Electrochemical Properties of Di-ferrocenes Linked by π-Bridge

π-다리로 연결된 디-페로센의 전기화학적 특성

Ferrocene (C10H10Fe: bis(η5-cyclopentadienyl)iron: Fc) is known as a good electron donor molecule showing one reversible redox cycle.1 This is one of the reasons why ferrocene is widely used as the main component of electronic, optical and biologically-active materials. Especially, multi-ferrocenyl compounds are frequently studied in the research field of intra-molecular electron-transfer processes. Di-ferrocene compounds linked by a polyethylene moiety (Fc(CH = CH)mFc: dFcEm) are the typical system studied by many researchers.2 The number and the geometry of ethylene linkage are the critical factors which control the efficiency of electrontransfer in this system. In this study, we prepared and spectroscopically characterized a di-ferrocenyl compound in which two ferrocenyl groups are linked by an enone moiety (Fc-C(O)CH = CH-Fc: Fc-Fc). The electron-transfer behavior of this compound was investigated electrochemically and compared with that of 1,2-diferrocenylethylene (dFcE). This is a part of series of studies of ferrocenyl chalcones.3

 

EXPERIMENTAL

General Methods

Acetylferrocene (ActFc), ferrocenecarboxaldehyde (FcAld) and HPLC-grade organic solvents were commercially purchased (Aldrich) and used as received

The FT-IR spectra were recorded on a MIDAC FT-IR spectrometer within the range of 4000 ~ 400 cm-1. The UV-Vis spectra were measured on an HP 8452A diode array spectrophotometer. The 1H NMR spectra were obtained on a Bruker Avance 500 using CDCl3 as a solvent. The mass spectra were obtained on a JMS-700 Mstation by fast atom bombardment (FAB). The electrochemical studies were carried out at room temperature with a CHI 620A Electrochemical Analyzer (CHI Instrument Inc.) under the following conditions: 1.0 mM samples in MeCN containing 0.1 M n-Bu4N․BF4 using a Pt-button (r = 1 mm) working electrode, Ag/AgCl reference electrode and Pt-wire (ϕ = 1 mm) counter electrode at a scan rate of 50 mV s-1. The potentials were referenced to that of Fc/Fc+ (E1/2 = +0.464 V vs. Ag/AgCl).

Preparation of Fc-Fc

A mixture of ActFc (1 mmol, 228 mg), FcAld (1 mmol, 214 mg) and NaOH (5 mmol, 200 mg) was ground with an agate mortar and a pestle. This mixture was allowed to stand in a water bath (85 ℃) for 30 min. The final product was extracted with CH2Cl2 and the solution was dried with MgSO4. After filtration, the solvent was removed at reduced pressure and the final product was separated from the residue by column chromatography (SiO2, CH2Cl2). Yield: 63% (267 mg). Wine-red colored powder. Mp: 208~209℃. Elemental analysis: Cal. (Obs.) for C23H20Fe2O: C, 65.14 (65.28); H, 4.75 (4.75). FAB-MS(m/z, %): 424.1 (M+, 100), 359.1 ([M-C5H5]+, 17), 307.3 ([M-Fe(C5H5)+4H]+, 10). FT-IR (KBr, cm-1): 3089 (Cp C-H), 1664, 1641 (C=O), 1574, 1448, 1290, 1248 (C=C), 826 (Cp C-H), 674 (C-H), 512, 481 (Cp-Fe). 1H NMR (500 MHz, CDCl3, ppm): 7.71 (1H, CH, d, J = 15.4 Hz), 6.75 (1H, CH, d, J = 15.4 Hz), 4.88 (2H, C5H4, t, J = 1.9 Hz), 4.60 (2H, C5H4, t, J = 1.8 Hz), 4.56 (2H, C5H4, t, J = 1.9 Hz), 4.47 (2H, C5H4, t, J = 1.8 Hz), 4.20 (5H, C5H5, s), 4.19 (5H, C5H5, s). UV-vis (nm): 314, 382, 492 (MeCN); 250, 316, 386, 500 (CHCl3); 320, 386, 510 (EtOH); 322, 394, 488, 512 (MeOH).

Preparation of dFcE

1,2-Diferrocenylethylene (dFcE) was synthesized by the McMurry coupling method4 using FcAld and a low-valence titanium compound prepared in a TiCl4 and Zn powder mixture, as described in a previous report.5 This compound was characterized spectroscopically and confirmed to be identical with that prepared before.5

 

RESULTS AND DISCUSSION

Synthesis and Characterization

Fc-Fc was synthesized by solvent-free aldol condensation using ActFc and FcAld with NaOH as a base catalyst. The purification of Fc-Fc was easily achieved by column chromatography using CH2Cl2 as the eluent, giving rise to a moderate yield (63%). In this reaction, using stoichiometric amounts of the reactants is important to reduce the amount of side products. For example, the product (Fc-Fc) can react further with an excess of the acetyl reactant to produce 1,5-pentadione derivatives via Michael addition reaction.6 The purified product was characterized by elemental analysis together with the FAB-mass, FT-IR, 1H-NMR and UV-Vis spectroscopic methods. The FAB-MS data show the mother peak (M+) at 424.1 for Fc-Fc with 100% intensity and the successive loss of the Cp and FeCp moieties at m/z = 359.1 and 307.3, respectively. The ν(C=O) of ActFc was observed at 1663 cm-1, which shifts to a lower frequency (1641 cm-1) for Fc-Fc. This is due to the delocalization of the π-electrons on the carbonyl and ethylene moieties in the enon linkage. The NMR spectra measured at room temperature show two doublets at 7.71 ppm (J = 15.4 Hz) and 6.75 ppm (J = 15.4 Hz) together with other peaks from the ferrocenyl moieties, indicating that the ethylene moiety in the enon linkage is in the trans-conformation.7

Scheme 1.Syntheses of Fc-Fc and dFcE.

Statistically, four isomers (two s-cis and two s-trans conformational isomers) are possible as the reaction product, among which the minimized energy structures of Fc-Fc with the s-cis conformation8 are shown in Fig. 1. This may be one of the reasons why the Fc-Fc compound was precipitated as a reddish-brown powder, rather than being grown as single crystals, when it was recrystallized in many appropriate solvent pairs such as CH2Cl2/n-Hx, CH2Cl2/ether, CH2Cl2/EtOH, etc. Furthermore, the techniques of electrochemical oxidation using an n-Bu4N․PF6 electrolyte and chemical oxidation with I2, TCNQ or F4TCNQ were not effective to obtain a single crystal of the Fc-Fc charge-transfer salt, for the same reason.

Fig. 1.Energy-minimized structures of Fc-Fc with s-cis conformation calculated by using DMol3 method.

Electrochemical Study

The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) results for Fc-Fc and dFcE are shown in Fig. 2 and 3, respectively. The electrochemical parameters are listed and compared with those of the reactants in Table 1. Both complexes show two reversible and reproducible cycles on a repeated scan between 0~1.3 V, irrespective of the working electrodes (Pt, Au or glassy carbon), corresponding to the two redox processes of the ferrocenyl moieties (Fc+ ↔ Fc). The first half-wave potential of dFcE (E1/21 = 0.404 V) is smaller than that of Fc-Fc (E1/21 = 0.581 V) and even smaller than those of ActFc (E1/21 = 0.712 V) and FcAld (E1/21 = 0.752 V). Based on the results of the MO calculation9 on the simplified compounds (Fig. 4), in which the ferrocenyl moieties were replaced with cyclopentadienyl rings, the electrons in the HOMO are mainly located on the Cp-C(O)CH=CH- part of Cp-C(O)CH=CH-Cp, while they are on the Cp part of Cp-CH=CH-Cp. It can be inferred from this result that the E1/2 1 value of dFcE is smaller than that of Fc-Fc. On the contrary, the difference between E1/21 and E1/22 for dFcE (ΔEo = 173 mV) is slightly larger than that for Fc-Fc (165 mV). As the ΔEo value is closely related to the measure of repulsive energy between the two charged centers (that is, the Coulomb repulsion energy), it can be expressed by the comproportionation constant (Kc) of the successive redox processes of the di-ferrocenyl system, as described below:10

Fig. 2.The cyclic voltammogram (CV) scanned 10 times repeatedly between 0 V and +1.3 V and corresponding differential pulse voltammograms (DPV; inset) of the Fc-Fc compound (vs. Ag/AgCl).

Table 1.aThe samples are dissolved in MeCN containing 0.1 M n-Bu4N·BF4 electrolyte, and the potentials (in volt) are referenced to Fc/Fc+ (E1/2 = +0. 464 V vs. Ag/AgCl). bE1/2 = (Epa + Epc)/2. cΔEo = E1/22 - E1/21.

Fig. 3.The cyclic voltammogram (CV) scanned 5 times repeatedly between 0 V and +1.0 V and corresponding differential pulse voltammograms (DPV; inset) of the dFcE compound (vs. Ag/AgCl).

Fig. 4.HOMO of (A) Cp-C(O)CH=CH-Cp (N = 34, -11.920 eV) and (B) Cp-CH=CH-Cp (N = 29, -12.408 eV). These are the model compounds in which the ferrocenyl moieties in Fc-Fc and dFcE are replaced with cyclopentadienyl rings for the simplification of the calculation.

Table 2.aΔEo = (RT/F) ln Kc = (25.69) ln Kc at 298K.

At equilibrium,

where n1 = n2 = 1, ΔEo = E2o - E1o and Kc = exp[ΔEo/25.69] at 298K. This gives Kc = exp(165/25.69) ≈ 616 for Fc-Fc and Kc = exp(173/25.69) ≈ 841 for dFcE. These Kc values are compared with those of dFcEm (m = 1, 2 and 3)2b in Table 2. The results clearly show that the number of sp2 carbon atoms between the redox active ferrocenyl centers of the dFcEm system is inversely proportional to the Kc value. It is noteworthy that the Kc value for Fc-Fc (No. C(sp2) = 3) is closer to that of dFcE (No. C(sp2) = 2), than that of dFcE2 (No. C(sp2) = 4), even though the enone bridge contains an additional ketone moiety. In other words, we can surmise that the additional antibonding π-orbital on the ketone moiety does not significantly inhibit the intramolecular electron-transfer process. Moreover, it should also be pointed out that the solvents used in the CV measurement (CH3CN and CH2Cl2) do not affect the results seriously enough to cause a deviation from this trend.

In conclusion, it is demonstrated herein that the enone is as effective a bridging group as the ethylene moiety, in terms of its accommodation of electronic communication between the two redox active ferrocenyl groups.