Heterometallic CuIIFeIII and CuIIMnIII alkoxo-bridged complexes revealing a rare hexanuclear M6(μ-X)7(μ3-X)2 molecular core

Abstract

The novel hexanuclear complexes [Cu₄Fe₂(OH)(Piv)₄(tBuDea)₄Cl]·0.5CH₃CN (1) and [Cu₄Mn₂(OH)(Piv)₄(tBuDea)₄Cl] (2) were prepared through one-pot self-assembly reactions of copper powder and iron(II) or manganese(II) chloride with N-tert-butyldiethanolamine (H₂tBuDea) and pivalic acid (HPiv) in acetonitrile. Crystallographic studies revealed the uncommon molecular core type M₆(μ-X)₇(μ₃-X)₂ in 1 and 2, which can be viewed as a combination of two trimetallic M₃(μ-X)₂(μ₃-X) fragments joined by three bridging atoms. The analysis and classification of the hexanuclear complexes having a M₃(μ-X)₂(μ₃-X) moiety as a core forming fragment using data from the Cambridge Structural Database (CSD) were performed. Variable-temperature (1.8–300 K) magnetic susceptibility measurements of 1 showed a decrease of the effective magnetic moment value at low temperature, indicative of antiferromagnetic coupling between the magnetic centres (J_Fe–Cu/hc = −6.9 cm⁻¹, J_Cu–Cu/hc = −4.1 cm⁻¹, J_Fe–Fe/hc = −24.2 cm⁻¹). Complex 1 acts as a catalyst in the reaction of mild oxidation of cyclohexane with H₂O₂, showing the yields of products, cyclohexanol and cyclohexanone, up to 17% using pyrazinecarboxylic acid as a promoter. In the oxidation of cis-1,2-dimethylcyclohexane with m-chloroperoxybenzoic acid (m-CPBA), 70% of retention of stereoconfiguration was observed for tertiary alcohols. Compound 1 also catalyses the amidation of cyclohexane with benzamide. In all three catalytic reactions the by-products were investigated in detail and discussed.