Anisole hydrogenation with well-characterized polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters: Effects of added water and acid, plus enhanced catalytic rate, lifetime, and partial hydrogenation selectivity

Following a comprehensive look at the arene hydrogenation literature by soluble nanocluster catalysts, six key, unfulfilled goals in nanocluster arene hydrogenation catalysis are identified. To begin to address those six goals, well-characterized polyoxoanion- and tetrabutylammonium-stabilized Rh(0) nanoclusters have been synthesized by the reduction of the precisely defined precatayst [BU4N](5)Na-3[(1,5-COD)(RhP2W15Nb3O62)-P-.] with H-2 in propylene carbonate solvent. These Rh(0) nanoclusters are characterized by their stoichiometry of formation, transmission electron microscopy, and the two rate constants which characterize their mechanism of formation; previous studies in our laboratories have provided additional characterization of polyoxoanion-stabilized Rh(0) nanoclusters. Propylene carbonate solutions of the Rh(0) nanoclusters catalyze the hydrogenation of anisole (methoxybenzene) under mild conditions (22-78 degreesC, 30-40 psig H-2). Proton donors such as water or (HBF4Et2O)-Et-. are discovered to affect both nanocluster formation and nanocluster arene hydrogenation catalysis. Under identical conditions, the Rh(0) nanoclusters are 10-fold more active than a commercially available, oxide-supported 5% Rh/Al2O3 catalyst of the same average metal-particle size. A series of lifetime experiments shows that the Rh(0) nanoclusters are capable of at least 2600 total turnovers (TTO), a lifetime significantly longer than the similar to 100 TTO often seen for nanocluster arene hydrogenation catalysts, and a lifetime slightly better than the prior record of 2000 TTO for a literature nanocluster system. The present polyoxoanion-stabilized Rh(0) nanoclusters also display a record, albeit modest, 30% selectivity for the partial hydrogenation of anisole to I -methoxycyclohexene with an overall yield of up to 8% at higher temperatures. In comparison to the 5% Rh/Al2O3 catalyst, the polyoxoanion-stabilized nanoclusters yield a 4.7-fold higher maximum yield of I -methoxycyclohexene. Finally, the seven main findings of the present work are summarized, including how they address five of the aforementioned six main goals in nanocluster arene hydrogenation.