Diiron(III) complexes of tridentate 3N ligands as functional models for methane monooxygenases: Effect of the capping ligand on hydroxylation of alkanes

Abstract

A series of non-heme (μ-oxo)bis(μ-benzoato)-bridged diiron(III) complexes of the type [Fe2(O)(OBz)2(L)2]2+ (1–6), where OBz=benzoate, L=bis(pyridin-2-ylmethyl)amine (L1), N-((6-methylpyridin-2-yl-)methyl)(pyridin-2-yl)methanamine (L2), N,N-dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine (L3), (1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)-methanamine (L4), (1H-benzo[o]imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L5) and bis((1H-benzo[o]imidazol-2-yl)methyl)amine (L6), have been isolated and characterized by means of elemental analysis, spectral and electrochemical methods. They have been studied as catalysts for selective hydroxylation of alkanes using m-choloroperbenzoic acid (m-CPBA) as the oxidant. In acetonitrile/dichloromethane mixed solvent all the complexes display a d–d band characteristic of a triply bridged diiron(III) core, revealing that they retain their identity in solution. Upon replacing a donor atom on the capping ligand by a stronger donor, the E1/2 value of the one-electron FeIIIFeIII→FeIIIFeII reduction becomes more negative. All the complexes function as efficient catalysts for hydroxylation of cyclohexane, with 390–410 total turnover numbers and good alcohol selectivity (A/K, 9.3–12.8). Adamantane is selectively oxidized (3°/2°, 15.7–28.1) to 1-adamantanol and 2-adamantanol, along with a small amount of 2-adamantanone (Total TON, 336–437), and interestingly the 3N capping ligands with the pyridyl donor around the diiron(III) center lead to high 3°/2° bond selectivity.