Resonance Raman Spectra of Ferrylporphyrins and Related Compounds in Dioxygen Matrices

Article abstract of DOI:10.1021/j100400a007

Resonance Raman (RR) spectra are reported for three ferryl compounds: (TPP)FeO, (OEP)FeO, and (salen)FeO (where TPP=tetraphenylporphyrinato anion, OEP=octaethylporphyrinato anion, and salen=N,N'-ethylenebis(salicylideneaminato) anion). (TPP)FeO and (OEP)FeO were formed via laser photolysis (406.7-nm line) of cocondensation products of Fe(TPP) and Fe(OEP), respectively, with dioxygen at 15 K.In both cases, the ferryl stretching (ν(FeO)) bands appear at 852 cm^-1 as the O-O bond cleveage reaction proceeds and their intensities reach maxima after about 20 min of laser irradiation (1-2-mW power).This photolysis does not occur with other exciting lines.In the case of Fe(salen), similar photolysis occurs readily with laser lines in the range from 457.9 to 514.5 nm as evidenced by the appearance of the ν(FeO) at 851 cm^-1.In contrast, attempts to prepare (Pc)FeO (Pc=phthalocyanato anion) by similar procedures were not successful although all lines in the region from 406.7 to 676.4 nm were employed.Instead, these excitations produced the RR spectrum of Fe(Pc)O2 which exhibited the ν(Fe-O2) and δ(FeOO) at 488 and 279 cm^-1, respectively.The oxidation and/or spin state marker bands were observed at 1375 (band A), 392 (band E), and 1575 cm^-1 (band D) for (TPP)FeO and at 1379 (ν₄), 1507 (ν₃), and 1643 cm^-1 (ν₁₀) for (OEP)FeO.These frequencies indicate that the iron atom in the ferrylporphyrins is low-spin with formal oxidation state close to Fe(IV).Furthermore, the FeO stretching force constant obtained (5.32 mdyn/Angstroem) is much larger than the FeO single-bond stretching force constant (3.80 mdyn/Angstroem).On the basis of these and other results, we proposed to formulate the five-coordinate ferrylporphyrin as PFe^IV<*>O(2-) (P=porphyrin) which involves one ? and two ? bonds.The marked enhancement of the ν(FeO) relative to porphyrin core vibrations suggest the possibility of direct resonance excitation via an electronic transition involving FeO charge transfer which is located near 406.7 nm, the wavelength of the laser line used for excitation.