The binary Ph2PCl/GaCl3 system: A room-temperature molten medium for P-P bond formation

Article abstract of DOI:10.1002/ejic.200800733

An equimolar reaction mixture of Ph₂PCl and GaCl₃ at room temperature results in the formation of a melt M consisting of the donor-acceptor complex Ph₂PCl→GaCl₃ (1a), the chloro(diphenylphosphanyl)diphenylphosphonium cation (2a), and the counteranions [Ga_nCl_3n+1]- (n = 1, 2, 3). The melt has been characterized by Raman and NMR spectroscopy and is compared with the reaction mixture of Ph₂PCl and GaCl₃ observed in solution (CH ₂Cl₂). The melt provides a facile and reactive source of the diphenylphosphenium cation, as demonstrated with use for P-P bond insertion into (PhP)₅, to give the 2,3,4,5-tetraphosphanyl-1,4-diphosphonium dication (3), and reductive coupling with gallium metal to give the diadduct Cl₃GaPPh₂PPh2GaCl₃ (5). Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800733

SHORT COMMUNICATION
DOI: 10.1002/ejic.200800733
The Binary Ph2PCl/GaCl3 System: A Room-Temperature Molten Medium
for P–P Bond Formation
Jan J. Weigand,*[a] Neil Burford,[b] and Andreas Decken[c]
Dedicated to Professor Heinrich Nöth on the occasion of his 80th birthday
Keywords: Melt reaction / Phosphorus cations / Structure elucidation / Coordination complexes
An equimolar reaction mixture of Ph2PCl and GaCl3 at room
temperature results in the formation of a melt M consisting
of the donor–acceptor complex Ph2PClǞGaCl3 (1a), the chlo-
ro(diphenylphosphanyl)diphenylphosphonium cation (2a),
and the counteranions [GanCl3n+1]– (n = 1, 2, 3). The melt has
been characterized by Raman and NMR spectroscopy and
is compared with the reaction mixture of Ph2PCl and GaCl3
observed in solution (CH2Cl2). The melt provides a facile and
reactive source of the diphenylphosphenium cation, as dem-
onstrated with use for P–P bond insertion into (PhP)5, to give
the 2,3,4,5-tetraphosphanyl-1,4-diphosphonium dication (3),
and reductive coupling with gallium metal to give the diad-
duct Cl3GaPPh2PPh2GaCl3 (5).
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
taining the new cyclotetraphosphanyldiphosphonium dicat-
ion 3.[3] The melt medium provides a potentially versatile
approach to a wide range of new phosphanylphosphonium
salts. This melt is observed to react with elemental gallium
which affects a new reductive coupling P–P bond-forming
reaction. Herein we describe the characterization of the
Ph2(Cl)P/GaCl3 system using Raman and NMR spec-
troscopy.
Introduction
The reaction between halophosphanes and ECl3 (E = Al,
Ga) Lewis acids in organic solvents has been extensively
investigated.[1a–1f] Whereas the expected E–P Lewis acid–
base complex 1 is observed, other structural alternatives are
also formed,[1f] including coordination complexes of the
phosphenium cation, also described as phosphanylphos-
phonium cations 2, which contain P–P bonds.[2a–2c] We have
exploited the reactivity of phosphanylphosphonium cations
to develop new catena-phosphorus cations;[2c] however, a
more diverse chemistry is becoming apparent for the
Ph2(Cl)P/GaCl3 system in the absence of solvent, which is
a molten mixture at room temperature.
Results and Discussion
Room-temperature molten salts (RTMS), also known as
ionic liquids, have been media of interest for a number of
years. The equimolar mixture of Ph2PCl and GaCl3 at room
temp. (Scheme 1, I) gives an analogous homogeneous liquid
(melt) that has some features in common with RTMS.[4]
The mixture is essentially identical to that resulting from
an equimolar mixture of solid GaCl3 and solid 2a[GaCl4]
(Scheme 1, II) by stirring at room temp. over 20 min (Fig-
ure 1).
The melt has been characterized by 31P{1H} NMR (Fig-
ure 2) and Raman spectroscopy (Figure 3) as a mixture (M)
of the Lewis acid–base complex Ph2(Cl)P–GaCl3 (1a) and
the gallate, digallate, and trigallate salts of the chloro(di-
phenylphosphanyl)diphenylphosphonium cation (2a). The
room-temperature melt M is indefinitely stable but is ex-
tremely moisture-sensitive.
The Raman spectra of 2a[GaCl4] and of the melt M are
essentially identical in the region from 3200 to 500 cm–1 and
show only bands that belong to the phosphanylphosphon-
A heated mixture of 2 Ph2PCl, 2 GaCl3, and 4/5 (PhP)5
results in the quantitative formation of [Ph8P6][GaCl4]2 con-
[a] Institut für Anorganische und Analytische Chemie, Westfäli-
sche Wilhelms-Universität Münster,
Corrensstrasse 28/30, 48149 Münster, Germany
Fax: +49-251-83-36074
E-mail: jweigand@uni-muenster.de
[b] Department of Chemistry, Dalhousie University,
Halifax, NS, B3H 4J3, Canada
[c] Department of Chemistry, University of New Brunswick,
Fredericton, NB, E3A 6E2, Canada
Eur. J. Inorg. Chem. 2008, 4343–4347
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4343

J. J. Weigand, N. Burford, A. Decken
SHORT COMMUNICATION
Figure 1. Formation of the melt composition M.
Scheme 1. Reactions of Ph2PCl or phosphanylphosphonium cation
2a with GaCl3 and subsequent reactions of the molten mixture with
(PPh)5 or Ga.
Figure 2. The 31P{1H} NMR spectra of the melt M obtained by
the reactions I and II according to Scheme 1.
ium cation 2a. Figure 3 shows the Raman spectra below
500 cm–1 of 2a[GaCl4] and M (I, II) (Table 1). Salt
2a[GaCl4] exhibits a band at 348 cm–1, corresponding to the
A1 vibration of the tetrahedral GaCl4– species, which is very
weak in the spectra of the melts. Other features in the spec-
trum of 2a[GaCl4] at 153 cm–1 and in the region of 379–
390 cm–1 belong to the T2 mode of the anion and are not
evident in the spectra of the melts. In contrast, the Raman
spectra for the melts show weak broad features at 431 cm–1
troscopic proof of the separate existence of Ga3Cl10– are the
broad bands at approximately 360–390 cm–1 and at 120–
–
140 cm–1 as well as the observation of GaCl4 , which is con-
sistent with an equilibrium such as that outlined in Equa-
tion (1).[5]
–
–
–
2 Ga2Cl7 h Ga3Cl10 + GaCl4
(1)
and strong bands at 363 cm–1 and 139 cm–1, assigned to
All assignments are in agreement with experimental data
–
Ga2Cl7 , as well as a broad band at approximately 322 cm–1 from the literature.[5,6] As the heptachlorodigallate and de-
which is assigned to Ga3Cl10–. The most substantial spec- cachlorotrigallate ions, Ga2Cl7– and Ga3Cl10–, are observed
Table 1. Comparison of Raman bands observed in the 2a[GaCl4], M (I), and M (II) systems.[5,6]
2a[GaCl4][a]
M (I)[a]
M (II)[a]
Assignment[b]
–
ca. 430 (w, sh, vbr)
395 (m)
ca. 430 (w, sh, vbr)
394 (m)
Ga2Cl7
Ga3Cl10––, νasym[Ga–(Clt)3]
Ga2Cl7 , νas–ym[Ga–(Clt)3]
GaCl4 , ν3[T2]
385 (m)
385 (m)
[c]
[c]
380vw(sh)
348vs
–
–
–
363 (vs)
363 (vs)
348 (w)
363 (vs)
363 (vs)
348 (w)
Ga2Cl7 , νsym[Ga–(Clt)3]
Ga3Cl10–, νsym[Ga–(Clt)3]
–
GaCl4 , ν1[A1]
–
322 (w, br)
ca. 274 (w, br)
ca. 274 (w, br)
322 (w, br)
ca. 274 (w, br)
ca. 274 (w, br)
Ga3Cl10
–
Ga2Cl7 , ν[Ga–Clb–Ga]
Ga3Cl10–, ν[Ga–Clb–Ga]
[c]
[c]
–
149m
–
–
GaCl4 , ν4[T2]
–
138 (m)
129 (m)
137 (m)
130 (m)
Ga2Cl7 , δ[Ga–(Clt)3]
Ga3Cl10–, δ[Ga–(Clt)3]
[a] Intensity and appearance of bands are indicated by the following abbreviations: s = strong, m = medium, w = weak, v = very, br =
–
–
–
broad, sh = shoulder. [b] Assignments of bands for GaCl4 , Ga2Cl7 , and GanCl3n+1 are based on data from the Ga/GaCl3 system from
ref.[6] [c] Not observed due to overlapping of bands.
4344
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4343–4347

The Binary Ph2PCl/GaCl3 System
the same complex AAЈAЈЈAЈЈЈBBЈ spin system as pre-
viously reported; consistent with the P6 homocycle com-
posed of two phosphonium centers (1,4-positions) and four
phosphane centers.[3]
A further illustration of the synthetic value of the new
melt medium for use in P–P bond formation is demon-
strated by the reaction of the melt M with gallium metal.
Addition of benzene and stoichiometric amounts of gallium
metal to M [Scheme 2 (a)] produces a two-phase liquid sys-
tem. After stirring for 48 h, a white precipitate is formed
that has been isolated and characterized as 5, representing
a donor–acceptor complex of Ph2PPPh2 with two GaCl3
acids. Excess GaCl3 was removed by washing the precipitate
with benzene. Adjustment of the reaction according to
Scheme 2 (b) provides the same product; however, a higher
yield (isolated 94%) is obtained by using excess GaCl3. The
31P{1H} NMR signal for 5 in CD2Cl2 (δ = –2.1 ppm) is
shifted downfield relative to that of Ph2PPPh2 (δ =
–14.4 ppm).[7]
Figure 3. Raman spectra below 500 cm–1 for 2a[GaCl4] and of the
melt M obtained by the reactions I and II according to Scheme 1.
in the molten mixture, M is considered to be an acidic me-
dium.
The 31P{1H} NMR spectra of the melt M at room temp.
(Figure 4a) indicate the presence of the phosphanylphos-
1
phonium salt 2a (two doublets, δiso = 75.5, –0.9 ppm, JPP
= –381.0 Hz) and the complex 1a (δiso = 49.0 ppm). A tem-
perature profile up to 400 K (Figure 4a) results in a broad-
ening of the signals, which is reversible upon cooling, im-
plying an equilibrium process. Introduction of (PhP)5 into
the melt M (Figure 4b) has no influence on the 31P{1H}
NMR spectrum at room temperature as (PhP)5 does not
dissolve in the melt. At slightly elevated temperatures (ca.
320 K), signals corresponding to cation 4 are observed (δiso
≈ 22 and –38 ppm).[7,8] At 340 K and above, signals distinc-
tively characteristic of the hexaphosphorus dication 3 (δiso
Scheme 2. Formation of diphosphanes through a coordination (for-
mation of a donor–acceptor complex) and reduction process.
Compound 5 crystallizes in the orthorhombic space
≈ 12 and –60 ppm)[3] are observed, demonstrating 3 as the group, Pbca, with four formula units in the unit cell. The
thermodynamic sink for this medium.[3] In CD2Cl2, the solid-state structure of 5 is shown in Figure 5, and crystallo-
same reaction mixture quantitatively produces cation 4, graphic details are summarized in the Experimental Sec-
presumably due to a kinetic barrier in solution.[7] From the tion. In the solid state, the P–Pi [2.2400(9) Å; symmetry
cooled NMR sample, rod-like crystals were recovered from code: (i) –x + 2, –y + 1, –z +1] and P–C [1.806(2) and
the clear melt M and were identified as 3. The 31P{1H} 1.814(2) Å] bonds are in the typical range for P–P [2.17–
NMR spectrum of the dissolved crystals in CD2Cl2 showed 2.24 Å] and P–C [1.78–1.83 Å] single bonds.[9] The P–Ga
Figure 4. Variable-temperature 31P{1H} NMR spectra of the melt M without (a) and with the reactant (PhP)5 (b).
Eur. J. Inorg. Chem. 2008, 4343–4347
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4345

J. J. Weigand, N. Burford, A. Decken
SHORT COMMUNICATION
ANCE 500 [1H (500.13 MHz), 13C (125.76 MHz) chemical shift
bond length [2.4285(6) Å] in 5 is typical of phosphane–gal-
lane complexes (2.35–2.68 Å),[10] and is comparable to
those of symmetrically alkylated complexes, such as
referenced to δTMS = 0.00 ppm; 31P (202.46 MHz) to δH3PO4(85%)
=
0.00 ppm]. J values are reported in Hz. Melting points were re-
corded with an electrochemical melting-point apparatus in sealed
capillaries under dinitrogen and are uncorrected. Raman spectra
were obtained for powdered and liquid samples with a Bruker RFS
100 instrument equipped with an Nd:YAG laser (1064 nm,
172 mW, 1000 scans, 90° geometry). Elemental analyses were per-
formed with a Vario EL III CHNS elemental analyzer at the IAAC,
University of Münster, Germany.
Me3PǞGaCl3
[2.353(2) Å][11]
and
Me3PǞGaMe3
[2.455(4) Å].[12] Moreover, the environments for the phos-
phorus centres in 5 are essentially tetrahedral, adopting a
perfectly staggered conformation with the GaCl3 moieties
which are in a trans disposition.
NMR Studies of the Reaction of Ph2PCl or [Ph2P(Cl)PPh2][GaCl4]
(2a[GaCl4]) with GaCl3: In a typical experiment, GaCl3 (5 mmol)
was added to Ph2PCl (5 mmol) or [Ph2P(Cl)PPh2][GaCl4]
(2a[GaCl4]) (5 mmol) with stirring under argon at room temp. In
both cases, pale yellow viscous liquids were obtained within 20 min.
The 31P NMR spectra were recorded for the liquids by using
H3PO4 as external standard. 31P{1H} NMR (300 K): δ = –73.62
1
(d, 1 P), –0.90 (d, JPP = 381.0 Hz, 1 P, AX spin system) ppm.
NMR Studies of the Reaction of Ph2PCl, GaCl3 and (PhP)5: To
1.0 mL of the room-temperature melt medium M was added
(PhP)5 (70 mg) in a glove box. The mixture was transferred to an
NMR tube and sealed. The variable-temperature 31P{1H} NMR
spectra of the melt M and the mixture were recorded from room
temp. up to 400 K (see text).
Preparation of Ph4P2Ga2Cl6 (5): Benzene (5 mL) and Ga
(0.33 mmol, 23 mg) were added to a freshly prepared molten salt
mixture from Ph2PCl (1 mmol, 221 mg) and GaCl3 (1 mmol,
176 mg) under argon. The mixture was stirred for 48 h, and the
white precipitate formed was separated by filtration. The precipi-
tate was washed with benzene (3ϫ5 mL) to remove excess GaCl3
and pentane (2ϫ5 mL), dried under vacuum, and recrystallized
from a minimal amount of CH2Cl2 by pentane diffusion at –32 °C
to yield 6 as very moisture-sensitive X-ray quality crystals; 680
Figure 5. ORTEP plot of the solid-state structure of the diadduct
5. Thermal ellipsoids with 50% probability (hydrogen atoms are
omitted for clarity). Selected bond lengths [Å] and angles [°]: P1–
P1i 2.2400(9), P–Ga 2.4285(6), P–C1 1.806(2), P–C7 1.814(2), C1–
P–C7 109.37(8), C1–P–Pi 104.20(6), C1–P–Ga 111.08(6), C7–P–Ga
109.09(6), Pi–P–Ga 116.43(3); symmetry code: (i) –x + 2, –y + 1,
–z + 1.
1
(450) mg (94%); m.p. 120–126 °C (dec.) (GaCl3 sublimes off). H
Conclusions
NMR (CD2Cl2, 300 K): δ = 7.49–7.59 (m, 8 H, Ph), 7.69–7.79 (m,
1
12 H, Ph) ppm. 13C NMR (CD2Cl2, 300 K): δ = 117.8 (t, JPP
=
A room-temperature melt medium M was readily pre-
pared by the stoichiometric combination of Ph2PCl and
GaCl3, and characterized by Raman and NMR spec-
troscopy which show the presence of complex 1a and cation
3
20.0 Hz, 4 C), 130.7 (t, JPP = 5.3 Hz, 8 C), 134.7 (s, 4 C), 135.9
(t, JPP = 7.8 Hz, 8 C) ppm. 31P{1H} NMR (CD2Cl2, 300 K): δ =
2
–2.08 (br, 1 P) ppm. C24H20Cl6Ga2P2 (722.53): calcd. C 39.90, H
2.49; found C 39.62, H 1.87. The same reaction with 2/3 mmol
(117 mg) of GaCl3 gave 450 mg (62%) of 5.
–
2a with a variety of complex anions (Ga2Cl7 and
Ga3Cl10–). The melt M is an excellent medium for cyclo-
phosphane P–P insertion reactions and a reduction reaction
to give a Lewis acid–base diadduct. Furthermore, this ap-
Crystal Data: A suitable single crystal was coated with Paratone-
N oil, mounted by using a 20-micron cryo-loop and frozen in the
cold nitrogen stream of the goniometer. A hemisphere of data was
proach opens a new route to form diphosphanes through a collected with a Bruker AXS P4/SMART 1000 diffractometer (Mo-
Ka radiation, λ = 0.71073 Å) by using ω and θ scans with a scan
width of 0.3° and 10 s time. The detector distance was 5 cm. The
data were reduced (SAINT 7.23A, Bruker AXS, Inc., Madison,
Wisconsin, USA, 2006) and corrected for absorption (G. Sheldrick,
SADABS 2004, Bruker AXS, Inc., Madison, Wisconsin, USA,
2004). The structures were solved by direct methods and refined by
full-matrix least squares on F2 (G. Sheldrick, SHELXTL 6.14,
Bruker AXS, Inc., Madison, Wisconsin, USA, 2000). All non-hy-
drogen atoms were refined first by using isotropic and later aniso-
tropic thermal parameters for non-hydrogen atoms. Hydrogen
atoms were added to the structure models in calculated position
and refined by using a riding model. Ph4P2Ga2Cl6, FW = 722.48,
orthorhombic, space group Pbca, Z = 4, a = 15.893(3), b =
9.922(2), c = 18.239(4), V = 2876(1), F(000) = 1432, T = 173(1), µ
= 2.556 cm–1, 18575 reflections collected, 3273 reflections unique
(Rint = 0.0322). The final R1 value was 0.0231 and wR2 = 0.0606 (all
data). CCDC-695825 contains the supplementary crystallographic
coordination (formation of a donor–acceptor complex) and
reduction process.
Experimental Section
General Considerations: All reactions were carried out in either a
glove box or by using standard Schlenk techniques under argon.
Dichloromethane, benzene and pentane were dried in an MBraun
solvent purification system, degassed with three freeze-pump-thaw
cycles and stored under nitrogen and over molecular sieves prior
[13]
to use. (PhP)5 and [Ph2PPh2Cl][GaCl4] (2a[GaCl4])[2a] were pre-
pared as previously reported. Ph2PCl was freshly distilled prior to
use. Reagent-grade GaCl3 and Ga (99.999%) were used as received
from commercial suppliers. All new compounds, unless otherwise
stated, were fully characterized by 31P, 1H, and 13C NMR spec-
troscopy. Measurements were performed at 25 °C. Bruker AV-
4346
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 4343–4347

The Binary Ph2PCl/GaCl3 System
Ferguson, J. Am. Chem. Soc. 2003, 125, 14404–14410; c) C. A.
Dyker, N. Burford, Chem. Asian J. 2008, 3, 28–36.
[3] J. J. Weigand, N. Burford, M. Lumsden, A. Decken, Angew.
Chem. Int. Ed. 2006, 45, 6733–6737.
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.
[4] T. Welton, Chem. Rev. 1999, 99, 2071–2083.
[5] S. Ulvenlund, A. Whaetley, L. A. Bengtsson, J. Chem. Soc.,
Dalton Trans. 1995, 2, 245–254.
[6] S. Ulvenlund, L. Bengtsson-Kloo, K. Ståhl, J. Chem. Soc. Far-
aday Trans. 1995, 91, 4223–4234.
[7] N. Burford, C. A. Dyker, A. Decken, Angew. Chem. Int. Ed.
2005, 44, 2364–2367.
Acknowledgments
We thank the Alexander von Humboldt Foundation (Feodor
Lynen Return Fellowship for J. J. W.), the Fonds der Chemischen
Industrie (Liebig fellowship for J. J. W.) and the Natural Sciences
and Engineering Research Council of Canada for funding. J. J. W.
thanks Prof. F. Ekkehardt Hahn (WWU Münster) for his generous
support and advice.
[8] When the temperature of the reaction mixture did not exceed
330 K and the melt was quenched to room temperature within
seconds, 31P{1H} NMR spectra of the resulting yellow oil in
CH2Cl2 showed the presence of only cation 4,[7] the phosphan-
ylphosphonium cation 2a, and small amounts of dication 3.
[9] J. J. Weigand, S. D. Riegel, N. Burford, A. Decken, J. Am.
Chem. Soc. 2007, 129, 7969–7976.
[1] a) E. R. Alton, R. G. Montemayor, R. W. Parry, Inorg. Chem.
1974, 13, 2267–2270; b) C. W. Schultz, R. W. Parry, Inorg.
Chem. 1976, 15, 3046–3050; c) R. W. Kopp, A. C. Bond, R. W.
Parry, Inorg. Chem. 1976, 15, 3042–3046; d) M. G. Thomas,
C. W. Schultz, R. W. Parry, Inorg. Chem. 1977, 16, 994–1001;
e) F. Sh. Shagvaleev, T. V. Zykova, R. I. Tarasova, T. Sh. Sitdi-
kova, V. V. Moskva, Zh. Obshch. Khim. 1990, 60, 1775–1779;
f) N. Burford, T. S. Cameron, D. J. LeBlanc, P. Losier, S.
Sereda, G. Wu, Organometallics 1997, 16, 4712–4717.
[10] N. C. Norman, N. L. Pickett, Coord. Chem. Rev. 1995, 145, 27–
54.
[11] J. C. Carter, G. Jigie, R. Enjalbert, J. Galy, Inorg. Chem. 1978,
17, 1248–1254.
[12] J. A. Burns, W. T. Pennington, G. H. Robinsonm, Organometal-
lics 1995, 14, 1533–1535.
[13] W. A. Henderson, M. Epstein, F. S. Seichter, J. Am. Chem. Soc.
1963, 85, 2462–2466.
[2] a) N. Burford, P. J. Ragogna, J. Chem. Soc., Dalton Trans. 2002,
4307–4315; b) N. Burford, P. J. Ragogna, R. McDonald, M.
Received: July 24, 2008
Published Online: September 2, 2008
Eur. J. Inorg. Chem. 2008, 4343–4347
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4347