Isolation and single-crystal X-ray structure analysis of the catalyst–substrate host–guest complexes [C₆H₆⊂H₃Ru₃{C₆H₅(CH₂) _<i>n</i>OH}(C₆Me₆)₂(O)]⁺ (<i>n</i> = 2, 3)

The trinuclear arene-ruthenium cluster cations [H₃Ru₃{C₆H₅ (CH₂)_<i>n</i>OH}(C₆Me₆)₂ (O)]⁺ (<b>3</b>: <i>n</i>=2, <b>4</b>: <i>n</i>=3) have been synthesised from the dinuclear precursor [H₃Ru₂ (C₆Me₆)₂]⁺ and the mononuclear complexes [{C₆H₅(CH₂)_<i>n</i>OH}Ru(H₂O)₃]²⁺ in aqueous solution, isolated and characterised as the hexafluorophosphate or tetrafluoroborate salts. Both <b>3</b> and <b>4</b> are derivatives of the parent cluster cation [H₃Ru₃ (C₆H₆)(C₆Me₆)₂(O)]⁺ (<b>1</b>) which was found to catalyse the hydrogenation of benzene to give cyclohexane under biphasic conditions. The mechanism postulated for this catalytic reaction (‘supramolecular cluster catalysis’), involving the hydrophobic pocket spanned by the three arene ligands in 1, was based on the assumption that the substrate molecule benzene is hosted inside the hydrophobic pocket of the cluster molecule to form a catalyst–substrate host–guest complex in which the hydrogenation of the substrate takes place. With the analogous cluster cations <b>3</b> and <b>4</b>, containing a (CH₂)_<i>n</i>OH side-arm (<i>n</i>=2, 3) as substituent at the benzene ligand, it was possible to isolate the cationic host–guest complexes as the hexafluorophosphate or tetrafluoroborate salts. The single-crystal X-ray structure analyses of [C₆H₆⊂<b>3</b>][PF₆] and [C₆H₆⊂<b>4</b>][BF₄], compared to that of [<b>3</b>][PF₆] show that the substrate molecule benzene is indeed held inside the hydrophobic pocket of <b>3</b> and <b>4</b>, the angle between the metal (Ru₃) plane and the aromatic plane being 67° and 89°, respectively.