Intramolecular Nitrene Transfer via the CN Bond Cleavage of Acetonitrile to a μ₃-Alkylidyne Ligand on a Cationic Triruthenium Plane

The μ3-η2-iminoacyl-μ3-hexylidyne complex, [(Cp*Ru)3(μ3-CC5H11)(μ3-η2-HN-CMe)(μ-H)] (2a) (Cp∗ = η5-C5Me5), was prepared by the reaction of the μ3-η2:η2(⊥)-hexyne complex, [{Cp*Ru(μ-H)}3(μ3-η2:η2-nBuCCH)] (1a), with acetonitrile. Protonation of 2a yielded an equilibrated mixture of cationic μ3-η2-iminoacyl complexes, [(Cp*Ru)3(μ3-CC5H11)(μ3-η2-HN-CMe)(μ-H)2]+ (3a and 3b), which are positional isomers with respect to the bridging hydrides. While the μ3-η2-iminoacyl ligand in 2a was rigid at the NMR time-scale, in 3b it exhibited mobility, owing to the reduced back-donation from the cationic Ru3 core. In contrast to robust 2a, 3b underwent skeletal rearrangement upon thermolysis because of the mobility of the μ3-η2-iminoacyl ligand. The equilibrated mixture of 3a and 3b was transformed into an equilibrated mixture of μ3-ethylidyne-μ3-η2-1-iminohexyl complexes, [(Cp*Ru)3(μ3-CCH3)(μ3-η2-HN-CC5H11)(μ-H)2]+ (4a and 4b) at 140 °C, via the intramolecular nitrene transfer from the iminoacyl ligand to the μ3-hexylidyne carbon. Density functional theory (DFT) calculations suggest that the μ3-ethylidyne ligand in 4a, b leads to decreased steric repulsion with the surrounding Cp∗ groups, as compared with the μ3-hexylidyne ligand in 3a, b. The stabilization arising from the formation of the μ3-ethylidyne ligand is a possible driving force for the transformation.