Formation of Bisvinylidene Complexes through Consecutive Alkyne–Vinylidene Rearrangements

The ortho -, meta -, or para- bis-phenylethynyl benzenes (BPEBs), o -/ m -/ p -(PhC≡C) 2 –C 6 H 4, 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 –C 6 H 4 ] 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)–C 6 H 4 }Ph)(dppe)Cp] + isolated under these conditions increases accordingly. Comparison of transition states of the rearrangement process ( R TS ) 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.