Coordinative interaction between nitrogen oxides and iron-molybdenum POM Mo72Fe30

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Grzhegorzhevskii, M. O. Tonkushina, A. V. Fokin, K. Belova and A. Ostroushko, Dalton Trans., 2019, DOI:

10.1039/C8DT05125A.

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Eﬃcient extraction of sulfate from water using

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Coordinative interaction between nitrogen oxides and iron-

molybdenum POM Mo₇₂Fe₃₀

Kirill V. Grzhegorzhevskii,*^aMargarita O. Tonkushina,^aAndrei V. Fokin,^aKsenia G. Belova^{a, b}and

Received 00th January 20xx,

Accepted 00th January 20xx

Alexander A. Ostroushko^a

DOI: 10.1039/x0xx00000x

The process of adsorption of nitrogen monoxide and dioxide by the giant Keplerate nanocluster Mo₇₂Fe₃₀was studied in

detail under ambient condition and air/argon atmosphere. The obtained Raman and IR spectra showed that coordination of

NO_xto the Mo₇₂Fe₃₀leads to formation of the nitrate ions by sharing the bridged or terminal oxygen in FeO₆polyhedra with

adsorbed NO₂molecules. In accordance with elemental analysis and X-ray photoelectron spectroscopy, the composition of

produced complex was found as [POM-(NO₂)_x]·(NO₂)_y(where x=6, y=14±3). The carried thermal analysis revealed the

significant influence of NO_xcoordination on releasing the water molecules and decomposition of the constitutional acetate

ligands for Mo₇₂Fe₃₀. Furthermore, the performed measurements of temperature dependency of electron paramagnetic

resonance spectra for pure nanocluster and treated with NO₂allowed us to draw up a conclusion about the delocalization

of weak-bonded NO₂molecules in the pores of Mo₇₂Fe₃₀crystal at 25 °C. The opposite situation is observed under the

cryogenic temperatures. The localization of NO₂molecules occurs resulting in the distortion of FeO₆octahedra towards the

tetrahedral symmetry accompanied with appearing the signal at g-factor 4.3. The produced complex compound [POM-

(NO₂)_x]·(NO₂)_ypossesses a sufficiently NO₂capacity, water solubility and pH-depended decomposition, these are important

properties for potential NO_x-donor, which can be hypothetically applied in the biomedicine.

www.rsc.org/

giant POMs⁹. The reason of this especial behaviour of

molybdenum and tungsten cations is optimal combination of

1. Introduction

ionic radius and ability to strong metal-oxygen π-bonding^9,10

.

The coordinative interactions play a great role for scientific

progress in chemistry. Often, such interactions are related with

transition metals, which are the part of vitally important

proteins^1,2: pyruvate decarboxylase (Mn), cytochrome C (Fe),

heme oxygen carriers (Fe), B₁₂enzymes (Co), “blue” enzymes

(Cu), xanthine oxidase (Mo), insulin (Zn) etc. Also the influence

of coordination environment on the catalytic activity is well-

known for many transition metal oxides TiO₂, V₂O₅, Cr₂O₃, MnO₂

etc. and metal-organic complexes (e.g. Ziegler–Natta

catalyst)^3,4. Moreover, the significance of the coordinative

interactions of transition metals is difficult to overestimate for

the self-assembly processes including metal-organic

The requisite ion radius provides the increasing of coordination

number (C.N.) concerning oxygen ligands from (e.g.

4

tetrahedron of Cr⁶⁺) to 6 or 7 (octahedron and pentagonal

bipyramid of Mo⁶⁺or W⁶⁺). Whereas, in the oxygen polyhedra,

the strong Me-O π-bonding causes the shifting of metal cation

towards one of the vertices resulting in formation one short and

one long Me-O bonds in trans-position to each other. That

serves for implementation of Lipscomb’s rule, which commonly

forbids the existing more than two such vertex per one MeO₆

octahedra, and determines the polycondensation of smaller

POM to giant species containing tens or hundreds metal

atoms^8–10

.

frameworks

(MOF)

formation^5–7

.

Consequently,

an

Among the different shapes whom the giant POMs can possess

(toroidal¹¹- Mo₁₅₄, hedgehog-like¹²– Mo₃₆₈etc.), the fullerene-

like giant POMs (so called Keplerates), due to high-symmetric

isotropic structure, are being interest as an universal system for

studying the many kind of intermolecular interactions in the

inorganic-organic hybrid materials. The two most important

investigation of coordinative interactions of transition metals in

different compounds are indispensable for development of the

modern chemistry and biochemistry, especially. In this context,

the giant polyoxometalates (POM), as unique class of

coordination compounds, should be noted⁸. Despite of the

diversity coordinative abilities of transition metals, only few of

them – V, Nb, Ta, Mo and W – can form the polyoxometalate

structures and the latest two elements are capable to form

representatives

Mo₁₃₂=(NH₄)₄₂[Mo^VI₇₂Mo^V₆₀O₃₇₂(CH₃COO)₃₀(H₂O)₇₂] · (~300H₂O)

· (~10CH₃COONH₄) and Mo₇₂Fe₃₀[Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂

of

Keplerates^13,14

are

nanocluster

=

{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)}(H₂O)₉₁] · 150H₂O. Their highly

hydrophilic surface with ordered coordinative centres and inner

cavity, which is able to host-guest exchanging^15–19, were

differently functionalized with electrostatic, van-der-Waals

interactions and hydrogen bonding for variety applications:

a.

Institute of Natural Sciences and Mathematics, Ural Federal University named

after the B.N. Yeltsin, 620002, Ekaterinburg, Russia

^b.Institute of High Temperature Electrochemistry, 620137, Ekaterinburg, Russia

†Electronic

DOI: 10.1039/x0xx00000x

Supplementary

Information

(ESI)

available.

See

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selective ion-transport membranes²⁰, magnetic hollow of POM-NO_xassociates by the direct contact of NO_V₂_ies_wtr_Ae_rta_icm_{le O}w_ni_lit_nh_e

nanoparticles²¹, photoactive water splitting system²², magnetic nanocluster as sorbent.

DOI: 10.1039/C8DT05125A

resonance imaging (MRI) contrast agent²³, nanoreactor²⁴etc. In order to examine this strategy, we carried out the

The Mo^V-free Mo₇₂Fe₃₀POM, possessing the very low toxic investigation of an interaction between the Mo₇₂Fe₃₀crystals

effect^25,26for animals as compared with Mo₁₃₂(where the and the nitrogen oxides NO_x(NO₂and NO) adsorbed from the

Mo^V:Mo^VIratio is 60:72), was successfully used to gaseous phase in a dynamic mode. The obtained product was

transcutaneous iontophoretic delivery of bioactive molecules, studied with a vibration spectroscopy (IR and Raman), electron

which were associated with POM (vitamin B₁and human paramagnetic resonance (EPR), X-ray photoelectron

insulin²⁷). Furthermore, the stabilization of nanocluster spectroscopy (XPS), X-ray diffraction (XRD), elemental analysis,

structure was registered in the POM-protein complexes^28,29

.

UV-Vis spectroscopy, thermogravimetric analysis (TGA)

Therefore, taking into account the formation of stable combined with differential thermal analysis (DTA) and mass-

associates of Mo₇₂Fe₃₀(diameter is 2.5 nm) with human insulin spectrometry (MS) in argon (Ar) atmosphere. We registered a

and egg albumin²⁸, we focused our study on the iron- formation of new complex [POM-(NO₂)_x]·(NO₂)_y(where x=6,

molybdenum POM as promising nanocluster compound for y=14±3) which is stable during the 6 months observation under

potential biological applications.

the atmospheric conditions. In the complex POM-(NO₂)_x, the

Obviously, this association between biomolecules and Mo₇₂Fe₃₀obtained experimental data indicated to presence of NO_x

-

have to be strongly depended on the electrostatic forces and molecules in an form of NO₃anions. Moreover, additional

coordinative interactions. In order to control of such association amount (y) of weakly bonded NO₂is also registered. The joint

and predict the possible behaviour of Mo₇₂Fe₃₀in living system, usage of the abovementioned physicochemical methods

the model experiments should be carried out in vitro. Whereas allowed as to establishing the two possible sites in Mo₇₂Fe₃₀

the main contribution to association of POM macroanions with structure for such coordination of NO₂molecules. In addition,

proteins is conditioned by the electrostatic potential of surface POM-(NO₂)_xpotential activity in nitration reaction of 1,3-

groups³⁰, the binding Mo₇₂Fe₃₀with non-ionogenic low- dimethoxybenzene (1,3-DMB) was studied for deeper

molecular-weight substances is caused by the coordinative understanding the nature of interaction between POM and

interaction, e.g. the adsorption of 30 CO₂molecules per NO_x. Finally, we showed the ability to using of complex POM-

Keplerate nanocluster³¹.

(NO₂)_xas a potential source of NO_xfor its administration to the

One of such low-molecular-weight substances, whom crucial live systems.

role in biochemistry (in signal transduction and cytotoxicity)

was underrated until 1980, is nitric monoxide³². The effect of

2. Experimental section

General

nitric monoxide on the antiviral and antimicrobial activity

processes³³, the great cardiotropic action³⁴- this is why a lot of

modern investigations are focussed on the nitric oxide’s

influence to the human body and on the searching for new NO-

donor drugs³⁴, including inorganic porous NO-donors³⁵.

All chemicals were analytical reagent grade. The crystal sample

of Keplerate nanocluster POM Mo₇₂Fe₃₀was prepared in

accordance with published method¹⁴. The structure verification

of obtained POM was carried out by the comparison of IR and

In addition to biological meaning, the NO is very important part

of coordination chemistry of giant POM. As an by-product of

hydroxylamine oxidation during POM synthesis, the NO

molecule coordinates to the molybdenum atom through the

nitrogen forming a very important building unit: Mo(NO)O₆

pentagonal bipyramid¹⁰. This unit is a part of main building

block¹⁰– {Mo₁₇}= {Mo^VI₁₅[MoNO]³⁺₂O₅₈(H₂O)₂}^20-, based on

which the serial of first giant POMs were constructed³⁶– Mo₃₆,

Mo₅₇Fe₆, Mo₁₅₄. Furthermore, the nitrosyl (NO), nitrito (ONO),

nitro (NO₂) and nitrato (NO₃) complexes are registered for iron-

containing porphyrines^37–39, including myoglobin which is

mediately involved in the electron transfer of the respiratory

chain in the mitochondria⁴⁰. Also, NO spin trap are well-known

based on the Fe²⁺complexes with diethyldithiocarbamate or N-

methyl-D-glucamine dithiocarbamate⁴¹. Consequently, the

strong coordinative interaction between Mo₇₂Fe₃₀and nitrogen

oxides is hypothetically expected. According with data²⁷about

the application of Mo₇₂Fe₃₀as a transport agent for bioactive

substances, the investigation of nitrogen oxides association

with this POM could lead to an important bioinspired progress ,

in searching for NO-delivery systems, in particular.

Furthermore, here we suggest a simple approach to production

Raman spectra with reference data from literature^14,42

:

experimental data for Mo₇₂Fe₃₀are 1613m, 1532s, 1411s,

1342w, 1045w, 1032w, 965s/ 945s, 849s, 749vs, 624m, 547s,

435w in IR spectrum, and 969s(sh), 952vs, 909m(sh), 842m,

717m, 586vw, 518m, 452m, 370m, 240m in Raman spectrum.

For nitrogen dioxide production, we used the reaction:

Cu_{(thin powder)}+ 4HNO_{3(conc. 90°C)}→Cu(NO₃)₂+ 2NO₂+ 2H₂O.

The controlled NO₂evolving was achieved by the gradual adding

the hot nitrogen acid to cooper powder. The interaction

between POM and NO₂was realized with transmission of

freshly prepared gaseous nitrogen dioxide through the

Mo₇₂Fe₃₀powder in the Erlenmeyer flask (25 ml) under the

vacuum pump during 30 min, then the flask was purged with

argon for 1 min and left closed. The used laboratory setup was

consisted of reacting round-bottomed flask (250 ml) with

mounted dropping funnel for controlled NO₂production. Then,

under vacuum pump, obtained NO₂consistently went through

the cold-water condenser with receiving flask, glass tube with

mixture of zeolite dehumidifier (4Å pores) and freshly dried

Na₂SO₄(for removing of water vapor from a gaseous phase),

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and to Erlenmeyer flask (25 ml) with POM powder (0.17 g of the quadrupole mass spectrometer QMS 403 C with electron

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DOI: 10.1039/C8DT05125A

Mo₇₂Fe₃₀). After that, the gas-flow bubbled through the sodium beam ionization (100 eV) coupled to thermoanalyser STA 409

hydroxide solution (1 mol/l) for slake of acidic vapour. All joints Luxx (Netzsch). During the thermal analysis, the samples were

were sealed with a high vacuum grease. After the exposition heated from 50°C to 800°C with rate 10°C per minute. Before

under vacuum condition, the integrity of POM structure is measuring, the POM-NO sample was taken out from quartz

confirmed with IR and Raman spectroscopy of obtained POM- tube and stayed on air in order to achieve the equilibrium

NO_xassociates (Fig. 1, Fig.2). Important to note, that after the oxygen content.

POM exposition under NO₂flow and consequent staying in the

closed flask, some amount of nitrogen dioxide desorbed from

POM, during the 40 min, and coloured the atmosphere in

orange. The excess of NO₂was removed by purged the flask

with argon flow.

X-ray diffraction

The Mo₇₂Fe₃₀and POM-NO₂were investigated at 25°C on a

Bruker ADVANCE D8 diffractometer with Cu Kα radiation. The

post processing of the registered diffractogram was carried out

with CELREF Unit-Cell refinement software obtained from the

“Laboratoire des Materiaux et du Génie Physique de l'Ecole

Supérieure de Physique de Grenoble”. For procedure of cell

parameters’ refinement, we applied the crystallographic data of

pure Mo₇₂Fe₃₀from the reference paper¹⁴as a starting point:

The nitrogen monoxide was produced by the reaction:

2FeSO₄+ 2NaNO₂+ 3H₂SO₄→Fe₂(SO₄)₃+ 2NaHSO₄+ 2NO +2H₂O.

The reaction was performed under inert argon atmosphere

(0.0004% of O₂) in order to prevent NO oxidation to NO₂by air.

In the three-necked flask, the FeSO₄(120 mmol) aqueous

solution (100 ml), acidified with H₂SO₄(180 mmol), was heated

to 70-80°C, and aqueous solution of NaNO₂(100 mmol in 10 ml)

was dropwise added by the dropping funnel. The obtained

colourless NO was carried with argon flow to quartz tube with

Mo₇₂Fe₃₀powder, and then to Drexel bottle containing aqueous

solution of NaOH (1 M). The quartz tube with POM has inlet and

outlet gas-taps, which allowed to disconnecting the tube after

exposition nanocluster under NO atmosphere (during 30 min)

and measuring the vibrational spectra under inert conditions.

푅3

space group , a=55.1306(14) Å, c=60.193(2) Å.

X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) measurements were

performed using electron spectrometer K-Alpha+ (Thermo

Fisher Scientific, USA) with monochromatic Al Kα radiation

(1486 eV). The charge compensation system was applied, and

the vacuum was maintained at 10^-7mbar. The internal Au-

standard (Au4f, 84 eV) was used to spectrometer calibration.

The correction of spectrum scale was made with the C1s signal

(284.8 eV). Samples (Mo₇₂Fe₃₀and POM-NO₂) were placed at

the double-stick carbon tape. By means of optical system with

CCD-camera the place on the sample’ monocrystal surface was

selected for spectrum measuring. The X-ray beam was focused

into the 300 μm spot, and spectrum was collected with suitable

analyser pass energy: 200 eV for survey spectrum and 50 eV for

narrow spectral range.

Vibrational spectroscopy

Raman spectra of all samples were measured using the confocal

Raman microscope Alpha 300 AR (WiTec GmbH) at low power

of laser beam (He-Ne laser with excitation wavelength 633 nm)

in order to prevent uncontrollable destruction of the

nanoclusters⁴². In case of the POM-NO sample in quartz tube

with inert atmosphere, the focusing of laser beam behind the

tube surface allowed us to exclude SiO₂modes from

observation in spectrum. For another samples the spectra were

obtained under atmospheric conditions. Scattered light was

collected by the objective in backscattering geometry and

passed through the appropriate edge-filter. The spectrum was

detected by Andor Newton CCD camera with 1600200 pixels

matrix thermoelectrically cooled down to -60 °C. A 50 μm

diameter multimode optical fiber was used as a confocal

pinhole.

IR spectra of initial POM Mo₇₂Fe₃₀and obtained samples POM-

NO_xwere measured at room temperature under atmospheric

conditions using FT-IR spectrometer Nicolet 6700 (Thermo

Scientific) in dry KBr pellet, in the range 4000-400 cm^-1with

spectral resolution 2 cm^-1. Due to the contact of POM-NO

sample with air during the IR measurement, adsorbed NO

molecules were immediately oxidized to NO₂, as expected.

Electron paramagnetic resonance

The electron paramagnetic resonance spectra of Mo₇₂Fe₃₀and

POM-NO₂were registered using the Bruker EMX Plus

spectrometer with microwave frequency 9.4 GHz in the

magnetic field range from zero to 7000 G. The EPR-spectra of

empty quartz ampoule was measured as background signal. The

temperature variation of EPR spectrum was obtained by

stepwise collecting of EPR signal during the heating of sample

from 100 to 295 K. The cryogenic temperatures were achieved

with liquid nitrogen cooling. The crystal Cd_0.95Mn_0.05Te was

added in the quartz tube with POM-NO₂sample and was used

as an internal standard for investigation of temperature

dependency of EPR spectra.

Elemental analysis, solubility and extinction coefficient

The measuring of light elements (C, N, H) content in the POM-

NO₂sample was performed with automatic elemental analyzer

CHNS РЕ 2400 series III (Perkin Elmer Instruments). The

evaluation of the POM-NO₂solubility was made based on the

measuring the iron and molybdenum concentrations in the

TGA-DTA-MS

The thermogravimetric analysis combined with differential

thermal analysis and mass-spectrometry was carried out for

Mo₇₂Fe₃₀, POM-NO₂and POM-NO in argon atmosphere using

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solution. The latter was produced by the several hours stirring the theory of symmetry predicts the presence of three

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of crystal sample POM-NO₂in the distilled water at 37 °C. The vibrational modes: ν₁(A₁) = 1318 cm^-1is ^Dsy^Om^{I: 1}m^0.e¹⁰tr³i⁹c^/Cst⁸r^De^Tt⁰c⁵h¹i²n⁵g^A,

insoluble part of crystals was separated with centrifugation, and ν₂(A₁) = 750 cm^-1is bending vibration, ν₃(B₁) = 1618 cm^-1is

obtained solution was analyzed with atomic emission asymmetric stretching⁴⁴. In IR spectrum of gaseous NO₂these

spectrometer iCAP 6500 Duo (Thermo Scientific). Further, the modes have a different intensity: ν₁– weak, ν₂– strong, ν₃– very

UV-Vis spectra of obtained solution and aqueous solution of strong⁴⁴. The observed band at 1385 cm^-1does not match to any

-

pure Mo₇₂Fe₃₀were measured at 25 °C with spectrometer UV- of NO₂bands, but the same sharp peak is characteristic for NO₃

1800

(Shimadzu)

equipped

with

thermoelectrically ion (D_3hsymmetry) at the slightly shifted position^45,46– 1300-

1340 cm^-1. The performed investigation⁴⁶dedicated to

comparison of experimental and DFT-calculated (density

functional theory) IR spectra for microhydrated nitrate ions

temperature-controlled cell positioner.

Nitration of 1,2-dimethoxybenzene

-

allows us to attribute the band at 1385 cm^-1to signal of NO₃

The nitration procedure was performed with slightly modified

method from reference data⁴³. The 1.211 ml of 1,2-

dimethoxybenzene (1,2-DMB) in 4 ml of glacial acetic acid at

25°C was added to the 0.1g of Mo₇₂Fe₃₀crystals in the round-

bottom flask (flask 1), which was connected through gas-taps

with the mixing-chamber (flask 2) from one side, and with the

water-jet air pump from another. The obtained POM-

suspension was being stirred all the time. The gaseous mixture,

containing the NO – 4.95 and N₂– 95.05% (ratings), was put into

the mixing-chamber from the gas-cylinder. Then the flask 1 was

connected to the vacuum line while the requisite low pressure

would be achieved. Further, flask 1 was isolated from vacuum

line with help of the first gas-tap (GT-1), and the second gas-tap

(GT-2), connecting flask 1 and flask 2, was opened. The NO/N₂

mixture was distributed between both flasks, after that the flask

2 was connected through the third gas-tap (GT-3) with air for

several seconds. Then, the NO was oxidized to NO₂as it was

indicated by orange color of gaseous phase observed in the

flasks 1 and 2. The POM-suspension in glacial acetic acid with

1,2-DMB was stirred under this atmosphere during the 5 min,

after that the GT-1 was opened to removing the established

gaseous atmosphere by the vacuum line, and GT-2 was closed

at the end of this cycle. Than fresh portion of NO/N₂mixture

was admitted into the flask 2 and all cycle was repeated 30

times. The referent experiment was carried out by the same

manner without the Mo₇₂Fe₃₀crystals. During the conversion of

1,2-DMB into the 1,2-dimethoxy-4-nitrobenzen (1,2-DMNB) the

suspension became orange-yellow-colour. The content of 1,2-

DMNB was determined by the ¹H NMR spectra in CDCl₃

obtained with NMR spectrometer AVANCE 500 ( Bruker

BioSpin).

(H₂O)₃species which have the C_3hsymmetry and appear at 1372

-

cm^-1. In other spectra of microhydrated nitrate ions NO₃(H₂O)_n,

n=1-6, the vibrational modes are also observed in the

approximately similar region, but their sharp signal was split to

-

two peaks. The specific of NO₃(H₂O)₃hydrated ions is each of

-

three water molecules form H-bond with NO₃central ion

through the one hydrogen atom⁴⁶.

The possible reasons of nitrate ion formation are solvating the

NO₂by the nanocluster water or coordination of the NO₂to

some oxygen centres in Mo₇₂Fe₃₀structure. Furthermore, one

alternative scenario is theoretically possible. In previous study,

the bubbling of gaseous CO₂through the aqueous solution of

Keplerate Mo₁₃₂leaded to substitution of oxygens, belonged to

the acetate ligands in vertices of dimolybdenum bridge unit

V

-

[Mo₂O₄]²⁺, by CO₃ions³¹. This substitution was not only

-

accompanied with corresponding signals of CO₃stretching in IR

spectrum. The significant changes were observed for bands at

1545 and 1340 cm^-1related with acetate ligands. In our case, we

did not register any spectral features pointing out to the acetate

ligand exchange with NO₃^-ions (Fig. 1). Also the substitution of

aqua-ligands by the nitrate ions did not occur, as one can see on

the stretching and bending vibration of H₂O in IR spectra (Fig. 1,

Fig. S1†, see supporting information). Therefore, we may

conclude that differences of interaction between Keplerates

(Mo₁₃₂and Mo₇₂Fe₃₀) and gaseous molecules (CO₂and NO₂) are

caused both the nature of adsorption centres in POM and

conditions of components contact – bubbling through the POM

solution and exposition of POM crystal in gas flow, respectively.

In order to clarify, whether the free nitrate ions in POM-NO₂, we

studied the catalytic activity of Mo₇₂Fe₃₀in the nitration

reaction of 1,2-dimethoxybenzene (1,2-DMB) in the glacial

acetic acid at 25°C where the possess a free nitrate ions is

+

prerequisite for nitronium anions NO₂formation⁴³. It is well-

3. Results and discussion

3.1 The vibrations of adsorbed NO₂molecules in the POM

structures

known, that 1,2-DMB is easily nitrated substrate which

transforms to the one major product 1,2-dimethoxy-4-

nitrobenzen (1,2-DMNB)⁴⁷. The performed experiment

consisted of the periodical contact of freshly produced gaseous

NO₂with a mixture of 1,2-DMB and glacial acetic acid under

The IR spectra of Mo₇₂Fe₃₀treated with NO (called POM-NO)

and NO₂(called POM-NO₂) are presented in Fig. 1 (full-range IR

spectra in Fig. S1†, see supporting information). The integrity of

Mo₇₂Fe₃₀nanocluster structure after NO_xadsorption is

indicated by presence of all POM’s characteristic bands in the

fingerprint region below 1000 cm^-1. The major feature observed

for these spectra is appearance of new narrow strong band at

1385 cm^-1. The intensity of this band is well-correlated with

amount of adsorbed NO_x. For NO₂molecule (C_2vpoint group)

1

ambient condition. The H NMR spectra showed the presence

of POM did not change the yield of 1,2-DMNB. This result argues

to absence of free nitrate ions in POM crystals, and allows us to

suggest a coordination of NO₂to some oxygen centres of

Mo₇₂Fe₃₀. The nature of such centres is discussed below.

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Fig. 2. Raman spectra of solid samples Mo₇₂Fe₃₀, POM-NO (in

quartz tube), POM-NO₂obtained with 633 nm He-Ne laser

under ambient conditions.

Fig. 1. IR spectra of solid samples Mo₇₂Fe₃₀, POM-NO, POM-NO₂

in dry KBr pellets on air atmosphere.

The usage of Raman spectroscopy also confirmed the Mo₇₂Fe₃₀

structure integrity for the samples POM-NO and POM-NO₂(Fig.

2). The detailed bands attribution have been presented in

previous paper⁴², here, we want to note three early-

undescribed combination bands and overtones at 1424 =

2×715(±2) cm^-1, 1546 = 715+842(±10) cm^-1and 1622 =

715+911(±4) cm^-1. As a result of the NO₂absorption, the high-

frequency bands at 911 and 951 cm^-1shifted to 915 and 957 cm^-

¹, respectively. These bands are assigned to Mo-O(terminal)

stretching vibrations⁴², so the shifting is probably caused by the

interaction of corresponding Mo-O vertices with NO₂

molecules. In confirmation of this, the absence of intensive

shoulder at 975 cm^-1, assigned to stretching vibration ν(Mo-

O_terminal), is observed for POM-NO₂as compared to Mo₇₂Fe₃₀.

Furthermore, two weak signals, one sharp at 1050 cm^-1and one

wide at 1287 cm^-1, appeared in the POM-NO₂spectrum. First of

them could be assigned to the ν₁symmetry stretching of

3.2 TGA/DTA studies coupled with mass-spectrometry analysis of

gaseous phase

The obtained data about the weight loss, endothermic or

exothermic peaks, and corresponding m/q signals are

presented in table 1, Fig. S2† and Fig.S3† (see supporting

information) for all samples. During thermal decomposition,

above the 100 °C, the water molecules leave the nanocluster

first. Besides, at this temperature, POM loses its stability, and

only the fragments of nanocluster structure exist above this

temperature^16,42. In the previous study it was shown^16,42that a

molybdenum oxide (in particular, MoO₃) and Fe₂(MoO₄)₃are

formed as the final destruction products (DP) of Mo₇₂Fe₃₀in the

solid phase under air atmosphere.

The direct desorption of coordinated NO₂molecules from POM

crystals can be evaluated through the m/q=46 and m/q=30

signals, latter is much intensive and it is produced by the

electron-beam ionization of NO₂molecules⁵². For POM-NO

sample we observed only one weak signal of m/q=30 at 323 °C,

whereas for the POM-NO₂sample the several peaks of NO₂

evolving are presented (Table 1). The first main NO signal is

registered for POM-NO₂sample at 224 °C. This signal

accompanies with the strong exothermic effect at 231 °C and

could be attributed to releasing of weakly bonded NO₂. The

following NO₂releasing, observed at high temperatures (at 281,

349 and near 740°C), allow us to draw a conclusion about the

strong interaction (chemisorption) of the nitrogen dioxide with

DP of POM (also with an initial nanocluster Mo₇₂Fe₃₀, probably).

As a comparison, for transitional metal nitrates (nitrates of Fe,

Co, Ni, Mn, Cu, Zn), the decomposition occurs at lower

temperature^53,54(near 100-200°C). Furthermore, such high

temperature as for the signal of m/q=30 near 740°C for POM-

NO₂(which is characteristic for thermal decomposition of alkali

or alkali earth metals nitrates^55,56) may be attributed to release

of some constitutional NO_xfrom solid DP of POM-NO₂. This is

additional proof that NO₂molecules are not exist in the form of

free nitrate ions in the DP of Mo₇₂Fe₃₀.

-

hydrated ion NO₃, which is observed for anhydrous HNO₃as

weak intensity signal⁴⁸. Moreover, the appearing of such band

was predicted on the course of ab initio calculation of hydrated

nitrate ions in the reference paper⁴⁹. However, for anhydrous

HNO₃the most intensive band appears at 1294 cm^-1.

Consequently, in the POM-NO₂spectrum, this band has to

revealing⁴⁸along with weak band at 1050 cm^-1but it does not

occur on practice. Thus, in obtained spectrum (Fig.2), band at

1287 cm^-1corresponds to slightly shifted strong band of nitrate

ions^48,50, which should be observed at 1294 cm^-1. In such case,

the band at 1050 cm^-1has a different nature. Taking into

account foresaid, we suppose that appearing of this band is

caused by the stretching of some metal-oxygen terminal groups

involved in the NO₂coordination, instead of attribution of this

sharp peak at 1050 cm^-1to nitrate ion vibration by some

authors⁵¹.

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Table 1 TGA/DTA analysis of Mo₇₂Fe₃₀, POM-NO and POM-NO₂

Journal Name

TG (-)loss/(+)rise of weight, %

Stages

T^Vo^iet^wal^{Article Online}

Sample

m/q

DTA peaks (en/ex), °C

DOI: 10.1039/C8DT05125A

Mo₇₂Fe₃₀

18, 44

44

148(en)

226(en)

345(ex)

-17.4 (25-215 °C)

-8.85 (215-415 °C)

+0.35 (415-645 °C)

-0.32 (645-800 °C)

-26.22

44

410(ex)

~500(ex), 564(ex),

635(ex), 750(en)

106(en)

-

POM-NO

POM-NO₂

18

18, 44

30, 44,

44

228(en)

356(ex)

426(ex)

- 9.2 (25-205 °C)

-9.6 (205-375 °C)

+4 (435 – 800 °C)

-14.80

-25.22

538(ex), 638(ex),

691(ex)

-

18

129(en)

194(en)

231(ex)

354(ex)

541(ex)

740(ex)

-10.85 (25-180 °C)

-5.42 (180-225 °C)

-3.66 (225-340 °C)

-3.64 (340-385 °C)

+0.22 (385-610 °C)

-1.87 (610-800 °C)

18, 30, 44, 46

-

30

Notation: en/ex – endothermic or exothermic effect; m/q – ratio of mass to charge of the ion;

The treatment of nanocluster with NO_xinfluences on releasing 3.3 Coordinative sites for NO₂in the Mo₇₂Fe₃₀structure: XPS and

the water and CO₂molecules upon the heating. As it shown in EPR study

mass-spectrum of POM-NO₂(Fig.S3†, see supporting

XPS analysis

information), the signal of m/q=18 (corresponding to ionized

H₂O molecules) at 233 °C shifts ~20-30°C down and splits into

the two components as compared with pure nanocluster.

Furthermore, the NO₂absorption results in the appearing of

new shoulder of m/q=18 signals near the 280-290°C that is

significantly higher as compared with upper limit of water

release temperature for Mo₇₂Fe₃₀sample. This is probably

caused with the presence of the different types of structural

aqua-ligands involved in the process of NO₂coordination in

POM-NO and POM-NO₂samples. Due to the different kind of

Based on the vibrational spectroscopy and thermal analysis

data we determined that NO₂is adsorbed in the Mo₇₂Fe₃₀

structure forming NO₃species, which are not like a free nitrate

-

ions. However, the presumed coordinative interaction between

nitrogen dioxide and nanocluster has not been discovered yet.

For this purpose, we used the XPS analysis. We chose the POM-

NO₂sample for further examination due to significantly higher

content of NO₂than in the POM-NO that is important for

carrying out of valid measurements. The survey XPS spectrum

of Mo₇₂Fe₃₀and POM-NO₂sample can be found in the

supporting information (Fig. S4†). The selected regions of

binding energy (BE) for molybdenum 3d, 3p and iron 2p are

-

interactions of NO₃ions with such aqua-ligands (microhydrated

environment) the latter could release after desorption of the

major amount of nitrogen oxides, only.

In addition to water release, the CO₂evolving is also very

characteristic stage for thermal destruction of Keplerate

nanoclusters⁵⁷, which is caused with the acetate ions

decomposition and characterized via presence of ionized

particles with m/q=44 in the gaseous phase. The absorption a

small amount of nitrogen monoxide or dioxide leads to the

significant changes in the dynamic of CO₂releasing (Fig.S3†, see

supporting information). Here, we emphasized the most

important results of NOx adsorption by the pure Mo₇₂Fe₃₀: (1)

absence of early stage of CO₂releasing near 150°C for POM-NO

and POM-NO₂; (2) shifting the CO₂-peaks at 228 and 408°C to

236 and 431°C, respectively, and changing their intensity in

opposite directions for POM-NO; (3) drastically dropping all CO₂

signals intensity excepting the main peak at 354 °C for POM-

NO₂. The last one leads to release of major amount of CO₂at the

much higher temperature as a result of NO₂coordination with

the initial nanocluster or with the DP.

presented in Fig.

3 (the corresponding peaks’ data is

accumulated in Table S1, see supporting information). As well-

known¹⁴, in the pure nanocluster both metals (Mo and Fe) are

in the highest valence states – VI and III, respectively. That is in

line with obtained XPS data for molybdenum which 3d_3/2and

3d_5/2signals are observed at 236.11 and 232.95 eV as typical⁵⁸

for Mo^VI. Whereas for iron, there is a mixture of different states

of 2p_3/2-5/2binding electrons. Despite the satellite near 720

eV^59,60, two 2p_3/2signals, placed above the 710 (±0.3) eV, can be

assigned to the Fe^IIIvalence states^61,62: 711.6 and 715.49 eV.

First of them, with the larger area, is characteristic⁶¹for Fe₃O₄.

The signal, which is similar to the second one, was observed in

the spectrum of K₃FeF₆(714.6 eV)⁶³, where the iron surrounded

with the six fluorine atoms having the strong oxidation

properties. Consequently, the peak at 715.49 eV should

correspond to the oxidized iron atoms with less electron density

than for Fe^IIIobserved at 711.6 eV. This situation can be

explained if taking into account the fragment

{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)} located in the inner cavity of

6 | J. Name., 2012, 00, 1-3

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Mo₇₂Fe₃₀. This fragment coordinates to some vertices of iron or

molybdenum polyhedra through the oxygens. Due to the

different electronegativity of Mo^VIand Fe^III(2.27 and 1.96,

respectively⁶⁴), the binding of this fragment with Fe through the

bottom vertex of FeO₆octahedron should lead to shift the

bonding electrons toward the molybdenum resulting in the

additional oxidation of iron atom. More interesting is the

presence of a 2p_3/2signal at 709.31 eV corresponding to slightly

reduced⁶¹Fe^III, however, the share of this state is small (~13

times less than sum of Fe^III2p_3/2signals at 711.6 and 715.49 eV).

Hypothetically, the coordination of bidentate acetate ligands,

having the delocalized electrons of carboxyl group, to the iron

and molybdenum polyhedra could be a reason of partial

reduction of Fe^IIIdue to the increasing of electron density near

the iron atom.

View Article Online

DOI: 10.1039/C8DT05125A

The adsorption of NO₂molecules influences on the iron and

molybdenum valence states in the POM-NO₂sample,

significantly (Fig. 3). The molybdenum 3d_3/2signal splits into the

two states with higher and lower BE – 236.54 and 235.75 eV,

respectively. This relates with rearrangement of electron

density in the Mo-O bonds that indirectly points out to the

interaction between those oxygen atoms and NO₂molecules

-

forming NO₃anions. The presence of latter was directly

identified by the characteristic signal of N_1sat 406.76 eV. For

nitrate salts the N_1ssignal is typically observed^65,66at 407.2-

407.4 eV, whereas for the coordination compounds the NO₃

group reveals at 406.7-406.9 eV⁶⁷. Due to the deep vacuuming

of the POM-NO₂samples during the XPS analysis, we expected

that the strong bonded NO₂molecules should stay, whereas the

weak-bonded ones release.

Fig. 3. The XPS spectra of the Mo₇₂Fe₃₀and POM-NO₂samples.

-

Obviously, in order to forming a NO₃ion from NO₂, the nitrogen

As the result of nanocluster packing into the crystal, the large

pores (LP) are formed. Each POM is surrounded by the 4 LP. The

small pores (SP), belonging to the POM skeleton, represent the

regular hexagon with bridged μ-O oxygens in the vertices (Fig.

4b). Totally, each nanocluster possesses the 20 SP, but the

accessibility of them is different. The part of these SP are turned

inside the large pores: three SP from one side and three parallel

from another are oriented into the lateral large pores, one SP

on top and one SP at the bottom is oriented into the upper and

down LP (Fig. 4a), respectively. However, besides the SP

included into the LP, there are two SP, which should be

separately considered as additional kind of pores (SP_c-axis). They

are placed on the axis going through the crystal direction at hkl

(0 0 1) (Fig. 4b).

dioxide should be coordinated with additional oxygen atom.

Besides the foresaid molybdenum, the Fe-O bonds can also

participate in the coordination of NO₂. Indeed, for POM-NO₂

sample, the main signals of Fe 2p_3/2are slightly shifted

(711.6→711.89 and 715.49→715.19 eV), and the shoulder of

2p_3/2signal at 709.31 is absence. Therefore, due to the strong

oxidation properties of nitrogen dioxide, its introduction into

the nanocluster crystal leaded to decrease of the electron

density on some Fe^IIIcentres as compared with pure Mo₇₂Fe₃₀.

Thereby, both Mo-O and Fe-O bonds are involved into the NO₂

coordination. Consequently, the most probably to suppose that

NO₂is coordinated with the bridged oxygen in the fragment Mo-

μO-Fe. However, from the other side, in order to change the

electron density distribution of iron and molybdenum, it is not

required to locate the NO₂in direct proximity both of them. This

rearrangement could be conditioned indirectly through the

polarization of conjugated bonds.

Both types of pores, LP and SP_c-axis, spread through the whole

crystal as tunnels. Taking into account the van-der-Waals radius

of nitrogen and oxygen atoms (0.154 and 0.140 nm,

respectively⁶⁸), one NO₂molecule can completely occupy the

one SP. The additional stabilization of this host-guest

interaction is probably provided by the hydrogen bonding of

Model of NO₂adsorption

The 3D model illustrating the possible sites for NO₂coordination

in the Mo₇₂Fe₃₀crystal was designed on the basis of the above

discussed results and crystallographic data (Fig. 4). At the

beginning, we found the crystal directions corresponding to the

large and small pores: the first ones have a height of ~15 Å and

a width of ~7 Å (Fig. 4a), whereas the second ones have a

diameter of 5.6 Å (Fig. 4b).

-

NO₃with neighboring water molecules, also contained in SP,

-

forming the microhydrated environment NO₃(H₂O)₃. So, in

consistent with XPS data, the most probably coordination of

NO₂inside the SP is presented in the Fig. 4c. Thus, the small

pores on c-axis (small tunnel) is blocked after coordination with

guest NO₂molecules. This results in the main diffusion of NO₂

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molecules from the gaseous phase into the POM crystal through interaction of NO₂molecules with Mo₇₂Fe₃₀is re_Va_ieli_wze_Ad_rticb_ley_Ont_lh_ine_e

DOI: 10.1039/C8DT05125A

the LP. The alternative coordination of NO₂to the SP can realize three different manner: strong in-plane coordinative

through the interaction of nitrogen atom in NO₂with terminal- interaction inside the SP (chemisorption), weak-bonded out-of-

oxygen in FeO₆polyhedron (Fig. S5†). In this case, the NO₂plane interaction with SP and very weak-bonding in LP (physical

molecule is not placed in the plane of small pore that leads to adsorption). The last one does not contribute to the POM-NO₂

absence of the additional stabilization of such coordination composition due to the full desorption of extra nitrogen dioxide

(weak-bonded NO₂), making it is less favorable than interaction proceeding on air under the room temperature.

with μ-O oxygen. Regardless of in-plane or out-of-plane

EPR spectroscopy

coordination of NO₂to SP, in accordance with above discussed

structure of LP, each POM possesses only eight SP, which can

interact with guest NO₂molecules, whereas the other twelve SP

are unavailable due to the steric factors. Along with this

assumption, the XRD analysis (Fig. S6†) showed the slight

compression of POM crystal parameters after NO₂adsorption:

a,b= 55.2826(±0.2047) Å, c= 60.6225(±0.0508) Å for Mo₇₂Fe₃₀

sample, and a,b= 54.9836(±0.3478) Å, c= 60.0899(±0.1117) Å

for POM-NO₂sample. However, the low quality of XRD spectra

allow just to evaluate the cell parameters but does not give the

precise values.

Two things rouse the using of EPR spectroscopy to determine

the favorable coordination of NO₂in the POM structure. First,

the Mo₇₂Fe₃₀has a wide intensive signal at g=2.00 (3350 G)

conditioned by the collective resonance of high symmetric Fe^III

centers array (S=5/2)^16,70(Fig.5a), which could be sensitive to

incorporation of NO₂molecules into the crystal structure and

coordination them to the oxygen atoms connected with iron.

Second, NO₂, NO molecules, as well as NO₃ions, are also

paramagnetic and could be hypothetically identified by EPR

spectroscopy.

-

Therefore, we expected the maximal eight NO₂molecules

would strongly bond with one nanocluster. However, the ability

of additional storage of NO₂molecules in the LP without

coordination to μ-O oxygen or terminal-oxygen of SP was also

found during the visual observation of POM-NO₂. This sample

stayed in the closed flask immediately after saturation in the

NO₂stream. Later, inside the flask, an atmosphere, which was

primarily transparent, became orange due to desorption of very

weakly-bonded NO₂. Then, the flask, which was purged with

argon stream to colorless atmosphere, became the orange

again. This behavior repeated several times during the 3 hours

pointing out to presence of some extra adsorption capacity of

Mo₇₂Fe₃₀, which is owing to the tunnels formed by the LP in

POM crystal, most probably.

The carried out elemental analysis showed that 20±3 molecules

of NO₂are belonged to each Mo₇₂Fe₃₀in the POM-NO₂sample

(1.52±0.24 w.% of nitrogen). We used for calculation the

molecular weight of POM (18 648.85 g/mol), which includes the

contribution of 150 cluster water molecules¹⁴. However, based

on XPS data, because of the N 1s_1/2and Mo 3p_3/2signals are

placed inside the narrow spectral range width of 15 eV (Fig. 3),

we may directly calculate the ratio of nitrogen and molybdenum

in the POM-NO₂sample (after removing the weakly-bonded

NO₂during several hours vacuuming at 10^-7mbar) with

sufficient accuracy. After normalization of peaks area per value

of the photoionization cross-section upon 1487 eV (σ_N1s1/2=1.8

and σ_Mo3p3/2=5.94 in units of C 1s cross-section⁶⁹) we obtained

the Mo:N=11.5:1, that is close to the stoichiometric ratio

Mo₇₂:(NO₂)₆, if only strong bonded NO₂included. So, that only

six from eight SP of each POM are available for effective NO₂

adsorption.

The observed EPR spectra of POM-NO₂sample did not show the

features of an interaction between nitrogen dioxide and

Mo₇₂Fe₃₀at room temperature. However, during the

temperature variation from 295 K to 100 K, a new EPR signal

was registered. At that, the appearance of two very weak

signals (at g=4.72 and g=3.87), observed for pure Mo₇₂Fe₃₀at

cryogenic temperature, was not registered in the case of POM-

NO₂sample. The comparison of EPR spectra at different

temperatures for Mo₇₂Fe₃₀and POM-NO₂samples is presented

in the Fig.5. The full-range EPR spectra are shown in Fig. S7† (see

supporting information). In order to provide the sufficient

accuracy during the measurement of temperature variation of

EPR spectra, the crystal of Cd_0.95Mn_0.05Te with narrow sharp

signal at g-factor 2.00 was used as an internal standard (the

comparison of temperature behavior of integral intensity of EPR

signals for Cd_0.95Mn_0.05Te, Mo₇₂Fe₃₀and POM-NO₂is presented

in Fig. S8†, see supporting information).

For POM-NO₂sample, a new signal at g-factor 4.3 evidently

appears from 200 K (-73.15 °C) and continues to grow until 100

K (-173.15 °C). Below 120 K (-153.15 °C), the additional structure

of the EPR spectrum is revealed near g-factor 2.9 (2500-3000 G).

The observed behavior of the POM-NO₂samples under

temperature variation is a result of an interaction between pure

nanocluster and nitrogen dioxide (Fig.5).

The g-factor 4.3 is frequently observed for Fe^IIIions in the glass-

matrixes or in the crystals doped with iron, e.g. PbWO₄. In this

solid with scheelite structure, both cations (Pb^II, Fe^III) take a

position with tetrahedral symmetry⁷¹. The registered angular

dependency of such Fe^IIIcenters possess nearly isotropic

character, which is caused by the some distortion of double

oxygen’s tetrahedra. In the case of Mo₇₂Fe₃₀having the signal

with g=2.00, the trivalent iron is in the oxygen’s octahedra

(Fig.4). Consequently, we may assume, that observation of new

signal for POM-NO₂sample at g=4.3 is due to distortion of

oxygen’s polyhedra, which leads to appearing the symmetry

elements corresponding to tetrahedral position of Fe^IIIas in

foresaid case of PbWO₄(Fig.5b).

Furthermore, it is worth to note, that content of hydrogen

determined in the POM-NO₂was significantly lower than in the

Mo₇₂Fe₃₀: 1.65 w.% against 2.84 w.%, respectively. This

decreasing is conditioned with the cluster-water molecules

release after NO₂sorption. The calculated water loss was 120

from 150 H₂O molecules as compared with initial Mo₇₂Fe₃₀(see

Supporting Information). Thus, we may conclude, that

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View Article Online

DOI: 10.1039/C8DT05125A

Fig. 4. The model of the NO₂adsorption by the small pores (SP) oriented inward the large pore (LP) (a) or by the SP placed on the

c-axis of crystal (b). In both case the considered pores’ types spread through the crystal as a tunnel. (c) – the model of NO₂

coordination to the μ-O in bond Mo-O-Fe inside the SP (“in-plane coordination”). The purple and yellow colors correspond to the

Mo and Fe oxygen polyhedra, respectively.

The reason of this distortion is caused with NO₂coordination to approximately 20 FeO₆polyhedra could also interact with

the oxygen atoms: bridged oxygen (μ-O) in Mo-O-Fe bonds nitrogen dioxide by “out-of-plane” mechanism. This fact can

and/or terminal-oxygen (t-O) in vertex of FeO₆octahedron. It sufficiently explain the essential amount of weak-bonded NO₂

seems to be unlikely, that signal with g=4.3 is owing to exclusive molecules per one nanocluster: 14±3 molecules (excepting six

coordination of NO₂molecules with the μ-O atoms, because in strong bonded NO₂) determined from elemental analysis. Thus,

this case the discussed signal should reveal at room the POM-NO₂sample, produced by the simple exposition of

temperature, since at such coordination nitrogen dioxide is Mo₇₂Fe₃₀crystals in the NO₂stream, can be considered as a

strong-bonded in-plane of SP and its position does not depend potential NO_xdonor acting at room temperature due to the

on the temperature, significantly. The observation of this signal presence of delocalized NO₂molecules in the tunnels formed by

at the cryogenic temperatures points out that some adsorbed the large pores. Moreover, the six stoichiometric NO₂molecules

NO₂molecules, delocalized under the room temperature, have can be further liberated after decomposition of the Mo₇₂Fe₃₀in

become ordered (Fig.5c). The role of such delocalized NO₂the living organism²⁵.

molecules can play the weak-bonded NO₂molecules, which are

Biological application

coordinated above the SP (“out-of-plane” coordination through

the t-O) in the tunnel of LP that results in the mobility of such

NO₂at 25 °C (Fig.4a). Thus, the tetragonal distortion of Fe^III

polyhedron can be conditioned by the simultaneous

coordination of two molecules of nitrogen dioxide to the one

FeO₆octahedron: first NO₂is coordinated with the μ-O and the

second one with the t-O (Fig.5b).

This statement is in line with releasing of weak-bonded nitrogen

dioxide molecules under deep vacuuming displayed in the XPS

experiment (Fig.3) that results in the preservation of a

stoichiometry amount of NO₂: 6 molecules per one POM. Based

on the large pore geometry and because of the three FeO₆

polyhedra belong to each SP, we may suggest that

In the context of searching the modern NO-donor drugs and so-

called NO-delivery systems, the our obtained results concerning

the coordination of NO₂molecules in the Mo₇₂Fe₃₀crystal in the

form of NO₃^-ions may be interesting for such application. As it

was told above, the POM possess a high NO₂-capacity (20±3

molecules per one nanocluster), and its toxic effect for

mammalian is practically absent that was shown on the

laboratory rats in the previous study²⁵. Furthermore, the weak-

bonded NO₂molecules and the strong bonded ones can liberate

in the human body and hypothetically convert to the nitrogen

monoxide during the gradual POM destruction at the

physiological pH value (~7.2 or higher) of media⁷².

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a

b

Fig. 5. The temperature dependence of the EPR spectra (left wing) for Mo₇₂Fe₃₀and POM-NO₂samples: the signal position in field

scale (a) or g-factor scale (b). The presented atoms illustrate the distortion of FeO₆polyhedra from the octahedral to tetrahedral

type during the simultaneous coordination of two NO₂molecules to the μ-O and t-O oxygens atoms at cryogenic temperature. The

drawn model of tetragonal distortion of FeO₆octahedron was made with assumption, that bridged oxygen atoms are hindered to

changing of them position as compared with upper and bottom vertices.

This expectation is based on the data reported in literature^73,74plays an important role. However, despite what kind of the

about the possible biochemical route of NO production. So, the biochemical route of NO production would realize, the most

emergence of nitric oxide is depended on the primarily important question, which needs to be answered for

-

appearing of the NO₂ions, which are reduced to NO by the application of NO-delivery system in human body, it is a

members of mitochondria electron transport chain⁷³. In turn, solubility of this system. In the present study, the performed

the nitrite anions are product of R-ONO₂molecules’ reduction investigation of POM-NO₂solubility in the distilled water upon

by the mitochondrial aldehyde dehydrogenase (mtALDH) using the mixing several hours at 37 °C showed that concentration of

the thiol molecules as a cofactor of such process⁷³. Moreover, iron and molybdenum was 111.7 and 537.7 mg/L, respectively.

the NO from nitrate ions could be produced by the nitrate This corresponds to the molar ratio Mo:Fe=2.8 which is slightly

reductase⁷⁴, where the coordination of NO₃^-to the Mo^VIcentre different from ratio for pure Mo₇₂Fe₃₀(Mo:Fe=2.6). The

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calculated concentration of POM was 1.26 g/L (6.67×10^-5to the contribution of “out-of-plane” coordinativ_Ve_iewin_At_re_ticr_la_ec_Ot_ni_lo_inn_e

DOI: 10.1039/C8DT05125A

mol/L). That is lower than maximum reachable concentration of with small pores of Mo₇₂Fe₃₀, however, they could be

2.1 g/l based on the weight of POM-NO₂. For comparison, the delocalized at room temperature in the large pores. Indeed, this

concentration of pure Mo₇₂Fe₃₀can reach the 2.5 g/l. The delocalization was well-identified based on the measuring

registered UV-Vis spectrum of POM-NO₂solution is similar to temperature variation of EPR spectrum for POM-NO₂sample,

spectrum of pure Mo₇₂Fe₃₀(Fig. S9†) and the extinction where the novel signal of g=4.3 appears from the -50 °C

coefficients at the characteristic wavelength 325 nm are the achieving the highest intensity at cryogenic temperature near -

same for both samples: ε=2.93×10⁵L/(mol×cm) for POM-NO₂150 °C. The third state relates to the strongest type of NO₂

and ε=2.94×10⁵L/(mol×cm) for pure Mo₇₂Fe₃₀. Thus, we may bonding due to the coordination “in-plane” of small pores – it is

conclude, after treatment with NO₂the solubility of Mo₇₂Fe₃₀in a chemical adsorption.

water decreases because the dehydration of nanocluster, which Thus, obtained model of NO₂interaction with POM from

losses the 120 from 150 weak-bonded H₂O molecules - cluster gaseous phase allowed us to draw up a conclusion that

water (see section XPS analysis). Despite, the obtained suggested synthetic route, consisted of the simple contact of

concentration of POM-NO₂(1.26 g/l) corresponds to the 20(±3) Mo₇₂Fe₃₀with NO₂flow, can be used for production of new

times larger concentration of carried NO₂(1.13-1.5×10^-3mol/L), complex compound [POM-(NO₂)₆]·(NO₂)_14±3, which possesses a

that on three-order exceeds the traditional therapeutic high NO₂capacity and good solubility in aqueous media. Taking

concentration of nitroglycerin in tablets, approximately.

into account the possible mechanisms of pH-depended NO₂

liberation in the human body^73,74that accompanies the NO

formation (important signal molecule and mediator), the

synthesized POM-NO₂compound can be considered as a

promising NO-donor for the biological applications. However,

such assumption should be investigated additionally.

Conclusions

As a result of carried out experiments, two kind of samples with

different content of NO_xwere produced: POM-NO and POM-

NO₂. For all produced samples, in IR spectrum, the appearing of

a new sharp intensive band at 1385 cm^-1corresponds to the

coordination of NO_xspecies in the Mo₇₂Fe₃₀crystal as nitrate

Conflicts of interest

There are no conflicts to declare.

-

ions with microhydrated environment – NO₃(H₂O)₃having the

C_3hsymmetry, that is in line with theoretical calculation⁴⁶. The

analysis of Raman spectra, which well correlates with IR data,

additionally displayed the contribution of FeO₆polyhedra to the

NO_xcoordination by the presence of band at 1050 cm^-1. As we

might to conclude from TG-DTA-MS spectra, the thermal

decomposition of Mo₇₂Fe₃₀strongly depends on the NO and

NO₂treatment resulting in the increasing of release-

temperature of H₂O and CO₂molecules as compared with pure

POM. Revealing the several stages of water molecules release

from POM-NO and POM-NO₂samples, including a new high-

temperature signal m/q=18 at 289 °C especially, should be

attributed to the presence of different kinds of aqua-ligands,

related with NO₂coordination by the direct or indirect way, in

the structure of POM-NO₂crystal. Against the simple nitrates of

transitional metals, the relatively high temperature (224 °C) of

NO₂-evolving from POM-NO₂sample points out to the strong

interaction between nitrogen dioxide and nanocluster and/or

its destruction products.

Based on the XPS analysis, the stoichiometry composition of

POM-NO₂was determined as [POM-(NO₂)₆], whereas weak-

bonded NO₂molecules (14±3 NO₂per one POM as established

from the elemental C,H,N-analysis) were removed from this

sample under deep vacuum condition (10^-7mbar). Thus, we

found that NO₂molecules, adsorbed by the Mo₇₂Fe₃₀, could be

in three different states. First state corresponds to the very

weak bonded NO₂molecules, which are physically adsorbed in

the large pores of POM and remove under standard

atmosphere pressure without heating. The second state

pertains to the weak-bonded NO₂molecules, which are also

physically adsorbed in the large pores of POM. These

coordinated NO₂molecules possess higher bonding energy due

Acknowledgements

Authors express one’s thanks to Dr. habil. Vladimir A. Vazhenin

for great support in organization of EPR measurements and for

important notes concerning EPR spectra interpretation, and to

Dr. Mikhail Yu. Kornev for detailed discussion of NMR spectra.

This work was performed in frames of State Task by the RF

Ministry of Education and Science (project no.

4.6653.2017/8.9), and Competitiveness Enhancement Program

of Ural Federal University (founded by the Government of the

Russian Federation Act 211, contract № 02.A03.21.0006). The

equipment of Ural Centre for Shared Use “Modern

nanotechnology” Ural federal university was used.

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Article Doi

Coordinative interaction between nitrogen oxides and iron-molybdenum POM Mo72Fe30

DOI: 10.1039/c8dt05125a

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