Non-heme μ-Oxo- and bis(μ-carboxylato)-bridged diiron(iii) complexes of a 3N ligand as catalysts for alkane hydroxylation: stereoelectronic factors of carboxylate bridges determine the catalytic efficiency

Abstract

A series of non-heme (μ-oxo)bis(μ-dicarboxylato)-bridged diiron(III) complexes, [Fe₂(O)(OOCH)₂(L)₂]²⁺1, [Fe₂(O)(OAc)₂(L)₂]²⁺2, [Fe₂(O)(Me₃AcO)₂(L)₂]²⁺3, [Fe₂(O)(OBz)₂(L)₂]²⁺4, [Fe₂(O)(Ph₂AcO)₂(L)₂]²⁺5 and [Fe₂(O)(Ph₃AcO)₃(L)₂]²⁺6, where L = N,N-dimethyl-N′-(pyrid-2-ylmethyl)ethylenediamine, OAc⁻ = acetate, Me₃AcO⁻ = trimethylacetate, OBz⁻ = benzoate, Ph₂AcO⁻ = diphenylacetate and Ph₃AcO⁻ = triphenylacetate, have been isolated and characterized using elemental analysis and spectral and electrochemical techniques. They have been studied as catalysts for the selective oxidation of alkanes using m-chloroperbenzoic acid (m-CPBA) as the oxidant. Complexes 2, 3, and 4 possess a distorted bioctahedral geometry in which each iron atom is coordinated to an oxygen atom of the μ-oxo bridge, two oxygen atoms of the μ-carboxylate bridge and three nitrogen atoms of the 3N ligand. In an acetonitrile/dichloromethane solvent mixture all the complexes display a d–d band characteristic of the triply bridged diiron(III) core, revealing that they retain their identity in solution. Upon replacing electron-donating substituents on the bridging carboxylates by electron-withdrawing ones the E_1/2 value of the one-electron Fe^IIIFe^III → Fe^IIIFe^II reduction becomes less negative. On adding one equivalent of Et₃N to a mixture of one equivalent of the complex and an excess of m-CPBA in the acetonitrile/dichloromethane solvent mixture an intense absorption band (λ_max, 680–720 nm) appears, which corresponds to the formation of a mixture of complex species. All the complexes act as efficient catalysts for the hydroxylation of cyclohexane with 380–500 total turnover numbers and good alcohol selectivity (A/K, 6.0–10.1). Adamantane is selectively oxidized to 1-adamantanol and 2-adamantanol (3°/2°, 12.9–17.1) along with a small amount of 2-adamantanone (total TON, 381–476), and interestingly, the sterically demanding trimethylacetate bridge around the diiron(III) centre leads to high 3°/2° bond selectivity; on the other hand, the sterically demanding triphenylacetate bridge gives a lower 3°/2° bond selectivity. A remarkable linear correlation between the pK_a of the bridging carboxylate and TON for both cyclohexane and adamantane oxidation is observed, illustrating the highest catalytic activity for 3 with strongly electron-releasing trimethylacetate bridges.