Isolation and single-crystal X-ray structure analysis of the catalyst–substrate host–guest complexes [C 6H 6⊂H 3Ru 3{C 6H 5(CH 2) nOH}(C 6Me 6) 2(O)] + ( n=2, 3)

Abstract

The trinuclear arene-ruthenium cluster cations [H 3Ru 3{C 6H 5(CH 2) n OH}(C 6Me 6) 2(O)] + ( 3: n=2, 4: n=3) have been synthesised from the dinuclear precursor [H 3Ru 2(C 6Me 6) 2] + and the mononuclear complexes [{C 6H 5(CH 2) n OH}Ru(H 2O) 3] 2+ in aqueous solution, isolated and characterised as the hexafluorophosphate or tetrafluoroborate salts. Both 3 and 4 are derivatives of the parent cluster cation [H 3Ru 3(C 6H 6)(C 6Me 6) 2(O)] + ( 1) 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 3 and 4, containing a (CH 2) n OH side-arm ( n=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 [C 6H 6⊂ 3][PF 6] and [C 6H 6⊂ 4][BF 4], compared to that of [ 3][PF 6] show that the substrate molecule benzene is indeed held inside the hydrophobic pocket of 3 and 4, the angle between the metal (Ru 3) plane and the aromatic plane being 67° and 89°, respectively.