Synthesis, Characterization, and Reactivity of a Series of Homo-and Hetero-dinuclear Complexes based on an Asymmetric FloH Ligand System

In a previous communication we reported the site-directed generation of a heterodinuclear (FeCuII)-Cu-III complex (1) by using an asymmetric dinucleating ligand FloH. The iron(III) ion was introduced first on the preferential metal-binding site of the ligand that led to the formation of the thermodynamically favored five-membered chelate ring upon metal-binding. Copper(II) was introduced in the next step. The stepwise metalation strategy reported previously has now been extended to synthesize a series of heterodinuclear (FeMII)-M-III [M = Mn (2), Fe (3), Co (4), and Ni (5)] and (FeCuI)-Cu-II (1a) as well as the homodi-nuclear (CuCuI)-Cu-I (6) complexes. The complexes were characterized by X-ray crystallography (except for 1a and 6), and by a limited number of spectroscopic methods. Complex 1 with a labile solvent binding site at Fe-III reacted with H2O2 to form a transient intermediate that showed reactivity typical of metal peroxide complexes. The metal centers in the complexes 2-5 are coordinatively saturated, and hence they showed no reactivity with H2O2. Complex 1a reacted with O-2 via an intermolecular pathway to form a mu-oxo bridged tetrameric complex 1b, which was structurally characterized. This is in contrast to the homodinuclear (CuCuI)-Cu-I and heme (FeCuI)-Cu-II cores, which prefer an intramolecular pathway for O-2 activation.