<i>endo</i> and <i>exo</i> Coordination of Indanol: Synthesis, Isolation and Structural Characterisation of [H₃Ru₃ (<i>endo</i>-Indanol)(C₆Me₆)₂(O)]⁺ and [H₃Ru₃ (<i>exo</i>-Indanol)(C₆Me₆)₂ (O)]⁺ as Their Tetrafluoroborate Salts

The reaction of 2,3,4,7-tetrahydro-1<i>H</i>-inden-2-ol with ruthenium chloride hydrate in refluxing ethanol yields the chloro-bridged dinuclear complex [RuCl₂(indanol)]₂ (<b>1</b>). The mononuclear complex [Ru(indanol)(H2O)3]²⁺ (<b>2</b>), formed in situ from <b>1</b> in aqueous solution, reacts with the dinuclear complex [H₃Ru₂(C₆Me₆)₂]⁺ to give a trinuclear arene-ruthenium cluster as a mixture of two isomers, the cations [H₃Ru₃(<i>endo</i>-indanol)(C₆Me₆)₂(O)]⁺ (<b>3a</b>) and [H₃Ru₃(<i>exo</i>-indanol)(C₆Me₆)₂(O)]⁺ (<b>3b</b>), in a 1:1 ratio. The hydroxy function of the indanol ligand is oriented towards the μ3-oxo cap of <b>3a</b>, whereas the OH group is bent away from the metal skeleton of <b>3b</b>. These two isomers, which can easily be separated by silica-gel chromatography, were isolated and characterised as their tetrafluoroborate salts. The single-crystal X-ray structure analysis of [<b>3a</b>][BF₄] shows a strong intramolecular hydrogen bond between the μ3-oxo ligand and the hydroxyl function, which even persists in acetone solution, as demonstrated by NMR spectroscopy. On the other hand, the hydroxy function of <b>3b</b> was found to be free in the solid state as well as in solution, as shown by an X-ray crystal structure analysis and by NMR spectroscopy. The catalytic activities of the water-soluble trinuclear cations <b>3a</b> and <b>3b</b> for the hydrogenation of benzene to give cyclohexane under biphasic conditions are considerably different, the <i>exo</i> isomer <b>3b</b> being more active than the <i>endo</i> isomer <b>3a</b>.