Hexanuclear and undecanuclear iron(iii) carboxylates as catalyst precursors for cyclohexane oxidation

Abstract

Two multinuclear complexes [Fe₆(μ₃-O)₂(μ₄-O₂)L₁₀(OAc)₂(H₂O)₂]·2.625Et₂O·2.375H₂O (1) and [Fe^III₁₁Cl(μ₄-O)₃(μ₃-O)₅L₁₆(dmf)_2.5(H₂O)_0.5]·Et₂O·1.25dmf·3.8H₂O (2), where HL = 3,4,5-trimethoxybenzoic acid and dmf = dimethylformamide, have been prepared from trinuclear iron(III) carboxylates via their structural rearrangement in dimethylformamide or diethyl ether–dimethylformamide 9 : 1, respectively, and slow vapor diffusion of diethyl ether into the reaction mixture. Both compounds have been characterized by X-ray diffraction, optical, Mössbauer spectroscopy, and magnetic measurements. Complex 1 possesses a hexanuclear ferric peroxido–dioxido {Fe₆(O₂)(O)₂}¹²⁺ core unit, which adopts a recliner conformation, while complex 2 contains an unprecedented {Fe₁₁O₈Cl}¹⁶⁺ core, in which 9 ferric ions are six-coordinate and the remaining two are five-coordinate. Another structural feature of note of the undecanuclear core is the presence of a deformed cubane entity {Fe₄(μ₃-O)(μ₄-O)₃}⁴⁺. Both complexes act as catalyst precursors for the oxidation of cyclohexane to cyclohexanol and cyclohexanone with aqueous H₂O₂, in the presence of pyrazinecarboxylic acid. Remarkable TONs and TOFs (the latter mainly for 1) with concomitant quite good yields have been achieved under mild conditions. Moreover, 1 exhibits remarkably high activity in an exceptionally short reaction time (45 min), being unprecedented for any metal catalyzed alkane oxidation by H₂O₂. The catalytic reactions proceed via Fenton type chemistry.