Synthesis and structures of the first triiodoarsenate(III) anion, EtAsI3-, and zwitterions, (HPy)2As 2I6

Article abstract of DOI:10.1016/j.poly.2013.08.070

The first examples of organotriiodoarsenates(III) in the form of zwitterions, (3-HPy)₂As₂I₆ and (4-HPy) ₂As₂I₆, have been obtained from aqueous solutions, and their structures have been established by XRD. The dimeric molecules in the crystals are built from two square pyramidal (HPy)AsI ₄ units joined by 2 bridging I atoms. The crystal packing of both compounds is largely defined by interlayer As?I contacts, which fulfill the coordination sphere of the As atoms to 6 in the 4-HPy isomer and result in its characteristic red colour and lower solubility. These intermolecular interactions in the crystal structure can be suppressed by the inclusion of a solvent, showing them to be not very strong. The intramolecular bonding via the two bridging I atoms was proven to be rather weak as well, by the formation of the salt [Et₄N][EtAsI₃] with a monomeric triiodoarsenate(III) anion from acetonitrile solution. In the closely related compound p-H₂NC₆H₄AsI₂·HI, one iodide can be substituted for the OTs^- anion to give [p-H ₃NC₆H₄AsI₂]OTs, showing the iodide belongs to the outer sphere rather than being coordinatively bound to the As atom. The lengths of the trans-I-As-I bonds in the determined structures can be formalized by a semi-empirical modified Pauling equation, which relates them to the bond order, taking into account the antibonding influence of the 4s orbital of the As atom.

Full text of DOI:10.1016/j.poly.2013.08.070

Polyhedron 67 (2014) 115–121
Contents lists available at ScienceDirect
Polyhedron
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ꢀ
Synthesis and structures of the first triiodoarsenate(III) anion, EtAsI3 ,
and zwitterions, (HPy)2As2I6
Igor Yu. Ilyin a, Sergey N. Konchenko a,b, Alexander V. Virovets a, Natalia V. Kuratieva a,b
,
Nikolay A. Pushkarevsky a,b,
⇑
a Nikolaev Institute of Inorganic Chemistry SB RAS, Akademika Lavrentieva Ave. 3, 630090 Novosibirsk, Russia
b Department of Natural Sciences, Novosibirsk State University, Pirogova St. 2, 630090 Novosibirsk, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2013
Accepted 14 August 2013
Available online 11 September 2013
The first examples of organotriiodoarsenates(III) in the form of zwitterions, (3-HPy)2As2I6 and (4-HPy)2-
As2I6, have been obtained from aqueous solutions, and their structures have been established by XRD. The
dimeric molecules in the crystals are built from two square pyramidal (HPy)AsI4 units joined by 2 bridg-
ing I atoms. The crystal packing of both compounds is largely defined by interlayer Asꢁ ꢁ ꢁI contacts, which
fulfill the coordination sphere of the As atoms to 6 in the 4-HPy isomer and result in its characteristic red
colour and lower solubility. These intermolecular interactions in the crystal structure can be suppressed
by the inclusion of a solvent, showing them to be not very strong. The intramolecular bonding via the two
bridging I atoms was proven to be rather weak as well, by the formation of the salt [Et4N][EtAsI3] with a
monomeric triiodoarsenate(III) anion from acetonitrile solution. In the closely related compound p-H2-
NC6H4AsI2ꢁHI, one iodide can be substituted for the OTsꢀ anion to give [p-H3NC6H4AsI2]OTs, showing
the iodide belongs to the outer sphere rather than being coordinatively bound to the As atom. The lengths
of the trans-I–As–I bonds in the determined structures can be formalized by a semi-empirical modified
Pauling equation, which relates them to the bond order, taking into account the antibonding influence
of the 4s orbital of the As atom.
Keywords:
Iodoarsenates(III)
Crystal structure
Intermolecular interactions
Ligand lability
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
the increase of the atomic number of both elements. Quite the con-
trary, two iodo(methyl)arsenic(V) species, [MeAsI3]+ and [Me2-
Aryldihaloarsanes RAsX2 are readily obtained in the reactions of
arylarsonous acids with the corresponding concentrated HX acid.
In such reactions carried out in aqueous media, the corresponding
aryltrihaloarsenate anions are not separated in the form of stable
solids, despite the presence of Xꢀ in large concentrations [1]. On
the other hand, it is well known that the compounds RnAsX3ꢀn
(n = 0–2) act as weak Lewis acids and form complexes with other
nucleophiles, including halides. Such complex anions, e.g. of the
AsI2]+, are known as rare examples of As halogenated cations [6].
Here we describe the first structurally characterized triiodoarse-
nates(III), obtained as the zwitterions [HPyAsI3] (Py = 3 or
4-C5H4N). Most notably, the compounds have been crystallized di-
rectly from aqueous media. Their crystal packing is discussed in
view of intermolecular interactions and solubility; the possibility
of triiodoarsenate [RAsI3]ꢀ anion formation in organic solutions is
also considered.
nꢀ
nꢀ
types [AsmX3m+n
]
[2] and [RnAsnX3n
]
[3], are structurally well
studied and tend to readily form in polar organic solutions for
the combination of As with Cl and Br. Stable crystalline organohal-
oarsenates were isolated with such anions as [RAsCl3]ꢀ, [R2As2Cl5]-
ꢀ, and [R2As2Br6]2ꢀ [3]. No organofluoroarsenates(III) are known;
organofluorinated species are only reported as cations with higher
valent As, such as the only structurally characterized [MeAsF3]+ [4]
and [Ph2AsF2]+ [5]. Interestingly, organoiodoarsenates also remain
unknown to date, in spite of the fact that halide complexes of the
group 15 elements become more thermodynamically stable with
2. Experimental
2.1. General remarks
All the starting reagents are commercially available. EtAsI2 was
kindly donated by Prof. J. D. Woollins. DMSO-d6 for NMR experi-
ments was degassed in a vacuum and stored over 3 Å molecular
sieves. 4-Aminophenylarsonic acid was prepared according to a
published method [7]. Owing to the known instability of
pyridyldiazonium salts [8], a quick further reaction with arsonic
alkaline solutions was necessary. The IR spectra were recorded
on a Varian FTS 800 spectrometer. The NMR spectra were recorded
⇑
Corresponding author. Tel.: +7 383 316 58 31; fax: +7 383 330 94 89.
E-mail address: nikolay@niic.nsc.ru (N.A. Pushkarevsky).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.070

116
I.Yu. Ilyin et al. / Polyhedron 67 (2014) 115–121
3
3
for solutions in DMSO-d6 on Bruker Avance 300 and 500 spectrom-
eters (1H: 300.130; 500.129 MHz, 13C: 75.438; 125.757 MHz). The
chemical shifts (d, ppm) were referenced to the signals of the
solvent: dH = 2.50, dC = 39.5. The atom numbering in the NMR
spectra descriptions corresponds to that in Figs. 1, 2 and 4; the
assignments of peaks was made by a comparison to the spectra
of similar compounds [9].
(d, JH,H = 9 Hz, 2 H, C(3)–H and C(5)–H), 6.81 (d, JH,H = 9 Hz, 2 H,
C(2)–H and C(6)–H), 2.28 (s, 3 H, CH3). 13C{1H} NMR: 154.2 (s,
C(4)), 145.1 (s, C(11)), 138.1 (s, C(14)), 132.3 (s, C(3) and C(5)),
114.4 (s, C(2) and C(6)), 128.2 (s, C(12) and C(16)), 125.5 (s,
C(13) and C(15)), 117.8 (s, C(1)), 20.7 (s, CH3). Compound 1 shows
essentially the same signals in the 1H and 13C spectra as those of
the cation of 2.
2.2. Preparation
2.2.2. Synthesis of 3-I2AsC5H4N (3)
Solution of As2O3 (6.48 g, 32.8 mmol) and Na2CO3 (14.1 g, 133
mmol) in 40 mL of water was prepared. 3-Aminopyridine (2.87 g,
30.5 mmol) was dissolved in 20 mL of water, then conc. HCl (9.5
mL, 95 mmol) was added. The latter mixture was cooled to ꢀ10
°C and a solution of NaNO2 (2.27 g, 32.9 mmol in 4 mL of water)
was added over 1 min. Immediately afterwards the resultant bright
yellow solution was added to the stirred and cooled to 0 °C arsenite
solution. The resulting brick red mixture was stirred for 10 min at
room temperature, and then gradually heated to 70 °C. After cool-
ing, 7 ml of conc. HCl and 20 g of activated carbon were added, the
solution was heated to boiling and filtered. The filtrate was concen-
trated to 20 mL, filtered, and Na2S2O5 (3.00 g, 15.8 mmol), conc.
HCl (3 mL, 30 mmol) and NaI (9.21 g, 61.5 mmol) were added. After
thorough mixing, the orange precipitate that formed was separated
by filtration and dried in a vacuum over P2O5. For purification, the
crude product was mixed with 20 mL of water, 1.5 g of NaI, 2 mL of
conc. HCl and 0.2 g of Na2S2O5 were added, then after mixing the
precipitate was thoroughly sucked on a glass frit, dried in a
2.2.1. Synthesis of p-H2NC6H4AsI2ꢁHI (1) and [4-H3NC6H4AsI2][OTs] (2)
To a solution of 4-H2NC6H4AsO3H2 (1.82 g, 8.38 mmol) in 50 mL
of water conc. HCl (15.0 mL, 150 mmol), Na2S2O5 (1.0 g, 5.3 mmol)
and NaI (10.0 g, 66.7 mmol) were added. A yellow precipitate of 1
formed immediately. After some mixing, HOTsꢁH2O (30.0 g, 158
mmol) was added and the mixture was shaken well, whereupon
a thick orange precipitate initially formed which turned into a
yellow crystalline solid. The latter was filtered out and dried in a
vacuum desiccator over P2O5. Yield: 4.05 g (81%). Crystals suitable
for XRD were grown from the water-THF solution upon cooling.
Anal. Calc. for C13H14AsI2NO3S (593.1): C, 26.33; H, 2.38; N, 2.36.
Found: C, 26.2; H, 2.4; N, 2.3%. IR (KBr, cmꢀ1): 1594 (m), 1546
(s), 1493 (s) (m(Py ring)); 1223 (vs), 1212 (vs), 1168 (vs), 1125
(vs), 1062 (m), 1032 (vs), 1007 (vs), 818 (vs), 681 (vs), 592 (m),
567 (vs), 493 (s), 413 (m) (
m
(SO3) + d(C–H, aromatic)). 1H NMR:
3
10.4 (br. s, 3 H, NH3), 7.51 (d, JH,H = 9 Hz, 2 H, C(12)–H and
3
C(16)–H), 7.15 (d, JH,H = 9 Hz, 2 H, C(13)–H and C(15)–H), 7.45
Fig. 1. (A) Independent part of the crystal structure of 2, showing the atom numbering scheme; (B) packing of the molecules in the crystal, featuring NH3ꢁ ꢁ ꢁO3S interactions
(the hydrogen bonds are shown as dashed lines, H atoms are omitted).

I.Yu. Ilyin et al. / Polyhedron 67 (2014) 115–121
117
Fig. 2. Structure of the molecular dimers in the crystals of (A) 3ꢁMe2CO, (B) 4 and (C) 4ꢁ2THF.
vacuum desiccator with P2O5 and further in a dynamic vacuum.
Yield: 4.34 g (27%). Single crystals of the acetone solvate were ob-
tained by cooling of a saturated acetone solution at ꢀ25 °C. Anal.
Calc. for C5H5AsI3N (534.7): C, 11.2; H, 0.94; N, 2.62. Found: C,
10.9; H, 0.9; N, 2.5%. IR (KBr, cmꢀ1): 3468 (br, m) (
m(N–H)); 1606
(m), 1577 (m), 1512 (s), 1436 (s) (m(Py ring)); 770 (s), 663 (vs)
(d(C–H, aromatic)). 1H NMR: 15.36 (br. s, 1H, NH, chemical shift de-
pends on water amount), 9.07 (s, 1H, C(2)–H), 8.93 (d, 3JH,H = 6 Hz,
3
3
1H, C(4)), 8.89 (d, JH,H = 8 Hz, 1H, C(6)), 8.14 (dd, JH,H = 8 Hz,
3JH,H = 6 Hz, 1H, C(5)). 13C{1H} NMR: 150.9 (s, C(1)), 147.9 (s,
C(2)), 142.6, 141.4 (both s, C(4), C(6)), 126.2 (s, C(5)).
2.2.3. Synthesis of 4-I3AsC5H4NH (4)
4-Aminopyridine (1.50 g, 15.9 mmol) was dissolved in 39 mL of
water and then H2SO4 (15.6 g, 159 mmol) was added. The mixture
was cooled to 0 °C, and a solution of sodium nitrite (1.49 g, 21.6
mmol in 5 mL of water) was added in one portion. After 40 s, the
suspension was poured into a stirred solution of sodium arsenite
(2.70 g, 20.8 mmol) and Na2CO3 (3.6 g, 34.0 mmol) in 150 mL of
water, while keeping the temperature at 0 °C. The mixture was stir-
red for 20 min at room temperature and then heated to 60 °C for 5
min. Hydrochloric acid was added dropwise until pH = 5 was
reached, the mixture was cooled and filtered. The filtrate was con-
centrated to the half of the volume in vacuum, filtered, and acidat-
ed with hydrochloric acid until pH = 1. Then Na2SO3 (3.0 g, 24
mmol) and excess of NaI (5.10 g, 34.0 mmol) was added until the
complete formation of a purple crystalline precipitate of 4-I3AsC5-
H4NH. The product was filtered and dried in vacuum. Yield: 2.9 g
(35%). Crystals of 4 suitable for XRD were obtained after recrystal-
lization from water, with addition of some HCl and NaI to prevent
hydrolysis. Single crystals of THF solvate (orange–red) were ob-
tained by cooling of a saturated THF solution at ꢀ25 °C. Anal. Calc.
for C5H5AsI3N (534.7): C, 11.2; H, 0.94; N, 2.62. Found: C, 11.2; H,
Fig. 3. Packing of the molecular dimers in the structure of 4. The hydrogen atoms
are not shown. Intermolecular Asꢁ ꢁ ꢁI contacts within the sum of rW are shown as
dashed bonds.
to 2 ml volume. After complete evaporation of MeCN in a vacuum,
the formation of solely colourless crystals along with a yellow oil
was noticed. Data for 5: Anal. Calc. for C10H25AsI3N (614.9): C,
19.53; H, 4.10; N, 2.28. Found: C, 19.7; H, 4.0; N, 2.3%. IR (KBr,
cmꢀ1): 2976 (m), 2925 (w), 2865 (w), 1483 (s), 1474 (s), 1444
(m), 1435 (m), 1412 (m), 1390 (s), 1363 (s), 1315 (w), 1184 (s),
1169 (s), 1052 (m), 1019 (m), 995 (s), 801 (m, br), 782 (s), 711
(w), 546 (w).
2.3. X-ray crystallography
Single-crystal X-ray diffraction data for 3ꢁMe2CO and 4 were
1.1; N, 2.6%. IR (KBr, cm-1): 3454 (w) (
m(N–H)); 1605 (m), 1582
collected using graphite monochromatized Mo K
a radiation (k =
(m), 1472 (s) (
m
(Py ring)); 753 (s), 737 (vs) (d(C–H, aromatic)). 1H
0.71073 Å) at 293 K (for 3ꢁMe2CO) and 150 K (for 4) on a Bruker
3
NMR: 10.1 (br s, 1H, NH), 8.88 (d, JH,H = 5 Hz, 2H, C(3)–H and
C(5)–H), 8.51 (br s, w1/2 = 17 Hz, 2H, C(2)–H and C(6)–H). 13C{1H}
NMR: 173.8 (br s, C(1)), 139.6 (s, C(3), C(5)), 127.9 (s, C(2), C(6)).
X8 Apex CCD diffractometer equipped with a 4 K CCD area detec-
2.2.4. Reactions of EtAsI2 with Et4NI and Bun4NI. Synthesis of
[Et4N][EtAsI3] (5)
A solution of EtAsI2 (0.463 g, 1.29 mmol) in 3 mL of MeCN was
added to a mixture of Et4NI (0.346 g, 1.35 mmol) or Bun4NI (0.499
g, 1.35 mmol) with 3 mL of MeCN. An instant colour change to
darker orange was noticed, along with rapid dissolution of undis-
solved crystalline salt in the case of Et4NI. Crystals of 5 were ob-
tained by cooling the solution to ꢀ18 °C overnight; a sample for
XRD was taken from the bulk of crystals. Yield of 5, 0.301 g
(38%). The solution with Bun4NI did not afford crystals by cooling
at various temperatures down to ꢀ18 °C, even after concentration
Fig. 4. The structure of the [EtAsI3]ꢀ anion in the crystal of 5.

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I.Yu. Ilyin et al. / Polyhedron 67 (2014) 115–121
Table 2
Crystal data and refinement parameters for the compounds 2, 3ꢁMe2CO, 4, 4ꢁ2THF and 5.
2
3ꢁMe2CO
4
4ꢁ2THF
5
Empirical formula
Formula weight
T (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
13H14AsI2NO3S
C8H11AsI3NO
592.80
293(2)
monoclinic
C2/c
C5H5AsI3N
534.72
150(2)
C13H21AsI3NO2
678.93
150.0(2)
C10H25AsI3N
614.93
150.0(2)
tetragonal
I41/a
593.03
150.0(2)
monoclinic
P21/n
triclinic
triclinic
ꢀ
ꢀ
P1
P1
16.3094(4)
5.33700(10)
20.1737(6)
90
98.7550(10)
90
1735.52(7)
4
2.270
14.5660(4)
8.9547(3)
23.4675(6)
90
107.175(1)
90
2924.47(15)
8
2.693
7.8819(2)
8.6376(2)
9.4949(2)
63.571(1)
69.801(1)
64.731(1)
513.76(2)
2
8.5979(11)
11.0251(15)
11.5617(16)
99.350(3)
106.615(3)
105.032(4)
980.6(2)
2
13.5062(17)
13.5062(17)
40.126(5)
90
90
90
7319.6(16)
16
2.232
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc. (g cmꢀ3
)
3.457
2.299
Absorption coefficient
5.644
8.634
12.259
6.457
6.899
(mmꢀ1
)
Maximum and minimum 0.7463/0.5041
transmission
0.8463/0.5451
0.4405/0.1734
0.1919/0.2686
0.7479/0.4571
F(000)
1112
2128
468
628
4544
Crystal size (mm3)
h (°)
0.35 ꢂ 0.10 ꢂ 0.08
1.50–31.56
ꢀ18 6 h 6 24,
0.08 ꢂ 0.07 ꢂ 0.02
2.70–26.42
ꢀ18 6 h 6 18,
0.22 ꢂ 0.18 ꢂ ꢂ0.08
2.44–30.52
ꢀ9 6 h 6 11,
ꢀ12 6 k 6 12,
ꢀ11 6 l 6 13
5355
0.25 ꢂ 0.20 ꢂ 0.03
1.90–32.57
ꢀ10 6 h 6 13,
ꢀ16 6 k 6 16,
ꢀ17 6 l 6 17
9401
0.21 ꢂ 0.19 ꢂ 0.06
2.14–31.35
ꢀ17 6 h 6 19,
ꢀ11 6 k 6 19,
ꢀ37 6 l 6 56
10840
Index ranges
ꢀ7 6 k 6 4, ꢀ29 6 l 6 27 ꢀ11 6 k 6 11,
ꢀ28 6 l 6 29
Reflections collected
Independent reflections
Observed reflections
12138
5737
4212
11896
2997
2451
3102
2887
6931
5490
5546
3388
(I > 2r(I))
Rint
0.0257
100
0.0391
99.8
0.0155
99.3
0.0184
99.1
0.0395
99.8
Completeness to
h = 25.25 (%)
Data/restraints/
parameters
5737/0/192
2997/0/127
1.028
3102/0/92
1.107
6931/0/181
0.998
5546/0/142
0.999
Goodness-of-fit (GOF) on 0.864
F2
Final R indices [I > 2
R indices (all data)
r
(I)] R1 = 0.0456, wR2 = 0.1190 R1 = 0.0291, wR2 = 0.0613 R1 = 0.0192, wR2 = 0.0453 R1 = 0.0313, wR2 = 0.0622 R1 = 0.0424, wR2 = 0.0734
R1 = 0.0668, wR2 = 0.1351 R1 = 0.0413, wR2 = 0.0644 R1 = 0.0214, wR2 = 0.0459 R1 = 0.0461, wR2 = 0.0676 R1 = 0.0901, wR2 = 0.0874
Largest difference peak
2.013/ꢀ1.131
1.378/ꢀ0.764
0.947/ꢀ0.806
1.331/ꢀ0.882
0.193/ꢀ1.016
and hole (e Åꢀ3
)
tor. The / scan technique was employed to measure intensities.
The data for 2, 4ꢁ2THF and 5 were collected on a Bruker Apex
can even be prevented by adding a sulfite or pyrosulfite directly
into the reaction solution (Eq. 2) [13]. Thus the synthesis of the
aryldiiodides can be easily achieved in one stage.
DUO diffractometer with Mo Ka radiation using / and x scans of
narrow (0.5°) frames at 150 K. Absorption corrections for 2, 3ꢁMe2-
CO, 4 and 4ꢁ2THF were applied using the SADABS program [10]. The
absorption correction for 5 was applied analytically based on crys-
tal faces [10]. The crystal structures were solved by direct methods
and were refined by the full-matrix least squares technique with
the use of the SHELXTL package [11]. All atomic displacement param-
eters for non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were located in idealised positions and refined
isotropically according to the riding model. Data collection and
refinement parameters are summarized in Table 2.
ArAsO3H2 þ 4HI ! ArAsI2 þ I2 þ 3H2O
ð1Þ
ArAsO3 þ 2Iꢀ þ SO2 þ Hþ ꢀ ArAsI2 þ HSOꢀ þ H2O
ð2Þ
4
In all cases a large excess of iodide is present in the solution, so
triiodoarsenate anions [RAsI3]ꢀ could form when they are of suffi-
cient stability. When non-basic arene substituents are taken (e.g.
when Ar = phenyl with nitro, iodo, methoxy or benzoyl substitu-
ents [14]) only the neutral diiodoarsanes separate from the
reaction solution. In the case when an amino group is present as
the arene substituent, the corresponding hydriodides are
commonly separated [1]. In both cases the composition has been
verified by elemental analysis, but to our knowledge, no structural
information has been reported on hydriodides to date, so there is
no data as to whether the additional iodine anions are coordinated
to the As atom to form triiodoarsenates.
In our attempt to synthesize p-aminophenyldiiodoarsane by
treatment of aminophenylarsonic acid with an excess of iodide
upon acidification, a yellow precipitate was obtained. According
to the 1H and 13C NMR spectra, the compound most likely was
hydriodide 1 (Chart 1). For further implementation, the absence
3. Results and discussion
3.1. Synthesis
Compounds of the type Ar2AsI and ArAsI2 are the longest known
iodide organyls of arsenic [12]. The latter have become widespread
since they are readily obtained upon treatment of arylarsonic acids,
ArAsO3H2, with concentrated hydroiodic acid, where the acid acts
as a reductant (Eq. 1). The free iodine formed can be removed by
a following treatment of the solution with SO2, or the formation

I.Yu. Ilyin et al. / Polyhedron 67 (2014) 115–121
119
basal atoms (ꢃ0.1 Å). In the structures of the solvates 3ꢁMe2CO
and 4ꢁ2THF, the As– -I distances differ by 0.147 and 0.081 Å,
NH3
NH3
AsI2
SO3
l
respectively (Table 1), while in the solvent-free structure of 4 the
difference is noticeably larger (0.341 Å), thus making the pyrami-
dal environment rather unsymmetrical. Nevertheless, all the
distances between As and the furthest bridging halide, Asꢁ ꢁ ꢁI(3)’,
are sufficiently shorter than the sum of rW (Table 1) and lie in
the same length range as those found for binary iodoarsenates(III)
[19], and as such can be considered as normal coordination bonds
(see Supplementary Information for a more detailed discussion).
The dimeric structures and corresponding bond lengths are very
close to those of the anions in the known salts [Prn4N]2[As2I8]
[19a] and [Bun4N]2[As2I8] [19b], except for the larger deviation of
the As atom from the pyramid base in the latter compounds due
to the steric bulk of the apical I atom.
I
AsI2
2
1
H
N
H
N
[Et4N][EtAsI3]
5
AsI3
3
AsI3
4
Comparing the structures of 3 and 4 with those of the Cl and Br
congeners, one can note that the same dimers also exist in the case
of Br, [R2As2Br6]2ꢀ, whilst they are absent for Cl [3]. Thus, the As–X
bond strength expectedly rises in the row Cl–Br–I, as does the ten-
dency of As(III) to increase its coordination number to 5, along with
the increasing nucleophilicity of the halide atoms. The pyridine
rings are coplanar in 3ꢁMe2CO and 4, but they are only parallel in
4ꢁ2THF. For the analogous Sb-containing anions, [R2Sb2X6]2ꢀ
(R = aryl, alkyl), there are two series of structures known [20],
where the R groups can be placed cis or trans with respect to the
Sb2X6 plane. The anion [Ph2Sb2I6]2ꢀ was shown to form both cis
and trans isomers, depending only on the crystal environment
[20]. In our case only the trans position of the Py units was found.
These slight differences in the structures of the molecular
dimers can hardly explain the very distinct properties of the crys-
talline phase of 4, which is visibly much less soluble in water as
compared to 3, deep-red coloured, and precipitates upon slight
acidification (starting at pH = 3) as compared to the strongly acidic
conditions necessary for 3. Evidently, some stronger intermolecu-
lar interactions in 4 give a very sufficient contribution to the stabil-
ity of its crystalline phase. Upon a closer look at the structure one
can see two additional interactions between the dimeric units
(Fig. 3). First, there are additional Asꢁ ꢁ ꢁI contacts with the I atoms
placed trans to the Py group, with a distance of 3.7620(4) Å, still
slightly shorter than the sum of rW [15]. Secondly, close contacts
of the Py rings of neighboring units are evident, which are symmet-
rically parallel with a 3.64 Å interplane distance. Possibly the Py
Chart 1.
of additional iodide was strongly desired, so we attempted to sub-
stitute the iodide anion for another anion, which we succeeded on
adding an excess of p-toluenesulfonic acid to the reaction mixture.
Under such conditions, a similar yellow precipitate formed, for
which elemental analysis and solution NMR spectra confirm that
the substitution of one iodide anion took place to give [p-H3NC6H4-
AsI2][OTs], 2. A crystal structure was obtained for the latter com-
pound (Fig. 1).
For the synthesis of a diiodoarsane functionalized with a pyridyl
group, the analogous procedure was attempted with 4-pyridylar-
sonic acid. In this case the compound [4-HPyAsI3] (4) was formed
as a dark red precipitate, the structure of which was confirmed by
X-ray structural analysis (Fig. 2) as well as by NMR and analytical
data. Interestingly, even in the presence of a highly concentrated
solution of HOTs, the p-toluenesulfonate anion was not included
in the structure; later the reaction was shown to proceed equally
well with hydrochloric acid. Upon recrystallization from THF, the
crystalline solvate 4ꢁ2THF was obtained.
In the analogous synthesis of the 3-substituted pyridylarsonic
acid, on the contrary, a yellow precipitate of the iodide derivative
was formed which again contains the zwitterionic triiodoarsenate
complex [3-HPyAsI3] (3). The crystal structure was determined for
the compound recrystallized from acetone as the solvate 3ꢁMe2CO
(Fig. 2). The composition of the initial yellow polycrystalline com-
pound corresponds to neat 3, according to the elemental analysis
and NMR data.
rings experience sufficient p–p interactions and may be attracted
as dipoles, considering the positive charge at the N atom and their
antiparallel orientation. In total, the structure can be seen to be
composed of molecular layers spreading parallel to the [011]
plane, with more evident interactions within the layer than be-
tween them. It is noteworthy that neither of these interactions is
present in the solvate structures, which consist of sufficiently
3.2. Crystal structures
The crystal packing of the tosylate salt 2 (Fig. 1b) consists of
square pillars of cations and anions, bound by a coulombic attrac-
tion and hydrogen bonding between NH3 and SO3 groups. Thus,
the AsI2 groups are stacked over each other aside the pillar. Notably,
the distances from an As atom to the I atoms of the neighboring
molecule, 4.319(2) and 4.126(2) Å are well beyond the sum of the
van der Waals radii (rW, 3.83 Å for As + I [15]), so the As atoms do
not have close contacts to complete their coordination sphere. This
goes in contradiction with the previously known RAsI2 structures,
where the coordination sphere of the As atom is effectively fulfilled
either by neighboring I atoms (for MeAsI2 [16] and m-HOOCC6H4-
Table 1
Selected bond lengths (Å) and angles (°) for the compounds obtained.
2
3ꢁMe2CO
4
4ꢁ2THF
5
As–C(1)
As–I(1)
As–I(2)
As–I(3)
As–I(3)’a
1.957(4)
2.5545(7)
1.995(4)
2.018(3)
2.007(3)
1.965(5)
2.7094(6) 2.8019(3) 2.6891(5) 2.9889(8)
2.5848(17) 2.6531(6) 2.6262(3) 2.6639(4) 2.5835(7)
–
–
3.1441(6) 2.9632(3) 3.1336(5) 2.7811(8)
3.2909(6) 3.3039(3) 3.2147(5)
–
AsI2 [17]) or by a p
-conjugated unsaturated fragment (as in CpxAsI2
[18] or p-HOOCC6H4AsI2 [17]), in all the cases the contacts being
smaller than the sum of corresponding rW values(Table 1).
The structures of 3ꢁMe2CO, 4 and 4ꢁ2THF consist of similar
dimeric units, [HPyAsI3]2 (Fig. 2). Both As atoms in the dimers
are related by an inversion center; they possess a square-pyrami-
dal I4C environment and lie very close to the mean plane of the
C(1)–As–I(1) 100.35(14) 93.74(13) 89.29(7)
92.52(9)
94.94(8)
88.52(9)
92.86(1)
90.09(1)
87.73(18)
97.54(16)
92.75(18)
95.47(2)
99.03(2)
C(1)–As–I(2) 96.29(14)
93.38(12) 95.44(8)
91.28(13) 89.84(7)
C(1)–As–I(3)
I(1)–As–I(2)
I(2)–As–I(3)
I(1)–As–I(3)
–
99.70(2)
–
–
94.33(2)
89.94(1)
94.08(1)
91.97(1)
173.21(2) 173.95(1) 176.77(1) 165.29(2)
a
Inversion equivalent.

120
I.Yu. Ilyin et al. / Polyhedron 67 (2014) 115–121
separated dimers and solvent molecules. This means the effect of
solvent inclusion overcomes the additional coordination to the
6th position of the As atom and dipole–dipole interactions
between the units, which indirectly confirms that the latter two
interactions are rather weak. The easy precipitation of the corre-
sponding phase of 4 from aqueous solution is probably facilitated
by its sufficiently hydrophobic behaviour, analogously to the
known diiodoorganoarsanes, RAsI2.
It should be noted that the preference of the cation [p-H3NC6H4-
AsI2]+ to crystallize with OTsꢀ rather than to coordinate the Iꢀ an-
ion goes in contradiction with the HSAB principle, so the formation
of the structure of 2 is likely determined by the aforementioned
anionic and cationic groups attraction. The Py substituted arsanes,
3 and 4, are probably not capable of building strong enough bond-
ing with the tosylate anion; on the other hand, coordination of
iodide and protonation of the N atom makes the packing of the
zwitterionic molecules more stable under the same conditions.
solubilities and appearance, while the molecular dimers in the
crystal structures appear to be of very similar geometry. The
reason for such a difference is, most likely, specific interactions be-
tween the molecules in the crystals, which complete the coordina-
tion number of the As atoms to 6 with neighboring I atoms, along
with dipole-dipole interactions between the antiparallel Py rings.
The slight differences in the Py rings orientation in the 3- and 4-
substituted isomers might lead to a weakening of the overall inter-
action and result in the inclusion of solvent molecules into the
crystal structure. This also means that the coordination of a fifth
iodine atom to the central As atom is energetically only slightly
favourable. Separate experiments with Iꢀ coordination to EtAsI2
showed that the coordination of one Iꢀ anion to the As atom occurs
easily, but even the formation of molecular dimers, i.e. the coordi-
nation of another additional I atom, is unfavorable in the crystal-
line state. The Py-substituted As species can provide further
synthetic potential based on the coordination of the Py group to
the metal centers, as well as additional hydrogen bonding via the
N–H proton, which was shown in a recent publication to be
responsible for the formation of coordination polymer and zwitter-
ionic compounds with bis(4-pyridyl)ditelluride [21]. The
implementation of such potential is now in progress in our group.
ꢀ
3.3. Synthesis and structure of EtAsI3
It was of interest to check if the triiodidoarsenates can form
without the influence of zwitterion packing in the structure. We
tried the method implemented earlier for chloro- and bromo- ana-
logues [3], with the reactions of EtAsI2 with Et4NI and Bun4NI in
acetonitrile. In both cases the addition of Iꢀ led to a sufficient dark-
ening of the solution, which indicated the formation of [EtAsI3]ꢀ.
Crystals of [Et4N][EtAsI3] (5) have been obtained, for which the
structure was determined by means of X-ray diffraction (Fig. 4).
Rather interesting, the obtained structure does not contain dimers
bound by bridging iodides, which is more typical for bromides
(commonly crystallized as [R2As2Br6]2ꢀ) as compared with chlo-
rides. The structure of 5 is molecular, with the anions surrounded
solely by the neighboring cations. Taking into account the lone pair
on the As atom, its coordination sphere can be described as an
uncompleted distorted trigonal bypyramid, with the As–I(1) and
As–I(3) bonds forming an angle of 165.3° (Table 1). The As atom
lies nearly in the plane of the I atoms (deviating by 0.053 Å towards
the C atom). The As–I(2) bond, which does not have a trans-conge-
ner, is approximately 0.1 Å shorter than the terminal As–I bonds in
3 and 4. Nearly the same As–I bond lengths are observed as for
compound 2.
Acknowledgements
The authors are grateful to the Russian Foundation for basic
research (Grant Nos. 12-03-31530, 12-03-31759, 13-01-01088)
and the Federal target program ‘‘Kadry’’ (Contract No. 8631) for
financial support.
Appendix A. Supplementary data
CCDC 935858–935862 contains the supplementary crystallo-
graphic data for 2, 3ꢁMe2CO, 4, 4ꢁ2THF and 5. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2013.08.070.
In the analogous reaction with the Bun4N+ cation, a crystalline
phase could not be obtained. As a result of almost full evapora-
tion of the reaction mixture, only colourless crystals covered
with a yellow oil could be seen, most likely the starting com-
pounds separately. It is noteworthy that the structures of the
binary iodoarsenates [Bun4N][As2I8] and [Prn4N][As2I8] are known
[19b], where two I atoms are placed trans with respect to the
As2I6 plane, making them very similar to the dimeric molecules
of 3 and 4. Nevertheless, in our attempts to make salts with
[Bun4N]I no crystals were formed, despite the fact that the struc-
ture of [Bun4N][As2I8] has the same cation, and the anion vol-
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