Toward Functional Carboxylate-Bridged Diiron Protein Mimics: Achieving Structural Stability and Conformational Flexibility Using a Macrocylic Ligand Framework

A dinucleating macrocycle, H2PIM, containing phenoxylimine metal-binding units has been prepared. Reaction of H2PLM with [Fe-2(Mes)(4)] (Mes = 2,4,6-trimethylphenyl) and sterically hindered carboxylic acids, Ph3CCO2H or (ArCO2H)-C-Tol (2,6-bis(p-tolyl)benzoic acid), afforded complexes [Fe-2(PIM)-(Ph3CCO2)(2)] (1) and [Fe-2(PIM)((ArCO2)-C-Tol)(2)] (2), respectively. X-ray diffraction studies revealed that these diiron(II) complexes closely mimic the active site structures of the hydroxylase components of bacterial multicomponent mono-oxygenases (BMMs), particularly the syn disposition of the nitrogen donor atoms and the bridging mu-eta(1)eta(2) and mu-eta(1)eta(2) modes of the carboxylate ligands at the diiron(II) centers. Cyclic voltammograms of 1 and 2 displayed quasi-reversible redox couples at +16 and +108 mV vs ferrocene/ferrocenium, respectively. Treatment of 2 with silver perchlorate afforded a silver(I)/iron(III) heterodimetallic complex, [Fe-2(mu-OH)(2)(ClO4)(2)(PIM)-((ArCO2)-C-Tol)Ag] (3), which was structurally and spectroscopically characterized. Complexes 1 and 2 both react rapidly with dioxygen. Oxygenation of 1 afforded a (mu-hydroxo)diiron(III) complex [Fe-2(mu-OH)(PIM)(Ph3CCO2)(3)] (4), a hexa(mu-hydroxo)-tetrairon(III) complex [Fe-4(mu-OH)(6)(PIM)(2)(Ph3CCO2)(2)] (5), and an unidentified iron(III) species. Oxygenation of 2 exclusively formed di(carboxylato)diiron(III) compounds, a testimony to the role of the macrocylic ligand in preserving the dinuclear iron center under oxidizing conditions. X-ray crystallographic and Fe-57 Mossbauer spectroscopic investigations indicated that 2 reacts with dioxygen to give a mixture of (mu-oxo)diiron(III) [Fe-2(mu-O)(PIM)((ArCO2)-C-Tol)(2)] (6) and di(mu-hydroxo)diiron(III) [Fe-2(mu-OH)(2)(PIM)((ArCO2)-C-Tol)(2)] (7) units in the same crystal lattice. Compounds 6 and 7 spontaneously convert to a tetrairon(III) complex, [Fe-4(mu-OH)(6)(PIM)(2)((ArCO2)-C-Tol)(2)] (8), when treated with excess H2O.