Abstract
In a quest for novel persistent free radicals, two products were obtained from the reaction between 1,1-diphenylhydrazine and 4-chloro-7-nitrobenzofurazan: the expected 1-(4nitrobenzofurazan-7-yl)-2,2-diphenyl-hydrazine 3 formed via an SNAr process, and the unexpected 4,7-benzofurazandione-(bis-1,1-diphenylhydrazone) 4 formed from 3 by a further ipso substitution and dehydrogenation. Oxidation of 3 with solid PbO2 or solid KMnO4 at room temperature affords during the first few minutes a persistent mono-nitro-hydrazyl free radical 6, which reacts with NO2 and forms another persistent hydrazyl free radical 7 having two nitro groups. Two blue dimeric compounds, 11 and 13, were obtained by dimerisation of 6 and 7 respectively, and loss of N2O4, and their structures were determined by mass spectrometry and NMR spectra. Additionally, single crystal X-ray diffraction was used to ascertain the structures of compounds 4 and 13. Other novel derivatives were prepared either by N-alkylation of 3 with General Papers ARKIVOC 2011 (xi) 198-221 Page 199 ARKAT-USA, Inc. methyl iodide, which afforded compound 5, or by homolytic nitration of 3 with gaseous NO2 which yielded the 4,7-dinitrobenzofurazan derivative 8. When gaseous NO2 was used in a similar reaction, only one N-nitro group was present in the product 7. Persistent free radicals were obtained from 4, 11, or 13 with KMnO4. Cyclic voltammograms for 3 and 8 as well as electronic absorption spectra for all new compounds 3, 4, 5, 8, 11, and 13 are discussed.
Highlights
The extreme stability of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH)1,2 is due to a combination of steric and electronic factors
In a quest for novel persistent free radicals, two products were obtained from the reaction between 1,1-diphenylhydrazine and 4-chloro-7-nitrobenzofurazan: the expected 1-(4nitrobenzofurazan-7-yl)-2,2-diphenyl-hydrazine 3 formed via an SNAr process, and the unexpected 4,7-benzofurazandione-(bis-1,1-diphenylhydrazone) 4 formed from 3 by a further ipso substitution and dehydrogenation
Linnett’s double quartet theory7 allows a graphical representation of DPPH in terms of spins α and β: the first two resonance structures for the upper row of Chart 1 correspond to the Linnett structure shown in the lower row, a covalent bond or an unshared electron pair consists of two electrons with opposite spins α and β; the remaining three electrons with spins α, α and β are distributed between the two nitrogen atoms and their N−N bond
Summary
The extreme stability of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) is due to a combination of steric and electronic factors. Steric shielding by two ortho-nitro groups prevents its dimerization while the presence of an electron-donor diarylamino group and of a picryl electron-acceptor group provide a unique electronic combination. This “push-pull electronic effect” has been called “capto-dative effect”,4 or “merostabilization”.5. The present investigation had as initial objective the synthesis of analogs of DPPH having a 4-nitrobenzofurazan electron-withdrawing group, EWG (4-nitro-2,1,3-benzoxadiazole, abbreviated as NBD) instead of picryl. Such a group would have a lower steric shielding than picryl and a comparable electron-withdrawing effect, and it might impart interesting fluorescence properties. The large prisms could be solved by conventional techniques showing without doubt that 4 was present in this mixture of polymorphs
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