Bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester

Article abstract of DOI:10.1016/j.ica.2011.01.023

When bis(o-trifluoromethylphenyl)dithiophosphinic acid (1) is dissolved in methanol, crystals of bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester (2) are formed. The structure of dithiophosphinic methyl ester (2) has been characterized via single-crystal X-ray diffraction and multinuclear NMR studies. Compound 2 is remarkable in that the dithiophosphinic (PS₂) core is preserved during this transformation. The transformation to 2 suggests that the o-trifluoromethylphenyl groups on phosphorus assist in retaining the PS ₂ core, possibly by steric hindrance.

Full text of DOI:10.1016/j.ica.2011.01.023

Inorganica Chimica Acta 370 (2011) 519–522
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Bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester
John R. Klaehn a, , Dean R. Peterman a, Mason K. Harrup a, Thomas A. Luther a, Lee M. Daniels b
⇑
a Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID 83415-2208, USA
b Rigaku Americas Corporation, 9009 New Trails Dr., The Woodlands, TX 77381, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
When bis(o-trifluoromethylphenyl)dithiophosphinic acid (1) is dissolved in methanol, crystals of bis(o-
trifluoromethylphenyl)dithiophosphinic methyl ester (2) are formed. The structure of dithiophosphinic
methyl ester (2) has been characterized via single-crystal X-ray diffraction and multinuclear NMR stud-
ies. Compound 2 is remarkable in that the dithiophosphinic (PS2) core is preserved during this transfor-
mation. The transformation to 2 suggests that the o-trifluoromethylphenyl groups on phosphorus assist
in retaining the PS2 core, possibly by steric hindrance.
Received 13 September 2010
Received in revised form 22 December 2010
Accepted 9 January 2011
Available online 21 January 2011
Keywords:
Synthesis
Published by Elsevier B.V.
Characterization
Dithiophosphinic acid
Dithiophosphinic ester
X-ray crystal determination
1. Introduction
we describe the addition of a methyl to the structure of 1 after it
has been recrystallized from methanol.
Dithiophosphinic acid compounds are well-known for their
ability to selectively bind/chelate certain metal ions from aqueous
solutions. The most cited dithiophosphinic acid is Cyanex 301Ò for
the extraction and separation of many different metals [1–13].
Cyanex 301Ò is primarily used as an industrial metal extractant
for the first row transitional elements [1–6]; however, it has been
tested for the separation of actinides from lanthanides [7–13].
There are other dithiophosphinic acid derivatives that show com-
plexation behavior with metals [14–22], where some exhibit selec-
tive extraction of actinides(III) over lanthanides(III) [18–22].
Recently, a report has shown that an aromatic dithiophosphinic
2. Results and discussion
From the synthetic method, compound 1 is isolated as the
ammonium salt for long term storage; however, the ammonium
salt can be easily converted into 1 by hydrochloric acid. Compound
1 has been recrystallized from tetrahydrofuran and hexanes previ-
ously without any observable changes in structure. The best sol-
vent for recrystallization of 1 is from hexanes, and the isolated
free acid gives light green crystals with little odor. Typically, tetra-
hydrofuran gives an opaque crystalline solid, not suitable for X-ray
diffraction. However, prismatic crystals are formed from methanol
when the solvent is slowly evaporated at room temperature. A few
of these crystals were used for NMR spectroscopy, and a downfield
shift was observed, relative to 1, in the 31P NMR spectra as a singlet
at 69.3 ppm. The acidic proton resonance for 1 was not seen in the
1H NMR spectrum; however, a resonance corresponding to three
protons was observed as a singlet at 2.15 ppm. In addition, the
13C NMR spectrum exhibited an additional singlet at 14.2 ppm.
The remaining aromatic peaks did not show any significant shifts
in the 1H and 13C NMR spectra. Overall, the 1H and 13C NMR spectra
indicated that 1 changed from the original free acid to a substi-
tuted methyl group. The unexpected reactivity between the dithio-
phosphinic acid and methanol produces a new dithiophosphinic
ester, 2 (Fig. 2).
acid,
bis(o-trifluoromethylphenyl)dithiophosphinic
acid,
1
(Fig. 1), is capable of extracting actinides(III) over lanthanides(III)
with high selectivity [22]. Understanding how 1 maintains the
phosphorus–sulfur core provide clues to producing better dithio-
phosphinic acid extractants in the future.
Aromatic dithiophosphinic acid derivatives can be prepared by
several pathways: Higgins synthetic route [23], modified Kosolap-
off reactions [24–31], or phosphorus-halide nucleophilic additions
[22]. Compound 1 was synthesized by nucleophilic addition at
phosphorus [22], and this route provides the capability to maintain
substituent (ligand) regiochemistry. Compound 1 can be isolated in
good yields and can be recrystallized in hot hexanes. From these
crystals, the X-ray structure of 1 was determined. In this paper,
⇑
The single crystal X-ray determination provided the structure of
the new product, bis(o-trifluoromethylphenyl)dithiophosphinic
Corresponding author. Fax: +1 208 526 8541.
E-mail address: john.klaehn@inl.gov (J.R. Klaehn).
0020-1693/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.ica.2011.01.023

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J.R. Klaehn et al. / Inorganica Chimica Acta 370 (2011) 519–522
CF3
S
P
SH
CF3
Fig. 1. Bis(o-trifluoromethylphenyl)dithiophosphinic acid, (1).
methyl ester (2; Fig. 3). The structure of 2 was remarkable, because
the dithiophosphinic core (PS2) remained unchanged. Another
interesting feature of the structure, previously noted with 1, is
the relative orientation of the o-trifluoromethyl groups. Two fluo-
rines, each from a different o-trifluoromethyl group, are in close
proximity to the phosphorus at a distance of 2.923(3) Å (P1–F3)
and 3.220(3) Å (P1–F5). These distances are close enough to sug-
gest electronic interactions between the fluorines and the phos-
phorus center. However, the 31P NMR of 2 (singlet) implies that
this interaction is not observable in solution at room temperature.
Crystal data and refinement parameters are given in Table 1, and
the relevant bond angles and bond lengths are given in Tables 2
and 3. The phosphorus–sulfur bond distances in 2 are found to
be 2.0929(8) Å (P1–S1) and 1.9483(8) Å (P1–S2), which is remark-
ably close to 1 [P1–S1 = 1.9566(8) Å and P1–S2 = 2.0885(8) Å].
However, the ammonium salt of 1 showed different phosphorus–
Fig. 3. View of the structure of bis(o-trifluoromethylphenyl)dithiophosphinic
methyl ester, (2). Ellipsoids are drawn at the 50% probability level.
Table 1
Crystal data and structure refinement for bis(o-trifluoromethylphenyl)dithiophos-
phinic methyl ester, (2).
sulfur
bond
distances
[P1–S1 = 2.0018(5) Å
and
P1–
S2 = 1.9942(5) Å], because the ionic charge is delocalized nearly
equally between the sulfurs. This suggests that 2’s methyl group
on sulfur does not influence the phosphorus–sulfur bonds, when
compared to 1. The bond angles around the phosphorus are dis-
torted [114.75(10)° for C–P–C and 113.58(4)° S@P–S] from the typ-
ical tetrahedral arrangement, and these angles differ with those in
1 [113.53(10)° for C–P–C and 113.05(4)° S@P–S] and in the ammo-
nium salt of 1 [113.29(7)° for C–P–C and 113.20(2)° S@P–S]. The C–
P–C bond angles show the greatest differences among the three
compounds, and 2 shows the largest distortion around phosphorus
(over 1°). This suggests that 2’s methyl group on sulfur and the o-
trifluoromethyl groups might contribute to the observed distortion
through steric encumbrance, but electronic effects or crystal pack-
ing forces are possible, also.
Dithiophosphinic esters are not common in literature; however,
the formation of the dithiophosphinic ester [R2PS2R0 (R,R0 = alkyl,
aromatic)] is obvious when the reactions involve reactive groups
[32–35], like phosphorus–chloride with mercaptans or silicon–chlo-
ride with dithiophosphinic acid. These reactive compounds easily
form dithiophosphinic ester products. However, the formation of
the dithiophosphinic ester with organic alcohols has not been previ-
ously observed. Dithiophosphinic acids are not expected to react
with organic alcohols, since many of the dithiophosphinic acids
are recrystallized from these solvents. In this case, the reaction with
methanol was unexpected, and isolating crystals of 2 was even more
remarkable. The proposedtransformationsuggests that 1 is intimate
with methanol (Scheme 1), and it will protonate methanol at the hy-
droxyl position. One of the sulfurs on 1 then displaces water on the
protonated alcohol and bonds with the methyl group giving 2. The
described mechanism in Scheme 1 suggests a stepwise reaction
(SN1 type); however, a concerted (SN2 type) mechanism cannot be
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C15H11F6PS2
400.33
100(2)
0.71073
monoclinic
P21/n
14.1767(9)
8.8669(8)
15.1837(5)
90.00
119.9540(10)
90.00
1653.70(19)
4
1.608
0.475
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Dcalc (mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
)
808
Crystal size (mm3)
0.49 ꢁ 0.34 ꢁ 0.21
h Range for data collection (°)
Index ranges
3.10–27.48
ꢀ18 6 h 6 14
ꢀ11 6 k 6 11
ꢀ14 6 l 6 19
11322
3719 [Rint = 0.0454]
98.0%
0.9069 and 0.8007
Full-matrix least-squares on F2
3719/0/219
1.033
R1 = 0.0427, wR2 = 0.0964
R1 = 0.0603, wR2 = 0.1050
0.374 and ꢀ0.385
Reflections collected
Independent reflections
Completeness to h = 27.48°
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2
r
(I)]
R indices (all data)
Largest differences in peak and hole (e Åꢀ3
)
excluded. Due to the aromatic o-trifluoromethyl groups, the authors
postulate that steric encumbrance is one of the principal factors that
preserve the PS2 core of 2. Observance of this transformation sug-
gests that 1 is a strong acid, and dithiophosphinic acids that have
strong acid behavior towards organic alcohols have not been thor-
oughly studied. However, it is apparent that mixing organic alcohols
(like methanol) with 1 should be explored further, and it is feasible
that these dithiophosphinic acids can form other new dithiophos-
phinic ester derivatives.
CF3
S
P
S
CH3
CF3
Fig. 2. Bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester, (2).

J.R. Klaehn et al. / Inorganica Chimica Acta 370 (2011) 519–522
521
Table 2
Bond lengths (Å) for bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester, (2).
4. Experimental
S(1)–C(1)
S(1)–P(1)
S(2)–P(1)
P(1)–C(2)
P(1)–C(9)
F(1)–C(8)
F(2)–C(8)
F(3)–C(8)
F(4)–C(15)
F(5)–C(15)
F(6)–C(15)
C(2)–C(3)
C(2)–C(7)
1.816(3)
2.0929(8)
1.9483(8)
1.839(2)
1.850(2)
1.343(3)
1.344(3)
1.340(3)
1.348(3)
1.340(3)
1.341(3)
1.392(3)
1.415(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
1.391(3)
1.379(3)
1.387(3)
1.388(3)
1.496(3)
1.395(3)
1.410(3)
1.390(3)
1.379(3)
1.381(3)
1.392(3)
1.500(3)
4.1. Materials and general procedures
Compound 1 was generated by following a literature prepara-
tion [22]. The following solvents were obtained from commercial
sources and used without further purification: methanol (ACS
grade), hexanes and tetrahydrofuran was used directly from the
manufacturer (Aldrich). Proton, 31P{1H} and 13C{1H} NMR spectra
were recorded on a Bruker DMX 300WB spectrometer operating
at 7.04 T: 300 MHz (1H), 121 MHz (31P) and 75 MHz (13C).
C(7)–C(8)
C(9)–C(10)
C(9)–C(14)
C(10)–C(11)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(14)–C(15)
4.2. Crystal structure determination and refinement
X-ray diffraction data were collected using a Rigaku SCXmini
from Rigaku Americas Corporation. Intensity data were collected
in the range 3° < h < 27.5°; Lorentz and polarization corrections
were applied. The data were corrected for absorption by face-in-
dexed numerical integration using measured crystal dimensions.
The structure was solved by direct methods and refined by full-
matrix least-squares. All carbon, fluorine, sulfur, and phosphorus
atoms were refined with anisotropic thermal parameters. The
hydrogen atoms were located on a difference map, but calculated
hydrogen positions were used for structure refinement with isotro-
pic thermal parameters fixed at 1.2 times the carbon Uiso values.
The structures were solved by direct methods [36] and refined
via full-matrix least-squares routines [37].
Table 3
Bond angles (°) for bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester, (2).
C(1)–S(1)–P(1)
C(2)–P(1)–C(9)
C(2)–P(1)–S(2)
C(9)–P(1)–S(2)
C(2)–P(1)–S(1)
C(9)–P(1)–S(1)
S(2)–P(1)–S(1)
C(3)–C(2)–C(7)
C(3)–C(2)–P(1)
C(7)–C(2)–P(1)
C(4)–C(3)–C(2)
C(5)–C(4)–C(3)
C(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(6)–C(7)–C(2)
C(6)–C(7)–C(8)
C(2)–C(7)–C(8)
F(3)–C(8)–F(1)
F(3)–C(8)–F(2)
F(1)–C(8)–F(2)
102.35(9)
114.75(10)
109.10(7)
113.37(7)
108.02(7)
97.60(7)
113.57(4)
117.7(2)
116.79(16)
125.00(16)
121.7(2)
120.1(2)
119.2(2)
121.3(2)
119.9(2)
115.5(2)
124.7(2)
106.7(2)
106.0(2)
105.9(2)
F(3)–C(8)–C(7)
F(1)–C(8)–C(7)
F(2)–C(8)–C(7)
114.33(19)
112.4(2)
110.9(2)
C(10)–C(9)–C(14)
C(10)–C(9)–P(1)
C(14)–C(9)–P(1)
C(11)–C(10)–C(9)
C(12)–C(11)–C(10)
C(11)–C(12)–C(13)
C(12)–C(13)–C(14)
C(13)–C(14)–C(9)
C(13)–C(14)–C(15)
C(9)–C(14)–C(15)
F(5)–C(15)–F(6)
F(5)–C(15)–F(4)
F(6)–C(15)–F(4)
F(5)–C(15)–C(14)
F(6)–C(15)–C(14)
F(4)–C(15)–C(14)
117.9(2)
113.15(16)
128.86(17)
121.8(2)
119.7(2)
119.7(2)
121.4(2)
119.6(2)
117.2(2)
123.1(2)
106.61(19)
106.45(19)
105.75(19)
113.10(19)
112.4(2)
4.3. Isolation of bis(o-trifluoromethylphenyl)dithiophosphinic methyl
ester (2)
The isolated dithiophosphinic acid (1) was transferred to a vial
and immediately dissolved into methanol (250 mg/10 mL). Com-
pound 2 was crystallized from methanol by slow evaporation at
room temperature, which took about two weeks to form. The
transparent, hexagonal crystals were used for single crystal X-ray
crystallography and multinuclear NMR spectroscopy.
111.98(19)
3. Conclusions
When 1 was recrystallized in methanol, the solvent reacted
with 1 to form the dithiophosphinic methyl ester, 2. This reaction,
in which the methyl group is added to the dithiophosphinic struc-
ture and water is eliminated, has no observed precedent in recent
literature. The results suggest that the structure of 1 is stable dur-
ing this transformation, possibly due to the sterically encumbering
o-trifluoromethyl groups protecting the dithiophosphinic core
(PS2) core. Also, 1 is apparently a strong enough acid to displace
water, thus resulting in 2. The multinuclear NMR spectra are in
agreement with the structure of 2, and the X-ray proves that this
is 2’s structure. Interestingly, the X-ray structure shows that 2 re-
tains the relative orientation of the o-trifluoromethyl groups be-
hind phosphorus as seen in 1, and there is additional distortion
to the tetrahedral arrangement at phosphorus in 2 when compared
to 1. Overall, 2 is remarkable in that the dithiophosphinic (PS2) core
is retained during this transformation.
4.4. [(o-CF3C6H4)2PS2CH3] (2)
[M.p. = 80–90 °C, 31P NMR d (toluene-d8) = (s) 69.3; 1H NMR d
(toluene-d8) = (dd, JPH = 18.0 Hz, JHH = 9.0 Hz, 2H) 8.73, (t,
JPH = 18.0 Hz, JFH = 9.0 Hz, 2H) 8.73, (m, 3H) 0.91–1.10; 13C NMR d
(toluene-d8) = (multiplet, ipso) 137.1, (s, para) 134.9, (doublet,
JPC = 12.0 Hz, ortho) 133.8, (multiplet, meta) 131.2, (s, meta)
131.0, (doublet of quartets, JPC = 6.0 Hz, JCF = 38.4 Hz, ortho)
129.9, (quartet, JCF = 273.0 Hz, CF3) 123.8, (s, PS2CH3) 14.2].
Acknowledgements
This work was supported by the United States Department of
Energy and the Laboratory Directed Research and Development
(LDRD) program at the Idaho National Laboratory (INL) through
contract DE-AC07-05ID14517.
H
H
H
H
CF3
C
H
H
S
P
CH3
CF3
CF3
CF3
S-
P
C
H
H
S
P
O
S
P
H
+O
CH3OH
-H2O
SH
S
S
S
H
CF3
CF3
CF3
CF3
Scheme 1. Possible transformation to bis(o-trifluoromethylphenyl)dithiophosphinic methyl ester, (2).

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