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

Abstract

A dinucleating macrocycle, H[subscript 2]PIM, containing phenoxylimine metal-binding units has been prepared. Reaction of H[subscript 2]PIM with [Fe[subscript 2](Mes)[subscript 4]] (Mes = 2,4,6-trimethylphenyl) and sterically hindered carboxylic acids, Ph[subscript 3]CCO[subscript 2]H or ArTolCO[subscript 2]H (2,6-bis(p-tolyl)benzoic acid), afforded complexes [Fe[subscript 2](PIM)(Ph[subscript 3]CCO[subscript 2])[subscript 2]] (1) and [Fe[subscript 2](PIM)(Ar[subscript Tol]CO[subscript 2])[subscript 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 monooxygenases (BMMs), particularly the syn disposition of the nitrogen donor atoms and the bridging μ-η[superscript 1]η[superscript 2] and μ-η[superscript 1]η[superscript 1] 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[subscript 2](μ-OH)[subscript 2](ClO[subscript 4])[subscript 2](PIM)(Ar[superscript Tol]CO[subscript 2])Ag] (3), which was structurally and spectroscopically characterized. Complexes 1 and 2 both react rapidly with dioxygen. Oxygenation of 1 afforded a (μ-hydroxo)diiron(III) complex [Fe[subscript 2](μ-OH)(PIM)(Ph[subscript 3]CCO[subscript 2])[subscript 3]] (4), a hexa(μ-hydroxo)tetrairon(III) complex [Fe[subscript 4](μ-OH)[subscript 6](PIM)[subscript 2](Ph[subscript 3]CCO[subscript 2])[subscript 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 [superscript 57]Fe Mössbauer spectroscopic investigations indicated that 2 reacts with dioxygen to give a mixture of (μ-oxo)diiron(III) [Fe[subscript 2](μ-O)(PIM)(Ar[superscript Tol]CO[subscript 2])[subscript 2]] (6) and di(μ-hydroxo)diiron(III) [Fe[subscript 2](μ-OH)[subscript 2](PIM)(Ar[superscript Tol]CO[subscript 2])[subscript 2]] (7) units in the same crystal lattice. Compounds 6 and 7 spontaneously convert to a tetrairon(III) complex, [Fe[subscript 4](μ-OH)[subscript 6](PIM)[subscript 2](Ar[superscript Tol]CO[subscript 2])[subscript 2]] (8), when treated with excess H[subscript 2]O.