Rotation of Coordinated Acetylide Ligands on the Triangular Surface of Trinuclear Heterometallic Clusters

Convenient and widely applicable synthetic routes to the trinuclear heterometallic acetylide complexes LMMʹ₂(CO)₈(C≡CR) have been developed. These routes involve the reaction of metal acetylides LM(CO)₃(C≡CR) (L = Cp and Cp*; M = W and Mo; R = Ph, C₅H₄F, C₅H₄OMe, ^tBu, and ⁿPr) with Os₃(CO)₁₀(NCMe)₂ and with Ru₃(CO)₁₂. For the WOs₂ derivatives prepared (1‒3), the acetylide ligand adopts an asymmetric arrangement in which the acetylide C‒C vector is coordinated to one of the W–Os bonds. For all the WRu₂ derivatives (4–9), the acetylide ligand adopts both the asymmetric (with its C–C bond orthogonal to one of the W–Ru bonds) and the symmetric arrangement (with its C–C bond orthogonal to the unique Ru–Ru bond) and undergoes rapid interconversion in solution. For the MoRu₂ derivatives (10, 11), the acetylide favors the asymmetric form in both solution and the solid state; however, when the substituent R and the ligand L are replaced by a bulky tert-butyl group and Cp* ligand, respectively (13), the symmetric form becomes the dominant species. The dynamic ¹³C NMR studies suggest that the acetylide ligand of the WOs₂ derivatives is static but, in the asymmetric MoRu₂ derivatives (10, 11), the acetylide is fluxional and undergoes migration from one Mo–Ru edge to the other. The preference of the site selectivity for the acetylide ligand has also been studied by variation of the transition-metal atoms (M and M′), the accessory ligand (L), and the substituent (R). The structures of the complexes CpWOs₂(CO)₈(C≡CPh) (1), CpWRu₂(CO)₈(C≡CPh) (4), and CpMoRu₂(CO)₈(C≡CPh) (10) have been determined by single-crystal X-ray diffraction studies. Crystal data for 1: space group P2₁/c; a = 8.332 (3) Å, b = 14.543 (4) Å, c = 17.819 (5) Å, β = 94.46 (3)°, Z = 4; final R = 0.068, R_w = 0.090, and GOF = 1.768. Crystal data for 4: space group P2₁/n; a = 12.476 (1) Å, b = 13.216 (4) Å, c = 13.395 (4) Å, β = 97.99 (2)°, Z = 4; final R = 0.029, R_w = 0.027, and GOF = 1.583. Crystal data for 10: space group P2₁/c; a = 12.770 (4) Å, b = 8.188 (4) Å, c = 21.313 (4) Å, β = 91.26 (2)°, Z = 4; final R = 0.030, R_w = 0.031, and GOF = 2.34.