Spin-adduct of the P4?- radical anion during the electrochemical reduction of white phosphorus

Article abstract of DOI:10.1007/s11172-010-0103-6

The radical anion P₄^?- was detected and identified by the ESR method as a spin-adduct with nitrone during the electrochemical reduction of white phosphorus in the presence of a spin trap, viz., α-phenyl-N-tert-butylnitrone, in a special electrolysis cell with a helical platinum working electrode in the potentiostatic mode. The character of the behavior of P₄^?- and the spin trap during electrochemical reduction was monitored by cyclic voltammetry directly in the electrolysis cell, and the spin-adduct formed was detected by ESR.

Full text of DOI:10.1007/s11172-010-0103-6

Russian Chemical Bulletin, International Edition, Vol. 59, No. 2, pp. 466—468, February, 2010
466
•–
Spinꢀadduct of the P4 radical anion
during the electrochemical reduction of white phosphorus
M. K. Kadirov, Yu. G. Budnikova, K. V. Kholin, M. I. Valitov, S. A. Krasnov,
T. V. Gryaznova, and O. G. Sinyashin
A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843 2) 73 2253. Eꢀmail: kadirov2004@mail.ru
–
•
The radical anion Р4 was detected and identified by the ESR method as a spinꢀadduct
with nitrone during the electrochemical reduction of white phosphorus in the presence of a spin
trap, viz., αꢀphenylꢀNꢀtertꢀbutylnitrone, in a special electrolysis cell with a helical platinum
–
•
working electrode in the potentiostatic mode. The character of the behavior of Р4 and the
spin trap during electrochemical reduction was monitored by cyclic voltammetry directly in the
electrolysis cell, and the spinꢀadduct formed was detected by ESR.
Key words: white phosphorus, electrochemical reduction, ESR, radical anion, spinꢀadduct.
A solution of white phosphorus in benzene (~1.6•10–2 mol L–1
)
The problem of the selective cleavage of the Р—Р bonds
in a white phosphorus molecule is very important in the
chemistry of phosphorusꢀcontaining compounds.1,2 The
radical character of the cathodic reduction of white phosꢀ
phorus has been assumed earlier.3 However, such prodꢀ
ucts of activation of Р4 have not yet been detected. The
purpose of the present study was to observe the primary
products of cathodic reduction of a Р4 molecule. To create
a necessary concentration of paramagnetic species in the
resonator of an ESR spectrometer, we used a special cell
with the helical working electrode.4 The method of spin
traps5,6 based on the reaction of electrochemically inacꢀ
tive αꢀphenylꢀNꢀtertꢀbutylnitrone (1), resulting in the staꢀ
ble nitroxyl radical (Scheme 1), was used to detect radical
anion species. The stable nitroxyl radical can be identified
by ESR spectroscopy.
was purged with helium through a capillary immersed to the
bottom of the cell. The material of the working and auxiliary
electrodes was platinum, and Ag/AgNO3 (0.01 mol L–1) was
a reference electrode.
Measurements were carried out using the programꢀapparaꢀ
tus complex7 mounted on the basis of an analogꢀelectrochemical
setup with a PIꢀ50ꢀ1 potentiostat, a Prꢀ8 programmer, an
Xꢀrange ESR spectrometer (Radiopan), and Е14ꢀ440 analogꢀtoꢀ
digital and digitalꢀtoꢀanalog converter (LꢀCard), and a computer.
The WinSim 0.96 program (NIEHS) was used for ESR specꢀ
tra processing.
The images of the films were obtained on a HITACHI
TMꢀ1000 scanning electron microscope.
Results and Discussion
The curves obtained by cyclic voltammetry (CV) at the
Pt electrode for a solution of compound 1 in MeCN
(0.01 mol L–1) and a solution of white phosphorus in a benꢀ
zene—MeCN (1 : 1) mixture vs 0.1 M solution of Et4NBF4
are shown in Fig. 1. To exclude the formation of products
of phosphorus oxidation and hydrolysis, all procedures
were carried out in a thoroughly dehydrated solvent. It
should be mentioned that the compounds containing the
mobile hydrogen atom give no pronounced reduction peaks
on Pt down to potentials of the supporting electrolyte disꢀ
charge. The CV curves of a solution of white phosphorus
at a potential of –1.5 V contain the irreversible peak corꢀ
responding to its reduction. Thus, the reduction peak close
to the oneꢀelectron peak (estimated by the comparison
with benzophenone used as standard) relates to the elecꢀ
tron transfer to a white phosphorus molecule to form radꢀ
Scheme 1
Experimental
Benzene was dehydrated by distillation over sodium. αꢀPheꢀ
nylꢀNꢀtertꢀbutylnitrone (1) and Еt4NBF4 (Fluka) were used withꢀ
out additional purification. Acetonitrile was purified by triple
distillation over KMnO4 and P2O5, and Еt4NBF4 was dried in
vacuo for 2 days at 100 °С. Dimethylformamide was distilled,
kept for 12 h over calcined K2CO3, then distilled repeatedly over
CaH2, and stored over molecular sieves 3A calcined at 300 °С.
–
•
ical anions Р4
.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 456—458, February, 2010.
1066ꢀ5285/10/5902ꢀ0466 © 2010 Springer Science+Business Media, Inc.

Spinꢀadduct of P4•– radical anion
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 2, February, 2010
467
i/μA
1 is by ~1 V more negative than the reduction peak of Р4
(see Fig. 1). Therefore, this trap was used for the detection
of white phosphorus radical anions. The experimental ESR
spectrum of the spinꢀadduct detected upon the reduction
of white phosphorus (~1.6•10–2 mol L–1) in the potenꢀ
tiostatic mode at a potential of –1.5 В and a 1 М solution
of nitrone 1 in a benzene —MeCN (1 : 1) mixture at the Pt
electrode vs 0.1 М solution of Et4NBF4 is shown in Fig. 2.
0
1
–40
2
–80
–120
–160
a
–3.0 –2.5 –2.0 –1.5 –1.0 –0.5
E/V
Fig. 1. CV curves of a 0.01 M solution of nitrone 1 in МeCN
vs 0.1 М solution of Et4NBF4 (1) and white phosphorus solution
in a benzene—MeCN (1 : 1) mixture against the same backꢀ
ground after helium purging for 10 min (2) (T = 298 K, Pt
electrode, v = 0.1 V s–1).
1
2
3
b
4
5
3290 3300
3310
3320 3330 3340
3350 H/G
Fig. 2. Experimental (1) and simulated overall (2) ESR spectra
of the spinꢀadducts detected upon the reduction of white phosꢀ
phorus and a 1 М solution of nitrone 1 in a benzene—МeCN
(1 : 1) mixture vs 0.1 М solution of Et4NBF4 after helium purgꢀ
ing for 10 min and the ESR spectra of their constituent spinꢀ
adducts: 1•1• (3), 1•1´• (4), and 1•P4 – (5).
•
c
There is a large data bank on the
ESR spectra and magnetic resonance
characteristics of spinꢀadducts of
compounds belonging to various
classes.* In particular, the spinꢀadꢀ
duct of the shortꢀlived phosphorusꢀ
–
•
centered radical РНО2 and nitroꢀ
ne 1 was obtained8 by the electrochemical oxidation of
hypophosphite on the nickel electrode. The following magꢀ
netic resonance parameters of the spinꢀadduct were deꢀ
tected: аN = aP = 15.91 G, aН—СН = 1.99 G, аН—РН
3.21 G, g = 2.0060. However, no spinꢀadducts of the Р4
radical anion were studied. The reduction peak of nitrone
=
–
•
Fig. 3. View of the polyphosphorus films upon the electrochemꢀ
ical reduction of P4 in DMF vs 0.1 М solution of Et4NBF4 deꢀ
tected with a HITACHI TMꢀ1000 microscope with a magnifiꢀ
cation of 200 (a), 2000 (b), and 10 000 times (c).
* http://epr.niehs.nih.gov.

468
Russ.Chem.Bull., Int.Ed., Vol. 59, No. 2, February, 2010
Kadirov et al.
and aγꢀP were attributed to the phosphorus atom in the
βꢀposition and two equivalent phosphorus atom nuclei in
the γꢀpositions to the radical center, respectively.
During several first minutes after the beginning of elecꢀ
trolysis, the lines of the experimental spectrum begin to
broaden and the pattern changes gradually, indicating the
formation of polymer products.
a
The formation of polymer products upon the electroꢀ
chemical reduction of white phosphorus in DMF vs 0.1 М
solution of Et4NBF4 at Е = 2.1 V and СP = 5•10–2 mol L–1
4
is confirmed by the microscopic images of the electrode
surface (Figs 3 and 4). The view of the polyphosphorus
films depends on the electrolysis conditions and duration.
The polyphosphorus products formed at the Pt electrode
in DMF are rapidly dissolved and desorbed to the elecꢀ
trolyte volume. At the same time, on the lead cathode in
DMF they form a black sponge nanoporous film (see Fig. 3).
In a waterꢀcontaining electrolyte, a black polyphosphorus
conducting film is also formed, but the structure of polyꢀ
phosphides is filamentous and needleꢀlike in this case (see
Fig. 4). The thickness of the polymer filaments is ~1 μm.
b
Thus, the Р4 – radical anion was detected for the first
•
time in the potentiostatic mode by the ESR method as the
spinꢀadduct with αꢀphenylꢀNꢀtertꢀbutylnitrone in the elecꢀ
trochemical reduction of white phosphorus in the elecꢀ
trolysis cell with the helical platinum working electrode.
The present study confirms the radical character of the
intermediates of white phosphorus described earlier.3
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 07ꢀ03ꢀ00213)
and the Russian Academy of Sciences (Program No. 18).
c
References
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sa, O. J. Scherer, H. Sitzman, J. Organomet. Chem., 2004,
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4. M. K. Kadirov, Pat. RF 69 252; Byul. Izobret. [Invention
Bulletin], 2007, No. 34 (in Russian).
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Fig. 4. View of the polyphosphorus films upon the electrochemꢀ
ical reduction of P4 in an aqueous solution of HCl detected
with a HITACHI TMꢀ1000 microscope with a magnification of
2000 (a), 4000 (b), and 10 000 times (c).
The ESR spectrum also exhibits two spinꢀadducts of free
7. M. K. Kadirov, V. L. Odivanov, Yu. G. Budnikova, Pribory i
tekhnika eksperimenta [Experimental Instruments and Techꢀ
nique], 2007, 1, 151 (in Russian).
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Commun., 2002, 4, 293.
radicals of the spin trap 1•1• with the hyperfine coupling
constants (HFC) аN = 13.59 G, аН = 2.22 G and аN
=
= 18.60 G, аН = 4.00 G for the first and second adducts,
respectively, with the ESR linewidths δН = 0.65 G. The
–
•
ESR spectrum of spin adduct 1•Р4 has the following
parameters: aN = 14.10 G, aβ = 14.6 G, аН = 0.80 G,
Received May 5, 2009;
ꢀP
aγꢀP = 0.78 G, and δН = 0.65 G. The HFC constants aβꢀP
in revised form September 9, 2009