Synthesis of a trinuclear cation [H₃Ru₃(Fc-arene)(C₆Me₆)₂(O)]⁺ containing a ferrocenyl group tethered to an arene ligand

Ferrocene carboxylic acid 2-cyclohexa-1,4-dienyl-ethyl ester (<b>1</b>) is prepared from ferrocene carboxylic acid and 2-cyclohexa-1,4-dienyl-ethanol. This diene reacts with RuCl₃•<i>n</i>H₂O in refluxing ethanol to afford quantitatively [Ru(Fc-arene)Cl₂]₂ (<b>2</b>) (Fc-arene=ferrocene carboxylic acid phenethyl ester). The dinuclear complex <b>2</b> reacts with triphenylphosphine to give the mononuclear complex [Ru(Fc-arene)(PPh₃)Cl₂] (<b>3</b>). The trinuclear arene-ruthenium cluster cation [H₃Ru₃ (Fc-arene)(C₆Me₆)₂(O)]⁺ (<b>4</b>) is synthesised from the dinuclear precursor [H₃Ru₂ (C₆Me₆)₂]⁺ and the mononuclear complex [Ru(Fc-arene)(H₂O)₃]²⁺, accessible from <b>2</b> in aqueous solution. The water-soluble trinuclear cluster cation <b>4</b> catalyses the hydrogenation of benzene to give cyclohexane under biphasic conditions.