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

A series of non-heme (mu-oxo)bis(mu-dicarboxylato)-bridged diiron(III) complexes, [Fe-2(O)(OOCH)(2)(L)(2)](2+) 1, [Fe-2(O)(OAc)(2)(L)(2)](2+) 2, [Fe-2(O)(Me3AcO)(2)(L)(2)](2+) 3, [Fe-2(O)(OBz)(2)(L)(2)](2+) 4, [Fe-2(O)(Ph2AcO)(2)(L)(2)](2+) 5 and [Fe-2(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 mu-oxo bridge, two oxygen atoms of the mu-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 (FeFeIII)-Fe-III -> (FeFeII)-Fe-III 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 (gimel(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 degrees/2 degrees, 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 degrees/2 degrees bond selectivity; on the other hand, the sterically demanding triphenylacetate bridge gives a lower 3 degrees/2 degrees 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.