η³-Edge-Bridging versus η³-Face-Capping Coordination of a Conjugated Ynenyl Ligand on a Triruthenium Cluster Core

The reaction of the cluster complex [Ru3(μ-Η)(μ3-η2-apyr)(CO)9] (1; Hapyr = 2-aminopyrimidine) with 1,6-diphenoxy-2,4-hexadiyne in refluxing THF gives the trinuclear derivative [Ru3(μ3-η2-apyr)(μ-η3-PhOCH2CHCC⋮CCH2OPh)(μ-CO)2(CO)6] (2), which contains an edge-bridging ynenyl ligand. Heating 2 in refluxing toluene leads to an equilibrium mixture of compounds 2 and [Ru3(μ3-η2-apyr)(μ3-η3-PhOCH2CHCC⋮CCH2OPh)(CO)8] (3), in which the ynenyl ligand is in a face-capping position. A mechanistic proposal that accounts for the interconversion of 2 and 3 has been obtained from DFT calculations, which have also revealed that isomer 2 is slightly more stable than 3 (3.5 kcal/mol). The bonding of the ynenyl ligands of 2 and 3 to their corresponding triruthenium frameworks has been studied by MO calculations.