The importance of cluster fragmentation in the catalytic hydrogenation of phenylacetylene by PtRu₅ carbonyl cluster complexes

PtRu₅ (CO)₁₃ (PBu₃^t) (μ₃ - PhC₂ H) (μ₅ - C) (2) has been shown to be a catalyst precursor for the hydrogenation of PhC₂H to styrene and ethylbenzene. Three new organometallic products have been found in the catalyst solutions. These are Ru₅ (CO)₁₂ (μ₅ - C) [PtPBu₃^t] (μ₃ - PhC₂ H) (μ - H)₂ (4), Pt (PBu₃^t) (PhC₂ H) (CO) (5), and Ru₅(CO)₁₁(μ₄-CCHCPh)(μ₄-HC₂Ph)(μ₃-HC₂Ph) (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 PhC₂H 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 PBu₃^t ligand, one CO ligand and one μ₂-PhC₂H ligand. Compound 5 can be obtained directly from Pt (PBu₃^t)₂ by reaction with PhC₂H under an atmosphere of CO. Compound 6 was obtained from the reaction of Ru₅(CO)₁₅(μ₅-C) with PhC₂H in the presence of UV-Vis irradiation. Compound 6 contains three equivalents of PhC₂H; one is present as triply bridging PhC₂H 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.