Modeling dioxygen-activating centers in non-heme diiron enzymes: Carboxylate shifts in diiron(II) complexes supported by sterically hindered carboxylate ligands

General synthetic routes are described for a series of diiron(II) complexes supported by sterically demanding carboxylate ligands 2,6-di(p-tolyl)benzoate ((ArCO2-)-C-Tol) and 2,6-di(4-fluorophenyl)benzoate (Ar4-FPhCO2-). The interlocking nature of the m-terphenyl units in self-assembled [Fe-2(mu-O2CArTol)(2)(O2CArTol)(2)L-2] (L = C5H5N (4); 1-Melm (5)) promotes the formation of coordination geometries analogous to those of the non-heme diiron cores in the enzymes RNR-R2 and Delta9D. Magnetic susceptibility and Mossbauer studies of 4 and 5 revealed properties consistent with weak antiferromagnetic coupling between the high-spin iron(II) centers. Structural studies of several derivatives obtained by ligand substitution reactions demonstrated that the [Fe-2(O2CAr')(4)L-2] (Ar' = Ar-Tol; Ar4-FPh) module is geometrically flexible. Details of ligand migration within the tetracarboxylate diiron core, facilitated by carboxylate shifts, were probed by solution variable-temperature F-19 NMR spectroscopic studies of [Fe-2(mu-O2CAr4-FPh)(2)(O2CAr4-FPh)(2)(THF)(2)] (8) and [Fe-2(mu-O2CAr4-FPh)(4)(4-(BuC5H4N)-Bu-1)(2)] (12). Dynamic motion in the primary coordination sphere controls the positioning of open sites and regulates the access of exogenous ligands, processes that also occur in non-heme diiron enzymes during catalysis.