Solution Photochemistry of [p-(Dimethylamino)phenyl]pentazole

Article pubs.acs.org/JPCA

Solution Photochemistry of [p‑(Dimethylamino)phenyl]pentazole (DMAPP) at 193 and 300 nm B. Bazanov and Y. Haas* Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: The photochemistry of [p-(dimethylamino)phenyl]pentazole (DMAPP) at 193 nm and in the near UV is reported, with emphasis on the nature of the final stable products. The (dimethylamino)phenyl azide (DMAPA) is found as a major product in MeCN, but not in dichloromethane (DCM). (In this paper the acronyms DMAPP and DMAPA refer to the para isomers.) The photochemistry of DMAPA is also explored for comparison. The data obtained in MeCN solutions are consistent with the initial formation of the corresponding nitrene, but in DCM, different products are found, on the basis of NMR data. In the case of high reactant concentration in DCM (10−2 M), quantitative conversion of DMAPP and DMAPA is observed, indicating a high quantum yield. In contrast, MeCN solutions react much more slowly. A radical-type chain reaction mechanism is proposed to account for this observation. At high dilution, DMAPP is completely converted to products in both solvents. Possible mechanisms accounting for these results are discussed. An interesting finding was that photolysis at 193 nm did not lead to complete OPA dissociation, although it absorbs light at this wavelength. Interest in arylpentazoles has recently been revived due to the reports that the cyclo-pentazolate anion (cyclo-N5−) was formed in the gas phase from [4-(dimethylamino)phenyl]pentazole15 and 4-hydroxyphenylpentazole (HOPP).16 The allnitrogen cyclo-pentazolate anion was detected by mass spectrometry. Attempts to repeat this reaction thermally in the bulk have been unsuccessful,11 although ceric ammonium nitrate (CAN) oxidation of (4-methoxyphenyl)pentazole did yield products believed to indicate the prior formation and degradation of cyclo-N5−.10 This inference is in contrast with a previous paper12 that concluded that no direct observation of the anion in solution was demonstrated. In any case, at the present time there is a consensus that cyclo-N5− has not been isolated by reactions of aryl azides in the bulk. A considerable theoretical effort (ref 17 and references therein) accompanied the experimental work and was very useful in interpreting them. In view of the admittedly limited scope of photochemical experiments it is interesting to compare the photochemistries of DMAPP and DMAPA. The photochemistry of aryl azides is extensively documented, especially upon excitation in the near UV,18,19 whereas that of arylpentazoles is poorly known. This work was designed to document the thermally stable products of DMAPP and DMAPA and the effect of variables such as solvent, excitation wavelength, and concentration on the

1. INTRODUCTION Preparation of stable pentazole heterocyclic molecules was a challenge for a long time. Lifschitz1 (1915) believed he was the first to prepare one, but in the same year Curtius2 et al. showed convincingly that this was not the case in a paper entitled “The So-Called Pentazole Compounds of Lifschitz”. Curtius’ rebuttal discouraged workers in the field, and the first successful synthesis was published by Huisgen and co-workers in 1957 who extensively studied and characterized them in the late 1950s and early 1960s.3−5 These compounds spontaneously decompose at room temperature due to a rather low activation energy, typically 20 kcal/mol or less.6 The primary thermal reaction is generally believed to be extrusion of dinitrogen (N2) forming the corresponding azide.3,7 In contrast to the extensive thermal chemistry,8−12 the photochemistry of arylpentazoles has been reported only recently. Portius et al.13 studied the photochemistry of DMAPP in liquid solution using ultrafast IR spectroscopy. They found that irradiation in dichloromethane (DCM) at 310 or 320 nm (at the first absorption band) formed an excited state of DMAPP. Most of it (86%) returns to the ground state the rest forms DMAPA by extrusion of one N2 molecule, or the corresponding nitrene by extrusion of two. Geiger et al.14 reported the photochemistry of 4-oxidophenylpentazole (OPP) in water and in acetonitrile at its first absorption band (320 and 370 nm, respectively) and at 193 nm. Stable photoproducts were examined by NMR and UV−vis spectroscopies, revealing a fairly complex reaction sequence. In this case, the pentazole was found to be completely converted to products, the major one being the azide OPA. The latter was not photolyzed, presumably because irradiation was carried out at wavelengths not absorbed by it. © XXXX American Chemical Society

Special Issue: Markku Räsänen Festschrift Received: September 29, 2014 Revised: October 27, 2014

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dx.doi.org/10.1021/jp509815y | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Chart 1. Main Compounds Discussed in This Article: Names, Molecular Structures, and Abbreviations

reaction of triplet nitrene with the azide.20 Azepines are observed upon using appropriate scavengers such as primary or secondary amines.21,26 Their formation is readily rationalized if nitrene is initially formed. Triplet arylnitrenes are potent hydrogen atom abstractors, forming aniline derivatives22−25 In the case of DMAPP, Li et al.26 found that photolysis in hexane leads to formation of the azo compound in 92% yield, whereas adding diethylamine results in reducing the azo yield to 55% and to formation of p-(dimethylamino)aniline at 44% yield. Arylnitrenes and (heteroaryl)nitrenes are of current interest because of their use in photoaffinity labeling and photo crosslinking (PAL/PCL), a major technique for studying molecular interactions in biological systems. In a recent paper27 it was

product distribution. Due to its thermal lability DMAPP was irradiated at low temperatures (typically −20 °C) and so was DMAPA. A brief review of the photochemistry of aryl azides is called for, as a common mechanism appears to hold for many of them, carrying different substituent groups on the phenyl ring. The products formed will be referred to as the standard products. A key intermediate in the photochemistry of aryl azides is the corresponding nitrene,18 a highly reactive species formed by extrusion of N2. It may exist as a triplet or a singlet, the latter being extremely reactive and short-lived. Symmetric azo compounds are often found as stable end products. They may be formed either by recombination of two nitrenes or by a B



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reported that formation of the photochemical products is wavelength dependent, and that intermediates are powerful hydrogen and proton abstraction agents, leading to formation of aniline derivatives. To the best of our knowledge, the photochemistry of aryl azides in the far UV (for instance, by using an excimer ArF laser at 193 nm) has not been reported. In this communication some results on DMAPP and DMAPA excited at this wavelength are discussed and compared with near UV data. The purpose of this part is to check whether novel reaction pathways and products are revealed. The names, formulas and abbreviations of some proposed products are collected in Chart 1.

A Bruker Avance II 500 MHz NMR spectrometer was used for all NMR measurements. The concentration of DMAPP/ DMAPA required for NMR experiments was 10−2 M. A PerkinElmer CHN analyzer 2400 from the Microanalysis Lab of Hebrew University was used for determination of CHN content. 2.3. Irradiation Procedure. DMAPP and DMAPA were irradiated in several different solvents at −20 °C. Lowconcentration (LC) experiments were performed as follows: 2 mL of the appropriate solvent was loaded into a 10 mm quartz cuvette equipped with a magnetic stirrer; the cuvette was placed in the q-pod holder and allowed to equilibrate to the set temperature. A stock solution was prepared by dissolving about 1/2 mg of DMAPP or DMAPA in 2 mL of a precooled solvent in a 5 mL vial and kept in dry ice. From this stock solution about 20 μL was added to the cuvette. The irradiation of the lowconcentration solution lasted at most 1−2 min, and the resulting product mixture was analyzed by UV−vis spectroscopy only. High-concentration (HC) experiments were performed as follows: About 1.5 mL of the solvent (for NMR analysis, deuterated ones) was loaded into a 10 mm × 10 mm cuvette with a magnetic stirrer. The cuvette was placed in the q-pod holder and allowed to equilibrate to the set temperature. Roughly 1 mg of DMAPP was weighed and added to the cuvette. The irradiation of the high-concentration solution lasted up to 30 min, and the solution was analyzed by NMR spectroscopy or, after appropriate dilution, by UV−vis spectrometry. 1D proton and 2D COSY and HSQC spectra were used to identify the constituents of the product mixture. 2.4. Data Processing. H NMR was used to identify photoproducts and also to estimate relative yields by using integration of the H NMR peaks. Whenever possible, structures were compared with literature data or by running the spectra of authentic samples. In some cases, neither were available. In these cases use was made of ACD/NMR predictors that allowed fast and accurate prediction of NMR spectra, chemical shifts, and coupling constants. A large fraction of the irradiation products detected by H NMR method had a structure of p-(dimethylamino)phenyl with different functional groups in the para position to the dimethylamino group. Signals were found in the aromatic region, mostly in the 6−8.5 ppm shift range. H NMR spectra were calculated using existing programs, such as ACD/ Laboratories Software, Version 11.01, available in SciFinder (https://scifinder.cas.org/scifinder); for individual examples and references please see section 4. The UV−vis absorption spectrum of the same samples was measured in parallel. An 100× dilution was required for these measurements performed before and after irradiation. In most cases reactant bands were observed after irradiation solution, especially in the HC experiments. At these concentrations most of the light was absorbed in a thin layer next to the entrance window, so that only a small fraction of the rigorously stirred solution was illuminated. To reveal product bands overlapping the reactant ones, the spectral contributions of the reactants were subtracted. 2.5. Reagents and Solvents. N,N-Dimethyl-p-phenylenediamine dihydrochloride (Fluka, purris, ≥99.0%), sodium azide (Aldrich, purrum, ≥99.0%), sodium nitrite (Aldrich, ≥97.0%), hydrochloric acid (Sigma-Aldrich, 37% fuming), methanol (Biolab, HPLC supra gradient, ≥99.95%), chloro-

2. EXPERIMENTAL SECTION 2.1. Synthesis. DMAPP was synthesized by a slightly modified version of the Ugi and Huisgen3 method, as previously described.28 In a typical procedure, DMAPP was prepared as follows: 2.31 g of N,N-dimethyl-p-penylenediamine dihydrochloride (11 mmol) was dissolved in 19 mL of methanol in a vial cooled to −10 °C in a circulation reactor using methanol as the bath fluid. Four milliliters of hydrochloric acid were added to the vial followed by 2.22 g of sodium nitrite (32 mmol) dissolved in 5 mL of triple distilled water that was added slowly dropwise, keeping the reaction mixture continuously stirred. The mixture initially turned to deep purple; upon continued stirring for an additional hour, the solution became light brown. After the reaction mixture was cooled to −25 °C, 4.3 g of sodium azide (66 mmol) dissolved in 10 mL of purified water was slowly added dropwise. Intense gas evolution was immediately observed. The reaction mixture became viscous due to precipitation of light yellow solid and stopping the magnetic stirrer. A glass rod was manually used intermittently to continue stirring at 5 min intervals for an additional hour. The reaction mixture was filtered gravitationally, and the filtrate was washed with about 50 mL of methanol at −18 °C (to remove DMAPA that is also always formed in these syntheses). After the filtration DMAPP was transferred to a vial and dried under vacuum at −30 °C, DMAPA (p(dimethylamino)phenyl azide) was synthesized from DMAPP by allowing it to warm to room temperature. The conversion was quantitative after 30 min. 2.2. Equipment. A q-pod 2e temperature controlled cuvette holder system (Quantum North West) equipped with a magnetic stirrer was used for irradiation experiments purged with nitrogen to avoid water condensation from air. A USB4000 fiber optic spectrometer was used with a DH-2000-BAL light source for UV−vis absorption measurements, all supplied by Ocean Optics. The concentration of DMAPP/DMAPA required for UV experiments was about 10−4 M to avoid saturation. The photolysis light sources were a pulsed ArF excimer laser with variable repetition rate (Neweks Ltd., PSX100) and a 150 W xenon lamp fitted with a water filter and the appropriate low-pass and high-pass filters for the experiment (Hamamatzu E7536 with L2175). The spectrometer and light source were attached to the qpod perpendicular to the irradiation direction for the lowconcentration experiments. Absorbance measurement for the high-concentration experiments were carried out in an uncooled cuvette holder nearby following 100× dilution (measurements were taken as quickly as possible). C



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form (Sigma-Aldrich ACS spectroscopic grade, ≥99.8%), methylcyclohexane (Sigma-Aldrich, spectroscopic grade, ≥99%), ethanol absolute (Sigma-Aldrich, for HPLC, ≥99.8%), toluene (J. T. Bruker, ACS grade, ≥99.5%), 2methyltetrahydrofuran (Sigma-Aldrich, anhydrous, ≥99.0%), dichloromethane (Sigma-Aldrich, ACS reagent, ≥99.5%), acetonitrile (Fluka, for UV spectroscopy, ≥99.8%), methylene chloride-d2 (D, 99.9%, Cambridge Isotope Laboratories, Inc.), and acetonitrile-d3 (d, 99.8%, Cambridge Isotope Laboratories, Inc.) were used as received. Dichloromethane was dried over 4 Å molecular sieves and filtered with glass syringe containing molecular sieves through 0.2 μ PTFE filter immediately before use.

this work) nm for DMAPP and DMAPA, respectively). The light source was a xenon lamp, equipped with appropriate filters. In view of the large molecular absorption coefficients of DMAPP and DMAPA, the consumption of the reactants can be easily monitored using UV absorption spectroscopy at 10−4 M or less. The change in reactant concentration was followed by NMR for the high-concentration (HC) samples, but for the lowconcentration (LC) ones UV−vis spectroscopy had to be used. All irradiations were carried out at −20 °C unless otherwise noted. Figure 2 shows a typical proton NMR spectrum recorded after irradiation of DMAPP in MeCN. Most products could be readily assigned in this case, but in several other experiments, H atom signals were found in the aromatic region and are not yet conclusively assigned. The assignments are discussed in section 4. The extent of reaction, expressed as the percentage of the reactant remaining after 30 min irradiation is summarized in Table 1. Note the high reactivity of DMAPP and DMAPA in DCM upon 193 nm irradiation, where a quantitative elimination of the reactant (DMAPP or DMAPA) is observed. In MeCN the reactivity is medium for DMAPP (66% unreacted) and very small for DMAPA (94% remaining). However, DMAPP and DMAPA are completely converted to products in a short irradiation time under LC conditions. (Figure 3 shows the DMAPP spectrum before (Figure 3a) and during irradiation; Figure 3b after 15 s irradiation and Figure 3c after 30 s. Complete conversion is achieved within 30 s. Although in most LC experiments the reactant (DMAPP/ DMAPA) was quantitatively converted to the products in less than 2 min, in some cases the residual reactant absorption had to be subtracted (Figure 4). Figure 4a shows the spectrum after DMAPA absorption and Figure 4b before the subtraction. Figure 5 shows the UV−vis absorption spectrum of (left) ∼10−2 M solution of DMAPA in MeCN (Figure 5a) and DCM (Figure 5b) and (right) of a ∼10−4 M solution after irradiation at (top) 193 nm or (bottom) by the xenon lamp at 240−400 nm. Figure 6 shows the UV−vis absorption spectrum: left panels of a ∼10−2 M solution of DMAPP in MeCN (Figure 6a) and in DCM (Figure 6b); right panels of a ∼10−4 M solution after irradiation (top) at 193 nm or (bottom) by the xenon lamp at 240−400 nm. Figure 7 exhibits the UV−vis absorption spectrum of DMAPP solution in DCM:MeOH (1:1) irradiated at 340−400 nm. Figure 7a shows an HC sample irradiated and measured after 100× dilution; Figure 7b, an LC sample irradiated and measured as is. The product peaks around 414 nm and 250−260 nm are assigned to azo compounds and quinone diimine (QDI), respectively (see below for justification). Addition of a proton donor (e.g., MeOH) to DCM led to the appearance of an absorption band at 450−600 nm, typical of semiquinones (SQ), Figure 7. 3.2. Product Distribution. Table 2 summarizes the results obtained under HC conditions using H NMR analysis. Molecular oxygen is known to affect the photochemistry of some azides;29,19 runs of argon-saturated solutions yielded the same results as aerated ones, in both DMAPP and DMAPA experiments. Thus, it is assumed that O2 is unimportant in this case.

3. RESULTS 3.1. Reactant Consumption. Previous work indicated that an important photochemical reaction channel of DMAPP is the extrusion of a nitrogen molecule (N 2 ) to form the corresponding azide. An azide was also found to be an important product of 4-oxidophenylpentazole (OPP) photolysis.14 These results suggest that a possible mechanism of arylpentazole photochemical reactions is initial formation of the corresponding azide, followed by azide photolysis, which was extensively studied. It was therefore considered worthwhile to investigate DMAPP and DMAPA photolysis under similar conditions. The results in this paper pertain only to products that are stable at moderately low temperatures; their analysis was typically carried out at −20 to −30 °C. Several solvents were tested in preliminary runs. It was found that the yield and product distribution were strongly solvent dependent. In this report we focus on two solvents only, MeCN and DCM. They were chosen because the compounds dissolve easily in them, their absorption at 193 nm is smaller than that of other solvents (MeCN is almost transparent at this wavelength), and they have different polarities and are liquid at −30 °C. DCM was used by Portius et al. in their study on DMAPP photochemistry,13 allowing a comparison. Figure 1 shows the absorption spectra of DMAPP and DMAPA in MeCN solution in the range 190−600 nm. As both molecules absorb at 193 nm, experiments were run in two wavelength regimes: at 193 nm, using an ArF laser, and at the first strong absorption band (peaking at 329 nm (ε = 14 900 M−1 cm−1)4 and 283 nm (ε = 19 300 M−1 cm−1, measured in

Figure 1. UV absorption spectra of (a) DMAPA and (b) DMAPP in MeCN (∼10−4 M). D



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Figure 2. Aromatic region of the H NMR spectrum of ∼10−2 M DMAPP solution in MeCN recorded after irradiation at 193 nm.

Table 1. Percentage of the Reactant Remaining after 30 min of Irradiation, HC Conditions (H NMR Data) solvent

reactant

source

MeCN DCM MeCN DCM

DMAPP DMAPP DMAPA DMAPA

193 193 193 193

nm nm nm nm

% unreacted

reactant

source

66.2 0 93.5 0

DMAPP DMAPP DMAPA DMAPA

340−400 340−400 240−400 240−400

% unreacted nm nm nm nm

51.8 77.2 23.1 7.6

Figure 4. UV−vis absorption spectrum of DMAPA ∼10−4 M solution in MeCN after 2 min irradiation at 193 nm, (a) after and (b) before DMAPA absorption subtraction.

−4

Figure 3. UV−vis absorption spectrum of a ∼10 M DMAPP solution in MeCN recorded at some time intervals after irradiation by a 193 nm laser, (a) after 0, (b) after 15 and (c) after 30 s.

offered here as an incentive for further research. The possible production (Table 2) of PDMAAN (p-(dimethylamino)acetonitrile) is particularly intriguing, as it requires that the C−N bond connecting the two rings must be cleaved and that it cannot be formed from the nitrene. It is also found upon DMAPA photolysis in MeCN. This and some other products, though minor, may indicate that photochemical methods are capable of producing novel molecules, and warrants further study. For more details and justification please see in the Supporting Information.

4. ASSIGNMENT Table 3 summarizes the proposed assignments of molecules observed by H NMR and UV spectroscopy. The molecules DMAPD, DMAPDA, CDMAPA, PDMAAN, PCMDMA, and PDCMDMA are proposed on the basis of the NMR spectra that are found to be very similar to (though not identical with) the observed ones. All are rather similar to each other, and to that of dimethylaniline. They were suggested by calculations of H NMR spectra in the SDBS database. It is assumed that their structures are similar, and that they are parasubstituted dimethylanilines. This speculative assignment is E



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Figure 5. UV−vis absorption spectrum of DMAPA solution in (a) MeCN and (b) DCM ∼10−2 M (left) and ∼10−4 M (right) irradiated at 193 nm (top), with a xenon lamp at 240−400 nm (bottom), at −20 °C, for 30 min. Solvent and DMAPA spectra subtracted.

5. DISCUSSION 5.1. General Comments. The primary process in the photochemistry of aromatic azides is the generation of singlet nitrene, which can undergo intersystem crossing to the triplet. The latter usually reacts to form azo compounds.19,20,26 As shown in Table 2, DMAPA is the major product of DMAPP photolysis in MeCN and also in DCM upon 240−400 nm excitation. In the latter solvent, irradiation at 193 nm produces no DMAPA. The azo compound b-DMAAB is an important product in the photochemistry of DMAPA upon UV excitation in MeCN, but of minor significance in the case of DMAPP. bDMAAB is formed most probably under HC conditions by a bimolecular reaction, as there are many DMAPP or DMAPA molecules around. When the DMAPA/DMAPP initial concentration is low, the nitrene appears to preferentially subtract a proton from the solvent forming QDI42,43 (Figures 5 and 6). Under HC conditions, only a small fraction of the reactant is consumed; in contrast, when the DMAPA/DMAPP initial concentration is low, complete conversion occurs. Nitrenes, which were shown to react with H atom donors to form aniline derivatives,22 are likely to subtract a hydrogen atom from the solvent forming a PPD (dimethylphenylenediamine) monomer. DMAPA, formed from irradiated DMAPP, may lose another N2 molecule if irradiated at an appropriate absorption band to form the nitrene.18,44 It does not absorb light in the 340−400 nm range used to photolyze DMAPP and thus accumulates in the cell. The nitrene is highly reactive at room temperature and can be observed at ambient temperatures only by using picosecond spectroscopy. In low-temperature glasses its motion is hindered, making it easier to observe. Figure 8 shows the UV

spectrum obtained upon DMAPP irradiation at 340−400 nm in a low-temperature 2-MTHF matrix (83 K). The spectrum is very similar to that reported by Kobayashi and assigned to a nitrene.45 It disappears at higher temperatures. The main products in DMAPP photolysis at 193 nm in LC conditions are b-DMAAB and QDI (Figure 3). The latter is probably an intermediate in the formation of aniline derivatives and requires a hydrogen (or proton) donor. It appears to be stable at −20 °C, at which the spectra were obtained. On the other hand, irradiation in DCM yields a completely different product distribution, except that DMAPA is a major product of DMAPP photolysis in this solvent as well. Under HC conditions, DMAPA is the major product, and much DMAPP is not reactive even upon 30 min irradiation. In contrast, DMAPA appears to be nonreactive under 193 nm radiation, but highly reactive in the near UV. In MeCN the major products are the azo compound and QDI, and in DCM QDI, PDMAT, and PPD, the azo is a very minor product (Table 2). Bearing in mind that DCM absorbs (weakly) at wavelengths below 240 nm, it is proposed to term it a “Reactive” solvent, in contrast with MeCN which is “inert”. A plausible cause for the difference is partial dissociation of DCM to form chlorine atoms that can extract a hydrogen atom from the phenyl moiety of DMAPP or DMAPA to create a radical that scavenges a chlorine atom from DCM to form a chlorinated molecule. We have not been able to find experimental evidence for the products of DMAPP and DMAPA formed upon irradiation at 193 nm. Our assignments are based on calculated spectra (Table 3). The net result is substitution of a C−N bond by a C−C one. This makes sense thermodynamically, as the C−C bond (ca. 348 kJ/mol) is stronger than the C−N one (ca. 308 kJ/mol). F



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Figure 6. UV−vis absorption spectrum of DMAPP solution in (a) MeCN and (b) DCM HC samples (left, ∼10−2 M 30 min photolysis) and LC samples (right, ∼10−4 M 1−2 min photolysis) irradiated at 193 nm (top), or with a filtered xenon lamp light at 340−400 nm (bottom). Solvent and DMAPP spectra subtracted.

Table 2. Product Distribution (%) of Irradiated HC Samples (Main Products)a DMAPP irradiation wavelength 193 nm

340−400 nm

DCM

MeCN

DCM

MeCN

DMAPA: 0 CDMAPA: 21.1 U1: 78.9b

DMAPA: 21.0 b-DMAAB: 6.6 PPD: 3.1 DMAPD: 3.1

DMAPA: 22.8

DMAPA: 44.4 b-DMAAB: 0.6 PPD: 1.4 DMAPD: 0.8 QDI: 0.7 PDMAAN: 1.1

DMAPA

Figure 7. UV−vis absorption spectrum of DMAPP solution in DCM:MeOH (1:1) irradiated at 340−400 nm. HC sample irradiated, (a) measured after 100× dilution; LC sample irradiated and (b) measured (10−4 M). The peaks marked as DMAPA, DMAPDA, and SQ are the proposed products (Chart 1). Solvent and DMAPP spectra subtracted.

irradiation wavelength 193 nm

5.2. Mechanism. MeCN: Low vs High Concentration. At 193, LC main products are azo and QDI. DMAPA seems to appear as a shoulder on the 250−260 nm band. The HC main products are (NMR) DMAPA (21.0%), b-DMAAB (azo) (6.6%), DMAPD (3.1%), and PPD (phenylene) (3.1%). The UV spectrum is different from the LC one: there is a band close to the azo band, and another (or several) that appears to broaden the QDI band. These results may be rationalized by assuming that DMAPA is formed in both concentration ranges, but most of it is

340−400 nm

DCM

MeCN

DCM

MeCN

CDMAPA: 94.6 U2: 5.4c

b-DMAAB: 3.3 PPD: 2.0 PDMAAN: 1.2

PPD: 12.0 PCMDMA: 7.2 QDI: 35.3 PDMAT: 32.4 PDCMDMA: 5.5

b-DMAAB: 35.3 PPD: 4.3 DMAPD: 0.7 QDI: 14.5 PDMAAN: 12.1

a Yet unassigned species are labeled as U1, U2, etc. bHas two doublets in aromatic region at 8.0,6.8 ppm. cHas two doublets in aromatic region at 8.3, 7.1 ppm.

consumed in LC. The nitrene, formed from the extrusion of N2, is converted to an iminoradical I by H atom transfer; scavenge of another hydrogen atom forms PPD (Scheme 1). G



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Table 3. Proposed Assignments of Photoproducts of DMAPP and DMAPA Photolysis Compared to Literature Data or Computed Spectra NMR Shifts in ppm, ε (at Band Max in M−1 cm−1) compound PPD

experimental data

literature or computed data

1

H NMR: 6.62, 6.53, 2.72 (CD3CN) C NMR: 115, 41 (CD3CN) UV−vis: 239 nm (CH3CN/H2O) 13

SQ QDI

DMAPD

13

1

H NMR: 6.92, 6.41, 2.26 (CD3CN) UV−vis: ∼450−580 nm (CD2Cl2:CD3OD) 1 H NMR: 6.85, 6.45, 2.11 (CD3CN) UV−vis: ∼250−260 nm (CD3CN) 1

H NMR: 8.07, 6.68, 3.08(CD3CN) C NMR: 110,40 (CD3CN)

13

b-DMAAB

1

DMAPDA

1

H NMR: 7.71, 6.79, 3.15(CD3CN) UV−vis: ∼420, 460 nm (CH3CN) H NMR: 6.84, 6.69, 2.83 (CD2Cl2:CD3OD) C NMR: 119, 114 (CD2Cl2:CD3OD) UV−vis: ∼730 nm (CD2Cl2:CD3OD) found by CHN analyzer: C, 46.02; H, 4.39; N, 25.45; Cl, 24.14 1 H NMR: 7.77, 7.16, 3.11(CD2Cl2) 1 H NMR: 7.25, 6.44 (CH3CN) 1 H NMR: 7.10, 6.50 (CD2Cl2) 1 H NMR: 7.21, 6.52 (CD2Cl2) 1 H NMR: 7.25, 6.60 (CD2Cl2) 13

CDMAPA PDMAAN PCMDMA PDCMDMA PDMAT

H NMR: 6.71−6.64, 2.82 (CDCl3)30 C NMR: 115.5, 42.0 (CDCl3)30 UV−vis: 242, 307 nm (buffer pH 8)31 ε ∼ 9600 M−1 cm−1, ε ∼ 2000 M−1 cm−1 UV−vis: ∼460 nm (buffer pH 8)25 ε ∼ 5900 M−1 cm−1 1 H NMR: 6.81, 6.33 (PCE)32 UV−vis: ∼257, 266 nm (MeCN:H2O)25 ε ∼ 26 900 M−1 cm−1, ε ∼ 26 300 M−1 cm−1 1 H NMR: 7.8, 6.8, 3.033 13 C NMR: 113, 42 UV−vis: 441 nm (CH3CN/H2O)34 1 H NMR: 7.83−7.80, 6.77−6.75, 3.06 (CDCl3)35 UV−vis: ∼422, 457 nm (CH3OH)36 ε ∼ 19 800 M−1 cm−1, ε ∼ 22 100 M−1 cm−1 1 H NMR: 6.85, 6.67, 2.77 (DMSO-d6)37 13 C NMR: 118.13, 114.35 UV−vis: ∼730 nm (water)38 calcd for C8N4H8Cl2: C, 41.58; H, 3.49; N, 24.25; Cl, 30.68 1 H NMR: 7.10 (d) measured for 1-azido-2,3-dichlorobenzene39,40 (CDCl3) 1 H NMR: 7.30, 6.64 measured for trans-p-(dimethylamino)cinnamonitrile32,35 1 H NMR: 7.2, 6.728 1 H NMR: 7.3, 6.728 1 H NMR: 7.1, 6.741 (CDCl3) 1

The azo compound (bis(dimethylamino)azobenzene in this case) is formed by a reaction between the nitrene and DMAPP or DMAPA (Scheme 2). At 340−400 nm irradiation, the main LC product is QDI; very little azo is observed. In HC samples, the main products are DMAPA (44.4%) and DMAPD (3.1%). Explanation: The azo specimen is not formed because the nitrene is consumed by the reaction to form QDI. This reaction requires a hydrogen or proton donor. Although MeCN is not a good one, in this case it may be active enough. Some residual water is another possible donor. In LC, all water is consumed and QDI is formed. In HC, assuming that the source of water (or another proton donor) is from the solvent, supply is much more abundant, reducing the efficiency of the azo-forming channel and enhancing QDI formation in the case of DMAPA,

Figure 8. DMAPN UV−vis absorption spectrum, produced by irradiation, xenon lamp at 340−400 nm of DMAPP in 2-MTHF glass, 83 K.

Scheme 1. Proposed Mechanism for PPD Formation

H



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Scheme 2. Proposed Mechanism for b-DMAAB Formation

Scheme 3. Proposed Mechanism for QDI and SQ Formation

Scheme 4. Mechanism Proposed for DMAPDA Formation

dependent mechanism. DCM is known to to dissociate in the VUV (due to a weak absorption extending to 240 nm) to form chlorine atoms and CH2Cl radical.46,47 The absorption cross section of CH2Cl2 at 193 nm is 3.35 × 10−19 cm2 molecule−1 ; ε = 900 M−1 cm−1.48 The quantum yield of Cl atoms formation is 1. The radicals may react with DMAPP or DMAPA and start a chain reaction.49

but not of DMAPP (Table 2). This difference may be due to a fast IC channel to the ground state, which is much more effective in DMAPP than in DMAPA. This point will be discussed in a separate publication. QDI is known to react with PPD to form 2 molecules of SQ.42 Scheme 3 shows a possible sequence. Scheme 4 depicts a possible mechanism leading to DMAPDA. However, ammonia was not detected with the currently used analytic tools. DCM: Low vs High Concentration. Upon 193 irradiation, under LC conditions, the main products are an absorber at 330 nm, which is not DMAPP (H NMR evidence); DMAPP is completely consumed (Figure 6 and Table 2). UV−vis of that product differs slightly from that of DMAPP, the same UV−vis spectrum observed at HC upon 193 irradiation; at this point we cannot come up with a plausible assignment. There are two unassigned compounds formed at a high quantum yield and designated in Table 2 as U1 (78.9% DMAPP in DCM at 193 nm) and U2 (5.4% DMAPA in DCM at 193 nm) that may absorb in that range. In the case of 340−400 nm excitation, the LC main products are QDI and SQ when methanol is added but no azo (bDMAAB) is found; for the HC samples, the main products are DMAPA (44.4%), b-DMAAB (0.6%), PPD (1.4%), QDI (0.7%), and PDMAAN (1.1%). No unassigned lines are observed. Under HC conditions an absorption band adjacent to that of the azo molecule band and of comparable intensity is found in the UV spectrum; it may be due to DMAPD. This molecule can formally be formed from b-DMAAB, but a reaction of this kind is not known for azo compounds. Another option is a reaction between two PPD molecules with the elimination of NH2. 5.3. Solvent and Wavelength Effects. In concentrated solutions, only 34% of DMAPP reacts upon 193 nm excitation in MeCN, but the molecule is completely consumed in DCM (Table 2). This large difference may indicate a radical chain mechanism operating in DCM but not in MeCN. Excitation in the 340−400 nm range leads to consumption of about 50−70% in both solvents. This indicates an excitation wavelength

ArH + Cl• → Ar •+HCl Ar •+CH 2Cl 2 → ArCl + CH 2Cl•

Thus, in DCM it is possible to form molecules such as CDMAPA, CDMAPP, PCMDMA, PDCMDMA, and PDMAT, as proposed. At this point the data are not extensive enough to permit a detailed mechanism. Under certain conditions, MeCN can act as a nucleophile in photocemical reactions and create a new C−C bond in an olefin.50 In the case of nitrene, a different situation holds, but energetically it is possible to replace a C−N bond by a C−C one. These speculations must be further explored. Our attempts to use HPLC and HPLC−MS methods to shed more light on the system, did not result yet in a conclusive outcome.

6. SUMMARY The photochemistry of DMAPP in MeCN and DCM solutions is reported under various conditions. DMAPA is the most abundant product, but in MeCN a large fraction of DMAPP remains unreacted even after lengthy irradiation in concentrated solutions (∼10−2 M). Under 193 nm excitation, secondary photolysis yields also an azo compound, [(dimethylamino)phenyl]diazene and p-(dimethylamino)phenylenediamine. In all identified products the CN bond connecting the phenyl ring with the pentazole one is retained, after losing two N2 molecules. However, NMR evidence points to production of other molecules, which can only be formed if this bond is broken; Additional research is required on this point. Irradiation at wavelengths longer than 340 nm, results in a similar product distribution, with a larger azide fraction, probably due to the fact that the azide does not absorb light in I



Article

(7) Müller, R.; Wallis, J. D.; Philipsborn, W. V. Direct Structural Proof for the Pentazole Ring System in Solution by 15N NMR Spectroscopy. Angew. Chem. 1985, 97, 513−515. (8) Butler, R. N.; Fox, A.; Collier, S.; Burke, L. A. Pentazol Chemistry: the Mechanism of the Reaction of Aryldiazonium Chlorides with Azid Ion at −80 °C: Concerted versus Stepwise Formation of Arylpentazols, Detection of a Pentazene Intermediate, a Combined 1H and 15N NMR Experimental and ab initio Theoretical Study. J. Chem. Soc., Perkin Trans. 2 1998, 2243−2247. (9) Butler, R. N.; Stephens, J. C.; Burke, L. A. First Generation of Pentazole (HN5, Pentazolic Acid), the Final Azole, and a Zinc Pentazolate Salt in Solution: A new N-Dearylation of 1-(pmethoxyphenyl) Pyrazoles, a 2-(p-methoxyphenyl) Tetrazole and Application of the methodology to 1-(p-methoxyphenyl) pentazole. Chem. Commun. 2003, 8, 1016. (10) Butler, R. N.; Hanniffy, J. M.; Stephens, J. C.; Burke, L. A. A Ceric Ammonium Nitrate N-dearylation of N-p-Anisylazoles Applied to Pyrazole, Triazole, Tetrazole, and Pentazole Rings: Release of Parent Azoles. Generation of Unstable Pentazole, HN(5)/N(5)(−), in Solution. J. Org. Chem. 2008, 73, 1354−1364. (11) Benin, V.; Kaszynski; Radziszewski, J. G. Arylpentazoles Revisited: Experimental and Theoretical Studies of 4-Hydroxyphenylpentazole and 4-Oxophenylpentazole Anion. J. Org. Chem. 2002, 67, 1354−1358. (12) Schroer, T.; Haiges, R.; Schneider, S.; Christe, K. O. The Race for the First Generation of the Pentazolate Anion in Solution is Far From Over. Chem. Commun. 2005, 1607−1609. (13) Portius, P.; Davis, M.; Campbell, R.; Hartl, F.; Zeng, Q.; Meijer, A. J. H. M.; Towrie, M. Dinitrogen Release from Arylpentazole: A Picosecond Time-Resolved Infrared, Spectroelectrochemical, and DFT Computational Study. J. Phys. Chem. A 2013, 117, 12759−12769. (14) Geiger, U.; Haas, Y.; Grinstein, D. The Photochemistry of an Aaryl Pentazole in Liquid Solutions: The Anionic 4-Oxidophenylpentazole (OPP). J. Photochem. Photobiol. A: Chem. 2014, 277, 53−61. (15) Ö stmark, H.; Wallin, S.; Brinck, T.; Carlqvist, P.; Claridge, R.; Hedlund, E.; Yudina, L. Detection of Pentazolate Anion (cyclo-N5−) from Laser Ionization and Decomposition of Solid p-dimethylaminophenylpentazole. Chem. Phys. Lett. 2003, 379, 539−546. (16) Vij, A.; Pavlovich, J. G.; Wilson, W. W.; Vij, V.; Christe, K. O. Experimental Detection of the Pentaazacyclopentadienide (Pentazolate) Anion, Cyclo-N5−. Angew. Chem., Int. Ed. Engl. 2002, 41, 3051− 3054. (17) Perera, S. A.; Gregusova, A.; Bartlett, R. J. First Calculations of 15 N-15N J Values and New Calculations of Chemical Shifts for High Nitrogen Systems: a Comment on the Long Search for HN5 and its Pentazole anion. J. Phys. Chem. A 2009, 113, 3197−201. (18) Gritsan, N. P.; Platz, M. S. Kinetics, Spectroscopy and Computational Chemistry of Arylnitrenes. Chem. Rev. 2006, 106, 3844−3867. (19) Gritsan, N.; Pritchina, E. The Mechanism of Photolysis of Aromatic Azides. Russ. Chem. Rev. 1992, 61, 500−516. (20) Budyka, M. F.; Kantor, M. M.; Alfimov, M. V. The Photochemistry of Phenyl Azide. Russ. Chem. Rev. 1992, 61, 25−39. (21) Huisgen, R.; Vossius, D.; Appl, M. Die Thermolyse des Phenylazids in Primären Aminen; die Konstitution des Dibenzamils. Chem. Ber. 1958, 91, 1−12. (22) Iddon, B.; Meth-Cohn, O.; Scriven, E. F. V.; Suschitzky, H.; Gallagher, P. T. Developments in Arylnitrene Chemistry: Syntheses and Mechanisms. Angew. Chem., Int. Ed. 1979, 18, 900−917. (23) Scriven, E. F. V. Current Aspects of the Solution Chemistry of Arylnitrenes. In Reactive Intermediates; Abramovitch, R. A., Ed.; Plenum Press: New York, 1982. (24) Scriven, E. F. V., Ed. Azides and Nitrenes; Academic Press: New York, 1984. (25) Wentrup, C. Reactive Molecules; Wiley: New York, 1984; Chapter 4. (26) Li, Y. Z.; Kirby, J. P.; George, M. W.; Poliakoff, M.; Schuster, G. B. 1,2-Didehydroazepines from the Photolysis of Substituted Aryl Azides: Analysis of Their Chemical and Physical Properties by Time-

this range. These results indicate the intermediacy of a nitrene radical, in agreement with the well-known photochemistry of aromatic azides. In DCM DMAPP is completely consumed under similar irradiation periods, even in concentrated solutions, yielding a quite different product distribution. The assignment of the products is not unambiguous yet, but NMR evidence shows that the phenyl ring is retained and strong signals are observed in the aromatic region. Comparison with calculated spectra indicates that in many products the former phenylpentazole CN bond is replaced by a CC bond. Because DCM absorbs at 193 nm, it is proposed that in this case radicals formed from DCM decomposition, such as Cl• and CH2Cl• start a chain reaction that leads to quantitative conversion of DMAPP to products. The reaction pattern and mechanism in DCM have yet to be elucidated; replacement of the azido group by radical attack, in competition with N2 extrusion to form the nitrene is a possible reaction route in this solvent. The photochemistry of DMAPA at 193 nm is also studied. It is found that in spite of the fact that DMAPA absorbs strongly at 193 nm, only a small fraction reacts; 93.5% remains intact, compared to 66% in the case of DMAPP. In contrast, near UV excitation results in higher conversion. These results indicate a fast internal conversion to the ground state directly from energy levels near 6.4 eV.

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ASSOCIATED CONTENT

S Supporting Information *

Use of H NMR to assign products of DMAPP photolysis, H NMR spectra of ∼10−2 M DMAPP solution irradiated at 193 nm in MeCN and in DCM, H NMR spectra of ∼10−2 M DMAPP solution irradiated at 340−400 nm in MeCN and at 240−400 nm in DCM, H NMR spectra of ∼10−2 M DMAPA solution irradiated at 193 nm in MeCN and in DCM, H NMR spectra of ∼10−2 M DMAPA solution irradiated at 340−400 nm in MeCN and at 240−400 nm in DCM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Y. Haas. E-mail: [email protected]. Tel: +97226585067. Notes

The authors declare no competing financial interest.

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REFERENCES

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Solution Photochemistry of [p-(Dimethylamino)phenyl]pentazole

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