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.