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 KOtBu (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.