Degradation of CYANEX 301 in Contact with Nitric Acid Media

Aug 16, 2012 - Idaho National Laboratory, 2525 Fremont Avenue, Idaho Falls, Idaho 83415, ... containing 50:50 mixtures of the [R;R] and [S;S] diastere...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/IECR

Degradation of CYANEX 301 in Contact with Nitric Acid Media Philippe Marc,† Radu Custelcean,† Gary S. Groenewold,‡ John R. Klaehn,‡ Dean R. Peterman,‡ and Lætitia H. Delmau*,† †

Chemical Separations Group, Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS-6119, Oak Ridge, Tennessee 37831-6119, United States ‡ Idaho National Laboratory, 2525 Fremont Avenue, Idaho Falls, Idaho 83415, United States S Supporting Information *

ABSTRACT: The nature of the degradation product obtained upon contacting CYANEX 301 (bis(2,4,4-trimethylpentyl)dithiophosphinic acid) with nitric acid has been elucidated and found to be a disulfide derivative. The first step to the degradation of CYANEX 301 in toluene has been studied using 31P{1H} NMR after being contacted with nitric acid media. The spectrum of the degradation product exhibits a complex multiplet around δP = 80 ppm. A succession of purifications of CYANEX 301 has resulted in single crystals of the acidic form and the corresponding ammonium salt. Unlike the original CYANEX 301, which consists of a complex diastereomeric mixture displaying all possible combinations of chiral orientations at the 2-methyl positions, the purified crystals were shown by single-crystal X-ray diffraction to be racemates, containing 50:50 mixtures of the [R;R] and [S;S] diastereomers. The comparison between the 31P {1H} NMR spectra of the degradation products resulting from the diastereomerically pure CYANEX 301 and the original diastereomeric mixture has elucidated the influence of the isomeric composition on the multiplicity of the 31P {1H} NMR peak. These NMR data indicate the initial degradation leads to a disulfidebridged condensation product displaying multiple resonances due to phosphorus−phosphorus coupling, which is caused by the inequivalence of the two P atoms as a result of their different chirality. A total of nine different NMR resonances, six of which display phosphorus−phosphorus coupling, could be assigned, and the identity of the peaks corresponding to phosphorus atoms coupled to each other was confirmed by 31P {1H} homodecoupled NMR analysis.

■

INTRODUCTION CYANEX 301 is a commercial solvent-extraction reagent, mainly composed of bis(2,4,4-trimethylpentyl)dithiophosphinic acid (Figure 1A). CYANEX 301’s application to the extraction

oxidized to CYANEX 272 when equal volumes of 5 M nitric acid and 2.5 g/L of CYANEX 301 in toluene are shaken for 15 min followed by 10 days of nonagitated contact at room temperature. The accepted consensus is that formation of the oxoderivatives by contact with nitric acid is preceded by an oxidative coupling of two molecules of CYANEX 301, resulting in the formation of a disulfide intermediate having the structural formula (R2(PS)SS(SP)R2, where R = 2,4,4-trimethylpentyl).4−7 The formation of a disulfide bridge by oxidation of two dithiophosphinic acid molecules has been reported for other dithiophosphinic acids having different pendent groups at phosphorus. Using 31P NMR analysis, Peters8 has observed the formation of such product in the case of the dibutyl derivative, Kuchen and Hertel9 reported the observation of bridged dithiophosphinic acids with the diethyl and dipropyl derivatives, and Beleaga et al.10 have observed the disulfide starting with diphenyldithiophosphinic acid. The hypothesis of the formation of a disulfide bridge between two molecules of CYANEX 301 was considered by Sole,11 where CYANEX 301 (0.5 M in xylene) was contacted with equal volumes of different nitric acid concentrations. They observed formation of oxo-derivatives and suggested that the disulfide intermediate was also being formed; however, their IR

Figure 1. CYANEX 301 (A), CYANEX 302 (B), and CYANEX 272 (C).

and separation of many metals has been reported previously. Wieszczycka and Tomczyk1 provide a good review of these applications. The stability of CYANEX 301 has been the object of numerous studies, where various reagents and conditions have been used which correspond to potential industrial conditions. A broad variety of observed degradation products with CYANEX 301 have been shown in literature. For example, Bhattacharyya2 states that the degradation produced both the mono-oxo and dioxo derivatives (CYANEX 302 (Figure 1B and CYANEX 272 (Figure 1C) when contacted with nitric acid media, but without providing any evidence. Menoyo3 report the formation of CYANEX 302 as an intermediate, which is finally © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13238

March 22, 2012 July 27, 2012 August 16, 2012 August 16, 2012 dx.doi.org/10.1021/ie300757r | Ind. Eng. Chem. Res. 2012, 51, 13238−13244

Industrial & Engineering Chemistry Research

Article

slight-greenish oil. The flask was transferred to the refrigerator to crystallize the product (several days). The final product formed slight-greenish, prismatic crystals [mp = 34−36 °C; 31P NMR (600 MHz); δ (d8-toluene) = (s) 64.4 ppm [R;R], [S;S] isomers]. Degradation Experiments. The degradation of the products was realized in a temperature-controlled area at 25 °C using a rotating wheel. Each sampling was realized using a pipet, and the sampled volume was fixed at 500 μL. A volume of 1 mL of 0.1 M CYANEX 301 in toluene was contacted to the same volume of 1 M nitric acid. NMR Analysis. The NMR experiments were run either on a Bruker Avance 400 or on a Bruker Avance III 400. The latter instrument was used for the 31P {1H} homodecoupled experiments. Both have operating frequencies of 400.13 MHz (1H) and 161.96 MHz (31P). Nuclear magnetic resonance (NMR) spectra for following the purification of CYANEX 301 were acquired on a Bruker Avance III 600 MHz spectrometer with a magnetic field strength of 14.093 T corresponding to operating frequencies of 600.13 MHz (1H), 564.63 MHz (19F), 242.92 MHz (31P), 152.92 MHz (13C), and 60.82 MHz (15N). All deuterated solvents were from Aldrich. All samples were analyzed in reference to a coaxial insert containing 0.1 M H3PO4 in D2O. The signal associated to the phosphorus nuclei of the phosphoric acid was set to 0 ppm, and the deuterated water was used for the lock. Electrospray Ionization Mass Spectrometry (ESI-MS) Analysis. Diluted solutions of CYANEX 301 were analyzed using a Bruker (Billerica, MA) micrOTOF-QII, which is an electrospray ionization mass spectrometer (ESI-MS) configured such that initially sprayed ions are transmitted from the ESI-MS source to a high resolution time of flight (TOF) via serially oriented ion funnels, a hexapole, a quadrupole, and finally a collision cell. Samples were introduced to the ESI-MS by direct infusion at a rate of 3 μL·min−1, with the temperature of the ESI source capillary maintained at 180 °C. The ESI potential was maintained at 4.5 kV in these experiments. Primary standards of CYANEX 301 were prepared in toluene to a concentration of 40 mM, which was then diluted 1−100 with MeOH to generate a 400 μM solution for ESI-MS analysis. Experiments to evaluate the effect of contact with 3 M HNO3 were conducted by adding 1 mL of the 40 mM CYANEX 301 primary standard to the same volume of acid, and stirring at a modest rate. Periodically, 10 μL of the organic layer was withdrawn, mixed with 990 μL of MeOH, and analyzed. XRD Analysis. Single-crystal X-ray data were collected on a Bruker SMART APEX CCD 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. All non-hydrogen atoms were refined anisotropically. The C−H hydrogen atoms were placed in idealized positions using a riding model and were refined isotropically. The N−H atoms of the ammonium salt, and the S−H atoms of the acidic form were located from the difference Fourier maps and refined isotropically. A summary of crystallographic data for both the ammonium salt (1) and the acidic form (2) are listed in Table 1. Compound Isolation for Elemental Analyses. Five milliliters of purified Cyanex 301 at 1 M in toluene was placed in contact with an equal volume of 1 M HNO3 for 1 h as described above in the Degradation Experiments section. The

analyses did not extend to a frequency low enough to observe the diagnostic S−S stretching vibration. In a subsequent study, Sole and Hiskey12 probably observed the disulfide using fast atom bombardment mass spectrometry, which showed abundant ions at m/z values corresponding to the protonated and Cu+-cationized molecule. However the ion identifications and their relationship to the disulfide in the original sample were not emphasized. Thus, the formation of disulfide by condensation of two molecules of CYANEX 301 remains a commonly accepted fact, but no study clearly identifying this product has been published yet.

■

EXPERIMENTAL SECTION Reagents. The commercial extractant CYANEX 301 was kindly supplied by Cytec Canada Inc., the assay in bis(2,4,4trimethylpentyl)dithiophosphinic acid being around 75−80 %. High-purity toluene was provided by EMD and was used as a solvent. For the synthetic isolation of ultrapurified CYANEX 301, the solvents, tetrahydrofuran (anhydrous), petroleum ether (ACS grade), hexanes (ACS grade), and toluene (ACS grade) were obtained from Aldrich, along with sodium sulfate (anhydrous, granular) and hydrogen chloride (puriss, fuming 36.5−38 %). Nitric acid was ultrapure JT Baker Ultrex II, with an assay of 68 % in nitric acid, and diluted in ultrapure water obtained from a Milli-Q Gradient A10 system. ACS grade phosphoric acid (assay 85 %) used for the coaxial insert was provided by Mallinckrodt and diluted in deuterated water 99.9 atom % D, provided by Aldrich. Purification of CYANEX 301. Commercial CYANEX 301 was purified using the recrystallization of the ammonium salt in toluene, followed by the reacidification of the purified salt. The final obtained product exhibits a purity of more than 99 % for the combined isomers.13 Ultrapurification of the CYANEX 301 ammonium salt, resulting in a 50:50 racemic mixture of [R;R], [S;S]: 5.0 g (0.015 mol) of CYANEX 301 ammonium salt was transferred to a 100 mL round-bottom flask. Also, 80 mL of a 1:1 toluene and hexanes (ACS grade) solution was added to the flask and heated to near boiling. Small quantities of THF (anhydrous, no inhibitor, 4−5 mL) was introduced to slowly dissolve the remaining salt. Afterward, another 10−20 mL of hexanes was introduced to the mixture while heating near boiling, so that no precipitate was detected. The flask was removed from heating, quickly capped, and cooled to room temperature. The next day, a small portion of crystalline solids were seen at the bottom of the flask. [Note: precipitation of the salt was limited to 2 days since the other isomers will coprecipitate.] The solution was transferred to another flask, and the salt was washed two times with hexanes. The ultrapurified salt was dried under a nitrogen gas stream that gives 1.7 g of ultrapurified product. [31P NMR (600 MHz); δ (d6-acetone) = (s) 64.75 ppm (major peak; [R;R], [S;S] isomers); (s) 64.86 ppm (very minor peak; [R;S], [S;R] isomers)]. The ultrapurified compound exhibited an enantiomeric purity of about 95 % in [R;R] and [S;S]. Acid Regeneration. A 125 mL separatory funnel was filled with 50 mL of PET ether and 25 mL of 4.0 M hydrochloric acid. A small amount of diethyl ether (anhydrous) was needed to dissolve the remaining salt. After the salt was dissolved, the organic layer was isolated and the water layer was washed two more times with 25 mL of PET ether. All organic extracts were combined and dried with anhydrous Na2SO4. The organic solution was filtered and placed under reduced pressure. The flask was purged with nitrogen gas, and the pure product is a 13239

dx.doi.org/10.1021/ie300757r | Ind. Eng. Chem. Res. 2012, 51, 13238−13244

Industrial & Engineering Chemistry Research

Article

The comparison between the spectra of the organic phases, after contact with nitric acid and CYANEX 301, exhibits a complete change in CYANEX 301. A close-up on the complex multiplet obtained at δP ≈ 80 ppm for the new product is presented in Figure 3. The chemical shift and multiplicity of

Table 1. Crystallographic Data for 1 and 2 Formula M Crystal size [mm] Crystal system Space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z T (K) ρcalcd [g·cm−3] 2θmax [deg] μ [cm−1] Reflns collected Independent reflns Parameters Rint R1,a wR2b (I > 2σ(I))

1

2

C16H38NPS2 339.56 0.18 × 0.15 × 0.02 Monoclinic C2/c 8.3942(8) 9.6626(10) 26.242(3) 90 90.562(2) 90 2128.4(4) 4 173(2) 1.060 56.58 0.320 8353 2648 106 0.0540 0.0548, 0.1322

C16H35PS2 322.53 0.35 × 0.14 × 0.08 Triclinic P̅1 11.8398(12) 12.0290(12) 14.8387(15) 75.089(2) 82.148(2) 89.399(2) 2022.4(4) 4 173(2) 1.059 56.68 0.332 25477 10021 367 0.0467 0.0552, 0.1364

a R 1 = ∑(|F 0 | − |F c |)/∑|F0 |. ∑[w(F02)2]}1/2.

b

Figure 3. Spectrum obtained after the degradation of purified CYANEX 301.

wR 2 = {∑[w(F 0 2 − F c 2) 2 ]/

this peak provide evidence that CYANEX 302 and 272 do not correspond to the observed product. This peak has been reported previously in the literature by Chen14 when CYANEX 301 is degraded under irradiation, but without identifying the corresponding compound. The presence of the disulfide was substantiated by ESI-MS results. Benchmark analyses of CYANEX 301 prior to HNO3 contact showed abundant ions at m/z 323, 345, and 367 that correspond to protonated and natiated versions of the intact molecule, i.e., [CyxH 2 ] + , [CyxHNa]+ , and [CyxNa 2 ] + , respectively (Figure 4a), where Cyx represents the conjugate base of CYANEX 301. The ion at m/z 667 corresponds to [(CyxH)2Na]+ and noncovalent cluster species such as these are typical in ESI-MS analyses. Na+ functions as a cationization agent and is present as an adventitious contaminant in most ESI-MS experiments. Surprisingly, the ESI-MS spectrum of

organic phase was then separated by centrifugation and checked by NMR to ensure that the compound was fully degraded. The toluene was evaporated at room temperature using a rotavap. Room temperature was maintained to avoid any alteration of the disulfide. The yellowish oily residue is taken back twice in dichloromethane (about 100 mL each time) to remove the last traces of toluene. A small fraction of the oil after the last dichloromethane treatment is dissolved into toluene and an NMR spectrum is obtained to confirm that no changes had occurred. The rest of the oil is packed under argon and sent for elemental analyses.

■

RESULTS AND DISCUSSION Degradation of Purified CYANEX 301. One milliliter of 0.1 M purified CYANEX 301 in toluene was contacted with 1 M nitric acid for 3 h, and then a sample of the organic phase was analyzed using 31P {1H} NMR. The spectrum is presented in Figure 2, with spectra of 0.1 M purified CYANEX 301 in pristine toluene, and of commercial CYANEX 302 and 272.

Figure 2. 31P {1H} NMR spectra of commercial CYANEX 302 and 272, purified CYANEX 301, and degraded CYANEX 301.

Figure 4. (a) Positive ion ESI-MS spectrum of purified CYANEX 301. (b) Spectrum acquired after 18 h of contact with 3 M HNO3. 13240

dx.doi.org/10.1021/ie300757r | Ind. Eng. Chem. Res. 2012, 51, 13238−13244

Industrial & Engineering Chemistry Research

Article

purified CYANEX 301 contained ions corresponding to the disulfide couple Cyx−Cyx, specifically [(Cyx−Cyx)H]+, [(Cyx−Cyx)Na]+, and [(Cyx−Cyx)2Na]+ at m/z 643, 665, and 1307. These ions represent about 20 % of the total CYANEX 301-derived ions in the analysis and appear to be induced by the electrospray process. The alternative explanation is that the disulfide is present as an impurity in the purified CYANEX 301; however, this has been ruled out since the 31P NMR spectrum of the pure compounds displayed a single resonance corresponding to the intact acid. We hypothesize that the disulfide is formed by transient oxidizing conditions arising in the ESI-MS droplets, and more studies are underway to understand the phenomenon, which underscores the great susceptibility of the dithiophosphinates to oxidative coupling. Difficulty encountered in totally eliminating the ESI-induced disulfide did not preclude observation of the dramatic conversion of CYANEX 301 to the disulfide upon contact with 3 M HNO3. The ESI-MS analysis acquired after 18 h of contact showed abundant ions corresponding to the disulfide couple (Figure 4b), and ions corresponding to intact CYANEX 301 were not observed. Spectra collected at intermediate times showed a mixture of intact CYANEX 301 and the disulfide, with the latter growing in intensity with increasing contact time. It is worthwhile noting that the protonated version of the coupled disulfide degradation product at m/z 643 was probably the same species observed by Sole and Hiskey in their earlier fast atom bombardment mass spectrometry analysis.11 XRD Analysis of Diastereomerically Purified CYANEX 301. The ammonium salt of CYANEX 301 (1) crystallized from toluene/hexanes in the monoclinic C2/c space group, with half of the molecule in the asymmetric unit. Both the phosphinate anion and the ammonium cation sit on the crystallographic C2 axis. As a result, the phosphinate moiety displays C2 symmetry, whereas NH4+ displays 2-fold disorder. Figure 5a shows an ORTEP representation of 1, with 50 % probability thermal ellipsoids. The two alkyl groups of the phosphinate anion display identical chirality at the 2-methyl positions ([R,R] or [S,S]). The crystal is overall a racemate, containing a 50:50 mixture of the [R,R] and [S,S] enantiomers. Anions of the same chirality ([R,R] or [S,S]) form N−H···S hydrogen-bonds with the ammonium cations, resulting in two-dimensional layers in the crystallographic ab plane (Figure 5b). There are two different N−H···S hydrogen bonds, of which one is bifurcated. The observed H···S contact distances and N− H−S angles are 2.281, 2.456 Å (bifurcated) and 171.7°, 137.5° (bifurcated), respectively. Finally, layers of opposite chirality alternate along the crystallographic c-axis, packing via van der Waals interactions (Figure 5c). The acidic form of the ultrapurified CYANEX 301 (2) crystallized in the triclinic P̅ 1 space group, with two crystallographically unique molecules of 2 in the asymmetric unit. Figure 6a shows an ORTEP representation (50 % probability) of one of the two crystallographically unique molecules (the second one is virtually identical). As in the previous structure, three different diastereomers are possible: [R;R], [S;S], and [R;S]. The crystal of 2 is a racemate, containing a 50:50 mixture of the [R;R] and [S;S] enantiomers. Molecules of the same chirality ([R;R] or [S;S]) form S−H···S hydrogen-bonds resulting in one-dimensional chains (Figure 6b). There are two different S−H···S hydrogen bonds, with

Figure 5. Crystal structure of 1. (a) Ortep representation with 50 % ellipsoids. (b) Homochiral hydrogen-bonded layer. (c) Packing of layers with alternate chiralities.

H···S contact distances and S−H−S angles of 2.523, 2.524 Å and 171.2°, 172.9°, respectively. Hydrogen-bonded chains of similar chirality ([R;R] or [S;S]) pack along the crystallographic c-axis through van der Waals interactions between the end tert-butyl groups (Figure 6c). Finally, chains of opposite chirality alternate along the crystallographic b-axis (Figure 6d). Degradation of Ultrapurified CYANEX 301. Using the same degradation conditions, a sample of the 0.1 M of ultrapurified CYANEX 301 in toluene was contacted with 1 M 13241

dx.doi.org/10.1021/ie300757r | Ind. Eng. Chem. Res. 2012, 51, 13238−13244

Industrial & Engineering Chemistry Research

Article

time scale, due to the fast-exchange process of the proton on the −SH group and with the S group, and hence, the phosphorus atoms cannot be asymmetric. In the case of a disulfide, the two thio groups are nonequivalent. This results in the phosphorus atoms bearing four potentially different substituents. If the stereodescriptors of the carbons of the alkyl chains borne by each phosphorus nucleus are nonequivalent, the phosphorus atoms will exhibit asymmetric character just like carbon atoms do, and the 31P peak in the spectra of the disulfide will be split. Phosphorus Atoms Asymmetry in the Disulfide. In order to describe this asymmetry of the phosphorus nuclei, an adapted version of the Cahn, Ingold, and Prelog rules has been used to ascribe the 2-methyl stereodescriptors to the phosphorus atoms. It was hypothesized that the sulfur linked with a double bond to the phosphorus is the atom determining the direction for the allocation of a stereodescriptor. The priority of the other substituents is determined using the rules applied for carbon, i.e., in the case of two identical alkyl chains with different 2-methyl stereodescriptors for the asymmetric carbons on the chains, R has the priority over S. In the disulfide, each phosphorus atom can be assigned three different stereodescriptors. Indeed, if the two alkyl chains have the same stereodescriptor on their asymmetric carbon atoms, it will not be possible to give the priority to one chain on the other, and the phosphorus is in this case nonasymmetric (denoted as NAs in the rest of the text). In the other cases, once the priority of each substituent is defined, the phosphorus atoms will be assigned an R or S descriptor, depending on the rotational order of the substituents. The different stereodescriptors assigned to each phosphorus atom, depending on the asymmetry of the carbons, are presented in Table 2. Refining the Different Combinations of Asymmetric Phosphorus Atoms. All the different combinations of asymmetric 2-methyl pendent groups have been sorted

Figure 6. Crystal structure of 2. (a) Ortep representation with 50 % ellipsoids. (b) Homochiral hydrogen-bonded chain. (c) Homochiral packing of chains into layers. (d) Packing of layers with alternate chiralities.

nitric acid, and the organic phase was analyzed by 31P {1H} NMR. The acquired spectrum (Figure 7, black trace) exhibits a

Table 2. Phosphorus Atoms Asymmetry as a Function of Carbon Asymmetry

Figure 7. Comparison between the 31P {1H} NMR spectra obtained for degraded CYANEX 301 samples. The purified sample is represented by the green trace, with intense multiplet peaks. The ultrapurified sample is the black trace, predominantly a singlet.

tremendous difference when compared to that of the purified CYANEX 301 (green trace), in that a near-complete loss of multiplicity of the peak is seen. The 31P NMR spectrum of the degraded, ultrapurified compound is predominantly a singlet that corresponds to the central peak of the multiplet observed previously in the degraded purified CYANEX 301. The doublets can still be observed, but at much lower intensity, and are due to traces of [R;S] and [S;R] isomers that remain in the ultrapurified mixture. The comparison between the two 31P NMR spectra shows the effect of the asymmetric 2-methyl stereocenters on the multiplicity of the 31P peak in the NMR spectrum of the degradation product (Figure 7). Identification of the Degradation Product, Explanation of the 31P {1H} NMR Spectra. The 31P {1H} NMR spectra are consistent with the formation of a disulfide bridge between two molecules of CYANEX 301. In the case of intact CYANEX 301, the two thio groups are equivalent at the NMR

Phosphorus 1a

Phosphorus 2b

P asymmetryc

[R;R] [R;R] [R;R] [R;R] [S;S] [S;S] [S;S] [S;S] [R;S] [R;S] [R;S] [R;S] [S;R] [S;R] [S;R] [S;R]

[R;R] [S;S] [R;S] [S;R] [R;R] [S;S] [R;S] [S;R] [R;R] [S;S] [R;S] [S;R] [R;R] [S;S] [R;S] [S;R]

{NAs;NAs} {NAs;NAs} {NAs;S} {NAs;R} {NAs;NAs} {NAs;NAs} {NAs;S} {NAs;R} {R;NAs} {R;NAs} {R;S} {R;R} {S;NAs} {S;NAs} {S;R} {S;S}

a

The letters between square brackets represent the 2-methyl stereodescriptors of the carbon atoms on the alkyl chains borne by P1, respectively [R1;R2]. bThe letters between square brackets represent the 2-methyl stereodescriptors of the carbon atoms on the alkyl chains borne by P2, respectively [R3;R4]. cThe letters between braces represent the 2-methyl stereodescriptors of the phosphorus atoms in the disulfide, respectively {P1;P2}. 13242

dx.doi.org/10.1021/ie300757r | Ind. Eng. Chem. Res. 2012, 51, 13238−13244

Industrial & Engineering Chemistry Research

Article

Table 3. Study of the Different Combinations of Asymmetric Phosphorus Atoms and Formation Probability Case {NAs;NAs} {NAs;R} and {R;NAs} {NAs;R} and {S;NAs} {R;R} {S;S} {R;S} and {S;R}

Probability of formationa

Comments Made of two enantiomers {[S;S];[S;S]} and {[R;R];[R;R]} and their diastereoisomers, which are in fact one molecule {[R;R];[S;S]} (a rotation gives {[S;S];[R;R]}) Made of only two enantiomers {[R;S];[R;R]} and {[R;S];[S;S]}, these two possibilities are equivalent (considering one enantiomer, a rotation can change {NAs;R} in {R;NAs}, and inversely) Made of only two enantiomers {[S;R];[R;R]} and {[S;R];[S;S]}, these two possibilities are equivalent (considering one enantiomer, a rotation can change {NAs;S} in {S;NAs}, and inversely) Each of these two possibilities is made with only one molecule (it is impossible to distinguish the two phosphorus when they have the same stereodescriptor); {R;R} and {S;S} are “double” diastereoisomers: considering alkyl chains and phosphorus These two possibilities are in fact only one molecule. The couples {[R;S];[R;S]} and {[S;R];[S;R]} are equivalent

4 4 4 1 1 2

a The formation probability is the number of combinations of 2-methyl asymmetric pendent groups leading to given possible 31P NMR coupling of two phosphorus atoms. The total number of combinations is 16.

considering their impact on the phosphorus atoms. The symmetry of the disulfide dictates that some combinations are identical. Therefore, the number of combinations contributing to each of the possible disulfide-coupled, phosphorus products has been determined (see Table 3). This evaluation assumes that the initial mixture is racemic, and the condensation of two molecules of CYANEX 301 is equiprobable regardless of the chirality of the 2-methyl positions on the alkyl chains. Explanation of the 31P {1H} NMR Spectrum of Degraded CYANEX 301, Attribution of the Peaks. The assignment of the resonances in the 31P NMR spectrum to a particular enantiomer has been done assuming that: • There is a coupling between two phosphorus atoms in the disulfide when their 2-methyl stereodescriptors are different. • Considering a given (R, S, or NAs) phosphorus atom, the impact on its chemical shift when bridged to a enantiomerically different phosphorus is all the more important as the difference of the chemical shifts of the associated singlet (if the phosphorus atom is R, then the associated singlet is the one corresponding to {R;R}) is important. • While the mixture is supposedly racemic, the integration of the peaks must fit the formation probability of the considered couple. The attribution of the different peaks on the 31P NMR spectrum is presented in Figure 8. It has not been possible to ascribe a side to the R and S phosphorus. Instead, they are presented with the letters X and Y as a reminder of the ambiguity. Validation of the 31 P NMR Coupling between Phosphorus Nuclei. To ensure that the multiplicity of some 31P NMR peaks (doublets) was a consequence of coupling with other phosphorus atoms present in the disulfidebridged molecule and to validate the identification of the coupled phosphorus nuclei, 31P homodecoupled NMR was used: this method is commonly used in 1H NMR but is relatively unusual in 31P NMR. The result of this homodecoupling is that the peak corresponding to the irradiated phosphorus will disappear, and if the multiplicity is really a result of a coupling with another phosphorus nucleus, one of the other doublets will become a singlet, corresponding to the coupled phosphorus atoms. The results of these experiments are presented in Figure 9. The red arrow indicates the frequency of irradiation (peak annihilated due to the irradiation

Figure 8. Attribution of the peaks for the 31P {1H} NMR spectrum of degraded purified CYANEX 301.

of the corresponding phosphorus), and the green one points out the peak becoming a singlet. Elemental Analysis of the Degraded CYANEX 301. In order to validate that the degradation product of CYANEX 301 is mostly a disulfide, it has been isolated, and sent for elemental analysis. The result and comparison with the theory of this analysis are presented in Table 4. While the percentages appear to depart from the expected values, it is important to point out that the analysis was done on a degraded compound and not a pure compound. These results are actually consistent with the hypothesis of the formation of a disulfide bridge, since the majority of the sulfur remains. The missing 4.25 % on the total mass can probably be assigned to some residual solvent.

■

CONCLUSION The aim of this work was to study the degradation product(s) of CYANEX 301 [bis(2,4,4-trimethylpentyl)dithiophosphinic acid] after contacted with nitric acid. Both purified and ultrapurified CYANEX 301 were used for these analysis. The purified CYANEX 301 contained three stereoisomers resulting from the presence of two chiral carbon centers on the pendant alkyl groups [R;R], [R;S], [S;S]. The ultrapurified CYANEX 301 was isolated by further recrystallization. The ultrapurified product gave crystals shown by single crystal X-ray diffraction to be 50:50 mixture of the [R;R] and [S;S] diastereomores. 31P NMR analyses were performed on samples of CYANEX 301 13243

dx.doi.org/10.1021/ie300757r | Ind. Eng. Chem. Res. 2012, 51, 13238−13244

Industrial & Engineering Chemistry Research

Article

Figure 9. Validation of the coupling between phosphorus nuclei using homodecoupling NMR analysis.

■

ACKNOWLEDGMENTS This research was sponsored by the Office of Nuclear Energy, U.S. Department of Energy. P.M. and L.H.D. want to thank Mike Brown from Bruker Biospin, The Woodlands, TX, for writing the phosphorus homodecoupling pulse program.

Table 4. Elemental Analysis of Degraded CYANEX 301 Calculated mass percentages Carbon Phosphorus Hydrogen Sulfur Oxygen Nitrogen Total a

59.77 9.63 10.66 19.94 0.00 0.00 100.00

% % % % % % %

Analyzed mass percentages

Deltaa

58.65 % 8.61 % 9.92 % 18.57 % Not analyzed