Hydrogenation of trans-Cinnamaldehyde with Hydrido-Carbonyl Osmium(II) Complexes of Chelating Phosphine Ligands

A series of new hydridocarhonyl osmium(II) complexes, OsHCl(CO)(PPh₃)(L-L)[L-L=Ph₂P(CH₂) _nPPh₂ (n=1 (1), 2 (2), 3 (3), cis-Ph₂PCH=CHPPh₂ (4), and Fe(η⁵-C₅H₄PPh₂)₂ (5)] has been synthesized from OsHCl(CO)(PPh₃)₃ and chelating diphosphines. These complexes have been characterized by IR. ¹H NMR and elemental analysis. The catalytic activities of these complexes both for the transfer hydrogenation of trans-cinnamaldehyde with 2-propanol as the hydrogen donor, and for the selective hydrogenation of trans-cinnamaldehyde with H₂, have been examined. Complexes (1)-(5) were shown to have higher selectivities for the transfer hydrogenation of the C=O bond of aldehyde than for the transfer hydrogenation of the C=C bond of aldehyde. The selectivities for the transfer hydrogenation with 2-propanol as well as for the hydrogenation with H₂ have been found to decrease in the order 3>5>2>4>1. Complex (3) has shown to possess almost 90% of the selectivity to cinnamyl alcohol for transfer hydrogenation. It is also found that there is a correlation between the v(CO) of each complex and the hydrogenation, of the C=O bond of trans-cinnamaldehyde. Overall, the selectivities with the complexes (1)-(5) are greater for the transfer hydrogenation with 2-propanol than for the hydrogenation with H₂.