Electrochemical Decomplexation of Phosphine-Pentacarbonyltungsten Complexes: The Phosphole Case

Article abstract of DOI:10.1021/om030671g

Summary: Electrochemical reduction of phosphole P-W-(CO)₅ complexes affords the free phospholes in good yields. This technique allows the decomplexation of a 2-ethoxy-4-keto-4,5-dihydrophosphole, which has been characterized by X-ray crystal structure analysis of its P-sulfide.

Full text of DOI:10.1021/om030671g

Organometallics 2004, 23, 1961-1964
1961
Notes
Electr och em ica l Decom p lexa tion of
P h osp h in e-P en ta ca r bon yltu n gsten Com p lexes: Th e
P h osp h ole Ca se
Dmitry G. Yakhvarov,† Yulia H. Budnikova,† Ngoc Hoa Tran Huy,‡
Louis Ricard,‡ and Franc¸ois Mathey*,‡
Institute of Organic and Physical Chemistry of RAS, Arbuzov Street, 8,
420088, Kazan, Russia, and Laboratoire “He´te´roe´le´ments et Coordination” UMR CNRS 7653,
DCPH, Ecole Polytechnique, 91128 Palaiseau Cedex, France
Received November 24, 2003
Summary: Electrochemical reduction of phosphole P-W-
(CO)5 complexes affords the free phospholes in good
yields. This technique allows the decomplexation of a
2-ethoxy-4-keto-4,5-dihydrophosphole, which has been
characterized by X-ray crystal structure analysis of its
P-sulfide.
imidazole.3,4 In a variant of this first technique, pyri-
dinium tribromide is used as the halogenation reagent
and 2,2′-bipyridine for the displacement.5
Another method relies on the displacement of the P
ligand by a chelating bidentate diphosphine such as Ph2-
PCH2CH2PPh2 and, of course, can only be applied to the
decomplexation of monodentate ligands. Reported ex-
amples include bifunctional phospholes6 and 1-chloro-
phosphirenes.7
In tr od u ction
The transient electrophilic terminal phosphinidene
complexes [R-P-M(CO)5] (M ) Cr, Mo, W) combine easy
access, large range of substituents, and an impressive
array of reactions with organic or organometallic mol-
ecules.1,2 The original access through thermal decom-
position of the appropriate 7-phosphanorbornadiene
complexes is still the major route to these carbene-like
species. As such, they already constitute a powerful tool
for making new organophosphorus structures. These
structures are obtained as their P-M(CO)5 complexes,
and hence, to be fully effective, this chemistry needs to
be coupled to efficient decomplexation techniques. In-
deed, while the complexing groups are useful for tuning
the electrophilicity and the lifetime of the phosphinidene
moiety and increasing the stability and tractability of
the reaction products, they must be removed if useful
ligands or biologically active molecules are the final
targets of the synthetic work.
In practice, the most effective complexing group is the
pentacarbonyltungsten unit. As might be expected, this
is also the one that gives the strongest bond to the P
atom. Presently, only three decomplexation techniques
have been applied to its removal with some success. The
oldest method involves an oxidation of W(0) to W(II) by
iodine, which weakens the P-W bond, followed by a
displacement of the phosphorus ligand by 1-methyl-1H-
The third group of methods involves oxidative condi-
tions. Trimethylamine oxide has been shown to be the
reagent of choice for the conversion of [P-W(CO)5]
complexes into the corresponding phosphoryl com-
pounds.8
As might be guessed, all these methods suffer from
severe limitations and only a restricted number of
products resulting from this phosphinidene chemistry
have been isolated as free species. Hereafter, we propose
a completely different decomplexation technique, which
works under reductive conditions and thus nicely supple-
ments the earlier methods.
Resu lts a n d Discu ssion
We decided to investigate the electrochemical proper-
ties of [R3P-W(CO)5] complexes by cyclic voltammetry
(CV) using a series of phosphole complexes as the model
compounds. In all cases on the CV curves, only one two-
electron irreversible reduction wave was observed. A
typical curve is shown in Figure 1. The potentials and
peak currents are presented in Table 1. As can be seen
from the data, all these potentials are in the range of
the reduction potential of [W(CO)6] under the same
(3) Marinetti, A.; Mathey, F.; Fischer, J .; Mitschler, A. J . Chem. Soc.,
Chem. Commun. 1984, 45.
* Corresponding author. New address: UCR-CNRS J oint Research
Chemistry Laboratory, Department of Chemistry, University of Cali-
fornia Riverside, Riverside, CA 92521-0403.
(4) For a recent example, see: van Eis, M. J .; Zappey, H.; de Kanter,
F. J . J .; de Wolf, W. H.; Lammertsma, K.; Bickelhaupt, F. J . Am. Chem.
Soc. 2000, 122, 3386.
(5) Marinetti, A.; Mathey, F.; Fischer, J . J . Am. Chem. Soc. 1985,
107, 5001.
(6) Espinosa-Ferao, A.; Deschamps, B.; Mathey, F. Bull. Soc. Chim.
Fr. 1993, 130, 695.
† Institute of Organic and Physical Chemistry of RAS.
‡ Laboratoire “He´te´roe´le´ments et Coordination” UMR CNRS 7653.
(1) Review: Mathey, F.; Tran Huy, N. H.; Marinetti, A. Helv. Chim.
Acta 2001, 84, 2938.
(2) Review: Lammertsma, K.; Vlaar, M. J . M. Eur. J . Org. Chem.
2002, 4, 1127.
(7) Deschamps, B.; Mathey, F. Synthesis 1995, 941.
(8) Tran Huy, N. H.; Mathey, F. J . Org. Chem. 2000, 65, 652.
10.1021/om030671g CCC: $27.50 © 2004 American Chemical Society
Publication on Web 03/16/2004

1962 Organometallics, Vol. 23, No. 8, 2004
Notes
Ta ble 2. Electr och em ica l P r op er ties of F r ee
P h osp h oles (5 × 10-3 M a ceton itr ile solu tion , sca n
r a te 100 MV s-1) a n d Decom p lexa tion Yield s of
P h osp h ole Com p lexes (* con ver sion a s m ea su r ed
by in tegr a tion of th e 31P NMR p ea k s)
F igu r e 1. CV of 1-phenyl-3,4-dimethylphospholepentacar-
bonyltungsten complex.
Ta ble 1. Red u ction P oten tia ls a n d P ea k
Cu r r en ts fr om th e CV Cu r ves of
P h osp h ole-P en ta ca r bon yltu n gsten Com p lexes
(5 × 10-3 M a ceton itr ile solu tion , sca n r a te
Sch em e 1
100 m V s-1
)
electrochemical conditions (Epred ) -2.19 V, Ipred ) -8.1
µA). Moreover, two electrons are transferred at these
potentials. To determine the evolution of the initially
formed dianionic species, we decided to repeat these
experiments on a preparative scale. Monitoring of the
solution by 31P NMR spectroscopy showed the complete
disappearance of the resonance for the 1-phenyl-3,4-
dimethylphosphole P-W(CO)5 complex at +9.7 ppm and
the appearance of a single P-containing product at -0.8
ppm. This product was isolated by column chromatog-
raphy in 86% yield and identified as the free phosphole.
Thus the overall electrochemical reduction scheme
corresponds to eq 1.
from the desired free phosphorus heterocycle. Similar
reductive electrochemical decomplexations were carried
out with a series of phosphole pentacarbonyltungsten
complexes (Table 2). As can be seen, the reaction is quite
general and no problem arises from the electrochemical
reduction of the free phospholes, which occurs at more
negative potentials than the reduction of the corre-
sponding complexes.
We were eager to apply this new and mild electro-
chemical decomplexation technique to an original case.
For that purpose, we decided to repeat a former experi-
ment where the reaction of an excess of ethoxyacetylene
with the transient phosphinidene complex [PhP-W(CO)5]
led to a variable mixture of the symmetrical 3,4-bis-
(ethoxy)phosphole complex 1 (δ 31P -9.5 ppm in CDCl3)
together with the 1-phenylphosphirane complex 2 (δ 31P
-190.1 ppm in CDCl3), whose proportion increased with
the temperature (Scheme 1).9 The reaction was carried
out at 110 °C for 28 h. To our surprise, we were unable
to get 1. Instead, a mixture of 2 (δ 31P -188.2 ppm in
The presence of tungsten was detected in the anodic
compartment of the electrochemical cell by the intense
blue color of its oxidation products. This allows a useful
preliminary separation of tungten-containing species
(9) Tran Huy, N. H.; Mathey, F. Phosphorus Sulfur Silicon 1990,
47, 477.

Notes
Organometallics, Vol. 23, No. 8, 2004 1963
Finally, numerous electrochemical studies of [M(CO)n-
(PR3)6-n] complexes (M ) Cr, Mo, W) have been de-
scribed in the literature,10 but as far as we know, they
were all directed toward the characterization of the
oxidation products, inter alia the 17e cationic species.
From another standpoint, Kochi and others have stud-
ied electrochemical reductions of several 18-electron
carbonyl complexes that dissociate a CO ligand upon
electron transfer.11
Exp er im en ta l Section
NMR spectra were recorded on a multinuclear Bruker
AVANCE 300 MHz spectrometer operating at 300.13 for 1H,
75.47 for 13C, and 121.50 MHz for 31P. Chemical shifts are
expressed in parts per million (ppm) downfield from internal
tetramethylsilane (1H and 13C) and external 85% aqueous H3-
PO4(31P). Mass spectra were obtained at 70 eV with an HP
5989B spectrometer by the direct inlet method. All electro-
chemical experiments were carried out under a dry argon
atmosphere. Acetonitrile was purified by distillation over
P4O10. Supporting electrolyte Bu4NBF4 was recrystallized from
ethyl acetate, melted, and dried in a vacuum. Cyclic voltam-
mograms were recorded in CH3CN in the presence of Bu4NBF4
(0.3 M) on a gold electrode (working surface 0.2 mm2) in a
thermostatically controlled three-electrode electrochemical cell
(substrate concentration 5 × 10-3 M). Saturated calomel was
used as the reference electrode. Platinum wire was used as
an auxiliary electrode. Curve recording was performed at
constant potential scan rate (100 mV s-1). Preparative elec-
F igu r e 2. ORTEP drawing of one molecule of 6. Thermal
ellipsoids enclose 50% of the electronic density. Main bond
lengths (Å) and angles (deg): P(1)-S(1) 1.9392(5), P(1)-
C(1) 1.817(1), P(1)-C(4) 1.831(1), P(1)-C(7) 1.804(1), O(1)-
C(2) 1.219(2), O(2)-C(4) 1.335(2), O(2)-C(5) 1.458(2),
C(1)-C(2) 1.513(2), C(2)-C(3) 1.461(2), C(3)-C(4) 1.348-
(2); C(7)-P(1)-C(1) 108.06(6), C(7)-P(1)-C(4) 105.82(6),
C(1)-P(1)-C(4) 92.07(6), C(7)-P(1)-S(1) 115.47(4), C(1)-
P(1)-S(1) 118.31(5), C(4)-P(1)-S(1) 114.21(4), C(4)-O(2)-
C(5) 115.4(1), C(2)-C(1)-P(1) 106.8(1), O(1)-C(2)-C(3)
123.9(1), O(1)-C(2)-C(1) 122.1(1), C(3)-C(2)-C(1) 113.9-
(1), C(4)-C(3)-C(2) 113.9(1), O(2)-C(4)-C(3) 129.2(1),
O(2)-C(4)-P(1) 117.8(1), C(3)-C(4)-P(1) 113.1(1).
trochemical reductions were performed in
a preparative
electrochemical cell with a separation of the anodic and
cathodic compartments. A carbon-glass electrode (working
surface 60 cm2) was used as the working electrode. The
electrolyses were carried out at room temperature in galvano-
static regime.
toluene) and a new product (3) (δ 31P +11.8 ppm in
toluene) was formed. During the chromatographic work-
up, 3 hydrolyzed to give a new more stable product (4)
(δ 31P +0.03 ppm in CDCl3) (Scheme 1).
E lect r och em ica l Decom p lexa t ion of (1-P h en yl-3,4-
d im eth ylp h osp h ole)p en ta ca r bon yltu n gsten . (1-Phenyl-
3,4-dimethylphosphole)pentacarbonyltungsten (0.1 g, 0.2 ×
10-3 mol) was dissolved in acetonitrile (20 mL) containing Bu4-
NBF4 (0.3 M). Then a constant current of 10.72 mA was passed
through the solution for 1 h. Complete conversion of the
complex into the corresponding free phosphole was observed
by 31P NMR spectroscopy. The product was purified by
chromatography on silica gel using hexane/dichloromethane,
9:1, as the eluent. The yield of isolated product was 84%.
Syn th esis of Com p lex 4. A solution of 2,3-dimethyl-5,6-
bis(methoxycarbonyl)-7-phenyl-7-phosphanorbornadiene com-
plex3 (1.0 g, 1.53 × 10-3 mol) and 1 mL of a 40% solution of
ethoxyacetylene in hexane (ca. 4 × 10-3 mol) was heated at
110 °C for 24 h in toluene (10 mL). After evaporation, the
residue was chromatographed on silica gel, with hexane/
diethyl ether (1:9) as the eluent: 0.28 g of 4 was isolated as a
slightly yellow powder (yield 34%).
The identification of 4 mainly relied on 13C NMR
spectroscopy and mass spectrometry. At m/z 544, the
highest mass corresponds to the formulation. The 13C
spectrum in CDCl3 shows a P-CH2 resonance at 43.57
(1J C-P ) 27 Hz), a OCH2 at 70.38 (3J C-P ) 5.2 Hz), a
dCH at 111.55 (2J C-P ) 12 Hz), a (Ph)C-P at 132.77
(1J C-P ) 36.7 Hz), a (EtO)C-P at 186.87 (1J C-P ) 48
2
Hz), and carbonyls at 195.05 (d, J C-P ) 6.8 Hz, cis-
WCO), 197.27 (s, cyclic CO), and 198.46 (d, 2J C-P ) 23.3
Hz, trans-WCO). We have no obvious explanation of why
the insertion of the second molecule of alkyne into the
initially formed phosphirene gave the symmetrical
product 1 in our former experiments and the unsym-
metrical product 3 in the present case, although the
formation of 3 appears to be more logical from an
electronic standpoint.
Electr och em ica l Decom p lexa tion of (1-P h en yl-2-eth -
oxy-4-k et o-4,5-d ih yd r op h osp h ole)p en t a ca r b on ylt u n g-
st en (4). (1-Phenyl-2-ethoxy-4-keto-4,5-dihydrophosphole)-
pentacarbonyltungsten (4) (0.2 g, 0.37 × 10-3 mol) was
dissolved in acetonitrile (20 mL) containing Bu4NBF4 (0.3 M).
A constant current of 10 mA was passed through the solution
for 2 h. Then sulfur (0.3 g) was added to the solution. After
Anyhow, we applied the electrochemical decomplex-
ation technique to complex 4. Only one major irrevers-
red
ible reduction wave was observed (Ep
) -2.04 V,
red
Ip ) -5.6 µA). Once again, the process corresponds
to the transfer of two electrons. Monitoring the reaction
mixture by 31P NMR spectroscopy showed the formation
of the decomplexation product 5 at -31.5 ppm. The
product was reacted in situ with sulfur to give in 63%
overall yield from 4 the stable sulfide 6, which was fully
characterized by X-ray crystal structure analysis (Figure
2). This successful decomplexation shows that this
electrochemical technique is compatible with carbonyl
and vinyl ether functionalities.
(10) See for example: Wimmer, F. L.; Snow, M. R.; Bond, A. M. Inorg.
Chem. 1974, 13, 1617. Bond, A. M.; Colton, R.; J ackowski, J . J . Inorg.
Chem. 1975, 14, 274. Bond, A. M.; Darensbourg, D. J .; Mocellin, E.;
Stewart, B. J . J . Am. Chem. Soc. 1981, 103, 6827. Bagchi, R. N.; Bond,
A. M.; Brain, G.; Colton, R.; Henderson, T. L. E.; Kevekordes, J . E.
Organometallics 1984, 3, 4.
(11) See for example: Ohst, H. H.; Kochi, J . K. J . Am. Chem. Soc.
1986, 108, 2897. Hershberger, J . W.; Kochi, J . K. J . Chem. Soc., Chem.
Commun. 1982, 213. Narayanan, B. A.; Amatore, C.; Casey, C. P.;
Kochi, J . K. J . Am. Chem. Soc. 1983, 105, 6351.

1964 Organometallics, Vol. 23, No. 8, 2004
Notes
stirring for 45 min, the solvent was evaporated under vacuum.
The product 6 was purified by chromatography on silica gel
using hexane/ether, 9:1, as the eluent. The yield of isolated
2894 with I > 2σ(I). Goodness of fit on F2 1.087; R(I >
2σ(I)) ) 0.0352, wR2 ) 0.1029(all data); maximum/minimum
residual density 0.305(0.052)/-0.360(0.052) e Å-3. Data were
collected on a KappaCCD diffractometer at 150.0(1) K with
Mo KR radiation (λ ) 0.71073 Å). Full details of the crystal-
lographic analysis are described in the Supporting Informa-
tion.
1
product was 63% (59 mg). 31P NMR (CDCl3): δ 35.6. H NMR
(CDCl3): δ 1.37 (t, 3H, J ) 7.0 Hz, CH3), 3.17 (m, 2H, P-CH2),
4.13 (q, 2H, O-CH2), 6.04 (d, 1H, J ) 31.2 Hz, CHd), 7.25-
7.59 (m, 3H, Ph), 7.79-7.87 (m, 2H, Ph). 13C NMR (CDCl3): δ
1
14.24 (s, CH3), 44.55 (d, J C-P ) 57.6 Hz, P-CH2), 70.38 (d,
2
3J C-P ) 7.1 Hz, O-CH2), 114.92 (d, J C-P ) 18.1 Hz, dCH),
Ack n ow led gm en t. One of the authors (D.G.Y.)
thanks CNRS for the financial support of his stay in
Palaiseau.
1
1
128.16 (d, J C-P ) 80.5 Hz, P-C(Ph)), 180.12 (d, J C-P ca. 81
Hz, P-C(OEt)), 193.68 (d, 2J C-P ) 7.7 Hz, CdO). MS (EI): m/z
(ion, relative abundance) 252 (M+, 19), 251 (100).
Cr ysta llogr a p h ic Da ta for 6. M ) 252.25 g/mol; mono-
clinic; space group P21/n; a ) 14.127(1) Å, b ) 6.204(1) Å, c )
14.761(1) Å, â ) 108.001(1)°, V ) 1230.4(2) Å3; Z ) 4; D )
1.362 g cm-3; µ ) 0.375 cm-1; F(000) ) 528. Crystal dimensions
0.20 × 0.16 × 0.10 mm. Total reflections collected 5896 and
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for 6. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM030671G