Persistance of the Cyclopnane Radical Anions and its Relation to Structure

Abstract

SummaryReactions of [2,]cyclophanes (N = 2,. . .6) with solvated electrons in 1,2-dimethoxyethane at 193 K have been studied by ESR. and ENDOR. spectroscopy. All but the two most highly bridged cyclophanes ( N = 5 and 6) are reduced to paramagnetic species under these conditions. Whereas the radical anions of [2.2]-paracyclophane and p3] (1,2,4)-and p4] (1,2,4,5)cyclophanes are sufficiently persistent to be characterized by their hyperfine data, those of the remaining five cyclophanes undergo a rapid cyclization to the radical anions of4,5,9,10-tetrahydropyrenes. These have been identified as the unsubstituted tetrahydropyrene (from [2.2]-metacyclophane and [23] (1,2,3)cyclophane), the 2,7-dimethyl-derivative (from [2,] (1,3,5)-and p4] (1,2,3,5)cyclophanes) and the 1,8-dimethyl-derivative (from [24] (1,2,3,4)cyclophane). The persistence of the cyclophane radical anions seems to depend on the numbers, nmeta and npa,a, of the meta-and para-positions of the bridging ethano groups in the two benzene rings. The prerequisite for the radical anion to be persistent is nmeta 6 npara.

[2.2]Paracyclophane-Bridged Mixed-Valence Compounds: Application of a Generalized Mulliken−Hush Three-Level Model

“…While the oxidation of unsubtituted [2.2]paracyclophane to the corresponding radical cation is irreversible in solution, 12 the radical anion of [2.2]paracyclophane is more persistent. [13][14][15][16] Badger and Brocklehurst reported absorption spectra of both the radical anion and the radical cation of [2.2]paracyclophane which were generated by γ-radiolysis of glassy solutions at low temperatures. 17 The absorption spectrum of the radical cation was compared with the photoelectron spectrum of neutral [2.2]paracylophane by Heilbronner et al 18 By ESR and ENDOR spectroscopy it was shown that a substitution of [2.2]paracyclophane by electron donating groups such as methyl or methoxy substituents leads to a stabilization of the corresponding radical cations.…”

“…In this context, the investigation of charged cyclophane systems is of great interest for studying charge-transfer processes over the [2.2]paracyclophane moiety. While the oxidation of unsubtituted [2.2]paracyclophane to the corresponding radical cation is irreversible in solution, the radical anion of [2.2]paracyclophane is more persistent. − Badger and Brocklehurst reported absorption spectra of both the radical anion and the radical cation of [2.2]paracyclophane which were generated by γ-radiolysis of glassy solutions at low temperatures . The absorption spectrum of the radical cation was compared with the photoelectron spectrum of neutral [2.2]paracylophane by Heilbronner et al By ESR and ENDOR spectroscopy it was shown that a substitution of [2.2]paracyclophane by electron donating groups such as methyl or methoxy substituents leads to a stabilization of the corresponding radical cations. ,− By these methods, a strong intramolecular electronic coupling between the two aromatic subunits was found which is in agreement with the interpretation of the absorption signal of the [2.2]paracyclophane radical cation as a charge resonance band by Badger and Brocklehurst .…”