Synthesis and characterisation of phosphido-bridged hexaosmium "raft" clusters; crystal structure of OS₆H₂(CO)₁₈(μ₃-PPh)

Treatment of the activated "raft" cluster Os₆(CO)₂₀(NCMe) with PR¹R²H (R¹R²Ph; R¹Ph, R²H) at room temperature affords the phosphine-substituted cluster Os₆(CO)₂₀(PR¹R²H) (R¹R²Ph (1); R¹Ph, R²H (1)) in quantitative yield. Thermolysis of 1 in toluene gives a single orange-brown product characterised as Os₆(CO)₁₆(μ-PPhO₂) (3). This complex is also obtained from the thermolysis of Os₆(CO)₁₇(PPh₂H), which has the bicapped tetrahedral metal framework. Thermolysis of 2 gives two products, characterised as Os₆H₂(CO)₁₈(μ₃-PPh) (4) and the known complex Os₅(CO)₁₅(μ₄-PPh) (5). The structure of 4 has been established by a single-crystal X-ray diffraction study. The metal framework consists of a central Os₃ triangle each side of which is bridged by another Os atom to give an arrangement of four triangles sharing three common edges. Three of these triangles are approximately coplanar, but the fourth, which is asymmetrically capped by the phosphorus atom of the μ₃-PPh group, is tilted by 72.4° from the Os₅ plane. Complex 4 crystallises in space group P 1 - with a 9.733(2), b 10.555(1), c 17.585(2) Å, α 95.87(1), β 94.47(2), γ 105.60(1)°, Z = 2; 4591 observed data with F > 4σ(F) were refined by blocked full-matrix least squares to R = 0.080, R_w = 0.079.