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 [M^II(dppe)Cp]⁺ (M^II = Ru^II, Fe^II). 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 (E_A) for o-BPEB (E_A = 107 kJ/mol) compared to the p-BPEB analogue (E_A = 80 kJ/mol). This steric impost on the double rearrangement was further quantified by buried volume calculations (V_B^total) 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.