Development of Novel Carbon-Carbon Bond Formation Reactions on Mixed-Metal Clusters

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Treatment of Cp^*Ru (CO)_2Cl (1, Cp^* =η5-C_5Me_5) with NaCo (CO)_4 (2) produces the metal-metal bonded heterobimetallic complex Cp^*Ru (CO)_2 (μ_2-CO) Co (CO)_3 (4). Stoichiometric head-to-tail dimerization of p-tolylacetylene at the bimetallic site of 4 proceeds to form the dinuclear metallacyclopentadiene complex Cp^* (CO) Ru{η2 : η4-(μ_2-C (Tol) CHC (Tol) CH} Co (CO)_2 in which two alkyne molecules are coupled to form a ruthenacyclopentadiene core which coordinates to the Co (CO)_2 fragment. On the other hand, catalytic head-to-head coupling of methyl propiolate in the presence of 4 yields (E)-(MeO_2C) CH=CH (C*CCO_2Me). On the other hand, reaction of 1 with 2 equiv of [PPN][Rh (CO)_4](2, PPN+=(Ph_3P)_2N+) quantitatively produced the trinuclear mixed-metal cluster (Cp^*Ru)_2RhCl(CO)_6 (5), which was selectively converted into the pentanuclear cluster (Cp^*Ru)_3Rh_2Cl(CO)_9 (6) having a Ru-Rh-Ru-Rh-Ru array. Substitution of the CO ligands in 5 with ButNC proceeded at the Ru center to give {Cp^*Ru (CNBut)}(Cp^*Ru)RhCl (CO)_5(7) or {Cp^*Ru(CNBut)}_2RhCl (CO)_4 (8) depending upon the reaction conditions, whereas the Cl ligand at the Rh center in 5 was readily displaced upon treatment with AgPF_6 to form [(Cp_*Ru)_2 {Rh(MeCN)}(CO)_6]PF_6, (9・PF_6). The molecular structures of 4, 5, 6, 7, 8, and 9・PF_6 have been determined by X-ray crystallography.