Synthesis and characterisation of 5, 5′-bistetrazolate salts with alkali metal, ammonium and imidazolium cations

Job/Unit: Z13150

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Date: 21-05-13 16:49:20

Pages: 14

ARTICLE

DOI: 10.1002/zaac.201300150

Synthesis and Characterisation of 5,5’-Bistetrazolate Salts with Alkali Metal,

Ammonium and Imidazolium Cations

Lars H. Finger,^[a]Fabian G. Schröder,^[a]and Jörg Sundermeyer*^[a]

Dedicated to Professor Heinrich Nöth on the Occasion of His 85th Birthday

Keywords: Alkali metals; 5,5’-Bistetrazolate salts; Crystal structures; Coordination chemistry; Ionic liquids

Abstract. We report on the synthesis and characterisation of the alkali

nium (8) 5,5’-bistetrazolate. Among the presented compounds are the

metal 5,5’-bistetrazolate (BT^2–) salts Li₂BT (1), Na₂BT (2), K₂BT (3), first reversibly melting 5,5’-bistetrazolate salts (10, 11) and also the

Rb₂BT (4) and Cs₂BT (5) as well as the mono ammonium (9) and the first ionic liquid on the basis of 5,5’-bistetrazole (11). All substances

mono (10) and bis(1-ethyl-3-methylimidazolium) (11) salts of 5,5’-

bistetrazole. The crystal structures of the hydrates of the alkali metal

salts (1h–4h) and hydrate free (9) to (11) are presented. Additionally

were investigated by NMR and IR spectroscopy, elemental analysis

and combined TGA/DSC measurements. Corresponding to our main

motive, the evaluation of the coordination behaviour of 5,5’-bistetra-

we present structures of the monocaesium 5,5’-bistetrazolate (6) and zole, we give a short summary of the metal 5,5’-bistetrazole salts struc-

alternative modifications of the known salts barium (7h) and diammo-

turally characterised so far.

Intrigued by the first detailed description of the rare earth

metal salts, we wanted to further investigate the coordination

behaviour of 5,5’-bistetrazole. We chose the alkali metal salts

as a first model system. Our special interest concerned the

question if a chelating coordination mode is dependent on the

cation size. In addition we specifically wanted to analyse the

sodium and the barium salt of 5,5’-bistetrazole which have

been utilised in the synthesis of the free acid and the rare earth

metal salts, respectively. During our literature research we

were surprised to find that the monoammonium salt of 5,5’-

bistetrazole is regarded a promising gas generating compound

used in airbag systems.^[5b]We wished to prepare and investi-

gate this compound to evaluate in which way it deviates from

similar substances. Finally we were interested if the 5,5’-biste-

trazolate anion can be employed in organic salts with weakly

coordinating cations. The specific aim was to prepare unusual

ionic compounds in which an isolated 5,5’-bistetrazolate di-

anion is present.

Introduction

The first synthesis of 5,5’-bistetrazole (H₂BT) was already

reported at the beginning of the 20th century.^[1]The potential

of this nitrogen-rich compound as an explosive was noticed

soon.^[2]Friederich developed a novel synthesis for the title

compound which no longer utilised handling of the highly haz-

ardous reactants cyanogen and hydrazoic acid in aqueous solu-

tions.^[3]This procedure was improved and simplified by Nel-

son et al.^[4]Whereas 5,5’-bistetrazole and its derivatives have

been intensively studied regarding their energetic properties

and numerous patents claim their use in explosive composi-

tions,^[5]very little is known about the coordination behaviour

of this intriguing ligand. Steel et al. were the first to publish

three transition metal complexes in 1995 and presented also

the crystal structure of the free acid.^[6]The group of Klein

prepared and characterised several rare earth metal salts,^[7]

Klapötke et al. presented a series of alkaline earth metal salts^[8]

and the first structurally characterised transition metal salt.^[9]

Combinations of 5,5’-bistetrazole with nitrogen-rich cations

such as ammonium, hydrazinium, guanidinium and related

have been investigated most recently by Klapötke et al. and

also Shreeve et al.^[10]

We wish to report herein on the synthesis and characterisa-

tion including single crystal XRD structure determination of a

series of alkali metal salts, the monoammonium salt and two

1-ethyl-3-methylimidazolium salts of 5,5’-bistetrazole. Ad-

ditionally we want to present alternative crystal structure modi-

fications of the previously described barium and diammonium

5,5’-bistetrazolate.

* Prof. Dr. J. Sundermeyer

Fax: +49-64-21-2825711

E-Mail: jsu@chemie.uni-marburg.de

[a] Fachbereich Chemie

Results and Discussion

Philipps-Universität Marburg

Hans-Meerwein-Str. 4

Synthesis

35032 Marburg, Germany

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/zaac.201300150 or from the au-

thor.

The high acidity of 5,5’-bistetrazole^[11]allows the prepara-

tion of alkali metal salts starting from the respective metal

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L. H. Finger, F. G. Schröder, J. Sundermeyer

ARTICLE

hydroxide or carbonate. This route was also employed for the

barium salt. The dilithium (1h) and dirubidium (4h) salts crys-

tallised from hot water with two, the disodium (2h) and barium

(7h) salts with five water molecules per molecular unit

(Scheme 1) and were characterised by XRD. The dipotassium

(3) and dicaesium (5) salts usually crystallised without hy-

drating water molecules in the structure. Both compounds have

not afforded publishable structure solutions so far, though. In-

stead the single crystal structures of dipotassium 5,5’-bistetra-

zolate dihydrate (3h) and monocaesium 5,5’-bistetrazolate

(CsHBT, 6) are presented. In each case also the water-free

phase was prepared and investigated. Water-free Li₂BT is

highly hygroscopic.

Scheme 2. Synthesis of monoammonium (9) and mono (10) and di

(11) (1-ethyl-3-methyl-imidazolium) 5,5Ј-bistetrazolate.

SIR2004^[15]or SUPERFLIP,^[16]refined with SHELXL-97^[17]

and finally validated using PLATON^[18]software, all within

the WinGX^[19]software bundle. Absorption corrections where

either applied within WinGX (multi-scan^[20]or gaussian^[21]) or

beforehand within the APEX2 software (multi-scan).^[22]

Graphic representations were created using Diamond 3.^[23]C-

bound hydrogen atoms were constrained to parent site; hydro-

gen atoms on heteroatoms were located in the Fourier map

and refined independently. In all graphics the displacement el-

lipsoids are shown for the 50% probability level, hydrogen

atoms are shown with arbitrary radius. Selected crystal data

and experiment parameters are listed in Table 1 and Table 2.

Crystallographic data (excluding structure factors) for the

structures reported in this paper have been deposited with the

Cambridge Crystallographic Data Centre as supplementary

publication no. CCDC-927399, -927400, -927401, -927402,

-927403, -927404, -927405, -927406, -927407, -927408. Cop-

ies of the data can be obtained free of charge on application

to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax:

+44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk; http://

www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi].

Scheme 1. Synthesis of alkali metal 5,5Ј-bistetrazolates (3) and (5),

the hydrates (1h), (2h) and (4h) and barium 5,5Ј-bistetrazolate penta-

hydrate (7h).

Monoammonium 5,5’-bistetrazolate ((NH₄)HBT, 9) was

prepared by reacting equimolar amounts of 5,5’-bistetrazole

and diammonium 5,5’-bistetrazolate ((NH₄)₂BT, 8). To our

surprise the crystallisation of this mixture by cooling a boiling

aqueous solution to room temperature yielded again (NH₄)₂BT

(8) as evidenced by ¹³C-NMR spectroscopy, elemental analysis

and XRD unit cell determination. Only if the remaining solu-

tion is decanted and cooled further to 4 °C, the monoammon-

ium 5,5’-bistetrazolate crystallises.

Mono(1-ethyl-3-methylimidazolium)

5,5’-bistetrazolate

((EMIM)HBT, 10) was prepared by combining barium 5,5’-

bistetrazolate and 1-ethyl-3-methylimidazolium bisulfate in

water. Bis(1-ethyl-3-methylimidazolium) 5,5’-bistetrazolate

(EMIM₂BT, 11) was synthesised by reacting 5,5’-bistetrazole

with EMIM carbonate in dry methanol (Scheme 2). The prod-

uct EMIM₂BT is again highly hygroscopic.

Dilithium 5,5’-bistetrazolate dihydrate (Li₂BT·2H₂O, 1h)

¯

crystallised in the triclinic space group P1 with a single mole-

cule in the unit cell and half a molecule in the asymmetric

unit. The bond lengths in the tetrazole moiety are in the same

range as values which have been found in the free 5,5’-biste-

Molecular Structures

The data collection for the single crystal structure determi- trazole acid^[6b]and published 5,5’-bistetrazolate salts.^[7–10]

nations was performed with a Stoe IPDS-II or a Bruker

The N2–N3 distance is generally the shortest whereas N1–

D8 QUEST diffractometer by the X-ray service department of N2 and N3–N4 are the longest bonds. The observed bond

the Fachbereich Chemie, University of Marburg. The Stoe lengths lie between typical values for N–N single (1.366 Å)

IPDS-II device is equipped with a Mo-K_αX-ray source (λ = and N=N double (1.300 Å) bonds in heterocyclic aromatic sys-

0.71073 Å), graphite mono-chromator and active imaging tems. The observed C–N distances show evenly good agree-

plate. Stoe IPDS software (X-AREA) was used for data collec- ment with bond lengths in common aromatic heterocycles

tion, cell refinement and data reduction, respectively.^[12]The (1.331 Å (pyrazole), 1.338 Å (pyridine)).^[24]A tabular com-

D8-QUEST is equipped with a Mo-K_αX-ray micro source parison of bond lengths and angles in the bistetrazole moiety of

(Incotec), a fixed chi goniometer and a PHOTON 100 CMOS all presented structures is given in the Supporting Information.

detector. Bruker software (Bruker Instrument Service, APEX2,

The bistetrazole moiety in structure 1h is perfectly planar

SAINT) was used for data collection, cell refinement and data and builds almost planar chains in which bistetrazolate and

reduction.^[13]The structures were solved with SIR-97,^[14]lithium ions are alternating (Figure 1). The lithium atoms are

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5,5’-Bistetrazolate Salts with Alkali Metal, Ammonium and Imidazolium Cations

Table 1. Selected crystallographic data for compounds 1h to 4h and 6.

Li₂BT·2H₂O (1h)

Na₂BT·5H₂O (2h)

K₂BT·2H₂O (3h)

Rb₂BT·2H₂O (4h)

CsHBT (6)

Formula

C₂H₄Li₂N₈O₂

186.01

C₂H₁₀N₈Na₂O₅

272.16

C₂H₄K₂N₈O₂

250.33

Monoclinic

P2₁/c

colourless block

0.21 ϫ 0.09 ϫ 0.09

3.8505(4)

11.3156(12)

9.7354(10)

90

93.006(9)

90

423.59(8)

2

1.963

11.08

C₂H₄N₈O₂Rb₂

343.08

Monoclinic

P2₁/c

colourless block

0.33 ϫ 0.13 ϫ 0.09

4.0884(2)

11.3281(7)

9.9483(6)

90

93.510(5)

90

459.88(5)

2

2.478

106.32

C₂H₁Cs₁N₈

270.02

Monoclinic

Pc

colourless block

0.32 ϫ 0.12 ϫ 0.05

4.6222(4)

8.6949(6)

16.2896(17)

90

94.123(8)

90

652.98(10)

4

2.747

56.11

FW /g·mol^–1

Crystal system

Space group

Colour, habit

Triclinic

¯

P1

colourless block

0.83 ϫ 0.61 ϫ 0.41

7.2089(6)

7.4623(6)

10.7752(8)

81.333(3)

76.478(3)

67.604(3)

519.85(7)

2

Crystal size /mm 0.13 ϫ 0.08 ϫ 0.07

a /Å

b /Å

c /Å

α /°

β /°

3.3677(8)

7.3623(2)

7.5007(2)

82.897(18)

87.120(19)

80.761(18)

182.06(4)

1

1.679

1.38

94

100(2)

γ /°

V /Å³

Z

D_calc/g·cm^–3

μ /cm^–1

F(000)

T / K

1.739

2.23

280

100(2)

252

100(2)

324

100(2)

496

100(2)

θ range /°

h, k, l range

Refl. Coll.

Refl. Indep.

Refl. I Ͼ 2σ(I)

2.74 : 26.69

–4:4, –9:9, –9:9

1476

762

602

2.96 : 27.17

–9:8, –9:9, –13:13

6166

2310

1952

2.76 : 26.72

–4:4, –12:14, –12:12

2267

893

778

2.73 : 26.72

–5:5, –14:14, –12:12

5725

973

888

2.34 : 26.67

–5:5, –10:10, –18:20

3188

2313

2152

Data/ rest./ param. 762 / 2 / 72

2310 / 1 / 194

0.0616

0.0532

0.1386

1.047

893 / 0 / 72

0.0244

0.0224

0.0546

0.987

973 / 0 / 72

0.0575

0.0210

0.0492

1.047

2313 / 4 / 209

0.0473

0.0266

0.0629

0.999

R_int

0.0287

0.0349

0.0872

0.968

R₁(obs)

wR₂(all)

GooF (F²)

CCDC

927399

927400

927401

927402

927403

Table 2. Selected crystallographic data for compounds 7h and 8 to 11.

BaBT·5H₂O (7h)

(NH₄)₂BT (8)

(NH₄)HBT (9)

(EMIM)HBT (10)

(EMIM)₂BT (11)

Formula

C₂H₁₀BaN₈O₅

363.52

C₂H₈N₁₀

172.18

Orthorhombic

Pban

colourless block

0.18 ϫ 0.16 ϫ 0.14

9.1301(10)

10.7631(13)

3.6660(6)

90

360.25(8)

2

1.587

C₂H₅N₉

155.15

Monoclinic

P2₁/c

colourless needle

0.66 ϫ 0.10 ϫ 0.07

4.5136(6)

8.4692(12)

15.522(2)

90

94.320(6)

90

591.67(14)

4

1.742

1.37

C₈H₁₂N₁₀

248.28

Monoclinic

P2₁/n

colourless block

0.17 ϫ 0.15 ϫ 0.12

4.5978(3)

18.9191(11)

13.0052(7)

90

94.586(4)

90

1127.65(12)

4

1.462

1.05

C₁₄H₂₂N₁₂

358.44

Monoclinic

P2₁/n

colourless block

0.27 ϫ 0.23 ϫ 0.16

9.8020(6)

7.9782(6)

12.0450(8)

90

108.716(5)

90

892.13(10)

2

1,334

0.92

FW /g·mol^–1

Crystal system

Space group

Colour, habit

Triclinic

¯

P1

colourless block

Crystal size /mm 0.21 ϫ 0.13 ϫ 0.08

a /Å

b /Å

c /Å

α /°

β /°

7.4404(7)

8.1615(7)

9.2594(9)

88.638(8)

73.671(7)

88.933(7)

539.40(9)

2

2.238

37.07

348

100(2)

γ /°

V /Å³

Z

D_calc/g·cm^–3

μ /cm^–1

F(000)

T / K

1.24

180

100(2)

320

100(2)

520

100(2)

380

100(2)

θ range /°

h, k, l range

Refl. Coll.

Refl. Indep.

Refl. I Ͼ 2σ(I)

2.29 : 26.68

–9:9, –10:10, –11:11

6259

2289

2073

2.93 : 26.70

–10:11, –13:13, –4:4

2935

392

319

2.63 : 27.08

–5:4, –10:10, –19:19

3538

1302

1025

1.90 : 26.71

–5:5, –23:20, –16:14

7731

2383

1470

2.34 : 26.72

–10:12, –12:10, –15:15

7475

1882

1355

Data/ rest./ param. 2289 / 12 / 185

392 / 0 / 37

0.0382

0.0273

0.0661

0.939

1302 / 0 / 124

0.0448

0.0517

0.0970

1.211

2383 / 0 / 169

0.0528

0.0337

0.0696

0.781

1882 / 0 / 120

0.0429

0.0339

0.0843

0.893

R_int

0.0471

0.0226

0.0547

0.989

R₁(obs)

wR₂(all)

GooF (F²)

CCDC

927404

927405

927406

927407

927408

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coordinated in a tetrahedral fashion by two oxygen and two

nitrogen atoms with bond angles ranging from 104.1(1)° to

115.5(1)°. In the crystal structure the chains are stacked and

connected over lithium oxygen bonds, the stacks (Figure 2) are

held together by strong hydrogen bonds (O1^IV···N3:

2.785(2) Å).

Figure 3. Dimeric units in disodium 5,5Ј-bistetrazolate pentahydrate

(2h); selected bond lengths /Å and angles /°: C1–C2^I: 1.463(2), C1–

N1: 1.341(2), N1–N2: 1.349(2), N2–N3: 1.320(2), N3–N4: 1.354(2),

C1–N4: 1.338(2), C2–N5: 1.342(2), N5–N6: 1.351(2), N6–N7:

1.315(2), N7–N8: 1.346(2), C2–N8: 1.338(2), N1–Na1: 2.408(2), N8–

Na1: 2.450(2), N7–Na2: 2.556(2), O1–Na1: 2.377(2), O4–Na1:

2.473(2), O5–Na1: 2.351(2), O5^I–Na1: 2.380(2), O1–Na2: 2.345(2),

O2–Na2: 2.355(2), O3–Na2: 2.357(2), O3^II–Na2: 2.524(2), O4–Na2:

2.407(2), O3^II···N2: 2.968(2), N1–Na1–N8: 178.0(1), N7–Na2–O3^II:

I

II

147.0(1); symmetry operations: = 1–x, –y, 1–z, = –x, –y, 2–z.

Figure 1. Chain structure in dilithium 5,5Ј-bistetrazolate dihydrate

(1h); selected bond lengths /Å and angles /°: C1–C1^I: 1.468(3), C1–

N1: 1.335(2), N1–N2: 1.347(2), N2–N3: 1.316(2), N3–N4: 1.345(2),

C1–N4: 1.335(2), N1–Li1^II: 2.060(3), N2–Li1: 2.034(3), O1–Li1:

1.991(3), O1^III–Li1: 2.012(3), O1^II···N4^I: 2.786(2), O1^IV···N3:

2.785(2), O1–Li1–N2: 115.5(1), N2–Li1–N1^II: 111.0(1), O1–Li1–

O1^III: 114.6(2), N1^II–Li1–O1^IV: 104.1(1); symmetry operations: ^I= 1 –

II

III

IV

x, –y, 2 –z, = 1 –x, 1 –y, 2 –z, = –1+x, y, z, = 2 –x, 1 –y, 1 –z.

Figure 4. Stacked structure of dimeric units in Na₂BT·5H₂O (2h), hy-

¯

drogen atoms are omitted for clarity; viewing direction [120].

The potassium salt exhibits a special behaviour concerning

crystallisation. In multiple attempts the water-free substance

could be crystallised. This could be proven by IR spectroscopy,

elemental analysis and XRD single crystal analysis. The re-

sulting structure solution clearly evidences the water-free

phase but does not allow any discussion of bond lengths due

to bad crystal quality. In one instance the chosen crystal turned

out to be a dihydrate of the respective salt whose solution is

presented here. Dipotassium 5,5’-bistetrazolate dihydrate (3h)

crystallised in the space group P2₁/c with four asymmetric

units consisting of half a molecule per unit cell. The potassium

atom is coordinated by five nitrogen and three oxygen atoms

forming a doubly capped trigonal prism (Figure 5).

In one case the potassium atom is coordinated in a distorted

chelating fashion (between N4^IIIand N1^V) with an angle of

57.24(3)° at the central atom. The potassium atom is not in

plane with the chelating NCCN fragment, though. Potassium

nitrogen distances range from 2.916(1) Å (N2^II) to 2.977(1) Å

(N2) and are in good agreement with distances found in the

potassium tetrazolate.^[25]The K1–N1^Vdistance (3.211(1) Å) is

considerably longer than typical K–N bonds but can still be

regarded as a coordinating bond. Similar distances have been

Figure 2. Chain stacking in the crystal structure of dilithium 5,5Ј-bis-

tetrazolate dihydrate (1h); view along the b axis.

¯

Na₂BT·5H₂O (2h) crystallised in the space group P1 with

two molecular units per elementary cell and one molecule per

asymmetric unit. The sodium atoms are six fold coordinated

in an octahedral fashion. Na1 is surrounded by two nitrogen

and four oxygen atoms, Na2 by one nitrogen atom and five

oxygen atoms, the coordination polyhedron around Na2 is con-

siderably distorted. Sodium nitrogen distances range from

2.408(2) Å (N1–Na1) to 2.556(2) Å (N7–Na2), sodium oxygen

bond lengths lie between 2.345(2) Å (O1–Na2) and 2.473(2) Å

(O4–Na1). Dimeric entities prevail in which two bistetrazolate

moieties are linked by two sodium atoms (Figure 3). When

viewing along the line which is connecting the dimers (direc-

¯

tion [120]) a stacked structure appears (Figure 4).

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5,5’-Bistetrazolate Salts with Alkali Metal, Ammonium and Imidazolium Cations

Figure 7. Coordination of the rubidium atoms in dirubidium 5,5Ј-biste-

trazolate dihydrate (4h); selected bond lengths /Å and angles /°: C1–

C1^I: 1.457(4), C1–N1: 1.340(3), N1–N2: 1.347(3), N2–N3: 1.317(3),

N3–N4: 1.346(2), C1–N4: 1.340(3), Rb1–N2: 3.101(2), Rb1–N2^II:

3.046(2), Rb1–N4^III: 3.035(2), Rb1–N4^IV: 3.087(2), Rb1–N1^V:

Figure 5. Coordination of the potassium atoms in dipotassium 5,5Ј-

bistetrazolate dihydrate (3h); selected bond lengths /Å and angles /°:

C1–C1^I: 1.457(3), C1–N1: 1.335(2), N1–N2: 1.351(2), N2–N3:

1.315(2), N3–N4: 1.348(2), C1–N4: 1.341(2), K1–N2: 2.977(1), K1–

N2^II: 2.916(1), K1–N4^III: 2.922(1), K1–N4^IV: 2.949(1), K1–N1^V:

3.211(1), K1–O1: 2.760(1), K1–O1^VI: 2.892(1), K1–O1^VII: 2.860(1),

O1^VI···N3^II: 2.823(2), O1^VII···N1^V: 2.844(2), N4^III–K1–N1^V: 57.24(3);

3.238(2), Rb1–O1: 2.941(2), Rb1–O1^VI

: :

3.016(2), Rb1–O1^VII

2.997(2), O1^VI···N3^II: 2.844(3), O1^VII···N1^V: 2.872(3), N4^III–Rb1–N1^V:

I

II

III

55.63(5); symmetry operations: = –x, –y, –z, = 1+x, y, z, = 1+x,

IV

V

VI

0.5–y, 0.5+z,

= x, 0.5–y, 0.5+z, = –x, 0.5+y, 0.5–z,

= 1 –x,

I

II

III

VII

symmetry operations: = 1–x, 1–y, –z, = 1+x, y, z,

= x, 0.5–y,

1–y, –z,

= –1+x, y, z.

IV

V

VI

0.5+z, = 1–x, 0.5–y, 0.5+z, = 1–x, –0.5+y, 0.5–z, = 2–x, –y, –z,

VII

= –1+x, y, z.

the related salts of 1H-tetrazole^[25]and 5-aminotetrazole^[27]

with respect to alkali metal nitrogen and alkali metal oxygen

bond lengths.

observed in potassium phenanthroline complexes.^[26]The

packing in the crystal structure is dominated by a wave like

topology (viewing along the c axis) in which layers of 5,5’-

bistetrazolate dianions alternate with layers consisting of po-

tassium atoms and water molecules (Figure 6).

Figure 8. Wave-like structure in dirubidium 5,5Ј-bistetrazolate dihy-

drate (4h); hydrogen atoms are omitted for clarity.

Figure 6. Wave-like topology in dipotassium 5,5Ј-bistetrazolate dihy-

drate (3h); view along the c axis; hydrogen atoms are omitted for

clarity.

Dicaesium 5,5’-bistetrazolate could be crystallised from

aqueous solution in the form of colourless blocks. However,

XRD experiments led to several analogous data sets for which

Dirubidium 5,5’-bistetrazolate dihydrate (4h, Figure 7) crys- no reasonable solution could be generated so far. In one case

tallised in the monoclinic space group P2₁/c with four asym- instead of the dicaesium salt a crystal of monocaesium 5,5’-

metric units consisting of half a molecule per unit cell. It is bistetrazolate (6) was investigated. Compound 6 crystallised in

isostructural, although according to the XRD results enantio- the monoclinic space group Pc with two asymmetric entities

meric, to the potassium salt.

per unit cell (Figure 9). The asymmetric unit consists of two

The Rb–N distances range from 3.035(2) Å (Rb1–N4^III) to molecular units. PLATON suggests the space group P2₁/c on

3.238(2) Å (Rb1–N1^V) in the longest case. Rubidium oxygen the basis of an additional inversion centre. However, neither

distances deviate between 2.941(2) Å (Rb1–O1) and in P2₁/c nor in P2/c, a reasonable refinement is possible, and

3.016(2) Å (Rb–O1^VI). As in the potassium salt (3h) a wave- the expected extinction of reflexes for the 2₁axis is not pres-

like structure is visible, when viewing along the c axis (Fig- ent. The suggestion by PLATON has therefore to be attributed

ure 8). The tilting angle between the 5,5’-bistetrazolate moie- to a pseudo inversion centre. Additionally a partial mix of

ties is higher, though (59.1° in (4h) compared to 52.3° in (3h)). enantiomers is present in the structure; a TWIN refinement has

The presented structures show an overall good agreement with been applied resulting in a freely refined BASF value of

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0.3935. The asymmetric unit consists of two caesium atoms

and two 5,5’-bistetrazole moieties with a single proton remain-

ing in each case.

Figure 12. Molecular unit of barium 5,5Ј-bistetrazolate pentahydrate

(7h); selected bond lengths /Å and angles /°: C1–C2: 1.459(4), C1–

N1: 1.342(4), N1–N2: 1.341(4), N2–N3: 1.322(4), N3–N4: 1.352(4),

C1–N4: 1.334(4), C2–N5: 1.341(4), N5–N6: 1.348(4), N6–N7:

1.319(4), N7–N8: 1.343(4), C2–N8: 1.335(4), N5–Ba1: 2.971(3), N6^I–

Ba1: 2.961(3), N7^II–Ba1: 2.946(3), O1–Ba1: 2.782(2), O2–Ba1:

2.700(3), O3–Ba1: 2.871(2), O3^II–Ba1: 2.805(2), O4–Ba1: 2.860(2),

O4^I–Ba1: 2.816(3), O1···N1: 2.782(4), N5–Ba1–N6^I: 116.2(1), N6^I–

I

Ba1–N7^II: 113.8(1); symmetry operations: : 2–x, 2–y, 1–z, ^II: 1–x, 2–

Figure 9. Molecular structure of caesium 5,5Ј-bistetrazolate (6), the

isolated atoms N11^I, N12^Iand N14^IIbelong to respective tetrazole

rings which are not displayed to avoid an overcrowding of the picture;

selected bond lengths /Å and angles /°: N1–Cs1: 3.183(9), N5–Cs1:

3.318(9), N6^I–Cs1: 3.337(7), N14^II–Cs1: 3.388(8), N15^III–Cs1:

3.400(10), N3^II–Cs1: 3.268(9), N12^I–Cs1: 3.221(8), N11–Cs1:

3.333(8), N10–Cs2: 3.308(10), N11^I–Cs2: 3.257 (8), N3^II–Cs2:

3.251(6), N4^III–Cs2: 3.295(9), N8^III–Cs2: 3.205(9), N14^IV–Cs2:

3.341(8), N13^V–Cs2: 3.293(8), N6^VI–Cs2: 3.322(9), N7^VI···N9:

2.728(11), N16^III···N2^II: 2.709(11), N1–Cs1–N5: 53.6(2), N4^III–Cs2–

y, 1–z, ^III: –1+x, y, z.

I

III

N8^III: 53.9(2); symmetry operations: :1+x, y, z, ^II: x, 1–y, 0.5+z,

:

1+x, 1–y, 0.5+z, ^IV: 1+x, 2–y, 0.5+z, ^V: x, 2–y, 0.5+z, ^VI: 1+y, 1+y, z.

The coordination polyhedra around the caesium atoms ap-

proximate doubly capped trigonal prisms. Cs–N distances

range from 3.183(9) Å to 3.400(10) Å and compare well to

related substances.^[25,28]In two cases the 5,5’-bistetrazolate li-

gand coordinates in a chelating fashion (N1–Cs1: 3.183(9) Å,

N5–Cs1: 3.318(9) Å, N4^III–Cs2: 3.295(9) Å, N8^III–Cs2:

3.205(9) Å). In these cases the pentacycle from two carbon,

two nitrogen and one caesium atom is almost planar. In com-

parison to the potassium (3h) and rubidium salts (4h) the che-

lating angles are further decreased (N1–Cs1–N5: 53.6(2)°,

N4^III–Cs2–N8^III: 53.9(2)°]. It is noteworthy that in multiple

cases not only a single atom of a respective tetrazole ring lies

in bonding distance to the caesium atom, but two neighbouring

atoms show very similar distances (most prominent: N12^I–

Cs1: 3.221(8) Å and N11^I–Cs1: 3.225(8) Å). A η²-bonding sit-

uation might be considered, but the unchanged bond lengths

in the tetrazole rings contradict a participation of the π-orbitals

in the coordination. The same phenomenon was observed with

the caesium tetrazolate^[25]and the caesium cyanotetrazol-

ate.^[28]

Figure 10. Hydrogen-bonded chains of 5,5Ј-bistetrazolate in the crys-

tal structure of caesium 5,5Ј-bistetrazolate (6); view along the a axis.

Figure 11. Crystal structure of caesium 5,5Ј-bistetrazolate; view along

[210]; stacked chains are tilted against each other by 90°.

In the crystal structure hydrogen bonded chains of

Barium 5,5’-bistetrazolate has been described previously as

5,5’-bistetrazolate (N7^VI···N9: 2.728(11) Å, N16^III···N2^II: a tetrahydrate.^[8,29]We found it to crystallise as a pentahydrate

¯

2.709(11) Å) prevail, between which the caesium atoms are (7h) in the space group P1 with two molecular entities per

located (Figure 10). The chains are tilted by 90° with respect to unit cell. The structure is very similar to the already known

neighbouring chains and arranged in stacks (viewing direction modification. The barium atom is coordinated ninefold in a

[210]). Within the stacks the chains are shifted with respect to triply capped trigonal prism by six oxygen and three nitrogen

each other so that every fifth chain is analogous (Figure 11).

atoms (Figure 12). O–Ba distances range from 2.700(3) Å

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5,5’-Bistetrazolate Salts with Alkali Metal, Ammonium and Imidazolium Cations

(O2–Ba1) to 2.871(2) Å (O3–Ba1), nitrogen barium bond Na⁺(1.16 Å, CN6) are not coordinated in a chelating fashion,

lengths are 2.971(3) Å (N5–Ba1), 2.961(3) Å (N6^II–Ba1) and the larger alkali metal ions are chelated by the 5,5’-bistetrazol-

2.946(3) Å (N7^III–Ba1).

ate moiety. In the case of the potassium and the rubidium salt,

Zigzag chains of barium atoms which are connected over the chelation is less pronounced, as the metal atoms are not in

two water molecules (the trigonal prisms share edges) and the plane with the 5,5’-bistetrazolate moiety. So far all atoms ex-

bistetrazole moieties filling the cavities between the chains are cept Ba²⁺which have an ionic radius above 1.26 Å are che-

a main motive in both crystal structures (Figure 13a). The ad- lated; the ones with smaller radii are not. We are convinced

ditional water molecule in the present structure does not coor- that this effect is of course on the one hand affected by the

dinate to the barium atom. Its position in the crystal becomes increasing Lewis acidity of the cations with decreasing radius

evident when the structure is viewed along the a axis (Fig- and therefore preferred coordination by water, but on the other

ure 13b). Situated between chains of barium atoms and the ad- hand can also be attributed to the rigid nature of the bicyclic

jacent bistetrazole moieties it connects the chains over strong ligand. Very small cations considerably increase the chelating

hydrogen bonds (O5···N2: 2.815(4) Å, O5···O1: 2.775(3) Å). angle and thereby decrease the C–N–M angle in the metalla-

These are not the only hydrogen bonds, almost every OH-bond pentacycle. Open chelating ligands like oxalic acid can easily

is participating in a strong network of H-bonds constraining balance this by decreasing the distance between the coordinat-

the crystal structure.

ing atoms which is less easily possible for the bicyclic 5,5’-

bistetrazolate. A considerable decrease of the C–N–M angle

will lead to less orbital overlap between the anionic lone pairs

of the nitrogen atoms and the metal orbitals, which we believe

to be the reason why small atoms are chelated less likely. The

only exception to this trend is the copper(II) salt.^[9]Despite

having a very small ionic radius of 0.87 Å (CN6) it is chelated

by 5,5’-bistetrazole. The differences in bond lengths indicate,

though, that only one of the chelating atoms forms a strong

ionic bond (2.027(4) Å) whereas the other one only forms a

weak coordinative bond (2.760(5) Å). The chelating angles N–

M–N follow a distinct and natural trend by increasing with

the cation becoming smaller. In Table 3 the ionic radii of the

respective metal ions are opposed to the coordination mode

and if applicable the chelating angle.

The structure of the diammonium 5,5’-bistetrazolate (8) was

described previously with a monoclinic cell.^[10a]During our

own independent research we were able to confirm this struc-

ture, but also observed a second orthorhombic modification. In

this case (NH₄)₂BT (8) crystallised in the space group Pban

with two molecules in the unit cell but only a quarter molecule

in the asymmetric unit (Figure 14). In contrast to the known

structure in which the bistetrazolate moiety is perfectly planar,

here the tetrazole rings are strongly tilted against each other

(N1–C1–C1^I–N^IV: 35.6(1)°).

In both crystal structures a layered topology is prominent.

Viewing along the c axis a picture almost indistinguishable

from the previously reported structure appears (Figure 15a).

Bistetrazolate moieties are connected over the ammonium ions

via a hydrogen-bonding network. Only when viewing the

structure along the b axis a considerable difference becomes

visible. Whereas in the structure described by Klapötke et al.

the bistetrazolate anions are ordered in a stair like fashion here

they are completely coplanar in each layer (Figure 15b).

The monoammonium salt of 5,5’-bistetrazole crystallised in

the monoclinic space group P2₁/c with four molecules per unit

cell. The asymmetric unit consists of one ammonium cation

and two symmetrically non-equivalent tetrazole rings. The re-

maining proton is disordered over the positions H1 and H2

with site occupation factors of roughly 50% (Figure 16). The

Figure 13. Crystal structure of 7h; a) Zigzag chains of barium atoms;

view in b direction; b) barium chains and adjacent bistetrazolate anions

connected over the fifth water molecule; view in a direction; hydrogen

atoms other than those on O5 are omitted for clarity.

When comparing the ionic radii of the metal atoms in 5,5’-

bistetrazole salts structurally characterised so far and correlat-

ing those to the coordination mode and potential chelating an-

gle an almost fully consecutive picture forms. Within the rare

earth metal salts a chelating coordination is observed for La,

Ce, Nd, Sm and Eu.^[7a]The ionic radii for the respective coor-

dination number range from 1.26 Å (Eu³⁺, CN9) to 1.36 Å

(La³⁺, CN9).^[30]Among the alkaline earth metal salts only with

strontium (ionic radius 1.32 Å, CN6) a chelating coordination bond lengths in the tetrazole moieties are consistent with the

is observed. Whereas the small cations Li⁺(0.73 Å, CN4) and previous structures.

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Table 3. Comparison of ionic radii and coordination modes in known 5,5Ј-bistetrazole salts, ions ordered according to ionic radius.

Ion (CN)^[a]

Be²⁺(4) Li⁺(4)

Mg²⁺(6)

Cu²⁺(6) Ca²⁺(6) Er³⁺(8) Na⁺(6) Tb³⁺(8) Th⁴⁺(9)

Eu³⁺(9)

Ionic radius /Å^[30]

Coord. mode

Chelating angle /° N–M–N

Ion (CN)

0.41

none

0.73

single

0.86

none

0.87

chelate

72.7

1.14

single

1.14

none

1.16

single

1.18

none

1.23

none

1.26

chelate

62.9

Sm³⁺(9) Nd³⁺(9) Sr²⁺(6)

Ce³⁺(9) La³⁺(9) Ba²⁺(9) K⁺(8)

Rb⁺(8) Cs⁺(8)

Ionic radius /Å

Coord. mode

Chelating angle /° N–M–N

1.27

chelate

62.7

1.30

chelate

62.2

1.32

chelate

64.1, 62.8 61.2

1.34

chelate

1.36

chelate

60.8

1.61

single

1.65

chelate chelate

57.2 55.6

1.75

1.88

chelate

53.6, 53.9

[a] Original literature: For Be, Mg, Ca, Sr and Ba see Ref. [8], for Cu see Ref. [9], for Er, Tb, Eu, Sm, Nd, Ce and La see Ref. [7a], for Th

see Ref. [7b], Li, Na, K, Rb and Cs are part of this work.

Figure 16. Molecular structure of ammonium 5,5Ј-bistetrazolate (9);

selected bond lengths /Å: C1–C1^I: 1.463(4), C1–N1: 1.329(3), N1–

N2: 1.358(3), N2–N3: 1.310(3), N3–N4: 1.347(3), C1–N4: 1.327(3),

Figure 14. Molecular unit of diammonium 5,5Ј-bistetrazolate (8); se-

C2–C2^II: 1.453(4), C2–N5: 1.337(3), N5–N6: 1.337(3), N6–N7:

lected bond lengths /Å and angles /°: C1–C1^I: 1.461(3), C1–N1:

1.318(3), N7–N8: 1.334(3), C2–N8: 1.345(3); symmetry operations:

1.337(1), N1–N2: 1.345(1), N2–N2^II: 1.318(2), N3···N1: 2.949(1),

^I:–x, 1–y, 1–z, ^II: –x, –y, 1–z.

N3^III···N2: 2.938(1), N1–C1–C1^I–N1^IV: 35.55(5), symmetry opera-

I

tions: : 1.5–x, 0.5–x, z; ^II: x, 0.5–y, –z; ^III: 1–x, 1–y, 1–z; ^IV: 1,5–x,

y, –z.

Figure 17. Crystal structure of monoammonium 5,5Ј-bistetrazolate (9);

view along the a axis; chains of BT monoanions connected over H-

bonds; only one position of the disordered hydrogen atom is displayed.

Figure 15. Crystal structure of diammonium 5,5Ј-bistetrazolate; a) hy-

drogen bonding; view in c direction; b) layered structure; view along

the b axis.

The structure is analogous to the structure of the caesium

5,5’-bitetrazolate (6). The positions of the caesium atoms are

As in the structure of CsHBT (6) the chains of bistetrazolate

occupied by ammonium ions. The bistetrazole moieties build anions are stacked and shifted with respect to each other so

hydrogen bonded chains (N4···N6: 2.679(3) Å), between which that every fifth layer is analogous. The stacks are tilted in a

the ammonium ions form a comparatively weaker hydrogen perpendicular fashion with respect to each other which be-

bonding network (donor acceptor distances between comes evident when the structure is viewed along the [210]

3.037(3) Å (N9···N8) and 3.065(3) Å (N9···N7), Figure 17).

direction (Figure 18).

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5,5’-Bistetrazolate Salts with Alkali Metal, Ammonium and Imidazolium Cations

Figure 18. Stacked layer structure of ammonium 5,5Ј-bistetrazolate,

neighboured stacks are tilted against each other by 90°; viewing direc-

tion [210]; only one position of the disordered hydrogen atom is dis-

Figure 20. Crystal structure of (EMIM)HBT (10); view along the

a axis.

played.

(EMIM)HBT (10) crystallised in the space group P2₁/n with

four molecules per unit cell (Figure 19). The bond lengths in

the bistetrazolate anion correspond well to the previous struc-

tures the imidazolium cation shows almost identical values,

when compared to common compounds.^[31]

Figure 21. Molecular structure of Bis(1-ethyl-3-methylimidazolium)

5,5Ј-bistetrazolate; hydrogen atoms are omitted for clarity; selected

bond lengths /Å: C1–C1^I: 1.460(2), C1–N1: 1.345(2), N1–N2:

1.354(2), N2–N3: 1.320(2), N3–N4: 1.355(2), C1–N4: 1.336(2), C2–

N5: 1.330(2), C2–N6: 1.327(2), C3–N6: 1.378(2), C3–C4: 1.350(2),

I

C4–N5: 1.385(2); symmetry operation: : –x, –y, –z.

Figure 19. Molecular structure of (EMIM)HBT (10); carbon bound

hydrogen atoms are omitted for clarity; selected bond lengths /Å: C1–

C2:1.454(2), C1–N1: 1.334(2), N1–N2: 1.354(2), N2–N3: 1.315(2),

N3–N4: 1.353(2), C1–N4: 1.335(2), C2–N5:1.324(2), N5–N6:

1.366(2), N6–N7: 1.299(2), N7–N8: 1.349(2), C2–N8: 1.334(2), C3–

N9: 1.329(2), C3–N10: 1.329(2), C4–N10: 1.373(2), C4–C5: 1.350(2),

C5–N9: 1.386(2).

In this case the monoanion of the 5,5’-bistetrazolate does

not form hydrogen-bonded chains as in compounds (6) and (9)

but hydrogen-bonded dimers (N8···N4: 2769(2) Å]. The dimers

are stacked in a direction and separated from neighbouring

stacks by imidazolium cations (Figure 20).

Bis(1-ethyl-3-methylimidazolium)

5,5’-bistetrazolate

Figure 22. Crystal structure of bis(1-ethyl-3-methylimidazolium) 5,5Ј-

bistetrazolate; view along the a axis; hydrogen atoms are omitted for

clarity.

((EMIM)₂BT, 11) crystallised in the monoclinic space group

P2₁/n with two molecules in the unit cell; the asymmetric unit

consists of half a molecule (Figure 21). All bond lengths corre-

spond well to previous structures. The bistetrazolate dianion

being isolated from any further bonding contacts does not alter

its conformation. In the crystal structure stacks of molecular

units prevail when viewing along the a axis (Figure 22).

Thermal Analysis

Combined TGA and DSC measurements were conducted

with a Netzsch STA 409 CD under inert gas conditions (Ar,

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40 mL·min^–1) in aluminium oxide crucibles with a hole in the drated and dehydrated salts. The hydrated salts show strong

lid. The applied heating rate was 10 K·min^–1. Probe masses and broad stretching modes of the OH bonds involved in hy-

were in the range of 5.5 to 6.8 mg. The lithium salt (1h) loses drogen bonds roughly between 2500 and 3700 cm^–1and below

two equivalents of crystal water at 165 °C, the sodium salt (2h) 900 cm^–1the corresponding bending modes. The latter are

is dehydrated at 63 °C but the amount differs distinctively from often partly obscuring the vibrations of the BT fragment. The

the values observed in the crystal structure and the elemental water-free substances show a very concise spectrum built up

analysis. Instead of a weight loss of 33.1% corresponding to of seven major bands all lying within the fingerprint region

the pentahydrate, only a loss of 13.5% was observed. This at about 700 to 1400 cm^–1. All bands slowly shift to lower

indicates a crystal water content of one to two equivalents. It wavenumbers when proceeding from lithium to caesium as

is assumed, that water was lost during storage and while evac- counterion. A distinct effect of the atomic weight of the coun-

uating the oven in order to ensure an inert gas atmosphere. terion onto the lattice vibrations of the BT-fragment can be

Among the alkali metal salts only the caesium salt (5) decom- observed. A comparison of the region from 680 to 1400 cm^–1

poses below 400 °C (392 °C). The other salts show decomposi- of the water-free alkali metal salts of 5,5’-bistetrazole are pre-

tion at onset temperatures of 416 (Rb₂BT), 424 (K₂BT), 453 sented in Figure 23. The barium salt roughly fits into this trend

(Na₂BT) and 478 °C (Li₂BT). These values coincide with the taking into account that the atomic weight of the barium atom

observations of Klapötke et al. who found no decomposition has to be considered with 50% as only one barium atom is

of the alkaline earth metal salts below 400 °C.^[8]In all cases present per BT moiety.

the decomposition is not complete at the stated temperatures

but only a fraction of the total mass is lost. Upon continued

heating the residues decompose in a stepwise manner but

rather gradually so that no further definite decomposition tem-

peratures can be determined. Thermal decomposition is com-

plete at close to 1000 °C. The monoammonium salt (9) shows

decomposition at an onset temperature of 282 °C, the diammo-

nium salt (8) measured for direct comparison showed decom-

position at 290 °C. This value corresponds well to the tempera-

ture observed by Chavez et al. (280 °C, the difference may be

due to a higher heating rate in our case).^[29]The monoammo-

nium and the diammonium salt show no large difference con-

cerning thermal behaviour. If there is a specific and consider-

able advantage of the monoammonium 5,5’-bistetrazolate over

the diammonium salt it has to affect different properties. The

DSC curve of (EMIM)HBT (10) shows an endothermic peak

at 147 °C, which is not accompanied by a weight loss in the

TGA curve and can therefore be attributed to the substance

melting. Decomposition takes place at 266 °C. (EMIM)₂BT

even melts at just 95 °C and can therefore be classified as an

ionic liquid. The decomposition of the substance follows at

Figure 23. Comparison of the IR spectra of the water-free alkali metal

5,5Ј-bistetrazolates in the region from 680 to 1400 cm^–1.

Conclusions

250 °C. The melting points were redetermined with a slower

heating rate (2 K·min^–1) rate with a Büchi Melting Point B540.

The melting point of 10 was determined as 145.5 °C,

(EMIM)₂BT (11) melted at 92.5 °C. The melting points stayed

unchanged over three temperature cycles between room tem-

perature and 10 °C above the melting point. To the best of our

knowledge these are the first bistetrazolate salts exhibiting a

distinct melting point. Compound 11 is a rare example of an

ionic liquid containing a dianion, nevertheless having low lat-

tice energy. The TGA of 11 reveals an abrupt loss of 80%

weight at an onset of 249.6 °C in connection with an exother-

mic peak in the DSC (see electronic supplement). Only small

amounts of substance remain after the first decomposition.

The alkali metal salts 1 to 5 of the nitrogen-rich diacid 5,5’-

bistetrazole were synthesised and characterised by elemental

analysis, carbon NMR and IR spectroscopy and TGA and DSC

measurements. The corresponding hydrates 1h to 4h and the

mono caesium 5,5’-bistetrazolate (6) were structurally charac-

terised by means of XRD single crystal analyses. Additionally,

a second crystal modification of the known barium 5,5’-biste-

trazolate (7h) was presented. The compounds have been ana-

lysed with respect to the coordination behaviour of the bicyclic

ligand and the results compared with all metal 5,5’-bistetrazol-

ates structurally characterised so far. A consecutive trend be-

tween chelation and the cation size was found and attributed

to the Lewis acidity of the cation and the geometric limitations

of the rather rigid ligand. Furthermore the monoammonium

5,5’-bistetrazolate (9), and mono- (10) and bis(1-ethyl-3-meth-

IR Spectroscopy

IR spectra were recorded with a Bruker ALPHA FT IR spec- ylimidazolium) 5,5’-bistetrazolate (11) were synthesised and

trometer with platinum ATR sampling as the pure substance. fully characterised including single crystal structure determi-

There is a strong difference between the spectra of the hy- nation. The diammonium 5,5’-bistetrazolate (8) is additionally

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5,5’-Bistetrazolate Salts with Alkali Metal, Ammonium and Imidazolium Cations

(ATR): 417 (s), 742 (w), 1040 (w), 1064 (w), 1168 (w), 1209 (w),

presented in a different crystal modification as it was known

so far. No special properties could be attributed to the

monoammonium salt 9, to be used in airbag technology, setting

it apart from the diammonium salt (8), so far. The imidazolium

salts are the first reversibly melting 5,5’-bistetrazolate com-

pounds, the bis(1-ethyl-3-methylimidazolium) 5,5’-bistetrazo-

late (11) being the first ionic liquid on the basis of 5,5’-bis-

tetrazolate dianions.

1324 (w), 1346 (w) cm^–1.

Disodium 5,5’-bistetrazolate pentahydrate (2h): Sodium hydrogen

carbonate (1.38 g, 16.5 mmol) and 5,5’-bistetrazole (1.14 g,

8.24 mmol) were suspended in water (5 mL). The solids dissolved

slowly accompanied by gas evolution. After the gas evolution had

stopped, the mixture was heated to obtain a clear solution. The solvent

was evaporated to dryness and the residue recrystallised from a mini-

mum amount of hot water (ca. 3 mL) obtaining single crystals suitable

for XRD. The sodium salt crystallised as a pentahydrate in 60% yield

(1.35 g, 4.96 mmol).

Experimental Section

C₂H₁₀N₈Na₂O₅(272.13 g·mol^–1) C 8.73 (calcd. 8.83), H 3.38 (3.70),

N 41.11 (41.18)%; ¹³C NMR (75.5 MHz, D₂O): δ = 154.5 (s, CN₄)

ppm. IR (ATR): 499 (m, sh), 579 (s, br), 730 (m), 1022 (m), 1047

(w), 1148 (m), 1186 (m), 1307 (s), 1326 (s), 1628 (s), 2600–3600 (s,

br) cm^–1; TGA (10 K·min^–1): 63 (dehydr.), 453 (decomp.) °C; DSC

(10 K·min^–1): 85 (endoth.), 464 (exoth.) °C.

All chemicals and solvents were used as received unless stated other-

wise. Methanol, acetonitrile and diethyl ether were dried and purified

according to common procedures^[32]and stored over 3 Å molecular

sieves until use. Elemental analyses were carried out by the service

department for routine analysis and mass spectrometry with a vario

MICRO cube (Elementar). NMR spectra were recorded in automation

with a Bruker Avance 300 spectrometer at room temperature. IR spec-

tra were recorded with a Bruker APLPHA FT-IR spectrometer with

Platinum ATR-sampling. Hygroscopic substances were investigated on

an identical device inside a glove box (type MB 150 B-G, mBraun).

TGA/DSC measurements were performed with a Netzsch STA 409 CD

in aluminium oxide crucibles, with an argon flow rate of

40 mL·min^–1and a heating rate of 10 K· min^–1; TGA decomposition

points are given as onset temperature, DSC data is given as peak value.

Melting points were determined with a Büchi Melting Point B540.

Drying in fine vacuum at 90 °C yielded the water-free disodium salt

(2).

C₂N₈Na₂(182.06) C 12.99 (calcd. 13.19), N 60.52 (61.55)%. IR

(ATR): 734 (s), 1015 (s), 1049 (m), 1148 (m), 1182 (m), 1308 (s),

1345 (s) cm^–1.

Dipotassium 5,5’-bistetrazolate (3): Potassium carbonate (1.65 g,

12.0 mmol) and 5,5’-bistetrazole (1.50 g, 10.9 mmol) were suspended

in water (15 mL) and after the gas evolution had ceased the mixture

was heated to reflux. The solvent was evaporated at room temperature

and the residue recrystallised from boiling water (4.5 mL). Single crys-

tals suitable for XRD were obtained but did not lead to publishable

solutions. In one case the investigated crystal turned out to be the

corresponding dihydrate 3h. The product was dried in air at room tem-

perature yielding 1.42 g (6.63 mmol, 61%).

Caution! 5,5’-Bistetrazole and its derivatives are potential explosives

and sensitive to heat, shock, friction and electrical spark. Although

no unintended decomposition was observed during our experiments

cautious handling of all substances and protective gear like safety

shields, gloves, earthed equipment and earplugs are strongly recom-

mended. Furthermore the handling and storage of larger quantities

should be avoided.

C₂K₂N₈(214.27 g·mol^–1) C 11.14 (calcd. 11.21), N 52.42 (52.30)%;

¹³C NMR (75.5 MHz, D₂O): δ = 154.7 (s, CN₄) ppm. IR (ATR): 732

(s), 1009 (s), 1042 (m), 1140 (m), 1170 (m), 1302 (s), 1334 (s) cm^–1;

TGA (10 K·min^–1): 424 (decomp.) °C; DSC (10 K·min^–1): 429 (ex-

oth.) °C.

5,5’-Bistetrazole was prepared following the procedure invented by

Friederich^[3]and optimised by Nelson et al.^[4]Diammonium 5,5’-biste-

trazolate (8) was synthesised according to Hyoda et al.^[33]and later on

in a way also described by Klapötke et al.^[10a]Analysis for compound

8: TGA (10 K·min^–1): 290 °C (decomp.); DSC (10 K·min^–1): 305 °C

(exoth.)

Dirubidium 5,5’-bistetrazolate dihydrate (4h): Rubidium carbonate

(1.32 g, 5.72 mmol) was suspended in water (15 mL) and 5,5’-bistetra-

zole (790 mg, 5.72 mmol) was added. After the gas evolution had

ceased the water was evaporated on a hot plate and the slightly yellow

residue recrystallised from a minimum amount of water (3 mL). Single

crystals suitable for XRD could be obtained. The X-ray structure in-

cludes two water molecules per molecular unit which are lost while

drying in air at room temperature. During the drying process the crys-

tals transform from colourless translucent needles to a colourless pow-

Dilithium 5,5’-bistetrazolate dihydrate (1h): Lithium hydroxide

monohydrate (301 mg, 7.18 mmol) and 5,5’-bistetrazole (496 mg,

3.59 mmol) were suspended in water (5 mL) and the mixture heated

to reflux. The clear hot solution was filtered and the solvents evapo-

rated to dryness. The raw product (654 mg, 3.52 mmol, 98%) was

recrystallised from a minimum amount of water yielding 48%

(319 mg) of 1h and crystals suitable for XRD.

C₂H₄Li₂N₈O₂(185.99 g·mol^–1) C 12.80 (calcd. 12.92), H 2.05 (2.17), der. Water-free rubidium 5,5’-bistetrazolate (4) was obtained in 62%

N 59.80 (60.25)%; ¹³C NMR (75.5 MHz, [D₆]DMSO): δ = 155.4 (s,

CN₄) ppm; ¹³C NMR (75.5 MHz, D₂O): δ = 154.5 (s, CN₄) ppm. IR

(1034 (m), 1066 (w), 1169 (w), 1212 (m), 1317 (w), 1345 (m),

2600–3400 (m, br) cm^–1; TGA (10 K·min^–1): 165 (dehydr.), 478 (de-

comp.) °C; DSC (10 K·min^–1): 182 (endoth.), 479 (exoth.) °C.

yield (1.10 g).

C₂Rb₂N₈(307.02 g·mol^–1) C 7.75 (calcd. 7.82), N 36.36 (36.50)%; ¹³

NMR (75.5 MHz, D₂O): δ = 154.7 (s, CN₄) ppm. IR (ATR): 733 (m),

1009 (s), 1040 (m), 1136 (m), 1169 (m), 1296 (s), 1325 (s) cm^–1; TGA

(10 K·min^–1): 416 (decomp.) °C; DSC (10 K·min^–1): 424 (exoth.) °C.

C

Drying the product at 180 °C in fine vacuum yielded the water-free

salt (1). The substance is highly hygroscopic and upon air contact ab-

sorbs two equivalents of water within minutes.

Dicaesium 5,5’-bistetrazolate (5): Caesium carbonate (2.16 g,

8.96 mmol) and 5,5’-bistetrazole (760 mg, 5.50 mmol) were dissolved

in water (15 mL) and the solution was evaporated to dryness after

stirring for 30 minutes. The residue was recrystallised from a small

amount of hot water and the product was isolated in 70% yield (1.55 g,

C₂Li₂N₈(149.96 g·mol^–1) C 15.72 (calcd. 16.02), N 74.63 (74.72)%;

¹³C NMR (75.5 MHz, [D₆]DMSO): δ = 155.6 (s, CN₄) ppm. IR

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L. H. Finger, F. G. Schröder, J. Sundermeyer

ARTICLE

3.86 mmol). Single crystals could be obtained but did not lead to a

publishable solution so far.

3-methylimidazolium bisulfate (356 mg, 1.71 mmol) in water (20 mL).

The suspension was stirred for three days and then filtered. The solu-

tion was evaporated to dryness and the residue recrystallised from a

hot mixture of acetone and water. Single crystals suitable for XRD

were obtained by layering a concentrated solution in acetonitrile with

diethyl ether. Yield: 220 mg (0.87 mmol, 52%).

C₂Cs₂N₈(401.90 g·mol^–1) C 6.09 (calcd. 5.89), N 27.92 (27.88); ¹³C

NMR (75.5 MHz, D₂O): δ = 154.6 (s, CN₄) ppm. IR (ATR): 733 (m),

1006 (s), 1035 (m), 1111 (w), 1120 (w), 1130 (m), 1169 (m), 1290 (s),

1314 (s) cm^–1; TGA (10 K·min^–1): 392 (decomp.) °C; DSC

(10 K·min^–1): 402 (exoth.) °C.

C₈H₁₂N₁₀(248.25 g·mol^–1) C 38.48 (calcd. 38.71), H 4.80 (4.87), N

55.60 (56.42)%; ¹H NMR (300.1 MHz, [D₆]DMSO): δ = 1.40 (t, 3 H,

³J(H,H) = 7.2 Hz, CH₂CH₃), 3.85 (s, 3 H, Me), 4.19 (q, ³J_H,H= 7.2 Hz,

2 H, CH₂CH₃), 7.70 (s, 1 H, H_Im), 7.78 (s, 1 H, H_Im), 9.15 (s, 1 H,

H_Im), 16.19 (br. s, 1 H, H_BT) ppm; ¹³C NMR (75.5 MHz, [D₆]DMSO):

δ = 15.0 (s, CH₂CH₃), 35.7 (s, CH₃), 44.1 (s, CH₂CH₃), 121.9 (s, C_Im),

123.5 (s, C_Im), 136.3 (s, C_Im), 149.4 (s, CN₄) ppm. IR (ATR): 412 (w),

483 (s), 627 (s), 650 (s), 703 (m), 758 (s), 866 (m), 890 (m), 999 (m),

1017 (m), 1029 (m), 1092 (m), 1161 (s), 1281 (m), 1341 (s), 1465 (w),

1560 (w), 1574 (w), 1617 (w), 2300–2700 (w, br), 3112 (w), 3141 (w)

cm^–1; TGA (10 K·min^–1): 266 (decomp.) °C; DSC (10 K·min^–1): 147

(endoth.), 282 (exoth.) °C; M. P. (2 K·min^–1): 145.5 °C.

Barium 5,5’-bistetrazolate pentahydrate (7h): Barium hydroxide

octahydrate (3.88 g, 12.3 mmol) and 5,5Ј-bistetrazole (1.70 g,

12.3 mmol) were dissolved in boiling water (100 mL). The solution

was filtered hot and slowly cooled to room temperature. The product

crystallised in 79% yield (3.55 g, 9.77 mmol), single crystals suitable

for XRD analysis could be obtained.

Pentahydrate: C₂H₁₀BaN₈O₅(363.49 g·mol^–1) C 6.61 (calcd. 6.61), H

2.20 (2.77); N 31.00 (30.83)%; ¹³C NMR (75.5 MHz, D₂O): δ = 155.4

(s, CN₄) ppm. IR (ATR): 400–850 (s, br), 1021 (s), 1048 (m), 1121

(w), 1151 (m), 1181 (s), 1308 (s), 1327 (s), 1650 (m), 2700–3600 (s,

br) cm^–1. The comparatively high deviation of the hydrogen value may

indicate a partial mixture of e.g. the penta- and tetrahydrate. This is

not confirmed by the nitrogen value, though. Drying the Substance in

air at room temperature yielded the dihydrate after four days.

Bis(1-ethyl-3-methylimidazolium) 5,5’-bistetrazolate (11): A solu-

tion of 5,5’-bistetrazole (349 mg, 2.53 mmol) in dry methanol (10 mL)

was slowly added to a solution of 1-Ethyl-3-methylimidazolium

methyl carbonate (938 mg, 5.04 mmol) in dry methanol (5 mL). Slow

gas evolution was observed and the mixture was heated to 50 °C for

sixteen hours before the solvent was removed in vacuo. The residue

was recrystallised by layering a concentrated solution in dry acetoni-

trile with dry diethyl ether. Large crystals suitable for XRD formed

within hours, crystallisation was complete after a few days. After a

second recrystallisation from dry acetonitrile at –30 °C 560 mg

(1.56 mmol, 62%) of the pure product were obtained.

Dihydrate: C₂H₄BaN₈O₂(309.45 g·mol^–1) C 7.70 (calcd. 7.76), H 1.26

(1.30), N 35.94 (36.21)%. IR (ATR): 542 (s, br), 571 (s, sh), 675 (m,

br), 725 (m), 819 (w, br), 1019 (m), 1047 (w), 1085 (w), 1143 (w),

1186 (w), 1218 (w), 1310 (m), 1339 (m), 3057 (w, br) 3523 (w, br)

cm^–1.

Drying the product at 120 °C in fine vacuum for four hours yielded

the water-free salt.

C₁₄H₂₂N₁₂(358.41 g·mol^–1) C 47.10 (calcd. 46.92), H 6.21 (6.19), N

46.91 (46.90)%; ¹H NMR (300.1 MHz, [D₆]DMSO): δ = 1.37 (t, 3 H,

C₂BaN₈(273.42 g·mol^–1) C 8.79 (calcd. 8.79), N 41.54 (40.98)%. IR

(ATR): 728 (m), 1021 (s), 1057 (m), 1085 (w), 1143 (s), 1191 (m),

1312 (m), 1340 (s) cm^–1.

3

³J_H,H= 7.1 Hz, CH₂CH₃), 3.89 (s, 3 H, Me), 4.25 (q, J_H,H= 7.1 Hz,

2 H, CH₂CH₃), 7.76 (s, 1 H, H_Im), 7.85 (s, 1 H, H_Im), 9.67 (s, 1 H,

H_Im) ppm; ¹³C NMR (75.5 MHz, [D₆]DMSO): δ = 15.2 (s, CH₂CH₃),

35.5 (s, CH₃), 44.0 (s, CH₂CH₃), 121.9 (s, C_Im), 123.5 (s, C_Im), 136.8

(s, C_Im), 156.0 (s, CN₄) ppm. IR (ATR): 621 (m), 651 (s), 732 (s), 739

(s), 800 (m), 850 (w), 894 (m), 1006 (w), 1027 (m), 1094 (w), 1156

(s), 1293 (m), 1315 (m), 1428 (w), 1453 (w), 1561 (m), 1659 (w),

2978 (w), 3053 (m), 3136 (w) cm^–1TGA (10 K·min^–1): 250 (decomp.)

°C; DSC (10 K·min^–1): 94.5 (endoth.), 298 (exoth.) °C; M. P.

(2 K·min^–1): 92.5 °C.

Ammonium 5,5’-bistetrazolate (9): Diammonium 5,5’-bistetrazolate

(554 mg, 3.22 mmol) and 5,5’-bistetrazole (445 mg, 3.22 mmol) were

dissolved in water (25 mL), the solution heated to reflux and then

evaporated to dryness. The residue was recrystallised from a minimum

amount of hot water (ca. 5 mL) so that an aqueous solution at 115 °C

in a closed flask was saturated. The solution was then slowly cooled to

room temperature yielding needles of diammonium 5,5’-bistetrazolate

(201 mg, 1.17 mmol, 36% of the starting material. C₂H₈N₁₀

(172.15 g·mol^–1) C 13.86 (calcd. 13.95), H 4.55 (4.68), N 80.98

(81.36)%). The supernatant solution was transferred to a second flask

and slowly cooled to 4 °C whereupon the desired product crystallised

in 60% yield (302 mg, 1.95 mmol). Single crystals suitable for XRD

were obtained.

Supporting Information (see footnote on the first page of this article):

Table with bond lengths, bond angles and torsion angles in the bistetra-

zolate anions of compounds, 1h to 4h, 6, 7h and 8 to 11; TGA and

DSC Curve of (EMIM)₂BT (11).

C₂H₅N₉(155.12 g·mol^–1) C 15.65 (calcd. 15.49); H 2.92 (3.25), N

80.64 (81.27)%; ¹³C NMR (75.5 MHz, D₂O): 149.4 (s, CN₄) ppm. IR

(ATR): 426 (m), 476 (s), 687 (m), 851 (s, br), 1009 (s), 1070 (m),

1105 (m), 1224 (w), 1282 (m), 1335 (w), 1377(s), 1409 (m), 1528

(w), 2100–3300 (m, br); TGA (10 K·min^–1): 282 (decomp.) °C; DSC

(10 K·min^–1): 289 (exoth.) °C. The deviation of the nitrogen value

in the elemental analysis may be attributed to the product not being

completely pure. The tendency of equimolecular solutions of diammo-

nium 5,5’-bistetrazole and the free acid to yield the diammonium salt

upon crystallisation makes it not possible to purify the product by

recrystallisation.

Acknowledgments

Financial support by the Studienstiftung des deutschen Volkes and the

Fond der Chemischen Industrie is gratefully acknowledged. We thank

Jens Eußner (M. Sc.) for his introduction into and continued help with

TGA/ DSC measurements. We are very thankful to Dr. Klaus Harms

for fruitful discussions and valuable advice regarding XRD single crys-

tal structure determination.

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Received: March 12, 2013

Published Online: ᭿

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L. H. Finger, F. G. Schröder, J. Sundermeyer

ARTICLE

L. H. Finger, F. G. Schröder, J. Sundermeyer* .............. 1–14

Synthesis and Characterisation of 5,5’-Bistetrazolate Salts with

Alkali Metal, Ammonium and Imidazolium Cations

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Synthesis and characterisation of 5, 5′-bistetrazolate salts with alkali metal, ammonium and imidazolium cations

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