Tetrakis[2,4-bis(trifluoromethyl)phenyl]diphosphoxane: An anhydride of a phosphinous acid

Article abstract of DOI:10.1002/anie.200801025

Steric and electronic characteristics of the bulky, strongly electron-withdrawing 2,4-(CF₃)₂C₆H₃ group are ideally suited to stabilize the unusual form of a diphosphoxane, R₂POPR₂, the anhydride of a phosphinous acid. The high-yield synthesis and first structural characterization of a tetraorganyl diphosphoxane are described (see structure; C gray, H white, O red, F yellow, P green). (Figure Presented)

Full text of DOI:10.1002/anie.200801025

Communications
DOI: 10.1002/anie.200801025
Acid Anhydrides
Tetrakis[2,4-bis(trifluoromethyl)phenyl]diphosphoxane: An
Anhydride of a Phosphinous Acid**
Berthold Hoge* and Boris Kurscheid
Diorganylphosphinous acids, R2POH, are generally unstable
with respect to their tautomeric counterparts R2P(O)H
[Eq.(1)].In 1960, Burg and Griffiths demonstrated that the
As known for phosphinous acids, the anhydride can also
be stabilized by strongly electron-withdrawing substituents
such as the CF3 group.[1] Although tetrakis(trifluoromethyl)-
diphosphoxane, (CF3)2P-O-P(CF3)2, and (CF3)2POH are
stable with respect to their corresponding phosphane oxide
tautomers, the colorless liquids with high vapor pressure are
highly reactive and immediately deflagrate on contact with
air.
While sterically demanding groups destabilize the phos-
phinous acid form, anhydrides can be stabilized by bulky tBu
groups, as demonstrated in 1976 by Veits and co-workers.[8]
On the basis of these results, the electron-withdrawing and
sterically demanding 2,4-bis(trifluoromethyl)phenyl group is
expected to be ideal for the stabilization of the unusual form
of diphosphoxane derivatives, the anhydrides of phosphinous
acids.
unusual form of a phosphinous acid can be stabilized by the
strongly electron-withdrawing trifluoromethyl group.[1]
Bis(trifluoromethyl)phosphinous acid, (CF3)2POH, exists
in the gas phase as a mixture of two rotational isomers, and it
is to date the only example of a neutral phosphinous acid that
is stable with respect to its tautomeric phosphane oxide
structure.[2]
Very recently Cowley and co-workers succeeded in the
stabilization of a phosphinous acid derivative as a b-diketi-
minato-supported hydroxyphosphenium cation.[3]
The reaction of 1,3-bis(trifluoromethyl)benzene with n-
butyllithium gives a mixture of 2,6- and 2,4-bis(trifluorome-
thyl)phenyl lithium, (Scheme 1).Treatment of PCl with an
3
The claimed[4] bis(pentafluorophenyl)phosphinous acid,
(C6F5)2POH, has been shown to exist in the solid state as well
as in CHCl3, toluene, and CH3CN exclusively as the phos-
phane oxide.[5] The phosphinous acid tautomer can only be
observed in solvents with high donor numbers (e.g. DMF,
THF).[5] Substituents with a stronger electron-withdrawing
effect than the C6F5 group, for example, the p-tetrafluoropyr-
idyl group,[6] effect a further stabilization of the phosphinous
acid tautomer,[5] while the increased steric demand of a 2,4-
bis(trifluoromethyl)phenyl group (2,4-(CF3)2C6H3) favors the
phosphane oxide tautomers.[7]
Scheme 1. Synthesis of bis[2,4-bis(trifluoromethyl)phenyl]diethylamino-
phosphane (1).
The anhydrides of phosphinous acids, R2P-O-PR2, exhibit
a similar behavior.In general, these compounds are less stable
than the corresponding diphosphane monoxides, R2P(O)-PR2
[Eq.(2)].
excess of the lithiated bis(trifluoromethyl)phenyl derivatives
leads to the formation of a complex product mixture of
differently substituted 2,6- and 2,4-bis(trifluoromethyl)phe-
nylphosphanes.[9] To achieve a selective synthesis of bis[2,4-
bis(trifluoromethyl)phenyl]phosphane derivatives, a bulky
substituent has to be attached to the phosphorus center to
avoid the attack of the more sterically demanding 2,6-
bis(trifluoromethyl)phenyl moiety.According to this idea,
we investigated the reaction of dichlorodiethylaminophos-
phane, Cl2PNEt2, with a mixture of 2,6- and 2,4-bis(trifluoro-
methyl)phenyl lithium, which allowed the selective synthesis
of the aminophosphane derivative 1.[10]
[*] Dr. B. Hoge, B. Kurscheid
Institut für Anorganische Chemie
Universität zu Köln, Greinstrasse 6, 50939 Köln (Germany)
Fax: (+49)221-470-5196
E-mail: b.hoge@uni-koeln.de
Homepage: http://www.hoge.uni-koeln.de/
Upon treatment with concentrated hydrochloric acid, 1 is
selectively converted into the corresponding chlorophos-
phane 2 (Scheme 2), which is hydrolyzed by aqueous THF
to the phosphane oxide 3 (83%).Compound 3 reacts with an
[**] We are grateful to Prof. Dr. D. Naumann and Prof. Dr. G. Meyer for
their generous support. The Merck company, Darmstadt, is
acknowledged for financial support. We want to thank Dr. W. Tyrra
for helpful discussions.
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Figure 1. Experimental and calculated 31P{1H} NMR spectrumof 5.
couplings could be determined.After iteration of the 31P
2
and 19F NMR resonances, the absolute value of the JP, P
coupling is calculated to be 172.3 Hz.
As expected, the diphosphoxane 5 is sensitive to hydrol-
ysis.NMR spectroscopic investigations in CDCl
always
3
allowed the detection of the phosphane oxide 3 as the
product of hydrolysis.However, colorless crystals of the
diphosphoxane 5 can be handled for short periods in ambient
atmosphere without significant decomposition.
The diphosphoxane 5 crystallizes in the monoclinic space
group C2/c (No.15) with one molecule in the asymmetric unit.
The molecular structure exhibits a twofold synperiplanar
arrangement of the phosphorus lone pairs with respect to the
À
opposite P O bond, resulting in a molecular structure with
approximate C2 symmetry (Figure 2).The trifluoromethy-
Scheme 2. Synthesis of tetrakis[2,4-bis(trifluoromethyl)phenyl]diphos-
phoxane (5).
excess of elemental sodium to the corresponding phosphinite
salt 4.[10]
Treatment of 4 with the chlorophosphane 2 allows the
selective synthesis of the tetraorganyl diphosphoxane deriv-
ative 5 (Scheme 2).The isomeric diphosphane monoxide 6 is
formed only as a minor product, which could be identified by
its characteristic resonances in the 31P NMR spectrum at d =
39.0 ppm for the oxygen-bonded phosphorus atom and at d =
À32.5 ppm for the trivalent phosphorus atom. The doublet
1
splitting of 247 Hz matches the expected range of JP, P
couplings.In the absence of acids, no rearrangement of the
diphosphane monoxide 6 and diphosphoxane 5 has been
observed.
Figure 2. Molecular structure of diphosphoxane 5. Selected bond
lengths [pm] and angles [8]: P1-O1 165.6(3), P2-O1 166.1(4), P1-C1
185.3(3), P1-C2 186.7(4), P2-C3 184.8(4), P2-C4 187.1(4); P1-O1-P2
119.7 (2), C1-P1-C2 100.0(2), C3-P2-C4 99.4(2).
The 19F NMR spectrum of 5 in CDCl3 exhibits two
resonances, one singlet at d = À63.3 ppm for the CF3 group
in para position and one multiplet at d = À57.1 ppm for the
CF3 group in ortho position.The multiplet splitting is caused
by the coupling to two magnetically non-equivalent phos-
phorus atoms.The 19F and the 31P NMR spectra (Figure 1)
both show higher-order resonances of an [AX6]2 spin system
lated nitrogen derivative (CF3)2N-O-N(CF3)3 shows a com-
parable structural motif.[11] The P O bond lengths of the
À
4
6
(A = 31P and X = 19F) with JP, F and JP, F couplings of + 60.8
and + 5.4 Hz, respectively. Because the long-range F,F
coupling of the ortho CF3 groups along the P-O-P unit is
diphosphoxane 5 (165.6(3) and 166.1(3) pm) are in the typical
À
range of P O single bonds.Bis(trifluoromethyl)phosphinous
À
acid, (CF3)2POH, exhibits a comparable P O separation of
4
6
close to zero, only the relative signs of the JP, F and JP, F
166.1(4) pm in the gas phase.[2]
Angew. Chem. Int. Ed. 2008, 47, 6814 –6816
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Communications
The last refinement cycles included atomic positions for all atoms,
anisotropic thermal parameters for all non-hydrogen atoms, and
isotropic thermal parameters for all hydrogen atoms.Intensity data
for 5 (C32H12F24OP2; 930.36 gmolÀ1): T= 298 K, colorless transparent
plate, monoclinic, space group C2/c (No.15), a = 2468.7(1), b =
934.5(6), c = 3188.4(2) pm, b = 94.897(5)8, V= 7.328 nm3, Z = 8,
The strong electron withdrawal and high steric demand of
the 2,4-(CF3)2C6H3 group are ideally suited to stabilize the
unusual form of a diphosphoxane—the anhydride of a
phosphinous acid.The higher steric demand of the 2,6-
(CF3)2C6H3 group has, to date, prevented the selective
synthesis of a diphosphoxane derivative bearing one 2,4-
and one 2,6-(CF3)2C6H3 group at each phosphorus atom.
1calcd = 1.686 gcmÀ3
, , F(000) = 3664; 45929
m(MoKa) = 0.269 cmÀ1
reflections with 3.12 < 2q < 54.28, 8000 independent reflections in
structure solution and refinement for 532 parameters, R1 (I ꢀ 2s(I)) =
0.078, wR2 (all data) = 0.228, w = 1/{s2(Fo2) + [0.1666(F2o + 2Fc2)/3]2}.
CCDC-678308 contains the supplementary crystallographic data for
this paper.These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif
Experimental Section
5: Bis[2,4-bis(trifluoromethyl)phenyl]phosphane oxide (1.2 g,
2.6 mmol) in 1,2-dimethoxyethane (15 mL) is treated for two hours
at room temperature with an excess of elemental sodium.After
filtration under inert conditions, chlorobis[2,4-bis(trifluoromethyl)-
phenyl]phosphane (1.1 g, 2.1 mmol) in diethyl ether (30 mL) is added
to the light-orange solution.After stirring for three minutes at room
temperature, hexane (200 mL) is added to complete the precipitation
of inorganic salts.Subsequent filtration under inert conditions and
removal of the solvents in vacuum yielded 5 (1.7 g, 1.8 mmol; 86%).
Single crystals were obtained on recrystallization from diethyl ether
at À308C.Elemental analysis (%) calcd for 5: C 41.31, H 1.30; found:
C 41.14, H 1.35. MS (20 eV; R = C6H3(CF3)2): m/z (%): 930 (100)
[M+], 911 (10) [M+ÀF], 861 (4) [M+ÀCF3], 670 (26) [PR3+], 473 (100)
[M+ÀPR2]. 1H NMR (300 MHz, CDCl3, 258C, TMS): d = 7.4–8.0 ppm
(m). 13C NMR (75 MHz, CDCl3, 258C, TMS): d = 141.7 (m, C1), 133.8
Received: March 3, 2008
Published online: July 25, 2008
Keywords: anhydrides · diphosphoxanes · phosphinite ·
.
phosphinous acid · phosphorus
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[2] B.Hoge, P.Garcia, H.Willner, H.Oberhammer,
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[3] Z.Lu, M.Findlater, A.H.Cowley, Chem. Commun. 2008, 184 –
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1
(q, JF,C = 272.8 Hz, C8). 19F NMR (188 MHz, CDCl3, 258C, CFCl3):
186.
d = À63.3 (s, 12F), À57.1 ppm (m, X part of an [AX6]2 spin system
with A = 31P). 31P NMR (81 MHz, CDCl3, 258C, H3PO4): d =
105.6 ppm (m, A part of an [AX6]2 spin system with X = 19F).
Data collection for X-ray structure determination was performed
on a STOE IPDS II diffractometer using graphite-monochromated
MoKa radiation (71.073 pm). The data were corrected for Lorentz and
polarization effects.A numerical absorption correction based on
crystal-shape optimization was applied for all data (X-Shape 2.01,
Crystal Optimisation for Numerical Absorption Correction, STOE &
Cie GmbH, Darmstadt, 2001).The programs used herein are Stoeꢀs
X-Area (X-Area 1.16, Stoe & Cie GmbH Darmstadt, 2003), including
X-RED and X-Shape for data reduction and numerical absorption
correction (X-RED32 1.03,Stoe Data Reduction Program , Stoe & Cie
GmbH, Darmstadt, 2002), and X-Step32 program (X-STEP32 1.06f,
Stoe & Cie GmbH, Darmstadt, 2000), including SHELXS-97 (G.M.
Sheldrick, SHELXS-97, University of Göttingen, 1998) and
SHELXL-97 (G.M. Sheldrick, SHELXL-97, University of Göttin-
gen, 1997) for structure solution and refinement.All hydrogen atoms
were placed in idealized positions using a riding model.
[4] D.D.Magnelli, G.Tesi, J.U.Lowe, W.E.McQuistion,
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[7] B.Hoge, unpublished results.
[8] V.L. Foss, Yu.A. Veits, V.A. Solodenko, I.F. Lutsenko,
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