Donor-Stabilized Cations and Imine Transfer from N-Silylphosphoranimines

Article abstract of DOI:10.1021/ja039549z

A series of donor-stabilized N-silylphosphoranimine salts [DMAP·PCl₂=NSiMe₃]⁺X^- (DMAP = 4-(dimethylamino)pyridine) were prepared by the reaction of Cl₃P=NSiMe₃ with DMAP in the presence of silver salts AgX (X = OSO₂CF₃, BF₄, and SbF₆). Repeating the reaction in the absence of AgX gave the chloride salt [DMAP·PCl₂=NSiMe₃]Cl which has been shown to be in equilibrium with free DMAP and Cl₃P=NSiMe₃. Attempts to stabilize a N-silylphosphoranimine cation with phosphine donors led to unexpected imine transfer chemistry. For example, Cl₃P=NSiMe₃ reacts with phosphines, R₃P (R = ⁿBu and Ph), to produce the metathesis products PCl₃ and R₃P=NSiMe₃ which subsequently react together to afford the N-phosphinophosphoranimines R₃P=N?PCl₂ and ClSiMe₃ as a byproduct. Copyright

Full text of DOI:10.1021/ja039549z

Published on Web 02/05/2004
Donor-Stabilized Cations and Imine Transfer from N-Silylphosphoranimines
Eric Rivard, Keith Huynh, Alan J. Lough, and Ian Manners*
UniVersity of Toronto, Department of Chemistry, DaVenport Laboratories, 80 St. George Street,
Toronto, Ontario, Canada M5S 3H6
Received November 11, 2003; E-mail: imanners@chem.utoronto.ca
N-Silylphosphoranimines R3PdNSiR′3 are of considerable inter-
est as model species with phosphorus-nitrogen multiple bonds1
and as polymer precursors.2 Cationic N-silylphosphoranimines
[R2PdNSiR′3]+ are currently unknown but have been considered3
as possible intermediates in the formation of polyphosphazenes4
via thermally induced condensation polymerization and would be
expected to exhibit interesting reactivity. In this communication
we report on our attempts to prepare the cation [Cl2PdNSiMe3]+
and the discovery of an unusual imine transfer reaction involving
phosphorus centers.
Initial attempts to generate an N-silylphosphoranimine cation
involved the reaction of Cl3PdNSiMe3 (1)5 with the halide
abstractor Ag[OTf] (OTf ) OSO2CF3) in CH2Cl2. In lieu of
obtaining the triflato derivative (TfO)PCl2dNSiMe3 (2), the
quantitative formation of poly(dichlorophosphazene) [Cl2PdN]n and
Figure 1. Molecular structure of [3]OTf with thermal ellipsoids at the
30% probability level. All hydrogen atoms are omitted for clarity.
1
TfOSiMe3 was detected by H, 19F, and 31P NMR spectroscopy.
Attempts to trap 2 by performing the reaction in arene solvents
(C6D6 and toluene)6 or at low temperature (-70 °C) gave similar
results. Use of substoichiometric quantities (10-20 mol %) of Ag-
[OTf] led to the incomplete consumption of 1 (ca. 70%) over a
period of 2 weeks to yield [Cl2PdN]n and cyclic phosphazene
oligomers [Cl2PdN]x, x ) 3-5. These results suggest that if 2 is
indeed generated initially, it rapidly oligomerizes due to the labile
OTf group at phosphorus.
with the closest anion-cation contact [4.0 Å: N(3)-F(1)] still well
outside the sum of the van der Waals radii for the constituent atoms.
From the structural data presented, two plausible resonance
contributors, I and II, help describe the bonding within [3]+:
Encouraged by the successful application of coordinating pyridine
derivatives to stabilize cationic phosphorus(III)7 and, in some cases,
phosphorus(V) centers,7c,d,8 we repeated the reaction of 1 and
Ag[OTf] in the presence of the strong donor 4-(dimethylamino)-
pyridine (DMAP). Upon combining these reagents in CH2Cl2 the
immediate formation of a white precipitate (AgCl) was observed.
Analysis of the reaction mixture by 31P NMR spectroscopy indicated
that the clean conversion of 1 (δ ) -54 ppm) had occurred to
yield a new product with a downfield-shifted resonance at -40
ppm. This species was isolated as colorless needles (mp ) 98-
100 °C) and identified as the novel cationic phosphoranimine salt,
[DMAP‚PCl2dNSiMe3]OTf, [3]OTf by X-ray diffraction (Figure
1). Despite the existence of numerous (>500) different phosphor-
animine derivatives, [3]OTf represents, to our knowledge, the first
crystallographically characterized cationic member of this family.9
The X-ray study of [3]OTf 10 revealed that a very short internal
phosphazene [P(1)-N(3)] bond length of 1.490(3) Å was present,11
which approaches those observed within the triply bonded imino-
phosphenium cations RNtP+ (1.46-1.49 Å);12 consequently, a
wide P-N-Si bond angle of 144.1(2)° is observed. The DMAP
ligand is strongly bound to the phosphorus center in [3]OTf and
lies at a distance of 1.713(2) Å [P(1)-N(1)]. As a point of reference,
the P-NDMAP distances within the bis(imino)phosphonium adduct
[(DMAP)2P(dNMes*)2]+ (4) (Mes* ) 2,4,6-tBu3C6H2)8a are quite
elongated [1.812(4) and 1.830(4) Å]. These data suggest that the
[Cl2PdNSiMe3]+ cation, if isolated in free form, would be highly
electrophilic. The OTf- counterion in [3]OTf exists as a spectator
Interestingly, the reaction of DMAP with 1 also gave [3]+ in
the absence of a halide abstractor; however, the reaction times were
appreciably longer (2 h vs 5 min). Remarkably, the product, [3]Cl,
slowly reconverts into 1 and DMAP in the solid state, with 95%
conversion after 14 d at 20 °C. This process is reversible as
dissolution of the products in CH2Cl2 regenerated [3]Cl after a few
hours. The equilibrium between [3]Cl and the precursors 1 and
free DMAP was further confirmed by the addition of [Ph3PdNd
PPh3]Cl, which led to complete retroconversion of [3]Cl after 1.0
equiv of Cl- was added. It is likely that the recombination of [3]+
and Cl- in the solid state is promoted by their close proximity in
this phase.
The analogous salts [3]BF4 and [3]SbF6 were prepared from
DMAP, 1, and either Ag[BF4] or Ag[SbF6] in CH2Cl2. While the
hexafluoroantimonate salt [3]SbF6 proved indefinitely stable in
solution, [3]BF4 slowly decomposed within 3 days to give a mixture
of DMAP‚BF3, the cyclic phosphazene [Cl2PdN]3, and presumably
volatile FSiMe3 (Scheme 1). One possible route which explains
the formation of phosphazene oligomers involves the initial
formation of (undetected) FCl2PdNSiMe3 (5) via the addition of a
BF4--derived fluoride ion to [3]+, followed by the combination of
the BF3 and DMAP byproducts to give DMAP‚BF3. Condensation
of 5 with the elimination of FSiMe3 could then produce [Cl2Pd
N]3. Repeated attempts to generate 5 independently from 1 and
9
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J. AM. CHEM. SOC. 2004, 126, 2286-2287
10.1021/ja039549z CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
is intimately linked with the nature of the counterion. Attempts to
prepare a phosphine-donor analogue uncovered an unusual imine
transfer reaction. Studies directed at elucidating the mechanism and
scope of the imine transfer reaction, and the isolation of the hitherto
unknown cation [R3P‚PCl2dNSiR′3]+ are in progress.
Acknowledgment. E.R. is grateful to NSERC for a Postgraduate
Fellowship (1999-2003) and the University of Toronto for an Open
Fellowship (2003). I.M. thanks NSERC for funding and the
Canadian Government for a Canada Research Chair.
Supporting Information Available: Experimental details for the
synthesis and characterization of new compounds (PDF, CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
various fluorinating agents (e.g., Ag[BF4]) gave highly complicated
reaction mixtures consistent with the formation of partially fluori-
nated phosphazene oligomers [F2-yClyPdN]x. The increased stability
of [3]SbF6 supports the delivery of fluoride to [3]+ as the initial
step in the decomposition of [3]BF4. The SbF6- ion is particularly
resistant to fluoride ion transfer, and consequently, is often
employed to facilitate the isolation of highly reactive cations (e.g.,
S42+).13
The successful isolation of the amine donor-stabilized phos-
phoranimine cation [3]+ prompted us to attempt the synthesis of
an analogous phosphine-stabilized cation [R3P‚PCl2dNSiMe3]+ (6).
Such a species would complement the burgeoning area of P(III)
f P(III) coordination chemistry.14 Surprisingly, when 1 was allowed
to combine with phosphines PR3 (R ) Ph and nBu) in CH2Cl2 (25
°C, 24 h and 6 d), the known N-phosphinophosphoranimines
R3PdN-PCl2 (R ) Ph and nBu; 7 and 8) were formed along with
a stoichiometric quantity of ClSiMe3. Monitoring the reaction by
1H and 31P NMR spectroscopy revealed the initial formation of
PCl3 and R3PdNSiMe3 prior to the formation of 7 and 8 (Scheme
2). This observation is striking as the initial step in the reaction
can be formally regarded as an example of an imine-transfer reaction
involving two phosphorus centers.
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Scheme 2
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This process may indeed involve the formation of a phosphine-
stabilized cation 6, followed by imine transfer to R3P via a three-
membered intermediate or transition state (with PCl3 as a byprod-
uct). However, we have yet to be able to detect 6, and the direct
attack of the phosphine at nitrogen also needs to be considered.
Although the delivery of imine functionality to phosphines is well-
known using organoazide reagents (Staudinger reaction),15 other
examples of such a transformation still remain very rare.16,17
In summary, the successful synthesis of a novel donor-stabilized
N-silylphosphoranimine cation has been reported, whose stability
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