On the reaction of triaryl trithioarsenites, (ArS)3As, with iodine

Article abstract of DOI:10.1080/10426500500272178

Iodine in dry nonprotic solvents oxidizes triaryl trithioarsenites, (ArS)₃ As, to As(III) iodide, AsI₃, and disulfides, ArSSAr. The reaction most likely involves arylsulfenyl iodide, ArSI, as an intermediate. With triphenyl and tris(

Full text of DOI:10.1080/10426500500272178

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Phosphorus, Sulfur, and Silicon
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On the Reaction of Triaryl
Trithioarsenites, (ArS)3As, with
Iodine
Maria D. Markaki a & Panayiotis V. Ioannou a
a Department of Chemistry , University of Patras ,
Patras, Greece
Published online: 15 Aug 2006.
To cite this article: Maria D. Markaki & Panayiotis V. Ioannou (2006) On the Reaction
of Triaryl Trithioarsenites, (ArS)3As, with Iodine, Phosphorus, Sulfur, and Silicon and
the Related Elements, 181:4, 921-929, DOI: 10.1080/10426500500272178
To link to this article: http://dx.doi.org/10.1080/10426500500272178
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Phosphorus, Sulfur, and Silicon, 181:921–929, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500500272178
On the Reaction of Triaryl Trithioarsenites, (ArS)3As,
with Iodine
Maria D. Markaki
Panayiotis V. Ioannou
Department of Chemistry, University of Patras, Patras, Greece
Iodine in dry nonprotic solvents oxidizes triaryl trithioarsenites, (ArS)3As, to
As(III) iodide, AsI3, and disulfides, ArSSAr. The reaction most likely involves aryl-
sulfenyl iodide, ArSI, as an intermediate. With triphenyl and tris(4-chlorophenyl)
trithioarsenites, AsI3 is prepared in very good yields. When the disulfide, which is
produced, has MeCONH or NH CMe2 OH groups, then it acts as a Lewis
base towards AsI3 forming complexes with stoichiometry 2AsI3·3ArSSAr. Probable
coordination modes of the AsI3 are discussed.
Keywords Arsenic triiodide; arsenic triiodide as Lewis acid; diphenyl aryldithioarson-
ites; disulfides; iodine; triaryl trithioarsenites
INTRODUCTION
We have recently been studying the effect of the oxidizing element octa-
sulfur on dithioarsonites [L-As(SPh)2, L = Ar or R]1 and on triaryl2 and
trialkyl3 trithioarsenites [(ArS)3As and (RS)3As], spurred by the (erro-
neous) report4 that (p-MePhS)3As is oxidized by octasulfur in refluxing
carbon disulfide to the ester (p-MePhS)3As S. We have found that the
previously mentioned As(III) esters are not oxidized to the As(V) ones
probably because the As(III) is not nucleophilic towards octasulfur.
Next, we studied the air oxidation of triaryl5 and trialkyl3
trithioarsenites, and we found that electron-withdrawing groups at the
para position of the phenyl group tend to favor the oxidative decompo-
sition compared to those having electron-donating groups, according to
Eq. (1):
2(ArS)3As + 3/2O2 → As2O3 + 3ArSSAr
(1)
Aliphatic trithioarsenites bearing a terminal COOH were decom-
posed by air (Eq. 1), while those bearing a COO− group were
Received March 24, 2005; accepted April 12, 2005.
Address correspondence to Panayiotis V. Ioannou, University of Patras, Department
of Chemistry, Patras, Greece. E-mail: ioannou@chemistry.upatras.gr
921

922
M. D. Markaki and P. V. Ioannou
resistant to air oxidation. The esters As(glutathione)3, As(cysteine)3,
and As(penicillamine)3 were found to be air oxidized at a slower rate
than their parent thiols. These and other6 oxidative decomposition re-
sults have been rationalized by assuming dioxygen binding to As(III).
The adduct then decomposes to the observed products.
Stronger oxidizing elements other than octasulfur and dioxygen are
the halogens. The action of iodine, dissolved in ethanol, on the ester 1
with an unspecified stoichiometry was reported7 to give diphenyl disul-
fide, but the fate of the arsenic was not determined.
FORMULAS 1–7
In this article, we report our results on the action of iodine and, in one
case, bromine on the trithioarsenites 1–7. We found that the reaction
of iodine with the esters 1–7 is simpler in nonprotic solvents; arsenic
triiodide is produced that either precipitates out or forms Lewis adducts
when the produced disulfide has certain nitrogen and/or oxygen donor
atoms.
RESULTS AND DISCUSSION
The reaction of 1 with iodine in methanol under various stoichiometries
gave a complex mixture of products, among them diphenyl disulfide,
as revealed by TLC analyses. However, in dry nonprotic solvents, e.g.,
chloroform, acetone, ether, 1 reacted cleanly with iodine giving, by TLC,
only two compounds with the stoichiometry of Eq. (2):
2(PhS)3As + 3I2 → 2AsI3 + 3PhSSPh
(2)
The best solvent for the isolation of AsI3 was ether, in which the triio-
dide is sparingly soluble (1:500).8 The yields, 85–90%, compare favor-
ably with those reported for its preparation in aqueous media8a,9 using
As2O3/HCl/KI. Because the ester 1 is easily prepared and is stable to
storage,10 the reaction (2) can be useful for preparations of milligram
amounts of arsenic triiodide. The ester 2 gave AsI3 (75–80%), while

The Preparation and Lewis Acid Behavior of AsI3
923
from the reaction of 3 with iodine, pure bis(4-nitrophenyl) disulfide,
was isolated.
The most likely mechanism for the reaction (2) is shown in Scheme 1.
It is not easy to say which of the two intermediates, the tetracoordinated
(ArS)3AsI+I− or the pentacoordinated (ArS)3As(I2), is preferred.
SCHEME 1
The mechanism suggests that arylsulfenyl iodide ArSI is an interme-
diate, and we sought a preparation of phenylsulfenyl iodide to test this
hypothesis. However, arylsulfenyl iodides are only accessible in spe-
cial instances,11 and the use of phenylsulfenyl iodide in the literature12
is questionable. We therefore prepared phenylsulfenyl bromide,13 con-
taminated by diphenyl disulfide, by reacting diphenyl disulfide with
bromine,14 and reacted it with 1 with a 3:1 stoichiometry. The reaction
was very fast, and 1 was not detected by TLC. Therefore, there are
strong indications that phenylsulfenyl iodide is an intermediate in the
reaction (2), and its reactivity explains the complex mixture of products
obtained when the reaction (2) was done in methanol.
The oxidation of the trithioarsenite 1 with bromine gave diphenyl
disulfide and AsBr3, which could not be separated. Overall, iodine
and bromine can therefore oxidize trithioarsenites to AsX3 plus disul-
fides, and the use of the solid iodine is more convenient for preparative
purposes.
Iodine should oxidize esters 4–7 to AsI3 and disulfide. The gum pro-
duced from 6 could not be crystallized, while the powdery solids isolated
from the reaction of 4, 5, and 7 with iodine were the Lewis adducts 8–10,
in which each AsI3 acts as a Lewis acid formally coordinated to three
donor atoms. Because AsI3 is not coordinated to the S S of diphenyl
disulfide, it follows that it must be coordinated to N and/or O atoms in
8–10, and it is not extracted by carbon disulfide in which it is soluble
(1:18).8

924
M. D. Markaki and P. V. Ioannou
FORMULAS 8–10
The first adduct of AsI3, acting as a Lewis acid, AsI3·3PhNH2, was
prepared by H. Schiff in 1863 and other earlier examples8a are AsI3·xpy
(x = 2, 3), AsI3·xNH3 (x = 3, 4, 12), and AsI3·Me2(Ph)As. More re-
cent work on the Lewis acid behavior of AsI3 includes the reaction
with Lewis bases having N,15−17 P,16,18 and As16,18 as well as O,17,19,20
S,21 and Se21 donor atoms. In some instances,18,19,21 the determina-
tion of the structure of the complexes by X-ray crystallography was
possible.
Bis(4-acetamidophenyl) disulfide in the solid state must have inter-
molecular hydrogen bonding, forming transoid-transoid “polymer”22 be-
cause strong (at 3294 cm−1) and weaker (at 3171, 3108 cm−1) bands for
N H stretching and very strong amide I (at 1662 cm−1) and amide II (at
1540 cm−1) bands are seen in its solid phase IR spectrum. The IR spec-
trum of the black 8 is quite similar to that of the disulfide except that
the bands are at 3304, 1654, and 1534 cm−1, respectively. These data
indicate that the “polymeric” structure of the bis(4-acetamidophenyl)
. . .
disulfide is retained in 8, having a C O H N hydrogen-bonding net-
work, which is slightly perturbed by AsI3. From the composition of 8
and its IR spectrum, the AsI3 molecule seems to be bonded to the lone
pairs of three nitrogen atoms. Then, each arsenic will have a N3I3 en-
vironment. However, the brown 1:1 complexes of AsI3 with acetamide
and with N, N-dimethylacetamide have, by IR, arsenic coordinated to
carbonyl oxygen.20
The solid state IR spectrum of 9 is quite simple, does not re-
semble that of bis(4-aminophenyl) disulfide,23 and does not contain

The Preparation and Lewis Acid Behavior of AsI3
925
decomposition products that were found with AsI3·3PhNH2, which in
ether gave PhNH2·HI and I-As(NHPh)2.15 While from the mass bal-
ance and percent analysis six acetone molecules must be present, in
the IR spectrum we find a very weak carbonyl peak at 1704 cm−1. This
fact, coupled with the absence of NH2 vibrations at 3418, 3335, and
3212 cm−1 of the disulfide23 and the presence of two very strong but
not too broad bands at 3444 and 2876 and a much weaker peak at 2575
cm−1, indicates that the amino group of the disulfide has been added
to the carbonyl group of acetone to give a geminal aminoalcohol (N-
substituted hemiaminal), which did not dehydrate to a Schiff base most
likely because the electron pair on nitrogen is not free to initiate the
dehydration. Therefore, the AsI3 must be coordinated to the nitrogen
and, from the composition, each arsenic must have a N3I3 environment.
The band, then, at 3444 cm−1, must be due to intermolecularly hydro-
gen bonded OH groups, and the band at 2876 cm−1 must be due to a
stretching vibration of an elongated and, thus, weaker N H bond. The
elongation is caused by the “donation” of the nitrogen electron pair to
the arsenic.
The solid state IR spectrum of the black 10 differs from that of
2,2´-dithiobis(pyridine N-oxide).23 Of the C C vibrations at 1463 and
1422 cm−1 of the free ligand, the one at 1422 cm−1 was less intense
in 10. The N O stretching vibrations at 1249 and 1230 cm−1 of the
free ligand were less intense in 10, and a new vibration at 1182 cm−1
appeared that is attributed to N OAs stretching. The N O bending
vibration moved from 838 to 832 cm−1 in 10. The coordination of re-
lated ligands showed that, e.g., when pyridine N-oxide binds to plat-
inum(II), the N O stretching is shifted from 1242 to 1235 cm .
−1 24 When
2-mercaptopyridine N-oxide, which exists mainly as the tautomeric 1-
hydroxypyrid-2-thione with βCH at 1263 cm−1 and N O stretching at
1112 and 1081 cm
lantimony(V), the N O stretching appears at 1197 cm
,
−1 25 is coordinated in a bidentate mode to tetrapheny-
.
−1 26 The stretch-
ing vibrations of the N O of the free ligand, 2,2ꢀ-bipyridine N, Nꢀ-
dioxide (BipyO2), at 1262 and 1255 appeared at 1265 and 1210 cm−1 in
the reddish-brown complex AsI3·2BipyO2, while the N O bending vi-
brations of the free ligand at 852 and 842 cm−1 appeared at 842 cm−1 in
the complex.17 From the composition of 10 and its IR spectrum, a biden-
tate coordination of the ligand to give, for example, a 9-membered ring
and an O3I3As3− core with a high charge on arsenic is not probable.
We propose that 2,2ꢀ-dithiobis(pyridine N-oxide) most probably acts,
through its O−, as a monodentate ligand, thus lowering the charge on
the arsenic atom compared to the bidentate coordination, and, more-
over, the positive charge on nitrogen, N+ O As−, can be dissipated
through the S S bridge to the other pyridine N-oxide ring via their

926
M. D. Markaki and P. V. Ioannou
canonical structures.27 Whether 10 is a single compound is an open
question in the absence of crystallographic data.
Arsenic triodide is quite stable in water; in fact, it can be recovered
unchanged from the solution9 and has been used in medicine for the
treatment of tuberculosis in pigs, of anaemia from hereditary syphilis,
of arthritis and certain dermatitises, but it is toxic.8a Given the effective-
ness of arsenic(III) oxide (As2O3, arsenoxR) in treating acute promyelo-
cytic leukemia in man,28 arsenic triiodide and its complexes, like 8–10,
should be examined in place of arsenic(III) oxide. Based on the behav-
ior of (ArS)3As and I2, the reaction of iodine with thioesters of arson-
ous and arsinous acids, L-As(SPh)2 and L2As-SPh, should give L-AsI2
and L2As-I (L = R or Ar) and PhSSPh. From these iodides, a wide
range of organoarsenic compounds can be prepared. Such studies are
underway.
EXPERIMENTAL
The aromatic trithioarsenites have been prepared by literature meth-
ods as follows: 1,10 3, 5, and 6,2 2, 4, and 7.5 Silica gel 60 H (Merck)
was used for Thin Layer Chromatography (TLC). AR grade solvents
have been dried over A4 molecular sieves and de-aerated by boil-
ing, flushing with nitrogen, stoppering, and cooling to r.t. TLC was
run on microslides using the appropriate disulfides2,5,10 as standards.
The visualization was effected by iodine vapors followed by spraying
with 35% sulfuric acid and charring. Compounds containing an ArS
group initially gave colored spots and on charring left a faint black
spot. For melting points, an Electrothermal (model 9100) apparatus
was used, and IR spectra were obtained on a Perkin-Elmer (model
16PC FT-IR) spectrometer. Elemental analyses were obtained through
the Centre of Instrumental Analyses, University of Patras, Patras,
Greece.
The Reaction of the Trithioarsenites 1–7 With Iodine
Triphenyl Trithioarsenite, 1
To a solution of 1 (201 mg, 0.5 mmol) in ether (6 mL), iodine (191
mg, 0.75 mmol) was added and stirred at r.t. for 1 h. Centrifugation and
washing with ether (2 × 2 mL) gave arsenic triiodide (204 mg, 90%) as
an orange solid having m.p. 141◦C (lit.8a 138.6, 140.7, 146◦C). IR (KBr):
3431 s, broad, 1636 w, broad, 595 m, broad. Evaporation of the dark red
supernatant gave a light orange solid (169 mg), which was diphenyl
disulfide (by TLC and IR) contaminated by arsenic triiodide.

The Preparation and Lewis Acid Behavior of AsI3
927
Tris(4-Chlorophenyl) Trithioarsenite, 2
The reaction of 2 (101 mg, 0.2 mmol) with iodine (73 mg, 0.3 mmol)
in ether (1 mL), as previously mentioned, gave arsenic triiodide (72 mg,
78%), m.p. 141–142◦C.
Tris(4-Nitrophenyl) Trithioarsenite, 3
To a suspension of 3 (0.25 mmol) in dichloromethane (3 mL), io-
dine (0.375 mmol) was added. At once, the ester 3 dissolved giving a
dark brown solution from which an orange solid precipitated. Evapo-
ration, drying in vacuum, addition of carbon disulfide (3 mL), centrifu-
gation, and washing with carbon disulfide (1 × 2 mL) left the disulfide
(99 mg, 86%), m.p. 183–184◦C (lit.29 173–181◦C; lit.30 191◦C). Its IR
(KBr) was the same as reported.23 From the supernatant, arsenic tri-
iodide (120 mg) contaminated by the disulfide (by TLC and IR) was
obtained, m.p. 140◦C.
Tris(4-Acetamidophenyl) Trithioarsenite, 4
The ester 4 (0.147 mmol) and iodine (0.221 mol) in acetone (2 mL)
after 1 h of stirring and evaporation gave a cherry-red gum. The addition
of carbon disulfide (3 mL) and boiling transformed the gum to a black
solid. Centrifugation and washing with carbon disulfide (2 × 2 mL) left 8
as a black powder (121 mg, 88%). It is soluble in acetone (yellow solution)
and in methanol (orange solution), sparingly soluble in chloroform, and
insoluble in ether and carbon disulfide. M.p. 148–149◦C. Calculated for
C48H48N6O6S6I6As2 (M 1908.58): C, 30.20; H, 2.53; N, 4.40; S, 10.08%.
r
Found. C, 30.41; H, 2.48; N, 4.33; S, 10.55%. IR (KBr): 3303, s, sharp,
3112 m, 1654 vs, 1594 s, 1534 vs, 1490 ms, 1396 ms, 1320 m, 1117 w,
1078 w, 1045 w, 1006 m, 816 s, 672 mw, 604 mw.
Tris(4-Aminophenyl) Trithioarsenite, 5
The addition of iodine (0.375 mmol) to a solution of 5 (0.25 mmol)
in acetone (3 mL) immediately precipitated a brown-orange gum. Af-
ter 3 h of stirring at r.t., evaporation and drying in a vacuum gave
a sticky, colored foam. It was covered with ether and stirred at r.t.
overnight. Centrifugation and washing with ether (1 × 2 mL) gave 9
as a light yellow-brown solid (246 mg, 98%) insoluble in carbon disul-
fide and ether, sparingly soluble in chloroform (yellow), and soluble in
methanol (yellow). M.p.: 73◦C shrinks, 92◦C turns red, 99–101◦C melts
to a red oil. Calculated for C54H72N6O6S6I6As2 (M 2004.79): C, 32.35;
r
H, 3.62; N, 4.19; S, 9.60%. Found: C, 31.97; H, 3.15; N, 4.69; S, 9.55%.
IR (KBr): 3444 vs, slightly broad, 2876 vs, slightly broad, 2575 m, 1704

928
M. D. Markaki and P. V. Ioannou
vw, 1618 s, 1565 m, 1488 vs, 1406 w, 1374 w, 1313 w, 1206 w, 1122 w,
1017 w, 808 s, 612 w, 495 w.
Tris(2-Pyridyl) Trithioarsenite, 6
The reaction of 6 (0.25 mmol) suspended in dichloromethane (3 mL)
with iodine (0.375 mmol) gave a cherry-red oil that could not be induced
to crystallize and was not studied further.
Tris(2-pyridyl N-Oxide) Trithioarsenite, 7
To a suspension of 7 (0.82 mmol) in acetone (5 mL), iodine (1.23
mol) was added. At once, the ester dissolved and a new solid formed.
After 2 h of stirring, evaporation and drying in vacuum gave a black
crystalline solid. The addition of carbon disulfide (3 mL), centrifugation,
and washing with carbon disulfide (1 × 2 mL) gave 10 as a black-brown
powder (677 mg, 99%). It is soluble in methanol (yellow) and acetone
(orange), sparingly soluble in chloroform, and insoluble in ether. M.p.
105–108◦C. Calculated for C30H24N6O6S6I6As2 (M 1668.21): C, 21.69;
r
H, 1.45; N, 5.04; S, 11.53%; Found: C, 21.48; H, 1.02; N, 5.36; S, 11.52%.
IR (KBr): 3444 m, slightly broad, 3090 m, 1590 w, 1554 m, 1460 vs, 1418
s, 1248 m, 1230 sh, 1182 m, 1142 m, 1084 w, 1039 w, 828 m, 758 s, 704
m, 578 s.
The Reaction of Triphenyl Trithioarsenite, 1, With Bromine
The reaction of 1 (0.315 mmol) and bromine (0.473 mmol) in ether (6 mL)
gave a yellow solution from which the arsenic tribromide and diphenyl
disulfide could not be separated due to similar solubilities.8a Hydrolysis
of the mixture in acetone gave As2O3 (16 mg, 50% recovery) and impure
diphenyl disulfide (114 mg), which was recrystallized from petroleum
ether, m.p. 59–60◦C (lit.31 61–62◦C).
The Reaction of 1 With Phenylsulfenyl Bromide
To a solution of bromine (103 mg, 0.625 mmol) in dichloromethane,
diphenyl disulfide (136 mg, 0.625 mmol) was added, and the red solution
was left at r.t. for 2 h. TLC (petroleum ether) showed the presence of
disulfide (R 0.46) and a spot as R 0.0 (due to the hydrolysis of PhSBr
f
f
on the plate). The evaporation left an oil (202 mg, expected 236 mg
PhSBr). To the oil in dichloromethane (3 mL), solid 1 (0.35 mmol) was
added. At once, the solution decolorized to a nearly colorless solution.
TLC (petroleum ether) showed only the presence of disulfide and the
absence of 1 (R 0.10).
f

The Preparation and Lewis Acid Behavior of AsI3
929
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