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][Ru<INF>3</INF>(μ-Cl)(μ-PhCCPh)(CO)<INF>9</INF>] (<BO>1</BO>) 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, Ru<INF>3</INF>(μ-PhCCPh)(CO)<INF>7</INF>(dppm) (<BO>2</BO>), or its 48-e CO adduct, Ru<INF>3</INF>(μ-PhCCPh)(CO)<INF>8</INF>(dppm) (<BO>3</BO>). Whereas the CO-induced conversion of <BO>2</BO> into <BO>3</BO> 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 <BO>2</BO> is reported, revealing a perpendicular conformation of the alkyne relative to the metal triangle. The high reactivity of <BO>2</BO> 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, Ru<INF>3</INF>(μ-PhCCPh)(CO)<INF>6</INF>(dppm)<INF>2</INF> (<BO>4</BO>) (53% yield). Reaction of <BO>2</BO> with hydrogen gas (1 atm, 25 °C, 10 min) yields the dihydrido species, Ru<INF>3</INF>(μ-H)<INF>2</INF>(μ-PhCCPh)(CO)<INF>7</INF>(dppm) (<BO>5</BO>) (89% yield) existing as a mixture of two isomers differing in the orientation of the alkyne relative to the edge-bridging dppm ligand. Complex <BO>2</BO> reacts with a terminal alkyne like phenylacetylene under mild conditions to afford a mixture of the “fly-over” type compound Ru<INF>3</INF>{μ-HCC(Ph)C(O)(Ph)CCPh}(dppm)(CO)<INF>6</INF> (<BO>6</BO>) (57% yield) and the diruthenacyclopentadiene derivative Ru<INF>2</INF>{μ-HCC(Ph)(Ph)CCPh}(μ-dppm)(CO)<INF>4</INF> (<BO>7</BO>) (20% yield). The structure of <BO>6</BO> reveals the occurrence of a disymmetric edge-bridging dialkenyl ketone ligand HC&dbd;C(Ph)C(O)(Ph)C&dbd;CPh, resulting from regioselective coupling between the two alkynes and a carbonyl group. The formal unsaturation of <BO>6</BO> 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 <BO>6</BO>. 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, Ru<INF>3</INF>(μ-H){μ-CC(Ph)C(O)(Ph)CCPh}(dppm)(CO)<INF>6</INF> (<BO>8</BO>) (94% yield). The X-ray structure of <BO>8</BO> is reported. Unsuccessful attempts to release the organic moiety from the cluster core are described. The reaction of a THF solution of <BO>6</BO> with CO in a reactor [P(CO) = 10 atm, T = 80 °C] leads to the new binuclear “fly-over” species Ru<INF>2</INF>{μ-HCC(Ph)C(O)(Ph)CCPh}(CO)<INF>6</INF> (<BO>9a</BO>), 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 Ru<INF>2</INF>{μ-HCC(C<INF>3</INF>H<INF>7</INF>)C(O)(Ph)CCPh}(CO)<INF>6</INF> (<BO>9b</BO>) (resulting from the coupling between diphenylacetylene and 1-pentyne) is reported.