Thermal analysis of manganese(II) complexes of general formula (Bu4N)2[MnBrnCl4-n]

Article abstract of DOI:10.1016/j.tca.2008.10.003

Thermal decomposition of compounds consisting of tetrahalogenomanganese(II) anions, [MnBr_nCl_4-n]^2- (n = 0-4), and a tetrabutylammonium cation has been studied using the DSC, TG-FTIR, TG-MS and DTA techniques. The measureme

Full text of DOI:10.1016/j.tca.2008.10.003

Thermochimica Acta 481 (2009) 46–51
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Thermal analysis of manganese(II) complexes of general formula
(Bu4N)2[MnBrnCl4−n
]
E. Styczen´ a, D. Wyrzykowskia, M. Gazdab, Z. Warnkea,∗
a Faculty of Chemistry, University of Gdan´sk, Sobieskiego 18, 80-952 Gdan´sk, Poland
b Faculty of Physics and Mathematics, Technical University of Gdan´sk, Narutowicza 11/12, 80-952 Gdan´sk, Poland
a r t i c l e i n f o
a b s t r a c t
2−
]
Article history:
Received 14 August 2008
Received in revised form 1 October 2008
Accepted 6 October 2008
Available online 17 October 2008
Thermal decomposition of compounds consisting of tetrahalogenomanganese(II) anions, [MnBrnCl4−n
(n = 0–4), and a tetrabutylammonium cation has been studied using the DSC, TG-FTIR, TG–MS and DTA
techniques. The measurements were carried out in an argon and static air atmospheres over the temper-
ature ranges 173–450 K (DSC) and 300–1073 K (TG). Solid products of the thermal decomposition were
identified by FT-FIR spectroscopy as well as X-ray powder diffractometry.
© 2008 Elsevier B.V. All rights reserved.
Keywords:
Tetrahalogenomanganeses(II)
Thermal decomposition
1. Introduction
facilitating correct interpretation of the thermal decomposition
steps of the inorganic constituent.
Owing to frequent occurrence of the chloride and bromide
ligands in metallocomplexes, their spectroscopic characteristics,
involving mostly metal–ligand frequencies, have inspired a num-
ber of research workers. Apart from earlier reports on the [CoX4]2−
[1], [NiX4]2− [2], [FeX4]2− [3] and [MnX4]2− [4] (where X = Cl−,
Br−) complexes, those with the same organic cation were for
the first time prepared and spectroscopically characterized by
Clark and Dunn [5]. Apart from the spectroscopic studies, con-
siderable attention has been focused on structural analysis [6–16]
and magnetic properties [17–22]. Additionally, the (Bu4N)2[MX4]
(M = Mn2+, Co2+, Ni2+, Cd2+; X = Cl−, Br−) salts have been found
to melt neat at 373 K to mobile, electrically conducting liquids
[23–25].
This work is a continuation of our investigations into tetra-
halogenometallates(II) [27,28], but this time a manganese(II) ion is
the centre of co-ordination and a tetrabutylammonium ion, Bu4N+,
acts as a counter-ion.
To investigate phase transformations, DSC curves have been
recorded over the range 174–450 K. By inspection of the TG, DTG
and DTA curves and using the FTIR, FT-FIR and MS techniques as
well as the results of the X-ray powder diffractometry of poly-
crystalline samples, products of successive steps of the thermal
decomposition of the (Bu4N)2[MnBrnCl4−n] compounds have been
identified. As previously, the measurements were run in argon and
static air.
However, to the best of our knowledge, there are no reports on
thermal decomposition of compounds with the tetrahalogenomet-
allate(II) anions. Just this was the reason which prompted us to
embark on these studies. Our interest has been focused on ther-
mal stability of the metal–ligand bond in the complex anion.
Our studies have now been extended over the synthesis of some
2. Experimental
The manganese(II) complex salts were obtained by a procedure
described in the literature, by mixing together stoichiometric quan-
tities of a manganese(II) halide with tetrabutylammonium chloride
and/or bromide in ethanol [2].
novel compounds with mixed-ligand anions of general formula
2−
[MBrnCl4−n
]
(n = 1–3). The tetrahedral geometry of the anions
2.1. Synthesis of (Bu4N)2[MnCl4]
was stabilized by cations of quaternary ammonium salts whose
thermal decomposition had previously been reported [26]. Thus,
MnCl2 (0.05 mol) was dissolved in a small quantity of ethanol
and the solution was added to ethanolic solution of Bu4NCl (0.1 mol)
(found: C, 56.8; N, 4.1; H, 10.9. Calcd. for (Bu4N)2[MnCl4], C, 56.3;
N, 4.1; H, 10.7%).
∗
Corresponding author. Tel.: +48 58 5235360; fax: +48 58 3410357.
E-mail address: warnke@chem.univ.gda.pl (Z. Warnke).
0040-6031/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2008.10.003

E. Styczen´ et al. / Thermochimica Acta 481 (2009) 46–51
47
Table 1
2.2. Synthesis of (Bu4N)2[MnBr4]
Temperature and enthalpy changes for physical transformations of the man-
ganese(II) complexes as taken from the DCS curve.
(Bu4N)2[MnBr4] was also prepared as described, by using MnBr2
(0.05 mol) and Bu4NBr (found: C, 44.7; N, 3.0; H, 8.7. Calcd. for
(Bu4N)2[MnBr4], C, 44.7; N, 3.2; H, 8.5%).
Complexes
T
␴
[K]
ꢁH [kJ/mol]
Tm [K]
ꢁHm [kJ/mol]
␴
(Bu4N)2[MnCl4]
(Bu4N)2[MnBrCl3]
(Bu4N)2[MnBr2Cl2]
(Bu4N)2[MnBr3Cl]
(Bu4N)2[MnBr4]
326
351
352
351
324
–
338
379
364
370
379
17.8a
3.4
7.6
19.8
17.4
28.0
26.3
7.8
2.3. Synthesis of (Bu4N)2 [MnBrCl3]
4.8
MnCl2 (0.05 mol) was dissolved in a small quantity of ethanol
andthe solutionwas added toanequimolar mixture(0.05 mol each)
of Bu4NCl and Bu4NBr in ethanol (found: C, 52.9; N, 4.4; H, 10.1.
Calcd. for (Bu4N)2[MnBrCl3], C, 52.9; N, 3.9; H, 10.0%).
T
, Temperature of phase transformation; ꢁH , enthalpy of phase transformation;
␴
␴
Tm, melting point of the compound; ꢁHm, enthalpy of melting.
a
ꢁH = ꢁH + ꢁHm
.
␴
Table 2
Results of analysis of the decomposition products obtained in the inert atmosphere.
2.4. Synthesis of (Bu4N)2[MnBr3Cl]
Complex
Step Temperature range [K] DTGmax [K] Mass loss [%]
(Bu4N)2[MnBr3Cl] was prepared as described, but MnBr2
(0.05 mol) was used as a manganese halide (found: C, 47.4; N, 3.4;
H, 9.1. Calcd. for (Bu4N)2[MnBr3Cl], C, 47.1; N, 3.4; H, 8.9%).
(Bu4N)2[MnCl4]
1
2
3
4
440–553
553–620
620–695
695–980
523
583
658
657
51
26
4
9
2.5. Synthesis of (Bu4N)2[MnBr2Cl2]
(Bu4N)2[MnBrCl3]
1
2
3
4
493–588
588–638
638–698
780–998
540
606
675
949
49
25
5
MnCl2 (0.05 mol) was dissolved in
a small quantity of
ethanol and the solution was added to an ethanolic solution
of Bu4NBr (0.01 mol) (found: C, 49.9; N, 4.2; H, 9.6. Calcd. for
(Bu4N)2[MnBr2Cl2], C, 49.9; N, 3.7; H, 9.4%).
17
(Bu4N)2[MnBr2Cl2]
(Bu4N)2[MnBr3Cl]
(Bu4N)2[MnBr4]
1
2
3
488–598
598–678
803–1028
543
598
990
48
27
21
2.6. Instrumental
1
2
3
498–600
600–648
781–990
540
624
970
49
25
22
The FT-FIR spectra were recorded on a BRUKER IFS 66 spec-
trophotometer in PE over the 650–50 cm−1 range.
1
2
3
480–583
583–650
650–955
536
607
909
40
25
23
The TG-FTIR analyses in argon (Ar 5.0) were run on a Netzsch TG
209 apparatus coupled with a Bruker FTIR IFS66 spectrophotome-
ter (range 298–1073 K, corundum crucible, sample mass ca. 12 mg,
heating rate 15 K/min, flow rate of the carrier gas 18 mL/min).
The TG-DTA measurements in helium were carried out on SDT
2960 (TA Instruments) apparatus (range 290–1273 K, corundum
crucible, sample mass ca. 4–5 mg, heating rate 5 K/min). The SDT
apparatus was connected on-line with the quadrupole mass spec-
trometer QMD 300 Thermostar (Balzers).
The course of thermal analysis was broken at points correspond-
ing to the main steps of decomposition and the residues in the
crucible were quickly cooled in the stream of argon. This enabled
to analyze the residues at strictly pre-determined steps of decom-
position. The analysis was carried out using the FTIR spectroscopic
techniques as well as the X-ray powder diffractometry.
change of the orientation of the hydrocarbon chains of the tetra-
butylammonium cation in the crystal lattice of a compound, which,
in turn, effectively change the environment of the [MnBrnCl4−n
ions [10,24,25]. It is also worth noting that all those transformations
are irreversible. Upon cooling of a sample, corresponding peaks
in the DSC curves are missing. In addition, upon repeated heat-
ing, peaks due to the transformations are missing as well. It is also
interesting to note that around 280 and 230 K small endothermic
transformations emerge amounting to ca. 2 kJ/mol which might be
due to conversion of the initial metastable phase to a stable one.
2−
]
The presence of crystalline phases was checked by X-ray diffrac-
tion with the use of Philips X’Pert diffractometer system. The XRPD
patterns were recorded at room temperature with Cu K␣ radia-
tion (ꢀ = 1.540 Å). Qualitative analysis of the diffraction spectra was
carried out with the ICDD PDF database [29].
Table 3
Results of the analysis of the decomposition products obtained in air.
Complex
Step Temperature range [K] DTGmax [K] Mass loss [%]
(Bu4N)2[MnCl4]
1
2
3
453–573
573–673
673–873
518
613
853
53
27
13
3. Results and discussion
Results of the thermal analysis of the complexes are compiled
in Tables 1–3.
(Bu4N)2[MnBrCl3]
(Bu4N)2[MnBr2Cl2]
(Bu4N)2[MnBr3Cl]
(Bu4N)2[MnBr4]
1
2
3
473–583
583–673
673–883
523
653
873
54
21
13
The (Bu4N)2[MnBrnCl4−n
] complexes have been found to
undergo phase transformation preceding melting (DTA and DSC
curves). Only with (Bu4N)2[MnCl4] the phase transformation
occurs almost simultaneously with the melting as shown by over-
lapping peaks in the DSC curve. This precludes determination of
the enthalpies of both transformations. Endothermic phase trans-
formations occurring on heating of the complexes reveal existence
of various polymorphic forms. At the phase transition tempera-
ture, bulky counter-ions execute exochoric motions. A change in
the temperature of the environment results most probably in the
1
2
3
473–578
578–673
673–863
523
653
833
43
30
18
1
2
3
453–593
593–683
683–893
533
653
863
45
29
18
1
2
3
473–573
573–673
673–873
533
653
853
37
31
17

48
E. Styczen´ et al. / Thermochimica Acta 481 (2009) 46–51
Fig. 3. FTIR spectra of the volatile products of the decomposition of (Bu4N)2[MnCl4]
at 531 K and (Bu4N)2[MnBr4] at 546 K, in argon.
Fig. 1. DSC of the (Bu4N)2[MnBr3Cl] complex.
An exception is provided by (Bu4N)2[MnBr3Cl] (Fig. 1) where upon
repeated heating a new peak emerges due to exothermic transfor-
mation (H = −20.3 kJ/mol).
the spectra (ca. 450–650 K). A comparison of the FTIR spectra of
(Bu4N)2[MnCl4] and (Bu4N)2[MnBr4] reveals both similarities (in
the 3000–1400 cm−1 range) and differences (below 1400 cm−1) in
the composition of the volatiles. As these are mixtures of com-
pounds, their identification based on the IR spectra only turned out
to be difficult. Helpful in this matter proved ionic current profiles.
In the case of the bromochloromanganates(II), the ionic currents
enable also to establish the sequence of volatilization of particular
butyl halides. Thus, during the decomposition of (Bu4N)2[MnCl4]
and (Bu4N)2[MnBrCl3] (Fig. 4) one of the first volatiles released
Bis(tetrabutylammonium) tetrahalogenomanganates(II) are rel-
atively stable in the molten state. Their thermal decomposition
occurs at a temperature by more than 100 K higher than their melt-
ing points.
It should also be noticed that the pathway of the thermal trans-
formations of the tetrahalogenomanganates(II) is affected by the
kind of the halide ligand in the coordination sphere of Mn(II). For
instance, upon heating of (Bu4N)2[MnCl4] and (Bu4N)2[MnBrCl3]
there appear four thermal decomposition steps, whereas with the
remaining complexes only three steps have been recorded. Ther-
mal curves of exemplary compounds exhibiting either four or three
endothermic steps are shown in Fig. 2.
In the FTIR spectra of the volatile decomposition products
(Fig. 3), bands due to stretching C–H vibrations of the methy-
lene and methyl groups (around 3000 cm−1) as well as the
C–N one (1450 cm−1) could be identified. Bands below 690 cm−1
assignable to the C–X (X = Br, Cl) bond are missing in the
undecomposed compound this indicating unambiguously degra-
dation of the cation over the temperature range of recording
+
+
+
is BuCl. Peaks at m/z = 27 and 41 are due to the C2H3 and C3H5
ions derived from fragmentation of the molecular ion, C4H8
(m/z = 56). Respective curves have identical shapes as a function
of temperature and the intensity of particular ionic currents cor-
respond to a BuCl spectrum filed in a spectrum library [30]. With
the remaining compounds, BuBr is volatilized. Moreover, during
the decomposition of all the bromochloromanganates(II) studied,
Bu3N was detected among the gaseous products. A similar pat-
tern of the decomposition of (Bu4N)+ has previously been reported
[26,31–33].
Taking into account both the MS and FTIR spectra, the following
equations describing thermal decomposition of the compounds in
Fig. 2. TG and DTA traces of (Bu4N)2[MnBrCl3] (a) and (Bu4N)2[MnBr3Cl] (b) in argon.

E. Styczen´ et al. / Thermochimica Acta 481 (2009) 46–51
49
Fig. 4. Ionic current profiles recorded during TG–MS analysis of (Bu4N)2[MnBrCl3]
in the inert atmosphere.
Fig. 5. TG and DTA curves of (Bu4N)2[MnCl4] in the air atmosphere.
solid product. Accordingly, supplemented equations for the decom-
position of the first two compounds can be written as follows:
the inert gas atmosphere can be suggested:
(Bu4N) [MnCl4] 440−−→553 K (Bu4N)[MnCl3]
2
(Bu4N) [MnCl4] 440−−→553 K (Bu4N)[MnCl3]
−Bu N,BuCl
3
2
−Bu N,BuCl
3
553−620−K→,620−695 KMnCl2 + CxHyNz695−−→980 KMn, C
(1)
(2)
(3)
(4)
(5)
553−620 K
1 Bu3N : MnCl2
+
1 MnCl2 + CxHyNz
−Bu N,BuCl
3
−→
2
2
−1/2Bu N,BuCl
3
620−−→695 KMnCl2 + CxHyNz695−−→980 KMn, C
(6)
495−585 K
(Bu4N)2[MnBrCl3]−Bu−N→,BuBr(Bu4N)[MnCl3]
−1/2Bu
N
3
3
585−638−K→,638−698 KMnCl2 + CxHyNz790−−→995 KMn, C
495−585 K
(Bu4N)2[MnBrCl3]−Bu−N→,BuBr(Bu4N)[MnCl3]
−Bu N,BuCl
3
3
585−638 K
1 Bu3N : MnCl2
+
1 MnCl2 + CxHyNz
(Bu4N) [MnBr2Cl2] 488−−→595 K (Bu4N)[MnBr2Cl]
−→
2
2
2
−1/2Bu N,BuCl
3
−Bu N,BuCl
3
638−−→698 KMnCl2 + CxHyNz790−−→995 KMn, C
(7)
595−−→645 K MnBr2 + CxHyNz802−−→1027 KMn, C
−1/2Bu
N
3
−Bu N,BuCl
3
An additional proof for the presence of MnCl2 is a peak at 910 K
in the DTA trace indicating its melting (Tm
= 923 K).
495−600 K
MnCl
(Bu4N)2[MnBr3Cl]−Bu−N→,BuBr(Bu4N)[MnBr2Cl]
2
3
600−−→650 K MnBr2 + CxHyNz781−−→990 KMn, C
−Bu N,BuCl
3
478−583 K
(Bu4N)2[MnBr4]−Bu−N→,BuBr(Bu4N)[MnBr3]
3
−→ MnBr2 + CxHyNz650−−→955 KMn, C
583−650 K
−Bu N,BuBr
3
where CxHyNz is the undecomposed organic entity, Mn and C are
elemental manganese and its carbide, respectively.
With (Bu4N)2[MnCl4] and (Bu4N)2[MnBrCl3] interesting are
additional steps over the range 620–695 and 638–698 K, respec-
tively, revealing formation of a stable solid phase. This is, most
probably, a Bu3N:MnCl2 adduct resulting from interaction of the
volatile tributylamine with solid MnCl2. Upon further heating
the adduct is decomposed leaving behind stable manganese(II)
chloride. With the remaining (Bu4N)2[MnBrnCl4−n], with n = 2–4,
formation of a similar adduct is less probable owing to the steric
hindrance. For this reason, additional decomposition steps of these
compounds are missing. At the second step, MnBr2 appears as the
Fig. 6. XRPD pattern of (Bu4N)2[MnCl4] heated up to 900 K (1). For comparison,
reflections due to Mn2O3 (2) and Mn3O4 (3) are shown.

50
E. Styczen´ et al. / Thermochimica Acta 481 (2009) 46–51
Fig. 7. The FT-FIR spectra of (Bu4N)2[MnBr2Cl2] (1) and its decomposition products at various temperatures: T = 573 K (2) and T = 653 K (3) in argon.
Thermal decomposition of the complexes up to 650 K is
air) in the final products of decomposition of (Bu4N)2[MnBrnCl4−n
complexes (Fig. 6).
]
endothermic and independent of the oven temperature. A solid
residue left at this step is manganese(II) halide. Above 650 K the
decomposition is affected by oxygen present as indicated by a vari-
ety of energetic effects seen in the DTA curves recorded in argon
(Fig. 2) and in the air (Fig. 5).
A peak in the DTA curve (Fig. 5) is indicative of an exothermic
course of the last step. This can be interpreted as being due to oxi-
dation of the manganese(II) halide to manganese oxides, Mn2O3
and Mn3O4 as evidenced in the XRPD spectra of the sinters (Fig. 6).
A closer examination of the FT-FIR spectra of the sinters sampled
at particular temperatures reveals differences among successive
decomposition products (Fig. 7).
Thus, in the solid product of the first decomposition step of
(Bu4N)2[MnBr2Cl2] there are still bands due to stretching vibrations
of the Mn–Cl and Mn–Br bonds at 280 and 210 cm−1, respectively,
which are missing in the products of the next step. However, on
the basis of solely the Far-IR bands it was difficult to identify the
final product. More helpful proved to be the X-ray powder patterns
of the sinters which confirmed the presence of MnBr2 in the sin-
ters of the second step of decomposition of (Bu4N)2[MnBr2Cl2] as
well as the elemental manganese (in argon) and its oxides (in the
It is worth noting that inspection of the XRPD patterns revealed
also MnCl2 in a sinter of (Bu4N)2[MnBr2Cl2] sampled at 923 K
(together with a peak due to its melting). These findings suggest the
decomposition of (Bu4N)2[MnBr2Cl2] (Eq. (3)) to occur to a minor
extent according the following reaction pathway:
(Bu4N)2[MnBr2Cl2]−Bu−N→,BuBr(Bu4N)[MnBrCl2]−Bu−N→,BuBrMnCl2
3
3
(8)
An additional proof in favour of the suggested pathway of
decomposition of the manganese(II) complexes is a comparison of
the percentile experimental and calculated mass losses (Table 4).
Owing to the rapid progress of the first two decomposition
steps, it was difficult to determine final temperature of the first
step and that of the onset of the other. In this situation the readings
of the mass losses during the two steps might be burdened with
a small error. Just for this reason the mass losses taken from the
TG curves of the first step might be slightly larger than those
calculated. It is, however, much more easily to spot a point in the
Table 4
Percentile experimental and calculated mass losses.
Complex
Step
Argon
Air
Mass loss [%]
Found
Total mass loss [%]
Mass loss [%]
Found
Total mass loss [%]
Calcd
Found
Calcd
Calcd
41
Found
–
Calcd
(Bu4N)2[MnCl4]
(Bu4N)2[MnBrCl3]
1
2
3
4
51
26
4
41
28
13
10
–
77
81
90
–
69
82
92
53
–
27
13
41
7
80
93
82
89
9
1
2
3
4
49
25
5
45
25
13
10
–
74
79
96
–
70
83
93
54
45
–
–
21
13
38
7
75
88
83
90
17
(Bu4N)2[MnBr2Cl2]
(Bu4N)2[MnBr3Cl]
(Bu4N)2[MnBr4]
1
2
3
48
27
21
36
36
21
–
75
96
–
72
93
43
30
18
36
36
18
–
73
91
–
72
90
1
2
3
49
25
22
40
34
19
–
74
93
–
74
96
45
29
18
40
34
17
–
74
92
74
91
1
2
3
40
25
23
38
38
18
–
65
88
–
76
94
37
31
17
38
38
15
–
68
85
–
76
91

E. Styczen´ et al. / Thermochimica Acta 481 (2009) 46–51
51
TG curve corresponding to the formation of the manganese(II)
Acknowledgement
halide (i.e. the end of either the second or third step). At that
temperature total mass losses are comparable, thus indicating the
validity of the suggested transformations. Small differences can be
due to incomplete degradation of the organic matter (CxHyNz) that
becomes completely combusted only during the next step as evi-
denced by larger calculated and found mass losses corresponding
to the final step. Similar total mass losses of the whole complex
are also indicative of the presence of the suggested solid residues.
In this case, small differences arise from unidentified particular
components of the mixture, i.e. manganese carbide and elemental
manganese in argon and Mn2O3 and Mn3O4 in air. This makes
difficult a precise calculation of the total mass loss.
This research was supported by the Polish State Committee for
Scientific Research under grants DS/8232-4-0088-8 and BW/8000-
5-0454-8.
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