Polynitrogen stability

Keywords Valence electron rule, Metal ring, Metal cluster, AN +2 valence electron rule, 8/V +6 valence electron rule, 6N +14 valence electron rule, Pentagon stability, Cyclopentaphosphane, Hydronitrogen, Polynitrogen, Triazene, 2-Tetrazene, Tetrazadiene, Pentazole, Hexazine, Nitrogen Oxide, Disiloxane, Disilaoxirane, 1,3-Cyclodisiloxane, Metallacycle, Inorganic heterocycle... 

The introduction of powerful electron-withdrawing substituents (NO2, CN, a polynitrogen heterocycle) at the a-C atom of AN stabilize anions of AN, thus facilitating their generation. At the same time, stabilization of anions of AN leads to a decrease in their reactivity and such anions act as milder nucleophilic agents. It will be seen from the following that this leads to an increase in the contribution of C-alkylation. 

Christe and co-workers505 have reported the synthesis [Eq. (4.143)] and full characterization of the homoleptic N5+ polynitrogen cation. Both salts are surprisingly stable and decompose at —70°C. Crystal structure of the fluoroantimonate salt 217 is in excellent agreement with calculated values (MP2/6-31G 440 and B3LYP506). The N5+ cation has a V-shaped structure of C2v symmetry and its exceptional stability is largely due to resonance stabilization. 

The availability of ever faster computers is responsible for the continuously improving quality in the calculations of the stability and the properties of pentazole compounds. Azidopentazole has been identified as the most stable of the known Ns polynitrogen compounds and considerable work has gone into the question whether a pentazole salt of the Ns+ ion would be stable. 

At the same time, many theoretical calculations have been carried out over the last two decades to find new polynitrogens. Computational studies have been able to assist and encourage experimental work to find and identify unknown polynitrogens by providing information about molecular structures and kinetic stabilities on the potential energy surface with respect to N2. Many hypothetical structures of polynitrogens (from N4 to N60) have been predicted by quantum-mechanical calculations, which have led to the... 

For N9, both neutral and cationic forms have open-chain C2v structures [64]. DFT calculations have also shown that barrier of the reaction, N9- N6+ N3, is predicted to be about 32 kcal/mol, whereas the barrier of the reaction, N9+ - N7+ + N2, is 2.1 kcal/mol. This suggests that neutral N, is kinetically more stable than the N9+ cation with respect to dissociation. Also, the relative kinetic stability of N9 is completely opposite to other oddnumbered polynitrogens such as the N5 and N7 species, where ionic forms are kinetically more stable than the neutral forms. 

Calculations of polynitrogens larger than N,2 have determined relative energetics and vibrational frequencies of possible isomers, but not activation barriers. Future work should explore the kinetic stabilities of these larger polynitrogens on their potential energy surfaces to validate their possible application as energetic materials. 

Under ambient conditions, the stability of Ns was an essential requisite for synthesis of polynitrogen salts. The first example, Ns AsF6 salt, tended to explode, whereas Ns SbF6 and Ns Sb2Fir proved more stable. Studies of the stability Ns salts at higher pressures and temperatures may aid discovery of other polynitrogen species. It is found that if Ns SbF6 is pressurized to 4.5 GPa and heated, only when the temperature exceeds 205 C, does the... 

There are varied examples of polynitrogen bases that form bidentate cyclic structures with metal cations. The most clear-cut cases of such chelate effects are encountered with polyamines, among which ethylenediamine is the best-studied case. The main studies, concerning the cations of Li, Na, K, Mg, A1 and Cu, are cited in ref. [54], The combined spectroscopic and ab initio study of copper complexes shows bidentate cyclic structures for the four iV-methyl-substituted derivatives of ethylenediamine, but a hydrogen bond stabilized monodentate sUiicture for the Cu+/ethylenediamine is also possible [54],... 

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