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

Abstract The hexapyrrole‐α,ω‐dicarbaldehydes 1 a and 1 b were metallated with CuII, NiII, and PdII 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 C2‐symmetric form or a heterohelical open Ci‐symmetric form in solution. A theoretical study suggested that the closed form of 1 aPd was stabilized by the Pd–Pd interaction. Compound 1 aPd 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.