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

Abstract

Treatment of 1 0 ,1 000 -bis(ethynyl)biferrocene (1) with 2 equiv of FeCl(η 2 -dppe)(Cp*) (2) in the presence of [H 4 N]PF 6 produced the vinylidene complex [1 0 ,1 000 -((Cp*)(η 2 -dppe)FedCdCH) 2 bfc][PF 6 ] 2 (3) (97%, Cp*=η 5 -C 5 Me 5 ; dppe=1,2-bis(diphenylphosphino)ethane; bfc=1 0 ,1 000 -biferrocenyl, (η 5 -C 5 H 4 ) 2 Fe) 2 ), while neutral tetranuclear 1 0 ,1 000 -((Cp*)(η 2 -dppe)Fe-CtC) 2 bfc (4) was obtained from the reaction of 3 with KO t Bu (68%). The IR, 1 H, 13 C, and 31 P NMR, cyclic voltammetry, M€ ossbauer, and UV-vis data were obtained for 4. When reacted with 2 equiv of [FcH]PF 6 or TCNQ, complex 4 provided 4[X] 2 (Fc = 1-ferrocenyl, (η 5 -C 5 H 5 )Fe(η 5 -C 5 H 4 ), X = PF 6 , 89%; X = TCNQ = 7,7,8,8tetracyanoquinodimethane, 91%), and 4[PF 6 ] was obtained by reacting 4 with 4[PF 6 ] 2 in a 1:1 molar ratio (76%). Organometallics 4[PF 6 ] 3 (93%) and 4[PF 6 ] 4 (87%) were obtained from the reaction of 4[PF 6 ] 2 with 1 or 2 equiv of Ag [PF 6 ]. 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 M€ ossbauer spectroscopy of 4[PF 6 ] n (n=0-4) allowed the stepwise observation of the four metal-centered oxidations and established the localized character of the mixedvalence complexes. EPR spectra of 4[PF 6 ] 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 mixedvalent species 4[PF 6 ] 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. § Part of the Dietmar Seyferth Festschrift. In honor of Prof. Dr. h.c. mult. Dietmar Seyferth for his outstanding contribution as editor to Organometallics.

Synthesis and Properties of a Mixed‐Valence Compound with Single‐Step Tunneling and Multiple‐Step Hopping Behavior

“…Accordingly, the second oxidation wave, which corresponds to the sequential oxidation of the second iron end group, is shifted toward positive potential ( E 0 2 – E 0 1 = 0.12 V). The wave separation corresponding to the iron‐centered events is smaller than that in the bis(iron) 7 ( E 0 2 – E 0 1 = 0.26 V)9 but much larger than that in 8 ( E 0 2 – E 0 1 = 0.00 V) 22. Apparently, the TTF core acts as a better electronic coupler than the bfc unit.…”

“…The incorporation of functional groups into the bridge between the redox centers and control of the molecular topology open routes to functional molecules with oriented electron‐transfer capabilities,14,15 switching abilities,16 multipath connections,12,17 optical activities,18,19 or magnetic properties 5,12,20. Recently, it was shown that the 1′,1″′‐biferrocenyl (bfc) unit in the family of complexes [Cp*(dppe)Fe–C≡C–bfc–C≡C–Fe(dppe)Cp*][PF 6 ] n ( 8 [PF 6 ] n , n = 0–4) acts as a molecular relay and mediates electron transfer between the two ends of the molecule by hopping processes 21,22…”

Ferrocenyl‐Pyrenes, Ferrocenyl‐9,10‐Phenanthrenediones, and Ferrocenyl‐9,10‐Dimethoxyphenanthrenes: Charge‐Transfer Studies and SWCNT Functionalization

“…For neutral 5 b and 12 , as expected, no absorptions between 800 and 3000 nm were detected, whereas mono‐oxidation of 5 b and 12 ( E ≈500 mV vs. Ag/Ag + ) gave mixed‐valent species [ 5 b ] + and [ 12 ] + and hence a broad absorption within this area (800–3000 nm) was observed. A further increase in the potential resulted in a decrease of this band, which is characteristic for intervalence charge transfer excitations (Figure , Figures SI24 and SI25 in the Supporting Information) . Please note that the low Δ E °′ values ( 5 b : 100 mV, 12 : 80 mV) and therefore the small comproportionation constants ( 5 b : K c =70.0, 12 : K c =29.9) lead, in the best case, to a mixture consisting of an equilibrium of 73 % mixed‐valent monocationic [ 12 ] + as well as 13.5 % neutral 12 and di‐cationic species [ 12 ] 2+ each.…”

“…Af urtheri ncrease in the potentialr esulted in ad ecrease of this band, which is characteristic for intervalencec harget ransfer excitations (Figure 9, Figures SI24 and SI25 in the Supporting Information). [85][86][87][88][89] Please note that the low DE8' values (5b:1 00 mV, 12:8 0mV) and therefore the small comproportionation constants( 5b: K c = 70.0, 12: K c = 29.9) lead, in the best case, to am ixture consistingo fa ne quilibrium of 73 %m ixed-valent monocationic [12] + as well as 13.5 %n eutral 12 and di-cationic species [12] 2 + each. As the composition of this equilibrium is not determinable by using an OTTLE cell, the extinctionv alues are associated with considerable uncertainties.…”

Functionalised Biferrocene Systems towards Molecular Electronics

“…We attribute all redox features to the Fe 2+ /Fe 3+ couples . The first redox event is the Fe 2+ /Fe 3+ of one of the ferrocene centres at a lower potential, followed by the second redox event at a higher potential.…”