Aquasoluble iron(III)-arylhydrazone-β-diketone complexes: Structure and catalytic activity for the peroxidative oxidation of C5–C8 cycloalkanes

Abstract

The aquasoluble FeIII complexes [Fe(H2O)3(L1)]∙4H2O (3) and [Fe(H2O)3(L2)]∙3H2O (4), bearing the basic forms of 5-chloro-3-(2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hydrazinyl)-2-hydroxy-benzenesulfonic acid (H3L1, 1) and 3-(2-(2,4-dioxopentan-3-ylidene)hydrazinyl)-2-hydroxy-5-nitrobenzenesulfonic acid (H3L2, 2), were synthesized and fully characterized including by X-ray crystal structural analysis. In the channels of the water-soluble 3D networks of 3 and 4, the uncoordinated water molecules are held by oxygen atoms of the carbonyl and sulfonyl groups, and by the water ligands. The FeIII coordination environment resembles that in the active sites of some mononuclear non-heme iron-containing enzymes. The complexes show a high catalytic activity for the peroxidative oxidation (with aqueous H2O2) of C5–C8 cycloalkanes to the corresponding alcohols and ketones under mild conditions. The effects of various factors, such as amounts of oxidant, catalyst and HNO3 additive, were investigated allowing to reach overall yields of ca. 25% and turnover numbers (TONs) up to 290. The catalytic reactions proceed via both oxygen- and carbon-radicals as shown by radical trap experiments.