Reactions of phosphine oxides with bromophosphoranimines; Synthesis and unusual rearrangements of O-donor stabilized phosphoranimine cations

Article abstract of DOI:10.1021/ic201360p

Reaction of phosphine oxides R₃P=O [R = Me (1a), Et (1c), ⁱPr (1d) and Ph (1e)], with the bromophosphoranimines BrPR′R″P=NSiMe₃ [R′ = R″ = Me (2a); R′ = Me, R″ = Ph (2b); R′ = R″ = OCH₂CF₃ (2c)] in the presence or absence of AgOTf (OTf = CF₃SO₃) resulted in a rearrangement reaction to give the salts [R₃P=N= PR′R″O-SiMe₃]X (X = Br or OTf) ([4]X). Reaction of phosphine oxide 1a with the phosphoranimine BrPMe₂=NSiPh₃ (5) with a sterically encumbered silyl group also resulted in the analogous rearranged product [Me₃P=N=PMe₂O-SiPh₃]X ([8]X) but at a significantly slower rate. In contrast, the direct reaction of the bulky tert-butyl substituted phosphine oxide, ^tBu₃P=O (1b) with 2a or 2c in the presence of AgOTf yielded the phosphine oxide-stabilized phosphoranimine cations [^tBu₃P= O?PR′₂=NSiMe₃]⁺ ([3]⁺, R′ = Me (d), OCH₂CF₃ (e)). A mechanism is proposed for the unexpected formation of [4]⁺ in which the formation of the donor-stabilized adduct [3]⁺ occurs as the first step.

Full text of DOI:10.1021/ic201360p

ARTICLE
pubs.acs.org/IC
Reactions of Phosphine Oxides with Bromophosphoranimines;
Synthesis and Unusual Rearrangements of OꢀDonor Stabilized
Phosphoranimine Cations
Martin Bendle,† Keith Huynh,‡ Mairi F. Haddow,† and Ian Manners*,†
†School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, U.K.
‡Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada
S Supporting Information
b
ABSTRACT: Reaction of phosphine oxides R3PdO [R = Me
i
(1a), Et (1c), Pr (1d) and Ph (1e)], with the bromopho-
sphoranimines BrPR0R00PdNSiMe3 [R0 = R00 = Me (2a); R0 =
Me, R00 = Ph (2b); R0 = R00 = OCH2CF3 (2c)] in the presence or
absence of AgOTf (OTf = CF3SO3) resulted in a rearrangement
reaction to give the salts [R3PdNdPR0R00OꢀSiMe3]X (X = Br
or OTf) ([4]X). Reaction of phosphine oxide 1a with the
phosphoranimine BrPMe2dNSiPh3 (5) with a sterically en-
cumbered silyl group also resulted in the analogous rearranged
product [Me3PdNdPMe2O-SiPh3]X ([8]X) but at a significantly slower rate. In contrast, the direct reaction of the bulky tert-butyl
t
substituted phosphine oxide, Bu3PdO (1b) with 2a or 2c in the presence of AgOTf yielded the phosphine oxide-stabilized
phosphoranimine cations [tBu3PdO PR02dNSiMe3]+ ([3]+, R0 = Me (d), OCH2CF3 (e)). A mechanism is proposed for the
3
unexpected formation of [4]+ in which the formation of the donor-stabilized adduct [3]+ occurs as the first step.
Scheme 1
’ INTRODUCTION
Phosphoranimines, R3PdNR0 have a diverse chemistry and
have been the subject of considerable attention in the field of
main group chemistry.1,2 N-Silylphosphoranimines (XRR0Pd
NSiMe3, X = halogen, alkoxy; R, R0 = alkyl, aryl, aryloxy, and Cl)
are of particular interest as precursors to form polyphosphazenes
[RR0PdN]n.3ꢀ15 The use of these species as monomers com-
plements the alternative ring-opening polymerization (ROP)
approach from hexachlorocyclotriphosphazene [Cl2PdN]3 which
affords polydichlorophosphazene, [Cl2PdN]n, at 200ꢀ250 °C16,17
or at room temperature using trialkylsilylium carborane initiators.18,19
For example, the PCl5-catalyzed polymerization of Cl3PdNSiMe3 is
a living process and provides access to [Cl2PdN]n with molecular
weight control and narrow polydispersities, compared to the broad
molecular weight distributions afforded by the high temperature ROP
of [Cl2PdN]3.10,11,20ꢀ23 Moreover, although the ROP route yields a
wide variety of polyphosphazenes with RO-, RN(H)-, and RR0N-
substituents following nucleophilic replacement of the halogen
groups in [Cl2PdN]n,16,17,24ꢀ29 only partial substitution is possible
with alkyl or aryl groups. This limitation is a consequence of the chain
cleavage reactions that take place on treatment of [Cl2PdN]n with
Grignard or organolithium reagents,17,30,31 but was overcome by the
development of the thermal condensation polymerization of N-silyl
phosphoranimines. This route yields poly(alkyl/aryl)phosphazenes
through the use of presubstituted phosphoranimines, which can be
synthesized with a wide range of alkyl and aryl groups.3ꢀ6,8,9,12,32ꢀ36
The mechanism of this route has been tentatively proposed to involve
formation of a highly reactive cationic phosphorus(V)ꢀnitrogen
species through heterolytic cleavage of a P-X (X = halogen, OR, R =
alkyl, aryl) bond in the monomer to generate an active cation,
[RRPdNSiMe3]+.9 This highly Lewis acidic species would be
expected to be reactive toward the monomer and initiate a chain-
growth condensation polymerization of the phosphoranimine
CF3CH2OP(RR0)dNSiMe3, at temperatures exceeding 200 °C
(Scheme 1).
The proposed intermediacy of cationic phosphoranimine
species in the polymerization mechanism has made their gen-
eration and isolation of key relevance. Phosphoranimine cations
of the form [P(RR0)dNSiMe3]+ (R, R0 = halogen, alkyl, aryl,
Received: June 24, 2011
Published: September 19, 2011
r
2011 American Chemical Society
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Scheme 2
BrMe2PdNSiMe3 (2a). Phosphine oxides R3PdO (1) were reacted
with bromophosphoranimines (2), with the expectation that
donor-stabilized cation salts [3]Br would be formed (Scheme 4),
as has previously been observed for N- and P-donor ligands such as
DMAP and PMe3.37,38,40,41 Indeed, when BrMe2PdNSiMe3 (2a)
was reacted with 1 equiv of Me3PdO (1a) in CH2Cl2 for 10 min at
25 °C, 31P{1H} NMR revealed consumption of the starting
materials and the formation of a new species (72% yield by 31P{1H}
NMR integration) with resonances at δ = 35.7 and 95.0 ppm, 2JPP
=
28.8 Hz, presumed to be the anticipated48 cation [3a]+ (Scheme 1).
However, in addition to the (initial) major product, several other
species were also observed by 31P{1H} NMR (Figure 1). Moreover,
the concentration of [3a]+ decreased after 60 min, and this was
accompanied by the increase in concentration of a new product with
resonances at δ = 25.0 and 40.9 ppm (2JPP = 20.9 Hz). After 10 h,
only trace quantities of the species giving rise to the initially observed
resonances at δ= 35.7 and 95.0 ppm were present and after 5 d, 92%
conversion to the product with two resonances at δ = 25.0 and 40.9
ppm was detected. Notably, no signal corresponding to poly-
(dimethylphosphazene) [Me2PdN]n (δP = 8 ppm)8 was observed.
Purification of the product to allow isolation as a crystalline material
was hindered by the presence of small quantities of impurities. To
circumvent this problem, the reaction was repeated in the presence
of 1 equiv of AgOTf to replace the anticipated Brꢀ counterion
(precipitated as AgBr) in the product with [OTf]ꢀ. 31P{1H} NMR
of the filtered reaction mixture after 60 min showed complete
consumption of starting materials, as well as the formation of all
species observed in the aforementioned reaction without AgOTf.
Interestingly, the exchange of the anion also resulted in a more rapid
conversion of the initial product (presumed to be [3a]+) with
31P{1H} NMR resonances at δ = 37.3 and 94.0 ppm (2JPP = 28.6
Hz) to the final product (δ = 25.0 and 39.8 ppm, 2JPP = 17.3 Hz)
(72% yield after 60 min), with no significant increase in conversion
beyond 96% after 24 h. Along with decrease in the reaction time, use
of the triflate anion permitted isolation of the final product as a white
crystalline solid allowing characterization of the product by single-
crystal X-ray diffraction. Surprisingly, the X-ray study showed the
product to be [4a]OTf, thereby suggesting that the targeted (and
initially formed) cation [3a]+ had undergone a structural rearrange-
ment (Figure 2, Scheme 4).
(b). Reaction of 1a with BrMePhPdNSiMe3 (2b) and Br-
(CF3CH2O)2PdNSiMe3 (2c). To gain insight into the scope of
this unexpected rearrangement process we studied analogous
reactions where the substituents on the phosphoranimine sub-
strate were varied. The possibility that increased steric bulk might
allow the stabilization of other intermediates during the rearran-
gement process led us to study the unsymmetrically substituted
phosphoranimine BrMePhPdNSiMe3 (2b) as a substrate. On
reaction of 1a and 2b (25 °C, CH2Cl2), the resultant 31P{1H}
NMR spectrum after 60 min revealed the formation of similar
species to those formed in the reaction of 1a with 2a with
comparable coupling constants and patterns, including reso-
nances at δ = 24.0 and 95.7 ppm (2JPP = 29.8 Hz), likely to be
due to [3b]+, and a further species with resonances at δ = 28.5
and 29.1 ppm (2JPP = 11.8 Hz), attributed to the rearranged
product [4b]+ (Scheme 4). Complete consumption of [3b]+ was
observed after 2 d and conversion to [4b]+ reached 89%, with no
further change after 5 d. As for the reaction with 2a, no
Scheme 3
alkoxy) would be expected to be highly electrophilic, and are
therefore likely to require significant stabilization by a suitable
donor to permit their isolation (Scheme 2). Reaction of DMAP
(4-dimethylaminopyridine) with Cl3PdNSiMe3 has been shown
to afford the donor-stabilized cation [DMAP PCl2PdNSiMe3]+.37
3
This chemistry has been extended to a series of other nitrogen-
donors, and substituted phosphoranimines, XP(RR0)dNSiMe3
(X = Cl, Br; R, R0 = Me, Ph, OCH2CF3),37,38 as well as a DMAP-
stabilized hexacationic cyclophosphazene synthesized from the
reaction of DMAP with [Cl2PdN]3.39 Attempts to transpose this
chemistry to phosphines successfully gave the phosphine-stabilized
cations, [R00 PꢀP(RR0)dNSiMe3]+ for a number of alkyl-, aryl-,
3
and alkoxy-substituted phosphoranimines.40,41 However, reaction of
phosphines (R3P, R = Me, Ph) with the trichloro-substituted
phosphoranimine (Cl3PdNSiMe3) resulted in an unexpected
dehalogenation reaction to give R3PdNꢀPCl2 via a complex
mechanism that involves a number of reductive chlorination and
condensation steps.42
Interestingly, attempts to further diversify the chemistry to
phosphite-stabilized cations via the reaction of the phosphorani-
mines BrP(RR0)dNSiMe3 (R = Me, R0 = Ph) with both stoichio-
metric and substoichiometric amounts of phosphite (RO)3P (R =
Me, Et, Ph) at room temperature gave high molecular weight
polymer [RR0PdN]n (Mw = 1.5ꢀ8.0 ꢁ 105 g molꢀ1; PDI =
1.7ꢀ6.5) (for an example, see Scheme 3).40,43 Our initial investiga-
tions into the mechanism of this reaction have revealed the
presence of an induction period,44 a result that poses intriguing
questions as to the nature of the true catalyst or initiator in this
system. There exists a possibility that a small amount of BrSiMe3
(likely to be present as an impurity in the monomer) may act as a
co-catalyst, reacting with the phosphite to give species bearing a
PdO bond via an Arbuzov-type rearrangement reaction.45,46 The
product of such a reaction may be the true initiator. This has
prompted us to investigate the reactions of phosphine oxides
R3PdO with phosphoranimines to assess their ability to stabilize
the phosphoranimine cations or act as polymerization initiators.47
Herein we report our studies of the reactions of a series of
phosphine oxides with phosphoranimines, which has led to the
isolation of the targeted O-stabilized phosphoranimine cations as
well as their participation in some unusual rearrangement reactions.
’ RESULTS AND DISCUSSION
resonances corresponding to polymer, [MePhPdN]n (δP
=
1.8 ppm)12,40 were detected. Repeating the reaction in the
presence of 1 equiv of AgOTf, made no significant difference
1. Reactions of Phosphine Oxides (1aꢀe) with Bromopho-
sphoranimines (2aꢀc). (a). Reaction of Me3PdO (1a) with
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Scheme 4
Figure 1. 31P{1H} NMR spectrum of the reaction between 1a and 2a in CH2Cl2 after 10 min. Signals A and B correspond to the cations [3a]+ and [4a]+
respectively, C and D were unidentified intermediates (see Supporting Information).
to the rate of the reaction, but did permit more complete
conversion to [4b]+ (ca. 100%) after 10 days.
To further investigate the effect of varying the phosphorani-
mine, Br(CF3CH2O)2PdNSiMe3 (2c) was also reacted with 1a.
The use of species 2c, with particularly electron-withdrawing
CF3CH2O substituents as a substrate, has previously been shown
to afford a significant increase in the stability of the respective
P- and N-donor stabilized cations compared to alkyl-substituted
phosphoranimines.38,41 31P{1H} NMR analysis of the reaction of
1a with 2c (25 °C, CH2Cl2) after 60 min showed incomplete
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Scheme 5
Figure 2. Thermal ellipsoid plot of [4a]OTf at the 50% probability
level. All hydrogen atoms have been omitted for clarity.
consumption of starting materials, and formation of a new
species with resonances consistent with the structure [3c]+,
2
2
δ = ꢀ26.3 (d, JPP = 32.0 Hz), 105.2 ppm (d, JPP = 32.0 Hz),
and a species at δ = 2.5 (d, 2JPP = 23.7 Hz), 25.4 ppm (d, 2JPP = 23.7
Hz), presumed to be [4c]+. Species [3c]+ was no longer observed
after 24 h, and the conversion to [4c]+ increased to 83% after 10
days. Significant 31P NMR peak broadening and the formation of
inseparable oils on workup prompted us to repeat the reaction in
the presence of a stoichiometric quantity of AgOTf. For similar
reasons, the final product could not be purified.
(c). Effect of Changing the Phosphoranimine Silyl Substitu-
ent; Synthesis and Reactivity of BrMe2PdNSiPh3 (5). Next, we
investigated the effect of replacing the SiMe3 group on the
phosphoranimine nitrogen center with the more bulky SiPh3
analogue. On the basis of previous reactivity studies,42,49,50 it was
expected that this sterically encumbered phenyl-substituted silyl
moiety might prevent or slow the rearrangement. We therefore
prepared the new phosphoranimine, BrMe2PdNSiPh3 (5) via
the oxidative bromination of the N-silylaminophosphine 6 with
Br2 in CH2Cl2 (Scheme 5). An X-ray crystallographic study was
undertaken of the crystalline product which, along with multi-
nuclear NMR, confirmed the formation of 5 (Figure 3).
The reaction of 1a with 5 (25 °C, CH2Cl2) appeared
analogous to the reaction of 1a with the trimethylsilylphosphor-
animine (2a), but with a significantly slower conversion to the
final product of 66% after 5 d: a similar set of solution species,
analogous to those observed in the reaction of 1a and 2a were
identified by 31P{1H} NMR, including [7]+ and [8]+, the
triphenyl-substituted analogues of [3a]+ and [4a]+, respectively.
Importantly, the formation of [7]+ occurred at a similar rate to
the formation of [3a]+ from 2a whereas the subsequent rearran-
gement to [8]+ was a slower process compared to the conversion
of [3a]+ to [4a]+. Performing the reaction of 1a with 5 in the
presence of AgOTf further slowed the reaction, with 45%
conversion to [8]OTf after 5 d, but permitted isolation of the
product in a form suitable for single-crystal X-ray diffraction. The
result of the X-ray study and further multinuclear NMR analysis
confirmed that the product, [8]OTf, was the analogue of
Figure 3. Thermal ellipsoid plot of 5 at the 50% probability level. All
hydrogen atoms have been omitted for clarity.
[4a]OTf with SiPh3 substituents in place of an SiMe3 group at
nitrogen (Scheme 4, Figure 4).
(d). Reaction of the Sterically Encumbered Phosphine Oxide,
tBu3PdO (1b), with Phosphoranimines 2a and 2c. Next, we
investigated the analogous chemistry using a bulky phosphine
oxide, tBu3PdO (1b) with the aim of isolating intermediates in
the rearrangement reaction. Phosphoranimine 2a was therefore
treated with 1 equiv of 1b (25 °C, CH2Cl2). Analysis of the
reaction solution by 31P{1H} NMR after 10 min revealed about
2% conversion to a single product displaying two resonances
(δ= 32.0 and 113.3 ppm, 2JPP = 66.3 Hz). On the basis of the similar
shifts and an increase in the coupling constant (attributed to the
stronger PdOfP+ interaction afforded by the more strongly
electron donating phosphine oxide), this product appeared
analogous to the intermediate [3a]+ formed in the reaction of
2
1a with 2a (with resonances at δ = 35.7 and 95.0 ppm, JPP
=
28.8 Hz). However, no further reaction beyond 5% was observed
between 1b and 2a after 40 min. This was presumed to be a result
of an equilibrium resulting from competitive nucleophilic attack
of the bromide counterion to displace the sterically encumbered
phosphine 1b. Similar equilibria have been previously detected
for the formation of P- and N-donor stabilized phosphoranimine
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Figure 4. Thermal ellipsoid plot of [8]OTf at the 50% probability level.
All hydrogen atoms have been omitted for clarity.
Figure 6. Thermal ellipsoid plot of [3e]OTf at the 50% probability
level. All hydrogen atoms and additional molecules in the asymmetric
unit have been omitted for clarity.
Reaction of 1 equiv of 1b with the trifluoroethoxy-substituted
phosphoranimine Br(CF3CH2O)2PdNSiMe3 (2c) (25 °C,
CH2Cl2) in the presence of 1 equiv of AgOTf also yielded
a single product by 31P{1H} NMR after 16 h with δ = 30.5
(d, 2JPP = 67 Hz), 118.6 ppm (d, 2JPP = 67 Hz). This species was
also isolated and characterized as an O-donor-stabilized salt of a
phosphoranimine cation [3e]OTf by single crystal X-ray diffrac-
tion (Figure 6, Scheme 4).
i
(e). Reaction of R3PdO (R = Et, Pr, Ph, 1cꢀe) with
Phosphoranimine BrMe2PdNSiMe3 (2a). To examine the scope
t
of the reaction beyond Me- and Bu-substituted phosphine
i
oxides, Et3PdO (1c), Pr3PdO (1d), and Ph3PdO (1e) were
Figure 5. Thermal ellipsoid plot of [3d]OTf at the 50% probability
level. All hydrogen atoms have been omitted for clarity.
also reacted with 2a. We hoped that the range of steric and
electronic properties intrinsic to 1cꢀe would allow us to identify
one or more of the other intermediates in the observed rearran-
gement process. Reaction of 1c with 2a (25 °C, CH2Cl2)
appeared to proceed similarly to that observed between 1a and
2a, with the observation of comparable resonances by 31P{1H}
NMR and 85% conversion to [4d]+ after 5 d. This suggested that
the increase in steric bulk from Me to Et was not sufficient to
afford any significant stabilization of the initial adduct, [3f]+, or
any of the other potential intermediate species in the reaction
pathway. The rearrangement was confirmed by repeating the
reaction with 1 equiv of AgOTf (which gave a slight decrease in
rate), and this led to the isolation [4d]OTf, which was char-
acterized by multinuclear NMR and single crystal diffraction
(Figure 7).
cations from the free donor ligand and the halogenophosphor-
animine substrate.38,41
To drive the equilibrium in favor of [3d]+ the reaction was
repeated in the presence of 1 equiv of AgOTf to facilitate the
removal of the bromide counterion as AgBr. As anticipated,
31P{1H} NMR analysis of the reaction mixture formed by 1b and
2a in the presence of 1 equiv of AgOTf after 16 h showed the
quantitative formation of a single species with no significant
differences in shift and coupling constants compared to the
product of the reaction in the absence of the silver salt. After
workup and recrystallization, the product was confirmed to be
the phosphine oxide-stabilized phosphoranimine salt [3d]OTf
(Scheme 4) by X-ray diffraction (Figure 5).
Reaction of 1d with 2a (25 °C, CH2Cl2) also preceded as for
1a and 1c, but with an apparent equilibrium between the starting
materials and initial adduct [3g]+. A second set of resonances,
In light of this result, we attempted to explore the generality of
this reaction with respect to other phosphoranimine substrates.
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consistent with the formation of the rearranged product ([4e]+,
comprising 4% of phosphorus-containing species) was observed
by 31P{1H} NMR after 1 h. Further monitoring showed 24%
conversion to [4e]+ after 5 d, but at a rate slower than apparent
side reactions to afford unidentified products. Repeating the
reaction between 1d and 2a in the presence of 1 equiv of AgOTf
gave almost quantitative conversion of starting materials to [3g]+
after 1 h, but appeared to further slow the subsequent formation
of [4e]+ with only about 2% conversion after 5 d.
In the reaction of 1e with 2a (25 °C, CH2Cl2) the consump-
tion of starting materials was very slow with only 4% conversion
to [4f]+ after 5 days, but no resonances that could indicate the
formation of [3h]+ were observed, despite detection of small
amounts of other species analogous to those observed in the
reaction of 1a with 2a. This could be explained by the bulky
nature and poor donor ability of 1e which would disfavor the
formation of [3h]+ with Brꢀ as the counteranion. When the
reaction of 1e with 2a was repeated in the presence of 1 equiv of
AgOTf for 1 h, the resultant 31P{1H} NMR showed complete
consumption of starting materials after 1 h to give 47% [3h]+ and
44% [4f]+, with [3h]+ no longer observed after 10 h, and
conversion to [4f]+ maximizing at 91% after 24 h. This faster
conversion (44% in 1 h compared to 4% in 5 d without AgOTf)
supported the hypothesis that [4]+ was formed from [3]+ with
the resultant overall rate of the rearrangement reaction depen-
dent on the concentration of [3]+.
The adduct [3]+ was also further destabilized when the phosphine
oxide possessed relatively electron withdrawing phenyl groups (1e).
2. Discussion of X-ray Crystallographic and NMR Charac-
terization Data for OꢀDonor Stabilized Cations [3]+ and
Rearranged Cations [4]+. (a). Structural Characterization of the
Phosphine Oxide-Stabilized Phosphoranimine Cations [3]+. Se-
lected examples of the cations [3]+ were studied by X-ray
crystallography (Tables 1 and 2) and confirmed the reported
atom connectivity. Selected bond lengths and angles for the
O-donor stabilized cations [3]+ are given in Table 1 (see also
Tables 2 and 3). The P(2)ꢀO(1) distances show no significant
variation, suggesting that the phosphine oxide PꢀO bond length
was relatively insensitive to changes in substituents on the
phosphoranimine. The OꢀP(1) distance was found to be more
sensitive to substituent variation, with the bond length in [3d]+
(1.641(4) Å) being significantly greater than for [3e]+
Table 2. Crystal Data and Structural Refinement for
[3d]OTf, [3e]OTf, and 5
[3d]OTf
[3e]OTf
5
empirical formula
C
18H42F3-
C20H40F9-
C20H21-
NO4P2SSi
515.62
orthorhombic
Pbca
NO6P2SSi
683.62
BrNPSi
414.35
monoclinic
Cc
formula weight
crystal system
space group
a (Å)
monoclinic
P21/c
These results show that 1cꢀe exhibit reactivities toward
phosphoranimine 2a that are in-between those of 1a and 1b,
with rates of rearrangement from [3]+ to [4]+ decreasing with
increasing size of the phosphine oxide substituents. Equilibria
were also observed between the starting materials and [3]+ for the
sterically more demanding phosphine oxides 1b and 1d, presumably
because of unfavorable interactions which favor significant
phosphine oxide loss from [3]+ and the recoordination of Brꢀ.
9.2967(3)
17.1918(7)
33.2588(12)
90
24.9510(12)
17.4531(8)
22.1048(9)
90
16.395(2)
9.2190(12)
13.7655(17)
90
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
90
93.280(2)
90
105.461(6)
90
90
5315.7(3)
8
9610.3(7)
12
2005.3(4)
4
Z
DC (mg mꢀ3
)
1.289
1.417
1.372
μ (Mo KR), mmꢀ1 0.331
0.324
2.191
F(000)
2208
4272
848
θ range (deg)
index ranges
2.37ꢀ24.76
2.17ꢀ24.88
2.56ꢀ23.12
ꢀ10 e h e 11 ꢀ30 e h e 27 ꢀ23 e h e 23
ꢀ20 e k e 18 ꢀ21 e k e 21 ꢀ13 e k e 13
ꢀ40 e l e 32
25295
ꢀ18 e l e 26
72757
ꢀ19 e l e 19
5427
reflns colled
independent
temp (K)
GOF on F2
R, % (I > 2σ[I])a
Rw, %b
4867
17598
5427
100(2)
100(2)
1.077
100(2)
1.056
1.093
6.93
10.28
2.49
16.03
25.40
6.33
peak/hole (e/Å3)
a R = ∑||Fo| ꢀ |Fc||/∑|Fo|. b Rw = {∑w(Fo ꢀ Fc2)2/∑w(Fo ) }
0.469/ꢀ0.540
1.403/ꢀ0.580
0.453/ꢀ0.373
Figure 7. Thermal ellipsoid plot of [4d]OTf at the 50% probability
level. All hydrogen atoms have been omitted for clarity.
2
2 2 1/2
.
Table 1. Selected Bond Lengths and Angles for the Phosphine Oxide-Stabilized Phosphoranimine Salts [3d]OTf and [3e]OTf,
R3P(2)ꢀOꢀP(1)R0R00dNꢀSiMe3
P(2)ꢀO(1) [Å]
O(1)ꢀP(1) [Å]
P(1)dN(1) [Å]
N(1)ꢀSi(1) [Å]
P(2)ꢀO(1)ꢀP(1) [deg]
P(1)ꢀN(1)ꢀSi(1) [deg]
[3d]OTf
[3e]OTf
1.571(4)
1.584(5)
1.580(5)
1.568(5)
1.641(4)
1.586(5)
1.564(5)
1.574(5)
1.523(5)
1.471(7)
1.466(7)
1.481(7)
1.687(5)
1.696(7)
1.710(7)
1.684(7)
153.0(3)
159.7(5)
174.4(4)
176.6(5)
145.8(3)
170.0(5)
162.0(5)
168.6(5)
a Structural parameters for [3e]OTf are given for all three molecules in the asymmetric unit.
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Table 3. Crystal Data and Structural Refinement for [4a]OTf, [4a]PF6, [4d]OTf, and [8]OTf
[4a]OTf
[4a]PF6
[4d]OTf
[8]OTf
empirical formula
formula weight
crystal system
space group
a (Å)
C9H24F3NO4P2SSi
389.38
C8H24F6NOP3Si
385.28
C12H30F3NO4P2SSi
431.46
C24H30F3NO4P2SSi
575.58
triclinic
monoclini
C2/c
orthorhombic
Pbca
monoclinic
P21/c
P1
8.3809(3)
8.7810(4)
13.9100(6)
73.781(2)
76.827(2)
76.643(2)
941.61(7)
2
13.9242(17)
10.7663(14)
24.733(3)
90
13.8576(12)
13.4460(13)
23.067(2)
90
18.5098(19)
9.7109(8)
16.3446(15)
90
b (Å)
c (Å)
R (deg)
β (deg)
104.734(2)
90
90
107.282(6)
90
γ (deg)
V (Å3)
90
3585.8(8)
8
4298.0(7)
8
2805.3(5)
4
Z
DC (mg mꢀ3
)
1.373
1.427
1.334
1.363
μ (Mo KR), mmꢀ1
0.443
0.448
0.395
0.322
F(000)
408
1600
1824
1200
θ range (deg)
index ranges
2.54ꢀ32.05
ꢀ12 e h e 12
ꢀ12 e k e 12
0 e l e 20
5936
2.42ꢀ27.51
ꢀ18 e h e 11
ꢀ13 e k e 13
ꢀ32 e l e 32
15927
2.30ꢀ27.61
ꢀ18 e h e 18
ꢀ8 e k e 17
ꢀ30 e l e 30
51399
2.30ꢀ7.39
ꢀ22 e h e 21
0 e k e 11
0 e l e 19
4445
reflns colled
independent
temp (K)
5936
4111
4993
4445
100(2)
100(2)
100(2)
100(2)
GOF on F2
R, % (I > 2σ[I])a
Rw, %b
1.073
1.029
1.060
1.149
4.21
3.91
2.88
4.89
10.73
10.19
8.15
13.27
peak/hole (e/Å3)
a R = ∑||Fo| ꢀ |Fc||/∑|Fo|. b Rw = {∑w(Fo ꢀ Fc2)2/∑w(Fo ) }
0.682/-0.831
2
1.013/-0.796
0.619/-0.251
0.370/-0.496
2 2 1/2
.
the N-donor (1.47ꢀ1.53 Å)38,52 and phosphine-donor (1.48ꢀ
1.53 Å)41 stabilized phosphoranimine cations previously reported.
This could be explained by a negative hyperconjugation interac-
tion,53 whereby an increased donation of the nitrogen lone-pairs
into the σ* orbitals of the phosphoranimine PꢀOR0 bonds
was induced by the more electron-withdrawing substituents in
[3e]+. This may also explain the wider bond angle at N and O
observed for [3e]+ (162.0(5)ꢀ170.0(5)° and 159.7(5)ꢀ
176.6(5)°, respectively) compared to [3d]+ (145.8(3)° and
153.0(3)°), resulting from the presence of less stereochemically
active lone pairs because of involvement in overlap with the
empty phosphorus-substituent σ* orbitals.
(b). Structural Characterization of the Rearranged Cationic
Products [4]+. X-ray studies were performed on several examples
of salts of the cations [4]+ (Tables 3 and 4). Selected bond
lengths and angles for the rearranged cations ([4]+ and [8]+) are
given in Table 4, including [4a]PF6 (see Figure 10), isolated
from the reaction of 1a and 2a in the presence of 1 equiv of
AgPF6. In all of the salts listed in Table 4, the P(2)ꢀN(1) bond
lengths (1.570(4)ꢀ1.5876(17) Å) were consistently elongated
when compared to the P(1)ꢀN(1) bond lengths (1.555(4)ꢀ
1.5670(17) Å), and all possess bond lengths in the range for
phosphorusꢀnitrogen double-bonds (1.54 to 1.58 Å).51 This
suggested that the positive charge was distributed between both
phosphorus atoms within the PꢀNꢀP moiety (as is generally
observed in the [R3PdNdPR3]+ cations)51 and therefore, that
structures A and B (Figure 9) both contribute to the bonding
description of [4]+, but the weighting of B is larger. Although the
Figure 8. Resonance structures for [3]+.
(1.564(5)ꢀ1.586(5) Å), suggesting that a stronger interaction
between the phosphine oxide and the phosphoranimine was
present in the latter case. This could be easily explained by the
presence of the more electron-withdrawing OCH2CF3 substitu-
ents (compared to Me) creating an increased electron deficiency
at the phosphoranimine phosphorus center and inducing a
stronger bond between the phosphine oxide and the phosphor-
animine phosphorus. In the case of [3d]+ the O(1)ꢀP(1) bond
was longer than the P(2)ꢀO(1) bond (1.641(5) vs 1.571(4) Å),
supporting the bonding model of a formally phosphine oxide
oxygen lone pair donating to the cationic phosphorus center
(structure B, Figure 8). By contrast, in [3e]+ these bonds were
approximately the same length, suggesting that the interactions
between both phosphorus centers and the phosphine oxide
oxygen were more equivalent, and implying that the bonding
arrangement was closer to that of structure A in Figure 8.
The phosphoranimine PdN bond lengths in the phosphine
oxide-stabilized cations, [3]+ (1.466(7)ꢀ1.523(5) Å) were
shorter than the typical values (1.54ꢀ1.58 Å),51 particularly in
the case of [3e]+, and were comparable to those observed for
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a
Table 4. Selected Bond Lengths and Angles for the Rearranged Salts [4a]OTf, [4a]PF6 , [4d]OTf, and [8]OTf, [R3P(2)-Nd
P(1)R0R00-O-SiR† ]X
3
P(2)ꢀN(1) [Å]
N(1)ꢀP(1) [Å]
P(1)ꢀO(1)[Å]
O(1)ꢀSi(1) [Å]
P(2)ꢀN(1)ꢀP(1) [deg]
P(1)ꢀO(1)ꢀSi(1) [deg]
[4a]OTf
[4a]PF6
[4d]OTf
[8]OTf
1.580(2)
1.561(2)
1.5597(17)
1.5615(16)
1.5607(10)
1.578(3)
1.6806(17)
1.6623(16)
1.6738(10)
1.678(3)
145.35(16)
137.74(12)
140.86(8)
146.2(3)
144.77(12)
153.77(12)
137.11(6)
131.8(2)
1.5876(17)
1.5838(11)
1.570(4)
1.5670(17)
1.5605(11)
1.555(4)
a Synthesized in an analogous reaction to [4a]OTf, see Figure 10.
Scheme 6
Figure 9. Resonance structures for [4]+.
PꢀP couplings were generally greater for [3]+ and [7]+ (28.8 to
66.6 Hz) than for rearranged cations [4]+ or [8]+ (9.9 to 33.7
Hz). The values for [3]+ and [7]+ were also always significantly
larger than for [4]+ or [8]+ when the substituents at P and Si were
the same. In a number of examples it was also possible to observe
2JSiP couplings by both 31P{1H} NMR (as satellites) and 29Si-
{1H} NMR, allowing an assignment of the two 31P resonances as
either internal or terminal.
The variations in chemical shifts between [3]+ or [7]+ and
[4]+ or [8]+ were also informative: from 31P{1H} NMR the
PdO-stabilized phosphoranimine cations [3]+ and [7]+ all
possess a 31P resonance at high field (ꢀ30.9 to 41.4 ppm,
attributed to the phosphoranimine phosphorus) and one at
low field (58.7 to 118.5 ppm) for the O-donor phosphorus
center. In contrast, the rearranged cations ([4]+ and [8]+)
possess two 31P{1H} NMR resonances at a comparatively similar
chemical shift (17.3 to 42.9 ppm).
Figure 10. Thermal ellipsoid plot of [4a]PF6 at the 50% probability
level. All hydrogen atoms have been omitted for clarity.
The 29Si NMR resonance attributed to the SiR3 group in the
isolated compounds also showed a strong sensitivity to the
element to which it was bound (N or O). 29Si NMR resonances
were observed with negative chemical shift values (ꢀ26.5
to ꢀ3.8 ppm) when attached to a nitrogen atom of a phosphor-
animine (2) or phosphoranimine cation ([3]+),38 while the
rearranged product ([4]+) showed a positive shift at significantly
lower field (24.9 to 24.7 ppm), when attached to an oxygen atom.
3. Reactivity of Phosphine Oxide-Stabilized Cations [3]+
and Rearranged Cations [4]+. (a). Reactions of [3d]+ and [4a]+
with DMAP. The possible involvement of cations of type [3]+ and
[4]+ in the phosphite-initiated polymerization of phosphorani-
mines XR2PdNSiMe3 (X = Cl or Br) prompted us to explore
some key aspects of their chemistry. The structures of [3]+ and
[4]+ suggested that the former would be expected to react with
PꢀNꢀP and PꢀOꢀSi bond angles show considerable variation,
there appeared to be similar (PꢀOꢀSi) or greater (PꢀNꢀP)
sensitivity to the crystal packing (from a change in counterion),
than to a change in substituents on the silyl- or terminal phosphorus-
groups. This result indicated that the bond angles at O and N
were relatively flexible, and their solid-state geometric param-
eters were not a reliable indicator of any variation in bonding
character for these species.
(c). NMR Data for O-Stabilized Cationic Phosphoranimines
([3]+) and Rearranged Products ([4]+). Multinuclear NMR
analysis provided complementary evidence for the formulation
of the cationic products as the donor-stabilized phosphoranimine
cations [3]+ and [7]+, or rearranged products [4]+ and [8]+.
Analysis of coupling interactions between various nuclei sup-
ported the assignments (vide supra): observed 31P{1H} NMR
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Scheme 7
δ = 24.1 and 114.7 ppm (2JPP = 67.7 Hz) and release of 2a (δ =
6.9), presumably because of formation of the cation [tBu3Pd
OfPMePhdNSiMe3]+. Heating the solution to 60 °C gave no
further reaction, and raising the temperature to 160 °C resulted
in decomposition, but with no 31P{1H} NMR evidence for the
formation of [MePhPdN]n after redissolution of the non-
volatile products in CDCl3.
4. Mechanistic Studies of the Rearrangement of [3]+ to
[4]+. The transformation of [3]+ to [4]+ is mechanistically
intriguing, with the rearrangement requiring connectivity trans-
formations from PꢀNꢀSiR3 to PꢀOꢀSiR3 and from R3Pd
OꢀP to R3PꢀNꢀP. A number of examples of reactions invol-
ving similar Me3Si group migrations have been reported and are
clearly relevant to the rearrangement from [3]+ to [4]+.54ꢀ56 In
these reports, an initially formed species containing a P(=O)-
NR0ꢀSiR3 moiety rearranges to give the corresponding R0Nꢀ
PꢀOꢀSiR3 connectivity. In some of these examples, an inter-
molecular reaction involving a [1,3]-silyl migration is proposed;
however, experimental evidence supporting this mechanism is
limited. Although we have not been able to fully elucidate the
mechanism for the conversion of [3]+ to [4]+ some important
observations have been made that give useful information and
allow us to tentatively propose a mechanism for this process.
(a). Influence of the Counterion in the Rearrangement of
[3]+ to [4]+. We obtained results that allowed a comparison of
the rates of rearrangement from [3a]+ to [4a]+ as a function of
the counteranions of varying nucleophilicity (Brꢀ, [OTf]ꢀ and
[BPh4]ꢀ). This was achieved by addition of a stoichiometric
amount of the respective silver salt (AgX, X = OTf or BPh4)
to a 1:1 mixture of 1a and 2a (25 °C, CH2Cl2). This gave the
immediate precipitation of a white solid (AgBr) along with
complete consumption of 1a and 2a observed by 31P{1H}
NMR. The reaction was also performed in the absence of added
Ag salt and also in the presence of 5 equiv of [Ph4P]Br to provide
further comparisons. These anion variations had a significant
influence on the rates of both the consumption of [3a]+ and the
formation of [4a]+. A faster rate of consumption of [3a]+ was
generally observed for the less coordinating anions with the rate
for the consumption fastest for [BPh4]ꢀ and [OTf]ꢀ compared to
Brꢀ and the case of excess Brꢀ, with 3%, 16%, 38%, and 47% of [3a]+
remaining, respectively, after 1 h. However, the significant slowing of
the rearrangement for the reaction of 2a with the bulky phosphine
oxide 1d (and between 1a and 5 where the latter has a bulky silyl
group) on exchanging the bromide counterion for triflate suggested
the influence of the anion on the rearrangement was complex.
(b). Effect of Varying the Phosphine Oxide (1) on the
Rearrangement of [3]+ to [4]+. With the exception of the highly
sterically encumbered phosphine oxide 1b (which did not form
[4]+) all of the phosphine oxides with smaller and less electron
donating substituents (1a,c-e) formed [3]+ on reaction with 2a,
which then apparently subsequently rearranged to [4]+. Con-
sideration of the rates of these reactions (when triflate was used
as the counteranion to ensure a high initial conversion to [3h]+)
allowed for a number of interesting observations. For the series of
reactions of 1aꢀ1e with 2a it was clear that the rate of
rearrangement was sensitive to the nature of the substituent on
the phosphine oxide. There was a significant decrease in rate as
the size and electron donating capability of phosphine oxide
substituents increased (92%, 18%, and 0% conversion to [4]+
after 5 h for 1a, 1c, and 1d respectively, with conversion for 1d
only reaching about 4% after 10 d) and for the most sterically
encumbered and strongest donor (1b) the rearrangement was
strongly donating ligands such as DMAP to displace the more
weakly bound R3PdO ligand whereas the latter should display
different reactivity.38 Samples of [3d]OTf and [4a]OTf were
each treated with a 2-fold excess of DMAP in solution at 25 °C.
Reaction of [3d]OTf (prepared in situ in CDCl3) with
DMAP led to complete conversion to the DMAP-stabilized
phosphoranimine cation [9]+,38 and the release of 1 equiv of 1b
was detected by 31P{1H} and 1H NMR. In contrast, a solution
of [4a]OTf and a 2-fold excess of DMAP showed no reaction
after 4 days, or when heated to 160 °C with 1 equiv of DMAP
(Scheme 6).
(b). Thermal Stability of [3d]+ and Reactivity of [3d]+ and
[4a]+ toward BrMePhPdNSiMe3. Given that variation of the
phosphine oxide or phosphoranimine substituents failed to
permit isolation of any intermediates in the rearrangement of
[3]+ to form [4]+, the reactivity of [3d]+ was explored at elevated
temperature. However, when [3d]OTf was heated at 60ꢀ150 °C
in 1,2-dichlorobenzene decomposition to uncharacterized
products was detected by 31P{1H} NMR.
We also explored whether either of the cations of type [3]+
and [4]+ would function as initiators for the polymerization of
the phosphoranimine BrMePhPdNSiMe3 (2b). Reaction of
[4a]OTf with a 10-fold excess of 2b in 1,2-dichlorobenzene
resulted in 42% conversion to a number of unidentified species
after 14 h at 60 °C by 31P{1H} NMR. Complete decomposi-
tion was observed after heating to 160 °C for 5 h, but no 31P
NMR resonances that could be assigned to the formation of
[MePhPdN]n (δP = 1.8 ppm)12,40 were detected. Reaction of
[3d]OTf with a 10-fold excess of 2b in 1,2-dichlorobenzene at
25 °C only resulted in 98% conversion of the former to a new
species observed by 31P{1H} NMR with two resonances at
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Inorganic Chemistry
ARTICLE
halted completely (vide supra). Interestingly, a rate of rearrange-
ment almost equivalent to that observed for 1a was detected
(83% conversion after 5 h) when a bulky phosphine oxide with
relatively electron-withdrawing phenyl groups (1e) was used, and
implied that the donor ability of the phosphine oxide has the
most significant influence on the rearrangement of [3]+ to [4]+.
Another significant result came from the reaction between 1e
and 2a, which appeared to strongly disfavor the formation of
[3h]+ to the extent that it could not be detected by 31P{1H}
NMR which showed only a very slow consumption of starting
materials to give [4]+, with only 9% conversion in 10 d. This was
accelerated when the initial conversion of 1e and 2a to [3h]+ was
increased from being undetectable to 100% by exchange of
bromide for triflate, to give 91% conversion in 24 h. This
substantial increase in rate of rearrangement, strongly supported
the hypothesis that the formation of [4]+ was via [3]+, with the
rate conversion to [4]+ strongly dependent on the concentration
of [3]+.
encumbered phosphine oxide (1b), which permitted the isola-
tion and characterization of the first examples of this new type of
stabilized phosphoranimine cation ([3d]+ and [3e]+), these
initial species were found to undergo a rearrangement to the
cations [4]+ or [8]+. A mechanism for this rearrangement was
suggested and involves a phosphine migration and silyl group
transfer, and is consistent with observed dependencies on the
electronic nature of the phosphine oxide and whether the silyl
group is -SiMe3 or -SiPh3. No evidence was found in these studies
that supported the intermediacy of the cations [3]+ or [4]+ in the
chain growth polycondensation of bromophosphoranimines to
form polyphosphazenes.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details (.pdf)
b
and crystallographic information files (.cif). This material is
available free of charge via the Internet at http://pubs.acs.org.
(c). Influence of the Silyl Group in 2a and 5. Changing the
Me3Si-substituent on the phosphoranimine 2a to the more
sterically encumbered Ph3Si group on 5 gave a significant
decrease in the rate of the rearrangement, particularly when
the bromide counterion was replaced with triflate. The change
from 2a to 5 gave an increase in the time taken for the
consumption of [3]+ when the anion was Brꢀ or [OTf]ꢀ of
around 10 or 100 times, respectively. These results strongly
suggested that the silyl group is intimately involved in the
rearrangement reaction, as expected.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ian.manners@bristol.ac.uk.
’ ACKNOWLEDGMENT
M.B. thanks the EPSRC for funding, K.H. thanks NSERC for a
Postgraduate Fellowship (2005-2008) and I.M. would like to
thank the Canadian Government for a Canada Research Chair at
Toronto and the support of a European Union Marie Curie Chair
and a Royal Society Wolfson Research Merit Award.
(d). Proposed Mechanism. The results obtained and previous
studies of somewhat related species54ꢀ56 allowed us to tenta-
tively propose a mechanism for the formation of [4]+ after 1 and
2 initially react to form [3]+ (Scheme 7). The cation [3]+ could
undergo an intramolecular rearrangement whereby the phos-
phoranimine nitrogen nucleophilically attacks the phosphorus
center of the ligated phosphine oxide. This could generate an
intermediate ([10]+, Scheme 7) that is presumably transient and
therefore was not observed by NMR. A further rapid rearrange-
ment could then give [4]+. Alternative pathways involving the
intermediate [10]+ might be expected to generate side products
that were not structurally characterized. The complex sensitivity
of the rate of the rearrangement to the nature of the counteranion
could be a consequence of weak coordination at the cationic
phosphorus center in [3]+ and at the phosphorus or silicon sites
in [10]+ and the resulting influence on the activation energy for
the various reaction steps. The electronic nature of the substit-
uents on the phosphine oxide might be expected to affect the rate
of the rearrangement. The observed reduction in rearrangement
rate for the electron donating substituents could be due to the
lower electrophilicity of the phosphine oxide phosphorus center
in [3]+,57 possibly making attack by the phosphoranimine
nitrogen less favorable. Significantly, SiMe3 groups have a greater
established migratory aptitude than their SiPh3 analogues,42,49,50
and this is consistent with the relative rearrangement rates
detected in the cases of the reactions of 1a with 2a and 5.
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’ SUMMARY
A range of phosphine oxides R3PdO (R = tMe, Et, iPr and Ph;
1aꢀe) have been shown to react with bromophosphoranimines
BrR0R00PdNSiMe3 (2) and BrMe2PdNSiPh3 (5) to give a range
of O-donor-stabilized phosphoranimine cations ([3]+ or [7]+).
With the exception of those formed by the most sterically
10301
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