Triruthenium–iridium clusters containing alkyne ligands : synthesis, structure, and catalytic implications of [(µ-H)IrRu₃(CO)₁₁(µ₃-η²-PhC≡CPh)] and [IrRu₃(CO)₁₀(µ₄-η²-PhC≡CPh)(µ-η²-PhC=CHPh)]

The mixed-metal cluster [HIrRu</sub>3</sub>(CO)₁₃] <b>1</b> reacts with one equivalent of disubstituted alkynes RC≡CR to give [HIrRu₃(CO)₁₁(µ₃-η2-RC≡CR)] (R = Ph <b>2</b>; R = Me <b>3</b>), with a second equivalent of the alkyne the clusters [IrRu₃(CO)₁₀(µ₄-η2-RC≡CR)(µ-η2-RC=CHR)] (R = Ph <b>4</b>; R = Me <b>5</b>) are obtained. The single-crystal X-ray structure analyses of <b>2</b> and <b>3</b> show these clusters to have a tetrahedral Ru3Ir framework containing the alkyne ligand coordinated in a parallel fashion over the Ru₃ face of the metal skeleton. In contrast, the clusters <b>4</b> and <b>5</b> consist of a butterfly arrangement of the Ru₃Ir framework with the alkyne ligand coordinated to all four metal atoms, giving an overall octahedral Ru₃IrC₂ skeleton, as demonstrated by the single-crystal structure analysis of <b>4</b>. Cluster <b>1</b> is an excellent catalyst for the hydrogenation of diphenylacetylene to give stilbene (catalytic turnover number 990 within 15 min), clusters <b>2</b> and <b>4</b> are also catalytically active but seem to represent side-channels of the catalytic cycle.