Different nature of the interactions between anions and HAT(CN)6: From reversible anion-π complexes to irreversible electron-transfer processes (HAT(CN)6 = 1,4,5,8,9,12-Hexaazatriphenylene)

We report experimental evidence indicating that the nature of the interaction established between HAT(CN)₆, a well-known strong electron acceptor aromatic compound, with mono- or polyatomic anions switches from the almost exclusive formation of reversible anion-π complexes, featuring a markedly charge transfer (CT) or formal electron-transfer (ET) character, to the quantitative and irreversible net production of the anion radical [HAT(CN)₆]^•- and the dianion [HAT(CN)₆]^2- species. The preferred mode of interaction is dictated by the electron donor abilities of the interacting anion. Thus, weaker Lewis basic anions such as Br^– or I^– are prone to form mainly anion-π complexes. On the contrary, stronger Lewis basic F^– or -OH anions display a net ET process. The ET process can be either thermal or photoinduced depending on the HOMO/LUMO energy difference between the electron donor (anion) and the electron acceptor (HAT(CN)₆). These ET processes possibly involve the intermediacy of anion-π complexes having strong ET character and producing an ion-pair radical complex. We hypothesize that the irreversible dissociation of the pair of radicals forming the solvent-caged complex is caused by the reduced stability (high reactivity) of the radical resulting from the anion.