Influence of electronic and structural effects on the oxidative behavior of nickel porphyrins.

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E(1/2) for the second oxidation varies considerably with the type of supporting electrolyte. E(1/2) values for the first reduction to give a porphyrin pi-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a pi-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (Delta/Ox(2) - Ox(1)/) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of Delta/Ox(2) - Ox(1)/ is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO(4)(-) or PF(6)(-)) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating Delta/Ox(2) - Ox(1)/ and thus the binding affinity of the anion to the porphyrin dication. One is the type of pi-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.

Synthesis and Electrochemical Studies of a Series of Fluorinated Dodecaphenylporphyrins.

Dodecaphenylporphyrins with varying degrees of fluorination of the peripheral phenyl rings (F(x)()DPPs) were synthesized as model compounds for studying electronic effects in nonplanar porphyrins, and detailed electrochemical studies of the chloroiron(III) complexes of these compounds were undertaken. The series of porphyrins, represented as FeDPPCl and as FeF(x)()DPPCl where x = 4, 8 (two isomers), 12, 20, 28, or 36, could be reversibly oxidized by two successive one-electron transfer steps in dichloromethane to give pi-cation radicals and pi-dications, respectively. All of the compounds investigated could also be reduced by three electrons in benzonitrile or pyridine. In benzonitrile, three reversible reductions were observed for the unfluorinated compound FeDPPCl, whereas the FeF(x)()DPPCl complexes generally exhibited irreversible first and second reductions which were coupled to chemical reactions. The chemical reaction associated with the first reduction involved a loss of the chloride ion after generation of [Fe(II)F(x)()DPPCl](-). The second chemical reaction involved a conversion between the initially generated Fe(II) porphyrin pi-anion radical and the final Fe(I) porphyrin reduction product. In pyridine, three reversible one-electron reductions were observed with the second reduction affording stable Fe(II) porphyrin pi-anion radicals for all of the complexes investigated.