Understanding the structural properties of a homologous series of bis-diphenylphosphine oxides

Article abstract of DOI:10.1002/1521-3765(20000703)6:13<2338::AID-CHEM2338>3.0.CO;2-N

A homologous series of bis-diphenylphosphine oxides (C₆H₅)₂PO-(CH₂)_nPO(C ₆H₅)₂ (with n = 2-8; denoted 2-8] have been investigated to explore the effects of a range of competing and cooperative intermolecular and intramolecular interactions on the structural properties in the solid state. The important factors influencing the structural properties include intramolecular aspects such as the conformation of the aliphatic chain and the intramolecular interaction between the two P=O dipoles in the molecule, and intermolecular aspects such as long-range electrostatic interactions (dominated by the arrangement of the P=O dipoles), C-H ... O interactions, C-H ... π interactions and π ... π interactions. Compounds 3 and 5 could be crystallized only as solvate co-crystals (3 · water and 5 · (toluene)₂], whereas the crystal structures of all the other compounds contain only the bis-diphenylphosphine oxide molecule. The crystal structures have been determined from single-crystal X-ray diffraction data, with the exception of 7 (which has been determined here from powder X-ray diffraction data) and 4 (which was known previously). The compounds with even n represent a systematic structural series, exhibiting characteristic, essentially linear P=O ... P=O ... P=O dipolar arrays, together with C-H ... O and C-H ... π interactions. For the compounds with odd n, on the other hand, uniform structural behaviour is not observed across the series, although certain aspects of these crystal structures contribute in a general sense to our understanding of the structural properties of bis-diphenylphosphine oxides. Importantly, for the compounds with odd n, there is "frustration" with regard to the molecular conformation, as the preferred all-anti conformation of the aliphatic chain gives rise to an unfavourable parallel alignment of the two P=O dipoles within the molecule. Clearly the importance of avoiding a parallel alignment of the P=O dipoles becomes greater as n decreases. Local structural aspects (investigated by high-resolution solid-state ³¹P NMR spectroscopy) and thermal properties of the bis-diphenylphosphine oxide materials are also reported.

Full text of DOI:10.1002/1521-3765(20000703)6:13<2338::AID-CHEM2338>3.0.CO;2-N

FULL PAPER
Understanding the Structural Properties of a Homologous Series of
Bis-diphenylphosphine Oxides
Patrizia Calcagno, Benson M. Kariuki, Simon J. Kitchin, James M. A. Robinson,
Douglas Philp,* and Kenneth D. M. Harris*[a]
Abstract: A homologous series of bis-di-
phenylphosphine oxides (C6H5)2PO-
(CH2)nPO(C6H5)2 (with n ˆ 2 ± 8; denot-
ed 2 ± 8] have been investigated to
explore the effects of a range of com-
peting and cooperative intermolecular
and intramolecular interactions on the
structural properties in the solid state.
The important factors influencing the
structural properties include intramolec-
ular aspects such as the conformation of
the aliphatic chain and the intramolec-
5 ´ (toluene)2], whereas the crystal struc-
tures of all the other compounds contain
only the bis-diphenylphosphine oxide
molecule. The crystal structures have
been determined from single-crystal
X-ray diffraction data, with the excep-
tion of 7 (which has been determined
here from powder X-ray diffraction
data) and 4 (which was known previous-
ly). The compounds with even n repre-
sent a systematic structural series, ex-
hibiting characteristic, essentially linear
structural behaviour is not observed
across the series, although certain as-
pects of these crystal structures contrib-
ute in a general sense to our under-
standing of the structural properties of
bis-diphenylphosphine oxides. Impor-
tantly, for the compounds with odd n,
there is ªfrustrationº with regard to the
molecular conformation, as the prefer-
red all-anti conformation of the aliphatic
chain gives rise to an unfavourable
ˆ
parallel alignment of the two P O di-
ˆ
ˆ
ˆ
ˆ
ular interaction between the two P O
P O ´´´ P O ´´´ P O dipolar arrays, to-
poles within the molecule. Clearly the
importance of avoiding a parallel align-
À
À
dipoles in the molecule, and intermolec-
ular aspects such as long-range electro-
static interactions (dominated by the
gether with C H ´´´ O and C H ´´´ p
interactions. For the compounds with
odd n, on the other hand, uniform
ˆ
ment of the P O dipoles becomes great-
er as n decreases. Local structural as-
pects (investigated by high-resolution
solid-state 31P NMR spectroscopy) and
thermal properties of the bis-diphenyl-
phosphine oxide materials are also re-
ported.
ˆ
arrangement of the P O dipoles),
À
À
C H ´´´ O interactions, C H ´´´ p inter-
actions and p ´´´ p interactions. Com-
pounds 3 and 5 could be crystallized
only as solvate co-crystals (3 ´ water and
Keywords: phosphine oxides ´ sol-
id-state structures ´ structure eluci-
dation ´ structural rationalization
that act between functional groups, longer range electrostatic
interactions, and intramolecular (conformational) energies.
The presence of certain functional groups can lead to
predictable local interactions, which may promote character-
istic crystal packing motifs in one-, two- and perhaps even
three-dimensions. In this regard, the use of hydrogen bonding
as a basis for the design and construction of crystals has been
particularly exploited.[6±17] Electrostatic interactions, which
are significant over long range, may also be an important
determinant of the overall packing arrangements observed in
crystals. Understanding the structural properties of molecular
crystals is further complicated if the molecule in question has
intramolecular degrees of freedom, in which case the molec-
ular conformation must also be considered directly in ration-
alizing the crystal structure. In this regard, we emphasize that
the optimal crystal structure does not necessarily contain the
molecule in the conformation that is optimal for the isolated
moleculeÐthe observed crystal structure represents an over-
all compromise between several competing and cooperating
Introduction
The molecular packing arrangements observed in crystals are
the outcome of a balance among several different non-
covalent forces, which may represent varying degrees between
cooperativity and competition.[1±5] In order to understand the
optimal structural arrangements observed in crystals, which
correspond to minima in total energy, we require an under-
standing of the interplay between short-range intermolecular
forces (such as hydrogen bonding, dispersion and repulsion)
[a] Dr. D. Philp, Prof. Dr. K. D. M. Harris, P. Calcagno, Dr. B. M. Kariuki,
Dr. S. J. Kitchin, J. M. A. Robinson
School of Chemistry
University of Birmingham
Edgbaston, Birmingham B15 2TT (UK)
Fax : (‡44)121-414-7473
E-mail: D.Philp@bham.ac.uk
K.D.M.Harris@bham.ac.uk
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Chem. Eur. J. 2000, 6, No. 13

2338±2349
factors, including the intramolecular (conformational) energy
and the energies of the different types of intermolecular
interactions discussed above.
In seeking to understand the role played by intramolecular
and intermolecular factors in controlling crystal packing
arrangements, there is continuing interest in the properties
ˆ
of phosphine oxides. The P O group in phosphine oxides is a
ˆ
P O groups in the molecule, whereas for the compounds in
which n is an odd number (denoted the ªodd seriesº), the all-
anti conformation leads to a parallel alignment of the two
good hydrogen-bond acceptor, and phosphine oxides are well
known to form complexes in solution[18, 19] and in the solid
state.[11, 12, 20±31] In this context, triphenylphosphine oxide
(TPPO) has been used[24] to facilitate co-crystal formation,
ˆ
P O groups. For the even series, we may expect that the all-
anti conformation should be adopted, as the antiparallel
ˆ
based on the ability of the P O group to form strong
ˆ
alignment of the P O groups represents a favourable intra-
molecular dipolar interaction. However, for the odd series,
the parallel alignment of the P O groups leads to an
unfavourable intramolecular electrostatic interaction, and
becomes increasingly unfavourable as the aliphatic chain
becomes shorter. For the odd series, it is clear that a
compromise must be reached between the favourable all-anti
conformation of the aliphatic chain (which forces the
unfavourable parallel alignment of the P O dipoles) and
the avoidance of a parallel alignment of the P O dipoles
(which requires a distortion of the molecule from the
favourable all-anti conformation of the aliphatic chain). The
need to avoid a parallel alignment of the P O dipoles may be
particularly important for short aliphatic chains, which
correspond to a shorter distance and stronger interaction
hydrogen bonds to ªguestº molecules. Although attempts to
rationalize molecular crystal structures are often focused
entirely on the nature of short-range interactions between
neighbouring molecules, longer range electrostatic interac-
tions may also play a significant role for molecules with large
multipole moments. In this regard, the strong dipolar charac-
ˆ
ˆ
ter of the P O group in phosphine oxides may be expected to
have a significant influence on the structural properties of
these materials, both with regard to short-range interactions
(such as hydrogen bonding) and long-range electrostatic
interactions (which may approximate to dipolar arrays of the
ˆ
ˆ
ˆ
P O groups).
ˆ
Although many hydrogen-bonded co-crystals of TPPO
have been reported,[24, 28, 29, 32] these studies have focused on
co-crystals based on N H ´´´ O and O H ´´´ O interactions,
and only few cases involve significant C H ´´´ O interac-
tions.[30, 31] For this reason, we set out to explore systems in
which C H ´´´ O interactions may be formed with TPPO. As
the P O group is a good hydrogen-bond acceptor, a C H ´´´ O
interaction between TPPO and a good C H donor (such as
CHCl3, CH(NO3)3, C C H or NC H) may be expected to
produce short C H ´´´ O contacts that are comparable in
strength and geometric characteristics to traditional strong
interactions such as O H ´´´ O and N H ´´´ O. As part of this
work,[30] we investigated a co-crystal in which a (TPPO)2 ±
water aggregate, based on the hydrogen bonded P O ´´´
H O H ´´´ O P unit, forms an exceptionally short C H ´´´ O
À
À
ˆ
between the two P O dipoles in the molecule.
À
In understanding the crystal structures of these compounds,
these intramolecular considerations must again be balanced
against intermolecular factors, recognizing that different
molecular conformations may be able to propagate different
combinations of intermolecular interactions and to promote
different three-dimensional packing arrangements. The bis-
diphenylphosphine oxide molecules have four phenyl rings,
and edge-to-face (C H ´´´ p) and face-to-face (p ´´´ p) inter-
actions, as well as C H ´´´ O interactions, may have an
important influence on the crystal packing. It is also important
to consider the direct interactions between the P O dipoles of
different molecules, as these long-range electrostatic inter-
actions may promote specific packing arrangements. In the
present context, we recognize that, for a one-dimensional
array of dipoles, a parallel co-axial arrangement provides the
most favourable electrostatic interaction.
À
ˆ
À
À
ꢀ À
À
À
À
À
À
À
ˆ
ˆ
À À
ˆ
À
hydrogen bond with a molecule of 1,4-diethynylbenzene (the
À
ꢀ À
C H ´´´ O interaction involves a C C H group of the 1,4-
diethynylbenzene molecule as the donor and the oxygen atom
of the water molecule (shown above) as the acceptor). In this
À
aggregate, the polarization of the O H bonds of the water
À
ˆ
molecule is promoted by the O H ´´´ O P interactions with
the TPPO molecules, enhancing the negative polarization of
the oxygen atom of the water molecule.
Results and Discussion
In order to explore systematically the factors that affect the
structural properties of phosphine oxides in the solid state for
cases in which no traditional strong hydrogen-bond donors
are present, we focus here on the family of solid bis-
diphenylphosphine oxides (C6H5)2PO(CH2)nPO(C6H5)2 (n ˆ
1 ± 8 ; 1 ± 8). For these molecules, two important intramolec-
ular features are the conformation of the aliphatic (CH2)n
chain and the dependence of the relative orientations of the
Preliminary remarks
The compounds in the odd series exhibit markedly different
physical properties from those in the even series. For example,
members of the odd series have good solubility in all solvents
tested (toluene, benzene, acetone, chloroform, dichlorome-
thane) whereas members of the even series are significantly
soluble only in the halogenated solvents of those considered.
Another property that differs markedly between the odd and
even series is the melting temperature, which is substantially
lower for the crystalline materials of the odd series than those
ˆ
P O dipoles on the chain length. The aliphatic chain prefers
an all-anti conformation. For the compounds in which n is an
even number (denoted the ªeven seriesº), the all-anti
conformation leads to an antiparallel alignment of the two
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D. Philp, K. D. M. Harris et al.
FULL PAPER
of the even series (as discussed below, many of the crystalline
materials of the odd series are actually solvates). In view of
these observations and the discussion in the Introduction, it is
reasonable to expect major differences in the structural
properties between the even and odd series.
In the present work, the crystal structures of compounds 2,
6 and 8 within the even series have been determined from
single-crystal X-ray diffraction data, and relevant crystallo-
graphic data are summarized in Table 1. The crystal structure
of 4 has been reported previously.[33] As discussed below in the
section on the structural properties of the even series, there
are strong similarities between the crystal structures of the
compounds in the even series (particularly for 4, 6 and 8).
For the compounds in the odd series, only 7 was found to
crystallize without the formation of a solvate co-crystal. For 3
and 5, crystals suitable for single-crystal X-ray diffraction
studies were obtained by crystallization from toluene, but
were found to be solvate co-crystals with stoichiometries 3 ´
water and 5 ´ (toluene)2. The 3 ´ water co-crystal was obtained
reproducibly by crystallization from toluene, and presumably
relies on residual amounts of water present in the solvent. In
the present work, the crystal structures of 3 ´ water and 5 ´
(toluene)2 have been determined from single-crystal X-ray
diffraction data and the crystal structure of 7 has been
determined directly from powder X-ray diffraction data.
Relevant crystallographic data are summarized in Table 3. A
solvate co-crystal of 1 with benzene (stoichiometry 1 ´ (ben-
zene)0.25) has been reported previously.[34] As the structural
properties of these co-crystals are influenced not only by
intrinsic characteristics of the bis-diphenylphosphine oxide
molecules but also by the presence of the solvent molecules, it
is not surprising that uniform structural behaviour in not
observed across the set of crystal structures for the odd series.
Nevertheless, certain aspects of these crystal structures
contribute in a general sense to our understanding of the
structural properties of bis-diphenylphosphine oxides, as
discussed below in the section on the structural properties
of the odd series.
Structural properties of the even series
In the crystal structure of 2, there is one molecule in the
asymmetric unit. This molecule is disordered (Figure 1 a;
Table 1) between two orientations related approximately by
1808 rotation about the long axis of the molecule (approx-
imately the crystallographic c axis). The proportions of the
two molecular orientations are 88.7% and 11.3%. From the
average crystal structure, we cannot assess a priori whether
the two molecular orientations represent two different
ordered domains (situation A) or whether there is a uniform
distribution (subject to the probabilities quoted above) of the
two disordered molecular orientations throughout the crystal
(situation B). Nevertheless, consideration of the intermolec-
ular interactions discussed below strongly suggests that
situation A, with ordered domains containing only the major
orientation and ordered domains containing only the minor
orientation, should be more favourable than situation B, in
which individual molecules may adopt the minor orientation
while surrounded by neighbouring molecules in the major
Figure 1. The crystal structure of 2 showing a) the asymmetric unit, b) the
À
crystal packing arrangement viewed along the b axis (with C H ´´´ p
À
interactions shown by dotted lines) and c) short C H ´´´ O contacts.
orientation. The crystal structure for a domain comprising
only the major orientation is shown in Figure 1b and is
discussed below.
ˆ
The (CH2)2 chain has the anti conformation, with the P O
groups in a given molecule oriented antiparallel to each other.
ˆ
In the crystal structure, P O groups of neighbouring mole-
cules are aligned approximately along the b axis (periodic
repeat distance 5.8Š), forming linear arrays of the type
ˆ
ˆ
ˆ
P O ´´´ P O ´´´ P O, with O ´´´ P distances 4.34 Š and 4.39 Š
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Bis-diphenylphosphine Oxides
2338±2349
Table 1. Crystallographic data for compounds in the even series.[a]
series (reflected mainly by a
progressive increase in the
Compound
2
4
6
8
length of the b axis) and a slight
difference in the conformation
of the aliphatic chain in the case
of 8. There are also some com-
mon features between the unit
cell dimensions of 2 and those
of 4, 6 and 8, suggesting some
common packing motifs. For
example, all structures in the
even series have a unit cell axis
of length about 5.8Š ( b axis for
2; c axis for 4, 6 and 8). In view
crystal system
crystal size [mm3]
space group
R[I > 2s(I)]
wR2
a [Š]
b [Š]
c [Š]
a [8]
monoclinic
0.06 Â 0.08 Â 0.2
Cc
0.111
0.2381
16.308(4)
5.817(13)
23.199(5)
90
102.260(6)
90
2151(5)
4
1.329
triclinic
±
P1
±
±
8.862(1)
12.517(2)
5.826(1)
102.67(1)
104.22(1)
79.71(1)
592.5(3)
1
triclinic
triclinic
0.05 Â 0.2 Â 0.15
0.05 Â 0.2 Â 0.5
Å
Å
Å
P1
P1
0.0443
0.1253
0.0594
0.1443
8.865(7)
13.354(6)
5.841(4)
101.42(2)
104.07(3)
83.52(3)
655.9(8)
1
8.8029(14)
14.123(2)
5.9130(9)
99.070(4)
104.602(7)
84.254(3)
701.1(2)
1
b [8]
g [8]
V [Š3]
Z
1 [gcmÀ3
]
1.285
1.227
1.219
of the similar structural proper-
ties of 4, 6 and 8, we discuss
these compounds together.
In the crystal structures of 4
[a] The data for 4 are taken from reference [33]. For 4, the unit cell axes {a, b, c} shown above have been
transformed from those {apub, bpub, cpub} quoted in reference [33] to facilitate comparison with the unit cells of 6
and 8, with a ˆ bpub; b ˆ cpub; c ˆ apub].
(Figure 2), 6 (Figure 3) and 8
ˆ
ˆ
(for the two independent P O groups) and P O ´´´ P angles
167.38 and 165.98 respectively. These linear arrays are
presumably mediated by electrostatic interactions between
(Figure 4), the asymmetric unit comprises half the molecule,
with the two halves of the molecule related by a centre of
À
inversion at the mid-point of the central C C bond. In 4 and 6,
ˆ
the P O dipoles. For disorder situation B discussed above,
molecules in the minor orientation would exist within an array
of molecules of the major orientation, representing a defect
ˆ
ˆ
(P O ´´´ O ˆ P ´´´ P O) that is highly unfavourable on electro-
static grounds. On this basis, disorder model A appears more
plausible than disorder model B, as discussed above. As
À
ˆ
shown in Figure 1c, there are also relatively short C H ´´´ O P
interactions between neighbouring molecules along the b axis
À
(involving aliphatic C H bonds). Within the ac plane,
neighbouring molecules interact through edge-to-face
À
(C H ´´´ p) interactions between phenyl rings. Of the four
independent rings in a given molecule, two rings act as
acceptors (p) for these interactions and two rings act as
À
donors (C H). The H ´´´ p(centroid) distances for these
interactions are in the range 3.00 Š to 3.65 Š.
In summary, the crystal structure of 2 may be considered in
ˆ
ˆ
terms of linear dipolar arrays of P O ´´´ P O interactions
Figure 2. The crystal structure of 4 viewed along the c axis with short
À
along the b axis, together with C H ´´´ O interactions and
À
À
C H ´´´ p contacts shown.
C H ´´´ p interactions. Distances and angles relating to these
interactions are given in Table 2. There is no evidence for any
significant p ´´´ p interactions between adjacent phenyl rings
in this structure.
From inspection (see Table 1) of the unit cell dimensions for
4, 6 and 8, it is clear that these materials have strong structural
similarities. As discussed below, these structures are essen-
tially identical, except for the requirement to accommodate
an extra (CH2)2 unit in the aliphatic chain on moving along the
the aliphatic chains have the all-anti conformation. In 8, the
majority of the aliphatic chain has the all-anti conformation,
but with some deviation in the middle portion of the chain. In
ˆ
4, 6 and 8, the two P O groups in a given molecule are strictly
ˆ
antiparallel. The P O groups of all molecules are aligned
approximately along the c axis (periodic repeat distance ca.
ˆ
5.8Š), and the P O groups of adjacent molecules form linear
ˆ
ˆ
ˆ
arrays of the type P O ´´´ P O ´´´ P O. These arrays are
presumably promoted by the favourable electrostatic inter-
Table 2. Selected information on intermolecular interactions in the crystal
structures of compounds in the even series.
ˆ
actions between the P O dipoles. The geometrical character-
istics of these arrays are remarkably constant for all members
À
ˆ
ˆ
Compound
C H ´´´ O
P O ´´´ P O
À
ˆ
C ´´´ O [Š]
C H ´´´ O [8]
O ´´´ P [Š]
P O ´´´ P [8]
of the even series (including 2), with O ´´´ P distances in the
ˆ
range 4.34 Š to 4.47 Š and P O ´´´ P angles in the range 163.78
2
3.60
3.59
3.50
3.52
169.4
168.5
175.3
171.84.39
4.47
4.39
4.34
4.39
165.9
167.3
163.7
to 167.38 (see Table 2). In addition, adjacent molecules along
4
6
8
À
ˆ
the c axis are linked by C H ´´´ O P interactions (involving a
164.4
164.1
À
C H bond of the CH2 group that is a with respect to the
3.58175.4
phosphine oxide group).
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D. Philp, K. D. M. Harris et al.
FULL PAPER
Figure 3. The crystal structure of 6 viewed along the c axis with short
À
C H ´´´ p contacts shown.
À
Each phenyl ring is involved in two edge-to-face (C H ´´´ p)
interactions with phenyl rings from different neighbouring
À
molecules, in one case acting as the donor (C H) and in the
other case acting as the acceptor (p). In total, each molecule
forms such interactions with four neighbours, giving rise to
À
networks of C H ´´´ p interactions in the ab plane (H ´´´
p(centroid) distances in the range 3.08Š to 3.38Š). There
are no significant p ´´´ p interactions in any of the structures in
the even series (pairs of phenyl rings that may appear in
Figures 2, 3 and 4 to have p ´´´ p interactions are actually at
different ªheightsº along the c axis).
In the crystal structure of 8, the central part of the aliphatic
chain is disordered between two different conformations, with
71.5% in the major component (Figure 4a). For both the
major and minor components, the C-C-C-C dihedral angle for
Figure 4. The crystal structure of 8 showing a) disorder in the molecular
conformation and b) the packing arrangement for a domain comprising
only the major component of the disordered structure, viewed along the c
À
axis, with short C H ´´´ p contacts shown.
À
À
the C C bond adjacent to the central C C bond is approx-
imately gauche (828 for the major component and 728 for the
minor component) (note that the C-C-C-C dihedral angle for
À
À
the next C C bond away from the central C C bond deviates
slightly from anti (1648 for the major component and 1528 for
the minor component)). The major and minor conformations
differ only in the positions of the two central carbon atoms of
the aliphatic chain, and all other atoms (including the
diphenylphosphine oxide groups) are in identical positions.
À
The C-C-C-C dihedral angle for the central C C bond is 1808
(as required by the fact that there is an inversion centre at the
midpoint of this bond).
Figure 5. The crystal structure of 1 ´ (benzene)0.25 viewed along the a axis.
Structural properties of the odd series
The crystal structure of 1 ´ (benzene)0.25 (Figure 5; Table 3) has
been reported previously.[34] The asymmetric unit comprises
two molecules of 1 and half a molecule of benzene. For each
rings are almost parallel with perpendicular distances be-
tween the planes of about 3.24 Š and 3.27 Š. Several
ˆ
À
molecule of 1 in the asymmetric unit, the P O bonds within
intermolecular edge-to-face (C H ´´´ p) interactions are ob-
ˆ
ˆ
the molecule are almost antiparallel with O P ´´´ P O dihe-
served between molecules of 1, with H ´´´ p(centroid) dis-
dral angles of 169.78 and 177.88. In each molecule, two phenyl
tances in the range 2.95 Š to 3.23 Š. In addition, the two
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Bis-diphenylphosphine Oxides
2338±2349
Table 3. Crystallographic data for compounds in the odd series.[a]
one toluene molecule). The
crystal structure is shown in
Figure 7. The aliphatic chain
has the all-anti conformation,
Compound
1 ´ (benzene)0.25
3 ´ water
5 ´ (toluene)2
7
crystal system
crystal size [mm3]
space group
R[I > 2s(I)]
wR2
triclinic
±
P1
±
±
monoclinic
0.3 Â 0.25 Â 0.5
P21/a
0.0512
0.1602
monoclinic
0.2 Â 0.3 Â 0.5
P21/m
0.1438
0.3797
5.755(3)
37.488(10)
8.816(5)
90
102.80(5)
90
monoclinic
[b]
Å
ˆ
with the P O groups aligned
P21/n
[b]
essentially parallel to each oth-
er. The packing of the mole-
cules of 5 creates two types of
channel parallel to the a axis,
which are occupied by toluene
molecules. In one type of chan-
nel (denoted A), the channel
walls are constructed only from
phenyl rings (from four differ-
ent molecules of 5 in the pro-
jection on the bc plane shown
in Figure 7). In the other type
[b]
a [Š]
b [Š]
c [Š]
a [8]
b [8]
g [8]
V [Š3]
9.246(2)
13.390(2)
19.642(5)
74.14(2)
78.42(2)
84.59(2)
2290(2)
4
7.920(2)
19.484(8)
15.582(7)
90
92.17(2)
90
2403(2)
4
1.2781.174
12.560(1)
10.203(1)
22.889(2)
90
105.53(1)
90
1855(2)
2
2826(1)
4
Z
1 [gcmÀ3
]
1.265
1.176
[a] The data for 1 ´ (benzene)0.25 are taken from reference [34]. [b] Structure determined from powder X-ray
diffraction data (Rp ˆ 0.038, Rwp ˆ 0.05).
molecules of 1 in the asymmetric unit interact through
À
À
C H ´´´ O interactions involving C H bonds of the CH2
À
group. Each molecule of 1 has intramolecular C H ´´´ O
À
interactions (involving phenyl C H), with C ´´´ O distances
in the range 3.17 Š (C H ´´´ O angle 173.98) to 3.46 Š
À
À
(C H ´´´ O angle 166.58). This structure does not contain
ˆ
ˆ
ˆ
any linear P O ´´´ P O ´´´ P O dipolar arrays.
In the crystal structure of 3 ´ water (Figure 6), the asym-
metric unit comprises one molecule of 3 and one molecule of
water. The water molecules may be considered to occupy
channels (along the a axis) formed by molecules of 3. In the
À
À
aliphatic chain, one C C bond is gauche and the other C C
ˆ
bond is anti, through which the two P O bonds are brought
ˆ
towards an antiparallel alignment (O ˆ P ´´´ P O dihedral
angle 1488). Ab initio calculations (HF/6-31G) indicate that
the conformation observed in the crystal structure is more
stable than the all-anti conformation (which contains parallel
À1
ˆ
P O dipoles) by 2.5 kcalmol . For each molecule of 3 in the
crystal structure, one P O group points towards the water
channel and participates in O H ´´´ O hydrogen bonding with
the water molecules to form hydrogen-bonded chains along
ˆ
À
the a axis (Figure 6b). In these arrays, each molecule of water
ˆ
is hydrogen-bonded to two P O groups with approximately
À
ˆ
linear O H ´´´ O interactions. The other P O group in each
molecule of 3 does not participate in hydrogen bonding. Short
À
C H ´´´ O contacts may also be identified between the oxygen
À
atom of the water molecule and C H bonds of two
neighbouring phenyl rings, and between the oxygen atom of
À
the water molecule and a C H bond of the aliphatic chain.
Interactions between different phenyl rings comprise both an
À
edge-to-face (C H ´´´ p) contact (H ´´´ p(centroid) distance
3.07 Š) and p ´´´ p interactions (perpendicular distance be-
tween planes ca. 3.6 Š; interfacial contact area ca. 1/3 of
ˆ
each ring). This structure does not contain any linear P O ´´´
ˆ
ˆ
P O ´´´ P O dipolar arrays.
Crystals of 5 ´ (toluene)2 were found to be unstable and it
was necessary to keep the sample inside a sealed capillary
during the single crystal X-ray diffraction measurement in
order to avoid loss of solvent and consequent degradation of
the crystal. The asymmetric unit comprises half a molecule of
5 and disordered toluene molecules (representing a total of
Figure 6. The crystal structure of 3 ´ water viewed along the a axis (the
direction of the channel) (a) and showing the hydrogen-bonded chains
along the a axis (horizontal) (b).
Chem. Eur. J. 2000, 6, No. 13
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2343

D. Philp, K. D. M. Harris et al.
FULL PAPER
À
À
the acceptor in C H ´´´ p interactions from phenyl C H bonds
of toluene molecules in both channels (H ´´´ p(centroid)
distances 3.37 Š and 3.54 Š) and to act as the donor in a
À
C H ´´´ p interaction to the toluene molecule in one channel
(H ´´´ p(centroid) distance 3.31 Š). In the channels of type A,
the methyl group of the toluene molecule is directed towards
a phenyl ring of 5 (H ´´´ p(centroid) distance ca. 3.8Š).
In the crystal structure of 7 (Figure 8a), the asymmetric unit
comprises one molecule, which adopts a conformation with
one gauche bond (C-C-C-C dihedral angle 57.78) in the (CH2)7
chain and with the other bonds in this chain close to anti. This
Figure 7. The crystal structure of 5 ´ (toluene)2 viewed along the a axis (the
direction of the channels). Some disordered toluene molecules have been
omitted for clarity.
of channel (denoted B), the channel walls are constructed
from the aliphatic chain and two phenyl rings of one molecule
of 5 and the aliphatic chain of another molecule of 5 (as shown
projected on the bc plane in Figure 7). In the channels of type
A (which are rather elongated in cross-section), the toluene
molecules are disordered with 50% occupancy between two
orientations, with the same positions occupied by the carbon
atoms of the phenyl rings of the two molecular orientations;
the two methyl positions give rise to a ªp-xylene-likeº
disordered molecule in the average structure. In the channels
of type B, the toluene molecules are disordered with 50%
occupancy between two orientations related by 1808 rotation
about the normal to the molecular plane; the positions of the
carbon atoms of the phenyl rings of the two molecular
orientations do not coincide in this case, although they are
located in the same plane (for clarity, only one molecular
orientation in the channels of type B is shown in Figure 7).
The low stability of these crystals could be due to facile escape
of the toluene molecules from the channels, leading to
collapse of the host channel structure.
Figure 8. The crystal structure of 7 viewed along the b axis (a) and showing
À
the short C H ´´´ O contacts in the structure (b).
ˆ
conformation of the (CH2)7 chain brings the two P O groups
ˆ
ˆ
in the molecule into almost parallel orientations (O P ´´´ P O
ˆ
dihedral angle 5.88). As shown in Figure 8b, one P O group is
ˆ
ˆ
The structure of 5 ´ (toluene)2 contains linear P O ´´´ P O
À
involved in two C H ´´´ O interactions with a neighbouring
ˆ
ˆ
´´´ P O dipolar arrays (O ´´´ P distance 4.32 Š; P O ´´´ P angle
164.268) along the a axis, directly analogous to those found in
the crystal structures of compounds in the even series. This
motif corresponds again to a characteristic unit cell axis of
À
molecule (involving one aliphatic C H bond and one phenyl
C H bond), whereas the other P O group is involved in one
C H ´´´ O interaction (with a phenyl C H bond). The
structure also contains several C H ´´´ p interactions, but does
À
ˆ
À
À
À
À
length about 5.8Š. In addition, C H ´´´ O interactions (C ´´´ O
ˆ
ˆ
ˆ
not contain any linear P O ´´´ P O ´´´ P O dipolar arrays.
À
distance 3.48Š; C H ´´´ O angle 157.68) may be identified
between adjacent molecules along the a axis, as in the structures
of the even series. Edge-to-face interactions are observed
between the phenyl rings of adjacent molecules of 5 in the bc
plane. Although the toluene molecules are disordered within
their molecular planes, each molecule of 5 appears to act as
Thermal analysis
The melting temperatures measured for all materials studied
here are shown in Figure 9. We focus first on the structures
that do not contain solvent molecules (i.e. the compounds in
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Chem. Eur. J. 2000, 6, No. 13

Bis-diphenylphosphine Oxides
2338±2349
with the loss of toluene) followed by a sharp endotherm with
onset at 1248C (assigned as melting of the de-solvated solid
phase of 5).
Structural characterization of the materials obtained fol-
lowing removal of water from 3 ´ water and removal of toluene
from 5 ´ (toluene)2 represents an interesting area for future
studies. An understanding of the phase behaviour of these
materials (for example, on driving the solvent from the
crystals and following the reintroduction of the same or
different co-crystal molecules under controlled conditions)
may reveal appropriate conditions for the preparation of
unsolvated crystalline samples of 3 and 5 appropriate for
structure determination from single crystal or powder dif-
fraction data, allowing direct structural comparisons with the
unsolvated crystal structures of the compounds in the even
series and 7. In addition it is relevant to note that, as
demonstrated recently for another system,[46] de-solvated
crystals produced in TGA experiments may be used as seeds
for subsequent crystal growth of the de-solvated phase.
Figure 9. Measured melting temperatures (filled squares) for 2, 4, 6, 7 and
8 and melting/decomposition temperatures (filled triangles) for 3 ´ water
and 5 ´ (toluene)2. The melting/decomposition behaviour for 3 ´ water and
5 ´ (toluene)2 is discussed fully in the text (the temperatures shown
correspond to loss of solvent from these crystal structures).
Solid-state NMR spectroscopy
It has been shown previously[35] that high-resolution solid-
state 31P NMR spectroscopy is a sensitive probe of the local
structural properties of co-crystals containing triphenylphos-
phine oxide. In this light, we have extended our character-
ization of the structural properties of the bis-diphenylphos-
phine oxides 2 ± 8 to include high-resolution solid-state 31P
NMR spectroscopy, allowing an assessment of the extent to
which this technique is sensitive to the contrasts and
similarities in the structural properties of the materials in this
family.
The isotropic peaks in the high-resolution solid state 31P
NMR spectra of 2, 3 ´ water, 4, 5 ´ (toluene)2, 6, 7 and 8 are
shown in Figure 10, and relevant data are summarized in
Table 4. In general, the results are consistent with the crystal
structures reported above. The high-resolution solid state 31P
NMR spectrum of 2 comprises two partially resolved peaks
the even series (2, 4, 6, 8) and 7). In the even series, the
melting temperature decreases (in a fairly linear manner) as n
increases, and is mirrored by a gradual decrease in the density
of the crystals as n increases. The melting temperature of 7
falls below the line established for the even series. For each of
the compounds in the even series and 7, the differential
scanning calorimetry (DSC) thermogram shows only an
endothermic process, which is assigned as the melting
transition.
The melting behaviour of 3 ´ water and 5 ´ (toluene)2 is less
straightforward to define, as the first process that occurs on
heating is decomposition of the co-crystal through loss of the
included solvent molecules. Rapid heating on a melting point
apparatus shows visible evidence of liquid at about 828C for
3 ´ water and at about 458C for 5 ´ (toluene)2, probably
corresponding to loss of solvent from these crystal structures.
The resulting (de-solvated) solid phases melt subsequently at
about 1438C for 3 and at about 1248C for 5 (only these
transitions are observed under conditions of slow heating on a
melting point apparatus).
On heating 3 ´ water in a thermogravimetric analysis (TGA)
experiment, a total mass loss of 3.7% is observed, consistent
with essentially complete loss of water (the theoretical
amount of water in the crystal structure is 3.9%). This loss
of water takes place in two steps, beginning (for heating rate
2KminÀ1) at about 558C (mass loss 2.0%) and at about 798C
(mass loss 1.7%). DSC at heating rate 10KminÀ1 shows a very
broad endotherm beginning around 508C (presumably asso-
ciated with the loss of water) followed by a sharp endotherm
with onset at 1408C (assigned as melting of the de-solvated
solid phase of 3). For 5 ´ (toluene)2, TGA shows a similar two-
step mass loss, with the total mass loss (25%) comparable to
the theoretical amount of toluene (28%) in the crystal
structure. At a heating rate of 2KminÀ1, the two mass losses
begin at about 408C (8%) and about 55 8C (17%). DSC at a
heating rate of 10KminÀ1 shows two comparatively narrow
exothermic peaks in the same temperature region (associated
Figure 10. High-resolution solid-state 31P NMR spectra for 2, 3 ´ water, 4,
5 ´ (toluene)2, 6, 7 and 8.
Chem. Eur. J. 2000, 6, No. 13
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2345

D. Philp, K. D. M. Harris et al.
FULL PAPER
Table 4. Isotropic chemical shifts (d), linewidths (Dn; full-width at half
maximum) and relative integrated intensities (I) of the high-resolution
solid-state 31P NMR spectra of the bis-diphenylphosphine oxide materials
studied. The linewidths and intensities for 2, 3 ´ water and 7, and the
isotropic chemical shifts for 2, were obtained by peak deconvolution.
understanding of the relationships between local structural
properties and solid state NMR properties will focus on
understanding the relationship between the anisotropy of the
31P chemical shift tensor and local interactions involving the
ˆ
P O group. The range of materials studied here, with their
Compound
2
3 ´ water
4
5 ´ (toluene)2
6
7
8
range of interactions and local structural situations involving
d [ppm]
30.9
29.6
185
55
32.4
37.6
100
51
29.1
32.1
29.0 29.2 30.3
27.7
ˆ
P O groups, provide a good opportunity for systematic
fundamental studies in this area.
Dn [Hz]
I [%]
75
100
39
100
156
100
49
49
51
85
99
45
49
Conclusion
with approximately equal intensities, consistent with having
one molecule of 2 (and thus two inequivalent 31P nuclei) in the
asymmetric unit. Some additional intensity is observed at
d ꢁ 32, and may arise from the minor component in the
disordered crystal structure. For 4, 6 and 8, the high-resolution
solid-state 31P NMR spectrum comprises a single isotropic
peak, consistent with having one half molecule (i.e. one 31P
environment) in the asymmetric unit. The isotropic chemical
shifts for 4, 6 and 8 are similar, reflecting the similar local
environments of the 31P sites in the crystal structures.
However, the slightly higher isotropic chemical shift for 8 in
comparison with 4 and 6 is somewhat surprising in view of the
very similar local structural properties in these materials.
Nevertheless, the isotropic chemical shift for 8 is close to those
for 2, which also shares several local structural features in
common with 4, 6 and 8.
In summary, the bis-diphenylphosphine oxide compounds in
the even series represent a systematic structural series,
ˆ
ˆ
ˆ
characterized by essentially linear P O ´´´ P O ´´´ P O dipo-
lar arrays (corresponding to a 5.8Š unit cell axis), and
À
À
augmented by C H ´´´ O and C H ´´´ p interactions. Com-
pounds 4, 6 and 8 in particular have virtually identical crystal
ˆ
ˆ
ˆ
structures. The persistence of the P O ´´´ P O ´´´ P O dipolar
arrays in these structures (and in the structure of 5 ´ (to-
luene)2) emphasizes the important role that long range
electrostatic interactions may play in influencing crystal
structures, in addition to the much more localized types of
interactions (such as hydrogen bonding) that are usually the
focus of the structural rationalization and design of crystals.
In contrast, the compounds in the odd series do not exhibit
uniform structural behaviour across the series, although these
crystal structures nevertheless contribute to our general
understanding of the structural properties of bis-diphenyl-
phosphine oxides. The propensity for the compounds in the
odd series (with the exception of 7) to form solvate co-crystals
alludes to the difficulties in forming crystal structures in which
different packing factors are simultaneously optimizedÐ
clearly, the formation of co-crystals creates the opportunity
for the formation of new types of interactions (such as
The high-resolution solid-state 31P NMR spectrum of 3 ´
water comprises two isotropic peaks separated by more than
5 ppm, consistent with having one molecule of 3 in the
asymmetric unit. The significant separation between the
peaks reflects significantly different local environments. The
peak at d ˆ 37.6 can be assigned to the 31P site involved in
ˆ
À
P O ´´´ H O hydrogen bonding, as the formation of strong
hydrogen bonds is known to shift isotropic 31P resonances to
higher frequency.[35] Indeed, the value of the isotropic
À
O H ´´´ O hydrogen bonding in the case of 3 ´ water) and new
packing arrangements (such as the channel structure with
ˆ
chemical shift for this P O group is entirely consistent with
ˆ
ˆ
ˆ
the value expected, according to the correlations developed
linear P O ´´´ P O ´´´ P O dipolar arrays in the case of
5 ´ (toluene)2) which would otherwise not occur.
previously,[35] for a P O group engaged in two strong hydro-
gen bonds.
ˆ
An important difference between the odd and even
compounds concerns the molecular conformation. Thus, while
the even compounds can (as observed in their crystal
structures) adopt a conformation in which the favoured all-
anti conformation of the aliphatic chain also corresponds to a
The single peak in the high-resolution solid-state 31P NMR
spectrum of 5 ´ (toluene)2 is consistent with having one half
molecule of 5 in the asymmetric unit. The isotropic chemical
shift is significantly higher than those for compounds in the
even series. We recall that 5 ´ (toluene)2 and the compounds in
ˆ
favourable relative alignment of the two P O dipoles in the
ˆ
ˆ
ˆ
the even series all exhibit P O ´´´ P O ´´´ P O arrays, al-
though other aspects of the crystal structures are substantially
different, and are presumably responsible for the significant
differences in isotropic chemical shifts.
molecule, this opportunity does not exist for the odd
compounds. Thus, for the compounds in the odd series, the
preferred all-anti conformation of the aliphatic chain gives
rise to an unfavourable parallel alignment of the two P O
dipoles within the molecule, with the importance of avoiding a
ˆ
The high-resolution solid state 31P NMR spectrum of 7
contains two isotropic peaks, consistent with having one
molecule of 7 in the asymmetric unit. The separation between
these peaks (1.5 ppm) reflects significant differences, high-
ˆ
parallel alignment of the P O dipoles becoming greater as the
length of the aliphatic chain becomes shorter. This ªfrustra-
tionº with regard to the molecular conformation clearly
complicates the formation of a well-defined structural series,
particularly as the degree of frustration depends on chain
length, and it is not surprising that different members of the
odd series find different ways of relieving this frustration. In
light of these facts, the contrasting thermal properties and
ˆ
lighted above, in the local interactions involving the two P O
groups in this structure.
The high-resolution solid state 31P NMR spectra discussed
above are clearly in good agreement with the known local
structural properties of the materials studied. A more detailed
2346
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Chem. Eur. J. 2000, 6, No. 13

Bis-diphenylphosphine Oxides
2338±2349
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publication nos. CCDC-141361 (2),
CCDC-141362 (3 ´ water), CCDC-141363 (5 ´ (toluene)2), CCDC-141364
(6), and CCDC-141365 (8). Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK
(fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Thermal analysis: Differential scanning calorimetry (DSC) was carried out
on a Perkin Elmer Pyris I instrument under helium purge and thermog-
ravimetric analysis (TGA) was carried out using a Perkin Elmer TGA6
instrument under nitrogen. In both cases, the samples were heated at either
2 or 108C/min. Melting temperatures were determined using an Electro-
thermal 9200 melting point apparatus and are uncorrected.
melting points (see section on thermal analysis and Figure 9)
between the odd and even series is not surprising.
Finally, we note that for appropriate values of n, com-
pounds in the family (C6H5)2PO(CH2)nPO(C6H5)2 are poten-
ˆ
tial receptors (through the pair of P O hydrogen bond
acceptors) for ªguestº molecules such as water and amines
containing an appropriate pair of hydrogen bond donors.
Binding of such molecules to the bis-diphenylphosphine
oxides may be expected to occur in a highly selective manner,
depending on the length (n) of the aliphatic chain, and may
form the basis of selective molecular sensors. For such
applications, the bis-diphenylphosphine oxide molecules
may either be applied directly in the solid state or grafted
on to appropriate substrates, with the further requirement
that some appropriate physical property of the materials
should vary in a well-defined manner as the concentration of
bound species changes (corresponding to changes in the
concentration of the ªguestº molecules in the external
environment). Clearly the fundamental understanding of
structural properties of bis-diphenylphosphine oxides devel-
oped in the present paper represents an essential foundation
upon which to design materials for such applications. We are
currently exploring the potential of the bis-diphenylphos-
phine oxides with regard to such applications.
Spectroscopy: Infrared spectra were recorded for ground polycrystalline
samples dispersed in KBr disks, using a Perkin Elmer Paragon 1000 FT-IR
spectrometer at ambient temperature with resolution 2 cmÀ1. Solution state
1H NMR spectra were recorded on
a Bruker AC 300 (300 MHz)
spectrometer using residual CHCl3 present in deuterated chloroform as
the internal reference. Solution-state 31P NMR spectra were recorded on a
Bruker AC 300 (121 MHz) spectrometer. Solution-state 13C NMR spectra
were recorded on a Bruker AC 300 (75 MHz) spectrometer using the
PENDANT sequence. Mass spectra were recorded by using fast atom
bombardment mass spectrometry on a VG ZabSpec mass spectrometer.
Solid-state NMR spectroscopy: High-resolution solid-state 31P NMR
spectra were recorded on a Chemagnetics CMX-Infinity 300 spectrometer
(121.5 MHz) using either a Chemagnetics 3.2 mm double resonance probe
or a 4 mm triple resonance probe. Cross-polarisation (31P 1H), magic-
angle sample spinning (MAS) and high-power 1H decoupling were used.
Typically the spinning frequency was (7000 Æ 4) Hz and the decoupling
field strength was about 62 kHz, with recycle delays in the range 30 s to
420 s. Chemical shifts are reported relative to the 31P resonance in aqueous
H3PO4 (85%) as an external standard. All spectra were recorded at 258C.
Except in the case of 5 ´ (toluene)2, all samples were ground to powders
before loading into the zirconia rotors. The sample of 5 ´ (toluene)2 was not
ground, in view of the known susceptibility of this material to undergo
decomposition by loss of toluene.
Experimental Section
Structure determination: Single crystals were grown by slow evaporation of
solutions containing each of the bis-diphenylphosphine oxide compounds
in a suitable solvent. Chloroform was used for 2, 6 and 8, toluene was used
for 3 and 5, and a dichloromethane/hexane mixture was used for 7. All
crystal structures with the exception of 7 were determined from single
crystal X-ray diffraction data. Crystals of 7 suitable for conventional single
crystal X-ray diffraction could not be grown, and the structure of 7 was
determined instead from powder X-ray diffraction data.
Synthesis: DMF was dried over 4 Š molecular sieves. THF was distilled
from sodium/benzophenone and was used immediately. All other solvents
and reagents were used as supplied. Thin-layer chromatography (TLC) was
performed on aluminium plates coated with Merck Kieselgel 60 F254. Flash
column chromatography was performed using Kieselgel 60 (0.040 ±
0.063 mm mesh Merk 9385).
Preparation of the hexa-P-phenyl-P,P'-alkyl-di-phosphonium bromides
9 ± 15
Synchrotron microcrystal X-ray diffraction data were recorded for a
microcrystal of 2 on station 9.8at the SRS (Daresbury Laboratory), with
l ˆ 0.6875 Š and using a Siemens SMART CCD detector and goniometer
system. The crystal structure was solved by direct methods using the
General procedure: A mixture of the appropriate dibromoalkane (1 equiv)
and triphenylphosphine (2 equiv) were heated under reflux for 10 h in
dried DMF (50 mL). In a typical experiment, 5 g of the starting dibromide
was used. The solvent was removed in vacuo and the products were
recrystallised from CH2Cl2/hexane to afford colourless solids (55 ± 86%).
[36]
[37]
program SHELXS86
and refined using SHELXL93.
Single-crystal X-ray diffraction data for 3 ´ water, 5 ´ (toluene)2 and 6 were
recorded using graphite-monochromated CuKa1 radiation on a Rigaku
R-Axis II diffractometer with rotating anode source and image plate
detector. Data were recorded for two orientations of each crystal. In each
orientation, image plate scans were recorded with crystal rotation in 10 ±
158 frames about one axis over a total range of 1808. The crystal-to-detector
distance was 55 ± 76 mm and the exposure time was 30 ± 50 minutes per
frame. The same instrument was used to record single crystal X-ray
diffraction data for 8, but MoKa radiation was used and the image plate
scans covered 1808 with crystal rotation recorded in 48 frames. Data were
processed using standard R-Axis II software.[38] The crystal structures were
solved by direct methods[39] and refined using SHELXL93.[37]
Hexa-P-phenyl-P,P'-ethanediyl-bis-phosphonium bromide 9: Dibromo-
ethane (5 g, 26.6 mmol) and triphenylphosphine (13.96 g, 53.2 mmol)
afforded
9
(13.8g, 73%). M.p. > 2908C (from CH2Cl2/hexane); MS
‡
(FAB ‡ ): m/z (%): 633 ([M À Br] , 2), 475 (5), 289 (100), 262 (16), 183
(8); 1H NMR (300 MHz; CDCl3): d ˆ 4.20 (d, JP, H ˆ 4.4 Hz, 4H; PCH2),
2
7.62 ± 7.82 (m, 18H; ArH), 7.95 ± 8.08 (m, 12H; ArH); 13C NMR (75 MHz;
1
CDCl3): d ˆ 56.7 (PCH2CH2), 116.5 (d, JP, C ˆ 87 Hz; C1 Ph), 130.6, 134.6,
135.3.
Hexa-P-phenyl-P,P'-propanediyl-bis-phosphonium bromide 10: Dibromo-
propane (5 g, 24.7 mmol) and triphenylphosphine (12.98g, 49.5 mmol)
afforded 10 (11.7 g, 66%). M.p. > 2908C (from CH2Cl2/hexane); MS
‡
The crystal structure of 7 was solved from powder X-ray diffraction data
using the Genetic Algorithm method in our program GAPSS[40] and refined
using standard Rietveld refinement techniques in the program GSAS.[41]
Full details of this structure determination are described elsewhere.[42]
(FAB ‡ ): m/z (%): 647 ([M À Br] , 17), 565 (6), 489 (10), 289 (100);
1H NMR (300 MHz; CDCl3): d ˆ 1.80 ± 1.99 (m, 2H, PCH2CH2), 4.54 ± 4.73
(m, H, PCH2), 7.52 ± 8.05 (m, 30H, ArH); 13C NMR (75 MHz; CDCl3): d ˆ
1
1
17.7 (PCH2CH2), 22.3 (d, JP, C ˆ 54.2 Hz; PCH2), 117.7 (d, JP, C ˆ 87.4 Hz,
C1; Ph), 130.3 (d, 2JP, C ˆ 6.0 Hz; C2 and C2' Ph), 134.0 (d, 3JP, C ˆ 4.0 Hz; C3
and C3' Ph), 134.8(C4 Ph).
Powder X-ray diffraction patterns for 2, 3 ´ water, 4, 5 ´ (toluene)2, 6, 7 and 8
were recorded on a Siemens D5000 diffractometer using Ge-monochro-
mated CuKa1 radiation and a linear position-sensitive detector covering 88
in 2q. These powder diffraction patterns confirmed that, in all cases, the
single crystals used for structure determination from single crystal
diffraction data were representative of the bulk polycrystalline samples.
Hexa-P-phenyl-P,P'-butanediyl-bis-phosphonium bromide 11: Dibromo-
butane (5 g, 23.1 mmol) and triphenylphosphine (12.1 g, 43.6 mmol)
afforded 11 (10.7 g, 63%). M.p. > 2908C (from CH2Cl2/hexane); MS
‡
(FAB ‡ ): m/z (%): 661 ([M À Br] , 55), 579 (52), 503 (24), 317 (77), 289
Chem. Eur. J. 2000, 6, No. 13
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2347

D. Philp, K. D. M. Harris et al.
FULL PAPER
(100), 262 (58); 1H NMR (300 MHz; CDCl3): d ˆ 1.71 ± 1.93 (m, 4H;
(5), 154 (16); 1H NMR (300 MHz; CDCl3): d ˆ 1.90 ± 2.11 (m, 2H;
PCH2CH2), 2.43 ± 2.56 (m, 4H; PCH2), 7.35 ± 7.75 (m, 20H; ArH);
PCH2CH2), 3.62 ± 3.85 (m, 4H; PCH2), 7.68± 8.02 (m, 30H; ArH); 13C NMR
1
2
1
(75 MHz; CDCl3): d ˆ 22.0 (d, JP, C ˆ 51.3 Hz; PCH2), 23.1 (d, JP, C
ˆ
ˆ
13C NMR (75 MHz; CDCl3): d ˆ 14.9 (PCH2CH2), 30.2 (d, JP, C ˆ 71.0 Hz;
1
2
20.1 Hz; PCH2CH2), 118.1 (d, JP, C ˆ 86.1 Hz, C1; Ph), 130.4 (d, JP, C
PCH2), 128.7 (d, 2JP, C ˆ 10.8Hz; C2 and C2 ' Ph), 130.7 (d, 3JP, C ˆ 9.3 Hz; C3
3
1
12.6 Hz; C2 and C2' Ph), 134.1 (d, JP,C ˆ 10.1 Hz; C3 and C3' Ph), 134.7
and C3' Ph), 131.7 (C4 Ph), 132.7 (d, JP,C ˆ 98.3 Hz; C1 Ph); 31P NMR
(C4 Ph).
(121.5 MHz; solid-state CP MAS): d ˆ 32.5, 37.7; 31P NMR (121.5 MHz;
CDCl3): d ˆ 32.7.
Hexa-P-phenyl-P,P'-pentanediyl-bis-phosphonium bromide 12: Dibromo-
pentane (5 g, 21.7 mmol) and triphenylphosphine (11.38g, 43.4 mmol)
afforded 12 (10.9 g, 67%). M.p. > 2908C (from CH2Cl2/hexane); MS
Tetra-P-phenyl-P,P'-butanediyl-bis-phosphine oxide 4: Phosphonium bro-
mide 11 (5 g, 6.7 mmol) and NaOH 30% (50 mL) afforded 4 (2.74 g, 88%).
M.p. 256.2 ± 256.68C (from chloroform) (lit. 259 ± 2608C[45]); MS (FAB ‡ ):
‡
(FAB ‡ ): m/z (%): 675 ([M À Br] , 91), 593 (24), 517 (35), 331 (100),
‡
‡
m/z (%): 481 ([M ‡ Na] , 100), 459 ([MH] , 53); 1H (300 MHz; CDCl3):
d ˆ 1.62 ± 1.81 (m, 4H; PCH2CH2), 2.15 ± 2.35 (m, 4H; PCH2), 7.35 ± 7.76 (m,
20H; ArH); 13C NMR (75 MHz; CDCl3): d ˆ 23.1 (PCH2CH2), 29.5 (d,
289 (82), 262 (39); 1H NMR (300 MHz; CDCl3): d ˆ 1.75 ± 2.25 (m, 6H;
PCH2CH2CH2), 3.62 ± 3.85 (m, 4H; PCH2), 7.58± 8.02 (m, 30H, ArH);
1
13C NMR (75 MHz; CDCl3): d ˆ 22.0 (PCH2CH2CH2), 22.6 (d, JP, C
ˆ
1JP, C ˆ 71.8Hz; PCH 2), 128.7 (d, JP, C ˆ 11.6 Hz; C2 and C2' Ph), 130.7 (d,
2
2
1
50.7 Hz; PCH2), 31.4 (d, JP, C ˆ 17.5 Hz; PCH2CH2), 118.2 (d, JP, C
ˆ
3JP, C ˆ 9.2 Hz; C3 and C3' Ph), 131.8(C4 Ph), 132.8(d, JP, C ˆ 98.1 Hz; C1
1
2
86.1 Hz, C1; Ph), 130.4 (d, JP, C ˆ 12.5 Hz; C2 and C2' Ph), 133.9 (d,
Ph); 31P NMR (121.5 MHz; solid-state CP MAS): d ˆ 29.0; 31P NMR
(121.5 MHz; CDCl3): d ˆ 32.3.
3JP, C ˆ 9.9 Hz; C3 and C3' Ph), 134.8(C-4).
Hexa-P-phenyl-P,P'-hexanediyl-bis-phosphonium bromide 13: Dibromo-
hexane (6 g, 24.6 mmol) and triphenylphosphine (12.9 g, 49.2 mmol)
afforded 13 (10.5 g, 55%). M.p. > 2908C (from CH2Cl2/hexane); MS
Tetra-P-phenyl-P,P'-pentanediyl-bis-phosphine oxide 5: Phosphonium bro-
mide 12 (5 g, 6.6 mmol) and NaOH 30% (50 mL) afforded 5 (2.5 g, 78%).
M.p. for 5 ´ (toluene)2 (crystallized from toluene) 123.6 ± 123.88C (from
slow heating), whereas fast heating resulted in the appearance of liquid at
about 458C (see section on thermal analysis for a full discussion) (lit. 124 ±
‡
(FAB ‡ ): m/z (%): 689 ([M À Br] , 100), 607 (28), 531 (41), 345 (65),
289 (50), 262 (72); 1H NMR (300 MHz; CDCl3): d ˆ 1.54 ± 1.95 (m, 8H,
PCH2(CH2)2), 3.65 ± 3.91 (m, 4H, PCH2), 7.62 ± 7.95 (m, 30H, ArH);
‡
1268C[45]); MS (FAB ‡ ): m/z (%): 473 ([MH] , 100), 271 (5), 229 (4), 215
1
13C NMR (75 MHz; CDCl3): d ˆ 22.4 (d, JP, C ˆ 50.3 Hz, PCH2), 22.3
(7), 154 (5); 1H NMR (300 MHz; CDCl3): d ˆ 1.45 ± 1.71 (m, 6H;
PCH2(CH2)2), 2.12 ± 2.28(m, 4H; PCH 2), 7.35 ± 7.85 (m, 20H; ArH);
2
1
(PCH2CH2CH2), 29.1 (d, JP, C ˆ 16.4 Hz, PCH2CH2), 118.2 (d, JP, C
ˆ
2
85.9 Hz; C1 Ph), 130.5 (d, JP, C ˆ 12.4 Hz; C2 and C2' Ph), 133.7 (d,
1
13C NMR (75 MHz;CDCl3): d ˆ 20.9 (PCH2CH2), 29.3 (d, JP, C ˆ 71.9 Hz;
3JP, C ˆ 9.8Hz; C3 and C3 ' Ph), 134.9 (C4 Ph).
PCH2), 31.9 (PCH2CH2CH2), 128.6 (d, 2JP,C ˆ 11.5 Hz; C2 and C2' Ph), 130.7
(d, 3JP, C ˆ 9.1 Hz; C3 and C3' Ph), 131.7 (C4 Ph), 132.9 (d, 1JP, C ˆ 97.9 Hz; C1
Ph); 31P NMR (121.5 MHz; solid-state CP MAS): d ˆ 32.1; 31P NMR
(121.5 MHz; CDCl3): d ˆ 33.8.
Hexa-P-phenyl-P,P'-heptanediyl-bis-phosphonium bromide 14: Dibromo-
heptane (2.88 g, 11.7 mmol) and triphenylphosphine (5.85 g, 22.3 mmol)
afforded 14 (7.5 g, 86%). M.p. > 2908C (from CH2Cl2/hexane); MS
‡
(FAB ‡ ): m/z (%): 703 ([M À Br] , 100), 621 (34), 545 (39), 359 (77),
311 (20), 289 (46); 1H NMR (300 MHz; CDCl3): d ˆ 1.52 ± 2.08(m, 10H,
PCH2(CH2)3), 3.63 ± 3.85 (m, 4H, PCH2), 7.63 ± 7.95 (m, 30H, ArH);
Tetra-P-phenyl-P,P'-hexanediyl-bisphosphine oxide 6: Phosphonium bro-
mide 13 (5 g, 6.5 mmol) and NaOH 30% (50 mL) afforded 6 (2.8g, 89%).
M.p. 196.9 ± 197.28C (from chloroform) (lit. 196 ± 1988C[45]); MS (FAB ‡ ):
3
13C NMR (75 MHz; CDCl3): d ˆ 22.1 (d, JP, C ˆ 4.5 Hz; PCH2CH2CH2),
‡
‡
m/z (%): 509 ([M ‡ Na] , 67), 487 ([MH] , 32); 1H NMR (300 MHz;
1
2
22.5 (d, JP, C ˆ 50.3 Hz; PCH2), 27.3 (PCH2(CH2)2CH2), 29.6 (d, JP, C
ˆ
ˆ
1
2
CDCl3): d ˆ 1.31 ± 1.44 (m, 4H; PCH2CH2CH2), 1.48± 1.66 (m, 4H;
16.4 Hz; PCH2CH2), 118.1 (d, JP, C ˆ 85.9 Hz; C1 Ph), 130.4 (d, JP, C
3
PCH2CH2), 2.13 ± 2.27 (m, 4H; PCH2), 7.38± 7.76 (m, 20H; ArH);
12.4 Hz; C2 and C2' Ph), 133.7 (d, JP, C ˆ 9.8Hz; C3 and C3 ' Ph), 134.9
13C NMR (75 MHz; CDCl3): d ˆ 21.3 (PCH2CH2CH2), 29.6 (d, JP, C
ˆ
1
(C4 Ph).
2
2
72.2 Hz; PCH2), 30.5 (d, JP,C ˆ 14.3 Hz; PCH2CH2), 128.6 (d, JP, C
ˆ
Hexa-P-phenyl-P,P'-octanediyl-di-phosphonium bromide 15: Dibromooc-
tane (1.55 g, 5.7 mmol) and triphenylphosphine (3 g, 11.4 mmol) afforded
15 (3.46 g, 76.3%). M.p. > 2308C (from CH2Cl2/hexane); MS (FAB ‡ ): m/z
3
11.5 Hz; C2 and C2' Ph), 130.7 (d, JP, C ˆ 9.1 Hz; C3 and C3' Ph), 131.6
(C4 Ph), 133.0 (d, 1JP, C ˆ 97.3 Hz; C1 Ph); 31P NMR (121.5 MHz; solid-state
CP MAS): d ˆ 29.0; 31P NMR (121.5 MHz; CDCl3): d ˆ 32.7.
‡
(%): 717.2 ([MH À Br] , 100), 635 (43), 559 (41), 373 (41), 289 (53), 262
(73); 1H NMR (300 MHz; CDCl3): d ˆ 1.18± 1.35 (m, 4H; P(CH 2)3CH2),
1.52 ± 1.75 (m, 8H, PCH2CH2CH2), 3.54 ± 3.78(m, 4H, PC H2), 7.61 ± 7.95
Tetra-P-phenyl-P,P'-heptanediyl-bis-phosphine oxide 7: Phosphonium bro-
mide 14 (4 g, 5.1 mmol) and NaOH 30% (50 mL) afforded 7 (2.5 g, 98%).
‡
3
(m, 30H; ArH); 13C NMR (75 MHz; CDCl3): d ˆ 22.3 (d, JP, C ˆ 3.0 Hz;
M.p. 90 ± 918C (from CH2Cl2/hexane); MS (FAB ‡ ): m/z (%): 501 ([MH] ,
100), 299 (6), 215 (6), 154 (11); 1H NMR (300 MHz; CDCl3): d ˆ 1.35 ± 1.45
(m, 6H, PCH2CH2(CH2)2), 1.55 ± 1.70 (m, 4H; PCH2CH2), 2.13 ± 2.28(m,
4H, PCH2), 7.40 ± 7.79 (m, 20H; ArH); 31C NMR (75 MHz; CDCl3): d ˆ
1
P(CH2)2CH2), 22.7 (d, JP, C ˆ 50.8Hz; PCH 2), 28.0 (P(CH2)3CH2), 29.8(d,
1
2JP, C ˆ 16.4 Hz; PCH2CH2), 118.2 (d, JP, C ˆ 85.9 Hz; C1 Ph), 130.5 (d,
3
2JP, C ˆ 12.4 Hz; C2 and C2' Ph), 133.7 (d, JP, C ˆ 10.2 Hz; C3 and C3' Ph),
1
2
4
21.3, 28.5, 29.6 (d, JP, C ˆ 71.7 Hz; PCH2), 30.6 (d, JP, C ˆ 14.7 Hz;
135.0 (d, JP,C ˆ 2.8Hz; C4 Ph).
2
3
PCH2CH2), 128.6 (d, JP, C ˆ 11.4 Hz; C2 and C2' Ph), 130.7 (d, JP, C
ˆ
Preparation of the tetra-P-phenyl-P,P'-alkenyl-bis-phosphine oxides 2 ± 8
1
9.1 Hz; C3 and C3' Ph), 131.7 (C4 Ph), 133.1 (d, JP,C ˆ 98.3 Hz; C1 Ph);
31P NMR (121.5 MHz; solid-state CP MAS): d ˆ 27.8, 30.4; 31P NMR
(121.5 MHz; CDCl3): d ˆ 32.9.
General procedure: The salt was heated under reflux for 10 h in 30%
aqueous NaOH (50 mL). The mixture was cooled and diluted. The product
was extracted with CH2Cl2 (2 Â 20 mL). The organic layer was dried with
MgSO4 and filtered. The solvent was then removed under reduced pressure
to afford colourless solids, that were recrystallised from CH2Cl2/hexane.
Tetra-P-phenyl-P,P'-octanediyl-bis-phosphine oxide 8: Phosphonium bro-
mide 15 (2 g, 2.5 mmol) and NaOH 30% (25 mL) afforded 8 (0.97 g,
75.9%). M.p. 172.4 ± 172.88C (from chloroform) (lit. 170.5 ± 1728C[44]); MS
Tetra-P-phenyl-P,P'-ethanediyl-bis-phosphine oxide 2: Phosphonium bro-
mide 9 (5 g, 6.9 mmol) and NaOH 30% (50 mL) afforded 2 (52.5 mg, 1.7%
(low due to difficulties during purification)). M.p. 268.4 ± 268.58C (from
‡
‡
(FAB ‡ ): m/z (%): 537 ([M ‡ Na] , 100), 515 ([MH] , 39), 313 (9), 201
(10); 1H NMR (300 MHz; CDCl3): d ˆ 1.11 ± 1.44 (m, 8H;
P(CH2)2CH2CH2), 1.45 ± 1.68(m, 4H; PCH 2CH2), 2.12 ± 2.36 (m, 4H;
PCH2), 7.32 ± 7.84 (m, 20H; ArH); 13C NMR (75 MHz; CDCl3): d ˆ 21.3 (d,
3JP, C ˆ 3.96 Hz, PCH2CH2CH2), 28.7 (P(CH2)3CH2), 29.6 (d, 1JP, C ˆ 71.7 Hz,
‡
chloroform) (ref. 268± 269 8C[43]); MS (FAB ‡ ): m/z (%): 453 ([M‡Na] ,
‡
100), 431 ([MH] , 75), 229 (29), 176 (12), 154 (8); 1H NMR (300 MHz;
2
CDCl3): d ˆ 2.50 (d, JP, C ˆ 2.5 Hz, 4H; PCH2), 7.39 ± 7.56 (m, 12H; ArH),
2
2
PCH2), 30.8(d, JP,C ˆ 14.7 Hz), 128.6 (d, JP, C ˆ 11.8Hz; C2 and C2 ' Ph),
1
7.64 ± 7.75 (m, 8H; ArH); 13C NMR (75 MHz; CDCl3): d ˆ 21.6 (d, JP, C
ˆ
3
4
130.7 (d, JP, C ˆ 9.6 Hz; C3 and C3' Ph), 131.6 (d, JP, C ˆ 2.8Hz; C4 Ph),
74.1 Hz; PCH2), 128.8, 130.8, 131.1 (d, JP, C ˆ 100.7 Hz; C1 Ph), 132.1; 31P
NMR (121.5 MHz; solid-state CP MAS): d ˆ 30.7; 31P NMR (121.5 MHz;
CDCl3): d ˆ 32.9.
1
133.1 (d, JP, C ˆ 97.7 Hz; C1 Ph); 31P NMR (121.5 MHz; solid-state CP
1
MAS): d ˆ 30.3; 31P NMR (121.5 MHz; CDCl3): d ˆ 33.3.
Tetra-P-phenyl-P,P'-propanediyl-bis-phosphine oxide 3: Phosphonium
bromide 10 (5 g, 6.8mmol) and NaOH 30% (50 mL) afforded 3 (3 g,
98%). M.p. for 3 ´ water (crystallized from toluene) 142.9 ± 143.28C (from
slow heating), whereas fast heating resulted in the appearance of liquid at
about 828C (see section on thermal analysis for a full discussion) (lit. 142 ±
Acknowledgement
We are grateful to EPSRC, HEFCE and the University of Birmingham for
financial support. Dr R L Johnston is thanked for his involvement in the
‡
1438C[44]); MS (FAB ‡ ): m/z (%): 445 ([MH] , 100), 367 (3), 307 (4), 201
2348
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Chem. Eur. J. 2000, 6, No. 13

Bis-diphenylphosphine Oxides
2338±2349
work on structure solution from powder diffraction data, and Professor W.
Clegg and Dr. S. Teat are thanked for assistance in connection with the
microcrystal X-ray diffraction experiments on Station 9.8at Daresbury
Laboratory.
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Received: October 8, 1999 [F2075]
Chem. Eur. J. 2000, 6, No. 13
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0947-6539/00/0613-2349 $ 17.50+.50/0
2349