Hydroformylation of styrene and 1-octene catalyzed by binuclear and oligomer rhodium(I) complexes containing the bis- p-phosphinito ligands [( p-Ph 2POC 6H 4) 2X] (X=O, CMe 2, S)

Abstract

The reactions of rhodium(I) substrates with the ligands [( p-Ph 2POC 6H 4) 2X] (X=O, 1; X=CMe 2, 2; X=S, 3) have been studied with the aim to obtain rhodium(I) complexes to utilize as precatalysts in hydroformylation of olefins. The reactions of [Rh(CO) 2Cl] 2 with ligands 1–3, in a Rh to ligand 1:1 molar ratio, in toluene, at room temperature afford the products [Rh(CO)(Cl)(μ-L)] 2 (L= 1–3). The reactions of [Rh(acac)(CO) 2] with 2 are strongly dependent on the ligand nature and experimental conditions. The product formed in a 1:0.5 Rh to ligand molar ratio is dimer with the metal centers held together by the ligand, while the product formed in a 1:1 molar ratio is oligomer. The in situ catalytic systems formed either by [Rh(CO) 2Cl] 2 and ligands 1–3 or by [Rh(acac)(CO) 2] and 2, at variable ligand-to-metal molar ratio, have been employed in the hydroformylation of styrene and 1-octene. Almost quantitative conversion of styrene was achieved with the catalytic system formed by [Rh(acac)(CO) 2] and 2, in a 1:0.5 molar ratio, operating at 60°C and 40 atm. The chemoselectivity of the reaction was very high being the linear (L) and the branched (B) aldehydes about 99.9% of the reaction products. The terminal aliphatic olefin 1-octene was hydroformylated with lower conversion in the aldehydes with respect to styrene and isomerization and hydrogenation of the double bond occurred in great extent. Lowering the temperature, the conversion of 1-octene increases but the chemoselectivity drastically decreases owing to the formation of more isomerization products. The results have been explained considering both the flexibility and the number of phosphorus atoms (considering each of them as a monodentate ligand) coordinated to each metal center in the trigonal bipyramidal hydridorhodium intermediates formed, namely {[Rh(H)(CO) 3] 2(μ- 2)} (1:0.5 [Rh(acac)(CO) 2] to 2 molar ratio) and the oligomer [Rh(H)(CO) 2(μ- 2)]n (1:1 [Rh(acac)(CO) 2] to 2 molar ratio). In the presence of an excess of ligand 2 the conversion in the aldehydes drastically decreases owing to the rigidity of the intermediate hydridorhodium oligomer species formed.