Catalytic Selective Oxidation of Primary and Secondary Alcohols Using Nonheme [Iron(III)(Pyridine-Containing Ligand)] Complexes

The selective oxidation of different primary and secondary alcohols to carbonyl compounds by hydrogen peroxide was found to be catalysed in conversion ranging from good to excellent by an iron(III) complex of a pyridine-containing macrocyclic ligand (Pc-L), without the need of any additive. The choice of the counteranion (Cl, Br, OTf) appeared to be of fundamental importance and the best results in terms of selectivity (up to 99%) and conversion (up to 98%) were obtained using the well characterised [Fe(III)(Br)2(Pc-L)]Br complex, 4c. Magnetic moments in solid state, also confirmed in solution (d6-DMSO) by Evans NMR method, were calculated and point out to an iron metal centre in the high spin state of 5/2. The crystal structure shows that the iron(III) centre is coordinated by the four nitrogen atoms of the macrocycle and two bromide anions to form a distorted octahedral coordination environment. The catalytic oxidation of benzyl alcohol in acetonitrile was found occurring with better conversions and selectivities than in other solvents. The reaction proved to be quite general, tolerating aromatic and aliphatic alcohols, although very low yields were obtained for terminal aliphatic alcohols. Preliminary mechanistic studies are in agreement with a catalytic cycle promoted by a high-spin iron complex.