Study of Redox-induced Reactions of Vinylidene and bis-Vinylidene Complexes of Manganese

Abstract

Oxidative dehydrodimerization of some phenylvinylidene complexes of manganese is studied by cyclic voltammetry. In the case of (η⁵-C₅H₅)(CO)₂Mn=C=C(H)Ph, the process occurs as the homolysis of the C_β–H bond in the radical cation of {(η⁵-C₅H₅)(CO)₂Mn=C=C(H)Ph}^+· and the dimerization of intermediate σ-phenylethinyl cation [(η⁵-C₅H₅)(CO)₂Mn–C≡C–Ph]⁺ to a binuclear dication of bis-carbine type (η⁵-C₅H₅)(CO)₂Mn⁺≡C– C(Ph)=C(Ph)–C≡Mn⁺(CO)₂(η⁵-C₅H₅). The reduction of the latter leads to binuclear bis-vinylidene complex (η⁵-C₅H₅)(CO)₂Mn=C=C(Ph)–C(Ph)=C=Mn(CO)₂(η⁵-C₅H₅). Oxidative dehydrodimerization of complexes (η⁵-C₅R₅)(CO)(L)Mn=C=C(H)Ph (R = H, L = PPh₃; R = Me, L = CO) occurs through the immediate C_β–C_β coupling of radical cations {(η⁵-C₅R₅)(CO)(L)Mn=C=C(H)Ph}^+· and yields binuclear dication bis-carbine complexes (η⁵-C₅R₅)(CO)(L)Mn⁺≡C–C(H)(Ph)–C(H)(Ph)–C≡Mn⁺(CO)(L)(η⁵-C₅R₅), whose reduction leads to neutral compounds (η⁵-C₅H₅)(CO)₂Mn=C=C(Ph)–C(Ph)=C=Mn(CO)(L)(η⁵-C₅H₅). Complex (η⁵-C₅H₅)(CO)₂Mn=C=C(Ph)–C(Ph)=C=Mn(CO)₂(η⁵-C₅H₅) undergoes the oxidation-induced nucleophilic addition of water, forming cyclic bis-carbene product with a bridge heterocyclic ligand (μ-3,4-diphenyl-2,5-dihydro-2,5-diylidene)-bis-(η⁵-cyclopentadienyldicarbonyl manganese).