Binding of Catechols to Iron(III)-Octaethylporphyrin: An Experimental and DFT Investigation

Abstract

The synthesis, X-ray structure, and properties of high-spin, five-coordinate catecholate complexes of (octaethylporphyrinato)iron(III), FeIII(OEP)(L) (L: catecholate monoanion), are reported here for the first time. In these complexes, catechol binds in an η1-fashion as an axial ligand, which is supported by DFT calculations. The Fe–O–C angles of FeIII(OEP)(Hcat), FeIII(OEP)(4-tBu-Hcat), FeIII(OEP)(4-NO2-Hcat), and FeIII(OEP)(sal) are 119.5, 125.1, 122.2, and 124.3°, respectively. FeIII(OEP)(Hcat) has the smallest Fe–O–C angle in addition to the smallest dihedral angle of 26.2° between the planes of the porphyrin and axial catechol ligand among all phenolate complexes of iron(III)–porphyrins. In comparison to those of FeIII(OEP)(OPh) and FeIII(OEP)(4-tBu-Hcat), the Fe–O bond in FeIII(OEP)(Hcat) is elongated by 0.064 and 0.038 Å, respectively. This is due to the H-bonding interactions in FeIII(OEP)(Hcat) and not caused by steric hindrance. In the 1H NMR spectra of the complexes, the signals of the ortho- and para-protons of catechol are shifted upfield, whereas those of the meta-protons are shifted downfield. The alternating shift pattern observed is due to negative and positive spin densities on the catechol carbon atoms and is indicative of π-spin delocalization on the catecholato ligand. Electrochemical data reveal that the complexes undergo three one-electron oxidations and a single one-electron reduction. Based on spectroelectrochemical and DFT studies, the first oxidation is assigned to a catechol-to-semiquinone transformation and the second and third oxidations are found to be porphyrin-ring-centered.