Supramolecular cluster catalysis: benzene hydrogenation catalyzed by a cationic triruthenium cluster under biphasic conditions.

Organometallic catalysis most often proceeds through a catalytic cycle that involves the coordination of the substrate, either by ligand substitution or by oxidative addition, transformation of the coordinated substrate, and liberation of the product, either by decoordination or by reductive elimination.[1] Classical examples that have been studied in great detail are the hydrogenation of olefins with Wilkinson×s catalyst[2] and the carbonylation of methanol with rhodium iodide (Monsanto Process).[3] The complete characterization of the intermediates of the latter process and the proposal of a well-established catalytic cycle represents one of the triumphs of organometallic chemistry.[4] In all these reactions, the elementary steps of the catalytic process are believed to occur within the first coordination sphere of the organometallic catalyst.[5] We now have reasons to believe that organometallic catalysts may transform a substrate without prior coordination, the interactions between both partners entirely relying on weak intermolecular contacts. Although hydrogen transfer from a catalyst molecule to a substrate via a merely hydrogen-bonded catalyst ± substrate complex has already been considered as the mechanism of ketone transfer hydrogenation reactions,[6] catalytic transformations by host ± guest interactions and molecular recognition are generally accepted only in enzymatic catalysis.[7] The water-soluble organometallic cluster cation 1 (see Scheme 1), accessible from [( -C6H6)Ru(H2O)3] with [( 6C6Me6)2Ru2( 2-H)3] in aqueous solution and isolated as the BF4 salt,[8] was found to catalyze the hydrogenation of aromatic substrates under biphasic conditions. An unusually high catalytic activity of 1 was observed for the hydrogenation of ethylbenzene. From the reaction mixture the cluster cation 2 could be isolated as the BF4 salt (Scheme 1).[9] Ru Ru