Reactivity of [Ru2(μ-η1:η1-NCPh2)(μ-η1:η2-PhCCHPh)(CO)6] with Alkynes. Insertion Reactions of Nonactivated Alkynes into Ru−C and Ru−N Bonds

Abstract

The μ-1-azavinylidene−μ-alkenyl diruthenium complex [Ru2(μ-η1:η1-NCPh2)(μ-η1:η2-PhCCHPh)(CO)6] (1) reacts with excess 1-phenyl-1-propyne or 3-hexyne in 1,2-dichloroethane at reflux temperature to give cis-stilbene and the metallacyclic derivatives [Ru2{μ-η3:η4-R1CCR2CR1CR2NCPh(C6H4)}(μ-CO)(CO)4] (2, R1 = Ph, R2 = Me; 3, R1 = R2 = Et). These compounds contain ligands that result from the ortho metalation of a phenyl group of the original 1-azavinylidene ligand, the release of cis-stilbene, the insertion of the corresponding alkyne molecule into a Ru−N bond, and the insertion of a second alkyne molecule into a Ru−C bond. The reaction of compound 1 with phenylacetylene in 1,2-dichloroethane at reflux temperature gives cis-stilbene, [Ru2{μ-η1:η2-NCPh(C6H4)}(μ-η2:η4-PhCCHCHCPh)(CO)5] (4), and [Ru{η5-PhCCHCPhCHCPhCHNCPh(C6H4)}(CO)2] (5). The hydrocarbyl ligand of 4, which can be described as a 1,4-diphenylbuta-1,3-diene-1,4-diyl ligand, arises from a head-to-head dimerization of phenylacetylene, whereas the organic ligand of 5 results from an unprecedented insertion of phenylacetylene into a Ru−N bond in addition to a head-to-tail trimerization of phenylacetylene. Thermolysis of compound 1 in refluxing 1,2-dichloroethane gives the tetranuclear derivative [C48H30N2O8Ru4] (6) and the binuclear product [Ru2(μ-η1:η1-NCPh2)(μ-η3:η4-PhCHCPhCPhCPh)(CO)4] (7). The molecular structure of complex 6 remains unknown. Compound 7 contains a bridging dienyl ligand (with a phenyl substituent η2-coordinated to a Ru atom) that arises from the coupling of diphenylacetylene with the diphenylalkenyl ligand of 1. Mechanistic proposals that account for the formation of complexes 2−7 from compound 1 are given. The new C−N bonds of compounds 2, 3, and 5 are remarkable because they have been formed by migratory insertion of weakly electrophilic alkynes into Ru−N bonds, a process that has hitherto been very seldom observed only with internal alkynes and never with terminal alkynes.