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)

Journal Article · Wed Jun 24 00:00:00 EDT 1992 · Inorganic Chemistry; (United States)

DOI:https://doi.org/10.1021/ic00039a007· OSTI ID:6915615

Rodgers, K R; Reed, R A; Spiro, T G ^[1]; Su, Y O ^[2]

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1. Princeton Univ., NJ (United States)
2. 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), Vol. 31:13; ISSN 0020-1669
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
59 BASIC BIOLOGICAL SCIENCES
IRON COMPLEXES
ELECTRON SPIN RESONANCE
NUCLEAR MAGNETIC RESONANCE
RAMAN SPECTROSCOPY
AQUEOUS SOLUTIONS
DEUTERIUM
ELECTROCHEMISTRY
HYDROXIDES
PH VALUE
PORPHYRINS
TEMPERATURE RANGE 0065-0273 K
VALENCE
VOLTAMETRY
CARBOXYLIC ACIDS
CHEMISTRY
COMPLEXES
DISPERSIONS
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
ISOTOPES
LASER SPECTROSCOPY
LIGHT NUCLEI
MAGNETIC RESONANCE
MIXTURES
NUCLEI
ODD-ODD NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
RESONANCE
SOLUTIONS
SPECTROSCOPY
STABLE ISOTOPES
TEMPERATURE RANGE
TRANSITION ELEMENT COMPLEXES
400201* - Chemical & Physicochemical Properties
550200 - Biochemistry