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Binding of Catechols to Iron(III)–Octaethylporphyrin: An Experimental and DFT Investigation
Author(s) -
Chaudhary Arvind,
Patra Ranjan,
Rath Sankar Prasad
Publication year - 2010
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201000707
Subject(s) - chemistry , catechol , ligand (biochemistry) , crystallography , steric effects , delocalized electron , dihedral angle , porphyrin , bond length , stereochemistry , electrochemistry , molecule , photochemistry , crystal structure , hydrogen bond , electrode , biochemistry , receptor , organic chemistry
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 η 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‐ t Bu‐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‐ t Bu‐Hcat), the Fe–O bond in Fe III (OEP)(Hcat) is elongated by 0.064 and 0.038 Å, respectively. This is due to the H‐bonding interactions in Fe III (OEP)(Hcat) and not caused by steric hindrance. In the 1 H 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.