EXAFS studies of molecular geometries of some CoII and CoIII porphyrins

Abstract

Co K-edge EXAFS data of some (porphinato)cobalt(III) complexes with amine ligands in chloroform solution are reported. Data for the following compounds were collected: chloro(tetraphenylporphinato)cobalt(III)(TPPCoCl) complexed with pyridine (A), isoquinoline (B) and cyclohexylamine (C) and 4-methylpiperidine complexed with the sterically hindered chloro(tetra-o-dichlorophenylporphinato)cobalt(III)(TCIPCoCI)(D) and chloro(tetra-o-difluorophenylporphinato)cobalt(III)(TFPCoCl)(E). In addition to the four N atoms from the macrocyclic porphyrin ligand, the cobalt atom is coordinated to either one amine and a chlorine atom (A and B) or two axial amine ligands with the chloride as a counterion (C–E). An interesting feature is the longer Co—Cl distances in A and B relative to those in the solid state. The Co—N distances are represented by a single composite distance ranging from 1.91(1) to 1.95(1)Å. The Co—N bond lengths for the ortho-substituted porphyrins were found to be similar to those in the less hindered TPP complexes. The EXAFS for the cobalt(II) compounds TCIPCo (F) and TFPCo (G) gave distances similar to those of the crystal structure of TPPCo. The equatorial Co—N distances depend more on the conformation of the porphyrin core than on the cobalt oxidation state, as demonstrated by the similarity between the Co^III—N and Co^II—N distances.