1′,1′′′-Bis(ethynyl)biferrocenyl-Bridged Fe(dppe)Cp* Units: Synthesis, Solid-State Structures, and Electronic Couplings

Treatment of 1′,1′′′-bis(ethynyl)biferrocene (1) with 2 equiv of FeCl(η²-dppe)(Cp*) (2) in the presence of [H₄N]PF₆ produced the vinylidene complex [1′,1′′′-((Cp*)(η²-dppe)FeCCH)₂bfc][PF₆]₂ (3) (97%, Cp* = η⁵-C₅Me₅; dppe = 1,2-bis(diphenylphosphino)ethane; bfc = 1′,1′′′-biferrocenyl, (η⁵-C₅H₄)₂Fe)₂), while neutral tetranuclear 1′,1′′′-((Cp*)(η²-dppe)Fe−CC)₂bfc (4) was obtained from the reaction of 3 with KO^tBu (68%). The IR, ¹H, ¹³C, and ³¹P NMR, cyclic voltammetry, Mössbauer, and UV−vis data were obtained for 4. When reacted with 2 equiv of [FcH]PF₆ or TCNQ, complex 4 provided 4[X]₂ (Fc = 1-ferrocenyl, (η⁵-C₅H₅)Fe(η⁵-C₅H₄), X = PF₆, 89%; X = TCNQ = 7,7,8,8-tetracyanoquinodimethane, 91%), and 4[PF₆] was obtained by reacting 4 with 4[PF₆]₂ in a 1:1 molar ratio (76%). Organometallics 4[PF₆]₃ (93%) and 4[PF₆]₄ (87%) were obtained from the reaction of 4[PF₆]₂ with 1 or 2 equiv of Ag[PF₆]. For the purpose of reasonable solubility the triflate salts of tri- and tetracationic species 4[OTf]₃ (94%) and 4[OTf]₄ (95%) were synthesized instead of the hexafluorophosphate salts. The structures of 3, 4, and 4[TCNQ]₂ in the solid state were determined by single-crystal X-ray structure analysis. IR and Mössbauer spectroscopy of 4[PF₆]_n (n = 0−4) allowed the stepwise observation of the four metal-centered oxidations and established the localized character of the mixed-valence complexes. EPR spectra of 4[PF₆]_n (n = 1, 3) proved the formation of two radicals, one centered on the half-sandwich termini and the other centered on the (ethynyl)biferrocene bridge, indicating that the mediating state is sufficiently low lying to be populated at 66 K. The NIR spectra of the mixed-valent species 4[PF₆]_n (n = 1, 2) and 4[OTf]₃ showed the presence of two bands, and their characteristic parameters are in accord with an electron transfer involving intermediate states.