Studies on the chemistry of tetraamminezinc(II) dipermanganate ([Zn(NH 3)4](MnO4)2): Low-temperature synthesis of the manganese zinc oxide (ZnMn2O4) catalyst precursor

Article abstract of DOI:10.1002/hlca.200890180

Tetraamminezinc(II) dipermanganate ([Zn(NH₃)₄] (MnO₄)₂ ; 1) was prepared, and its structure was elucidated with XRD-Rietveld-refinement and vibrational-spectroscopy methods. Compound 1 has a cubic lattice consisting of a 3D H-bound network built from blocks formed by four MnO₄^- anions and four [Zn(NH ₃)₄]²⁺ cations. The other four MnO ₄¹ anions are located in a crystallographically different environment, namely in the cavities formed by the attachment of the building blocks. A low-temperature quasi-intramolecular redox reaction producing NH ₄NO₃ and amorphous ZnMn₂O₄ could be established occurring even at 100°. Due to H-bonds between the [Zn(NH ₃)₄]²⁺ cation and the MnO₄ ^- anion, a redox reaction took place between the NH₃ and the anion; thus, thermal deammoniation of compound 1 cannot be used to prepare [Zn(NH₃)₂](MnO₄)₂ (contrary to the behavior of the analogous perrhenate (ReO₄^-) complex). In solution-phase deammoniation, a temperature-dependent hydrolysis process leading to the formation of Zn(OH)₂ and NH₄MnO₄ was observed. Refluxing 1 in toluene offering the heat convecting medium, followed by the removal of NH₄NO₃ by washing with H₂O, proved to be an easy and convenient technique for the synthesis of the amorphous ZnMn₂O₄.

Full text of DOI:10.1002/hlca.200890180

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Helvetica Chimica Acta – Vol. 91 (2008)
Studies on the Chemistry of Tetraamminezinc(II) Dipermanganate
([Zn(NH3)4](MnO4)2): Low-Temperature Synthesis of the Manganese Zinc
Oxide (ZnMn2O4) Catalyst Precursor
by Istva´n E. Sajo´ a), La´szlo´ Ko´tai*a), Ga´bor Kereszturya), Istva´n Ga´csa), Gyçrgy Pokolb),
Ja´nos Kristo´fc), Bojan Soptrayanovd), Vladimir M. Petrusevskid), Daniel Timpue), and
Pradeep K. Sharmaf)
a) Chemical Research Center, Hungarian Academy of Sciences, P. O. Box 17, HU-1525 Budapest
(e-mail: kotail@mail.chemres.hu)
b) Department of Inorganic and Analytical Chemistry, Budapest University of Technology and
Economics, Pf. 91, HU-1521 Budapest
c
´
) University of Pannonia, Department of Analytical Chemistry, Egyetem u. 10, HU-8200 Veszprem
d) University Sv Kiril Metodij, PMF, Institute Hemija, POB 162, Skopje 1001, Macedonia
e) Petru Poni Institute of Macromolecular Chemistry, Aleea Gr. Ghica Voda, 41A, Iasi 700487, Romania
f) Department of Chemistry, J. N. V. University, Jodhpur, 342005, Jodhpur, India
Tetraamminezinc(II) dipermanganate ([Zn(NH3)4](MnO4)2; 1) was prepared, and its structure was
elucidated with XRD-Rietveld-refinement and vibrational-spectroscopy methods. Compound 1 has a
cubic lattice consisting of a 3D H-bound network built from blocks formed by four MnOꢀ4 anions and
four [Zn(NH3)4]2þ cations. The other four MnOꢀ4 anions are located in a crystallographically different
environment, namely in the cavities formed by the attachment of the building blocks. A low-temperature
quasi-intramolecular redox reaction producing NH4NO3 and amorphous ZnMn2O4 could be established
occurring even at 1008. Due to H-bonds between the [Zn(NH3)4]2þ cation and the MnO4ꢀ anion, a redox
reaction took place between the NH3 and the anion; thus, thermal deammoniation of compound 1 cannot
be used to prepare [Zn(NH3)2](MnO4)2 (contrary to the behavior of the analogous perrhenate (ReOꢀ4 )
complex). In solution-phase deammoniation, a temperature-dependent hydrolysis process leading to the
formation of Zn(OH)2 and NH4MnO4 was observed. Refluxing 1 in toluene offering the heat convecting
medium, followed by the removal of NH4NO3 by washing with H2O, proved to be an easy and convenient
technique for the synthesis of the amorphous ZnMn2O4.
1. Introduction. – Spinel-type ZnMn2O4 compounds are widely used materials, e.g.,
as industrial catalysts in hydrocarbon processing, in chemical and electronic industries,
as well as in other areas of technology [1]. The preparation of the reactive solid-state
precursors to obtain the highly active defectous ZnMn2O4 structures at low temper-
atures might be accomplished by mixing Zn and Mn at atomic level. One of the possible
candidates, [Zn(NH3)4](MnO4)2 (1) was discovered by Klobb [2]. Müller et al. [3]
studied its IR spectrum and powder X-ray diffractogram, but its detailed structure has
not been brought to light yet. Our efforts were directed toward developing a
preparation technique to obtain pure compound 1 and to study its structure and
properties by means of XRD-Rietveld-refinement, vibrational spectroscopy, and
thermal methods.
2. Experimental. – General. Freshly prepared pure materials were used in all measurements. Solid-
state IR spectra: BioRad-Digilab FTS-45-FT IR spectrometer in the 4000 – 400 cmꢀ1 region and BioRad-
ꢀ 2008 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta – Vol. 91 (2008)
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Digilab FTS-40-FIR spectrometer in the 400 – 40 cmꢀ1 region; nujol mulls at r.t. Low-temp. IR
measurements (ꢀ1008): Perkin-Elmer 2000-FT IR spectrometer in the 4000 – 400 cmꢀ1 region; AgCl
pellets. Raman spectra: Nicolet 950-FT-Raman spectrometer equipped with a Nd :YAG laser (1064 nm
line) for excitation in the 4000 – 100 cmꢀ1 region of Raman shifts in a mixture with KBr; during
measurements, the compacted powdered sample of 1 was rotated to avoid overheating and
decomposition. X-Ray powder diffraction measurements: Philips PW-1050-Bragg-Brentano parafocus-
ing goniometer equipped with a secondary beam graphite monochromator and proportional counter;
scan recording in step mode by using CuKa radiation at 40 kVand 35 mA tube power; diffraction-pattern
evaluation by full profile fitting techniques; Rietveld-refinement parameters: wavelength 1.5406 Š, 2q
range 10 – 1128, number of data points 5100, profile function Pearson VII, Rwp ¼ 0.184, and weighting
scheme 1/y0. Thermal studies: Derivatograph-type simultaneous thermoanalytical equipment (Hungar-
ian Optical Works, Budapest), equipped with a selective H2O monitor and a gas-titrimetric apparatus
(NH3 was absorbed and determined as static pH titration with 0.1m HCl at pH 5); under N2; heating rate
2.58/min. Thermogravimetry (TG)/MS measurements: a STD-2960 simultaneous DTA/TGA (DTA ¼
differential thermal analysis; TA instruments) and Thermostar-GSD-200-Q-MS (Balzers) device; 58
heating rate in a He flow. Differential-scanning calometry (DSC): Mettler-Toledo TA4000 instrument and
Perkin-Elmer Pyris-Diamond DSC; under N2; from ꢀ 50 to 3008 and ꢀ 100 to 258 with 38/min or 108/min
heating rate, resp. Although accidental explosion of compound 1 did not occur in our experiments, it
should be noted that the MnOꢀ4 complexes of reducing ligands are potentially hazardous explosives, and
they have to be handled with care.
Tetraamminezinc(2þ) Permanganate (1:2) ([Zn(NH3)4](MnO4)2; 1). A 25% aq. NH3 soln. (600 ml)
was added to 400 ml of an aq. soln. of ZnSO4 · 7 H2O (23 g, 80 mmol), and cooled to þ 58. After addition
of a sat. (at 58) aq. KMnO4 soln. (1 l), the mixture was cooled to þ 28 [2]. The resulting dark purple
crystalline solid was filtered off and washed with a small amount of cold H2O: 4.2 g (13.5%) of 1. Purple
microcrystals.
Tetraamminecopper(2þ) Permanganate (1:2) ([Cu(NH3)4](MnO4)2). [Cu(NH3)4](MnO4)2 was
prepared according to the method described previously [4].
Manganese Zinc Oxide (Mn2ZnO4) (ZnMn2O4). Compound 1 (4.0 g) was carefully dried at 408 and
then mixed with toluene (100 ml). The suspension was refluxed under intensive stirring for 2 h. The solid
residue was filtered off, washed successively with H2O, EtOH, and Et2O and then dried in vacuo at r.t.:
black powder (1.1 g, 42%).
3. Results and Discussion. – 3.1. Synthesis and Hydrolysis of [Zn(NH3)4](MnO4)2
(1). Dark purple microcrystalline 1 could be prepared by the reaction of saturated
aqueous [Zn(NH3)4]SO4 solution and KMnO4 in the presence of an excess of NH3 by
cooling the saturated solution from þ 5 to þ 28 (Eqn. 1). Higher NH3 concentration or
a larger temperature gradient decreased the purity of the formed 1. Compound 1 is
slightly soluble in H2O (0.91 g/100 ml at 198) and stable in the solid state and under dry
conditions. However, in wet form, fast decomposition occurs, especially when exposed
to light. Compound 1 decomposes in aqueous solution, partially with the formation of
MnO2 and O2, but (like in the case of the analogous Cu-compound), a temperature-
dependent hydrolysis process [4][5] also occurs, and Zn(OH)2 and NH4MnO4 are
formed (Eqn. 2). Evaporation of excess NH3 at room temperature in vacuum or by
mild heating at atmospheric pressure leads to the formation of small needle-like single
crystals of NH4MnO4 embedded in a brown amorphous matrix1).
[Zn(NH3)4]SO4 (aq.) þ 2 KMnO4 (aq.) ! [Zn(NH3)4](MnO4)2 (s) þ K2SO4 (aq.) (1)
[Zn(NH3)4](MnO4)2 þ 2 H2O ! Zn(OH)2 þ 2 NH3 þ 2 NH4MnO4
(2)
1
)
The XRD plot of purple crystalline material NH4MnO4 is available upon request from L. K.

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The reaction of Eqn. 2 is a temperature-dependent hydrolysis reaction of the complex
cation [6]. This means dissociation of the [Zn(NH3)4]2þ ion into Zn2þ and NH3, which is
then followed by protonation of a part of the liberated NH3, namely with the formation
of NHþ4 and OHꢀ ions. These temperature-dependent equilibrium processes are
reversible. However, removal of NH3 shifts the equilibrium toward the formation of
Zn(OH)2 precipitate, thus NHþ4 and MnOꢀ4 ions accumulate in the solution. Slow
evaporation of this solution under ambient conditions leads to pure single crystalline
NH4MnO4 identified by the XRD method1). Since the partial vapor pressure of NH3
above the aqueous solution of compound 1 is higher than the vapor pressure of H2O
[6], slow evaporation with mild heating or the removal of NH3 under vacuum can also
be used to complete the reaction of Eqn. 2. Due to this reaction, in spite of the existence
of H2O-insoluble [Zn(NH3)2](ReO4)2 [7], the preparation of the analogous MnO4ꢀ
complex, [Zn(NH3)2](MnO4)2, was not succesfull in the partial deammoniation
experiments of the aqueous solution of 1, neither in the room-temperature nor in the
higher-temperature deammoniation experiments.
3.2. X-Ray Studies. Due to the decomposition reaction during the crystallization
periods, our efforts to grow single crystals of 1 were unsuccessful. Therefore, a Rietveld
refinement of the XRD data was performed (Fig. 12)). Compound 1 crystallizes in a
close packed cubic structure of [Zn(NH3)4]2þ ions with MnO4ꢀ anions occupying all the
octahedral interstices and half the tetrahedral interstices. Comparison of its lattice
parameters with the analogous values of [Zn(NH3)4](ClO4)2 (2) [8] and
[Zn(NH3)4](ReO4)2 (3) [3] is shown in Table 1.
Table 1. Lattice Parameters of Isomorphous [Zn(NH3)4](MnO4)2 (1), [Zn(NH3)4](ClO4)2 (2), and
[Zn(NH3)4](ReO4)2 (3)
1
2
3
Crystal system
Space group
Lattice constants [Š]
Cell volume [Š3]
Z
cubic
cubic
cubic
F-43m (216)
a ¼ 10.335(10)
1103.9
F-43m (216)
a ¼ 10.240
1073.7
4
F-43m (216)
a ¼ 10.53
1167.6
4
4
D [g/cm3]
2.251
2.056
3.743
The main feature of the structure is a three-dimensional ZnꢀNꢀH ··· OꢀMn H-
bonded network built from block-like structural motifs of 4-4 [Zn(NH3)4]2þ and MnO4ꢀ
ions. The stoichiometry of this building element indicates that only one of the two
MnOꢀ4 (Type 1) takes part in the 3D network. The other MnO4ꢀ (Type 2) is captured in
the cavities enclosed by the connection of the tetramer building blocks of the 3D-
network (Fig. 23)). The N ··· O distances and the strength of the H-bond in the 3D-
networks formed either by the Type 1 MnOꢀ4 in 1 or by the Type 1 ClO4ꢀ in compound 2
are almost the same. The H-bond formed by the Type 2 MnOꢀ4 might be only a bit
2
)
)
Powder X-ray data of [Zn(NH3)4](MnO4)2 (1) and ZnMn2O4 are available upon request from L. K.
The plots of the projection of the structure of [Zn(NH3)4](MnO4)2 (1) and of its H-bonds are
available upon request from L. K.
3

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stronger than the H-bond formed by the Type 2 ClOꢀ4 in the appropriate ClO4ꢀ salt. The
MnOꢀ4 anions located in the cavities (Type 2) are bound with NH3 H-atoms (d(O(2) ···
N) ¼ 3.025 Š). Therefore, their free rotation is hindered within the cavities, although
their freedom is higher than that of the Type 1 MnOꢀ4 anions. Thus, refinement can be
performed with the best result at higher Biso for O-atoms of Type 2 MnOꢀ4 than for O-
atoms of Type 1 MnOꢀ4 . From the comparison of the N ··· O atomic distances in 1 and 2
(Table 2), it can be concluded that there is no significant difference between the
strengths of the H-bonds in 1 and in 2. The structure and packing of 1 in the solid state
can be seen in Fig. 23).
Table 2. Atomic Distances [Š] in the Solid Compounds [Zn(NH3)4](MnO4)2 (1) and [Zn(NH3)4]-
(ClO4)2 (2)
1
2 [8]
ZnꢀN
2.016(24)
1.486(22)
1.450(20)
1.192a)
2.014
1.418
1.366
1.190
3.175
3.067
M(1)ꢀO
M(2)ꢀO
NꢀH
N ··· O(1)
N ··· O(2)
3.176(35)b)
3.025(33)b)
a) NꢀH Atomic distances were only estimated and not refined. b) N ··· O Distances were used to
evaluate the strength of H-bonds.
3.3. Vibrational Spectroscopy Results. Knowing its crystal structure, the vibrational-
spectra analysis of compound 1 can be performed. The result of factor-group analysis
for the tetrahedral MnOꢀ4 anion and the ZnN4 fragment of the tetrahedral
[Zn(NH3)4]2þ cation is shown in Table 3. All four vibrations of these tetrahedral
species are Raman active, but only the two triply degenerated vibrations are IR active.
Table 3. Factor-Group Analysis of the MnOꢀ4 Anion and the ZnN4 Fragment of the [Zn(NH3)4]2þ Cation.
Space group: F 4-3m ¼ T2d (Z ¼ 4, ZB ¼ 1).
Free-ion symmetry
Site symmetry
Factor group
Activity
Assignment
Td
A1
E
F2
F2
Td
A1
E
F2
F2
Td
A1
E
F2
F2
Raman
Raman
Raman, IR
Raman, IR
ns(MnO4, ZnN4)
dE (MnO4, ZnN4)
nas(MnO4, ZnN4)
dF 2 (MnO4, ZnN4)
Permanganate Vibrations. The assignment of the anion vibrations of 1 can be found
in Table 44). Since there are two crystallographically different MnOꢀ4 anions in the
cubic lattice, Raman bands should appear twice for all four (ns, dE, nF 2, and dF 2)
4
)
The results of ab initio quantum-chemical frequency calculations of the anion and cation of
[Zn(NH3)4(MnO4)2 (1) are available upon request from L. K.

Helvetica Chimica Acta – Vol. 91 (2008)
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Fig. 2. Crystal structure of [Zn(NH3)4](MnO4)2 (1). a) H-Bonds in 1 and b) packing of 1. Light grey
tetrahedrons mean the [Zn(NH3)4]2þ cations; dark grey tetrahedrons mean the two types of MnO4ꢀ
anions (Mn1: cavity-embedded, Mn2: 3D-network-forming MnOꢀ4 ).

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Table 4. Assignment of Vibrational Frequencies of the MnOꢀ4 Anion Observed in the IR (Nujol) and
Raman Spectra (KBr) of [Zn(NH3)4](MnO4)2 (1) at RoomTemperature
IR (diffuse reflection)
Raman (diffuse reflection)
n˜ [cmꢀ1
]
Type
n˜ [cmꢀ1
]
Type
ns(MnO4)
dE (MnO4)
nas(MnO4)
836w
2a)
841vs
836vs
346m
342 (sh)
914 (sh)
908m
1b)
2a
2a)
1b)
345m
2a)
910vs
900vs
903m
897m
892 (sh)
388m
381 (sh)
dF 2 (MnO4)
388w (sh)
381m
Combination bands
ns þ nasc) (MnO4)
ns þ nasc) (MnO4)
1749w
1728w
b
a) Cavity-embedded type of MnO4ꢀ anions (tentative assignment). ) 3D-Network-bound type of MnO4ꢀ
anions (tentative assignment). c) The components of the nas stretching.
vibrational normal modes. Since the degeneration of the F2 bands in this symmetric
crystallographical environment does not cease, splitting of the nas asymmetric stretching
bands into 3 þ 2 bands (or into 2 ꢁ 3 bands with overlapping two neighboring bands) is
an unusual phenomenon. An inconsistency was found in the IR spectra either (Figs. 3
and 4). Like in the case of the corresponding ClOꢀ4 complex 2, both of the forbidden IR
bands (ns and dE) appear as a singlet in the IR spectrum. This and the splitting of at least
one of the triply degenerated nas bands into three components in the Raman spectrum
show a lowering of symmetry of at least one type of the MnOꢀ4 anion environments. The
lowering of symmetry may appear in case of the other type of MnOꢀ4 as well, due to the
splitting of both kinds of the nas Raman bands; however, only one kind of the forbidden
IR bands appears. There are several possible reasons for the appearance of the
forbidden IR bands of tetrahedral oxo anions [9]. One of them is a dynamic lattice
distortion as it was supposed in the case of compound 2 [8], or some orientational effect
which was observed several times in NꢀH H-bonded compounds, e.g., in NH4ClO4
[10]. Although both the dynamic lattice distortion and the MnOꢀ4 orientation depend
on the temperature, but with opposite sign. Decreasing the temperature can freeze one
orientation, and this should increase the intensity of the forbidden band. In the case of a
dynamic lattice distortion, however, the decreasing temperature decreases the extent of
distortion, because the anisotropic thermal motions are slowing down. Based on these
considerations, liquid-N2-temperature IR studies were performed to study the effect of
temperature on the intensity of the forbidden ns(MnꢀO) bands of compound 1. To
disclose the effect of possible phase transitions on the low-temperature IR-spectral
features, a low-temperature differential-scanning-calometry (DSC) study was also
performed. The DSC results confirmed the absence of any phase transitions. Since the
decrease of temperature affects the band widths, the integrated intensities of each

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Fig. 3. Room-temperature IR spectrum of [Zn(NH3)4](MnO4)2 (1) in AgCl matrix
Fig. 4. Low-temperature (ꢀ1008) IR spectrumof [Zn(NH 3)4](MnO4)2 (1) in AgCl matrix
group of bands were calculated for each spectrum, and the intensity ratios I(nas;
MnꢀO)/I(ns;
MnꢀO)
were
compared.
The
analogous
Cu-complex
[Cu(NH3)4](MnO4)2 (4) [4] has no dynamic lattice distortion or favored MnO4ꢀ
orientation; therefore, compound 4 was also studied to compare the effect of cooling on
the intensity ratios of the bands. The change in the ratio I(nas; MnꢀO)/I(ns; MnꢀO) of
4 was less than 20% during cooling from room temperature to liquid-N2 temperature

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Helvetica Chimica Acta – Vol. 91 (2008)
(Table 5). In the case of 1, however, this change was more than 150%. Since the
intensity of the ns band increases with decreasing temperature, the dynamic lattice
distortion as a possible reason for the appearance of the forbidden IR and splitting of
the degenerated Raman bands seems to be less probable than the effect of the MnO4ꢀ
anion orientation. Based on this consideration, the forbidden ns and dE IR bands
probably belong to the cavity-embedded MnOꢀ4 (Type 2). The appropriate Raman
bands of this type of MnOꢀ4 were assigned on the basis of the wavenumbers of the given
Raman and the forbidden IR bands. The other Raman bands in each pair of bands were
attributed to the 3D network-bound MnOꢀ4 (Type 1). Unambiguous assignment of the
F2 bands of each MnOꢀ4 type was not possible because the nas band in the low-
temperature IR spectrum was split into only three bands (901, 910, and 921), and dF 2
was out of the measurement range (4000 – 400 cmꢀ1) of the low-temperature IR device.
Table 5. Comparison of Integrated Intensities I(n) [%] of Room-Temperature and Liquid-N2-Temper-
ature Forbidden ns(MnꢀO) IR Bands of [Zn(NH3)4](MnO4)2 (1) with the ns(MnꢀO) of the Analogous
[Cu(NH3)4](MnO4)2 (4) in AgCl Matrix
1
4
258
ꢀ 1008
258
ꢀ 1008
I(ns)
0.052
0.133
0.684
0.839
I(nas)
14.350
14.377
35.395
36.283
I(nas)/I(ns)
276 (253%)
109 (100%)
51.7 (119%)
43.4 (100%)
Two combination bands were also assigned to the MnOꢀ4 anion. These are the ns þ
nas bands, when two components of nas gave separated combinations. These bands were
assigned to the MnOꢀ4 anion because similar bands and band structures were also
observed in the diffuse-reflection IR spectra of KMnO4 and NH4 MnO4 [11].
Tetraamminezinc(II) Cation Vibrations. The [Zn(NH3)4]2þ complex cation has 17
atoms, and there are 11 Raman-active (3A1 þ 8E) and 21 (F2) IR- and Raman-active
bands that can be taken into consideration among the 45 possible vibrational modes.
All vibrations belonging to the coordinated NH3 are IR- and Raman-active. The
assignment of the vibrational bands belonging to the complex cation are given in
Table 6. The bands in the IR spectra at 421, 388 (Table 4), and 381 cmꢀ1 (Table 4) may
belong to any of the ns(ZnN4), nas(ZnN4), and dF 2(MnO4) modes. To assign these
bands, ab initio quantum-chemical calculations were performed on the MnOꢀ4 and
[Zn(NH3)4]2þ ion4). Also the IR data of 1 were compared with those of the analogous
[Zn(NH3)4](ClO4)2 (2) and [Zn(NH3)4](ReO4)2 (3) complexes, and of an isotope-
substituted derivative, [Zn(15NH3)4](ReO4)2 [3][7][8][12].
A band appearing at 421 cmꢀ1 was observed in the spectra of each isomorphous
compound 1 – 3 [3][8][12]. Since all of the four normal modes of the ClOꢀ4 were
assigned, and dF 2(ClO) appeared at 630 cmꢀ1 and dF 2(MnO4) below 400 cmꢀ1
(Table 4), this band definitely belonged to one of the ZnꢀN vibrations. The
quantum-chemical calculations (DFT//BLYP/TZCVP) showed that the value of the
ns(ZnN4) frequency is higher than the value of the nas(ZnN4) frequency. Although
ns(ZnN4) is IR-inactive and should not be observed, it was detected as a weak band at

Helvetica Chimica Acta – Vol. 91 (2008)
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Table 6. Assignment of [Zn(NH3)4]2þ Ion Frequencies in the IR (Nujol) and Raman Spectra (KBr) of
Compound 1 at RoomTemperature
n˜ [cmꢀ1
]
Species
IR (diffuse reflection)
Raman (1064 nm)
ZnꢀNH3 Vibrations:
ns(NH)
A1
3253m
3258vw
3163vw
ds(NH)
ns(ZnN4)
nas(NH)
dE(NH)
1r(NH)
A1
A1
E
E
E
1219s
–
3329vs
1612m
716w
690w
–
–
3326vw
–
–
695vw
ZnꢀN Vibrations:
ns(ZnN4)
dE(ZnN4)
nas(ZnN4)
dF 2(ZnN4)
A1
E
F2
F2
451w
232w
421m
179s
–
–
426w
–
Combination and overtone bands:
2dE(NH)
3193
1401
1384
848
–
–
–
–
1r þ 1r(NH)a)
21r(NH)a)
2nas(ZnN4)
a) The components of the 1r NꢀH rocking stretching.
451 cmꢀ1 (the band at 421 cmꢀ1 is believed to arise from nas(ZnN4)). The forbidden
ns(ZnN4) band also appeared at 432 and 411 cmꢀ1 in the spectra of the isomorphous
compounds 2 and 3, respectively. However, only one band (nas at 403 cmꢀ1) was found in
the IR spectra of [Zn(15NH3)4](ReO4)2, but its Raman spectrum contained two bands
(411 and 403 cmꢀ1), which were unambiguously assigned to the appropriate Raman-
active ns(ZnN4) and nas(ZnN4) modes [12]. Appearance of the forbidden ns(ZnN4) and
dE(ZnN4) bands in the IR spectrum of 1 is the consequence of the interaction between
the encaged MnOꢀ4 (Type 2) and the NH3 H-atoms of the cations. The dE(ZnN4) was
assigned to the band at 232 cmꢀ1, which is near to the 221 cmꢀ1 band of compound 2 [8].
The IR-active asymmetrical deformation band dF 2(ZnN4) was observed at 179 cmꢀ1 as
a strong and broad band, located close to the appropriate value of 2 (178 cmꢀ1) [8] and
3 (191 cmꢀ1) [3]; for the isotope-labelled (15NH3) analogous ReOꢀ4 complex, this band
appeared at 174 cmꢀ1 [12]. Generally, the IR absorptions of 1 measured by Müller et al.
[3] in KBr were located at higher frequencies than in our spectrum obtained in the
diffuse-reflection mode. The low-temperature IR measurements of 1 in AgCl showed
four bands (462, 448, 436, and 423 cmꢀ1) which probably arose from the singlet of
ns(ZnN4) and a triplet of nas(ZnN4). The Raman spectra of 1 at room temperature
contained only one assignable band (nas(ZnN4) at 426 cmꢀ1, weak); the other three
Raman-active modes could not be detected due to their low intensity.
Based on these considerations and the quantum-chemical calculations (G94//
B3LYP/6-311G and DFT//BLYP/TZVP), the other two bands at 388 and 381 cmꢀ1 do

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not belong to the ZnN4 vibrations but may belong to the two different MnOꢀ4 anions
and can be assigned as a pair of the dF 2(MnO4) bands (see above, Table 4).
3.4. Thermal Studies. To avoid explosion-like decomposition, Al2O3 was added to 1
(10% of 1 in a-Al2O3) to perform thermogravimetry (TG)/MS measurements.
Decomposition under inert atmosphere occurred in 2 þ 1 steps. On the basis of mass-
loss data (residue was 64.20%), the amorphous product was estimated to have a
[ZnO þ Mn2O3] formula (theoretical value, 64.40%). The first decomposition step
occurred at 1078; however, the decomposition peak temperature varied in the 100 –
1308 range depending on the heating rate. This temperature was lower than the
deammoniation temperature of the analogous ReOꢀ4 salt 3 (150 – 1958, at which the
intermediate [Zn(NH3)2](ReO4)2 complex [7] was formed). The thermo-gas titrimetric
investigation indicated that 2 mol of NH3 were released in the first decomposition step,
and no further NH3 formation in the second or third step was observed. The
unambiguous features of this decomposition process were the formation of H2O in the
first two steps and the lack of O2 evolution in the entire thermal-decomposition process.
Mainly N2O and its fragmentation products were detected in the second step, although
some N2 and other NOx by-products could also be detected in the other decomposition
steps, as also observed in the case of compound 4 [4]. The weight loss in the third step
was 5.4%, which can be attributed to the decomposition of some previously formed by-
products or ZnMn2O4þx nonstoichiometric phases. IR Studies performed on the white
crystals obtained from the aqueous extract of the X-ray amorphous thermal-
decomposition intermediates formed in the first decomposition step unambiguously
confirmed the formation of NH4NO3. The final decomposition product was heated up
to 5008. XRD of the residue showed only the presence of the tetragonal modification of
ZnMn2O4. Summarizing these results, the main decomposition process of compound 1
can be illustrated with Eqn. 3. Dissimilarly from the corresponding ReOꢀ4 salt 3 [7], the
diammine complex [Zn(NH3)2](MnO4)2 did not form. The second decomposition step
(Tpeak 2318) was a ZnMn2O4-catalyzed decomposition reaction of NH4NO3 (Eqn. 4).
[Zn(NH3)4](MnO4)2 ! ZnMn2O4 þ NH4NO3 þ 2 NH3 þ H2O
NH4NO3 ! N2O þ 2 H2O
(3)
(4)
Since the thermal-decomposition temperature of the NH4NO3 formed according to
Eqn. 3 was lower than the decomposition temperature of pure NH4NO3 (2608) [13],
and not only N2O but other NOx products were also formed, the ZnMn2O4 probably
catalyzed this decomposition reaction. DSC Studies showed that all decomposition
steps were exothermic. The reaction heat DH was ꢀ 169 kJ/mol in the first step
(Eqn. 3), which was lower than the appropriate reaction heat of compound 4 (DH ¼
ꢀ290 kJ/mol). The reaction heat of the second decomposition step (Eqn. 4) could
not be determined because of the explosion-like decomposition of NH4NO3. The
explosion-like decomposition of NH4NO3 also confirmed the suggested catalytic effect
of the formed ZnMn2O4.
The lack of an endothermic effect of NH3 liberation from the complex cation
[Zn(NH3)4]2þ indicated that the exothermic solid-phase quasi-intramolecular redox
reaction between the NH3 ligand and the MnOꢀ4 anion started before the deammo-

Helvetica Chimica Acta – Vol. 91 (2008)
1657
niation step of the complex cation. Due to this reaction, the [Zn(NH3)2](MnO4)2
complex could not be prepared by partial deammoniation of complex 1. Since
[Zn(NH3)2](ReO4)2 was prepared in this way [7], the higher redox activity of MnO4ꢀ
may be one of the reasons for this redox reaction. On the other hand, this redox
reaction provided a possibility for the preparation of ZnMn2O4-type materials. To
control the reaction rate, the use of toluene as a liquid heat-convecting medium (the
boiling point of toluene is almost the same as the decomposition temperature of 1)
proved to be suitable. After refluxing for 2 h, the filtered residue was washed with H2O,
then dried, and finally heated up to 5008. The crystallization process of ZnMn2O4
(tetragonal modification) as a function of the temperature was monitored by XRD (see
Fig. 52)). Fig. 5 shows that the formal [ZnO þ Mn2O3] product formed during thermal
decomposition consisted of a highly defectous amorphous ZnMn2O4 spinel-like
compound which could be crystallized by heat treatment up to 5008. The amorphous
decomposition product is a potential catalyst and can be applied as an additive in
various industrial processes and products [1].
Fig. 5. XRD of a) the amorphous formal [ZnO þ Mn2O3] product formed from [Zn(NH3)4](MnO4)2 (1)
at 1008 in toluene, and b) tetragonal ZnMn2O4 formed at 5008
4. Conclusions. – [Zn(NH3)4](MnO4)2 (1) has a cubic lattice consisting of a 3D H-
bound network of the building blocks of 4-4 [Zn(NH3)4]2þ cations and MnO4ꢀ anions.
The residual MnOꢀ4 anions are located in crystallographically different environments.
The temperature-dependent orientation of the MnOꢀ4 anion is the reason for the
appearance of forbidden IR bands and splitting of the degenerated Raman bands. The

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presence of the H-bonds between the complex cation and the anion results in a low-
temperature intramolecular redox reaction with the formation of NH4NO3 and an
amorphous highly defectous ZnMn2O4 around 1008. In solution-phase deammoniation,
a temperature-dependent hydrolysis process occurs, and Zn(OH)2 and NH4MnO4 are
formed. Dissimilarly from the analogous ReOꢀ4 complex, [Zn(NH3)2(MnO4)2] cannot
be obtained by thermal deammoniation of compound 1. This is due to the higher redox
activity of the MnOꢀ4 anion. Thermal treatment of compound 1 in toluene as heat-
convecting medium leads to an easy and controlled preparation of the highly defectous
amorphous ZnMn2O4 catalyst precursor.
5. Supplementary Material. – The following supplements are available upon request from L. K.: Far-
IR spectrum of [Zn(NH3)4](MnO4)2 (1) in nujol. Raman spectrum of 1 in KBr. CIF File of the X-ray
study of 1. IR Spectra of the white crystals (NH4NO3) isolated from the aq. extract of the product formed
in the first decomposition step of 1. TG Results of 1 under He (58/min; diluted with Al2O3). TG/DTG/
DTA and H2O-Detector study of 1 under N2 (58/min; diluted with Al2O3). TG/MS Plot for NH3 (and its
fragments) evolved during thermal decomposition of 1. See also Footnotes 1 – 4.
´
´
The authors wish to thank Dr. Bela Pukanszky for the high-temperature-range DSC measurements.
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Received April 7, 2008