N. C. Gonnella et al.
(Bruker-Biospin) operating at 162.1 MHz for 31P. A Bruker H/X
CP/MAS probe equipped with a 4-mm rotor was used to acquire
all spectra.
Upon standing at atmospheric conditions for 16 h, dichlorot-
riphenylphosphorane was hydrolyzed to a product where the
phosphorous chemical shift is at 42.97 ppm. Since the chem-
ical shift of commercially obtained triphenylphosphine oxide
appeared at 28.65 ppm, the downfield shift of about 15 ppm
suggested a significant change in this hydrolyzed product. While
multiple polymorphs of TPPO are known to exist, their preparation
and characterization have not been reported.[20] However, be-
cause of the significant downfield shift from triphenyl phosphine
oxide, it was proposed that the unknown peak may be due to
the hydrochloride salt of TPPO as opposed to an alternate poly-
morph form. To confirm this hypothesis, the hydrochloride salt of
TPPO was prepared according to an internally developed proce-
dure. Initial attempts to prepare the hydrochloride salt of TPPO,
based upon a known procedure,[21] were unsuccessful. It was
found, however, that the desired product could be reproducibly
prepared from a ‘slurry-to-slurry’ conversion in which TPPO was
added to anhydrous 2M ethereal HCl under argon. The resultant
slurry was allowed to age overnight at ambient temperature to
yield the desired product as a solid after filtration under an inert
atmosphere. The 31P NMR spectrum showed the unknown peak
to be the hydrochloride salt of TPPO with the monohydrate as a
minor component (Fig. 2).
Solid state 31P NMR data were collected for DCTP obtained from
two different commercial batches (Batch 1 and Batch 2). The rotors
were packed in dry nitrogen atmosphere and 31P ssNMR spectra
were collected. The phosphorous ssNMR data showed a higher
composition of DCTP for the material from Batch 2, having only
3% conversion to the hydrolyzed form. The material from Batch 1,
however, showed about 8% conversion to the hydrolyzed form.
The percent of reagent and hydrolyzed impurity was determined
from the relative integrated intensities of the phosphorous peaks.
These data were in agreement with its use as a synthetic reagent
where complete conversion to product was consistently found
for material showing lower levels of hydrolysis product (internal
communication).
Samples were packed into 4-mm Zirconia rotors and sealed
with Kel-F caps. Preparation occurred either under atmospheric
conditions or under dry nitrogen. Spectra were acquired using
variable amplitude cross polarization,[4–7] magic angle spinning
(MAS)[8,9] and high power proton decoupling with two phase
pulse modulation (TPPM).[10–13] Contact time of 3 ms was used to
acquire all spectra. The MAS frequency was 15 kHz and the proton
decoupling field was approximately 49 kHz. Recycle delays, based
upon saturation of 31P signal intensity, were sufficiently long to
allow full relaxation of 31P signal. Phosphorous chemical shifts are
quoted with respect to phosphoric acid.
Results and Discussion
The31PNMRchemicalshiftsofdichlorotriphenylphosphorane,and
its hydrolyzed products obtained from both ssNMR and solution
NMR are given in Table 1. The ssNMR data show that dichloro-
triphenylphosphorane exists in the pseudo-trigonal bipyramidal
form as evidenced by the peak at −43.44 ppm and in the ionic
quasi-phosphonium salt appearing at 63.62 ppm.[14,15] Multiple
peaks for the dichlorotriphenylphosphorane at −43.44 ppm and
63.62 ppm are due to 31P, 35,37Cl dipolar coupling in 31P MAS
spectra.[16,17] Measured 31P–35Cl isotropic (J) scalar couplings for
the peaks at −43.44 and 63.62 ppm were approximately 230 and
317 Hz with quadrupole-perturbed dipolar and anisotropic scalar
distortions of approximately +14 Hz and −179 Hz, respectively.[18]
Because of the phosphorane’s susceptibility to hydrolysis on
exposure to moisture, this compound required handling in a
dry atmosphere. When NMR rotors were packed under a dry
nitrogen atmosphere, the monohydrate phosphine oxide, at
27.04 ppm, was present in the phosphorane spectrum as the
only hydrolyzed product. The identity of the monohydrate was
confirmed via a preparation, according to a known procedure.[19]
The 31P NMR data, comparing the dichlorotriphenylphosphorane
spectrum with the hydrolyzed monohydrate spectrum are shown
in Fig. 1. The hydrolyzed product was also evident from the loss of
dipolarcouplingofchlorinewithphosphorousthatischaracteristic
of a 31P–35Cl bond.
When the rotors were packed in the laboratory atmosphere,
the exposure to air for a short duration of several minutes
increased the amount of hydrolyzed product to about 30%. This
hydrolyzedproductconsistedofthemonohydrateat27.04 ppm[19]
and hydrochloride salt at 42.97 ppm. Spectra of DCTP prepared
underdrynitrogenatmosphereandunderlaboratoryatmospheric
conditions are shown in Fig. 1 (b–d).
A comparison of solution NMR with solid state NMR was
also carried out. The 31P NMR solution spectrum of the
dichlorotriphenylphosphorane was difficult to acquire because
of the almost instant conversion to the hydrolyzed form upon
dissolution in solvent. This was found to be the case in DMSO
where the chemical shift of the phosphorane showed complete
conversion to the hydrolyzed form with the 31P chemical shift at
39.96 ppm which is identical with the 31P solution NMR chemical
shift of the purchased TPPO. In benzene, however, it was possible
to trap the covalent form long enough to get a spectrum of the
dichlorotriphenyphosphorane (Fig. 3). The spectrum showed that
the covalent dichlorotriphenylphosphorane phosphorous atom
resonates at −29.6 ppm and ionic dichloride at 69.8 ppm. Peaks
consistent with the hydrolyzed forms appear between 38.8 and
40.8 ppm. Upon standing in solution, complete conversion to the
TPPO was observed. All solution NMR chemical shifts appeared
downfield relative to that in the solid state.
Table 1. Phosphorous-31 NMR chemical shifts for dichlorotri-
phenylphosphorane and hydrolyzed products
Compound
ss NMR δ31P/(ppm)a
solution NMR δ31P/(ppm)
Ph3PCl+Cl−
Ph3PCl2
63.62
−43.44
28.65
69.8b
−29.8b
Ph3PO
38.96c
(38.87–40.81)b
(38.87–40.81)b
40.81b
Ph3PO · H2O
Ph3PO · HCl
27.04
42.97
44.40d
a Referenced from 85% H3PO4.
b Dichlorotriphenylphosphorane and hydrolyzed products (Batch 1)
acquired in benzene-d6.
c Triphenylphosphine oxide (purchased material) spectrum acquired in
DMSO-d6.
d Hydrolyzed product of dichlorotriphenylphosphorane (Batch 1) after
standing in benzene-d6 for 1 h.
Unlike solution NMR, the solid state phosphorous NMR spectra
showthatdichlorotriphenylphosphoranedoesnotrapidlyconvert
c
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2009, 47, 461–464