Electrochemistry and spectral characterization of oxidized and reduced (TPPBr(x)())FeCl where TPPBr(x)() is the dianion of beta-brominated-pyrrole tetraphenylporphyrin and x varies from 0 to 8

Abstract

The electrochemistry and spectroelectrochemistry of (TPPBr(x)())FeCl (TPPBr(x)() is the dianion of beta-brominated-pyrrole tetraphenylporphyrin and x = 0-8) were examined in PhCN containing tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. Each compound undergoes two reversible to quasireversible one-electron oxidations and either three or four reductions within the potential limits of the solvent. The two oxidations occur at the conjugated porphyrin pi ring system, and DeltaE(1/2) between these two electrode reactions increases as the molecule becomes more distorted. The overall reduction of each compound involves the stepwise electrogeneration omicronf an iron(II), iron(I), and iron(I) pi anion radical. An equilibrium between chloride-bound and chloride-free iron(II) forms of the porphyrin is observed with association of the anionic ligand being favored for compounds with x > 5. Singly reduced (TPPBr(x)())FeCl (x = 0 to x = 6) forms both mono- and bis-CO adducts in CH(2)Cl(2). Only the mono-CO adduct is observed for (TPPBr(7))FeCl, and there is no binding at all of CO to (TPPBr(8))FeCl. The nu(CO) of both the mono- and bis-adducts increases with increase in the number of Br groups, but in a nonlinear fashion which is explained in terms of two competing effects. One is the electron-withdrawing affinity of the Br substitutents and the other the nonplanarity of the macrocycle.