Formation of Bisvinylidene Complexes through Consecutive Alkyne–Vinylidene Rearrangements

The ortho-, meta-, or para-bis-phenylethynyl benzenes (BPEBs), o-/m-/p-(PhC≡C)2–C6H4, were converted into bis(vinylidene) complexes by two consecutive 1,2-migration reactions within the coordination sphere of half-sandwich complexes [MII(dppe)Cp]+ (MII = RuII, FeII). The yield of the novel bis(vinylidene) complexes o-/m-/p-[{Cp(dppe)Ru═C═C(Ph)}2–C6H4]2+ is affected by the spatial proximity of the substituents and decreases from 90% (para) to 0% (ortho). The yield of the intermediate monovinylidene species [Ru(═C═C{o-/m-/p-(Ph–C≡C)–C6H4}Ph)(dppe)Cp]+ isolated under these conditions increases accordingly. Comparison of transition states of the rearrangement process (RTS) obtained from computational methods (DFT) confirmed an increased activation barrier (EA) for o-BPEB (EA = 107 kJ/mol) compared to the p-BPEB analogue (EA = 80 kJ/mol). This steric impost on the double rearrangement was further quantified by buried volume calculations (VBtotal) of the vinylidene moieties when located around the central phenylene ring in ortho-, meta-, or para-positions, which decreases from 43–46% (o-), to 22–31% (m-), and 16% (p-). Moving from p-BPEB to 1,9-bis(phenylethynyl)anthracene or 1,1′-bis(phenylethynyl)ferrocene motifs reduced the yields of the bisvinylidenes from 90% to 66% and 40%, respectively. Electrochemical measurement confirmed that the nature of the positive charges in monovinylidene complexes {Ru(═C═CArAr′)(dppe)Cp}+ and {Ru(dppe)Cp}+ fragments is not repulsive, indicating that substrates with more than two alkynyl functionalities can be converted.