Oxidative dehydrodimerization of manganese vinylidene complexes (η5‐C5R5)(CO)(L)MnCCHPh (R = Me, L = CO; R = H, L = PPh3)

The oxidative dehydrodimerization of the phenylvinylidene manganese complexes (η5‐C5R5)(CO)(L)MnCCHPh (5: R=Me, L=CO; 6: R=H, L=PPh3) into the corresponding bis‐vinylidene compounds (η5‐C5R5)(CO)(L)MnCCPhCPhCMn(CO)(L)(η5‐C5R5) (7: R=Me, L=CO; 8: R=H, L=PPh3) was studied by cyclic voltammetry and chemical experiments. It was found that dehydrodimerization of 5 and 6 proceeds via direct C(β)C(β) coupling of the radical cations 5 +· and 6 +· to give dicationic bis‐carbyne complexes [(η5‐C5R5)(CO)(L)Mn≡CCPhHCPhHC≡Mn(CO)(L)(η5‐C5R5)]2+ (10 ++: R=Me, L=CO; 11 ++: R=H, L=PPh3), which can be converted into bis‐vinylidene complexes 7 and 8 respectively by reduction with (η‐C6H6)2Cr. In this process, binuclear 19‐electron bis‐carbyne compounds 10 ·· and 11 ·· undergo further transformation into the dimers 7 and 8 via homolysis of C(β)H bonds of the bis‐carbyne ligand, thus exhibiting rare examples of reductive activation of CH bonds in 19‐electron transition metal carbyne complexes.