Gallium(III)-containing, sandwich-type heteropolytungstates: Synthesis, solution characterization, and hydrolytic studies toward phosphoester and phosphoanhydride bond cleavage

Article abstract of DOI:10.1021/acs.inorgchem.6b01030

The gallium(III)-containing heteropolytungstates [Ga₄(H₂O)_10-- (β-XW₉O₃₃)₂]^6- (X = As^III, 1; Sb^III, 2) were synthesized in aqueous acidic medium by reaction of Ga³⁺ ions with the trilacunary, lone-pair-containing [XW₉O₃₃]^9-. Polyanions 1 and 2 are isostructural and crystallized as the hydrated sodium salts Na₆[Ga₄(H₂O)₁₀(β-AsW₉O₃₃)₂]·28H₂O (Na-1) and Na₆[Ga₄(H₂O)₁₀(β-SbW₉O₃₃)₂]·30H₂O (Na-2) in the monoclinic space group P21/c, with unit cell parameters a = 16.0218(12) ?, b = 15.2044(10) ?, c = 20.0821(12) ?, and β = 95.82(0)°, as well as a = 16.0912(5) ?, b = 15.2178(5) ?, c = 20.1047(5) ?, and β = 96.2(0)°, respectively. The corresponding tellurium(IV) derivative [Ga₄(H₂O)₁₀(β-TeW₉O₃₃)₂]^4- (3) was also prepared, by direct reaction of sodium tungstate, tellurium(IV) oxide, and gallium nitrate. Polyanion 3 crystallized as the mixed rubidium/sodium salt Rb2Na2[Ga₄(H₂O)₁₀(β- TeW₉O₃₃)₂]·28H₂O (RbNa-3) in the triclinic space group P with unit cell parameters a = 12.5629(15) ?, b = 13.2208(18) ?, c = 15.474(2) ?, α = 80.52(1)°, β = 84.37(1)°, and γ = 65.83(1)°. All polyanions 1-3 were characterized in the solid state by single-crystal XRD, FT-IR, TGA, and elemental analysis, and polyanion 2 was also characterized in solution by 183^W NMR and UV-vis spectroscopy. Polyanion 2 was used as a homogeneous catalyst toward adenosine triphosphate (ATP) and the DNA model substrate 4-nitrophenylphosphate, monitored by ¹H and ³¹P NMR spectroscopy. The encapsulated gallium(III) centers in 2 promote the Lewis acidic synergistic activation of the hydrolysis of ATP and DNA model substrates at a higher rate in near-physiological conditions. A strong interaction of 2 with the P-O bond of ATP was evidenced by changes in chemical shift values and line broadening of the ³¹P nucleus in ATP upon addition of the polyanion.

Full text of DOI:10.1021/acs.inorgchem.6b01030

Article
pubs.acs.org/IC
Gallium(III)-Containing, Sandwich-Type Heteropolytungstates:
Synthesis, Solution Characterization, and Hydrolytic Studies toward
Phosphoester and Phosphoanhydride Bond Cleavage
Balamurugan Kandasamy,†,⊥ Stef Vanhaecht,‡,⊥ Fiona Marylyn Nkala,† Tessa Beelen,‡ Bassem S. Bassil,†,§
Tatjana N. Parac-Vogt,‡ and Ulrich Kortz*,†
†Department of Life Sciences and Chemistry, Jacobs University, 28725 Bremen, Germany
‡Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001, Heverlee, Belgium
§Department of Chemistry, Faculty of Sciences, University of Balamand, P.O. Box 100, Tripoli, Lebanon
S
* Supporting Information
ABSTRACT: The gallium(III)-containing heteropolytungstates [Ga4(H2O)10-
(β-XW9O33)2]6− (X = AsIII, 1; SbIII, 2) were synthesized in aqueous acidic
medium by reaction of Ga3+ ions with the trilacunary, lone-pair-containing
[XW9O33]9−. Polyanions 1 and 2 are isostructural and crystallized as the hydrated
sodium salts Na6[Ga4(H2O)10(β-AsW9O33)2]·28H2O (Na-1) and
Na6[Ga4(H2O)10(β-SbW9O33)2]·30H2O (Na-2) in the monoclinic space group
P21/c, with unit cell parameters a = 16.0218(12) Å, b = 15.2044(10) Å, c =
20.0821(12) Å, and β = 95.82(0)°, as well as a = 16.0912(5) Å, b = 15.2178(5) Å, c
= 20.1047(5) Å, and β = 96.2(0)°, respectively. The corresponding tellurium(IV)
derivative [Ga4(H2O)10(β-TeW9O33)2]4− (3) was also prepared, by direct
reaction of sodium tungstate, tellurium(IV) oxide, and gallium nitrate. Poly-
anion 3 crystallized as the mixed rubidium/sodium salt Rb2Na2[Ga4(H2O)10(β-
TeW O ) ]·28H O (RbNa-3) in the triclinic space group P1 with unit cell
̅
9
33
2
2
parameters a = 12.5629(15) Å, b = 13.2208(18) Å, c = 15.474(2) Å, α = 80.52(1)°, β = 84.37(1)°, and γ = 65.83(1)°.
All polyanions 1−3 were characterized in the solid state by single-crystal XRD, FT-IR, TGA, and elemental analysis, and
polyanion 2 was also characterized in solution by 183W NMR and UV−vis spectroscopy. Polyanion 2 was used as a homogeneous
catalyst toward adenosine triphosphate (ATP) and the DNA model substrate 4-nitrophenylphosphate, monitored by 1H and 31
P
NMR spectroscopy. The encapsulated gallium(III) centers in 2 promote the Lewis acidic synergistic activation of the hydrolysis
of ATP and DNA model substrates at a higher rate in near-physiological conditions. A strong interaction of 2 with the P−O bond
of ATP was evidenced by changes in chemical shift values and line broadening of the 31P nucleus in ATP upon addition of the
polyanion.
predominantly in dimeric, sandwich-type structures, namely,
INTRODUCTION
■
the Weakley-type [M4(B-α-XW9O34)2]n−,5 the Knoth-type [M3-
Polyoxometalates (POMs) are a large subclass of inorganic
compounds, which can frequently be prepared by self-assembly
of oxo-anions of early transition metals in high oxidation states
(MoVI, WVI, VV) in aqueous, acidic medium.1 The chemical and
physical properties of POMs such as shape, size, composition,
charge, acidity, redox behavior, and solubility can be easily
modified.2 The structural and compositional versatility of these
nanosized, molecular inorganic anions translates to amazing
notable applications in various research domains, such as
homogeneous and heterogeneous catalysis, materials science,
and medicine.3
́
(A-α/β-XW9O34)2]n−,6 the Herve-type [M3(B-α-XW9O33)2]n−,7
and the Krebs-type [M4(B-β-XW9O33)2]n−.8 Sometimes un-
expected structures can be obtained, mainly depending on the
reaction conditions.9
In 1997, Krebs’ group reported a sandwich-type structure series com-
prising two lone-pair-containing trilacunary 9-tungstoantimonate(III)
units, two 3d transition metal centers, and two extra tungsten
centers, with the general formula [M2(H2O)6(WO2)2(β-
SbW9O33)2](14−2n)− (Mn+ = Mn2+, Fe3+, Co2+, Ni2+), by reaction
of the transition metal ions with [Sb2W22O74(OH)2]12− in
aqueous acidic medium.8a Further isostructural derivatives such
as [M2(H2O)6(WO2)2(β-BiW9O33)2](14−2n)− (Mn+ = Fe3+, Co2+,
Ni2+, Cu2+, Zn2+), [(VO(H2O)2)2(WO2)2(β-BiW9O33)2]10−, and
[Sn1.5(WO2(OH))0.5(WO2)2(β-XW9O33)2]10.5− (X = Sb, Bi)8b,c
Keggin-type polyanions such as [PW12O40]3− are of special
interest for POM researchers, due to their ability to form
several lacunary (vacant) derivatives by loss of one or more
WO6 addenda units, such as [PW11O39]7− or [PW9O34]9−.1a
These species can be considered as inorganic ligands pos-
sessing nucleophilic oxygen atoms at the lacunary site, which
can coordinate to suitable metal ions.4 Such work has resulted
Received: April 28, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b01030
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Article
have also been reported by the same group. The first tetra-
substituted structure of the Krebs-type, [Mn4(H2O)10
(β-TeW9O33)2]8−, was reported in 1998.10 Some additional deriv-
atives with other occupancies in the four guest metal sites are also
known, such as [M3(H2O)8(WO2)(β-TeW9O33)2]8− (M = Ni2+,
Co2+), [(Zn(H2O)3)2(WO2)1.5(Zn(H2O)2)0.5(β-TeW9-
O33)2]8−, and [(VO(H2O)2)1.5(WO(H2O)2)0.5(WO2)0.5(VO-
(H2O))1.5(β-TeW9O33)2]7−.11
Our group has also worked extensively in this area, and
we reported [Fe4(H2O)10(β-XW9O33)2]n− (X = AsIII, SbIII,
n = 6, X = SeIV, n = 4),12a [In4(H2O)10(β-AsW9O33H)2]4−, and
[In4(H2O)10(β-SbW9O33)2]6−,12b as well as [Al4(H2O)10
(β-XW9O33H)2]4− (X = AsIII, SbIII),12c which were all obtained
by reaction of the respective lone-pair-containing trilacunary POM
precursor with the corresponding metal ions. More recently,
we reported the mono- and di-rare-earth-containing 21- and
20-tungstoantimonates(III) [Ln(H2O)4Sb2W21O72(OH)]10−
(Ln = Yb3+, Lu3+) and [Ln2(H2O)8Sb2W20O70]8− (Ln = Yb3+,
Lu3+, Y3+), by reaction of [Sb2W22O74(OH)2]12− with Ln3+ at
different pH.13 Some other derivatives have also been prepared
and their catalytic properties studied, such as water oxidation,14
epoxidation of alkenes,12c,15 Mannich-type reactions,16 and bio-
inspired oxygenation.17
Phosphoester bonds are characterized by a high stability, as
they play an important role in protecting our genetic material.18
However, in the last decades much attention has been paid to
the controlled cleavage of this relatively inert bond as a vital
step in several biochemical techniques. Being inspired by
nature, most researchers look toward the use of metal ion-
containing catalysts as new artificial phosphoesterase. Many
reports in the literature can be found on the cleavage of DNA
model substrates by structurally different metal ion com-
plexes.19 The Parac-Vogt group has already extensively explored
the biological activity of POMs toward the cleavage of phos-
phoester bonds in numerous DNA and RNA model
substrates.20 Initially, several isopolyanions were studied for
the kinetics and underlying mechanism of the hydrolysis of
DNA model substrates. In the case of heptamolybdate
[Mo7O24]6−, hydrolysis of the phosphoester bond results
from destabilization by incorporation of the substrate into the
POM framework.20a Decavanadate [V10O28]6− was also found
to catalyze the hydrolysis reaction by a fast and reversible dis-
sociation of the POM structure, after which the substrate could
bind and be activated by the smaller POM fragments.20b In
different studies, several metal-ion-substituted heteropolytung-
states were screened for phosphoesterase activity against DNA
and RNA model substrates.20f−i In this approach, the expected
activity results from the Lewis-acidic properties of the incor-
porated metal ion, while the POM acts as a stabilizing ligand,
keeping the metal ion in its active form.
Very little work has been carried out on gallium-containing
POMs to date, and in the few known compounds gallium is
largely present as a tetrahedrally coordinated primary hetero-
atom, such as in the Keggin-type compounds H5GaW12O40 and
[α-GaW9O34H2]9−, respectively.21 The latter was employed to
synthesize the trivanadium-substituted derivative [α-GaW9-
V3O40]8−, as based on solution studies (51V and 183W NMR).22
Solution studies of other isostrucural derivatives were also
reported, such as [β-GaW9M3O40]n− (M = Mn2+, Co2+, Ni2+,
Fe3+, V5+).23 In 2005, Krebs’ group structurally characterized
the Weakley-type dimers [M4(H2O)2(GaW9O34)2]14− (M = Cu2+,
Zn2+) by single-crystal XRD.24 Interaction of gallium(III) salts
such as Ga(NO3)3 or GaCl3 with monolacunary Keggin-type ions
[α-XW11O39]n− (X = BIII, n = 9; SiIV, GeIV, n = 8; PV, AsV, n = 7),
trilacunary Keggin-type ions [α/β-XW9O34]10‑ (X = SiIV, GeIV),
and monolacunary Wells−Dawson-type ions [X2W17O61]10−
(X = PV, AsV) has been studied by several research groups,
and the products were investigated by 183W NMR spectros-
copy and other analytical techniques (FT-IR, FAB mass
spectrometry, electrochemistry, and thermogravimetric anal-
ysis).25 Patzke’s group synthesized two gallium(III)-containing
tungstosilicates(IV), [Ga6(H2O)3(α-SiW9O35(OH)2)2]10− and
[Ga4(H2O)2{α-SiW10O38}2]12−, and studied their solid-state
and solution properties.26
Here we report on the synthesis and structural charac-
terization of gallium(III)-containing heteropolyanions of the
Krebs-type and their activity as homogeneous catalysts for the
hydrolysis of the DNA model substrates 4-nitrophenylphos-
phate (NPP) and adenosine triphosphate (ATP).
EXPERIMENTAL SECTION
■
General Procedures. All reagents were used as purchased without
further purification. The trilacunary POM precursor salts Na9[B-α-
AsW9O33]·19.5H2O and Na9[B-α-SbW9O33]·19.5H2O were synthe-
sized according to the literature, and their purity was confirmed by
FT-IR.8a,27 The infrared (IR) spectra for the solid samples were
obtained on KBr pellets using a Nicolet Avatar 370 FTIR
spectrophotometer. The solution 183W NMR spectra of the obtained
compounds were recorded on a 400 MHz JEOL ECX instrument at
room temperature, using 10 mm tubes. The solution 1H and 31P NMR
spectra were recorded on a Bruker Avance 400 MHz spectrometer at
room temperature using 5 mm tubes. The respective resonance fre-
quencies were 399.78 MHz (1H), 161.975 MHz (31P), and 16.69 MHz
(
183W). The 1H, 31P, and 183W chemical shifts are respectively
referenced to TMSPA-d4, trimethylphosphate, and 1 M Na2WO4(aq).
Thermogravimetric analyses were carried out on a TA Instruments
SDT Q600 instrument, using 10−30 mg of sample in 100 μL alumina
pans under a 100 mL/min flow of nitrogen; the temperature was ramped
from 20 to 1000 °C at a rate of 5 °C/min. Elemental analyses were
performed at Institut des Sciences Analytiques, Villeurbanne, France.
X-ray Crystallography. Single crystals of Na-1, Na-2, and RbNa-3
were mounted on a Hampton cryoloop in light oil for data collection at
100 K. Indexing and data collection were measured on a Bruker D8
SMART APEX II CCD diffractometer with kappa geometry (graphite
monochromator, λMo Kα = 0.710 73 Å. Data integration was performed
using SAINT.28 Routine Lorentz and polarization corrections were
applied. Multiscan absorption corrections were performed using
SADABS. Direct methods (SHELXS) successfully located the tungsten
atoms, and successive Fourier syntheses (SHELXL) revealed the
remaining atoms.29 Refinements were full matrix least-squares against
|F|2 using all data. In the final refinement, the Ga, As, Sb, Te, W, Na,
and Rb atoms were refined anisotropically, whereas the O atoms and
disordered countercations were refined isotropically.
Synthesis of Na6[Ga4(H2O)10(β-AsW9O33)2]·28H2O (Na-1).
GaCl3 (1.080 mL of a 0.514 M aqueous solution, 0.555 mmol) was
directly added to Na9[B-α-AsW9O33]·19.5H2O (0.737 g, 0.250 mmol)
in 20 mL of water. The pH of the resulting colorless solution was
adjusted to 3 by using 2 M HCl. The solution was then heated to 80 °C
for 1 h, then allowed to cool to room temperature. Colorless crystals were
formed after 2 to 3 weeks with a yield of 0.47 g (33%). IR: 3408(vs),
965(vs), 908(w), 826(s), 689(s), 472(m), 430(w) Anal. Calcd (found):
As 2.67 (2.75), Ga 4.96 (4.97), Na 2.45 (2.36), W 58.91 (58.51).
Synthesis of Na6[Ga4(H2O)10(β-SbW9O33)2]·30H2O (Na-2).
GaCl3 (1.080 mL of a 0.514 M aqueous solution, 0.555 mmol) was
directly added to Na9[B-α-SbW9O33]·19.5 H2O (0.715 g, 0.25 mmol)
in 20 mL of water. The pH of the resulting colorless solution was
adjusted to 3 by using 2 M HCl. The solution was then heated to 80 °C
for 1 h, then allowed to cool to room temperature. The color of the
reaction mixture then changed to pale yellow, and colorless crystals were
formed after 2 to 3 days with a yield of 0.72 g (51%). IR: 3420(vs),
B
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Article
958(s), 897(w), 814(s), 668(s), 470(m), 420(vw) Anal. Calc (found):
Sb 4.24 (4.32), Ga 4.85 (4.69), Na 2.40 (2.35), W 57.59 (56.79).
Synthesis of Rb2Na2[Ga4(H2O)10(β-TeW9O33)2]·28H2O (RbNa-3).
TeO2 (61.28 mg, 0.384 mmol) was dissolved in a minimum amount of
concentrated HCl and added directly to Na2WO4·2H2O (1.140 g,
3.456 mmol) in 10 mL of water. The pH of the solution was adjusted
to 5.8 by using 4 M NaOH, and the reaction mixture was heated to
70 °C for 10 min, and then allowed to cool to room temperature.
Ga(NO3)3 (0.197 g, 0.768 mmol) was directly added and the solu-
tion was further stirred at room temperature for 1 h. Addition of
0.5 mL of 0.5 M RbCl and slow evaporation at room temperature
resulted in colorless single crystals after 2 to 3 weeks, with a yield
of 0.83 g (37%). IR: 3418(vs), 974(vs), 880(vw), 819(vs), 746(m),
700(s), 662(m), 489(m). Anal. Calcd (found): Te 4.55 (4.42),
Ga 4.81 (4.62), Rb 2.95 (2.97), Na 0.80 (0.93), W 57.04 (56.74).
Polyanions 1−3 could also be synthesized using Ga(NO3)3 instead
of GaCl3.
Hydrolysis Experiments. Hydrolysis experiments were performed
by mixing compound Na-2, dissolved in hot 100 mM LiCl in D2O,
with the substrate. TMSPA-d4 (trimethylsilylpropionic acid-d4) as a
water-soluble reference was added to samples needing 1H NMR
characterization. After mixing, the pD value of the solutions was
adjusted by the addition of DCl or NaOD. The pD value was calcul-
ated using the formula pD = pH + 0.41. The reaction mixtures were
heated in a heating block or oil bath, and the reactions followed
by means of 1H or 31P NMR spectroscopy after different time
intervals. The integration of the peaks corresponding to start and
end products was used to calculate the progress and kinetics of the
reaction.
Figure 1. Combined polyhedral/ball-and-stick representation of
[Ga4(H2O)10(B-β-XW9O33)2]n− (X = As, 1; Sb, 2; n = 6, X = Te, 3;
n = 4). Color code: WO6, red octahedra; Ga, blue; As, Sb, Te yellow;
O, red.
n = 6, X = SeIV, n = 4), [In4(H2O)10(β-AsW9O33H)2]4−,
[In4(H2O)10(β-SbW9O33)2]6−, and [Al4(H2O)10(β-XW9-
O33H)2]4− (X = AsIII, SbIII),12 strongly suggesting that tetra-
substituted Krebs-type POMs are most stable within this
charge range. Contrary to the aluminum(III) and indium(III)
analogues, no protonation sites were identified within the
POM skeleton of 1−3, as checked by bond valence sum cal-
culations, which showed that all terminal ligands of the gallium
atoms are water molecules, and confirmed the expected oxida-
tion states of the heteroatom (AsIII, SbIII, TeIV), GaIII, and WVI
centers.32
Polyanions 1−3 have similar Ga−O bond lengths, which fall
in the range 1.892(7)−1.957(7) Å for 1, 1.900(7)−1.988(7) Å
for 2, and 1.897(9)−2.025(10) Å for 3, respectively. As
expected, the average AsIII−O bond lengths (1.799(6) Å) in 1
are shorter than TeIV−O (1.893(9) Å) in 2 and SbIII−O
(1.999(7) Å) in 3. On the other hand, the corresponding hetero-
atom distances As···As (ca. 5.98 Å) and Sb···Sb (ca. 5.61 Å) are
between those of our reported [Al4(H2O)10(β-XW9O33H)2]4−
(X = As, Sb) (As···As = 5.92 Å, Sb···Sb = 5.87 Å), [In4(H2O)10(β-
AsW9O33H)2]4− (As···As = 6.26 Å), and [In4(H2O)10(β-
SbW9O33)2]6− (Sb···Sb= 5.87 Å), reflecting the size differences
of the incorporated guest ions GaIII, AlIII, and InIII, respectively.
Selected bond lengths and angles of polyanions 1−3 are shown in
Table 1, and crystal data are shown in Table S1 (Supporting
Information).
The solid-state FT-IR spectra (Figure S1) of compounds
Na-1, Na-2, and RbNa-3 show very similar absorbance peaks in
the polyanion metal-oxo region. All three compounds showed
bands around 960(s), 895(w), and 820(s), corresponding
to W−O terminal as well as corner- and edge-shared WO6
stretching frequencies, respectively. The thermal stability of the
three salts Na-1, Na-2, and RbNa-3 was also investigated by
thermogravimetric analysis (TGA). All three compounds show
one main weight loss step until ca. 400 °C, which is attributed
to the loss of crystal and coordinated water molecules. No sig-
nificant weight loss was observed above 400 °C, and the com-
pounds decomposed eventually to their respective metal oxides
at higher temperatures (Figure S2).
RESULTS AND DISCUSSION
■
The respective interaction between the lone-pair-containing
trilacunary POM precursors [XW9O33]9− (X = AsIII, SbIII) with
Ga3+ ions in slightly acidic aqueous medium resulted in the
formation of the two polyanions [Ga4(H2O)10(β-AsW9O33)2]6−
(1) and [Ga4(H2O)10(β-SbW9O33)2]6− (2), which were iso-
lated in crystalline form as hydrated sodium salts Na6[Ga4-
(H2O)10(β-AsW9O33)2]·28H2O (Na-1) and Na6[Ga4(H2O)10-
(β-SbW9O33)2]·30H2O (Na-2), respectively. Both salts crystal-
lize in the monoclinic space group P21/c and are isomorphous.
For tellurium(IV), the stoichiometric reaction of TeO2 or
2−
TeO3 and Na2WO4 is known to produce the trilacunary
[TeW9O33]8− unit, as reported by Pope and co-workers.30 We
adapted this approach in order to prepare the tellurium(IV)
analogue [Ga4(H2O)10(β-TeW9O33)2]4− (3), which crystallized
as mixed rubidium/sodium salt Rb2Na2[Ga4(H2O)10(β-
TeW O ) ]·28H O (RbNa-3) in the triclinic space group P1.
̅
9
33
2
2
Polyanions 1−3 are isostructural and of the Krebs-type,
which consists of two equivalent {β-XW9O33} units linked by
four octahedrally coordinated Ga3+ ions, resulting in a structure
with idealized C2h symmetry (Figure 1). The four Ga3+ ions are
arranged in pairs at two positions: an outer position with three
terminal aqua ligands and an inner one with two. Krebs and
co-workers have previously suggested a formation mechanism
of the structure involving insertion, isomerization, and
dimerization.8a As a matter of fact, both [α-SbW9O33]9− and
[β-SbW9O33]9− isomers were identified in aqueous solution by
the same group, with the “α” isomer dominating at neutral
pH and the “β” one in acidic pH.8a The α-based Herve-
́
type
sandwich dimer, which forms in neutral pH, supports this
hypothesis, as we have also previously reported.7,31
It is worth noting that the charges of polyanions 1 (6−),
2 (6−), and 3 (4−) fall within the range of our previously
reported Krebs-type compounds containing trivalent metal
cations, namely, [Fe4(H2O)10(β-XW9O33)2]n− (X = AsIII, SbIII,
C
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Inorganic Chemistry
Article
of phosphoester bonds by metal ion complexes. Typically, the
metal ions possess at least two available coordination sites
in order to display any phosphoesterase activity, where one
allows the coordination of the substrate while the other one
serves to deliver a coordinated water molecule, which serves as
a nucleophile in the hydrolysis reaction.33 The crystal structure
of polyanion 2 showed that the inner gallium(III) ions have two
water molecules and the outer gallium(III) atoms contain three
water molecules attached. It is very apparent that 2 has many
available coordination sites, which can be easily substituted
by the substrate. Together with several possible cooperative
gallium(III) ions, this compound seems ideally suited for
hydrolysis of phosphoester bonds and might enhance the rate
of the hydrolysis. In our study, we focused our attention on
compound 2 toward the hydrolysis of phosphoester bonds.
For this purpose we used 4-nitrophenylphosphate, which is
commonly used as a substrate in enzymatic studies,34 as well as
a model substrate in studies focusing on the development of
artificial metallonucleases.35 NPP is very resistant to hydrolysis,
with a half-life of 135 days at pH 5.0 and 50 °C in the absence of
a catalyst.20d
Table 1. Selected Bond Lengths and Angles for Polyanions
1−3
1
2
3
As−O 1.799(6)
Sb−O 1.999(7)
Te−O 1.893(9)
As−As 5.977(14)
Sb−Sb 5.607(9)
Te−Te 5.702(13)
Ga1−O3A 1.900(7)
Ga1−O4A 1.892(7)
Ga1−O8A 1.946(7)
Ga1−O8B 1.924(7)
Ga1−O1G1 2.055(7)
Ga1−O2G1 2.068(7)
Ga2−O7A 1.924(7)
Ga2−O9A 1.934(7)
Ga2−O8T 1.957(7)
Ga2−O1G2 1.977(7)
Ga2−O2G2 1.950(7)
Ga2−O3G2 2.092(7)
Ga1−O2A 1.903(7)
Ga1−O6A 1.900(7)
Ga1−O7A 1.988(7)
Ga1−O7B 1.970(7)
Ga1−O1G1 2.024 (7)
Ga1−O2G1 2.015(7)
Ga2−O3A 1.933(7)
Ga2−O7T 1.955(7)
Ga2−O4G2 1.928(7)
Ga2−O1G2 1.975(7)
Ga2−O2G2 1.947(8)
Ga2−O3G2 2.101(7)
Ga1−O5A 1.921(8)
Ga1−O9A 1.894(8)
Ga1−O2A 1.928(8)
Ga1−O2B 1.950(8)
Ga1−O1G1 1.995(9)
Ga1−O2G1 2.033(9)
Ga2−O4A 1.938(10)
Ga2−O2T 2.034(8)
Ga2−O4G2 1.985(8)
Ga2−O1G2 1.926(13)
Ga2−O2G2 1.947(11)
Ga2−O3G2 1.962(9)
Ga1−O3A−W3 144.3(4) Ga1−O2A−W2 141.6(4) Ga1−O5A−W5 142.6(5)
Ga1−O4A−W4 143.2(4) Ga1−O6A−W6 143.5(4) Ga1−O9A−W9 140.9(5)
Ga1−O8A−W8 173.7(4) Ga1−O7A−W7 175.7(4) Ga1−O2A−W2 172.2(5)
Ga1−O8B−W8 175.5(4) Ga1−O7B−W7 177.6(4) Ga1−O2B−W2 177.7(5)
Ga2−O7A−W7 136.9(4) Ga2−O3A−W3 136.9(4) Ga2−O4A−W4 133.4(5)
Ga2−O9A−W9 134.3(4) Ga2−O4G2−W5 134.9(4) Ga2−O4G2−W7 134.1(5)
Ga2−O8T−W8 165.2(4) Ga2−O7T−W7 165.9(4) Ga2−O2T−W2 168.7(6)
The solution behavior of all three polyanions 1−3 was also
studied by 183W NMR. The 183W NMR spectrum of a freshly
prepared solution of Na-2 in 0.1 M HCl showed five signals
at −118, −132, −144, −152, and −170 ppm, respectively, with
relative intensities 2:2:2:2:1 (Figure 2), which is fully consistent
Figure 3. General model of hydrolysis of 4-nitrophenylphosphate (NPP).
To explore the hydrolytic activity of polyanion 2, the
hydrolysis reaction with NPP was carried out and monitored by
1H NMR spectroscopy. In addition, the hydrolysis of ATP was
followed by 31P NMR spectroscopy. The solubility of 2 is poor
in water, so we performed all reactions in the presence of LiCl
in D2O in order to increase its solubility. In a typical experi-
ment, a reaction mixture was prepared containing 0.5 mM poly-
anion 2 and 1 mM NPP (1:2 ratio) in 100 mM LiCl in D2O, and
then heated to 60 °C. The reaction mixture was kept at this
1
temperature, and H NMR spectra were taken at certain time
intervals. More detailed 1H NMR (Figure 4) spectra showed that
Figure 2. 183W spectrum of polyanion 2 in 0.1 M HCl (* possible
impurity signal).
with the C2h symmetry of polyanion 2 in the solid state. The
arsenic analogue Na-1 showed insufficient stability in 0.1 M
HCl during the measurement time, as based on multiple peaks
in the spectrum that could not be assigned, whereas the solu-
bility of RbNa-3 in water was too low, even after countercation
exchange.
Additionally, the UV−vis spectrum of Na-2 displays an
absorption at 256 nm, which is mainly attributed to charge
transfer of the WO bonds in 2. Time-dependent UV−vis
spectra of Na-2 were also measured, and no significant change
was observed after 24 h. As based on the above solution studies,
we concluded that polyanion 2 is stable enough for performing
meaningful hydrolytic studies of phosphoester bonds.
Hydrolysis of Phosphoester Bond. Over the last two
decades, several studies were performed to explore the hydrolysis
1
Figure 4. H NMR spectra of the hydrolysis of 1 mM NPP with
0.5 mM 2 at pD 5 and 60 °C.
complete hydrolysis of NPP occurred within 24 h. During the
reaction a gradual disappearance of the doublets assigned to
NPP can be seen, while two new doublets appear, which can be
assigned to the newly formed hydrolysis product p-nitrophenol
(NP). The upfield shift of the NP resonances can be explained
by the presence of a hydroxyl group, instead of the phosphate
group in NPP.
1
On the basis of the H NMR integration values, the per-
centage of NPP at different time intervals was calculated and
D
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Article
the natural logarithm of the concentration of starting product as
a function of time was fitted to a first-order decay function to
obtain the observed rate constant (Figure S3). However, the
kinetic experiments in which the dependence of the reaction
rate on catalyst concentration is studied could not be reliably
performed due to the very low solubility of the polyanion salt.
The hydrolysis reaction in the absence of polyanion 2 was also
performed in order to correct for the self-hydrolysis of NPP.
The NPP hydrolysis reaction in the presence of 2 was carried
out at different pD values, and the rate constants are
summarized in Table 2.
Zr4−WD2, which is slightly larger than that found for 2 in this
study (1.08 × 10−3 min−1 at pD 6). This might be due to the
higher coordination number and stronger Lewis acidity of ZrIV,
which would result in a better activation of the substrate.
In order to investigate the true need for a POM as ligand for
the Lewis-acidic metal ions, the reactions were also performed
in the presence of the simple GaCl3 salt. However, even in
very small concentrations of GaCl3, precipitates formed in
1
the solution, resulting in the disappearance of H NMR peaks
in the spectrum (Figure 6). The exact composition of the
Table 2. Rate Constants for the Spontaneous Hydrolysis
(kuncat) of 1 mM NPP and in the Presence of 0.5 mM 2 at
60 °C (kobsd) at Various pD Values
a
pD
kobsd
kuncat
kcorr (min−1
)
4
5
6
7
8
1.38 × 10−3
1.36 × 10−3
1.26 × 10−3
3.59 × 10−4
5.28 × 10−5
1.33 × 10−3
6.93 × 10−4
1.81 × 10−4
7.83 × 10−5
4.95 × 10−5
5.00 × 10−5
6.67 × 10−4
1.08 × 10−3
2.81 × 10−4
3.30 × 10−6
Figure 6. 1H NMR spectra of 1 mM NPP in the presence of different
a
concentrations of GaCl3.
kcorr was obtained by subtracting the kuncat value from kobsd
.
precipitate that forms upon addition of GaCl3 is unclear, but
based on 1H NMR it is likely that a complex between GaIII and
NPP is formed. The complete disappearance of the aromatic
signals of NPP suggests an interaction of the phosphate group
with the GaIII ion. On the other hand, no precipitates or gel-
type materials were formed in the NMR tubes when we used
polyanion 2 for hydrolysis, suggesting that the four GaIII ions in
2 are strongly coordinated to the trilacunary Keggin units and
not released in the solution during the catalytic reaction.
Nevertheless, it was difficult to proof the solution stability of 2
unequivocally. Due to the low solubility of Na-2 combined with
the low sensitivity of the 183W nucleus (14.3% natural abundance),
the 183W NMR measurements at conditions pertinent to the
catalytic reactions were not very informative. The 71Ga NMR
spectrum of the reaction mixture was also recorded, but the low
Figure 5. pD dependence of the hydrolysis rate of 1 mM NPP in the
presence of 0.5 mM 2 at 60 °C.
1
sensitivity (0.05 relative to H) and quadrupolar nature of the
71Ga nucleus (spin 3/2) resulted in very broad peaks, which
could not be conclusively analyzed. In addition, we measured
UV−vis spectra of 2 in the presence of NPP, but the UV
bands of the POM at 256 nm overlap with the strong UV band
of the NPP substrate, which absorbs in the same range, and
hence the UV−vis technique was also not very informative.
Indirect evidence for the solution stability of 2 was obtained
by precipitation of the catalyst after the reaction by addition of
tetrabutylammonium bromide (TBABr) and subsequently taking
the IR spectrum of the obtained TBA salt of 2 (Figure S4).
Hydrolysis of ATP. Adenosine triphosphate, known as a
molecular unit of currency of intracellular energy transfer, consists
of a sugar molecule with three linked phosphate groups and was
used as a substrate in hydrolytic studies screened by 31P NMR
spectroscopy. ATP possesses two phosphoanhydride bonds in its
molecular structure, and two steps are required for complete hydro-
lysis. Different concentrations of POM as well as ATP were used in
order to understand the solution dynamics of ATP hydrolysis.
31P NMR spectra were taken at different time intervals during the
hydrolysis of ATP, in the presence and absence of polyanion 2.
As can be seen in Figure 7a, there is one triplet at −21.92 ppm
and two doublets at −9.73 and −10.45 ppm; these peaks
were unambiguously assigned to the central phosphorus (Pβ),
Figure 5 shows a bell-shaped pH reactivity profile, which is
often observed in phosphate cleavage reactions promoted by
metal complexes36 and indicates that deprotonation of the two
acidic metal-bound species results in opposite effects. The first
deprotonation leads to a reactivity increase and is commonly
associated with the formation of an active nucleophile (i.e., a
metal-bound hydroxide), whereas the second deprotonation
results in an anionic species that forms an inactive complex with
the polyanion.36
Interestingly, the isostructural Al derivative [(Al4(H2O)10(β-
12c
SbW9O33)2]6− shows a comparable reactivity toward NPP.
At pD 6 and 7 rate constants of 7.33 × 10−4 min−1 and
4.14 × 10−4 min−1, respectively, were observed, which is similar
to the rate constants obtained for polyanion 2. This is not too
surprising, considering the identical structure and charge of the
two polyanions and comparable Lewis acidity of GaIII and AlIII.
A comparison of the catalytic activity of 2 with that of the
zirconium-containing 32-tungsto-4-phosphate [Zr4(P2W16-
O59)2(μ3-O)2(OH)2(H2O)4]14− (Zr4−WD2), which also con-
tains four strong Lewis acidic metal ions (ZrIV) and shows
hydrolytic activity toward NPP,37 revealed that both poly-
anions exhibit comparable activity. At pD 6.4 and 50 °C the
observed rate constant of 5.06 × 10−3 min−1 was calculated for
E
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Inorganic Chemistry
Article
Figure 8. 31P NMR spectra of reaction mixtures containing different
concentrations of ATP, polyanion 2, and GaCl3. The values in brackets
represent the chemical shift changes compared to a sample containing
20 mM ATP in the absence of 2.
Figure 7. (a) 31P NMR of ATP hydrolysis (20 mM) using 2 mM
polyanion 2 in LiCl/H2O. (b) Adenosine triphosphate structure.
(c) Possible stepwise hydrolysis of ATP in the presence of 2.
concluded that 2 shows a very strong affinity only for Pγ and Pβ,
and the GaIII ions in 2 are selectively coordinated to these,
resulting in the activation and hence hydrolysis of the terminal
phosphoanhydride bond between Pβ and Pγ. The same reaction
was carried out with free GaIII ions instead of polyanion 2;
surprisingly, no chemical shift changes were observed, which is
probably due to the lack of selectivity of free GaIII ions toward
the substrate. In this study, we propose that two gallium(III)
centers from the same polyanion are involved in the catalysis,
and probably one GaIII ion in polyanion 2 is coordinated to the
terminal oxygen of the Pγ, while a nearby GaIII ion in the struc-
ture coordinates to the oxygen atom of Pβ, thereby activating
the phosphoanhydride bond between these two phosphorus
atoms. This hypothesis needs to be further supported by
theoretical molecular modeling calculations, but the available
experimental data indicate that polyanion 2 has some specific,
selective properties and performs well as an effective catalyst for
the hydrolysis of phosphoanhydride bonds. Very small and
broader peaks also appeared during the hydrolysis, which is
probably due to a different and stronger complex between the
POM and ATP.
terminal phosphorus (Pγ), and inner phosphorus (Pα)
respectively. In the first stage of hydrolysis reaction, we noticed
that first ATP is converted to adenosine diphosphate (ADP) due
to the appearance of a singlet at 0.86 ppm and the disappearance
of a triplet at −21.85 ppm. The former one is attributed to
the presence of free phosphate, while the latter one is due to the
disappearance of the signal for the central phosphorus (Pβ) in
ATP, respectively. After almost all the ATP has been converted to
ADP, a new singlet around 1.15 ppm starts to appear in the
31P NMR spectrum, which can be attributed to adenosine
monophosphate (AMP). This is the result of the hydrolysis of the
remaining phosphoanhydride bond in ADP, which releases AMP
and another free phosphate. As it is well known that ATP itself
undergoes self-hydrolysis in solution, a reaction mixture lacking 2
was monitored at different time intervals to investigate the
catalytic power of the polyanion. Only 8% of ATP was converted
to ADP in the absence of 2 in the first 20 h, whereas 75%
ATP hydrolysis occurred in the presence of the polyanion.
The ATP−ADP conversion magnitude slightly increased when
the time increased. In total, 19% ATP−ADP self-hydrolysis
occurred in the first 40 h, while the hydrolysis yield reached up to
93% in the presence of 2. This clearly indicates that polyanion 2
acts as an effective homogeneous catalyst toward the hydrolysis of
the phosphoanhydride bonds in the ATP structure. From the data
obtained, a general model of hydrolysis of ATP is proposed
(Figure 7c).
In order to find out the origin for the selectivity of the
phosphoanhydride bond hydrolysis in ATP, distinct reaction
mixtures were prepared containing different concentrations
of polyanion 2 (Figure 8). When 1 mM 2 was present in the
reaction mixture, the Pγ and Pβ chemical shift values changed
about 0.16 and 0.19 ppm, respectively, in comparison with the
spectrum of pure ATP, whereas the Pα values shifted upfield
only by 0.05 ppm. The same effect exactly doubled when the
concentration of 2 was increased from 1 mM to 2 mM. With
further increases of the concentration of 2, the chemical shift
values of Pγ and Pβ were affected more, which points toward
a fast equilibrium on the NMR time scale for the complexa-
tion between POM and ATP. From this experiment it can be
CONCLUSIONS
■
In conclusion, we have successfully synthesized the three
gallium-containing, sandwich-type heteropolytungstates
[Ga4(H2O)10(β-XW9O33)2]n− (X = As, 1; Sb, 2; n = 6; Te, 3;
n = 4) in aqueous acidic medium. Polyanions 1−3 have been
characterized in the solid state by FT-IR spectroscopy,
elemental analysis, TGA, and single-crystal XRD. The solution
properties of 2 were studied by 183W NMR and UV−vis
spectroscopy, and its catalytic activity toward the hydrolysis of
the DNA model substrate NPP and ATP at near physiological
pH was followed by 31P NMR. Similar biological studies of
other related POM derivatives as well as DFT studies are in
progress, and the results will be reported elsewhere.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.6b01030.
■
S
X-ray crystallographic data (CIF)
F
DOI: 10.1021/acs.inorgchem.6b01030
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
X-ray crystallographic data (CIF)
Article
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4, 62−68.
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́
X-ray crystallographic data (CIF)
Solid-state FT-IR and thermograms for Na-1, Na-2, and
RbNa-3 (PDF)
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̈
̈
AUTHOR INFORMATION
■
Pohlmann, H.; Dickman, M. H.; Rosu, C.; Pope, M. T.; Krebs, B.
Inorg. Chem. 1999, 38, 2688−2694. (d) Krebs, B.; Droste, E.;
Piepenbrink, M.; Vollmer, G. C. R. Acad. Sci., Ser. IIc: Chim. 2000, 3,
205−210.
Corresponding Author
*E-mail: u.kortz@jacobs-university.de.
Author Contributions
́ ́ ́
(9) (a) Kortz, U.; Jeannin, Y. P.; Teze, A.; Herve, G.; Isber, S. Inorg.
⊥B. Kandasamy and S. Vanhaecht contributed equally.
Chem. 1999, 38, 3670−3675. (b) Kortz, U.; Isber, S.; Dickman, M. H.;
Ravot, D. Inorg. Chem. 2000, 39, 2915−2922. (c) Kortz, U.; Matta, S.
Inorg. Chem. 2001, 40, 815−817. (d) Bassil, B. S.; Kortz, U.; Tigan, A.
S.; Clemente-Juan, J. M.; Keita, B.; de Oliveira, P.; Nadjo, L. Inorg.
Chem. 2005, 44, 9360−9368. (e) Nsouli, N. H.; Ismail, A. H.;
Helgadottir, I. S.; Dickman, M. H.; Clemente-Juan, J. M.; Kortz, U.
Inorg. Chem. 2009, 48, 5884−5890. (f) Chen, L.; Shi, D.; Zhao, J.;
Wang, Y.; Ma, P.; Wang, J.; Niu, J. Cryst. Growth Des. 2011, 11, 1913−
1923. (g) Chu, L.; Zhou, B.; Wang, C.; Zhao, Z.; Su, Z.; Yu, K. Solid
State Sci. 2011, 13, 488−491.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
U.K. thanks the German Research Foundation (DFG, KO-
2288/20-1) and Jacobs University for research support.
T.N.P.V. thanks KU Leuven and FWO Flanders for financial
support. The COST Actions CM1203 (PoCheMoN) and
CM1006 (EUFEN) are acknowledged. S.V. thanks COST
Action CM1006 (EUFEN) for a short-term scientific mission
at Jacobs University and Agency for Innovation by Science
and Technology (IWT) for a doctoral fellowship. Figure 1 was
generated using Diamond version 3.2 software (copyright,
Crystal Impact GbR).
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120, 7252−7259.
(11) Limanski, E. M.; Drewes, D.; Droste, E.; Bohner, R.; Krebs, B. J.
Mol. Struct. 2003, 656, 17−25.
(12) (a) Kortz, U.; Savelieff, M. G.; Bassil, B. S.; Keita, B.; Nadjo, L.
Inorg. Chem. 2002, 41, 783−789. (b) Hussain, F.; Reicke, M.;
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DOI: 10.1021/acs.inorgchem.6b01030
Inorg. Chem. XXXX, XXX, XXX−XXX