Edge-bridging and face-capping coordination of alkenyl ligands in triruthenium carbonyl cluster complexes derived from hydrazines:: Synthetic, structural, theoretical, and kinetic studies

The reactions of the triruthenium cluster complex [Ru-3( mu-H)(mu(3)-eta(2)-HNNMe2)(CO)(9)] (1; H2NNMe2=1,1-dimethylhydrazine) with alkynes (PhCdropCPh, HCdropCH, MeO(2)CCdropCCO(2)Me, PhCdropCH, MeO(2)CCdropCH, HOMe(2)CCdropCH, 2-pyCdropCH) give tri-nuclear complexes containing edge-bridging and/or face-capping alkenyl ligands. Whereas the edge-bridged products are closed triangular species (three Ru-Ru bonds), the face-capped products are open derivatives (two Ru-Ru bonds). For terminal alkynes, products containing gem (RCCH2) and/or trans (RHCCH) alkenyl ligands have been identified in both edge-bridging and face-capping positions, except for the complex [Ru-3(mu(3)-eta(2)-HNNMe2)(mu(3)-eta(3)- HCCH-2-py)(mu-CO)(CO)(7)], which has the two alkenyl H atoms in a cis arrangement. Under comparable reaction conditions (1:1 molar ratio, THF at reflux, time required for the consumption of complex 1), some reactions give a single product, but most give mixtures of isomers (not all the possible ones), which were separated. To determine the effect of the hydrazido ligand, the reactions of [Ru-3( mu-H)(mu(3)-eta(2)-MeNNHMe)(CO)(9)] (2; HMeNNHMe = 1,2-dimethylhydrazine) with PhCdropCPh, PhCdropCH, and HGdropCH were also studied. For edge-bridged alkenyl complexes, the Ru-Ru edge that is spanned by the alkenyl ligand depends on the position of the methyl groups on the hydrazido ligand. For face-capped alkenyl complexes, the relative orientation of the hydrazido and alkenyl ligands also depends on the position of the methyl groups on the hydrazido ligand. A kinetic analysis of the reaction of 1 with PhCdropCPh revealed that the reaction follows an associative mechanism, which implies that incorporation of the alkyne in the cluster is rate-limiting and precedes the release of a CO ligand. X-ray diffraction, IR and NMR spectroscopy, and calculations of minimum-energy structures by DFT methods were used to characterize the products. A comparison of the absolute energies of isomeric compounds (obtained by DFT calculations) helped rationalize the experimental results.