Tetrahydroxy-<i>p</i>-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-<i>p</i>-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 (<b>1</b>) and [{SiW₁₂O₄₀}{Cu₂(bpy)₂(H₂O)(ox)}₂]·16H₂O (<b>5</b>) (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 <b>1</b> and <b>5</b>. 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 <b>1</b> 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 <i>equatorial</i><i>−</i><i>axial</i> Cu₂(μ²-O)₂ rhomboid core is formed. On the other hand, the inorganic−metalorganic hybrid compound <b>5</b> 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 <b>1</b>, also with respect to DFT calculations of the exchange coupling constant.