Hydrogenation of Trimetallic Clusters of the Iron Triad Containing Bridging Carbyne Ligands. Reductive Cleavage of a Triply Bridging Carbyne

The clusters HM³(μ-COMe)(CO)₁₀ (1) (a, M = Fe; b, M = Ru; c, M = Os), prepared by methylation of the corresponding HM₃(μ-CO)(CO)₁₀ monoanion, react with hydrogen to give H₃M₃(μ₃-COMe)(CO)₉ (2). This process may be reversed under carbon monoxide. For these and other related carbyne-containing clusters the relative stabilities of HM₃(μ-CX)(CO)_10-nL_n and H₃M₃(μ₃-CX)(CO_9-nL_n are found to depend upon (i) the metal, (ii) the carbyne substituent X, and (iii) the ligands L. Thus, although 2a is unstable under ambient conditions, reverting nearly quantitatively to la, the substituted derivative H₃Fe₃(μ₃-COMe)(CO)₇(SbPH₃)₂ (5a) can be isolated in fair yield by hydrogenation of la in the presence of triphenylantimony. Similarly, although HRu₃(μ-CN(Me)CH₂Ph)(CO)₁₀ (4) does not react with hydrogen to give a stable product, in the presence of 4 equiv of triphenylantimony H₃Ru₃(μ₃-CN(Me)CH₂Ph)(CO)₆-(SbPH₃)₃ (6) can be prepared in good yield. These observations are rationalized in terms of the relative importance of M₃-CX and C-X π-bonding interactions and the relative metal-carbonyl and metal-hydrogen bond strengths. Under more severe conditions reductive cleavage of the carbyne ligand as CH₃X can be achieved. At 130 °C and 3.5 MPa of 1:1 carbon monoxide-hydrogen, 2b decomposes to dimethyl ether and Ru₃(CO)₁₂. This process represents overall reduction of a carbonyl ligand by molecular hydrogen and is made possible by the activation of this carbonyl by methylation. The trends observed for cluster hydrogenation may have implications for potential Lewis acid activation of cluster-bound carbonyl ligands.