Griffith model bonding in dioxygen complexes of manganese porphyrins

Journal Article · Wed Jul 02 04:00:00 EDT 1980 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00534a006· OSTI ID:6928027

Hanson, L K ^[1]; Hoffman, B M

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1. Brookhaven National Lab., Upton, NY

The binding of O/sub 2/ to Mn(II) porphyrins greatly alters their optical and EPR spectra. The optical spectra of these pentacoordinate complexes are transformed from the normal to hyper type with a split Soret band typical of Mn(III) porphyrins. EPR spectra indicate a spin change from S = 5/2 to S = 3/2, and /sup 17/O substitution reveals little unpaired spin density on the O/sub 2/. Contradictory interpretations of the electronic and geometric structures of these complexes have recently been advanced: analysis of the EPR data supports a d/sup 3/ Mn/sup IV/O/sub 2//sup 2 -/ with O/sub 2/ bound in the Griffith mode (edge-on, parallel to the porphyrin plane) whereas ab initio calculations favor a d/sup 2/..pi..* Mn/sup III/O/sub 2//sup -/ with O/sub 2/ bound in the Pauling mode (end-on, bent). Charge iterative extended Hueckel calculations on both Griffith and Pauling models of oxymanganese porphine are reported here in which the O-O, Mn-O, and out-of-plane Mn distances and the O-O orientation above the porphinato plane were varied. For all the calculations, the porphine ring as well as the metal transfers considerable charge density onto the O/sub 2/, suggesting that a formal charge description of these complexes solely in terms of the O/sub 2/Mn core is an oversimplification. The resultant wave functions were used to calculate /sup 55/Mn and /sup 17/O hyperfine splittings. Griffith models with three unpaired electrons occupying orbitals of predominantly d character give excellent agreement with the observed /sup 55/Mn and /sup 17/O values. The optical hyperspectra can be ascribed to porphine ..pi.. ..-->.. d/sub x/,O/sub 2/..pi..* charge-transfer transitions that mix with the porphine ..pi.. ..-->.. ..pi..* Soret transitions. All the experimental results can be explained in terms of a d/sup 3/ configuration and Griffith binding of the dioxygen but not Pauling binding.

• DOE Contract Number: EY-76-C-02-0016
• OSTI ID: 6928027
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 102:14; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
400301 -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
CARBOXYLIC ACIDS
COMPLEXES
DATA
ELECTRON SPIN RESONANCE
EVEN-ODD NUCLEI
EXPERIMENTAL DATA
HETEROCYCLIC ACIDS
HETEROCYCLIC COMPOUNDS
INFORMATION
INTERMEDIATE MASS NUCLEI
ISOTOPE EFFECTS
ISOTOPES
LABELLED COMPOUNDS
LIGHT NUCLEI
MAGNETIC RESONANCE
MANGANESE 55
MANGANESE COMPLEXES
MANGANESE ISOTOPES
MOLECULAR STRUCTURE
NUCLEI
NUMERICAL DATA
ODD-EVEN NUCLEI
ORGANIC ACIDS
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN 17
OXYGEN COMPLEXES
OXYGEN ISOTOPES
PORPHYRINS
RESONANCE
SPECTRA
STABLE ISOTOPES
TRANSITION ELEMENT COMPLEXES