General Route to Olefins and Polyenes Having Metal Termini through the Palladium-Catalyzed Self-Dimerization of Bimetallic Fischer Carbenes

Pd-catalyzed self-dimerization of group 6 bimetallic Fischer carbenes has been successfully achieved. Through these reactions, a series of novel bimetallic olefins have been prepared, in reactions where no side products other than oxidation of the metal carbene complexes have been observed. CpM-substituted bimetallic olefins and polyenes (M = Mn, Re, Fe) and metallocenyl (M = Fe, Ru) substituted bimetallic olefins are available in acceptable yields. The experiments show that the E/Z selectivity in these processes is modulated by the steric hindrance of the metallic moiety next to the carbene center. The UV−vis spectra of CpM-substituted bimetallic olefins (M = Mn, Re, Fe) show an absorption around 350 nm attributed to a MLCT band, whereas metallocenyl (M = Fe, Ru) substituted alkenes show strong absorptions between 250 and 350 nm, together with a smaller absorption around 400 nm, which correspond to LF and MLCT bands, respectively. A small solvatochromic effect for both groups of compounds was observed. The electrochemistry of CpM(CO)2R-substituted olefins shows that Mn, Re, and Fe derivatives display irreversible oxidations. In contrast, metallocenyl-subtituted complexes show reversible or quasi-reversible processes. Ferrocene derivatives display two one-electron oxidations, which correspond to the formation of a mixed-valence compound.