Technical Comments

Response to Comment on “Synthesis and characterization of the pentazolate anion cyclo-N5 in (N5)6(H3O)3(NH4)4Cl”

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Science  16 Mar 2018:
Vol. 359, Issue 6381, aas8953
DOI: 10.1126/science.aas8953

Abstract

Huang and Xu argue that the cyclo-N5 ion in (N5)6(H3O)3(NH4)4Cl we described in our report is theoretically unfavorable and is instead protonated. Their conclusion is invalid, as they use an improper method to assess the proton transfer in a solid crystal structure. We present an in-depth experimental and theoretical analysis of (N5)6(H3O)3(NH4)4Cl that supports the results in the original paper.

In their comment, Huang and Xu (1) bring up the point that the ellipsoid of the O2 atom in the asymmetry unit plot of the pentazolate salt was much larger than those of the remaining atoms in (N5)6(H3O)3(NH4)4Cl, and conclude that there should actually be less electron density than assigned in the model. They claim the O2 atom could also be a nitrogen atom. However, they ignore the anisotropic displacement parameters of the disordered atoms; thus, such a conclusion is meaningless. Huang and Xu also calculated separate ion pair N5⋅⋅⋅H3O+ energetics for the solid crystal structure and drew the conclusion that HN5 should exist in place of N5.

In our paper (2), the (N5)6(H3O)3(NH4)4Cl crystal containing the stabilized pentazolate anion was prepared by the rupture of the C–N bond in 3,5-dimethyl-4-hydroxyphenylpentazole through treatment with m-chloroperbenzoic acid (m-CPBA) and ferrous bisglycinate [Fe(Gly)2]. Although the high-quality x-ray crystallographic data provided the visualized proof of the structure of the pentazolate salt, confirmation of its structure and understanding of the cation-anion interactions have puzzled and intrigued researchers. Initial attempts via nuclear magnetic resonance (NMR), mass spectrometry (MS), and infrared (IR) spectroscopy to confirm the structure of (N5)6(H3O)3(NH4)4Cl were complicated. Fortunately, the expected single crystals were obtained by low-temperature evaporation of a solution in ethanol and ethyl acetate. At the very early stage, HN5 was considered the most likely structural unit in pentazolate salt on the basis of NMR and MS spectra. However, the x-ray structure determination suffered from charge balance and the cyclo-N5 showed high symmetry with five equal N-N bond lengths. We concluded that the cyclo-N5 should exist in its anionic form rather than its acid form, HN5. The non-hydrogen atoms were identified by their peak weights and sources from starting materials. Cl atoms gave the highest peak weights; N atoms gave the lowest. After all the non-hydrogen atoms were confirmed and refined, all the H atoms were smoothly located from Q maps from the high-quality crystal data. Each N atom was bonded to four H atoms, which gave an NH4+ cation. Each O atom was coordinated by six atoms. After searching similar ions and considering electric neutrality, the O atom was assigned as H3O+ with disordered H atoms with half occupancy. There were two oxygen atoms in the ellipsoid plot of (N5)6(H3O)3(NH4)4Cl, one a bit larger, the other cigar-shaped. The difference can be attributed to the disorder of the water molecules in the crystal structure, rather than to incorrect atom assignment (3).

Shown in Fig. 1, A and B, are two 15N NMR spectra of (N5)6(H3O)3(NH4)4Cl in which the essential difference is the 15N labeling in their starting material, 3,5-dimethyl-4-hydroxyphenylpentazole. The labeling effect with 15N isotope can be discerned by analyzing the spectra (4). The unlabeled pentazolate salt shows a lone visible 15N signal resonant at –356.18 ppm, whereas the labeled pentazolate salt has the same signal at –358.61 ppm. Both signals are reasonably assigned to NH4+ (5). The chemical shift difference of ~2.4 ppm comes from the 15N labeling of the NH4+ cation, which indicates that the NH4+ cation is derived from the N atom bonded to the aryl ring in aryl pentazole through oxidative decomposition.

Fig. 1 Spectra.

(A) 15N NMR spectrum of (N5)6(H3O)3(NH4)4Cl. (B) 15N NMR spectrum of (N5)6(H3O)3(NH4)4Cl with labeled atom at C–N bond position. (C) EPR spectrum of [Fe(Gly)2]/m-CPBA/DMPO mixtures at –25°C.

As far as the source of Cl is concerned, the only chlorine source is m-chloroperbenzoic acid. We concluded that the Cl ion must come from the dechlorination reaction of m-CPBA (6). Previous studies of iron-catalyzed oxidation reactions (7) indicate that the m-CPBA might react with ferrous glycinate and initiate a free radical process responsible for the cleavage of the C–N bond and the generation of Cl ion. The isolation of 2,6-dimethyl-1,4-benzoquinone from the reaction mixture supports the C–N bond cleavage pathway. In order to explore this free radical–based hypothesis, we recorded low-temperature electron paramagnetic resonance (EPR) spectra (Fig. 1C), which showed a strong EPR signal when mixing [Fe(Gly)2], m-CPBA, and DMPO (dimethyl pyridine N-oxide). In summary, excess m-CPBA plays a dual role as the oxidant to initiate a free radical pathway combining with [Fe(Gly)2] and the source of chloride.

To evaluate the thermodynamic stability of (N5)6(H3O)3(NH4)4Cl, we used density functional theory calculations to quantify the electrostatic interactions among the constituent ions in pentazolate salt (8). Taking the lattice parameters and atomic coordinates from single-crystal x-ray diffraction analysis as input, the geometry optimization was performed on the basis of the conjugate gradient method. One approximation for the hydronium ions is that the hydrogen atoms are fixed at three occupancy sites, whereas they practically occupy six resonance states at 50% probability level. The simulated structure was considered as finally optimized when the residual forces were less than 0.03 eV/Å and the stress components were less than 0.01 GPa. The simulated structure (Fig. 2A) shows satisfactory agreement with the characterized structure by x-ray diffraction, with the discrepancy of the volume at 0.4% and the discrepancy of lattice vectors at ~1%. At the equilibrium state of the crystal, each chloride ion is stabilized by four ammonium ions. In Fig. 2B, the four ammonium ions around the chloride ion form a tetrahedron, resulting in four N-H⋅⋅⋅Cl hydrogen bonds (2.44 Å and 8.02 kcal/mol). All the residual hydrogen atoms affiliated with ammonium ions directly contact the pentazolate anion, and the N⋅⋅⋅H-N hydrogen bond is 2.10 Å and 12.06 kcal/mol. All the hydrogen atoms affiliated with hydronium ions interact with their neighboring pentazolate anion, resulting in two types of N⋅⋅⋅H-O hydrogen bonds, the first being 2.17 Å and 13.26 kcal/mol and the second being 2.26 Å and 11.81 kcal/mol. With the aid of the electrostatic interactions among the ions, the entire pentazolate crystal is thermodynamically stable. Binding energy is defined as the difference between total energy of constituent ions in the free state and total energy of the crystal. The binding energy of (N5)6(H3O)3(NH4)4Cl crystal is calculated to be 1000 kcal/mol, much higher than the binding energy of NaCl (187.9 kcal/mol) (9).

Fig. 2 Calculations.

(A) Calculated lattice vector lengths (Å), angles (°), and volumes (Å3) of the primitive cell of pentazolate salt, along with results from experiments. (B) Hydrogen bonds stabilizing (N5)6(H3O)3(NH4)4Cl at equilibrium state. Crystal orbital Hamilton population (COHP), the result of multiplying the electron density of states by the overlap element of the Hamiltonian, shows the contribution of a specific contact to band energy. The integrated value of COHP at band energy directly quantifies the interactive strength (S, in kcal/mol). (C) Proton transfer between cyclo-N5 ion and hydronium ion/ammonium ion in crystal and in vacuum. ΔEprotonated-unprotonated is the total energy difference of the systems between the protonated state and the unprotonated state. The state with lower total energy is the thermodynamically more stable one. Color code: sapphire, N; red, O; green, Cl; white, H.

To better understand the basicity of cyclo-N5 ion, a known amount of pentazolate salt can be dissolved in deionized water at ambient temperature, enabling the pH of the solution to be accurately and easily determined by pH meter. When 50 mg of (N5)6(H3O)3(NH4)4Cl was dissolved in 5 ml of deionized water (1.71 × 10−2 mol/liter), the pH of the solution was measured to be ~5.55 at ambient temperature (22.3°C). In contrast, under the identical concentration of 1.71 × 10–2 mol/liter, the pH of aqueous ammonium chloride is 5.51 (10), hence the solution of (N5)6(H3O)3(NH4)4Cl is mildly acidic. The measured pH values for different concentrations of (N5)6(H3O)3(NH4)4Cl solutions are listed in Table 1. The results lead us to conclude that the correctly calculated equilibrium constant of cyclo-N5 ion is Embedded Image = 8.425 ± 0.002, indicating that it is more basic than acetate ion (Embedded Image = 9.24) (10).

Table 1 The pH value of pentazolate salt solutions.

Specific amounts of (N5)6(H3O)3(NH4)4Cl (from 10 to 50 mg) were dissolved in 5 ml of deionized water at 22.3°C. The obtained solutions were analyzed using a pH meter with a pH probe.

View this table:

In general, NH3 has a stronger affinity for the proton than for H2O (11). If the HN5 instead of cyclo-N5 were present in the crystal of (N5)6(H3O)3(NH4)4Cl, the cyclo-N5 ion should accept protons from H3O+ rather than NH4+. On the basis of this hypothesis, we constructed a crystal structure model (Fig. 2C) in accordance with (N5)6(H3O)3(NH4)4Cl except that one proton had shifted from the hydronium ion to the cyclo-N5 ion (with the H-N bond length set at 0.95 Å). Subsequently, the geometry optimization of the created structure was performed on the basis of the conjugate gradient method. One unanticipated finding was that the proton on the cyclo-N5 ion quickly transferred to the nearest H2O molecule to form H3O+. The total energy was lowered by 2.74 kcal/mol, implying that the cyclo-N5 ion is thermodynamically more stable than HN5 in the crystal structure. The highly symmetrical forces in the crystal also support the cyclo-N5 ion in place of HN5.

References and Notes

Acknowledgments: Supported by National Natural Science Foundation of China grants 11604017 and 21772092, the JASMIN infrastructure for parallel computing, Fundamental Research Funds for the Central Universities grant 30917011101, and the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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