Coordination controlled atropoisomerism in phenanthroline-strapped porphyrins: A swinging affair

Abstract

A phenanthroline-strapped porphyrin bearing two pyridine side arms attached to opposite meso positions was designed as a potential cytochrome c oxidase model and its zinc and copper(I)-zinc complexes were prepared. In the zinc porphyrin complex, the pyridine arms underwent a rapid exchange of their coordination to the zinc atom on the open face of the strapped-porphyrin, as demonstrated by variable temperature NMR studies. Only the β2 atropoisomer was observed for the zinc complex because of the rapid exchange. This swinging motion no longer occurred when Cu I was bound within the phenanthroline strap. In this Cu I / Zn II complex, the αβ atropoisomer predominated and one pyridine was coordinated to the zinc atom on the proximal side and the other pyridine was bound to the copper on the distal face. This work shows that allostery can be effective to control atropoisomery in phenanthroline strapped porphyrin as heme protein models.