Formation of Bisvinylidene Complexes through Consecutive Alkyne–Vinylidene Rearrangements

The <i>ortho</i>-, <i>meta</i>-, or <i>para-</i>bis-phenylethynyl benzenes (BPEBs), <i>o</i>-/<i>m</i>-/<i>p</i>-(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 <i>o</i>-/<i>m</i>-/<i>p</i>-[{Cp(dppe)RuCC(Ph)}₂–C₆H₄]²⁺ is affected by the spatial proximity of the substituents and decreases from 90% (<i>para</i>) to 0% (<i>ortho</i>). The yield of the intermediate monovinylidene species [Ru(CC{<i>o</i>-/<i>m</i>-/<i>p</i>-(Ph–CC)–C₆H₄}Ph)(dppe)Cp]⁺ isolated under these conditions increases accordingly. Comparison of transition states of the rearrangement process (^<b>R</b><b>TS</b>) obtained from computational methods (DFT) confirmed an increased activation barrier (<i>E</i>_A) for <i>o</i>-BPEB (<i>E</i>_A = 107 kJ/mol) compared to the <i>p</i>-BPEB analogue (<i>E</i>_A = 80 kJ/mol). This steric impost on the double rearrangement was further quantified by buried volume calculations (<i>V</i>_B^total) of the vinylidene moieties when located around the central phenylene ring in <i>ortho</i>-, <i>meta</i>-, or <i>para-</i>positions, which decreases from 43–46% (<i>o</i>-), to 22–31% (<i>m</i>-), and 16% (<i>p</i>-). Moving from <i>p-</i>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.