Resonance Raman studies of dioxygen binding to ortho-substituted tetraphenyl- and tetranaphthyl-porphyrinatoiron(II) derivatives with a covalently linked axial imidazole

Abstract

Resonance Raman spectra with 457.9 nm laser excitation have been recorded for the deoxy and oxy complexes of ortho-substituted tetraphenyl- and tetranaphthyl-porphyrinatoiron(II) derivatives with a covalently linked proximal imidazole. The intramolecular imidazole co-ordination gives high-spin iron(II) states in the deoxy forms of these complexes, which are revealed by the appearances of the indicative ν₈ bands of the porphyrin ring at 367–375 cm^–1. Their Fe–N (imidazole) stretching modes were directly detected at 201–223 cm^–1, and are discussed in relation to the Fe–N (imidazole) bond strength and the geometry of the imidazole co-ordination. As a result, a significant tilting of the imidazole ring plane from the Fe–N (imidazole) vector is proposed for the double-side encumbered derivatives. The dioxygenation was monitored by the upshift of the porphyrin skeletal band (ν₄). The dioxygen adducts of the double-side encumbered porphyrins showed high iron–dioxygen stretching frequencies (579 and 580 cm^–1) relative to those of oxyhemoproteins and other dioxygenated porphyrinatoiron(II) complexes generally observed in the range 568–573 cm^–1. These high frequencies are considered to reflect the decreased Fe–O–O angle induced by the narrow ester-linked cavity. For complexes of 5,10,15,20-tetrakis(o-pivalamidophenyl)porphyrinatoiron(II) with a covalently attached imidazolyl group at the β-pyrrole position the ν(Fe–N_ε) and ν(Fe–O₂) frequencies were found to be fairly similar to those of the same complexes with externally added imidazole ligands. This indicates that the covalent linkage between the imidazole and the porphyrin periphery is flexible and long enough to allow the formation of stable O₂-adduct species.