Some Ruthenium Derivatives of Penta-1,4-diyn-3-one

The reaction between Ru(C≡CH)(dppe)Cp* (1) and oxalyl chloride affords ({Ru(dppe)Cp*}C≡C)₂CO (2) in 72% yield. Methylation (MeOTf) of 2 occurs first on the carbonyl oxygen, affording [({Ru(dppe)Cp*}C≡C)2C(OMe)]OTf ([3]OTf). A second methylation of [3]⁺ on the alkynyl Cβ proceeds slowly, affording [{Cp*(dppe)Ru}CCMeC(OMe)CC{Ru(dppe)Cp*}][OTf]₂ ([4][OTf]₂), whereas protonation of [3]+ occurs readily to give crystallographically characterized [{Cp*(dppe)Ru}CCHC(OMe)CC{Ru(dppe)Cp*}][OTf]₂ ([5][OTf]₂). The molecular structures of [3]OTf and [5][OTf]2 suggest that polarization by the CO group results in significant contributions from the alkynyl-allenylidene or alkynyl-carbyne mesomers, respectively. Reaction of 2 in refluxing MeOH containing [NH4]PF6 results in partial methanolysis to give Ru{C≡CC(O)CH═CH(OMe)}(dppe)Cp* (6). Knövenagel condensation of 2 with CH₂(CN)₂ affords {[Ru(dppe)Cp*]C≡C}₂C═C(CN) ₂ (7). The related asymmetric complex {Cp*(dppe)Ru}C≡C[C═C(CN)₂]C≡CC≡C{Ru(dppe)Cp*} (8) was obtained from the reaction between Ru{C≡CC(CN)═C(CN)₂}(dppe)Cp* and lithiated Ru(C≡CC≡CH)(dppe)Cp*. Single-crystal structural determinations of 2, [3]OTf, [5][OTf]₂, 6, 7, and 8 are reported, together with a supporting computational study of relevant electronic structures