Resonance Raman and magnetic resonance spectroscopic characterization of the Fe(I), Fe(II), Fe(III), and Fe(IV) oxidation states of Fe(2-TMPyP)[sup n+] (aq)
- Princeton Univ., NJ (United States)
- National Taiwan Univ., Taipei (Taiwan)
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 spectroscopic methods. These pH values were chosen because they are below and above the pK[sub a] values of the Fe(II) (11.2), Fe(III) (11.0), and Fe(IV) (10.0) complexes. The 2-TMPyP[sup 2+] ligand stabilizes four iron oxidation states, I-IV, in aqueous solution. ESR data of the highly reduced complex (g[perpendicular] = 2.32 and g[parallel] = 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 ([minus]0.740 V at pH 9 and [minus]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[sup II]-OH stretch in a model heme complex is reported and assigned to a band at 464 cm[sup [minus]1] on the basis of its 20-cm[sup [minus]1] downshift in H[sub 2][sup 18]O. The Fe(IV/III) potentials at pH 9 and 12 are among the least positive ever reported for porphyrin complexes. For solutions with pH [gt] pK[sub a[Fe(IV)]], an Fe(IV) complex can be chemically or electrochemically generated and is stable for hours at room temperature. 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.
- DOE Contract Number:
- FG02-88ER13876
- OSTI ID:
- 6915615
- Journal Information:
- Inorganic Chemistry; (United States), Journal Name: Inorganic Chemistry; (United States) Vol. 31:13; ISSN 0020-1669; ISSN INOCAJ
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
400201* -- Chemical & Physicochemical Properties
550200 -- Biochemistry
59 BASIC BIOLOGICAL SCIENCES
AQUEOUS SOLUTIONS
CARBOXYLIC ACIDS
CHEMISTRY
COMPLEXES
DEUTERIUM
DISPERSIONS
ELECTROCHEMISTRY
ELECTRON SPIN RESONANCE
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
HYDROXIDES
IRON COMPLEXES
ISOTOPES
LASER SPECTROSCOPY
LIGHT NUCLEI
MAGNETIC RESONANCE
MIXTURES
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ODD-ODD NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PH VALUE
PORPHYRINS
RAMAN SPECTROSCOPY
RESONANCE
SOLUTIONS
SPECTROSCOPY
STABLE ISOTOPES
TEMPERATURE RANGE
TEMPERATURE RANGE 0065-0273 K
TRANSITION ELEMENT COMPLEXES
VALENCE
VOLTAMETRY