2,3,5,6-tetrafluoro-p-phenylenebis(phosphanes) - preparation and structure of an electron-poor P-RF-P linker

Article abstract of DOI:10.1002/ejic.200901042

A general synthetic access to functional 2,3,5,6-tetrafluorop- phenylenebis(phosphanes) is presented, which should open the way to linear phosphorus-based conjugated systems incorporating an electron-deficient aryl moiety. 1,4-Bis[bis(diethylamino) phosphanyl]-2,3,5,6-tetrafluorobenzene and l,4-bis(dichlorophosphanyl)-2,3,5,6-tetrafluorobenzene have been synthesized in good yields and were characterized by spectroscopic methods and X-ray crystallography.

Full text of DOI:10.1002/ejic.200901042

SHORT COMMUNICATION
DOI: 10.1002/ejic.200901042
2,3,5,6-Tetrafluoro-p-phenylenebis(phosphanes) – Preparation and Structure of
an Electron-Poor P–RF–P Linker
Andreas Orthaber,[a] Ferdinand Belaj,[a] Jörg H. Albering,[b] and Rudolf Pietschnig*[a]
Keywords: Phosphorus / Phosphanes / Fluorinated ligands / Electron-deficient compounds
A general synthetic access to functional 2,3,5,6-tetrafluoro-
Bis[bis(diethylamino)phosphanyl]-2,3,5,6-tetrafluorobenzene
p-phenylenebis(phosphanes) is presented, which should and 1,4-bis(dichlorophosphanyl)-2,3,5,6-tetrafluorobenzene
open the way to linear phosphorus-based conjugated sys-
tems incorporating an electron-deficient aryl moiety. 1,4-
have been synthesized in good yields and were characterized
by spectroscopic methods and X-ray crystallography.
been reported, where the fluorine atoms in higher fluori-
nated precursors are partially replaced by isolobal PR2
units.[21–23] The latter are, however, not available for further
functionalization at the phosphorus atoms, except for the
transformation to the corresponding phosphoranes.[24,25]
A general route towards the synthesis of dichlorophos-
phanes includes the reaction of the corresponding R–M (M
= Li or MgBr; R = organic substituent) compound with an
excess of PCl3, if steric protection of R is sufficient to pre-
vent multiple substitution at the phosphorus atom. Re-
cently, we investigated the reaction of 2,3,5,6-F4C6HLi with
PCl3 and observed the formation of a mixture of multiple-
substituted phosphanes [PCln(C6F4H)3–n] even if a 10-fold
excess of PCl3 was used.[26] To circumvent this problem, we
treated the “protected” chlorobis(diethylamino)phosphane
ClP(NEt2)2 with 2,3,5,6-F4C6Li2 at a low temperature.
Preparation of the latter dilithio compound is not trivial.
On the one hand the temperature must not raise above
–50 °C at any time (to prevent the instantaneous formation
of highly reactive aryne species), and on the other hand
LiCl elimination does not take place at temperatures below
–70 °C. Owing to incomplete lithiation, the monosubsti-
tuted tetrafluorobenzene derivative is always obtained as a
byproduct, but can be removed from the desired product by
crystallization (Scheme 1).
Introduction
Phosphanes – especially dichlorophosphanes – are im-
portant precursors in the synthesis of σ2λ3-phosphanes. In
the last decades general synthetic routes to diphosphenes,[1]
iminophosphanes,[2] phosphaalkenes[3,4] and phospholes[5]
have been established, which employ dichlorophosphanes as
[P+] synthons. The parent p-phenylenebis(dichlorophos-
phane) has been prepared in 1962,[6] and the 2,3,5,6-tetra-
methyl-[7] and -tetraaryl-substituted[4] congeners later on.
In 2002, Gates et al. reported the synthesis of a 2,3,5,6-
tetramethyl-p-phenylenebis(phosphaalkene) and polymeric
derivatives thereof.[7] Until now only little is known about
electron-deficient bis(phosphaalkenes),[8–10] although many
electron-rich low-coordinated phosphanes have been re-
ported in the last decades.[11–13] A special focus of experi-
mental and theoretical work was set on push-pull-substi-
tuted low-coordinated phosphanes, especially iminophos-
phanes,[14,15] diphosphenes,[16,17] and phosphaalkenes.[9] Re-
cently, the twist angle of diphosphenes was studied theore-
tically, but a detailed explanation highlighting the influence
of the electron-accepting groups remains still elusive.[18,19]
The incorporation of C6F5 groups into phospholes was
achieved by Baumgartner and co-workers, and they ex-
plained their stabilizing effect on the LUMO level by means
of DFT calculations.[20]
Results and Discussion
In the literature the syntheses of fully alkylated or aryl-
ated 2,3,5,6-tetrafluoro-p-phenylenebis(phosphanes) have
[a] University of Graz, Department of Chemistry,
Schubertstr. 1, 8010 Graz, Austria
Scheme 1. Synthesis of 1 and 2 starting from 2,3,5,6-F4C6H2. Reac-
tion conditions: (a) nBuLi/thf, –80 °C; (b) ClP(NEt2)2, –35 °C; (c)
HCl(g)/Et2O, 0 °C.
Fax: +43-316 380 9835
E-mail: rudolf.pietschnig@uni-graz.at
[b] Graz University of Technology, Institute of Chemistry and
Technology of Materials,
Compound 1 is obtained as a white solid showing a sin-
gle 31P NMR resonance at δ = 84.9 ppm as well as a single
resonance in the 19F NMR spectrum at δ = –141.6 ppm in
Stremayrgasse 16, 8010 Graz, Austria
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200901042.
34
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 34–37

2,3,5,6-Tetrafluoro-p-phenylenebis(phosphanes)
solution, which shows no resolved coupling between 19F Crystallization from CDCl3 yields crystals of 2 suitable for
and 31P. Single crystals of 1 are obtained by crystallization X-ray diffraction (Figure 2). The central perimeter of 2 is
from a cold 1,2-dimethoxyethane (dme)/pentane solution. almost planar [max. deviation 0.003(1) Å] and shows signif-
In the crystal structure, the molecules are arranged around icantly smaller deformation than in 1 [C–C–C angle ranging
centers of symmetry. The benzene ring shows a significant from 116.4(1) to 122.1(1)°]. The P–C bond length is
in-plane deformation with C–C–C angles of only 112.6(2)° 1.843(1) Å, which is slightly shorter than 1. This is attrib-
at C1 and a mean value of 123.7(2)° at the fluorine-substi- uted to the lower conjugation with the aryl group of the P–
tuted atoms C2 and C3 (Figure 1). Nevertheless, the ben- Cl bonds compared to the P–N bonds. The central perime-
zene ring is planar [max. deviation of 0.005(10) Å of the C ter is bisecting the Cl–P–Cl angle [100.86(2)°]. Angles be-
atoms from the l.s. plane (l.s. = least-squares)], whereas the tween the l.s. plane and the P–Cl bonds are quite similar
atoms P1, N1, and N2 are lying 0.32(1), –0.81(1), and [49.71(2) and 51.16(2)°]. Each of the molecules has almost
–0.15(1) Å above/below the l.s. plane through the C atoms, C2h symmetry, and the PCl2 groups are thus arranged in an
respectively. Consequently, the molecules show significant alternating fashion. The structure shows P–Cl distances of
deviations from C2h symmetry presumably due to steric 2.0529(4) and 2.0543(4) Å, which is in the normal range for
and/or electronic interaction of the nitrogen atoms with the a single bond between these elements. The packing shows
central aryl moiety. Examples for the latter are known from short intermolecular contacts F1···C3 of 2.964(1) Å, which
the literature.[27] The P–C bond is rather long [C1–P1 is 0.21 Å shorter than the sum of the van der Waals radii.
1.867(2) Å] compared to undistorted P–C bonds. Out of the
eight structurally characterized bis[(dialkylamino)aryl]-
phosphanes reported so far, also 2,3,5,6-F5C6P(NiPr2)2
shows an elongated P–C bond [1.896(5) Å], which is even
longer than in 1.[28] Similar to 1, the only other structurally
characterized P,PЈ-bis(dialkylamino)-substituted p-phenyl-
enebis(phosphane), i.e. 1,4-bis[(diethylamino)phosphanyl]-
2,5-dimethylbenzene,[29] shows an out-of-plane angle of the
P–C bonds of 5.2(1)°, which is only half of that observed
for 1 [10.0(1)°]. In addition, the P–C bond [P1–C1
1.842(1) Å] is longer than average, yet shorter than in 1.
Figure 2. ORTEP drawing of 2. Ellipsoids are drawn at 50% prob-
ability level.
Interestingly, 1 is stable under ambient conditions towards
moisture and air over weeks.
Conclusions
We report the preparation of a 2,3,5,6-tetrafluoro-p-
phenylenebis(dichlorophosphane) in good overall yield
from cheap starting materials in a straightforward reaction
protocol. This molecule is likely to become a key compound
in the synthesis of polymeric low-coordinated phosphanes
incorporating strongly electron-withdrawing aryl linkers.
Further experimental and theoretical work will be needed
for a better understanding of the bonding situation and the
resulting changes in the electronic properties of perfluori-
nated arylphosphanes.
Figure 1. ORTEP drawing of 1. Thermal ellipsoids are drawn at
Experimental Section
50% probability level. Hydrogen atoms are omitted for clarity.
General: Reactions were carried out under argon by using modified
Schlenk techniques or in a glovebox. Solvents were degassed and
requires prolonged reaction times. After complete reaction, dried by using a PureSolv solvent purification system. nBuLi and
Acidic cleavage with HCl(g) of the diethylamino groups
diethylamine were purchased from Sigma Aldrich and used as re-
ceived. PCl3 (from Sigma Aldrich) was distilled prior to use.
1,2,4,5-tetrafluorobenzene was obtained from ABCR and used as
received. ClP(NEt2)2 was prepared according to a modified pro-
cedure by King et al.[30] Lithiation of the tetrafluorobenzene was
carried out according to a modified procedure by Harper et al.[31]
NMR spectra were recorded with a Varian Unity Inova 400 and a
Bruker Avance 300, operating at 1H frequencies of 399.983 MHz
and 300.132 MHz, respectively. 19F NMR spectra for 1 were re-
corded with a Bruker Avance 500 spectrometer (470.740 MHz for
the ammonium salt is removed by filtration, and the spec-
troscopically pure product is obtained after removal of the
solvent. The coupling patterns in the NMR spectra of 2 are
characterized by higher-order spin systems for which in the
simplest case an AAЈX2XЈ2 spin system can be expected by
assuming unhindered rotation of the phosphane units. The
AX and AXЈ coupling constants have been estimated to 59
and 12 Hz, respectively. Owing to hindered rotation around
the P–C bonds at room temperature, the molecule shows in
fact a lower symmetry and a more complex spin system. 19F). Chemical shifts are given in ppm, referenced externally to
Eur. J. Inorg. Chem. 2010, 34–37
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
35

A. Orthaber, F. Belaj, J. H. Albering, R. Pietschnig
SHORT COMMUNICATION
TMS, TMS, H3PO4 (85%), and CF3C6H5 for 1H, 13C, 31P, and
19F, respectively. Mass spectrometry was performed with an Agilent
5975C mass spectrometer equipped with a direct injection unit (DI-
EI).
IϾ2σ(I) and wR2 = 0.0551 for all 2006 unique reflections, GOF
= 1.087. CCDC-749006 (for 1) and -749007 (for 2) contain 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.
Preparation of 1: CAUTION: The dilithio intermediate in the prep-
aration of 1 is highly reactive and prone to LiF eliminination at
temperatures above –50 °C under the formation of very reactive
aryne species! To a cooled solution of 1,2,4,5-tetrafluorobenzene
(7.61 g, 50.7 mmol) in thf (ca. 50 mL) was added nBuLi (hexane
solution, 1.6 , 70 mL, 112 mmol) over a period of 1 h by main-
taining the temperature strictly below –75 °C. The suspension was
stirred at this temperature for 3 h, until chlorobis(diethylamino)-
phosphane (24.23 g, 115 mmol) [dissolved in thf (50 mL)] was
slowly added and the mixture warmed to –50 °C and stirred at this
temperature for another 3 h. The suspension was allowed to reach
room temp. overnight. After filtration, the solvent was removed in
vacuo to yield the crude product as a viscous oil. Crystallization
from 1,2-dimethoxyethane (dme) at –80 °C yielded a white solid
product (15.16 g). Recrystallization from pentane/dme gave crystals
suitable for X-ray diffraction. Yield 60% (15.16 g, 30.4 mmol).
C22H40F4N4P2 (498.52): calcd. C 53.00, H 8.09; found C 52.65, H
8.45. 1H NMR (400 MHz, CDCl3, 25 °C): δ = 3.05 (m, 8 H, CH2),
Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic details and selected bond lengths for 1 and
2.
Acknowledgments
Financial support by the Austrian Science Fund (FWF) (Grants
P18591-B03 and P20575-N19) and the EU-COST Action CM0802
“PhoSciNet” is gratefully acknowledged.
[1] M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, T. Higuchi, J.
Am. Chem. Soc. 1981, 103, 4587–4589.
[2] E. Niecke, W. Flick, Angew. Chem. Int. Ed. Engl. 1973, 12, 585–
586.
[3] S. Shah, J. D. Protasiewicz, Chem. Commun. 1998, 1585–1586.
[4] S. Shah, T. Concolino, A. L. Rheingold, J. D. Protasiewicz, In-
org. Chem. 2000, 39, 3860–3867.
[5] P. J. Fagan, W. A. Nugent, J. Am. Chem. Soc. 1988, 110, 2310–
2312.
3
1.02 (t, JH,H = 7.1 Hz, 12 H, CH3) ppm. 13C NMR (75.5 MHz,
3
CDCl3, 25 °C): δ = 14.73 (d, JP,C = 1.1 Hz, 4 C, CH2), 44.17 (d,
[6] E. M. Evleth, L. D. Freeman, R. I. Wagner, J. Org. Chem. 1962,
2JP,C = 19.1 Hz, 4 C, CH2), 121.05 (m, 2 C, Caryl-P), 145.40 (dm,
1JC,F = 252.6 Hz, 4 C, Caryl-F) ppm. 31P NMR (121.5 MHz,
CDCl3, 25 °C): δ = 84.9 (br. m) ppm. 19F NMR (CDCl3,
470.7 MHz, 25 °C): δ = –141.6 (br. m) ppm. M.p. 92 °C (from
dme). MS (DI-EI): m/z (%) = 426.2 (100) [M – NEt2], 104.1 (95)
[PNEt], 175.1 (72) [PNEt2], 498.4 (35) [M+], 355.1 (21) [M –
(NEt2)2].
27, 2192–2197.
[7] V. A. Wright, D. P. Gates, Angew. Chem. Int. Ed. 2002, 41,
2389–2392.
[8] M. Yam, C.-W. Tsang, D. P. Gates, Inorg. Chem. 2004, 43,
3719–3723.
[9] J. D. Watts, D. J. Watts, M.-J. Huang, J. Phys. Chem. A 2009,
113, 1886–1891.
[10] K. B. Dillon, H. P. Goodwin, J. Organomet. Chem. 1994, 469,
Preparation of 2: Solid 1 (707 mg, 1.4 mmol) was dissolved in di-
ethyl ether (150 mL) and the mixture cooled with an ice bath. An-
hydrous HCl(g) was bubbled through the solution for 1 h. After
filtration and removal of the solvent, the product was obtained as
a white crystalline material. Recrystallization from cold (–10 °C)
CDCl3 afforded crystals suitable for X-ray diffraction. Yield 75%
(374 mg, 1.1 mmol). A second crop [10% (49 mg, 0.1 mmol)] was
obtained by washing the ammonium salt with diethyl ether
(20 mL). C6Cl4F4P2 (351.82): calcd. C 20.48, H 0.00; found C
20.19, H 0.25. 13C NMR (75.5 MHz, CDCl3, 25 °C): δ = 123.58
125–128.
[11] C. Moser, M. Nieger, R. Pietschnig, Organometallics 2006, 25,
2667–2672.
[12] R. Pietschnig, E. Niecke, Organometallics 1996, 15, 891–893.
[13] R. Pietschnig, E. Niecke, M. Nieger, K. Airola, J. Organomet.
Chem. 1997, 529, 127–133.
[14] W. W. Schoeller, T. Busch, E. Niecke, Chem. Ber. 1990, 123,
1653–1654.
[15] K. Miqueu, J.-M. Sotiropoulos, G. Pfister-Guillouzo, V. L.
Rudzevich, H. Gornitzka, V. Lavallo, V. D. Romanenko, Eur.
J. Inorg. Chem. 2004, 2289–2300.
1
2
1
[16] K. B. Dillon, H. P. Goodwin, J. Organomet. Chem. 1992, 429,
(dt, JP,C = 84.5, JC,F = 16.8 Hz), 146.56 (dm, JC,F = 262.2 Hz)
ppm. 31P NMR (121.5 MHz, CDCl3, 25 °C): δ = 133.8 (higher-
169–171.
[17] M. Scholz, H. W. Roesky, D. Stalke, K. Keller, F. T. Edelmann,
J. Organomet. Chem. 1989, 366, 73–85.
[18] T. L. Allen, A. C. Scheiner, Y. Yamaguchi, H. F. Schaefer III,
J. Am. Chem. Soc. 1986, 108, 7579–7588.
[19] H.-L. Peng, J. L. Payton, J. D. Protasiewicz, M. C. Simpson, J.
Phys. Chem. A 2009, 113, 7054–7063.
[20] Y. Ren, Y. Dienes, S. Hettel, M. Parvez, B. Hoge, T.
Baumgartner, Organometallics 2009, 28, 734–740.
[21] S. Sasaki, Y. Tanabe, M. Yoshifuji, Bull. Chem. Soc. Jpn. 1999,
72, 563–572.
[22] H. G. Horn, H. J. Lindner, Chem. Ztg. 1988, 112, 195–200.
[23] L. I. Goryunov, J. Grobe, V. D. Shteingarts, B. Krebs, A. Linde-
mann, E.-U. Würthwein, C. Mück-Lichtenfeld, Chem. Eur. J.
2000, 6, 4612–4622.
[24] L. I. Goryunov, V. D. Shteingartz, J. Grobe, Russ. J. Org.
Chem. 2004, 40, 757–758.
[25] R. Weiss, R. May, B. Pomrehn, Angew. Chem. Int. Ed. Engl.
1996, 35, 1232–1234.
[26] A. Orthaber, F. Belaj, R. Pietschnig, J. Organomet. Chem.,
DOI: 10.1016/j.jorganchem.2009.10.025.
order “tt”, JP,F = 59, JP,F = 12 Hz) ppm. 19F NMR (CDCl3,
3
4
3
282.4 MHz, 25 °C): δ = –129.4 (higher-order “dd”, JP,F = 59.3,
4JP,F = 12.5 Hz) ppm. M.p. 85 °C (from CDCl3). MS (DI-EI): m/z
(%) = 351.8 (100) [M, Cl4 pattern], 314.9 (61) [M – Cl, Cl3 pattern],
100.9 (36) [PCl2, Cl2 pattern], 210.9 (33) [M – 4 Cl], 248.9 (6) [M –
PCl2, Cl2 pattern].
X-ray Crystallography: The structures were solved by direct meth-
ods using SHELXS and refined with SHELXL. For 2, absorption
of twinned crystals was treated by using TWINABS correction.
Crystal data for 1 (pentane/dme): C22H40F4N4P2, M = 498.52, mo-
noclinic space group P21/c, a = 10.401(3), b = 9.786(3), c =
13.144(6) Å, β = 102.67(4)°, V = 1305.3(8) Å3, Z = 2, dc
=
1.268 gcm–3 µ = 0.212 mm–1, T = 95 K, R1 = 0.0498 for 2184 ob-
served reflections with IϾ2σ(I) and wR2 = 0.1239 for all 2835
unique reflections, GOF = 1.031. Crystal data for 2 (CDCl3):
C6Cl4F4P2, M = 351.80, monoclinic space group P21/c, a =
8.2844(8), b = 10.3702(10), c = 6.4688(6) Å, β = 90.690(3)°, V =
555.70(9) Å3, Z = 2, dc = 2.102 Mg/m3, µ = 1.370 mm–1, T =
101(2) K, R1 = 0.0209 for all 1877 observed reflections with
[27] R. Weiss, F. G. Pühlhofer, J. Am. Chem. Soc. 2007, 129, 547–
553.
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Eur. J. Inorg. Chem. 2010, 34–37

2,3,5,6-Tetrafluoro-p-phenylenebis(phosphanes)
[28] P. W. Dyer, J. Fawcett, M. J. Hanton, R. D. W. Kemmitt, R. [31]
Padda, N. Singh, Dalton Trans. 2003, 104–113.
[29] S. D. N. Stephan, S. A. Reiter, S. D. Nogai, H. Schmidbaur, Z.
Naturforsch., Teil B 2005, 60, 511–519.
[30] R. B. King, P. M. Sundaram, J. Org. Chem. 1984, 49, 1784–
1789.
R. J. Harper, E. J. Soloski, C. Tamborski, J. Org. Chem. 1964,
29, 2385–2389.
Received: October 27, 2009
Published Online: November 24, 2009
Eur. J. Inorg. Chem. 2010, 34–37
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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