Isopolyoxometalates of antimony: Arylstibonic acids [H8(RSb) 12O28] and derived dodecanuclear polyoxostibonates [M 2H10- x(RSb)12O30]x-, M = Na or K

Article abstract of DOI:10.1021/om1008692

Electrospray ionization mass spectrometry shows arylstibonic acids, p-XC₆H₄SbO₃H₂ (X = methyl, nitro, or chloro), and 1-naphthylstibonic acid exist in MeCN as dodecanuclear aggregates [H₈(RSb)₁₂O₂₈]. With KOH or NaOH these form polyoxometalate ions [M₂H₈(RSb)₁₂O ₃₀]^2- (M = K or Na) with a hexagonal antiprismatic array of the Sb atoms, as shown by X-ray crystallographic structures for three new examples. There is one firmly encapsulated 10-coordinate M⁺ ion within an inorganic 12-crown-6 moiety {Sb₆O₆} in the hexagonal channel and one six-coordinate M⁺.

Full text of DOI:10.1021/om1008692

6518 Organometallics 2010, 29, 6518–6526
DOI: 10.1021/om1008692
Isopolyoxometalates of Antimony: Arylstibonic Acids
[H8(RSb)12O28] and Derived Dodecanuclear Polyoxostibonates
[M2H10-x(RSb)12O30]x-, M = Na or K
Brian K. Nicholson,*,† Christopher J. Clark,‡ Cody E. Wright,† and Tania Groutso§
†Chemistry Department, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand,
‡Bioengineering Technologies, Plant and Food Research, Ruakura Research Centre, Private Bag 3123,
Hamilton 3240, New Zealand, and §Chemistry Department, University of Auckland, PO Box 92019,
Auckland, New Zealand
Received September 7, 2010
Electrospray ionization mass spectrometry shows arylstibonic acids, p-XC6H4SbO3H2 (X =
methyl, nitro, or chloro), and 1-naphthylstibonic acid exist in MeCN as dodecanuclear aggregates
[H8(RSb)12O28]. With KOH or NaOH these form polyoxometalate ions [M2H8(RSb)12O30]2- (M=
K or Na) with a hexagonal antiprismatic array of the Sb atoms, as shown by X-ray crystallographic
structures for three new examples. There is one firmly encapsulated 10-coordinate Mþ ion within an
inorganic 12-crown-6 moiety {Sb6O6} in the hexagonal channel and one six-coordinate Mþ.
Introduction
stibonic acids and now report that under appropriate con-
ditions these acids form polyoxostibonate cages with intrigu-
ing structures. Herein we describe the mass spectrometric
behavior of the acids RSbO3H2 [R=p-ClC6H4, p-MeC6H4,
p-O2NC6H4, and 1-naphthyl] and present the characteriza-
tion and structural determinations of Naþ or Kþ salts of
high-nuclearity polyoxyanions formed by them.7 Prelimi-
nary details of the present work have been communicated
earlier.8
Related examples of organopolyoxostibonates, or mixed
oxometalates incorporating organo-antimony, have been
reported only recently, prepared under different conditions
from those used and described herein.9,10
Arylstibonic acids, of nominal formula RSbO3H2, have
been known for over 100 years.1 However, in contrast to the
simple molecular arylarsonic acids, their precise form is
unknown since they invariably exist as amorphous powders
with limited solubility in aqueous solution, except when it is
alkaline. Their titration behavior is complicated and vari-
able.2 It is generally concluded that they are polymeric
species involving five- or six-coordinate Sb with μ-O link-
ages,3 with early Mossbauer studies favoring trigonal-bipyr-
amidal geometry.4 A recent crystal structure determination5
of a sterically hindered compound, 2,6-Mes2C6H3SbO3H2,
showed a molecular dimer with a central Sb2O2 core and five-
coordinate Sb, but the bulky R groups in this instance
prevent higher aggregation; thus the result cannot be used
as a specific basis for deducing the arrangement in stibonic
acids with smaller aryl groups.
Experimental Section
Preparation of the arylstibonic acids was carried out essen-
tially using the procedure established by Doak and Steinman.11
For convenience a typical synthesis is included below. No
melting point data are given, as these substances do not have
well-defined melting or decomposition temperatures.11 For the
preparation of the derivatives of the acids the crude yield
(contaminated with other salts) will be essentially quantitative.
However the crystalline material from the slow evaporation of
ESI-MS is a powerful method for determining species in
solution,6 so we decided to investigate the oligomerization of
*To whom correspondence should be addressed. E-mail: b.nicholson@
waikato.ac.nz.
(1) (a) Doak, G. O.; Freedman, L. D. Organometallic Compounds of
Arsenic, Antimony and Bismuth; Wiley: New York, 1970. (b) Dunning, F.;
Reid, E. E. J. Am. Chem. Soc. 1926, 48, 2959. (c) Riddell, W.; Basterfield, S.
Trans. R. Soc. Can., Ser. 3 1929, 23, 45.
(2) (a) Schmidt, H. Justus Liebigs Ann. Chem. 1920, 421, 174. (b) Schmidt,
H. Ber. 1922, 55B, 697.
(3) Doak, G. O. J. Am. Chem. Soc. 1946, 68, 1991.
(7) Early attempts at characterizing salts of aryl stibonic acids are
described in: Fargher, R. G.; Gray, W. H. J. Pharmacol. 1921, 18, 341.
See also: Gray, W. H.; Lamb, I. D. J. Chem. Soc. 1938, 401.
(8) Clark, C. J.; Nicholson, B. K.; Wright, C. E. Chem. Commun.
2009, 923.
(4) Bowen, L. H.; Long, G. G. Inorg. Chem. 1978, 17, 551.
(5) Beckman, J.; Finke, P.; Hesse, M.; Wettig, B. Angew. Chem., Int.
Ed. 2008, 47, 9982.
(6) (a) Henderson, W.; McIndoe, J. S. Mass Spectrometry of Inor-
ganic, Coordination and Organometallic Compounds; J. Wiley and Sons:
New York, 2005. (b) Henderson, W.; Nicholson, B. K.; McCaffrey, L. J.
Polyhedron 1998, 17, 4291. (c) Long, D.-L.; Streb, C.; Song, Y.-F.; Mitchell,
S.; Cronin, L. J. Am. Chem. Soc. 2008, 130, 1830. (d) Alyea, E. C.; Craig,
D.; Dance, I.; Fisher, K.; Willett, G.; Scudder, M. CrystEngComm 2005, 7,
491. (e) Miras, H. N.; Wilson, E. F.; Cronin, L. Chem. Commun. 2009, 1297.
(9) (a) Gouzerh, P.; Proust, A. Chem. Rev. 1998, 98, 77. (b) Beckmann,
J.; Heek, T.; Takahashi, M. Organometallics 2007, 26, 3633. (c) Beckmann,
J.; Hesse, M. Organometallics 2009, 28, 2345. (d) Chandrasekhar, V.;
Thirumoorthi, R. Organometallics 2009, 28, 2637. (e) Ali, S.; Baskar, V.;
Muryn, C. A.; Winpenny, R. E. P. Chem. Commun. 2008, 6375.
(10) (a) Jami, A. K.; Prabhu, S. R.; Baskar, V. Organometallics 2010,
29, 1137. (b) Prabhu, M. S. R.; Jami, A. K.; Baskar, V. Organometallics
2009, 28, 3953. (c) Piedra-Garza, L. F.; Dickman, M. H.; Moldovan, O.;
Breunig, H. J. Inorg. Chem. 2009, 48, 411.
(11) Doak, G. O.; Steinman, H. G. J. Am. Chem. Soc. 1946, 68, 1987.
r
pubs.acs.org/Organometallics
Published on Web 11/08/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 23, 2010 6519
MeCN is variable, depending on how long the solution is left to
concentrate, how much noncrystalline compound precipitates
out, etc. Crystals are initially highly solvated with lattice water,
some of which at least is lost as soon as the crystals are collected.
This makes a formal yield somewhat difficult to define. ESI-MS
was carried out on a Bruker MicrOTOF instrument, operating
under standard conditions in negative ion mode, with samples
made up in MeCN immediately before infusion. Assignment of
ions was aided by matching the characteristic patterns generated
by the 121Sb (57%) and 123Sb (42%) isotopes. Peaks are reported
as the m/z with the greatest intensity in the isotopic envelope.
Preparation and Characterization of p-ClC6H4SbO3H2.
4-Chloroaniline (6.38 g, 0.05 mol) was dissolved in ethanol
(125 mL) in a 500 mL wide-mouth conical flask, surrounded
by an ice-bath, with an efficient stirrer. Concentrated sulfuric
acid (2.7 mL, 5 g) and SbCl3 (11.4 g, 0.05 mol) were added. Once
the latter had completely dissolved, a solution of sodium nitrite
(3.5 g in 5 mL of water) was added to initiate diazotization. This
thick mixture was stirred for 30 min. Cuprous bromide (1 g) was
added and the ice-bath removed. As the mixture warmed,
nitrogen evolved spontaneously. Stirring was continued for 24 h
to ensure complete nitrogen evolution. Steam distillation was
used to remove the alcohol. The residue of crude stibonic acid
was collected on a Buchner funnel, washed with water, and
air-dried.
The crude acid was dissolved in concentrated hydrochloric
acid (ca. 1 L) and filtered, and a solution of pyridine (5 mL) in
concentrated HCl (20 mL) added. The precipitated pyridinium
salt, [pyH][ClC6H4SbCl5], was collected on a sintered glass filter,
washed several times with concentrated hydrochloric acid, and
left to dry. It was dissolved in the minimum volume of dilute
sodium carbonate solution (ca. 2 L of a 1% (w/v) solution) and
filtered. The free acid was obtained by the dropwise addition of
dilute hydrochloric acid while stirring rapidly. The precipitate
was collected by filtration and washed thoroughly with water
acidified with a few drops of dilute hydrochloric acid. After
air drying the yield was 12.3 g, 87%, assuming a formula of
p-ClC6H4SbO3H2.
p-O2NC6H4SbO3H2. ESI-MS showed that Naþ was strongly
incorporated even after two diffusion purifications. Anal. Found:
C 26.11; H 2.76; N 4.76. Calcd for C6H6NO5Sb: C 24.52;
H 2.06; N 4.76; Calcd for C72H56N12O52Sb12: C 25.56; H 1.67;
N 4.96; Calcd for NaC72H55N12O52Sb12: C 25.4; H 1.63; N 4.94.
ESI-MS (m/z, assignment, (calc), intensity, R = p-O2NC6H4):
1700.535 [NaH5(RSb)12O28]2-, (1700.514), 100%; 1146.688,
[Na2H5(RSb)12O29]3-, (1146.671), 100%; 1139.359, [NaH6(RSb)12-
O29]3-, (1139.344), 80%. IR (KBr disk, cm-1): 3400 (s, br), 3190 (s,
br), 1630 (m), 1597 (m), 1576 (m), 1516 (s), 1477 (w), 1390 (m), 1355
(s),1315(w) 1279(w),1105(m),1068(m),1014(m),937(w),854(s),
738 (s), 710 (s), 684 (s), 600 (w), 533 (w), 466 (m).
Preparation and Characterization of (1-naphthyl)SbO3H2.
This was prepared following the same procedure from R-naphthyl-
amine (7.16 g, 0.05 mol) in ethanol (200 mL). The crude acid was
dissolved in 1:1 MeOH/conc HCl (1.5 L) to form the pentachlor-
ostibonate salt. The precipitation after dissolving in Na2CO3 solu-
tion was with dilute nitric acid. The product, (1-naphthyl)SbO3H2,
was obtained as an off-white solid (20%), which ESI-MS showed
contained very little Naþ, so it was not purified further. Anal.
Found: C 38.8; H 2.55. Calcd for C10H9O3Sb: C 40.2; H 3.03%;
Calcd for C120H92Sb12O28: C 41.9; H 2.7%. ESI-MS (m/z, assign-
ment, (calc), intensity, R=naphthyl): 3442.413, [H7(RSb)12O28]-,
(3442.422), 100%. IR (KBr disk, cm-1): 3401 (s, br), 3052 (s),
1656 (m), 1623 (w), 1590 (m), 1558 (w), 1504 (s), 1384 (s), 1335 (m),
1300 (w), 1263 (m), 1212 (w), 1166 (w), 1136 (w), 1058 (w), 1023 (m),
953 (w), 796 (s), 768 (s), 745 (m), 680 (m), 618 (w), 514 (w), 468 (m),
407 (w).
Preparation of Crystals of K4[H8(p-ClC6H4Sb)12O30] 58H2O.
3
p-Chlorophenylstibonic acid (244 mg, 0.86 mmol) was dissolved
in water (50 mL) containing KOH (0.6 mL of 2 mol L-1, pH=
12.0). Potassium nitrate (50 mg, 0.5 mmol) was dissolved in
25 mL of the alkaline stibonate solution (giving overall 0.43 mmol
of Sb, 1.1 mmol Kþ), and the clear solution was left to evaporate to
dryness. The white residue crystallized from a MeCN (10 mL)/H2O
(0.2 mL) solution by slow evaporation, forming colorless block
crystals. IR (KBr disk, cm-1): 3370 (s, br), 1631 (s, br), 1572 (m),
1477 (s), 1384 (s), 1183 (w), 1090 (s), 1066 (s), 1013 (s), 821 (s), 726
(s), 654 (s), 603 (s), 491 (s), 457 (w).
The product was further purified by dissolving a sample (2 g)
in a mixture of water and concentrated NH3(aq) (2:1, ca. 300 mL)
in a plastic beaker. The open beaker was placed in a closed
desiccator containing glacial acetic acid. The acid diffused into
the solution over several days, precipitating the stibonic acid as
an off-white powder, which was collected by gravity filtration.
Recovery was 80-90%. Anal. Found: C 26.93; H 2.46. Calcd for
C6H6ClO3Sb: C 25.44; H 2.14; Calcd for C72H56Cl12O28Sb12:
C 26.56; H 1.73. ESI-MS (m/z, assignment, (calc), intensity, R=
p-ClC6H4): 3275.719, [NaH6(RSb)12O28]-, (3275.738), 18%;
3253.744, [H7(RSb)12O28]-, (3253.756), 100%; 2157.189 [H6-
Preparation of Crystals of Rb0.67Na2.33[H9(p-MeC6H4Sb)12-
O30] 20H2O. p-Tolylstibonic acid (221 mg, 0.84 mmol) was
3
dissolved in water containing NaOH (0.6 mL of 2 mol L-1
,
pH 11.2). A 10 mL aliquot of this solution was combined with
RbI (43 mg, 0.2 mmol) in water (1 mL) (ratio of Sb:Na:Rb
17:24:20), and the clear solution was left to evaporate to dryness.
The resulting white solid was crystallized from MeCN/H2O
solution (4:1 v/v, 5 mL) by slow evaporation to give clear,
truncated square-pyramidal crystals after three weeks. IR
(KBr disk, cm-1): 3414 (s, br), 3015 (w), 2920 (w), 1638 (m),
1593 (m), 1492 (s), 1472(w), 1455 (m), 1392 (m), 1308 (w), 1210
(w), 1186 (m), 1074 (s), 1018 (w), 974 (w), 891 (w), 803 (s), 726
(m), 658 (s), 605 (m), 581 (m), 488 (s), 450 (m).
(RSb)16O36]2-, (2157.162), 9%; 1648.353, [Na2H4(RSb)12O28]2-
,
(1648.356), 4%; 1637.361, [NaH5(RSb)12O28]2-, (1637.365), 18%;
1626.366 [H6(RSb)12O28]2-, (1626.375), 18%. IR (KBr disk,
cm-1): 3400 (s, br), 3190 (s), 1634 (m), 1573 (m), 1477 (s), 1383
(s), 1182 (w), 1090 (s), 1067 (s), 1013 (s), 947 (w), 814 (s), 728 (s),
663 (s), 602 (w), 489 (s).
Preparation of Crystals of [Ph4P][Na2H9(p-MeC6H4Sb)12-
O30] xH2O. p-Tolylstibonic acid (221 mg, 0.84 mmol) was dis-
3
solved in water (50 mL) containing NaOH (0.6 mL of 2 mol L-1
,
Preparation and Characterization of p-MeC6H4SbO3H2. Sim-
ilarly, from p-toluidine a white powder was obtained in 79%
initial yield. Anal. Found: C 33.35; H 3.49. Calcd for C7H9O3Sb:
C 31.98; H 3.45; Calcd for C84H92O28Sb12: C 33.51; H 3.08. ESI-MS
(m/z, assignment, (calc), intensity, R = p-MeC6H4): 3008.434,
[H7(RSb)12O28]-, (3008.419), 100%; 1993.619 [H6(RSb)16O36]2-
(1993.604), 3%; 1503.719 [H6(RSb)12O28]2-, (1503.706), 18%. IR
(KBr disk, cm-1): 3401 (s, br), 3200 (s), 1635 (m), 1593 (m), 1493
(m), 1447 (w), 1395 (s), 1310 (w), 1280 (w), 1210 (w), 1187 (m), 1073
(m), 1018 (w), 802 (s), 743 (s), 667 (s), 600 (w), 486 (s), 460 (w).
Preparation and Characterization of p-O2NC6H4SbO3H2.
This was prepared from p-nitroaniline using the same method,
except that the crude acid was dissolved in a 1:1 mixture of
MeOH and concentrated HCl to form the pyridinium salt. The
product was an off-white powder, crude yield 92% calculated as
pH 11.2). A 10 mL aliquot of this solution was combined with a
solution of [Ph4P]Br (84 mg, 0.2 mmol) in water (5 mL), giving a
fine white suspension, which was left to evaporate to dryness.
The resulting powder was crystallized from MeCN (4 mL)/H2O
(0.2 mL) by slow evaporation to give colorless crystals within
four weeks.
ESI-MS of the crystals redissolved in MeCN: m/z 1525.689,
calc for [H4Na2(p-MeC6H4Sb)12O28]2- 1525.688; m/z 3390.494,
calc for [H4Na2 (p-MeC6H4Sb)12O28þPh4P]- 3390.507.
X-ray Crystal Structure Determinations. Crystals were rapidly
transferred from the mother liquor onto the diffractometer and
cooled immediately to ca. 90 K, since they invariably lost crys-
tallinity on exposure to air, presumably due to loss of lattice
solvent (water and/or MeCN). Data were collected on a Bruker
Apex II CCD diffractometer and processed routinely, including

6520 Organometallics, Vol. 29, No. 23, 2010
Nicholson et al.
absorption corrections using a multiscan method (SADABS).12
Because of the relatively poor diffraction, the size of the mole-
cules, and the presence of extensive lattice water molecules, the
refinements were not routine. Each was treated differently, as
presented below. Structures were solved using the SHELXS97
program13 and refinement on Fo2 was with SHELXL9713 oper-
ating under the WinGx interface.14
others clearly having partial occupancy or loosely held posi-
tions. Although some of them refined with unrealistic tempera-
ture factors, this appeared to model the included solvent
reasonably well since PLATON15 indicated only small voids left
in the unit cell. All non-H atoms were treated anisotropically, but
H atoms were not included given the poor quality of the data.
Crystal and refinement data: C84H124Na2.66O50Rb0.67Sb12, Mr
˚
Solution and Refinement for K4[H8(p-ClC6H4Sb)12O30
]
3498, monoclinic, space group P21/n, a = 17.7337(5) A, b =
˚
3
˚
58H2O. The structure was solved by direct methods to reveal
the 24 Sb atoms in the asymmetric unit. Subsequent difference
maps revealed the remainder of the complex anions and the Kþ
cations. Further analysis gave the positions of 77 O atoms from
the lattice water molecules, and these refined cleanly with iso-
tropic temperature factors. Another 39 O atoms were located in
sensible positions, but were less well defined; thus these were
refined with a common, isotropic temperature factor. All the
included water molecules formed a reasonable H-bonded net-
24.9225(8) A, c=27.4111(9) A, β=98.190(2)°, U=11991.3(6)
A , Z=4, Dc=1.938 g cm-3, μ(Mo KR)=2.95 mm-1, F(000)=
3
˚
6756. T=89(2) K. Crystal size 0.22 ꢀ 0.12 ꢀ 0.12 mm3. Total
data 125 562, unique data 21 102 (Rint 0.098), observed (I >
2σ(I)) data 11 852, θ range=2-25°. Refinement converged with
R1=0.0970 (I > 2σ(I)), wR2=0.2878 (all data), GoF=1.082,
-3
˚
Δe=3.4/-2.1 e A
Solution and Refinement for [Ph4P][Na2H9(p-MeC6H4Sb)12-
.
O30] xH2O. Unfortunately the crystals diffracted weakly, giv-
3
ing data with an average I/σ(I) of only 2.2. The structure was
solved by direct methods (SHELXS97) to give the positions of
the 12 Sb atoms. A subsequent difference map showed all the
anion O atoms and the two Naþ cations. With further cycles the
tolyl rings and the Ph4Pþ cation were located, but were not well
defined. The phenyl carbon atoms of the cation were constrained
as rigid hexagons using the AFIX 66 option of SHELXL97, while
the tolyl rings were constrained using the SAME, SIMU, and
DELU options.
work with sensible O O distances. There are probably only a
few extra water molecules unaccounted for because of the
3 3 3
3
relatively small (ca. 600 A , 4%) remaining void volume in the
˚
cell. The aryl-ring H atoms were included in calculated posi-
tions, but the H atoms of the water molecules and the eight that
must be associated with each of the anions for charge neutrality
were not included.
Crystal and refinement data: C72H144Cl12K4O88Sb12, Mr 4446.6,
˚
triclinic, space group P1, a=20.3732(6) A, b=23.0650(7) A, c=
˚
The four H2O molecules coordinated to Naþ cations were
well defined, as was one lattice water that links two anions
through H bonding. However the rest of the lattice solvent
(presumably H2O though it may also include MeCN) was very
diffuse and could not be modeled sensibly.
Without the solvent included the refinement converged with
R1=0.166, wR2=0.435. To remove the diffuse solvent contribu-
tion, the SQUEEZE16 routine of PLATON was invoked. This
allowed refinement to give R1=0.125 and wR2=0.346.
While the results were sufficient to demonstrate that the anioþn
has the same structure as that found for the PhCH2NMe3
example in our earlier report,8 and the Rbþ example above, the
quality of the determination precludes any detailed analysis of
bond parameters.
˚
34.213(1) A, R= 87.386(2)°, β= 86.265(2)°, γ= 83.649(2)°; U=
3
15932.5(8) A , Z=4, Dc=1.854 g cm-3, μ(Mo KR)=2.4 mm-1
,
˚
F(000)=8632. T=89(2) K. Crystal size: 0.44 ꢀ 0.28 ꢀ 0.20 mm3.
Total data 359 194, unique data 72 182 (Rint 0.070), θrange 1-27.5°.
Refinement converged with R1=0.0918 (I > 2σ(I)), wR2=0.2430
-3
˚
(all data), GoF=1.085, Δe=3.1/-4.9 e A
.
Solution and Refinement for Rb0.67Na2.33[H9(p-MeC6H4Sb)12-
O30] 20H2O. The crystals diffracted weakly, giving data with an
3
average I/σ(I) of only 3.6. The structure was solved by direct
methods to give the positions of the 12 Sb atoms. A subsequent
difference map showed all the anion O atoms and the three
cations. These were refined as mixed Naþ/Rbþ with tied occu-
pancy factors summing to 1.0 and with temperature factors
constrained to be the same. Ultimate refinement showed only
Naþ/Rbþ(3) had significant (50%) contribution from Rbþ, the
other two cation sites being solely Naþ. With further cycles the
tolyl rings were located, but some were clearly disordered. The
aryl rings were constrained as rigid hexagons using AFIX 66,
with temperature factors constrained by the SIMU and DELU
options of SHELXL97.13
Crystal and refinement data: C108H123Na2O35PSb12, Mr=3519.01,
˚
orthorhombic, space group Pcab, a=28.5319(7) A, b=29.5235(7)
3
-3
˚
˚
˚
A, c=38.2815(10) A, U=32246.9(14) A , Z=8, Dc=1.450 g cm
,
μ(Mo KR)=2.05 mm-1, F(000)=13 600. T=90(2) K. Crystal size
0.44 ꢀ 0.29 ꢀ 0.13 mm3. Total data 27 4 394, unique data 25 318
(Rint 0.133), observed (I > 2σ(I)) data 14 781, θ range = 2-24°.
(Data are calculated from the refinement against the SQUEEZED
data, so ignore contributions from the diffuse lattice solvent.)
Refinement converged with R1 = 0.125 (I > 2σ(I)), wR2 = 0.346
Four H2O molecules coordinated to Naþ cations were well-
defined. In addition there were another 16 lattice water mole-
cules included in the refinement, some reasonably well-defined,
-3
˚
(all data), GoF=1.101, Δe=3.4/-1.8 e A
.
Scheme 1. Preparation of Aryl Stibonic Acids by the Method of
Doak and Steinman11
Results and Discussion
Preparation of Arylstibonic Acids 1-4. Arylstibonic acids
are traditionally prepared using the Scheller reaction, as
modified by Doak and Steinman (Scheme 1).11 The key to
the procedure is the conversion of the initial crude acid
(which contains Sb2O3 as an impurity, among others) to a
well-defined pyridinium salt [pyH][RSbCl5], which can be
purified by recrystallization.
Hydrolysis of this salt under mildly alkaline conditions
precipitates the acid as an amorphous powder, which can be
collected and dried. The purity of the arylstibonic acids thus
formed was assessed by Doak and Steinman only by ele-
mental analysis for Sb, varying amounts of water of crystal-
lization being assigned to achieve matching data. Given the
(12) Blessing, R. H. Acta Crystallogr. 1995, A51, 33.
(13) Sheldrick, G. M. SHELX97, Programs for Crystal Structure
Analysis; University of Gottingen: Germany, 1997.
(15) Spek, A. L. Acta Crystallogr. 1990, A41, C34.
(14) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(16) Van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194.

Article
Organometallics, Vol. 29, No. 23, 2010 6521
Figure 1. Negative ion ESI mass spectrum of p-tolylstibonic acid in MeCN, showing the dominant peak assigned to
[H7(MeC6H4Sb)12O28]- (calcd m/z 3008.419). The inset shows the characteristic isotope envelope for the parent ion arising mainly
from the two isotopes of Sb.
strong affinity that the acids appear to have for cations (see
below), it seems likely that material prepared by the pub-
lished method would have contained some Naþ from the hy-
drolysis step. To avoid this, we have added an extra purifica-
tion step that involves dissolving the acid in the minimum
amount of aqueous NH3 and allowing CH3COOH to slowly
diffuse in to precipitate the acid. This works successfully
with p-ClC6H4 and p-MeC6H4, examples 1 and 2, but the
p-O2NC6H4 example 3 still contained significant amounts of
Naþ even after two such purification cycles. Modification of
the preparation by using a sequence of NH3(aq) in place of
the Na2CO3, followed by HCl for the hydrolysis of the
pyridinium (p-nitrophenyl)pentachlorostibonate, gave ma-
terial that was free of Naþ adducts in the ESI-MS, but the
overall purity of the sample was compromised. Further at-
tempts to purify it led to incorporation of adventitious
sodium, showing a particularly strong affinity for complexation
with this example. In contrast, (1-naphthyl)SbO3H2 (4) showed a
much lower affinity for Naþ and did not need the extra purifica-
tion step to provide ESI-MS spectra with little contribution from
adducts. Compounds 1-3 are well known, but the 1-naphthyl
compound 4 has not been characterized previously.
Elemental analysis cannot reliably distinguish between the
nominal mononuclear formula RSbO3H2 and the polynuclear
form [H8(RSb)12O28] that is indicated from the mass spectra (see
below), though there is a better match for the latter in each case.
ESI-MS of Arylstibonic Acids. Samples of each of the
arylstibonic acids were dissolved in MeCN and examined
using ESI-MS. In every case, the dominant peaks could be
assigned from mass measurements and from the distinctive
isotope patterns to the anions derived from a parent acid of
formula [H8(RSb)12O28]. These were mainly [H7(RSb)12O28]-
and/or the corresponding doubly charged species [H6-
(RSb)12O28]2-. Figure 1 shows a typical spectrum.
Interestingly, the p-Me and 1-naphthyl examples gave
mainly the 1- ions, while the p-Cl compound gave both 1-
and 2- ions in a ratio of ca 2:1. In contrast, the p-NO2 species
gave mainly 2- and 3- ions, which incorporated Naþ ions,
such as [NaH5(RSb)12O28]2-, showing the difficulty of com-
plete purification for this compound. The specificity for the
dodecanuclear aggregation for all these derivatives was
remarkable. Other minor peaks appeared to arise from
Sb11, Sb13, Sb16, and Sb28 species, but these were barely signif-
icant. The main peaks were usually accompanied by satellites
corresponding to species where one or more Hþ had been
replaced by Naþ to give, for example, [NaH6(RSb)12-
O28]-. For very fresh samples these were relatively minor
and presumably arose either from traces of Naþ carried over
from the preparation or from adventitious Naþ from the
mass spectrometer source. However if the samples were left
in standard glassware for any length of time, the Naþ species
increased as cations were leached from the glassware, in-
dicating a strong affinity for complexation by the acid
cluster. As noted above, the p-NO2 example had an especially
high affinity for cations. There is clearly some dependence on
the relative electron-withdrawing nature of the R groups
controlling the charge on the ions, and for all examples the
sodium salts showed a greater tendency to form the 2- or 3-
ions than did the parent acids, which appeared mainly as 1-
ions under the same conditions.
The [NaxHy(RSb)12O28]z- aggregates appeared indefi-
nitely stable in MeCN solution, with ESI-MS unchanged

6522 Organometallics, Vol. 29, No. 23, 2010
Nicholson et al.
Table 1. Negative Ion ESI-MS Data for Arylstibonic Acids in MeCN Obtained from the NCI Chemotherapeutics Repositorya
NCI code
formula
m/z
assignment
-
NSC13735
4-Cl-3-O2NC6H4SbO3H2
3793.64
2051.72
1896.28
3402.04
3380.07
1827.98
1818.98
1689.52
[H7(RSb)12O28
[H6(RSb)13O30
[H6(RSb)12O28
]
]
]
2-
2-
-
NSC13742
NSC13743
2-O2NC6H4SbO3H2
3-O2NC6H4SbO3H2
[NaH6(RSb)12O28
]
-
[H7(RSb)12O28
[H6(RSb)13O30
[H8(RSb)12O29
[H6(RSb)12O28
]
]
]
]
2-
2-
2-
NSC13746
NSC13748
NSC13776
NSC13778
NSC13782
Na[4-HO3SC6H4SbO3H]
no significant high-mass
peaks in MeCNb
1683.18 (in MeCN)
1675.87 (in H2O)
no significant high mass
peaks in MeCNb
no significant peaks in
MeCN or H2O
3-
4-(H2N)O2SC6H4SbO3H2
[NaH4(RSb)16O36
[H5(RSb)16O36
]
3-
]
4-[(HOC2H4)NH]O2SC6H4SbO3H2
3-(HO2CCHCH)C6H4SbO3H2
4-(HOC2H4NH)C(O)C6H4SbO3H2
no significant peaks in
MeCN or H2O
a The provenance of the archived samples is largely unknown, and the samples may not be pure (R. H. Shoemaker, personal communication).
b [RSbO3H.nH2O]- signals in low-mass region in H2O.
after 24 h in MeCN in the absence of a source of cations.
Furthermore, a mixture of MeCN solutions of the p-Cl- and
p-Me-phenylstibonic acids showed peaks corresponding to
the individual dodeca-clusters with no scrambling to give
mixed species even after a week at room temperature.
MeCN was the solvent of choice for running ESI-MS.
MeOH could also be used and gave the same species initially,
but slow development of peaks arising from esterification
gave ions such as [H5(RSb)12O27(OMe)]2-. 1,2-Dichloroeth-
ane or MeCN/H2O (1:1) gave the same species as in MeCN,
but overall ionization efficiency was lower, while tetrahy-
drofuran caused breakdown of the cluster units.
or MeCN, the p-sulfamoylphenylstibonic acid uniquely gave
signals assigned to an Sb16 cluster derived from [H8(RSb)16-
O36], with only traces of Sb12 species. The other examples in
H2O gave only low mass ions associated with mainly mono-
mers [RSbO3H]- and related dimers.
In summary, it appears that, rather than variable poly-
meric species, solid arylstibonic acids are well-defined ag-
gregates of formula [H8(RSb)12O28] and that these dissolve
intact in MeCN solutions. Other nuclearities, especially Sb13
and Sb16 species, are often present but are only minor con-
tributors, except for the one example described above. In
water, these clusters are initially intact but hydrolyze to give
monomeric RSbO3H-, and this breakdown is enhanced at
higher pH.
Unfortunately we have been unable as yet to grow single
crystals of any of these acids for full structural characteriza-
tion, but the possible structures can perhaps be deduced from
the structures of salts derived from the acids, as described
below. We note that formally the species [H8(RSb)12O28] is
the equivalent of the well-known isopolytungstate [H2W12-
O40]6- [with the R group on Sb(V) a surrogate for the O on
W(VI)],19 providing a possible analogy for structural predic-
tion. There is also comparison with the Keggin ion structural
isomers such as [PMo12O40]3-, which would also indicate a
cuboctahedron, dodecahedral core for the Sb atoms in
arylstibonic acid clusters if the R-isomer was to be formed.
However analogies between polyoxometalates with main
group (Sb) and transition metals (Mo or W) may be com-
plicated by the recent theoretical findings that imply that
the stability order of [PW12O40]3- Keggin species (R > β >
γ > δ > ε) is completely reversed for [Mn(MeSb)12O28]6-
aggregates.20
These results initially seemed to conflict with reports that
arylstibonic acids could be analyzed by HPLC/ESI-MS,
giving peaks assignable to monomeric Sb species.17 However
these chromatographic analyses were carried out by dissol-
ving the solid acids in 0.05 M NaOH at 45 °C for 30 min
before eluting with aqueous NH4OAc at pH 9. These condi-
tions apparently served to hydrolyze the Sb12 aggregates to
monomeric species. To confirm this supposition, a sample of
p-chlorophenylstibonic acid was dissolved in 0.05 M NaOH
under the reported conditions. This solution then gave peaks
in the low mass region assignable to [RSbO3H nH2O]- (m/z
3
282.899, 300.911, and 318.923, respectively, for n=0-2) and
related dimers and trimers.
During the course of the present study, Rishi et al. de-
scribed the inhibition of DNA binding to B-ZIP dimers, using
12 arylstibonic acids that were sourced from the National
Cancer Institute archives.18 To extend our ESI-MS investi-
gation, we requested some of the same samples and analyzed
these in MeCN. Results are summarized in Table 1 and
generally indicate that these examples also aggregate. The
2-nitro-, 3-nitro-, and 3-nitro-4-chlorophenylstibonic acids all
showed clean ions from the equivalent Sb12 species discussed
above, although interestingly, reasonably abundant signals
from Sb13 clusters were also present for the latter two
examples.
Structure of K2[K2H8(p-ClC6H4Sb)12O30] 58H2O. Crys-
3
tals of a potassium salt of p-chlorophenylstibonic acid
suitable for X-ray structural analysis were grown from
MeCN/H2O solution. These were found to correspond to
the tetrapotassium salt K4[H8(p-ClC6H4Sb)12O30].
The other compounds listed in Table 1 gave no equivalent
peaks in MeCN, possibly because of low solubility. In H2O
€
(19) (a) Long, D.-L.; Abbas, H.; Kogerler, P.; Cronin, L. J. Am.
Chem. Soc. 2004, 126, 13880. (b) Wang, J.; Ren, Q.; Zhao, J. J. Coord.
Chem. 2008, 61, 192. (c) Zavalij, P.; Guo, J.; Whittingham, M. S.; Jacobson,
R. A.; Pecharsky, V.; Bucher, C. K.; Hwu, S. J. J. Solid State Chem. 1996,
123, 83.
(17) (a) Simmons, T. L.; McCloud, T. G. J. Liq. Chromatogr. Relat.
Technol. 2003, 26, 2041. (b) Akee, R., unpublished data, 2010.
(18) Rishi, V.; Oh, W. J.; Heyerdahl, S. L.; Zhao, J. F.; Scudiero, D.;
Shoemaker, R. H.; Vinson, C. J. Struct. Biol. 2010, 170, 216.
(20) Zhang, F. Q.; Guan, W.; Zhang, Y. T.; Xu, M. T.; Li, J.; Su,
Z. M. Inorg. Chem. 2010, 49, 5472.

Article
Organometallics, Vol. 29, No. 23, 2010 6523
Figure 2. Hexagonal antiprismatic arrangement of the 12 Sb atoms in the [K2H8(p-ClC6H4Sb)12O30]2- anion. The same arrangement
is found for the two sodium-containing anions.
Figure 3. Two views of the [K2H8(p-ClC6H4Sb)12O30]2- anion, with the Kþ ions omitted and only the ipso carbon atoms of the aryl
rings included. Purple = Sb, red = O, gray = C.
The lattice contains two independent anions in the asymmetric
unit, but these have very similar structures; hence the following
discussion is based on average parameters. Figures 2-4 show
views of the anion. The core unit consists of 12 six-coordinate
Sb atoms, each with one p-ClC6H4 group attached and five O
atoms. The basic arrangement of the Sb12 unit is an irregular
hexagonal antiprism (Figure 2).
The lower Sb6 array is planar, while the upper Sb6 array is
a puckered chair arrangement. The Sb atoms are linked by 30 O
atoms, divided into 18 double-bridging, six triple-bridging, and
six terminal ones (presumably Sb-OH). The overall symmetry
of the oxostibonate framework (stripped of the attached aryl
rings and cations) is C3v (Figure 3a). There must be an extra two
Hþ associated with the cluster from charge considerations, but
their positioning is undefined.
while those from the puckered array of Sb atoms are splayed
out to form a bowl-shaped cavity (Figure 4), which may
function as a cavitand.
There are four distinct sites for the potassium cations.
Each of the anions has a firmly encapsulated Kþ. This sits
within the puckered hexameric face, coordinated in a planar
hexagonal array by six of the μ-O atoms from the anion
(average O Kþ 2.79 A). There are further interactions
˚
3 3 3
þ
˚
with three of the interlayer μ3-O atoms (O K 2.72 A), and
3 3 3
the coordination of this cation is completed by a loosely held
H2O molecule sitting above the face (O...Kþ 3.47 A). This
˚
generates a 10-coordinate Kþ in total. For this site the anion
is behaving as an inorganic crown ligand, as illustrated in
˚
Figure 5, with cross ring O...O distances of 5.46 A, compared
with ca 5.55 A in 18-crown-6 Kþ complexes.21
˚
The aryl rings are of two types. Those attached to Sb atoms
in the planar array form three pairs arranged face-to-face,
For each anion there is a second Kþ sitting below the other
face of the hexagonal antiprism, coordinated to three μ-O

6524 Organometallics, Vol. 29, No. 23, 2010
Nicholson et al.
Figure 4. Full structure of the [K2H8(p-ClC6H4Sb)12O30]2- anion, showing the coordination of three different Kþ ions (the fourth Kþ
is not closely associated with the anion). Purple = Sb, green = Cl red = O, yellow = K, light gray = C.
atoms (average O Kþ 2.62 A) from the anion, with three
˚
3 3 3
water molecules (average O Kþ 2.48 A) making up octa-
˚
3 3 3
hedral coordination.
The third Kþ in each case is within the bowl-shaped cavity
generated by the protruding aryl rings above the puckered
six-membered face. There are small differences in this case
between the two independent anions in the asymmetric unit.
There are two interactions with μ-O from the anion and three
coordinated H2O molecules (one or two of which are also
weakly bridging to the Kþ ion in site 1). In addition there is a
weak interaction with a Cl atom from a p-ClC6H4 group
from the adjacent anion (Cl Kþ 3.49/3.64 A), and the
˚
3 3 3
K
þ is positioned so that it lies above the face of one of the
˚
p-ClC6H4 rings, (Kþ ring plane 3.32/3.37 A). This inter-
þ 3 3 3
Figure 5. Cross-section of the 10-coordinate Kþ site in the
[K2H8(p-ClC6H4Sb)12O30]2- anion, showing the 12-crown-6
action places the K to one side of the cavity, breaking the
C3v symmetry of the rest of the anion.
The fourth Kþ in each case is in a general lattice position,
˚
{Sb6O6} feature. The O O cross-ring distance is 5.46 A. The
3 3 3
each coordinated by five H2O (average Kþ OH2 distances
same arrangement is found for the Naþ derivatives, with O
O
distances of ca. 5.31 A. Purple = Sb, red = O, yellow = K (or Na).
3 3 3
3 3 3
˚
˚
2.78 A), and each is positioned symmetrically above one of
the aryl rings of an anion (Kþ ring-plane distances 3.75
3 3 3
˚
and 3.25 A for K(4) to the ring attached to Sb(6) and K(8) to
so there are probably a few extra molecules not accounted
for, but overall the solvent molecules are unusually well
behaved.
the ring attached to Sb(16), respectively).
The remainder of the structure consists of solvent within
the lattice. A total of 116 H2O molecules in the asymmetric
Structure of Rb0.67Na0.33[Na2H9(p-MeC6H4Sb)12O30] 20H2O.
3
unit were located and refined, and these gave sensible O
distances for an H-bonded array, linking the anions and cat-
ions together. Analysis gives ca. 4% remaining void volume,
O
The structure of a mixed Rbþ/Naþ salt was determined and
was found to be closely related to the Kþ salt discussed
above. The same core hexagonal antiprism was found, with
three cation sites similar to those already described. Despite
the crystals growing from a solution where the Rbþ:Naþ
ratio was ca. 1:1, refinement showed that the two sites most
3 3 3
(21) (a) Cambridge Crystallographic Data Base, 2010. (b) Ariga, K.;
Kumitake, T. Supramolecular Chemistry-Fundamentals and Applica-
tions; Springer-Verlag: Heidelberg, 2006.

Article
Organometallics, Vol. 29, No. 23, 2010 6525
It consists of separated Ph4Pþ cations and the same [(RSb)12-
O30] anion as found for the other two examples. In this case
there are only two Naþ cations, one occupying the 10-coordi-
nate inorganic crown site and one on the opposite face with
six-coordination made up from three μ-O atoms and three
H2O. This latter Naþ is again involved in linking two anions
together through the same cubic arrangement found for the
Rbþ compound (cf. Figure 6). The third cation site found for
the Kþ and Rbþ compounds is now vacant, which allows for
overall C3v symmetry to be retained for the anions.
The bond parameters are very similar to those of the Rb
˚
compound, with O O cross-ring distances of 5.28 A, and
3 3 3
again the Naþ is displaced by 0.83 A from the least-squares
˚
plane of the six crown-type oxygen atoms.
Discussion
Figure 6. Cubane-like H-bonding moiety that links two anions
via the six-coordinate Naþ ions in all of the known sodium
When the acids [H8(RSb)12O28] are dissolved in alkaline
solution, salts are formed. If KOH is used, crystals contain-
ing a tetra-anion are formed, namely, [K4H8(RSb)12O30]. If
NaOH is used and other cations are added, then salts of a
trianion are formed, including so far [PhCH2NMe3][Na2H9-
0
derivatives of arylstibonic acids. Ow and Ow are coordinated
water molecules, O and O0 are μ-oxygen atoms from the anion
framework.
(RSb)12O30],8 [Ph4P][Na2H9(RSb)12O30], and Rb0.67Na0.33
-
closely associated with the anion were occupied only by Naþ
2
cations, with the third site having /3:1/3 mixed Rbþ:Naþ
occupancy.
[Na2H9(RSb)12O30]. As an interesting historical note, as
early as 1921 Fargher and Gray isolated solids from the reac-
tion of arylstibonic acids with NaOH and found Na:Sb ratios
of up to 1:3, consistent with our present results, though they
were unable to provide an explanation.7
Once again there is one cation, Na(1), that is firmly encap-
sulated in the crown face of the anion. In this case the O
O
3 3 3
þ
þ
˚
is 5.31 A (cf. 5.46 A for the larger K example), and the Na
˚
˚
The anions found in the crystal structures correspond to
a parent acid of formula [H12(RSb)12O30], which contrasts
with the formula for the precursor acids indicated from the
ESI-MS study of [H8(RSb)12O28]. Possibly formation of the
salts has concomitantly added two H2O to the core unit, pre-
sumably via opening of two Sb-O-Sb bridges to generate
sites for coordination of the tightly held cations. Alterna-
tively, two H2O may be readily lost in the mass spectrometer
from an acid of formula [H12(RSb)12O30], since the major
peaks in the mass spectra of the salts correspond to two H2O
fewer than found by X-ray crystal structure; for example, for
[Ph4P][Na2H9(RSb)12O30] the most intense peak corresponded
to [Na2H4(RSb)12O28]2-, though [Na2H8(RSb)12O30]2- was
also present.
is displaced by 0.83 A from þthe O6 plane toward the interior
˚
of the anion (0.61 A in the K example). There are three links
to μ3-O atoms from the middle of the anion, and there is a
single H2O attached on the upper site. The second Naþ
occupies the lower Sb6 face, again coordinated to three μ-O
from the anion framework, with a further three H2O mole-
cules completing six-coordination. This unit is involved in an
aesthetically pleasing link to the equivalent site on an adja-
cent anion via the H-bonded cubic arrangement illustrated in
Figure 6, with Naþ-O-H O-Naþ bridges. We are un-
3 3 3
aware of a precedent for this type of linking of Naþ, which
would more usually be joined by μ-OH2 bridges, an arrange-
ment that may be precluded here by the bulk of the anion.
The third cation site is in the pocket formed by the aryl
rings and contains mixed Rbþ/Naþ (2:1). It is linked to Na(1)
by a bridging H2O ligand and is coordinated to two μ-O from
the anion. The remaining sites around the cation are occu-
pied by three of the p-tolyl rings which sandwich the cation
Despite differing aryl groups and different crystal packing,
the structures of all of these derivatives (five independent
Naþ species, two independent Kþ ones) show a remarkably
conserved anion configuration. Interestingly, this geometry
was first reported by Baskar et al. as one subunit of a very
complex compound assigned the formula [Na21(PhSb)48-
O114] 46H2O 4MeCN, which made it unclear as to the
˚
with distances of 3.30, 3.49, and 3.55 A from the cation to the
least-squares planes of the rings, indicating weak π-bonding.
This type of π-interaction between aryl rings and group 1
cations is now well-established for other systems.22
The lattice water in this structure was less well-defined.
Twenty H2O were located, and these refined reasonably well
to give a H-bonding network holding the main units to-
gether. However there still remained significant electron
density that could not be modeled sensibly.
3
3
overall charge state, though there were three Naþ cations
associated with the Sb12 subunit.23 For all of the known
examples the basic unit is a hexagonal antiprism with one
puckered and one planar array of μ-O linked Sb atoms,
with overall C3v symmetry. The distance between the least-
˚
squares planes through the two Sb6 arrays is 3.0 A, and the
˚
Sb Sb distances in the planar array are shorter (ca. 3.15 A)
Structure of (Ph4P)[Na2H9(p-MeC6H4Sb)12O30](H2O)4
3 3 3
3
þ
˚
than those in the puckered array (ca. 3.64 A for Na salts,
xH2O. This crystal structure was of poor quality, with lattice
water that could not be modeled successfully. It is less
reliable than the others, but the overall features are clear.
3.67 A for Kþ ones).
˚
The Sb-O (terminal) bond lengths average around 1.99 A
in all examples, consistent with being single bonds and hence
˚
Sb-OH groups. The doubly bridging O atoms have similar
(22) (a) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303.
(b) Dougherty, D. A. In Encyclopaedia of Supramolecular Chemistry;
Atwood, J. L.; Steed, J. W., Eds.; Marcel Decker: New York, 2004; p 214.
(c) Pink, M; Sieler, J. Inorg. Chim. Acta 2007, 360, 1221. (d) Shukla, R.;
Lindeman, S. V.; Rathore, R. Chem. Commun. 2009, 5600.
(23) Baskar, V.; Shanmugan, M.; Helliwell, M.; Teat, S. J.; Winpenny,
R. E. P. J. Am. Chem. Soc. 2007, 129, 3042.

6526 Organometallics, Vol. 29, No. 23, 2010
Nicholson et al.
˚
Sb-O distances (1.97-2.00 A), while the triply bridging ones
For all the Naþ-containing examples the anions are linked
through the cubic {O3Na(H2O)6NaO3} H-bonded units
(Figure 6), despite different crystal packing interactions.
However this is not conserved for the Kþ compound, where
the O3K(H2O)3 moiety links indirectly to the adjacent anion
via a network of H-bonded water molecules.
We have obtained crystals prepared from Ca(OH)2,
though the quality was such that only the basic core geome-
try could be determined. The same hexagonal-antiprism
structure as found for the Kþ or Naþ species was indicated,
with Ca2þ ions in the two usual sites. This arrangement is
obviously favored for these medium-sized cations.
The compounds reported herein represent new polyoxo-
metalates that have a number of novel features: (i) they are
formed from alkaline solutions, whereas the polyoxometa-
lates formed by Mo, W, V, etc., are formed under acid con-
ditions;26 (ii) the anions form with an open structure, which
generates a hexagonal channel; (iii) there is a very strong
affinity for cations, particularly in the 10-coordinate site,
which acts as an inorganic crown moiety; (iv) the same
structure is adopted for Ph, p-ClC6H4, p-MeC6H4, and
p-O2NC6H4 derivatives, with either Kþ and Naþ counter-
ions, and probably also with Ca2þ; (v) the structural integrity
of the anionic clusters is maintained in solution since ESI
mass spectra indicate Sb12 species dominate.
We note that Baskar et al. have very recently characterized
a Sb16 polyoxostibonate from p-chlorophenylstibonic acid in
the presence of dimethylpyrazole, under thermal conditions
in nonaqueous solvents,10 so the aggregation processes of
organostibonic acids can apparently be controlled using
different templating cations and bases and different condi-
tions. In future papers we will report results of our studies
with other metal ions.
˚
are longer, as expected (ca. 2.1 A).
Two cation sites are intimately associated with the anion.
With the first, one hexagonal face acts as a {Sb6O6} crown
ligand, providing six in-plane O atoms for bonding with a
further three framework oxygens attaching below the cation
and a water molecule making up the tenth site above the
cation, the polyoxometalate framework effectively acting as
a corand.24 For the Kþ example the cation is slightly less
displaced from the hexagonal plane than in the Naþ case, and
the cross-ring O O distances have expanded a little to ac-
3 3 3
commodate the larger cation. The presence of a crown ligand
moiety contained within the structure of a discrete poly-
oxometallate molecule appears unusually rare, but the {Sb6O6}
entity is closely similar, albeit slightly smaller, to those found
in niobates {Nb6O6} and the pores of large cluster molecules
{Fe3W3O6}, {Mo6O6}, {Mo3V3O6}, and {Mo4VKO6}.25 All
contain a ring of six oxygen atoms that are essentially co-
˚
planar to within 0.013-0.083 A, as in our case, and cross-
˚
ring O O distances that range from 5.502 to 5.639 A.
3 3 3
Cations (Kþ or NH4þ) ensnared by these ligands may be as
˚
little as 0.271 or as far as 1.523 A from the plane of the oxygen
atoms.
In contrast, the other hexagonal face is attached to the
cation by only three framework oxygen atoms, with a further
three terminal H2O molecules completing the coordination.
Whereas either Naþ or Kþ can occupy these sites, there was
no exchange with Rbþ when crystallization was carried out
in the presence of the larger cation.
When the remaining cations are large organic ones
(PhCH2NMe3þ, Ph4Pþ), these are separate from the anions,
occupying general lattice sites. However if there is a third
small cation available, this is found in a pocket formed by the
aryl groups above the puckered hexagonal face. This is
attached to two μ-O from the framework and is bridged by
a H2O molecule to the high-coordinate cation in site 1. Re-
maining interactions are with the faces of the aryl rings
in weak π-type bonding.22 These interactions displace the
cation from the C3v axis toward one side of the pocket. In the
case of the Kþ example, the fourth cation is only loosely
associated with the outside of the anion through a weak
π-interaction with an aryl ring.
Acknowledgment. We thank Dr. Robert H. Shoemaker
and the Drug Synthesis and Chemistry Branch, Develop-
mental Therapeutics Program, Division of Cancer Treat-
mentand Diagnosis, National Cancer Institute, Bethesda,
MD, for generously supplying samples of arylstibonic
acids from their archives.
Supporting Information Available: Full details of the crystal
structure determinations, cif files, and extra structure dia-
grams are available free of charge via the Internet at http://
pubs.acs.org.
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