Binding of catechols to iron(III)-octaethylporphyrin: An experimental and DFT investigation

摘要

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 ?· ~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-NO_2-Hcat), and Fe~(III)(OEP) (sal) are 119.5, 125.1, 122.2, and 124.3°, respectively. Fe ~(III)(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 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 ? respectively. This is due to the H-bonding interactions in Fe~(III)(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. The synthesis, X-ray structures, and properties of new heme analogues Fe ~(III)(OEP)(L) (L: Hcat, 4-NO_2-Hcat, 4-tBu-Hcat, and sal) are reported, in which catechol binds in an ?·~1-fashion as an axialligand. Spectroelectrochemical studies of one- and two-electron oxidations are also reported. Experimental findings are supported by DFT calculations.