Explosive Werner-type cobalt( III ) complexes †‡

A series of potentially explosive Werner-type cobalt( III ) complexes comprising the anions azotetrazolate, nitrotetrazolate, picrate and dipicrylamide were prepared via simple metathetical routes. Treatment of [Co(NH 3 ) 5 NO 2 ]Cl 2 , trans -[Co(NH 3 ) 4 (py)NO 2 ]Cl 2 (py = pyridine), trans -[Co(NH 3 ) 4 (NO 2 ) 2 ]Cl, and [Co(NH 3 ) 5 N 3 ]Cl 2 with equimolar amounts of disodium azotetrazolate, (Na 2 C 2 N 10 ·5H 2 O, 1 ), in aqueous solutions a ﬀ orded new cobalt( III ) azotetrazolate salts [Co(NH 3 ) 5 NO 2 ](C 2 N 10 )·2H 2 O ( 2 ), trans -[Co(NH 3 ) 4 (py)- NO 2 ](C 2 N 10 )·2H 2 O ( 3 ), trans -[Co(NH 3 ) 4 (NO 2 ) 2 ] 2 (C 2 N 10 ) ( 4 ), and [Co(NH 3 ) 5 N 3 ](C 2 N 10 )·H 2 O ( 5 ) in moderate to excellent yields (46 – 88%). Similar treatment of trans -[Co(NH 3 ) 4 (NO 2 ) 2 ]Cl with 1 equiv. of sodium 5-nitrotetrazolate dihydrate (= NaNT, 6 ) a ﬀ orded the novel cobalt( III ) 5-nitrotetrazolate derivative trans - [Co(NH 3 ) 4 (NO 2 ) 2 ](NT)·H 2 O ( 7 ) as orange, rectangular prismatic crystals in 64% yield. Two complex cobalt( III ) picrates, trans -[Co(NH 3 ) 4 (NO 2 ) 2 ](picrate)·H 2 O ( 9 ) and [Co(NH 3 ) 5 N 3 ](picrate) 2 ( 10 ), were prepared in a similar manner from the corresponding chloride precursors and equimolar amounts of sodium picrate. The reaction of trans -[Co(NH 3 ) 4 (NO 2 ) 2 ]Cl with sodium dipicrylamide (= NaDPA) in a 1 : 1 molar ratio gave the ﬁ rst cobalt( III ) dipicrylamide, trans -[Co(NH 3 ) 4 (NO 2 ) 2 ](DPA)·H 2 O ( 12 ). Finally, the highly explosive, dark blue-green dichroitic non-electrolyte complex mer -[Co(en)(py)(N 3 ) 3 ] ( 13 ) was formed upon treatment of [Co(en)(py) 2 (NH 3 )Cl]Cl 2 ·H 2 O with excess NaN 3 in hot water (93% yield). The molecular and crystal structures of 2 , 3 , 4 , 5 , 7 , 9 , 10 , 12 , and 13 were determined by single-crystal X-ray di ﬀ raction. In the solid state, all compounds comprised extensive hydrogen-bonded supramolecular networks. Repre-sentative studies of the energetic properties (impact and friction sensitivity, combustion) revealed that some of the new compounds can be classi ﬁ ed as primary explosives.


Introduction
Primary explosives are highly sensitive explosive compounds which are used to initiate large amounts of secondary explosives such as 2,4,6-trinitrotoluene (= TNT) 1 in initiating devices like primers and detonators for commercial and military applications.Historically, one of the first practical primary explosives was mercury fulminate, Hg(CNO) 2 ("Knallquecksilber"), which has been widely used for almost 100 years. 2 Later, mercury fulminate was replaced by lead(II) azide, Pb(N 3 ) 2 , 3 and several forms of lead styphnate (= LS, Scheme 1) 4 which have been generally used as primary explosives until today.However, lead(II) azide shows severe disadvantages such as highly toxic hydroazoic acid, HN 3 , can be formed under certain conditions, 3 and the use of all these materials is always associated with lead pollution in the environment. 5For this reason, the search for environmentally acceptable ("green") primary explosives is currently a hot topic in this field. 6One of the most useful approaches is the development of nitrogenrich energetic compounds based on tetrazole and tetrazine derivatives. 7A highly promising candidate that has come out of these research efforts is the recently reported copper(I) 5-nitrotetrazolate (= DBX-1). 8Easily prepared DBX-1 has been shown to be one of the best "drop-in" lead(II) azide replacements to date due to its high thermal stability and comparable safety and performance.
Several Werner-type cobalt(III) complexes comprising 5-nitrotetrazolate and related ligands have also been synthesized and reported to have primary explosive properties.In particular the complexes shown in Scheme 2, pentaammine(5cyanotetrazolato-N 2 )cobalt(III) perchlorate (CP), 9 pentaammine (4,5-diaminotetrazole-N 2 )cobalt perchlorate, 10 and tetraamminebis(5-nitrotetrazolato-N 2 )cobalt(III) perchlorate (BNCP), 11 were shown to have suitable energetic properties.However, their perchlorate content makes them unacceptable as alternative primary explosives because perchlorate has various adverse health effects. 12ther potentially explosive Werner-type cobalt(III) complexes have frequently been described in the literature without any practical use.These comprise mainly some long-known azido complexes.While several series of complexes such as [Co-(NH 3 ) 5 N 3 ] 2+ , 13 cis-and trans-[Co(NH 3 ) 4 (N 3 ) 2 ] + , 14 and cis-and trans-[Co(en) 2 (N 3 ) 2 ] + (en = ethylene-1,2-diamine) 15 are often thermally quite stable (with the exception of some azides and perchlorates), there are several cases of highly explosive species.For example, several salts of the [Co(N 3 ) 6 ] 3− anion were found to be quite sensitive to impact and friction, 16 and the compound trans-tetraammine-diazido-cobalt(III)-trans-diammine-tetraazidocobaltate(III), [Co(NH 3 ) 4 (N 3 ) 2 ][Co(NH 3 ) 2 (N 3 ) 4 ], is described as a "dangerous detonator", but its energetic properties have not been studied in detail. 17Also notable is the (in)famous green non-electrolyte complex mer-[Co(NH 3 ) 3 (N 3 ) 3 ].It has been reported that this compound is highly impact-sensitive and can even explode upon grinding under water. 18n this contribution we report the synthesis, structural characterization, and energetic properties of a series of new explosive Werner-type cobalt(III) complexes.Our synthetic protocol involves combination of complex cobalt(III) cations with various nitrogen-rich or oxygen-rich, sensitive, secondary high explosive anions.For the present study, azotetrazolate, 5-nitrotetrazolate, picrate, and dipicrylamide were chosen as suitable representative energetic anions.Unlike the cobalt(III) complexes depicted in Scheme 2, the new compounds reported here contain the energetic anions in the outer sphere and not directly coordinated to the central Co 3+ cation.

Azotetrazolates
The azotetrazolate dianion, (C 2 N 10 2− ), has been found to be extremely useful in the design of new nitrogen-rich energetic salts. 19The disodium salt Na 2 C 2 N 10 •5H 2 O (1) is the most readily accessible azotetrazolate precursor described in the literature.It was first prepared by Thiele more than 100 years ago by oxidation of aminotetrazole with KMnO 4 in boiling sodium hydroxide solution (Scheme 3).This rather harsh method provides disodium azotetrazolate as its pentahydrate in the form of large, bright yellow crystals. 20ew Werner-type cobalt(III) complexes containing the azotetrazolate anion were synthesized according to Scheme 4 by metathetical reactions of chloride precursors with equimolar amounts of 1.As starting materials, several easily accessible penta-and tetraamminecobalt(III) chlorides containing nitro and azido ligands were chosen ([Co(NH 3  (5, 88%) crystallized within 24 h upon undisturbed standing of the reaction mixtures.The nitro complexes 2-4 formed yellow to orange crystals, whereas the azido complex 5 crystallized in the form of dark red rods.Recrystallization from hot water was possible but not necessary, as the products directly obtained from the original reaction mixtures were already quite pure.Routine characterization was mainly based on IR data and elemental analyses.As expected, these data provided only some basic information on the nature of the products.Thus they will be briefly discussed here only for compound [Co(NH 3 ) 5 NO 2 ](C 2 N 10 )•2H 2 O (2) as a typical example.Elemental analysis was consistent with the formation of the expected product as the dihydrate.The IR spectrum showed two intense absorptions which are characteristic for the free azotetrazolate dianion.A strong band around 730-740 cm −1 can be assigned to the asymmetrical C-NvN stretching mode of the azo group, whereas a strong band around 1390-1400 cm −1 is attributable to the asymmetrical N-CvN stretching mode of the ring.These characteristic bands are more or less the same in all compounds containing uncoordinated azotetrazolate dianions. 19In the IR spectrum of 2, the two bands appear at 740 and 1402 cm −1 .While 1 H NMR data were unavailable for the new complexes 2-5, the presence of uncoordinated azotetrazolate dianions could also be verified by the 13 C NMR data, measured in DMSO-d 6 solutions.In all four cases, the signal of the carbon atom of the azotetrazolate dianion appeared in the 13 C NMR spectra at δ = 173.1,which is characteristic of saltlike azotetrazolates. 19In the 13 C NMR spectrum of 3 the signals of the coordinated pyridine could be observed at δ = 8. 4, 7.8, and 7.3.All four new compounds were also structurally characterized by single-crystal X-ray diffraction.In all cases, suitable single-crystals were obtained directly from the original reaction mixtures.A summary of the crystallographic data and refinement parameters for all nine crystal structures reported in this study is given in Table 1.Fig. 1-12 show photographs of the single-crystals, ORTEP drawings of the molecular structures including selected bond lengths and angles, as well as packing diagrams of the hydrogen-bonded networks in the solid state.

Picrates
It is well established that anhydrous picric acid tends to be unstable, and its impact and friction sensitivities are higher than those of TNT.Nevertheless, numerous organic and inorganic picrate salts have been prepared and their crystal struc-tures were studied. 26Shreeve et al. reported mono and bridged azolium picrates as energetic salts.7c,27 Picrates of Werner-type cationic cobalt(III) ammine complexes have also been frequently reported, but mainly with respect to studying their solubility and/or crystallinity.A typical example is a publication by Ephraim in which the solubility of various complex cobalt(III) picrates was studied. 28These included e.g.[Co-(NH 3 ) 5 H 2 O]( picrate) 3 , [Co(NH 3 ) 4 (H 2 O) 2 ]( picrate) 3 , [Co(NH 3 ) 5 X]-( picrate) 2 (X = Cl, Br, I, NO 2 , NO 3 ) as well as cis-and trans-[Co-(NH 3 ) 4 (NO 2 ) 2 ](picrate).Although the energetic properties of these compounds were not explicitly studied, it was noted that combustion analyses proved difficult due to regularly occurring explosions. 28In the course of our study, two Werner-type complex cobalt(III) picrates were prepared as outlined in Scheme 7.
Sodium picrate, prepared in situ by neutralizing picric acid with NaOH, was subsequently treated with either trans-[Co- 9) and [Co(NH 3 ) 5 N 3 ]( picrate) 2 (10), which precipitated from the concentrated aqueous solutions, were isolated by filtration and purified by recrystallization from a minimum amount of hot water.Complex 9 was obtained in 57% yield as large, orange blocks (Fig. 16), whereas complex 10 (87% yield) formed dark red rods (Fig. 19).Complex 9 has already been mentioned in the early work by Ephraim. 28Notably, this author described no less than four different crystal forms for this compound, depending on the recrystallization conditions: (1) yellow, hair-like crystals of several centimeters in length; (2) glistening needles and platelets of rhombic shape; and (3) thick, heavy prism-like crystals.As can be seen in Fig. 16, we apparently obtained only the latter sort of crystals.Both the IR and 13 C NMR data of 9 and 10 showed the typical values for the picrate anion.Both complexes were also structurally characterized by single-crystal X-ray diffraction (Table 1, Fig. 16-21 and ESI ‡).
The dinitro complex 9 crystallizes in the triclinic space group P1 ˉ.Fig. 17 shows that the unit cell contains two cations, two anions and two water molecules.All bond lengths and angles in the individual components show non-significant deviations from those reported in the literature. 21,22Fig. 18 illustrates the supramolecular hydrogen-bonded crystal structure of 9.The two water molecules, the ammine and nitro ligands of the trans-[Co(NH 3 ) 4 (NO 2 ) 2 ] + cation, and the nitro groups of the picrate anion all participate in the complex hydrogen-bonded network.The azido derivative 10 also crystallizes in the triclinic space group P1 ˉ, but has no water of crystallization.As can be seen in Fig. 21, all ligands in the [Co-(NH 3 ) 5 N 3 ] 2+ dication as well as the oxygen atoms of the phenoxide and nitro groups of the picrate anions participate in the N-H⋯O and N-H⋯N hydrogen bonds forming the supramolecular network (see ESI ‡ for full details).

Dipicrylamides
Dipicrylamine (2,2′,4,4′,6,6′-hexanitrodiphenylamine, Scheme 8) combines several interesting structural features in that it contains six nitro groups which are flexible and can interact and   4), Co-N(3) 1.9537 (10), Co-N(3)#2 1.9537 (10), Co-N(3)#3 1.9537 (10), Co-N(3)#4 1.9537 (10), C(1)-N(4) 1.438(6), O(4)-N(4)-C(1) 117.44 (16).(#1 −x + 1, −y + 0, z + 0; #2 −x + 0, y + 0, z + 0; #3 x, −y + 1, z; #4 −x + 0, −y + 1, z + 0).adjust in the crystal lattice and in that it has a secondary amine group which can be deprotonated with alkali and alkaline earth metal hydroxides to form water soluble salts.In the resulting dipicrylamide anion (= DPA − ), partial delocalization of the negative charge mediated by the aromatic rings is possible, which may facilitate coordination of the oxygen atoms of the nitro groups with suitable metal ions. 29he ammonium salt of dipicrylamine, also known as Aurantia or Imperial Yellow, was discovered in 1874 by Gnehm and used as a yellow colorant for leather, wool, and silk until the early 20th century. 30However, this use has been terminated due to the highly toxic and explosive nature of dipicrylamine. 31Dipicrylamine can also be used for the extraction of K + ions from sea bittern (a mixture of K + , Na + , and Mg 2+ ).32a A related study carried out with a mixture of K + , Rb + , and Cs + revealed that the Cs + ion shows maximum selectivity towards DPA − .32b In fact, it has been reported that DPA − can be used for the recovery of Cs + from radioactive wastes. 33Only recently, the structural chemistry of alkali metals and alkaline earth metals as well as ammonium and azolium dipicrylamides has been investigated in detail.All these compounds display interesting hydrogen-bonded supramolecular structures in the solid state. 29 Werner-type cobalt(III)-ammine complex of dipicrylamide was prepared according to Scheme 9 by treatment of trans-[Co-(NH 3 ) 4 (NO 2 ) 2 ]Cl with 1 equiv. of in situ-prepared NaDPA (Scheme 9).
Complex 12 crystallizes in the triclinic space group P1 ˉ.
Fig. 23 shows that the unit cell contains two crystallographically independent trans-[Co(NH 3 ) 4 (NO 2 ) 2 ] + cations and two DPA anions.The structural parameters (bond lengths and angles) in the dipicrylamide anions show only little differences when compared to those reported for the alkali, alkaline earth metals, ammonium or azolium salts of dipicrylamide. 29 The non-electrolyte complex mer-[Co(en)( py)(N 3 ) 3 ] ( 13) Well known from the early studies on Werner-type cobalt(III) azido complexes is the non-electrolyte complex mer-[Co-(NH 3 ) 3 (N 3 ) 3 ].The highly explosive material was first reported by Linhard and Weigel in 1950. 18 This dark blue-green compound was found to form in various occasions when aquo-or azido-cobalt(III) ammine complexes are treated with excess sodium azide.13) was isolated in 93% yield as a dark blue-green crystalline solid (Scheme 10).As expected for a non-electrolyte complex, the solubility of 13 in water is very low, although the resulting very dilute solutions show an intense dark green color.This material was found to be unpredictably and dangerously explosive.It should thus be prepared and handled only in very small amounts and with utmost care.Compound 13  was even too dangerously explosive to be fully characterized by IR and elemental analysis.Attempts to obtain an IR spectrum of 13 were unsuccessful, because an explosion occurred upon grinding of ca.2-3 mg with ca. 100 mg of KBr.As a derivative of the unknown cobalt triazide, compound 13 has a high nitrogen content of ca.52% which certainly accounts for its highly explosive nature.Despite all the dangerous properties, it proved possible to structurally verify the nature of 13 by singlecrystal X-ray diffraction.Even this task was met by major obstacles.Recrystallization from hot water was impossible due to the low solubility of 13.Thus small portions of the originally obtained material were spread on glass sample holders and searched under a microscope for suitable crystals.It turned out that most of the crystals were strongly intergrown, with most of them having the appearance of Christmas trees.Notably, like mer-[Co(NH 3 ) 3 (N 3 ) 3 ], 18 the crystals of 13 were blue-green dichroitic.Finally, a small blue-green crystal fragment was found to be suitable for X-ray diffraction (Table 1 and Fig. 25 and 26; see also ESI ‡).As shown in Fig. 25, the three azide ligands in 13 are arranged in meridional positions as in the parent non-electrolyte mer-[Co(NH 3 ) 3 (N 3 ) 3 ]. 18The Co-N bond lengths to the two opposing azide ligands are identical (Co(1)-N(4) 1.946(2), Co(1)-N(2) 1.948(2) Å), whereas that to the azide ligands trans to an amino group of ethylenediamine is slightly longer (Co(1)-N( 7) 1.962(2) Å).The packing diagram of 13 shows a polymeric hydrogen-bonded network (Fig. 26).In this case, supramolecular association of the complex molecules occurs only via N⋯H-N hydrogen bonds between terminal azide nitrogen atoms and the NH 2 groups of the coordinated ethylenediamine.

Energetic properties of the new cobalt(III) complexes
The methods for testing primary explosives have just been summarized in a very informative short review article by Mehta et al. 35 In addition to a simple combustion test, the impact and friction sensitivity were tested according to established BAM methods using a BAM drophammer and BAM friction tester (Fig. 27 and 28).1c The initial test results of the energetic properties of seven cobalt(III) complexes prepared in the course of this study are listed in Table 2.Not included in the table is the dipicrylamide derivative 12 because it was found to be insensitive to both impact and friction.Also not  included is the non-electrolyte complex mer-[Co(en)( py)(N 3 ) 3 ] ( 13).This complex was found to be too sensitive to be handled safely.All tests were repeated six times over a storage period of 30 days.These test series revealed no significant deviations from the originally measured values, indicating a good long-term stability of the title compounds.Moreover, the drophammer tests were repeated in the temperature range of −55 °C to 20 °C in order to get an impression of the low-temperature performance.For example, hunting and sports ammunition is expected to show good performance at temperatures down to −25 °C, whereas military ammunition should function even at temperatures as low as −55 °C.Thus, drophammer tests of all seven compounds were performed at −55 °C, −30 °C, 0 °C, 10 °C, and 20 °C.In all cases these tests revealed an unexpected increase of the impact sensitivity by 1-2 J at temperatures below 0 °C which remained constant to −55 °C.
Although this effect may be small, it is significant and repro-  ducible.In summary, all energetic cobalt(III) complexes showed a stable performance over the entire temperature range of 75 degrees.
Both picrates 9 and 10 can also be classified as primary explosives.With values of 8 (9) and 7.5 (10) they are slightly less sensitive to impact than lead(II) styphnate or lead(II) azide and at the same time much less friction-sensitive.However, both samples exploded upon ignition with a flame.With 27.10% (9) and 30.53% (10) the nitrogen content is still quite good, while the negative oxygen balance can be explained by the high carbon content of these picrates.In view of the easy accessibility of these and related complexes, such Werner-type cobalt(III) picrates certainly merit further investigation as alternative energetic materials.As mentioned above, the dipicrylamide complex 12 was found to be insensitive to both impact and friction (>40 J and >360 N, resp.).This is in agreement with recent findings by Zhou et al. who reported that the impact sensitivities of various ammonium and azolium dipicrylamides are in the range of that of the secondary explosive TNT.29c In view of the known high toxicity of dipicrylamine, 29c,30 this energetic anion does not appear to be a useful alternative for lead(II)-containing explosives anyway.In contrast, the non-electrolyte complex 13 could not be safely tested as even minor samples of 2-3 mg could explode in an unpredictable manner.(10), N(4)-Co(1)-N( 10) 174.59 (10).

Conclusions
In summarizing the work reported here, we have investigated the synthesis, structures, and energetic properties of a series of novel Werner-type cobalt(III) ammine complexes comprising the energetic anions azotetrazolate, nitrotetrazolate, picrate and dipicrylamide in the outer sphere, i.e. not directly coordinated to the central Co 3+ ion.The title compounds are accessible in a straightforward manner by simple salt metathesis reactions using readily available starting materials.Also easily prepared, though unpredictably and dangerously explosive, is the new dark blue-green non-electrolyte complex mer-[Co(en)( py)(N 3 ) 3 ] (13).The crystal structures of all new complexes are characterized by extensive hydrogen-bonded networks.In addition to the structural authentication by X-ray diffraction, the energetic properties of these explosive Wernertype cobalt(III) complexes have been tested.The tests revealed that several of the title complexes can be classified as primary explosives.Particularly promising in that respect are the compounds 5 and 10 which contain the [Co(NH 3 ) 5 N 3 ] + cation, as

Single crystal X-ray crystallography
The intensity data of 2, 3, 4, 5, 7, 10, and 13 were collected on a Stoe IPDS 2 T diffractometer with MoK α radiation.The data were collected with the Stoe XAREA 39 program using ω-scans.The space groups were determined with the XRED32 39 program.The intensity data of 9 and 12 were registered on an Oxford Diffraction Nova A diffractometer using mirror-focused CuK α radiation.Absorption corrections were applied using the multi-scan method.The structures were solved by direct methods (SHELXS-97) and refined by full matrix least-squares methods on F 2 using SHELXL-97. 40autionary note All compounds described in this study are sensitive energetic materials which should only be synthesized and manipulated on a small (<250 mg) scale using proper safety equipment including thick leather gloves and jackets, face shields and blast screens, ear plugs and plastic or Teflon laboratory equipment.Care should be taken not to extensively dry or heat these materials as the anhydrous complexes are even more sensitive and prone to explosion on manipulation.In particular, the non-electrolyte complex 13 is extremely sensitive to impact and friction and should be handled with utmost care.
gen bonding in all crystal structures reported here can be found in the ESI.‡NitrotetrazolatesExplosive materials comprising the 5-nitrotetrazolate anion, [CN 4 NO 2 ] − (= NT), play an important role in the chemistry and technology of nitrogen-rich energetic materials.Various salts 21 and transition metal complexes 22 containing nitrotetrazolate have already been prepared and tested.Recently, we described
The best synthetic method (94% yield) involves the reaction of [Co(NH 3 ) 4 (H 2 O) 2 ](ClO 4 ) 3 with a large excess of NaN 3 .Almost quantitative yields of mer-[Co-(NH 3 ) 3 (N 3 ) 3 ] can also be obtained in a simple manner by airoxidation of a mixture of CoSO 4 •7H 2 O, (NH 4 ) 2 SO 4 and NaN 3 in aqueous ammonia solution.It was noted that even small amounts of mer-[Co(NH 3 ) 3 (N 3 ) 3 ] could lead to dangerous detonations upon heating grinding or impact.Detonations were even observed when larger crystals were ground under water.18We now succeeded in the preparation and structural characterization of a new non-electrolyte complex formally derived from cobalt(III) triazide.To an aqueous solution of [Co(en)-( py) 2 (NH 3 )Cl]Cl 2 •H 2 O 34 was added a ca.20-fold excess of NaN 3 and the reaction mixture was heated on a steam bath until all the water had evaporated.During the reaction a color change from red to dark green occurred and the smell of pyridine became evident.Unreacted sodium azide was easily removed by repeated washing of the dark residue with cold water.After drying in air, the new non-electrolyte complex mer-[Co(en)( py)-(N 3 ) 3 ] (

Fig. 27
Fig. 27 BAM drophammer setup used in this study.

Fig. 28
Fig. 28 BAM friction test setup used in this study.