Less sensitive oxygen-rich organic peroxides containing geminal hydroperoxy groups †

Organic energetic materials usually contain carbon, hydrogen, nitrogen, and oxygen, and tend to be nitrogen rich to increase the energy content through formation of highly stable dinitrogen upon detonation. While the explosive nature of organic peroxides is widely recognized, due to the presence of weak O–O bonds (45–50 kcal mol ), detailed energetic materials properties have only been reported for triacetone triperoxide (TATP), diacetone diperoxide (DADP), hexamethylene triperoxide diamine (HMTD), and methyl ethyl ketone peroxide (MEKP). The high sensitivities of TATP, DADP, HMTD, and MEKP toward impact, friction, and other stimuli have precluded civilian and military energetic materials applications due to safety concerns. These high sensitivities, coupled with the widely publicized use of TATP by terrorists, have likely discouraged broader exploration of organic peroxides as energetic materials. To allow practical applications, it will be necessary to identify organic peroxides that combine high energy contents with reduced sensitivities toward stimuli. In this direction, a recent report demonstrated that cocrystals of DADP and 1,3,5-triiodo-2,4,6-trinitrobenzene (TITNB) have reduced impact sensitivity compared to both pure DADP and TITNB, because of stabilizing I O close contacts in the cocrystals. Herein, we describe the synthesis, structure, and energetic materials properties of five new organic compounds (1–5) that each contain two geminal methylene bis(hydroperoxy) moieties. These compounds have oxygen to carbon ratios ranging from 0.80 to 1.33. Four of the new compounds are significantly less sensitive toward impact and friction than TATP, and the detonation velocity and detonation pressure of one compound are higher than those of 2,4,6-trinitrotoluene. Geminal hydroperoxides 1–5 were synthesized by treating the corresponding ketones or aldehyde with 30–50 wt% H2O2 in the presence of iodine (method A, 1, 3–5) or concentrated HCl (method B, 2) as a catalyst using published general procedures for geminal hydroperoxides (Scheme 1). Compounds 1–5 were characterized with H and C NMR spectroscopy, infrared spectroscopy, and elemental analyses. Additionally, X-ray crystal structures of 4 and 5 H2O were determined. A low resolution X-ray crystal structure of 1 Et2O confirmed the molecular structure. Solvates 1 Et2O and 5 H2O were used only for the crystallography experiments; unsolvated 1–5 were used for all other measurements. Attempts to prepare the geminal hydroperoxides derived from cyclohexane-1,3,5-trione, cyclohexane-1,2,3,4,5,6hexaone, and benzene-1,3,5-tricarbaldehyde led to violent gas evolution, likely due to the instability of the products.

3][4] The high sensitivities of TATP, DADP, HMTD, and MEKP toward impact, friction, and other stimuli have precluded civilian and military energetic materials applications due to safety concerns. 2These high sensitivities, coupled with the widely publicized use of TATP by terrorists, 2 have likely discouraged broader exploration of organic peroxides as energetic materials.To allow practical applications, it will be necessary to identify organic peroxides that combine high energy contents with reduced sensitivities toward stimuli.In this direction, a recent report demonstrated that cocrystals of DADP and 1,3,5-triiodo-2,4,6-trinitrobenzene (TITNB) have reduced impact sensitivity compared to both pure DADP and TITNB, because of stabilizing IÁ Á ÁO close contacts in the cocrystals. 6rein, we describe the synthesis, structure, and energetic materials properties of five new organic compounds (1-5) that each contain two geminal methylene bis(hydroperoxy) moieties.These compounds have oxygen to carbon ratios ranging from 0.80 to 1.33.Four of the new compounds are significantly less sensitive toward impact and friction than TATP, and the detonation velocity and detonation pressure of one compound are higher than those of 2,4,6-trinitrotoluene.
The thermal behaviour was studied with thermogravimetry.Compounds 1-5 show onsets of thermal decomposition between 98 and 117 1C (Table 1).CBS-4M electronic enthalpies were calculated with the Gaussian09 software package to obtain heat of formation values. 9The heat of formation values are all exothermic, ranging from À703.6 to À418.2 kJ mol À1 .Compound 4 has the most positive heat of formation.
A perspective view of 4 is shown in Fig. 1.This is the only compound among 1-5 for which unsolvated single crystals could be grown.Compound 4 has a crystalline density (1.648 g cm À3 at 100 K) that is slightly lower than those of orthorhombic (1.704 g cm À3 at 123 K) and monoclinic (1.713 g cm À3 at 100 K) 2,4,6-trinitrotoluene (TNT). 10Since the formula weights of 4 and TNT are similar (Table 1), 4 packs nearly as efficiently as TNT in the solid state.TNT does not contain any strong hydrogen bonds, and only van der Waals forces are present. 10By contrast, the lattice of 4 contains intermolecular O-HÁ Á ÁO hydrogen bonds, where the hydrogen atom on O1 is donated to O4 0 and the hydrogen atom on O4 is donated to O1 0 .The oxygen-oxygen distance in this interaction is 2.701 Å.This configuration results in O1 and O4 being both hydrogen bond donors and acceptors.Additionally, there are close contacts between O2 and O2 0 (2.912 Å) and C3-H5 0 (2.896 Å).These contacts are within or at the edge of the van der Waals radii for OÁ Á ÁO (3.04 Å) and CÁ Á ÁH (2.80 Å). 11 Recent studies of energetic materials have shown that such close contacts are attractive because the dispersion forces are larger than the repulsive Coulombic forces. 12Dissociation energies of OÁ Á ÁO close contacts are similar to those of weak hydrogen bonds (3-13 kJ mol À1 ). 12 Table 1 gives energetic test results for 1-5, with TNT and TATP for comparison.Impact, friction, and electrostatic discharge sensitivities were determined with a BAM drop hammer, a BAM friction tester, and an electrostatic discharge tester using standard test methods. 13Sensitivity classifications are based on the ''UN Recommendations on the Transport of Dangerous Goods''. 14Energetic performance was calculated using the EXPLO5 V6.02 software. 15Compounds 1-5 are ''very sensitive'' toward impact, 14 with values ranging from o1 to 3 J.They are ''extremely sensitive'' toward friction, 14 with values of 5 N for 1-3 and o5 N for 4 and 5.The electrostatic discharge sensitivity values for 1-5 are much greater than electrical discharges that can be created by the human body (r0.02J 1 ), so they can be safely handled.
The calculated detonation velocities of 1-5 range from 6150 to 7130 m s À1 (Table 1).The increase in detonation velocities in going from 1 to 3 parallels the increasing oxygen to carbon ratios and increasing crystalline density.Compound 4 has the highest detonation velocity (7130 m s À1 ) and the highest crystalline density (1.648 g cm À3 at 100 K) among 1-5.
3][4] Compound 5 is much more sensitive than 1-4, with values similar to those of TATP.The higher sensitivity of 5 may arise from its high oxygen to carbon ratio of 1.33.The calculated detonation velocities for 1-5 are much higher than that of TATP, most likely due to the higher crystalline densities and greater oxygen to carbon ratios.The calculated detonation velocity and detonation pressure of 4 are higher than those of TNT (Table 1).Thus, 4 is highly energetic.Typical primary energetic materials  This journal is © The Royal Society of Chemistry 2015 have impact and friction sensitivities of r4 J and r10 N, respectively, but must be safe enough to handle. 1 Compounds 1-4 have sensitivity values in this range, and are the first organic peroxides that might be safely used as primary explosives.][4] Despite their less sensitive nature, the thermal decomposition temperatures of 1-4 will need to be increased to allow use as primary energetic materials.Interestingly, the impact
(1-3 J) and friction (B5 N) sensitivity values for 1-4 are similar, and do not vary with the nature of the organic framework and increasing O : C ratios from 1-4.This lack of a trend is consistent with the O-O linkages being the ''trigger bonds'' that initiate decomposition upon cleavage.The solid state structure of 4 reveals intermolecular O-HÁ Á ÁO hydrogen bonds, as well as several OÁ Á ÁO and CÁ Á ÁH close contacts.The hydrogen bonds and attractive close contacts may serve to stabilize the labile O-O bonds and buffer them toward shock, thereby reducing the sensitivities of 1-4.Intermolecular IÁ Á ÁO close contacts also lead to reduced sensitivity in cocrystals of DADP and TITNB. 6For comparison, the solid state structures of highly sensitive DADP and TATP lack O-HÁ Á ÁO hydrogen bonds and OÁ Á ÁO close contacts, and contain only very weak OÁ Á ÁH and CÁ Á ÁH interactions.3b The stronger hydrogen bonds and close contacts are likely important stabilizing features in 1-4.Finally, this work demonstrates that careful manipulation of organic peroxide structures can lead to compounds with useful energetic materials properties.The authors acknowledge generous support from the Office of Naval Research (Grant No. N00014-12-1-0526 to C.H.W., Grant No. N00014-12-1-0538 to T.M.K.).

Table 1
Sensitivities and energetic performance of 1-5, TNT, and TATP a BAM drophammer.b BAM friction.c Electrostatic discharge sensitivity.d Oxygen balance for CO 2 .e Decomposition temperature from DTA (5 1C min À1 ).f Room temperature density estimation without solvent.g Calculated molar enthalpy of formation.h Total energy of detonation.i Detonation pressure.j Detonation velocity.k Volume of detonation products.l Values from ref. 3h and 16. m Values from ref. 10.