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The growing effort to design new sustainable synthetic methodologies, based on readily available and environmentally friendly transition metals, has boosted research on iron complexes. This review article focuses on C-C-bond-forming reactions occurring at bridging ligands in diiron complexes, aimed at evidencing distinctive aspects and advantages associated with the presence of two adjacent iron centres. A number of diiron-mediated C-C-bond-forming reactions reported in the literature, including nucleophilic and electrophilic additions and insertion and cycloaddition reactions, have been accumulated over the years, which, together with more recent developments, indicate that diiron complexes might provide promising alternatives to precious metals in the challenging field of metal-promoted C-C bond formation.

Reactions of the vinylalkylidene complexes [Fe2{μ-η1:η3-CRCHCH(NMe2)}(μ-CO)(CO)(Cp)2] (R = SiMe3, 1a; Tol, 1b; CO2Me, 1c, Tol = 4-MeC6H4) with HCCR′ (R′ = CPh2OH, CO2Me, Ph) lead to the formation of a mixture of 1,3-disubstituted ferrocenes [1-R-3-R′-Fc] (5, 7, 9, 11) and 1,2,4-trisubstituted ferrocenes [1-NMe2-2-R′-4-R-Fc] (4, 6, 8, 10) (R = SiMe3, R′ = CPh2OH, 4 and 5; R = SiMe3, R′ = CO2Me, 6 and 7; R = SiMe3, R′ = Ph, 8 and 9; R = Tol, R′ = CPh2OH, 10 and 11). The polysubstituted Cp ring of the ferrocenyl products results from the cycloaddition of the alkyne with the bridging vinylalkylidene ligand and involves the cleavage of one of the two substituents (H or NMe2) on the vinyl moiety, as well as of the Fe−Fe bond. The cycloaddition reaction has been extended to a variety of bridging vinylalkylidene diiron complexes with different substituents and functionalities on the C3 bridging ligand, and to different alkynes. Thus, the vinylalkylidene complex 1b reacts with HCCPh, yielding a mixture of four different ferrocenes, due to the absence of regioselectivity in the alkyne cycloaddition: [1-NMe2-2-Ph-4-Tol-Fc] (12), [1-Tol-3-Ph-Fc] (13), [1-NMe2-3-Ph-4-Tol-Fc] (14), and [1-Tol-2-Ph-Fc] (15). Conversely, the reactions with symmetric alkynes do not produce regioisomers: treatment of 1b and 1c with R′CCR′ (R′ = Et, Ph) affords a mixture of 1,2,3-trisubstituted ferrocenes [1-R-2-R′-3-R′-Fc] (17, 19, 21, 23) and tetrasubstituted ferrocenes [1-NMe2-2-R′-3-R′-4-R-Fc] (16, 18, 20, 22) (R = SiMe3, R′ = Et, 16 and 17; R = Tol, R′ = Et, 18 and 19; R = Tol, R′ = Ph, 20 and 21; R = CO2Me, R′ = Ph, 22 and 23). Likewise, the vinylalkylidene complexes [Fe2{μ-η1:η3-C(X)CHCH(R)}(μ-CO)(CO)(Cp)2] (R = CO2Me, X = NMe2, 2a; R = CN, X = NMe2, 2b; R = CO2Me, X = SMe, 3a; R = CN, X = SMe, 3b) react with alkynes HCCR′ (R′ = Tol, Ph), yielding a mixture of trisubstituted ferrocenes [1-X-2-R′-4-R-Fc] (24, 26, 28) and [1-X-3-R′-4-R-Fc] (25, 27, 29) (X = NMe2, R = CO2Me, R′ =Tol, 24 and 25; X = NMe2, R = CN, R′ =Tol, 26 and 27; X = SMe, R = CO2Me, R′ = Ph, 28 and 29). Finally, the reactions of 2a and 3b with PhCCPh afford the tetrasubstituted ferrocenes 22 and [1-SMe-2-Ph-3-Ph-4-CN-Fc] (30), respectively. The crystal structures of 4 and 11 have been determined by X-ray diffraction studies.

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