Synthesis and hydrogenation of iridium complexes with bidentate and water-soluble phosphine ligands: developing novel water-soluble catalysts for hydrogenation of unsaturates

The hydrogenation of unsaturated carbon-carbon bonds catalyzed by transition metal hydrido complexes has received considerable attention in the literature. Environmental interests have placed considerable emphasis on switching these processes into aqueous media. The dimeric complex [Ir(COD)Cl]₂ (COD = 1,5-cyclooctadiene) has been found to undergo bridge-splitting reactions with bidentate phosphine ligands to produce coordinatively unsaturated, 16 electron complexes of the form [Ir(COD)(P-P)]Cl (P-P = bidentate phosphine). Two bidentate ligands were examined: 1,2-bis(dimethylphosphino)ethane [DMPE] and 1,2-bis(diethylphosphino)ethane [DEPE]}. These compounds have been found to be both water soluble and reactive towards molecular hydrogen at room temperature, qualities that make them ideal for serving as catalysts for the aqueous hydrogenation of unsaturates.

Water-soluble ligands were also allowed to react with [Ir(COD)Cl]₂. In these cases, the product was analogous to the [Ir(COD)(PMe₃)₃]Cl complex. Two water-soluble phosphines were examined: 1,3,5-triaza-7-phosphaadamantane [PTA] and tris(hydroxy-methyl)phosphine [THP]. When these complexes were hydrogenated in water, the PTA complex underwent an oxidative addition of molecular hydrogen with the loss of COD to form the facial isomer of Ir(H)₂(PTA)₃Cl. However, the THP made both meridional and facial isomers of Ir(H)₂(THP)₃Cl upon hydrogenation. The facial was determined to be the kinetically favored product while the meridional was the thermodynamically favored product.

The complexes Ir(COD)(DMPE)Cl, Ir(COD)(DEPE)Cl, and Ir(H)₂(PTA)₃Cl were determined to be excellent aqueous hydrogenation catalysts. They catalyzed the reduction allyl alcohol to n-propanol under relative mild conditions.