Unligated diruthenium(II,II) tetra(trifluoroacetate): The first X-ray structural study, thermal compressibility, Lewis acidity, and magnetism

The title compound, [Ru₂(O₂CCF₃) ₄] (1), has been obtained without any exogenous ligands and crystallized by deposition from the gas phase at 170°C. Its crystal structure has been determined for the first time to confirm an infinite chain motif built on axial Ru⋯O interactions of the diruthenium(II,II) units. The X-ray diffraction studies at variable temperatures showed no phase transitions in the range of 295-100 K but revealed a significant decrease in the volume per atom from 14.2 to 13.3 ų. This noticeable thermal compressibility effect is discussed in connection with the solid-state packing of the [Ru₂(O₂CCF₃)₄] _∞ chains. The highly electrophilic character of the diruthenium(II,II) units has been shown by the gas-phase deposition reaction of [Ru₂(O₂CCF₃)₄] with an aromatic donor substrate, namely [2.2]paracyclophane (C₁₆H₁₆). As a result of the above reaction, a new arene adduct [Ru₂(O ₂CCF₃)₄·C₁₆H₁₆] (2) has been isolated in crystalline form. It has an extended one-dimensional (1D) chain structure comprised of alternating building units and based on the rare bridging mode of [2.2]paracyclophane, [Ru₂(O₂CCF ₃)₄·(μ₂-η²: η²-C₁₆H₁₆)]_∞. The magnetic susceptibility of 1 and 2 has been measured and compared in the range of 1.8-300 K. In addition, in the course of synthesis of 1 by the carboxylate exchange reactions, a new mixed-carboxylate diruthenium(II,II) core complex [Ru₂(O₂CCF₃)₃(O₂CC ₂H₅)] (3), bearing no exogenous ligands, has also been isolated and structurally characterized. It exhibits an interesting polymeric structure in which the ruthenium(II) centers selectively form axial interdimer contacts with the O-atoms of the propionate groups only.