Nickel(II) meso-tetraphenyl-homoporphyrins, -secochlorins, and -chlorophin:: Control of redox chemistry by macrocycle rigidity

This paper presents the first electrochemical study of Ni(II) secochlorins, chlorophin, and homoporphyrins and demonstrates the influence of macrocycle-rigidity on the site of electroreduction. Oxidations and reductions were investigated by cyclic voltammetry. The measured electrode potentials were found to be dependent on the nature of the substituents attached to the porphyrinic moiety and on the ring flexiblitiy. The voltammetric behavior of these molecules when employed as catalysts for the electrochemical catalytic debromination of trans-1,2-dibromocyclohexane was used to determine whether reduction peaks were due to a metal-based (formation of catalytically active Ni(I) complexes) or ligand-based (formation of catalytically less active pi-anion radical) reduction. The results showed that the homoporphyrins formed ligand-based reduction products. The homoporphyrins are locked into a nonplanar conformation stabilizing the small Ni(II) ion which results in their inability to accommodate the larger Ni(I) ion. In contrast, the electronically quite similar but conformationally flexible chlorin and secochlorin complexes formed Ni(I) complexes upon electrochemical reduction. Our findings shed further light on the structural features required of porphyrinic cofactors such as factor F-430 to undergo metal-centered reduction events in their catalytic cycles. The results also provide a blue-print for synthetic porphyrinic NL(II) complexes to be utilized for electrochemical catalysis.