and Bis(1‐hydroxytetrazol‐5‐yl)triazene

Jul. 29, 2024

and Bis(1‐hydroxytetrazol‐5‐yl)triazene

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For the first time, an adequate selective synthesis, circumventing the formation of 2&#;hydroxy&#;5H&#;tetrazole, of 1&#;hydroxy&#;5H&#;tetrazole (HTO), as well as the synthesis of bis(1&#;hydroxytetrazol&#;5&#;yl)triazene (H 3 T) are reported. Several salts thereof were synthesized and characterized which resulted in the formation of new primary and secondary explosives containing the 1&#;oxidotetrazolate unit. Molecular structures are characterized by single&#;crystal X&#;ray diffraction, 1 H and 13 C NMR, IR, and elemental analysis. Calculation of the detonation performance using the explo5 code confirmed the energetic properties of 1&#;hydroxy&#;5H&#;tetrazole. The detonation properties can be adjusted to the requirements for those of a secondary explosive by forming the hydroxylammonium ( 6 ) or hydrazinium ( 7 ) salts, or to meet the requirements of a primary explosive by forming the silver salt 4 , which shows a fast DDT on contact with a flame. The sensitivities of all compounds towards external stimuli such as impact, friction, and electrostatic discharge were measured.

The reaction of 5&#;amino&#;1&#;hydroxytetrazole (5&#;ATO) with nitrous acid is strongly dependent on the equivalents of HNO 2 . By applying one equivalent and copper(0)/ethanol it is possible to perform a dediazonization, resulting in 1&#;hydroxytetrazole &#; the highlight of this work. By applying half an equivalent of HNO 2 it is possible to obtain the azo&#;coupled product bis(1&#;hydroxytetrazol&#;5&#;yl)triazene. Both compounds as well as salts thereof are fully characterized in this work.

When applying the systems analogously to 5&#;ATO, the dediazonization selectively produces 1&#;hydroxy&#;5H&#;tetrazole, thus representing the first adequate selective synthesis for this long over&#;due compound, whereas following Hofmann et al. results in the formation of bis(1&#;hydroxytetrazol&#;5&#;yl)triazene, the twice N&#;oxidized bis(tetrazol&#;5&#;yl)triazene. Therefore, we report the first adequate procedure to obtain 1&#;hydroxy&#;5H&#;tetrazole without the need of protection groups and elaborate workups. Additionally, the first synthesis of bis(1&#;hydroxytetrazol&#;5&#;yl)triazene is reported. Both acids as well as salts thereof are prepared and are characterized by single crystal X&#;ray diffraction experiments, complemented by NMR and IR spectrometry as well as thermal and physical analysis.

Independently from the applied reduction procedure, the dediazonization always starts with a diazotization of the amine, which, for several azoles, can instantly explode and therefore should never be isolated. It can also couple with other compounds and this well&#;known behavior is exploited to produce azo dyes, such as Methyl red (2&#;(N,N&#;dimethyl&#;4&#;aminophenyl) [20] or Tartrazine (trisodium 1&#;(4&#;sulfonatophenyl)&#;4&#;(4&#;sulfonatophenylazo)&#;5&#;pyrazolone&#;3&#;carboxylate). [21] Lesser known are triazenes, which result from the coupling of a diazonium cation with an amine. In Hofmann et al. published the first synthesis of sodium bis(tetrazol&#;5&#;yl)triazene by diazotization of amino guanidinium nitrate with sodium nitrite, intermediately producing 5&#;aminotetrazole which then further reacts to the triazene species. [22]

The first synthesis was reported by Pallazzo in (Figure ), [13] however, the addition of hydrazoic acid to sodium fulminate is not suitable nor desired for a synthesis on gram&#;scale due to the high risks involved when working with fulminates. In , Bettinetti et al. successfully synthesized 1&#;hydroxy&#;5H&#;tetrazole by the addition of hydrazoic acid to nitrolic acids, which too is not desired for the work on gram&#;scale. [14] Bettinetti et al. also described the silver salt of 1&#;hydroxytetrazole, yet an extensive characterization is missing. In , Begtrup et al. [15] used Oxone® to oxidize 1,5H&#;tetrazole. This idea was then followed by Giles et al. [16] in for oxidizing ethyl tetrazole&#;5&#;carboxylate. Both procedures result in an isomeric mixture of the 1&#;hydroxy&#; and 2&#;hydroxy&#;tetrazole derivatives, requiring further protection (for the oxidized 1H&#;tetrazole), as well as separation of the isomers and subsequent secession of the protecting group. In a previous paper in , we showed that the addition of hydroxylamine to a solution of cyanogen azide results in a cyclization forming 5&#;amino&#;1&#;hydroxytetrazole (5&#;ATO). [17] In , Henry et al. showed that 1,5H&#;tetrazole can be obtained by the elimination of the amino group of 5&#;amino&#;1H&#;tetrazole through diazotization followed by subsequent reduction of the diazonium cation. These so&#;called hydro&#;dediazonizations are performed by boiling a diazonium cation in acidic ethanol. However, this procedure usually results in ethoxy&#;dediazonization. [18] Kornblum et al. later replaced the step in which reduction with acidic ethanol occurs by using hypophosphorous acid as the reducing agent instead. [19]

Tetrazoles are a class of heterocycles that exhibit a wide variety of possible applications. A prominent representative is Losartan, which is included in the WHO's List of Essential Medicines [1] as a treatment for hypertension. [2] In addition to pharmaceutical uses, tetrazoles show great potential for application as energetic materials with high nitrogen contents like in copper(I) nitrotetrazolate (DBX&#;1).[ 3 , 4 ] The high endothermic heat of formation of tetrazoles (e. g. +236 kJ mol &#;1 for 1,5H&#;tetrazole) is advantageous for possible applications as energetic materials, since a large amount of energy is released upon detonation. [5] General protocols to further increase the enthalpy of formation of tetrazoles include N&#;oxidation by Oxone®, [6] HOF, [7] or H 2 O 2 , [8] which also has the advantage of increasing the oxygen balance and crystal density. [9] In , Klapötke et al. [10] obtained dihydroxylammonium&#;5,5&#;&#;bistetrazolyl&#;1,1&#;&#;diolat (TKX&#;50) through the N&#;oxidation of 5,5&#;&#;bistetrazole followed by formation of the hydroxylammonium salt. This compound possesses excellent detonation properties while being thermally stable (221 °C) and moderately sensitive towards impact (20 J) and friction (120 N), thus being stable enough to be safely used as a high&#;performing secondary explosive. TKX&#;50 surpasses properties of 1,3,5&#;Trinitro&#;1,3,5&#;triazinan (RDX) and 1,3,5,7&#;Tetranitro&#;1,3,5,7&#;tetrazocan (HMX), which have long been used as the main charge in detonation devices. Even though the 5,5&#;&#;bistetrazole has been known since , [11] it took nearly 100 years until its potential for use as an explosive was discovered in the compound TKX&#;50. It is therefore surprising that 1,5H&#;tetrazole has been known since , [12] but complete characterization of the corresponding N&#;oxide, namely 1&#;hydroxy&#;5H&#;tetrazole, is missing.

Results and Discussion

Synthesis

Warning! The synthetic work described in this section involves the handling of very sensitive intermediates (diazotetrazole&#;1&#;N&#;oxide) and products (e.&#;g., silver salt 4). Proper protective measurements and equipment must be used!

The starting material 5&#;ATO is readily available from the reaction of cyanogen azide and hydroxylamine. [17] 1&#;Hydroxy&#;5H&#;tetrazole (1) can be obtained by dissolving 5&#;ATO in semi&#;concentrated sulfuric acid (40%) and diazotizing with sodium nitrite, while keeping the temperature below 5&#;°C (Scheme&#; ). The diazotization solution is then added to a mixture of ethanol and elemental copper and stirred at 55&#;°C for 2&#;hours. Due to the intermediate formation of 5&#;diazonium&#;1&#;hydroxytetrazole, the ratio of 5&#;ATO to sulfuric acid (40%) should be quite low. A ratio of 1&#;:&#;10 was used for the synthesis of all herein investigated compounds, as a higher concentration of 5&#;diazonium&#;1&#;hydroxytetrazole leads to micro&#;detonations within the reaction solution. An attempt to reduce the amount of acid to a ratio of 1&#;:&#;2 resulted in a violent detonation of the whole reaction solution, destroying the round&#;bottom flask. Compound 1 was extracted into DCM, and the ammonium salt (5) was subsequently precipitated by passing gaseous ammonia through the organic phase. The sodium (2) and potassium (3) salts were obtained by refluxing a solution of the corresponding carbonate and 5 in water, followed by evaporating to complete dryness, extracting the residue with ethanol, and precipitating the salts by adding diethyl ether. The silver salt (4) can be precipitated by adding silver nitrate to a solution of 1 in water and filtering off the solid. The hydroxylammonium (6) and hydrazinium (7) salts were obtained by adding the corresponding base to a solution of 1 in ethanol and precipitating with diethyl ether. Due to coextraction of sulfuric acid after dediazonization pure 1 is obtained by dissolving 3 in 2&#;m hydrochloric acid, extracting with ethyl acetate, and removing the solvent in vacuo. Bis&#;(1&#;hydroxytetrazol&#;5&#;yl)triazene 8 was obtained as a monohydrate by dissolving 5&#;ATO in concentrated hydrochloric acid followed by diazotization with half an equivalent of sodium nitrite, adjusting to pH>10 with sodium hydroxide, and extraction into ethyl acetate. Compound 8 crystallized by slow evaporation of the solvent. Salts 9&#;14 were precipitated by adding the corresponding hydroxide (9, 10), carbonate (11&#;13), or free base (14) dissolved in the minimal volume methanol to a solution of 8 in ethyl acetate. The copper ammonium salt 15 was obtained by precipitating the copper salt of 8 using copper sulfate followed by recrystallization from concentrated ammonia.

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When preparing compound 8, it is important to make sure any trace amounts of the diazonium cation are removed by stirring under basic conditions (pH>10), since it is possible to co&#;extract it into the organic solvent. When removing the solvent in vacuo, a detonation occurred on slightly touching the solid with a plastic spatula, leading to the person performing the experiment being injured.

NMR spectroscopy

NMR spectroscopy was performed in DMSO&#;d6, D2O or acetone&#;d6 and spectra are depicted in the Supporting Information. The 1H NMR spectrum of compound 1 in DMSO&#;d6 shows one signal at δ=9.48&#;ppm for the proton attached to the carbon atom of the tetrazole ring. The 13C NMR of compound 1 in DMSO&#;d6 shows one signal at δ=137.7&#;ppm, which is shifted upfield compared to the starting material 5&#;ATO (δ=150.5&#;ppm). Deprotonation of 1 leads to a shift in the signals observed in the 1H as well as 13C NMR spectra. For example, the 1H NMR of potassium 1&#;oxido&#;5H&#;tetrazolate (3) in D2O shows one signal at δ=8.54&#;ppm, which is shifted upfield compared to the free acid 1. Additionally, the signal in the 13C NMR of 3 in D2O is shifted downfield to δ=164.5&#;ppm. The 13C NMR of compound 8 in DMSO&#;d6 shows one signal at δ=150.0 attributing both carbon atoms (Figure&#; ). Compared to the parent molecule, 5&#;ATO (δ=150.5&#;ppm), the carbon atoms of 8 are, contrary to 1, not drastically shifted. Deprotonation of 8 results in a downfield shift of the signal. The 13C NMR of the tripotassium salt (11) in D2O shows one signal at δ=153.2&#;ppm. The influence of deprotonation of 8 on the 13C NMR signal is not as prominent as that observed for compound 1.

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Physicochemical properties

Thermal behavior. The thermal behaviors of compounds 1&#;7 are shown in Figure&#; . Compound 1 shows a decomposition temperature of 186&#;°C (Figure&#; ), and two endothermic events at 80&#;°C and 96&#;°C, which TGA indicates as corresponding to a phase transition and/or melting point. Compound 1, 1&#;hydroxy&#;5H&#;tetrazole, exhibits a higher thermal stability compared to the similar compound 5&#;amino&#;1&#;hydroxytetrazole (105&#;°C), [17] and is almost identical with that of 1&#;aminotetrazole (182&#;°C). [23] Compound 2 shows loss of water at 110&#;°C and a decomposition temperature of 273&#;°C, which is the highest of all of the 1&#;hydroxy&#;5H&#;tetrazole salts reported in this work (Figure&#; ). The potassium (3) and the silver (4) salts, which are both free of water, show decomposition temperatures of 236&#;°C and 211&#;°C, respectively. Both of these salts detonate violently on reaching their critical temperatures. The silver salt (4) also immediately detonates on contact with a flame. Compound 5 shows an endothermic event at 180&#;°C, which corresponds to the loss of ammonia due to evaporation, as indicated by the onset of mass loss in the TG. The endothermic event seamlessly evolves into an exothermic decomposition at 188&#;°C, at which temperature a significant mass loss of 85.5&#;wt.% occurs as shown by the TG (Figure&#; ). The same behavior is observed for the hydroxylammonium salt 6. Figure&#; shows an endothermic event at 115&#;°C, which evolves into the first of two exothermic decomposition events at 159&#;°C. TGA measurements revealed the first decomposition to correspond to the decomposition of hydroxylamine accompanied by a weight loss of 24.9&#;wt.%. The second exothermic event with onset at 203&#;°C is attributed to the decomposition of the residual 1&#;hydroxy&#;5H&#;tetrazole.

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Compound 7 shows an exothermic decomposition temperature of 213&#;°C, with two endothermic events in addition observed at 80&#;°C and 103&#;°C. The TG measurement of 7 shows a mass loss of 13&#;wt.% with onset at 103&#;°C, which corresponds to the evaporation of hydrazine. Therefore, it can be assumed that the exothermic event actually corresponds to thermal decomposition of residual neutral compound 1, rather than the hydrazinium salt itself. Figure&#; shows that bis(1&#;hydroxytetrazol&#;5&#;yl)triazene monohydrate (8) is thermally stable up to 95&#;°C. Despite being a monohydrate, no endothermic event which could be attributed to a loss of water was observed in the DTA spectrum of 8. Due to stabilization by crystal water, 8 decomposes immediately on reaching a temperature (95&#;°C) high enough to remove the crystal water molecule. The alkaline salts 9&#;12 all show the presence of endothermic events between 85&#;135&#;°C corresponding to the loss of water in each compound (Figure&#; ). In general, these compounds are very thermally stable, with decomposition temperatures of between 292&#;°C (12) and 335&#;°C (10). Interestingly, the triguanidinium salt 13 shows no endothermic event in the DTA, which confirms loss of water already occurring at room temperature. This agrees with the measured elemental analysis, which fits perfectly with the values calculated for the anhydrous salt, which shows a thermal stability of up to 222&#;°C. The hydroxylammonium salt 14 shows an endothermic event at 104&#;°C which evolves into an exothermic decomposition, a behavior similar to that of compound 5 (Figure&#; ). This can be explained by the evaporation of hydroxylamine occurring, which results in the formation of an unstable residue that immediately decomposes at 148&#;°C. Interestingly, compound 15 shows no loss of water until the onset of decomposition at 198&#;°C, indicating strong coordinative bonds of the aqua ligands to the copper(II) cation as well as a high stability of the crystal water.

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Heats of formation, sensitivity, and detonation parameters. The calculated and measured explosive properties of all compounds are listed in Tables&#; and . Heat of formations were calculated by applying the atomization method using room temperature CBS&#;4&#;M enthalpies.[ 24 , 25 , 26 , 27 , 28 ] A detailed explanation of the calculation can be found in the Supporting Information. Compound 1 shows a comparable heat of formation (252&#;kJ&#;mol&#;1) to its parent molecule 5&#;amino&#;1&#;hydroxytetrazole (256&#;kJ&#;mol&#;1). However, 1 possesses a slightly lower density than its parent compound, which results in a minor reduction of the detonation velocity (&#;m&#;s&#;1) and detonation pressure (269&#;kbar) compared to 5&#;ATO (&#;m&#;s&#;1, 298 kbar). While the impact sensitivity of 1 (10&#;J) is comparable to that of 5&#;ATO (10&#;J), the friction sensitivity is higher for 1 (28&#;N) than for 5&#;ATO (108&#;N), meaning 1 is more sensitive towards external stimuli than 5&#;ATO. In contrast, the hydroxylammonium (6) salt of 1&#;hydroxy&#;5H&#;tetrazole shows a higher heat of formation (259&#;kJ&#;mol&#;1 (6)) compared to the corresponding salt of 5&#;ATO (Hx+: 227&#;kJ&#;mol&#;1). Compound 6 shows a very high detonation velocity (&#;m&#;s&#;1) compared to compound 5 (&#;m&#;s&#;1), which is due to the higher density of 6 1.67&#;g&#;cm&#;3 (5: 1.40&#;g&#;cm&#;3). Although compound 7 shows a slightly lower density (1.60&#;g&#;cm&#;3) than compound 6, the significantly higher enthalpy of formation of 7 (346&#;kJ&#;mol&#;1) results in its even higher detonation velocity of &#;m&#;s&#;1. Compounds 6 and 7 both outperform even HMX in terms of detonation velocity. While compound 5 is completely insensitive (IS>40&#;J, FS>360&#;N), compounds 6 (IS=6&#;J, FS 240&#;N) and 7 (IS=26&#;J, FS>360&#;N) show higher impact sensitivities, but crucially, are comparable to (6) or less sensitive (7) than HMX. The monohydrate sodium salt (2) is completely insensitive towards external stimuli, whereas the potassium (3) salt is more sensitive (IS=4&#;J, FS=54&#;N). The silver salt (4) is the most sensitive out of all of the compounds, and has to be classified as extremely sensitive, with an impact sensitivity below 1&#;J and a friction sensitivity of 1&#;N. The silver salt shows the characteristics of a primary explosive, immediately detonating on contact with a flame (Figure&#; ).

Table 1

&#;

2&#;a

2&#;b

HMX[q]

Formula

CH2N4O

CH3N4O2Na

CHN4OK

CHN4OAg

CH5N5O

CH5N5O2

CH6N6O

C4H8N8O8

M [g&#;mol&#;1]

86.05

126.05

124.14

192.91

103.09

119.08

118.10

296.16

IS [J][a]

>40

7 [31]

FS [N][b]

>360

240

>360

112 [31]

ESD [mJ][c]

960

<0.28

740

200 [31]

ρ [g&#;cm&#;3][d]

1.63

1.81

1.78

1.88

3.47

1.40

1.67

1.60

1.91

N [%][e]

65.11

44.45

45.13

29.04

67.94

58.81

71.16

37.84

ΩCO [%][f]

&#;18.59

&#;12.36

&#;32.22

&#;8.29

&#;38.80

&#;20.15

&#;40.64

0.0

ΩCO2 [%][g]

&#;37.19

&#;25.39

&#;45.11

&#;16.59

&#;54.33

&#;33.59

&#;54.19

&#;21.61

Tendo [°C][h]

80, 96

110

&#;

180

115

80, 103

&#;

Texo [°C][i]

186

273

236

211

188

159, 203

213

275

ΔfH 0 [kJ&#;mol&#;1][j]

252

&#;366

&#;364

&#;

207

259

346

ΔfH 0 [kJ&#;kg&#;1][k]

&#;

441

&#;

369

explo5&#;v6.05.04

&#;

&#;ΔExU 0 [kJ&#;kg&#;1][l]

&#;

Tdet [K][m]

&#;

V0 [L&#;kg&#;1][n]

469

451

456

426

&#;

549

461

485

401

PCJ [kbar][o]

269

189

182

187

&#;

213

331

319

378

Vdet [m&#;s&#;1][p]

&#;

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Table 2

&#;

Formula

C2H5N11O3

C2H12N11O8Li3

C2H12N11O8Na3

C2H6N11O5K3

C2H6N11O5Rb3

C5H18N20O2

C2H12N14O5

C2H10N12O5Cu

M [g&#;mol&#;1]

234.14

339.01

387.16

381.45

520.55

390.34

312.22

345.73

IS [J][a]

>40

FS [N][b]

>360

288

>360

128

288

ρ [g&#;cm&#;3][c]

1.80

1.67

1.71

1.97

2.55

1.59[p]

1.79[p]

1.96

N [%][d]

66.66

45.45

39.80

40.39

29.60

71.77

62.81

48.62

ΩCO [%][e]

&#;10.38

&#;7.08

&#;6.20

&#;25.17

&#;9.22

&#;49.19

&#;15.37

&#;13.88

ΩCO2 [%][f]

&#;24.23

&#;16.52

&#;14.46

&#;33.56

&#;15.37

&#;69.69

&#;25.62

&#;23.14

Tendo [°C][g]

&#;

122

135

99, 121

&#;

123

&#;

Texo [°C][h]

100

311

339

294

292

222

146

205

ΔfH° [kJ&#;mol&#;1][i]

734

&#;

158

464

&#;53

ΔfH° [kJ kg&#;1][j]

&#;

559

&#;38

explo5&#;v6.05.04

&#;

&#;ΔExU 0 [kJ&#;kg&#;1][k]

&#;

Tdet [K][l]

&#;

V0 [L&#;kg&#;1][m]

429

476

474

420

&#;

501

437

402

PCJ [kbar] [n]

362

213

173

143

&#;

201

364

249

Vdet[m&#;s &#;1 ] [o]

&#;

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Due to this behavior, the initiation capability of 4 towards Pentaerythrityltetranitrat (PETN) was tested, whereby it was found that 50&#;mg of 4 were able to initiate 200&#;mg of pressed PETN (Figure&#; ). Compound 2&#;a shows a low detonation velocity of &#;m&#;s&#;1, which is attributed to its low heat of formation as well as being a monohydrate. The orthorhombic form 2&#;b exhibits a slightly lower detonation velocity (&#;m&#;s&#;1) than triclinic 2&#;a, due to the lower density of 2&#;b. Interestingly, the potassium salt 3 exhibits an even lower detonation velocity than 2&#;a/b, even though it has a higher density (1.88&#;g&#;cm&#;3). Due to the high heat of formation of compound 8 (734&#;kJ&#;mol&#;1), it shows an outstanding detonation velocity of &#;m&#;s&#;1, as well as a very high detonation temperature (&#;K), which even outperforms HMX. However, in addition to its low thermal stability (decomposition temperature of only 100&#;°C), compound 8 is also very sensitive towards impact (<1&#;J) and friction (4&#;N), preventing application of 8 as a high&#;performance secondary explosive.

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Complete deprotonation of 8 forming the alkali salts 9 (Li+), 10 (Na+), and 11 (K+) results in a drastic decrease in energetic performance. This is due to the presence of a large amount of crystal water which results in highly exothermic enthalpy of formations (&#;&#;&#;&#;kJ&#;mol&#;1). Additionally, the absence of protons, not included as crystal water, leads to a decrease in detonation velocity within the series of increasing cation weight. Compound 13, crystallizes as a trihydrate, but completely loses its crystal water by drying in air at room temperature, as confirmed by elemental analysis as well as IR spectroscopy. Therefore, the heat of formation and detonation properties were calculated for the anhydrous compound, showing an endothermic heat of formation (158&#;kJ&#;mol&#;1). The rather low density for 13 of 1.59&#;g&#;cm&#;3, detonation velocity of &#;m&#;s&#;1 and detonation pressure of 201 kbar means it lies in the range of the other alkali salts. Compound 14 was obtained as an amorphous solid which is thermally stable up to 146&#;°C. It has an endothermic heat of formation of 464&#;kJ&#;mol&#;1 and density of 1.79&#;g&#;cm&#;3, with a calculated detonation velocity of &#;m&#;s&#;1, which surpasses that of HMX. Compound 14 has a calculated detonation pressure of 364&#;kbar as well as moderate sensitivities to external sources (IS=4&#;J, FS=128&#;N), which are comparable to those of HMX.

Identification of the Structure of Triethanolamine ...

Associated Data

Supplementary Materials

Supporting Information

OPEN-11-e-s001.pdf

(1.2M)

GUID: 3DF-EF84-4F56-A9F0-EF3E

Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request.

Abstract

Triethanolamine (TEOA) is one of the most commonly used sacrificial agents in photocatalysis. Due to its more complex structure compared to, for example, ethanol, and its sacrificial role in photocatalysis, it gives a mixture of products. The structures of these molecules are not usually analyzed. Herein, we obtain and isolate the products of TEOA and N&#;tert&#;butyl diethanolamine oxygenation under photocatalytic conditions with &#;15&#;% yield, and followingly characterized them by NMR and mass spectroscopy. The reaction is mediated by potassium poly(heptazine imide) (K&#;PHI) in the presence of O2 and affords formyl esters of β&#;hydroxyethylene formamides from the corresponding ethanolamines.

Keywords:

carbon nitride, formamide, oxygenation, photocatalysis, triethanolamine

Whatever happened to TEOA? Triethanolamine (TEOA) is one of the most commonly used electron donors in carbon nitride photocatalysis. Due to its &#;sacrificial&#; role and more complex structure compared to simple alcohols, TEOA produces a complex mixture of products. Photocatalytic oxygenation of TEOA with O2 from air, mediated by carbon nitride photocatalysis, gives di formyl ester of N,N&#;di(β&#;hydroxyethylene) formamide among others products.

Introduction

Photocatalysis facilitates chemical reactions through photoinduced electron transfer. Except for redox neutral reactions,[ 1 , 2 , 3 , 4 ] photocatalysis relies on sacrificial agents &#; organic and inorganic compounds as well as inorganic ions capable of accepting or donating electrons to the reaction mixture to enable a desired transformation. In net&#;oxidative reactions, O2, [5] S8,[ 6 , 7 ] S2O8 2&#;, [8] and Ag+, [9] for example, have been used to extract photogenerated electrons from the excited photocatalyst and therefore promote oxidation of a substrate by the hole residing in the valence band (VB). In net&#;reductive processes, compounds with low oxidation potential, such as alcohols, [9] tertiary amines [10] and biomass, [11] quench the photogenerated hole and therefore promote reduction of a substrate by the electron temporarily stored in the conduction band (CB). However, sacrificial agents also possess plenty of other often undesirable roles. [12] For example, one&#;electron oxidation and deprotonation of an aliphatic alcohol gives a C&#;centred radical. It is a strong reductant, which upon injection of an electron into the conduction band of a semiconductor photocatalyst alters its photophysical properties. On the other hand, it is also a strong oxidant, which, for instance, in the hydrogen evolution reaction, competes with H+ for photogenerated electrons. [13]

Carbon nitride materials are an emerging class of photocatalysts and have been applied in full water splitting, [15] as well as in proton reduction [16] and water oxidation half reactions, [17] and also in numerous organic reactions, [18] just to name a few. Among the sacrificial electron donors, triethanolamine (TEOA) is one of the most commonly used in carbon nitride photocatalysis. The scope of reactions where TEOA is employed ranges, from H2 production, [19] CO2 reduction to either HCOOH [20] or CO, [21] to the synthesis of cyclopentanoles [10] and γ,γ&#;dichloroketones from enones. [22] The last two reactions are mediated by potassium poly(heptazine imide) (K&#;PHI, Figure&#; a), which is also used in this work.

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Due to its low oxidation potential of +0.64&#;V vs. SCE in CH3CN [10] and its more complex structure compared to simple aliphatic alcohols, such as ethanol and benzylalcohol (which, under anaerobic conditions, give aldehydes [23] and, in the presence of O2, aldehydes and carboxylic acids [24] ) oxidation of TEOA produces a greater variety of products. The pathway of TEOA oxidation depends on the conditions. Similar to aliphatic alcohols in non&#;aqueous anaerobic media, it was suggested that the product of TEOA oxidation is an aldehyde (Figure&#; b, aldehyde path).[10],[14] In aqueous media, typically in the presence of O2, TEOA forms an iminium cation which, upon hydrolysis, accompanied by the N&#;C bond cleavage gives diethanolamine and glycolaldehyde. [14] Nevertheless, the &#;aldehyde pathway&#; could be complicated by formation of an iminium cation which, followed by intramolecular attack of a hydroxyl group, gives oxazolidine. [10]

Ghosh et&#;al. synthesized N,N&#;dialkylformamides from trialkylamines &#; compounds structurally similar to TEOA, using Eosin Y as a homogeneous photocatalyst and O2 from air. [25] Non&#;photocatalytic oxygenation of tertiary aliphatic amines with O2 accompanied by C&#;C bond cleavage applied to the synthesis of N,N&#;dialkylformamides was reported by Li et&#;al. [26]

The products of TEOA oxidation might react with substrates or target compounds and decrease the selectivity of a photocatalytic process. Therefore, knowing the structure of TEOA oxidation products is essential for designing photocatalytic reactions and identifying those in which using TEOA would be detrimental.

Herein, we study oxygenation of TEOA and N&#;tert&#;butyl diethanolamine by K&#;PHI in the presence of O2 as the terminal oxidant. We isolate formyl esters of the corresponding N,N&#;di(β&#;hydroxyethylene) formamides (Figure&#; b) and confirm their chemical structures by 1H and 13C&#;NMR spectroscopy and mass spectrometry.

Results and Discussion

A mixture of TEOA (0.05&#;mmol), K&#;PHI (5&#;mg) in CH3CN (2&#;mL) was stirred under an air atmosphere (O2 20 vol.%) and illumination with 465&#;nm photons for 24&#;h. After solvent concentration in vacuum, 1H&#;NMR spectroscopy of the reaction mixture in CDCl3 revealed a complex composition, while compound 2&#; a was the major product (15&#;%, Scheme&#; ). Upon reaction mixture work&#;up and purification, 2&#; a was isolated, and the identity of its chemical structure was unambiguously confirmed by counter synthesis, namely by reacting diethylamine with formic acid (see Experimental Section for details). Similar to N,N&#;dimethylformamide, due to the high rotational barrier around the R2N&#;C(O)H bond, the 1H&#;NMR spectrum of 2&#; a shows four methylene groups appearing as four triplets. Under the same conditions, commercial N&#;tert&#;butyl diethanolamine gave formamide 2&#; b with a comparable 15&#;% yield. As reported earlier, ( n C6H13)3N and ( n C8H17)3N gave N,N&#;dialkylformamides with relatively low 35&#;38&#;% yields, [25] which points at the high reactivity of aliphatic amines and their &#;sacrificial&#; role in photocatalysis. Therefore, using ethanolamines as substrates in photocatalysis remains a challenge to selectively access products of their oxygenation and formylation.

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Substituted pentane&#;1,5&#;diol 3 that lacks a tertiary amine moiety gave lactone 4, while aldehyde 5 was not formed (Scheme&#; ). These results clearly indicate that, when the substrate lacks a NR3 moiety, as expected, the hydroxyl group is oxidized at first instance, followed by nucleophilic attack of the adjacent hydroxyl group and dehydrogenation of hemiacetal to lactone, which is likely to be a photocatalytic process as well. [5]

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A tentative mechanism of TEOA oxygenation&#;formylation is shown in Figure&#; . Overall, our results and those reported earlier by König et&#;al. confirm that oxidation of TEOA and N&#;tert&#;butyl diethanolamine proceeds at the nitrogen lone pair rather than at the primary alcohol. [25] Given that the oxidation potential of aliphatic tertiary amines is in general lower (E ox(TEOA)=+0.64&#;V vs. SCE in CH3CN [10] ) than that of aliphatic alcohols (E ox(PhCH2OH) >+2.2&#;V vs. SCE in CH3CN [27] ), single electron transfer (SET) from the nitrogen lone pair with the formation of intermediate I is thermodynamically more feasible. Earlier reports indicate that the rate of hole quenching in K&#;PHI and related semiconductors by 4&#;methylbenzyl alcohol is 1.43×107&#;M&#;1&#;s&#;1. [23] The relatively low CB potential in K&#;PHI (&#;0.75&#;V vs. SCE [28] ) is at least partially, responsible for the thermodynamic stability of K&#;PHI radical anion.[ 29 , 30 , 31 ] Therefore, reductive quenching of the K&#;PHI excited state, similar to earlier reports, [32] rather than oxidative quenching might be operative in the present case as well. Similar to the cross&#;dehydrogenative coupling of tetrahydroisoquinolines with various nucleophiles employing O2 as the electron acceptor, [33] intermediate I is first converted into the iminium cation II via hydrogen atom transfer (HAT) to O2 .&#;. Formation of intermediate III is suggested based on oxygenation of the benzylic position in tetrahydroisoquinolines with O2. [34] Therein, however, lactams are obtained with high selectivity upon elimination of H2O from organic hydroperoxide III, which is likely due to the stabilizing effect of the aromatic ring. Ethanolamines 1 are missing any conjugation, which could stabilize the intermediate IV. As such, cleavage of the C&#;C bond in a β&#;position of the peroxide IV moiety leads to the intermediate V. Due to the low O&#;O bond dissociation free energy (BDFE) in organic peroxides (&#;44&#;kcal&#;mol&#;1 [35] ) such as in tentative intermediate III, its cleavage produces a highly reactive hydroxyl radical that likely to enable side reactions and therefore reduces selectivity towards 2. Intermediate V reacts with one, in case of 1&#; b, or two equivalents of formic acid, in case of 1&#; a, and produces 2. The mismatch between the number of HCOOH molecules produced upon C&#;C bond cleavage in a substrate (one molecule) and a number of HCOOH molecules required to convert V derived from TEOA into ester 2&#; a (two molecules) might be an additional reason for the low selectivity toward 2&#; a. In the 1H&#;NMR spectrum of the crude reaction mixture, we observed several peaks at &#;8.03&#;ppm, which could be assigned to formic acid.

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Conclusion

Despite the presence of few electron&#;rich sites in triethanolamine, that is, the NR3 moiety and three primary alcohols, the initial SET from triethanolamine to the photoexcited K&#;PHI takes place at the nitrogen atom with subsequent formation of the iminium cation. The generality of this pathway was illustrated for two substrates, triethanolamine and N&#;tert&#;butyl diethanolamine. The products of their oxygenation are formyl esters of the corresponding β&#;hydroxyethylene formamides.

Experimental Section

Chemicals. Diethanolamine (&#;99&#;%) was purchased from Fluka. Triethanolamine (&#;99&#;%), 5&#;aminotetrazole monohydrate (97&#;%), and formic acid (&#;95&#;%) were purchased from Sigma&#;Aldrich. CH3CN (hypergrade for LC&#;MS) was purchased from Merck. LiCl (&#;99&#;%) and KCl (&#;99.5&#;%) were purchased from Carl Roth. N&#;tert&#;butyl diethanolamine (>97&#;%) was purchased from TCI. All chemicals were used as received.

Methods. 1H and 13C&#;NMR spectra were recorded on an Agilent 400&#;MHz (at 400&#;MHz for Protons and 101&#;MHz for Carbon&#;13) NMR spectrometer. Chemical shifts are reported in ppm downfield from the CHCl3 residual peak: 7.26&#;ppm in 1H&#;NMR and 77.2&#;ppm in 13C&#;NMR. An Agilent Network GC System coupled with Agilent Inert Mass Selective detector (electron ionization) was used for reaction mixture composition analysis and to obtain mass spectra of the products.

Potassium poly (heptazine imide) (K&#;PHI) was synthesized according to the previously described procedure with some adaptation. [10] A mixture of lithium chloride (3.71&#;g), potassium chloride (4.54&#;g) and 5&#;aminotetrazole monohydrate (1.65&#;g) was ground in a ball mill for 5&#;min at a shaking rate of 25&#;s&#;1. The reaction mixture was transferred into a porcelain crucible and covered with a lid. The crucible was placed in the oven and heated under constant nitrogen flow (15&#;L&#;min&#;1) and atmospheric pressure at the following temperature regime: heating from room temperature to 550&#;°C within 4&#;h, annealing at 550&#;°C for 4&#;h. After completion of the heating program, the crucible was allowed to cool slowly to room temperature under nitrogen flow. The crude product was removed from the crucible, washed with deionized water (100&#;mL) for 3&#;h in order to remove salts, and separated by centrifugation (&#;rpm, 10&#;min). The solid was redispersed in deionized water (1.5&#;mL) and separated by centrifugation (&#;rpm, 5&#;min). Redispersion&#;centrifugation was repeated 3&#;times in total. The solid was dried in a vacuum oven (20&#;mbar) at 65&#;°C overnight.

Photocatalytic oxidation of ethanolamines

A mixture of ethanolamine (0.05&#;mmol) and K&#;PHI (5&#;mg) in CH3CN (2&#;mL) was loaded into 4&#;mL vial and sealed with a rubber septum. A balloon filled with air (O2 20&#;vol.%) was connected, via a needle, to the reaction mixture head space through the septum. The reaction mixture was stirred under illumination with a 465&#;nm LED (electrical power 50&#;W) for 24&#;h. K&#;PHI was separated by centrifugation. The solution was concentrated in vacuum (50&#;°C, 150&#;mbar), the residue was dissolved in CDCl3 and analyzed by NMR spectroscopy. For product separation, CDCl3 was concentrated in vacuum (50&#;°C, 150&#;mbar), followed by purification by HPLC (JASCO LC&#; series equipped with CAPCELL PAK MG II C18 20&#;mm l. D.x250&#;mm column, CH3CN 10&#;mL&#;min&#;1).

(Formylazanediyl)bis(ethane&#;2,1&#;diyl) diformate 2&#;a

1H&#;NMR (400&#;MHz, CDCl3) δ 8.08 (s, 1H), 8.05 (s, 1H), 8.04 (s, 1H), 4.32 (t, J=5.4&#;Hz, 2H), 4.28 (t, J=5.3&#;Hz, 2H), 3.64 (t, J=5.5&#;Hz, 2H), 3.60 (t, J=5.3&#;Hz, 2H). 13C&#;NMR (101&#;MHz, CDCl3) δ 163.5, 160.7, 160.5, 61.3, 60.7, 47.0, 41.9. MS (70&#;eV): m/z (%): 160.0 (2) [M&#;C(O)H]+, 143.1 (100) [M&#;C(O)OH]+. Molecular ion not observed.

2&#;(N&#;tert&#;butylformamido)ethyl formate 2&#;b

1H&#;NMR (400&#;MHz, CDCl3) δ 8.47 (s, 1H), 8.06 (s, 1H), 4.30 (t, J=6.6&#;Hz, 2H), 3.58 (t, J=6.6&#;Hz, 2H), 1.39 (s, 9H). 13C&#;NMR (101&#;MHz, CDCl3) δ 162.4, 161.0, 61.5, 55.7, 39.8, 29.8. MS (70&#;eV): m/z (%): 173.1 (69) [M]+, 158.1 (100) [M&#;CH3]+.

Synthesis of (formylazanediyl)bis(ethane&#;2,1&#;diyl) diformate 2&#;a from diethanolamine

A solution of diethanolamine (0.35&#;g, 3&#;mmol) in formic acid (3&#;mL) was stirred at reflux for 2&#;h. Formic acid was distilled off in vacuum (50&#;°C, 50&#;mbar). The residue was dissolved in CH2Cl2 (5&#;mL), washed with NaHCO3 solution. Organic phase was separated, dried over anhydrous Na2SO4, concentrated in vacuum. Yield: 0.51&#;g, 90&#;%. Colorless oil. 1H&#;NMR (400&#;MHz, CDCl3) δ 8.08 (s, 1H), 8.05 (s, 1H), 8.04 (s, 1H), 4.32 (t, J=5.4&#;Hz, 2H), 4.28 (t, J=5.3&#;Hz, 2H), 3.64 (t, J=5.4&#;Hz, 2H), 3.60 (t, J=5.3&#;Hz, 2H). 13C&#;NMR (101&#;MHz, CDCl3) δ 164.8, 160.9, 160.7, 61.1, 60.5, 47.4, 42.3.

Conflict of interest

A patent WO// has been filed by Max Planck Gesellschaft zur Förderung der Wissenschaften E.V. in which O.S. is listed as a co&#;author.

1.&#;

Supporting information

Supporting Information

Click here for additional data file.(1.2M, pdf)

Acknowledgements

O. S. and Y. Z. thank Max Planck Society, China Scholarship Council (CSC No. ) and Prof. Antonietti for financial support of the research.

Notes

O. Savateev, Y. Zou, ChemistryOpen , 11, e.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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