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

摘要

A dinucleating macrocycle, H2PIM, containing phenoxylimine metal-binding units has been prepared. Reaction of H2PIM with [Fe2(Mes)4] (Mes = 2,4,6-trimethylphenyl) and sterically hindered carboxylic acids, Ph3CCO2H or Ar^TolCO2H (2,6-bis(p-tolyl)benzoic acid), afforded complexes [Fe2(PIM)(Ph3CCO2)2] (>1) and [Fe2(PIM)(Ar^TolCO2)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 multi-component monooxygenases (BMMs), particularly the syn disposition of the nitrogen donor atoms and the bridging μ-η¹η² and μ-η¹η¹ 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, [Fe2(μ-OH)2(ClO₄)₂(PIM)(Ar^TolCO₂)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 [Fe₂(μ-OH)(PIM)(Ph₃CCO₂)₃] (>4), a hexa(μ-hydroxo)tetrairon(III) complex [Fe₄(μ-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 ⁵⁷Fe Mössbauer spectroscopic investigations indicated that >2 reacts with dioxygen to give a mixture of (μ-oxo)diiron(III) [Fe₂(μ-O)(PIM)(Ar^TolCO₂)₂] (>6) and di(μ-hydroxo)diiron(III) [Fe₂(μ-OH)₂(PIM)(Ar^TolCO₂)₂] (>7) units in the same crystal lattice. Compounds >6 and >7 spontaneously convert to a tetrairon(III) complex, [Fe₄(μ-OH)₆(PIM)₂(Ar^TolCO₂)₂] (>8), when treated with excess H₂O.