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

The synthesis, X-ray structure, and properties of high-spin, five-coordinate catecholate complexes of (octaethylporphyrinato)iron(III), Fe-III(OEP)(L) (L: catecholate monoanion), are reported here for the first time. In these complexes, catechol binds in an eta(1)-fashion as an axial ligand, which is supported by DFT calculations. The Fe-O-C angles of Fe-III(OEP)(Hcat), Fe-III(OEP)(4-tBu-Hcat), Fe-III(OEP)(4-NO2-Hcat), and Fe-III-(OEP)(sal) are 119,5, 125.1, 122.2, and 124.3 degrees, respectively. Fe-III(OEP)(Hcat) has the smallest Fe-O-C angle in addition to the smallest dihedral angle of 26.2 degrees between the planes of the porphyrin and axial catechol ligand among all phenolate complexes of iron(III)-porphyrins. In comparison to those of Fe-III(OEP)(OPh) and Fe-III(OEP)(4-tBu-Hcat), the Fe-O bond in Fe-III(OEP)(Hcat) is elongated by 0.064 and 0.038 angstrom, respectively. This is due to the H-bonding interactions in Fe-III(OEP)(Hcat) and not caused by steric hindrance. In the H-1 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 it-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.