Formation of Triradical Trications from Hexaquaternary Salts Derived from 4,4'-Bipyridine

We recently reported the preparation of tetraquaternary salts derived from 4,4'-bipyridine of the type (1; R = Me or Et, n = 2 ,3 or 4) [1], and showed that they were reversibly reduced to diradical dicat­ ions (2; R = Me or Et, n = 2, 3 or 4) by the uptake of two electrons. Reversible redox systems of this type are of interest as components of electrochromic memory display devices [2—5], as bielectronic mediators [6, 7] and as components of redox active membranes [8]. In a different area of application they have very recently been found to be of great interest as intercalators of D N A [9]. We now report on an extension of this research programme involv­ ing the preparation of the two hexaquaternary salts 1,1 '-bis-[2-(l'-methyl-[4,4'-bipyridinium]-l-yl)ethyl]-4,4'-bipyridinium hexaperchlorate (3; n = 2) and l,l'-bis-[3-(l'-methyl-[4,4'-bipyridinium]-l-yl)propyl]-4,4'-bipyridinium hexaperchlorate (3; n = 3) and their behaviour on polarographic reduction in aqueous solution. The hitherto unknown diquaternary salt 1,1'-bis(2-bromoethyl)-4,4'-bipyridinium dibromide (4; n = 2, X = Br) was considered to be a suitable starting material for the synthesis of the hexaquaternary salt (3; n = 2). Reaction of 4,4'-bipyridine, however, with 1,2-dibromoethane usually results in the forma­ tion of polyviologens [10, 11] or the diquaternary salt (5) [1] depending on reaction conditions. The mono­ quaternary salt, l-(2-bromoethyl)-4,4'-bipyridinium bromide (6) has also been isolated but in an impure state [12]. In our hands reaction of 4,4'-bipyridine with excess 1,2-dibromoethane under reflux pro­ duced a precipitate which proved to be a mixture of