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.

Roles of Metal Ions in Foldamers and Other Conformationally Flexible Supramolecular Systems

“…[Pd 2 (L17−4H)] complex was differently bridged and more flexible. Other dinuclear double-hairpin structures have also been reported by Setsune and co-workers with hexapyrrole dicarbaldehyde ligands, with Cu(II) and Ni(II), 71 with similar results also reported by Xie and co-workers. 72…”

“…The authors were able to employ the same base/acid stimuli to deprotonate this site and switch coordination modes, although the end result in this case is still a double hairpin, the resultant [Pd 2 ( L17 –4H)] complex was differently bridged and more flexible. Other dinuclear double-hairpin structures have also been reported by Setsune and co-workers with hexapyrrole dicarbaldehyde ligands, with Cu(II) and Ni(II), 71 with similar results also reported by Xie and co-workers. 72 …”

“…59 Another conversion from an extended-type conformation (a double hairpin) to helical form has been achieved with square planar palladium(II) by Setsune and co-workers. 60,61 For example, they formed a hexapyrrole sequence capped with aldehydes (H 4 L8, Figure 6 top, X = O), allowing imine condensations, aided by zinc p-tert-butylbenzoate, to alter the terminal groups. This adopted an extended conformation due to alternating NH−H hydrogen bonds between pyrrole groups (Figure 6a).…”

Synthesis, Structure, and Properties of Helical Bis-Cu(II) Complex of Linear Hexapyrrolic Ligand

“…The ESR spectrum of complex 1-Cu indicated the presence of two noninteracting Cu­(II) ions in slightly different coordination environments. Generally, the bis­(Cu­(II)) complexes stabilized with hexapyrrolic ligands are ESR-inactive due to weak/strong antiferromagnetic coupling between the two Cu­(II) metal. , However, in 1 - Cu the two Cu­(II) ions were far from each other and remained noninteracting. The ESR spectra of 1-Cu are simulated considering two independent copper metal nuclei with equal weightage.…”

Bis(naphthobipyrrolyl)methene-derived hexapyrrolic BODIPY as a single-molecule helicate with near-infrared emission

“…4,10 a Also, their metal-coordinating and anion-binding abilities can induce formation of helical conformations in oligomers. 11 And they exhibit intense absorption and emission in the visible and NIR regions. Research is continuing apace to find new NIR-active helical dyes.…”