Palladium activation of white phosphorus
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 6, June, 2010
1117
from 31P NMR data. We found that the formation of
H3PO3 from white phosphorus in the presence of cationic
palladium complexes with PPh3 is a catalytic process actiꢀ
vated by PPh3 molecules released from cationic complex 2
(see Scheme 1).
Scheme 1
Thus, the reactions of cationic palladium(II) complexꢀ
es with white phosphorus involve coordination of the P4
molecule followed by hydrolysis to phosphorous acid as
the major product.
Experimental
All experiments were carried out under dry nitrogen in stanꢀ
dard Schlenk ware. Freshly distilled solvents were used; THF
was distilled over Na/benzophenone. White phosphorus was puꢀ
rified with a solution of potassium bichromate in conc. H2SO4
followed by recrystallization from DMF. The resulting phosꢀ
phorus was melted (50 °C) and rolled into beads while stirring
with a magnetic bar and then cooling. Tributylphosphine oxide
used as the internal standard for determination of the concentraꢀ
tions of the components in solution was prepared by oxidation of
Bu3P with a twofold excess of H2O2 in toluene at 20 °C. The
complex [(PPh3)2PdBr2] was synthesized as described earlier.14
NaBPh4 (Across Organics) was employed as purchased.
31P NMR spectra were recorded on a Bruker Avanceꢀ400
highꢀresolution spectrometer (161.9 MHz) in THF at ~20 °C.
MALDI mass spectra were recorded on a Bruker Daltonics
ULTRAFLEX III MALDIꢀTOF/TOF mass spectrometer with
the transꢀ2ꢀ[3ꢀ(4ꢀtertꢀbutylphenyl)ꢀ2ꢀmethylpropꢀ2ꢀenylidene]ꢀ
malononitrile (DCTB) matrix. The palladium and phosphorus
contents of the precipitates obtained were determined using inꢀ
ductively coupled plasma mass spectrometry (ICPꢀMS) on
a Perkin—Elmer Elan DRC II mass spectrometer (USA) and
atomic absorption spectroscopy (AAS) on a Carl Zeiss AAS1
spectrometer.
sphere of the palladium complex. This is primarily due to
the fact that a coordinated white phosphorus molecule
substantially increases the electron density on the central
metal atom, which weakens the coordination bond beꢀ
tween palladium and triphenylphosphine and facilitates
the release and exchange of the latter for white phosphorus.
These exchange interactions between PPh3 and white
phosphorus in the coordination sphere of palladium are
confirmed by the broadened signal of free PPh3 in the
31P NMR spectrum.
According to elemental analysis data, the black preꢀ
cipitate contains palladium phosphides with a molar P : Pd
ratio of 5 : 2; no other phosphorus compounds were deꢀ
tected in the reaction mixture. However, after a small
amount of deaerated water was added and the resulting
solution was stirred in an inert atmosphere for 24 h, the
supernatant contained phosphorous acid as the major
To confirm the formation of H3PO3 and H3PO4, small
amounts of these acids were added to the reaction mixture and
their concentrations were determined from the increased inꢀ
tegral intensities of the corresponding signals in the 31P NMR
spectrum.
1
product (31P NMR, δ: 2.70, d, JP,H = 667 Hz). The
minor products included H3PO4 (δ 0.00) and H4P2O7
(δ 4.46). The latter was completely converted into H3PO4
upon addition of a small amount of 2 N aqueous HCl to
the reaction mixture. We found that the formation of
H3PO3 results from hydrolysis of the coordinated white
phosphorus molecule in the coordination sphere of pallaꢀ
dium. Earlier,12,13 this has been noted for some complexes
of Group VIII metals.
The MALDI mass spectrum of the supernatant conꢀ
tains the ion peaks with m/z 892 and 630 corresponding to
the molecular ions [Pd(PPh3)3]+ and [Pd(PPh3)2]+, reꢀ
spectively. This confirms the structures of the complexes
[Pd(PPh3)3] and [Pd(PPh3)2] in solution, with palladium
in the zero oxidation state. Therefore, the oxidation of
white phosphorus into phosphorous acid is accompanied
by the partial reduction PdII → Pd0. The latter palladium
remains in solution because of its recoordination by free
PPh3 present in the reaction mixture, which is evident
Reaction of the complex [(PPh3)2PdBr2] with white phosphoꢀ
rus. Sodium tetraphenylborate (0.0164 g, 0.048 mmol) was addꢀ
ed to a solution of [(PPh3)2PdBr2] (0.019 g, 0.024 mmol) and
Bu3PO (0.01 g, 0.048 mmol) as the internal standard in THF
(5 mL). The mixture was stirred at 50 °C for 1 h. The 31P NMR
spectrum (THF) of the mixture exhibited a signal at δ 27.57 due
to the cationic complex [(PPh3)2Pd(THF)2]2+. Then a 0.06 M
solution of white phosphorus (0.4 mL) in THF (Pd : P4 = 1 : 1)
was slowly added dropwise and the reaction mixture was stirred
at 50 °C for 1 h. Deaerated water (2 μL) was added to the resultꢀ
ing solution. The 31P NMR spectrum (THF) of the solution
showed the signals of H4P2O7 (δ 4.46), H3PO3 (δ 2.70, d, 1JP,H
=
= 667 Hz), and H3PO4 (δ 0.00) with an integral intensity ratio of
1 : 3 : 1, respectively.
We are grateful to the Analytical Chemistry Center at
the A. E. Arbuzov Institute of Organic and Physical Chemꢀ
istry, Kazan Research Center of the Russian Academy of
Sciences for performing quantitative AAS analysis.
Kagirov
