The Dioxygen Adducts of Several Manganese(II) Porphyrins. Electron Paramagnetic Resonance Studies

The reversible dioxygen adduct of manganese porphyrin (Mn(Por)) is five coordinate and of intermediate spin state (S=3/2) unlike the adducts of cobalt, iron, and chromium porphyrins. We have prepared a series of Mn(Por)(02) complexes, where Por = para-substituted tetraphenylporphyrin ([T(p-X)PP]; X = F, H,η-BuO) or octaethylporphyrin (OEP), and have examined their magnetic properties. The EPR parameters of the four complexes are qualitatively similar, but do differ within narrow limits: -D~2-3.2 cm-1; E/D~-1/3)-A(55Mn)~52-54ËŸ10-4 cm-1; -B(55Mn)~86-88 ËŸ l0-4 cm-1. We have also remeasured the effects of 17O substitution, and at low temperature (~10 K) find that it is possible to obtain the value of the small l70 hfs constant: a(l7O)~2.3ËŸ10-4 cm-1. Solely on the basis of the magnetic resonance parameters obtained, it is possible to rule out most possible combinations of an odd-electron configuration and Mn-C2 binding geometry. These results and qualitative bonding considerations support the original description in terms of a Mnlv-022- valency formalism with a symmetric, edge-on dioxygen.