The reaction of some tertiary phosphines with two mole
equivalents of diiodine to produce the iodophosphonium triiodides
[R3PI]I3; influence of R in causing subtle variations in solid state
structures
Wendy I. Cross, Stephen M. Godfrey, Charles A. McAuliffe, Robin G. Pritchard,
Joanne M. Sheffield and Graeme M. Thompson
Department of Chemistry, University of Manchester Institute of Science and Technology,
Manchester, UK M60 1QD. E-mail: stephen.m.godfrey@umist.ac.uk
Received 18th May 1999, Accepted 5th July 1999
A variety of R3PI4 compounds have been synthesized and characterised by elemental analysis, NMR and visible
spectroscopy. Apart from Ph3PI4, all are reported for the first time. X-Ray crystallographic studies on two of these
materials, Pri3PI4 and (Prn N)3PI4, revealed different solid-state structures which are, in turn, different to both the two
2
polymorphs of Ph3PI4 and Fc3PI4 (Fc = ferrocenyl) the crystal structures of which have been previously reported. The
compound Pri3PI4 exists as [(IPPri3)2I3]I3, which contains two [IPPri3]ϩ cations independently weakly linked to one of
the two terminal iodine atoms of the same triiodide anion. The structure also contains a discrete triiodide anion. The
compound (Prn N)3PI4 exhibits very little anion–cation interaction. Of the two molecules present in the asymmetric
2
unit, one exhibits no anion–cation interaction, while the other shows only a very weak contact [d(I–I)]
(cation–anion) = 3.944(2) Å.
adduct have appeared quite recently. The first report of an
Introduction
iodophosphonium triiodide was by Gridunova et al.8 who
described R3PI4 (R = ferrocenyl). Additionally, Cotton and
Kibala9 reported that the product from the reaction of tri-
phenylphosphine with two equivalents of diiodine is solvent
dependent. If the reaction is performed in CH2Cl2 (high relative
permittivity) the compound [(IPPh3)2I3]I3 results. The crystal
structure of this material reveals parallel zigzag chains of
Although the formation of 1:1 tertiary phosphine dihalogen
adducts, R3PX2 (X = Cl, Br or I), has been recognised for over a
century1 it is only recently that their solid-state structures have
been elucidated. The geometrical nature of such compounds
has been shown to be dependent on X, the halide, the organo
substituents bound to the phosphorus and, in some cases, the
relative permittivity (polarity) of the solvent employed for their
preparation.2–6 The structure of the diiodo compounds,
R3PI2,2,3 has revealed the same topology, R3P–I–I, regardless of
the nature of R, although it has been noted that the organo
substituents do influence the iodine–iodine bond length in the
resultant compound. Thus, for example, Ph3PI2 exhibits d(I–I)
of 3.142(2) Å,2 whereas PhMe2PI2 shows d(I–I) of 3.409(2) Å.
The nature of R for a given PR3 will obviously affect its basicity
and thus its donor power towards I2, which is in turn reflected in
d(I–I) for the resultant charge-transfer complex. The analogous
bromide complexes, R3PBr2,4 do exhibit geometrical depend-
ence on R. Where R = Et an ionic compound results, [Et3-
PBr]Br, which contains a tetrahedral phosphorus centre; in
contrast, however, where R = C6F5, a trigonal bipyramidal
compound results (C6F5)3PBr2. A similar situation is observed
for the dichloro adducts, R3PCl2,5,6 being ionic, [R3PCl]Cl
(R = Prn or Pri) or trigonal bipyramidal [R3 = (C6F5)3 or
Ph2(C6F5)].
Despite the current interest in the 1:1 adducts formed
between tertiary phosphines and dihalogens, much less has
been reported concerning the 1:2, R3PX4 adducts, despite the
fact that these too have been known for over a century.1 The
formation of 1:2 tertiary phosphine tetraiodides, R3PI4, has
been extensively studied conductimetrically by Harris and co-
workers.7 Solutions of these species in acetonitrile gave con-
ductance values very close to that expected for a 1:1 electrolyte.
This observation led the authors to conclude that the adducts
were ionic, [Ph3PI]I3, in MeCN, the presence of the triiodide
anion being detected using visible spectroscopy. The only crys-
tal structures reported for a 1:2 tertiary phosphine–dihalogen
Ϫ
[(IPPh3)2I3]ϩ cations sandwiched between layers of I3 anions.
In contrast, however, the same reaction performed in toluene
produces the compound [IPPh3]I3. The crystal structure9 of this
material reveals a strong interaction between the [IPPh3]ϩ cat-
ion and a triiodide anion, the distance between the terminal
Ϫ
iodine atom of I3 and the iodine atoms of [IPPh3]ϩ being
3.551(1) Å. The individual [IPPh3]I3 units are further linked
into a polymer by weak interactions between the triiodine
anions [d(I–I) = 3.741(1) Å]. A similar structural arrangement
is also observed for the related compound triferrocenyl-
iodophosphonium triiodide8 in which the I–I cation–anion
interactions are 3.736(1) Å and the triiodide anions are again
weakly linked in the solid state, d(I–I) = 4.139(1) Å.
The interaction of the related compounds R2PI and RPI2
with diiodine has also recently been reported and a very inter-
esting variety of layer and helical type structures in the solid
state have been established via X-ray crystallography.10,11
Finally, of relevance to the present study is the recent report of
the crystal structure of bromotriphenylphosphonium tribrom-
ide, [BrPPh3]Br3.12 In contrast to the analogous iodine con-
taining species, described above, no cation–anion interactions
are observed, the compound existing as discrete [BrPPh3]ϩ
cations and tribromide anions.
In view of the renewed interest in the products formed from
the reaction of tertiary phosphines with dihalogens and the fact
that relatively little information is available concerning the
products formed from the reaction of two mole equivalents of
diiodine with PR3, we decided to investigate the products
formed from these reactions upon changing the R substituents
on the tertiary phosphine. Additionally, we were interested in
J. Chem. Soc., Dalton Trans., 1999, 2795–2798
2795
Cross, Wendy I.
