Experimental Detection of the Pentazole Anion, N5(-)

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REPORT DATE (DD-MM-YYYY) 2. REPORT TYPE 3. DATES COVERED (From To) Technical Paper 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER V L:U 11" 'I i >- ' -"5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER T'/,fM' 5e. TASK NUMBER fl-^S 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S) Air Force Research Laboratory (AFMC) AFRL/PRS 5 Pollux Drive Edwards AFB CA 93524-7048 11. SPONSOR/MONITOR'S NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited. 13. SUPPLEMENTARY NOTES 14. ABSTRACT 20021212 128 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT 18. NUMBER OF PAGES 19a. NAME OF RESPONSIBLE PERSON Leilani Richardson a. REPORT Unclassified b. ABSTRACT c. THIS PAGE /A Unclassified Unclassified 19b. TELEPHONE NUMBER (include area code) (661)275-5015 Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std. 239.18 ^ S-»*--/1*-* i\.<- r* 4 • " v... ~/)4} MEMORANDUM FOR PRS (In-House Contractor Publication) FROM: PROI (STINFO) 01 April 2002 SUBJECT: Authorization for Release of Technical Information, Control Number: AFRL-PR-ED-TP-2002-073 Ashwani Vij (ERC) et al., "Experimental Detection of the Pentazole Anion, N5'" Publication in Science (magazine) (Deadline: ASAP) (Statement A) 1. This request has been reviewed by the Foreign Disclosure Office for: a.) appropriateness of distribution statement, b.) military/national critical technology, c.) export controls or distribution restrictions, d.) appropriateness for release to a foreign nation, and e.) technical sensitivity and/or economic sensitivity. Comments: Signature _ Date 2. This request has been reviewed by the Public Affairs Office for: a.) appropriateness for public release and/or b) possible higher headquarters review. Comments: Signature Date . 3. This request has been reviewed by the STINFO for: a.) changes if approved as amended, b) appropriateness of references, if applicable; and c.) format and completion of meeting clearance form if required Comments: Signature Date 4. This request has been reviewed by PR for: a.) technical accuracy, b.) appropriateness for audience, c.) appropriateness of distribution statement, d.) technical sensitivity and economic sensitivity, e.) military/ national critical technology, and f.) data rights and patentability Comments: APPROVED/APPROVED AS AMENDED/DISAPPROVED PHILIP A. KESSEL Technical Advisor Space and Missile Propulsion Division Date Experimental Detection of the Pentazole Anion, N5~ Ashwani Vij,1* James G. Pavlovich,2 William W. Wilson,1 Karl O. Christe1'3* The pentazole anion has been generated from /jara-hydroxyphenylpentazole and identified by electrospray ionization mass spectrometry. Whereas at low collision voltages the />ara-phenoxypentazole anion undergoes stepwise N2 elimination generating the corresponding azide and nitrene, at high collision voltages the N5" anion is formed. Fragmentation of the pentazole anion produces the N3" anion as the principal negative ion. These experiments provide the first experimental proof for the existence of the pentazole anion. They also demonstrate that under suitable reaction conditions the C-N bond in a phenylpentazole can selectively be broken with conservation of the pentazole ring, thus providing a potential synthetic route to the pentazole anion. Nitrogen and oxygen are unique among the chemical elements. In contrast to the other elements, their homonuclear single bond energies are significantly less than one half of their double bond or one third of their triple bond energies. Consequently, homonuclear polynitrogen and polyoxygen species are thermodynamically highly unstable and the *ERC, Inc., Air Force Research Laboratory, Edwards Air Force Base, CA 93524, USA. department of Chemistry, University of California, Santa Barbara, CA 93106, USA. 3 Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661, USA. *To whom correspondence should be addressed. E-mail: karl.christe@Edwards.af.mil DISTRIBUTION STATEMENT A Approved for Public Release Distribution Unlimited . **-,/ j/fÜMl 0 3 -Q 2- ~ UOoLh number of known compounds is very limited. Due to the highly endothermic heats of formation, their syntheses and handling present great challenges. It is therefore no surprise that for oxygen only one metastable allotrope, i. e., ozone, is known and for nitrogen none are known that can be isolated in bulk, while most other elements can exist in the form of many stable allotropes. Polynitrogen compounds have been studied extensively for the last two decades. In view of the great experimental difficulties, most of the efforts have been limited to theoretical studies (1-9). The first major breakthrough in the synthesis area was achieved in 1999 with the synthesis of the N54" cation in the form of a marginally stable ASFö" salt (10). Subsequently, the thermally more stable N5+SbF6" was synthesized, and the crystal structure of Ns+Sb2Fn" was determined (11). Based on Born-Haber cycle considerations, the stability of an ionic salt is governed by four factors: the lattice energy, the electron affinity of the cation, the first ionization potential of the anion, and the activation energy barriers of the ions toward decomposition. Theoretical calculations from our and other (1,5,8,13-19) research groups predict that the unknown pentazole anion (see Figure 1) has a first ionization potential and activation energy barrier toward decomposition that might be high enough to provide a stabile salt. As a result, a worldwide effort is underway to synthesize this anion. Although the existence and stability of substituted pentazole ring compounds have been demonstrated successfully more than 40 years ago by Huisgen and Ugi (20-23) and substituted pentazoles have been well characterized (24-29), all attempts to prepare either the parent HN5 molecule (30,31) or its anion, N5", have so far been unsuccessful. In this paper, we wish to report the first experimental detection of this important anion. In our pursuit of the N5" anion, the following strategy was employed: (i) the use of Ugi-Huisgen-type, substituted phenylpentazoles as starting materials; (ii) the transfer of maximum negative charge to the pentazole ring by the use of highly electron donating susbstituents on the phenyl ring in /?ara-position to the pentazole substituent to increase the aromaticity and stability of the pentazole ring, while at the same time weakening the connecting C-N bond; (iii) the selective cleavage of the C-N bond while keeping the N-N bonds of the pentazole ring intact; and (iv) the use of an analytical method that is ideally suited for the generation and detection of anions. A similar approach for steps (i) - (iii) has recently been published, but attempts to cleave the C-N bond by ozonolysis were unsuccessful (32). The reasons, outlined above, prompted us to choose /?ara-hydroxy (32) or /?ara-dimethylamino (22) substituted phenylpentazoles as starting materials and negative ion electron spray ionization mass spectrometry (ESIMS) (33-35) as the analytical tool. The /?ara-dimethylaminophenylpentazole and />ara-hydroxyphenylpentazole were dissolved in strongly polar solvents, such as CH3CN or a mixture of CH3OH and CH2CI2, and infused into the spectrometer with a syringe pump. Desired negative ion peaks were mass-selected and subjected to secondary negative ion mass spectroscopy (MS/MS) at variable collision voltages. The most interesting results were obtained with ^ara-hydroxyphenylpentazole in CH3CN solution. An intense, parent - H, peak was observed at m/e =162 and massselected for subsequent MS/MS studies. Using a low collision voltage of -10 Volt, the m/e =162 peak (OC6H4N5") underwent step wise N2, N2, and CO loss, giving rise to intense peaks with m/e values of 134 (OC6H4N3"), 106 (OCeHUN), and 78 (C5H4N"), respectively. The loss of the first N2 molecule is due to the opening of the pentazole ring and produces the phenoxyazide ion. The second N2 loss occurs from the azido group and generates a nitrene. The nitrene nitrogen can readily insert into the phenyl ring, and the resulting seven-membered ring can undergo a facile CO extrusion giving a pyridine anion. Secondary fragmentation of the m/e =134 peak at a collision voltage of-30 Volt gave rise to intense peaks at m/e = 78 (C5H4N"), 52 (C3H2N"), and 50 (C3N"). The secondary fragmentation patterns of the m/e =106 and 78 peaks at collision voltages of75 Volt gave only a very intense m/e peak at 50 (C3N"). Using high collision voltages of about -75 Volt for the secondary MS of the m/e = 162 peak, however, gave a very different fragmentation pattern. The only peaks observed were m/e = 70, 52, 50, and 42 (see Figure 2). The m/e = 70 peak can only be due to N5", and the m/e = 52 and 50 peaks are due to C3H2N" and C3N", respectively, and, as shown above, result from the fragmentation of the pyridine anion. The m/e = 42 peak is due to the azide anion, N3". These results clearly demonstrate that at high collision voltages the pentazole anion is formed. The fragmentation of the m/e = 70 peak to N3" (and neutral N2 that is not observable in the negative ion spectrum) is in accord with the theoretically predicted decomposition pathway of N5" (8) and further supports its identification as N5". The N3" and N2 fragments have been calculated at the CCSD(T)/augcc-pVTZ level of theory to be 60 kJ/mol lower in energy than N5", with an energy barrier of 116 kJ/mol to the cycloreversion (8). These values indicate that bulk N5" salts should be manageable on a preparative scale. Furthermore, the formation of the N5" peak from a starting material containing a pentazole ring and the vibrational instability of open-chain N5" (8), establish beyond doubt that the observed N5" species must be the long-sought pentazole anion. In summary, our results constitute the first experimental detection of the pentazole anion and demonstrate that in suitably substituted phenyl-pentazoles the C-N bond can be cleaved while leaving the pentazole ring intact. Since the substituted phenylpentazoles are easily accessible, this approach holds great promise for the bulk synthesis of N5" salts, and experiments in this direction are in progress in our laboratories. Our results on N5 (10,11) and N5", together with the recent observations of the N4 molecule as a metastable species with a lifetime exceeding 1 microsecond (36), the observation of a new but illcharacterized poly-nitrogen species from a discharge generated nitrogen plasma (37), and exciting progress in high nitrogen compounds (29,38), indicate a bright future for experimental polynitrogen chemistry. References and Notes 1. R. J. Bartlett, Chem. Ind. 140 (2000), and references cited therein; a compilation of data for N2 to N10 can be found at http://www.qtp.ufl.edu/~bartlett/polynitrogen.pdf. 2. S. Fau, R. J. Bartlett, J. Phys. Chem. A 105, 4096 (2001). 3. M. Tobita, R. J. Bartlett, J. Phys. Chem. A 105, 4107 (2001). 4. T. M. Klapoetke, Angew. Chem. Int. Ed. 38, 2536 (1999), and references cited therein. 5. M. N. Glukhovtsev, H. Jiao, P. v. Rague Schleyer, Inorg. Chem. 35, 7124 (1996). 6. H. H. Michels, J. A. Montgomery, Jr., K. O. Christe, D. A. Dixon, J. Phys. Chem. 99, 187(1995). 7. 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Hegarty, J. Elguero, Polyhedron, 4, 1721 (1985). 18. V. A. Ostrovskii, G. B. Erusalimskii, M. B. Shcherbinin, Russ. J. Org. Chem. 31, 1284 (1995). 19. M. N. Glukhovtsev, P. v. R. Schleyer, C. Maerker, J. Phys. Chem. 97, 8200 (1993). 20. R. Huisgen, I. Ugi, Angew. Chem. 68, 705 (1956); Chem. Ber. 90, 2914 (1957). 21.1. Ugi, R. Huisgen, Chem. Ber. 91, 531 (1958). 22.1. Ugi, H. Perlinger, L. Behringer, Chem. Ber. 91, 2324 (1958). 23.1. Ugi, Angew. Chem. 73, 172 (1961). 24. J. D. Wallis, J. D. Dunitz, J. Chem. Soc, Chem. Commun. 910 (1983). 25. M. Witanowski, L. Stefaniak, H. Januszewski, K. Bahadur, G. A. Webb, J. Cryst. Mol. Struct. 5, 137(1975). 26. R. Mueller, J. D. Wallis, W. v. Philipsborn, Angew. Chem. Int. Ed. 24, 513 (1985). 27. R. N. Butler, S. Collier, A. F. M. Fleming, J. Chem. Soc, Perkin Trans. 2, 801 (1996). 28. R. N. Butler, A. Fox, S. Collier, L. A. Burke, J. Chem. Soc, Perkin Trans. 2, 2243 (1998). 29. A. Hammerl, T. M. Klapoetke, Inorg. Chem. 41, 906 (2002). 30. R. Janoschek, Angew. Chem. Int. Ed. 32, 230 (1993). 31. K. F. Ferris, R. J. Bartlett, J. Am. Chem. Soc. 114, 8302 (1992). 32. V. Benin, P. Kaszynski, J. G. Radziszewski, J. Org. Chem. 67, 1354 (2002). 33. M. Yamashita, J. B. Fenn, J. Chem. Phys. 88, 4451 (1984); C. M. Whitehouse, R N. Dreyer, M. Yamashita, J. B. Fenn, Anal. Chem. 57, 675 (1985). 34. R. B. Cole (ed), Electrospray Ionization Mas Spectrometry (Wiley-Interscience, New York, NY, 1997). 35. B. H. Lipshutz, K. L. Stevens, B. James, J. G. Pavlovich, J. P. Snyder, J. Am. Chem. Soc. 118, 6796 (1996). 36. F. Cacace, G. de Petris, A. Troiani, Science 295, 480 (2002). 37. J. P. Zheng, J. Waluk, J. Spanget-Larsen, D. M. Blake, J. G. Radziszewski, Chem. Phys. Lett. 328, 227 (2000). 38. D. E. Chavez, M. A. Hiskey, R. D. Gilardi, Angew. Chem. Int. Ed. 39, 1791 (2000). 39. This work was financially supported by the Defense Advanced Research Project Agency, the Air Force Office of Scientific Research, and the National Science Foundation. The authors are grateful to Drs. Robert Corley, Arthur Morrish, Don Woodbury, and Michael Berman for their steady support, to Prof. W. Kaska for the use of the ESIMS, and to Drs. Michael Gerken, Thorsten Schroer, Stefan Schneider, Ralf Haiges, and Ross Wagner for support and stimulating discussions. FIGURE CAPTIONS 1. Minimum energy structure of the planar D5h pentazole anion from refl, calculated at the CCSD(T)/aug-cc-pVTZ level of theory, bond length in Ä. 2. Negative ion mass spectrum of the mass-selected m/e =162 peak due to OC6H4N5", recorded at a collision voltage of -75 Volt. The peaks at m/e = 70 and 42 are due to the N5" anion and its decomposition product N3", respectively, while the peaks at 52 and 50 are the C3H2N" and C3N" fragments, respectively, resulting from the breakdown of the phenoxyazide anion. N N N 1.3294 N / N 10 o -3 CN 3 $ Sr - e> x' a a o a-co o -to _5S" To I o __§-^ a |o a go "a I CD x: o o E c/o ia m o 3~£ E oCN O T— CN CO O -SLO st en BLo 2: 1 o q _JLo o'- X O sz a. CN JD ft CD Q. E q CD co E o CN" O" 10 c E . '"' CN - a o co 05 T O l-o o d ^ -so rev -ä CM CO U =i_o o o o T T CN o 0 O H co co CO co T CO CN co o co co CO CN CM CN o CN CO o CO o -3- o CN O o o