Synthesis of a heterometallic trinuclear cluster of ruthenium and iridium containing a perpendicularly coordinated alkyne ligand and its dynamic behavior

Heterometallic clusters containing a perpendicularly coordinated alkyne ligand, (Cp*Ru)2(Cp*Ir)(μ3–η2:η2(⊥)-PhCCR)(μ-H)2 (6a, R = Me; 6b, R = Ph), were prepared by the reaction of a trinuclear heterobimetallic tetrahydrido complex of ruthenium and iridium, (Cp*Ru)2(Cp*Ir)(μ-H)3(μ3-H) (2), with internal alkyne. X-ray diffraction studies clearly showed that the alkyne ligand was preferentially attached to the Ru–Ru bond in a perpendicular manner rather than to the Ru–Ir bond. Although no detectable changes were observed in a VT-NMR study on 6a, VT-NMR studies on 6b implied that the alkyne ligand underwent a switchback motion at the heterometallic site. The presence of the minor isomer 8b, in which the alkyne ligand was attached to the Ru–Ir bond, was also proposed by the density functional theory (DFT) calculations. The energy difference between 6b and 8b is estimated at 4.1 kcal mol−1. These results suggest that the coordination site of the alkyne ligand was determined thermodynamically.