AgNO3?NH4NO3 – an enigmatic double-salt type “decomposition intermediate” of diamminesilver(I) permanganate

Article abstract of DOI:10.1002/zaac.202100098

The 1 : 1 type double salt of AgNO₃ and NH₄NO₃ (NH₄NO₃ ? AgNO_3, compound 1) was prepared and its Raman spectrum was evaluated. IR and far-IR spectra of compound 1 and its deuterated analog (ND₄NO₃ ? AgNO₃, compound 1-D) were also studied. We identified two types of nitrate ions, one had a relatively strong, whereas the other is only weakly hydrogen bonded to the surrounding ammonium ions. The thermal decomposition features of compound 1 and its formation mechanism from [Ag(NH₃)₂]MnO₄ (compound 2) at 80 °C with consecutive aqueous leaching of the decomposition residue has been elucidated. The solid phase decomposition product of compound 2 is presumably [Ag(NH₃)NO₃] (compound 3), which disproportionates into AgNO₃ and [Ag(NH₃)₂]NO₃ (compound 4). Compound 4 thermally decomposes in the solid phase into Ag+MnO_x, while in aqueous solution its hydrolysis results in NH₄NO_3, which crystallizes out with an equimolar amount of AgNO₃ formed during the disproportionation as compound 1.

Full text of DOI:10.1002/zaac.202100098

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.202100098
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
AgNO3·NH4NO3 – an enigmatic double-salt type
“decomposition intermediate” of diamminesilver(I)
permanganate
Kende Attila Béres,[a] Vladimir Petruševski,[b] Berta Barta Holló,[c] Péter Németh,[d, e]
Lara Fogaça,[a, f] Fernanda Paiva Franguelli,[a, f] Attila Farkas,[g] Alfréd Menyhárd,[h]
Imre Miklós Szilágyi,[f] and László Kótai*[a, i]
°
The 1:1 type double salt of AgNO3 and NH4NO3
(NH4NO3 ·AgNO3, compound 1) was prepared and its Raman
spectrum was evaluated. IR and far-IR spectra of compound 1
and its deuterated analog (ND4NO3 ·AgNO3, compound 1-D)
were also studied. We identified two types of nitrate ions, one
had a relatively strong, whereas the other is only weakly
hydrogen bonded to the surrounding ammonium ions. The
80 C with consecutive aqueous leaching of the decomposition
residue has been elucidated. The solid phase decomposition
product of compound 2 is presumably [Ag(NH3)NO3] (com-
pound 3), which disproportionates into AgNO3 and [Ag(NH3)2]
NO3 (compound 4). Compound 4 thermally decomposes in the
solid phase into Ag+MnOx, while in aqueous solution its
hydrolysis results in NH4NO3, which crystallizes out with an
equimolar amount of AgNO3 formed during the disproportiona-
tion as compound 1.
thermal decomposition features of compound
1 and its
formation mechanism from [Ag(NH3)2]MnO4 (compound 2) at
Introduction
(NH3)4](MnO4)2 (M=Cu, Zn, Cd) complexes[8–10] is known, how-
ever, in the case of compound 1, it is questionable why only the
double salt (compound 1) is formed instead of the expected
ammonium nitrate. Furthermore, thermal decomposition fea-
tures of compound 1 showed that compound 2 forms via an
unknown intermediate (compound 3), which transforms into
compound 1 during the aqueous leaching.[7] Neither AgNO3 nor
NH4NO3 is present in the evaporated aqueous leachate, which
raises that question how could form AgNO3 and NH4NO3 in 1:1
molar ratio?
A double salt of ammonium nitrate and silver nitrate (1:1,
NH4NO3 ·AgNO3, compound 1) was prepared first by Russel and
Maskelyne[1] from aqueous NH4NO3 and AgNO3 solutions, but its
properties (except the structure[2] and melting/solubility
diagrams[3–6]) have not been studied in detail. Compound 1 was
also isolated from the aqueous leachate of the thermal
decomposition product of [Ag(NH3)2]MnO4 (compound 2), but
the mechanism of its formation is unknown.[7] The selective
oxidation of one of the coordinated ammonia ligand into
ammonium nitrate during the thermal decomposition of [M-
In order to answer to these questions, the knowledge of the
properties of compound 1 is essential. There are no available
[a] K. A. Béres, L. Fogaça, F. Paiva Franguelli, L. Kótai
Institute of Materials and Environmental Chemistry, Research
Centre for Natural Sciences, ELKH, Magyar tudósok krt. 2, 1117
Budapest, Hungary
[f] L. Fogaça, F. Paiva Franguelli, I. M. Szilágyi
Department of Inorganic and Analytical Chemistry, Budapest
University of Technology and Economics, Műegyetem rakpart 3,
1111 Budapest, Hungary
E-mail: kotai.laszlo@ttk.hu
[g] A. Farkas
[b] V. Petruševski
Department of Organic Chemistry, Budapest University of Technol-
ogy and Economics, Műegyetem rakpart 3, 1111 Budapest,
Hungary
Faculty of Natural Sciences and Mathematics, Ss. Cyryl and
Methodius University, Skopje, Macedonia
[c] B. B. Holló
[h] A. Menyhárd
Department of Chemistry, Biochemistry and Environmental Protec-
tion, Faculty of Sciences, University of Novi Sad, Trg Dositeja
Obradovića 3, 21000 Novi Sad, Serbia
Department of Physical Chemistry and Materials Science Budapest
University of Technology and Economics, Műegyetem rakpart 3,
1111 Budapest, Hungary
[d] P. Németh
[i] L. Kótai
Institute for Geological and Geochemical Research, Research
Centre for Natural Sciences, ELKH, Budaörsi street 45, 1112
Budapest, Hungary
Deuton-X Ltd., Selmeci. U. 89, H-2030 Érd, Hungary
Supporting information for this article is available on the WWW
under https://doi.org/10.1002/zaac.202100098
[e] P. Németh
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Chemie published by Wiley-VCH GmbH. This is an open access
article under the terms of the Creative Commons Attribution Non-
Commercial NoDerivs License, which permits use and distribution
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is non-commercial and no modifications or adaptations are made.
Research Institute of Biomolecular and Chemical Engineering
University of Pannonia, Egyetem út 10, 8200 Veszprém, Hungary
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data about spectroscopic and thermal properties of
Ammonium ions
AgNO3.NH4NO3. Although its properties in solution and melt
phase were studied in detail,[3–6] the information about its solid
phase properties are missing.
The isolated ammonium ion has four normal modes, ν1(A1, νs), a
doubly degenerated ν2(E’,δs), and two triply degenerated ones,
ν3(F2,νas) and ν4(F2,δas). The site group is C1 in compound 1, the
point group isomorphic with the factor group is C2h, thus 36
(2×9 IR active (Au and Bu) and 2×9 Raman active (Ag and Bg)
internal modes exist (Figure 1).
The external modes of isolated tetrahedral ammonium ion,
Rxyz(F1) and Tyyz)(F2) decompose into 4×6=24 (2×3=6 IR (Au
and Bu) and 2×3=6 Raman (Ag and Bg) active translational (Tx,
Ty, Tz) and the same number and types of rotational (Rx, Ry, Rz))
modes (ESI Figure 4).
Therefore, compound
1
and its deuterated analog
(ND4NO3 ·AgNO3, compound 1-D) were synthesized and their
spectroscopic and thermal properties were studied in detail.
Based on the results, the mechanism that leads to the formation
of compound 1 from compound 2 could be elucidated.
Results and Discussion
Synthesis of compound 1 from AgNO3 and NH4NO3
The compound 1 could easily be crystallized out from the
AgNO3-NH4NO3 melt and AgNO3-NH4NO3-H2O solutions as
well.[3,4] Based on the ternary phase diagram constructed from
experimental Schreinemacker’s experimental data,[3] com-pound
1 could easily be prepared by crystallization of AgNO3 and
NH4NO3 solutions within the crystallization field of compound 1
(D=kcdeSL area) (ESI Figure 1). Based on the XRD data of the
compound 1 prepared with 1:1 stoichiometry neither AgNO3
nor NH4NO3 polymorphs could be found (PXRD Card No: 83-
1796, ESI Figure 2).
Nitrate ions
The nitrate ions in compound 1 have almost ideal D3h
symmetry. There are four normal modes of an isolated ion,
ν1(A’1,νs), ν2(A”2, π), and two doubly degenerated ones, ν3(E’, νas)
and ν4 (E’, δ). In principle, there are 2×6=12, altogether, 24
internal modes consisting of 6+6=12 IR (Au, Bu) and 6+6=12
Raman (Ag, Bg) modes.
The number of modes, however, due to two crystallo-
graphically different nitrate ions may be 48, and every normal
mode is active in Au, Bu (IR) and Ag, Bg (Raman) (Figure 2). The
external modes of isolated D3h nitrate ion (Rz (A’2), Tz(A”2), Txy(E’)
and Rxy(E”) decompose in the same way as the external modes
of the ammonium ions. Thus, 4*6 modes (Tx, Ty, Tz, Rx, Ry, Rz)
should appear, but due to the two crystallographically different
nitrate ions, the number of modes altogether may be 2×24=
48. (ESI Figure 5).
Synthesis of deuterated compound 1 (1-D)
The deuterated compound 1 (1-D) was prepared in two
different ways. Gradual deuteration of compound 1 in D2O with
the complete evaporation and removal of the equilibrium
protium content, using freshly prepared CaO as Ca(OH)x(OD)1À x
,
resulted in gradual increase of deuterium level[7,16] in compound
1 and four deuteration steps were needed to reach almost
complete deuteration of compound 1. For all these steps freshly
prepared CaO had to be used in a closed desiccator.
Silver ion
The silver ion is coordinated with four oxygen atoms of nitrates
and is treated as a translating single ion having only hindered
The synthesis performed in the abovementioned route
resulted in almost completely deuterated 1-D. Deuterium
chloride (35% in D2O) reacted with 1 equivalent of AgNO3 in
D2O and formed deuterated nitric acid (DNO3) and AgCl. After
separation of AgCl with filtering, DNO3 was neutralized with
1 equiv. of ND3 in D2O, and the formed D2O solution of ND4NO3
was combined with saturated D2O solution of 1 equiv. of
AgNO3. The solution containing AgNO3, and ND4NO3 was dried
in a closed desiccator over freshly prepared CaO. The deutera-
tion degree was confirmed with IR spectroscopy (ESI Figure 3).
Figure 1. Internal modes for ammonium ions in compounds 1 and
1-D.
Correlation analysis
The crystal structure of compound 1 is known[2] (monoclinic,
P21/a, Z=4). A striking feature of the structure is the presence
of the two crystallographically different nitrate groups that are
arranged in parallel layers (201). According to these structural
features, ESI Figures 4–6 shows the result of correlation analysis.
Figure 2. Internal modes for nitrate ions in compounds 1 and 1-D.
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translations. Due to C1 site symmetry and C2h factor group, 12
Ammonium-ion bands assignment
(2×3 IR (Au, Bu) and 2×3 Raman (Ag and Bg) translational modes
(Tx, Ty and Tz) should be observed both in the IR and Raman
spectra (ESI Figure 6).
The symmetric and antisymmetric vibrational bands of the
ammonium ion could easily be assigned,[17,18] comparing the IR
spectra of compound 1 and its deuterated form (Table 1,
Figure 1 and ESI Figure 4). The band system in the NÀ H
stretching region contains both the symmetric and antisymmet-
ric stretching bands at 3055 and 3203 cmÀ 1, respectively. The
intensities of components of the antisymmetric mode in the IR
spectrum are expected to be much higher than that of the
symmetric one because the mode is IR forbidden for the
undistorted ammonium ion. Based on this consideration, the
most intensive bands at 3286, 3203 and 3055 cmÀ 1 are assigned
to the components (F2) of νas, coinciding with νs(NÀ H)+ν5 and
δs(NÀ H)+δas (NÀ H) modes. A weak νs band is assigned at
2973 cmÀ 1. The νs(NÀ H)+ν5 combination could also be ob-
served in ammonium-ion containing oxometallate compounds
that have strong hydrogen bonds.[16,18] Considering the highest
wavenumber νL mode (198 cmÀ 1 in the far IR spectra), the
expected combination band wavenumber is ~3171 cmÀ 1.
Spectroscopic features of compounds 1and 1-D
+
All four modes of ammonium ions (NH4 and ND4+) and four
modes of nitrate ions of compound 1 and 1-D are Raman and
IR active due to the distortion of the Td and D3h symmetry of
isolated ammonium and nitrate ions, respectively. There are
two kinds of crystallographically distinguishable nitrate ions,
thus, two series of nitrate bands are expected and are found in
+
our data (Figure 3). The isotope substitution between NH4 and
+
ND4 ions results in a significant shift in IR band wavenumbers
(Figure 3 and ESI Figure 3 and 7) and can be used to distinguish
the overlapping NÀ H and NÀ O bands.
On deuteration, all of the stretching bands belonging to
+
NH4 ions disappear, and new bands appear in the 2700–
2200 cmÀ 1 region (ESI Figures 3 and 7). The highest wave-
number band at 2626 cmÀ 1 belongs to the ν(OÀ D) of surface
D2O absorbed on the sample 1-D. The complex band system
contains νas(NÀ D), νs(NÀ D) and the appropriate ammonium (νs +
ν5 and δs +δas and nitrate-ion (νs +νas) combination bands. The
most probable location of the νs(NÀ D) band is at 2350 cmÀ 1
(νNH/νND =1.27). The lowest wavenumber component of the
band system in the IR spectrum of 1-D at 2252 cmÀ 1 belongs to
the first overtone of δs(NÀ D) band, therefore, the new band
appearing at 1123 cmÀ 1 on deuteration of compound 1 belongs
to δas(NÀ D). Thus, the bands at 1406 and 1376 cmÀ 1 (δNH/δND
~1.24) are associated with the components of the triply
degenerated δas(NÀ H). It has to remark, that the nitrate
2δas(NÀ O) overtone bands also appear in this wavenumber
range, but the main components of the band system at
~1400 cmÀ 1 significantly change after deuteration of compound
1, thus they have to be associated with NÀ H modes (Figure 3).
The symmetric bending mode of ammonium ion δs(E) at
1758 and 1740 cmÀ 1 almost completely disappears on deutera-
tion of compound 1. Only a small peak at 1761 cmÀ 1 remains
back, which belongs to νs(NÀ O)+δ(NÀ O) combination mode of
the nitrate ion.[17,19] On deuteration of compound 1, the
appearing of NÀ D mode can be expected around 1300 cmÀ 1,
but it is covered by the wide and intense νas(NÀ O)(E’) nitrate
bands. Using the δas(NÀ H)/δas(NÀ D) value found for ammonium
chloride and the δas(NÀ D) for compound 1, the calculated
δas(NÀ H) wavenumber is found to be 1401 cmÀ 1, close to the
highest wavenumber component of the mixed band system
(1406 cmÀ 1) in the IR spectrum of compound 1.
Figure 3. IR spectra of compound 1 (blue) and compound 1-D (red)
at room temperature in the region of nitrate modes.
Table 1. Assignment of ammonium ion bands in the room
temperature IR spectra of compound 1 and 1-D.
Assignation
1
1-D
Normal modes/cmÀ 1
νas(F2)
νs +νL
3286, 3203, 3055
2508, 2454, 2387
δs(E)+δas(F2)
νs (A1)
δs(E)
2973
2350
1758, 1740
~1300[a]
1123
δas(F2)
1406,[b] 1381[a]
Combination and overtone bands/cmÀ 1
2δas
δs +δas
2853, wide
2252
~3121, 3167[b]
~2423[b]
[a] Covered by bands of other modes. [b] calculated ones.
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Nitrate-ion bands assignment
deuteration, the weak bands at 1771, 1763, 1753 and 1740 cmÀ 1
appears in the IR spectrum of 1-D (νs(NÀ O) (A’1))=1069+
δs(NÀ O) (E”)=704, and νs(NÀ O(A’1)=1036+δs(NÀ O)(E“)=730,
724 and 704 cmÀ 1 (Figure 3).
Two series of nitrate ion bands are expected and found in the
spectra of compounds 1 and 1-D. The assignment of four
normal modes and their combinations are given in Table 2.[20]
The IR forbidden νs mode becomes IR active and two sets of
singlet bands (A’1) appear. The two νs(NÀ O)(A’1) and the
2×2=4 νas(NÀ O) (E’) modes assigned in the IR spectra of
compound 1 (ESI Figure 3) show that the two nitrate environ-
ments are remarkably different. On deuteration of compound 1,
the NÀ O band positions and shapes significantly change for the
bands of the first nitrate ion type, whereas the band positions/
shapes for the second type nitrate remain almost unchanged
(Figure 3 and Table 1). This finding strongly suggests that only
one (the first) type of the nitrate ions are in hydrogen bond
interactions with ammonium ions. In the spectrum of com-
pound 1 between the two bands of the in-plane deformation
modes δ(NÀ O) (E“), the component at 723 cmÀ 1 splits into two
parts (730 and 724 cmÀ 1). In contrast, the component at
703 cmÀ 1 remains singlet in the spectrum of 1-D (Figure 3). The
out-of-plane deformation band/rocking mode of coordinated
Based on the above-mentioned results, the nitrate ion type
(I) is in a hydrogen bond interaction with ammonium ion while
the nitrate ion type (II) has not direct or more probably has only
a weak hydrogen-bond interactions with ammonium ions.
The AgÀ O modes (all related to the hindered translation of
silver cations) are generated by the bidentate coordination of
nitrate on silver and are the lattice modes appearing below
400 cmÀ 1. For non-deuterated sample the bands occur at 204,
115 and 82 cmÀ 1, whereas for deuterated sample they appear-
ing at 196, 136 and 95 cmÀ 1 were assigned to such modes.
Variable-temperature Raman spectroscopic results
Raman spectra of compound 1 were recorded at room temper-
°
ature and at À 150 C in the spectral range between 4000 and
100 cmÀ 1 (ESI Figure 8–11). The ammonium-ion stretching
bands were too week to evaluate at room temperature, but at
À
nitrate ion (π(A”2) and/or 1(NO3 …Ag+) at 811 cmÀ 1) splits into
two components (813, 800 cmÀ 1).
À 150 C the bands belong to νas(F2), νs(A1)+ν5 and δs(E)+δas(F2)
°
The δas(NÀ H)(F2) appears as a wide band at 1036 cmÀ 1 with
a shoulder (1080 cmÀ 1) in the IR spectrum of compound 1,
whereas in the spectrum of the deuterated form, a well-
separated band appears instead of the shoulder at 1069 cmÀ 1,
and the lower wavelength band component (1036 cmÀ 1)
remains unchanged. The appearance of νs +ν3 combination
bands (Table 2) show that the νs(A’1)=1080 and 1036 cmÀ 1
interact with the 1283/1232 and 1389/1283 cmÀ 1) components
of νas(E“) of the two nitrate ion types, respectively. The same
combinations in the IR spectrum of 1-D coincide with the NÀ D
stretching modes, but an νas(NÀ O)(E’) component (1192 cmÀ 1) is
combined with both νs(NÀ O) modes (1069 and 1036 cmÀ 1
respectively) and results in two weak combination bands at
2162 and 2142 cmÀ 1.
modes appeared as a complex band system with components
at 3292, 3192; 3121 and 3048 cmÀ 1 (ESI Figure 8).
The symmetric nitrate stretching modes (νs(A1)) appeared as
°
doublets both at room temperature and À 150 C (1048, 1041
and 1050, 1041 cmÀ 1, respectively) (ESI Figure 9). The antisym-
metric nitrate stretching mode (νas(E’)) wavenumber range
(1500–1200 cmÀ 1) contained more than four bands (2×2=4) at
°
À 150 C (Figure 4) due to the presence of NÀ H deformation
modes as well. The shift of these deformation modes towards
higher wavenumbers was in accordance with a bidentate
coordination of nitrate-ion to the silver (νas Raman shifts of the
nitrate ion in Cs or C2v appear at 1407–1390 cmÀ 1 in molten
AgNO3[21]).
The Raman shifts belonging to in-plane deformation mode
°
The νs(NÀ O)+δs(NÀ O) type combination bands appear in
(δ(E“)) appeared at 732, 725 and 708 cmÀ 1 at À 150 C (ESI
the range of δs(NÀ H) modes. With shifting the NÀ H bands on
Figure 9), which indicated that among the 2×2=4 expected
Table 2. Assignment of ammonium ion bands in the room
temperature IR spectra of compound 1 and 1-D.
Assignation
1
1-D
Normal modes/cmÀ 1
νas(NÀ O)(E’)
1389sh[a], 1283, 1262,
1232
1387sh, 1293, 1192
νs(NÀ O) (A1)
π(A“2) (out-of-
plane)
1080sh, 1036
1069, 1036
813,800[b]
811[b]
δs(E) (in-plane)
723, 703
730, 724, 704
Combination and overtone bands/cmÀ 1
ν1 +ν3
ν1 +δ
2417, 2362, 2314
2252
1771, 1763, 1753,
1740
1753[a], 1740[a]
[a] Covered by bands of other modes. [b] mixed band contains
rocking mode of coordinated nitrate.
Figure 4. Raman spectra of compound 1 in the range of 1500–
1200 cmÀ 1 at room temperature (blue line) and À 150 C (red line).
°
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°
nitrate bands of the two different nitrate ions, three bands
content of the eutectic melt with DTG maximum at 253 C.
°
appeared. The Raman inactive out-of-plane deformation (π)
Between ~270 and 370 C the decomposition slows down.
appeared at 822 and 823 cmÀ 1 (25 and À 150 C, respectively),
Above 370 C the next decomposition step corresponds to the
°
°
which confirmed the symmetry lowering (D3hÀ C2h). This mode
may be overlapped with the rocking mode of coordinated
ammonia.
silver nitrate degradation. The mass loss of 26.0% agrees well
with the calculated value for nitrate group loss (24.81%).
Generally, the mass losses are in accordance with the calculated
ones for NH4NO3 : AgNO3 =1:1 eutectic melt. The percentage of
final product (40.55%) is somewhat lower than that calculated
for the silver content of compound 1 (43.16%).
The silver-oxygen and lattice modes of the Raman spectra
found in the far-IR range (<400 cmÀ 1) (ESI Figure 9) appeared
as wide bands at 160 and 120 cmÀ 1 (25 C), which decomposed
°
°
at À 150 C into a complicated band system consisting of
The TG-MS results in inert atmosphere show that the first
mass loss belongs to the ammonium nitrate decomposition
components at 180, 162, 135, 127sh and 116 cmÀ 1. It is worth
mentioning that low-frequency lattice modes are as a rule
coupled to other modes with comparable frequency, having
the same symmetry.
with evolution of H2O (m/z=18 and 17, OH2 and OH+,
+
respectively) and simultaneous N2O formation (Figure 6 and ESI
Figure 13).
In the second decomposition step of compound 1 in inert
atmosphere, the usual thermal decomposition features of
AgNO3 are observed.[19] The solid reaction product is metallic
silver. Thus, NO (m/z=30) and O2 (m/z=32) evolution could be
detected (ESI Figure 13) by TG-MS. At the TG-DTG curve in inert
atmosphere two mass loss steps are observed with DTG peak
UV spectroscopic results
Compound 1 is colorless, thus it shows a wide band system
only in the UV range (ESI Figure 12) with two separable peaks at
341 and 347 nm, which might be associated with O2p-Ag5s LMCT
bands of the coordinated nitrates. The Ag4d–5s band, observed in
°
maxima at 417 and 452 C, whereas in air only one peak appears
°
at 439 C. The two-step process is probably associated with the
ammine type silver complexes, could not be identified, and the
two different parallel (e.g. catalytic and non-catalytic) reaction
routes of AgNO3 decomposition, the presence of air changes
and the amount/type of the (catalytically) active species.
The thermal effects of the decomposition processes of 1
were analyzed in N2 and O2 atmospheres (Figure 7). The first
!
S
allowed nitrate (π π) band appears as a very weak band at
!
À 1
S
215 cm , whereas the forbidden and weak π n transition of
nitrate ions cannot be observed.
°
transition of 1 was melting at 109 C onset in both atmospheres
Thermal studies on compound 1
in accordance with the values found by Zawidzki,[16]
Roozeboom[17] and Dingemans.[10] The melting heats were found
to be 26.42 and 26.36 kJ/mol, in N2 and O2, respectively. The
eutectic melt composition was ~1:1.[10] The first decomposition
step, attributed to ammonium nitrate decomposition from the
eutectic melt, was followed by endothermic effect in both
The thermal decomposition of compound 1 was followed in
°
argon and air. Up to 400 C the atmosphere only slightly affects
the decomposition mechanism, the aerial oxygen having only a
minor role in the process (Figure 5).
°
°
°
Above 400 C the degradation mechanism is different in
atmospheres with DSC peak minima at 253 C (N2) and 271 C
(O2). Both peak temperatures were significantly higher than the
argon and air. In argon atmosphere, compound 1 loses 33.4%
of its mass in a well-defined step. This mass loss can be
attributed to the decomposition of the ammonium nitrate
°
melting temperatures of NH4NO3 (Mp.=168 C) and AgNO3
°
(Mp.=209 C). Right after the decomposition of the ammonium
Figure 5. TG and DTG curves of compound 1 in argon and air.
Figure 6. TG-MS curves characteristic for water evolution.
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thermolysis.[22] The solid reaction product is metallic silver and
NO (m/z=30) and O2 (m/z=32) evolution could be detected by
TG-MS. There are two peaks appearing at the simultaneous TG-
°
DTG-DSC curve in inert atmosphere at 417 and 457 C peak
temperatures, whereas in air only one peak was observed at
°
444 C. The two-steps process probably belongs to two different
parallel (e.g. catalytic and non-catalytic) reaction routes of
AgNO3 decomposition and the presence of air changes the
amount/type of the (catalytically) active species.
Chemism of compound 1formation from [Ag(NH3)2]MnO4
(compound 2)
The oxygen content of permanganate ion is not enough to
complete the oxidation of one of the ammonias into nitrate
and the formation of AgMnO2 as the end product. However, the
°
primary decomposition step at 80 C is carried out with
formation of compound 3, which does not require incorpo-
ration of aerial oxygen. Furthermore, there is no elementary
Figure 7. DSC curves in N2 and O2 atmosphere
silver formation and the permanganate ion completely disap-
[7]
°
°
nitrate content of the eutectic, the residue was silver nitrate
melt. Although the decomposition reaction route of ammonium
nitrate was strongly dependent on the conditions and the
presence of additives, these processes were exothermic.[18] The
endothermicity of the processes might be attributed to the
following reasons:
(1) enrichment of the melt in AgNO3 may request energy to
keep the AgNO3 in molten state due to increasing the melting
point and
pear at 80 C, whereas, at 125 C, metallic silver forms. The IR
°
spectrum of the decomposition intermediate formed at 80 C
(ESI Figure 14) shows the presence of nitrate (1314, 1213, 1038,
820 or 707 cmÀ 1), coordinated ammonia (3324, 3265, 3174 cmÀ 1,
1745, 1366) and water (~3500 cmÀ 1, νOH ; ~1600 cmÀ 1, δOH2
)
nÀ
bands, and the complete absence of MnO4 (n=1–3) oxoan-
ions (missing νas(MnÀ O) bands for MVII, MVI or MV species).[23]
There is no MnO2 either, only lower manganese oxides could be
detected.[24] Figure 8 shows the XRD peaks of starting com-
°
(2) the heat evolved is absorbed to keep the liquid state.
pound 1, the intermediate
3
formed at 80 C and the
°
°
The different reaction heat values above 400 C showed that
decomposition product formed at 125 C. No metallic silver
°
the oxidation state of the end-products (e.g. N-oxides) was
definitely distinct in air and inert atmospheres, and the reaction
heat in inert atmosphere was more (1567.1 kJ/mol) while in air
was less endothermic (77.6 kJ/mol).
occurs is in the product formed at 80 C and intermediate 3 can
unambiguously be assigned as [Ag(NH3)2NO3 (Card No.C76-
1562).
Based on the results, we propose the following formation
mechanism for intermediate 3:
The evolution of N2 and O2 during the decomposition of
compound
1
was also expected, but they were hardly
1) During the decomposition reaction of compound 1 only
one of the ammonia ligand is oxidized with the permanganate
ion into nitrate in consistent with ammine complexes of
transition metals.[23,25] As a result, the permanganate anion
disappears, but equivalent amount of nitrate ion forms to
neutralize the charge of the silver ions remained back as Ag+.
The equation can be summarized as:
detectable because of the relatively high level of environmental
nitrogen and oxygen even after longer purging of the furnace
by argon (ESI Figure 13). The evolved N2O gave rise to a peak of
low intensity in the first decomposition step. The presence of
m/z=14 peak (N+), however, proved the formation of N+ as
N2O fragmentation product suggesting that the main decom-
position process was consistent with
½AgðNH3Þ2�MnO4 ! f½AgðNH3Þ�NO3 þ MnOg
NH4NO3 ! N2O þ 2 H2O þ 36:1 kJ=mol
Reychler described a brown compound with [Ag(NH3)NO3]
composition,[26] and later Kurilov reported it as a mixture of
[Ag(NH3)2]NO3 (compound 4) and AgNO3.[27] The primarily
formed monoammine complex (compound 3) is dispropor-
tionated into [Ag(NH3)2]NO3 (compound 4) and AgNO3, which
explains the presence of coordinated ammonia and nitrate
bands in the IR spectrum of the solid decomposition intermedi-
reaction heat. The decomposition processes according to
1
NH4NO3 ! N2 þ = O2 þ 2 H2O þ 118:4 kJ=mol
2
°
route generally observed at >850 C. Other reaction routes with
NO formation was unlikely due to presence of O2 and lack of
NO.
°
ate formed at 80 C.
The second decomposition step of compound 1 in inert
atmosphere shows the usual decomposition features of AgNO3
2) The compound 3 easily decomposes on heating above
100 C into metallic silver, N2 and NH3,[28] which explains Ag
°
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Figure 9. Comparison of the reaction products formed in the
decomposition of com- pound 2, and Ag+MnO (a), Ag+Mn3O4 (b)
or Ag+Mn2O3 (c).
The summary of the equations, the formation of AgMnO2
and compound 1 during the various experimental routes are
reported in Figure 10.
Figure 8. XRD of compound 2 and its decomposition products at
°
80 and 125 C.
[7]
°
appearance at 125 C reported in. The decomposition inter-
mediate of compound 2 contains compound 3 and MnO. The
mixture of compound 3 and MnO decomposes on heating in air
into Ag and MnOx (x=1 (MnO), 1.34 (Mn3O4) or 1.5 (Mn2O3)
mixture, which resulted in AgMnO2 due to 1:1 Ag:MnOx ratio.[7]
We found that the solid phase reaction of Ag+MnO, Ag+
Mn2O3 and Ag+Mn3O4 results in similar XRD patterns as the
decomposition of compound 2 (Figure 9).
3) The formation of pure compound 1 from the aqueous
leaching of the mixture containing compound 3 suggests that
AgNO3 and NH4NO3 must be formed with exactly 1:1 molar
ratio. Therefore, we propose the chemical formula for com-
pound 3 is [Ag(NH3)NO3]. In water, compound 3 can easily
disproportionate into AgNO3 and compound 4, whereas
compound 4 hydrolyses during the evaporation according to
the reported way[29,30] and give rise to insoluble Ag2O and
soluble NH4NO3.
2 ½AgðNH3ÞNO3� ¼ ½AgðNH3Þ2�NO3 þ AgNO3
1
AgNO3 AgðNH3Þ2�NO3 þ = H2O ¼ 1= Ag2O þ NH3 þ NH4NO3
Figure 10. Decomposition mechanism of compound 2 and the
formation of compound 3.
2
2
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microspectrometer, l 532 nm Nd-YAG and 785 nm diode laser, an
Conclusions
Olympus BX-40 optical microscope), and PXRD (Philips PW-1050
Bragg-Brentano parafocusing goniometer with Cu tube), TG-MS (TA
Instruments SDT Q600 thermal analyzer coupled to Hiden Analytical
HPR-20/QIC mass spectrometer), DSC (Perkin Elmer Diamond DSC).
A double salt of NH4NO3 ·AgNO3 (compound 1) has been
synthesized with aq. leaching of the 80 C thermal decomposi-
tion product of [Ag(NH3)2]MnO4. Its deuterated form (com-
pound-1D) was prepared from AgNO3 and DCl, by a reaction of
the formed DNO3 with ND3 in D2O. The formed ND4NO3 and
°
AgNO3 solution in D2O resulted in pure ND4NO3 ·AgNO3 double Acknowledgements
salt. IR and Raman spectroscopic studies showed that two
distinguishable types of nitrate ion having different strength of
hydrogen bonds with the ammonium ions.
The thermal decomposition of NH4NO3 ·AgNO3 was studied
in inert and oxidative atmospheres. We found that compound 1
The research within project VEKOP-2.3.2-16-2017-00013 was
supported by the European Union and the State of Hungary,
co-financed by the European Regional Development Fund.
B. B. H. acknowledges financial support (Grant No. 451-03-9/
2021-14/200125) of the Ministry of Education, Science and
Technological Development of the Republic of Serbia. P. N.
acknowledges financial support from János Bolyai Research
Scholarship and the ÚNKP-20-5-PE-7 New National Excellence
program of the Ministry for Innovation and Technology. The
research reported in this paper was also supported by the NRDI
K 124212 and an NRDI TNN_16 123631 as well as the BME
Nanotechnology and Materials Science TKP2020 IE grant of
NKFIH Hungary (BME IE-NAT TKP2020).
°
melted congruently at 109 C and the ammonium nitrate
content decomposed first without N2O formation, and the silver
nitrate decomposition occurred in the second decomposition
step.
The chemism of compound 1 formation from [Ag(NH3)2]
MnO4 (compound 2) has been elucidated. The first decom-
position step was a solid phase oxidation reaction of one of the
ammonia molecules by the permanganate ions into nitrate with
a formation of unstable [Ag(NH3)NO3] (compound 3) intermedi-
°
ate. This compound quickly decomposes into Ag above 100 C.
In the presence of water, however, compound 3 dispropor-
tionates into AgNO3 and [Ag(NH3)2]NO3. Hydrolyis of [Ag(NH3)2]
NO3 during evaporation of the solvent results in insoluble Ag2O
and soluble NH4NO3.
Keywords: ammonium silver nitrate · spectroscopy · double
salt · hydrolysis · thermal decomposition · redox reaction
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Experimental Section
Chemical grade silver nitrate, ammonia solution (25%), nitric acid
and the deuterated materials, ND3 in D2O ( 25% ND3 content), D2O
and DCl (all of them with 99.0% isotope content) were supplied by
Deuton-X Ltd, Hungary.
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8
© 2021 The Authors. Zeitschrift für anorganische und allgemeine Chemie
published by Wiley-VCH GmbH.
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Journal of Inorganic and General Chemistry
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
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Manuscript received: March 18, 2021
Revised manuscript received: April 19, 2021
Accepted manuscript online: April 22, 2021
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ARTICLE
K. A. Béres, V. Petruševski, B. B. Holló,
P. Németh, L. Fogaça, F. Paiva Fran-
guelli, A. Farkas, A. Menyhárd, I. M.
Szilágyi, L. Kótai*
1 – 10
AgNO3·NH4NO3 – an enigmatic
double-salt type “decomposition
intermediate” of diamminesilver(I)
permanganate