Rearrangement and scission of terminal alkynes in dimolybdenum complexes on reaction with ruthenium carbonyl: Formation of trinuclear vinylidene and hexanuclear bis(alkylidyne) clusters

The reaction of the dimolybdenum alkyne complexes Mo-2(CO)(4)(mu-R(1)C=CR(2))Cp(2) (Cp = eta-C5H5; R(1) = H, R(2) = H, Me, Ph, CO(2)Me; 1a-d) with Ru-3(CO)(12) in refluxing toluene or heptane affords reasonable yields of the orange mu(3)-vinylidene clusters [Mo2Ru(mu(3)-C=CHR(2))(CO)(7)Cp(2)] (2a-d). The crystal structure of Mo2Ru(mu(3)-C=CHMe)(CO)(7)Cp(2) (2b) has been determined and shows that the metal triangle is capped by a vinylidene ligand which is formally sigma-bound to the two molybdenum atoms and pi-bound to the ruthenium. Low to moderate yields of the blue-turquoise hexanuclear bis(alkylidyne) clusters Mo2Ru4(mu(3)-CR(1))(mu(3)-CR(2))(CO)(12)Cp(2) (3a-d) are formed in addition to the vinylidene clusters, especially if the reaction is carried out in heptane. The synthesis of 3a is particularly noteworthy, as it represents the first example of the scission of ethyne into two methylidyne fragments on a metal cluster. The disubstituted alkyne complex 1e (R(1) = R(2) = Me) leads to the corresponding hexanuclear cluster 3e in either solvent, but complexes of bulkier disubstituted alkynes 1f ang 1g (R(1) = R(2) = Et, CO(2)Me) do not give analogous products. The X-ray crystal structure of the cluster Mo2Ru4(mu(3)-CMe)(2)(CO)(12)Cp(2) . CH2Cl2 (3e . CH2Cl2) reveals an octahedral metal core in which the two MoCp fragments occupy adjacent positions, with the two alkylidyne groups formed by scission of the alkyne ligand capping the two Mo2Ru faces. Unusually for octahedral clusters, complexes of type 3 have 84 cluster valence electrons.