Dramatic effect of N-substituents in viologens on single electron transfer from tributylphosphine

A single electron transfer (SET) takes place from tributylphosphine (1a) to 1-alkyl-1'-methylviologens in acetonitrile containing a large amount of methanol under an argon atmosphere. In contrast, no SET takes place from 1a to viologens whose alkyl groups on the nitrogens are larger than the methyl group under the same conditions, 1a instead nucleophilically attacking the viologen to form a covalent adduct. This dramatic substituent effect is discussed in terms of SET occurring within a tight encounter complex formed between the phosphine and the viologen.