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ruthenium-promoted isomerization to the phosphine-like
maximum yield. Spontaneous disproportionation of I to PH3
and H3PO2 acid takes place even at low temperature and
hampers the isolation of H3PO as a pure product. Nonethe-
less, H3PO, the first defined compound of phosphorus in the
ꢀ1 oxidation state, was characterized in solution by NMR
spectroscopy and trapped in the coordination sphere of
ruthenium(II) by tautomerization to phosphinous acid,
H2P(OH), to afford stable organometallic cationic complexes.
Studies are in progress to explore the reactivities of both free
H3PO and coordinated H2P(OH).
tautomers PH(OH)2 and P(OH)3, respectively.[17] Thus, we
reasoned that a water-soluble version of complex 1 could
react with a water/ethanol solution of the electrogenerated
H3PO by stabilizing the putative phosphinous acid, H2P(OH),
assisting the energetically favored H3PO tautomerization at
ruthenium.
In keeping with this working hypothesis, reaction of crude
I with the ruthenium complexes [CpRu(tppms)2Cl][18] (2)
[Cp = cyclopentadienyl,
tppms = m-SO3C6H4PPh2(ꢀ)Na(+)
]
and [CpRu(pta)(CH3CN)2]PF6 (3)[19] (pta = 1,3,5-triaza-7-
phosphaadamantane) gave as products the organometallic
complexes [CpRu(tppms)2{H2P(OH)}]PF6 (4) and [CpRu-
(pta)(CH3CN){H2P(OH)}]PF6 (5) after simple workup
(Scheme 3; see the Supporting Information for details).[20]
Experimental Section
CAUTION: Both white phosphorus and phosphine are very toxic,
flammable, and hazardous compounds (see the Supporting Informa-
tion).
H3PO was electrogenerated in a 50 mL home-made electro-
chemical cell (see Supporting Information for details).[22] White
phosphorus (70 mg, 0.56 mmol) was suspended in H2O/EtOH 2:1
(30 mL) and warmed to 608C under nitrogen to ensure dissolution
(ca. 30 min). HCl (0.5 mL, 2m) was then added to the mixture and an
electrical current with intensity i = 150 mA (current density
5.0 mAcmꢀ2) was applied for about 30 min while keeping the
temperature constant at 608C. During the experiment, the mixture
became cloudy and a yellowish suspension formed. Further addition
of HCl (1.5 mL, 2m) and vigorous stirring at 608C gave an almost
colorless solution. The overall potential on the cell was ꢀ1.5–2.5 V.
A
31P{1H} NMR spectrum of an aliquot showed complete
conversion of P4 into PH3, H3PO, and H3PO2, with signals at
d(PH3) = ꢀ243.05 ppm (s); d(H3PO) = ꢀ17.55 ppm (s); d(H3PO2) =
4.97 ppm (s). The relative amount depends from several factors such
as reaction time, temperature, and acidity. In the experiment
described above, the highest production of H3PO was obtained with
the following ratios: PH3 (15.1%), H3PO (68.8%), and H3PO2
(16.1%).
Scheme 3. Synthesis of [CpRu(tppms)2{H2P(OH)}]PF6 (4) and [CpRu-
(pta)(CH3CN){H2P(OH)}]PF6 (5) from H3PO trapping.
Synthesis of [CpRu(tppms)2{H2P(OH)}]PF6 (4): [CpRu-
(tppms)2Cl] (2, 28 mg, 0.0301 mmol) was dissolved in dry MeOH
(15 mL) and reacted with one equiv of solid TlPF6 (10.5 mg,
0.0301 mmol) at room temperature under stirring. After 4 h, the
solution was filtered by cannula to remove TlCl, giving a clear yellow
solution of [CpRu(tppms)2(CH3OH)]PF6. A H2O/EtOH 2:1 solution
(14.0 mL) of H3PO, prepared as described above, was transferred to a
Schlenk flask and evaporated under vacuum to half of the volume to
remove the solvent and the volatiles (PH3). The resulting colorless
solution, containing only H3PO (70%) and H3PO2 (30%), was then
added dropwise to the solution of [CpRu(tppms)2(CH3OH)]PF6, and
the stirred suspension was warmed to 508C for 1 h, after which a clear
yellow solution was obtained. Evaporation of the solvent to dryness
left a yellow solid, which was washed with Et2O (2 ꢀ 5 mL) and
filtered under vacuum. Yield 19.6 mg (60%, based on complex 2).
The formation of complexes 4 and 5 confirmed that the
electrogenerated phosphine oxide may be trapped before
disproportionation by coordination of its tautomer,
H2P(OH), to ruthenium. Both compounds were characterized
by ESI-MS, IR and NMR spectroscopy. The 31P{1H} NMR
spectrum contains a triplet centered at d = 70.3 ppm for 4
(2JPP = 51.3 Hz), while that of 5 has a doublet at d = 74.1 ppm
(2JPP = 63.0 Hz) for the coordinated phosphinous acid. Switch-
1
ing off the proton decoupler gave triplets with identical JPH
couplings (366.0 Hz), thus providing confirmation for a
H2P(OH) coordinated species. Complexes 4 and 5 are the
first compounds incorporating phosphinous acid and may be
related to the crystallographically authenticated sulfur ana-
logue, that is, [CpRu(PPh3)2{H2P(SH)}]PF6 (6), obtained by
controlled hydrolysis of the P4S3 dinuclear sandwich complex
[{CpRu(PPh3)2}2(m,h1:1-Pap,Pbas-P4S3)](PF6)2.[21] Both 4 and 5
are air-stable orange microcrystalline materials that share
most of their chemicophysical properties with 6 and the other
known [CpRu(PPh3)2{PH(3ꢀx)(OH)x}]+ derivatives (x = 2,
3).[17]
In conclusion, the elusive and poorly investigated phos-
phine oxide H3PO (I) has been generated by simple electro-
chemical methods based on a two-step process involving first
the electroreduction of white phosphorus to PH3 in an acidic
water/ethanol mixture at a lead electrode, followed by PH3
oxidation at the zinc anode yielding H3PO in about 70%
H3PO2
is
left
unreacted,
as
formation
of
[CpRu-
(tppms)2{PH(OH)2}]PF6 (4c) was not observed. See the Supporting
Information for an independent synthesis of 4c and [CpRu(TPPMS)2-
(PH3)]PF6 (4b).
1H NMR (400.13 MHz, CD3OD, 208C): d = 4.6 (s, Cp, 5H), 6.9–
8.2 ppm (m, aromatic, 28H). The signal for to the two H2P(OH)
protons could not be directly observed as it is masked by the aromatic
1
proton signals, but is was identified indirectly by H,31P HMQC 2D-
NMR, which correlated the phosphorus resonance with a doublet at
ca. 7.5 ppm (1JHP = 366.0 Hz; Supporting Information, Figure S6).
2
31P{1H} NMR (161.97 MHz, CD3OD, 208C): d = 70.3 (t, JPP
=
51.3 Hz, H2P(OH)), 46.7 (d, 2JPP = 51.3 Hz, tppms), ꢀ145.1 ppm
1
(sept, JPF = 705.9 Hz, PF6); 31P NMR (161.97 MHz, CD3OD, 208C):
d = 70.3 (tt, 1JHP = 366.0 Hz, 2JPP = 51.3 Hz, H2P(OH)), 46.7 (br d,
2JPP = 51.3 Hz, tppms), ꢀ145.1 ppm (sept, 1JPF = 705.9 Hz, PF6). IR
5372
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 5370 –5373