The solubilization of transition-metal complexes into aqueous media for use in biphasic catalytic systems

Abstract

The water-soluble phosphine complex of ruthenium(II), RuCl2(PTA)4, has been prepared by the reduction of RuCl3 in ethanol in the presence of the air-stable phosphine 1,3,5-triaza-7-phosphaadamantane. RuCl2(PTA)4 is an effective catalyst for the regioselective conversion of unsaturated aldehydes to unsaturated alcohols using a biphasic aqueous/organic medium with sodium formate as teh source of hydrogen, thus affording facile catalyst recovery and recycle. Both aromatic and aliphatic aldehydes were reduced to the corresponding alcohols. The formate ion was shown to directly be the hydrogen source by deuterium labelling experiments. The rate of hydrogenation of benzaldehyde was found to be first-order in catalyst concentration, first-order in substrate concentration, and independent of formate concentrations >2.5M. The reduction process was greatly retarded by the presence of excess phosphine ligand. The apparant activation energy determined was 23.9 kcal-mol. A catalytic cycle was proposed which involves the rapid formation of a ruthenium hydride formate complex followed by phosphine dissociation and aldehyde addition in a slower step. Subsequent formation of a metal alkoxide intermediate and protoantion completes the cycle. Recycling experiments demonstrated the catalyst to be quite robust. A series of 1,3,5-triaza-7-phosphaadamantane complexes of group VI metal carbonyl complexes were also prepared. Reaction of M(CO)4(NHC5H10)2 with PTA in dichloromethane yields the complexes of group VI metal carbonyl complexes were also prepared. Reaction of M(CO)4(PTA)2(M=W, Mo), while the reaction of W(CO)3(C5H5N)3 with PTA in THF yields W(CO)3(PTA)3. The following complexes were characterized by X-ray crystallography: RuCl2(PTA)4, RuCl2(PTA)2(PTAH)2*2Cl, RuCl4(PTAH)2*Cl, W(CO)4(PTA)2, Mo(CO)3(PTAH)3*3Cl, 1,3,5-triaza-7-phosphaadamentane oxide.