Carbonate encapsulation from dissolved atmospheric CO2 into a polyoxovanadate capsule

Article abstract of DOI:10.1039/c9dt01103j

An aqueous synthesis, involving the reduction of the VO₃^- anion in a mild alkaline pH in the presence of α-Bi₂O₃, led to the formation of a fully reduced polyoxovanadate (POV) capsule, with CO₃^2- anion encapsulation in its internal cavity, in the compound [Na₆(H₂O)₂₄][H₈VIV15O₃₆(CO₃)]·3N₂H₄·10H₂O (1). This CO₃^2- anion encapsulation, the source of which is absorbed aerial CO₂ in the pertinent aqueous alkaline reaction mixture, occurs only in the presence of α-Bi₂O₃. Compound 1 crystals, upon exposure to HCl acid vapor, exclude CO₂ gas that can react with the Grignard reagent (PhMgBr) to form triphenylcarbinol and benzoic acid; during this solid-vapor interface reaction, compound 1 itself transforms into an amorphous material that includes the Cl^- anion but could not be characterized unambiguously. Thus, we have synthesized a chloride ion (Cl^-) encapsulated compound [Na₁₀(H₂O)₂₄][H₃VIV15O₃₆(Cl)]·6H₂O (2) in a direct synthesis protocol, which has been characterized by crystallography as well as by other spectroscopic methods. Compounds 1 and 2, each having fifteen vanadium(iv) centers, exhibit interesting magnetism in their solid states. The temperature-dependent magnetic susceptibilities for compounds 1 and 2 have been recordred at 0.1 T in the temperature range of 3-300 K. The temperature-dependent magnetic susceptibilities of compounds 1 and 2 are shown in the form of χ_Mvs. T and their product χ_MT vs. T plots.

Full text of DOI:10.1039/c9dt01103j

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Carbonate encapsulation from dissolved
atmospheric CO2 into a polyoxovanadate
Cite this: DOI: 10.1039/c9dt01103j
capsule†‡
Sateesh Mulkapuri,
Sathish Kumar Kurapati
and Samar K. Das
*
−
An aqueous synthesis, involving the reduction of the VO3 anion in a mild alkaline pH in the presence of
α-Bi2O3, led to the formation of a fully reduced polyoxovanadate (POV) capsule, with CO32− anion encap-
2−
sulation in its internal cavity, in the compound [Na6(H2O)24][H8VI1V5O36(CO3)]·3N2H4·10H2O (1). This CO3
anion encapsulation, the source of which is absorbed aerial CO2 in the pertinent aqueous alkaline reac-
tion mixture, occurs only in the presence of α-Bi2O3. Compound 1 crystals, upon exposure to HCl acid
vapor, exclude CO2 gas that can react with the Grignard reagent (PhMgBr) to form triphenylcarbinol and
benzoic acid; during this solid–vapor interface reaction, compound 1 itself transforms into an amorphous
material that includes the Cl− anion but could not be characterized unambiguously. Thus, we have syn-
thesized a chloride ion (Cl−) encapsulated compound [Na10(H2O)24][H3VI1V5O36(Cl)]·6H2O (2) in a direct
synthesis protocol, which has been characterized by crystallography as well as by other spectroscopic
methods. Compounds 1 and 2, each having fifteen vanadium(IV) centers, exhibit interesting magnetism in
their solid states. The temperature-dependent magnetic susceptibilities for compounds 1 and 2 have
been recordred at 0.1 T in the temperature range of 3–300 K. The temperature-dependent magnetic sus-
ceptibilities of compounds 1 and 2 are shown in the form of χM vs. T and their product χMT vs. T plots.
Received 13th March 2019,
Accepted 5th May 2019
DOI: 10.1039/c9dt01103j
rsc.li/dalton
known to have well-defined internal cavities for
encapsulation.22–25 All these shell-containing POV clusters,
Introduction
Polyoxometalates (POMs), the all-inorganic metal–oxygen capable of encapsulating a guest species, have a common
cluster species having wide structural diversity1–14 (in size as feature of the presence of a square-pyramidal {O4VvO} polyhe-
well as in shape) and diverse applications (in catalysis,4,5 dron, which is believed to have a tendency to form the cage or
photochemistry,5
electrochemistry,4–11
medicine,12 shell.23–27,30–49 The presence of the guest spices, such as
magnetism,13–20 proton conduction,21 etc.), exhibit interesting halide, pseudohalide, inorganic, or organic anions in the con-
host–guest chemistry.21–29 Complete encapsulation of a guest cerned synthesis reaction mixture, plays a key role in the
molecule/ion in the internal cavity of the host POM cluster is isolation of the resulting host–guest compounds. The guest-
−
of considerable interest because it represents a unique supra- anionic-species like Cl−, Br−, I−, CN−, CO32−, NO−, NO2−, N3
,
molecular core–shell entity, in which the guest species influ- PO43−, HCOO−, CH3COO−, SH−, SCN−, SO32−, SO42−, ClO4− are
ences the electronic properties of the resulting host–guest known to be encapsulated by the following shell-containing
system.21–49 In this regard, among POMs, polyoxovanadates POV hosts: [V12O32(X)]5− (X = NO−, CN−, OCN−, NO2−, NO3
,
−
(POVs) draw special attention, because a number of POVs are HCO2−, CH3COO−, Cl−, Br−, I−);31–34 [V12O32(X)]4− (X = CO2,
CH3CN, DCM, CH3NO2, CH3Br);26,30 [V15O36(CO3)]7− 24,25
[V15O36(X)]6− (X = Cl−, Br−, I−);35,36 [V16O38(X)]n− (X = F−, Cl−,
Br−, CN−, H2O; n = 3, 4);37–45 [V16O39(Cl)]6− 45 [V16O42(Cl)]8− 42
[V18O42(X)]n− and [V18O44(X)]n− (X = H2O, Cl−, Br−, I−, SO4
SO32−, VO43−, N3−, NO2−, NO3−, SH−, HCOO−; n = 5–15);46–49
[V22O54(X)]7− (X = ClO4−, N3−); [V30O74{(V4O4)O4}]10− 46
Thus,
;
School of Chemistry, University of Hyderabad, P.O. Central University,
Hyderabad – 500046, India. E-mail: skdas@uohyd.ac.in, samar439@gmail.com;
Fax: +91-40-2301-2460; Tel: +91-40-2313-4853
;
;
2−
,
†We would like dedicate this article to Prof. V. Chandrasekhar on the occasion
of his 60th birthday.
.
chloride anion (Cl−) encapsulation was found in diverse cage-
containing POV systems, as reported by Müller,35 Peng,37
Khan,42 Hayashi43 and their co-workers. Among these host–
guest systems, carbonate anion (CO32−) encapsulation by the
mixed-valence POV cluster anion [VV7 V8IVO36(CO3)]7− has drawn
‡Electronic supplementary information (ESI) available: Bond distances and
bond angles tables, PXRD patterns, TGA-DTA, TGA-IR, TGA-Mass, BVS calcu-
lations, carbonate ion confirmation tests, volumetric redox titrations for
vanadium centers and reports of elemental analysis. CCDC 1839257 (2). For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9dt01103j
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2−
our special attention, because the CO3 anion can be derived ceptibilities were measured in the temperature range of
from CO2 gas (CO2 + OH− → CO3 + H+), which is an impor- 3–300 K on a Quantum Design VSM SQUID. ICP-OES elemen-
2−
tant component in our earth-surface atmosphere in the tal analyses were performed on an ICP-OES Varian 720ES. XPS
context of global warming.50,51 Müller and coworkers reported analyses of the powdered samples were performed on an
the first mixed-valent POV encapsulated carbonate compound Omicron Nanotech ESCA+ (Oxford Instruments, Germany).
Li7[VV7 V8IVO36(CO3)] ∼ca.·39H2O in 1990, which was synthesized Powder X-ray diffraction (PXRD) patterns of 1 and 2 were
in the presence of Li2CO3.24 Four years later, Yamase and recorded on a Bruker D8-Advance diffractometer using graph-
Ohtaka prepared the same mixed-valent host–guest compound ite monochromated Cu Kα1 (1.5406 Å) and Kα2 (1.54439 Å)
K5H2[VV7 V8IVO36(CO3)]·14.5H2O (but with different cations), radiation.
which was synthesized by photolyzing an aqueous CH3OH
4−
Synthesis of the compound
solution mixture of [V4O12
]
in the presence of K2CO3.25
[Na6(H2O)24][H8VI1V5O36(CO3)]·3N2H4·10H2O (1)
There are reports on the interactions between CO2 and
POMs.52–57
3.66 g of NaVO3 was added to 100 mL of hot water
Herein, we report a unique aqueous synthesis of a POM-based (temp. 85–90 °C) in a 250 mL conical flask to give a clear col-
host–guest system [Na6(H2O)24][H8VI1V5O36(CO3)]·3N2H4·10H2O (1), ourless solution. 0.31 g of solid α-Bi2O3 and 0.72 g of solid
in which the fully reduced POV capsule captures dissolved NaOH were added subsequently to the above solution and
aerial CO2 from an alkaline aqueous synthesis mixture in the immediately covered with a watch glass and stirred for 30 min
form of carbonate anion. The absorption of aerial CO2 by an on a hot plate (temp. 85–90 °C). Afterwards, 2.5 g of
aqueous alkaline solution is not surprising. But, the fact that N2H5·HSO4 was added in small portions (to avoid aggressive
the encapsulation of this dissolved aerial CO2 into the internal effervescence) to the above reaction mixture. After the addition
cavity of the POV capsule does not happen ‘just like that’ of N2H5·HSO4, the colour of the solution changed from yellow
without the presence of some amount of α-Bi2O3 in the concerned to dark green, and this was stirred for another 15 min and
reaction mixture, is surprising. We could isolate compound 1 cooled to room temperature. The solution was filtered, and the
from an aqueous alkaline solution of sodium vanadate under filtrate was left undisturbed for 24 hours in an open atmo-
reducing condition, only in the presence of α-Bi2O3. The com- sphere. The dark brown coloured precipitate formed, was fil-
pound [Na6(H2O)24][H8VI1V5O36(CO3)]·3N2H4·10H2O (1), upon tered off and the dark green coloured filtrate was kept undis-
exposure to HCl vapor, excluded carbonate in the form of CO2, turbed for crystallization. After 10 days, the dark green crystals
whereby compound 1 transformed into an amorphous substance, along with a minute amount of unidentified precipitate settled
which could not be characterized unambiguously, but was found and were filtered, washed three times with a small amount of
to contain chlorine (from elemental analysis). We, therefore, syn- ice-cold water (∼20 mL), washed twice with 20 mL of an
thesized
a
chloride-included
compound
[Na10(H2O)24] ethanol and ice-cold water mixture (1 : 1 ratio) and dried in air
[H3VI1V5O36(Cl)]·6H2O (2), in which the same POV cluster capsule, at room temperature. Yield: 2.21 g (50% based on vanadium).
as found in compound 1, encapsulates chloride anion in its Anal. calcd % for V15Na6O79C1H88: V, 33.81; Na, 6.09; C, 0.55;
cavity, as observed from its crystal structure determination.
H, 3.61, N, 3.76. Found: V, 34.96 (ICP-OES); Na, 6.58 (ICP-OES);
C, 0.61(CHN); H, 3.98; (CHN); N, 3.72; (CHN).
Synthesis of the compound [Na10(H2O)24][H3VI1V5O36(Cl)]·6H2O (2)
Experimental
3.66 g of NaVO3 was added to 100 mL of hot water
(temp. 85–90 °C) in a 250 mL conical flask to give a colourless
solution. 0.31 g of solid α-Bi2O3 and 0.72 g of solid NaOH were
added subsequently to the above solution and immediately
covered with a watch glass and stirred for 30 min on a hot
plate (temp. 85–90 °C). 2.5 g of N2H5·HSO4 was added in small
portions (to avoid effervescence) to the resulting reaction
All the chemicals were of analytical grade and used as received.
Deionised water was used for all the experiments. Compounds
1 and 2 were synthesized by conventional wet synthesis
methods under ambient atmosphere.
Physical measurements
Elemental analysis was performed using a FLASH EA series mixture. After the addition of N2H5·HSO4, the colour of the
1112 CHNS analyser. A Bruker Tensor II equipped with solution changed from yellow to dark green, and this was
Platinum ATR (Attenuated Total Reflectance) accessory was stirred for another 15 min. 4.5 g of solid NaCl was added to
used to collect the FT-IR spectra. A TA instrument Q600 TG/ the reaction mixture, and the stirring was continued for
DTA coupled with a Bruker Tensor II ATR-IR was used to 10 more minutes. The reaction contents were cooled to room
record TGA-FTIR measurements. TG analyses were carried out temperature, and the solution was filtered. The filtrate was
on a PerkinElmer – STA 6000, and TG-Mass analyses were per- kept undisturbed in an open atmosphere; after 4 days, black-
formed on
a
PerkinElmer
–
STA 6000 coupled with
a
colored crystals along with a minute amount of unidentified
PerkinElmer–Clarus SQ8S mass spectrometer. Raman spectra precipitate were obtained, filtered, washed three times with a
were recorded on a Wi-Tec alpha 300AR laser confocal optical minimum amount of ice-cold water (∼20 mL), washed twice
microscope (T-LCM) facility equipped with a Peltier-cooled with 20 mL of an ethanol and ice-cold water mixture (1 : 1
CCD detector using a 785 nm Argon ion laser. Magnetic sus- ratio) and dried in air at room temperature. Yield: 2.35 g
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Table 1 Crystal data and structure refinement parameters for com-
pounds 1 and 2
(55.5% based on vanadium from the product). Anal. calcd %
for V15Na10O66ClH63: V, 35.56; Na, 10.70; Cl, 1.65. Found: V,
35.53 (ICP-OES); Na, 10.58 (ICP-OES); Cl, 1.68 (ICP-OES).
1
2
Carbonate ion confirmation test
Empirical formula
Formula weight
T (K)/λ (Å)
Crystal system
Space group
a/Å
b/Å
c/Å
CH88N6Na6O73V15
2254.77
100(2)/0.71073
Hexagonal
ˉ
P62c
ClH63Na10O66V15
2148.93
Test 1. 0.1 M calcium hydroxide solution was added drop-
wise to (0.1 mmol) 5 mL of the solution of compound 1 (deep
green solution). Cloudy white precipitation slowly formed,
which in turn confirmed the presence of the CO32− ion in com-
pound 1.
Test 2. In a 15 mL test tube, 5 mL of a 0.1 mmol solution of
compound 1 was treated with 5 mL of 0.1 M hydrochloric acid.
An immediate gas effervescence was observed along with the
generation of a bright blue-coloured solution. The efferves-
cence confirmed the presence of carbon dioxide in the form of
carbonate, and the generation of the bright blue colour in the
solution indicated the presence of vanadium in the +4 oxi-
dation state.
293(2)/0.71073
Hexagonal
ˉ
P62m
12.9910(6)
12.9910(6)
23.1099(12)
90
90
120
3377.5(4)
2
2.130
2.140, 2086
1.085
0.0285/0.0769
0.018(6)
2.384/−0.705
13.0800(18)
13.0800(18)
11.160(2)
90
90
120
1635.5(5)
1
2.094
2.237, 1000
1.065
0.0585/0.1753
0.02(9)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
P calcd (g cm−3
)
μ (mm−1), F (000)
Goodness-of-fit on F2
R1/wR2 [I > 2σ(I)]
R1/wR2 (all data)
Largest diff. peak/hole (e Å−3
)
1.005/−0.797
CO2 exclusion from compound 1 in a gas–solid interface reac-
tion followed by Grignard reaction
Single-crystal X-ray diffraction data of 1 and 2 were collected at
100 K and 293 K, respectively, on a Bruker Smart Apex III
single-crystal X-ray diffractometer equipped with a CCD area
detector system, graphite monochromator and a Mo-Kα fine-
focus sealed tube (λ = 0.71073 Å). The scans were recorded
with a ω scan width of 0.3°. Data reduction was performed
using SAINT PLUS,58 and empirical absorption corrections
using equivalent reflections were performed by the program
2.2 g (1 mmol) of compound 1, having encapsulated carbonate
anion, was taken in a three-neck round-bottom flask and
reacted with dry HCl vapours at 5 °C. The generated CO2 was
passed through a dry tetrahydrofuran (THF) solution of tri-
methylamine to trap the unreacted HCl vapours and allowed
to react with 2 mL of freshly prepared 0.1 M PhMgBr reagent
in 10 mL of THF at −80 °C. The reaction mixture was mixed
with water and the pH was adjusted to 6.3 with 1 M aqueous
HCl solution and then extracted with diethyl ether. The resul-
tant reaction mixture was analysed through HRMS to identify
the resulting products.
SADABS.59
Structure
solutions
were
done
using
SHELXT-2014,60 and full-matrix least-squares refinement was
carried out using SHELXL-2018.61 All of the non-H atoms were
refined anisotropically. Compound 1 crystallizes in the hexag-
ˉ
onal P62c space group and compound 2 crystallizes in the hex-
Manganometric redox titrations of compounds 1 and 2
ˉ
agonal P62m space group. The important crystallographic data
Prior to the preparation of solutions, the solvents used were
saturated with UHP nitrogen gas to remove dissolved oxygen.
0.05 N potassium permanganate solution (KMnO4) was pre-
pared and standardized with 0.05 N ammonium ferrous sul-
phate ((NH4)2Fe(SO4)2·6H2O) in 0.2 N sulphuric acid solution.
The titrations were carried out completely under nitrogen
atmosphere at room temperature to prevent the interaction of
air with the vanadium(IV) ion solution. The 0.05 N solution
was freshly prepared by dissolving the corresponding
vanadium(IV) compound in 0.2 N H2SO4 solution. 10 mL of the
of compounds 1 and 2 were given in Table 1. The details of the
bond distances and angles corresponding to compounds
1 and 2 were provided in the ESI Tables S1 and S2‡ containing
the supplementary crystallographic data for compounds 1 and
2; CCDC 1839257‡ and CSD 434477 contain the supplementary
crystallographic data for compounds 1 and 2, respectively.
Results and discussion
above solution was transferred into an Erlenmeyer flask and The synthesis of the deep green-colored compound
0.2 mL of 85% H3PO4 (syrupy phosphoric acid) as catalyst and [Na6(H2O)24][H8VI1V5O36(CO3)]·3N2H4·10H2O (1) was carried out
2 drops of ferroin as the redox indicator were added. The in an aqueous medium under ambient conditions. The syn-
mixture was titrated against standardized 0.05 N potassium thesis involves NaVO3 as the vanadium precursor, N2H5·HSO4
permanganate solution (KMnO4). The endpoint was realized as the reducing agent, a small quantity (10 mol%) of α-Bi2O3
by a colour change from red to pale green. The experiment was and solid NaOH (to adjust the pH to 8.5). Compound 1 (which
repeated for two concurrent values. The obtained results and does not contain any bismuth) cannot be synthesized in
calculations are given in the ESI (section S8‡).
absence of α-Bi2O3 from the concerned reaction mixture. It is
also important to mention that no external CO3 source was
used for the preparation of 1, and the spontaneous absorption
2−
X-ray crystallography
Single crystals of compounds 1 and 2, suitable to collect diffr- of atmospheric CO2 is the only possible source of encapsulated
action data, were obtained directly from the reaction mixture. CO32− in 1. Thus, α-Bi2O3 plays an important role in the encap-
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sulation of dissolved atmospheric CO2 (in the alkaline mean plane of the concerned basal square plane of the four
aqueous synthesis mixture) into the cavity of the {V15} capsule oxygen atoms. This implies that the interactions between peri-
in 1. It is known that α-Bi2O3 shows a weak synergetic inter- pheral vanadium atoms and the central carbonate ion are
action with CO2 in an alkaline solution.62–65 Based on this, we strong enough to consider these as coordinate covalent bonds.
speculated that in the present aqueous alkaline synthesis of The overall topology of the POV cluster (compound 1) is com-
compound 1, α-Bi2O3 establishes a reversible equilibrium with parable to that of the mixed-valent POV cluster in the com-
absorbed atmospheric CO2: α-Bi2O3 + CO2 ⇋ (BiO)2CO3, and pound, Li7[VV7 V8IVO36(CO3)] ∼ca.·39H2O, reported by Müller and
supplies the carbonate anion to compound 1 for its encapsula- co-workers.24
2−
tion. In order to understand the survival of Bi2O3 in the con-
Since we did not supply any source of CO3
anions
cerned reaction mixture before and after crystallization, we per- (included in the cavity of the {V15} capsule in compound 1) in
formed FESEM studies along with elemental mapping on the the concerned reaction mixture and it is the aerial CO2 dis-
reaction mixture by analyzing the sample solution after one solved in alkaline reaction mixture that becomes encapsulated
day and after four days of reaction. (In this synthesis, crystalli- in the internal cavity of compound 1, we characterized the
zation begins after two days of reaction.) FESEM images of the guest carbonate anion using IR and Raman spectral studies
reaction mixture collected after one day showed the presence along with TGA-MS and TGA-IR studies besides single-crystal
of some flake-shaped particles decorated on sphere-shaped X-ray crystallography. The presence of carbonate can be rea-
vanadium-containing particles. In the case of reaction mixture lized by infrared (IR) bands observed at 1468 and 1406 cm−1
collected after four days, FESEM images showed the presence that can be assigned to stretching frequencies of the carbonate
of a similar type of flake-shaped particles, but on needle- ion (Fig. 2a).
2−
shaped vanadium-containing crystals (since the crystallization
The presence of CO3 was also realized with the help of
process had already started). The elemental map of these online coupled TGA-Infrared (Fig. 2b and Fig. S5, ESI‡) and
flake-shaped particles of both instances showed mainly the TGA-Mass spectral analysis (Fig. 2c and Fig. S7, ESI‡). When
existence of bismuth and oxygen. Thus, it was determined that the temperature of the sample reached 350 °C, CO2 stretching
the solution contained some amount of bismuth- and oxygen- vibrations at 2295 cm−1 and 2345 cm−1 appeared (i.e., the
2−
containing compounds that could be Bi2O3. The existence of decomposition of CO3 as CO2). The complete disappearance
Bi(OH)3 (in the alkaline reaction mixture) and elemental of CO2 vibrations occurs at around 445 °C. The TGA-Mass
bismuth (under reducing condition) could not be ruled out spectra were also consistent with the TGA-IR data and showed
(see section S1, ESI‡ for FESM images and concerned elemen- a mass loss response, corresponding to CO2 at a similar temp-
tal maps).
erature region (Fig. 2c and Fig. S7, ESI‡). The qualitative ana-
2−
The crystal structure of 1 reveals that an encapsulated lysis test for the CO3 ion was also performed and consistent
2−
CO3 anion was surrounded by 15 vanadium and 21 oxygen positive results were obtained (section S5, Fig. S8, ESI‡).
atoms spherically in the fully reduced POV capsule anion
Any carbonate salt, upon reacting with a mineral acid, liber-
[H8VI1V5O36(CO3)]6−. The carbonate anion was stabilized by ates CO2 gas (CO3 + 2H+ → CO2 + H2O), but not always with
coordination with three vanadate units {V–O(CO32−) = 2.30 Å} an acid vapor. Remarkably, the crystals of the compound
in the cavity of the POV capsule. The POV cluster displays 12 [Na6(H2O)24][H8VI1V5O36(CO3)]·3N2H4·10H2O (1) (having a car-
{VO5} square pyramidal units and three distorted octahedral bonate anion per formula unit) underwent gas–solid interface
2−
2−
{VO6} units that were formed via coordination with the central reaction with HCl vapor with the exclusion of CO3 from the
2−
encapsulated CO3 anion, as shown in Fig. 1. Each of these POV capsule as CO2 gas, resulting in the formation of an amor-
three vanadium centers, coordinated to the central CO32−, lie phous solid (eqn (1)), as confirmed from its PXRD studies
about 0.39 Å above the mean plane of the four equatorial (section S6, ESI‡).
oxygen atoms. On the other hand, in each of twelve VO5
½Na6ðH2OÞ ꢀ½H8VIV O36ðCO3Þꢀ ꢁ 3N2H4 ꢁ 10H2Oð1; solidÞ
centers, the vanadium is located about 0.631 Å above the
þ HCl ð2v4aporÞ 1!5 CO2 þ an amorphous solid
ð1Þ
This exclusion of carbonate as CO2 gas can simply be moni-
tored by IR spectroscopy. The IR bands, observed at 1468 and
1406 cm−1 for 1, disappear upon exposing the crystals of 1 to
HCl vapor, resulting in the formation of an amorphous sub-
stance in a solid-to-solid transformation (eqn (1)). We could
not unambiguously characterize the resulting amorphous com-
pound, formed by a gas–solid reaction (eqn (1)). However,
ICP-OES elemental analyses of this resulting amorphous solid
showed the presence of chlorine. The excluded CO2 vapor from
the encapsulated carbonate, as usually, can be reacted with
Grignard reagent, PhMgBr to form triphenylmethanol and
the carbonate encapsulated cluster cage, [H8V1IV5O36(CO3)]6− (compound 1). benzoic acid (section S7, ESI‡) (Scheme 1).
Fig. 1 Thermal ellipsoidal and polyhedral plots (with 60% probability) of
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Scheme 1 Conversion of CO2, excluded from compound 1, to triphe-
nylmethanol and benzoic acid by reacting with PhMgBr.
was found to contain chlorine (ICP-OES), we intended to syn-
thesize a chloride anion-encapsulated compound through
direct synthesis. Thus, we performed a synthesis similar to the
synthesis of compound 1 but added an excess amount of NaCl
at the end. We indeed isolated and structurally characterized a
Cl− ion included compound [Na10(H2O)24][H3V15O36(Cl)]·6H2O
(2), in which a chloride anion was found to be encapsulated in
the internal cavity of the same {V15} cluster found in com-
pound 1. The crystal structure of this chloride anion-encapsu-
lated cluster cage is shown in Fig. 3. As far as compositions are
concerned, the compound 2 POV cluster anion encapsulates a
Cl− in its cavity (Fig. 3) and is tri-protonated in contrast to the
2−
compound 1 POV cluster anion, which encapsulates a CO3
anion in its cavity and is protonated by 8 H+. The Cl− ion at
the center of the cluster shell (compound 2) does not have any
significant interactions with the cluster cage and hence is
stabilized by electrostatic interactions.
The presence of the same {V15} cluster cage in compound 2
can also be established by comparing its Raman signal with
that of compound 1. As shown in Fig. 4, the Raman signals of
compounds 1, 2 and the known mixed-valent compound
Li7[V7VV8IVO36(CO3)] ∼ca.·39H2O24 are comparable. More specifi-
cally, the Raman bands corresponding to V–O(µ2-O/µ3-O)
stretchings, observed for
200–650 cm−1, are comparable to those found for the iso-
structural mixed-valent compound
Li7[VV7 VI8VO36(CO3)]
1
and
2
in the range of
∼ca.·39H2O.24 The Raman spectral results are consistent with
the IR spectral results (Fig. 2a, vide supra). Careful analysis
shows an extra but small feature at around 800 cm−1 in the
Raman spectra of 1 as well as in the mixed-valent compound
(both are carbonate anion-containing compounds), which is
not present in the Raman spectrum of 2 (chloride anion
encapsulated compound 2). We can thus assign this band at
around 800 cm−1 in the Raman spectra of both compounds to
the Raman active A′ mode of D3h carbonate.66
Fig. 2 (a) FTIR spectra of compounds 1, 2 and the Müller mixed-valent
compound Li7[VV7 VIVO36(CO3)] ∼ca.·39H2O.24 (prominent carbonate
8
stretching bands of 1 and the Müller mixed-valent compound were
located at 1468 and 1406 cm−1, whereas compound 2 show none of
these bands in that region). (b) TGA-IR spectral profile of compound 1
displaying stretching bands corresponding to CO2 evolution as a result
2−
of CO3 decomposition in 1. (c) TGA-Mass spectrum of compound 1
2−
displaying a peak corresponding to CO2 evolution as a result of CO3
decomposition in 1.
As we could not unambiguously characterize the amor-
phous compound formed in the gas–solid reaction (as it loses
Fig. 3 Polyhedral and thermal ellipsoidal plots (with 60% probability) of
crystallinity during its solid-to-solid state formation), and it the chloride-encapsulated cluster cage, [H3VI1V5O36(Cl)]10− (in compound 2).
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Fig. 4 Raman spectra of compounds 1 and 2 along with the Müller’s
mixed-valent compound Li7[VV7 V8IVO36(CO3)] ∼ca.·39H2O.24
There are only very few organic-free fully reduced polyoxova-
nadate (POV) cluster-containing compounds; interestingly, all
of them were reported by Müller’s group.13,19,20,47 All these
fully reduced organic-free POV compounds were reported to
exhibit remarkable magnetism. Compounds 1 and 2 (present
work) are new additions to this rare class of organic free fully
reduced vanadium-based magnetic materials. Thus, we have
recorded magnetic susceptibilities of compounds 1 and 2 at
0.1 T in the temperature range of 3–300 K. Fig. 5 shows the
magnetic behaviors of compounds 1 and 2 in the form of χM
Fig. 5 χM vs. T and χMT vs. T plots of compounds 1 and 2: temperature
dependences of χM and χMT.
versus T and the product χMT versus T plots, respectively. The detailed account of magnetic interactions, including theore-
χMT value of compound 1 at 300 K is 5.758 cm3 K mol−1 tical studies, in compounds 1 and 2 are not possible at this
(6.786μB), which is very close to the total spin-only value stage and will be reported later in detail.
5.626 cm3 K mol−1 (6.708μB), expected for 15 non-interacting
The bond valence sum calculations conclude that, all
electrons. In the case of compound 2, the corresponding value vanadium centers in 1 and 2 are in the +4 oxidation state in
is 4.104 cm3 K mol−1 at 300 K, which is significantly less com- contrast to the vanadium centers of same cluster cage contain-
pared to the total spin-only value (6.708μB, expected for 15 ing the compound Li7[VV7 V8IVO36(CO3)] ∼ca.·39H2O (mixed-
non-interacting electrons). As shown in Fig. 5, as the tempera- valent compound),24 which has 7 vanadium atoms in the +5
ture decreases, the χMT value of 1 decreased linearly and oxidation state and 8 vanadium atoms in the +4 oxidation
reached 2.36 cm3 K mol−1 at 42 K. The value decreased rapidly state. Accordingly, the bond distances around the vanadium
below 42 K and reached 1.104 cm3 K mol−1 at 3 K. This behav- centres in
1 and 2 are quite different from those in
iour does not follow Curie and Curie–Wiess law and seems to Li7[VV7 VI8VO36(CO3)] ∼ca.·39H2O24 and other mixed-valent
be the antiferromagnetic interactions of individual spins in 1. {V18O42} systems.40 The bond valence sum calculations (BVS)
On the other hand, the χMT value of 2 is constant when the on 1 and 2 (section S8, ESI‡) furnishes that the relevant BVS
temperature is varied from 300 K to 250 K, but decreased line- values are in the ranges of 3.40–4.22 (Avg. = 3.89) and
arly to 0.39 cm3 K mol−1 with the decrease of temperature 3.87–4.28 (Avg. = 4.11) for 1 and 2, respectively. The average V–
from 250 K to 3 K. The χMT value of 0.39 cm3 K mol−1 at 3 K O terminal bond distances (1.62–1.64) are significantly longer
for 2 is close to zero and infers an S = 0 ground state for 2. in 1 and 2 compared to the same distances reported for mixed-
Thus, the magnetic susceptibilities of 1 as a function of temp- valent and fully oxidized polyoxovanadates.67 This observation
erature are considerably different from 2 as a function of temp- is also consistent with the +4 oxidation state of the vanadium
erature. This clearly suggests that the encapsulated and co- centers in 1 and 2. In order to support the BVS calculations,
ordinated carbonate anion in 1 as the guest facilitates a we recorded the X-ray photoelectron spectra (XPS) of com-
different magnetic pathway compared to the magnetic pounds 1 and 2, whereby O 1s response at 530 eV was taken as
exchange pathways, shown by compound 2 having non-co- the reference. Fig. 6 (see also, section S9, ESI‡) shows the
ordinated encapsulated Cl− anions as the guest. However, the partial XPS plots of 1 and 2. Both compounds display a signal
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Fig. 7 UV-Vis absorption (solution) spectra of 1 and 2 in water.
Fig. 6 Core level X-ray photoelectron spectroscopy of compounds 1
and 2.
in the range of 514.96–515.73 eV for V 2p3/2 with FWHM (full
width at half maximum) of 2.73–4.27 eV, which is character-
istic for the vanadium(IV) oxidation state.68 Another XPS signal
with relatively weak intensity was also observed in the range of
522.50–523.12 eV corresponding to V 2p1/2. The volumetric
redox titrations of nitrogen gas purged aqueous solutions of 1
and 2 using standardized KMnO4 solution, and ferrion as an
indicator also revealed the presence of 15 electrons per
formula unit of each compound, 1 and 2 (section S10, ESI‡).
Apart from the above-mentioned BVS, XPS, and volumetric
redox titration confirmations, the presence of the V (IV) oxi-
dation states of 1 and 2 can be realized with the help of d–d
transitions in their UV-Vis spectra. For this, 0.1 M lit−1 aqueous
solutions (based on the V4+ metal ion, 0.1 M per mL) of com-
pounds 1 and 2 were used to record UV-vis absorption (solu-
Fig. 8 Thermogravimetric analyses plots of compounds 1 and 2.
2−
with the decomposition of the CO3 ion. However, the cluster
showed much thermal stability up to 495 °C and then decom-
poses. In contrast, compound 2 showed gradual weight loss in
the temperature range of 50 to 400 °C, corresponding to the
water of crystallization and remained stable up to 600 °C.
Conclusions
tion) spectra. The d–d transitions corresponding to V4+ (d1) ion In conclusion, even though it is known that an alkaline
in the region of 600 to 1200 nm with an absorption maxima at aqueous solution absorbs aerial CO2 gas and that aqueous
964 nm for 1 and at 995 nm for 2 (Fig. 7) were observed.69,70 polyoxometalate chemistry had long been known (for over 200
The bands below 400 nm are perhaps due to ligand-to-metal years), not a single polyoxometalate compound had been
charge transfer ‘LM-CT’ (O → V) transitions.
reported until today, in which the absorbed aerial CO2 was
The thermal stability of the compounds 1 and 2 was exam- encapsulated. We made this inclusion (as the carbonate) poss-
ined by TGA analysis under helium gas atmosphere (Fig. 8). ible by using some amount of α-Bi2O3 in the relevant reaction
The TGA analysis revealed that compound 1 shows two-stage mixture. We characterized this confined carbonate anion by
dissociation of lattice water and hydrazine molecules as well as IR, TGA-Mass, and TGA-IR spectral studies including single-
sodium coordinated water molecules, followed by CO2 evolution crystal X-ray crystallography. We have to admit that even
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though the terminal source of this carbonate encapsulation is
air, this will not solve the global warming problem (as one big
cluster anion encapsulates only one carbonate anion).
However, the present host–guest system is unique in the sense
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Author contributions
The manuscript was written through the contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Conflicts of interest
19 D. Gatteschi, L. Pardi, A. L. Barra, A. Müller and J. Döring,
Nature, 1991, 354, 463–465.
The authors declare no competing financial interests.
20 A.-L. Barra, D. Gatteschi, L. Pardi, A. Müller and J. Döring,
J. Am. Chem. Soc., 1992, 114, 8509–8514.
Acknowledgements
21 P. Yang, M. Alsufyani, A.-H. Emwas, C. Chen and
N. M. Khashab, Angew. Chem., Int. Ed., 2018, 57, 13046–13051.
22 A. Müller, S. K. Das, P. Kögerler, H. Bögge,
M. Schmidtmann, A. X. Trautwein, V. Schünemann,
E. Krickemeyer and W. Preetz, Angew. Chem., Int. Ed., 2000,
39, 3413–3417.
The authors thank SERB, DST, Government of India (Project
No.
EMR/2017/002971)
for
their
financial
support. S. Mulkapuri thanks, UGC, New Delhi, India, for the
fellowship and S. K. Kurapati thanks the UGC, New Delhi,
India for awarding the Dr D. S. Kothari Post-Doctoral
Fellowship (Award Letter No. F.4-2/2006 (BSR)/CH/15-16/0074).
We also thank DST-FIST, UGC-SAP, and UPE-II.
23 A. Müller, H. Reuter and S. Dillinger, Angew. Chem., Int. Ed.
Engl., 1995, 34, 2328–2361.
24 A. Müller, M. Penk, R. Rohlfing, E. Krickemeyer and
J. Döring, Angew. Chem., Int. Ed. Engl., 1990, 29, 926–927.
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