A re-investigation of arsenoacetic acid, (AsCH2COOH)n

Article abstract of DOI:10.1016/j.jorganchem.2013.07.056

Detailed spectroscopic data have been obtained for arsonoacetic acid, As(CH₂COOH)O₃H₂, and its barium and sodium salts. The X-ray crystal structure of the free acid is isomorphous with phosphonoacetic acid. Reduction gave the As(I) compound arsenoacetic acid, (AsCH₂COOH)_n which was shown by ESI-MS to contain cyclic species based on As-As bonds, with n mainly 3-6. The X-ray crystal structure of the hexamer was determined as the pyridine solvate and shown to have a hexacyclic As₆ ring in a puckered chair conformation, with -CH ₂COOH groups in equatorial sites, each H-bonded to a pyridine molecule in the lattice.

Full text of DOI:10.1016/j.jorganchem.2013.07.056

Journal of Organometallic Chemistry 745-746 (2013) 80e85
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A re-investigation of arsenoacetic acid, (AsCH2COOH)n
,
a,
Brian K. Nicholson a , Peter S. Wilson b, Adelle Nancekivell a
*
a Chemistry Department, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand
b Institute for Applied Ecology New Zealand, School of Applied Sciences, Auckland University of Technology, Private Bag 92006, Auckland 1142, New Zealand
a r t i c l e i n f o
a b s t r a c t
Article history:
Detailed spectroscopic data have been obtained for arsonoacetic acid, As(CH2COOH)O3H2, and its barium
and sodium salts. The X-ray crystal structure of the free acid is isomorphous with phosphonoacetic acid.
Reduction gave the As(I) compound arsenoacetic acid, (AsCH2COOH)n which was shown by ESI-MS to
contain cyclic species based on AseAs bonds, with n mainly 3e6. The X-ray crystal structure of the
hexamer was determined as the pyridine solvate and shown to have a hexacyclic As6 ring in a puckered
chair conformation, with eCH2COOH groups in equatorial sites, each H-bonded to a pyridine molecule in
the lattice.
Received 23 May 2013
Received in revised form
10 July 2013
Accepted 18 July 2013
Keywords:
Arsenic
Cyclopolyarsine
Ó 2013 Elsevier B.V. All rights reserved.
X-ray crystal structure
Electrospray ionisation mass spectrometry
1. Introduction
its chemistry, so we herein report the results of a re-investigation
using modern techniques.
Interest in the chemistry of organo-arsenic(I) chemistry can be
traced to the historically important drug Salvarsan (also known as
Arsphenamine) which Ehrlich introduced as a cure for syphilis in
1910 [1]. This compound, produced by reduction of 3-amino-4-
hydroxyarsonic acid, was originally assigned the structure 1a based
on analogies with aryleazo compounds. As]As double bonds,
however, are only isolable with extremely bulky substituents [2],
therefore polymeric or cyclic structures were subsequently pro-
posed [3]. In support of the cyclic forms, recent ESI-MS studies on
Salvarsan have shown that it consists of (RAs)n rings 2, n ¼ 3e8,
with n ¼ 5 and 6 being the most abundant [4]. For other derivatives,
structural studies have now defined cyclic species (RAs)n for n ¼ 4,
5, and 6 which all show puckered rings with AseAs distances of ca
2. Experimental
ESI-MS was carried out on a Bruker MicrOTOF instrument,
operating under standard conditions in negative ion mode, with
samples made up in H2O or 1:1 CH3CN:H2O (with the addition of
pyridine), immediately before infusion. Peaks are reported as the
m/z with the greatest intensity in the isotopic envelope. NMR
spectra were recorded on a Bruker Avance 400 machine in D2O or
pyridine-d5, and IR spectra were from a PerkineElmer Spectrum
100 run as KBr disks. Arsenoacetic acid was handled under an at-
mosphere of nitrogen using standard Schlenk techniques.
ꢀ
ꢀ
2.46 A and AseAseAs angles of around 90 [5].
2.1. Synthesis of barium arsonoacetate, Ba3[As(CH2COO)O3]2
A compound of this type that is still unresolved is arsenoacetic
acid, the product of reduction of arsonoacetic acid 3. This was first
reported in 1923 by Palmer [6] and was assigned the structure 1b.
Even now it is represented as such in the Merck Index [7] where it is
noted that it has had veterinary use as a tonic for horses. It has been
patented as a fire retardant [8a] and as a treatment for premen-
strual syndrome in women [8b] and has been used for treating
chronic fatigue syndrome in horses [8c]. The original structure 1b is
clearly unlikely but there have been no modern studies concerning
The barium salt of arsonoacetic acid was prepared using the
Meyer reaction [9], as adapted by Palmer [6]. Arsenic trioxide (10 g,
51 mmol) was added to a hot aqueous solution of NaOH (10 g,
0.4 mol in 30 mL H2O) and cooled to room temperature. Chloro-
acetic acid (4.8 g, 51 mmol) was added and stirred for 1 h. The clear
solution was acidified with glacial acetic acid (16 mL) and, after
cooling to 40 ꢀC, the precipitated excess arsenic trioxide was
filtered by suction and washed with water. The filtrate was poured
into a hot solution of BaCl2$2H2O (18.5 g, 76 mmol) in H2O (60 mL)
and stirred for 5 min. Barium arsonoacetate was filtered by suction
after the solution was allowed to stand overnight, and washed
thoroughly with water. Yield 30 g, 75%. IR: (cmꢁ1) 3403(vs, br)
* Corresponding author. Fax: þ64 7 838 4219.
E-mail address: b.nicholson@waikato.ac.nz (B.K. Nicholson).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.07.056

B.K. Nicholson et al. / Journal of Organometallic Chemistry 745-746 (2013) 80e85
81
2994(w) 2946(w) 2430(m) 1582(vs) 1421(s) 1390(s) 1328 (w)
1269(m) 1176(m) 1100(m) 942(m) 882(vs, br) 774(w) 708(s)
685(m) 626 (m) 556(w) 466(m) 419(m).
(<1%) calc for [(AsR)10 ꢁ H]ꢁ 1338.341; 1204.409 (1%) calc for
[(AsR)9 ꢁ H]ꢁ 1204.406; 1070.473 (<1%) calc for [(AsR)8 ꢁ H]ꢁ
1070.471; 952.528 (4%) calc for [(AsR)7O ꢁ H]ꢁ 952.531; 876.517
(<1%) calc for [(As7R6) ꢁ 2H]ꢁ 876.515; 802.602 (100%) calc for
[(AsR)6 ꢁ H]ꢁ 802.602; 742.581 (2%) calc for [(As6R5) ꢁ 2H]ꢁ
742.580; 668.667 (50%) calc for [(AsR)5 ꢁ H]ꢁ 668.667; 608.646
(8%) calc for [As5R4 ꢁ 2H]ꢁ 608.646; 534.732 (13%) calc for
[(AsR) ꢁ H]ꢁ 534.732; 474.711 (6%) calc for [As4R3 ꢁ 2H]ꢁ 474.711;
400.798 (12%) calc for [(AsR)3 ꢁ H]ꢁ 400.797.
2.2. Synthesis of sodium arsonoacetate Na3[As(CH2COO)O3]$H2O
Barium arsonoacetate (22 g, 28 mmol) was added to a hot so-
lution of anhydrous Na2SO4 (10.7 g, 75 mmol) in H2O (50 mL) and
stirred for 1 h at room temperature. Barium sulphate was filtered
and the filtrate concentrated using a rotary evaporator. The product
crystallised as the monohydrate (12.95 g, 85%) [6]. 1H NMR: [D2O]
2.5. X-ray crystal structure determinations
d
3.16 (s, CH2) ppm; 13C{1H} NMR: [D2O]
d 42.6 (s, CH2), 172.6 (s,
COOꢁ) ppm. IR (cmꢁ1) 3321(vs) 3245(s) 2934(w) 2742(m) 2399(m)
1712(m) 1601(vs) 1408(vs) 1376(vs) 1335 (w) 1255(m) 1191(m)
1118(s) 935(s) 891(vs) 834(s) 793(w) 735(s) 699(m) 620(s) 542(m)
434(m) 405(w).
Crystals were obtained by slow evaporation of an aqueous so-
lution for 3 and by cooling a saturated pyridine solution from room
temperature to 4 ꢀC for 4. Data were collected on a Bruker Apex II
CCD diffractometer with Mo-Ka X-rays. Corrections for absorption
were carried out using a multi-scan procedure. Structures were
solved using the SHELXS97 program [12] and refined on Fo2 with
SHELXL97 [12] operating under the WinGx interface [13]. Analysis
of hydrogen bonding was performed with Mercury [14]. Diagrams
were generated with ORTEP-III [15] and CrystalMaker [16]. Crystal
data and refinement details are summarized in Table 1.
2.3. Synthesis of arsonoacetic acid As(CH2COOH)O3H2
As described by Palmer [6], barium arsonoacetate (5.4 g,
7 mmol) was stirred in H2SO4 solution (1.2 mL conc. H2SO4, 24 mL
H2O) for 5 h and precipitated barium sulphate was filtered by
suction. Solvent was removed in vacuo over conc. H2SO4 until
crystallisation commenced. The free acid was extracted into abso-
lute ethanol (6 mL) and remaining inorganic material filtered. Pe-
troleum spirits (6 mL) was added to the filtrate, which was dried
over concentrated H2SO4 in vacuo until crystallisation commenced.
Although a little unusual, the method was followed as described.
The final product was washed with petroleum spirits (1.54 g, 60%).
Higher yields of better purity were obtained following a method
described by Rozovskaya et al. [10] and Sparkes and Dixon [11].
Barium arsonoacetate (1.3 g, 5.04 mmol) was stirred for
30 min with a sulfonated polystyrene resin (AmberliteR IR-120,
10 mL wet volume) in its Hþ form. The solution was filtered and
water removed using a rotary evaporator leaving a white powder,
pure arsonoacetic acid (0.50 g, 80%). Found: C, 13.27; H, 2.78%;
As(CH2COOH)O3H2 requires: C, 13.06; H, 2.74%. 1H NMR: [D2O]
3. Results and discussion
3.1. Syntheses and characterisation
The syntheses are summarised in Scheme 1. Preparation of
barium arsonoacetate was carried out using the Meyer reaction [9],
as modified by Palmer [6] wherein an alkaline solution of As2O3
was reacted with ClCH2COOH, the product then being precipitated
as the barium salt, Ba3[As(CH2COO)O3]2. This was readily converted
to the more soluble sodium salt Na3[As(CH2COO)O3] by stirring in
an aqueous solution of Na2SO4 and removing BaSO4 by filtration.
The barium salt was also the precursor for arsonoacetic acid 3.
The original method reported by Palmer in which Ba3[As(CH2COO)
O3]2 was reacted with H2SO4, followed by recrystallization
from EtOH gave reasonable yields of the free acid [6]. However,
an alternative route [10,11] involving stirring a suspension of
d
3.73 (s, CH2) ppm. 13C{1H} NMR: [D2O]
d 39.3 (s, CH2), 168.0 (s,
COOꢁ) ppm. IR: (cmꢁ1) 3500e2600(vs, br) 2999(w) 2936(w)
2404(m) 1882 (m) 1688(vs) 1449(w) 1389(w) 1306(m) 1239 (w)
1215 (w) 1139(m) 1115(w) 993(m) 891(vs, br) 853(m) 795(s) 776(s)
662(m) 423(s). ESI-MS: [H2O] m/z 550.804 (15%) calc for [3M ꢁ H]ꢁ
550.798; 532.793 (16%) calc for [3M ꢁ H2O ꢁ H]ꢁ 532.787; 366.867
Table 1
Crystal data and refinement details.
Compound
As(CH2COOH)O3H2
As6(CH2COOH)6$6C5NH5
(52%) calc for [2M
ꢁ
H]ꢁ 366.863; 348.856 (29%) calc for
[2M ꢁ H2O ꢁ H]ꢁ 348.852; 260.875 (9%) calc for [C2As2H7O5]ꢁ
260.873; 182.929 (100%) calc for [M ꢁ H]ꢁ 182.927; 138.940 (30%)
calc for [M ꢁ COOH]ꢁ 138.938, where M ¼ As(CH2COOH)O3H2.
Formula
Mr
C2H5AsO5
183.98
C42H48As6N6O12
1278.38
T (K)
121(2)
90(2)
Crystal system
Space group
Orthorhombic
P212121
6.0904(3)
7.7557(4)
10.4713(5)
90
90
90
494.62(4)
4
2.471
Triclinic
P-1
ꢀ
2.4. Synthesis of arsenoacetic acid (AsCH2COOH)n
a (A)
10.1456(1)
11.6233(1)
12.4298(2)
113.948(1)
92.378(1)
106.716(1)
1261.85(3)
1
ꢀ
b (A)
ꢀ
c (A)
Arsenoacetic acid was prepared based on the synthesis by
Palmer [6]. Sodium arsonoacetate (12.5 g, 50 mmol) was added to a
cold solution of conc. H2SO4 (22.5 mL), H3PO2 (47.6 mL, 50% in H2O)
and H2O (100 mL). It was left to stand at room temperature under
an N2 atmosphere for three days. The orange precipitate was
filtered under nitrogen, washed with water, and dried in vacuo over
conc. H2SO4. The filtrate was allowed to stand under N2 for another
two days, and a second crop was obtained. Combined fractions
were 4.70 g, 70%. Found: C,17.40; H, 2.26%; (AsCH2COOH)n requires:
a
b
g
(deg)
(deg)
(deg)
3
ꢀ
V (A )
Z
r
(g cmꢁ3
)
1.682
3.989
m
(mmꢁ1
)
6.80
Size (mm3)
F (000)
0.40 ꢂ 0.56 ꢂ 0.60
0.29 ꢂ 0.20 ꢂ 0.17
360
636
qmax (deg)
32.6
28.1
Refln collected
Tmax, min
Unique reflns (Rint
13,348
37,809
C, 17.93; H, 2.26%. 1H NMR: [Pyridine-d5]
d
3.59 (s, CH2) ppm; 13C
0.1718, 0.1057
1764(0.0548)
0.0190
0.5504, 0.3908
6128(0.0384)
0.0221
{1H} NMR: 26.9 (s, CH2), 174.1 (s, COOH) ppm. IR: (cmꢁ1) 3410(m,
d
)
br) 2944(m, br) 2638(m) 2558(w) 1681(vs) 1438(m) 1287(s)
1132(m) 1096(m) 948(m) 824(w) 766(w) 658(w) 491(m). ESI-MS:
[1:1 CH3CN:H2O] (with the addition of a small drop of pyridine)
m/z 1472.275 (<1%) calc for [(AsR)11 ꢁ H]ꢁ 1472.276; 1338.342
R1 [I > 2s(I)]
wR2 (all data)
0.0507
1.188
þ0.38/ꢁ1.20
0.0466
1.035
þ0.45/ꢁ0.30
GOF on F2
ꢀꢁ3
Final
D
e (e A
)

82
B.K. Nicholson et al. / Journal of Organometallic Chemistry 745-746 (2013) 80e85
R
R
As
As
As
R
R
As
AsR
n
As
R
1a R = 3-amino-4-hydroxyphenyl
1b R = CH2COOH
3-amino-4-hydroxyphenyl
n = 0 - 5
2
R =
O
R
R
HO
As
As
R
As
CH2COOH
As
As
R
As
R
As
HO
R
3
4 R = CH2COOH
__________________________________________________________________________________
Na3AsO3 + ClCH2COOH
[As(CH2COO)O3]3-
Ba2+
Ba3[As(CH2COO)O3]2
H2SO4
Na3[As(CH2COO)O3]
Na2SO4
H3PO2
[As(CH2COOH)O3H2]
(AsCH2COOH)n
(4)
(3)
Scheme 1.
Ba3[As(CH2COO)O3]2 with an ion-exchange resin in its acid form
was more convenient and gave better yields of pure As(CH2COOH)
O3H2, 3. This was fully characterised by elemental analysis and by
The compound is not very soluble in common solvents so NMR
spectra were recorded in pyridine-d5 to give signals at
3.59 (1H,
CH2) and
26.9 and 174.1 (13C, CH2 and COOH respectively).
d
d
NMR spectroscopy which showed singlet signals at
d
3.73 for CH2 in
Although the mass spectral studies (see below) show that the acid
contains a range of rings, (AsCH2COOH)n, there was no apparent
splitting of the NMR peaks, suggesting that the resonance fre-
quencies of the {AsCH2COOH} group are insensitive to its ring size.
The main region of the ESI-MS of arsenoacetic acid in MeCN:-
H2O:pyridine (1:1:0.1) is shown in Fig. 1, and peaks and full
assignment are listed in Section 2.4. The main peaks are unam-
biguously assigned as [(RAs)n ꢁ H]ꢁ for n ¼ 3e11, R ¼ CH2COOH,
which are undoubtedly derived from the corresponding cyclic
species. Relative intensities suggest that the abundances are in the
order n ¼ 6 > 5 > 4 > 3 with others only very minor. Other peaks
present were from a series [AsmRmꢁ1 ꢁ 2H]ꢁ for m ¼ 4e7. It is not
apparent what the structure of these minor arsenic-rich species
might be, but are possibly responsible for the orange colour of the
sample. One other significant peak that consistently showed is
[(AsR)7O ꢁ H]ꢁ which is presumably a cyclic compound with seven
As and one O making up the ring. While this is not unexpected, the
equivalent oxygenated rings are not seen for any other nuclearities.
Note that the smaller ring sizes do not arise from fragmentation
of larger ones in the mass spectrometer. Relative intensities of
the 1H and 39.3 (CH2) and 168.0 (COOH) in the 13C spectra. ESI-MS
d
of an aqueous solution gave the expected ions in negative ion
mode; [M ꢁ H]ꢁ was the dominant peak, with others arising from
aggregates ([2M ꢁ H]ꢁ and [3M ꢁ H]ꢁ) and from condensation
processes ([2M ꢁ H2O ꢁ H]ꢁ and [3M ꢁ H2O ꢁ H]ꢁ). One other
significant peak of interest corresponded to [M ꢁ CO2 ꢁ H]ꢁ, arising
from decarboxylation of the acid. This unusual fragmentation un-
der mild electrospray conditions can be understood from the pre-
sumed structure of the resulting ion, [H2O3As]CH2]ꢁ which is an
arsenic ylide so would have some stability. The crystal structure of 3
was determined by X-ray methods, see below.
The As(I) compound arsenoacetic acid was prepared by reduc-
tion of Na3[As(CH2COO)O3] with H3PO2 at room temperature under
a nitrogen atmosphere [6]. This gave an insoluble orange powder of
the crude product which appeared pure from elemental analysis
and spectroscopic data. However, the pure acid is pale yellow, so
the orange colour indicates some minor component which could
not be identified. The solid arsenoacetic acid is readily oxidised
and even when stored under nitrogen it slowly darkens in colour.

B.K. Nicholson et al. / Journal of Organometallic Chemistry 745-746 (2013) 80e85
83
Fig. 1. An ESI-MS (ꢁve ion mode) of arsenoacetic acid, (AsCH2COOH)n in MeCN:H2O:pyridine (1:1:0.1) in the m/z 350e850 range. The peaks assigned to [(RAs)n ꢁ H]ꢁ for n ¼ 3e6
are indicated. The peaks at m/z 608.646 and 474.711 match [(Rmꢁ1Asm) ꢁ 2H]ꢁ for m ¼ 5 and 4 respectively but are of unknown structure and probably arise from impurities.
peaks remain constant under a range of conditions for any given
sample, and when fragmentation does commence under very high
exit-voltage conditions, the processes do not generate different ring
sizes. Rather extrusion of {R2As} and loss of {CO2} occurs amongst
other complicated pathways. Interconversion of ring sizes was also
shown not to occur in MSeMS studies of Salvarsan rings in our
earlier study [4].
These ESI-MS results clearly show that, as with most other
organo-arsenic(I) compounds [3e5], arsenoacetic acid exists as a
mixture of Asn rings, with the hexamer and pentamer the most
abundant. There can no longer be any justification for formulating
arsenoacetic acid as the diarsene 1b.
arsonoacetic acid. The CeOeH coupled stretch at 1287 cmꢁ1 and
the CeOeH deformation at 948 cmꢁ1 are relatively more intense
than for the salts. There are no intense peaks between 700 and
900 cmꢁ1, consistent with the absence of As]O and AseO groups.
The AseC stretch at 491 cmꢁ1 is at higher frequency than observed
for the arsonoacetic and for the salts.
In an attempt to purify the arsenoacetic acid, a concentrated
solution in pyridine was cooled and produced some pale-yellow
crystals suitable for X-ray crystal structure analysis. As discussed
below, these proved to be of the hexamer, which fractionally
crystallised from the mixture as the pyridine solvate. Attempts to
grow crystals of the other ring sizes were not successful.
The main features in IR spectra of all the compounds examined
could be readily assigned, based on the literature [17]. The barium
and sodium salts of arsonoacetic acid were similar, as expected.
Broad OeH stretches around 3300e3400 cmꢁ1 are associated with
lattice water and possibly with incompletely ionised acid groups. A
distinctive band for each at 2400 cmꢁ1 can be ascribed to either
overtones [18] or more likely to eOH stretching under strong H-
bonding conditions [19]. The carboxylate C]O stretch is lower for
the Ba2þ salt (1582 cmꢁ1) than for the Naþ example (1601 cmꢁ1).
Other major peaks are around 1400 cmꢁ1 (CH2 bending),1260 cmꢁ1
(CeOeH coupled stretch), 1100 cmꢁ1 (CeC stretch) and 950 cmꢁ1
(OeH deformation). The AseO peaks will be amongst the strong
peaks in the 700e890 cmꢁ1 region, which is more clearly resolved
for the Naþ salt. The AseC stretch gives rise to the peaks at
434 cmꢁ1 (Naþ) and 419 cmꢁ1 (Ba2þ).
3.2. X-ray crystal structures of 3 and 4
The structure of arsonoacetic acid is illustrated in Fig. 2, with
selected bond parameters listed in the caption. The compound is
isomorphous with the corresponding phosphonoacetic acid [20]
and the structures differ only in the longer AseO, As]O and Ase
C for 3 as a consequence of the larger size of As compared with P.
The arsenic atom is close to tetrahedral, linked to three O atoms and
to the acetic acid moiety by an AseC bond. The H atoms on O(1) and
O(2) were located in the refinement, leaving O(3) as the formally
doubly-bonded one. However, all the AseO distances are very
similar because there is extensive H-bonding between molecules.
As]O(3) acts as a double H-acceptor, from a carboxylate OeH(4) of
one molecule and from an AseOeH(1) of another. The carboxylate
C]O acts as an acceptor towards the AseOeH(2) of an adjacent
molecule. The phosphonoacetic acid shows the same lattice in-
teractions [20], though in this case the P]O bond remains signif-
icantly shorter than the PeOH ones.
The structure of the arsenoacetic acid hexamer 4 as the pyridine
solvate is shown in Figs. 3 and 4. Molecules lie on a crystallographic
inversion centre and consist of a puckered, cyclic As6 ring with a chair
conformation. Each As carries a eCH2COOH group in an equatorial
position. Structurally characterised (RAs)6 molecules previously re-
ported are those with R ¼ Ph, p-tolyl and p-MeOC6H4 [5aec] so the
presentexampleisthefirsthexamerwithanon-aromaticsubstituent.
For the arsonoacetic acid 3 the OeH stretching region from 3500
to 2300 is extremely broad, as expected given the extensive strong
H-bonding network found in the crystal structure determination.
The C]O stretch at 1688 cmꢁ1 is at higher frequency compared
with the equivalent in the Ba2þ and Naþ salts, indicating a non-
ionised carboxylic acid group. The strong AseO peaks are around
890 and 785 cmꢁ1, and the AseC stretch at 423 cmꢁ1
For the arsenoacetic acid, the IR spectrum is much simpler. The
OeH stretching region is again very broad, from 3400 to 2600 cmꢁ1
.
,
consistent with the extensive H-bonding that would be expected
for the solid. The C]O stretch at 1681 cmꢁ1 is similar to that for the

84
B.K. Nicholson et al. / Journal of Organometallic Chemistry 745-746 (2013) 80e85
Fig. 2. The molecular structure of arsonoacetic acid, 3. Selected bond parameters are
ꢀ
bond lengths (A): As(1)eO(1) 1.7013(13), As(1)eO(2) 1.6461(12), As(1)eO(3)
1.7072(13), As(1)eC(1) 1.9185(17); bond angles (deg): OeAseO (av.) 107.0(1)ꢀ, OeAseC
(av.) 111.9(1)ꢀ.
Other (RAs)n rings with a sp3-C substituent for which structures
are known are either pentamers (R ¼ CH3, CH2SiMe3) or tetramers
(R ¼ tBu, CF3) [5]. For 4 the AseAs bond lengths average 2.4594(3)
which is essentially the same as in other published examples. How-
ever, the ring in 4 is more puckered than that in (PhAs)6, with AseAse
As angles averaging 88.61(1)ꢀ for 4 and 91.0ꢀ for the aryl example. The
CeAseAs bonds are 98.88(5)ꢀ, so the As atoms are by no means
tetrahedral, with the lone pair having a strong steric effect.
Fig. 4. A representation of the structure of (AsCH2COOH)6$6C5H5N, showing the H-
bonding interactions between the pyridine molecules and the carboxyl groups.
ꢀ
Average distances (A) are: OeH 0.85(3), H/N 1.77(3), O/N 2.624(2).
The acetic acid moieties are arranged so they alternate above
and below the As6 ring, giving idealised C3v symmetry. This allows
them to form H-bonds to six pyridine molecules trigonally arranged
above and below, as shown in Fig. 4. The carboxyl H atoms were
located in the X-ray determination so the structure is better
described as COOH groups H-bonding to pyridine, rather than
COOꢁ groups acting as H-acceptors from pyridinium cations, but
the H/N interactions are strong as indicated by distances that are
towards the shorter end of the range found for other systems [21].
4. Conclusion
Arsonoacetic acid has the same crystal structure as the phos-
phorus equivalent, phosphonoacetic acid. On reduction this gives
the As(I) species arsenoacetic acid which is a mixture of (AsCH2
-
COOH)n rings with the n ¼ 6 and 5 the main species. The hexamer
contains an As6 ring which is strongly puckered in a chair confor-
mation, with the substituents in equatorial sites.
Acknowledgements
We thank Dr Jan Wikaira, University of Canterbury, and Dr Tania
Groutso, University of Auckland, for collection of X-ray intensity
data sets.
Appendix A. Supplementary material
Fig. 3. The structure of a molecule of the arsenoacetic acid hexamer. Selected bond
CCDC 939836 and 939837 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
ꢀ
parameters are bond lengths (A): As(1)eAs(2) 2.4566(3), As(2)eAs(3) 2.4589(3),
As(1)eAs(3)0 2.4628(3), AseC (av.) 1.999(2); bond angles (deg): As(1)eAs(2)eAs(3)
89.228(9), As(2)eAs(3)eAs(1)0 87.856(9), As(2)eAs(1)eAs(3)0 88.753(8), CeAseAs (av.)
98.88(5).

B.K. Nicholson et al. / Journal of Organometallic Chemistry 745-746 (2013) 80e85
85
Chem. 623 (1997) 75;
(d) (Me3SiCH2As)5: R.L. Wells, C.Y. Kwag, A.P. Purdy, A.T. McPhail, C.G. Pitt
Polyhedron 9 (1990) 319;
(e) (CH3As)5: J.H. Burns, J. Waser J. Am. Chem. Soc. 79 (1957) 859;
(f) (ButAs)4: O. Mundt, G. Becker, H.J. Wessely, H.J. Breunig, M. Kischkel
Z. Anorg. Allg. Chem. 486 (1982) 70;
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.07.056.
(g) (CF3As)4: N. Mandel, J. Donohue Acta Crystallogr. D27 (1971) 476;
(h) (MesitylAs)4: A.J. Roening, J.J. Davidson, S.N. MacMillan, J.M. Tanski,
R. Waterman Dalton Trans. (2008) 4488.
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