Article pubs.acs.org/JPCB
Photochemistry of Aryl Pentazoles: para-Methoxyphenylpentazole U. Geiger and Y. Haas* Institute of Chemistry, The Hebrew University, Jerusalem 9190401, Israel S Supporting Information *
ABSTRACT: The photolysis of para-methoxyphenyl pentazole (MeOPP) in methylcyanide (MeCN), investigated in the far UV (FUV) and near UV (NUV) is compared with the photolysis of para-methoxyphenyl azide (MeOPA). The main photoproduct of MeOPP is MeOPA, which, due to further photon absorption, yields mostly 4,4′dimethoxyazobenzene as the final thermally stable product. The overall product quantum yield of both MeOPP and MeOPA is about 0.5 due to competition with internal conversion (IC) to S0 implying ultrafast nonreactive processes. Both pentazole and azide yield only 4,4′-dimethoxyazobenzene (trans and cis isomers, trans first) upon FUV photolysis in dilute solutions, whereas in the NUV, though the azo isomers are still the major products, other species are also formed. The yields of these products are 17% and 10% for the azide and the pentazole, respectively. The difference in yield indicates pentazole photoreactions that do not proceed via the azide. The products observed upon NUV photolysis may be the precursors of tars commonly produced in azide photochemistry. The exclusive production of azo compounds in the FUV indicates a rapid route to the triplet nitrene, generally considered to be the precursor of azo products.
■
INTRODUCTION Aryl pentazoles (see Figure 1 for the structure of methoxyphenyl pentazole) have been known since the 1950s, when
ionization on para-dimethyl amino phenyl pentazole (DMAPP).8 Theory predicts that the five membered nitrogen ring is a possible building block of stable all-nitrogen compounds9,10 making the pentazolate anion the key to unlocking pure nitrogen chemistry−if synthesized in large quantities. Attempts to produce the pentazolate anion in the bulk by means of thermal- and electrochemistry were largely unsuccessful,11,12 with the possible exception of cerium ammonium nitrate oxidation13,14 yielding products indicative of N5− production, but without directly detecting the anion itself. The claim was refuted by a subsequent paper.15 A theoretical study of the system16 showed that N5− could be produced in the gas phase by electronic excitation and therefore photochemistry might be used in the bulk. All aryl pentazole dissociation reactions must compete with the thermodynamically favorable dinitrogen expulsion and production of the corresponding aryl azide, implicating that photochemical reaction paths of these materials might avoid the well-known photochemistry of the azide, characterized by the formation of a nitrene.17,18 The solution phase photochemistry of OPP was examined by us in a previous paper19 and that of DMAPP by Portius et al.20 and by us.21 In these cases no C−N bond cleavage was observed and all photochemistry occurred via the azide-nitrene route.
Figure 1. UV−vis spectra of MeOPP (solid) and MeOPA (dashed), 14 μM in MeCN at −27.5 °C. TDDFT calculated data of the first strong UV absorption bands optically are shown as red lines, solid for MeOPP, and dashed for MeOPA (see section 3.5 and Table S2 for calculation details).
they were first synthesized by Ugi and Huisgen.1−6 Cleavage of the CN bond connecting the phenyl and the pentazole rings in these materials was considered as a possible route to prepare polynitrogen compounds such as pentazolic acid (HN5) or the cyclopentazolate anion (N5−). Interest in aryl pentazoles increased after they were used by two groups to experimentally prepare the pentazolate anion in the gas phase: Christe et al. using high energy gas phase collisions on para-oxido phenyl pentazole (OPP)7 and Ö stmark et al. using laser desorption © 2015 American Chemical Society
Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: October 30, 2014 Revised: February 7, 2015 Published: February 8, 2015 7338
DOI: 10.1021/jp5108813 J. Phys. Chem. B 2015, 119, 7338−7348
Article
The Journal of Physical Chemistry B
(ppm): 4.72 (2H, t, wide), 1.71 (2H, p), 1.43 (2H, sextet), 0.97 (3H, q)). 2.2. Synthesis of MeOPP and MeOPA. MeOPP was synthesized according to Butler et al.14 as advised by Ö stmark et al.23 with slight modifications: A 100 mL beaker was immersed in a circulator cooled acetone bath. The circulator chiller was set to −5 °C to achieve 0 °C in the reaction vessel. A 0.40 g (3.2 mmol) sample of para-anisidine was dissolved in 2.05 mL of methanol, and 0.53 mL of (6.2 mmol) 37% HCl was added dropwise while stirring. A 0.42 mL (3.6 mmol) sample of n-butyl nitrite was added dropwise and the reaction was allowed 20 min to complete. The color first changes to deep purple and later to red. The temperature was reduced to below −20 °C, and the solution was diluted with 5.6 mL of a 1:1 water/methanol mixture. A 45 mL aliquot of the chilled hexanes was added to form a two-phase system and 0.228 g (3.5 mmol) of sodium azide dissolved in 1.8 mL of a 3:2 water/ methanol mixture was slowly injected into the lower phase. The reaction was allowed to run for 30 min and the resulting solid product was filtered in a cooled sintered glass filter and washed with 50 mL of a 3:2 water/methanol mixture cooled to dry ice temperature. This solid was dried in vacuum while cooling to below −20 °C. NMR indicated that the product was MeOPP contaminated with about 10% MeOPA. Yield: (70 ± 20) mg ((11 ± 3)% based on MeOPP only) with (7.9 ± 0.8)% MeOPA impurity; no other contaminants were detected with proton NMR. Proton NMR (MEOPP): 8.09 (doublet, 2H), 7.22 (doublet, 2H), and 3.91 (singlet, 3H) ppm. 2.3. Photolysis of MeOPP and MeOPA. All irradiations were performed in a q-pod 2e temperature controlled cell holder with magnetic stirring (Quantum North West) at −27.5 °C and 750 rpm, in a 10 mm × 10 mm quartz cell. The excitation sources were either a 150 W xenon lamp fitted with a water filter, an appropriate optical filter, and a concentrator lens (Hamamatsu E7536 with L2175) or a pulsed ArF excimer laser (Neweks Ltd., PSX-100) with a diverging lens before the sample. The lens to sample distance in the laser setup was set to allow 2.5 mW of laser power to illuminate ∼1 cm2 of the sample. The ArF laser and the Xe lamp, when used with a 235− 400 nm filter will be denoted as FUV (far UV) and NUV (near UV), respectively. A Bruker Avance II 500 MHz NMR spectrometer was used for all NMR measurements. For low concentrations experiments (LC, ∼10−5 M), 1−5 mg of MeOPP was dissolved in 1 mL of MeCN cooled to −40 °C. A 5−20 μL aliquot of this liquor diluted in 2 mL of MeCN was used for irradiation. Products of LC experiments were analyzed using UV−vis absorption only: a DH-2000 light source and a USB4000 spectrometer (Ocean Optics) were connected via fiber optics at a perpendicular angle to the excitation orientation. Measurements were done a few seconds after irradiation to allow equilibration and mixing. For medium concentrations (MC, ∼10−3 M), ∼1 mg of MeOPP was dissolved in cooled CD3CN (0.75 mL, −40 °C) in a small vial. A 240 μL aliquot of this solution was added to 0.75 mL of CD3CN in the cuvette, at the working temperature. Then, 40 μL of the irradiated solution was diluted in 160 μL of MeCN in a 2 mm optical path cell to affect a X25 optical dilution. A spectrum was collected using a different light source (DH-2000-BAL, Ocean Optics) that allows recording spectra down to 225 nm only. No cooling was needed, as the measurement was fast enough to permit neglect of thermal reactions.
Azides, and even more so, pentazoles, are high energy density materials (HEDMs). On dissociation, they release a large amount of energy, much of it in translational energy of the products, and therefore they are considered as potential propellants and explosives. The source of their extreme exothermicity is the fact that the triple NN bond is very stable (EDIS = 945 kJ//mol, 226 kcal/mol): nearly six times the single NN bond (EDIS = 160 kJ, 38.4 kcal/mol) and about twice the NN double bond (EDIS = 456 kJ, 109 kcal/mol). Therefore, a polynitrogen system based on single and double NN bonds is extremely metastable and tends to spontaneously release dinitrogen (N2) accompanied by large energy release. The barrier to N2 expulsion of these compounds is relatively small, 30−50 kcal/mol for organic azides and about 20−25 kcal/mol for the most stable pentazoles. para-Methoxyphenyl pentazole (MeOPP) is a relatively stable phenyl pentazole that can be stored in dry ice temperature for months and manipulated at moderate temperatures (< −20 °C) for many hours with no noticeable thermal degradation. This pentazole derivative was studied less often than DMAPP or OPP because of its poorer thermal stability and the difficulty of separating it from the azide after synthesis (both DMAPP and OPP can be purified by washing or recrystallizing with methanol−water mixtures while MeOPP cannot). MeOPP was, however, the aryl pentazole investigated in the first claims for the thermal production of N5− in solution.13,14 It is also expected to have a much lower contribution from its quinoidic structure and so a weaker C− N bond with a bond order closer to 1, compared to DMAPP and OPP. The photochemistry of the few aryl pentazoles studied thus far is reminiscent of the photochemistry of the corresponding azides due to either the conversion of the pentazoles to the azides as a first step or the direct formation of phenyl nitrenes from the pentazoles.19,20 The fate of the nitrenes produced from the pentazoles or the azide depends on the environment, substrate concentration, and oxygen availability. The common stable end products of aryl azide photochemistry are substituted azobenzene derivatives (nicknamed azo compounds in this paper), anilines, and azepines.18 Any difference between the product distributions of pentazole vs azide having the same aryl component is an indication of a photochemical path available to pentazoles and not to azides. This article presents a preliminary investigation of the photochemical end products of para-methoxypentazole (MeOPP) and its corresponding azide (MeOPA) in acetonitrile. As C−N bond cleavage is predicted to proceed via a conical intersection starting from a highly excited electronic state,16 the effects of short excitation wavelength are compared with those of longer wavelengths. Similar experiments with OPP19 did not discern any qualitative excitation wavelength effects. The choice of solvent was due to the wide spectral window of acetonitrile and its small absorption at 193 nm.
1. EXPERIMENTAL SECTION 2.1. Materials. p-Anisidine (Alfa Aesar, 99%), HCl (Sigma, 37%), NaNO2 (Aldrich, 97%), NaN3 (Aldrich, purum), MeOH (BioLab, HPLC supra gradient), hexanes (Sigma-Aldrich, 98.5%+), acetonitrile (Bio Lab inc., HPLC ultragradient grade), trifluoroethanol (Alfa Aeasar, >99.5%) and acetonitrile-d3 (Cambridge Isotope Laboratories, %D > 99.8%) were used as received. N-Butyl nitrite was synthesized according to Noyes22 and analyzed using proton NMR (CD3OD, shift 7339
DOI: 10.1021/jp5108813 J. Phys. Chem. B 2015, 119, 7338−7348
Article
The Journal of Physical Chemistry B In high concentrations experiments (HC, ∼10−2 M), 3−10 mg of MeOPP were dissolved in MeCN or CD3CN (1.5 mL) directly in the cuvette at the working temperature. For UV−vis absorption measurements a sample (5 μL) of the irradiated solution was taken and diluted in 245 or 495 μL of MeCN in a 2 mm optical path cell to allow an X250 or X500 optical dilution. A spectrum was collected without cooling but quickly enough to avoid significant thermal reactions. UV−vis absorption was performed using equipment as for the medium concentration experiments. MeOPA was produced by heating MeOPP to 30 °C for 30 min and then cooling to −27.5 °C prior to excitation. Complete conversion was obtained as the first order reaction half-life of this reaction was measured at 25 °C and found to be 2.4 min. The proton NMR shift of CD3CN was measured relative to TMS at −35 °C and found to be 1.97 ppm. This was later used to reference all reported spectra. A frequency ratio Ξ of 25.145020% was used to reference the carbon shifts. 2.4. Analysis of UV−vis Spectra. UV−vis spectra were analyzed under the assumption that at any time the recorded spectrum is a linear combination of a set of several component spectra, that is, At,λ = Σim= 1 εi,λCi,t where At,λ is the absorbance at wavelength λ and time t assuming an optical path of 1 cm, m is the number of materials, εi,λ is the extinction coefficient of material i at wavelength λ, and Ci,t is the concentration of material i at time t. This assumption allows us to extract the spectra of unidentified substances as long as they exhibit distinct kinetic behavior. The full details of the analysis method used are given in the Supporting Information (S1). A shortcoming of this method is that the spectra produced by it are not necessarily those of pure materials if different products follow similar kinetics. 2.5. Error Analysis. A major contribution to the errors in estimating yields is due the unavoidable thermal degradation of MeOPP while manipulating the samples. Small variations in temperature change the MeOPA content of the solutions. In addition, spectral contributions by species with closely related kinetic behavior lead to errors in the estimated concentrations of the products due to spectral contributions by species having closely related kinetic behavior. However, these errors are largely canceled when the ratios of concentrations are considered. Therefore, concentration ratios should be regarded as much more precise than absolute values of the concentrations themselves. The latter turn up to have rather large error bars, as found upon repetitive runs. 2.6. TD-DFT Calculations. All calculations were performed at the B3LYP/CC-pVDZ level using the Gaussian software package.24
3.1. LC Photolysis Results. The UV−vis spectra of LC MeOPP and MeOPA solutions in MeCN irradiated in the far and near UV are shown in Figure 2. In both cases any residual
Figure 2. UV−vis spectra of irradiated MeOPP (solid line) and MeOPA (dashed) in MeCN: (A) FUV experiment, (B) NUV experiment; 28 μM solutions, 60 s irradiation at −27.5 °C.
reactant peak is overlapped by other absorption bands and a few new absorption bands appear: 357 and 250 nm for the FUV irradiation; and 449, 308, and 251 nm in the case of NUV irradiation. The spectrum of FUV irradiated MeOPP can be satisfactorily reproduced by adding two new absorbing compounds, in addition to MeOPP and MeOPA. The spectra of the added compounds fit well the literature spectra of the trans and cis isomers of 4,4′-dimethoxyazobenzene,25 as shown in Figure 3.
Figure 3. UV−vis spectra in MeCN ensuing from the analysis for the trans- (green) and cis- (orange) azo isomers.
To better distinguish between the two azo spectra (i.e., to attain a better kinetic difference between the two) the FUV-irradiated MeOPA solution was further irradiated at 360 nm and then at 280 nm using the Xe lamp equipped with appropriate interference filters. This procedure is known to cause trans → cis and then cis → trans isomerization26 and therefore allows better spectral separation (see Supporting Information, Figure S3 for the resulting spectra). The vanishing absorption of the cis-azo spectrum at ∼380 nm is an artifact due to the imperfect kinetic separation between it and the trans isomer spectrum and some linear mixing between the two spectra. The contributions of these factors are estimated to be smaller than 5%, based on many repetitions.
3. RESULTS Figure 1 displays the UV−vis absorption spectra of MeOPP and MeOPA. The UV−vis spectrum of the MeOPP synthesis product without subtraction of the 8% MeOPA spectrum is shown in Supporting Information, Figure S1. The proton NMR spectrum of both compounds can be seen in Supporting Information, Figure S2, the 8% MeOPA contamination was determined using these NMR data. Supporting Information, Table S1 lists all the compounds mentioned in the paper with the abbreviations used. Literature UV−vis absorption spectra and NMR data (ppm) are presented when available. 7340
DOI: 10.1021/jp5108813 J. Phys. Chem. B 2015, 119, 7338−7348
Article
The Journal of Physical Chemistry B
consistent with a mechanism in which the MeOPP’s initial reaction upon FUV irradiation is the formation of MeOPA by nitrogen extrusion. Absorption of another photon detaches a second N 2 molecule forming p-methoxyphenyl nitrene (MeOPN), which then forms trans-azo via a bimolecular reaction with either MeOPP, MeOPA, or with another nitrene molecule.18 When operated at 2.5 mW, the 193 nm laser delivers 4.0 nmols of photons per second. The initial absorbance of the solution at 193 nm was 0.294, 91% of which may be attributed to MeOPP (the extinction coefficients of MeOPP and MeOPA at 193 nm are 17 000 and 23 00 M−1 cm−1, respectively, and the MeOPA contamination was ∼8%), which means that 1.9 nmol of photons was absorbed by MeOPP during the first second of irradiation. At the same time 0.95 nmol of MeOPP was decomposed (the total volume of the solution was 2 mL and the concentration decreased from 15.25 to 14.78 μM) so that the quantum yield of the reaction was 0.50 ± 0.08. As no other products were detected it is assumed that all unreacted excited molecules relax to the ground state without producing the azide. A similar analysis of the irradiation of MeOPA leads to a QY of 0.60 ± 0.09. The spectrum obtained upon NUV irradiation of MeOPP is not the same as that obtained upon FUV excitation. In addition to MeOPP, MeOPA, and the two azo isomers, the spectrum of at least one more component is needed for a good fit of the data. The spectrum, due to yet unidentified compound(s) designated as U1 (solid line, Figure 5), shows two sharp peaks
The spectral data were converted to concentrations using the measured extinction coefficients of MeOPP (12 000 M−1 cm−1 at 275 nm) and MeOPA (13 000 M−1 cm−1 at 256 nm). The conversion to concentrations is based on assuming that no other products except MeOPA and the two azo compounds contribute (as indicated by the good fit to the spectral data), on the conservation of mass and on the fact that production of one azo molecule consumes two pentazole ones (the aryl equivalence of azo is twice its concentration). The maximum deviation from the conservation of mass, as measured by the sum of the aryl-equivalent concentrations at any given time, is lower than 1 μM (6% of the 17 μM solution used); this figure is used as a lower limit to the error of the analysis method. The analysis resulted in spectra (Figure 3) and kinetic traces (Figure 4) in which the cis isomer constituted (50 ± 7)% of the azo
Figure 5. Normalized (maximum intensity set to 1) UV−vis spectra of U1 from the NUV irradiation of MeOPP (solid line) and U2 from the irradiation of MeOPA (dashed).
Figure 4. (A) Kinetic traces following a FUV irradiation of MeOPP (17 μM total, 15.5 μm MeOPP) in MeCN. (B) Blowup of initial stages of irradiation without the MeOPP trace. Analysis data are given as points and lines denote exponential (MeOPP) or biexponential fits. The decay time of MeOPP (assuming exponential decay) is 41 s.
at 204 and 232 nm, a broad and flat absorption band at 280− 312 nm, and another broad absorption at ∼445 nm with a shoulder toward the blue. It may represent more than one product, having the same time-dependent characteristics. U1 is obtained by averaging the spectra during the first 60 s of irradiation. As the reaction advances MeOPP concentration is reduced, becoming eventually smaller than MeOPA concentration (Figure 6). If the pentazole and the azide yield different products, the structure of U1 should be a function of time. Attempts to obtain the spectrum after a much shorter time (
