New and Selective Routes to Functionalized Biferrocenes and Terferrocenes by [3 + 2] Cycloadditions of Alkynes with Bridging C₃ Ligands in Diiron Complexes

The diiron bridging enaminoalkylidene complex [Fe2{μ-η1:η3-C(Fc)CHCH(NMe2)}(μ-CO)(CO)(Cp)2] (2), containing a ferrocenyl group on the bridging ligand, reacts with HCCR, affording the corresponding 3-R-substituted 1,1′′-biferrocene (R = CPh2OH, 4a; R = Tol, 4b; R = CO2Me, 4c; R = CH2OH, 4d). In a related reaction, the μ-vinylaminoalkylidene complex [Fe2{μ-η1:η3-C(NMe2)CHCH(CO2Me)}(μ-CO)(CO)(Cp)2] (1a), upon treatment with ethynylferrocene, leads to the formation of 2-NMe2-4-CO2Me-1,1′′-biferrocene (4f). Conversely, the μ-enaminoalkylidene complex [Fe2{μ-η1:η3-C(Tol)CHCH(NMe2)}(μ-CO)(CO)(Cp)2] (1b) reacts with ethynylferrocene, affording a mixture of monosubstituted biferrocene 4b and disubstituted 2-NMe2-4-Tol-1,1′′-biferrocene (4e), in comparable yields. Reaction of 2 with ethynylferrocene (HCCFc) leads to the formation of a mixture of 1,3-terferrocene (5a) and the 5-NMe2-substituted 1,3-terferrocene (5b). Investigation of the reactivity of the vinyliminium complex [Fe2{μ-η1:η3-C(Fc)CHCNMe2}(μ-CO)(CO)(Cp)2][SO3CF3] (3) with HCCCPh2OH is also reported: the reaction affords selectively the 3-NMe2-4-R-disubstituted 1,1′′-biferrocene (6). The molecular structure of 5a has been determined by X-ray diffraction studies.