Stereochemistry and chiroptical properties of bimetallic single helicates of hexapyrrole-α,ω-dicarbaldimines with high diastereoselectivity

Bimetallic complexes of hexapyrrole-alpha,omega-dicarbaldimines consisting of a pair of four-coordinate metal sites adopt a helical closed C-2-symmetric form or sigmoidal open forms depending on whether the 2,2'-bipyrrole subunit at the center of the hexapyrrole chain takes cis- or trans-conformation. X-ray crystallography of a bisNi complex having N-[(R)-1-cyclohexylethyl]carbaldimine units at both ends of the hexapyrrole chain revealed a non-symmetric heterohelical open form where the metal coordination sites of opposite helical sense sit on opposite sides of the central 2,2'-bipyrrole subunit. BisPd complexes preferred a closed C-2 form and a steric bulk at the 3,3'-position of the 2,2'-bipyrrole subunit improved the helical sense bias. A bisPd complex with N-[(R)-1-cyclohexylethyl]carbaldimine units adopts a helical closed C-2 form exclusively with full bias for a P-helical sense. These bimetallic single stranded helicates were reversibly oxidized to pi-cation radicals at 0.1 similar to 0.3 V vs. a ferrocene/ ferrocenium couple and spectroelectrochemistry revealed remarkable absorption and CD spectral changes in the Vis-NIR region.

Automated summary

This study discusses bimetallic complexes of hexapyrrole-alpha, omega-dicarbaldimines and their structure-property relationships. The complexes can adopt different structural forms depending on the conformation of the 2,2'-bipyrrole subunit, and reversible oxidation and spectroscopic changes were observed in experimental conditions.