Resonance Raman Spectra of Nitridoiron(V) Porphyrin Intermediates Produced by Laser Photolysis

Article abstract of DOI:10.1021/ja00187a010

Nitridoiron(V) porphyrins, NFeOEP (OEP, octaethylporphinato anion), NFeTPP (TPP, tetraphenylporphinato anion), and NFeTMP (TMP, tetramesitylporphinato anion), were produced by laser irradiation of thin films of the corresponding azido complexes at ca. 30 K.Formation of the nitrido complexes was detected by the appearance of the ν(Fe<*>N) (ν, stretching vibration) at 876 cm^-1 for the OEP and TPP complexes and at 873 cm^-1 for the TMP complex in resonance Raman (RR) spectra.These assignments were confirmed by observed frequency shifts due to (56)Fe/(54)Fe and (14)N/(15)N isotopic substitutions.Frequencies of structure-sensitive bands known for OEP and TPP complexes suggest the Fe(V) state for the nitrido complex and rule out the possibility of ?-cation-radical formation.Although the spin state cannot be determined definitively by vibrational spectroscopy, the relatively small Fe<*>N stretching force constant (5.07 mdyn/Angstroem) favors the high-spin (d_xy)¹(d_xz)¹(d_yz)¹ configuration that was found for isoelectronic oxomanganese(IV) porphyrins.In the case of the OEP complex, RR spectroscopy provides evidence for a reaction scheme in which NFe^VOEP is first formed by laser photolysis of the N3Fe^IIIOEP complex due to low-power irradiation (1-60 mW in the 406.7-514.5-nm range) and subsequently converted to the μ-nitrido dimer (FeOEP)2N by local heating, which occured when high laser power (220 mW at 413.1 nm) was applied to the sample.This dimer exhibits the symmetric Fe-N-Fe stretching band at 438 cm^-1, which shows expected shifts upon (56)Fe/(54)Fe and (14)N/(15)N substitutions.Formation of the μ-nitrido dimer is also confirmed by the positions of the structure-sensitive bands in the high-frequency region.The above reaction scheme resembles the autoxidation process of oxyiron porphyrins at low temperature which yields ferrylporphyrins as the intermediate and the μ-oxo dimer as the final product at room temperature.