From Allenes to Edge-Bridging Allyl Ligands or Face-Capping Alkenyl Ligands on a Triruthenium Hydrido Carbonyl Cluster: An Experimental and DFT Computational Study

Terminal allenes, H2CCCR1R2 (R1 = R2 = Me; R1 = SiMe3, R2 = Me; R1 = CO2Et, R2 = H;
R1 = Cy, R2 = H), react with the hydrazido-capped
hydrido carbonyl triruthenium complex [Ru3(μ-H)(μ3-κ2-HNNMe2)(CO)9] (1) in THF at reflux temperature to give the allyl-bridged
derivatives [Ru3(μ-κ3-H2CCHCR1R2)(μ3-κ2-HNNMe2)(μ-CO)2(CO)6], as
mixtures of syn-allyl and anti-allyl
isomers when R1 ≠ R2. These reactions
also produce a small amount of an alkenyl-capped derivative, [Ru3(μ3-κ2-HNNMe2)(μ3-κ2-MeCCHR1)(μ-CO)2(CO)6], when monosubstituted
allenes (R2 = H) are used. Density functional theory calculations
have shown that the reactions that give edge-bridging allyl or face-capping
alkenyl derivatives from compound 1 and allene are multistep
processes. In both cases, the first step, a bimolecular allene for
CO substitution, is rate-limiting. The way the allene approaches the
cluster complex determines the formation of an allyl or alkenyl product,
since this fact decides the final destiny of the hydride ligand, which
can be transferred to the central C atom of the allene to give an
allyl product or to the terminal CH2 fragment of the allene
to give an alkenyl product.