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 reduced in tetrabutylammonium perchlorate (TBAP)+CH3CN at the dropping mercury electrode to the biradical 3 in an EC-type reaction, consisting of a reversible charge transfer 3+++2e−=3op, followed by intramolecular cyclization (3op being the open form of the biradical) to cyclomers. Depending on the time scale, different cyclomers are formed: in the sub-millisecond time range, the coupled chemical reaction results in the reversible formation of fast forming species 3ff, cyclomers with π-type interactions (cis-cyclomer B). In the time range of seconds, the slow forming species 3sf with σ-type interactions (trans-cyclomer) are dominant. The forward and backward rate constants of the reaction 3ff=3sf are determined by the reverse pulse polarographic method: kf=3.0 s−1, kb=1.0 s−1. In the presence of metallic ions, 1:1 biradical–metal complexes are formed reversibly. The stoichiometry and formation constants are determined by the reverse pulse polarographic method: K3Li/op=1.2×104 l mol−1; K3Mg/op=1.1×105 l mol−1.