Structure of the radical cations of N,N'-polymethylene-syn-1,6:8,13-diimino[14]annulenes: an ESR and ENDOR study

Modeling EPR parameters of nitrogen containing conjugated radical cations

“…In this family, a large range in the experimental values 89-91 of the hfccs for the nitrogen nuclei was observed, going from 6.33 G for radical 43 to 25.7 G for radical 46, that is, an increase of around 20 G when the chain goes from n ¼ 1 to n ¼ 4. 89 The high thermodynamic and kinetic stability, as well as the size of the 14 N coupling constants, which is unusually large for a p-radical cation extending over 14 C atoms, strongly suggest that 45-47 radical cations are N-centered paramagnetic species with a N-N intramolecular three electron s bond. In radicals 45 and 46, the high experimental a iso ( 14 N) value is accomplished by a spectacular increase in the radical cation stability, allowing crystalization.…”

“…Radical cation 47, that could be considered as having a very long polymethylene chain joining the N atoms, shows larger a iso ( 14 N) value (26.6 G). [89][90][91]93,94 In order to study the nature of the N-N interaction, the theoretical data obtained from the DFT calculations were analyzed in depth and compared to the experimental data. [89][90][91]93 Perchlorate salt of radical cation 47 was also isolated but not sufficiently stable for an Xray crystallographic structure analysis.…”

Organic Radical Ions as Studied by Epr Spectroscopy

“…This rather crude procedure has been superseded by more refined ones, in which the oxidation is carried out in nitromethane, dichloromethane, or l,l,l,3,3,3-hexafluoropropano-2ol with a variety of reagents including trifluoroacetic acid (51), SbCl s (52), and A1C1 3 (53), as well as salts of Ag + (54), Hg 2+ (55), Tl 3+ (56,141), Pb 4+ (57), Co 3+ (58), and tris(p-bromophenyl)ammomumyl cation (59,60) . In particular, A1C1 3 in dichloromethane proved to be the system of choice for hydrocarbons (61), thiaheterocycles (62), and organosilicon compounds (63), while tris(pbromophenyl)ammoniumyl hexa-chloroantimonate in this solvent was more appropriate for some nitrogen-containing compounds (64). UV-irradiation enhances the oxidizing power of various reagents, so that derivatives of benzene (65) and polyenes (66), as well as those of azulene (67) and cyclooctatetraene (68) could be converted to their radical cations.…”

“…Typical examples of [n]annulene derivatives with n obeying the Huckel rule of stability are l,6-methano [10]annulene (9), 1,6:8,13-bridged [14]annulenes 10 and 11, 10a,10b-dimethyl-10a,10b-dihydropyrene (12), and bisdehydro[n]annulenes with n = 14,18,22, and 26 (13). The radical ions of alkyl-substituted benzenes (16,23,26,65,111,119) and of [n]annulene derivatives (23,38,61,64,118,(120)(121)(122)(123) have been extensively studied by EPR spectroscopy since 1958 (16) and 1964 (38), respectively.…”

“…In turn, the effect of the perturbation can be determined, which for alkylbenzenes and bridged [n]annulenes is usually classified as inductive, homoconjugative, and/or hyperconjugative. Interestingly, the radical cations of the diimino-bridged [14]annulenes (11) are N-centered, with a three-electron N-N o-bond (for m>3, % = 1.7-2.6 mT) (64), while in the corresponding anions (a N < 0.1 mT), as in both radical ions of 10, the spin population is almost entirely accommodated in the nperimeter (61,121). For the radical anions of 9-11, the coupling constants of the protons at the perimeter are strongly affected by deviations of the latter from planarity and can serve as a measure of such deviations (121).…”