Conformational Changes and Redox Properties of Bimetallic Single Helicates of Hexapyrrole‐α,ω‐dicarbaldehydes

The hexapyrrole‐α,ω‐dicarbaldehydes 1 a and 1 b were metallated with Cu II , Ni II , and Pd II to give bimetallic complexes where a pair of 3 N+O four‐coordinate metal planes are helically distorted and the central 2,2′‐bipyrrole subunit adopts a cis or trans conformation. X‐ray crystallographic analysis of the bisCu complex revealed a closed form with a cis ‐2,2′‐bipyrrole subunit and an open form with a trans ‐2,2′‐bipyrrole subunit. The bisPd complexes took a closed form both in the solid state and in solution. They are regarded as single helicates of two turns and the energy barrier for the interchange between an M helix and a P helix was remarkably influenced by the bulky 3,3′‐substituent of the central 2,2′‐bipyrrole subunit. Although the bisNi complexes adopt a closed form in the solid state, they exist as a homohelical open C 2 ‐symmetric form or a heterohelical open C i ‐symmetric form in solution. A theoretical study suggested that the closed form of 1 a Pd was stabilized by the Pd–Pd interaction. Compound 1 a Pd was reversibly oxidized by one electron at 0.14 V versus ferrocene/ferrocenium (Fc/Fc + ) and this oxidized species showed Vis/NIR absorption bands at λ =767 and 1408 nm.