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Rearrangement and Scission of Terminal Alkynes in Dimolybdenum Complexes on Reaction with Ruthenium Carbonyl:  Formation of Trinuclear Vinylidene and Hexanuclear Bis(alkylidyne) Clusters

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posted on 01.10.1996 by Harry Adams, Louise J. Gill, Michael J. Morris
The reaction of the dimolybdenum alkyne complexes Mo2(CO)4(μ-R1C⋮CR2)Cp2 (Cp = η-C5H5; R1= H, R2 = H, Me, Ph, CO2Me; 1ad) with Ru3(CO)12 in refluxing toluene or heptane affords reasonable yields of the orange μ3-vinylidene clusters [Mo2Ru(μ3-CCHR2)(CO)7Cp2] (2ad). The crystal structure of Mo2Ru(μ3-CCHMe)(CO)7Cp2 (2b) has been determined and shows that the metal triangle is capped by a vinylidene ligand which is formally σ-bound to the two molybdenum atoms and π-bound to the ruthenium. Low to moderate yields of the blue-turquoise hexanuclear bis(alkylidyne) clusters Mo2Ru43-CR1)(μ3-CR2)(CO)12Cp2 (3ad) 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 (R1 = R2 = Me) leads to the corresponding hexanuclear cluster 3e in either solvent, but complexes of bulkier disubstituted alkynes 1f and 1g (R1 = R2 = Et, CO2Me) do not give analogous products. The X-ray crystal structure of the cluster Mo2Ru43-CMe)2(CO)12Cp2·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.