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.

From Models of Hemoproteins to Self‐Assembled Molecular Wires

“…EPR spectroscopy studies on [ 7 FeCl] and [ 9 FeCl] showed a pentacoordinated high‐spin iron(III) state that was consistent with decoordination of the chloride and an imidazole coordination geometry in [ 7 FeCl] similar to that of [ 9 Zn]. Whereas an exogenic ligand is able to enter the phen pocket to bind on the distal side, the coordination of a sixth pendant ligand induces some geometrical constraints in the framework …”

“…EPR spectroscopy studies on [7 FeCl] and[ 9 FeCl] showedapentacoordinated high-spin iron(III) state that was consistent with decoordination of the chloride [24] and an imidazole coordination geometry in [7 FeCl] similart ot hat of [9 Zn].W hereas an exogenic ligand is able to enter the phen pocket to bind on the distal side, the coordination of as ixth pendant ligand induces some geometrical constraints in the framework. [25] In general, the behavior of the modelsi nt he ferrous state was difficult to anticipate, aside from the high reactivityo ft he parents tructures lacking built-in axial bases. [22] In models [8 Fe] + -[10 Fe] + + ,w hen the ferrous state is generatedi nt he absence of copper(I) in the phen strap, the pyridine moiety is able to enter the strap and act as as ixth ligand for the iron(II), which significantly lowers its affinity for oxygen to render the iron(II) complex air stable.…”

A question of flexibility in cytochrome c oxidase models

“…However, although the iron-copper complex [1Fe II Cu I ] + catalyzed the reduction of oxygen to water by a predominant four-electron mechanism, our attention was caught by the apparent lability of the proximal pyridyl ligand that was observed during titration experiments with various imidazoles (vide infra) and assigned to tension in the pendant ligands. To reduce this 4 constraint, model 2 (Figure 1) [17] was designed such that the proximal and distal ligands moved independently of one another. In addition, the alkyl chains, from the meso-phenyl rings to the picoline ligands, were lengthened by one methylene unit to provide a fully flexible scaffolding.…”