Structural dependence of the reagent Ph3PCl2 on the nature of the solvent, both in the solid state and in solution; X-ray crystal structure of trigonal bipyramidal Ph3PCl2, the first structurally characterised five-coordinate R3PCl2 compound

Article abstract of DOI:10.1039/a800820e

The very delicate structural balance of Ph₃PCl₂ when prepared in diethyl ether solution is illustrated by its X-ray crystallographic study; unlike the ionic species, [Ph₃PCl⁺...Cl^-... ⁺ClPPh₃]Cl, which prevails in dichloromethane solution, the non-solvated molecular species Ph₃PCl₂ is formed in diethyl ether which is the first example of a trigonal bipyramidal R₃PCl₂ compound to be structurally characterised, and this may have an effect on the chlorinating ability of the reagent.

Full text of DOI:10.1039/a800820e

View Article Online / Journal Homepage / Table of Contents for this issue
Structural dependence of the reagent Ph3PCl2 on the nature of the solvent,
both in the solid state and in solution; X-ray crystal structure of trigonal
bipyramidal Ph3PCl2, the first structurally characterised five-coordinate
R3PCl2 compound
Stephen M. Godfrey,† Charles A. McAuliffe, Robin G. Pritchard and Joanne M. Sheffield
Department of Chemistry, University of Manchester Institute of Science and Technology, Manchester, UK M60 1QD
The very delicate structural balance of Ph3PCl2 when
prepared in diethyl ether solution is illustrated by its X-ray
crystallographic study; unlike the ionic species,
[Ph3PCl+···Cl2···+ClPPh3]Cl, which prevails in dichlorome-
thane solution, the non-solvated molecular species Ph3PCl2
is formed in diethyl ether which is the first example of a
trigonal bipyramidal R3PCl2 compound to be structurally
characterised, and this may have an effect on the chlorinat-
ing ability of the reagent.
It therefore occurred to us that a different structural
modification of Ph3PCl2 could be exhibited in solvents of low
relative permittivity (low polarity). One reason for this was the
fact that [Ph3PCl···Cl···ClPPh3]Cl·2CH2Cl2 contains a dichloro-
methane solvent molecule in the structure,24 suggesting non-
innocent behaviour, and although no bonding interactions
between the solvent and the compound are observed, there
nevertheless remains the fact that d+ hydrogens on the
dichloromethane point towards the Cl2 ions giving a suspicion
of long range electrostatic interaction, and, therefore, an implied
influence on the structure adopted. Considering the widespread
use of Ph3PCl2 as a chlorinating agent, the structural nature of
the reagent is of great importance since the structure may
influence its efficacy as a chlorinating agent and, possibly, the
mechanism of chlorination. Consequently, the choice of solvent
employed for a given chlorination reaction utilising Ph3PCl2
may be of fundamental importance.
Although there are a number of previous reports concerning
compounds of stoichiometry R3PCl2, which are mainly focused
on Ph3PCl2, they are predominantly concerned with the nature
of such species in solution, using 31P NMR spectroscopy.1–9 All
such studies, performed in acetonitrile or dichloromethane
solution, concluded that the compounds are ionic, [R3PCl]Cl.
Similarly, conductivity studies of Ph3PCl2 in acetonitrile
solution led Harris and coworkers10,11 to conclude that the
compound is ionic, [Ph3PCl]Cl, since values close to those
expected for a 1:1 electrolyte were recorded. However,
Arzoumandis13 investigated the structural nature of Ph3PCl2 in
haloform solvents and concluded, from cryostatic and vibra-
tional spectroscopic studies, that 1:1 Ph3PCl2·YCX3 (Y = H,
D; X = Cl, Br) adducts were formed. It was reasoned that these
species were molecular dimeric entities which contain six-
coordinate phosphorus atoms.
Triphenylphosphine dichloride was prepared by us from the
direct reaction of triphenylphosphine with dichlorine in diethyl
ether in a 1:1 stoichiometric ratio [eqn. (1)].
PPh3 + Cl2 room temp.→ Ph3PCl2
(1)
N2,Et2O
The resultant white powder, which formed almost imme-
diately upon addition of the dichlorine, was recrystallised from
diethyl ether solution to produce a large quantity of colourless
crystals on standing at room temperature for ca. 1 week. The
melting point of the crystals was determined to be 118–119 °C
{cf. 160–161 °C for [Ph3PCl···Cl···ClPPh3]Cl·2CH2Cl2}. A
crystal was chosen for analysis by single crystal X-ray
diffraction. Interestingly, the structure‡ of Ph3PCl2 is shown to
be the sole example of a molecular trigonal bipyramidal R3PCl2
compound, Fig. 1, and not the ionic structure,
[Ph3PCl···Cl···ClPPh3]Cl·2CH2Cl2 which prevails in dichloro-
methane solution. This result is important for two reasons:
firstly, the very delicate balance between ionic and covalent
forms for Ph3PCl2 is clearly illustrated, in the more polar
CH2Cl2 an ionic structure is adopted whereas in diethyl ether a
molecular form is revealed. Secondly, the acute solvent
dependency of the structure of this reagent is clearly shown, and
other workers utilising the reagent for chlorination reactions
may find different rates and/or products which are dependent
solely on the polarity of the solvent chosen. The structure of
Ph3PCl2 contains two crystallographically independent mole-
cules in the asymmetric unit which exhibit quite different P–Cl
bond lengths. In one molecule, d(P–Cl) are quite similar, being
2.252(2) and 2.262(2) Å, however differences are observed in
the other, 2.280(2) and 2.225(1) Å. Both molecules also exhibit
slight distortions from regular trigonal bipyramidal geometry.
These distortions and the asymmetry of the d(P–Cl) may arise
from the ease of ionisation of the molecule. However,
asymmetric bonds in multiple halide systems are not un-
Studies concerning the solid-state structure of R3PCl2
compounds are rare, and studies concerning R3PCl2 (R = Me,
Ph) again concluded an ionic structure, [R3PCl]Cl.13–15
There is renewed interest in the structural nature of
compounds of stoichiometry, R3EX2 (E = P, As, Sb). We have
established the solid-state molecular charge transfer ‘spoke’
structure for Ph3PX2, viz. Ph3P–X–X, (X2 = Br2,16 I2,17–19
IBr20), whereas other workers have shown that Ph3PF2 is
trigonal bipyramidal21,22 A Raman spectroscopic study23 of
Ph3PCl2 prepared in toluene solution suggested that two
structural modifications could exist, an ionic form, [Ph3PCl]Cl,
prepared by bubbling dichlorine gas through a toluene solution
of PPh3 and, possibly, a trigonal bipyramidal form, prepared by
passing a stream of dichlorine over the surface of a toluene
solution of PPh3.
Until very recently, no single crystal X-ray crystallographic
data was available for any compound of stoichiometry R3PCl2.
However, a crystallographic study24 of the compound prepared
from the reaction of PPh3 and dichlorine in dichloromethane
solution revealed an unusual dinuclear ionic compound,
[Ph3PCl···Cl···ClPPh3]Cl·CH2Cl2. The long Cl···Cl contacts are
3.279(6) Å (van der Waals radius for dichlorine is 3.6 Å). The
solution 31P{H} NMR of this species, recorded in CDCl3 and
CD3CN gave single resonances at d 65.5 and 66.5, respectively,
these values being very similar to the values quoted by previous
workers and indicating that the simple ionic species [Ph3PCl]Cl
prevails in solution for these solvents, and the long range Cl···Cl
contacts are broken, as expected.
Chem. Commun., 1998
921

View Article Online
at d 65.5 is observed and the former resonance at d 247.0
disappears.
C(11)
C(7)
C(15)
C(16)
In conclusion, the solvent of preparation is critical in
determining the structure of Ph3PCl2. A molecular form persists
in solvents of low polarity which is converted into an ionic form
in solvents of higher polarity. Which structure is adopted will
almost certainly have an effect on the chlorinating ability of the
reagent, and, possibly, the nature of any products formed.
We are grateful to the EPSRC for a research studentship (to
J. M. S.).
C(12)
Cl(2)
C(10)
C(14)
C(13)
C(21)
C(17)
P(1)
C(9)
C(22)
C(23)
C(18)
C(20)
C(19)
C(8)
Cl(4)
C(26)
Cl(1)
C(27)
C(28)
C(29)
C(1)
C(24)
P(2)
C(25)
C(30)
C(6)
C(2)
Notes and References
C(31)
Cl(3)
C(32)
† E-mail: Stephen.M.Godfrey@umist.ac.uk
C(5)
C(3)
¯
‡ Crystal data: Triclinic, space group P1 (no. 2) a
=
9.408(2),
C(36)
C(4)
b
=
11.579(6), c
=
16.203(2) Å, a
=
95.23(3), b
=
103.09(2),
C(33)
C(34)
g = 111.10(3)°, U = 1574.7(1) Å3, Z = 4, Dc = 1.405 g cm23, m = 5.04
cm21, F(000) = 688. The structure analysis is based on 5280 reflections
C(35)
(Mo-Ka 2qmax
= 49.9), 5222 observed [I > 2s(I)], 379 parameters
Fig. 1 X-Ray crystal structure of trigonal bipyramidal Ph3PCl2 (two
crystallographically independent molecules are present in the asymmetric
unit). Selected bond lengths (Å) and angles (°): P(1)–Cl(1) 2.280(2),
P(1)–Cl(2) 2.225(1), P(2)–Cl(3) 2.262(2), P(2)–Cl(4) 2.252(2), C(7)–P(1)–
C(1) 123.5(2), C(7)–P(1)–C(13) 118.2(2), C(1)–P(1)–C(13) 118.2(2)
Cl(2)1–P(1)–Cl(1) 176.09(6), C(19)–C(25) 120.0(2), C(19)–P(1)–C(31)
123.6(2), C(25)–P(2)–C(31) 116.42, CI(4)–P(2)–Cl(3) 176.20(6).
Absorption correction (min., max. transmission 0.81, 1.00). The structure
was solved by direct methods and refined by full-matrix least squares. Final
residual R1 = 0.0762, wR2 = 0.2137. Final residuals (all data) R1 = 0.0941,
wR2 = 0.2681. CCDC 182/773.
1 G. A. Wiley and W. R. Stine, Tetrahedron, Lett., 1967, 2321.
2 D. B. Denney, D. Z. Denney and B. C. Chang, J. Chem. Soc., Dalton
Trans., 1976, 1243.
3 K. B. Dillon, R. J. Lynch, R. N. Reeve and T. C. Waddington, J. Chem.
Soc., Dalton Trans., 1976, 1243.
4 C. Brown, M. Murrany and R. Schmutzler, J. Chem. Soc. C, 1970,
876.
5 E. L. Muetterties and W. Mahler, Inorg. Chem., 1965, 4, 119.
6 E. G. Schnell and E. G. Rochow, J. Am. Chem. Soc., 1956, 78, 1084.
7 F. Seel, K. Rudolph and R. Budenz, Z. Anorg. Allg. Chem., 1965, 341,
196.
8 R. Bartsh, O. Stelzer and R. Schmuttzler, Z. Naturforsch., Teil B, 1981,
36, 1349.
9 R. Appel and H. Scholer, Chem. Ber., 1977, 110, 2382.
10 A. D. Beveridge and G. S. Harris, J. Chem. Soc., 1966, 520.
11 G. S. Harris and M. F. Ali, Tetrahedron Lett., 1968, 37.
12 G. G. Arzoumandis, Chem. Commun., 1969, 520.
13 K. B. Dillon and T. C. Waddington, Spectrochim. Acta, Part A, 1971,
27, 2381.
common. The classic example is I32, where asymmetric I–I
bonds are ascribed to the influence of surrounding molecules or
ions. Our recent work has shown even more dramatic examples
of autosolvation in R3PCl2 systems.25 In the case of Ph3PCl2
described here, comparison of the four Cl environments shows
that the closest approaches between Cl and H in adjacent
molecules are 2.92, 3.05, 2.83 and 2.82 for Cl(1), Cl(2), Cl(3)
and Cl(4), respectively. As P–Cl(4) is the shortest P–Cl bond, it
would appear that phenyl rings are exerting their influence on
d(P–Cl). We have previously observed this phenomenon with
interaction of Cl with d+ hydrogens on propyl chains.25 Caution
must be exercised when discussing E–X (E = P, As, Sb, Bi;
X = Br, Cl) bond lengths however, since considerable
asymmetry has already been illustrated from crystallographic
14 A. Finch, P. N. Gates and A. S. Muir, J. Raman Spectrosc., 1988, 19,
91.
26
studies e.g. Ph3BiCl2 [d(Bi–Cl) 2.529–2.615 Å], [Me3CH-
CH2]3AsBr2,27 [d(As–Br) 2.530–2.596 Å], Ph3SbCl2 [d(Sb–Cl)
2.382–2.490 Å].28
15 J. Goubeau and R. Baumgartner, Z. Electrochem., 1960, 64, 598.
16 N. Bricklebank, S. M. Godfrey, A. G. Mackie, C. A. McAuliffe and
R. G. Pritchard, J. Chem. Soc., Chem. Commun., 1992, 355.
17 S. M. Godfrey, D. J. Kelly, A. G. Mackie, C. A. McAuliffe,
R. G. Pritchard and S. M. Watson, J. Chem. Soc., Chem. Commun.,
1991, 1163.
18 N. Bricklebank, S. M. Godfrey, A. G. Mackie, C. A. McAuliffe,
R. G. Pritchard and P. J. Kobryn, J. Chem. Soc., Dalton Trans, 1993,
101.
19 N. Bricklebank, S. M. Godfrey, H. P. Lane, C. A. McAuliffe,
R. G. Pritchard and J. M. Moreno, J. Chem. Soc., Dalton Trans., 1995,
2421.
20 N. Bricklebank, S. M. Godfrey, C. A. McAuliffe and R. G. Pritchard,
J. Chem. Soc., Dalton Trans., 1993, 2261.
21 F. Weller, D. Nuszhar, K. Dehncke, F. Gingl and J. Strahle, Z. Anorg.
Allg. Chem., 1991, 602, 7.
22 K. M. Doxsee, E. M. Hannawait and T. J. R. Weakley, Acta
Crystallogr., Sect. C, 1992, 48, 1288.
23 M. A. H. A. Al-Juboori, P. N. Gates and A. S. Muir, J. Chem. Soc.,
Chem. Commun., 1991, 1270.
24 S. M. Godfrey, C. A. McAuliffe, R. G. Pritchard and J. M. Sheffield,
Chem. Commun., 1996, 2521.
The solution structure of Ph3PCl2 in low-polarity solvents is
also of importance since the trigonal bipyramidal structure of
Ph3PCl2 could simply be a solid-state phenomenon, i.e. the
molecule could auto-ionise in any given solvent, which has
already been illustrated25 when Ph3PCl2 is dissolved in CH2Cl2.
Deuterated ether, C2D6O is not really available; however, we
dissolved a sample of crystalline Ph3PCl2 prepared in Et2O in
deuterated benzene, C6D6, i.e. a non-polar solvent. A single
resonance was observed in the NMR spectrum at d 247, very
different
to
that
observed
for
the
ionic
[Ph3PCl···Cl···ClPPh3]Cl·CH2Cl2 which exhibited resonances
at d 65.5 or 66.5 (recorded in CH2Cl2 and CH3CN, re-
spectively). This value of d 247 is also completely different to
any previously reported value for a sample of Ph3PCl2 which,
prior to this work, has only been studied by 31P{H} NMR
spectroscopy in solvents of quite high polarity. This value of
d 247 is however comparable to analogous difluorophosphor-
anes, R3PF2, which are known to retain a molecular five-
coordinate geometry in solution, e.g. MePh2PF2 (d 243.2) and
Ph3PF2 (d 258.1).
25 S. M. Godfrey, C. A. McAuliffe, R. G. Pritchard, J. M. Sheffield and
G. M. Thompson, J. Chem. Soc., Dalton Trans., 1997, 4823.
26 D. M. Hawley and G. Ferguson, J. Chem. Soc. A, 1968, 2539.
27 J. C. Pazic and C. George, Organometallics, 1989, 8, 482.
28 S. P. Bone, M. J. Begley and D. B. Sowerby, J. Chem. Soc., Dalton
Trans., 1992, 2085.
The only 31P{H} NMR study of a compound of stoichiometry
R3PCl2,27 which was claimed to be trigonal bipyramidal is
(C6F5)3PCl2, which gave a single resonance at d 2110.29
Clearly, therefore, Ph3PCl2 retains a molecular trigonal
bipyramidal structure in solvents of low polarity. Addition of
CH2Cl2 to the C6D6 solution of Ph3PCl2 ionises the molecule to
produce [Ph3PCl···Cl···ClPPh3]Cl·2CH2Cl2, since a resonance
29 H. J. Emeleus and J. M. Miller, J. Inorg. Nucl. Chem., 1966, 28, 622.
Received in Basel, Switzerland, 23rd July 1997; Revised manuscript
received 30th January 1998; 8/00820E
922
Chem. Commun., 1998