Generation of tricyanomethyl spin adducts of α-phenyl-N-tert-butylnitrone (PBN) via non-conventional mechanisms

Abstract

Spin adducts, formally derived from tricyanomethyl radical attachment to α-phenyl-N-tert-butylnitrone (PBN; IUPAC name: N-benzylidene-tert-butylamine N-oxide), have been generated by various methods, such as oxidation of a mixture of tricyanomethanide ion and PBN by tris(4-bromophenyl)aminium ion or bromine, photo-oxidation of a mixture of tricyanomethane and PBN with 2,4,6-tris(4-methoxyphenyl)pyrylium ion as a sensitizer, or photolysis of chlorotricyanomethane and PBN at –30 °C, the low temperature being necessary to avoid fast concurrent cycloaddition with PBN. Both the C- and N-connected spin adducts, (NC)₃C-PBN˙ and (NC)₂CCN–PBN˙, have been characterized, as has an aminoxyl formed by elimination of hydrogen cyanide from the former species, (NC)₂CC(Ph)N(O˙)Bu^t. For comparison, similar experiments have been performed using carbamoyldicyanomethanide ion and carbamoylchlorodicyanomethane and the spin adduct H₂NCO(CN)₂C-PBN˙ has been characterized.

The redox properties of tricyanomethanide ion, carbamoyldicyanomethanide ion, chlorotricyanomethane and carbamoylchlorodicyanomethane have been studied by cyclic voltammetry. For chlorotricyanomethane, the redox reactivity has also been evaluated by its propensity to generate radical cations from aromatic substrates ArH in 1,1,1,3,3,3-hexafluoropropan-2-ol. Tricyanomethanide ion has E[(CN)₃C˙/(CN)₃C^–]_rev at 1.35 V (vs. Ag/AgCl) in acetonitrile while chlorotricyanomethane with E_pc at about 0.6 V and a capability to oxidize compounds with redox potentials up to ca. 1.5 V to radical cations, emerges as belonging to the strongest neutral organic electron acceptors.