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 macrocycl e-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 por phyrinic moiety and on the ring flexiblitiy. The voltammetric behavior of t hese molecules when employed as catalysts for the electrochemical catalytic debromination of trans-1,2-dibromocyclohexane was used to determine whethe r reduction peaks were due to a metal-based (formation of catalytically act ive Ni(I) complexes) or ligand-based (formation of catalytically less activ e pi-anion radical) reduction. The results showed that the homoporphyrins f ormed ligand-based reduction products. The homoporphyrins are locked into a nonplanar conformation stabilizing the small Ni(II) ion which results in t heir inability to accommodate the larger Ni(I) ion. In contrast, the electr onically quite similar but conformationally flexible chlorin and secochlori n complexes formed Ni(I) complexes upon electrochemical reduction. Our find ings shed further light on the structural features required of porphyrinic cofactors such as factor F-430 to undergo metal-centered reduction events i n their catalytic cycles. The results also provide a blue-print for synthet ic porphyrinic NL(II) complexes to be utilized for electrochemical catalysi s.