NMR spectroscopy and structural characterization of dithiophosphinate ligands relevant to minor actinide extraction processes

Article abstract of DOI:10.1039/c1dt11637a

Synthetic routes to alkyl and aryl substituted dithiophosphinate salts that contain non-coordinating PPh₄⁺ counter cations are reported. In general, these compounds can be prepared via a multi-step procedure that starts with reacting secondary phosphines, i.e. HPR₂, with two equivalents of elemental S. The synthetic transformation proceeds by oxidation of the phosphine followed by insertion of S into the H-P bond. This approach was used to synthesize a series of dithiophosphinic acids that were fully characterized, namely HS₂P(p-CF₃C₆H ₄)₂, HS₂P(m-CF₃C₆H ₄)₂, HS₂P(o-MeC₆H₄) ₂ and HS₂P(o-MeOC₆H₄)₂. Although the insertion step was found to be much slower than the oxidation reaction, the formation of (NH₄)S₂PR₂ from HPSR₂ occurred rapidly upon addition of NH₄OH. Subsequent cation exchange reactions proceeded readily with PPh₄Cl in water, under air and at ambient conditions to provide analytically pure samples of [PPh₄][S₂PR₂] (R = p-CF₃C ₆H₄, m-CF₃C₆H₄, o-CF ₃C₆H₄, o-MeC₆H₄, o-MeOC₆H₄, Ph, and Me, 1b-7b, respectively), which were characterized by elemental analysis, multinuclear NMR, and IR spectroscopy. In addition, S₂PPh₂^- and dithiophosphinates with ortho-substituted aryl groups (3b-6b) were characterized by X-ray crystallography. As opposed to the acids, which have short PS double bonds and long P-SH single bonds, the metric parameters for the S atoms in S ₂PR₂^- are equivalent. In addition, the presence of large non-coordinating PPh₄⁺ cations guard against intermolecular P-S...X interactions and ensure that the P-S bond is isolated. These S₂PR₂^- anions, which can be prepared in large quantities and isolated in crystalline form, are attractive for spectroscopic and theoretical studies because the P-S interaction can be probed independently in the absence of intermolecular interactions.

Full text of DOI:10.1039/c1dt11637a

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PAPER
NMR spectroscopy and structural characterization of dithiophosphinate
ligands relevant to minor actinide extraction processes
Scott R. Daly,a John R. Klaehn,b Kevin S. Boland,a Stosh A. Kozimor,*a Molly M. MacInnes,a
Dean R. Petermanb and Brian L. Scotta
Received 30th August 2011, Accepted 21st October 2011
DOI: 10.1039/c1dt11637a
Synthetic routes to alkyl and aryl substituted dithiophosphinate salts that contain non-coordinating
+
PPh4 counter cations are reported. In general, these compounds can be prepared via a multi-step
procedure that starts with reacting secondary phosphines, i.e. HPR2, with two equivalents of elemental
S. The synthetic transformation proceeds by oxidation of the phosphine followed by insertion of S into
the H–P bond. This approach was used to synthesize a series of dithiophosphinic acids that were fully
characterized, namely HS2P(p-CF3C6H4)2, HS2P(m-CF3C6H4)2, HS2P(o-MeC6H4)2 and
HS2P(o-MeOC6H4)2. Although the insertion step was found to be much slower than the oxidation
reaction, the formation of (NH4)S2PR2 from HPSR2 occurred rapidly upon addition of NH4OH.
Subsequent cation exchange reactions proceeded readily with PPh4Cl in water, under air and at ambient
conditions to provide analytically pure samples of [PPh4][S2PR2] (R = p-CF3C6H4, m-CF3C6H4,
o-CF3C6H4, o-MeC6H4, o-MeOC6H4, Ph, and Me, 1b–7b, respectively), which were characterized by
-
elemental analysis, multinuclear NMR, and IR spectroscopy. In addition, S2PPh2 and
dithiophosphinates with ortho-substituted aryl groups (3b–6b) were characterized by X-ray
crystallography. As opposed to the acids, which have short P S double bonds and long P–SH single
-
bonds, the metric parameters for the S atoms in S2PR2 are equivalent. In addition, the presence of
+
large non-coordinating PPh4 cations guard against intermolecular P–S ◊ ◊ ◊ X interactions and ensure
-
that the P–S bond is isolated. These S2PR2 anions, which can be prepared in large quantities and
isolated in crystalline form, are attractive for spectroscopic and theoretical studies because the P–S
interaction can be probed independently in the absence of intermolecular interactions.
-
R2PS2 ) are uniquely capable of selectively removing actinides
Introduction
from lanthanides via liquid/liquid extraction.11–22 Although there
are many variables that contribute to effective separations9,23
–
It has been proposed that nuclear energy is one of the few
immediate and viable options in terms of meeting increasing
global energy demands without increasing anthropogenic CO2
emissions with respect to fossil fuel combustion.1–3 However, the
safe transportation, storage and disposal of spent nuclear fuel
pose significant environmental concern, which has reinvigorated
research to develop new strategies for processing spent nuclear
fuel. Hence, in order to meet future needs, advanced separation
technologies are being developed to recycle the residual energy
content of the spent fuel, minimize nuclear waste production and
alleviate proliferation concerns associated with reprocessing.4–6
One significant challenge in achieving these goals involves sepa-
rating trivalent actinides from lanthanides, a separation hampered
by the similar chemical properties of the 5f - and 4f -elements.7–10
In certain cases, some soft-donor ligands (e.g., dithiophosphinates,
solubility, hydration energies, extraction kinetics, and synergistic
effects to name a few – it has been proposed that successful
separation processes rely on the ability of an extractant to
selectively bind actinides over lanthanides. It is additionally
assumed that this binding affinity is heavily influenced by subtle
differences in metal–ligand orbital mixing, such that more covalent
actinide-ligand bonds lead to selectivity for extracting the actinides
over the lanthanides.10,24–27
One of the largest separation factors reported to date was
observed for a dithiophosphinic acid that has ancillary aryl groups
with trifluoromethyl substituents, namely HS2P(o-CF3C6H4)2
(Scheme 1).28 Although this molecule exhibits outstanding selec-
tivity for removing americium from europium, many variables
associated with the extraction procedure must be rigorously
controlled. One such factor is associated with the CF3 substitution
pattern on the aryl ring. For reasons not well understood, when
the CF3 substituent is in the ortho position the compound
exhibits exceptional selectivity for actinides in the presence of
aLos Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
E-mail: stosh@lanl.gov
bIdaho National Laboratory, Idaho Falls, Idaho, 83415, USA
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commercial sources. Anhydrous solvents used to prepare HS2P(o-
MeC6H4)2 starting from PSCl3 were purchased from Aldrich and
used as received. H2O was purified to 18.2 MW-cm resistivity using
a Thermo-Scientific Barnstead Nanopure or Millipore Nanopure
water purification systems. Deuterated solvents were used as
received. PPh4Cl (Acros), Na2S·9H2O (Aldrich), sulfur (Aldrich),
PSCl3 (Aldrich), NaS2O4 (Aldrich), (NH4)2CO3 (Fisher), conc.
HCl (Fisher), conc. H2SO4 (Aldrich) and HS2PPh2 (Alfa Aesar)
were used as received.
Elemental analyses were carried out by Midwest MicroLab,
LLC (Indianapolis, IN). The infrared spectra were recorded on
a Nicolet Magna-IR System 750 spectrometer as KBr pellets. The
1H, 31P and 19F NMR data were obtained using a Bruker Avance
or Bruker DMX 300WB NMR spectrometers at 300 MHz, 121
MHz and 282 MHz, respectively. Chemical shifts are reported in
d units (positive shifts to high frequency) relative to TMS (1H),
H3PO4 (31P), or CFCl3 (19F).
Scheme 1 HS2P(m-CF3C6H4)2 (2a, left) and HS2P(o-CF3C6H4)2 (3a,
right).
lanthanides.28 However, if the CF3 substituent is moved to the
meta position, the separation factors are dramatically decreased
(Scheme 1).29
In order to better understand the steric and electronic dif-
ferences between CF3-substituted HS2PR2 compounds (R = p-
CF3C6H4, m-CF3C6H4, o-CF3C6H4; 1a–3a, respectively), we have
initiated spectroscopic and theoretical investigations using S K-
edge X-ray absorption spectroscopy (XAS) and time-dependent
density functional theory (TD-DFT) to study a carefully se-
lected family of dithiophosphinic acids and their corresponding
M(S2PR2)x complexes.30 These compounds were chosen because
the identity of the aryl substituents can be systematically modified,
which enables the effects of the ancillary substituents on the
P–S bond to be methodically probed in a step-wise fashion.
In addition, the dramatic difference in separation factors for
these diaryldithiophosphinic acids makes this series ideal for
identifying potential correlations between electronic structure
and extraction selectivity. However, preliminary S K-edge XAS
analyses on the HS2PR2 acids were complicated by the sulfur atoms
being in different chemical environments. In addition, analyses
of M(S2PR2)x coordination complexes were difficult because the
isolated S–P bonding interactions were not well defined. To
overcome these problems, we report here the synthesis, NMR
spectroscopy and structural characterization of a series of new
Bis(p-trifluoromethylphenyl)dithiophosphinic acid,
HS2P(p-CF3C6H4)2 (1a)
Compound 1a was prepared using the method previously de-
scribed for 3a from HP(p-CF3C6H4)2 (1.00 g, 3.10 mmol) and
S (0.208 g, 6.49 mmol) to yield 0.492 g of 1a (41.0%), as confirmed
by NMR spectroscopy. Crystals used for X-ray diffraction and
microanalysis were grown from concentrated hexane solutions and
collected as colorless plates. Anal. Calcd for C14H9F6PS2: C, 43.5;
H, 2.35; P, 8.02; F, 29◦.5. Found: C, 43.8; H, 2.47; P, 8.13; F, 29.5.
1H NMR (CDCl3, 20 C): d 2.62 (br s, SH, 1H), 7.76 (dd, 3JHH = 8
Hz, 4JPH = 3 Hz, meta-H, 4H), 8.09 (dd, 3JHH = 8 Hz, 3JPH = 14 Hz,
ortho-H, 4H). 31P NMR (C◦DCl3, 20 ◦C): d 53.7 (p, 3JPH = 14 Hz).
1
19F{ H} NMR (CDCl3, 20 C): d -63.7 (s, CF3). IR (cm-1): 522 w,
549 m, 598 m, 629 m, 668 s, 707 vs, 732 w, 782 w, 832 m, 849 w,
904 w, 960 w, 1015 s, 1062 vs, 1108 m, 1127 vs, 1140 s, 1174 s, 1286
sh, 1320 vs, 1398 s, 1609 m, 2374 br s, 3047 w, 3069 w, 3096 w.
Bis(m-trifluoromethylphenyl)dithiophosphinic acid,
HS2P(m-CF3C6H4)2 (2a)
-
non-coordinating [PPh4][S2PR2] salts that contain isolated S2PR2
anions required for obtaining interpretable S K-edge XAS data.
Compound 2a was prepared using the method previously de-
scribed for 3a from HP(m-CF3C6H4)2 (2.987 g, 9.28 mmol) and
S (0.911 g, 28.4 mmol) to yield 0.926 g of 2a (25.8%), as confirmed
by NMR spectroscopy. Crystals used for X-ray diffraction and
microanalysis were grown from concentrated hexane solutions and
collected as colorless plates. Anal. Calcd for C14H9F6PS2: C, 43.5;
H, 2.35; P, 8.02; F, 29.5. Found: C, 43.5; H, 2.35; P, 7.89; F, 29.5.
1H NMR (CDCl3, 20 ◦C): d 2.45 (br s, SH, 1H), 7.65 (td, 3JHH = 8
Hz, 4JPH = 3 Hz, meta-H, 2H), 7.82 (d, 3JHH = 8 Hz, para-H, 2H),
Experimental section
General considerations
The phosphines HP(o-MeC6H4)2 and HP(o-MeOC6H4)2 were
prepared by reduction of the corresponding phosphine oxides with
diisobutylaluminum hydride according to literature methods.31
The CF3-substituted diarylphosphines, HP(CF3C6H4)2 and the
bis(trifluoromethylphenyl)dithiophosphinic acids were prepared
using the methods previously described for HS2P(o-CF3C6H4)2
(3a) and (NH4)S2P(o-CF3C6H4)2.28 The NaS2PMe2·2H2O salt was
prepared as previously described.32 The Grignard reagent o-
MeC6H4MgBr was prepared as a 1.0 M solution in diethyl ether
from Mg0 (Aldrich) and 2-bromotoluene (Aldrich). Toluene used
for the oxidation of phosphines with sulfur was dried using sodium
and benzophenone, vacuum distilled and degassed by three freeze-
pump-thaw cycles before use. All oxidation reactions were per-
formed under argon in toluene using standard Schlenk techniques.
Subsequent manipulations involving HS2PR2 or (NH4)S2PR2 were
carried out in air with solvents that were used as received from
8.11 (dd, 3JHH = 8 Hz, 3JPH = 14 Hz, ortho-H, 2H), 8.26 (d, 3JPH
=
15 Hz, ortho-H, 2H). 31P NMR (CDCl3, 20 ◦C): d 53.8 (p, JPH = 15
◦
1
Hz). 19F{ H} NMR (CDCl3, 20 C): d -63.2 (s, CF3). IR (cm-1):
465 w, 497 w, 510 w, 536 m, 581 w, 618 sh, 630 m, 645 s, 650 m,
664 w, 677 s, 695 s, 720 s, 798 s, 809 m, 853 w, 905 m, 914 m, 929
m, 941 m, 995 w, 1072 vs, 1108 vs, 1123 vs, 1144 vs, 1167 vs, 1177
sh, 1229 w, 1250 sh, 1275 s, 1310 vs, 1326 vs, 1418 s, 1429 s, 1479
w, 1604 m, 2329 br s, 3047 sh, 3061 w.
Bis(o-tolyl)dithiophosphinic acid, HS2P(o-MeC6H4)2 (4a)
Solid sulfur (0.359 g, 11.2 mmol) was added to a Schlenk flask
that had been charged with a solution of HP(o-MeC6H4)2 (1.17 g,
2164 | Dalton Trans., 2012, 41, 2163–2175
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5.13 mmol) in toluene (40 mL) against a strong argon purge. The
flask was equipped with a reflux condenser and after the mixture
was heated at reflux (3 h), the sulfur completely dissolved. The
solution was cooled to room temperature and aqueous NH4OH
was added dropwise via syringe causing a thick white precipitate
of (NH4)S2P(o-MeC6H4)2 to form. The mixture was stirred for
30 min at room temperature and then filtered. The white solid
was washed with hexanes (3 ¥ 10 mL) and allowed to evaporate to
dryness in air. The dried solid was transferred to a beaker equipped
with a stir bar. To the beaker was added Et2O (50 mL) followed by
aqueous 4.0 M HCl (25 mL). The mixture was vigorously stirred
until most of the solid dissolved and had been extracted into the
organic phase. The Et2O layer was separated and the aqueous
solution was washed with Et2O (2 ¥ 25 mL). The Et2O fractions
were combined, dried over Na2SO4 and evaporated to dryness
under vacuum, which yielded 4a as an off-white semi-crystalline
solid that was confirmed by NMR spectroscopy (1.163 g, 81%).
Crystals used for X-ray diffraction and microanalysis were grown
from concentrated Et2O solutions and collected as colorless plates.
Anal. Calcd for C14H15PS2: C, 60.4; H, 5.43; P, 11.1. Found: C,
brown solution were removed by filtration, and the solvent was
evaporated to dryness under vacuum to reveal a yellow solid. The
yellow solid was washed with hot toluene and allowed to evaporate
to dryness in air. The resulting NaS2P(o-MeC6H4)2 was assayed by
31P NMR spectroscopy (d 59.9; d6-acetone) and converted to the
corresponding acid, 4a, by addition to 4 M HCl (50 mL) and
Et2O (150 mL), followed by vigorous mixing. The Et2O layer was
collected and transferred to a 250 mL Erlenmeyer flask charged
with a magnetic stir bar. To the flask (NH4)CO3 (4.5 g, 58 mmol)
was added followed by toluene (50 mL). The flask was heated to
boiling and the ether was removed. The solids were collected by
filtration and allowed to evaporate to dryness in air. The 31P NMR
spectrum of the solid revealed a single peak (d 57.5; d6-acetone)
corresponding to (NH4)S2P(o-MeC6H4)2 (6.8 g, 77%).
In order to obtain HS2P(o-MeC6H4)2 in high purity using this
synthetic route, we have found it necessary to first isolate the
(NH4)S2P(o-MeC6H4)2 salt and then convert it back to the acid
by addition of HCl and extraction with Et2O using the procedure
detailed in the preceding paragraph (Scheme 2). The conversion
of (NH4)S2P(o-MeC6H4)2 prepared by this method to HS2P(o-
60.6; H, 5.31; P, 11.0. H NMR (CDCl3, 20 ◦C): d 2.17 (s, Me,
MeC6H4)2, 4a, was confirmed by comparison of the H and 31P
1
1
6H), 7.18 (dd, 4JPH = 7 Hz, 3JHH = 7 Hz, meta-H, 2H), 7.36–7.48
NMR spectra to those collected for the single crystals described
in the previous synthesis of 4a.
(m, meta,para-H, 4H), 8.39 (ddd, 3JPH = 18 Hz, 3JHH = 8 Hz, 4JHH
= 2 Hz, ortho-H, 2H). 31P NMR (CDC◦l3, 20 ◦C): d 55.2 (t, 3JPH
=
1
18 Hz). 31P{ H} NMR (d6-acetone, 20 C): d 52.1. IR (cm-1): 454
w, 495 m, 513 w, 537 m, 552 m, 561 sh, 642 vs, 678 s, 687 s, 708 m,
713 m, 764 s, 769 s, 798 m, 833 w, 882 w, 991 w, 1033 w, 1065 m,
1133 m, 1160 m, 1196 m, 1278 m, 1378 w, 1429 m, 1451 s, 1465 m,
1559 w, 1588 w, 2372 br, 2923 w, 2970 m, 3002 w, 3034 w, 3060 w.
Bis(o-methoxyphenyl)dithiophosphinic acid, HS2P(o-MeOC6H4)2
(5a)
Solid sulfur (0.264 g, 8.23 mmol) was added to a Schlenk flask
that had been charged with a solution of HP(o-MeOC6H4)2 (0.968
g, 3.93 mmol) in toluene (50 mL) against a strong argon purge.
The flask was equipped with a reflux condenser and the mixture
was heated to reflux overnight. The solution was allowed to cool
to room temperature, during which time large colorless prisms of
5a slowly formed (0.981 g, 80%). Anal. Calcd for C14H15O2PS2: C,
54.2; H, 4.87; P, 9.98. Found: C, 54.7; H, 4.90; P, 9.53. 1H NMR
(CDCl3, 20 ◦C): d 3.61 (s, MeO, 6 H), 6.87 (dd, 4JPH = 6 Hz, 3JHH
= 8 Hz, meta-H, 2H), 7.11 (td, 4JPH = 3 Hz, 3JHH = 8 Hz, meta-H,
Alternative synthesis of HS2P(o-MeC6H4)2 (4a)33
The following reactions were carried out under a nitrogen atmo-
sphere, unless otherwise indicated. PSCl3 (1.8 mL, 18 mmol) and
anhydrous Et2O (150 mL) were added to a 500 mL three-neck
round bottom flask. The Grignard reagent o-MeC6H4MgBr (1.0
M in Et2O, 75 mL, 75 mmol) was added to an addition funnel via
syringe and then slowly added to the PSCl3 solution, which had
been cooled to 0 ◦C. Once the addition was finished, the mixture
was allowed to warm to room temperature and stirred overnight.
The reaction was then cooled to 0 ◦C and H2SO4 (75 mL, 10 wt%)
was added. Subsequently, an additional aliquot of H2O (100 mL)
was added, and the cloudy mixture became transparent. This
solution was transferred to a 500 mL separatory funnel and the
organic layer was collected. The aqueous layer was washed with
Et2O (2 ¥ 50 mL) and the organic fractions were combined and
dried over anhydrous Na2SO4. The Et2O was removed by a rotary
evaporator and the crude diphosphine disulfide [PS(o-MeC6H4)2]2
was isolated as a yellow oil (7.3 g). The identity and purity of
the [PS(o-MeC6H4)2]2 intermediate was confirmed by 31P NMR (d
77.3; CDCl3) and used without further purification.
The [PS(o-MeC6H4)2]2 compound was transferred to a 100 mL
flask equipped with a magnetic stir bar, gas inlet adapter, and
water-jacketed condenser. Anhydrous dioxane (60 mL) was added
to the flask, followed by sulfur (0.50 g, 16 mmol) and Na2S·9H2O
(3.7 g, 16 mmol). The reaction mixture was heated to reflux and
the immiscible yellow oil initially changed to dark-brown and
then slowly dissolved as the reflux continued overnight. After
cooling to room temperature, solids that precipitated from the
3
3
2H), 7.48 (t, JHH = 8 Hz, para-H, 2H), 8.13 (ddd, JPH = 18◦Hz,
3JHH = 8 Hz, 4JHH = 2 Hz, ortho-H, 2H). 31P NMR (CDCl3, 20 C):
d 47.6 (t, 3JPH = 18 Hz). IR (cm-1): 689 s, 738 m, 764 vs, 776 s, 800
s, 858 w, 874 w, 1014 s, 1021 sh, 1045 m, 1075 s, 1133 s, 1160 s,
1180 m, 1239 s, 1252 m, 1276 vs, 1292 sh, 1428 vs, 1455 s, 1475 vs,
1573 m, 1586 s, 1645 w, 2417 br, 2833 m, 2902 w, 2935 m, 2966 m,
3008 m, 3050 w, 3063 w, 3084 w.
Tetraphenylphosphonium bis(p-trifluoromethylphenyl)dithiophos-
phinate, [PPh4][S2P(p-CF3C6H4)2] (1b)
Compound 1b was prepared as described for 2b from freshly
isolated (NH4)S2P(p-CF3C6H4)2 (0.070 g, 0.17 mmol) and PPh4Cl
(0.066 g, 0. 18 mmol). Crystallization from Et2O/MeCN yielded
thin needles (0.046 g, 37%). Anal. Calcd for C38H28F6P2S2: C, 63.0;
H, 3.89; P, 8.55; F, 15.7◦. Found: C, 63.0; H, 3.92; P, 8.54; F, 15.6.
1H NMR (CD3CN, 20 C): d 7.59 (dd, 4JPH = 2 Hz, 3JHH = 8 Hz,
meta-H, 4 H), 7.64–7.77 (m, PPh4, 16H), 7.88–7.94 (m, PPh4, 4H),
3
3
1
8.26 (dd, JPH = 13 Hz, JHH = 8 Hz, ortho-H, 4H). 31P{ H, 13C}
◦
NMR (CD3CN, 20 C): d 25.0 (s, PPh4), 61.1 (s, S2PAr2). 19F{ H}
NMR (CD3CN, 20 ◦C): d -64.4 (s, CF3). IR (cm-1): 529 m, 575 m,
605 m, 615 w, 629 m, 671 s, 690 s, 700 s, 725 s, 752 m, 760 w, 778 w,
1
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834 sh, 839 m, 975 w, 997 m, 1016 m, 1058 s, 1108 s, 1124 s, 1135
sh, 1160 s, 1244 w, 1288 sh, 1320 vs, 1355 sh, 1394 m, 1438 s, 1484
m, 1575 w, 1585 w, 1604 m, 2990 sh, 3006 w, 3033 sh, 3050 m.
20 ◦C): d 25.0 (s, PPh4), 60.4 (s, S2PAr2). IR (cm-1): 465 w, 528 m,
569 s, 615 w, 641 vs, 669 w, 677 sh, 687 s, 722 s, 753 s, 764 w, 802
m, 843 w, 850 w, 976 w, 996 m, 1022 sh, 1030 w, 1063 m, 1107 s,
1129 m, 1160 m, 1190 m, 1263 m, 1279 m, 1311 m, 1336 m, 1376
m, 1436 s, 1452 m, 1467 m, 1483 m, 1566 sh, 1584 m, 1817 w, 2918
w, 2952 w, 2974 w, 3005 w, 3037 w, 3053 m.
Tetraphenylphosphonium bis(m-trifluoromethylphenyl)dithiophos-
phinate, [PPh4][S2P(m-CF3C6H4)2] (2b)
To a solution containing (NH4)S2P(m-CF3C6H4)2 (0.105 g, 0.260
mmol) in water (15 mL) was added a solution of PPh4Cl (0.0986
g, 0.260 mmol) in water (15 mL) in air. A white precipitate
immediately formed. The mixture was stirred for 30 min and then
filtered. The white solid was washed with water (3 ¥ 5 mL) followed
by EtOH (3 ¥ 3 mL) and allowed to evaporate to dryness in air.
The solid was dissolved in 2 mL MeCN and filtered into a 20 mL
vial. The vial was placed in a jar with Et2O (20 mL). Thin needles
formed slowly over a week by vapor diffusion and 2b was isolated
as single crystals (0.083 g, 44%). Anal. Calcd for C38H28F6P2S2: C,
63.0; H, 3.89; P, 8.55; F, 15.7◦. Found: C, 63.1; H, 3.98; P, 8.31; F,
15.8. 1H NMR (CD3CN, 20 C): d 7.47 (t, 3JHH = 8 Hz, meta-H,
2H), 7.57 (d, 3JHH = 8 Hz, para-H, 2H), 7.64–7.78 (m, PPh4, 16H),
Tetraphenylphosphonium bis(o-methoxyphenyl)dithiophosphinate,
[PPh4][S2P(o-MeOC6H4)2] (5b)
Compound 5b was prepared as described for 2b from freshly
isolated (NH4)S2P(o-MeOC6H4)2 (0.048 g, 0.15 mmol) and PPh4Cl
(0.057 g, 0.15 mmol). Crystallization from Et2O/MeCN yielded
colorless prisms (0.039 g, 41%). Anal. Calcd for C38H34O2P2S2: C,
70.4; H, 5.28; P, 9.55. Found: C, 70.3; H, 5.28; P, 9.66. 1H NMR
(CD3CN, 20 ◦C): d 3.29 (s, MeO, 6H), 6.73 (dd, 4JPH = 5 Hz, 3JHH
=
3
8 Hz, meta-H, 2H), 6.98 (tt, JHH = 8 Hz, meta-H, 2H), 7.24 (t,
3JHH = 8 Hz, para-H, 2H) 7.64–7.78 (m, PPh4, 16H), 7.88–7.95
3
3
4
(m, PPh4, 4H), 8.47 (ddd, JPH = 17 Hz, JHH = 8◦Hz, JHH = 2
Hz, ortho-H, 2H). 31P{ H, 13C} NMR (CD3CN, 20 C): d 25.0 (s,
1
3
3
7.88–7.95 (m, PPh4, 4H), 8.31 (dd, JPH = 12 Hz, JHH = 7 Hz,
PPh4), 53.3 (s, S2PAr2). IR (cm-1): 529 m, 560 m, 585 m, 616 w, 641
vs, 676 m, 692 s, 721 s, 753 s, 761 sh, 777 m, 799 m, 844 sh, 854 w,
931 w, 995 m, 1027 s, 1044 m, 1068 m, 1104 vs, 1127 m, 1157 m,
1171 m, 1182 m, 1238 s, 1263 s, 1294 w, 1315 m, 1338 w, 1429 s,
1470 s, 1570 s, 1581 s, 2830 m, 2902 m, 2927 m, 2957 w, 2976 sh,
2989 w, 3001 w, 3044 m, 3054 w, 3071 w.
3
1
ortho-H, 2H), 8.48 (d, JPH = 13 Hz, ortho-H, 2H). 31P{ H, 13C}
◦
NMR (CD3CN, 20 C): d 25.0 (s, PPh4), 61.1 (s, S2PAr2). 19F{ H}
1
◦
NMR (CD3CN, 20 C): d -64.3 (s, CF3). IR (cm-1): 527 m, 560
m, 615 w, 637 w, 646 s, 669 s, 689 s, 701 m, 710 w, 724 s, 754 m,
801 m, 807 w, 909 w, 916 w, 929 w, 996 m, 1028 w, 1070 s, 1094 sh,
1109 vs, 1118 sh, 1128 sh, 1160 s, 1171 m, 1189 sh, 1274 m, 1306
m, 1324 vs, 1415 m, 1440 s, 1484 m, 1586 w, 1597 w, 3003 w, 3017
w, 3047 m.
Tetraphenylphosphonium diphenyldithiophosphinate,
[PPh4][S2PPh2] (6b)
Compound 6b was prepared as described for 2b from freshly
isolated (NH4)S2PPh2 (0.100 g, 0.374 mmol) and PPh4Cl (0.130 g,
0.350 mmol). Crystallization from Et2O/MeCN yielded colorless
blocks (0.097 g, 48%). Anal. Calcd for C36H30P2S2: C, 73.4; H,
5.14; P, 10.5. Found: C, 73.4; H, 5.18; P, 10.5. 1H NMR (CD3CN,
20 ◦C): d 7.21–7.30 (m, 4H), 7.64–7.78 (m, PPh4, 16H), 7.88–7.95
Tetraphenylphosphonium bis(o-trifluoromethylphenyl)dithiophos-
phinate, [PPh4][S2P(o-CF3C6H4)2] (3b)
Compound 3b was prepared as described for 2b from freshly
isolated (NH4)S2P(o-CF3C6H4)2 (0.112 g, 0.278 mmol) and PPh4Cl
(0.107 g, 0.290 mmol). Crystallization from Et2O/MeCN yielded
colorless prisms (0.074 g, 37%). Anal. Calcd for C38H28F6P2S2: C,
63.0; H, 3.89; P, 8.55; F, 15.7. Found: C, 63.0; H, 4.08; P, 8.75; F,
1
(m, PPh4, 4H), 8.04–8.11 (m, 4H). 31P{ H, 13C} NMR (CD3CN,
20 ◦C): d 25.0 (s, PPh4), 62.6 (s, S2PPh2). IR (cm-1): 486 w, 528 s,
563 s, 614 m, 662 s, 693 vs, 699 sh, 724 s, 758 m, 854 w, 931 w, 996
m, 1027 w, 1069 sh, 1088 m, 1108 vs, 1166 w, 1185 w, 1282 w, 1304
w, 1315 w, 1339 w, 1395 sh, 1435 vs, 1469 m, 1484 m, 1570 sh, 1586
m, 2995 w, 3025 w, 3041 sh, 3051 m.
◦
1
16.0. H NMR (CD3CN, 20 C): d 7.44–7.77 (m), 7.88–7.95 (m,
PPh4), 9.20 (dd, 3JPH = 18 Hz, 3JHH = 8 Hz, ortho-H). 31P{ H, 13C}
1
◦
NMR (CD3CN, 20 C): d 25.0 (s, PPh4), 67.6 (s, S2PAr2). 19F{ H}
NMR (CD3CN, 20 ◦C): d -56.6 (s, CF3). IR (cm-1): 482 w, 527 m,
561 m, 586 w, 598 w, 616 w, 639 m, 653 s, 690 s, 702 m, 725 s, 754
m, 769 m, 855 w, 887 w, 965 w, 997 m, 1032 s, 1088 s, 1109 vs, 1124
sh, 1135 sh, 1165 sh, 1175 s, 1255 m, 1312 vs, 1335 w, 1382 w, 1436
s, 1472 sh, 1484 m, 1572 w, 1589 m, 2862 w, 2976 w, 3010 sh, 3020
w, 3052 m, 3085 sh, 3425 br.
1
Tetraphenylphosphonium dimethyldithiophosphinate,
[PPh4][S2PMe2] (7b)
To a slurry of NaS2PMe2·2H2O (0.105 g, 0.570 mmol) in MeCN
(15 mL) was added a solution of PPh4Cl (0.207 g, 0.552 mmol) in
MeCN (15 mL). The mixture was stirred overnight, filtered, and
evaporated to dryness under vacuum to yield a beige solid. The
solid was dissolved in MeCN (2 mL) and the resulting solution
was filtered into a 20 mL vial. The vial was placed in a jar with
Et2O (20 mL). Long ivory columns slowly formed over a week by
vapor diffusion. Yield: 0.127 g (49%). Anal. Calcd for C26H26P2S2:
Tetraphenylphosphonium bis(o-tolyl)dithiophosphinate,
[PPh4][S2P(o-MeC6H4)2] (4b)
Compound 4b was prepared as described for 2b from freshly
isolated (NH4)S2P(o-MeC6H4)2 (0.123 g, 0.416 mmol) and PPh4Cl
(0.156 g, 0.420 mmol). Crystallization from Et2O/MeCN yielded
colorless prisms (0.093 g, 36%). Anal. Calcd for C38H34P2S2: C,
74.0; H, 5.56; P, 10.0. Found: C, 74.0; H, 5.61; P, 10.1. 1H NMR
(CD3CN, 20 ◦C): d 2.03 (s, Me, 6H), 6.96 (dd, 4JPH = 6 Hz, 3JHH = 7
Hz, meta-H, 2H), 7.18–7.28 (m, meta,para-H, 4H), 7.64–7.78 (m,
1
C, 67.2; H, 5.64; P, 13.3. Found: C, 67.2; H, 5.63; P, 13.5. H
NMR (CD3CN, 20 ◦C): d 1.79 (d, 2JPH = 12 Hz, CH3, 6H), 7.64–
1
7.78 (m, PPh4, 16H), 7.89–7.95 (m, PPh4, 4H). 31P{ H, 13C} NMR
(CD3CN, 20 ◦C): d 25.0 (s, PPh4), 51.1 (s, S2PMe2). IR (cm-1): 502
w, 527 m, 608 s, 618 m, 690 s, 706 w, 724 s, 753 w, 759 w, 838 w,
854 w, 895 s, 926 m, 934 m, 994 m, 1017 w, 1028 w, 1070 w, 1109
PPh4, 16H), 7.88–7.95 (m, PPh4), 8.73 (ddd, 3JPH = 16 Hz, 3JHH
=
4
3
1
7 Hz, JHH = 2 Hz, ortho-H, 2H). 1P{ H, 13C} NMR (CD3CN,
2166 | Dalton Trans., 2012, 41, 2163–2175
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Scheme 2 Synthetic method for HS2PR2 and [PPh4][S2PR2].
vs, 1160 w, 1174 w, 1186 w, 1269 m, 1279 w, 1314 sh, 1322 w, 1338
w, 1413 w, 1435 s, 1481 s, 1571 sh, 1584 m, 2908 m, 2980 m, 3004
w, 3031 m, 3046 w, 3069 w, 3080 w.
program.36 Decay of reflection intensity was not observed as
monitored via analysis of redundant frames. The structure was
solved using direct methods and difference Fourier techniques.
All hydrogen atom positions were idealized and were allowed to
ride on the attached carbon atoms. For 1a, 4a and 5a the acidic
proton was located in the difference map and refined isotropically.
The final refinement included anisotropic temperature factors on
all non-hydrogen atoms. Structure solution, refinement, graphics
and creation of publication materials were performed using
SHELXTL.37 Data collection and refinement details are listed in
Table 1.
Crystallographic details
Single crystals obtained from hexanes (1a, 2a), diethyl ether (4a),
or toluene (5a) were mounted in a nylon cryoloop with Paratone-
N oil under argon gas flow. The data were collected on a Bruker
D8 diffractometer, with APEX II charge-coupled-device (CCD)
detector and cooled to 120(1) K using an American Cryoindustries
low temperature device. Single crystals of 3b–6b, obtained by
vapor diffusion of diethyl ether into acetonitrile solutions, were
treated identically. The instrument was equipped with graphite
Results and discussion
˚
monochromatized MoKa X-ray source (l = 0.71073 A) and a 0.5
Synthesis of HS2PR2 and [PPh4][S2PR2]
mm monocapillary. A hemisphere of data was collected using w
scans, with 10–30 s frame exposures and 0.5◦ frame widths. Data
collection and initial indexing and cell refinement were handled
using APEX II software.34 Frame integration, including Lorentz-
polarization corrections and final cell parameter calculations were
carried out using SAINT+ software.35 The data were corrected
for absorption using redundant reflections and the SADABS
The diaryldithiophosphinic acids required to prepare the dithio-
phosphinates shown in Fig. 1 were synthesized as presented in
Scheme 2. This transformation proceeds by oxidation of the cor-
responding secondary phosphine with elemental sulfur in refluxing
toluene followed by insertion of a second equivalent of sulfur into
the P–H bond. Using 31P NMR spectroscopy, we observed that
Table 1 Crystallographic data for H2SPR2 and [PPh4][S2PR2] complexes collected at 120(1) K
1a
2a
4a
5a
3b·Et2O0.5
4b
5b
6b
Formula
C14H9F6PS2
386.30
monoclinic
P21/c
11.111(1)
10.653(1)
13.573(2)
90
95.761(1)
90
C14H9F6PS2
386.30
triclinic
P-1
9.724(6)
13.297(8)
14.279(9)
115.729(7)
108.308(7)
90.064(7)
1558.5(17)
4
C14 H15PS2
278.35
monoclinic
P21/n
6.726(3)
15.693(6)
13.180(5)
90
102.276(4)
90
C14H15O2PS2 C40H33F6O0.5P2S2 C38H34P2S2
C38H34O2P2S2 C36H30P2S2
FW (g mol-1)
crystal system
space group
310.35
monoclinic
P21/n
7.548(4)
15.329(9)
12.623(7)
90
99.797(7)
90
1439.2(14)
4
761.72
triclinic
P-1
616.71
orthorhombic
Pbca
14.608(2)
14.870(2)
28.308(3)
90
648.71
triclinic
P-1
588.66
monoclinic
P21/n
14.493(2)
13.180(2)
17.082(2)
90
108.978(2)
90
3085.8(7)
4
˚
a (A)
13.124(3)
16.147(4)
17.906(4)
95.293(3)
107.609(3)
90.123(3)
3599.4(15)
4
9.138(2)
11.861(3)
16.465(4)
75.181(3)
74.268(3)
70.463(3)
1591.6(6)
2
˚
b (A)
˚
c (A)
a (deg)
b (deg)
90
90
g (deg)
3
˚
V (A )
1598.4(3)
4
1.605
1359.2(9)
4
1.360
6149.1(11)
8
Z
rcalc (g cm-3)
m (mm-1)
q range
1.646
1.432
0.475
1.406
0.299
1.332
1.354
0.302
1.267
0.300
0.488
0.500
0.484
0.305
2.44–25.24◦
2890/0/212
2.24–25.69◦
2.05–24.81◦
2.11–25.13◦ 1.83–25.27◦
4871/0/179 20544/0/914
2.00–25.38◦
5631/0/381
1.85–25.35◦ 1.99–25.20◦
5770/0/400 5532/0/362
data/restraints/
parameters
5785/1/421 4017/0/162
goodness-of-fit on F2 1.040
1.076
0.1015
0.3128
0.929
0.0470
0.1026
0.897
0.0602
0.1371
0.741
0.0528
0.0925
1.049
0.0399
0.0964
0.998
0.0462
0.1134
1.028
0.0397
0.0943
R1 [I > 2s(I)]a
0.0457
0.1135
wR2 (all data)b
largest diff. peak and 0.611/-0.380 1.130/-0.746 0.373/-0.340 0.327/-0.398 0.465/-0.449
0.499/-0.395
0.376/-0.471 0.292/-0.296
-3
˚
hole (e·A )
2
2
a R1 = R |Fo| - |Fc| | / | R|Fo| for reflections with Fo > 2 s(Fo2). b wR2 = [Rw(Fo - Fc2)2/R(Fo2)2]1/2 for all reflections.
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most time-efficient method to prepare (NH4)S2PR2 for subsequent
transformations.
The (NH4)S2PR2 salts were used to prepare the CF3C6H4 sub-
stituted HS2PR2 compounds HS2P(p-CF3C6H4)2, 1a and HS2P(m-
CF3C6H4)2, 2a, as previously reported for HS2P(o-CF3C6H4)2,
3a,28–29 and contributed here is their characterization by elemental
analysis, multinuclear NMR and IR spectroscopy. In addition,
although 2a has been synthesized previously,29 we analyzed single
crystals of 1a and 2a (grown from hexanes) by X-ray crystallog-
raphy for the first time (Fig. 2 and 3). While the synthesis of
the CF3-substituted ammonium salts appears nearly quantitative
by 31P NMR spectroscopy, the subsequent conversion of these
salts to the acids 1a–3a occurs in considerably lower yield (26–
41%). Ammonium salts were also used to prepare highly pure
crystalline solids of two new HS2PR2 compounds, namely HS2P(o-
MeC6H4)2, 4a and HS2P(o-MeOC6H4)2, 5a. These compounds are
similar to 1a–3a in that they can be prepared in high purity but
differ in that they are significantly less soluble in toluene, which
has an appreciable effect on their syntheses. For example, it is
more convenient to prepare 4a and 5a without increasing the
rate of sulfur insertion into the P–H bond by adding ammonium
hydroxide. Although this results in longer reaction times it is
attractive because upon sulfur insertion the dithiophosphinic acids
4a and 5a precipitate from the cooled toluene reaction mixtures as
highly pure microcrystalline solids and in greater yields (~80%).
Subsequent crystallization from diethyl ether (4a) or hot toluene
(5a) provides single crystals suitable for X-ray analysis (Fig. 4
and 5).
Fig. 1 Numbering convention used for reported complexes.
the oxidation step is relatively fast while the insertion reaction
is slow, ranging from days to weeks at elevated temperatures.
For instance, the reaction of two equivalents of elemental sulfur
with HP(p-CF3C6H4)2 resulted in formation of the PV oxidation
product within a few hours. In contrast, insertion of sulfur into
◦
the P–H bond was only ~50% complete after a week at 110 C.
A more time efficient method can be accessed by adding NH4OH
to the reaction mixture after oxidation to the phosphine sulfide
is complete, similar to previous reports for some (NH4)E2PR2
compounds, where E = S or Se.38–39 Quantitative consumption of
the HSP(p-CF3C6H4)2 reagent and concomitant precipitation of
the (NH4)S2P(p-CF3C6H4)2 salt occurred rapidly after addition
of ammonium hydroxide (Scheme 2). Our reported synthesis
in toluene is attractive in comparison to previous preparative
methods that use EtOH and H2O solvents because addition of
the NH4OH after phosphorus oxidation causes the (NH4)S2PR2
to precipitate and be separated from toluene soluble impurities.38–39
Although we have tried other syntheses, this approach provided the
In the course of exploring the reaction chemistry of the
dithiophosphinic acids, we identified an alternative synthesis of
HS2P(o-MeC6H4)2 (4a), as outlined in Scheme 3.33 Treatment of
PSCl3 with an excess of the (o-MeC6H4)MgBr Grignard reagent
Fig. 2 Hydrogen bonded dimer of HS2P(m-CF3C6H4)2, 2a. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
represented as spheres. Hydrogen atoms attached to carbon have been omitted from the figure. (S: orange, P: purple, C: black, F: green, H: cyan).
2168 | Dalton Trans., 2012, 41, 2163–2175
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Fig. 3 Molecular structure of HS2P(p-CF3C6H4)2, 1a. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
represented as spheres. The hydrogen bonded fragment is not shown.
Fig. 4 Molecular structure of HS2P(o-MeC6H4)2, 4a. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
represented as spheres. The hydrogen bonded fragment is not shown.
yielded the diphosphine disulfide [PS(o-MeC6H4)2]2, as previously
reported for certain tetraalkyldiphosphine disulfides.33,40 The P–P
bond was then cleaved with Na2S and S in subsequent reactions
to yield NaS2P(o-MeC6H4)2.32–33 Treatment of the sodium salt
with HCl provides HS2P(o-MeC6H4)2, 4a, as expected. The iso-
lated HS2P(o-MeC6H4)2 contains some residual impurities when
prepared using this method, but these can be removed in a
subsequent purification step by converting the HS2P(o-MeC6H4)2
to (NH4)S2P(o-MeC6H4)2 and then reforming the acid using HCl.
The (NH4)S2PR2 salts appear to be versatile starting materials
because they provide convenient synthetic precursors for the
dithiophosphinic acids and are susceptible to salt metathesis
reactions. The latter reaction pathway was exploited to exchange
anions are isolated. We observed that the (NH4)S2PR2 salts reacts
with PPh4Cl via cation exchange reactions in water, under air, and
at ambient temperatures (Scheme 2). This reaction seems quite
general and provided here is the synthesis and characterization of
an entire family of [PPh4][S2PR2] compounds that contain various
groups bound to the central phosphorus atom, specifically R = p-
CF3C6H4, m-CF3C6H4, o-CF3C6H4, o-MeC6H4, o-MeOC6H4, Ph
and Me; 1b–7b, respectively (Fig. 1).
NMR spectroscopy
Compounds 1a–5a (Fig. 1) were characterized in CDCl3 at room
temperature by 1H NMR spectroscopy. The S–H resonances
for HS2P(p-CF3C6H4)2, 1a, HS2P(m-CF3C6H4)2, 2a, HS2P(o-
CF3C6H4)2, 3a, were identified as broad singlets at d 2.62,
+
the NH4 cation, which is capable of H-bonding with anions, for
+
-
the large non-coordinating PPh4 group that ensures the S2PR2
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Fig. 5 Molecular structure of HS2P(o-MeOC6H4)2, 5a. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
represented as spheres. The hydrogen bonded fragment is not shown.
Scheme 3 Alternative synthetic method for HS2PR2.33
2.45 and 2.95.28 In contrast, the S–H resonances in HS2P(o-
MeC6H4)2, 4a and HS2P(o-MeOC6H4)2, 5a, were too broad to
be observed under these conditions at room temperature. Only
sharp singlets were identified in this region of the spectra at d
2.17 and 3.61 and are assigned to the methyl and methoxy groups,
respectively.
on their isotopic splitting, while for 4a the remaining aryl
hydrogen resonances fall between d 7.36–7.48 and cannot be
resolved.
The [PPh4][S2PR2] salts, 1b–7b, have poor solubility in CDCl3, so
direct NMR comparisons to the acids, 1a–6a, were not possible.
1
Instead, 1b–7b were characterized using H NMR spectroscopy
For the CF3 substitituted acids 1a–3a, the aryl portion of the
spectrum contains resonances that are well resolved and readily
interpreted based on the isotopic splitting distributions and peak
integrations. For example, peaks attributed to ortho- and meta-
protons in 1a were identified as doublets of doublets and assigned
based on their splitting with the phosphorus atom, which is
at room temperature in CD3CN. The obtained spectra reveal
only subtle differences compared to spectra of the associated
dithiophosphinic acids. For example, the S–H proton resonances
are absent and new aryl multiplets that correspond to the
tetraphenylphosphonium cations are observed at d 7.64–7.78 and
+
7.88–7.95. The sum of these PPh4 peak areas show the expected
3
4
-
1
around 14–15 Hz for JPH (ortho) and 3 Hz for JPH (meta). The
proton resonances for the aryl groups in 4a and 5a reveal more
1 : 1 ratio with S2PR2 . In addition, the H NMR spectrum of
[PPh4][S2PMe2], 7b, is unique in comparison to the aryl substituted
compounds, 1b–6b, in that the methyl resonance at d 1.79 is split
by the phosphorus atom.
1
pronounced coupling and were subsequently assigned using H–
1H COSY NMR experiments. The ortho-protons in 4a and 5a were
attributed to the doublets of doublets of doublets at d 8.39 and
8.13. This peak assignment was confirmed by the characteristic
splitting pattern that stems from coupling to the 31P (3JPH = 18 Hz),
the adjacent meta-hydrogen atoms (3JHH = 8 Hz) and four-bond
W-coupling with the para-hydrogen atoms (4JHH = 2 Hz).41 The
resonance for the meta-protons adjacent to the ortho-substituents
in 4a and 5a were found at d 7.18 and 6.87 as doublets of doublets
arising from four bond coupling with 31P (4JPH = 6–7 Hz) and
the para-hydrogen atoms (3JHH = 7–8 Hz). For 5a, resonances
associated with the remaining meta- and para-hydrogen atoms
are located between d 7.11 and 7.48 and are assigned based
The 31P NMR spectra provides the most convenient and diag-
nostic method for characterizing the HS2PR2 and [PPh4][S2PR2]
compounds and the results are summarized in Fig. 6. The
31P NMR proton coupled spectra for the CF3C6H4 substituted
HS2PR2 compounds, 1a and 2a, reveal pentets (3JPH = 14–15 Hz)
due to phosphorus coupling with the four ortho-protons on the aryl
rings. Similarly, the 31P NMR resonances for 4a and 5a appear as
triplets (3JPH = 18 Hz) at d 55.2 and 47.6, respectively. While the
4JPH-coupling of the meta-hydrogen atoms can be observed in the
1H NMR spectra of 1a–5a, this smaller coupling is obscured by
the more pronounced 3JPH-coupling in the 31P NMR spectra.
2170 | Dalton Trans., 2012, 41, 2163–2175
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the phosphorus chemical shift, since the 31P peak positions for
1b and 2b are similar to that of [PPh4][S2PPh2], 6b. Overall, this
observation is not limited to the PPh4+ salts and similar trends are
observed for the conjugate acids (Fig. 6).
Compounds 1b–3b were also characterized by 19F NMR spec-
troscopy and the results were compared with the conjugate acids,
1a–3a. In each case only a single 19F resonance was observed
and it was consistently assigned to the CF3 aryl substituents. As
opposed to the 31P NMR spectra, the 19F NMR resonances for
the S2PR2- anions, 1b–3b, are shifted slightly upfield compared to
their corresponding acids, 1a–3a. Again, the data for 3b, which
contains o-CF3C6H4 groups, is unique with the single 19F NMR
resonance shifted downfield (d -56.6) relative to 1b and 2b, which
have peaks with similar chemical shifts at d -64.4 and -64.3. The
19F NMR resonances of the conjugate acids 1a–3a follow this same
trend and combined with the 31P NMR data, these results suggest
that the o-CF3C6H4 compounds, 3a and 3b, have distinct electronic
properties.
Fig. 6 31P NMR shifts for the HS2PR2 and [PPh4][S2PR2] complexes
collected in CDCl3 and CD3CN, respectively. The 31P NMR shift for
HS2PPh2 and HS2P(o-CF3C6H4)2 in CDCl3 were previously reported.28,46
The proton-decoupled 31P NMR spectra for [PPh4][S2PR2], 1b–
7b (Fig. 1), all contain a single resonance at d 25.0 that corresponds
to the phosphorus nuclei in the PPh4+ cation. In addition, there is
a single peak attributable to dithiophosphinate, which appears at
lower field relative to the corresponding acids (Fig. 6). However,
given that the acids were analyzed in CDCl3 and the bases in
CD3CN, it is unclear to what extent these differences in chemical
shifts can be attributed to chemical vs. solvent effects. Overall, the
phosphorus resonances for compounds with o-CF3C6H4 groups,
3a and 3b, are the most positive (i.e. shifted to low field), which
is consistent with the electron-withdrawing properties of the o-
CF3 substituent. For example, it is well-accepted in aromatic
substitution chemistry and corroborated by density functional
calculations that CF3 substituents on phenyl rings inductively
produce a partial positive charge at adjacent ring positions
(Scheme 4).42 It seems likely that for 3b, the o-CF3 ring substituent
decreases electron density at the ipso-carbon (carbon attached to
P), causing the 31P resonance to shift to lower field. Consistent
with this interpretation, the electron donating aryl substituents in
the ortho-ring positions (Me and MeO) have the opposite effect
and systematically push the phosphorus resonance to higher field.
Structural studies of HS2PR2
The formation of the acids 1a, 2a, 4a and 5a was confirmed by
single-crystal X-ray diffraction; Fig. 2–5 and Tables 1 and 2. The
structures are similar to those observed for other dithiophosphinic
acids in that the coordination environment around the phosphorus
atom is pseudo-tetrahedral and the P–SH bond is considerably
longer than the adjacent phosphorus-sulfide double bond (Table
2).43 In addition, in each structure the S–H protons hydrogen bond
with the P S functional group in an adjacent molecule in the unit
cell (Fig. 2).
Selected bond distances and angles are provided in Table 2,
along with those previously reported for HS2P(o-CF3C6H4)2, 3a,28
and HS2PPh2, 6a.43 Although several trends in bond length are
observed that suggest the ortho-substituted dithiophosphinic acids
3a–5a are distinct compared to the other acids, the magnitude
˚
of these differences are small (~0.01 A) and their significance
is difficult to evaluate. More profound changes in the metrical
parameters are associated with the S–P–S and RH4C6–P–C6H4R
angles, which for 3a–5a are larger compared to those in 1a, 2a
and 6a. These marked differences for the ortho-substituted di-
aryldithiophosphinic acids likely reflect steric repulsions between
the ortho substituents and the sulfur atoms, which are clearly
evident in the structures of the anions (vide infra).
Hence, the 31P chemical shift for S2P(o-MeOC6H4)2 , which has
-
the most electron-donating aryl group, is almost identical to that
-
of S2PMe2 , 7b, which contains electron donating methyl groups
directly attached to phosphorus. This NMR data also suggests
that the meta- and para-CF3 aryl substituents have little effect on
Structural studies of [PPh4][S2PR2]
Structures of the phosphonium salts [PPh4][S2P(o-CF3C6H4)2],
3b, [PPh4][S2P(o-MeC6H4)2], 4b, [PPh4][S2P(o-MeOC6H4)2], 5b
and [PPh4][S2PPh2], 6b, were obtained by single-crystal X-ray
diffraction; Fig. 7–10 and Tables 1 and 3. These structures confirm
deprotonation of the acid, as reflected by the nearly identical S–P
bond distances and the presence of the tetraphenylphosphonium
cation (Table 3). Most importantly, the structural data reveals that
the sulfur atoms do not engage in any intermolecular interactions.
X-ray quality crystals of the para- and meta-CF3 derivatives 1b
and 2b have yet to be obtained.
Although all of the bond distances determined for compounds
3b–6b are quite similar (Table 3), the X-ray analysis shows a high
degree of structural variation among these molecules. As observed
Scheme 4 Inductive effects of ortho CF3, Me, and MeO substituents in
3a–5a.
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Table 2 Selected bond distances and angles for HS2PR2 complexes
˚
˚
˚
˚
HS2PAr2
H–S (A)
HS–P (A)
S–P (A)
P–C (A)
S–P–S (deg)
116.23(5)
117.3(2)
C–P–C (deg)
HS2P(p-CF3C6H4)2 (1a)
HS2P(m-CF3C6H4)2 (2a)
1.14(4)
1.17(5)
1.18(3)
1.40(4)
1.26(3)
1.25(2)
2.073(1)
2.074(3)
2.0885(8)
2.081(1)
2.087(2)
2.077(1)
1.949(1)
1.936(3)
1.9566(8)
1.951(1)
1.942(2)
1.954(1)
1.811(3)
1.815(3)
1.797(9)
1.802(9)
1.840(2)
1.849(2)
1.813(3)
1.815(3)
1.800(4)
1.812(4)
1.809(1)
1.809(1)
105.9(1)
105.6(4)
113.5(1)
107.3(1)
109.6(2)
104.21(6)
a
HS2P(o-CF3C6H4)2 (3a)
113.05(4)
113.16(6)
113.06(8)
108.16(2)
HS2P(o-MeC6H4)2 (4a)
HS2P(o-MeOC6H4)2 (5a)
b
HS2PPh2 (6a)
a Obtained from ref 46. b Obtained from ref 43.
Table 3 Selected bond distances and angles for [PPh4][S2PR2] complexes
a
a
˚
˚
PPh4S2PAr2
S–P (A)
P–C (A)
S–P–S (deg)
116.46(8)
C–P–C (deg)
111.0(2)
C–P–S (deg)
q (Scheme 5)
(PPh4)S2P(o-CF3C6H4)2 (3b)
1.977(2)
1.990(2)
[1.983(2)]
1.856(4)
1.864(4)
104.2(2)
110.4(2)
104.2(2)
110.8(2)
105.78(8)
108.95(8)
107.41(8)
110.10(8)
108.4(1)
109.4(1)
109.1(1)
109.4(1)
108.34(9)
108.36(9)
108.64(8)
108.89(9)
42.9◦
62.1◦
[52.5◦]
[1.860(6)]
(PPh4)S2P(o-MeC6H4)2 (4b)
(PPh4)S2P(o-MeOC6H4)2 (5b)
(PPh4)S2PPh2 (6b)
1.9895(9)
1.9948(9)
[1.992(1)]
1.835(2)
1.841(2)
[1.838(3)]
117.25(4)
115.65(5)
118.13(4)
106.9(1)
104.4(1)
103.5(1)
40.3◦
64.1◦
[52.2◦]
1.981(1)
1.998(1)
[1.989(2)]
1.833(3)
1.838(3)
[1.836(4)]
39.8◦
52.5◦
[46.2◦]
1.9764(9)
1.9770(9)
[1.976(1)]
1.831(2)
1.832(2)
[1.832(3)]
13.6◦
30.0◦
[21.8◦]
a Mean bond distances and angles are contained in brackets [ ]. Parenthetic values in the brackets represent the propagated error of the standard deviation.
for the HS2PR2 structures, 1a–5a, this variation is most easily
identified through bond angle comparisons, which suggests that
ortho-substituted aryl groups impart large differences in structure.
For example, the C–P–C bond angles are smallest for S2PPh2- and
are nearly perpendicular to the C–P–C plane, with an average
q angle of 21.8◦. This value is significantly smaller than the
average q angles observed for 3b–5b (q = 52.5◦, 52.2◦ and 46.2◦,
respectively). The different orientation of the phenyl groups in
the solid state structure of 6b compared to 3b–5b matches the
differences observed in the structures reported from gas-phase and
solution DFT calculations. The similarities between the calculated
and experimental structures combined with the absence of any
obvious solid-state interactions contributing to the structure of
6b suggests that the structural differences cannot be explained
by invoking crystal packing effects. Instead, it appears that the
increased rotation of the aryl groups in 3b–5b compared to 6b
can be attributed to intramolecular repulsions between the ortho-
substituents and the sulfur atoms, as previously described.42,44 For
example, steric repulsions are clearly reflected in the different
C–P–S angles for 3b–5b, whereas those in 6b are all close to
109◦ and reflect a near ideal tetrahedral arrangement around the
phosphorus atom. In contrast, the geometries for 3b–5b are best
described as a slightly squashed tetrahedra and comparison of the
angles between the S–P–S and C–P–C planes vary from 90◦ in 6b
to 85.6(1)◦ in 3b (Scheme 6). Overall, this analysis suggests that
-
largest for S2P(o-CF3C6H4)2 (Table 3). In addition, the phenyl
ring orientations for the ortho-substituted 3b–5b are quite different
-
than that of S2PPh2 since they are rotated around the P–C6H4R
bond. To evaluate the extent of this difference, we have defined
an angle q that is equal to zero when the plane defined by the
arene ring is perpendicular to the plane defined by the three
-
C–P–C atoms (Scheme 5). For S2PPh2 , 6b, the phenyl rings
Scheme 5 The q angle used to evaluate P–C6H4R bond rotation from a
perspective along the P–C bond.
2172 | Dalton Trans., 2012, 41, 2163–2175
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Fig. 7 Molecular structure of [PPh4][S2P(o-CF3C6H4)2], 3b. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
+
represented as spheres. The PPh4 cation has been omitted.
Fig. 8 Molecular structure of [PPh4][S2P(o-MeC6H4)2], 4b. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
+
represented as spheres. The PPh4 cation has been omitted.
Fig. 9 Molecular structure of [PPh4][S2P(o-MeOC6H4)2], 5b. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are
+
represented as spheres. The PPh4 cation has been omitted.
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Fig. 10 Molecular structure of [PPh4][S2PPh2], 6b. Ellipsoids are drawn at the 35% probability level, except for the hydrogen atoms, which are represented
+
as spheres. The PPh4 cation has been omitted.
The multi-nuclear NMR spectroscopic analysis also suggests
that the HS2P(o-CF3C6H4)2, 3a and [PPh4][S2P(o-CF3C6H4)2],
3b, compounds are unique. For example, 3a and 3b stand out
in comparison to the other compounds analyzed because their
31P and 19F NMR chemical resonances are shifted the furthest
downfield and vary dramatically compared to the other CF3
substituted diaryldithiophosphinates. While it is not clear if
this observation has any connection to binding selectivity, this
data suggests that the electronic structures of 3a and 3b, which
exhibit extraordinary separation factors (Scheme 1), are different
Scheme 6 Angular comparison of the S–P–S and R–P–R planes in 3b
and 6b.
compared to the other HS2PR2 and S2PR2- compounds, which do
not effectively extract actinides from lanthanides.29
The synthesis of the dithiophosphinates 1b–7b was carefully
deviation from idealized tetrahedral geometry around P increases
with increasing steric bulk of the arene ortho-substitutent, such
that Ph < o-Me < o-MeO < o-CF3.
designed to provide compounds containing non-coordinating
+
counter cations, namely PPh4 . The structural studies show that
in contrast to the conjugate acids, which have short P S and
long P–SH bonds, each of the [PPh4][S2PR2] compounds have
-
equivalent P–S bond lengths. In addition, for 1b–7b, the S2PR2
anion is isolated and the sulfur atoms are not bridging. This
makes these compounds especially attractive for spectroscopic
and theoretical studies because the P–S interaction can be probed
in the absence of intermolecular interactions. Moreover, the
phosphonium compounds can be prepared in large quantities and
isolated as highly pure single crystals.
Concluding remarks
To better understand how the ancillary substituents in the
HS2PR2 compounds affect actinide/lanthanide separations, we
have conducted the required synthesis, NMR spectroscopy and
structural analysis necessary to analyze and interpret future S K-
edge XAS measurements and TD-DFT calculations for a series
Overall, these NMR and structural results, taken together
with previous theoretical and experimental studies,42,44–45 suggest
-
of HS2PR2 and S2PR2 compounds. While all of the protonated
HS2PR2 molecules are structurally similar, it is clear based on
-
that the HS2P(o-CF3C6H4)2 and S2P(o-CF3C6H4)2 compounds
the crystallographic data that the ortho-substituted conjugate
have distinct spectroscopic and structural properties. These
observations are particularly noteworthy given the exceptional
selectivity of HS2P(o-CF3C6H4)2 for americium in the presence
of europium.28 We are currently conducting S K-edge X-ray
absorption spectroscopy experiments and time dependent-density
functional theory calculations to better understand the effects of
electronic structure on the binding selectivity of minor actinides
over lanthanides in separations science.
-
-
bases S2P(o-CF3C6H4)2 , 3b, S2P(o-MeC6H4)2 , 4b and S2P(o-
-
MeOC6H4)2 , 5b, are unique. Specifically, there is a dramatic
structural change associated with the orientation of the aryl groups
for 3b–5b in comparison to S2PPh2 , 6b. The magnitude of the
-
structural variations correlates with the steric bulk of the ortho-
substitutent and likely stems from steric repulsion with the sulfur
atoms.44 This geometric variation is most pronounced for S2P(o-
-
CF3C6H4)2 , 3b (which has the largest substituent in CF3), is
-
intermediate for S2P(o-MeC6H4)2 , and is least obvious for S2P(o-
Acknowledgements
-
MeOC6H4)2 ; the methoxy substituent has additional degrees of
rotational freedom that allow the methyl group to rotate away and
minimize steric interactions (Fig. 5 and 9).
This work was supported by the US Department of Energy,
Office of Basic Energy Sciences, Division of Chemical Sciences,
2174 | Dalton Trans., 2012, 41, 2163–2175
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Geosciences, and Biosciences (Heavy Element Chemistry Pro-
gram), the US Department of Energy, Office of Nuclear Energy
(Fuel Cycle R&D Program), the US Department of Energy, Office
of Science (SULI Science Undergraduate Internship Program)
(MacInnis) and the Glenn T. Seaborg Institute Postdoctoral
Fellowship (Daly). Los Alamos National Laboratory is operated
by Los Alamos National Security, LLC, for the National Nuclear
Security Administration of US Department of Energy under
Contract DEAC52-06NA25396.
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©