Evidence for variable metal‐radical spin coupling in oxoferrylporphyrin cation radical complexes

Abstract

Oxoferrylporphyrin cation radical complexes were generated by m-chloroperoxybenzoic acid oxidation of the chloro and trifluoromethanesulfonato complexes of tetramesitylporphyrinatoiron(II1) [(TMP)Fe] and the trifluoromethanesulfonato complex of tetra(2,6-dichlorophenyl)porphyrinatoiron(IlI) [TPP(2,. Coupling between ferry1 iron (S = 1) and porphyrin radical (S' = 1/2) spin systems was investigated by Mossbauer and EPR spectroscopy. The oxoferrylporphyrin cation radical systems generated from the TMP complexes show strong ferromagnetic coupling. Analysis of the magnetic Mossbauer spectra, using a spin Hamiltonian explicitly including a coupling tensor J , suggests an exchange-coupling constant J > 80 cm-'. The EPR spectra show non-zero rhombicity, the origin of which is discussed in terms of contributions from the usual zero-field effects of iron and from iron-radical spin-dipolar interaction. A consistent estimate of zero-field splitting parameter D = + 6 cm-' was obtained by EPR and Mossbauer measurements. EPR and Mossbauer parameters are shown to be slightly dependent on solvent, but not on the axial ligand in the starting (TMP)Fe complex. In contrast to the TMP complex, the oxoferrylporphyrin cation radical system generated from [TPP(2,6-C1)FeOS02CF3] exhibits Mossbauer and EPR spectra consistent with weak iron-porphyrin radical coupling of IJI = 1 cm-'.High-valency oxoheme complexes, formally two-electron oxidized above the ferric state, have been of considerable interest because of their importance in the chemistry of catalase and peroxidase enzymes [l, 21 and because they play a putative role in monooxygen transfer to organic substrates by cytochrome P450 [3, 41. Efforts to model biochemical function and to characterize better the electronic structure of these intermediates have resulted in the synthesis of oxoheme complexes in the same oxidation state as the enzymatic transients [5, 61 and in mimicking some of the biochemical functions of cytochrome P450 [5 -151. The sum of the physicochemical evidence marshalled to date best describes the complexes as oxoferrylporphyrin cation radical complexes [5, 16 -291. Despite the potential importance of the synthetic complexes for the elucidation of the electronic structures of the biological transients, only the oxoferrylporphyrin cation radical system [(TMP)Fe = 0 1 +Cl-, derived from tetramesitylporphyrinatoiron(II1) chloride [(TMP)FeCl], has been the object of serious efforts at characterization (complexes like [(TMP)Fe = O](Cl-), the counter ions are written in parentheses to indicate uncertainty as to the exact nature of their association with the iron(1V) center}. A large variety of spectroscopic techniques has been used for investigation of this species [6,16,18, 231. Strong ferromagnetic coupling between the electron spins of iron and the tetramesitylporphyrin radical has been demonCorrespondenre to A.

Elucidation of a Redox-Mediated Reaction Cycle for Nickel-Catalyzed Cross Coupling

“…Computational studies based on the crystal structure gave a calculated absorption spectrum (Figure S16) that is in qualitative agreement with the experimental spectra shown in Figure B. The frozen solution X-band EPR spectrum at 77 K in CH 3 CN (Figure C) is characteristic of a S = 3/2 species. , Quinuclidine and excess (dtbbpy)NiCl 2 were essential in forming this compound since their absence in the reaction between Ni(0) and Ni(II) led to a decomposition product (Figures S11 and S12), consistent with the increased stability observed by TA spectroscopy (Figure S2).…”

“…The frozen solution X-band EPR spectrum at 77 K in CH 3 CN (Figure 2C) is characteristic of a S = 3/2 species. 19,20 Quinuclidine and excess (dtbbpy)NiCl 2 were essential in forming this compound since their absence in the reaction between Ni(0) and Ni(II) led to a decomposition product (Figures S11 and S12), consistent with the increased stability observed by TA spectroscopy (Figure S2).…”

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

“…50,52 However, Krik and co-workers later reported that Cmpd I of this system could in fact be generated in nonpolar solvents under the same experimental conditions, in the presence of axially coordinating anions (such as triflate). 586 This work clearly reveals the prominent effect of axial ligation on O–O bond scission of the Fe III – m -CPBA adduct of TMP (Scheme 14). Furthermore, Nam and co-workers have reported on the disparities of [(X)(TPFPP)Fe III ]-facilitated (also see Figure 41, section 2.2; X = axial ligand) oxygenation of hydrocarbons using H 2 O 2 as the oxidant, where the feasibility and/or the yields of epoxidation and hydroxylation reactivities were observed to be strongly affected depending on the identity of X.…”

Reaction profile of the last step in cytochrome P-450 catalysis revealed by studies of model complexes

“…This intermediate was identified as an 1 A few exceptional cases with much longer bond lengths were also reported, but are attributed to either experimental pitfalls or unconventional data analysis oxoiron(IV) porphyrin cation radical complex, [(TMP c 7)Fe IV (O)] c (1), based on UV-vis [16], NMR [16], Mössbauer [16], EPR [17], resonance Raman (RR) [18,19], and EXAFS [20] spectroscopies. Similar to compounds I of the various enzymes, complex 1 is a cationic complex with an additional axial ligand trans to the oxo ligand.…”