Olefin hydrogenation using diimine pyridine complexes of Co and Rh
Q. Knijnenburg, A.D. Horton, H. van der Heijden, T.M. Kooistra, D.G.H. Hetterscheid, J.M.M. Smits, B. de Bruin, P.H.M. Budzelaar, A.W. Gal
JOURNAL OF MOLECULAR CATALYSIS A-CHEMICAL 2005, 232, 1-2, 151-159

Abstract:
Square-planar cobalt diimine pyridine complexes LCoR (L=2,6-[RN=CMe](2)C5H3N; R=n-C6H13 for L h,,, 2,6-(i-Pr)(2)C6H3 for L dip) are active in the hydrogenation of monosubstituted and disubstituted olefins; sterically more hindered trisubstituted olefins do not react. For the (LCo)-Co-dip system, a diamagnetic hydride intermediate was observed, although a small amount of paramagnetic product is also formed upon reaction of (LCoR)-Co-dip with H-2. DFT studies suggest a traditional hydrogenation cycle starting with LCoH, except that intermediate LCo(R)(H-2) transfers a hydrogen atom directly from H-2 to the alkyl group in a sigma-bond metathesis step, without going through a discrete Co-III. intermediate. Autoclave experiments show that conversion is not linear in catalyst intake. Diffusion limitation was ruled out as an explanation, and we propose a concentration-dependent catalyst decay. At low catalyst intake conversion rates up to 2 x 104 (mol octene/mol Colbar/h) can be reached. Reducing the steric bulk at the imine positions (L-dip -> L-hex), or changing the metal from cobalt to rhodium, do not alter the activity or specificity of the hydrogenation much. For the (LCo)-Co-hex and (LRh)-Rh-dip systems, no diamagnetic products corresponding to (LCoH)-Co-dip were observed.