Synthesis and characterisation of saturated and unsaturated triruthenium clusters containing electronically symmetrical and asymmetrical alkynes

The synthesis and characterisation of μ3-η2-alkynyl triruthenium clusters, [Ru3(μ3-η2-R1-4-C6H4CCR2)(μ-dppm)(μ-CO)(CO)7] (1, saturated), [Ru3(μ3-η2-R1-4-C6H4CCR2)(μ-dppm)(CO)7] (2, unsaturated) and [Ru3(μ3-η2-R1-4-C6H4CCR2)(μ-dppm)(PPh3)(CO)7] (3, saturated) containing symmetrical and asymmetrical alkynes in which R1 and R2 are electron donor or electron withdrawing groups in the para position of the aromatic ring(s) or R2 is ferrocenyl, are reported. Clusters 1 were obtained from the reactions of [PPN][Ru3(μ-Cl)(CO)10] with R1-4-C6H4CCR2 and dppm. Clusters 1 were successfully decarbonylated to give unsturated clusters 2, with the exception of the FcCCC6H4-4-NO2 containing cluster, which is stable. Novel adducts 3 were obtained in high yields by addition of PPh3 to unsaturated clusters 2. Clusters 1–3 were characterised by analytical and spectroscopic data, and structures were proposed on the basis of systematic 31P NMR studies and correlations with X-ray structural data of related compounds available in the literature. Saturated compounds 1 contain a CO and a dppm ligands bridging the same edge, which is also parallel to the μ3-η2-alkyne, as opposed to the structure previously proposed for the PhCCPh and other derivatives, and established by X-ray crystallography for the PhCCCCPh cluster derivative, in which the dppm ligand bridges a different edge. Unsaturated compounds 2 exhibit the same structure established for the PhCCPh derivative in the solid state, with the alkyne bonded in the μ3-η2-mode perpendicular to the Ru2 edge supported by the dppm ligand. Because the dppm phosphorus chemical shifts were sensitive to the alkyne electronic asymmetry, it was possible to show that clusters containing electronically asymmetrical alkynes exist in two inseparable isomeric forms, which differ with respect to the alkyne orientation. Similarly to their osmium analogues, saturated compounds 3 exist as inseparable mixtures of isomers that differ with respect to the position of the bridging CO and dppm ligands, and in the cases of asymmetrical alkyne derivatives, also with respect to the orientation of the alkyne. This work has established, therefore, that μ-CO and dppm ligand positions respective to the μ3-η2-alkyne in saturated clusters 1 and 3 are sensitive both to the nature of the coordinated alkyne and to the presence of a PPh3 in place of a CO ligand on the metal frame.