Ferrocenes Containing a Pendant Propargylic Chain Obtained via Addition of Propargyl Alcohol to µ-Vinyliminium Ligands in Diiron Complexes

The diiron bridging vinyliminium complexes [Fe2{µ-η1-η3-C=;N(Me)2C(R')=C(R'')}(µ-CO)(CO)(Cp)2][SO3CF3] (R' = H, R''; = SiMe3, 3a; R' = H, R'' = Tol =4-MeC6H4, 3b; R' = Me, R'' = Me, 3c; R' = SPh, R'' = Me, 3d; R' = H, R'' = Fc = [Fe(C5H4)(Cp)], 6e) react with propargyl alcohol (HC≡CCH2OH), in refluxing toluene, affording the polysubstituted ferrocenes as mixtures of two isomeric forms: [1-NMe2-2-R'-3-R''-5-CH2OCH2C≡CH-Fc] (R' = H, R'' = SiMe3, 6a; R' = H, R'' = Tol, 6b; R' = Me, R'' = Me, 6c; R' = SPh, R'' = Me, 6d, R' = H, R'' = Fc, 6e) and [1-NMe2-2-R'-3-R''-4-CH2OCH2C≡CH-Fc] (R' = H, R'' = SiMe3, 7a; R' = H, R'' = Tol, 7b; R' = Me, R'' = Me, 7c; R' = SPh, R'' = Me, 7d) in overall yields of about 55–65%. Formation of the functionalized cyclopentadienyl in the ferrocene products takes place through the assembly of two propargyl units with the bridging vinyliminium ligand: one alkynol is incorporated by a [3 + 2] cycloaddition with the bridging C3 ligand; a second alkynol unit gives rise to a pendant chain through -OH substitution. Investigations show that the substitution step is catalyzed by the parent diiron complex itself or by a mononuclear iron fragment (likely the Fp+ complex). The pendant propargyl chain has been exploited to connect the ferrocene to other molecular fragments: in particular, the reaction of 6a with 4-biphenyl azide, by copper-catalyzed azide–alkyne cycloaddition (CuAAC), leads to the formation of the triazole-functionalized ferrocene [1-NMe2-2-CH2OCH2-N3(C6H4Ph)C2H-4-SiMe3-Fc] (12). Moreover, 6a reacts with Co2(CO)8, affording the complex [Co2{µ-η2-HC≡CR}(CO)6] (13), (HC≡CR = 6a), where the alkyne adopts a η2 coordination to a dicobalt hexacarbonyl fragment. The molecular structure of 7a has been determined by X-ray diffraction studies.