Bimetallic Hydroformylation Catalysis
Bimetallic transition metal complexes based on a new binucleating tetratertiary-phosphine ligand system (Et2PCH2CH2)(Ph)PCH2P(Ph)(CH2CH2PEt2), eLTTP, have been prepared and characterized. The Rh(I) bimetallic complexes meso- and racemic-Rh2Cl2(CO)2(eLTTP) have been synthesized and characterized and the racemic structure characterized. The hydroformylation activity of the norbornadiene derivative, Rh2(norb)2(CO)2(eLTTP)2+(norb = norbornadiene), with respect to simple 1-alkenes and functionalized alkenes such as vinyl acetate are reported. The Rh2(eLTTP) catalyst system is unusual for several reasons: 1) excess phosphine ligand is not required to maintain catalyst stability; 2) unusually high normal/branched product aldehyde ratios are seen in the hydroformylation of 1-alkenes (18:1 normal to branched, at 120 psi and 80°C); 3) the hydroformylation catalysis is markedly faster than that seen for monometallic model system Rh(norb)(depe)+ (depe = Et2PCH2CH2PEt2) and 4) the vinyl acetate hydroformylation reaction does not sufrer from the extensive product, reactant and catalyst decomposition reactions typically seen for Rh(I)/PR3 catalyzed systems. The enhanced rate for the bimetallic verses monometallic system is believed to be due to bimetallic cooperativity in the form of intramolecular hydride transfer and elimination of the product aldehyde.