[[abstract]]Seventy percent of the ferrocene moieties in double-stranded polybisnorbornenes containing linearly aligned ferrocene linkers are oxidised and each of the neighbouring monomeric units in these polymers may strongly interact with each other; the oxidised form of 3c has been shown to be antiferromagnetic.[[notice]]補正完畢[[incitationindex]]SCI[[incitationindex]]EI[[booktype]]紙

Lin, Cheng-lan

Seventy percent of the ferrocene moieties in double-stranded polybisnorbornenes containing linearly aligned ferrocene linkers are oxidised and each of the neighbouring monomeric units in these polymers may strongly interact with each other; the oxidised form of 3c has been shown to be antiferromagnetic.補正完畢SCIEI紙

Tamkang University Institutional Repository

Electrochemical oxidation of double-stranded polybisnorbornenescontaining linearly aligned ferrocene linkerswCheng-Lan Lin, Hui-Chun Yang, Nai-Ti Lin, I-Jui Hsu, Yu Wang* andTien-Yau Luh*Received (in Cambridge, UK) 27th May 2008, Accepted 16th June 2008First published as an Advance Article on the web 30th July 2008DOI: 10.1039/b808958bSeventy percent of the ferrocene moieties in double-strandedpolybisnorbornenes containing linearly aligned ferrocene linkersare oxidised and each of the neighbouring monomeric units inthese polymers may strongly interact with each other; theoxidised form of 3c has been shown to be antiferromagnetic.There has been ever burgeoning interest on the electro-chemistry of macromolecules containing multiple electroactivecenters.1–4 Multielectron transfer is common in these polymersand their voltammetric curves are occasionally similar to thoseof one-electron transfer reactions.1 The electrochemical beha-viour of these polymers depends on whether interactionsbetween these electroactive groups are present or not.1,2 Forexample, in the oxidation of poly(vinylferrocene) 1, all theferrocene groups are oxidised at one potential and it is believedthat there is little interaction between these iron-containingsandwich pending groups.1 On the other hand, when theferrocene moieties are part of the polymeric backbone sepa-rated by diﬀerent types of linkers, two reversible oxidationwaves are in general observed.2 Similar behaviour is alsofound in ferrocene-containing dendrimers.3 We recently re-ported the ﬁrst DNA-like double-stranded bispolynorbor-nenes 2 using covalent ferrocene linkers.5 The spacingoccupied by each of the monomeric unit is about 5.5 A˚ basedon scanning tunneling microscopic images of 25 and on theX-ray structure of a related monomer.6 Unlike those ferro-cene-containing polymers in the literature, which are some-what ﬂuxional, 2 is more rigid and the ferrocene moieties in 2are aligned essentially in eclipsed manner with a ﬁxed distance.It is envisaged that interactions between the ferrocene units in2 might be reﬂected by their electrochemical behaviour.Since the oxidation potentials of the ferrocenedicarboxylateand the 4-aminobenzyl moieties in 2 were very close, and the4-aminobenzyl moiety in 2 would be very labile and suscep-tible to hydrolysis,7 we decided to tackle the electrochemistryof 3 containing a linear array of ferrocene derivatives. Theferrocene moiety and 4-aminobenzoate groups in 3 wouldshow very diﬀerent oxidation potentials. In addition, theaminobenzoate moiety would be more stable than the amino-benzyl entity under various conditions. The syntheses of 3essentially followed a similar strategy as described for 25 andthe details will be described elsewhere.Results of the cyclic voltammetry of 3a–c are summarised inTable 1, and the cyclic voltammograms are shown in Fig. 1. Aredox pair around 100 mV (with reference to the ferrocene–ferrocenium couple) was assigned to the redox of ferrocenemoieties in 3. The irreversible electrochemical oxidation of theaminobenzoate moiety in 3 gave an anodic peak around650 mV. As shown in Table 1, the ipc/ipa ratios were close tounity (where ipc and ipa are the cathodic and anodic peakcurrents, respectively) indicating that the redox behaviour ofthe ferrocene moieties in 3 would be reversible (Fig. 1, solidline).In order to investigate the oxidation characteristics of ferro-cene moieties in 3, a ﬁxed potential at 350 mV was employed forbulk electrolysis. The results are listed in Table 2. It is interestingto note that about 70% of ferrocene moieties in 3 were oxidised.When single stranded polymer 4 was used for the electrolysisTable 1 Cyclic voltammetric results of 3 in CH2Cl2 containing 0.1 MBu4NPF6Compound Epaa/V Epca/V DEp/V ipc/ipa3a 0.112 0.078 0.034 0.843b 0.062 0.022 0.040 0.943c 0.147 0.071 0.076 0.99a Peak potentials determined from cyclic voltammograms with poten-tial scans involving the redox reactions of the ferrocene moieties only.The electrochemical potentials are reported relative to the ferrocene/ferrocenium redox couple measured after every experiment.Department of Chemistry, National Taiwan University, Taipei,Taiwan 106. E-mail: tyluh@ntu.edu.tw; wangyu@ntu.edu.tw;Fax: +886-2-2364-4971; Tel: +886-2-2363-6288w Electronic supplementary information (ESI) available: Details ofelectrochemical experiments and frontier orbitals from the DFTcalculations. See DOI: 10.1039/b808958b4484 | Chem. Commun., 2008, 4484–4486 This journal is c The Royal Society of Chemistry 2008COMMUNICATION www.rsc.org/chemcomm | ChemCommPublished on 30 July 2008. Downloaded by Tamkang University Libraries on 07/04/2015 07:21:13. View Article Online / Journal Homepage / Table of Contents for this issueexperiment, a similar percentage of ferrocene groups wereoxidised. It is noteworthy that the pending ferrocene moietiesin 4 have been shown to align essentially in the same manner asthose of 3. As shown in Table 1, diﬀerences of some of the redoxpeak potentials of these polymers, DEp, were somewhat smallerthan 59 mV. It is noteworthy that each of these apparent redoxpairs would represent a summation of multiple electron transferprocesses in each of these polymers. Because of such strongoverlaps among individual redox processes, the apparent DEp, ofthese polymers might appear to be smaller than 59 mV.It is well documented that the ferrocene moieties in 1 werefully oxidised under similar conditions.1 We have reexaminedthe octamer 1 and found complete oxidation of the ferrocenemoieties. Apparently, the relative orientation of ferrocenemoieties in 1 and those in 3 and 4 would be very diﬀerent.Electron transfer numbers of the ferrocene moieties in 3b, 3cand 4 were also examined by the chronoamperometric methodon an microelectrode,1a and the results are also summarised inTable 2. On the average, there were around 70% of the totalferrocene units in these polymers were electrochemically oxi-dised during a potential-step process. The results obtained bythis protocol were consistent with those acquired by bulk-electrolysis experiments described above.The oxidised 3c was subjected to magnetic measurements,and the magnetic susceptibilities at diﬀerent temperatures thusobtained are shown in Fig. 2. The molar magnetic suscept-ibility, wM, was calculated based on the contribution of tenferrocene-containing monomers; the spin only wMT value ofone unpaired electron (S=1/2) should be 0.375 cm3 K mol1.8Accordingly, the measured wMT value at 295 K of 2.62 cm3 Kmol1 would suggest that roughly seven ferrocene out of every10 monomers being oxidised. In other words, there would be70% of the ferrocenium moieties, which is supposed to haveone unpaired electron, in the oxidised 3c. This result agreednicely with those from the electrochemical measurements.Moreover, the wMT increases with temperature from 2 to300 K, which indicates the strong antiferromagnetic couplingamong the oxidised monomers.9The oxidation behaviours of 3 and 4 are apparently verydiﬀerent from those of 11 and the sulfur- or silylene-spacedferrocene copolymers.2 The structural uniqueness of 3 and 4may play a pivotal role in this electrochemical oxidationprocess. That 70% of the ferrocene moieties in 3 and 4 wereoxidised suggest that each of the neighbouring monomericunits may strongly couple with each other, which can beevidenced by the antiferromagnetic interactions observed inmagnetic susceptibility measurements. However, due to theelectrostatic repulsion, the ferrocene linkers in 3 and 4 cannotbe fully oxidised. It is noteworthy that the oxidised form of 3and 4 appeared to be fairly stable under ambient conditions.Fig. 1 Cyclic voltammograms of (a) 3a, (b) 3b and (c) 3c in CH2Cl2containing 0.1 M Bu4NPF6. Scan rate = 100 mV s1. The adsorptionof oxidised polymers onto the electrode surface may result in theappearance of larger ipc of the ferrocene moieties.Table 2 Electron transfer ratios of the ferrocene moieties in thepolymers estimated by bulk-electrolysis (BE) experiments and bychronoamperometry at a microelectrode (CM)Electron transfer ratioa (%)Mn np BE CM3a 13 700 19 75 —3b 31 100 40 68 703c 31 200 36 69 —3c 37 600 43 — 744 1 700 8 105 —4 11 900 20 69 70a The percentage of ferrocene units in a polymer that undergo electrontransfer during the bulk-electrolysis process.Fig. 2 Magnetic susceptibility of the oxidised 3c as a function oftemperature. The solid (w) and open circle (wMT) are experimental databased on ten monomeric units after correction on diamagnetism andnormalization on the applied magnetic ﬁeld.This journal is c The Royal Society of Chemistry 2008 Chem. Commun., 2008, 4484–4486 | 4485Published on 30 July 2008. Downloaded by Tamkang University Libraries on 07/04/2015 07:21:13. View Article OnlineAs depicted earlier, the ferrocene moieties in 3 may bealigned coherently in essentially eclipsed manner with thespacing between neighbouring ferrocene moieties about5.5 A˚. In order to realise the charge distribution of the oxidisedform of 3, DFT calculations were undertaken on a modelhaving six ferrocenes aligned linearly with a separation of5.5 A˚ between the neighbouring ferrocenes, 5. Each of theferrocene moieties was assumed in D5h point group and a C2vsymmetry was imposed on 5.z The calculation of neutral 5 wascarried out in the restricted closed shell level. Four to sixelectrons were then removed from the HOMO to HOMO2of the neutral 5 to give the corresponding oxidised 5n+ (n =4–6) and the charge densities on each ferrocene moiety in theseions were obtained.z The Hirshfeld charge analysis10 at Featoms on 5 and 5n+ are summarised in Table 3. It is interestingto note that removal of electrons from 5 would yield unevencharge distribution among Fe atoms in oxidised forms of 5n+.The majority of the charge is located at the terminal ferrocenemoieties with minor charge resided at the interim ferrocenemoieties. These results indicate the charges are delocalised andaccumulated at the terminal ones to minimise the overallCoulombic interactions. The molecular orbital diagrams forHOMO to HOMO5 and LUMO to LUMO+1 are providedin the ESI.w Extensive oxidation up to the removal of ﬁve andsix electrons from 5 may, however, result in too much chargelocalised at the terminal ferrocene moieties which may some-what destabilise the system.In summary, we have demonstrated the unique electroche-mical properties of the ferrocene-containing double strandedbispolynorbornenes 3 and related polymer 4. The antiferro-magnetic character of the stable oxidised 3c suggested strongcoupling between the neighbouring ferrocene/ferroceniummoieties. The unusual structural feature and distinctiveredox characteristics of these polymers may be useful forfuture applications in catalysis as well as in optoelectronicapplications.This work was supported by the National Science Counciland the National Taiwan University of the Republic of China.We thank Professor Allen J. Bard for helpful discussions andNCHC for the support on the hardware and software formolecular orbital calculations.Notes and referencesz The calculations were carried out by Amsterdam Density Functional2006.01 (ADF)11 with the generalised gradient approximation (GGA).The corrections of LDA and GGA parts were taken from VWN12 andBP.13,14 The SCALAR ZORA was used for relativistic corrections.The triple zeta with two polarization function, TZ2P, Slater type basissets were used for all atoms.1 (a) J. B. Flanagan, S. Margel, A. J. Bard and F. Anson, J. Am.Chem. Soc., 1978, 100, 4248; (b) P. J. Peerce and A. J. Bard,J. Electroanal. Chem., 1980, 114, 89; (c) D. Albagli, G. Bazan, M.S. Wrighton and R. R. Schrock, J. Am. Chem. Soc., 1992, 114,4150; (d) M. Mora´n, C. M. Casado, I. Cuadrado and J. Losada,Organometallics, 1994, 12, 4327; (e) A. L. Crumbliss, D. Cooke, J.Castillo and O. Wisian-Neilson, Inorg. Chem., 1994, 32, 6088.2 (a) P. F. Brandt and T. B. Rauchfuss, J. Am. Chem. Soc., 1992,114, 1926; (b) D. A. Foucher, B. Z. Tang and I. Manners, J. Am.Chem. Soc., 1992, 114, 6246; (c) J. M. Nelson, H. Rengel andI. Manners, J. Am. Chem. Soc., 1994, 115, 7035; (d) M. T. Nguyen,A. F. Diaz, V. V. Dement’ev and K. H. Pannell, Chem. Mater.,1994, 5, 1389; (e) D. L. Compton and T. B. Rauchfuss, Organo-metallics, 1994, 13, 4367; (f) C. M. Casado, I. Cuadrado, M.Mora´n, B. Alonso, B. Garcı´a, B. Gonza´lez and J. Losada, Coord.Chem. Rev., 1999, 185–186, 53; (g) G. E. Southard and M. D.Curtis, Organometallics, 2001, 20, 508; (h) R. W. Heo, J.-S. Park,J. T. Goodson, G. C. Claudio, M. Takenaga, T. A. Albright andT. R. Lee, Tetrahedron, 2004, 60, 7225; (i) P. Park, A. J. Lough andD. A. Foucher, Macromolecules, 2002, 35, 3810; (j) X.-J. Wang,L. Wang, J.-J. Wang and T. Chen, Electrochim. Acta, 2007,52, 3941.3 (a) I. Cuadrado, C. M. Casado, B. Alonso, M. Mora´n, J. Losadaand V. Belsky, J. Am. Chem. Soc., 1997, 119, 7613; (b) S. Nalte, J.Ruiz, J.-C. Blais and D. Astruc, Chem. Commun., 2000, 417; (c) S.Nalte, J. Ruiz, V. Sartor, R. Navarro, J.-C. Blais and D. Astruc,Chem.–Eur. J., 2000, 6, 2544.4 (a) P. Ceroni, V. Vicinelli, M. Maestri, V. Balzani, M. Gorka, S.-K.Lee, P. Dragut and F. Vo¨gtle, Collect. Czech. Chem. Commun.,2004, 68, 1541; (b) F. Marchioni, M. Venturi, P. Ceroni, V.Balzani, M. Belohradsky, A. M. Elizarov, H.-R. Tseng and J. F.Stoddart, Chem.–Eur. J., 2004, 10, 6361; (c) K. Krishnamoorthy,R. R. Dasari, A. Nantalaksakul and S. Thayumanavan, Chem.Commun., 2007, 739.5 (a) H.-C. Yang, S.-Y. Lin, H.-C. Yang, C.-L. Lin, L. Tsai, S.-L.Huang, I.-W. P. Chen, C.-h. Chen, B.-Y. Jin and T.-Y. Luh,Angew. Chem., Int.Ed., 2006, 45, 726; (b) N.-T. Lin, S.-Y. Lin,S.-L. Lee, C.-h. Chen, C.-H. Hsu, L. P. Hwang, Z.-Y. Xie, C.-H.Chen, S.-L. Huang and T.-Y. Luh, Angew. Chem., Int. Ed., 2007,46, 4481.6 W.-Y. Lin, M. G. Murugesh, S. Sudhakar, H.-C. Yang, H. Tai,C.-S. Chang, Y.-H. Liu, Y. Wang, I.-W. P. Chen, C.-h. Chen andT.-Y. Luh, Chem.–Eur. J., 2006, 12, 324.7 Y.-Y. Lai, N.-T. Lin, Y.-H. Liu, Y. Wang and T.-Y. Luh,Tetrahedron, 2007, 63, 6051.8 O. Kahn, Molecular Magnetism, VCH, New York, 1993.9 For a precedented antiferromagnetic linear single-stranded ferro-cene-containing polymer, see: J. M. Nelson, P. Nguyen, R. Peter-sen, H. Rengel, P. M. Macdonald, A. J. Lough, I. Manners, N. P.Raju, J. E. Greedan, S. Barlow and D. O’Hare, Chem.–Eur. J.,1997, 3, 573.10 F. L. Hirshfeld, Theor. Chim. Acta, 1977, 44, 129.11 (a) G. te Velde, F. M. Bickelhaupt, S. J. A. van Gisbergen, C. F.Guerra, E. J. Baerends, J. G. Snijders and T. Ziegler, J. Comput.Chem., 2001, 22, 931; (b) C. Fonseca Guerra, J. G. Snijders,G. te Velde and E. J. Baerends, Theor. Chim. Acta, 1998, 99, 391.12 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 1980, 58, 1200.13 A. D. Becke, Phys. Rev. A, 1988, 38, 3098.14 J. P. Perdew and W. Yang, Phys. Rev. B, 1986, 33, 8800.Table 3 Hirshfeld charge analysis on 5 and 5n+ (n = 4–6)Compound Fe Fe Fe Fe Fe Fe5 0.0814 0.0867 0.0866 0.0866 0.0867 0.081454+ 1.2577 0.3469 0.1252 0.1252 0.3469 1.257755+ 1.6106 0.3590 0.1255 0.1255 0.3590 1.610656+ 1.9636 0.3710 0.1259 0.1259 0.3710 1.96364486 | Chem. Commun., 2008, 4484–4486 This journal is c The Royal Society of Chemistry 2008Published on 30 July 2008. Downloaded by Tamkang University Libraries on 07/04/2015 07:21:13. View Article Online

Chemical Communications

Electrochemical Oxidation of Double-Stranded Polybisnorbornenes Containing Linearly Aligned Ferrocene Linkers

Lin, Cheng-Lan

National Taiwan University Repository

Electrochemical oxidation of double-stranded polybisnorbornenes containing linearly aligned ferrocene linkers

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Electrochemical Oxidation of Double-Stranded Polybisnorbornenes Containing Linearly Aligned Ferrocene Linkers

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