First Study of the Thermal and Storage Stability of Arenediazonium Triflates Comparing to 4-Nitrobenzenediazonium Tosylate and Tetrafluoroborate by Calorimetric Methods

Article abstract of DOI:10.1021/acs.oprd.9b00307

Herein, for the first time, using isothermal flow calorimetry and differential scanning calorimetry (DSC)/thermal gravimetric analysis (TGA), we have determined the thermal decomposition energies for the number of solid arenediazonium triflates comparing to 4-nitrobenzene tosylate and 4-nitrobenzentetrafluoroborate. The kinetics of thermal decomposition, activation energies, and half-lives of the studied diazonium salts (DSs) were found. Using gas chromatography-mass spectrometry (MS) and liquid chromatography-MS, we have elucidated the products formed during thermolysis of the investigated DSs. By density functional theory quantum chemical calculations at the B3LYP/aug-cc-pVDZ level of theory, we simulated the thermodynamics of decomposition reactions proceeding via substitution of the diazonium group by corresponding nucleophiles. The method applied predicted the decomposition energies of all the studied compounds fairly precise, except for 2-nitrobenzene diazonium triflate. It has been found that 4-nitrobenzene diazonium triflate has increased storage stability under normal conditions comparing to the corresponding tosylate and tetrafluoroborate. The experimental and theoretical results demonstrated that comparing to DSC/TGA, isothermal flow calorimetry more adequately reflects the energetics of the thermal decomposition of DSs and their storage stability under normal conditions.

Full text of DOI:10.1021/acs.oprd.9b00307

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Full Paper
The first study of the thermal and storage stability of
arenediazonium triflates comparing to 4-nitrobenzenediazonium
tosylate and tetrafluoroborate by calorimetric methods
Alexander Bondarev, Evgeny Naumov, Assiya Kassanova, Elena
Krasnokutskaya, Ksenia Sergeevna Stankevich, and Victor D. Filimonov
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.9b00307 • Publication Date (Web): 13 Sep 2019
Downloaded from pubs.acs.org on September 15, 2019
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The first study of the thermal and storage stability of
arenediazonium triflates comparing to 4-
nitrobenzenediazonium tosylate and
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tetrafluoroborate by calorimetric methods
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Alexander A. Bondarev *, Evgeny V. Naumov , Assiya Zh. Kassanova , Elena A.
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Krasnokutskaya , Ksenia S. Stankevich , Victor D. Filimonov *
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Department of Biomedicine, Altai State University, Barnaul, Russian Federation
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S. Toraighyrov Pavlodar State University, Pavlodar, Kazakhstan
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The Kizhner Research Center,National ResearchTomsk Polytechnic University, Tomsk,
Russian Federation
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TABLE OF CONTENTS GRAPHIC
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ABSTRACT
Herein, for the first time, using isothermal flow calorimetry and DSC/TGA, we have determined
the thermal decomposition energies for the number of solid arenediazonium triflates comparing
to 4-nitrobenzene tosylate and 4-nitrobenzentetrafluoroborate. The kinetics of thermal
decomposition, activation energies, and half-lives of the studied diazonium salts were found.
Using GC-MS and LC-MS, we have elucidated the products formed during thermolysis of the
investigated diazonium salts. By DFT quantum chemical calculations at B3LYP/aug-cc-pVDZ
level of theory we simulated the thermodynamics of decomposition reactions proceeding via
substitution of diazonium group by corresponding nucleophiles. The method applied predicted
the decomposition energies of all the studied compounds fairly precise, except for 2-
nitrobenzene diazonium triflate. It has been found that 4-nitrobenzene diazonium triflate has
increased storage stability under normal conditions comparing to the corresponding tosylate and
tetrafluoroborate. The experimental and theoretical results demonstrated that comparing to
DSC/TGA, isothermal flow calorimetry more adequately reflects the energetics of the thermal
decomposition of diazonium salts and their storage stability under normal conditions.
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KEYWORDS
arenediazonium salts; stability; isothermal flow calorimetry; quantum chemical calculation;
thermolysis
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INTRODUCTION
Aromatic diazonium salts (DSs) are versatile synthetic blocks widely used in fine organic
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synthesis and industry. Beyond that, DSs have been increasingly applied in development of
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macro- and nanoscale composite materials due to their reactivity towards metal and non-metal
surfaces.2a
Nevertheless, such disadvantages as a poor storage stability in solid state and a propensity to
explosive decomposition upon heating, photo-irradiation or mechanical stress limit the
preparation and usage of DSs, especially, on an industrial scale.1a, 2b-d Several approaches to the
stabilization of hazardous DSs, allowing for the minimization of risks associated with the
processes of their production and utilization, are available. These include, for instance,
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preparation of polymer-supported DSs, freezing of benzenediazonium chlorides up to -84 °C,
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g
and application of flow-chemistry techniques for DS chemistry. Besides, to diminish the hazard
level of DSs, twelve rules of handling these compounds were formulated.2b
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Recently, we have synthesized arenediazonium tosylates ArN TsO (ADTs)
and
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trifluoromethane sulfonates ArN TfO (ADTfs) that showed major advantages over traditional
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diazonium salts. While being surprisingly stable in solid state, they retained high reactivity in a
vast array of diazonium chemistry reactions including the formation of aromatic iodides and
bromides,3a,3b,4a-f azides 3b,4g and boronic acids,3b and Pd-catalyzed C-C-cross-compling.3b,4h-j
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Additionally, it has been shown that ADTs can be successfully used for introducing F isotope
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into the aromatic ring, carrying out azo-coupling with ethyl-methyl acetoacetate, substituting
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the diazonium group with the triethoxysilyl moiety, and covalently grafting aromatic groups to
carbonized metal nanoparticles 5d and graphene.5e
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The thermal decomposition energies of some ADTs and ADTfs determined by DSC/TGA, in
most cases were found to be below 800 J/g. 3a,b, 4g Therefore, according to the safety criteria of
the United Nations Economic Commission for Europe (UNECE), they can be referred as
compounds that can be transported safely.6 However, the products of their thermal
decomposition have not been studied and remain unknown. Moreover, DSC/TGA provides
information about thermal decomposition energy at increased temperatures and does not reflect
decomposition processes taking place under normal conditions. To comprehensively assess the
possibility of safe use of ADTs and ADTfs in the laboratory and on an industrial scale, it is
necessary to determine reliable quantitative characteristics of their storage stability in a solid
state and thermal decomposition energies. Such evaluations should be done using various
methods and comparing to other types of DSs.
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Noteworthy, the DS decomposition in solution has been extensively investigated for a long
time (see, for example,1a,g), whereas not many studies cover stability and safety of DSs in the
solid state. The paucity of quantitative data, describing the decomposition of solid DSs, reflects
the lack of reliable generally accepted procedures for measuring the above-mentioned properties.
A comparative study of the stability of the solid DSs (chlorides, tetrachlorozincates and
tetrafluoroborates) was reported in 7a,c, however the decomposition products were not given. The
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investigation of the thermal decomposition of N- and N-substituted arenediazonium chlorides
and tetrafluoroborates has shown that isotope effect is insensitive to the nature and position of
substituents in the aromatic ring and the nature of the counterion.7b The stability test of
arenediazonium chlorides has demonstrated that the sensitivity to detonation decreases from
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ortho-, through meta-, to para- substitution. It was shown that the nature of the substituent in
the aromatic nucleus has a pronounced effect on DS stability as the nitro derivatives were
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significantly more sensitive to impact than the chlorine derivatives. The decrease in detonation
sensitivity with an increase in molecular weight was observed, which was associated with a
decrease in the specific value of the energy released per unit mass. The authors noted that the
detonation sensitivity of DSs depends on many factors such as the size and shape of the crystals,
as well as the presence of impurities. No correlation between detonation sensitivity and thermal
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a
stability was found. The values of the thermal decomposition energies of some ADTs and
ADTfs, determined by DSC/TGA, do not correlate with the structural features of the diazonium
cation or the nature of counterion. 3a,b, 4g The commonly occurred term “the storage stability of
DS” has not yet been described quantitatively. To address this issue, in most cases the ability of
DSs to be stored without changes for a certain time is indicated. Besides, to the best of our
knowledge, up to the present, there are no theoretical methods for predicting the energies of
thermal decomposition of DSs based on their chemical structure.
Our work aims to comprehensively address the challenges associated with the thermal and
storage stability assessment of DSs. To achieve that, we for the first time have studied the
kinetics and thermodynamics of thermal decomposition of arenediazonium triflates 1a-d
comparing to 4-nitrobenzenediazonium tosylate 2 and tetrafluoroborate 3 by DSC/TGA and
isothermal flow calorimetry. Additionally, we have endeavored to develop the criteria for the
evaluation of the storage stability of an array of diazonium salts with various counterions and
substituents. The electron-withdrawing nitro group and the electron-donating methoxy group
were chosen as substituents in the aromatic core of ADTfs 1a-d for the following reasons. First,
7a
according to , diazonium salts with NO - moieties are the most explosive, therefore they
2
represent the highest threshold of these properties. Secondly, DSs with NO - and MeO-
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substituents in the aromatic ring differ sharply in their properties,3a,b i.e. these two examples
should cover the widest range of properties studied.
We also aimed to determine the possibility to apply DFT quantum chemical calculations for
the theoretical evaluation of the DS thermal decomposition and clarification of its mechanism.
To the best of our knowledge, DFT methods have not been previously used for these purposes.
To investigate the mechanism, a GC-MS and LC-MS study of the decomposition products of
DSs 1a-d, 2, 3 was carried out. The obtained results are valuable for both applied and theoretical
field of diazonium chemistry. On the one hand, they allow to assess the stability, capabilities and
limitations of DSs for industrial use. On the other hand, they provide the better understanding of
the mechanisms of DS thermal decomposition and allow to establish the structure-stability
relationship.
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EXPERIMENTAL SECTION
Arenediazonium triflates 1a-d and 4-nitrobenzenediazonium tosylate 2 were synthesized
according to the procedure described previously.3a,b 4-Nitrobenzenediazonium tetrafluoroborate
3
was purchased from Aldrich (CAS № 456-27-9). All samples used in calorimetric studies were
dried in vacuum for 48 h. This procedure is recommended in the study of the decomposition of
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a
arendiazonium chlorides .
The DSC/TGA runs were made in argon atmosphere using open sample pans on Q600 SDT
-1
instrument (TA Instruments), a heating rate of 5 °C·min , and a temperature range of 20-600 °C.
A typical sample size was 10 mg.
Heat flow was measured under isothermal conditions in nitrogen atmosphere using TAM III
microcalorimeter (TA Instruments). The experiments were conducted according to the
conventional approach that is used for calorimetric studies of the safety of high-energy materials.
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Sample was put in a glass beaker placed in a standard calorimeter ampoule made from
Hastelloy with a volume of 1 mL. The ampoule was evacuated, then purged with nitrogen, argon
or air depending on experimental conditions and sealed. The heat flow was measured at three
different temperatures: 75, 80, and 85 °C. We conducted all flow calorimetry experiments at
temperatures significantly lower than the melting points in order to adequately approximate
kinetic curves to normal conditions. The isothermal test was carried out until the heat flux
decreased below 2 μW. This value is less than 1% of the maximum heat flux and corresponds to
the conversion degree α> 0.99. Time was varied from 1 to 50 days depending on sample and
temperature. The acquired experimental curves were approximated using the model of an
autocatalytic process and the Arrhenius equation. The experimental data were processed using
TAM Assistant Software v1.3.0.153. Gnuplot 4.5 9a and R Statistics v3.3.3 9b were used for
mathematical processing, statistical analysis, and dependencies building.
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The DS decomposition products were studied by GC-MS on an Agilent 7890A\5975C
instrument. The typical sample size was 50 mg. Samples were heated in a thermostat at 85 ° C
for 14 days. Then an aqueous solution of KI was added to the sample to convert the
undecomposed diazonium salts into the corresponding volatile aryl iodides,3a,b the products were
extracted with ethyl acetate and organic layer was filtered through a silica pad. The obtained
ethyl acetate extracts were then analyzed by GC-MS.
All LC-MS experiments were carried out on a high-resolution time-of-flight mass spectrometer
Agilent LC-1260 MS QTOF 6530 equipped with electrospray ionization source (ESI) and
atmospheric pressure chemical ionization source (APCI). A chromatographic method was
developed using a Zorbax Eclipse Plus column (C18, 2.1x50 mm, 1.8 micron). The following
gradient elution with water as “A” and acetonitrile as “B” was used at a flow rate of 0.25
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mL/min: 0-40 min, 0% B  100% B followed by isocratic elution with B for 20 min. The
operating parameters of the ESI-QTOF-MS were: gas flow rate, 9 L/min (N ); drying gas
2
temperature, 350 °C; nebulizer, 35 psi; sheath gas flow, 11 L/min; sheath gas temperature,
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00 °C, capillary, 3500 V; skimmer, 65 V; octopole RF voltage, 750 V; fragmentor 150 V;
energy of collision 20 eV. The operating parameters of the APCI-TOF-MS were: gas flow rate,
6 L/min (N ); drying gas temperature, 300 °C; nebulizer, 35 psi; vaporizer temperature, 400 °C;
2
capillary, 3500 V; skimmer, 65 V; octopole RF voltage, 750 V; fragmentor, 150 V; energy of
collision 20 eV. For the LS-MS experiments, the DS decomposition products were dissolved in a
water : acetonitrile mixture (1:1 v/v) at a concentration of 1 mg/mL. The volume of sample
injected was 5 µL. Acquired LS-MS spectra were processed using the OpenMS 2.0 software
package.10
For the theoretical study of suggested DS decomposition routes the quantum-chemical
calculations were performed using Kohn−Sham density functional theory (DFT), global-hybrid
11
GGA functional B3LYP and aug-cc-pVDZ basis set in Gaussian 09 software package. At the
first step, the geometry of all molecules participating in the reactions was optimized. To prove
the nature of the stationary points, the harmonic frequency calculations were done. Thereafter,
the vibrational frequencies and thermodynamic corrections were calculated at normal conditions
(
25 ° C, 1 atm) and at temperatures used for isothermal decomposition experiment (75, 80 and 85
° C).
RESULTS AND DISCUSSION
DSC/TGA results
Our study of the thermal and storage stability of arenediazonium triflates 1a-d, 4-
nitrobenzenediazonium tosylate 2 and tetrafluoroborate 3 has begun from collecting DCS/TGA
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data. Fig. 1-6 show DSC/TGA curves of thermal decomposition of DSs 1-3. It can be seen that
upon heating all the investigated DSs decompose with energy and weight loss. The temperatures
and decomposition energies are summarized in Table 1.
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For the DSs 1a, 1b, 1d, 3 additional low-temperature endothermic effects are observed in the
temperature range close to their melting point, whereas DSs 1c, 2 decomposed only
exothermically (Fig. 1-6, Table 1). Note that DS 2 decompose giving two exothermic peaks (Fig.
5
). The first one at 69.37 °C is characterized by a small amount of heat released (24.47 J/g) and
can be caused by the baseline issue, or crystallization from amorphous phase. While for the
second one, located at 146.6 °C, the heat release of 323.0 J/g is observed. In the case of DSs 1a,
1b, 1d the endothermic peaks are not accompanied by a weight loss and are probably associated
with the rearrangement of the crystal lattice.
The weight loss upon heating DS 3 in the endothermic process, starting at 58.7 °C is 28.5%,
which corresponds exactly to the elimination of volatile BF (28.6%) according to reaction (1):
3
It might seem that the loss of BF at 58.7 °C indicates that intermediate 3a undergoes
3
+
-
decomposition at 146.5 °С (equation 1). However, upon thermal decomposition of PhN BF , a
2
4
small endothermic peak at 63 °C was also observed on the DSC/TGA curves. The appearance of
this peak was explained by the removal of water bound to DS via hydrogen bonds. 12 It cannot be
applied to DS 3 thought, as the weight loss of DS 3 at 58.7 °C corresponds to 5.2 mol of water
per 1 mol of DS 3. Should it be such a large amount of water, DS 3 would have to be partially
dissolved, but not crystalline. Hence, there are contradictions in explaining the cause of the
+
-
endothermic peak appearance during the heating of DS 3 and PhN BF . Obviously, this issue
2
4
requires further special studies. Therefore, at present, we can propose equation 1 only as a
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hypothesis, partially consistent with the results of quantum-chemical modeling of DS
decomposition reactions.
The major weight loss upon heating of DSs 1-3 occurs in exothermic processes, which is
clearly associated with the formation of volatile decomposition products (Fig. 1-6).
Table 1. Temperatures and decomposition energies of diazonium salts 1-3 according to
DSC\TGA experiments data.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Endothermic process
Т, ° C Н, J/g (kJ/mol)
Exothermic process
Т, ° C Н, J/g (kJ/mol)
Mp, (° C)
Diazonium salts


+
-
2-NO С H N TfO 1a 104.1
73.49 (21.98)
43.6 (10.04)
-
143.3 -753.4 (-225.3)
111.2 -840.4 (-251.3)
116.4 -219.9 (-65.7)
110
2
6
4
2
+
-
3-NO С H N TfO 1b 102.5
108-109
104
2
6
4
2
+
-
4-NO С H N TfO 1c -
2
6
4
2
+
-
4-MeOС H N TfO
88.4
102.2 (29.05)
6
4
2
1
36.6 -328.9 (-93.5)
94-97
1
4
4
d
-
-
-
69.37 -24.47 (-7.8)
46.6 -323.0 (-103.7)
146.5 -229.2 (-54.3)
+
-
-NO С H N TsO 2
132
144
2
6
4
2
1
+
-
-NO С H N BF 3 58.7
2
6
4
2
4
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3
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5
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9
0
1
2
3
4
5
6
7
8
9
0
+
-
Figure 1. DSC/TGA thermograms of thermal decomposition of 2-NO С H N TfO 1a. The
2
6
4
2
temperature difference is shown in red; the sample weight is shown in green; the heat flow is
shown in blue.
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3
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5
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8
9
0
1
2
3
4
5
6
7
8
9
0
+
-
Figure 2. DSC/TGA thermograms of thermal decomposition of 3-NO С H N TfO 1b. The
2
6
4
2
temperature difference is shown in red; the sample weight is shown in green; the heat flow is
shown in blue.
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3
4
5
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8
9
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1
2
3
4
5
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9
0
+
-
Figure 3. DSC/TGA thermograms of thermal decomposition of 4-NO С H N TfO 1c. The
2
6
4
2
temperature difference is shown in red; the sample weight is shown in green; the heat flow is
shown in blue.
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2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
-
Figure 4. DSC/TGA thermograms of thermal decomposition of 4-MeOС H N TfO 1d. The
6
4
2
temperature difference is shown in red; the sample weight is shown in green; the heat flow is
shown in blue.
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1
2
3
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9
0
+
-
Figure 5. DSC/TGA thermograms of thermal decomposition of 4-NO С H N TsO 2. The
2
6
4
2
temperature difference is shown in red; the sample weight is shown in green; the heat flow is
shown in blue.
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+
-
Figure 6. DSC/TGA thermograms of thermal decomposition of 4-NO С H N BF 3. The
2
6
4
2
4
temperature difference is shown in red; the sample weight is shown in green; the heat flow is
shown in blue.
The exothermic effects Hexoterm of DSs 1-3 thermal decomposition are the most important for
the DS safety evaluation. As can be seen from Table 1, these effects strongly depend on the
nature and position of the substituent in the benzene ring and partly on the nature of counterions.
Among salts 1a-d with a triflate counterion, the energy release during decomposition is the
highest for 3-nitro-derivative 1b and decreases noticeably in the row 1b>1a>1d>1c. Among DSs
having the same 4-nitrobenzene diazonium cation and different counterions, the energy release
decreases in the row 2c>1c≈3. Therefore, the highest heat release is observed for DS with a TsO-
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2
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2
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counterion. However, a change in the counterion nature affects exothermic decomposition
process substantially less than the position and type of substituents in the benzene ring.
Isothermal flow calorimetry results
0
1
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9
0
1
2
3
4
5
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With DSC/TGA data in our hands we moved on to isothermal flow calorimetry analysis of
DSs studied. As the stability of DS could be affected by the presence of impurities, which are
often cannot be controlled by conventional analytical methods,1a,7a we first decided to evaluate
the reproducibility of thermal decomposition parameters measured. To do that we took samples
of ADTS 2 from three synthetic batches obtained under identical conditions (samples 1-3) and
samples of ADTS 2 purified by single and double precipitation of sample 3 from acetic
acid/ether solutions (repricipitation 1 and 2). During the isothermal decomposition the following
parameters have been determined: the integral enthalpy, initial and maximum heat flows,
decomposition rate constants, and initial product concentrations approximated by equation 2.
Additionally, we have varied the purged gas and studied sample decomposition in air, argon and
nitrogen atmosphere. The results show that the collected data are consistent, and the results do
not depend statistically significantly on the degree of sample purification. Even though there is a
slight tendency to the reduction of the initial heat flow depending on the number of
reprecipitations, the differences between the first and second reprecipitation are minimal. No
qualitative and statistically significant differences were observed when decomposition was
conducted in air, argon and nitrogen atmosphere. The average statistical deviation (RMD) for the
rate constants (according to equation 2) and the enthalpy was 5%. Therefore, the further
isothermal flow calorimetry experiments were carried out in a nitrogen atmosphere and after a
single reprecipitation of the initial DS.
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Table 2. The reproducibility of the DS 2 thermal decomposition parameters measured at 85
°
C depending on synthetic batch and a number of reprecipitations.
3
-1
H, kJ·mol-1 P0, mW/g
Sample
k·10 , s

1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Sample 1 Nitrogen (reprecipitation 1)
0.2723 ± 0.0364 234.2 ± 3.0
31.2 ± 8.1
Sample 2 Nitrogen, (source, reprecipitation
0
0
.2568 ± 0.0371 243.6 ± 20.7 12.7 ± 3.5
1
)
Sample 3 Nitrogen, (source, reprecipitation
.2639 ± 0.0283 235.5 ± 7.0 10.1 ± 3.5
1
)
Sample 3 Air, reprecipitation 2
Sample 3 Argon, reprecipitation 2
Sample 2, 3 Nitrogen,
0.2683 ± 0.0333 229.0 ± 28.9 8.7 ± 8.5
0.3119 ± 0.0352 242.3 ± 8.0 12.8 ± 2.4
0
.2608 ± 0.0184 238.9 ± 7.8 11.2 ± 2.3
(source, reprecipitation 1, 2)
Sample 4 Nitrogen, source
0.2589 ± 0.0340 241.7 ± 16.2 12.0 ± 2.0
0.2608 ± 0.0492 235.1 ± 17.9 10.2 ± 6.3
0.2898 ± 0.0221 238.1 ± 6.8 11.3 ± 2.3
Sample 4 Nitrogen, reprecipitation 1
Sample 4 Nitrogen, reprecipitation 2
General statistics
0.2745 ± 0.0140 237.7 ± 4.6 13.8 ± 3.3
k – rate constant,

H - integral enthalpy, P – initial heat flow
0
о
о
Table 3 and Fig. 7-10 show the results of isothermal decomposition of DSs 1-3 at75 С, 80 С,
о
о
and 85 С. At 75 С, the maximum heat flow values obtained for the 4-nitrobenzene diazonium
salts 1c, 2, 3 almost do not depend on the counterion nature. However, it can be noted that the
heat flow curve acquired during DS 1c decomposition is much steeper. Additionally, the Pmax
о
values obtained for this DS at 80 and 85 С are significantly higher than those for DS 2 and 3.
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The Pmax values found for 4-methoxybenzenediazonium triflate 1d are noticeably smaller
comparing to 4-nitrobenzenediazonium triflate 1c. Among the nitrobenzenediazonium triflates
1a-c, the maximum heat flow decreases from para-1c, through meta-1b, to ortho-1a substitution.
The values of Pmax are important not only for mathematical modeling of the reaction kinetics, but
also for quantitative description of the compound safety that is essential for practical application.
Table 3. Integral enthalpy and maximum heat flow values found during isothermal
decomposition of diazonium salts 1-3.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
∆
H, kJ/mol
Pmax, mW/g
Diazonium salt
o
o
o
o
o
o
75 C
414.0
227.9
200.0
80 C
85 C
75 C
80 C
85 C
+
-
-
2
3
-NO С H N TfO (1a)
386.0
230.0
235.1
183.2
232.4
156.0
396.0
225.4
250.0
106.0
231.0
147.0
0.705
6.57
1.49
2.33
2
6
4
2
+
-NO С H N TfO (1b)
14.15
47.89
2.97
28.8
2
6
4
2
+
-
4-NO С H N TfO (1c)
20.12
1.34
119.5
5.37
2
6
4
2
+
-
4
-MeOС H N TfO (1d) 183.1
6
4
2
+
-
4
-NO С H N TsO (2)
253.0
173.0
21.10
20.02
34.82
31.63
64.60
66.87
2
6
4
2
+
-
4-NO С H N BF (3)
2
6
4
2
4
∆
H – integral enthalpy, Pmax – maximum heat flow value
To investigate the kinetics of isothermal decomposition (Fig. 7-10), we approximated the
experimental heat flow curves with kinetic equation for autocatalytic reactions (2) that
qualitatively describes the heat flow over time dependency. The following main parameters were
determined: k - rate constant, and P , Pmax - values of the initial and maximum heat flows in the
0
autocatalytic reaction equation 2. Table 4 shows calculated kinetic parameters found as result of
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approximation of the experimental heat flow curves of DSs 1a-1d, 2, 3. The following kinetic
equation describing autocatalytic reaction was applied: a A → c C
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
,
-1
where α – conversion degree; k(T) – rate constant, [s ]; f(α) – the kinetic model in differential
form; A , A – initial and current concentration of DS, which for solid-phase reactions is measured
0
in mol/g units; С , С – initial and current concentrations of products, [mol/g]; a, c –
0
stoichiometric coefficients, for the processes studied a/c=1; P, P – current and initial heat flow;
0
ΔH – reaction enthalpy; B - is the pre-exponential factor, a constant for each chemical reaction;
Ea - an experimentally determined parameter that indicates the sensitivity of the reaction rate to
−1
−1
temperature; R - the universal gas constant (8.315 J∙K ∙mol ); T - the absolute temperature (in
K).
1
3
The kinetic equation 2 is used to model solid phase reactions , and can be expressed in terms
of the conversion degree α (equation 3). The kinetic curves for the reactions studied are best
described by equation 4 at m=1 and n=1. The temperature dependence of the reaction rate is
expressed by the Arrhenius equation 5. The reaction enthalpy ΔH is calculated by integrating a
curve approximating the experimental heat flux (equation 6), in the time interval (0, ∞). The
half-life corresponds to the conversion degree of 0.5. Due to the symmetry of the approximating
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kinetic curve 4, the half-life coincides with the maximum heat flux (in the case of more complex
kinetics, this rule is not fulfilled).
Figure 7 shows the experimental heat flow curves describing the isothermal decomposition of
DSs 1c, 2 and 3 at 85 ° C. As can be seen, the half-life of DSs depends on the counterion: for
tetrafluoroborate 3 it is 2h, for triflate 1c – 4h, and for tosylate 2 – 6h.
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9
0
Figure 7. The heat flow (P) during isothermal decomposition of nitrobenzenediazonium salts:
tetrafluoroborate 3, tosylate 2, and triflate 1c at 85 ° C.
The experimental heat flow curves describing the isothermal decomposition of salt 1a are more
complex and, therefore, differ from the curve characteristic for the autocatalytic process. At the
initial stage, a decrease in the heat flow is observed, which is associated with a higher rate of
endothermic process. Then, the heat flow is increased indicating the predominance of the
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exothermic process. The presence of the endothermic process is consistent with the DSC/TGA
data (Table 1). To describe the kinetics of this reactions, we used the model of two sequential
and two parallel autocatalytic processes. The deconvolution results are given in detail in
Supporting Information 1s. Table 4 shows the parameters of the heat flow kinetics calculated
when isothermal decomposition reaction is approximated by a single autocatalytic process
corresponding to the main exothermic stage. This stage is the most important as it determines the
main characteristics of safety and stability of DSs.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 4. Kinetic parameters of the isothermal decomposition of DSs 1-3 (P – initial heat flow
0
value, k – rate constant calculated according to the equation 2).
3
-1
P0, mW/g
k·10 , s
Diazonium salt
7
5o
80o
85o
75o
80o
85o
+
-
-
2
3
4
4
4
-NO С H N TfO 1a 40.6
88.2
11.0
13.1
147.0
44.6
0.00087 0.00184 0.00254
0.03048 0.06545 0.14339
0.10562 0.21057 0.53446
0.00739 0.01548 0.04504
0.08995 0.16775 0.32057
0.07851 0.14520 0.30432
2
6
4
2
+
-NO С H N TfO 1b 0.94
2
6
4
2
+
-
-NO С H N TfO 1c 9.0
48.4
2
6
4
2
+
-
-MeOС H N TfO 1d 14.78 32.00
113.8
41.5
6
4
2
+
-
-NO С H N TsO 2
19.57 27.5
392.4 896.0
2
6
4
2
+
-
4-NO С H N BF 3
1804.2
2
6
4
2
4
The half-life of 4-nitrophenyldiazonium triflate 1c is significantly less than the half-life of 4-
methoxyphenyldiazonium triflate 1d: the values found are 4 h and 16 h, respectively. At the
same time, the heat flow observed during decomposition of 4-nitrophenyldiazonium triflate 1c is
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much higher than that of 4-methoxyphenyldiazonium triflate 1d (Fig. 8): 119.5 mW/g for 1c
versus 5.37 mW/g for 1d (Fig.8).
0
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0
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0
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0
1
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9
0
1
2
3
4
5
6
7
8
9
0
Figure 8. The heat flow (P) during isothermal decomposition of 4-nitrobenzenediazonium 1c
and 4-methoxybenzenediazonium 1d triflates at 85 ° C.
The substitution pattern in aromatic ring has a pronounced effect on the DS stability. Among
DS with triflate counterion, the longest half-life time at 85° C has ortho- derivative 1a (62 h).
The meta- derivative 1b is less stable (11 h) and para- derivative 1c is the least stable (4 h). The
maximum heat flow values follow the opposite pattern and decrease in a row 1c > 1b > 1a. The
results discussed are presented in Tables 3 and 4 and Fig.9.
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0
1
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9
0
1
2
3
4
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6
7
8
9
0
Figure 9. The heat flow (P) during isothermal decomposition of 2-, 3- and 4-
nitrobenzenediazonium triflates 1a-c at 85 ° C.
The analysis of the kinetic data obtained at different temperatures followed by approximation
with the Arrhenius equation allowed us to find the kinetic parameters of DS decomposition
reactions occurring at 25 ° C. The obtained results are presented in Table 5 and Fig. 10. The
arenediazonium triflates 1b-d have higher activation energies comparing to tosylate 2 and
tetrafluoroborate 3. However, 2-nitrobenzenediazonium triflate is out of this line as it has the
lowest activation energy among the DS studied.
Table 5. The calculated kinetic parameters of DS 1-3 decomposition reactions occurring at 25 °
C.
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-1
Diazonium salt
k298 ·10 , s
Ea, kJ/mol
111.4
+
-
2-NO С H N TfO 1a
2
6
4
2
1.39
+
-
3
4
4
-NO С H N TfO 1b
2 6 4 2
2.90
159.7
0
1
2
3
4
5
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7
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9
0
1
2
3
4
5
6
7
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9
0
1
2
3
4
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8
9
0
1
2
3
4
5
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9
0
1
2
3
4
5
6
7
8
9
0
+
-
-NO С H N TfO 1c
2
6
4
2
4.45
173.0
+
-
-MeOС H N TfO 1d
6
4
2
0.049
48.91
18.75
187.1
+
-
4-NO С H N TsO 2
2
6
4
2
131.7
+
-
4
-NO С H N BF 3
2 6 4 2 4
140.3
о
k298 – rate constant at 298 K; Ea — activation energy, kJ/mol
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0
1
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9
0
1
2
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8
9
0
Figure 10. Results of the approximation of DS 1-3 decomposition reactions with the Arrhenius
equation
Based on the approximation data, we have modeled the process of decomposition of the
investigated DS over large time period (Fig. 11-12). According to the results, the stability of
arenediazonium triflates depends on the substitution pattern. In particular, at 25 °C 3-
nitrobenzenediazonium triflate has the longest half-life time of 83 years, whereas 2-
nitrobenzenediazonium triflate has the shortest half-life time of 25 years (Fig. 11). The nature of
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counterion affects greatly the DS stability (Fig. 12). Thus, 4-nitrobenzenediazonium tosylate 2
and tetrafluoroborate 3 have close half-life times of 4.5 years. Whereas 4-nitrobenzenediazonium
triflate 1c is much more stable with a half-life time of 46 years and a significantly lower
maximum heat flow. Note that the effect of counterion on DS stability becomes noticeable only
at low temperatures (25 °C), while at elevated temperatures (during DSC/TGA or isothermal
flow calorimetry at 75-85 °C) these differences disappear. This fact, as well as the values of
activation energies, suggest that the stability of DS under normal storage conditions is largely
determined by the strength of the crystal lattice. At higher temperatures, after the destruction of
the crystal lattice, the speed and energy of the process are likely to be influenced by both the
nature of the Ar-N2+ diazonium cation and the presence and nature of nucleophiles in the
immediate environment.
0
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0
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Figure 11. The heat flow (P) over the time dependency built upon modelling of the kinetic
о
curves of the decomposition of 2-, 3- and 4-nitrobenzenediazonium triflates 1a-c at 25 С.
1
1
1
1
1
1
1
1
1
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0
1
2
3
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8
9
0
1
2
3
4
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8
9
0
Figure 12. The heat flow (P) over the time dependency built upon modelling of the kinetic
curves of the decomposition of 4-nitrobenzenediazonium tosulate 2, triflate 1c and
о
tetrafluoroborate 3 at 25 С.
Thus, considering all the data, we can draw several conclusions. First of all, DSC/TGA alone
cannot serve as a reliable criterion for assessing the thermal stability and safety of DSs, since in
this case decomposition occurs at higher temperatures and is accompanied by intense
evaporation of low-molecular-weight reaction products. In addition, during DSC/TGA analysis
DSs decompose at different temperatures, which makes the analysis and comparison of the data
complicated. Therefore, the study of DS thermal stability should be complemented by a more
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