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(PF 6 ) 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 1 2+ and 2 2+ could be prepared by oxidation with 2 equiv of ferrocenium hexafluorophosphate and isolated as the PF6 – salts 1­(PF 6 ) 2 and 2­(PF 6 ) 2 at low temperature, the monocations 1 + and 2 + could only be detected and studied as comproportionated mixtures of 1, 1­(PF 6 ), 1­(PF 6 ) 2 and 2, 2­(PF 6 ), 2­(PF 6 ) 2 . A combination of EPR spectroscopy, IR and NIR spectroelectrochemistry, and DFT quantum chemical calculations reveal subtle distinctions in the electronic structures of 1­(PF 6 ) n and 2­(PF 6 ) 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(PF 6 ), in which the spin density is similarly distributed along the branch of the molecule trans to the ferrocenyl group, the spin density in 2­(PF 6 ) is more extensively, but not fully, delocalized. Further analysis of the ESR, NIR, and IR spectra reveals that charges are essentially localized in 1­(PF 6 ) and 1­(PF 6 ) 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(PF 6 ) and 2­(PF 6 ) 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.