The first gold(I) complexes based on thiocarbamoyl-pyrazoline ligands: Synthesis, structural characterization and photophysical properties

Polyhedron 63 (2013) 9–14

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The first gold(I) complexes based on thiocarbamoyl-pyrazoline ligands: Synthesis, structural characterization and photophysical properties Andressa Ferle a, Lucas Pizzuti a, Simone D. Inglez a, Anderson R.L. Caires a, Ernesto S. Lang b, Davi F. Back b, Alex F.C. Flores b, Amilcar M. Júnior c, Victor M. Deflon d, Gleison Antônio Casagrande c,⇑ a

Grupo de Pesquisa em Síntese e Caracterização Molecular, Universidade Federal da Grande Dourados – UFGD, 79.804-970 Dourados, MS, Brazil Departamento de Química, Universidade Federal de Santa Maria – UFSM, 97.105-900 Santa Maria, RS, Brazil Instituto de Química, Universidade Federal de Mato Grosso do Sul – UFMS, Av. Senador Filinto Muller 1555, 79.074-460 Campo Grande, MS, Brazil d Instituto de Química de São Carlos, Universidade de São Paulo, CP 780 São Carlos, SP, Brazil b c

a r t i c l e

i n f o

Article history: Received 23 May 2013 Accepted 8 July 2013 Available online 15 July 2013 Keywords: Gold Gold(I) luminescence Thiocarbamoyl-pyrazoline Gold(I) complexes

a b s t r a c t Novel gold(I) complexes based on thiocarbamoyl-pyrazoline ligands were synthetized for the first time and their structural/photophysical properties were determined. Two new monocationic complexes of the type [(Ph)3PAu(L)]PF6 (L = 1-thiocarbamoyl-5-(4-halophenyl)-3-phenyl-pyrazoline) were prepared by reacting (Ph)3PAuCl and ligands in the presence of KPF6 in a 1:1:1 molar ratio in a MeOH/CH2Cl2 (1:1) medium in good yields. The two cationic units from both complexes possess similar structural behaviour around the Au(I) atom which presents a two-coordinate and an almost linear coordination geometry. It was found good correlation between absorption and emission spectra and the photophysical properties were described. The prepared complexes (1 and 2) are representative examples of Au(I)/d10 compounds that present bluish luminescence in the solid state at room temperature when excited at kex = 320 nm. The bluish emission that appears at ca. 415 nm for both cationic complexes was tentatively assigned to the LMCT excitations arising from sulphur based orbitals to the Au(I) atom (S ? Au–P). Besides X-ray studies, the complete structural characterization of the prepared compounds also includes elemental analyses, IR spectroscopy, 1H and 13C NMR, UV–Vis absorption and luminescence studies. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The study of pyrazole and its derivatives have been presented as a promising research area due to their rich and extensive coordination chemistry and large area of application [1]. It is evidenced that this class of compounds possess a broad spectrum of important biological and pharmaceutical activities such as antimicrobial, antihypertensive, antitumor, anti-inflammatory and antidepressant activities [2]. It was found that some of these activities may be present as well as enhanced upon metalation of these type of ligands to an appropriate metallic centre. For example, Azan and co-workers have demonstrated that Pd(II) complexes containing 1-thiocarbamoyl-pyrazole derivatives have shown better antiamoebic activity than their corresponding free ligands [3a]. A similar behaviour of this family of complexes was reported by Netto and co-workers [3b]. They observed that the antitumor activity against three different cancer cell lines was enhanced upon coordination of tiocarbamoyl-pyrazole to the Pd(II) centre. Coordination complexes containing substituted pyrazoles and other metallic ions such as

⇑ Corresponding author. Tel.: +55 67 3345 3547; fax: +55 67 3410 2072. E-mail address: [email protected] (G.A. Casagrande). 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.07.007

Cu(I), Cu(II), Co(II), Ni(II), Zn(II) and Hg(II) were successfully prepared and characterized and they have presented interesting structural, optical and magnetic properties [4]. Some examples of gold(III) complexes containing 4,5-dihydropyrazole-carbothioamide derivatives were recently published in the literature by Wang and co-workers [5a]. These studies have indicated that gold(III) complexes containing this type of ligand might be a promising source of metal-based antitumor agents once they have shown higher cytotoxicity than cisplatin against HeLa (cervix carcinoma) cell line. Gold(I) and gold(III) complexes with thioamide ligands also have shown in vitro cytotoxicity against leiomyosarchoma cells [5b]. Recently, phosphine–gold(I) complexes incorporating emissive mercaptopteridine ligands were tested against four different adenocarcinoma cell lines (MCF7, A549, PC3 and LOVO). The results showed that the prepared compounds have presented impressive anti-proliferative activities (IC50 < 5 lM) against the cells above mentioned [5c]. Materials chemistry is another research area that has attracted the attention of some research groups concerning the preparation and characterization of new luminescent materials. Currently, there has been a developing interest in luminescent metal complexes because of their possible application as dopant emitters in organic emitting device technology. In addition, gold(I) complexes can be used as good energy donors for sensitization of

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visible and NIR luminescence of Eu(III) and Yb(III) ions as well as environmental sensors for the detection of volatile organic vapors (VOCs) for example [6]. Pioneering studies involving luminescent gold(I) complexes were published by Bruce and Fackler. These studies can be divided in two parts: firstly is concerning the mono and dinuclear gold(I)–phosphine derivatives. In this case the luminescence emissions were dominantly attributed to the metalcentered (MC) excitations and secondly refers to the mixed phosphine–gold(I)–S(thiolate) derivatives in which the luminescence emissions were assigned to the LMCT (S ? Au(I)) excitations in nature [7]. In a particular condition, significant gold(I)  gold(I) interactions can perturb the orbital energies involved in the ligant to metal charge transfer (LMCT) transition and consequently occurs a red-shift on the emission energy [7e]. Recently, mononuclear phosphine–gold(I), binuclear phosphine–gold(I) and mixed oxidation states phosphine–gold(I) and gold(III) complexes based on pyridine-thiolate and benzimidazole-thiolate ligands were synthesized and structurally characterized. These compounds have shown interesting photophysical and structural features as luminescence in the visible region, quenching of gold(I) luminescence caused probably by gold(I)  gold(III) interactions and presence of short gold(I)  gold(I) contacts in the solid state for example [8]. DFT studies also have shown a good agreement between theoretical and experimental UV-Vis absorption/excitation spectra of derivatives containing S–Au(I)–P bonds in their structures [8b]. Finally, similar behaviour was found in correlated compounds like phosphine–Au(I)–S(benzimidazolethiolate) where the maxima absorption in the UV-Vis spectra (from 310 to 317 nm) were assigned as a mixture of a benzimidazolethiolate IL and LMCT (S ? Au(I)) transitions from the sulphur atom to the Au(I) atom [8c]. From this perspective, gold(I) complexes represent potential targets of opportunity, owing to their intense, long lived luminescence in the solid state with a wide range of emission energies [9]. On this way, our research group started studying new luminescent systems based on gold(I)–S(thiocarbamoyl-pyrazoline) derivatives aiming future application of these molecules in materials or medicinal chemistry. In order to enrich the literature and to contribute to the development of the gold(I) chemistry, we present in this paper the synthesis, structural characterization and some photophysical properties of the first synthesized gold(I) complexes based on 1-thiocarbamoyl-pyrazoline ligands. 2. Experimental 2.1. Chemicals and measurements The starting materials (Ph)3PAuCl and the ligands L1 and L2 were prepared according to methods already reported in the literature [10]. KPF6 and solvents (AR grade) were obtained commercially and used in the synthesis without further purification. The solvent (CH2Cl2 spectroscopic grade) used in the UV–Vis experiments was obtained commercially from AldrichÒ. Elemental analyses (CHN) were performed using a Perkin–Elmer 2400 analyser. FT-IR spectra were acquired on a JASCO-4100 spectrophotometer using KBr pellets and the solution/solid state UV–Vis experiments were acquired on a Cary 50 UV–Vis spectrophotometer (200– 800 nm) using quartz cuvettes. The solid state emission spectra were acquired on a thin and uniform layer of the crystalline solids prepared between two quartz plates and using a Cary Eclipse/Varian Fluorimeter equipped with the solid state accessory. Melting points were determined using an Instruterm DF-3600 apparatus and are uncorrected. 1H and 13C NMR spectra were recorded on a Bruker DPX400 spectrometer in CDCl3 (400 MHz for 1H and 100 MHz for 13C) using TMS as internal standard. The X-ray data were collected using a Bruker APEX II CCD area-detector diffractometer with graphite-monochromatised Mo Ka radiation.

The crystal structures of the complexes were solved using the SHELX package [11]. All non-hydrogen atoms were refined with anisotropic displacement parameters, and the hydrogen atoms were included at their theoretical ideal positions. More detailed information about the structure determinations is given in Table S1. 2.2. Preparation of the complexes The synthesis of the complexes 1 and 2 were performed in accordance with the methodologies already published by our research group [10c,d]. Complex 1: To a solution of (Ph)3PAuCl 0.0494 g (0.1 mmol) in MeOH/CH2Cl2 (1:1 v/v), KPF6 0.0186 g (0.1 mmol) was added to promote the Au–Cl bond cleavage. After 15 min. 0.0644 g (0.12 mmol) of the ligand (L1) was added to this solution that was stirring at 55 °C for 1.5 h. Afterwards, the colorless solution obtained was filtered off and after one day, prismatic colorless crystals were isolated from the mother solution. The same procedure was applied to prepare the complex 2. Complex 1: (Yield 61% based on gold(I) precursor used; M.p. 178–180 °C; Anal. Calc. for C35H33AuF7N3OP2S: C, 44.78; H, 3.53; N, 4.49. Found: C, 44.93; H, 3.69; N, 4.45%. IR (KBr, m/cm1): 3354–3476 [m(N–H)], 1507–1421 [m(C@N)], 1360 [m(C@S)], 842 [m(P–F)], 542 [m(Carom.–F)]. 1H NMR (400 MHz, CDCl3): d = 7.78– 7.76 (m, 2 H, Ar), 7.66 (bs, 2 H, NH), 7.57–7.39 (m, 18 H, Ar), 7.18–7.15 (m, 2 H, Ar), 7.01–6.97 (m, 2 H, Ar), 6.93 (bs, 1 H, NH), 5.81 (dd, 1 H, J = 3.6 Hz, J = 11.2 Hz), 4.01 (dd, 1 H, J = 11.2 Hz, J = 18.2 Hz), 3.28 (dd, 1 H, J = 3.6 Hz, J = 18.2 Hz) ppm. 13C{1H} NMR (100 MHz, CDCl3): d = 170.1, 162.4 (d, JCF = 247.5 Hz), 160.7, 136.0 (d, JCF = 3.5 Hz), 134.0 (d, JCP = 13.5 Hz), 132.4 (d, JCP = 1.7 Hz), 132.2, 129.7 (d, JCP = 11.8 Hz), 129.1, 129.0, 127.8, 127.4 (d, JCF = 8.3 Hz), 127.2, 116.2 (d, JCF = 21.9 Hz), 63.8, 44.1 ppm). Complex 2: (Yield 65% based on gold(I) precursor used; M.p. 150–152 °C; Anal. Calc. for C34H29AuClF6N3P2S: C, 44.38; H, 3.17; N, 4.56. Found: C, 44.22; H, 3.45; N, 4.45%. IR (KBr, m/cm1): 3378–3517 [m(N–H)], 1510-–1425 [m(C@N)], 1356 [m(C@S)], 840 [m(P–F)], 551[m(Carom.–Cl)]. 1H NMR (400 MHz, CDCl3): d = 7.75– 7.73 (m, 2 H, Ar), 7.53–7.40 (m, 18 H, Ar), 7.29–7.27 (m, 2 H, Ar), 7.15–7.13 (m, 2 H, Ar), 6.52 (bs, 2 H, NH), 5.90 (dd, 1 H, J = 3.7 Hz, J = 11.3 Hz), 3.92 (dd, 1 H, J = 11.3 Hz, J = 18.0 Hz), 3.21 (dd, 1 H, J = 3.8 Hz, J = 17.9 Hz) ppm. 13C{1H} NMR (100 MHz, CDCl3): d = 173.6, 158.1, 139.5, 134.0 (d, JCP = 13.3 Hz), 133.8, 132.3 (d, JCP = 2.2 Hz), 131.6, 129.8, 129.6 (d, JCP = 11.8 Hz), 129.2, 129.0, 127.3, 127.0, 63.3, 43.5, 29.7 ppm. 3. Results and discussion 3.1. Synthetic considerations The cationic complexes 1 ([(Ph)3PAu(L1)]PF6MeOH, where L1 = 1-thiocarbamoyl-5-(4-fluorophenyl)-3-phenyl-pyrazoline) and 2 ([(Ph)3PAu(L2)]PF6, where L2 = 1-thiocarbamoyl-5-(4-chlorophenyl)-3-phenyl-pyrazoline) were synthesized by the reaction of triphenylphosphinegold(I) chloride (as gold(I) precursor) with the ligands L1 and L2 respectively in the presence of KPF6 in a 1:1:1 molar ratio in a MeOH/CH2Cl2 (1:1) medium in accordance with the reaction proposed below: KPF6

ðPhÞ3 PAuCl !

KCL



 ðPhÞ3 PAu PF6 þ L ! ½ðPhÞ3 PAuðLÞPF6

L1 = 1-thiocarbamoyl-5-(4-fluorophenyl)-3-phenyl-pyrazolineL 2 = 1-thiocarbamoyl-5-(4-chlorophenyl)-3-phenyl-pyrazoline As general trend, the (Ph)3PAuCl molecule has, low reactivity against chloride (Cl) substitution reaction probably due to its strong Au–Cl bond [12]. To overcome this tendency, the formation

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of a reactive intermediate ([(Ph)3PAu]PF6) was required. It is achieved by the cleavage of the Au–Cl bond present in the precursor (Ph3PAuCl) promoted by the KPF6 addition followed by KCl elimination. On this way, the electrophilic gold(I) centre present in the [(Ph)3PAu]PF6 species formed in solution becomes able to undergo the coordination by the sulphur atom present in the thiocarbamoyl-moiety of the ligand. It is worth mentioning that cationic complexes like [(Ph)3PAu(L)]Cl could not be obtained when the reactions were carried out in the absence of KPF6 under the same conditions. The analysis of the recovered materials revealed only the presence of the gold(I) precursor and the free ligand.

3.2. Crystal structure description Aiming the simplification on the discussion involving the molecular structures of the prepared complexes and because both molecules are correlated by an isostructural crystallographic relation (monoclinic (P21/n) space group), we discuss in this text the main features of both molecules together. More detailed information about the crystal structures is given in the Supporting information (Table S1). The molecular and crystalline structure of 1 is shown in the Fig. 1. The Fig. 2 shows the molecular and crystalline structure of 2. The two prepared compounds differ one of another from the substituent bonded to the aromatic ring present in the ligand structure (Fluorine atom for 1 and Chlorine for 2). Both molecules can be described as salts complexes of gold(I) of the type [(Ph)3PAu(L)]PF6. The complex 1 has an additional methanol molecule as solvate in the crystalline structure packing. The cationic unit contains the Au(I) centre bonded to the triphenylphosphine moiety and the sulphur atom from the thiocarbamoylmoiety of the ligand. The analysis of the bond angles P(1)–Au(1)– S(1) of 169.45(3)° for 1 and 170.28(8)° for 2 confirms that the Au(I) atoms display a two-coordinate and an almost linear coordination geometry in agreement with other similar compounds already mentioned in the literature [7c,8a,b]. The Au(1)–P(1) and Au(1)–S(1) bond lengths of 2.265(8) Å and 2.299(8) Å respectively for 1 and 2.265(11) Å and 2.300(2) Å respectively for 2, are in accordance with the values recently reported by Pope [5c]. No significant intermolecular metallophilic interactions like (Au  Au) were found from the analysis of the crystalline structure packing presented for the prepared compounds.

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3.3. UV–Vis absorption studies Gold(I)–phosphine derivatives often show a strong absorption in the UV–Vis range. Broad absorption bands were observed in this region of the electromagnetic spectrum during the pioneering works performed by Fackler and Bruce [7a,e]. Recently, this behaviour was also observed by Roesky and co-workers in their studies involving a new family of gold(I) complexes based on paracyclophanediyldiphosphane ligands [13a]. Encouraged by these studies, our attention was focused on the absorption and emission profiles that the title compounds have been presented in solution as well as in the solid state with the goal of adding new qualitative information about this kind of system to the literature. In order to minimise the solvent effect on the absorption profiles, the solution spectra of the prepared compounds were acquired in dilute solutions (3.0  105 M) of CH2Cl2 as a non-coordinating solvent. The spectra are shown in the Fig. 3. In order to compare the absorption profiles presented by the synthesized compounds, we initially have studied the difference between the spectra of the free ligands and the prepared complexes. As shown in Fig. 3, the spectra of the complexes 1 and 2 are very similar and both have presented two intense absorptions in the UV–Vis range. The absorption shoulders that appear at ca. 260 nm for both complexes can be attributed to the intraligand (IL) p ? p⁄ transitions from the triphenylphosphine moiety [7e,8a]. On this way the absorption shoulders that appear at ca. 255 nm for both ligands are also assigned to the intraligand (IL) p ? p⁄ transitions from aromatic system present in the structure of the ligands. The absorption shoulders at ca. 320 nm for both complexes are tentatively assigned to an intraligand transition (in character) of the type n ? p⁄ involving the N–C@S moiety, once the free ligands showed the absorption shoulders in a similar spectral region (at ca. 325 nm). On the contrary, the gold(I) precursor ((Ph)3PAuCl) does not show any significant absorption in this region (see the absorption spectrum in Figure S1 in the Supporting information). Charge transfer excitations of the LMCT (S ? Au(I)– P) type, should not be ruled out for the complexes 1 and 2 in this region of the spectrum. LMCT excitations as above mentioned have been observed in UV–Vis experiments involving similar compounds and were assigned to the (S ? Au(I)–P) transitions [7e,c,8a]. Moreover, DFT studies involving phosphine–Au(I)–S(aromatic-thiolates) derivatives which contain the P–Au(I)–S bond axis

Fig. 1. Molecular structure of 1. The hydrogen atoms, MeOH solvate and the [PF6] anion were omitted for clarity. Thermal ellipsoids are drawn at 50% probability level.

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Complex 1 Ligand 1

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Fig. 2. Molecular structure of 2. The hydrogen atoms and the [PF6] anion were omitted for clarity. Thermal ellipsoids are drawn at 50% probability level.

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in their structures, have shown that the electronic transitions that appear in the UV–Vis region (from 311 to 359 nm) were assigned mainly as LMCT excitations arising from (S ? Au–P) orbitals once the occupied level (HOMO) is a sulphur-based orbital and the LUMO is strongly located on the Au–P bond axis [8b]. Supported by these studies, we believe that the electronic transitions at ca. 320 nm for 1 and 2 can thus be assigned mainly as LMCT (S(1) ? Au(1)–P(1)) with some mixture of intraligand charge transfer (ILCT) transitions. Additionally to the solution studies, UV–Vis absorption analyses were performed in the solid state in order to compare and evaluate the solvent effect on the molecular and electronic structure of the prepared compounds (see Fig. 4).

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Wavelength (nm) Fig. 3. Absorption spectra of the prepared compounds measured at 298 K in CH2Cl2 solution (3.0  105 M for all compounds).

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Fig. 4. Absorption spectra of the complex 1 and 2 measured at 298 K in CH2Cl2 solution (solid line) and in the solid state (dotted line).The data are normalised for comparison.

It can be seen from Fig. 4, that the UV–Vis spectra profiles do not show significant differences whether in solution or in the solid state confirming the low solvent effect on the absorption bands (it is evidence that the molecular structural features of these molecules also persist in solution). 3.4. Emission studies Emissive compounds involving d10 systems such as copper(I), silver(I) and gold(I) featuring interesting structural and photophysical properties have been studied and this research area still

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The solid state emission spectra of the complexes 1 and 2 differ from those of the corresponding free ligands. The complexes have shown intense bluish emission at ca. 415 nm while the free ligands have shown poor emission when excited at kex = 320 nm. This behavior is in accordance with our interpretation on the UV–Vis absorption measurements where the absorption shoulders at ca. 320 nm (for 1 and 2) were assigned mainly to the LMCT (S ? Au– P) excitations. From the analysis of the emission spectra of the free ligands it is reasonable to interpret that the ILCT transitions have not great contribution on the emission spectra of the complexes 1 and 2. Therefore, the bluish emission at ca. 415 nm for both complexes is also tentatively assigned to the LMCT (S ? Au–P) excitations as already mentioned in similar studies [8b]. Fig. 6 shows the solid state excitation and emission spectra for the complex 2. The solid state excitation spectrum for 2 resembles closely those measured in the UV–Vis experiments for the same complex. The excitation and emission spectra for 2 are almost exact mirror images of one another, suggesting that the emission and excitation processes are probably correlated to the same excitation/emission energy levels.

4. Conclusion

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Wavelength (nm) Fig. 5. Emission spectra of the complexes 1, 2 and the ligands L1 and L2 measured in the solid state at 298 K (kex = 320 nm).

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We have successfully synthesized the first examples of gold(I) complexes based on 1-Thiocarbamoyl-pyrazoline ligands in good yields. The use of KPF6 was required to promote the cleavage of the Au–Cl bond from the gold(I) precursor giving cationic gold(I) complexes stabilized by [PF6] counterions as crystalline solids. The prepared complexes have shown interesting structural and photophysical properties featuring bluish luminescence in the solid state at room temperature. Good correlations were found between the solution and solid state UV–Vis measurements and as a general trend any significant solvent effect on the absorption profiles was observed. The bluish emission for 1 and 2 at ca. 415 nm was mainly assigned to the LMCT (S ? Au–P) excitations arising from the sulphur atom to the Au–P bond axis once the free ligands are poorly luminescent when excited at 320 nm. The structural resemblance between the complexes herein described and other anticancer complexes already published in the literature [5], suggests that the presented molecules should be tested as new metal-based antitumor drugs. New studies involving Cu(I), Ag(I) and other mononuclear and binuclear gold(I) complexes based on the title ligands are ongoing.

Acknowledgments 300

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Wavelength (nm) Fig. 6. Solid state excitation (kem = 415 nm) and emission (kex = 320 nm) measured at 298 K. The data are normalised for comparison. (The complex 1 has presented a similar behaviour).

requires great advances and new challenges must be overcome [9,13]. On this way, our attention was focused particularly on the qualitative luminescence features of the prepared compounds in the solid state. It was noticed during the course of the experiments that the solid samples of the prepared complexes emit bluish light clearly visible to the naked eye in the dark room. Thus, the complexes 1 and 2 are representative examples of d10 compounds that present bluish luminescence at room temperature when excited at kex = 320 nm. Fig. 5 displays the emission spectra of the prepared compounds.

We gratefully acknowledge the Brazilian Research Agencies, CNPq (Projeto Universal 2010-2012) and FUNDECT-MS (Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado do Mato Grosso do Sul) and FAPESP for financial support. A. Ferle also thanks UFGD and CAPES for the scholarships.

Appendix A. Supplementary data CCDC 880826 and 880825 contains the supplementary crystallographic data for 1 and 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.poly.2013.07.007.

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