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

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.

Graphical abstractA series of diiron(III) complexes has been isolated and studied as catalysts for alkane hydroxylation reactions using m-CPBA as the oxidant. Interestingly, the 3°/2° bond selectivity observed for the hydroxylation of adamantane for the complex of the 3N capping ligand with two pyridyl donors (L1) is the highest among the present complexes, suggesting that the capping ligand plays an important role in determining the product selectivity.Figure optionsDownload full-size imageDownload as PowerPoint slide