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Electrochemical Properties of Di-ferrocenes Linked by π-Bridge
Electrochemical Properties of Di-ferrocenes Linked by π-Bridge
Journal of the Korean Chemical Society. 2009. Feb, 53(1): 79-83
Copyright © 2009, The Korean Chemical Society
  • Received : November 20, 2008
  • Published : February 20, 2009
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About the Authors
경인 손
선영 강
율의 오
동윤 노

Abstract
Keywords
Ferrocene (C 10 H 10 Fe: bis( η 5 -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) m Fc: 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 1 H NMR spectra were obtained on a Bruker Avance 500 using CDCl 3 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-Bu 4 N․BF 4 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 + ( E 1/2 = +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 CH 2 Cl 2 and the solution was dried with MgSO 4 . After filtration, the solvent was removed at reduced pressure and the final product was separated from the residue by column chromatography (SiO 2 , CH 2 Cl 2 ). Yield: 63% (267 mg). Wine-red colored powder. Mp: 208~209℃. Elemental analysis: Cal. (Obs.) for C 23 H 20 Fe 2 O: 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). 1 H NMR (500 MHz, CDCl 3 , ppm): 7.71 (1H, CH, d, J = 15.4 Hz), 6.75 (1H, CH, d, J = 15.4 Hz), 4.88 (2H, C 5 H 4 , t, J = 1.9 Hz), 4.60 (2H, C 5 H 4 , t, J = 1.8 Hz), 4.56 (2H, C 5 H 4 , t, J = 1.9 Hz), 4.47 (2H, C 5 H 4 , t, J = 1.8 Hz), 4.20 (5H, C5H5, s), 4.19 (5H, C 5 H 5 , s). UV-vis (nm): 314, 382, 492 (MeCN); 250, 316, 386, 500 (CHCl 3 ); 320, 386, 510 (EtOH); 322, 394, 488, 512 (MeOH).
- Preparation of dFcE
1,2-Diferrocenylethylene ( dFcE ) was synthesized by the McMurry coupling method 4 using FcAld and a low-valence titanium compound prepared in a TiCl 4 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 CH 2 Cl 2 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, 1 H-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
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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 conformation 8 are shown in . 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 CH 2 Cl 2 /n-Hx, CH 2 Cl 2 /ether, CH 2 Cl 2 /EtOH, etc. Furthermore, the techniques of electrochemical oxidation using an n-Bu 4 N․PF 6 electrolyte and chemical oxidation with I 2 , TCNQ or F 4 TCNQ were not effective to obtain a single crystal of the Fc-Fc charge-transfer salt, for the same reason.
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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 . 2 and 3 , respectively. The electrochemical parameters are listed and compared with those of the reactants in 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 ( E 1/2 1 = 0.404 V) is smaller than that of Fc-Fc ( E 1/2 1 = 0.581 V) and even smaller than those of ActFc ( E 1/2 1 = 0.712 V) and FcAld ( E 1/2 1 = 0.752 V). Based on the results of the MO calculation 9 on the simplified compounds ( . 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 E 1/2 1 value of dFcE is smaller than that of Fc-Fc . On the contrary, the difference between E 1/2 1 and E 1/2 2 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
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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).
The cyclic voltammetry parameters forFc-Fc,dFcEand their reactants.a
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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.
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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).
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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.
The comparison ofΔEo(mV) andKcvalues of the diferrocenyl compounds.
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aΔEo = (RT/F) ln Kc = (25.69) ln Kc at 298K.
At equilibrium,
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where n 1 = n 2 = 1, ΔEo = E2o - E1o and K c = exp[ ΔEo /25.69] at 298K. This gives K c = exp(165/25.69) ≈ 616 for Fc-Fc and K c = exp(173/25.69) ≈ 841 for dFcE . These K c values are compared with those of dFcEm (m = 1, 2 and 3) 2b in 2 . The results clearly show that the number of sp 2 carbon atoms between the redox active ferrocenyl centers of the dFcEm system is inversely proportional to the K c value. It is noteworthy that the K c value for Fc-Fc (No. C( sp 2 ) = 3) is closer to that of dFcE (No. C( sp 2 ) = 2), than that of dFcE2 (No. C( sp 2 ) = 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 (CH 3 CN and CH 2 Cl 2 ) 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.
Acknowledgements
This work was supported by a Research Grant from Seoul Women's University (2007).
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