RESONANCE RAMAN AND MAGNETIC-RESONANCE SPECTROSCOPIC CHARACTERIZATION OF THE FE(I), FE(II), FE(III), AND FE(IV) OXIDATION-STATES OF FE(2-TMPYP)N+(AQ)

Four oxidation states of aqueous meso-5,10,15,20-tetrakis(2-N-methylpyridyl)porphinatoiron {Fe(2-TMPyP)(aq)} have been characterized at pH 9 and 12 via resonance Raman (RR), NMR, and ESR spectroscpic methods. These pH values were chosen because they are below and above the pK(a) values of the Fe(II) (11.2), Fe(III) (11.0), and Fe(IV) (10.0) complexes. The 2-TMpyp2+ ligand stabilizes four iron oxidation states, I-IV, in aqueous solution. The porphyrin core size marker band frequencies in the RR spectra are consistent with the metal ion radius increasing from Fe(IV) to Fe(II), but decreasing again for Fe(I). The Fe(I) core size is smaller than that of the Fe(II) species because Fe(I) is four-coordinate and low spin, whereas Fe(III) is five-coordinate and high spin. ESR data of the highly reduced complex (g(perpendicular-to) = 2.32 and g(parallel-to) = 2.00) clearly demonstrate that Fe(II) reduction gives Fe(I) and not the porphyrin pi-anion radical at pH 9 and 12. This is the first Fe(I) complex to be observed in aqueous solution, and the potentials of the Fe(II/I) couples (-0.740 V at pH 9 and -0.763 V at pH 12) are among the most positive of any yet observed for a porphinato complex. The first observation of a Fe(II)-OH stretch in a model heme complex is reported and assigned to a band at 464 cm-1 on the basis of its 20-cm-1 downshift in (H2O)O-18. The Fe(II)-OH adduct is five-coordinate and high spin on the basis of its RR and NMR spectra. The Fe(IV/III) potentials at pH 9 and 12 are among the least positive ever reported for porphyrin complexes. For solutions with pH > pK(a)[Fe(IV)], an Fe(IV) complex can be chemically or electrochemically generated and is stable for hours at room temperature. The low-frequency RR spectrum of this species exhibits an Fe(IV)=O stretch at 763 cm-1, which was assigned on the basis of its 31-cm-1 downshift in (H2O)-O-18. This Fe(IV) complex pins its stability from coordination of the ferryl iron by an axial hydroxide ligand. For solutions with pH < pK(a)[Fe(IV)], a transient Fe(IV) species is generated. At both pH values, the Fe(IV) complexes are converted to porphyrin pi-cation radical species at 120 K, as evidenced by their broad ESR lines near g = 2. However, the RR data indicate that an e(g) ground state is thermally accessible and that the primary species in the room temperature solution is an Fe(IV) complex. The six-coordinate Fe(IV) species shows an upfield beta-pyrrole H-2 NMR isotropic shift (-9.9 ppm) that is consistent with previously character six-coordinate porphinatoiron(IV) complexes in nonaqueous solution. Resonance Raman, NMR, and ESR data for the various oxidation states and states of axial ligation in the Fe(2-TMPyP) complexes suggest that the unique ability of this porphyrin ligand to stabilize both Fe(I) and Fe(IV) oxidation states is primarily due to the electrostatic influence of the N-methylpyridinium moieties at the porphyrin periphery, as opposed to perturbation of the interaction between metal ion and tetraaryiporphyrin pi-orbitals. The stabilization of such a wide range of oxidation states is unprecedented in iron porphyrin chemistry and demonstrates that it is possible to modulate the coordination and redox chemistry of model hemes through variations in the electrostatic potential in which it resides without severely perturbing the intrinsic properties of the metalloporphine moiety.