Reverse pulse polarography for EC processes. Electrochemistry of 1,1 '-trimethylene-bis-(4-carbomethoxypyridinium) ion

The 1,1 '-trimethylene-bis-(4-carbomethoxypyridinium) ion (3(+ +)) is reduc ed in tetrabutylammonium perchlorate (TBAP)+ CH3CN at the dropping mercury electrode to the biradical 3(..) in an EC-type reaction, consisting of a re versible charge transfer 3(+ +) + 2e(-) = 3(op), followed by intramolecular cyclization (3(op) being the open form of the biradical) to cyclomers. Dep ending on the time scale, different cyclomers are formed: in the sub-millis econd time range, the coupled chemical reaction results in the reversible f ormation of fast forming species 3(ff), cyclomers with pi -type interaction s (cis-cyclomer B). In the time range of seconds, the slow forming species 3(sf) with sigma -type interactions (trans-cyclomer) are dominant. The forw ard and backward rate constants of the reaction 3(ff) = 3(sf) are determine d by the reverse pulse polarographic method: k(f) = 3.0 s(-1), k(b) = 1.0 s (-1). In the presence of metallic ions, 1:1 biradical-metal complexes are f ormed reversibly. The stoichiometry and formation constants are determined by the reverse pulse polarographic method: K-3Li/op = 1.2 x 10(4 1)mol(-1); K-3Mg/op = 1.1 x 10.5 1 mol(-1). (C) 2000 Elsevier Science B.V. All rights reserved.