Issue 28, 2016

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 E1/2 value of the one-electron FeIIIFeIII → FeIIIFeII 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 (λmax, 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 pKa 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.

Graphical abstract: 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

Supplementary files

Article information

Article type
Paper
Submitted
18 Mar 2016
Accepted
01 Jun 2016
First published
10 Jun 2016

Dalton Trans., 2016,45, 11422-11436

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

M. Balamurugan, E. Suresh and M. Palaniandavar, Dalton Trans., 2016, 45, 11422 DOI: 10.1039/C6DT01059H

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