The importance of cluster fragmentation in the catalytic hydrogenation of phenylacetylene by PtRu 5 carbonyl cluster complexes

Abstract

PtRu 5 ( CO ) 13 ( PBu 3 t ) ( μ 3 - PhC 2 H ) ( μ 5 - C ) ( 2 ) has been shown to be a catalyst precursor for the hydrogenation of PhC 2H to styrene and ethylbenzene. Three new organometallic products have been found in the catalyst solutions. These are Ru 5 ( CO ) 12 ( μ 5 - C ) [ PtPBu 3 t ] ( μ 3 - PhC 2 H ) ( μ - H ) 2 ( 4 ) , Pt ( PBu 3 t ) ( PhC 2 H ) ( CO ) ( 5 ) , and Ru 5(CO) 11(μ 4-CCHCPh)(μ 4-HC 2Ph)(μ 3-HC 2Ph) ( 6). Compounds 4– 6 have been synthesized independently and structurally characterized and each one has been tested independently for its ability to produce hydrogenation of PhC 2H catalytically. Compound 4 contains an open square-pyramidal cluster of five ruthenium atoms with one platinum atom bridging an edge of the cluster. It is structurally related to 2 but contains one less CO ligand and two hydrido ligands formed by the addition of one equivalent of hydrogen to the metal cluster. It can be obtained directly from 2 by reaction with hydrogen in the presence of trimethylamine oxide. Compound 5 is a tricoordinated mononuclear platinum complex containing one PBu 3 t ligand, one CO ligand and one μ 2-PhC 2H ligand. Compound 5 can be obtained directly from Pt ( PBu 3 t ) 2 by reaction with PhC 2H under an atmosphere of CO. Compound 6 was obtained from the reaction of Ru 5(CO) 15(μ 5-C) with PhC 2H in the presence of UV–Vis irradiation. Compound 6 contains three equivalents of PhC 2H; one is present as triply bridging PhC 2H ligand; one is a quadruply bridging ligand; the third one has formed a bond to the carbido ligand in the center of the metal cluster to form a novel tetra-metallated allyl ligand. Compound 5 has the highest catalytic activity of all three compounds and is believed to be responsible for the vast majority of the catalytic hydrogenation produced from the solutions of 2. Compound 4 is transformed into 5 under the conditions of catalysis.