Tetrahydroxy-p-benzoquinone as a Source of Polydentate O-Donor Ligands. Synthesis, Crystal Structure, and Magnetic Properties of the [Cu(bpy)(dhmal)]₂ Dimer and the Two-Dimensional [{SiW₁₂O₄₀}{Cu₂(bpy)₂(H₂O)(ox)}₂]·16H₂O Inorganic−Metalorganic Hybrid

The use of tetrahydroxy-p-benzoquinone as a slow source of dihydroxymalonato and oxalato ligands led to the isolation under open-air mild reaction conditions of five different compounds, two of them prepared for the first time:  [Cu(bpy)(dhmal)]₂ (1) and [{SiW₁₂O₄₀}{Cu₂(bpy)₂(H₂O)(ox)}₂]·16H₂O (5) (bpy, 2,2‘-bipyridine; dhmal, dihydroxymalonate; ox, oxalate). A possible mechanism for the oxidation of the benzoquinone to give the croconate dianion, which undergoes further ring-opening oxidation to decompose into dihydroxymalonate and oxalate, is proposed. All compounds have been characterized by elemental analysis, thermogravimetry, infrared spectroscopy, and powder X-ray diffraction. Single-crystal X-ray diffraction, electron paramagnetic resonance, and magnetic susceptibility measurements have been performed for compounds 1 and 5. A complete band assignment of the experimental FT-IR spectra is given through comparison with the ones calculated using density functional theory (DFT). The neutral dimer 1 constitutes the first structurally characterized example of a transition metal−dhmal complex, and it contains two copper atoms bridged by two dihydroxymalonato ligands acting in a μ²-κ³O,O‘,O‘ ‘:κ¹O coordination fashion, so that an equatorial−axial Cu₂(μ²-O)₂ rhomboid core is formed. On the other hand, the inorganic−metalorganic hybrid compound 5 shows a two-dimensional arrangement of Keggin polyanions linked by one of the Cu atoms of the oxalate cationic dimers to give layers parallel to the (101̄) plane, the remaining ox−Cu−bpy fragments acting as interlamellar separators. In both cases, magnetic and EPR results are discussed with respect to the crystal structure of the compounds and, for compound 1, also with respect to DFT calculations of the exchange coupling constant.