Self-Activation of a Cluster-Bound Alkyne toward Carbon−Carbon Bond Forming Reactions

The methanol-catalyzed elimination of Cl^- from the “activated” anionic species [PPN][Ru₃(μ-Cl)(μ-PhCCPh)(CO)₉] (1) in the presence of bis(diphenylphosphino)methane (dppm) constitutes a rational high-yield route (>90%) to either the unique unsaturated 46-e (alkyne)triruthenium cluster, Ru₃(μ-PhCCPh)(CO)₇(dppm) (2), or its 48-e CO adduct, Ru₃(μ-PhCCPh)(CO)₈(dppm) (3). Whereas the CO-induced conversion of 2 into 3 is complete within few seconds at 25 °C under 1 atm of CO, the reverse transformation takes 1 h at 80 °C. The X-ray structure analysis of 2 is reported, revealing a perpendicular conformation of the alkyne relative to the metal triangle. The high reactivity of 2 is substantiated by a high chemical reactivity toward 2-e donors. Its reaction with 1 equiv of dppm (25 °C, 3 h) leads to the bis-dppm-substituted complex, Ru₃(μ-PhCCPh)(CO)₆(dppm)₂ (4) (53% yield). Reaction of 2 with hydrogen gas (1 atm, 25 °C, 10 min) yields the dihydrido species, Ru₃(μ-H)₂(μ-PhCCPh)(CO)₇(dppm) (5) (89% yield) existing as a mixture of two isomers differing in the orientation of the alkyne relative to the edge-bridging dppm ligand. Complex 2 reacts with a terminal alkyne like phenylacetylene under mild conditions to afford a mixture of the “fly-over” type compound Ru₃{μ-HCC(Ph)C(O)(Ph)CCPh}(dppm)(CO)₆ (6) (57% yield) and the diruthenacyclopentadiene derivative Ru₂{μ-HCC(Ph)(Ph)CCPh}(μ-dppm)(CO)₄ (7) (20% yield). The structure of 6 reveals the occurrence of a disymmetric edge-bridging dialkenyl ketone ligand HCC(Ph)C(O)(Ph)CCPh, resulting from regioselective coupling between the two alkynes and a carbonyl group. The formal unsaturation of 6 is masked by a weak interaction between the terminal C−Ph bond of the organic chain and one of the metal centers. Facile loss of this interaction is induced by mild thermolysis of 6. As a consequence, free rotation of the organic moiety around the metal−metal edge brings the opposite end of the organic chain (i.e., the C−H bond) close to the opposite face, thereby favoring CH activation to convert the alkenyl end into a vinylidene. This leads to quantitative formation of the vinylidene alkenyl ketone derivative, Ru₃(μ-H){μ-CC(Ph)C(O)(Ph)CCPh}(dppm)(CO)₆ (8) (94% yield). The X-ray structure of 8 is reported. Unsuccessful attempts to release the organic moiety from the cluster core are described. The reaction of a THF solution of 6 with CO in a reactor [P(CO) = 10 atm, T = 80 °C] leads to the new binuclear “fly-over” species Ru₂{μ-HCC(Ph)C(O)(Ph)CCPh}(CO)₆ (9a), thereby indicating that elimination of the edge-bridging dppm and cluster fragmentation are more favorable than elimination of a free ketone from the intact cluster. The X-ray structure of Ru₂{μ-HCC(C₃H₇)C(O)(Ph)CCPh}(CO)₆ (9b) (resulting from the coupling between diphenylacetylene and 1-pentyne) is reported.