The reactivity of a series of diiron(III) complexes (1-5) of the tetradentate ligand bpmen (N,N '-dimethyl-N,N '-bis(2-pyridylmethyl)ethane-1,2-diamine) has been investigated. Three new diiron(III) complexes (3-5) with fluoride and acetate ligands have been isolated and structurally characterized. Complex Fe-2(III)(mu-O)(mu-OH)(bpmen)(2) (1), which contains a mu L-oxo, mu-hydroxo diiron(III) diamond core, was found to catalyze the epoxidation of cyclooctene with hydrogen peroxide, but its mu-oxo, mu-acetate counterpart Fe-2 (III) (mu-O)(mu-OAc)(bpmen)(2) (2) is unreactive. The effect of additives such as hydrofluoric acid, tetrabutylammonium fluoride, acetic acid and acetate on the reactivity of I and 2 is reported. Complexes 3-5 with two monodentate ligands (F-or OAc-) attached to the bpmen-supported mu-oxo bridged diiron(III) core do not activate H2O2 for oletin epoxidation. (c) 2006 Elsevier B.V. All rights reserved.