Self-Assembled 3D Heterometallic Cu^II/Fe^II Coordination Polymers with Octahedral Net Skeletons: Structural Features, Molecular Magnetism, Thermal and Oxidation Catalytic Properties

The new three-dimensional (3D) heterometallic Cu^II/Fe^II coordination polymers [Cu₆(H₂tea)₆Fe(CN)₆]_n(NO₃)_2n·6nH₂O (1) and [Cu₆(Hmdea)₆Fe(CN)₆]_n(NO₃)_2n·7nH₂O (2) have been easily generated by aqueous-medium self-assembly reactions of copper(II) nitrate with triethanolamine or N-methyldiethanolamine (H₃tea or H₂mdea, respectively), in the presence of potassium ferricyanide and sodium hydroxide. They have been isolated as air-stable crystalline solids and fully characterized including by single-crystal X-ray diffraction analyses. The latter reveal the formation of 3D metal−organic frameworks that are constructed from the [Cu₂(μ-H₂tea)₂]²⁺ or [Cu₂(μ-Hmdea)₂]²⁺ nodes and the octahedral [Fe(CN)₆]⁴⁻ linkers, featuring regular (1) or distorted (2) octahedral net skeletons. Upon dehydration, both compounds show reversible escape and binding processes toward water or methanol molecules. Magnetic susceptibility measurements of 1 and 2 reveal strong antiferromagnetic [J = −199(1) cm⁻¹] or strong ferromagnetic [J = +153(1) cm⁻¹] couplings between the copper(II) ions through the μ-O-alkoxo atoms in 1 or 2, respectively. The differences in magnetic behavior are explained in terms of the dependence of the magnetic coupling constant on the Cu−O−Cu bridging angle. Compounds 1 and 2 also act as efficient catalyst precursors for the mild oxidation of cyclohexane by aqueous hydrogen peroxide to cyclohexanol and cyclohexanone (homogeneous catalytic system), leading to maximum total yields (based on cyclohexane) and turnover numbers (TONs) up to about 22% and 470, respectively.