REACTION OF DIOXYGEN WITH A CROSS-CONJUGATED CARBON-CARBON DOUBLE-BOND IN A BIS-MACROCYCLE DIIRON COMPOUND

Dioxygen at atmospheric pressure attacks a cross-conjugated carbon-car bon double bond in a diiron complex to form two, like, keto macrocycli c iron(II) complexes. This reaction occurs with high yield in both sol ution and in the solid state. A dioxetane intermediate is, therefore, invoked. The rate of the reaction is very dependent on the nature of t he axial ligands on the low-spin iron(II) ions in the bimetallic compl ex. The rate is at least a factor of 10(4) faster with DMF ligands tha n with CH3CN axial ligands. This rate dependence is explained by stabi lization of a peroxo biradical transition state en route to a dioxetan e intermediate. The keto-macrocyle product has the carbonyl group conj ugated with a beta-diimine in a six-membered chelate ring. The conform ation of this keto macrocycle is fixed on the NMR time scale and the s pectra of all ten non-equivalent protons in the complex can be unambig uously assigned. The keto beta-diimine ligand is an excellent pi-accep tor as indicated by the high Fe(II) to Fe(III) oxidation potential of the compound and by the Mossbauer spectrum, which shows a low value fo r the center shift and a high value for the quadrupole splitting param eter. (C) 1997 Elsevier Science S.A.