Control of chemoselectivity in hydrogenations of substituted nitro- and cyano-aromatics by cluster-derived ruthenium nanocatalysts

Catalyst precursors 1 and 2, made by ion-pairing [H₃Ru ₄(CO)₁₂]^- with NR4+ groups of functionalized MCM-41 and water-soluble poly(diallyldimethylammonium chloride), PDADMAC, respectively, have been evaluated for the chemoselective hydrogenation of nitro- and cyano-benzaldehydes. They are found to be inert toward -NO₂ and -CN groups, but active for the reduction of -CHO and >C=C< functionalities. Thus, 3, 4-nitrobenzaldehyde, nitrostyrene, and 3-cyanobenzaldehyde are hydrogenated with full selectivity to the corresponding nitrobenzylalcohols, 1-ethyl-3-nitrobenzene, and 3-cynaobenzylalcohol, respectively. No such chemoselectivity is observed either with PDADMAC-RuCl ₄ (3) or with (5%)Ru-Al₂O₃, where both the functional groups are hydrogenated. Kinetic analyses have been carried out for the hydrogenation of 4-nitrobenzaldehyde with 2. Existence of an induction time and two competitive equilibriums followed by the product-forming rate-determining step are inferred from the empirically derived rate expression. The kinetic results, structural evidences, and previous work strongly suggest that the observed chemoselectivity is probably a result of the absence of multiple crystal planes, differing in Miller indices, in the cluster-derived catalysts.