Chapter 6
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Synthesis of a Novel Series of Nitrogen-Containing Ligands for Use as Water Remediators, All Incorporating Long-Chain Aliphatic Moieties Justin Pothoof, Michele Bhagwagar, Grace Nguyen, Sara Tinawi, Sara Makki, and Mark A. Benvenuto* Department of Chemistry & Biochemistry, University of Detroit Mercy, 4001 W. McNichols Rd., Detroit, Michigan 48221-3038, United States *E-mail:
[email protected].
A series of symmetrical, highly multi-dentate podand ligands, all incorporating terminal, long-chain aliphatic moieties and a centrally-positioned series of nitrogen atoms, have been synthesized and characterized. These ligands were produced in a continuing effort to find effective, useful, inexpensive water remediators that are able to function as chelates and remove metal ions indiscriminantly from aqueous solutions. The ligands were complexed with a variety of metal ions, to determine the solubility of the resulting complexes in water. They were also examined to determine if they could retain metal cations in non-aqueous solvent.
Introduction Throughout the history of civilization, humankind has almost always lived near water. Even desert dwellers knew exactly where water was to be found, precisely because the presence of water meant life. Yet for as long, brackish water, saline water, and water that has in any other way been contaminated with some foreign matter has been of concern to people, precisely because it is either difficult or impossible to use (1–12). Saline water is certainly still useful for fishing and for waterborne traffic, but is routinely far too salty to drink.
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What can be called this degradation of water has become a more acute problem in the last 75 years than it has for almost all of the recorded history prior to that time, because of the explosion in human population worldwide (13–16). For example, the loss of the Aral Sea is one of the largest man-made catastrophes dealing with a freshwater source that has ever been recorded (17, 18). The pollution of the Gulf of Mexico near the mouth of the Mississippi River with agricultural waste and farm run-off is another large-scale degradation of a significant part of the world’s water that has become a major regional concern (4, 19). And the recently enhanced salinity of the aptly named Dead Sea, as its waters are used by companies in Israel and Jordan for mineral extraction, is a further issue of concern for peoples of the region who have used the sea for centuries (20). In these three cases and several more, the presence of larger numbers of people near water sources, and depending on water, stresses those water sources more than at any previous time in history. Clearly, the need for materials to clean polluted or degraded water is important. Numerous different chealting molecules have been used in some way to extract unwanted ions or organic matter from water, with ethylene diamine tetraacetic acid (EDTA) being arguably the widest known chemical used for this purpose, at least on a small scale (21–26). But the existence of a class of chelators does not prevent further, similar molecules from being produced and examined in the hope that they too may function as inexpensive, efficient remediators of environmentally degraded waters. This paper is the result of a research project utilizing undergraduate researchers, in which a novel series of ligands have been produced, under ambient temperature and pressure conditions, resulting in organic molecules having three or five nitrogen atoms in their central portion. These nitrogen atoms act to form a dative bond non-specifically with a variety of metal ions, forming coordination complexes. They function much as molecules such as EDTA and other established chelators, and should prove useful in remediating water containing a wide variety of cations.
Results and Discussion Synthesis and Characterization of Ligands The amines diethylene triamine (N3) and tetraethylene pentaamine (N5), as well as the two aldehydes: octanal and dodecanal, were all purchased from Aldrich and used as received. A single 1H NMR was run of each starting material to ensure purity of the material. Solvents, monoglyme and toluene, were purchased commercially and used without further purification. To synthesize the target ligands, two molar equivalents of an aldehyde were reacted with one molar equivalent of an amine at room temperature (25°C), with either monoglyme or toluene as a solvent, with mechanical stirring, for approximately 16 hours. Starting solutions were uniformaly clear, with the resulting solutions after stirring being very slightly yellow. Figure 1 shows the basic reaction chemistry for the production of the target ligands. 82
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Figure 1. General route for ligand synthesis. The reaction schmeme illustrated in Figure 1 was used to produce Ligands 1 and 2, and represents a wider scheme that can be used to produce numerous other long-chain ligands. Figure 2 shows both ligands that were produced in this study, and Table 1 indicates the sample sizes and molar ratios of materials used in the syntheses.
Figure 2. Shortest (Ligand 1) and longest ligand (Ligand 2). The ligands were examined by 1H NMR, and as expected, displayed a very cluttered, crowded aliphatic region. The reaction though, a Schiff’s base condensation, produces molecules that in these cases contain two imine sites. These double bonds are the only functionality the target ligands have besides the nitrogen atoms, and importantly, the imine proton appears as a singlet in the region δ 8.2 – 8.8. This is definitely different from the aldehydic proton of the starting material, which appears slightly above δ 11.0. It is also several ppm away from the -CH2- saturated methylene signals from those protons two bonds away from each nitrogen atom. In short, the imine singlet of each target ligand serves as a diagnostic that the ligand has been produced. The presence of the imine singlet, and the complete loss of the aldehyde singlet, were used as indicators that the ligands formed quantitatively. Evaluation of Ligand Complexing Abilities In a traditional manner, for each trial, a solution of Ligand 1 or 2 was added to a separatory funnel, then an aqueous solution of a metal salt was added (results are summarized in Table 2). Precipitation of material essentially occurred upon contact, with only a few seconds of shaking of the separatory funnel. Simply adding a sample of a dry salt to the non-aqueous solution of either Ligand 1 or 2 produced some unexpected results. This experimental approach was chosen because it was felt that it would be straightforward to separate any precipitate, dry it, and determine the overall conversion to the target coordination complex. But it can be seen in Table 2 that four of the five metal salts added directly to the non-aqueous ligand solution of Ligand 2 stayed in solution. While such 83
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behavior is not unheard of, it is not common. Reported examples of organic ligands that solvate ionic materials in non-aqueous solutions always involve more complex ligands than those produced here (27–29). It appears then that in the case of Ligand 2 – a molecule that can be termed α,ω-bis-dodecyl-tetraethylene-pentaamine – a very simply chelator has been found that will hold hundredth molar concentrations of several cations in non-aquoeous solution.
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Experimental Section Diethylene triamine and tetraethyelene pentaamine were purchased from Aldrich and used with no further purification, as were octanal and dodecanal. Solvents were purchased and used without distillation. The formation of all ligands was performed in monoglyme, and in toluene, with the amine first measured out and dissolved in the solvent. The aldehyde was added only after the amine was completely solvated. All solutions were stirred for a minimum of 16 hours, during which time minimal color changes occurred from clear to a very pale yellow. Samples of each ligand were dried by rotary evaporation, followed by a minimum of 16 hours on a Schlenk line, and solvated in CDCl3 for characterization via NMR. Both ligands were examined by 1H NMR using a Jeol 300 MHz instrument, at 25°C. Table 1 lists the amounts of amine and aldehyde used in production of Ligands 1 and 2.
Table 1. Ligand Synthetic Data Amine
Amine mass, g (mole)
Aldehyde, g (mole)
Molar mass of amine (g/mol)
Product – Ligand Number
Diethylenetriamine (N3)
1.104 (0.011)
2.533 (0.021), octanal
103.0
1
Tetraethylenepentaamine (N5)
2.031 (0.011)
3.962 (0.022), dodecanal
184.32
2
Two different techniques were used to form metal-ligand coordination complexes. The first technique involves dissolving in distilled water a stoichiometric equivance of a metal salt to the ligand solution, then adding this to a separatory funnel containing the stoichiometric equivalence of the ligand in non-aqueous solvent. In all such trials, a precipitate formed immediately. The second technique involved measuring out the proper amount of a dry metal salt, routinely a molar equivalence to the ligand solution in non-aqueous solvent. The dry salt was then added to the ligand solution directly. In several cases, the coordination complex that formed remained in solution. Results are summarized in Table 2. 84 Roberts-Kirchhoff and Benvenuto; Environmental Chemistry: Undergraduate and Graduate Classroom, Laboratory, and Local ... ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
For both techniques, 0.044 M solutions of Ligand 1 were used, and 0.033 M solutions of Ligand 2 were used. This allowed for an easy-to-measure, highly reproducible set of starting samples and experimental size.
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Table 2. Metal – Ligand Complex Formation Ligand
Ligand mass, g (mole x10-3)
Salt mass, g (mole x 10-3)
Visible Result, technique 1
Visible Result, technique 2
1
0.118 (1.144)
0.198 (0.726) Fe(ClO4)2·H2O
Brown precipitate
Dark precipitate
1
0.118 (1.144)
0.279 (1.02) ZnClO4·6H2O
Grey precipitate
Grey precipitate
1
0.118 (1.144)
0.283 (0.870) Pb(C2H3O2)2
Grey precipitate
Grey precipitate
1
0.118 (1.144)
0.187 (0.511) Blue-brown Co(ClO4)2·6H2O precipitate
Brown precipitate
1
0.118 (1.144)
0.058 (0.142) Grey precipitate Nd(NO3)3·6H2O
Grey precipitate
2
0.103 (0.198)
0.0504 (0.198) Fe(ClO4)2·H2O
Brown precipitate
Dark brown solution
2
0.103 (0.198)
0.073 (0.198) ZnClO4·6H2O
Grey precipitate
Yellowish solution
2
0.103 (0.198)
0.072 (0.198) Brown precipitate Co(ClO4)2·6H2O
2
0.103 (0.198)
0.075 (0.198) Pb(C2H3O2)2
2
0.103 (0.198)
0.088 (0.198) Whitish Nd(NO3)3·6H2O precipitate
Grey precipitate
Brown solution Whitish-grey precipitate White solution
Conclusions This duo of multi-dentate ligands is both very easy and straightforward to synthesize, requiring no specialized reaction apparatus. Because of a single, diagnostic imine peak in an otherwise cluttered 1H NMR, the ligands can be characterized without ambiguity, and without having to resolve the aliphatic region of the 1H NMR spectrum. The ligands produced here all have minimal functionality, yet possess enough that they have proven to be excellent chelators. Their syntheses have been accomplished by multiple undergraduate researchers. The formation of metal-ligand complexes that precipitate readily from water indicate that these ligands may find use as an inexpensive form of water remediator. The fact that one of these ligands is capable of solvating metal salts into non-aqueous media when dry metal salts were added directly to non-aqueous solutions of the ligands was unexpected. It is an intriguing phenomenon however, 85
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one that may hold promise for the use of these ligands in previously unexpected applications.
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