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

International audienceThe 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. © 2018 American Chemical Society