Immobilized rhodium hydrogenation catalysts

The showcase catalyst for olefin hydrogenations is Wilkinsonʹs complex ClRh(PPh3)3 (1). Here, the Wilkinson-type catalyst ClRh[Ph2P(CH2)3Si(OEt)3]3 (2), trans-ClRh(CO)[Ph2P(CH2)3Si(OEt)3]2 (3), and the chelate complex ClRh(PPh3)[Ph2PCH2CHOHCH2PPh2] (4) have been synthesized using the bifunctional ligands Ph2P(CH2)3Si(OEt)3 (5) and Ph2PCH2CHOHCH2PPh2 (6), and fully characterized. The dihydride complex 4(H)2 has been generated in situ and characterized with NMR. Complexes 2, 3, and 4 have been immobilized on silica, giving 2i, 3i, and 4i, respectively, which have been studied by solid-state NMR. The catalytic activities of 2i–4i for the hydrogenation of 1-dodecene (7), 2-cyclohexen-1-one (8), and 4-bromostyrene (9) are compared with those of the homogeneous analogs 2 and 3, as well as with 1. While there are no substantial differences between 1 and 2 concerning TON and TOF, 2i shows reversed activity for the three test samples: dodecene gives the lowest TOF, followed by bromostyrene and cyclohexenone. However, TON and maximal yield are the same for 1, 2, and 2i. In contrast to 1 and 2, the immobilized catalyst 2i can be recycled seven times, 4i three times. Catalysts 3 and 3i give maximal TOF for the hydrogenation of 7, followed by 8 and 9. The yields are below 100% for the homogeneous catalyst 3, but immobilization (3i) gives maximum yields for all substrates.