Addition of alkynes at bridging vinyliminium ligands in diiron complexes: Unprecedented diene formation by enyne-like metathesis

The zwitterionic bridging vinyliminium complex [Fe2{μ-η1:η3-C(Tol)double bond; length as m-dashC(CS2)Cdouble bond; length as m-dashN(Me)2}(μ-CO)(CO)(Cp)2] (5a) undergoes the addition of two equivalents of MeO2C–Ctriple bond; length of mdashC–CO2Me affording the bridging bis-alkylidene complex [Fe2{μ-η1:η3-C(Me)C{C(CO2Me)C(CO2Me)CSC(CO2Me)C(CO2Me)S}CNMe2}(μ-CO)(CO)(Cp)2] (6). One alkyne unit inserts into a C–CS2 bond of the bridging ligand, with consequent rearrangement that leads to the formation of a diene. The reaction shows analogies with the enyne metathesis. The second alkyne is incorporated into the bridging frame via cycloaddition at the thiocarboxylate function, affording a 1,3-dithiolene. The complexes [Fe2{μ-η1:η3-C(R′)double bond; length as m-dashC(CS2)Cdouble bond; length as m-dashN(Me)(R)}(μ-CO)(CO)(Cp)2] (R = Xyl, R′ = Tol, 5b; R = p-C6H4OMe, R′ = Me, 5c; Xyl = 2,6-Me2C6H3), treated with MeO2C–Ctriple bond; length of mdashC–CO2Me and then with HBF4, undergo the cycloaddition of the alkyne with the dithiocarboxylate group and protonation of the dithiocarboxylate carbon, affording the complexes [Fe2{μ-η1:η3-C(R′)double bond; length as m-dashC{C(H)SC(CO2Me)C(CO2Me)S}Cdouble bond; length as m-dashN(Me)(R)}(μ-CO)(CO)(Cp)2][BF4] (R = Xyl, R′ = Tol, 7a; R = p-C6H4OMe, R′ = Me, 7b), respectively. The X-ray molecular structure of 6 has been determined.