Unligated Diruthenium(II,II) Tetra(trifluoroacetate):  The First X-ray Structural Study, Thermal Compressibility, Lewis Acidity, and Magnetism

The title compound, [Ru<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>4</sub>] (<b>1</b>), 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 Å<sup>3</sup>. This noticeable thermal compressibility effect is discussed in connection with the solid-state packing of the [Ru<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>4</sub>]<sub>∞</sub> chains. The highly electrophilic character of the diruthenium(II,II) units has been shown by the gas-phase deposition reaction of [Ru<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>4</sub>] with an aromatic donor substrate, namely [2.2]paracyclophane (C<sub>16</sub>H<sub>16</sub>). As a result of the above reaction, a new arene adduct [Ru<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>4</sub>·C<sub>16</sub>H<sub>16</sub>] (<b>2</b>) 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, [Ru<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>4</sub>·(μ<sub>2</sub>-η<sup>2</sup>:η<sup>2</sup>-C<sub>16</sub>H<sub>16</sub>)]<sub>∞</sub>. The magnetic susceptibility of <b>1</b> and <b>2</b> has been measured and compared in the range of 1.8−300 K. In addition, in the course of synthesis of <b>1</b> by the carboxylate exchange reactions, a new mixed-carboxylate diruthenium(II,II) core complex [Ru<sub>2</sub>(O<sub>2</sub>CCF<sub>3</sub>)<sub>3</sub>(O<sub>2</sub>CC<sub>2</sub>H<sub>5</sub>)] (<b>3</b>), 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.