[4-(N,N-dimethylamino)phenyl]diphenylphosphine oxide and its partial oxides: An isomorphous crystal series

Article abstract of DOI:10.1071/CH03089

The title compound [4-(N,N-dimethylamino)phenyl]diphenylphosphine oxide (1) has been synthesized and its single-crystal X-ray structure determined. The structures of two 'partial' oxides of the parent compound, (2) and (3) (50% and 25% oxidized), isolated from the intermediate phosphine reaction product have also been determined, yielding an unusual isomorphous crystal series. An apparent P-O bond shortening with increasing oxidation is also observed across the series (1)-(3) (1.480(4), 1.351(4), 1.280(4) A respectively) and is considered an artifact of the structure refinement. The packing in the crystals shows primary one-dimensional chains with weak head-to-tail associations between the oxide oxygen (three-centre) and two hydrogen atoms from separate methyl groups of the N-substituted amine (range C-H...O 3.115(4)-3.356(8) A), extended into a sheet structure by peripheral inter-chain C-H (aromatic)...O (oxide) associations.

Full text of DOI:10.1071/CH03089

CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2003, 56, 1135–1139
[4-(N,N-Dimethylamino)phenyl]diphenylphosphine Oxide and
its Partial Oxides:An Isomorphous Crystal Series
A
B,D
C
C
Daniel E. Lynch, Graham Smith,
Karl A. Byriel and Colin H. L. Kennard
A
School of Science and the Environment, Coventry University, Coventry CV1 5FB, United Kingdom.
School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane 4001, Australia.
Department of Chemistry, The University of Queensland, Brisbane 4067, Australia.
B
C
D
Author to whom correspondence should be addressed (e-mail: g.smith@qut.edu.au).
The title compound [4-(N,N-dimethylamino)phenyl]diphenylphosphine oxide (1) has been synthesized and its
single-crystal X-ray structure determined. The structures of two ‘partial’ oxides of the parent compound, (2) and
(3) (50% and 25% oxidized), isolated from the intermediate phosphine reaction product have also been determined,
yielding an unusual isomorphous crystal series. An apparent P–O bond shortening with increasing oxidation is
also observed across the series (1)–(3) (1.480(4), 1.351(4), 1.280(4) Å respectively) and is considered an artifact
of the structure refinement. The packing in the crystals shows primary one-dimensional chains with weak head-
to-tail associations between the oxide oxygen (three-centre) and two hydrogen atoms from separate methyl groups
of the N-substituted amine (range C–H · · · O 3.115(4)–3.356(8) Å), extended into a sheet structure by peripheral
inter-chain C–H (aromatic) · · · O (oxide) associations.
Manuscript received: 30 April 2003.
Final version: 4 August 2003.
Introduction
incorporation of a lattice water molecule in order to balance
[
6]
It can be difficult to produce linear, one-dimensional (1-D)
hydrogen-bonded arrays in organic molecules without the
presence of secondary associations, which usually cross-link
these chains. There are several examples of adducts hav-
ing 1-D arrays which utilize disubstituted molecules with
interactive sites, but the design of single molecules for
this task usually requires one hydrogen-bond donor group
substituted in an opposing direction to a hydrogen-bond
acceptor element. However, neither donor nor acceptor
group should preferentially bind to itself. For this rea-
son carboxylic acids are not suitable as the donor element
because of their tendency to form dimers, usually across
an inversion centre (although there are exceptions such as
the hydrogen-bonding network. The oxygen atom of tri-
phenylphosphine oxide (tppo) has also proved to be a useful
hydrogen-bond acceptor but thus far only molecules con-
taining strong hydrogen-bond donor elements (e.g. mineral
[
7]
[8]
acids, carboxylic acids, amines and amides (including
[
1]
[9]
[10]
sulfonamides and ureas), and phenols and carbinols )
have provided examples of adducts with this compound.
With these, the presence of a simple non-linear single
[
2]
[11]
acceptor (1 : 1) hydrogen-bonding association
D
(graph set
[
12]
) is found between the proton donor group and the
tppo oxygen acceptor. Exceptions to this simple type of
[
3]
association among the tppo adducts are the double accep-
[
11]
tor types, with N,N-dimethyl-o-phenylenediamine,
3,4,5,6-tetrachloro-o-catechol,
with
[
3,4]
[10]
the relatively rare catena arrangement
which is essen-
and the analogous com-
[
5]
tially a 1-D array). (2-Carboxyphenoxy)acetic acid also
exhibits a single linear array, propagated through the acetic
acid (O–H) and the carbonyl oxygen of the 2-carboxy group.
Suitable donor groups include alcohols (O–H) and secondary
amines (N–H). The latter is ideally the donor element in
hydrogen-bonding associations, but in practice can prove
to be the acceptor element as well, hence producing sec-
ondary interactions. Either carbonyl oxygen or heterocyclic
nitrogen atoms are most suitable for the acceptor element
while nitro groups may also be considered, although they
also participate in secondary associations but more often are
non-associated. It is unwise to have too many hydrogen-bond
acceptor groups because these increase the probability of
pound with pentafluorophenol (where the second donor
is fluorine), and the lone triple acceptor example with
[
9]
p-toluenesulfonamide. The hydrate examples with tere-
[
8]
phthalic acid and adamantane carboxylic acid and with
[
13]
the diacetylene
(where structure extension involves
the water molecules) are exceptional but nevertheless
involve standard single acceptor (water) O–H · · · O (tppo)
associations.
The hydrogen-bond donor capabilities of dimethylamino
[9,14]
substituents have been previously demonstrated
on
groups which would normally be considered weaker
hydrogen-bond acceptors than the phosphine oxide. For this
reason we decided to synthesize a simple triarylphosphine
©
CSIRO 2003
10.1071/CH03089
0004-9425/03/111135

1
136
D. E. Lynch et al.
[
17]
oxide analogue containing a single para-dimethylamino-
substituted phenyl group and to investigate the hydrogen-
bonding array of the resultant solid using single crystal X-ray
methods. Molecules containing opposing diphenylphosphine
bromide complex with bis(diphenylphosphino)methane,
in which the occupancy factor for O was 30%. With the
Pd(ii) chloride complex with a similar diphosphine
[
18]
the
pendant arm was 50% oxidized. The phenomenon has
also been observed in structures of uncoordinated phos-
phines such as 1,1,1-tris(diphenylphosphinomethyl)propane
where the three oxygen atoms had occupancies of 11.3,
ꢀ
oxide and dimethylamino groups linked via biphenyl-4,4 -
ꢀ
ꢀ
diyl-, trans-stilbenyl-4,4 -diyl-, and azobenzene-4,4 -diyl-
[15]
bridges have been previously synthesized but no structural
data have been reported for these.
[
19]
In another example, the triphenylphos-
13.5, and 39.0%.
phine oxide/triphenylmethanol dimer,
[
20]
Reported in this paper are the synthesis and the single-
crystal X-ray structure of [4-(N,N-dimethylamino)phenyl]-
diphenylphosphine oxide (dappo) (1) (the 100% oxidized
or ‘full oxide’), together with those of the unusual ‘partial’
oxides of this compound, the ‘half-oxide’(2) (50% oxidized)
and the ‘quarter-oxide’ (3) (25% oxidized), formed in the
preparation of the parent phosphine (dapp) by partial facile
oxidation.
the two molecules
are related by pseudo-symmetry across a crystallographic
inversion centre (50% occupancy) while with the 50% oxi-
dized form of 1,4-bis(diphenylphosphino)butane, the two
phosphine centres (one oxidized and the other unoxidized)
are also 50% disordered across a crystallographic inver-
[
21]
sion centre.
This last example, 1-(diphenylphosphino)-
4-(diphenylphosphinoxido)butane, dppbo (B), is similar to
the present series, compounds (1)–(3), in being isomorphous
with the parent 1,4-bis(diphenylphosphinoxido)butane,
Discussion
[
22]
dppbo (A).
The crystal structures of the ‘full-oxide’(1) (shown in Fig. 1),
the ‘half oxide’ (2) (in which the oxygen has 50% occu-
pancy) and the ‘quarter-oxide’ (3) (in which the oxygen
has 25% occupancy) reveal an unusual isomorphous series.
The comparative cell dimensions for the three (Table 1),
of which (1) and (3) were obtained at low temperature,
An unusual structural feature is that within the set pro-
ceeding (1)–(3), the P–O bond distance shortens from the
[
23]
expected value from the structures of tppo
(1.483(2)
and 1.484(1) Å in the two monoclinic modifications and
1.479(2) Å in the orthorhombic modification) and with the
[
24]
tppo hemihydrates (1.484(1), 1.484(1) Å), and the adducts
3
[8]
show an expected volume contraction (54 Å ) in the cell
of tppo with compounds such as carboxylic acids where
for that pair. The phenomenon whereby partial oxidation of
the P in aryl phosphines has a small but significant effect
on the cell volume has precedence among crystallograph-
ically characterized examples in the Cambridge Structural
the molecules are linked by simple linear O–H· · ·O hydro-
[
8]
gen bonds (1.489 Å over eight examples ). This ‘normal’
P–O distance is found in (1) (1.480(4) Å) and compares with
1.351(4) in (2) to 1.280(4) Å in (3), the decrease paralleling
[
16]
Database (CSD), as indicated by the following. Heinze
reported that selective oxidation of the phosphorus centres
in uncoordinated ‘dangling arms’ of a multidentate phos-
phine ligand (triphos) in a series of Co(ii) complexes occurs
quite readily, but in the presence of dioxygen, reaction con-
ditions quite unlike those employed in our syntheses. The
first reported example of partial (incomplete) oxidation of
the pendant-arm phosphine-P was in the Re(iii) carbonyl
Table 1. Comparative cell data for compounds (1), (2), and (3)
Parameter
(1)
(2)
(3)
CCDC reference
Formula
Mr
206052
206053
206054
C20H20NOP C20H20NO0.50P C20H20NO0.25P
321.34
313.34
309.34
Crystal class
Space group
a [Å]
monoclinic
monoclinic
monoclinic
P2 /n
1
P2 /n
1
P2 /n
1
10.405(14)
10.491(7)
10.289(5)
b [Å]
c [Å]
8.672(12)
18.84(3)
96.469(16)
1689(4)
1.263
8.857(3)
18.71(1)
96.11(3)
1729(2)
1.204
4
0.159
293(2)
50
8.841(5)
18.04(1)
95.038(7)
1635(1)
1.257
◦
C33
C32
C34
C35
α [ ]
3
V [Å ]
Dc [g cm
Z
−3
]
4
4
µ (MoKα) [mm 1
Temp. [K]
−
]
0.167
168(2)
38
0.170
168(2)
53
C31
C36
C13 C12
C141
N141
◦
2
θmax [ ]
C14
P1
Crystal size [mm]
0.45 × 0.40
0.50 × 0.40
× 0.25
3222
3034
0.031
1821
0.0421
0.131
0.75
0.40 × 0.40
× 0.35
18724
3275
0.035
2507
0.0358
0.091
1.08
×
0.35
O1
C21
C11
Total data
Unique data
Rint
8681
1360
0.086
1154
0.060
0.136
1.05
C142
C15 C16
C26
N [I > 2.0σ(I)]
A
R1
C22
A
wR2
S
C25
C24
A
A, BB
0.0647, 4.22 0.146, 0.00
0.0533, 0.0501
C23
A
2
2 2
2 2 ½
R1 = (ꢀ|Fo| − |Fc|)/ꢀ|Fo|; wR2 = (ꢀw(F − F ) /ꢀw(F ) ) ;
o
c
o
2
2 2
½
Fig. 1. Molecular conformation and atom numbering scheme for (1).
S = {ꢀ [w(F − F ) ]/(n − p)} .
2 2 2
o
c
[
30]
B
−1
2
2
Atoms are shown as 30% probability ellipsoids.
w = [σ (F ) + (AP) + BP] , where P = [(maxF , 0) + 2(F , )]/3.
o
o
c

[
4-(N,N-Dimethylamino)phenyl]diphenylphosphine Oxide and its Partial Oxides
1137
◦
O occupancy.This contraction in bond length is very unusual
but is consistent with observations for the same set of pre-
(100%), compare 114.8 (50%)) indicated that the P moves
towards the O rather than vice versa. An almost identical
phenomenon is observed for this series (Table 2) where the
comparative mean C–P–C angles (106.1 (100%), compare
[18–20]
viously considered structures
phosphine groups. The values in (1)–(3) compared with the
having partially oxidized
[
20]
◦
distances for the other examples are as follows: 1.378(3)
103.9 (50%)) and C–P–O angles (112.7 (100%) compare
114.6 (50%)) show a further parallel change proceeding to
(3) (25%) (102.5 and 115.7 , respectively). This trend in the
[18]
◦
and 1.39(2) Å
(both ‘50% oxides’); 1.399(4) (11.3%);
[16]
◦
1
.295(14) (13.5%); 1.368(5) (39.0%).
This unusual phe-
nomenon had not been addressed previously in the litera-
ture before our 50% 1,4-bis(diphenylphosphinoxido)butane
C–P–C cone angles is similar to the angle change in pro-
ceeding from phosphine oxide to phosphine. In the structure
refinements with partial oxygen occupancy the bulky phenyl
groups retain essentially identical sites in their isomorphous
lattices and the P atom is pushed towards the (missing or par-
tially present) oxygen, decreasing the cone angle at P. The
result is that the bond between the weighted mean phospho-
rus position and the partially occupied oxygen site appears
[21]
example
where it was considered that the bond short-
ening was an artefact of the crystallographic refinement
process since the physical characteristics of the P–O bond
3
1
(
P NMR shift and ν(P O) infrared stretching frequency)
[
25]
did not change for this pair, e.g. for dppbo (A) (100%)
and dppbo (B) (50%) , ν(P O) was 1185 and 1182 cm ,
respectively, comparing with 1190 for (1) and 1195 cm for
tppo.
[21]
−1
−
1
[27]
shorter. The effect is clearly an artefact of the refinement
[26]
Moreover, a contraction of the C–P–C bond angles
process as suggested previously and could only be resolved
by treating the P O moiety as two different weighted entities.
The molecules of all three polymorphs (1)–(3) form linear
hydrogen-bonded chains which propagate through a three-
centre (about O1) C–H · · · O association involving hydrogen
atoms from two different methyl substituents on the amine
in dppbo (A) and dppbo (B) (mean 105.9 (100%), compare
◦
1
03.5 (50%)) and an expansion C–P–O angles (mean 112.7
Table 2. Comparative P–O bond distances and mean P cone angles
◦
◦
Compound %O P–O [Å] C–P–C [ ] C–P–O [ ] Reference
a
a
group (C121–H12 · · · O1 , C122–H14 · · · O1 ; a = − ½ + x,
(
(
(
1)
2)
3)
100 1.480(4)
50 1.351(4)
25 1.280(4)
100 1.481(2)
50 1.377(4)
106.1
103.9
102.5
105.9
103.5
112.7
114.6
115.7
112.7
114.8
This work
This work
This work
[22]
½
− y, − ½, + z; Fig. 2a). The range for these C · · · O dis-
tances is 3.115(4)–3.381(8) Å (Table 3) and is analogous
to the double acceptor O–H · · · O and N–H · · · O associa-
tions found with small incidence among the tppo adducts.
dppbo (A)
dppbo (B)
[21]
(a)
(b)
Fig. 2. (a) Head-to-tail chain extension showing the C–H · · · O hydrogen bonding as broken lines. The
view is down the b cell direction. (b) Peripheral inter-chain C–H · · · O hydrogen bonding.
Table 3. Hydrogen bond lengths [Å]
a
a
b
b
Compound
C141–H142 · · · O1
C142–H144 · · · O1
C32–H32 · · · O1
C33–H33 · · · O1
(
(
(
1)
2)
3)
3.381(8)
3.326(6)
3.232(5)
3.356(8)
3.335(6)
3.115(4)
3.337(9)
3.343(6)
3.161(5)
3.273(9)
3.276(6)
3.150(5)
a = − ½ + x, ½ − y, −½ + z; b = 1½ − x, ½ + y, ½ − z.

1
138
D. E. Lynch et al.
The chains extend across the ac cell diagonal and are weakly
linked peripherally through C–H (aromatic) · · · O three-
No absorption corrections were applied. The structures were solved by
direct methods and refined using full-matrix least-squares (SHELX-
[
29]
b
b
97 ), with anisotropic thermal parameters for all non-hydrogen atoms.
Hydrogen atoms were either included in the refinement with coordinates
and isotropic thermal parameters refined or were included at calculated
positions and treated as riding models. Occupancy values for the oxy-
gen atoms in (2) and (3) were obtained by least-squares refinement with
centrehydrogenbonds(C32–H32 · · · O1 , C33–H33 · · · O1 ;
b = 1½ − x, ½ + y, ½ − z; Fig. 2b), range 3.150(5)–
3
.343(6) Å, giving a two-dimensional sheet structure.
2
Conclusions
the isotropic thermal parameter set at 0.04 Å for the low temperature
2
structure (3) and 0.06 Å for the room temperature structure (2), after
An isomorphous crystal series exists for the variable oxide
compound dappo having the formula (for O of 1.00, 0.50,
which these atoms were allowed to refine anisotropically.
Note: A previous room temperature data set collected for compound
(1) using the Nonius CAD-4 diffractometer gave a poor refinement
0
.25), suggestive of a continuous series. An apparent P–O
residual and this prompted the low temperature CCD collection. This in
turn gave a more satisfactory residual which was however considerably
poorer than those for the partial oxides (2) and (3) and is consistent with
an unexplained incidence of twinning and poor morphology with crys-
tals of (1). The initial data set does provide a set of comparative room
temperature unit cell parameters (a 10.518(4), b 8.735(1), c 19.044(7) Å,
bond shortening is observed with decreasing oxygen occu-
pancy. This is a previously observed phenomenon among
phosphine oxides that is not supported by spectroscopic evi-
dence and is considered an artefact of the crystallographic
refinement procedure. The compound is found to form
an extended two-dimensional C–H · · · O hydrogen-bonded
array, indicating its added potential compared with tppo as a
crystallizing aid in crystal chemistry.
◦
3
β 96.64(2) , V 1737.9(9) Å ).
Acknowledgments
The authors acknowledge financial support from the School
of Science and the Environment of Coventry University,
the Centre for Instrumental and Developmental Chemistry
of the Queensland University of Technology, the Australian
Research Council, and the University of Queensland. The
University of Canterbury is thanked for the use of low tem-
perature CCD diffractometer facilities, while G. J. Langley
Experimental
Materials
[
15]
Butyl-
Dapp. Dapp was synthesized using the literature procedure.
◦
lithium (7.6 mL, 1.6 M in hexane) was added at ca. −60 C to a solution
−
2
of 4-bromo-N,N-dimethylaniline (2.44 g, 1.22 × 10 mol) in anhy-
drous tetrahydrofuran (THF) (ca. 50 mL) under argon. After 15 min,
−
3
chlorodiphenylphosphine (2.18 mL, d 1.194 g cm ) was slowly added,
then the mixture was allowed to warm to room temperature and remain
with stirring for 2 h. After hydrolysis with water (40 mL), the product
was extracted with ether. Drying of the ether layer with magnesium
sulfate and then evaporation under reduced pressure yielded a light yel-
low powder. Large colourless prisms of both the ‘half-oxide’(2) and the
(Southampton University) is thanked for ES-MS data
acquisition.
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1] For example: (a) M. C. Etter, D. A. Adsmond, D. Britton, Acta
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◦
(
2 mL, 30 wt-% solution in water) under stirring for 15 min.The mixture
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1
999, 55, 87.
[
[
[
2] (a) M. C. Etter, Acc. Chem. Res. 1990, 23, 120. (b) M. C. Etter,
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−
1
[
2M + H]). ν (KBr)/cm 1190 (s, P O).
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7
75.
−1
tions of ca. 1–10 µg mL acetonitrile solutions and recorded in positive
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Cell data and general details of data collection and structure refine-
ment for compounds (1)–(3) are given in Table 1. X-ray diffraction
data for compound (2) were collected at ambient temperature on an
Enraf–Nonius CAD-4 diffractometer while those for (1) and (3) were
collected at low temperature (168(2) K) on a Siemens P4 diffractometer
equipped with a SMART 1 K CCD area detector. Crystal monochrom-
atized MoKα X-radiation (λ 0.71073 Å) was used for all compounds.
Collection ranges were: (1): −9 ≤ h ≤ 9, −7 ≤ k ≤ 7, −17 ≤ l ≤ 17; (2):
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0
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−
22 ≤ l ≤ 22. No significant crystal decomposition was in evidence for
any compound. Data were corrected for Lorentz and polarization effects,
in the case of the CCD data, applied to three-dimensionally integrated
[
28]
diffraction spots, the data being processed using the program SAINT.

[
4-(N,N-Dimethylamino)phenyl]diphenylphosphine Oxide and its Partial Oxides
1139
(
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