Stabilization of sulfenyl(poly)selenide ions in N,N-dimethylacetamide

Article abstract of DOI:10.1039/b200638n

Whereas redox processes resulted from the reactions PhSe^-/S₈ or PhSe₂Ph/S₃^·-, mixed anions RSSe_y^- (R = Ph, PhCH₂; y = 1-3) were obtained by the slow addition of solid selenium to thiolate ions in N,N-dimethylacetamide. The RS^- + n Se reactions, which were investigated by spectroelectrochemistry, led initially (n = 1) only to the formation of RSSe^- ions. These species oxidized into RS₂R faster than RS^- on a gold electrode, with the simultaneous electrodeposition of very reactive microcrystals of selenium. On a preparative scale, the substitution of RSSe^- ions (R = CH₃, Ph) on alkyl halides yielded RSSeR′ compounds (R′ = PhCH₂, CH₃, respectively) which greatly disproportionated. Further additions of Se (n = 2, 3) to RS^- ions led to RSSe₂^- and RSSe₃^- in equilibrium with RS₂R and mixtures of Se_x^2- polyselenide ions (x = 4,6; 6,8). Visible spectra of RSSe₂^- and RSSe₃^- ions were calculated from the study of the backward reactions RS₂R + Se_x^2- (x = 4, 6).

Full text of DOI:10.1039/b200638n

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Stabilization of sulfenyl(poly)selenide ions in N,N-dimethylacetamide
Abdelkader Ahrika, Julie Robert, Meriem Anouti and Jacky Paris*
Laboratoire de Physicochimie des Interfaces et des Milieux R e´ actionnels, UFR Sciences et
Techniques, Parc de Grandmont, 37200, Tours, France. E-mail: paris@univ-tours.fr
Received (in Montpellier, France) 16th January 2002, Accepted 27th May 2002
First published as an Advance Article on the web 5th September 2002
ꢀ
ꢁ
ꢀ
yꢀ
, mixed anions RSSe
Whereas redox processes resulted from the reactions PhSe /S
8
or PhSe
2
Ph/S
3
(
dimethylacetamide. The RS + n Se reactions, which were investigated by spectroelectrochemistry, led initially
R ¼ Ph, PhCH ; y ¼ 1–3) were obtained by the slow addition of solid selenium to thiolate ions in N,N-
2
ꢀ
ꢀ
ꢀ
(
electrode, with the simultaneous electrodeposition of very reactive microcrystals of selenium. On a preparative
n ¼ 1) only to the formation of RSSe ions. These species oxidized into RS
2
R faster than RS on a gold
ꢀ
0
scale, the substitution of RSSe ions (R ¼ CH , Ph) on alkyl halides yielded RSSeR compounds
3
0
ꢀ
(
ions led to RSSe
R ¼ PhCH
2
, CH
3
, respectively) which greatly disproportionated. Further additions of Se (n ¼ 2, 3) to RS
ꢀ ꢀ 2ꢀ
3 2 x
2
and RSSe
in equilibrium with RS
R and mixtures of Se
polyselenide ions (x ¼ 4,6;
ꢀ
ꢀ
6
RS
,8). Visible spectra of RSSe and RSSe3 ions were calculated from the study of the backward reactions
2
2ꢀ
2
R + Se
x
(x ¼ 4, 6).
We previously reported that solid selenium slowly reacts with
ꢀ
Surprisingly, analogous anodic behaviours have been reported
ꢀ1 ꢀ
5
and ArSSe species, suggesting fast hetero-
RSe selenolate ions in N,N-dimethylacetamide (DMA), a
with RSe
2
ꢀ
4ꢀ
ꢀ
ꢀ
dipolar aprotic medium, yielding successively RSe
1
ions
and RSe ions
geneous reactions between RSe or ArS ions and electrogen-
erated solid selenium. This hypothesis is reconsidered in the
present paper which is mainly devoted to the expected stabili-
y
ꢀ
[
disproportionate [eqn. (2)] into diselanes and Se
eqn. (1); R ¼ Ph, PhCH
2
; y ¼ 2–4]. RSe
3
2ꢀ
polysele-
2
x
ꢀ
ꢀ
nide ions which had been characterized (x ¼ 4, 6, 8):
zation of RSSe
The reactions RS /Se and PhSe /S, RS R/Se
x
y
(y q 1; R ¼ Ph, PhCH
2
) and PhSeS ions.
(x ¼ 4, 6,
ꢀ
ꢀ
2ꢀ
ꢀ
ꢀ
2
RSe þ ðy ꢀ 1Þ SeðsÞ ! RSey
2
ð1Þ
ð2Þ
ꢁ
ꢀ
8
) and PhSe
2
Ph/S were therefore followed by UV-visible
3
RSey Ð RSe2R þ Sex2ꢀ
ꢀ
spectrophotometry coupled with voltammetry (CV and rotat-
ing gold disc electrode). Natural selenium-containing com-
pounds have been the subject of extensive studies because of
Reactions (1) are similar to those observed between sulfur and
ꢀ
3
7
,8
thiolates leading to RSy ions (R ¼ alkyl, y ¼ 2–5). How-
their possible cancer chemopreventive properties. ‘Selenenyl
0
ꢀ
ever, very little is known about ‘mixed anions’ such as RSSe
y
sulfides’ RSSeR identified in Allium volatiles from speciation
ꢀ
ꢀ
+
9
experiments, are most frequently prepared by reactions
or RSeSy (y q 1): a variety of selenenyl thiolates (RSeS Li )
resulting from the addition of one sulfur unit to lithium alkyl
0
between thiols (RSH) and selenenyl halides (R SeX, X ¼ Br,
ꢀ
+
selenolates (RSe Li ) in THF were characterized in situ by
1
Cl). These species are also obtained by mixing the symmetri-
0
7
7
4
Se NMR at 193 K; however ‘RSeS readily underwent
ꢀ
0
0
cal products RS
tion (8):
2
R and R Se
2
R of the usual disproportiona-
1
1
internal redox-reactions below room temperature’ [eqn. (3)],
ꢀ
0
0
0
and RS + Se (or Te) processes were described as ineffective
in THF.
2 RSSeR Ð RS R þ R Se R ð8Þ
Our spectroelectrochemical results on the stabilization of
2
2
4
RSeS ! RSe2R þ S22ꢀ
ꢀ
ð3Þ
ꢀ
2
RSSe ions were then applied on a preparative scale, to two
ꢀ
typical alkylations of RS (R ¼ CH
3
, Ph) + Se solutions.
Using UV-visible absorption spectrophotometry, we recently
showed that the stoichiometric addition of sulfur to selenolate
ꢀ
ꢀ
ions 2-NO C H Se
2
(ArSe , lmax ¼ 520 nm; [S] /
ad
6
4
ꢀ
ꢀ
Results and discussion
[ ¼ 1) yielded ’85% of ArSeS ions (lmax ¼ 728
nm); these species being partly oxidized in the presence of
excess sulfur [eqn. (5)]:
ArSe ]
0
5
x2ꢀ
x2ꢀ
ꢀ
ꢀ
5
S
, Se , RS and RSe ions in DMA
There is now general agreement concerning the nature of poly-
sulfide ions in dipolar aprotic media.
ꢀ
ꢀ
2
ArSe þ S2 Ð 2 ArSeS
ð4Þ
ð5Þ
12,13
In N,N-dimethyl-
acetamide, sulfur reduces in two bielectronic steps with
respect to S on a gold rotating electrode [waves R and R ,
ꢀ
2ꢀ
2
ArSeS þ 3 S2 Ð Ar2Se2 þ S8
8
1
2
ꢀ
ꢀ
While the formation of ArSe2 as in eqn. (4) (ArSe + Se) was
ꢀ
13
E1/2(R ) ¼ ꢀ0.40 V vs. reference, E (R ) ¼ ꢀ1.10 V]. The
1
1/2
2
ꢀ
complete, the conversion of ArS into ArSSe only reached
2
electrolysis at controlled potential on the plateau of R occurs
1
5
0%.
2ꢀ
via the disproportionation (9) of red S
8
ions (lmax1 ¼ 515 nm,
2ꢀ
It has now been established that RS ions oxidize into
RS R faster than RS ions on a gold electrode, according to
8
515
3
ꢀ1
ꢀ1
8
360
e
¼ 3800 dm mol cm ; lmax2 ¼ 360 nm, e ¼ 9000
ꢀ
3
ꢀ1
ꢀ1
ꢁ
ꢀ
radical-anion
2
dm mol cm ), up to the stable blue S
3
3
,6
eqns. (6) and (7):
3
617
3
ꢀ1 ꢀ1
cm ) in equili-
(
lmax ¼ 617 nm, e ¼ 4100 dm mol
2
ꢀ
in a minor proportion. In our
opinion cyclooctasulfur is in equilibrium with the reactive S
ꢀ
ꢀ
brium with its dimer S
6
2
RS þ S2 Ð 2 RS2
ð6Þ
ð7Þ
2
ꢀ
ꢀ
1
3
2
RS2 ꢀ 2 e ! RS2R þ S2
molecules, thus appearing in equations such as (9).
DOI: 10.1039/b200638n
New J. Chem., 2002, 26, 1433–1439
1433
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Table 1 Redox processes occuring at a Se-coated gold disc electrode
in DMA and related peak potentials vs. reference Ag/AgCl, KCl sat. in
DMA–NEt ClO (0.1 mol dm ) from CV at a scan rate of 100 mV s
Table 2 Electrochemical and spectrophotometric characteristics of
R and RY species (Y ¼ Se, S) in DMA—E1/2 at a rotating gold
ꢀ
RY
disc electrode vs. reference
2
ꢀ3
ꢀ1
4
4
a
Peaks (potential/V)
ꢀ
Redox process
RY
2
RY R
ꢀ
82ꢀ
62ꢀ
42ꢀ
32ꢀ
0
0
0
0
a
8
3
2
3
2
Se(s) + 2e Ð Se
R
1
R
2
R
3
R
4
R
5
(ꢀ0.49)
(ꢀ0.62)
(ꢀ0.89)
(ꢀ1.28)
(ꢀ1.55)
O
1
O
1
O
2
O
3
(ꢀ0.33)
(ꢀ0.33)
(ꢀ0.65)
(ꢀ1.0)
R, Y
E
1/2(R)/V
E
1/2(O)/V
l
max/nm
e
max
2
2
2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Se
Se
Se
Se
8
6
4
3
+ 2e Ð 4Se
ꢀ
Ph, Se
Ph, S
ꢀ0.76
ꢀ1.25
ꢀ1.55
ꢀ0.36
+0.02
ꢀ0.03
318
309
285
12 700
18 200
3850
+ 2e Ð 3Se
ꢀ
+ 2e Ð 4Se (?)
b
ꢀ
22ꢀ
2
PhCH , S
+ 2e ! 3Se (?)
a
3
ꢀ1
ꢀ1
b
a
i
e /dm mol cm
.
Shoulder.
R ¼ cathodic; O ¼ anodic.
2
ꢀ
ꢁꢀ
ꢁ
ꢀ
82ꢀ 82ꢀ
and A360 (S , and
S8 Ð 2 S3 þ S2
3
ð9Þ
ð10Þ
A
617(S
3
) decreased in favor of A515 (S
ꢀ
ꢁ
ꢀ
2ꢀ
ꢀ
ꢁꢀ
PhSe in part) according to calculated D[S3 ]/D[S8 ] values
S8 þ 8 e ! 8 S3
close to ꢀ2.6 ’ ꢀ8/3, and through the same isosbestic point
2
ꢀ
ꢀ1/3
ꢁ
ꢀ
62ꢀ
(
S
l ¼ 545 nm) as in the course of electrooxidation (13) of
S
8
and S
ions (S
3
Ð S ) oxidize (O
1
) and reduce (R
2
)
is
1/3
ꢀ
13
at the same potentials [E (O ) ¼ ꢀ0.20 V; E (R ) ¼
ions:
1
/2
1
1/2
2
ꢀ
1.10 V].
ꢁꢀ
8 S3 ! 3 S82ꢀ þ 2 eꢀ
However, as shown by the growth of the reduction wave of
ð13Þ
2
x
ꢀ
Prior to this study, Se
obtained by coulometric reduction (R
ions (x ¼ 8,6,4) were successively
–R steps) of weighed
1
3
2
amounts of grey selenium coating a large gold grid electrode.
Two further reduction steps detected on cyclic voltammograms
PhSe Ph [E (R) ¼ ꢀ0.76 V] from the first additions of diphe-
2
1/2
ꢁ
ꢀ
/
nyl diselenide, preceding the constant cathodic current of S
3
2
ꢀ
2ꢀ
2ꢀ
(
R
4
, R
5
, perhaps leading to Se
3
and Se
2
as in liquid
S8 ions [E1/2(R) ¼ ꢀ1.10 V] reaction (12) was not quantita-
1
4
ꢁ
ꢀ
3 0
ammonia) were not identifiable by our method because of
passivation phenomena on the gold electrode surface. The
redox processes summarized in Table 1 and the known UV-
tive. At the stoichiometric value [PhSe Ph] /[S ] ¼ 1/8 in
2
ad
ꢀ
1/3
the experimental conditions of Fig. 2, consumption of S
ions only reached 40%. S8 ions remained unreactive towards
2ꢀ
2
2ꢀ
visible spectra providing molar absorbance of the stable Se
ions (Fig. 1; x ¼ 8, lmax ¼ 648, 453, 398 nm; x ¼ 6,
max ¼ 598, 440 nm; x ¼ 4, lmax ¼ 550, 417 nm) were used
x
PhSe Ph since A always increased with addition of the sub-
2
strate in excess. Conversely, sulfur quantitatively reacted with
benzene selenolate ions in accordance with eqn. (14):
515
l
for quantitative data processing in this study.
ꢀ
2ꢀ
ꢀ
2
PhSe þ S8 ! PhSe2Ph þ S8
With the addition of sulfur to PhSe ions the spectra were the
ð14Þ
RY ions (Y ¼ Se, R ¼ Ph; Y ¼ S, R ¼ Ph, PhCH
2
) were
ꢀ
ꢀ3
ꢀ3
generated ([RY ] p 4 ꢂ 10
mol dm ) by electrolysis of
R species at a controlled potential of a gold electrode on
ꢀ
0
RY
2
ꢀ1
8
same as those observed when S was electrolyzed at 2 F mol
1
,15
the plateau of their bielectronic waves (Y ¼ Se,
13
S
8
for various [S
8
]
0
concentrations, regardless of [S]ad
ratio values between 0 and 1: the increases of absor-
/
3
,16
Y ¼ S ) according to previously described procedures
eqn. 11f]:
ꢀ
[PhSe ]
bances at 515 nm (S
0
[
2ꢀ
ꢁ
ꢀ
8
) and 617 nm (S ) because of the par-
3
f
tial disproportionation (9) with a negligible sulfur proportion
always confirmed (ꢃ6%) the conservation eqn. (15):
RY2R þ 2 eꢀ )b* 2 RYꢀ
ð11Þ
½S8ꢄ ¼ ½S82ꢀꢄ þ 1=2½S3
ꢁꢀ
ꢄ
ð15Þ
The electrochemical and spectrophotometric characteristics of
ꢀ
0
RY
2
R and RY species are summarized in Table 2.
At the same time, the reduction currents of PhSe Ph [E (R) ¼
2
1/2
ꢁ
ꢀ
2ꢀ
/S
8
increased, while the anodic wave of the latter [E1/2
ꢀ
0.76 V] and of
S
3
ions [E1/2(R) ¼ ꢀ1.10 V]
ꢁ
ꢀ
ꢀ
PhSe Ph/S
2
3
and PhSe /S
8
reactions
ꢀ
ꢁ
ꢀ
(
O) ¼ ꢀ0.20 V] progressively replaced that of PhSe ions
The addition of PhSe
2
Ph to S
3
ions resulted in instantaneous
changes in voltammograms and spectra (Fig. 2) which agreed
with equilibrium (12):
[
E
1/2(O) ¼ ꢀ0.36 V] up to stoichiometry (14).
PhSe2Ph þ 8 S3 ꢀ Ð 2 PhSe þ 3 S8
ꢁ
ꢀ
2ꢀ
ð12Þ
Fig. 2 Dependence of the UV-visible spectra on the addition of
ꢁ
ꢀ
ꢀ3
ꢀ3
solution.
(3); 0.23 (4); 0.32 (5);
diphenyl diselenide to a [S
3
]
0
¼ 6.06 ꢂ 10
mol dm
ꢁ
ꢀ
[
PhSe
0.50
nm min
2
Ph]ad/[S
3
]
0
¼ 0 (1); 0.063 (2); 0.12
5
3
/dm mol cm ) of Se
(3) ions in dimethylacetamide.
ꢀ1
ꢀ1
82ꢀ
Fig. 1 UV-visible absorption spectra (e
i
3
(6); 0.96 (7). Thickness of the cell ¼ 0.1 cm; scan rate ¼ 500
2
ꢀ
2ꢀ
ꢀ1
(
1), Se
6
(2) and Se
4
.
1
434
New J. Chem., 2002, 26, 1433–1439

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a
Table 3 DE1/2(O)/V variation of anodic half-wave potentials related
ꢀ
ꢀ
to the oxidations of RYZ and RY ions into RY R species
2
b
R ¼ Ph
R ¼ PhCH
2
R ¼ Ar
Y, Z ¼ S,S
Y, Z ¼ Se,Se
Y, Z ¼ S,Se
ꢀ0.23
ꢀ0.08
ꢀ0.47
ꢀ0.46
ꢀ0.06
ꢀ0.35
ꢀ0.50
ꢀ0.41
ꢀ0.68
a
ꢀ
ꢀ
b
DE1/2(O) ¼ E1/2(RYZ ) ꢀ E1/2(RY ). Ar ¼ 2-NO
2
C
6
H
4
.
a
c
ions into RS
1.40 V] totally shifted towards less anodic potentials (curve 2,
2
R [Fig. 4, curve 1, Ep ¼ +0.04 V, Ep ¼ ꢀ1.40
ꢀ
a
ꢀ
Ep ¼ ꢀ0.37 V). At the end of reaction (16) PhSSe had an
oxidation peak at ꢀ0.44 V (curve 3), and reversal of the vol-
tage scan direction resulted in the appearance of the sharp
Fig. 3 Changes in UV-visible spectra with the addition of selenium
/
c
2
ꢀ
ꢀ3
ꢀ3
reduction peak of electrodeposited Se [Ep (1) ¼ ꢀ0.49 V] fol-
powder to a [PhS ]
(RS )
0
¼ 2.87 ꢂ 10 mol dm solution. n ¼ (Se)ad
ꢀ
lowed by the subsequent cathodic peaks 2–5 of the polysele-
0
0
¼ 0 (1); 0.99 (2); 1.98 (3); 2.99 (4); 4.0 (5); recordings at equili-
0
brium except for (1) ! (2), A ¼ f(t), 0 < t < 95 min.
nide ions (Table 1), then associated anodic peaks (3 , 2 ).
ꢀ
This electrochemical behavior of RS ions in the presence of
selenium complies with the following electrocatalytic mechan-
ism [eqns. (16)–(18)], which is analogous to those previously
Thus, in contrast to the two successive steps which were evi-
denced in the course of the addition of sulfur to the least redu-
ꢀ
ꢀ3,6
ꢀ
ꢀ
1
reported for RS /RS
and RSe /RSe species:
2
2
ꢀ
ꢀ
5
Se (ArSe ) species, formation (4) of
cing 2-NO
2
C
ArSeS , then reversible oxidation (5) into ArSe Ar, only redox
H
6 4
ꢀ
ꢀ
2
RS ꢀ 2e ! RS2R
ð17Þ
ð16Þ
ð18Þ
ꢀ
2
ꢀ
ꢀ
RS þ SeðsÞ ! RSSe
RSSe ꢀ 2e ! RS2R þ 2 SeðsÞ
2
processes (12) and (14) resulted from the reactions PhSe Ph/
S3 and PhSe /S , respectively, without any observed stabi-
8
lization of PhSeS ions.
ꢁ
ꢀ
ꢀ
ꢀ
ꢀ
2
ꢀ
2ꢀ
2
ꢀ
ꢀ
2
Thus, RS , RSe
(
and RSSe ions oxidize into RY
R
ꢀ
Y ¼ S, Se) on a gold electrode, at a greater rate than RY
ꢀ
Stabilization and electrocatalytic oxidation of RSSe ions
ions, as shown by the differences between their respective
half-wave potentials of oxidation listed in Table 3. The occur-
rence of the catalytic processes from the addition of insoluble
When grey selenium powder (pellets, diameter ca. 10–30 mm)
was added to stirred solutions of PhS and PhCH2S ions at
ꢀ
ꢀ
ꢀ
selenium to RY ions (Y ¼ S, Se) implies fast heterogeneous
ꢀ
a ratio n ¼ (Se)ad/(RS )
0
¼ 1, the changes in UV-visible spec-
ꢀ
reactions such as in eqn. (16) between RY and the released
Se in the course of electrooxidation [e.g., eqn. (18)], although
tra and cyclic voltammograms were similar in both cases, as
illustrated in Figs. 3 and 4 (R ¼ Ph). In the course of the Se
consumption which required about 1.5 h up to n ¼ 1, two
absorption bands regularly increased with time: R ¼ Ph (see
ꢀ ꢀ
or RSSe were only obtained by direct addition (1) or
RSe
16) after 1.5 hour. The same schemes were tested on quantita-
2
(
tive electrolysis at a controlled potential of a large gold grid
Fig. 3, curves 1 ! 2), l
¼ 403 nm, l
¼ 260 nm, iso-
ꢀ
max1
max2
electrode (E ¼ ꢀ0.10 V), of a solution containing ArSe
sbestic point lis ¼ 272 nm; R ¼ PhCH , lmax1 ¼ 430 nm,
2
[
e
Ar ¼ 2-NO
2
Ph,
E
1/2(O) ¼ +0.16 V,
l
max ¼ 520 nm,
lmax2 ¼ 260 nm, l ¼ 324 nm, l ¼ 298 nm), in agreement
3
ꢀ1
ꢀ1 5
2ꢀ
is1
is2
ꢀ
max ¼ 1200 dm mol cm ] and ArSe [E1/2(O) ¼ ꢀ0.25
with the formation of only RSSe ions:
3
ꢀ1 ꢀ1 5
V, lmax ¼ 728 nm, emax ¼ 3450 dm mol
cm
]
ions:
ꢀ
ꢀ3
ꢀ3
ꢀ
ꢀ3
ꢀ1
RS þ SeðsÞ ! RSSeꢀ
ꢀ
ð16Þ
As soon as solid Se was added, and before any growth of
absorbance at about 400 nm, the oxidation current of RS
0
ꢀ
2
0
[ArSe ]
¼ 3.40 ꢂ 10
mol dm , [ArSe
]
mol dm [spectral changes in Fig. 5 as a function of z F mol
¼ 2.25 ꢂ 10
3
ꢀ ꢀ ꢀ
(
0 2 0
ArSe ) + (ArSe ) ]. As long as ArSe ions were in greater
ꢀ
Fig. 5 Spectral changes in the course of the electrooxidation at
ꢀ
ꢀ3
ꢀ3
ꢀ
ꢀ3
Fig. 4 Cyclic voltammograms of a [PhS ]
(0.14 mmol) solution (1) added with selenium powder (11 mg, 0.14
0
¼ 3.52 ꢂ 10 mol dm
E ¼ ꢀ0.10
V
vs. reference of
a
[ArSe ]
0
¼ 3.40 ꢂ 10
mol
Ph):
ꢀ
3
ꢀ
ꢀ3
ꢀ3
dm + [ArSe
2
]
z F mol (ArSe )
0
¼ 2.25 ꢂ 10 mol dm solution (Ar ¼ 2-NO
2
ꢀ1
ꢀ
ꢀ
mmol); Dt ¼ 2 min (2); 92 min (3). E vs. Ag/AgCl, KCl sat. in
0
2 0
+ (ArSe )
¼ 0 (1)–0.58 (6); 0.65 (7); 0.75 (8);
ꢀ3
ꢀ1
.
DMA–NEt ClO
4
4
0.1 mol dm . Scan rate ¼ 50 mV s
0.85 (9).
New J. Chem., 2002, 26, 1433–1439
1435

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ꢀ
2ꢀ
yꢀ
Formation of RSSe ions
concentration than initially added Se ([ArSe ] > [ArSe
z < 0.60, curves 1–6), the decrease in A was in accordance
]
0
,
5
20
ꢀ
ꢀ
Further additions of solid selenium to RSSe ions [R ¼ Ph,
with the oxidation of ArSe ions into ArSe
2
Ar (growth of
ꢀ
5
PhCH
2
, n ¼ (Se)ad/(RSe )
0
greater than 1] resulted in its total
its cathodic wave, E1/2 ¼ ꢀ0.69 V) at a potential (E ¼
ꢀ
consumption within 2 h (n ¼ 2), then 2.5 h (n ¼ 3), whereas
traces of solid Se remained unreactive for n ¼ 4 beyond 4 h.
The partial oxidation of the anionic solutions was shown by
ꢀ
0.10 V) only suited to that of ArSe
2
[eqn. (19)]:
ꢀ
2
ArSe2 ꢀ 2 eꢀ ! ArSe2Ar þ 2 SeðsÞ
Simultaneously A728 (ArSe ) remained at a constant value
because of the ‘instantaneous’ reaction between ArSe and
Se which was generated at the electrode surface. Then
ð19Þ
2
x
ꢀ
the appearance of Se
were detected by the simultaneous increase in their absorption
ions (x q 4). Mixtures of these species
2ꢀ
ꢀ
2ꢀ
2ꢀ
/Se
8
at around 625 nm (Se
5
6
, R ¼ Ph, Fig. 3, curves 3–5) or
2ꢀ 2ꢀ
8 2
2ꢀ
2ꢀ
90–620 nm (Se4 /Se6 or Se6 /Se , R ¼ PhCH ), and in
ꢀ
(
z > 0.60), curves 7ꢀ9), the consumption of ArSe (decrease
2
their reduction waves (RDE, x ¼ 8, 6, 4; E1/2 ¼ ꢀ0.55, ꢀ0.83,
in A728) resulted in the deposition of solid selenium on the gold
2
ꢀ
1.20 V), with greater currents at potentials close to those
of RS
R (R ¼ Ph, E1/2 ’ ꢀ1.25 V; R ¼ PhCH
1.55 V). At the same time another absorption band increased
grid, and the recovery of ArSe Ar (z ’ 1.05) according to its
2
2
2
, E1/2
’
3
ꢀ1
characteristic absorption at 378 nm (emax ¼ 7300 dm mol
ꢀ
ꢀ
1 5
cm ).
at about 405 nm (R ¼ Ph) or 380 nm (R ¼ PhCH
2
) which
Similarly, a gold foil (1 ꢂ 1 cm) was coated with grey sele-
2ꢀ
could not be related to Sex ions from the shape of their
own spectra (Fig. 1). Moreover, spectra and voltammograms
were the same when equilibrium was attained for the respective
ꢀ
nium by electrolysis (E ¼ ꢀ0.25 V) of PhSSe ions (about
0
.076 mmol), and then observed by scanning electron micro-
scopy (SEM). The SEM images shown in Figs. 6a–6b revealed
an epicentric crystallization with small dendrites, mostly 1 to
mm in length, which could explain the high reactivity of
Se-nucleophiles’ such as RY species (Y ¼ S, Se) towards
7–9
The recent interest in biochemistry of RSSeR species led
ꢀ
stoichiometries: RS + 2 Se (R ¼ Ph, Fig. 3, curve 3) and
2ꢀ
ꢀ
2ꢀ
RS R + Se ; RS + 3 Se (Fig. 3, curve 4) and RS R + Se
6
2
4
2
2
(
logous to those occurring in the course of the RSe + n Se(s)
see below Fig. 7, curve 5). All of these observations were ana-
ꢀ
‘
electrogenerated selenium.
ꢀ
2ꢀ
(
n q 2) and RSe
2
R + Se
x
(x ¼ 4, 6) reactions, which yielded
0
ꢀ
RSey ions (y ¼ 3, 4, maximal absorbances at 400–420 nm) in
ꢀ
us to examine the alkylation of two RSSe solutions on a pre-
parative scale as examples: CH SSe + PhCH Br and
PhSSe + CH I. The compositions of the mixtures of products
equilibrium with RSe
[
2
R and mixtures of polyselenide ions
ꢀ
1
2ꢀ
3
2
eqn. (2)]. Here again, the fast reactions RS R + Sex
2
ꢀ
3
(
R ¼ Ph, PhCH
2
; x ¼ 4, 6) were followed by calculations first
function of
] and [Se ] (598 < lmax
0
0
0
1
RSSeR , RS R, R Se R which were analyzed by H NMR and
2
2
of the polyselenide concentrations as
a
GC/MS (see Experimental) corresponded to significant dis-
proportionation of the expected selenenyl sulfides
CH SSeCH (’70%), as previously
reported for this class of rather unstable compounds
2ꢀ
2ꢀ
2ꢀ
82ꢀ
m ¼ [RS
48 nm), or [Se
deduced from experimental values of A598 and A648 or A598
2
R]ad/[Se
x
]
0
: [Se
6
<
2ꢀ
6
4
] and [Se
6
] (550 < lmax < 598 nm), were
3
2
Ph (’80%) and PhSSeCH
3
and A550 , respectively, by the use of the known e
i
molar
cm
1
1
[
eqn. (8)].
3
(dm mol
ꢀ1
ꢀ1
absorptivity (Fig. 1), e.g. eqn. (20), e
i
,
6
8
598
8
648
6
648
1
ꢃ4%): e5 ¼ 1750, e ¼ 2150;e ¼ 2500, e ¼ 1450.
98
Ai ¼ eiðSe6 ꢀÞ½Se62ꢀꢄ þ eiðSe82ꢀÞ½Se82ꢀꢄ
2
ð20Þ
The other concentrations [RS R] and [RSSe ], as well as the
ꢀ
2
y
ꢀ
average number y¯ of Se units in RSSe
culated by solving the conservation equations from [RS R]
y
chains, were then cal-
2
ad
2
ꢀ
and [Se
x
]
0
values. As an example, Fig. 7 shows the changes
in spectra for the reaction PhS Ph + Se . Two steps can be
2
ꢀ
2
6
distinguished:
(
of Se
i) for 0 < m p 0.42 (curves 1–2), the maximal absorbance
2ꢀ
2ꢀ
8
ions (A440 , A598) evolved towards that of Se (A398
6
,
A453 , A648) through three isosbestic points (l ¼ 398, 474 and
is
5
45 nm), according to the rough stoichiometry of eqn. (21):
PhS2Ph þ 2:4 Se6 ꢀ ! 2 PhSSe1:6ꢀ þ 1:4 Se82ꢀ
2
ð21Þ
Fig. 6 Scanning electron micrographs of electrodeposited selenium
on a gold foil from the oxidation of PhSSe ions (E ¼ ꢀ0.25 V).
Fig. 7 Changes in UV-visible spectra with the addition of diphenyl
solution:
ꢀ
2ꢀ
ꢀ3
ꢀ3
disulfide to a 2 [Se
6
]
0
¼ 1.15 ꢂ 10
mol dm
ꢀ
(
a) ꢂ1000; (b) ꢂ5000.
m ¼ [RS
2
R]ad/[Se
6
]
0
¼ 0 (1); 0.42 (2); 0.66 (3); 0.90 (4); 1.02 (5).
1
436
New J. Chem., 2002, 26, 1433–1439

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2
x
ꢀ
(x ¼ 4, 6) and RSꢀ + n Se (n ¼ 2, 3) depending on
Table 4 Calculated compositions of solutions at equilibrium for the reactions RS
initial conditions
2
R + Se
a
a
42ꢀ
62ꢀ
82ꢀ
yꢀ
Initial cond.
[Se
]
[Se
]
[Se
]
2
[(RS) ]
[(RSSe )]
y¯
2
ꢀ
[
[
[
[
[
[
[
[
(PhS)
ꢀ
2
]
0
¼ 2.70 + [Se
4
]
0
¼ 2.74
¼ 5.70
—
—
—
—
0.42
0.23
0.35
0.37
0.22
0.105
0.21
0.15
0.17
0.22
—
0.60
0.38
0.54
0.59
0.64
0.51
0.72
0.67
4.20
2.11
1.16
1.69
3.80
1.88
1.74
1.56
1.6
1.5
2.7
2.7
1.9
1.8
2.7
2.6
PhS ]
0
¼ 2.87 + [Se]
0
2ꢀ
(PhS)
ꢀ
2
]
0
¼ 1.12 + [Se
0
6
]
0
¼ 1.10
¼ 8.58
PhS ]
0
¼ 2.87 + [Se]
2
ꢀ
]
0
(PhCH
2
S)
2
]
0
¼ 2.54 + [Se
¼ 2.90 + [Se]
4
¼ 2.66
¼ 5.70
0.54
0.40
—
ꢀ
PhCH
(PhCH
2
S ]
0
0
5
—
5
5
5
2
6
ꢀ
]
0
2
S)
2
]
0
¼ 1.59 + [Se
¼ 1.54
0.47
0.44
5
0.20
0.22
ꢀ
PhCH
2
S ]
0
¼ 2.90 + [Se]
0
¼ 8.60
All concentrations are in mmol dm
—
5
5
a
ꢀ3
.
(
ii) for 0.42 < m p 1.0 (curves 2–5), the progressive displace-
Fig. 1 were subtracted from the experimental A values, 10
ꢀ ꢀ
nm apart; (iii) the known spectra of RSSe ions (RS + 1
2ꢀ
ment A648 ! A618 agreed with a partial recovery of Se
6
ꢀ
ions,
ions of
ꢀ
ꢀ
and with the formation at the same time of RSSe
y
Se, e
i
ꢃ 5%) led firstly to those of RSSe
2
(x ¼ 4), then RSSe
3
higher y¯ rank (Table 4, m ¼ 1, y¯ ¼ 2.7), giving a maximum
absorbance at about 412 nm. These results can be explained
by eqns. (22)–(24):
(x ¼ 6). The A ¼ f(l) curves are reported in Fig. 8 for R ¼ Ph,
whereas Table 5 summarizes the spectrophotometric charac-
; y ¼ 1–3). lmax
ꢀ
teristics of RSSe
wavelengths of RSSey ions were lower by ’25 nm than those
y
ions (R ¼ Ph, PhCH
2
ꢀ
2
RS2R þ Se6 ꢀ
Ð 2 RSSe3ꢀ
ꢀ
ð22Þ
ð23Þ
of homologous RSeSe
in both cases. The formation of RSSe ions could not be
y
species (y ¼ 1–3), with close e
y
values
1
ꢀ
f
4
ꢀ
2ꢀ
ꢀ
2ꢀ
2ꢀ
*
2
RSSe þ Se ꢀ ) 2 RSSe þ Se
proved from the addition of RS R disulfides to Se
8
ions,
3
6
2
8
2
b
because of the detection of solid selenium within the solutions
in the course of the reactions which entailed at first the shift
f
ꢀ
2ꢀ
ꢀ
2ꢀ
8
*
2
RSSe þ Se )b 2 RSSe þ Se
ð24Þ
2
6
2ꢀ
2ꢀ
2ꢀ
A648 (Se8 ) ! A630 (Se8 /Se6 ).
2
ꢀ
In the presence of Se6 ions in excess (m < 0.42), reactions
22) and (23) totally shift to the right, leading to a mixture
(
ꢀ
ꢀ
Conclusions
of RSSe and RSSe2 ions by equilibrium (24) ( y¯ ’ 1.6). With
2
6
ꢀ
displaces
further additions of RS
2
R, the consumption of Se
2
Whereas selenolate ions undergo redox exchanges to RSe R
equilibria (23) and (24) in reverse, increasing the y¯ value to 2.7
diselenides with sulfur in N,N-dimethylacetamide, selenium
adds to thiolates with the formation of the sulfur–selenium
bond in RSSe species. The latter reactions, and those leading
ꢀ
ꢀ
).
(
mixture RSSe
Similarly, three steps were observed from progressive
Ph to
2
/RSSe
3
ꢀ
changes in A ¼ f(l) curves during the addition of PhS
2
ꢀ
ꢀ
to RSe2 (RSe + Se) which we recently reported, are analo-
gous to the well known ‘S-nucleophilic processes’ affording
RS2 ions from thiolates and sulfur.
2
ꢀ
the more reducing Se
ions: (i) 0 < m p 0.35, A ! A
ꢀ 2ꢀ
4 6
4
550
598
2
and A417 ! A440 , and thus Se
! Se
according to the
ꢀ
nearly quantitative redox process (25).
ꢀ
ꢀ
ꢀ
In mixtures RS + RSSe , RSSe ions oxidize into RS
2
R
ꢀ
2
PhS2Ph þ 3 Se4 ꢀ ! 2 PhS þ 2 Se6
ꢀ
2ꢀ
faster than RS ions on a gold electrode, with a fast heteroge-
neous reaction between selenolate ions and electrogenerated
ð25Þ
0
(ii) 0.35 < m p 0.67, A598 ! A645 and A440 ! A450/A400 due to
further reaction of Se
accordance with the overall eqn. (26).
selenium. RSSeR alkylated selenenyl sulfides disproportionate
0 0
2
6
ꢀ
2ꢀ
ions which provided Se
8
ions in
to a large extent into symmetrical RS
pounds.
2 2
R and R Se R com-
yꢀ
RSSe ions (y ¼ 2, 3), which partly disproportionate into
PhS2Ph þ 1:5 Se4 ꢀ ! 2 PhSSe þ 0:5 Se8
2
ꢀ
2ꢀ
ð26Þ
2ꢀ
RS R and Se ions, result from the slow addition of solid
Se to RSSe ions. The same equilibria are readily obtained
2
x
ꢀ
(iii) 0.67 < m < 1.0
reactions of Se
by ‘Se-exchanges’ as in eqn. (24).
The calculated concentrations and average y¯ numbers at the
2
, A645 ! A615 and A450/A400 ! A407 ; the
2ꢀ
by the reactions between RS R and Sex ions.
2
2ꢀ
2ꢀ
6
/Se
8
ions finally led to y¯ ’ 1.6 (m ¼ 1)
ꢀ
end of slow reactions [RS ]
0
+ n (Se)ad (R ¼ Ph, PhCH
2
;
n ¼ 2, 3) and of the equivalent fast reactions
2
ꢀ
[
Based on [RS R] values at equilibrium compared with
RS
2
R]
0
+ [Se
x
]
0
(m ¼ 1; x ¼ 4, 6) are given in Table 4.
2
ꢀ
[
RS ]
0
or [RS
portionations of RSSe ions could therefore be roughly situ-
2
R]
0
(Table 4, R ¼ Ph and PhCH
2
), the dispro-
ꢀ
y
ated at ca. 25% (y ¼ 2) and 45% (y ¼ 3). These levels meet
ꢀ
ꢀ
1
those of RSe3 (30%) andRSe
Furthermore, the spectra of RSSe
(ꢅ45%) ions [eqn. (2)].
4
ꢀ
y
ions (y ¼ 2, 3) were
obtained over the wavelength range 350–600 nm as previously
ꢀ
ꢀ
1
ions: (i) RSSe /RSSe and
ꢀ
ꢀ
obtained for RSe
3
and RSe
4
2
ꢀ
ꢀ
RSSe2 /RSSe3 mixtures were assumed to give 1 < y¯ < 2
and 2 < y¯ < 3 respectively, with definite compositions linked
ꢀ
ꢀ
to y¯ values (e.g. y¯ ¼ 1.6, 40% RSSe and 60% RSSe ); (ii)
2
ꢀ
2
x
for the most accurate conditions [RS
2
R]
ions (x ¼ 6, 8 or 4, 6) calculated by
the use of concentrations in Table 4 and characteristics in
0
¼ [Se
] , the
0
2ꢀ
3
/dm mol
ꢀ1
ꢀ1
yꢀ
absorbances of Sex
Fig. 8 Calculated spectra (e
y ¼ 1–3, (1)–(3).
cm ) of PhSSe ions.
i
New J. Chem., 2002, 26, 1433–1439
1437

View Article Online
yꢀ
Table 5 Spectrophotometric characteristics of RSSe ions (y ¼ 1–3)
were rapidly analyzed without attempting to separate the indi-
0
0,11
in dimethylacetamide
vidual compounds because of the poor stability of the RSSeR
1
ꢀ
2ꢀ
3ꢀ
species.
R
RSSe
RSSe
RSSe
ꢀ ꢀ
+
a
Reaction of CH SSe ions with PhCH Br. CH S Na
0.814 g, 11.6 mmol), Se (0.909 g, 11.5 mmol), PhCH
Ph
l
max /nm
403
900
430
400
400
3000
375
405
3600
375
3 2 3
b c
(
2
Br
e
max
3
a
PhCH
2
l
e
max /nm
b c
max
(1.40 cm , 11.5 mmol). The composition of the mixture of pro-
ducts (1.97 g): CH SSeCH Ph (29%), (PhCH ) Se (57%),
2600
3000
3
2
2 2
2
(
3
CH
3
S)
2
(14%), was determined from d
H
(s, 2H) and d
H
(s,
a
b
3
/dm mol cm
ꢀ1
ꢀ1
c
lmax(y ¼ 2, 3) ꢃ 4 nm.
e
i
.
i
e (y ¼ 2, 3) ꢃ 15%.
H); (CH S) and (PhCH ) Se were identified by the use of
3
2
2 2
2
commercial samples which were added to the synthesized mix-
ture. Volatile (CH S) was assumed to have been greatly
reduced in the course of the solvent evaporation. The dispro-
3
2
Experimental
portionation level of CH SSeCH Ph (’80%) was thus calcu-
3
2
Materials and equipment
2 2 2
lated by reference to the only (PhCH ) Se proportion.
1
(
the mass spectra as a result of the known selenium extrusion
PhCH ) Se, which gave no H NMR signal, was detected in
2 2
N,N-Dimethylacetamide, grey selenium (99.999%, 100 mesh),
and all the organic compounds were purchased from Aldrich
except for diphenyl diselenide and dibenzyl mono-and di-sele-
nide (Acros Organics). Solvent purification and storage after
1
9
from benzylic diselenide under thermal conditions.
0
8
CH
3
SSeCH
2
Ph: d
H 5
2.24 (s, 3H), 4.08 (s, 2H); m/z 218 ( Se,
M , 4%), 91 (100), 65(16) and 39 (9). (PhCH ) S : d 3.80
+
ꢀ
3
addition of NEt
4
ClO
4
(Fluka, 0.1 mol dm ) as supporting
electrolyte have been reported elsewhere. Spectroelectro-
2 2
2
H
8
0
s, 4H); m/z 342 ( Se, M , 2%), 91 (100). (CH
(
m/z 262 ( Se, M , 7%), 91 (100).
+
1
7
(
3
S)
2
: d
H 5
2.39
+
6H, s; m/z 96 (M + 2, 11), 94 (M , 100%). (PhCH ) Se:
+
1
chemical equipments and electrodes, as well as the thermo-
2 2
8
0
+
ꢆ
17
statted (20.0 ꢃ 0.50 C) flow-through cell have previously
been described. All the potentials were referenced to Ag/AgCl,
KCl saturated in DMA–NEt ClO (0.1 mol dm ) electrode.
ꢀ3
ꢀ
ꢀ
+
3
I. PhS Li in THF (11 cm , 11
Reaction of PhSSe with CH
3
4
4
3
The Se-coated gold foil was observed by Scanning Electron
Microscopy (SEM FEG Gemini 982 Leo Microscope). The
micrographs were obtained in secondary electron image mode
3
mmol), Se (0.792 g, 10.0 mmol), CH I (0.80 cm , 12.8 mmol).
1
The products were identified both by H NMR with the use of
commercial samples of (PhS) , (CH Se) and PhSCH com-
pounds, and by GC-MS. The composition of the mixture
(1.79 g): PhSSeCH (32%), (PhS) (35%), (CH Se) (22%)
and PhSCH (11%, close to the initial ratio
PhS :PhSSe ¼ 1:10), was determined by combining the inte-
grals of d (s, 3H) with those of aromatic d , (PhS) (4 Ho)
2
3
2
3
(
accelerating voltage of 2 kV). The synthesized mixtures were
1
analyzed by H NMR spectroscopy (200.132 MHz, Bruker
AC 200) with CDCl as the solvent (Me Si as standard) and
GC-MS (Hewlett-Packard 5989 A, EI 70 eV).
3
2
3
2
3
4
3
ꢀ
ꢀ
H
H
2
and PhSSeCH (2Ho). It was in good agreement with the inte-
3
2
ꢀ
Generation of Sex ions
gration of the GC peaks. Here again, the disproportionation of
PhSSeCH (’70%) was evaluated from the respective propor-
2
x
ꢀ
ions were obtained by the
Accurate concentrations of Se
same method as recently reported: selenium was initially
deposited on large gold grid electrode [25 < w(Se)/
mg < 40] by the electrooxidation (E ¼ 0.0 V) of Sex solu-
tions ( x¯ ’ 6) which were themselves chemically generated in
DMA from the reduction of Se with hydrazine and sodium
3
1
,2
tions of PhSSeCH
3
and (PhS)
2
in the mixture. PhSSeCH
.46 (s, 3H), 7.51 (1Ho, Ar), 7.55 (1Ho, Ar); m/z 204 ( Se,
3
: d
H
8
0
2
M , 87%), 189 (57), 109 (100), 77 (51), 69 (37), 65 (76), 51
a
+
2ꢀ
(
2
43), 39 (49). (PhS) : d 7.45 (2Ho, Ar), 7.49 (2Ho, Ar); m/z
2
H
+
18 (M , 76%). (CH
80
(s, 6H); m/z 190 ( Se,
3
Se)
2
: d
H
2.67
5
+
+
1
8
M , 88%). PhSCH : d 2.44 (s, 3H); m/z 124 (M , 100%).
methoxide:
3
H
7
ꢀ
2ꢀ
1
2 SeðsÞ þ 4 MeO þ N2H4 ! 2 Se6 þ 4 MeOH þ N2
ð27Þ
The cathodic polarization of the Se-coated grid in DMA (40
References
3
1
A. Ahrika, J. Robert, M. Anouti and J. Paris, New J. Chem., 2001,
25, 741.
cm ) was then kept until the spectra and the related maximal
2ꢀ
absorbances of Sex (Fig. 1; x ¼ 8, E ¼ ꢀ0.55 V; x ¼ 6,
E ¼ ꢀ0.75 V; x ¼ 4, E ¼ ꢀ1.10 V) were attained. Concen-
2
3
A. Ahrika and J. Paris, New J. Chem., 1999, 23, 1177.
G. Bosser, M. Anouti and J. Paris, J. Chem. Soc., Perkin Trans. 2,
trated solutions of RS
2
R substrates in DMA (R ¼ Ph,
1
996, 1993.
3
2ꢀ
PhCH
2
; nmax ¼ 4 cm ) were progressively added to Se
x
ions.
4
C. K o¨ llemann, D. Obendorf and F. Sladsky, Phosphorus Sulfur
Relat. Elem., 1988, 38, 69.
In all cases, absorbances reached equilibrium within 1 min.
5
6
A. Ahrika, J. Auger and J. Paris, New J. Chem., 1999, 23, 679.
M. Bena ¨ı chouche, G. Bosser, J. Paris, J. Auger and V. Plichon,
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0
Syntheses of RSSeR compounds
7
E. Block, in Dietary Phytochemicals in Cancer Prevention and
Treatment, Plenum Press, New York, 1996, pp. 155–169 and refer-
ences cited therein.
C. Ip and D. J. Lisk, in Dietary Phytochemicals in Cancer Preven-
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J. J. Sullivan, J. Agric. Food Chem., 1994, 42, 2081.
(a) H. Rheinbolt and E. Giesbrecht, Liebigs Ann. Chem., 1950,
ꢀ
ꢀ
The CH SSe + PhCH Br and PhSSe + CH I reactions were
3
2
3
carried out according to the same procedure on a preparative
scale: solid sodium thiomethoxide (95%) and lithium thiophen-
oxide (1 mol dm in THF) of commercial origin were dis-
solved in 80 cm of deaerated DMA under an N2
8
9
0
ꢀ3
3
ꢀ
ꢀ
ꢆ
atmosphere. The RS solutions were stirred at 50 C with sele-
nium powder (Se:RS ¼ 1:1) which reacted within 3 hours.
1
1
98; (b) H. H. Sisler and N. K. Kotia, J. Org. Chem., 1971, 36,
Stoichiometric amounts of alkyl halides dissolved in DMA
(
ture to the yellow RSSe solutions. After filtration (0 C) of
1700; (c) J. L. Kice and T. W. S. Lee, J. Am. Chem. Soc., 1978,
100, 5094; (d ) M. Yoshida, T. Cho and M. Kobayashi, Chem.
Lett., 1984, 1109.
3
20 cm ) were then added dropwise (20 min) at room tempera-
ꢀ
ꢆ
3
the medium and addition of water (300 cm ), the products
11 (a) W. Mc Farlane, J. Chem. Soc. (A), 1969, 913; (b) V. A.
Potapov, S. V. Amosova, P. A. Petrov, L. S. Romanenko and
V. V. Keiko, Sulfur Lett., 1992, 15, 121.
were extracted with diethyl ether. The organic phase was thor-
oughly washed with water (elimination of residual DMA) and
dried over MgSO . After evaporation in vacuo, the mixtures
1
2
F. Gaillard and E. Levillain, J. Electroanal. Chem., 1995, 398, 77
and references cited therein.
4
1
438
New J. Chem., 2002, 26, 1433–1439

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13
14
15
G. Bosser and J. Paris, New J. Chem., 1995, 19, 391.
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