Cyclic Imide Dioximes: Formation and Hydrolytic Stability - Industrial

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Cyclic Imide Dioximes: Formation and Hydrolytic Stability Sung Ok Kang, Sinisa Vukovic, Radu Custelcean, and Benjamin P. Hay* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Poly(acrylamidoximes) play an important role in the uranium extraction from seawater. The present work reports solution studies of simple analogues to address the formation and stability of two binding sites present in these polymers, openchain amidoximes and cyclic imide dioximes, including: (1) conditions that maximize the formation of the cyclic form, (2) the existence of a base-induced conversion from open-chain to cyclic form, and (3) degradation under acid and base conditions.

1. INTRODUCTION There is an astonishing amount of 4 billion tons of uranium dissolved in Earth's oceans.1−4 This has led to the proposal to mine the oceans for uranium.5−16 Such technology requires adsorbent materials that are capable of sequestering very low concentrations of uranium (3 ppb) from seawater in the presence of higher concentrations of competing metal ions (sodium, calcium, iron, etc.).17 This challenge has motivated extensive experimental research on adsorbent development including spheres,18 membranes,19 fibers,20,21 and resins.22−25 Ion-exchange resins meet the requirements of high physical and chemical stability in seawater, rapid uptake of uranyl cation (UO22+), and economical loading capacity.22,25 Extensive screening tests of 200 organic polymers functionalized with a variety of chelating groups revealed that only poly(acrylamidoximes) were able to sequester UO22+ at the slightly alkaline pH 8.0−8.3 of seawater.24 The synthesis of the amidoxime functional group is accomplished by reaction of nitriles with hydroxylamine.26 This approach has been applied to the large-scale, cost-effective conversion of poly(acrylonitriles) to poly(acrylamidoximes). This polymeric conversion yields two different functional groups along the polymer: open-chain amidoximes and cyclic imide dioximes (Figure 1).24 Simple amidoximes are known to bind uranyl in aqueous solution,27,28 and recent data suggest an η2-binding mode in the polymer.29 In principle, the cyclic imide dioxime provides a tridentate binding site and is believed to play an important role in UO22+ uptake by the adsorbent.24 It is known that treatment of poly(acrylamidoximes) with 0.5 M KOH solution prior to its submersion to seawater enhances UO22+ extraction.6−11,30 Although it is believed that this enhancement is due to the conversion of open-chain amidoximes to cyclic imide dioximes,5−7,30 there is no direct evidence to support base-induced cyclization. In addition to exposure to base, these adsorbents are washed with 1 M HCl solution to elute adsorbed metals. This procedure is reported to decrease the extraction efficiency (6%) per cycle in poly(acrylamidoxime) adsorbents. It has been proposed that this decrease is due to the degradation of the cyclic binding sites.24 Even though it is known that cyclic imide dioximes are unstable under acidic conditions,26,31 detailed studies of the rate of degradation have not been reported. © 2012 American Chemical Society

Figure 1. Amidoximes and cyclic imide dioximes and their possible uranyl complexes in poly(acrylamidoximes).

To address issues regarding the formation and stability of cyclic imide dioxime and amidoximes occurring within the polymer, the behavior of simple analogues was investigated (Scheme 1). The present work reports a series of studies to address: (1) conditions that maximize the formation of the cyclic form, (2) the existence of a base-induced conversion from open-chain to cyclic form, and (3) degradation under acid and base conditions. The cyclic imide dioxime present in the polymer degrades rapidly in 1 M HCl at room temperature (pseudofirst-order, t1/2 = 0.9 h, k = 2.14 × 10−4 s−1). An analogue of this molecule, phthalimide dioxime, was much Received: Revised: Accepted: Published: 6619

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Scheme 1. Synthesis of Amidoxime and Cyclic Imide Dioxime from Nitriles

more stable (pseudofirst-order, t1/2 = 147 h, k = 1.31 × 10−6 s−1) under the same conditions. The results suggest simple and economic strategies to improve existing adsorbents. Figure 2. Evolution of the 1H NMR spectrum of 4 with time in DMSO-d6 at 130 °C (● = 4, and ★ = 5).

2. RESULTS AND DISCUSSION 2.1. Formation of Cyclic Form. Simple acetamidoxime 2 was synthesized from the reaction of acetonitrile with hydroxylamine,32−34 while hydroxylamine treatment of glutaronitrile 3 afforded both glutaro bisamidoxime 4 and cyclic form 5.31,35,36 Amidoxime 2 and bisamidoxime 4 showed very similar chemical shifts in 1H NMR. The amide and oxime hydrogens of the bisamidoxime 4 appeared at 5.34 and 8.72 ppm in DMSO-d6, respectively, and 5.35 and 8.70 ppm for 2. For cyclic imide dioxime 5, the imide and oxime signals are shifted downfield appearing at 8.37 and 10.02 ppm. In the 13 C NMR spectra, the amidoxime groups of ligands 2 and 4 were seen at 150.1 and 152.5 ppm, respectively, while the imidedioxime of the cyclic form 5 was observed slightly further upfield at 144.8 (see the Supporting Information, Figure S1). The identity of 5 was confirmed by single crystal X-ray structure determination (see the Supporting Information, Figure S7). It is known that the relative yields of 4 and 5 depend on the temperature of the reaction, where higher temperatures favors the cyclic form.24,26 In fact, the sublimation of 4 gives 5.31 This observation led us to investigate the conversion of bisamidoxime 4 to cyclic form 5 at high temperature. This transformation was complete in less than 3 h at 130 °C without any base present as seen in 1H NMR spectra in Figure 2. Therefore, thermal cyclization is a straightforward method for maximizing the yield of cylic form within poly(acrylamidoxime) adsorbents. 2.2. Stability under Alkaline Conditions. Washing of poly(acrylamidoxime) adsorbent by alkaline solution (2.5% potassium hydroxide) prior to its submersion in seawater is an established step to improve the adsorbent's capacity and is believed to convert open-chain amidoximes to the cyclic form (Scheme 2).5−7,30 This claim was tested by exposing bisamidoxime 4 to a solution of 1 M NaOD at various temperatures. Although no conversion to cyclic form 5 was seen at room temperature, a trace amount of 5 was observed after heating at 80 °C for an hour (Figure 3). After 2 days at 80 °C, 4 was completely degraded into glutarate 8 (see the Supporting Information, Figure S2), through intermediate 7. Because exposure to base does not convert open-chain amidoxime to the cyclic form, we questioned how the alkaline conditioning affects the adsorption efficiency of poly(acrylamidoximes). One explanation is that alkaline treatment

of poly(acrylamidoximes) simply deprotonates amidoximes (pKa ∼ 11.6) in polymer, enhancing UO22+ binding. However, this is unlikely because the conditioned poly(acrylamidoximes) containing amidoximates should equilibrate quickly when placed in seawater (pH 8.0−8.3) (acetamidoximes exist as a neutral species at pH 8.1−8.3).27 Alternatively, enhanced uranium uptake by alkaline-treated polymer has been attributed to increased hydrophilicity arising from amidoxime degradation.37 Simple amidoxime 2, cyclic imide dioxime 5, and its precursor, glutaronitrile 3, were subjected to hydrolysis in 1 M NaOD and monitored by NMR. Acetamidoxime 2 (0.4 M) was shown to be quite stable in 1 M NaOD solution for several months at room temperature. It took high temperature (80 °C) to accelerate hydrolysis. At 80 °C, 2 (0.4 M) was hydrolyzed to acetate in 2 days, and 5 (0.2 M) converted to glutarate in 2 days (see the Supporting Information, Figures S3 and S4). It took only 1 h to hydrolyze 3 (0.2 M) to 8 at 80 °C (see the Supporting Information, Figure S5). In all cases, the hydrolysis yields carboxylate products (Scheme 2). It is of note that slightly acidic α-hydrogens next to amidoxime/carboxylic acid groups can be replaced by deuterium in 1 M NaOD in D2O.38 Therefore, in the spectra of hydrolysis of 2 in 1 M NaOD, the signals of α-hydrogens at 1.56 (for 2) and 1.69 ppm (for its hydrolysis product 6) became smaller and revealed new broad signals of partially deuterated 2 and 6 at 1.54 and 1.67 ppm, respectively, appearing slightly upfield with time (see the Supporting Information, Figure S3A). No deuterium exchange was observed when nondeuterated 1 M NaOH in H2O was used (see the Supporting Information, Figure S3B). H/D exchange also occurred with 3−5 in 1 M NaOD solution. For 3, as expected, the more acidic α-hydrogens next to cyano groups are more rapidly replaced by deuterium relative to amidoxime/ carboxylate groups. As a result, only one singlet of median carbon hydrogens is observed in the 1H NMR spectra when treated with alkaline (see the Supporting Information, Figure S5). 2.3. Stability under Acidic Conditions. Amidoximes 2, 4, and 5 were exposed to 1 M DCl and monitored by NMR. Amidoxime 2 (0.4 M) and 4 (0.2 M) were shown to be stable in 1 M DCl at room temperature, several months for 2 and a 6620

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Scheme 2. Proposed Conversion of Poly(acrylamidoximes) to Poly(acrylimidedioximes) in Basic Solutions and Hydrolysis of Amidoximes 2 and 4 and Bisnitrile 3 in 1 M NaOD at 80 °C

through intermediate 9 (Figure 4)26,31 and ultimately to carboxylic acid. After 2 h at room temperature, two new sets of signals for 9 and 10 were observed (Figure 4b). Degradation to glutarimide 10 was confirmed by mass spectroscopy and a single-crystal X-ray structure determination. Crystals of 10 grown by slow evaporation of a saturated CHCl3/MeOH solution showed the same unit cell lattice constants as previously reported for this compound.39 The conversion of 5 into 10 was complete in less than 20 h at room temperature (Figure 4). 2.4. Acid-Resistant Cyclic Form. Degradation of the cyclic form in acid solution is due to the nucleophilic attack of water on the imino carbon. One way of slowing down the hydrolysis would be to lower the charge on the carbon. To demonstrate this proof-of-principle, we prepared ligand 12 from the phthalonitrile 11,40 which delocalizes the charge on the carbon by extending conjugation to the aromatic ring. The identity of phthalimide dioxime 12 was confirmed by single crystal X-ray structure determination (see the Supporting Information, Figure S8). Analogous to 5, the hydrolysis of 12 yields phthalimide 13 (Figure 5). As expected, 12 was much more resistant than 5 to acid-catalyzed hydrolysis. The reaction kinetics of hydrolyses of 5 and 12 were pseudofirst-order with the rate constants (k) of 2.14 × 10−4 and 1.31 × 10−6 s−1, respectively (see the Supporting Information, Figure S9−S12). The hydrolysis rate of 12 in 1 M DCl was substantially reduced (t1/2 ∼ 0.9 and 147 h for 5 and 12, respectively). This is true even though a highly

Figure 3. Evolution of the 1H NMR spectrum of bisamidoxime 4 (0.2 M) with time (a−g) at 80 °C in 1 M NaOD; the spectrum of cyclic form 5 is shown in h for comparison.

couple weeks for 4. With elevated temperature to 80 °C, both 2 and 4 degraded to carboxylic acid products (see the Supporting Information, Figures S3C and S6). Cyclic form 5 (0.2 M) was, however, very unstable in 1 M DCl solution, degrading into 10 6621

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Figure 4. Evolution of the 1H (left) and 13C NMR (right) spectrum of 5 (0.2 M) with time in 1 M DCl at room temperature (● = 5, △ = 9, and ★ = 10).

determining step is the nucleophilic attack of the imino carbons by water. The charges on imino carbons of 5 and 12 were calculated by natural bond orbital analysis,41−44 which revealed that 12 has lower charge on the imino carbon by 0.03 (+0.561 and +0.531 for 5 and 12, respectively). The transition states for the rate-determining step of hydrolysis were calculated for 5 and 12 (Figure 6).45−47 Consistent with the lower carbon charge, the energy barrier for 12 is 1.48 kcal/mol higher (37.06 and 38.54 kcal/mol for TS-5 and TS-12, respectively, in vacuum). It is also in accordance with Hammond's postulate that INT-12 is higher in energy than INT-5 by 1.61 kcal/mol (37.79 and 36.18 kcal/mol for INT-5 and INT-12, respectively).

3. CONCLUSIONS In summary, the formation and thermodynamic stability of cyclic imide dioximes as well as amidoximes were carried out using NMR experiments under acidic and alkaline conditions at room temperature and elevated temperatures. Contrary to the reported literature,5−7,30 alkaline treatment does not convert 4 to 5. However, this conversion is accomplished quantitatively by heating, suggesting a simple strategy to maximize the yield of cyclic forms in poly(acrylamidoximes). Amidoximes are stable at room temperature under strongly acidic (1 M HCl) or alkaline (1 M NaOH) conditions. However, with elevated temperature, they degrade to carboxylic acids. These results imply that enhanced uranium uptake after alkaline treatment of the polymer may be due to increased hydrophilicity arising from carboxylate formation.37 Cyclic imide dioxime 5 was unstable (t1/2 = 0.9 h) in 1 M HCl even at room temperature, degrading to a glutarimide 10 intermediate and ultimately to glutaric acid. On the other hand, phthalimide dioxime 12 took much longer to degrade (t1/2 = 147 h) under these conditions. Thus, although the uranium

Figure 5. Evolution of the 1H NMR spectrum of hydrolysis of 12 (2 mM) with time in 1 M DCl at room temperature to phthalimide 13 (● = 12, and ★ = 13).

dilute concentration (2 mM) was used due to the sparing solubility of 12 in 1 M DCl. Differences in rates of acid-catalyzed hydrolysis between cyclic imide dioximes 12 and 5 can be rationalized by comparing the charges on the imino carbons since the rate6622

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For heating experiments of the NMR samples, sealed NMR tubes were placed in an oil bath at 80 °C for a certain period of time, and NMR spectra were recorded at room temperature. All proton signals were referred to a TMS or TSP standard. 4.3. X-ray Experimental. Single crystals of 5 and 12 suitable for X-ray diffraction structural studies were obtained by the slow evaporation of a saturated EtOH solution of 5 and a MeOH/EtOH solution of 12, respectively. Single-crystal X-ray data for 12 were collected on a CCD-based X-ray diffractometer with fine-focus Mo Kα radiation (λ = 0.71073 Å), operated at 50 kV and 30 mA. The structure was solved by direct methods and refined on F2 using the SHELXTL software package. Absorption corrections were applied using SADABS, part of the SHELXTL package. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed in idealized positions and refined with a riding model, except the NH and the OH hydrogens, which were located from the difference Fourier maps and refined isotropically. For 5, diffraction data were collected on a CCD-based X-ray diffractometer equipped with Helios multilayer X-ray optics. X-rays were provided by a microfocus Cu rotating anode (λ = 1.54178 Å) generator operating at 45 kV and 60 mA. The structure was solved by direct methods and refined on F2 using the SHELXTL software package. Absorption corrections were applied using SADABS, part of the SHELXTL package. All nonhydrogen atoms were refined anisotropically. All hydrogen atoms were located from a single difference Fourier map and refined as independent isotropic atoms. 4.4. Modeling. B3LYP/6-31+G(d) calculations45 were carried out using NWChem software.46,47

Figure 6. Potential energy surface of acid-catalyzed hydrolytic degradation of cyclic forms 5 and 12. Charges on the imino carbon are +0.561 and +0.531 for 5 and 12, respectively (TS, transition state; and INT, intermediate).

binding affinity of 12 remains to be determined, the development of new adsorbents incorporating 12 should result in decreased degradation under acidic stripping conditions.

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4. EXPERIMENTAL SECTION 4.1. General Information. The chemicals used were purchased and were used without further purification. 1H and 13 C NMR spectra were measured at 400 MHz for 1H and 100 MHz for 13C. Compounds 2,32−34 4 and 5,31,35,36 and 1240 were obtained from the reaction of acetonitrile, glutaronitrile, and phthalonitrile, respectively, with hydroxylamine (hydroxylamine hydrochloride and 1.0 equiv of potassium hydroxide) in an aqueous ethanolic solvent according to standard methods.26 4.1.1. Acetamidoxime (2). 1H NMR (400 MHz, DMSO-d6): δ 8.70 (s, 1H), 5.35 (s, 2H), 1.62 (s, 3H). 13C NMR (400 MHz, DMSO-d6): δ 149.6, 16.6. 4.1.2. Glutaramidoxime (4). 1H NMR (400 MHz, DMSOd6): δ 8.72 (s, 2H), 5.34 (s, 4H), 1.94 (t, J = 7.4 Hz, 4H), 1.68 (quintet, J = 7.4 Hz, 2H). 13C NMR (400 MHz, DMSO-d6): δ 152.5, 30.3, 23.6. 4.1.3. Glutarimide Dioxime (5). 1H NMR (400 MHz, DMSO-d6): δ 10.02 (s, 2H), 8.37 (s, 1H), 2.35 (t, J = 6.0 Hz, 4H), 1.68 (quintet, J = 5.7 Hz, 2H). 13C NMR (400 MHz, DMSO-d6): δ 144.9, 25.2, 19.1. 4.1.4. Phthalimde Dioxime (12). 1H NMR (400 MHz, DMSO-d6): δ 10.60 (s, 2H), 9.26 (s, 1H), 7.67 (m, 2H), 7.52 (m, 2H). 13C NMR (400 MHz, DMSO-d6): δ 146.7, 132.0, 130.5, 121.0. 4.2. NMR Studies. The stability of amidoxime compounds was determined from 1H NMR measurements of amidoximes as a function of time in 1 M NaOD and 1 M DCl in D2O at room temperature and 80 °C. Amidoximes (0.2 M) were used for stability tests in 0.5 mL of 1 M NaOD/DCl with the exception of 2 (0.4 M) and 12 (2 mM) due to mono amidoxime group in 2 and the sparing solubility of 12 in D2O.

ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra of 2, 4, 5, 8, and 12. 1H NMR measurements of 2 in 1 M NaOD/NaOH/DCl, 3 and 5 in 1 M NaOD, and 4 in 1 M DCl with time at 80 °C. X-ray crystallographic data in CIF format for 5 and 12, and their ORTEP plots, rate constants, and half-life determinations of 5 and 12 in 1 M HCl. DFT coordinates and energies for 5, 12, TS-5, TS-12, INT-5, and INT-12. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 865-574-6717. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Fuel Resources Campaign in the Fuel Cycle Research and Development Program, Office of Nuclear Energy, U.S. Department of Energy (DOE), for support of this work. X-ray data collection for 10 and 12 was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. DOE. Diffraction data for 5 were collected at the University of Kansas with the X-ray diffractormeter purchased by the National Science Foundation, CHE-0923449.

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