Redox Properties of Ferrocenyl Ene-diynyl-Bridged Cp*(dppe)M–C≡C–1,4-(C6H4) Complexes

The complexes FcCH═C­{1,4-C≡C–C6H4–C≡CM­(dppe)­Cp*}2 (Fc = ferrocenyl (FeCp­(η-C5H4−); M = Fe (1), Ru (2)) were prepared from FcCH═C­{1,4-C≡C–C6H4–C≡CSiMe3}2 (3) via a desilylation/metalation protocol in good (2; 65%) to excellent (1; 97%) yield. The iron compound 1 could also be prepared in a stepwise fashion by desilylation of 3 to give FcCH═C­{1,4-C≡C–C6H4–C≡CH}2 (4), reaction with FeCl­(dppe)­Cp* to give the vinylidene complex FcCH═C­{1,4-C≡C–C6H4–CH═C═Fe­(dppe)­Cp*}2]­(PF6)2 (5­(PF6)2; 65%), and deprotonation. The cyclic voltammograms of 1 and 2 are characterized by an initial oxidation wave resulting from the overlap of two closely spaced oxidation processes, the potentials of which are sensitive to the identity of M, and a subsequent, one-electron-oxidation wave. Thus, while the dications 12+ and 22+ could be prepared by oxidation with 2 equiv of ferrocenium hexafluorophosphate and isolated as the PF6– salts 1­(PF6)2 and 2­(PF6)2 at low temperature, the monocations 1+ and 2+ could only be detected and studied as comproportionated mixtures of 1, 1­(PF6), 1­(PF6)2 and 2, 2­(PF6), 2­(PF6)2. A combination of EPR spectroscopy, IR and NIR spectroelectrochemistry, and DFT quantum chemical calculations reveal subtle distinctions in the electronic structures of 1­(PF6)n and 2­(PF6)n (n = 0–2). The HOMOs of 1 and 2 are more heavily distributed over the metal–diethynylbenzene arm trans to the ferrocenyl moiety. While one-electron oxidation of 1 gives 1­(PF6), in which the spin density is similarly distributed along the branch of the molecule trans to the ferrocenyl group, the spin density in 2­(PF6) is more extensively, but not fully, delocalized. Further analysis of the ESR, NIR, and IR spectra reveals that charges are essentially localized in 1­(PF6) and 1­(PF6)2 on the IR time scale, but ground-state exchange between the Fe­(dppe)­Cp* moieties can take place via the ferrocenyl moiety on the slower ESR time scale. For 2­(PF6) and 2­(PF6)2, optical charge transfer processes between the ferrocenyl moiety and the organometallic branches can also be observed, consistent with the increased coupling between the Ru­(dppe)­Cp* and Fc moieties that are linked by a linear conjugation pathway through the bridging-ligand backbone.