Carbonylation of 1-hexene catalyzed by cis-[Rh(CO)2(amine)2](PF6) complexes in aqueous tetrabutylammonium hydrogensulfate solutions

In aqueous solution of tetrabutylammonium hydrogensulfate (N(C4H9)4HSO4), rhodium complexes like cis-[Rh(CO)2(amine)2](PF6) (amine = pyridine, 4-picoline, 3-picoline, 2-picoline, 3,5-lutidine or 2,6-lutidine) promote the carbonylation of 1-hexene to heptanoic acid and heptanal under carbon monoxide atmosphere. Gaseous by-products (H2 and CO2) from the catalysis of the water–gas shift reaction (WGSR) are also observed. The catalytic activities for heptanoic acid and heptanal production depend on the nature of the coordinate amine to the rhodium center for [Rh] = 1 × 10−4 mol, [1-hexene] = 0.05 mol, 40 mL of water (2.2 mol); 2.4 g of N(C4H9)4HSO4 (5.6 wt.%), S/C = 500, P(CO) = 22 atm at 150 °C. Analyses of kinetic results for the Rh/4-picoline system (one of the more active and the most stable catalyst among tested) towards the carbonylation reaction for the organic products formation show a nonlinear dependence on total rhodium concentration and on N(C4H9)4HSO4 amount in the range of studies. The last result suggested that the salty medium stabilize the ionic Rh catalytic species formed under the reaction conditions, therefore enhancing the reactivity. The increase in P(CO) is accompanied by improvement in the catalytic activities of oxygenated products, then reaches a maximum and starts decreasing at higher P(CO). These data are discussed in terms of catalytic cycles bearing a common Rh–H catalytic species.