Arylarsonate- And Phosphonate-Capped Polyoxomolybdates, [(RC6H4As)2Mo6O24]n-and [(R′C6H4P)2Mo5O21]n-

Article abstract of DOI:10.1021/acs.inorgchem.1c00245

We report on the synthesis and structural characterization of four arylarsonate- and phosphonate-capped polyoxomolybdates that exhibit different organic substituents in the para position of the phenyl group. The reaction of arylarsonates (RAsO3, wherein R

Full text of DOI:10.1021/acs.inorgchem.1c00245

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Article
Arylarsonate- and Phosphonate-Capped Polyoxomolybdates,
[(RC6H4As)2Mo6O24]n− and [(R′C6H4P)2Mo5O21]n−
Paulami Manna, Saurav Bhattacharya, and Ulrich Kortz*
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ABSTRACT: We report on the synthesis and structural characterization of four
arylarsonate- and phosphonate-capped polyoxomolybdates that exhibit different
organic substituents in the para position of the phenyl group. The reaction of
arylarsonates (RAsO3, wherein R = 4-BrC6H4 or 4-N3C6H4) with molybdate in
aqueous pH 3.5 media resulted in the cyclic hexamolybdates
[(BrC6H4As)2Mo6O24]4− (Mo6As2La) and [(N3C6H4As)2Mo6O24]4−
(Mo6As2Lb), whereas the reaction of arylphosphonates (R′PO3, wherein R′ =
4-O2CC6H4 or 4-O2CC6H4CH2) with molybdate in aqueous pH 3 media
resulted in the cyclic pentamolybdates [(O2CC6H4P)2Mo5O21]6− (Mo5P2Lc)
and [(HO2CC6H4CH2P)2Mo5O21]4− (Mo5P2Ld), respectively. Polyanions
Mo6As2La and Mo6As2Lb comprise a ring of six MoO6 octahedra that is capped
on either side by an organoarsonate group, whereas Mo5P2Lc and Mo5P2Ld
consist of a ring of five MoO6 octahedra that is capped on either side by an
organophosphonate group, with the organic arms protruding away from the
metal−oxo core of the polyanions. All four polyanions Mo6As2La, Mo6As2Lb, Mo5P2Lc, and Mo5P2Ld have been characterized in the
solid state by single-crystal X-ray diffraction, IR spectroscopy, and thermogravimetric and elemental analysis and in solution by
1
multinuclear NMR (31P, 13C, and H). The synthetic procedure of (4-bromophenyl)arsonic acid, BrC6H4AsO3H2, is reported here
for the first time.
phosphonates,6 phosphites,7 and arsonates8 onto the surface of
INTRODUCTION
■
POMs. In particular, the organic modification of polyoxova-
nadates has been explored extensively, utilizing amino acids
and carboxylate-containing organic groups.4d−f,9 Various
organophosphonate and -arsonate-stabilized polyoxomolyb-
dates have also been reported, such as the pentamolybdate
family [X2Mo5O23]n−. In 1975, Pope et al. reported the
polyanion family [(RP)2Mo5O21]4− (R = H, CH3, C2H5, C6H5,
Polyoxometalates (POMs) are a rapidly growing class of
inorganic compounds because of the multitude of structures
and compositions along with impressive redox, photochemical,
optical, electronic, magnetic, thermal, and biological proper-
ties,1 rendering POMs important for a wide variety of
applications, in particular catalysis but also materials science,
biomedicine, electrochromism, sensor technology, and renew-
able energy.2 Polyoxotungstates, -molybdates, and -vanadates
are usually prepared via condensation reactions in aqueous
acidic media. The potential of POMs for pharmaceutical
applications due to their anticancer, antitumor, and antiviral
activity is well established, but frequently the selectivity toward
a specific target remains challenging.3 In this regard, the
covalent grafting of organic groups on the surface of POMs is
an elegant and promising strategy.4 This approach has several
advantages, such as precise structural control via the
attachment of functional groups (allowing one to fine-tune
steric hindrance and polarity), an increase in the stability of
POMs at physiological pH, and the possibility of binding to
proteins, tissues, and receptors. As the number of POMs with
the above features is rather small, it is of major interest to
rationally design and synthesize organically functionalized
derivatives of POMs that are water-soluble and stable.5
+
+
C2H4NH3 , p-CH2C6H4NH3 ), which are stable in aqueous
solution at pH 2.5−5.4g In 1988, Sasaki et al. reported the
crystal structure of the phosphite derivative
[(HP)2Mo5O21]4−,7b and in the same year, the crystal structure
of [(C6H5P)2Mo5O21]4− was reported by Lyxell and
Strandberg.7c In 2003, our group reported the first examples
of phosphonocarboxylate-functionalized polyoxo-5-molyb-
dates, [(O2C(CH2)nPO3)2Mo5O21]6− (n = 1, 2), respec-
tively.10
Received: January 25, 2021
Several strategies have been tested by researchers in this
regard over the years, such as the grafting amino acids,4b−e
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Then 5 mL of a 0.1 mM aqueous NaNO2 solution was added
dropwise, and the mixture was stirred for 15 min at 0 °C (solution 1).
In a separate beaker, 25 mL of water was heated to 80 °C and then
solid Na2CO3 (5.3 g, 50.0 mmol) and As2O3 (2.5 g, 10.0 mmol) were
added. Upon complete dissolution of the solids, CuSO4·5H2O (0.11
g, 0.44 mmol) was added. The resulting blue solution was allowed to
cool to room temperature and then cooled to 0 °C (solution 2).
Solution 1 was added dropwise to solution 2 at 0 °C over a period of
1 h under continuous high-speed stirring. A strong effervescence was
observed because of the release of N2 gas from the intermediate
diazonium salt as the reaction proceeded. When the addition was
complete, the reaction mixture was kept for 2 h at room temperature
under constant stirring. The resulting orange solution was filtered, and
the colorless filtrate was collected. The filtrate was adjusted to pH 7
with concentrated HCl and then filtered again. Subsequently, the
filtrate was adjusted to pH 1 with concentrated HCl to obtain a
colorless precipitate. The pH adjustments were accompanied by an
effervescence. The solution was left overnight at 4 °C, and then the
precipitate was filtered, washed with water, and dried in air. Yield:
4.78 g (85%). IR (2% KBr pellet, ν/cm−1): 1571 (sh), 1474 (sh),
1386 (sh), 1253 (m), 1208 (s), 1091 (s), 1064 (sh), 1009 (m), 901
In 1976, Stalick and Quicksall reported the structures of
[(CH3P)2Mo5O21]4− and [(NH3C2H4P)2Mo5O21]2−,7d and in
the same year, Pope et al. reported several organoarsonate-
functionalized polyoxo-6-molybdates, [(RAs)2Mo6O24]4− (R =
CH3, Ph, 4-NH2C6H4).11 In 1988, Liu et al. reported the
o r g a n o a r s o n a t e - c o n t a i n i n g 5 - m o l y b d a t e [ (n -
C3H7As)2Mo5O21]4−, and from the same reaction solution,
they also isolated the 6-molybdate [(n-C3H7As)2Mo6O24]4−.12
Now we decided to expand the family of organophosphonate
and -arsonate-containing penta- and hexamolybdates by
utilizing para-functionalized phenylphosphonates and -arso-
nates. Substitution of the phenyl ring in the para position with
biologically active groups such as azide (−N3) or carboxylate
(−COOH) would allow for the resulting polyanion to interact
post-synthetically with biological receptors. The azide func-
tional group is suitable for click reactions and the carboxylate
group can bind to amino groups, allowing for covalent
coupling with peptides, nucleotides, antibodies, etc. Therefore,
we were motivated to incorporate the appropriate para-
functionalized arylarsonate and -phosphonate groups into the
novel polyanions. Here we also report for the first time on the
synthesis of (4-bromophenyl)arsonic acid (see the Exper-
imental Section for details).
1
(w), 810 (s), 786 (s), 761 (m), 711 (s), 485 (m), 435 (s). H NMR
(400 MHz, DMSO-d6): δ 7.64 (d, 2H, Ar), 7.81 (d, 2H, Ar), 13C
NMR (400 MHz, DMSO-d6): δ 127.6, 131.1, 132.3, 133.1.
((NH2)3C)4[(BrC6H4As)2Mo6O24]·2H2O (Gua-Mo6As2La). Na2MoO4·
2H2O (0.774 g, 3.20 mmol) and BrC6H4AsO3H2 (0.280 g, 1.00
mmol) were dissolved in 5 mL of water at room temperature. The pH
of the reaction mixture was adjusted to 3.5 by adding 6 M H2SO4,
followed by reflux for 1 h. The reaction mixture was then filtered, and
the filtrate, upon cooling to room temperature, was added to an
aqueous solution of guanidium chloride at pH 3.5 [prepared by
dissolving 0.122 g of (NH2)3CCl in 20 mL of water and adjusting the
pH with diluted H2SO4]. This mixture was then heated at 80 °C until
the volume had reduced to 15 mL. Upon cooling to room
temperature, colorless block-shaped crystals of Gua-Mo6As2La were
formed that were filtered off and air-dried. Yield: 0.60 g (70% based
on Mo). Elem anal. Calcd for Gua-Mo6As2La: Mo, 33.90; As, 8.83;
Br, 9.41; C, 11.32; H, 2.14; N, 9.90. Found: Mo, 34.28; As, 8.47; Br,
9.24; C, 10.92; H, 2.06; N, 10.30. IR (2% KBr pellet, ν/cm−1): 1423
(s), 1421 (sh), 1383 (s), 1330 (s), 1321 (sh), 1043 (m), 1020 (m),
925 (w), 701 (sh), 689 (s), 650 (s), 617 (w), 602 (w), 518 (w), 501
(w).
EXPERIMENTAL SECTION
■
Materials and Physical Measurements. All chemicals were
received as reagent-grade from commercial sources and used without
further purification. The hetero groups (4-azidophenyl)arsonic acid,3a
(4-carboxyphenyl)phosphonic acid,13 and “(4-carboxybenzyl)-
phosphonic acid14 were synthesized following literature procedures
(structures shown in Scheme 1). We have developed a synthetic
Scheme 1. Structures of the Arylarsonates and
-Phosphonates Used in This Study
((NH2)3C)4[(N3C6H4As)2Mo6O24] (Gua-Mo6As2Lb). Polyanion
Mo6As2Lb was synthesized using the same procedure as that for
Mo6As2La but by using (4-azidophenyl)arsonic acid, N3C6H4AsO3H2
(0.243 mg, 1.00 mmol),3a instead of (4-bromophenyl)arsonic acid.
Yield: 0.64 mg (75% based on Mo). Elem anal. Calcd for Gua-
Mo6As2Lb: Mo, 36.29; As, 9.45; C, 12.12; H, 2.03; N, 15.90. Found:
Mo, 36.35; As, 9.64; C, 11.88; H, 2.03; N, 15.76. IR (2% KBr pellet,
ν/cm−1): 1423 (s), 1421 (sh), 1383 (s), 1330 (s), 1321 (sh), 1043
(m), 1020 (m), 925 (w), 701 (sh), 689 (s), 650 (s), 617 (w), 602
(w), 518 (w), 501 (w).
Rb3Na3[(O2CC6H4P)2Mo5O21]·5H2O (RbNa-Mo5P2Lc). Na2MoO4·
2H2O (0.30 g 1.25 mmol) was dissolved in 15 mL of water, followed
by the addition of (4-carboxyphenyl)phosphonic acid
(HO2CC6H4PO3H2; 0.10 g, 0.50 mmol) and KCl (0.11 g, 1.50
mmol). The reaction mixture was stirred at room temperature for 15
min, and then the pH of the clear solution was adjusted from 6 to 3 by
adding concentrated HCl(aq). Subsequently, the reaction mixture was
refluxed for 30 min. After the solution was cooled to room
temperature, 0.8 g of solid RbCl was added and dissolved by stirring.
The solution was then filtered into a 20 mL beaker, which was placed
in a larger flask containing some ethanol. The larger flask was closed
with a lid, and diffusion of ethanol vapor into the reaction solution led
to the formation of colorless crystals of RbNa-Mo5P2Lc after a few
days, which were filtered off and air-dried. Yield: 0.30 g (80% based
on Mo). Elem anal. Calcd for RbNa-Mo5P2Lc: Rb, 16.72; Na, 4.50;
Mo, 31.29; P, 4.04; C, 10.97; H, 1.18. Found: Rb, 17.12; Na, 4.29;
Mo, 31.38; P, 4.24; C, 10.55; H, 1.26. IR (2% KBr pellet, ν/cm−1):
1423 (s), 1421 (sh), 1383 (s), 1330 (s), 1321 (sh), 1043 (m), 1020
procedure for (4-bromophenyl)arsonic acid (Scheme 1). The NMR
spectra (1H, 13C, and 31P) were recorded at room temperature on a
JEOL ECX 400 MHz instrument at Jacobs University. The chemical
shifts are reported with respect to the references Si(CH3)4 (13C and
1H) and 85% H3PO4 (31P), respectively. The IR spectra were
recorded on KBr disks using a Nicolet-Avatar 370 spectrometer
between 400 and 4000 cm−1. Thermogravimetric analyses were
carried out using a TA Instruments Q600 device under the flow of N2
gas with a heating rate of 5 °C min−1 in the temperature range of 30−
800 °C. Elemental analyses were performed by Crealins, Villeurbanne,
France.
Synthesis. (4-Bromophenyl)arsonic Acid, BrC6H4AsO3H2. p-
Bromoaniline (3.44 g, 20.0 mmol) was dissolved in 25 mL of a 2
M HCl(aq) solution at 0 °C and allowed to stand at 0 °C for 10 min.
B
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Table 1. Crystal Data and Structure Refinement for Gua-Mo6As2La, Gua-Mo6As2Lb, RbNa-Mo5P2Lc, and CsK-Mo5P2Ld
Gua-Mo6As2La
Gua-Mo6As2Lb
RbNa-Mo5P2Lc
CsK-Mo5P2Ld
C16H8Cs3Mo5O31P2
(C16H24Cs3KMo5O30P2)
a
empirical formula
C16H32As2Br2N12Mo6O24
(C16H36As2Br2N12Mo6O26)
C16H32As2N18Mo6O24
C14H8Mo5Na2O29P2Rb3
(C14H18Mo5Na3O30P2Rb3)
a
fw, g mol−1
1661.83 (1697.81)
monoclinic
P2(1)/n
10.4611(10)
18.3773(16)
12.2541(10)
90
1586.07
monoclinic
P2(1)/n
10.4029(18)
18.112(3)
12.347(2)
90.00
1484.23 (1533.29)
monoclinic
P2(1)/c
1636.59 (1675.80)
monoclinic
C2/c
cryst syst
space group
a, Å
18.2365(15)
12.8646(10)
15.2918(13)
90
13.5121(16)
15.3292(18)
21.896(3)
90
b, Å
c, Å
α, deg
β, deg
γ, deg
volume, Å3
Z
calc, g cm−3
abs coeff, mm−1
108.029(4)
90
106.449(7)
90.00
103.492(2)
90
101.695(3)
90
2240.1(3)
2
2231.2(7)
2
3488.5(5)
4
4441.3(9)
4
D
2.464
2.361
2.826
2.448
4.969
3.207
6.130
3.959
F(000)
1584.0
1528
2788
3028
θ range for data
collection, deg
2.823−27.648
2.530−27.587
1.956−27.489
2.657−27.545
completeness to Θmax
,
99.4
99.4
99.7
99.7
%
index ranges
−13 ≤ h ≤ 13, −22 ≤ k ≤ 23, −15 ≤ −13 ≤ h ≤ 13, −23 ≤ k ≤ 23,
l ≤ 15 −16 ≤ l ≤ 16
22007 24434
−23 ≤ h ≤ 23, −16 ≤ k ≤ 13, −11 −17 ≤ h ≤ 17, −19 ≤ k ≤ 19,
≤ l ≤ 19 −28 ≤ l ≤ 28
22440 41052
reflns collected
indep reflns
R(int)
5176
5134
7956
5118
0.0682
0.0641
0.0527
0.0411
abs corrn
semiempirical from equivalents
5176/13/289
semiempirical from equivalents
5134/0/298
semiempirical from equivalents
7956/258/499
semiempirical from equivalents
5118/114/272
data/restraints/param
GOF on F2
1.044
1.150
1.036
1.182
b
c
R1, wR2 [I > 2σ(I)]
R1 = 0.0405, wR2 = 0.0992
R1 = 0.0512, wR2 = 0.1049
1.558 and −1.954
R1 = 0.0429, wR2 = 0.1145
R1 = 0.0469, wR2 = 0.1193
1.070 and −1.164
R1 = 0.0391, wR2 = 0.0823
R1 = 0.0559, wR2 = 0.0882
3.072 and −1.612
R1 = 0.0701, wR2 = 0.1765
R1 = 0.0769, wR2 = 0.1806
4.604 and −1.428
b
c
R1, wR2 (all data)
largest diff peak and
hole, e Å−3
a
b
c
The entries in parentheses are the actual formula units and weights as obtained from elemental analysis. R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 =
[∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
2
2
(m), 925 (w), 701 (sh), 689 (s), 650 (s), 617 (w), 602 (w), 518 (w),
501 (w).
SADABS program.15 The SHELX software package (Bruker) was used
to solve and refine the structures.16 The structures were solved by
direct methods and refined by the full-matrix least-squares method
(∑w(|Fo|2 − |Fc|2)2) with anisotropic thermal parameters for all heavy
atoms included in the model. Hydrogen atoms on the carbon atoms
were introduced in calculated positions and included in the
refinement riding on their respective parent atoms. It was impossible
to locate all sodium countercations via Fourier electron density
because of crystallographic disorder, which is a common problem in
POM crystallography, and the most disagreeable reflections were
removed by the SQUEEZE command in PLATON.17 Therefore, the
true number of countercations and crystal water molecules in the
compounds was determined by elemental analysis, and the resulting
formula units were used throughout the paper and in the CIF file for
overall consistency. The crystal data and structure refinement
parameters for the four compounds are summarized in Table 1.
Images of the crystal structures were generated by Diamond, version
3.2 (software copyright, Crystal Impact GbR). The data are provided
free of charge by The Cambridge Crystallographic Data Centre via the
deposition codes CCDC 2034543−2034546.
Cs3K[(HO2CC6H4CH2P)2Mo5O21]·5H2O (CsK-Mo5P2Ld). The poly-
anion Mo5P2Ld was synthesized by the same procedure as that used
for Mo5P2Lc but by using (4-carboxybenzyl)phosphonic acid
(HO2CC6H4CH2PO3H2; 0.108 g, 0.500 mmol) instead of (4-
carboxyphenyl)phosphonic acid. The reaction mixture was left at
room temperature in an open container for crystallization, and after 3
days, colorless crystals were obtained in roughly 30% yield (no
elemental analysis was performed on this material, but on the basis of
IR, it is clean Mo5P2Ld as a hydrated potassium−sodium salt). The
yield of the polyanion Mo5P2Ld could be increased significantly by
adding 0.5 g of solid CsCl to the reaction mixture after it had cooled
to room temperature and then keeping the 20 mL beaker inside a
larger, capped container with ethanol, analogously to Mo5P2Lc. After
2 days, colorless crystals of CsK-Mo5P2Ld were obtained. Yield: 0.32 g
(78% based on Mo). Using RbCl (as for the synthesis of Mo5P2Lc)
instead of CsCl resulted in the immediate formation of a colorless
precipitate (which was not further analyzed). Elem anal. Calcd for
CsK-Mo5P2Ld: Cs, 22.58; K, 2.21; Mo, 27.16; P, 3.51; C, 10.88; H,
1.94. Found: Cs, 22.37; K, 2.42; Mo, 27.17; P, 3.41; C, 10.19; H, 1.57.
IR (2% KBr pellet, ν/cm−1): 1423 (s), 1421 (sh), 1383 (s), 1330 (s),
1321 (sh), 1043 (m), 1020 (m), 925 (w), 701 (sh), 689 (s), 650 (s),
617 (w), 602 (w), 518 (w), 501 (w).
RESULTS AND DISCUSSION
■
Over the years, some organoarsonate-capped polyoxomolyb-
dates have been reported, such as the tetranuclear polyanion
X-ray Crystallography. Single crystals of RbNa-Mo5P2La, CsK-
Mo5P2Lb, Gua-Mo6As2Lc, and Gua-Mo6As2Ld were mounted on a
Hampton cryoloop with light oil, and then data were collected on a
Bruker Kappa X8 APEX II CCD diffractometer with a sealed Mo tube
and a graphite monochromator using Kα radiation (λ = 0.71073 Å) at
100 K. An empirical absorption correction was applied using the
18a,b
[(O3AsR)4Mo4O10]4−
and the dodecanuclear polyanion
[(O3AsR)4Mo12O34]4− (R = Ph, MeC6H4, 4-NH2C6H4,
C2H4OH, 4-HOC6H4, 4-HO2CC6H4, 3-O2N,4-HOC6H3, 4-
NCC6H4, 3-C2H4ON, 4-HOC6H3).18 The hexanuclear poly-
anion [(O3AsR)2Mo6O18]4− (R = CH3, Ph, 4-NH2C6H4) was
C
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reported by Pope’s group in 1976,11 and Liu et al. reported an
additional derivative (R = n-C3H7) in 1988.12 To date, no
further derivatives of this hexamolybdate structure have been
reported.
the range of 108.96−110.65(4)° and As−O bond lengths from
1.687 to 1.815(7) Å, as well as As−C bond lengths of 1.896−
2.142(8) Å. In the solid state, individual polyanions Mo6As2La
are surrounded by four guanidinium cations and two water
m o l e c u l e s , l e a d i n g t o t h e f o r m u l a u n i t
((NH2)3C)4[(BrC6H4As)2Mo6O24]·2H2O (Gua-Mo6As2La),
which represents a 3D network stabilized by hydrogen bonding
of the guanidinium cations with oxygen atoms of the
polyanion.
The reaction of (4-azidophenyl)arsonic acid
(N3C6H4AsO3H2; prepared according to ref 3a; see the
Experimental Section for details) with sodium molybdate
under the same conditions as those used for the synthesis of
M o 6 A s 2 L a r e s u lt e d i n t h e n o v e l p o l y a n i o n
[(N3C6H4As)2Mo6O24]4− (Mo6As2Lb; Figure 1b). The
structure of Mo6As2Lb is identical with that of Mo6As2La
except for the para substituent of the phenyl ring, which is N3
in the former and Br in the latter. Hence, the Mo−O and As−
O/As−C bond lengths in Mo6As2Lb and Mo6As2La are very
similar. Furthermore, Gua-Mo6As2La is isomorphous with
Gua-Mo6As2Lb, and hence they exhibit identical 3D solid-state
assemblies.
Being inspired by the previous results on arsonate-capped
polyoxomolybdates, we decided to also study organophosph-
onate-containing capping groups with molybdate ions in an
aqueous acidic solution.
The reaction of (4-carboxybenzyl)phosphonic acid with
sodium molybdate in an aqueous pH 3 solution for a 30 min
reflux resulted in [(O2CC6H4P)2Mo5O21]6− (Mo5P2Lc) in 80%
yield (Figure 1c). The novel polyanion Mo5P2Lc was isolated
as a hydrated mixed rubidium−sodium salt, RbNa-Mo5P2Lc,
which crystallized in the monoclinic system with the P21/c
space group (Table 1). The five MoO6 octahedra in Mo5P2Lc
are shared by a total of four edges and one corner, forming a
ring that is capped on each side by a (4-carboxyphenyl)-
phosphonate group (Figure 1c). This structure can be
considered to be the aryl analogue of our earlier-reported
[(O2CCH2PO3)2Mo5O15]6−.10
In Mo5P2Lc, the two carboxylate groups of (4-
carboxyphenyl)phosphonic acid remain deprotonated in the
polyanion structure, with one of them being weakly
coordinated to a Na+ ion (Na···OC = 2.21 Å) and the other
to a Rb+ ion (Rb···OC = 2.92 Å). In total, there are three
sodium and three rubidium ions balancing the negative charge
of each polyanion Mo5P2Lc. The countercations interact with
terminal oxygen atoms of {Mo5O21} units as well as crystal
water molecules, resulting in an overall 3D supramolecular
arrangement in the solid state. Good-quality single crystals of
polyanion Mo5P2Lc in decent yield were obtained in the
presence of rubidium cations using the ethanol vapor diffusion
technique. We discovered that the presence of K+ ions during
the synthesis allowed one to isolate the polyanion Mo5P2Lc in
better yield, although no potassium ions could be detected in
the final product RbNa-Mo5P2Lc. It is not evident what role K+
ions play during the formation and/or crystallization process of
Mo5P2Lc.
Figure 1. Combined polyhedral/ball-and-stick representation of (a)
Mo6As2La, (b) Mo6As2Lb, (c) Mo5P2Lc, and (d) Mo5P2Ld. Color
code: MoO6 octahedra, green; As, yellow; P, pink; O, red; Br, violet;
N, blue; C, gray. No hydrogen atoms are shown for clarity.
The reaction of (4-bromophenyl)arsonic acid with sodium
molybdate in an aqueous pH 3−3.5 solution at 1 h reflux
conditions resulted in [(BrC6H4As)2Mo6O24]4− (Mo6As2La) in
70% yield (Figure 1a). The novel polyanion Mo6As2La was
isolated as a hydrated guanidinium salt, Gua-Mo6As2La, which
crystallized in the monoclinic system with the P21/n space
group (Table 1). The six edge-shared MoO6 octahedra form a
ring, which is capped on each side by a (4-bromophenyl)-
arsonate group. Polyanion Mo6As2La exhibits (i) terminal
oxygen atoms with Mo−O bond lengths in the range of
1.702−1.715(5) Å, (ii) doubly bridging oxygen atoms with
Mo−O(Mo) bond lengths in the range of 1.894−1.937(5) Å,
and (iii) triply bridging oxygen atoms with Mo−O(As) bond
lengths in the range of 2.278−2.343(5) Å. The arsenic atoms
exhibit a tetrahedral geometry with O−As−O bond angles in
The reaction of (4-carboxybenzyl)phosphonic acid with
sodium molybdate under identical conditions as those for the
s y n t h e s i s o f M o
P
L
r e s u l t e d i n
5
2
c
[(HO2CC6H4CH2P)2Mo5O21]4− (Mo5P2Ld) in 78% yield
(Figure 1d). The novel polyanion Mo5P2Ld was isolated as a
hydrated mixed cesium−potassium salt, CsK-Mo5P2Ld, which
crystallized in the monoclinic system with the C2/c space
D
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group (Table 1). Overall, the structure of Mo5P2Ld is identical
with that of Mo5P2Lc, except that the phosphonate capping
group of the former has an extra degree of freedom because of
the methylene (CH2) group, which allows the planar aryl
group to move about a large cone angle and rotate at the same
time, whereas the aryl group in Mo5P2Lc can only rotate
around the P−C bond. The P−C−C angle for Mo5P2Ld in the
solid state is ca. 112°, whereas the P−C vector in Mo5P2Lc can
only rotate. It should be noted that Mo5P2Lc is fully
deprotonated, whereas Mo5P2Ld has both carboxylate groups
protonated, although both are synthesized at pH 3, which leads
to a charge difference of two units between the two polyanions.
It is likely that different solid-state packing effects are
responsible for this phenomenon, considering that both
polyanions have different countercations and crystallize in
different space groups.
Figure 3. 31P NMR spectra of Mo5P2Lc at different reaction
conditions: (i) free ligand HO2CC6H4PO3H2 at pH 6; (ii) reaction
mixture of Na2MoO4·2H2O, HO2CC6H4PO3H2 and KCl in water at
pH 6; (iii) reaction mixture after pH adjustment to pH 3 and 30 min
of stirring at room temperature; (iv) reaction mixture after 20 min of
reflux (pH 3); (v) reaction mixture after 5 min of microwave heating
at 70 °C (pH 3).
NMR SPECTROSCOPY
■
The 31P NMR spectra for RbNa-Mo5P2Lc and CsK-Mo5P2Ld
were collected in H2O/D2O at pH 3. The former exhibits a
singlet at 19.5 ppm, whereas the latter exhibits a singlet at 28.1
ppm (Figure 2). At the same pH, the free capping groups (4-
such a pH. After adjustment to pH 3 by concentrated HCl(aq)
and stirring at room temperature for 30 min, the reaction
mixture showed a clean singlet at 19.6 ppm, corresponding to
the polyanion Mo5P2Lc. Reflux of the reaction mixture for 20
min at pH 3 also resulted in a clean 31P NMR spectrum with a
singlet at 19.6 ppm. Furthermore, a very short 5 min reaction
in a microwave at 70 °C also resulted in a clean singlet at 19.6
ppm, indicating the smooth formation of Mo5P2Lc. We also
performed a solid-state reaction by simple mixing and grinding
of the solid reagents Na2MoO4·2H2O, HO2CC6H4PO3H2, and
KCl in a mortar, followed by dissolution in water (pH 6),
which gave two peaks at 12.9 and 20.3 ppm, respectively, in
analogy to the reaction mixture in aqueous solution at pH 6 as
described above. All of these results confirm that the polyanion
Mo5P2Lc is formed easily and cleanly in a pH 3 aqueous
solution.
CONCLUSIONS
■
We report on the synthesis and structural characterization of
BrC6H4AsO3H2 for the first time. When reacting
BrC6H4AsO3H2 with sodium molybdate in an aqueous acidic
medium, we obtained the bicapped hexamolybdate Mo6As2La.
When the same reaction was performed with N3C6H4AsO3H2
as the capping group, then the polyanion Mo6As2Lb was
formed. Both polyanions Mo6As2Lb and Mo6As2La are
isostructural, comprising a cyclic core of six edge-shared
MoO6 octahedra with two capping groups attached on
opposite sides.
The reaction of two arylphosphonates (RPO3, where R = 4-
O2CC6H4 and 4-O2CC6H4CH2) with sodium molybdate in
aqueous acidic media resulted in the cyclic pentamolybdates
Mo5P2Lc and Mo5P2Ld, respectively, which are capped on
either side by a arylphosphonate group. The 31P NMR spectra
for Mo5P2Lc and Mo5P2Ld exhibit singlets at 19.5 and 28.1
ppm, respectively, indicating that both polyanions are solution-
stable. In 2003, our group reported the first example of a 5-
molybdo-2-phosphonate functionalized with aliphatic phos-
phonocarboxylates, [(O2C(CH2)nPO3)2Mo5O21]6− (n = 1, 2),
respectively. The polyanions Mo5P2Lc and Mo5P2Ld presented
here are the first examples of Strandberg-type polyanions with
carboxyphenylphosphonate capping groups.
Figure 2. 31P NMR spectra of solid RbNa-Mo5P2Lc (top) and CsK-
Mo5P2Ld (bottom) dissolved in water at pH 3.
carboxyphenyl)phosphonic acid and (4-carboxybenzyl)-
phosphonic acid exhibit chemical shifts at 12.4 and 20.1
ppm, respectively. These results confirm the solution stability
of both polyanions Mo5P2Lc and Mo5P2Ld.
We also performed some control reactions at different
synthetic conditions monitored by 31P NMR to better
understand the formation mechanism of the polyanions
Mo5P2Lc and Mo5P2Ld from the respective starting materials
(Figure 3). The capping group for Mo5P2Lc as a free acid is
HO2CC6H4PO3H2, which exhibits a singlet in the 31P NMR
spectrum at 12.9 ppm in water at pH 6. The reaction mixture
of Na2MoO4·2H2O, HO2CC6H4PO3H2, and KCl in water (pH
6) results in two peaks at 12.9 and 20.3 ppm, corresponding to
the free phosphonic acid and polyanion Mo5P2Lc, respectively.
Such an observation indicates that the self-assembly of
Mo5P2Lc takes place already at room temperature after mixing
the starting materials in water (resulting in pH 6), but there is
still a significant amount of free phosphonic acid present at
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Our work demonstrates that the size of the heteroatom X (X
= P, As) and the corresponding X−O bond length differences
(P−O ∼ 1.5 Å vs As−O ∼ 1.7 Å) determine which POM
structure type is formed. For the two arylarsonate hetero
groups, cyclic hexamolybdates are formed, with a ring size
accommodating the longer As−O bonds, whereas for the two
arylphosphonate hetero groups, cyclic pentamolybdates are
formed, providing a ring size suitable for the shorter P−O
bonds.
For all four polyanions Mo6As2La, Mo6As2Lb, Mo5P2Lc, and
Mo5P2Ld, the functional groups in the para position of the aryl
groups (e.g., −N3, −Br, −COOH) can, in principle, be further
covalently conjugated to biomolecules (e.g., peptides, nucleo-
tides, antibodies, nanoparticles), thereby increasing the
capability of these materials for use in biomedical applications.
Our future work on such types of polyanions is geared in this
direction.
DEDICATION
■
́
Dedicated to Prof. Imre Toth on the occasion of his 70th
birthday.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00245.
■
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AUTHOR INFORMATION
Corresponding Author
■
Ulrich Kortz − Department of Life Sciences and Chemistry,
Jacobs University, Bremen 28759, Germany; orcid.org/
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Paulami Manna − Department of Life Sciences and Chemistry,
Jacobs University, Bremen 28759, Germany; orcid.org/
0000-0002-6063-3139
Saurav Bhattacharya − Department of Life Sciences and
Chemistry, Jacobs University, Bremen 28759, Germany;
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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