Stereochemical control of the diphosphine and alkyne ligands in triruthenium clusters: The effect of reversible CO loss/addition on the ligand distribution in [Ru3(µ3,η2-PhCCPh){µ-Ph2PCH(Me)PPh2}(CO)7,8

Abstract

Thermolysis of [Ru3{µ-Ph2PCH(Me)PPh2}(CO)10] (1) in the presence of diphenylacetylene affords the corresponding alkyne-substituted cluster [Ru3(µ3,η2-PhCCPh){µ-Ph2PCH(Me)PPh2}(CO)8] (2) and its unsaturated counterpart [Ru3(µ3,η2-PhCCPh){µ-Ph2PCH(Me)PPh2}(CO)7] (3), along with the alkyne-coupled complexes [Ru3{κ2-Ph2PCH(Me)PPh2}{µ3,η4-(CPh)4}(CO)4(µ-CO)2] (4) and [Ru2{µ-Ph2PCH(Me)PPh2}{µ,η4-(CPh)4}(CO)4] (5). The backbone-modified diphosphine in 2 has facilitated the growth of single crystals of the same and whose molecular structure was established by X-ray crystallography. The Ph2PCH(Me)PPh2 ligand bridges two ruthenium centers and occupies adjacent axial sites at one of the metallic faces. The diphosphine is situated trans to the alkyne ligand that caps the opposite metallic face. X-ray structural analysis of 3 reveals a perpendicular conformation of the alkyne relative to the metal triangle and a bridging diphosphine that coordinates the adjacent equatorial sites of the alkyne-bisected Ru–Ru bond. The kinetics for the conversion of 2 → 3 + CO under argon have been investigated over the temperature range 298–343 K in toluene. The calculated activation parameters [ΔHǂ = 13.3(3) kcal/mol; ΔSǂ = − 31(1) eu] are inconsistent with a rate-limiting step involving a dissociative loss of CO. Consistent with earlier reports on the related analog [Ru3(µ3,η2-PhCCPh)(µ-Ph2PCH2PPh2)(CO)7] by Lavigne and coworkers, the addition of CO to 3 (1 atm CO) at 258 K is rapid and affords 2 with a rate constant of 1.12(2) e−1s−1. Cluster 3 exhibits high reactivity toward 2e donors. Control experiments confirm that 3 reacts with additional PhCCPh (66 °C, 1 h) to give the diruthenium derivative 5 and that 4 also gives 5 in refluxing THF. The reaction of 3 with hydrogen gas (1 atm, 25 °C, 20 min) yields the dihydride [H2Ru3(µ3,η2-PhCCPh){µ-Ph2PCH(Me)PPh2}(CO)7] (6) in 85% yield. Clusters 2−6 were characterized by IR, 1H, and 31P NMR spectroscopies, and the solid-state structures of 2−5 were established by single-crystal X-ray crystallography. The bonding in clusters 2 and 3 and the thermodynamics for the equilibrium involving 2 ⇆ 3 + CO have been evaluated by electronic structure calculations.