N5- in Solution: Isotopic Labeling and Further Details of Its Synthesis by Phenyl Pentazole Reduction

Article abstract of DOI:10.1021/acs.jpca.7b04222

The cyclopentazolate anion, N₅^-, has been researched extensively over the years and detected in the gas phase more than a decade ago, but was only recently measured in solution. The process whereby aryl pentazole reduction leads to the production of N₅^- is still not fully understood. Here, the production of N₅^- in solution was investigated using isotopic labeling techniques while implementing changes to the synthesis methodologies. ¹⁵N labeled phenyl pentazole produced appropriately labeled phenyl pentazole radical anions and N₅^- which, upon collision induced dissociation, produced the expected N₃^- signals. Changing to higher purity solvent and less coated Na metal allowed for a much more rapid pace, with experiments taking less time. However, the best yields were obtained with heavily coated metal and much longer reaction times. Utilization of a vacuum line and ultrapure solvents led to no products being detected, indicating the importance of a sodium passivation layer in this reaction and the possibility that sodium is too strong a reducer. These findings can lead to better production methods of N₅^- and also explain past failures in implementing aryl pentazole reduction techniques. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.jpca.7b04222

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Article
5-
N in Solution: Isotopic Labeling and Further Details
of its Synthesis by Phenyl Pentazole Reduction
Boris Bazanov, Uzi Geiger, Dan Grinstein, Shmuel Welner, and Yehuda Haas
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04222 • Publication Date (Web): 15 Aug 2017
Downloaded from http://pubs.acs.org on August 18, 2017
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N5 in Solution: Isotopic Labeling and Further Details of its Synthesis by
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Phenyl Pentazole Reduction.
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Authors: Boris Bazanov, Uzi Geiger*, Dan Grinstein, Shmuel Welner, Yehuda Haas†.
Affiliation
B. Bazanov, U.Geiger*, D. Grinstein, S. Welner, Y. Haas:
Institute of Chemistry, Safra Campus
The Hebrew University of Jerusalem
9190401 Jerusalem (Israel)
E-mail: uzi.geiger@mail.huji.ac.il
phone: +972-54-5279783
† - deceased
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Abstract
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The cyclopentazolate anion, N5 , has been researched extensively over the years and detected
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in the gas phase more than a decade ago, but was only recently measured in solution. The
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process whereby aryl pentazole reduction leads to the production of N5 is still not fully
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understood. Here, the production of N5 in solution was investigated using isotopic labeling
techniques while implementing changes to the synthesis methodologies. 15N labeled phenyl
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pentazole produced appropriately labeled phenyl pentazole radical anions and N5 which, upon
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collision induced dissociation, produced the expected N3 signals. Changing to higher purity
solvent and less coated Na metal allowed for a much more rapid pace, with experiments taking
less time. However, the best yields were obtained with heavily coated metal and much longer
reaction times. Utilization of a vacuum line and ultra-pure solvents led to no products being
detected, indicating the importance of a sodium passivation layer in this reaction and the
possibility that sodium is too strong a reducer. These findings can lead to better production
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methods of N5 and also explain past failures in implementing aryl pentazole reduction
techniques.
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Introduction
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The first successful synthesis and identification of the pentazolate anion (N5 ) in solution was
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published several months ago in a succinct and introductory manner.1 In this paper we aim to
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elaborate on the synthesis of the N5 species, present more proof for its successful
identification, and report on some of the intriguing results that were accumulated since.
Ever since the synthesis of aryl pentazoles in the late 1950’s by Ugi and Huisgen2,3, attempts
were made to cleave the C—N bond of these compounds and produce the pentazolate anion. A
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major push to these experiments was made when N5 was identified in the gas phase by
collision induced dissociation of para oxidophenyl pentazole4 and then by laser desorption
ionization of para dimethylaminophenyl pentazole5. Yet, all past attempts of attaining the same
result in condensed phases by reductive, oxidative and photo- cleavage methods have failed.6–
11 Two possible exceptions are the CAN dearylation of para methoxyphenyl pentazole yielding
N3 which is probably a decomposition product of N5-,12–15 and more recently, the detection of a
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solid pentazolate salt by oxidative cleavage of 3,5-dimethyl,4-hydroxyphenyl pentazole16 which
could be further converted into cobalt complex 17.
An in-depth computational investigation of the link between the aryl pentazole radical anion
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and the formation of N5 in one of the gas phase experiments provided an indication that it
should also be present in solution.18 This led to the first experimental detection of the phenyl
pentazole radical anion (PPRA) and N5 in solution.1 Due to the brevity of the initial publication,
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some key aspects were not included in the report of this discovery, including identification
proof by means of isotopic labeling and MS/MS techniques. Further progress has also been
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made since the initial discovery, including changes to the synthesis method and observations
regarding its successful application, warranting this report. Regarding the latter, we have
observed a key parameter affecting the outcome of the procedure that may lead to serious
issues with reproducibility of results if left unknown to the community.
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Experimental
Warning! Phenyl pentazole is a sensitive explosive prone to detonation upon warming to
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room temperature. N5 , its salts, and compounds are expected to be both sensitive and
powerful explosives. All precautions should be used when dealing with such materials.
Materials and instruments
Tetrahydrofuran (THF, Analytical, Frutarom LTD. or anhydrous, Sigma Aldrich) was either dried
over activated A4 molecular sieves (Sigma) for at least 48 hours or refluxed over sodium
(Aldrich, ≥99.8% trace metal analysis) and benzophenone (Sigma, ReagentPlus, 99%) for at least
an hour under an atmosphere of nitrogen in a solvent still before use. Sodium metal (Aldrich,
cube in mineral oil, 99.9%) was cut under isooctane (Fluka, for UV spectroscopy) and wiped
from excess solvent before use. Aniline (Sigma-Aldrich, ACS reagent, > 99.5%), sodium nitrite
(Aldrich, 95%), sodium azide (Sigma, ReagentPlus, >99.5%), hydrochloric acid (ACS reagent,
37%), methanol (Biolab, HPLC grade, > 99.9%), and acetonitrile (Biolab, HPLC grade, > 99.9%)
were used as received. 15N-labeled sodium nitrite, aniline, and sodium azide were obtained
from Cambridge Isotope Laboratories Inc. and used analogously to the unlabeled materials.
MS data were recorded on an Agilent 6520 Accurate mass Q-TOF LC MS machine with either a
C18 column or no chromatography at all (analogous to ESI-MS):
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10 µL of the solution were immediately injected to a LC-MS using Hypersil BDS C18 column
(particle size 5 µm, length 100 mm, internal diameter 4.6 mm, #28105-104630) or only union (no
column). The following gradient separation method (for on column separation) or isocratic one
using 100% methanol as a mobile phase (for non-column method) was used:
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Time (min)
Acetonitrile
Water
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15
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Analysis of the eluent was performed on an Agilent 6520 Accurate mass Q-TOF LC MS operating
in the dual ESI negative ion mode using nitrogen as a drying gas. The LC-MS parameters were
slightly modified from the standard ones:
Parameter
Standard
On column separation
Non-column
experiment
experiment
Flow (mL/min)
Volume injected (µL)
Cooling of vile
0.5
0.5
0.25
10
3
No cooling
3500
350
10
Vial cooled
Vial cooled
2500
350
Vcap (V)
3000
300
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Gas temp. (ºC)
Drying gas (L/min)
Nebulizer (psig)
Fragmentor (V)
Skimmer (V)
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150
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100
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Octopole RF peak (V)
Collision energy
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750
0
750
0
10
Preparation for LC-MS analysis included the transfer of solutions to 2 mL HPLC vials while taking
care not to transport sodium pieces together with it. The vials were either kept cold with dry ice
on the auto-sampler or allowed to warm up to room temperature for other experiments. No
special care was taken to exclude air from the sample vials at this stage. It is noted that the
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total ion count signal peak for the methanol isocratic method started at 0.15-0.2 min and
usually the baseline values were regained after roughly 1 min. The signal between 0.25 and
0.75 min was therefore averaged to yield a better S/N ratio.
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A pure THF spectrum was measured the same way and used as background by subtraction from
all spectra.
MS/MS was performed with different collision energies and an ion selector bandpass with 1.3
m/z FWHM.
Synthesis of regular and 15N labeled phenylpentazole (PP)
Phenyl pentazole was synthesized as before.1 In a representative synthesis 1.86 g aniline, 4 mL
hydrochloric acid, and 25 mL methanol were loaded into a beaker with a magnetic stirring bar. The
beaker was immersed in an ice-water bath while stirring. 1.73 g sodium nitrite was dissolved in 10 mL
water and added dropwise while monitoring and controlling the temperature of the solution at
0-5 °C. Following the addition step, the solution was allowed to stand in the bath for 30 minutes and
then transferred to an acetonitrile-dry ice bath (-41 °C). 1.73 g sodium azide dissolved in 10 mL water
was added dropwise while monitoring and maintaining the temperature below -35 °C. After azide
addition the reaction was allowed 30 minutes to come to completion and the precipitated PP was
collected by cold filtration at -30 °C in a sintered glass filter and washed with a cold (-20 °C) 50%
water:methanol mixture. The solid product was collected into a small glass vial, which was placed on
activated 5A molecular sieves (Sigma) in a big glass vial and stored at -78 °C. ~0.56 g (20.5% yield from
aniline), ~90% purity with the rest being Phenyl azide by N.M.R. (1H-N.M.R: 8.18 (D), 7.73 (M), 15N-N.M.R
(tri-labeled, see below): 11.8 (D), -19.3 (M), -77.3 (M)).
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Three types of 15N labeled PP were prepared by utilizing labeled aniline, sodium nitrite, and terminally
labeled sodium azide. The incorporation of labeled reactants and their resulting isotopomers are
summarized in scheme 1. It should be noticed that the tri-labeled material is actually a mixture of two
isotopomers. The isotopically labeled PP is henceforth referred to as mono-labeled, bi-labeled, and tri-
labeled, and the 14N material is henceforth referred to as unlabeled.
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N
NaN3
NaN3
HCl, NaNO2
HCl, NaNO2
N
N
Cl-
Cl-
NH2
N+
N
N
N
N
unlabeled PP
N
N
N
15NH2
15N+
15N
N
mono-labeled PP
15N
15N
NaN3
N
N
Cl-
15N+15
N
N
bi-labeled PP
15N
N
N
15N
15N
15N
+
15N
N
15N
N
tri-labeled PP
Scheme 1: 15N labeled reactant use and the labeling groups of PP used in this study.
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Reduction of PP and liberation of N5
Method 1: About 1-3 mL THF, dried over molecular sieves, was placed in a small vial and cooled
to (-40) – (-78) °C. PP was weighed into the vial to form a solution with approximate
concentration of 0.01 M. A small piece of sodium was cut and washed with dry THF prior to
being loaded into the vial. The reaction mixture stored at -40 °C for at least 2 weeks and at
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-78 °C for at most 3 months prior to separation of the solution from the metal and injection into
the HPLC-MS. When needed, centrifugation was performed to receive clear solution. A
chromatographic column was used in this method with a mobile phase gradient transitioning
from H2O to MeCN then again to H2O, for five minute increments each.
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Method 2: Distilled THF and sodium metal were loaded into a small vial. The vial was cooled to
-10 °C in a freezer and a weighed amount of PP was dissolved in it. Due to the need for speedy
handling of PP, which may explode violently if left to warm to room temperature, the amount
of PP varied between different vials. The metal was cut to smaller pieces, exposing a fresh
metal surface, and the vial was left at -10 °C for an hour before the solvent was separated from
the metal and the vial was taken out of the freezer. The best signals were usually obtained from
samples left 2-2.5 hour at room temperature.
Method 3: Ace Glass Inc. glassware of the Airfree Schlenk type were used. THF was distilled
from a sodium-benzophenone still to an evacuated Straus flask, from which it was distilled
under vacuum when needed onto a sodium mirror in a 50 mL tube. Into a separate 50 mL tube
solid PP was loaded and kept under vacuum at -20 °C for 15-30 min. THF was distilled onto it
from the sodium mirror tube. Another sodium mirror was formed in a separate 50 mL tube,
onto which the PP THF solution was transferred (under vacuum). The reaction was carried out
for an hour at -10 °C in a freezer, after which the tube was vented and solution was drawn into
an HPLC vial for further analysis.
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Results and discussion
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Strong MS signals at m/z=147 and 70 were observed upon HPLC-MS injection of a reduced PP in
THF solution (according to method 1). These signals, with retention times of 7.9±0.7 and
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1.9±0.4 min, respectively and are due to PPRA and N5 ions (figure 1A-D). When changing to
mono-, di-, and tri-labeled PP, these signals shift, respectively, to m/z=148, 149, 150 for PPRA
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and to m/z=71, 72, 73 for N5 (figure 1E-J), all with the same retention times. Exemplary full
range mass spectra at the two relevant retention times are presented in the supporting
information (S1 and S2).
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Figure 1: Chromatograms of the N5 signal at m/z=70 (A) and of the PPRA signal at m/z=147 (B).
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MS signals of the N5 (C, E, G, I) and PPRA (D, F, H, J) signals: from unlabeled (C, D), mono-
labeled (E, F), di-labeled (G, H), and tri-labeled (I, J) reduced PP solutions.
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Method 1 for the reduction of PP produced signals of significant strength for PPRA and N5 , but
was flawed by long reaction times, large amounts of by-products from contaminant in the
solvent, and inconsistent results (success rate of 50-70 % to our estimation). Method 2 was
developed to overcome these drawbacks and incorporated better solvent purity and shorter
reaction times at higher temperatures. The results of method 2 are much more consistent and
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can be easily reproduced. Despite its benefits, this method yields much weaker signals of N5
and no PPRA signals (these are assumed to be below noise levels). Change in the detection
method accompanying reduction method 2 was intended to ameliorate some of these
difficulties.
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MS/MS measurements were performed on the N5 signal derived from differently labeled PP
reduced using method 2 (figure 2). The only negative anion produced by collision induced
dissociation of N5 is N3 with appropriate numbers of 15N atoms (scheme 2). This result supports
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the notion of a cyclic molecular species made up of symmetrically identical nitrogen atoms.
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However, it should be noted that if cyclo-N5 is produced at the cyclic configuration and later
isomerizes to a chain form, the same results will be obtained – only direct structural data (15N-
NMR, XRD) can fully elucidate the geometry of this molecular anion.
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Figure 2: MS/MS signals from the N5 signal of un- (A), mono- (B), bi- (C), and tri- (D) labeled PP
solutions in THF reduced by method 2.
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15N
15N
15N
N
N
N
N
N
15N
N
15N
N
-
-
-
-
N
N
N
N
N
N
15N
N
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-N2
-N2
-N2
-N2
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N-
N-
N-
15N-
N+
N-
m/z=43
N+
N-
N+
N-
N+
N-
m/z=42
m/z=42
m/z=42
100%
40%
20%
40%
(46%)
(17%)
15N-
N+
N-
m/z=43
(39%)
15N-
15N+
N-
m/z=44
15N-
N+ 15N+
N- N-
N-
m/z=43
60%
40%
(41%)
15N-
15N+
N-
40%
(40%)
15N-
15N+
15N-
(54%)
m/z=44
m/z=45
40%
20%
(42%)
(21%)
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Scheme 2: dissociation of differently labeled cyclo-N5 to N3 ions, the theoretical ratio of
product ions, and in parentheses the experimentally detected ratio of collision induced
dissociation products.
Involvement of the solid electrolyte interface passivating the sodium metal was suspected,
therefore an experiment using reduction method 2 and THF-d8 was performed. Any negatively
charged molecules produced directly from the solvent should have been easily spotted when
compared to a control experiment. No such results were found, nor was any difference from
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the control results measured using regular THF. We can therefore conclude that no soluble
anions were produced directly from the solvent and that the source of the passivation layer is
probably contaminants such as gases (O2, CO2) or products of the benzophenone-sodium
reaction, which remain after a single distillation.
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Reduction method 3 was initiated to better understand and control the parameters regulating
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the yields of the reaction. It yielded no detectable amounts of N5 in solution, but some N3 was
detected in these solutions, unlike in the other methods. Warming to room temperature did
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not change these results. This finding, together with the lower yield of N5 of method 2
compared to method 1, leads us to conclude that sodium metal by itself is too strong a reducer
and only the formation of a passivation layer (solid electrolyte interface, SEI)19 in the solvents of
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lesser purity facilitates the production of N5 . Such an outcome may indicate further reduction
of PPRA to the dianion, which may then decompose to yield the azide anion. Past failures to
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produce N5 by aryl pentazole reduction are also explained by this finding – the reducing agent
used, sodium in liquid ammonia,6 was too aggressive. It most probably prompted the
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production of the aryl pentazole dianion, which does not lead to N5 production. It also
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indicates a direction towards a quicker, cleaner production of N5 in solution by utilizing an
intrinsically less aggressive reducer. Further research of this issue is currently underway.
Conclusions
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Isotopically labeled N5 was produced in solution by reduction of labeled phenyl pentazole and
warming. The appropriate mass spectrometric signals were detected, including those of the
product azide anions upon MS/MS experiments. This provides strong evidence in support of our
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claim for the first solution synthesis and detection of N5 in solution. Variations in the synthetic
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procedure support the conclusion that sodium metal is, by itself, too strong a reducer for this
reaction and most probably produces the phenyl pentazole dianion, which decomposes to yield
the azide anion directly. It appears as though the main bottleneck for this reaction is long
reduction time needed when passivated sodium is used, a problem that might be ameliorated
by employing a gentler reducing agent.
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Dedication
This article is dedicated to the memory of the late Professor Yehuda Haas. Professor Haas was
the main power behind this research, pushing it forward in health and sickness. We will always
remember him fondly.
Acknowledgment
This research was not supported by any outside sources – only internal budgets were used.
Supporting Information. Exemplary full scale mass spectra.
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