was measured after the toluene had been rotary evaporated. The results suggest that Ligands 2 through 5 are effective in removing metal ions from aqueous solutions. A single experiment was performed using ligand 4 and cobalt(III) perchlorate to determine whether this also produced an insoluble precipitate in the aqueous phase of a water – toluene mixture. Again, the ligand to metal salt ratio was 1:1, and again a precipitate formed, this time a bright pink, which settled immediately to the bottom of the aqueous portion of the biphasic mixture. The formation of precipitates immediately upon mixing appears to be a straightforward indication that complexes have formed, and that they have low solubility in an aqueous solution. Ligand 1 does not appear to behave in similar fashion to ligands 2 – 5, perhaps because of steric hindrance produced by the proximity of the biphenyl terminal groups to any metal center, which may not occur in the four cases of the longer ligands. For the future, structures of the ligand metal complexes produced here will be examined – single crystal structures could be produced – and further insights will be gained into how these ligands form complexes, and how the biphenyl moieties affect and influence such structures when compared to ligands without such large end groups. But single crystal structures remain a means of characterization and understanding of a material in the solid phase, and this study was designed to indicate whether a complex formed, and whether it was soluble in aqueous solution.

Experimental Amines were purchased from Aldrich, biphenylaldehyde was generously provided by Mitsubishi Gas Chemicals, and solvents were purchased from Aldrich. The formation of ligands 1 – 5 were performed in absolute ethanol, with the aldehyde first solvated completely, then the amine (a liquid in all cases except N4, which was a whitish, crystalline solid) added to the solution. The resultant solutions were stirred for 16 h, at room temperature in the cases of Ligands 1 and 2, and at elevated temperature for Ligands 3 through 5. The solutions were then rotary evaporated to dryness, then further dried using standard Schlenk line apparatus. Analyses of the ligands by 1H NMR was via a Jeol 300 MHz instrument in CDCl3 at 25 °C. Table 1 lists the amounts of aldehyde and each respective amine used in these ligand formation reactions. 178 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Table 1. Ligand Synthetic Data Amine

Amine mass, g (mole)

Biphenylcarbaldehyde mass, g (mole)

Molar mass of amine (g/mol)

Product – Ligand Number

Ethylenediamine (N2)

0.50 (0.0083)

3.02 (0.0166)

60.10

1

Diethylenetriamine (N3)

0.50 (0.0048)

1.75 (0.0096)

103.17

2

Triethylenetetraamine (N4)

0.50 (0.0034)

1.24 (0.0068)

146.23

3

Tetraethylenepentaamine (N5)

0.50 (0.0026)

0.95 (0.0052)

189.30

4

Pentaethylenehexaamine (N6)

0.50 (0.0022)

0.80 (0.0044)

232.37

5

Complexation reactions with metal cations were carried out by producing aqueous solutions of a metal salt in distilled water, and solutions of each ligand in toluene. Total volume of mixing was 50 mL, with each phase comprising 25 mL of liquid. Toluene solutions were layered atop the aqueous solutions and if needed mechanically mixed for one minute. The results are shown in Table 2.

Table 2. Metal – Ligand Complex Formation Ligand

Ligand mass, g (mole × 10−4)

Cu(NO3)2•3H2O mass, g (mole × 10−4)

Visible Result

1

0.10 (2.58)

0.031 (1.29)

Biphase, minimal mixing

2

0.10 (2.32)

0.028 (1.16)

Blue precipitate in aqueous phase

3

0.10 (2.11)

0.051 (2.11)


4

0.10 (1.95)

0.047 (1.94)


5

0.10 (1.80)

0.043 (1.79)


179 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Conclusions This series of novel ligands is straightforward to synthesize and can be unambiguously characterized by 1H NMR. The results, the formation of solid complexes, suggest the ligands are capable of extracting metal ions from aqueous solutions, forming solid precipitates in all cases, except that of ligand 1. The complexes produced using ligands 2 through and including 5 exhibit low solubility in aqueous solutions, and precipitate immediately upon mixing of metal and ligand.


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181 Evans et al.; Trace Materials in Air, Soil, and Water ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Synthesis of a Series of Highly Multidentate Podand Ligands as

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