Preparation of the Cyclopentazole Anion in the Bulk: A Computational

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Preparation of the Cyclopentazole Anion in the Bulk: A Computational Study Uzi Geiger and Yehuda Haas* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel S Supporting Information *

ABSTRACT: The cyclopentazole anion (cyclo-N5−), calculated to be a stable species, was prepared in the gas phase but attempts to synthesize it in the bulk have so far been futile. An aryl pentazole radical anion was suggested as a promising precursor in the gas phase. It is shown computationally that the radical anion (which may be prepared by reduction of the phenyl pentazole neutral) may indeed be used to form the cyclopetazolate anion in the gas phase and in liquid solution, alongside and in competition with the extrusion of N2 to produce the corresponding azide. In the gas phase, the C−N dissociation yields are very low due to much more efficient detachment of an electron. In polar solvents, ionization is suppressed and the primary yields of the two competing reactions are similar. The reaction must be carried out at low temperatures and special measures have to be taken to avoid recombination of the nascent cyclo-N5− with the geminate phenyl radical. A possible remedy is to use a solvent that reacts efficiently with the phenyl radical by H atom transfer. Ostmark et al.11 proposed a possible mechanism in which the immediate precursor of cyclo-N5− is the DMAPP radical anion (DMAPP-RA), produced by the laser desorption ionization technique. In support of this proposal, the barrier for C−N cleavage in the radical anion was calculated to be 25.6 kcal/mol, only a little higher than the barrier for N2 extrusion (22.5 kcal/ mol).17 This paper reports quantum chemical calculations of these competing reactions in the gas phase and an extension to solvated aryl pentazoles. The formation of radical anions in solution may be achieved by reduction with a strong electron donor (such as an alkali atom) or electrochemically. In principle, this radical anion can be energized (thermally or photochemically) and lead to cyclo-N5− production in solution. The cleavage of radical anions18 was studied extensively in the gas phase mostly by mass-spectrometry methods19 and in solutions by methods such as stopped flow,20 pulse radiolysis,21 cyclic voltammetry,22 and flash-photolysis.23 Nonreversible reduction of DMAPP in solution electrochemical experiments was reported by Portius et al.24 but polymerization inhibited further analysis so that no indications for the production of cyclo-N5− were found. Quantum chemical methods are widely used for the calculations of thermochemical properties, including electron affinities.25 We have not been able to find previous data,

1. INTRODUCTION The cyclopentazole anion (cyclo-N5−) was calculated1−8 to be a relatively stable member of the all-nitrogen compound family. Activation barriers to form N2 and N3− are found in the 25−31 kcal/mol range. Using the LCAO-SCF-MO/4−31G theory level, the reaction is found to be concerted.2 Benin8 et al. calculate a lifetime of 2.2 days for cyclo-N5− (based on an activation barrier of ∼25 kcal/mol) at 0 °C in methanol. This indicates that cyclo-N5− is more stable than its possible precursors, aryl pentazoles (ArPs) and their radical anions (ArP-RAs). According to ref 9, aromaticity contributes to this stability. The successful detection of cyclo-N5− using ArPs as precursors verified the theoretical prediction of its being a stable species with a well-defined energy minimum. The molecule was recorded in the gas phase by mass spectroscopy with either para-oxidophenyl pentazole (OPP, a closed shell anion)10 or dimethylaminophenylpentazole (DMAPP, a neutral molecule)11 as precursors. Yields were not reported and despite numerous attempts cyclo-N5− was not yet prepared in the bulk.12−14 The mechanism of cyclo-N5− formation is not known. Thermal dissociation of the aryl pentazoles yields dinitrogen (N2) and the corresponding azide. The calculated gas phase barrier for this reaction is of the order of 20−28 kcal/mol, much smaller than for cleavage of the C−N bond required to produce cyclo-N5−, (86−98 kcal/mol).15 In liquid solutions, the measured barrier to produce N2 is about 20 kcal/mol.15,16 © XXXX American Chemical Society

Special Issue: William M. Gelbart Festschrift Received: March 2, 2016

A

DOI: 10.1021/acs.jpcb.6b02228 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B Scheme 1. Reactions Investigated in This Work

a

Reaction R3 was not considered in ref 11 but appears to be important due to its small barrier (the adiabatic ionization potential) in the gas phase.

The methyl pentazole radical anion (MeP-RA) was chosen as a model system in this research. MeP-RA is the smallest molecule that exhibits all reactions we aim to study, including C−N bond dissociation, N2 extrusion, and electron detachment. The benchmark method chosen was W1BD at the BMCCCSD geometries (W1BD//BMC-CCSD) and was shown to deliver very good agreement with experiment on a large number of molecules. 2.3. Solvents and Substituent Effects. Calculations began with the isolated molecule (i.e., corresponding to experiments in the gas phase). Solvent effects were modeled using the polarizable continuum model (PCM); because the anion is stabilized by polar solvents and the PCM does not include any solvent specific interactions, acetonitrile (MeCN) is a proper representative for a general polar solvent. Most detailed calculations were performed using this solvent. Several substituted pentazols were also studied (Scheme 2) in order to estimate the effects of strong and weak electron

calculated or experimental, on the electron affinities of aryl pentazoles. The three thermal reactions of PP-RA and its derivatives that are considered in this work were chosen based on their low reaction barriers (Scheme 1).

2. METHODS 2.1. Computational Details. HF, B3LYP, M06,26 CBS4M,27 CBS-QB3,28 G4(MP2),29 and W1BD30 calculations were performed with the Gaussian 09 program suite.31 MC3-BB,32,33 MC3-TS, 32 MC3-MPW, 32,33 MC-QCISD/3, 34,35 BMCCCSD, 36 BMC-QCISD, 36 MCG3/3, 34 MCG3-MPW, 32 G3SX,37 and G3SX(MP2)37 calculations were done using MLGAUSS38 interfaced with Gaussian 09. The B3LYP calculations were done as part of G4(MP2) calculations, using the GTbas3 basis set. The basis set for HF (aug-cc-pVTZ) was chosen to enable calculations of the larger systems; it includes diffusion function to enable good representation of anions as was the aug-cc-pVDZ basis set for M06. The search for transition states was usually started from an appropriate initial geometry guess using the standard methods and converged well. An exception was the TS2 geometry of MeP-RA, which had to be converged using the QST3 method.39,40 The following reaction parameters were calculated: ΔZPE, the difference in zero point energy; ΔS, difference in enthalpy, ΔEelec, difference in electronic energy, and ΔE, difference in electronic + ZP energy. Both total and activation parameters were calculated where appropriate. The integral equation formalism polarizable continuum model (IEFPCM) was used for estimating solvent field effects in DFT calculations.41 In this model, the solute cavity is created via a set of overlapping spheres. The default solvent parameters of Gaussian 09 were used. 2.2. Model System. The large number of nuclei and electrons of the studied systems made the use of high level methods prohibitively long and expensive. Forced to employ simpler ones, the validity of the computational methods used in this study was tested relative to highly accurate methods on a model molecule. (No direct experimental data is available in the literature for ArP-RAs).

Scheme 2. ArP-RAs Investigated in This Work and Their Acronyms

donating and withdrawing groups para and ortho to the pentazoale ring. Inserting these substituents in the ortho position allows also the examination of progressively larger steric effect. Data are presented for 2-naphtyl pentazol representing larger aromatic moieties.

3. RESULTS 3.1. Selecting an Approximate Computational Method Using W1BD//BMC-CCSD Theory As the Bench Mark. B

DOI: 10.1021/acs.jpcb.6b02228 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B W1 theory30 is often employed as a benchmark for more approximate methods in the absence of experimental data. Energies relevant to the pentazole RAs were calculated for MeP-RA using this method, as well as more approximate ones that can be employed for the larger phenyl pentazole RA. Results for two methods using M06 geometries are presented in Table 1; other properties as well as more details including

pentazole rings keep their planarity but in contrast with the ground state are not coplanar. This distortion makes a double bond contribution to the C−N bond unlikely, thus weakening it. Based on the structure of this TS, the fragments are likely to be rotationally excited. The Mulliken charge distributions in the minimum of the ground state and in the transition state geometries are shown in Figure 2. The overall charge distribution in the TS is similar to that of the minimum, yet the negative charge is distributed more equally in the TS than in the ground state (GS). All nitrogen atoms are negatively charged in the TS, as expected for the separated cyclo-pentazol anion (in which each atom carries −0.2 electronic charge) whereas in the minimum the N atom bonded to the phenyl ring carries a positive charge. 3.3. Energies Relevant to Gas and Solution Phase Reactions of PP-RA. The barriers for reactions R1, R2, and R3 of PP-RA were calculated for the isolated molecules using the Avg-C//M06 method and in MeCN by applying PCM. The results indicate that R3 (the electron detachment reaction) is expected to be kinetically dominant in the gas phase due its lower barrier. Dinitrogen extrusion and cyclo-N5− production are of minor importance yet because the barrier for R2 is only 2.8 kcal/mol lower than for R1; both are expected to be observable at room temperature. The PP-RA calculated data are given in Table 2 and Figure 3. R1 is found to be endothermic while R2 is strongly exothermic with 62.2 kcal/mol of difference between the two product sets (Figure 3). Therefore, the system’s thermodynamically favors R2 to a large extent. It is evident that if the product distribution is thermodynamically controlled, R1 and R2 are entirely negligible compared to R3 in the gas phase. Because cyclo-N5− is actually observed in the gas phase from two PP derivatives, the system is not completely determined by equilibrium. (N2 is also likely to be formed (via R2), but was not reported.) The experiments were conducted under low-pressure conditions10,11 in which back reactions are insignificant and the time span of the experiment is too short to allow thermodynamic equilibrium. In addition to suppressing the yields of R1 and R2, the low ionization barrier frustrates the direct detection of PP-RA itself; this may explain the failure to detect this species in ref 11. In order to estimate the relative yields, rate constants must be known. Using the Arrhenius approximation, pre-exponential factors must be available in addition to activation energies. In principle, these factors may be calculated from the entropy of activation; unfortunately the calculations were either computationally unfeasible or yielded large errors. A qualitative estimate may be obtained by noting that the entropy change due to electron detachment is generally small.19 As the activation entropy change due to unimolecular dissociation is usually also not large,47 the pre-exponential factor is therefore taken as the standard 1013 s−1 for the three reactions.

Table 1. Calculated Energies of MeP-RA Relative to the Ground-State Minimum Obtained Using Different Computational Methods (kcal/mol) and the Total Energy of MeP-RA (Ha)a quantity W1BD// BMC-CCSD Avg-C//M06 BMC-CCSD// M06 a

MeP-RA total energy

IP

ΔE‡1

ΔE‡2

ΔE1

ΔE2

−313.709

−4.6

11.8

20.1

−27.9

−6.6

−313.296 −313.196

−4.2 −4.0

11.3 10.5

20.6 20.9

−28.0 −29.6

−6.3 −6.5

IP is the adiabatic ionization potential including ZPE correction.

the effect of other calculation methods can be found in the Supporting Information (Tables S1, S2, and S3 and Figure S1). Some methods provided a reasonable approximation of the benchmark method (W1BD at the BMC-CCSD geometries) in the case of MeP-RA but a better one was obtained by using the average of the results of three methods, G3SX, CBS-QB3, and BMC-QCISD. This averaging method will be referred to as the Avg-C method in the following. Before proceeding to the detailed results, a check whether our modus operandi is compatible with the very large amount of experimental data on the dissociation of aryl (especially halide aryls) radical anions is necessary. The aryl pentazole radical anions are similar to the extensively studied halo-aryl ones: in both the extra electron is part of the π-system in the radical anion and becomes a σ-electron after dissociation. Thus, a curve crossing is found in both systems, and the final products are an aryl radical and an anion. In order to perform this examination, correlations found to be useful in the study of aryl radical ions kinetics42 were inspected. Figure S2 and Table S4 show the calculated reaction barriers versus the experimental rate constants for a number of chloro aryl radical anions (data from refs 20 and 42−46). The very good linear correlation obtained, as expected from the analysis presented in ref 42, supports the viability of the proposed model. 3.2. Properties of the Transition States. The minimum energies of the stable species as well as of the transition states of R1 and R2 were calculated and optimized. Transition states were identified by having one imaginary frequency. The R1 TS is of special interest for the problem at hand. The main coordinates leading to it are the C−N bond stretch and an out-of-plane dislocation of the rings, as shown in Figure 1. The phenyl and

Figure 1. PP-RA’s TS1M06 optimized geometry viewed from three directions. The length of the C−N bond and the angle between the ring planes and the C−N bond are noted. C

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Figure 2. Calculated atomic charges of PP-RA (A) and the PP-RA’s TS1 (B) in the gas phase (M06/cc-pVTZ).

making it much larger than the activation energies of the other two reactions. The electron ejection reaction can therefore be ignored in solution. As in the gas phase, the small difference between their energy barriers makes both routes observable simultaneously with ∼12% R1 products at room temperature. The second major effect is the lowering of ΔE1 to 14.8 kcal/ mol; the lower endothermicity of R1 and the small energy barrier difference between it and R2 envisage better prospects for the production of cyclo-N5− in solution than in the gas phase. However, in contrast with the gas phase case collisions increase the importance of thermodynamic effects. In addition, the small (∼11.7 kcal/mol) barrier for back reaction (i.e., recombination of cyclo-N5−, and the nascent phenyl radical) makes it highly probable, unless steps are taken to avoid the collision between the two fragments, especially in the solvent cage. It follows that although PP-RA is a possible precursor of cyclo-N5−, in solution special experimental conditions may be required to practically achieve it. One possible means is trapping the phenyl radical by a suitable scavenger. 3.4. Reactions of Substituted ArP-RA in Solutions. Tables S5 and S6 (Supporting Information) report the results obtained for some substituted PP-RAs, including ortho and

Table 2. Calculated Energies of PP-RA Relative to the GS Energy Minimum in the Gas Phase and in MeCN Using PCMa PP-RA total energy IP ΔE‡1 ΔE‡2 ΔE1 ΔE2

gas

MeCN

−504.741 16.4 27.4 24.6 24.9 −37.3

−504.817 59.2 26.5 25.3 14.8 −40.0

a

Avg-C//M06 data in kcal/mol, total energies in Hartree. IP is the adiabatic ionization potential of PP-RA and is considered to be the barrier for electron detachment (reaction R3).

The data predict a rather low yield of both R1 and R2 due to the expected dominance of R3, especially at low temperatures. Based on the activation barriers of Table 1 and assuming the same pre-exponential factors, the cyclo-N5− yield increases from 10−6 at 300 K to 10−3 at 600 K. In MeCN solutions, two major changes clearly stand out: the calculated IP of the radical anion increases by 42.8 kcal/mol

Figure 3. Energy level diagram of PP-RA reactions. Avg-C//M06/aug-cc-pVDZ values in kcal/mol. D

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4.2. Prospects for Solution Phase Synthesis of cycloN5−. In distinction with the neutral molecule, the results indicate that cyclo-N5− can be formed thermally from PP-RA in liquid solutions, as the reaction’s rate constant is similar to that of dinitrogen extrusion. However, it is apparent that special conditions must be met in order to materialize this option experimentally. The reaction must be carried out at low temperatures; first, to prevent rapid, uncontrollable decomposition of cyclo-N5−6,7 or the arylpentazole radical anion. Furthermore, the dinitrogen extrusion reaction is practically irreversible, whereas R1 results in two fragments that may recombine. Moreover, the recombination (backward) barrier is smaller than the forward one; at any given temperature, the back reaction may therefore dominate and reduce the resulting concentration of cyclo-N5− to the noise level. A scheme in which the system is energized briefly to start the reaction and cooled immediately after a short time of a few minutes might be useful. This may be achieved by irradiation with a pulsed light source; a TD-DFT calculation (M06/aug-cc-pVDZ) placed a strong transition at about 516 nm in the gas phase and 480 nm in acetonitrile. The possible product phenyl radical absorbs at 505−535 nm,49 so that a wavelength mainly leading to the excitation of PP-RA and not inducing the backward reaction may be found. 4.3. Practical Implementation. The backward reaction may make the scheme impractical. Its effect could be restricted if the phenyl radical is rapidly scavenged; because this radical is rather reactive (there are many examples of scavenging rate constants that are almost diffusion controlled) the method appears to be suitable. A caveat concerning this technique should be borne in mind: the scavenger should react rapidly with the phenyl radical but more slowly with PP-RA. This may be practical due to the different nature of PP-RA, where the radical center is delocalized over the π-system, whereas in the phenyl radical the lone electron is in a σ-orbital. Lin’s group reported rate constants for many phenyl radical reactions with several reactants.50,51 O2 reacts rapidly to convert phenyl to C6H5O2; at 296 K, the rate constant of the reaction is 1.4 × 10−11 cm3 molecule−1 s−1 = 8.4 × 109 lit mol−1 s−1.52,53 The reaction rate constant is almost independent of pressure of O2. The peroxyphenyl radical, which is rather inert, absorbs at 496.4 nm. NO reacts with phenyl radical to form nitrosobenzene, a stable molecule. The rate constant at 297 K is (1.97 ± 0.13) × 10−11 cm3/s.54 However, the best scavenger would be a suitable solvent. A possible interesting scavenger is acetone55 that displays a large rate constant (1.7 × 1010 cm3/(mol s) at room temperature and 45 Torr). Another major impediment in realizing preparation of cycloN5− is its separation and identification. Mass spectrometry, NMR (15N), and X-ray diffraction are all highly selective methods that can lead to unequivocal identification of the product. (UV, IR, or Raman spectroscopies are not sensitive enough for cyclo-N5− due to small cross sections.) Each of these methods involves nontrivial complications. Mass spectrometry was already used to monitor cyclo-N5−. For liquid phase reactions, the method of choice to transfer molecules to the gas phase and ionizing them is electrospray ionization. Some parts of the machine are heated (the injector, nebulizer, drying gas) and may reduce the sensitivity by decomposing cyclo-N5−; therefore, cold electrospray MS is more suitable for these ends.56

para substitution, and of NaP-RA. While most activation energies are only slightly affected by para substitution of PPRA, the calculated reaction enthalpy of R1 is reduced 30% upon changing the substituents from the most EWG (trifluoromethyl) to the most EDG (methoxy). Strong EDGs are therefore favorable for the synthesis of cyclo-N5−, both by the aforementioned effect and the increased stability of the neutral ArP starting materials. Minor differences are revealed on comparing phenyl to naphtyl pentazol. Thus, aromatic system enlargement does not substantially affect the character of the system. Changing substitution site from para to ortho also affects only minor changes to reaction parameters. In consequence, these calculations imply that substitution effects do not materially affect changes in the outcome of the ArP-RA’s reactions.

4. DISCUSSION 4.1. General Discussion and Comparison with Available Experimental Data. The data presented are based on a simplified model (commonly used in the radical anion dissociation studies): the same pre-exponential factor is assumed for the different reactions, a DFT-based calculation is used, and the N2 extrusion is assumed to be primarily a stepwise one. Nonetheless the model nicely reproduces experimental results (as the correlation in Figure S2 demonstrates) and reveals details of the reaction, such as the bending of the transition state in the C−N cleavage transition state, as in the C-Hal cleavage transition state of halo-aryl radical anion reactions (see refs 42−45 and Section 2 of the Supporting Information). It is therefore accepted as a viable model for the main features of the reactions. Glukhovtsev et al.9 calculated very large aromatic stabilization energy (ASE) for cyclo-N5− - 52.4 kcal/mol (compare with pyrrol’s ASE 25.5 kcal/mol). In the arylpentazoles and their radical anions, ASE of the N5 ring is smaller due to the formation of the C−N bond. It was argued48 that in the planar geometry, a certain double bond character is found for the C− N bond. In order to break the bond it is beneficial to convert it to a single bond. Because a double C−N bond between the two rings can be formed only if the rings are coplanar, an out-of plane distortion resulting in a bent structure is predicted for the transition state leading to C−N bond cleavage. This is indeed found in the calculated structures of TS1 (Figure 2). The calculations presented in this work support the hypothesis of Ostmark et al.11 that the radical anion might dissociate to yield cyclo-N5−. Moreover, it is shown that its yield in liquid solutions may be larger than in the gas phase. However, experimental implementation may be difficult, because the barrier to recombination of cyclo-N5− and the phenyl radical is only about 10 kcal/mol. Avoiding excessive loss of cyclo-N5− thus necessitates low temperatures. In ref 10, it was shown that cyclo-N5−is formed in the gas phase also upon activating the closed shell OPP, which is a substituted phenolate anion. Interestingly, cyclo-N5− was formed upon high energy collisions with an inert gas, while low energy ones led to N2 extrusion. No mechanism was advanced. In view of our results, a possible mechanism can be offered: high energy collision may affect the electronic structure of the pentazole phenolate anion, and move one electron from the oxygen atom to the pentazolophenol moiety. In the resulting biradical the barrier to C−N bond cleavage is reduced, as in the anion radicals allowing competition with N2 extrusion. E

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N NMR is another unequivocal identification method, which may be used directly in the solution. Its main disadvantage is poor sensitivity (for instance, by comparison to proton NMR). The best and most trustworthy method is X-ray diffraction. The required crystals, which may be very small, must be of high quality. In order to use this option, a crystallizable salt will be required. Lead azide and other highly sensitive explosives have been studied using X-ray diffraction, and the search for pentazole anion containing crystals may benefit from experience with the corresponding azides.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b02228. Results of different calculation methods for the MeP-RA model system. In-depth comparison of computational results and literature data of halo-aryl radical anions. Calculation results for substituted arylpentazole radical anions. (PDF)

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REFERENCES

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5. CONCLUSIONS AND SUMMARY The dissociation of phenyl pentazole radical anion to form cyclo-N5− and phenyl radical is shown computationally to be feasible in both gas phase and liquid solution. This holds also for several substituted aryl pentazoles and for 2-naphtyl pentazole. In the gas phase, the dominant thermal reaction is electron detachment due to the small ionization potential (∼16 kcal/mol) of the radical anion. If due to the dissociation of the anion radical, the experimental observation of the cyclo-N5− anion10,11 indicates that the system was reacting at a high temperature. The proposed mechanism predicts rather low yields of the product. The absence of the parent mass in the mass spectrum is due to the dominance of the electron detachment process. In solution, electron detachment involves a much higher barrier (∼59 kcal/mol in MeCN) due to the stabilization of the anion radical by the solvent and may be ignored. Both competing dissociation reactions, N2 extrusion and C−N bond cleavage, should be observed as they possess nearly the same barriers. Whereas nitrogen extrusion is irreversible, the C−N bond cleavage is reversible. Moreover, the barrier for the back reaction is smaller than that of the forward one (∼10 vs 25 kcal/mol) and may well take place rapidly in the solvent cage. This could be countered to a certain degree by scavenging the nascent phenyl radical. A suitable H atom donating solvent is proposed as the best choice.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support of internal funds of the Hebrew University is gratefully acknowledged. We are indebted to Boris Bazanov, M.Sc. for many fruitful discussions. F

DOI: 10.1021/acs.jpcb.6b02228 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.6b02228 J. Phys. Chem. B XXXX, XXX, XXX−XXX