Oxoferryl complexes of the halogenated (porphinato)iron catalyst: (Tetrakis(2,6-dichlorophenyl)porphinato)iron

Oxidation of (hydroxo)(tetrakis(2,6-dichlorophenyl)porphinato)iron(III) [TPP(2,6-Cl)Fe^IIIOH] by m-chloroperoxybenzoic acid (mCPBA) in THF or DMF yields the THF and DMF adducts of the oxoferryl complex TPP(2,6-Cl)Fe^IV=O. Addition of 10-fold molar excess of 1-methylimidazole (l-MeIm) to the TFF adduct gives the corresponding 1-MeIm complex. NMR of the DMF-d₇ adduct (200 MHz, DMF-d₇, -50 °C) shows proton resonances consistent with an oxoferryl S = 1 structure: 14 ppm (pyrrole H), 4.01 ppm (phenyl m- and p-H) in a ratio of 2:3. The UV-vis spectra (-40 °C) of the red solutions show the band at ~550 nm which is typical of the ferryl oxidation state: (THF) TPP(2,6-Cl)Fe^IV═O, λ_max(∊ X 10'³) 417 (169.5), 551 (10.6), 622 (3.4), 653 (1.4) nm; (DMF) TPP(2,6-Cl)Fe^IV=O, λ_max (∊ X 10^-3) 419 (157.6), 554 (11.1), 632 (3.5) nm; (l-Melm) TPP(2,6-Cl)Fe^IV==O, λ_max(∊ X 10^-3) 421 (157.5), 561 (13.2), 624 (3.3), 661 (1.8) nm. Resonance Raman bands in the region of the Fe═O stretch provide support for the oxoferryl structure with frequencies that correlate inversely with the strength of the sixth axial ligand: THF (841 cm^-1) < DMF (829 cm^-1) < 1-Melm (818 cm^-1). The Mössbauer spectra, similar to those of other ferryl S = 1 complexes, establish the ferryl oxidation state of iron. While quadrupole splittings vary inversely with axial ligand strength (THF (2 ².08 mm s^-1) < DMF (1.81 mm s^-1) < 1-Melm (1.35 mm s^-1)), isomer shifts are essentially constant. Resonance Raman and Mössbauer data are rationalized by a rearrangement of bonding electron density from Fe 3d_{x 2}__{y 2} to 3d_{z 2} with increasing axial ligand bonding. The Fe═O bond weakens, decreasing ν_Fe═o and the axial electric field gradient decreases, giving smaller quadrupole splitting. However, little net change in electron density at Fe results in constant δ.