Bis(trifluoromethyl)phosphinous acid (CF3)2P-O-H: An example of a thermally stable phosphinous acid - Synthesis, gas-phase structure, and rotational isomers

Article abstract of DOI:10.1002/chem.200501066

The bis(trifluoromethyl)-phosphinous acid, (CF₃):P-O-H, is the only known example of a thermally stable phosphinous acid. Although this compound has been known since 1960, little is known about the chemistry of this extraordinary compound: this might be due to the tedious, and in some part risky, synthesis that was originally published. An improved, simple, and safe synthesis that is based on the treatment of the easily accessible (CF ₃)₂PNEt₂ with at least three equivalents of p-toluene sulfonic acid, is presented. The reaction results in a complete conversion to the phosphinous acid, which is isolated in almost 90% yield. The compound exists in an equilibrium of two P-OH rotational isomers, a fact which is supported by quantum chemical calculations. The relative enthalpy difference of 6.4 kJ mol^-1, calculated at the B3PW91/ 6-311G(3d,p) level, is in excellent agreement with the experimental value of 5.9 kJ mol^-1, which was determined from the temperature dependence of the v(OH) bands of the two rotational isomers. The complete experimental vibrational spectra of both rotamers, their predicted vibrational spectra obtained by using quantum chemical calculations, and an attempt at photoinduced isomerization of matrix-isolated (CF₃)₂POH is presented. The experimental structure, obtained from an electron-diffraction study in the gas phase, is reproduced very well by ab initio and density functional theory (DFT) methods.

Full text of DOI:10.1002/chem.200501066

FULL PAPER
DOI: 10.1002/chem.200501066
Bis(trifluoromethyl)phosphinous Acid (CF3)2P-O-H: An Example of a
Thermally Stable Phosphinous Acid—Synthesis, Gas-Phase Structure, and
Rotational Isomers
Berthold Hoge,*[a] Placido Garcia,[b] Helge Willner,[b] and Heinz Oberhammer[c]
Dedicated to Professor Reint Eujen on the occasion of his 60th birthday
Abstract: The bis(trifluoromethyl)-
acid, which is isolated in almost 90%
yield. The compound exists in an equi-
from the temperature dependence of
the n˜(OH) bands of the two rotational
isomers. The complete experimental vi-
brational spectra of both rotamers,
their predicted vibrational spectra ob-
tained by using quantum chemical cal-
culations, and an attempt at photoin-
duced isomerization of matrix-isolated
(CF3)2POH is presented. The experi-
mental structure, obtained from an
electron-diffraction study in the gas
phase, is reproduced very well by ab
initio and density functional theory
(DFT) methods.
À À
phosphinous acid, (CF3)2P O H, is the
À
only known example of a thermally
stable phosphinous acid. Although this
compound has been known since 1960,
little is known about the chemistry of
this extraordinary compound; this
might be due to the tedious, and in
some part risky, synthesis that was orig-
inally published. An improved, simple,
and safe synthesis that is based on the
treatment of the easily accessible
(CF3)2PNEt2, with at least three equiv-
alents of p-toluene sulfonic acid, is pre-
sented. The reaction results in a com-
plete conversion to the phosphinous
librium of two P OH rotational iso-
mers, a fact which is supported by
quantum chemical calculations. The
relative
6.4 kJmolÀ1, calculated at the B3PW91/
6-311G(3d,p) level, is in excellent
enthalpy
difference
of
AHCTREUNG
agreement with the experimental value
of 5.9 kJmolÀ1, which was determined
Keywords:
calculations · isomers · phos
acid phosphorus
synthetic methods
density
functional
AHCTREUNG
·
·
Introduction
lent phosphorus atom is almost completely shifted to the
side of the phosphane oxide.[1]
Secondary phosphane oxides (SPOs) exhibit phosphane
As early as 1968, Chat and Heaton described the shift of
the equilibrium to the side of the phosphinous acid form on
coordination of the acid to transition-metal complexes
[Eq. (1)].[2]
À À
oxide (R2P(O)H)$phosphinous acid (R2P O H) tautomer-
ism (R = alkyl, aryl). The equilibrium between the pentava-
lent phosphane oxide and the phosphinous acid with a triva-
[a] Priv.-Doz. Dr. B. Hoge
Institut für Anorganische Chemie
Universität zu Kçln
Greinstraße 6, 50939 Kçln (Germany)
Fax : (+49)221-470-5196
E-mail: b.hoge@uni-koeln.de
[b] Dr. P. Garcia, Prof. Dr. H. Willner
Fachbereich C, Anorganische Chemie
Bergische Universität Wuppertal
Gaußstraße 20, 42097 Wuppertal (Germany)
[c] Prof. Dr. H. Oberhammer
Institut für Physikalische und Theoretische Chemie
Universität Tübingen
Due to their catalytic activity in homogeneous catalysis,
transition-metal complexes of phosphinous acids are cur-
72076 Tübingen (Germany)
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3567

rently of great interest, as was previously highlighted by Du-
brovina and Bçrner.[3]
The product (CF3)2POH can be separated in a 90% yield by
means of trap-to-trap condensation of the gaseous reaction
mixture. For the reactions and purification, safety precau-
As established by theoretical and experimental investiga-
tions, the electron-withdrawing effect of the organic groups
attached to the phosphorus atom strongly influences the
equilibrium distribution between the phosphane oxide and
phosphinous acid tautomers.[4,5] For the dimethyl derivative,
only the oxide isomer could be observed so far. This obser-
vation is supported by density functional theory (DFT) cal-
culations, which show that the corresponding phosphinous
acid is 42 kJmolÀ1 higher in energy. Depending on the elec-
tron-withdrawing effect of pentafluorophenyl and tetrafluoro-
tions are necessary, because (CF3)2POH, (CF3)2POP
and especially (CF3)2PCl react violently with air.
A
Herein, we present for bis(trifluoromethyl)phosphinous
acid, (CF3)2POH: 1) a safe, one step, and high-yield synthe-
sis; 2) a complete spectroscopic characterization; 3) a struc-
ture determination obtained from an electron-diffraction
study in the gas phase; and 4) a determination of the rota-
tional equilibrium of its two isomers in the gas phase.
À À
pyridinyl groups, the resulting phosphinous acids, R2P O H
(R = C6F5, C5NF4), are stabilized with respect to the corre-
sponding tautomeric oxide forms R2P(O)H by 1.7 and
10.5 kJmolÀ1, respectively. Based on this smaller energy dif-
ference, both derivatives exhibit a solvent-dependent equili-
brium between their tautomeric forms.[5] The phosphinous
À À
Results and Discussion
Trifluoromethylphosphorus derivatives are conveniently ac-
cessible by using the Ruppert procedure,[9] which was im-
proved for the synthesis of Et2NP(CF3)2 by Rçschenthaler
A
acid (CF3)2P O H is favored at the B3PW91/6-311G
A
and co-workers.[10] The minor air sensitive, and therefore
easily manageable, colorless liquid (CF3)2PNEt2 was synthe-
sized on a 100–200 g scale. In the presence of gaseous HBr,
the amino phosphane was transformed into (CF3)2PBr.[11]
The neat, volatile, colorless liquid, which explodes on con-
tact with air, hydrolyzed in the presence of water yielding
the target compound (CF3)2POH. To prevent the handling
level of theory by a zero-point-corrected energy difference
(DEZP) of about 14 kJmolÀ1 relative to the phosphane oxide
structure.
To date, the bis(trifluoromethyl)phosphinous acid,
(CF3)2P O H, is the only known example of a thermally
stable noncoordinated phosphinous acid. Although the
phosphinous acid was already synthesized in 1960 by Burg
and Griffiths,[6] little is known about the chemistry of this
extraordinary compound.
Virlichie and Dagnac described the sensitivity of the phos-
phinous acid towards hydrolysis,[7] evolving HCF3 as shown
in Equation (2):
À À
À
of (CF3)2PBr, the cleavage of the P N bond of (CF3)2PNEt2
and the following hydrolysis of the halogen derivative
(CF3)2PX can be performed in one step, by treating a solu-
tion of (CF3)2PNEt2 in CH2Cl2 with aqueous HCl, as shown
in Equation (3):
concd HCl
ðCF3Þ2PÀNEt2
ðCF Þ PÀOÀH
ð3Þ
ƒƒƒƒƒƒ!
3
2
À½NEt2H2ŠCl
However, the separation of (CF3)2POH from the organic
layer by means of fractional condensation was less success-
ful, yielding (CF3)2POH contaminated with variable
amounts of water. At room temperature the mixture
evolved HCF3, thus indicating a further hydrolysis step [cf.
Eq. (2)]. Finally, the remaining volatile (CF3)2POH can be
separated from the less volatile CF3P(O)(H)OH but the
yields are low.
In addition, Burg mentioned the formation of a white solid
on treatment of (CF3)2POH with NEt3, formulated as a
phosphinite salt, [NHEt3]P
(CF3)2O.[6,8]
A
À
One reason why the chemistry of the only known phos-
phinous acid has not been investigated in detail might be its
tedious, and in some part risky synthesis. The synthesis, pub-
lished by Burg and Griffiths,[8] starts with (CF3)2PI, obtained
by the autoclave reaction of CF3I with elemental phospho-
rus. The anhydride of the phosphinous acid, the tetrakis(tri-
À À
The cleavage of the P N bond of amino phosphanes can
also be performed by treatment with sulfonic acids.[12] In the
first step, on reaction of (CF3)2PNEt2 with sulfonic acid de-
rivatives, RS(O)2OH (R = CF3, C6H4CH3), an unstable
À À
mixed anhydride (CF3)2P O S(O)2R is formed which itself
has not been observed to date. Only the products of its rear-
À À
fluoromethyl)diphosphoxane (CF3)2P O P
(CF3)2, is ob-
rangement, the symmetric anhydrides (CF3)2P O P(CF3)2
and RS(O)2 O S(O)2R have been observed. The anhydride
(CF3)2P O P(CF3)2 reacts in the presence of sulfonic acids
to form (CF3)2POH and unstable (CF3)2P O S(O)2R, which
N
À À
tained in 80% yield on treatment of (CF3)2PI with silver car-
bonate. Before the product can be separated from the reac-
tion mixture by means of fractional condensation, the un-
reacted (CF3)2PI has to be converted to the removable
(CF3)2PCl by reaction with silver chloride.
À À
N
À À
À À
À À
in turn forms (CF3)2P O P(CF3)2 and RS(O)2 O S(O)2R
A
(Scheme 1). In summary, reaction of at least three equiva-
À À
The anhydride (CF3)2P O P
(CF3)2 represents a unique
lents of the nonvolatile p-toluene sulfonic acid, suspended in
compound showing no rearrangement to the diphosphane
monoxide, and can be transformed into (CF3)2PCl and
(CF3)2POH on reaction with gaseous HCl (86 h, 1008C).
the
less-volatile
solvent
1,6-dibromohexane
with
(CF3)2PNEt2, results in a quantitative yield of (CF3)2POH.
Because (CF3)2POH is the only volatile compound in the
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Chem. Eur. J. 2006, 12, 3567 – 3574

Thermally Stable Phosphinous Acid
FULL PAPER
Scheme 1. One-step synthesis of (CF3)2POH. R = CF3, C6H4CH3.
complex reaction mixture, the product can be easily separat-
ed in a 90% yield by using fractional condensation.
Figure 2. Rotational isomers of (CF3)2POH. The activation energy of the
interconversion was calculated at the B3PW91/6-311G
A
A multinuclear NMR spectroscopic investigation of the
obtained sample proved its purity. Surprisingly, the infrared
spectrum of the gaseous sample exhibits two bands in the
OH stretching region at n˜ = 3672 and 3598 cmÀ1 rather
than one at 3620 cmÀ1 as was described by Burg and co-
workers.[6,8] This discrepancy was already mentioned by
Dobbie and Straughan in 1971, who assumed that two rota-
tional isomers were responsible for the two different OH
stretching modes.[13] Stimulated by this assumption, the po-
tential energy[14] was calculated along the CPOH dihedral
angle. For a complete rotation of the OH group, the energy
scan (Figure 1) exhibits two minima. In the more-stable ro-
the STQN method.[14]
of 88 cmÀ1 is in good agreement with the experimental value
of 74 cmÀ1, which proves the coexistence of two rotational
isomers at room temperature. Due to the fast rotation of the
À
P OH group, a variable-temperature NMR spectroscopic
investigation of the neat product, as well as that of dilute
solutions in different kinds of solvents, did not allow the ob-
servation of rotational isomers.
Vibrational spectra: The first IR study on gaseous
(CF3)2POH was performed by Griffiths and Burg more than
40 years ago and its feasible phosphane oxide structure
(CF3)2P(O)H was excluded.[8] One n˜(OH) stretching vibra-
tion was reported at 3620 cmÀ1 and, based on the wavenum-
ber shifts upon deuteration, the bands at 1054 and 854 cmÀ1
À
were assigned as POH bending and P OH stretching
modes, respectively. Furthermore, the characteristic vibra-
tions of the (CF3)2P group were partly analyzed. Later,
Dobbie and Straughan attributed a doublet splitting of the
À
E H stretching and PEH bending modes in (CF3)2PEH
(E = O, S) to rotational isomerism.[13] A temperature effect
on the n˜(OH) bands in the IR spectrum of gaseous
(CF3)2POH was observed and an approximate value of
1 kcalmolÀ1 for the enthalpy difference between the con-
formers was estimated.
In the following, 1) the complete experimental vibrational
spectra of both rotamers, 2) their predicted vibrational spec-
tra obtained by quantum chemical calculations, and 3) an at-
tempt at photoinduced isomerization of matrix-isolated
(CF3)2POH will be presented.
Figure 1. Potential energy of (CF3)2POH[14] along the CPOH dihedral
angle.
tational isomer, the OH group is oriented cis relative to the
C-P-C bisector whereas it is oriented trans in the less-stable
rotational isomer (Figure 1). At the B3PW91/6-311G
A
The gas-phase IR and solid-phase Raman spectra of
(CF3)2POH are depicted in Figure 3. All vibrational data ob-
served in the gas phase, in a neon matrix, and for a liquid
and solid Raman sample are listed in Table 1. The experi-
mental wavenumbers are compared with predicted values
from quantum chemical calculations and a tentative assign-
ment of modes is given. By assuming Cs symmetry for the
rotamers of (CF3)2POH, the 27 fundamentals for each spe-
cies are represented as: Gvib = 15a’+12a’’, in which the a’
level of theory, the optimized rotational isomers, both with
Cs symmetry, exhibit a zero-point-corrected energy differ-
ence of 5.9 kJmolÀ1. The activation barrier for the intercon-
version calculated by using the synchronous transit-guided
quasi-Newton (STQN) method[14] at the same theoretical
level was found to be DEZP =12.6 kJmolÀ1 (Figure 2).
The difference between the two calculated OH valence
mode wavenumbers (n˜ = 3871 (trans) and 3783 cmÀ1 (cis))
Chem. Eur. J. 2006, 12, 3567 – 3574
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3569

B. Hoge et al.
sional, and 14 bending modes. The two n˜(OH) bands at 3672
and 3598 cmÀ1 are due to the two rotamers. Because the
weaker band at 3672 cmÀ1 increases with increasing temper-
ature at the expense of the band at 3598 cmÀ1, the weaker
band must belong to the less-stable species. Assignment to
cis or trans is in principle possible by analyzing the gas band
contours. However, all gas band contours were faded and
the less-stable species was found to be the trans rotamer by
using quantum chemical calculations.
Numerous equilibrium mixtures of rotamers have been
studied by using a trapped thermal effusive molecular beam
as an inert gas matrix.[15] If the barrier of rotation in the
molecule is greater than 5 kJmolÀ1, the conformational equi-
librium is retained in the matrix. The calculated barrier for
the isomerization of trans-(CF3)2POH!cis-(CF3)2POH
amounts to about 15 kJmolÀ1, independent of the level of
theory. Therefore, both rotamers should be seen in the IR
matrix spectra. However, to our surprise, quenching of dif-
ferent equilibrium mixtures from the gas phase in Ar or Ne
matrices at 12 or 6 K, respectively, yielded matrix-isolated
cis-(CF3)2POH exclusively in each case.
Figure 3. IR spectra of gaseous (CF3)2POH (a = 1.0 mbar, c =
4.4 mbar in a 200 mm path length gas cell at room temperature) and
Raman spectrum of solid (CF3)2POH at À1968C.
in-plane and a’’ out-of-plane modes are all IR and Raman
active. They can be divided into ten stretching, three tor-
Although (CF3)2POH is a
strong UV absorber (see
Figure 4), photolysis of the ma-
trices led neither to any iso-
mers nor to decomposition
products.
Table 1. Experimental and calculated fundamental wavenumbers (n˜ in cmÀ1) and band intensities (I) of cis-
and trans-(CF3)2POH.
IR gas (I)[a]
cis/trans
Ne matrix (I)[b]
Raman[c]
Calculated[d]
trans
Assignment[e]/Approximate
description of mode
cis
cis
cis
3672 (2.8)
3598 (14)
1211 (35)
1175sh (54)
1165 (100)
3873(46)
trans
n˜1
n˜(OH)
n˜(OH)
On closer inspection of the
IR spectra of the gaseous
sample at different tempera-
tures, several band shoulders
of variable intensity could be
recognized at n˜ = 1034, 845,
453, and 387 cmÀ1; these are
assigned to the POH motions
of the trans rotamer. For the
cis rotamer, 25 fundamentals
have been observed with n˜4/n˜16
and n˜19/n˜7 found to be acciden-
tally degenerated. The assign-
ment of all fundamentals is
based on comparison with the
calculated band position, in-
tensity, and symmetry class;
characteristic vibrations; as
well as the polarization of
some Raman bands of a liquid
sample. The bands in the range
n˜ = 600–100 cmÀ1 are more or
less coupled and the descrip-
tion of modes is in part arbi-
trary.
3592.7 (22)
1213.0 (81)
1175.7 (72)
1163.0 (100)
3784(27)
a’
a’
a’
a’
a’’
a’’
a’’
a’
1213w
1199w
1154m
1223(79)
1210
A
n˜2
n˜s(CF3)
N
1179
A
1174(95)
1162(57)
1156(80)
1143(56)
1114(37)
1083(38)
n˜3
n˜as
ACHTREUNG
1170(36)
1154(23)
1138(40)
1117(71)
n˜4
n˜as
ACHTREUNG
n˜16
n˜17
n˜18
n˜5
n˜s(CF3)
N
1149sh (27)
1120 (34)
1056 (9)
1034sh (4)
856 (20)
845sh
1146.0 (22)
1118.1 (50)
1057.6 (24)
n˜as
ACHTREUNG
n˜as
AHCTREUNG
d
d
ACHTREUNG
1083(58)
835(28)
trans
n˜6
ACHTREUNG
857.2 (36)
741.6 (1)
567.2 (4)
541.4 (3)
470.4 (39)
868s,p
845w
848(37)
a’
n˜(PO)
n˜(PO)
trans
n˜19
n˜7
741 (0.6)
750
745
563
544
542
520
C
744
740
564
542
540
519
C
a’’
a’
a’
ds
A
745vs,p
566 m
546m
ds(CF3)
ACHTREUNG
566 (1.8)
539 (0.9)
n˜8
das
das
ACHTREUNG
a’
n˜9
ACHTREUNG
a’’
a’’
a’’
n˜20
n˜21
n˜22
trans
n˜10
n˜23
trans
n˜11
n˜12
n˜24
n˜25
n˜13
n˜26
n˜14
n˜15
n˜27
n˜as
ACHTREUNG
524w
ACHTREUNG
465 (15)
453sh
482(48)
t
t
ACHTREUNG
461
450
U
ACHTREUNG
456.5 (7)
413.9 (15)
457s,p
452
422
U
a’
a’’
das
ACHTREUNG
408 (4.5)
387sh (2)
361 (2)
w
w
ACHTREUNG
360
A
ACHTREUNG
370m,p
281vs,p
241m
331(15)
359
277
274
229
187
160
123
77
C
a’
a’
1
ACHTREUNG
307 (0.5)
276
257
214
182
163
117
70
C
ACHTREUNG
a’’
a’’
a’
a’’
a’
1
1
1
1
d
ACHTREUNG
236 (1)
193 (1)
ACHTREUNG
184vw
ACHTREUNG
ACHTREUNG
134vw
85w
ACHTREUNG
a’
a’’
t
t
ACHTREUNG
17
A
15
A
ACHTREUNG
[a] Position and relative absorbance at band center. [b] Position at the most intensive matrix site. Intensity in-
Determination of the equilibri-
um: The clearly separated OH
stretching modes at n˜ = 3672
and 3598 cmÀ1 are suitable for
tegrated over all matrix sites belonging to the respective fundamental. [c] Solid at À1968C; “p” denotes polar-
ized band of
338.0 kmmolÀ1 [14]
w=out-of-plane deformation, 1=rocking.
a
liquid sample. [d] B3PW91/6-311G(3d,p); cis 100% = =
277.5 kmmolÀ; trans 100%
[e] Assignment for cis conformer. Types of vibration: n˜ =stretch, d=bending, t=torsion,
A
.
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Chem. Eur. J. 2006, 12, 3567 – 3574

Thermally Stable Phosphinous Acid
FULL PAPER
in which c and t denote cis and trans, x the molar fraction,
and K the equilibrium constant. In our experiment, x can be
replaced by the integrated absorbance A and the extinction
coefficient e to give Equation (6):
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
xc
xt
Ac
At
et
ln
¼ ln
þ ln
ð6Þ
ec
The A values can be measured directly from the spectra,
whereas the ratio of the extinction coefficients (ec /et) is
available only from experiments at different temperatures.
The measured data at seven different temperatures are gath-
ered in Table 2. The total concentration of both rotamers of
Figure 4. UV spectrum of gaseous (CF3)2POH (lmax = 206 nm). s = ab-
Table 2. Input data for the determination of the equilibrium constants.
sorption cross section.
T
p
A[a]
Ac/At
Kp
ln(Kp)
[K]
A
cis
trans
351.1
326.2
299.9
273.7
264.1
253.8
246.9
4.44
3.85
4.13
4.02
3.40
3.45
3.44
7.7668
8.0598
9.8155
1.8545
9.6301
9.4117
10.6461
2.7072
2.4706
2.4641
2.2140
1.6933
1.5257
1.6213
2.87
3.26
3.98
5.12
5.69
6.17
6.57
3.20
3.64
4.44
5.72
6.35
6.89
7.33
1.164
1.292
1.491
1.743
1.849
1.930
1.993
the evaluation of the chemical thermodynamics of the inter-
conversion process, shown in Equation (4):
trans-ðCF3Þ2POH Ð cis-ðCF3Þ2POH
ð4Þ
[a] Integrated absorbances for n˜(OH); n˜cis
3672 cmÀ1
=
3598 cmÀ1
, n˜trans =
In Figure 5, both gas-phase OH stretching bands at three
different temperatures are depicted. It can be seen that with
increasing temperature the absorption of the high frequency
band increases at the expense of the low frequency band.
Therefore, the peak at 3598 cmÀ1 must belong to the more
stable cis rotamer, in accordance with quantum chemical
calculations (see above).
.
(CF3)2POH in the gas cell is proportional to the pressure/
temperature ratio (p/T) and the individual concentrations
are proportional to the absorbance/extinction coefficient
ratio (A/e). Hence, at any temperature and pressure, Equa-
tion (7) is valid:
p
T
Ac At
ð7Þ
ꢀ
þ
ec
et
which can be rearranged to give Equation (8):
AtT
p
ðet=ecÞAcT
ð8Þ
ꢀ etÀ
p
The normalized integrated absorbances (AT)/p for the OH
stretching bands of both rotamers form a straight line, as
shown in Figure 6, for which the slope et /ec = 1.16. The two
points obtained at the highest and lowest temperature did
not fit the straight line well and have therefore not been
used for the regression. At the highest temperature some
decomposition of the sample occurred and at the lowest
temperature a little sample was deposited on the silicon win-
dows, which disturbed the total concentration of gaseous
(CF3)2POH in the IR beam. The ratio et /ec obtained from
quantum chemical calculations (1.70, cf. Table 1) is notice-
ably larger. The reason for the deviation remains unclear.
With the experimental et /ec ratio on hand, the absorbance
ratio can be converted into the molar ratio (Kp, Table 2) and
the data are plotted in Figure 7. The regression yields DHA =
Figure 5. IR spectra of gaseous (CF3)2POH in the n˜(OH) region at À26,
À9, and +518C.
The differences in enthalpy and entropy of both rotamers
were determined by the method of vanꢁt Hoff and Ulrich,
shown in Equation (5):
ꢀ
ꢁ
xc
xt
DHA DSA
ln Kx ¼ ln
¼ À
þ
ð5Þ
À5.92ꢁ0.14 kJmolÀ1 and DSA
=
À7.28 JmolÀ1 KÀ1, aver-
RT
R
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www.chemeurj.org
3571

B. Hoge et al.
At room temperature (228C) the composition in the equi-
librium amounts to 82% cis and 18% trans. These values
were used in the electron-diffraction study.
Structure analysis—electron diffraction: Averaged molecu-
lar intensities in the s ranges of 0.02–0.18 and 0.08–
0.35 pmÀ1 in steps (Ds) of 0.002 pmÀ1 (s = (4p/l)sinq/2, l =
electron wavelength, q = scattering angle) are shown in
Figure 8. The radial-distribution function (RDF) was de-
Figure 6. Plot of the integrated absorbances of the n˜(OH) IR bands of cis
(Ac) and trans (At) (CF3)2POH, normalized to the total concentration
(T/p). The slope represents the relative IR intensities of the n˜(OH)
bands.
Figure 8. Experimental and calculated molecular intensities for long
(above) and short (below) nozzle-to-plate distances and residuals.
rived by Fourier transformation of these intensities, by ap-
plying an artificial damping function exp
C
À
towards the orientation of the O H bond. The geometric
parameters of the prevailing cis form were refined by least-
squares fitting of the molecular intensities. The overall sym-
metry was constrained to Cs symmetry. Local C3v symmetry
À
with a possible tilt angle between the C3 axis and the P C
À
bond direction was assumed for the CF3 groups. The O H
bond length and P-O-H angle were not refined. Vibrational
amplitudes were collected in groups according to their cal-
Figure 7. Vanꢁt Hoff plot of the gas-phase equilibrium of cis/trans-
(CF3)2POH.
aged over the temperature range used. The uncertainty in
DHA is mainly due to errors in the determination of the tem-
perature and of Kp, whereas that for DSA is due to errors in
the proportionality factor and in DHA.
The reaction enthalpy, DHA, for the conversion of trans-
(CF3)2POH to cis-(CF3)2POH is in excellent agreement with
the
predicted
(B3PW91/6-311G
A
value
of
À6.40 kJmolÀ1. However, the calculated reaction entropy,
DSA, of À3.10 JmolÀ1 KÀ1 is lower than the experimental
value. The calculated entropy difference depends strongly
on the low frequency CF3 torsional vibrations in the two
conformers, with the torsions of the cis species being higher
in frequency on account of its intramolecular hydrogen
bridge. The torsional frequencies are predicted to be in the
range between 10 and 20 cmÀ1, and thus make contributions
to the entropy of about 30 JmolÀ1 KÀ1. Because the calcula-
tion of such low vibrational frequencies is not very reliable,
the calculated entropy difference is not reliable either.
Figure 9. Experimental radial-distribution function and difference curve.
The position of important interatomic distances are indicated by vertical
bars.
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Chem. Eur. J. 2006, 12, 3567 – 3574

Thermally Stable Phosphinous Acid
FULL PAPER
culated values. Geometric parameters of the trans conform-
er were tied to those of the cis form by using the calculated
differences. The ratio of both conformers was set to the
value derived from the vibrational data. With these assump-
tions, eight geometric parameters and ten vibrational ampli-
tudes (l1 to l10) were refined simultaneously. The following
correlation coefficients had absolute values larger than 0.7:
CF/FCF = 0.71, OPC/l8 = 0.75, tilt
A
Figure 10. Molecular model with atom numbering.
t
ACHTREUNG
are given in Table 3 (geometric parameters) and Table 4 (vi-
brational amplitudes) together with the calculated values,
and a molecular model is presented in Figure 10.
Experimental Section
Synthesis of bis(trifluoromethyl)phosphinous acid: p-Toluenesulfonic
acid monohydrate (20 g, 105 mmol), dried at room temperature for 12 h
at 5”10À4 mbar, was suspended in 1,6-dibromohexane (200 mL). After
degassing the resulting brown suspension for one hour under vacuum,
Table 3. Experimental and calculated geometric parameters for the cis
conformer.
[10]
(CF3)2PNEt2 (5.5 g, 23 mmol) was added using a syringe. After stirring
GED[a]
MP2/6-311G
G
B3LYP/6-311G
G
the suspension for 12 h at room temperature, the brown color disap-
peared. The volatile compounds were removed under dynamic vacuum
conditions by using three traps at À30, À78, and À1968C to separate
minor amounts of the solvent, (CF3)2POH and HCF3. The À788C trap
À
P O
À
P C
166.1(4)
189.5(3)
133.9(1)
97.0[d]
95.4(10)
99.8(15)
107.6(3)
112.2[d]
163.2
188.5
134.5
97.0
95.7
98.9
107.6
113.1
2.1
162.7
189.9
134.1
96.7
96.9
99.3
107.7
113.8
3.7
À
C F
contained 3.7 g (20 mmol, 88%) of
a colorless liquid identified as
À
O H
(CF3)2POH. The product can be stored for several months in sealed glass
ampoules at room temperature and exhibits no sign of decomposition.
NMR (neat, RT): 1H NMR: d = 4.4 ppm (s); 13C NMR: d = 125.7 ppm
C-P-C
O-P-C
F-C-F
P-O-H
(q, d, q) (1J
(P,C) = 30.0 Hz, 1J
A
U
tilt
A
2.9(12)
NMR: d = À69.3 ppm (d); 31P NMR: d = 77.9 ppm (sept) (2J
(P,F) =
t
A
À5.6(39)
À2.9
À3.9
83.1 Hz).
Vibrational spectra: Raman spectra of a liquid and a solid sample were
recorded on an FT-Raman spectrometer (Equinox 55/FRA 106sÀ1
[a] ra values in pm and 8. Error limits are 3s values and refer to the last
À
,
digit. [b] Tilt angle between C3 axis and P C bond direction in CPC
À
Bruker) using a 500 mW Nd:YAG laser (l = 1064 nm), a Si/CaF2 beam
splitter, and a liquid-nitrogen-cooled NIR detector D418-T. The liquid
sample was placed in a sealed 4 mm glass capillary and the solid sample
was condensed as a spot on a nickel finger at À1968C under high
vacuum. For each spectrum, 128 scans were averaged at an apodized res-
plane and away from the opposite P C bond. [c] Torsional angle of CF3
group. t
(CF3) = 08 for the exact staggered orientation. A negative value
implies shortening of the F2···F5 distance (see Figure 10 for atom num-
bering). [d] Not refined.
olution of 2 cmÀ1
.
Table 4. Interatomic distances and experimental and calculated vibra-
tional amplitudes (without distances involving hydrogen).[a]
Infrared spectra were measured with an IFS 66v FTIR spectrometer
(Bruker) employing resolutions of 1 or 2 cmÀ1 using a KBr/Ge or a 6 mm-
Mylar/Ge beamsplitter and DTGS detectors. For each spectrum, 64 scans
were averaged.
Distance
Ampl. (GED)
Ampl. (MP2)[a]
À
C F
À
P C
134
161
190
216
263
264–271
269
280
290
292
303
312
315
351
388
402
426
442
513
4.4(2)
4.4(2)
4.7(4)
5.7(2)
43.2 (254)
8.1(6)
8.8[b]
l1
l1
l2
l3
l4
l5
4.5
4.4
5.2
5.7
40.1
8.2
Infrared spectra of the gaseous sample in the temperature range À26 to
788C were obtained by using a temperature-controlled gas cell with an
optical path length of 20 cm. The double-walled glass cell equipped with
silicon windows (0.5 mm thick) and a Pt-100 temperature sensor was
placed inside the evacuated (<2 mbar) sample chamber of the spectrom-
eter. The cell was filled with (CF3)2POH vapor and evacuated from out-
side the spectrometer. Pressure control was accomplished by the use of a
Baratron pressure transducer (type 122A, 10 mbar, accuracy 10À3 mbar).
Temperature control was achieved to better than 18C by a flow of cooled
or heated air. In order to prevent systematic errors in the pressure read-
ings due to decomposition of (CF3)2POH or impurities from the cell, for
each experiment at every temperature, a new sample was taken from the
(CF3)2POH reservoir.
À
P C
F···F
F2···F5
P···F
C···O
8.8
7.8
C1···C2
F3···F6
O···F2
C1···F5
O···F1
C1···F6
F2···F6
O···F3
C1···F4
F1···F5
F1···F6
F1···F4
8.1(6)
l5
l4
l6
l6
l6
l6
l4
l7
l8
l9
l9
l10
43.2
A
41.0
40.8
42.6
45.2
43.6
98.8
15.7
11.9
22.5
24.3
10.0
15.0(16)
15.0(16)
15.0(16)
15.0(16)
43.2
10.5(18)
17.8(114)
A
Infrared spectra of matrix-isolated samples of equilibrium mixtures cis/
trans-(CF3)2POH/Ar (or Ne) at different temperatures were obtained by
trapping the molecular beam at 12 or 6 K, respectively. Details of the
matrix-isolation apparatus are given elsewhere.[17,18]
AHCTREUNG
13.9(55)
13.9(55)
9.3(30)
Electron diffraction: Electron diffraction intensities were recorded with a
Gasdiffraktograph KD-G2[19] at 25 and 50 cm nozzle-to-plate distances
and with an accelerating voltage of about 60 kV. The sample was cooled
to À188C and the inlet system and nozzle were at room temperature.
The photographic plates were analyzed with an Agfa Duoscan HiD scan-
ner and total-scattering-intensity curves were obtained from the TIFF-
file using the program SCAN3.[20]
[a] Values in pm, error limits are 3s values. For atom numbering see
Figure 10. Vibrational amplitudes were derived from a calculated (MP2/
6-31G
fined.
A
Chem. Eur. J. 2006, 12, 3567 – 3574
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
3573

B. Hoge et al.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
Acknowledgements
Dr. K. Glinka is acknowledged for helpful discussions. The Deutsche For-
schungsgemeinschaft and the Fonds der Chemischen Industrie are ac-
knowledged for financial support.
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Received: August 30, 2005
Published online: February 21, 2006
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Chem. Eur. J. 2006, 12, 3567 – 3574