Postprint version. Published in Inorganica Chimica Acta, Volume 263, Issue 1-2, October 15, 1997, pages 125-131. Copyright © 1997 Elsevier. The definitive version is available at http://dx.doi.org/10.1016/S0020-1693(97)05607-7.
Dioxygen at atmospheric pressure attacks a cross-conjugated carbon-carbon double bond in a diiron complex to form two, like, keto macrocyclic iron(II) complexes. This reaction occurs with high yield in both solution and in the solid state. A dioxetane intermediate is, therefore, invoked. The rate of the reaction is very dependent on the nature of the axial ligands on the low-spin iron(II) ions in the bimetallic complex. The rate is at least a factor of 104 faster with DMF ligands than with CH3CN axial ligands. This rate dependence is explained by stabilization of a peroxo biradical transition state en route to a dioxetane intermediate. The keto-macrocyle product has the carbonyl group conjugated with a β-diimine in a six-membered chelate ring. The conformation of this keto macrocycle is fixed on the NMR time scale and the spectra of all ten non-equivalent protons in the complex can be unambiguously assigned. The keto β-diimine ligand is an excellent π-acceptor as indicated by the high Fe(II) to Fe(III) oxidation potential of the compound and by the Mössbauer spectrum, which shows a low value for the center shift and a high value for the quadrupole splitting parameter.