Maximization of the energy capability level in transition metal complexes through application of 1-amino- and 2-amino-5: H -tetrazole ligands

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Szimhardt, M. H. H. Wurzenberger, M. Lommel, M. S. Gruhne and L. Zeisel, J. Mater. Chem. A, 2018, DOI:

10.1039/C8TA06326E.

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This comprehensive study features the synthesis, characterization and evaluation^DOo^If^{: 10}n^.1e⁰w^39/Ce⁸n^TAe⁰r⁶g³e²⁶ti^Ec

coordination compounds (ECC) based on two of the most powerful neutral tetrazoles, which have great

potential as lead-free primary explosives. In addition to its extensive physico-chemical characterization,

obtained complexes were tested in laser ignition and PETN initiation experiments to explore its

suitability as primary explosives. The properties of the very promising candidates can be fine-tuned by

variation of the implemented metal(II) salt and corresponding isomeric ligand.

1

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Maximization of the Energy Capability Level in Transition Metal

Complexes through Application of 1-Amino- and 2-Amino-5H-

tetrazole Ligands†

Received 00th January 20xx,

Accepted 00th January 20xx

Norbert Szimhardt, Maximilian H. H. Wurzenberger, Lukas Zeisel, Michael S. Gruhne, Marcus

Lommel, and Jörg Stierstorfer*

DOI: 10.1039/x0xx00000x

www.rsc.org/

Toxic lead compounds (especially lead azide and lead styphnate), reliable but highly harmful compounds, still dominate

the field of primary explosives used in most ignition and initiation systems to this day. Therefore, an extensive

investigation into energetic materials with similar performance and minimized toxicity led to the very promising strategy

of developing energetic coordination compounds (ECC). For the first time in literature, two isomeric amino-5H-tetrazoles

with sensitivities already in the range of primaries, which could be obtained via practicable, easily scalable synthesis,

proved to be very promising new energetic ECC ligands. By variation of the respective isomer (1-AT, 2-AT), metal(II) center

(Mn^II, Fe^II, Cu^II, Zn^II) and the corresponding counter-anion (Cl^–, NO₃^–, ClO₃^–, ClO₄^–, styphnate, trinitrophloroglucinate)

desired properties could be tailored gradually, yielding 20 new, highly energetic and easily accessible transition metal(II)

complexes. In addition to comprehensive characterization by XRD, IR, EA, solid-state UV/Vis, DTA and TGA, BAM standard

sensitivity measurements as well as ESD and ball drop impact sensitivities were determined. Classical initiation tests

(nitropenta filled detonator) validated the present approach to be extraordinarily promising for potential use in advanced

initiation systems. Beyond successfully achieving set goals in terms of sensitivity and toxicity, investigated complexes

match or even outperform lead-based primaries’ high performance and, more importantly, do not necessarily depend on

initiation by mechanical stimuli. Laser irradiation of the received primary explosives with near infrared light (NIR), yielded

initiation

reliably

and

safely,

further

extending

their

potential

for

future

applications.

In recent years, the demand for “green” chemicals, with re- and Restriction of Chemicals) regulations.³

spect to the invention, design and application of chemical The REACH regulations were established in June 2007 as a

products and processes, has grown continuously with the European Union-wide project to reduce the amount of harmful

objective of reducing or eliminating the use and generation of or at least suspected-to-be-harmful chemicals to which Euro-

hazardous substances.¹The wide and fast expanding field of pean citizens were exposed to in their everyday life. In fact,

“green” chemicals comprises biodegradable plastics, bio-based REACH is not restricted to industrial processes, and also covers

polymers and environmentally friendly explosives from differ- the chemicals used in everyday products such as paint, clean-

ent portfolios of sustainable, eco-sufficient materials feasible ing products or personal hygiene articels.⁴ECHA (European

of outclassing products based on petroleum feedstock or toxic Chemicals Agency) uses this decree to hold the chemical indus-

heavy metals such as lead, mercury, barium and others.²Inter- try responsible for collecting information about their manufac-

action between the chemical industry and academic research tured chemicals, while also assessing their hazards and poten-

plays a key part in this social and economic evolution and the tial risk to the environment. Based on the simple “one sub-

eventual success of green and sustainable materials. A guide- stance, one registration” principle manufacturers and import-

line relevant to applied research in this area is the candidate ers register their chemicals in conjunction with each other for

list of substances of very high concern for authorization (Annex later evaluation to clarify if these registered chemicals bear

XIV) from the REACH (Registration, Evaluation, Authorization any risk to the environment or health.⁵The subsequent au-

thorization process ensures that substances of very high con-

cern (SVHCs) are gradually replaced with other less dangerous

and less harmful alternatives – if possible and readily available.

As soon as the committee or the commission reaches an

agreement that the addition of a SVHC to the candidate list is

reasonable, the substance is added to the database.⁶

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Figure 1. List of requirements to be met for new potential primary explosives, which are compiled in our suitability evaluation diagram.

One of the many candidates mentioned on this list are lead coordination compounds are well-known with different te-

azide and lead styphnate, which are widely used as primary trazole ligands but surprisingly no complexes have been pre-

explosives. A total of 31 lead containing compounds appear on pared up to date with two of the most energetic and powerful

the REACH list, with all of them being suspected to be toxic for neutral ligands, 1-amino-5H-tetrazole (

1, 1-AT) and 2-amino-

reproduction and some also to be carcinogenic (e.g. lead 5H-tetrazole (

2

, 2-AT, Figure 2).^25–31These two isomers, which

chromate since 2009).⁷As lead azide and lead styphnate are proved to be non-toxic toward aquatic organisms (see 2.5) and

mainly used in small arm ammunition for military and com- have an incorporated hydrazine function (N_Tetrazole–NH₂) exhibit

mercial applications (mining, deconstruction works, etc.), an outstandingly high nitrogen content of 82.33% and were

there is a strong drive for substitution of these currently used- described almost 50 years ago for the first time in 1966 and

primary explosives. Scientists in many countries from all over 1969 respectively.^32,33Several recently published energetic

the world are therefore looking for possible alternatives and coordination compounds based on 1-methyl-5H-tetrazole as

replacements.^8–14Thought must be given to many different the ligand show a deficit in energetic performance.¹⁵Substitu-

factors (fast deflagration-to-detonation transition (DDT), com- tion of the methyl group by an amino group tremendously

patibility, energetic properties etc.) during the design, the increases the energy content of the molecule due to the addi-

synthesis and evaluation of potential new primary explosives. tional N–N single bond (Figure 2). The highly endothermic

The most important characteristics for suitable candidates character and the resulting driving force of nitrogen gas for-

include the sensitivities, the chemical and physical stability, mation, makes these tetrazoles exceptional great ligands,

low toxicity, low price, high performance and practicabilities which show potential in combination with oxygen-rich anions

according to industrial needs (Figure 1). The concept of ener- as primary explosives.

getic coordination compounds (ECC), containing nitrogen rich The obtained compounds easily outperform commonly used-

ligands with low or non-toxic metals (e.g. Mn, Fe, Cu, Zn), primaries like lead azide or lead styphnate through the maxi-

stands out from other competing strategies and fulfills all of mization of the energy content in the resulting transition metal

the desired criteria.^15–17One of the biggest advantages of this complexes.

promising approach is that the simple adjustment of one of

the three building blocks (metal, ligand, anion) allows exact

tailoring of the complexes in regard to the physico-chemical

properties. Another important benefit of energetic coordina-

tion compounds, making them highly promising and suitable

candidates for potential future applications, is their ignitability

by optical methods like laser irradiation.^18,19This approach

offers various advantages in comparison to conventional initia-

tion methods like short function times or the high isolation

toward other stimuli.²⁰

In particular endothermic tetrazoles have proven themselves

in the past as very promising ligands in energetic complexes.^21–

23

Their energetic character can easily be modified by various

Figure 2. Comparison of the per unit mass calculated enthalpies of formation for

methods such as alteration of the functional groups (alkyl

various monotetrazoles shows the highest values for 1- and 2-amino-5H-tetrazole. Gas

chains, nitro groups, nitramine units, hydrazinyl derivatives,

phase enthalpies of formation were calculated using the atomization method (∆_fH⁰

(g, M)

attached tetrazoles etc.) or their substitution pattern (N1-

substituted vs. N2-substituted vs. C5-substituted).^15,24Many

= H_(M)– ΣH⁰_(A)+ Σ∆_fH⁰_(A)) using Gaussian09 computed CBS-4M electronic enthalpies.

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In this comprehensive study, considerable efforts have been

made to implement the highly endothermic ligands 1-amino-

5H-tetrazole (1) and 2-amino-5H-tetrazole (2) into a large

number of promising coordination compounds, which show

great potential for possible future applications (optical and

classical initiation systems).

Results and Discussion

Synthesis

CAUTION! All investigated compounds are potentially explosive

energetic materials (the majority of the compounds lie in the

range of primary explosives), which show partly increased

sensitivities toward various stimuli (e.g. elevated temperatures,

impact, friction or electrostatic discharge). Therefore, proper

security precautions (safety glasses, face shield, earthed

equipment and shoes, leather jacket, Kevlar gloves, Kevlar

sleeves and ear plugs) have to be worn while synthesizing and

handling the described compounds. Especially the very sensi-

Scheme 1. a.) Synthesis of 1-amino-5H-tetrazole (1) and 2-amino-5H-tetrazole (2) by

amination of 1,5H-tetrazole by hydroxylamine-O-sulfonic acid (HOSA). b.) Synthesis of

2-amino-5H-tetrazolium perchlorate (3).

1-Amino-5H-tetrazole (

1) can be prepared (overall yield: 32 %),

in contrast to compound

2, directly and isomerically pure via a

simple and upscalable three-step synthesis (Scheme 2). In the

first step, one amino group of the hydrazine molecule is pro-

tected as benzhydrazone by treatment with benzaldehyde in a

condensation reaction. Particular attention should be given to

the low reaction temperature, the sequence of starting mate-

rials added and the reaction time to prevent formation of a

further reaction to the bisbenzhydrazone. The next step entails

the ring closure of the remaining free amino group of benzhy-

drazone in a [3+1+1] cyclization with triethyl orthoformate and

sodium azide carried out at 80 °C for 2.5 h under acidic condi-

tions. The resulting tetrazole immediately precipitates in the

form of a yellow-orange solid right after pouring into water.

The benzyl protection group is cleaved almost quantitatively in

the last reaction step under acidic conditions with hydrochloric

acid and the resulting benzaldehyde was removed using the

rotary evaporator at elevated temperatures under vacuum.

This reaction step has been greatly improved and simplified in

comparison to literature-reported procedures where water

steam distillation and distillation under vacuum are applied to

remove the emerging benzaldehyde.^32,35

tive compounds

4–

7

and 18–23 must be handled with great

care! An excess of ligand

2

has to be applied during the synthe-

sis of complex 18 in order to prevent the formation of unisola-

ble side species (most likely a diperchlorato or hexacoordinated

tetrazole complex), which tends to spontaneous violent de-

composition during crystallization.

In principle, the preparation of 1-amino-5H-tetrazole (1) can

be achieved via two different synthetic protocols. One way is

the amination of 1,5H-tetrazole, reported in 1969 for the first

time, which leads to an isomeric mixture of compound

2-amino-5H-tetrazole (

) as a side product.³³As of today, un-

fortunately no direct synthetic path has been developed for

this isomer, which in comparison to compound , is surprising-

1 and

2

1

ly more sensitive. The amination reaction of the in situ gener-

ated sodium tetrazolate proceeds via a nucleophilic substitu-

tion mechanism and has been performed with the well-known

amination reagent hydroxylamine-O-sulfonic acid (HOSA) in

water under reflux and a constantly readjusted pH value of 7–

8 according to a modified literature procedure (Scheme 1).^33,34

Instead of the very dangerous separation by distillation of the

isomeric mixture obtained, a much safer and more efficient

purification by flash column chromatography (EtOAc/DCM 5:1)

on silica gel was developed. The two isomers could be isolated

Scheme 2. Selective and isomerically pure preparation of 1-amino-5H-tetrazole (1) in a

in yields of 27% for compound

1 and 19% for the nitrogen-rich

three-step reaction starting from benzaldehyde and hydrazine monohydrate.

ligand 2. The relatively low yields can most likely be attributed

to the electron-poor character of the aromatic heterocycle and

its related low nucleophilicity. Treatment of the two amino-

Both isomers can easily be differed by infrared spectroscopy

(Figure S1) or proton and carbon NMR spectroscopy. Further-

5H-tetrazoles

1 and 2 with aqueous diluted acids (HNO₃and

1

more, two dimensional H, ¹⁵N NMR HMBC or proton coupled

HClO₄) resulted in ionic liquids and were therefore not been

investigated in more detail. The only exception was the proto-

¹⁵N NMR spectroscopy can be performed (Figure 3).

A large number of complexes based on various monotetrazoles

like 1,5-diaminotetrazole or 1-methyl-5H-tetrazole have been

described in literature but have insufficient power for any

possible application as a primary explosive.^16,30Owing to this

lack of power and to maximize the energetic performance,

new transition metal complexes were synthesized with even

more powerful endothermic ligands.

nated 2-amino-5H-tetrazolium perchlorate salt

(3), which

crystallized within 5 days and has been comprehensively char-

acterized (Scheme 1).

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accomplished through combination of aqueous solutions of

the relevant metal(II) salts and the ligand in the correspond-

1

ing stoichiometries at room temperature (Scheme 3).

Scheme 3. Synthesis of the transition metal(II) complexes 4–9 and 12 based on 1-

amino-5H-tetrazole (1) as ligand.

Water has been selected as the green solvent for all reaction

mixtures to ensure slow crystallization of the desired complex-

es and because of the good solubilities of the ligands and the

transition metal(II) salt starting materials in this solvent sys-

tem. All coordination compounds crystallized with decent

yields (32–79%) without inclusion of aqua ligands or crystal

water molecules resulting in a higher performance of the com-

plexes. The exclusive use of ethanol as solvent instead of water

leads to precipitation of the complexes in most cases in the

form of amorphous powders.

Scheme 4. Formation of complexes 10, 11 and 13–15 with varying metal(II) centers,

anions and ligands.

Figure 3. Proton coupled ¹⁵N NMR spectra and two dimensional ¹H, ¹⁵N-HMBC NMR

spectra of 1 and 2.

Analogous to our recently published work, an ethanolic solu-

tion of copper(II) chlorate was reacted with 1-amino-5H-

tetrazole (1) in order to synthesize the corresponding cop-

Our synthetic strategy comprises the preparation of cationic or

neutral complex units to increase the energetic character of

the coordination compounds and to ensure the integration of

oxidizing anions, facilitated through the application of neutral

basic nitrogen-rich ligands. In addition, the influence on the

coordination sphere and the physico-chemical properties were

investigated by alteration of the tetrazole isomers while main-

taining the metal(II) and anion building blocks. The synthesis of

per(II) chlorate coordination compound (Scheme 4).³⁶After a

few days and evaporation of the solvent, two different crystal-

line species appeared from the reaction mixture, which could

be picked and handled while still wet. X-ray diffraction analysis

revealed a complex species with six coordinating 1-AT ligands

(

10, Figure S9) and a species with four 1-AT ligands and two

coordinating chlorato anions (11, Figure S10). All attempts to

dry the compounds for further characterization failed due to

the divalent transition metal(II) complexes

4–9 and 12 was

4 | J. Name., 2012, 00, 1-3

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the extreme sensitivity and resulted in violent detonations. small amount of ice-cold ethanol when necessary in order to

Unfortunately, only a few single crystals suitable for X-ray remove unreacted starting materials and were then dried in air

diffraction of the manganese(II) complexes 13–15 (Scheme 4) overnight.

could be isolated in between amorphous bulk material with

indefinite composition and unreacted starting materials.

Crystal structures

Therefore, only their X-ray crystal structures, which are shown The nitrogen-rich ligand

in the SI can be presented. All attempted syntheses to achieve plexes were characterized by low temperature single crystal X-

a pure elemental analysis of the complexes were unsuccessful. ray diffraction. In contrast to ligand , 1-amino-5H-tetrazole (

The 2-amino-5H-tetrazole ( ) based coordination compounds could be solidified through controlled freezing with liquid

16

23 (Scheme 5) were synthesized in analogy to their 1- nitrogen. Its low melting point of 5 °C³⁵allowed the picking of

substituted isomers (easy distinguishable by IR, Figure S3) a single crystal at 0 °C for X-ray determination. The measure-

through a straightforward reaction of the corresponding met- ments of compound and 11 showed considerable structural

al(II) salts in the necessary stoichiometries in water or ethanol disorders within the tetrazole rings as well as in the perchlo-

at room temperature. An excess of ligand had to be applied rate ( ) and chlorate (11) anions. Nevertheless, they allow the

1, the perchlorate salt 3 and all com-

2

1)

2

–

4

2

4

during the synthesis of complex 18 in order to prevent the identification of the complexes’ composition. Elemental analy-

formation of an unisolable side species (most likely a diper- sis and infrared spectra comparison (Figure S2) verified the

chlorato or hexacoordinated tetrazole complex), which tended analogous structure of

to spontaneous violent decomposition during crystallization. It amino-5H-tetrazole) metal(II) perchlorate complexes

should also be noted, that the use of six equivalents of in- high degree of disorder observed for the copper(II) chlorate

stead of five during the synthesis of the complexes 19 20 complex 11 unfortunately allowed only an isotropic illustration

leads to the same monoaqua species and has no influence on (Figure S10) of the molecular structure. All complexes, except

the composition of the resulting coordination compounds. The compound , show an octahedral coordination sphere of the

preparation of the metal(II) perchlorate complexes 18 23 metal(II) center. Complexation of the ligands and always

yields for each metal(II) salt two different species depending takes place through the N4 nitrogen atom of the tetrazole ring,

largely on the applied stoichiometry and solvent system. In except for compounds and 19 which show bidentate amino-

addition to the aqua ligand containing coordination com- tetrazole ligands. The crystal datasets were uploaded to the

pounds 18

20, complex units with two additional cocrystallized CSD database³⁷and can be obtained free of charge with the

aminotetrazoles 21 23 can be isolated when an excess of lig- CCDC no. 1834855 ( ), 1834849 ( ), 1834847 ( ), 1834859 ( ),

and is used. Both species can easily be detected and distin- 1834851 ( ), 1834845 ( ), 1834844 ( ), 1834853 (10), 1834852

guished from each other by IR spectroscopy (Figure S4/5). In 12), 1834858 (13), 1834857 (14), 1834854 (15), 1834842 (16),

the case of the copper(II) perchlorate complexes 18 23, the 1834843 (17), 1834841 (18), 1834856 (19), 1834846 (20),

solvent system plays an important role in the formation of the 1834848 (21), 1834850 (22), 1838980 (23).

relevant complexes. Water leads to the diaqua species 18 Compound crystallizes in the form of colorless rods in the

whereas ethanol as solvent results in the formation of the orthorhombic space group P2₁2₁2₁with four formula units per

4

as in the other isotypic hexakis(1-

5–7

. The

2

–

9

–

1

2

8

-

1

3

5

6

7

8

9

(

/

,

1

cocrystallized compound 23

.

unit cell and a calculated density of 1.515 g cm⁻³at 143 K. All

non-hydrogen atoms lie within a plane with torsion angles

close to 0° (C1–N1–N2–N3 0.31(15)°, N5–N2–C1–N1 1.59(1)°

and N2–N1–C1–N4 0.12(16)°) and show bond lengths as well

as angles comparable to those of other 1-substituted tetrazole

compounds (Figure 4).³⁸

Figure 4. Molecular unit (left) and unit cell (right) of 1-AT (1). Thermal ellipsoids of non-

hydrogen atoms in all structures are set to the 50 % probability level. Selected bond

lengths (Å): N1–C1 1.3256(18), N1–N5 1.4000(17), N1–N2 1.3441(15), N2–N3

1.2986(19), N3–N4 1.370(2), N4–C1 1.3128(19); selected bond angles (°): N2–N1–N5

123.21(11), N2–N1–C1 109.17(11), N5–N1–C1 127.54(12), N1–N2–N3 105.71(11), N2–

N3–N4 110.82(12), N3–N4–C1 105.18(12).

Scheme 5. Synthesis of selected energetic coordination compounds 16–23 based on 2-

amino-5H-tetrazole (2) as ligand in combination with different metal(II) salts.

All coordination compounds were directly obtained from the

mother liquor and crystallized in satisfactory yields with excep-

tion of the cocrystallized species. The crystalline materials, all

suitable for X-ray diffraction, were filtered off, washed with a

The perchlorate salt

3, crystallizes in the form of colorless

blocks in the monoclinic space group P2₁/n with four formula

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units per unit cell and a calculated density of 1.963 g cm⁻³at rich ligands are bound to each metal(II) center and the per-

173 K. The distances between the ring atoms lie within the chlorate counter-anions are non-coordinating (Figures 6 and

range of partial double bonds and the torsion angle C1–N1– S6–8).

N2–N3 is very close to 0° (C1–N1–N2–N3 −0.5(2)°). Moreover, In each case, the three dimensional stability is guaranteed by

every bond length besides the N1–N2 distance is slightly several hydrogen bonds represented in Table S6, exemplarily

shorter in the non-coordinating tetrazole ring of 3. In addition illustrated for complex [Fe(1-AT)₆](ClO₄)₂(5). The unit cell of 5

to these results, the aromaticity of the heterocycle is proved is depicted in Figure 7 illustrating the two-dimensional packing

by the bond angles amongst the ring atoms. The bond distance in view along the crystallographic c-axis. The comparison of

between the amine's and the adjacent ring nitrogen is close to the investigated perchlorate complexes with 1-amino-5H-

those of the tetrazole's π-system. In regard to the N2's sp²- tetrazole as the coordinating ligand shows no major differ-

hybridization as part of an aromatic heterocycle, the bond ences besides the Jahn-Teller distorted Cu2 center in

6 (Figure

angles between N5, N1 and the neighboring ring atoms are S7) and the previously mentioned densities. Notwithstanding,

close to be the ideal 120°. The interesting conclusion, that the one might recognize the slight differences regarding the bond

whole aminotetrazolium cation appears as one plane molecule coordination distances between the ligands and the respective

when including the amino group at N5, can be made based on metal(II) center. While all complexes show disordered perchlo-

the torsion angle N5–N2–N3–N4 which almost amounts to rate anions, the manganese(II) compound

4 additionally pos-

180° (N5–N2–N3–N4 = 175.03(15)°). Due to the previously sesses strong disorders in the heterocyclic ligands (Figure S6).

mentioned sp²-hybridization of the atoms of this aromatic

heterocycle, the hydrogen atoms at N4 and C1 are part of the

molecule's plane. On the other hand, the hydrogen atoms of

the sp³-hybridized N5 are both orientated toward in the same

direction. The perchlorate counter-ion of the tetrazolium cati-

on is only marginally distorted exhibiting typical Cl–O bond

distances and bond angles based on their tetrahedral molecu-

lar structure (Figure 5). In summary, the resulting structural

parameters almost exclusively match data quoted in literature

of similar tetrazolium perchlorate salts.³⁹

Figure 6. Molecular unit of complex [Fe(1-AT)₆](ClO₄)₂(5). Selected bond length (Å):

Fe1–N14 2.185(3); selected bond angles (°): N14–Fe1–N14^vii88.88(11), N14–Fe1–N14ⁱⁱⁱ

91.12(12). Symmetry codes: (i) 1−y, x−y, z; (ii) 1−x+y, 1−x, z; (iii) −x, −y, −z; (iv) x−y, x, −z;

(v) −x+y, −x, z; (vi) −y, x−y, z; (vii) y, −x+y, −z.

Figure 5. Molecular unit of 2-AT_HClO₄(3). Selected bond lengths (Å): Cl1–O1

1.4369(14), N1–C1 1.318(2), N1–N2 1.333(2), N2–N3 1.289(2), N2–N5 1.369(2), N3–N4

1.331(2), N4–C1 1.322(2); selected bond angles (°): O1–Cl1–O3 109.82(8), O2–Cl1–O4

108.65(8), N2–N1–C1 101.68(14), N1–N2–N3 116.77(15), N1–N2–N5 121.17(15), N2–

N3–N4 101.33(15), N3–N4–C1 110.89(15), N1–C1–N4 109.33(15).

The bond lengths and angles of the coordinating ligands in the

complexes analyzed are in the same range as in the non-

coordinating ligand

sion in any of the following coordination compounds.

The metal(II) perchlorate compounds of 1-AT ( ) all crystal-

lize isotypically in the trigonal space group P−3 with densities

between 1.736 (

) and 1.918 g cm^–3) (Mn: 1.736 g cm^–3

(143 K); Fe: 1.766 g cm^–3(173 K); Cu: 1.918 g cm^–3(173 K); Zn:

1.783 g cm^–3(173 K)). All complexes consist of two distinguish-

able metal centers with one exhibiting an ideal coordination

polyhedron and the other being slightly distorted. Six nitrogen-

1 and are therefore not part of the discus-

Figure 7. Unit cell of [Fe(1-AT)₆](ClO₄)₂(5) in the direction of view along the c-axis.

4–7

The copper(II) chloride coordination compounds of 1-AT and 2-

AT both crystallize in the form of green blocks in similar mono-

4

(6

clinic space groups P2₁/n (

formula units per unit cell respectively. The calculated densi-

ties at 173 K vary only slightly (2.146 g cm⁻³

and

2.177 g cm⁻³

16) respectively). The two complexes show com-

8) and P2₁/c (16) with four and two

(8)

(

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pletely different coordination spheres, which are highly de- copper(II) coordination sphere with significant Jahn-Teller

pendent on the inserted ligand. 2-Amino-5H-tetrazole acts as a distortion, compound consists of a sevenfold coordinated

9

monodentate ligand in complex 16 with bridging chlorido metal(II) center leading to the coordination polyhedron of an

ligands, which are responsible for the assembly of 1D polymer- unusual pentagonal-like bipyramid (Figure 9). The equatorial

ic chains. The 1-AT ligand instead shows a bidentate behavior positions are occupied by two chelating nitrate anions and one

in compound

8

leading to the formation of 2D polymeric lay- equivalent of

1 forming a distinctly deformed plane. The ideal

ers. The copper(II) center of

8

is octahedrally coordinated and properties of a pentagonal bipyramid prescribe the bond an-

bridged to polymeric planes over two chlorido and two 1- gles between the equatorial ligands to be 72° – a requirement,

amino-5H-tetrazole ligands (Figure 8). The equatorial positions which is definitely not fulfilled for the investigated complex

are occupied by one bridging and one terminal equivalent of due to the nitrate anion's fixed structure. In addition, the axis

each, the chlorido and the 1-amino-5H-tetrazole ligand, gener- through the two-remaining amino-5H-tetrazole ligands in axial

ating a plane. The coordination octahedron exhibits a Jahn- positions is crooked, differing considerably from the ideal 180°.

Teller distortion along the Cl2ⁱⁱ–Cu1–N5 axis with N5 being part Another deviation from the perfect pentagonal bipyramid can

of the second bridging equivalent of 8. Moreover, the struc- be observed regarding the bond angles between axial and

ture of the complex is close to an ideal octahedron with the equatorial ligands partially varying by up to 10° from the ex-

maximum deviation of the bond angles between two cis- pected 90°. The nitrate complex 17 crystallizes in the form of

ligands from 90° being 5.76° and of two trans-ligands from blue plates in the monoclinic space group P2₁/n with two for-

180° being 4.41°. In contrast the molecular unit of 16 consists mula units per unit cell and

a calculated density of

of four bridging chlorido and two terminal 2-amino-5H- 2.169 g cm⁻³at 173 K. This coordination compound consists of

tetrazole ligands forming an octahedral coordination sphere an octahedrally coordinated copper(II) center including a

(Figure 8). Once again, the d⁹-copper center shows a significant straight, Jahn-Teller distorted N5ⁱⁱ–Cu1–N5^ivaxis between the

Jahn-Teller distortion along the Cl1–Cu–Cl1^ivaxis. The equato- axial 2-AT ligands (Figure 9). The equatorial positions of 17 are

rial coordination sites are occupied by two bridging chlorido occupied by the remaining two equivalents of

2 and the coor-

ligands and the remaining two equivalents of 2-AT building up dinating nitrate anions building up an almost ideal plane. Every

an almost ideal plane. Considering the findings that the bond bond angle of cis coordinating axial or equatorial ligands in 17

angles between ligands in cis positions are close to the ideal slightly differs from the ideal 90°.

90° and angles amongst molecules coordinating trans to each

other are exclusively 180°, the coordination polyhedron shows

multiple signs of being an ideal octahedron, despite the previ-

ously discovered distortions in regard to the bond lengths.

Figure 9. Molecular unit of [Cu(NO₃)₂(1-AT)₃] (9) (left) and the copper(II) coordination

environment of [Cu(NO₃)₂(μ-2-AT)₂] (17) (right). Selected bond lengths (Å) of 9: Cu–O1

1.987(3), Cu–O2 2.637(3), Cu–O4 1.990(3), Cu–O6 2.742(4), Cu–N13 2.287(4), Cu–N4

2.003(4), Cu–N9 1.999(4); selected bond angles (°) of (9): N4–Cu–N9 162.97(13), O1–

Cu–O2 54.03(11), O1–Cu–N13 84.56(13), O2–Cu–O6 80.11(10), O4–Cu–O6 51.81(11),

O4–Cu–N13 89.95(13), O1–Cu–N4 92.57(15), O1–Cu–N9 88.03(14), O2–Cu–N4

82.95(12), O2–Cu–N9 83.65(12), O4–Cu–N4 90.25(14), O4–Cu–N9 90.85(14), O6–Cu–

N4 87.65(14), O6–Cu–N9 79.72(13). Selected bond lengths (Å) of (17): Cu1–N5ⁱⁱ

2.4208(16), Cu1–O1 2.0339(12), Cu1–N4 1.9831(16); selected bond angles (°) of (17):

O1–Cu1–N4 87.42(6), O1–Cu1–N5ⁱⁱ81.45(5), O1–Cu1–N5^iv98.55(5). Symmetry codes of

(17): (i) −x, −y, −z; (ii) 0.5+x, 0.5−y, 0.5+z; (iii) 0.5−x, 0.5+y, 0.5−z; (iv) 0.5−x, −0.5+y,

0.5−z.

Figure 8. Copper(II) coordination environments of [CuCl(μ-Cl)(1-AT)(μ-1-AT)] (8) (left)

and [Cu(μ-Cl)₂(2-AT)₂] (16) (right). Selected bond lengths (Å) of 8: Cu1–Cl1 2.2854(6),

Cu1–Cl2 2.3122(6), Cu1–N9 2.0113(19), Cu1–N5 2.725(2); selected bond angles (°) of 8:

Cl1–Cu1–Cl2 177.96(2), Cl2–Cu1–N5 93.90(5), N5–Cu1–Cl1 84.55(5), N9–Cu1–Cl1

90.78(5), Cl2–Cu1–N9 90.64(5), . Symmetry codes of 8: (i) 0.5+x, 1.5−y, 0.5+z; (ii) 1.5+x,

1.5−y, 1.5+z. Selected bond lengths (Å) of 16: Cu–Cl1 2.8124(6), Cu–N4 2.0422(19);

selected bond angles (°) of 16: Cl1–Cu–N4 90.25(5), Cl1–Cu–Cl1ⁱⁱ91.30(2), Cl1^iv–Cu–N4

89.76(5), Cl1ⁱⁱ–Cu–N4 88.84(6). Symmetry codes of 16: (i) 2−x, 2−y, −z; (ii) x, 1+y, z; (iii)

2−x, 1−y, −z; (iv) x, −1+y, z.

The spatial order of the investigated copper(II) nitrate complex

(

17) reveals some very interesting observations: Each of the

Analogous to the chlorido complexes, the copper(II) nitrate

coordinating 2-amino-5H-tetrazole ligands possesses bridging

characteristics and thus causes cross-linking of adjacent cop-

per(II) centers.

coordination compounds

9

and 17 show a completely different

and 16, 1-

metal(II) coordination environment. In contrast to

8

AT now behaves monodentately, a fact leading to the for-

mation of closed coordination spheres, whereas the bidentate

behavior of 2-AT causes the building-up of 2D polymeric layers.

The extremely sensitive copper(II) perchlorate complex 18 is

the only compound which crystallizes in the tetragonal space

group P4₂/n with two formula units per unit cell and a calcu-

lated density of 1.947 g cm⁻³at 173 K. Once again, the Jahn-

Teller distortion along the O1–Cu–O1ⁱⁱⁱaxis induces significant

deviations between the coordination distances of axial and

equatorial ligands (Figure 10). The four equatorial 2-AT ligands

The very interesting copper(II) nitrate complex

9 crystallizes in

the form of blue rods in the monoclinic space group Cc with

four formula units per unit cell and a calculated density of

1.968 g cm⁻³at 173 K. Despite the usually observed octahedral

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form a crooked plane, a fact that causes the bond angles be- the structural parameters, besides the substitution of the aqua

tween axial and equatorial ligands to slightly differ from the ligand by one additional equivalent of

ideal 90°. On the other hand, the angles between cis amino-

5H-tetrazole ligands in the plane almost perfectly conform to

90°. The angles amongst two equatorial ligands coordinating

trans to each other are also influenced by the obliqueness of

the N1–N1ⁱⁱⁱ–N1ⁱⁱ–N1ⁱplane (N1–Cu–N1ⁱⁱ177.77(12)°). The

analogous manganese(II) complex 15 crystallizes in the form of

colorless blocks in the triclinic space group P−1 with two for-

2, could be observed.

mula units per unit cell and

a calculated density of

1.897 g cm⁻³at 173 K. Compared to compound 18 the Mn–N

bonds are slightly longer than the Cu–N ones with simultane-

ous shorter Mn–O bonds (Figure S14).

Figure 11. Molecular unit of [Zn(H₂O)(2-AT)₅](ClO₄)₂(20). Selected bond lengths (Å): Zn–

O1 2.083(3), Zn–N4 2.146(3), Zn–N9 2.197(3), Zn–N14 2.147(3), Zn–N19 2.166(3), Zn–

N21 2.160(3); selected bond angles (°): O1–Zn–N21 176.80(11), N4–Zn–N14 177.92(11),

O1–Zn–N4 87.56(12), O1–Zn–N9 86.13(11), O1–Zn–N14 91.08(12), O1–Zn–N19

89.52(11), N4–Zn–N9 90.45(11), N4–Zn–N19 90.29(11), N4–Zn–N21 89.63(11), N9–Zn–

N14 87.89(11).

The cocrystallized 2-AT molecules show no considerable devia-

tions from the previously examined coordinating

2 referring to

the bond lengths and bond distances. The octahedral coordi-

nation spheres of the d¹⁰and d⁶metal centers zinc(II) and

iron(II) show no Jahn-Teller distortion but differ slightly from

the ideal octahedron (Figure 12 and S18). Nevertheless, the d⁹

copper(II) center possesses a typical Jahn-Teller distortion

along the axial Cu–N bonds (Figure S19). The coordination

distances between the metal(II) center and the coordinating

ligand are roughly within the same range as for the com-

pounds 19 and 20 but appear to be somewhat longer than in

the previous cases. All perchlorate counter-anions show no

significant distortion.

Figure 10. Molecular unit of [Cu(H₂O)₂(2-AT)₄](ClO₄)₂(18). Selected bond lengths (Å):

Cu–O1 2.311(4), Cu–N1 2.025(2); selected bond angles (°): N1–Cu–N1ⁱⁱ177.77(12), O1–

Cu–N1 91.11(9), N1–Cu–N1ⁱⁱⁱ90.02(8). Symmetry codes: (i) y, 0.5−x, 0.5−z; (ii) 0.5−x,

0.5−y, z; (iii) 0.5−y, x, 0.5−z; (iv) 1−y, −0.5+x, −0.5+z.

The coordination polyhedrons built up by five equivalents of

2

are completed by one aqua ligand forming an octahedral coor-

dination sphere around the respective metal center (Figure 11

and S17). In both complexes, the M²⁺–N distances are signifi-

cantly longer than the particular M²⁺–O coordination distances

between the metal(II) center and the aqua ligands. Regarding

the bond angles of the metal(II) centers, it becomes clear that

there is no major distortion to be observed. Nevertheless,

none of the bond angles of either of these perchlorate com-

pounds conform the ideal values of 90° for cis coordinating

ligands and 180° for molecules coordinating trans to each

other. The isotypic complexes 19 and 20, two coordination

isomers containing an iron(II) (19) or alternatively a zinc(II) (20

)

metal center both crystallize in the form of colorless blocks in

the triclinic space group P−1 with two formula units per unit

cell and calculated densities of 1.855 g cm⁻³

(

19

20) at 173 K. On an additional note, the perchlo-

rate counter-anions containing Cl2 are significantly distorted.

The investigated complexes 21 23, all including cocrystallized

) and

Figure 12. Molecular unit of [Fe(2-AT)₆](ClO₄)₂·2 2-AT (21). Selected bond lengths (Å):

Fe1–N4 2.2191(16), Fe1–N9 2.1618(16), Fe1–N11 2.2135(16), Fe1–N19 2.1868(16),

Fe1–N24 2.2157(16), Fe1–N29 2.1629(16); selected bond angles (°): N4–Fe1–N19

175.41(6), N4–Fe1–N9 89.38(6), N4–Fe1–N11 86.35(6), N4–Fe1–N24 91.91(6), N24–

Fe1–N29 90.62(6).

1.884 g cm⁻³

(

–

2-amino-5H-tetrazole, crystallize in the triclinic space group

P−1 with two formula units per unit cell and calculated densi-

( (23) (Fe:

ties between 1.784 g cm⁻³21) and 1.815 g cm⁻³

Powder Diffraction

1.784 g cm⁻³(173 K); Cu: 1.815 g cm⁻³(143 K); Zn: 1.812 g cm⁻³

(173 K)). In comparison to the very similar coordination com-

pounds 19 and 20, once again, no major differences regarding

Due to the possible formation of different species or poly-

morphs, X-ray powder diffraction measurements were carried

out in addition to elemental analysis to ensure the purity of

the bulk material. Figure 13 shows the powder diffraction

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patterns for the compounds 5, 6, 9 and 17. The results prove

the purity of the nitrogen-rich coordination compounds. Small

deviations between the measured and the calculated data sets

can be observed in all diffractograms and are caused by the

temperature differences between the single crystal and pow-

der diffraction experiments. The larger differences in the in-

Figure 14. SEM image (350 x magnitude) and EDX spectrum of compound 5.

tensities for the iron(II) and copper(II) complexes

5 and 6 can

be attributed to the heavy disorders in the perchlorate moie-

ties (Figure 13A) and B)).

Figure 15. SEM images (250 x magnitude (left) and 800 x magnitude(right)) and EDX

spectrum of compound 6.

Figure 13. Powder diffraction experiments (comparison of the measured and calculated

data) for the complexes 5 (A), 6 (B), 9 (C) and 17 (D).

Scanning electron microscopy (SEM) and energy dispersive X-ray

spectroscopy (EDX)

Since sensitivity data are strongly dependent on the grain size

and crystal habitus (microscope images S20–25), scanning

electron microscopy and energy-dispersive X-ray spectroscopy

were

Figure 16. SEM image (4500 x magnitude) and EDX spectrum of compound 9.

performed to explore the compounds’ morphology but also

the chemical composition. The results of the measurements

for compounds

5

,

6

,

9

and 17 are shown in Figures 14–17.

forms up to 200 µm-sized

The iron(II) perchlorate complex

5

blocks beside plate-like crystals. Aside from insignificant trace

amounts of silicon, the EDX spectrum did not show any other

atoms present in the complex.

Figure 17. SEM image (650 x magnitude) and EDX spectrum of compound 17.

Sensitivities and thermal stability

Complex

the block-like crystals except for small spikes. Compared to

compound , no other elements, which are not included in the

6, illustrated in Figure 15 reveals the consistency of

The behavior of all compounds toward heating in a certain

range was investigated by differential thermal analysis (DTA) at

a heating rate of β = 5 °C min⁻¹. The observed endothermic

events (melting, dehydration or loss of coordinat-

ing/cocrystallized AT ligands) as well as other critical tempera-

5

complex composition, can be observed in the EDX spectrum.

The SEM image of complex 15 exposes an amorphous constitu-

tion in comparison to its microscope images. The phase-purity

is confirmed by the EDX spectrum.

tures are listed in Table 1. DTA plots of the compounds

1–3, 8,

Compound 17 shows a plate-like morphology and compared to

9,

12, 16–23, and additional information regarding the thermal

nitrate complex

9 a lower content of carbon and nitrogen,

stabilities can be found in the Figures S27–30 in the Supporting

Information.

which is consistent with the chemical compositions of the two

coordination compounds.

In accordance with our previous findings and literature re-

34

ports, higher vaporization temperatures T_vap.

,

(and melting

temperatures) in combination with lower exothermic decom-

position temperatures can be observed for 1-AT and 2-AT, and

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the more N1 substitution pattern is existent at the te- possible and leads to an unstable intermediate, which deto-

trazole.^40,43Both tetrazoles are liquids under ambient condi- nates immediately without significant endothermic signal.

tions and start to vaporize slowly at certain temperatures until An exceptional endothermic peak could be detected during

both compounds reach their respective exothermic decompo- the DTA measurement of the zinc(II) nitrate coordination com-

sition stages and deflagrate, as the TGA plot of both ligands pound 12 and was therefore investigated into more detail by

clearly illustrates (Figure S26). The thermal stabilities of most thermal gravimetric analysis together with complex

of the investigated compounds lie in the satisfying range of ence sample (Figure 19).

4 as refer-

175–180 °C (except

tion temperatures can be observed for the 2-amino-5H-

tetrazolium perchlorate salt ( , T_exo.= 122 °C) and for the cop-

per(II) nitrato complexes (T_exo.= 161 °C) and 17 (T_exo.

157 °C). Notably the poor thermal stabilities for the nitrato

coordination compounds and 17 are not surprising and a

3). Nevertheless, the lowest decomposi-

3

9

=

9

known issue in the literature.¹⁵

Table 1. Thermal stability measurements of 1–9, 12, and 16–23 by DTA.^a

T_endo.^b(°C)

T_exo.^c(°C)

1-AT (1)

—

182

197

122

195

187

174

189

175

161

180

171

157

173

159

200

169

192

157

2-AT (2)

2-AT_HClO₄(3)

122

—

[Mn(1-AT)₆](ClO₄)₂(4)

[Fe(1-AT)₆](ClO4)₂(5)

[Cu(1-AT)₆](ClO₄)₂(6)

[Zn(1-AT)₆](ClO₄)₂(7)

[CuCl(μ-Cl)(1-AT)(μ-1-AT)] (8)

[Cu(NO₃)₂(1-AT)₃] (9)

—

Figure 18.

containing metal(II) perchlorate complexes 4–7.

Comparison of the thermal stabilities (β = 5 °C min⁻¹

) of the 1-AT

—

[Zn(1-AT)₆](NO₃)₂(12)

[Cu(μ-Cl)₂(2-AT)₂] (16)

[Cu(NO₃)₂(μ-2-AT)₂] (17)

[Cu(H₂O)₂(2-AT)₄](ClO₄)₂(18)

[Fe(H₂O)(2-AT)₅](ClO4)₂(19)

[Zn(H₂O)(2-AT)₅](ClO₄)₂(20)

[Fe(2-AT)₆](ClO₄)₂∙ 2-AT (21)

[Zn(2-AT)₆](ClO₄)₂∙ 2-AT (22)

[Cu(2-AT)₆](ClO₄)₂∙ 2-AT (23)

115

—

110

—

Figure 19.

TGA measurements (β = 5 °C min⁻¹) of selected compounds 4 and 12.

132

106

96

While no change in weight after heating has been detected for

the manganese(II) complex until it decomposes, a gradual

4

108

loss of coordinating 1-AT molecules beginning at 110 °C up to

its decomposition point for coordination complex 12 can be

observed. It can be assumed that this endothermic loss leads

to the formation of an intermediate species, which later de-

composes exothermically.⁴¹

a

Onset temperatures at a heating rate of 5 °C min^{–1 b}Endothermic peak, which

.

indicates melting, dehydration, or loss of coordinating/cocrystallized AT ligands. ^c

Exothermic peak, which indicates decomposition.

Relatively high thermal stabilities have been determined for

the water-free 1-AT based metal(II) perchlorate complexes

By comparison of complexes with the same transition metal(II)

but with varying AT isomers or anions (except

9 and 17), no

(Figure 18), which increase in the following order Cu^II(T_exo.

174 °C) < Fe^II(T_exo.= 187 °C) < Zn^II(T_exo.= 189 °C) < Mn^II(T_exo.

=

general trend regarding their exothermic decomposition tem-

peratures can be drawn.

195 °C). Analogical complexes with 1-methyl-5H-tetrazole

instead of 1-amino-5H-tetrazole (methyl vs. amino group)

showed, interestingly exactly the same tendency, which could

possibly be a useful indicator during the planning and design of

future high-temperature stable energetic complexes.¹⁵Dehy-

dration (complex 18 and 20) or loss of co-crystallized 2-amino-

The sensitivities toward impact and friction were assessed

according to BAM standard methods (1 of 6) together with the

electrostatic discharge sensitivity for all compounds. In addi-

tion, the compounds have been categorized in accordance

with the “UN Recommendations on the Transport of Danger-

ous Goods” using the determined sensitivities. An overview of

the sensitivities is given in Table 2. The ligands as well as all

complexes have to be considered as very sensitive to all ap-

plied stimuli (impact, friction and ESD) and should therefore be

categorized as primary explosives. The majority of the synthe-

5H-tetrazoles during heating of 21–23 indicated by endother-

mic signals strongly agree with the identified X-ray crystal

structures. Drying of the iron(II) monoaqua complex 19 is not

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sized coordination compounds can even be described as “ex-

tremely sensitive” with values smaller than 10 N against fric-

tion. Implementation of non-oxidizing chlorido anions into the

complexes lowered the performance on the one hand but on

the other hand led to a desensitization of the coordination

compounds.

Figure 20.

Moment of detonation of compounds 6 and 9 during the ball drop

Table 2. Sensitivities toward impact, friction, and ESD of 1–9, 12 and 16–23 compared

impact test.

to lead styphnate and lead azide⁴²

.

compound

IS^a(J)

< 1

3

FS^b(N)

64

ESD^c(mJ)

—

The sensitivity (Figure 21) of the 1-AT perchlorate complexes

against ball drop impact rises in the following order Fe²⁺

(25 mJ) < Zn²⁺(16 mJ) < Mn²⁺(14 mJ) < Cu²⁺(< 4 mJ) and in the

following order toward friction Zn²⁺(0.5 N) ~ Mn²⁺(0.5 N) <

Fe²⁺(< 0.1 N) ~ Cu²⁺(< 0.1 N).

1^d

2^d

36

—

3

0.2

10

4

0.5

11^e

Table 3. Ball drop impact sensitivities for 4–7 and 9 ^a

.

5

< 0.1

0.5

12

6

5

4

5

6

7

9

7

12

IS (mJ)

14

25

< 4

16

6

8

60

50

a

ball drop impact sensitivity determined with the 1 of 6 method in accordance

9

3

2.25

6

10

with the MIL-STD 1751A (method 1016). For the testing a 30 mg sample size, a

drop weight of a 0.500-inch diameter steel ball (m = 8.35 g) and a steel ball

guidance track, which rose 32-inches vertically from a steel base were used.

12

< 1

< 2

< 1

1

620^e

16

40

10

17

2

10

The implementation of oxidizing anions like nitrates or per-

chlorates into the nitrogen-rich coordination compounds, in

comparison to halides, generally leads to increased sensitivi-

ties toward mechanical stimuli (Figure 22).

18

0.4

10

19

1.5

< 1

2

0.15

0.5

10

20

10

21

22

0.1

25

0.2

30

23

1

< 0.2

1.5

1.2^e

0.02–1.0

6–12

lead styphnate^f

lead azide^f

2.5–5

2.5–4

0.1–1

Determined at a grain size of < 100 μm were

3

,

6

,

8

,

9,

16

,

17, and 20. The

sensitivities of the remaining complexes were determined at grain sizes

a

between 100 and 500 μm. impact sensitivity according to the BAM drop-

b

hammer test (method 1 of 6); friction sensitivity according to the BAM

c

friction tester (method 1 of 6); electrostatic discharge sensitivity (OZM

ESD tester); Impact: insensitive > 40 J, less sensitive ≥ 35 J, sensitive ≥ 4 J,

very sensitive ≤ 3 J; Friction: insensitive > 360 N, less sensitive = 360 N,

sensitive < 360 N and > 80 N, very sensitive ≤ 80 N, extremely sensitive

≤ 10 N. As per the UN Recommendations on the Transport of Dangerous

d

e

Goods. from reference.³⁴measured on our new OZM Electric Spark

XSpark10 device. ^ffrom reference.⁴²

The sensitivity against ball drop impact is a very realistic and

safety-relevant parameter for compounds with a potential

industrial application. Therefore, and in order to get a better

understanding of their stability, the sensitivity toward ball

drop impact has been determined for the very promising wa-

Figure 21.

Comparison of the ball drop impact and friction sensitivities of the

metal(II) 1-AT perchlorates 4–7.

ter-free [M^II(1-AT)₆](ClO₄)₂coordination compounds

the copper(II) nitrato complex (Figure 20). An overview of

4–7 and

9

the experimental results is given in Table 3.

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ure 25). Additional details on the test procedures can be found

in the Experimental Section of the SI.

Figure 23.

Moment of detonation of compound 22 during the hot needle (HN)

initiation shown in sequence.

Figure 24.

Moment of detonation of complex 22 during the hot plate (HP) test

shown in sequence.

Figure 22.

Stabilities of the copper(II) chlorido and nitrato complexes 8/9 and

Positive initiation tests caused by a DDT of PETN indicated by a

hole in the witness plate and fragmentation of the copper shell

16/17 based on the isomeric amino-5H-tetrazoles.

(Figure 26 and Figure S49–52) were observed for 3, 5, 6, 9, 17–

The same trend can be observed when

2 is applied as a ligand

19, and 21–23.

instead of 1. Therefore, exact tailoring of the desired optical

and energetic properties can be easily accomplished by varia-

tion of the corresponding coordination environment, facilitat-

ing the synthesis of a wide range of different materials (Figure

S 31/32).

Primary explosive suitability evaluation

Hot plate and hot needle tests of the most promising com-

pounds were carried out in order to evaluate their applicability

as suitable primary explosives (Figure 23/24 and S33–48) and

to observe their behavior toward fast heating with and with-

out confinement. An outline of the experimental results is

given in Table 4. All compounds showed strong detonations

during the tests, which was indicated by the heavy defor-

mations of the used copper plate and needles which were

Figure 25.

PETN initiation capability test setup.

used in the analysis (except

only sharply).

9, 12, and 17, which deflagrated

Table 4. Overview of the hot plate and hot needle test results for 3–7, 9, 12, and 17–

23.^a

Figure 26.

Positive PETN initiation tests for the coordination compounds 5 and 6.

compound

HP

HN

det.

def.

det.

def.

compound

HP

HN

def.

det.

def.

det.

3

4

det.

def.

17

18

19

20

21

22

23

def.

det.

Energetic coordination compounds bearing nitrates as anions

are usually unable to initiate common secondary explosives,

due to their lack of performance. Thus, it is even more

5

6

astounding that both copper(II) nitrate complexes

9 and 17,

which show great promise for the future, were able to initiate

PETN. Negative test results, indicated by intact copper shells

and plates were surprisingly obtained for the perchlorate

7

9

12

complexes

4,

7

and 20. An overview of the test results can be

^adef.: deflagration; det.: detonation.

found in Table 5.

On the basis of the positive results of the hot needle and hot

plate tests, which gave a first indication of its suitability as a

primary explosive, the initiation capability for most of the

promising compounds toward pentaerythritol tetranitrate

(PETN) as the main charge in copper shells was explored (Fig-

12 | J. Name., 2012, 00, 1-3

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Journal Name

ARTICLE

a

tive copper(II) nitrate complexes

9

and 17 were successfully be

Table 5. Results of PETN initiation tests of 3–7, 9 and 17–23.

initiated and are therefore one of the rare examples of laser-

ignitable nitrate complexes in literature.^47,48

compound

initiation

compound

initiation

3

4

pos.

neg.

pos.

neg.

pos.

18

19

20

21

22

23

pos.

neg.

pos.

Table 6. Laser initiation test results.

5

compound

0.15 mJ

0.17 mJ

26 mJ

6

5

det.

—

det.

—

7

6

9

det.

—

17

16

17

18

19

21

23

x

^apos.: positive; neg.: negative.

x

det.

—

x

—

Toxicity

det.

x

—

In order to evaluate the toxicity of the synthesized complexes,

the toxicological impact on aquatic organisms of the two un-

—

coordinated nitrogen-rich ligands

1 and 2 was measured using

det.

—

the commonly known luminescence marine bacterium Vibrio

Fischeri NRRL-B-11177 strain.⁴³Both tetrazoles can be classi-

fied as non-toxic (toxicity level after 30 min exposure: very

toxic < 0.10 g/L; toxic 0.10–1.00 g/L; non-toxic > 1.00 g/L) with

half the maximal effective concentration values EC₅₀of

(–: not tested, x: no ignition, dec.: decomposition, deflag.: deflagration,

det.: detonation). Operating parameters: current I = 6–7 A; voltage U = 4 V;

theoretical maximal output power P_max= 45 W; theoretical energy E_max

0.15–26 mJ; wavelength λ = 915 nm; pulse length τ = 0.1–15 ms.

=

4.58 g/L for

1 and 3.87 g/L for 2 determined after an incuba-

tion time of 30 minutes. The values obtained lie within the

range of 1-methyl-5H-tetrazole, the methylated analogue of

1,5H-tetrazole¹⁵, and confirm the recently observed trend of

higher toxicities for 2-subsituted tetrazoles in comparison to

their 1-subsituted isomers.⁴⁴

Figure 27.

Moment of detonation of complex 18 (left) and destroyed lens and

Laser initiation

sample holder (right) caused by the tremendous shockwave.

The optical initiation of energetic materials has received con-

siderable attention in the last decades and a lot of research

has been done from many scientists all over the world.^18,44–46

Laser initiation has several advantages over classical ignition

methods (thermal or mechanical) and could therefore be of

great interest for industrial applications (e.g. laser ignitable

ammunition or optical detonators).²⁰All here investigated

compounds suffer from their increased sensitivities toward

mechanical stimuli making them highly suitable for application

as lead-free primaries but unfortunately not for any use in

laser ignition systems. Therefore, the focus of this contribution

was placed on more academic questions like the laser ignitibil-

ity of the complexes at different energy thresholds. Only se-

lected colored coordination compounds showed a response to

laser irradiation differing in the required energy input depend-

ing on the isomer/metal(II)/anion combination. A summary of

the performed experiments is given in Table 6. More details on

the experimental setup can be found in the Supporting Infor-

mation. All complexes investigated except the copper(II) chlo-

ride complex 16 detonated heavily under the employed set-

tings resulting in deformations of the sample holder and de-

struction of the optical lens and sapphire glass due to the

extreme powerful nature of the complexes (Figure 27, Figure

S53–57). The complexes exhibited similar initiation thresholds

and could be ignited at the lowest applicable initiation energy

settable in the parameters of the lasers. Even the very sensi-

UV-Vis spectroscopy

The UV-Vis spectra in the solid state, illustrated in Figure 28

and S58, were measured for selected laser-ignitable complexes

in the range of 350–1000 nm with regard to any possible cor-

relation between the laser ignitability and the absorption of

the compounds at the corresponding wavelength. The investi-

gated optical properties of the complexes are compiled and

summarized in Table 7. The examined energetic coordination

compounds showed typical absorptions in the NIR, visible and

ultra-violet region resulting from the corresponding d-d transi-

tions, which are highly dependent on the implemented 3d

metal(II), anion and isomeric ligand. The exact mechanism of

the underlying laser initiation process (e.g. thermal, electroni-

cal) is still questionable and has not been understood com-

pletely.^49,50

All investigated complexes show a moderate absorption in the

laser wavelength region of 915 nm, which could be a possible

explanation for the laser ignitibility of the coordination com-

pounds since none of the colorless complexes (Mn^IIor Zn^II)

could be initiated. The laser ignitibility is not only linked to its

absorption behavior but also to several other parameters like

the coordination sphere of the corresponding transition met-

al(II) or its electronic configuration. One plausible explanation

of the laser ignitibility of the complexes and imaginable initia-

J. Name., 2013, 00, 1-3 | 13

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ARTICLE

Journal Name

tion pathway could be the formation of several hot spots in sulfonic acid (HOSA). The obtained isomeric mixture was puri-

the material caused by photothermal excitations after irradia- fied by column chromatography, preventing the highly risky

tion. It should be emphasized that contrary to literature as- distillation mentioned in the literature and yielded the prod-

sumings, complexes bearing the nitrate anion could be ignit- ucts in satisfying yields. Due to the great potential of 1-AT as

ed.⁴⁷

ligand in powerful energetic transition metal(II) complexes, a

selective, upscalable and isomerically pure route toward this

highly endothermic compound has been developed. The re-

ceived nitrogen-rich aminoazoles were comprehensively char-

acterized by diverse analytical methods enclosing heteronu-

clear ¹⁵N NMR spectroscopy, X-ray studies and toxicity deter-

minations. The toxicity assessments toward aquatic organisms

revealed the non-toxicity of both heterocycles. Transition

metal(II) complexes based on 1-methyl-5H-tetrazole as ligand

have shown great promise in the past but had not enough

power to be appropriate candidates as lead-free primary ex-

plosives. As obvious and logical consequence, N-aminoazoles

with a higher nitrogen content exhibiting the powerful outer

N–N single bond have been incorporated into the equivalent

complexes resulting in a tremendous performance increase

releasing several of the most energetic coordination com-

pounds synthesized so far. The present study features the

preparation, analysis and evaluation of 20 new lead, nickel and

cobalt-free energetic complexes, which are built-up by two

isomeric aminotetrazoles, four different metals and six varying

anions. The synthesized complexes, which show a great coor-

dination sphere diversity depending on the inserted compo-

nents, can be prepared by an easy upscalable and low-cost

synthetic route. The strategy of energetic coordination com-

pounds (ECC), which enables the fine-tuning of the optical and

energetic properties through easy change of the building

blocks (metal, anion and ligand) has proved once again its

suitability in the difficult and challenging research area of lead-

free primary explosives. The synthesis of the metal(II) perchlo-

rate coordination compounds based on 2-amino-5H-tetrazole

were highly dependent on the reaction conditions like stoichi-

ometry or solvent system and yielded either aqua complexes

or co-crystallized species. Single X-ray studies in combination

with powder diffraction and electron microscopy of the auspi-

cious and mostly water-free complexes gave interesting in-

sights and allowed correlations between the observed struc-

ture and their sensitivity against mechanical stimuli. After

comparing all compounds, the following trends can be formu-

lated: i.) iron(II) and copper(II) complexes are more sensitive

than manganese(II) and zinc(II) coordination compounds; ii.)

the more oxidizing the anion, the more sensitive the com-

pound; iii.) complexes with 1-AT are more sensitive than those

with 2-AT, which can be attributed to the anhydrous nature of

those. Laser initiation experiments executed with NIR light at a

wavelength of 915 nm showed fast deflagration-to-detonation

transitions at extremely low energy thresholds for most of the

coordination compounds and destroyed large parts of the laser

setup due to the destructive power of the complexes. The

initiation capability toward pentaerythritol tetranitrate (PETN)

filled copper shell detonators was tested for several pre-

selected complexes (hot plate and hot needle experiments)

and demonstrated the enormous potential of the synthesized

complexes as lead-free primary explosive replacements. Be-

Table 7. Measured optical properties of complexes 5, 6, 8, 9, 16, 17–19, 21 and 23.

a

b

compound

M

color

λ_d-d

λ₉₁₅/λ_d-d

5

Fe^II(hs)

Cu^II

yellowish

blue

871

677

768

686

796

689

647

868

795

655

0.97

0.27

0.92

0.68

0.63

0.50

0.37

0.95

0.93

0.35

6

8

Cu^II

green

blue

9

Cu^II

16

17

18

19

21

23

Cu^II

green

blue

Cu^II

blue

Fe^II(hs)

Cu^II

yellowish

blue

a

Absorption intensity maximum wavelength, which can be assigned to electron

d-d transitions in the measured range of 350–1000 nm; ^bquotient of the absorp-

tion intensity at the laser wavelength and the intensity at the d-d absorption

wavelength

Additional studies in the future should be carried out to obtain

further information regarding the initiation process, which

should cover the influence of the complexes´ structure and the

type of the mechanism.

Figure 28.

Solid state UV-Vis spectra of selected copper(II) and iron(II) com-

plexes. The step in the absorption intensity at 800 nm is caused by a detector

change. The UV-Vis spectra have only qualitative character.

Conclusions

Two isomeric amino-5H-tetrazoles (1-amino-5H-tetrazole (

1, 1-

AT) and 2-amino-5H-tetrazole (2, 2-AT)) with sensitivities al-

ready in the range of primary explosives were synthesized in a

simple one step reaction starting from 1,5H-tetrazole and the

commercially available amination reagent hydroxylamine-O-

14 | J. Name., 2012, 00, 1-3

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11 D. Chen, H. Yang, Z. Yi, H. Xiong, L. Zhang, S. Zhu and G.

Cheng, Angew. Chem., Int. Ed., 2018, 57, 2081.

12 J.-G. Xu, C. Sun, M.-J. Zhang, B.-W. Liu, X.-Z. Li, J. Lu, S.-H.

sides the lead-free, but not completely environmentally friend-

ly perchlorate compounds, the nitrato complexes show excel-

lent properties and can therefore be considered as promising

primary explosive candidates.

Wang, F.-K. Zheng and G.-C. Guo, Chem. Mater., 2017, 29

9725.

,

13 M. H. V. Huynh, M. A. Hiskey, D. E. Chavez, D. L. Naud and R.

D. Gilardi, J. Am. Chem. Soc., 2005, 127, 12537.

14 M. H. V. Huynh, M. D. Coburn, T. J. Meyer and M. Wetz-

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Chem. Soc., 2016, 138, 4685.

Associated Content

Supporting Information

Compounds overview; IR spectroscopy of

DTA plots of 12 and 16 23; Column diagrams of the

complexes 18 23; Hot plate and hot needle tests; Initiation

1–9, 12, and 16–23;

1–

3,

8,

9,

–

capability tests; Laser ignition tests; UV-Vis spectra of 18

, 19,

21, and 23; Experimental part and general methods.

The Supporting Information is available free of charge on the

RSC Publications website.

19 I. A. Ugryumov, M. A. Ilyushin, I. V. Tselinskii and A. S. Ko-

zlov, Russ. J. Appl. Chem., 2003, 76, 439.

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national Pyrotechnics Seminar, Colorado, July, 2016.

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Conflicts of interest

The authors declare no conflict of interest.

23 M. A. Ilyushin, I. V. Bachurina, A. V. Smirnov, I. V. Tselinsky

and I. V. Shugalei, Cent. Eur. J. Energ. Mater., 2009, 7, 33.

Acknowledgements

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Financial support of this work by the Ludwig-Maximilian Uni-

versity of Munich (LMU). The authors would like to thank Prof.

Dr. Thomas M. Klapötke for not only providing the research

facilities but also for his scientific support. In addition, the

authors would like to thank Prof. Dr. Konstantin Karaghiosoff

for the measurement of various ¹⁵N NMR spectra, Prof. Dr.

Jürgen Evers for the recording of several X-ray powder diffrac-

tions, Mrs. Cornelia Unger for toxicity assessments as well as

Jasmin Lechner, Alexander Kremsmair and Philipp Spiess for

their great contribution to this work.

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Article Doi

Maximization of the energy capability level in transition metal complexes through application of 1-amino- and 2-amino-5: H -tetrazole ligands

DOI: 10.1039/c8ta06326e

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