Hexazine

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 effects of cyclic 6n electron conjugation have been found in the optimized geometries of pentazole 17 [102] and hexazine 18 [97], The N=N bond is longer than the isolated double bond in NH=NH. The N-N single bond in the tetrazadiene moiety is shorter than the single bond in NH NH. The bond lengths in 18 are nearly intermediate between those in NH NH and NH=NH. The aromatic character of pentazoles was supported by the effect of electron donating substituents on the thermodynamic and kinetic stabilization [103],... 

A recent theory of pentagon stability [68, 77] suggests thermodynanic stability of 17 and 18 relative to hexazine. Lone pair electrons in the molecular plane are promoted by the orbital phase continuity to delocalize in a cyclic manner through o bonds of five-membered rings (Scheme 6). The n-rt conjugations also contribute to the relative stability of 17. 

The D6h hexagonal structure of benzene is indeed related to the value of A JjS exceeding that of A is. The lower -resistance of hexazine... 

Hexazine (41), which is the final product of the successive azasubstitu-tion of benzene, possesses an even greater aromaticity than benzene—this is indicated by the values of RCI (83JOC1344) (Table VI), DRE [28.2 for... 

Like hexazine, s-tetrazine (39) and pentazine (40) are thermodynamically unstable to decomposition into HCN and N2 (see Table VII). 

However, in contrast to benzene (Table IX), hexazine is thermodynamically unstable with respect to decomposition into molecular nitrogen... 

Ab initio calculations show [83JA(105) 1760] that the D6/l structure of hexazine corresponds to a very shallow minimum on the PES. Hexazine has been detected experimentally by means of the low-temperature matrix isolation (80AG745) but, according to [83JMS(105)351], this result still requires verification. 

Since the aromaticity of hexazine and benzene is of about the same degree, evidently, the instability of the former must be related to some specificities of the cr-system. A detailed analysis of the factors that determine the instability of the nitrogen and phosphorus rings based on the... 

It is of interest to compare the present status of the ultimate azole and the ultimate azine, hexazine... 

Deprotonation from the azonium group leaves a lone pair of electrons on the nitrogen atom, and a neutral aza substituent. The known parent monocyclic azines (see Scheme la) include all the possible diazines and triazines, but only one tetrazine, the 1,2,4,5-isomer. Some 1,2,3,5-tetrazines have been reported, but only when heavily substituted or fused. Some aromatic bicyclic 1,2,3,4-tetrazines have been prepared (see Section 4.4.8.2.3) as well as reduced 1,2,3,4-tetrazines (see CHEC 2.21). No pentazines are known. All attempts to prepare hexazine also failed though several claims about fixation of the latter in a matrix have appeared. 

CATENATED NITROGEN LIGANDS PART II.1 TRANSITION METAL DERIVATIVES OF TRIAZOLES, TETRAZOLES, PENTAZOLES, AND HEXAZINE... 

Another index of delocalization, devised and widely applied by Schleyer et al.,163 is the nuclear-independent chemical shielding (NICS) value. When calculated above the ring, this value corresponds to the -contribution which is a significantly negative quantity for situations with aromatic delocalization . Scheme 21 shows the A and NICS values computed by Schleyer et al.141 for benzene, hexasilabenzene, and hexazine. The three species are seen to possess equally aromatic electron sextets, as indeed anticipated from the appreciable vertical resonance ener-... 

Catenated nitrogen ligands. Part II. Transition metal derivatives of triazoles, tetrazoles, pentazoles and hexazine. Adv. Inorg. Chem. Radioehem. 32, 171 (1988). 

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