Single Two-Electron Transfers and Successive One-Electron Transfers

Jan 4, 2011 - The IT bands in the near-IR spectra are rationalized in the framework of Marcus−Hush theory. The rigid and planar indacene platform bo...
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Organometallics 2011, 30, 1116–1121 DOI: 10.1021/om101123a

Single Two-Electron Transfers and Successive One-Electron Transfers in Biferrocenyl-Indacene Isomers§ Alessandro Donoli,† Annalisa Bisello,† Roberta Cardena,† Franco Benetollo,‡ Alberto Ceccon,† and Saverio Santi*,‡ †

Dipartimento di Scienze Chimiche, Universit a degli Studi di Padova, Via Marzolo 1, 35131 Padova, Italy, and ‡CNR, Istituto di Chimica Inorganica e delle Superfici, C.so Stati Uniti 4, 35127 Padova, Italy Received November 30, 2010

Novel biferrocenyl complexes of s- and as-dihydroindacenes have been prepared and the charge transfer properties of their mono- and dicationic derivatives, selectively generated by one-electron and two-electron oxidation, have been investigated. Mixed-valence cations are generated by chemical oxidation using acetylferricinium as an oxidant agent and monitored in the visible, IR, and near-IR regions. The IT bands in the near-IR spectra are rationalized in the framework of Marcus-Hush theory. The rigid and planar indacene platform bonded to two terminal redox groups displays a redox chemistry that can be switched from single two-electron transfers to two successive one-electron transfers by changing the supporting electrolyte from nBu4NPF6 to nBu4NB(C6F5)4.

Introduction Metallocene-based metallorganic frameworks having a π-conjugated system and displaying multielectron redox chemistry have attracted considerable attention because they exhibit interesting electrochemical, electronic, magnetic, and optical properties.1 In particular, the use of the FeII/ FeIII couple of ferrocene as charge (electron) carriers in conjugated organic chains is often mentioned as a potential application of these materials as molecular electronic devices.2 Mixed-valence compounds having two equivalent § Dedicated to Prof. J€urgen Heck on the occasion of his 60th birthday. *To whom correspondence should be addressed. E-mail: saverio. [email protected]. (1) (a) Fukino, T.; Fujita, N.; Aida, T. Org. Lett. 2010, 12, 3074. (b) Chebny, V. J.; Dhar, D.; Lindeman, S. V.; Rathore, R. Org. Lett. 2006, 8, 5041. (c) Manners, I. Science 2001, 294, 1664. (d) De Cola, L.; Belser, P. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol 5, p 97. (e) Paddow-Row, M. N. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol 2, p 179. (f) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (g) Santi, S.; Orian, L.; Donoli, A.; Bisello, A.; Scapinello, M.; Benetollo, F.; Ganis, P.; Ceccon, A. Angew. Chem., Int. Ed. 2008, 47, 5331. (2) (a) Debroy, P.; Roy, S. Coord. Chem. Rev. 2007, 251, 203. (b) Zhang, R.; Wang, Z.; Wu, Y.; Fu, H.; Yao, J. Org. Lett. 2008, 10, 3065. (c) Caballero, A.; Tarraga, A.; Velasco, M. D.; Espinosa, A.; Molina, P. Org. Lett. 2005, 7, 3171. (3) (a) Venkatasubbaiah, K.; Doshi, A.; Nowik, I.; Herber, R. H.; Reingold, A. L.; J€ akle, F. Chem.;Eur. J. 2008, 14, 444. (b) Santi, S.; Orian, L.; Durante, C.; Bencze, E. Z.; Bisello, A.; Donoli, A.; Ceccon, A.; Benetollo, F.; Crociani, L. Chem.;Eur. J. 2007, 13, 7933. (c) Wagner, M. Angew. Chem., Int. Ed. 2006, 45, 5916. (d) Nishihara, H. Bull. Chem. Soc. Jpn. 2001, 74, 19. (e) Barlow, S.; O'Hare, D. Chem. Rev. 1997, 97, 637. (4) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Allen, C. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357. (c) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10, 247. (d) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. (e) Crutchley, R. J. Adv. Inorg. Chem. 1994, 41, 273. (f) Demadis, K. D.; Haertshorn, C. M.; Meyer, T. J. Chem. Rev. 2001, 101, 2655. (g) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002, 31, 168. (h) D'Alessandro, D. M.; Keene, F. R. Chem. Soc. Rev. 2006, 35, 424. (i) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Coord. Chem. Rev. 2004, 248, 683. (j) Aguirre-Etcheverry, P.; O'Hare, D. Chem. Rev. 2010, 110, 4839.

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ferrocenes in different oxidation states are excellent benchmarks for the study of intramolecular electron transfer.3 The assessment of the electron transfer mechanism in mixed valence intermediates of conjugated biferrocenes has been mostly based on the Hush theory.4 In these studies, electrochemical and optical techniques are the most important tools to evaluate the magnitude of metal-metal electronic interaction. Much effort has been invested in the elucidation of the role that the nature of the spacer between two interacting units plays in governing the redox splitting ΔE (and the related comproportionation equilibrium constant, Kc) between successive electron transfer steps. As previously reported,5 caution is needed in the interpretation of electrochemical data due to the considerable dependence of redox potential and reversibility on the nature of solvent and supporting electrolyte. Moreover, it is important to take into account that a single redox process resulting from a small ΔE is not diagnostic of negligible communication. In fact, even in the case where the ΔE value is small and a single electrochemical wave is observed, a significant amount of mixed valence species is present in solution.6 A study on a series of diferrocenylpolyenes of general formula Fc(CHdCH)nFc (n = 1-6) showed that the mixed valence cations are (5) (a) Geiger, W. E.; Barriere, F. Acc. Chem. Res. 2010, 43, 1030. (b) D'Alessandro, D. M.; Keene, F. R. Dalton Trans. 2004, 3950. (c) Santi, S.; Orian, L.; Durante, C.; Bisello, A.; Benetollo, F.; Crociani, L.; Ganis, P.; Ceccon, A. Chem.;Eur. J. 2007, 13, 1955. (d) Santi, S.; Orian, L.; Durante, C.; Bencze, E. Z.; Bisello, A.; Donoli, A.; Benetollo, F.; Crociani, L.; Ceccon, A. Chem.;Eur. J. 2007, 13, 7933. (e) Diallo, A. K.; Daran, J.-C.; Carret, F.; Ruiz, J.; Astruc., D. Angew. Chem., Int. Ed. 2009, 48, 3141. (f) Bruna, S.; Gonzalez-Vadillo, A. M.; Nieto, D.; Pastor, C.; Cuadrado, C. Organometallics 2010, 29, 2796. (g) Hildebrandt, A.; R€uffer, T.; Erasmus, E.; Swarts, J. C.; Lang, H. Organometallics 2010, 29, 4900. (6) (a) Launay, J.-P. Chem. Soc. Rev. 2001, 30, 386. (b) Ribou, A.-C.; Launay, J.-P.; Sachtleben, M. L.; Li, H.; Spangler, C. W. Inorg. Chem. 1996, 35, 3735. (c) Hapiot, P.; Kispert, L. D.; Konovalov, V. V. V.; Saveant, J.-M. J. Am. Chem. Soc. 2001, 123, 6669. r 2011 American Chemical Society

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detectable and metal-metal electronic coupling exists even when the ΔE splitting is not observed (n = 4-6).6b If the mixed valence intermediate is detectable, a powerful probe for evaluating the magnitude of the metal-metal interaction in mixed valence compounds involves analysis of the intervalence charge transfer (IVCT) absorption bands in the nearIR region.4 In addition, if the energy gap between the metal group and the organic bridge is small, a ligand-to-metal charge transfer (LMCT) band appears in the visible region.7 Thus, the electronic properties in bimetallic molecules, such as polyaromatic biferrocenes, could result from either intramolecular metal-to-metal or ligand-to-metal electronic (charge) transfer. Herein, in order to get insight into such phenomena, we described (Scheme 1) the synthesis and the structural characterization of a family of mono- (1, 2) and bimetallic (3, 4) ferrocenyl complexes of the as- and s-dihydroindacenes and examined the electrochemical and optical properties of their mono- and dications generated by chemical oxidation.

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Scheme 1. Syntheses of Mono- (1, 2) and Bimetallic (3, 4) Ferrocenyl Complexes of the as- and s-Dihydroindacenesa

Experimental Section General Methods. All reactions and complex manipulations were performed in an oxygen- and moisture-free atmosphere utilizing standard Schlenk techniques or in a Mecaplex glovebox. Solvents were dried by reflux over the appropriate drying agent and distilled under a stream of argon. Ferrocene, tertbutyllithium solution, anhydrous ZnCl2 solution, N-bromosuccinimide (NBS), phosphorus pentoxide (P2O5), and PdCl2(PPh3)2 were Sigma Aldrich products. Acetylferricinium8 and mixtures of 1,4-, 3,4-, and 1,6-as-dihydroindacene9 and of 1,5- and 1,7-sdihydroindacenes10 were synthesized according to the published procedures. Microanalyses were performed at the Dipartimento di Scienze Chimiche, Universita di Padova. The X-ray structures were obtained by collecting the intensity data at RT using a Philips PW1100 single-crystal diffractometer (FEBO system) using graphite-monochromated (Mo KR) radiation, following the standard procedures. All intensities were corrected for Lorentz polarization and absorption.11 The structures were solved by direct methods using SIR-97.11 Refinements were carried out by full-matrix least-squares procedures (based on Fo2) using anisotropic temperature factors for all non-hydrogen atoms. The H-atoms were placed in calculated positions with fixed, isotropic thermal parameters (1.2Uequiv of the parent carbon atom). The calculations were performed with the SHELXL-97 program,11 implemented in the WinGX package.11 HRMS spectra were obtained using an ESI-TOF Mariner 5220 (Applied Biosystem) mass spectrometer with direct injection of the sample and collecting data in the positive mode. 1H and 13C NMR spectra (7) Zhu, Y.; Wolf, M. O. J. Am. Chem. Soc. 2000, 122, 10121. (8) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877. (9) Erden, I.; Xu, F. P.; Sadoun, A.; Smith, W.; Sheff, G.; Ossun, M. J. Org. Chem. 1995, 60, 813. (10) (a) Bell, W. L.; Curtis, V. C.; Eigenbrot, W., Jr.; Pierpont, J. L.; Robbins, C. G.; Smart, J. C. Organometallics 1987, 6, 266. (b) Trogen, L.; Edlund, U. Acta Chem. Scand. B 1979, 33, 109. (11) (a) North, A. T. C.; Philips, D. C.; Mathews, F. S. Acta Crystallogr. 1968, A24, 351. (b) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR-97. J. Appl. Crystallogr. 1999, 32, 115. (c) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of G€ottingen: Germany, 1997. (d) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (12) (a) Bax, A.; Subramanian, S. J. Magn. Reson. 1986, 67, 565. (b) Parella, T. Magn. Reson. Chem. 1998, 36, 467. (c) Ruiz-Cabello, J.; Vuister, G. W.; Moonen, C. T. W.; van Gelderen, P.; Cohen, J. S.; van. Zijl, P. J. Magn. Reson. 1992, 100, 282. (d) Willker, W.; Leibfritz, D.; Kerrsebaum, R.; Bermel, W. Magn. Reson. Chem. 1993, 31, 287. (e) Bax, A.; Summers, M. F. J. Am. Chem. Soc. 1986, 108, 2093. (f) Summers, M. F.; Marzilli, L. G.; Bax, A. J. Am. Chem. Soc. 1986, 108, 4285.

a Indacenes, dibromoindacenes, and complexes 1-4 are mixtures of isomers (see ref 18) differing from the double-bond position (- - -).

were obtained on a Bruker Avance DRX spectrometer (T = 298 K) operating at 400.13 and 100.61 MHz, respectively. The assignments of the proton resonances were performed by standard chemical shift correlation and 2D (NOESY and COSY) experiments. The 13C resonances were attributed through 2D-heterocorrelated COSY experiments (HMQC12 for the H-bonded carbon atoms, HMBC12 for the quaternary ones). CV experiments were performed in an airtight three-electrode cell connected to a vacuum/argon line. The reference electrode was a SCE (Tacussel ECS C10) separated from the solution by a bridge compartment filled with the same solvent/supporting electrolyte solution used in the cell. The counter electrode was a platinum spiral with ca. 1 cm2 apparent surface area. The working electrodes were disks obtained from a cross section of gold wires of different diameters (0.5, 0.125, and 0.025 mm) sealed in glass. Between successive CV scans the working electrodes were polished on alumina according to standard procedures and sonicated before use. An EG&G PAR-175 signal generator was used. The currents and potentials were recorded on a Lecroy 9310 L oscilloscope. The potentiostat was home-built with positive feedback loop for compensation of ohmic drop.13 Autolab PGSTAT 30 potentiostat/galvanostat (EcoChemie, The Netherlands) run by a PC with GPES software was used for the DPV experiments. The measurements were conducted in an airtight three-electrode cell, the same as used for the CV experiments. For DPV, we used a peak amplitude of 50 mV, a pulse width of 0.05 s, a 2 mV increment per cycle, and a pulse period of 0.1 s. UV-vis and near-IR absorption spectra were recorded with a Varian Cary 5 spectrophotometer. Preparation of the as-Dihydroindacene Bromides. To a stirred solution of as-dihydroindacenes (2 g, 13.0 mmol) and water (0.89 mL) in DMSO at 25 °C was added N-bromosuccinimide (13) Amatore, C.; Lefrou, C.; Pfl€ uger, F. J. Electroanal. Chem. 1989, 270, 43. (14) Ewen, I. M.; Ronnqvist, M.; Ahlberg, P. J. Am. Chem. Soc. 1993, 115, 3989.

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(4.6 g, 25.8 mmol).14 After 15 min the reaction mixture was poured into water (120 mL), and the crude product was extracted with diethyl ether (4  100 mL). The solvent was removed under vacuum. Precipitation with pentane from a CHCl3 solution of the crude products gave a bromohydrine white powder, which was used without any further purification (804 mg). A solution of as-dihydroindacene bromohydrine white powder (804 mg) in carbon tetrachloride (50 mL) was refluxed with phosphorus pentoxide (1.2 g, 8.4 mmol) for 2 h.15 After filtration, the solvent was removed under vacuum and the crude products eluted on a silica gel column using petroleum ether as eluent. Purification gave a mixture of 1,4-dihydro-2, 5-dibromo-, 3,4-dihydro-2,5-dibromo-, and 1,6-dihydro-2, 5-dibromo-as-indacenes. Yield: 512 mg, 1.6 mmol, 12% with respect to as-dihydroindacene. Anal. Calcd for C12H8Br2: C, 46.20; H, 2.58. Found: C, 46.42; H, 2.74. 1,4-Dihydro-2,5-dibromo-asindacenes: 1H NMR (CDCl3): δ 3.59 (m, 2H, H4), 3.62 (m, 2H, H1), 6.93 (m, 1H, H6), 6.94 (m, 1H, H3), 7.10 (qAB, 1H, J = 7.58 Hz, H7), 7.23 (qAB, 1H, J = 7.58 Hz, H8). 13C NMR (CDCl3): δ 43.66 (C4), 45.34 (C1), 116.58 (C7), 121.37 (C8), 123.72 (C2), 126.28 (C5), 130.34 (C3), 133.11 (C6), 138.55 (C3a), 139.27 (C6a), 139.54 (C3b), 142.87 (C8a). 3,4-Dihydro-2, 5-dibromo-as-indacenes: 1H NMR (CDCl3): δ 3.54 (m, 4H, H3,H4), 6.90 (m, 2H, H1, H6), 7.14 (s, 2H, H7, H8). 13C NMR (CDCl3): δ 43.58 (C3, C4), 133.19 (C1, C6), 118.78 (C7, C8), 123.10 (C2, C5), 136.94 (C3a, C3b), 141.34 (C6a, C8a). 1,6Dihydro-2,5-dibromo-as-indacenes: 1H NMR (CDCl3): δ 3.61 (m, 4H, H1,H6), 7.01 (m, 2H, H3, H4), 7.16 (s, 2H, H7, H8). 13C NMR (CDCl3): δ 45.31 (C1, C6), 119.20 (C7, C8), 125.20 (C2, C5), 130.55 (C3, C4), 135.75 (C3a, C3b), 141.14 (C6a, C8a). Preparation of the s-Dihydroindacene Bromides. To a stirred solution of s-dihydroindacenes (2 g, 13.0 mmol) and water (0.89 mL) in DMSO at 25 °C was added N-bromosuccinimide (4.6 g, 25.8 mmol).14 After 15 min the reaction mixture was poured into water (120 mL), and the crude product was extracted with diethyl ether (4  100 mL). The solvent was removed under vacuum. Precipitation with pentane from a CHCl3 solution of the crude products gave a bromohydrine white powder, which was used without any further purification (950 mg). Similarly to the preparation of the bromides of as-dihydroindacenes, a solution of s-dihydroindacene bromohydrine white powder (950 mg) in carbon tetrachloride (60 mL) was refluxed with phosphorus pentoxide (1.6 g, 11.3 mmol) for 2 h.15 After cooling to room temperature, the reaction mixtures were washed with a saturated NaHCO3 solution (140 mL) and extracted with diethyl ether (4  100 mL), and the ether layers dried over Na2SO4. After filtration, the solvent was removed under vacuum and the crude products eluted on a silica gel column using petroleum ether as eluent. Purification gave a mixture of 1, 5-dihydro-2,6-dibromo- and 1,7-dihydro-2,6-dibromo-s-indacenes. Yield: 536 mg, 1.7 mmol, 13% with respect to s-dihydroindacenes. Anal. Calcd for C12H8Br2: C, 46.20; H, 2.58. Found: C, 46.53; H, 2.80. 1,5-Dihydro-2,6-dibromo-s-indacenes: 1 H NMR (CDCl3): δ 3.57 (s, 4H, H1,H5), 6.89 (m, 2H, H3, H7), 7.29 (s, 2H, H4, H8). 13C NMR (CDCl3): δ 45.08 (C1, C5), 115.28 (C4, C8), 122.93 (C6), 132.78 (C3, C7), 124.17 (C2), 141.13 (C3a, C4a, C7a, C8a). 1,7-Dihydro-2,6-dibromo-s-indacenes: 1H NMR (CDCl3): δ 3.57 (s, 4H, H1,H7), 6.91 (m, 2H, H3, H5), 7.21 (s, 1H, H4), 7.35 (s, 1H, H8). 13C NMR (CDCl3): δ 45.08 (C1, C7), 111.94 (C4), 118.29 (C8), 132.78 (C3, C5), 122.93 (C6), 124.17 (C2), 139.45 (C7a, C8a), 142.52 (C3a, C4a). Preparation of the Mono- and Biferrocenyl Complexes of asDihydroindacenes (1 and 3). To a solution of ferrocene (900 mg, 4.8 mmol) in dry THF (10 mL) was added a pentane solution of tBuLi (3.8 mL, 1.7 M in pentane, 6.5 mmol) at 0 °C under an argon atmosphere, obtaining a dark orange solution. After stirring for 0.5 h, a solution of ZnCl2 in THF (9.6 mL, 0.5 M in THF, 4.8 mmol) was added. After 0.5 h, the reaction mixture (15) Porter, H. D.; Sute, C. M. J. Am. Chem. Soc. 1935, 57, 2023.

Donoli et al. was warmed to 25 °C and stirred for 1 h to give an orange suspension. To the resulting mixture was added a THF solution (10 mL) of as-dihydroindacene bromides (500 mg, 1.6 mmol) followed by the addition of a THF suspension (5 mL) of PdCl2(PPh3)2 (100 mg, 0.14 mmol). After stirring for 22 h, the mixture was poured into 50 mL of H2O, and the organic layer was separated. The water layer was extracted in Et2O (4  50 mL). The combined organic layers were dried over anhydrous Na2SO4. After filtration, the solvent was removed under vacuum to give a red-brown crude residue. Purfication by silica gel column chromatography (hexane/CH2Cl2, 4:1) of the crude residue gave a mixture of 1,4-dihydro-2-bromo-5-ferrocenyl-, 3,6-dihydro-2bromo-5-ferrocenyl-, 3,4-dihydro-2-bromo-5-ferrocenyl-, and 1,6-dihydro-2-bromo-5-ferrocenyl-as-indacenes (1) and a mixture of 1,4-dihydro-2,5-diferrocenyl-, 3,4-dihydro-2,5-diferrocenyl-, and 1,6-dihydro-2,5-diferrocenyl-as-indacenes (3). 1. Yield: 85 mg, 13%. Anal. Calcd for C22H17BrFe: C, 63.35; H, 4.11. Found: C, 63.32; H, 4.29. HRMS (ESIþ): m/z calcd for C22H17BrFe (Mþ), 415.9863; found, 415.9902. 1,4- and 3,6-Dihydro-2-bromo-5-ferrocenyl-as-indacenes (1a þ 1b, 10%). 1a: 1 H NMR (CD2Cl2): δ 3.68 (m, 4H, H1, H4), 4.09 (s, 5H, Cp), 4.34 (m, 2H, Hβ, Hβ0 ), 4.59 (m, 2H, HR, HR0 ), 6.83 (m, 1H, H6), 6.97 (m, 1H, H3), 7.11 (qAB, 1H, J = 7.70 Hz, H7), 7.26 (qAB, 1H, J = 7.60 Hz, H8). 13C NMR (CD2Cl2): δ 38.30 (C1), 38.68 (C4), 66.64 (CR, CR0 ), 69.39 (Cβ, Cβ0 ), 69.69 (CCp), 116.17 (C7), 121.58 (C8), 124.24 (C6), 133.26 (C2), 133.71 (C3), 145.16 (C6a), 145.88 (C5). 1b: 1H NMR (CD2Cl2): δ 3.68 (m, 4H, H3, H6), 4.09 (s, 5H, Cp), 4.34 (m, 2H, Hβ, Hβ0 ), 4.59 (m, 2H, HR, HR0 ), 6.83 (m, 1H, H4), 6.97 (m, 1H, H6), 7.11 (qAB, 1H, J = 7.70 Hz, H8), 7.26 (qAB, 1H, J = 7.60 Hz, H7). 13C NMR (CD2Cl2): δ 38.30 (C3), 38.68 (C6), 66.64 (CR, CR0 ), 69.39 (Cβ, Cβ0 ), 69.69 (CCp), 116.17 (C8), 121.58 (C7), 124.24 (C4), 133.26 (C2), 133.71 (C1), 139.64 (C8a), 145.88 (C5). 3,4-Dihydro-2-bromo5-ferrocenyl-as-indacenes (1c, 70%): 1H NMR (CD2Cl2): δ 3.63 (m, 4H, H3, H4), 4.09 (s, 5H, Cp), 4.33 (m, 2H, Hβ, Hβ0 ), 4.57 (m, 2H, HR, HR0 ), 6.81 (m, 1H, H6), 6.94 (m, 1H, H1), 7.15 (qAB, 1H, J = 7.80 Hz, H7), 7.16 (qAB, 1H, J = 7.80 Hz, H8). 13 C NMR (CD2Cl2): δ 38.21 (C4), 44.11 (C3), 66.64 (CR, CR0 ), 69.39 (Cβ, Cβ0 ), 69.69 (CCp), 81.23 (Cγ), 118.81 (C7, C8), 124.24 (C6), 133.69 (C2), 133.71 (C1), 137.57 (C3a,C3b), 140.87 (C8a), 144.45 (C6a), 145.63 (C5). 1,6-Dihydro-2-bromo5-ferrocenyl-as-indacenes (1d, 20%).: 1H NMR (CD2Cl2): δ 3.67 (m, 2H, H1), 3.70 (m, 2H, H6) 4.09 (s, 5H, Cp), 4.34 (m, 2H, Hβ, Hβ0 ), 4.59 (m, 2H, HR, HR0 ), 6.84 (m, 1H, H4), 6.97 (m, 1H, H3), 7.10 (qAB, 1H, J = 7.60 Hz, H8), 7.30 (qAB, 1H, J = 7.60 Hz, H7). 13C NMR (CD2Cl2): δ 38.30 (C1), 44.00 (C6), 66.64 (CR, CR0 ), 69.39 (Cβ, Cβ0 ), 69.69 (CCp), 116.17 (C8), 121.11 (C4), 121.95 (C7), 133.26 (C3a), 133.71 (C3), 133.80 (C2), 139.54 (C3b), 140.17 (C8a), 142.05 (C6a) 148.92 (C5). 3: Yield: 120 mg, 14%. Anal. Calcd for C32H26Fe2: C, 73.60; H, 5.02. Found: C, 73.56; H, 5.31. HRMS (ESIþ): m/z calcd for C32H26Fe2 (Mþ), 522.0733; found, 522.0630. 1,4-Dihydro-2,5-diferrocenylas-indacenes (3a, 70%): 1H NMR (CD2Cl2): δ 3.72 (m, 4H, H1, H4), 4.11 (s, 10H, Cp), 4.35 (m, 4H, Hβ, Hβ0 ), 4.62 (m, 4H, HR, HR0 ), 6.85 (m, 1H, H6), 6.94 (m, 1H, H3), 7.09 (qAB, 1H, J = 7.60 Hz, H7), 7.29 (qAB, 1H, J = 7.60 Hz, H8). 13C NMR (CD2Cl2): δ 38.52 (C4), 40.09 (C1), 66.75 (CR, CR0 ), 69.35 (Cβ, Cβ0 ), 69.70 (CCp), 81.50 (Cγ), 115.76 (C7), 121.82 (C8), 124.39 (C3, C6), 133.47 (C3b), 138.84 (C8a), 141.10 (C3a), 144.96 (C6a), 145.41 (C5), 147.67 (C2). 3,4-Dihydro-2,5-diferrocenylas-indacenes (3b, 30%): 1H NMR (CD2Cl2): δ 3.69 (m, 4H, H3, H4), 4.11 (s, 10H, Cp), 4.35 (m, 4H, Hβ, Hβ0 ), 4.61 (m, 4H, HR, HR0 ), 6.81 (m, 2H, H1, H6), 7.14 (s, 2H, H7, H8). 13C NMR (CD2Cl2): δ 40.08 (C3, C4), 69.40 (Cβ, Cβ0 ), 66.51 (CR, CR0 ), 69.70 (CCp), 81.87 (Cγ), 118.61 (C7, C8), 124.59 (C1, C6), 137.68 (C3a, C3b), 143.16 (C6a, C8a), 144.78 (C2,C5). 1,6Dihydro-2,5-diferrocenyl-as-indacenes (3c, 10%): 1H NMR (CD2Cl2): δ 3.68 (m, 4H, H1, H6), 4.12 (s, 10H, Cp), 4.35 (m, 4H, Hβ, Hβ0 ), 4.61 (m, 4H, HR, HR0 ), 7.01 (m, 2H, H3, H4), 7.20 (s, 2H, H7, H8). 13C NMR (CD2Cl2): δ 38.36 (C3, C6), 66.62

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Figure 1. X-ray structure of 1c. (CR, CR0 ), 69.25 (Cβ, Cβ0 ), 69.72 (CCp), 81.57 (Cγ), 119.08 (C7, C8), 122.09 (C3, C4), 137.75 (C6a, C8a), 141.40 (C3a, C3b), 146.45 (C2,C5). Preparation of the Mono- and Biferrocenyl Complexes of s-Dihydroindacenes (2 and 4). All the reactions were performed by using the same procedures followed for the synthesis of 1 and 3 and the same amounts of reagents. Purification by preparative TLC on silica gel (hexane/CH2Cl2, 4:1) of the crude residue gave a mixture of 1,5-dihydro-2-ferrocenyl-6-bromo- and 1,7-dihydro-2-bromo-6-ferrocenyl-s-indacenes (2) and a mixture of 1,5dihydro-2,6-diferrocenyl- and 1,7-dihydro-2,6-diferrocenyl-sindacenes (4). 2: Yield: 72 mg, 11%. Anal. Calcd for C22H17BrFe: C, 63.35; H, 4.11. Found: C, 62.98; H, 4.20. HRMS (ESIþ): m/z calcd for C22H17BrFe (Mþ), 415.9863; found, 415.9898. 1,5-Dihydro-2-bromo-6-ferrocenyl-s-indacenes (2a, 45%): 1H NMR (CD2Cl2): δ 3.61 (m, 4H, H1, H5), 4.12 (s, 5H, Cp), 4.38 (m, 2H, Hβ, Hβ0 ), 4.62 (m, 2H, HR, HR0 ), 6.72 (m, 1H, H7), 6.95 (m, 1H, H3), 7.28 (s, 1H, H8), 7.36 (s, 1H, H4). 13C NMR (CD2Cl2): δ 45.52 (C1, C5), 66.71 (CR, CR0 ), 69.60 (Cβ, Cβ0 ), 69.97 (CCp), 115.53 (C8), 115.59 (C4), 124.27 (C7), 133.38 (C3), 141.55 (C4a), 141.77 (C7a, C8a), 142.15 (C6), 143.22 (C3a), 146.45 (C2). 1,7-Dihydro-2-bromo-6-ferrocenyl-s-indacenes (2b, 55%): 1H NMR (CD2Cl2): δ 3.62 (m, 4H, H1, H7), 4.11 (s, 5H, Cp), 4.36 (m, 2H, Hβ, Hβ0 ), 4.60 (m, 2H, HR, HR0 ), 6.77 (m, 2H, H3, H5), 7.21 (s, 1H, H4), 7.44 (s, 1H, H8). 13C NMR (CD2Cl2): δ 39.80 (C1, C7), 66.71 (CR, CR0 ), 69.60 (Cβ, Cβ0 ), 69.97 (CCp), 112.12 (C4), 118.92 (C8), 124.29 (C3), 124.43 (C5), 141.77 (C3a, C4a), 142.72 (C8a), 144.79 (C7a). 4: Yield: 36 mg, 4%. Anal. Calcd for C32H26Fe2: C, 73.60; H, 5.02. Found: C, 73.68; H, 4.88. HRMS (ESIþ): m/z calcd for C32H26Fe2 (Mþ), 522.0733; found, 522.0701. 1,5-Dihydro-2,6-diferrocenyls-indacenes (4a, 55%): 1H NMR (CD2Cl2): δ 3.54 (m, 4H, H1, H5), 4.12 (s, 10H, Cp), 4.39 (m, 4H, Hβ, Hβ0 ), 4.64 (m, 4H, HR, HR0 ), 6.65 (m, 2H, H3, H7), 7.31 (s, 2H, H4, H8). 13C NMR (CD2Cl2): δ 39.11 (C1, C5), 66.00 (CR, CR0 ), 69.81 (Cβ, Cβ0 ), 70.09 (CCp), 82.25 (Cγ), 114.98 (C4, C8), 127.19 (C3, C7), 140.70 (C2, C6), 142.91 (C3a, C4a, C7a, C8a). 1,7-Dihydro2,6-diferrocenyl-s-indacenes (4b, 45%): 1H NMR (CD2Cl2): δ 3.65 (m, 4H, H1, H7), 4.10 (s, 10H, Cp), 4.34 (m, 4H, Hβ, Hβ0 ), 4.59 (m, 4H, HR, HR0 ), 6.80 (m, 2H, H3, H5), 7.19 (s, 1H, H4) 7.48 (s, 1H, H8). 13C NMR (CD2Cl2): δ 39.60 (C1, C7), 66.44 (CR, CR0 ), 69.37 (Cβ, Cβ0 ), 69.79 (CCp), 82.25 (Cγ), 124.51 (C3, C5), 111.65 (C4), 119.01 (C8), 139.54 (C3a, C4a), 144.48 (C7a, C8a) 145.76 (C2, C6).

Figure 2. Oxidative CVs in CH2Cl2 at 20 °C of 3 with (a) 0.1 M nBu4NPF6, (b) 0.1 M nNBu4TFAB, and (c) DPV with 0.1 M nBu4NTFAB. Oxidative CVs in CH2Cl2 of 4 with (d) 0.1 M nBu4NPF6, (e) 0.1 M nBu4NTFAB, and (f) DPV with 0.1 M nBu4NTFAB.

verted to bromohydrines. Then, dehydratation reaction with P2O5 in CCl4 at 80 °C gave the bromide mixtures.16 Subsequently, the Negishi reaction17 of excess FcZnCl (Fc = ferrocenyl), as-/s-dihydrodibromoindacenes, and PdCl2(PPh3) catalyst in THF at 25 °C for 22 h gave the π-conjugated complexes 1-4.18 In the molecular structure obtained for 3,4-dihydro-2bromo-5-ferrocenyl-as-indacene (1c) (Figure 1) the as-indacene group and the σ-bonded Cp ring of the ferrocene moiety are almost coplanar, as imposed by an operative π-electron resonance with a torsion angle about C10-C11 of 15-16°. Similar features were found for the structurally correlated (2-ferrocenyl)indene.19 Electrochemistry. Cyclic voltammograms (CVs) of 1-4 were recorded under argon in CH2Cl2/0.1 M nBu4NPF6. All the complexes show one oxidation wave in the range from 0 to 1 V vs SCE (Figure 2a,d and Supporting Information), which consistently met the chemical reversibility criteria in the range of scan rates of 0.1-50 V s-1 as they all showed cathodic/anodic peak current ratios of ia/ic = 1. Monometallic ferrocenyl complexes 1 and 2 showed a single reversible wave at a potential of Ep = 0.48 V vs SCE. Irrespective of the number of electroactive Fc groups, 3 and 4 showed only a single reversible wave at a potential of Ep = 0.48 and 0.49 V vs SCE, respectively, thus suggesting that the ferrocenyl groups in 3 and 4 are electrochemically indistinguished. We also recorded the CVs of 3 and 4 with nBu4NTFAB (TFAB = B(C6F5)4-) as the supporting electrolyte, whose weak ionpairing and nucleophilic properties have been previously described.5 Interestingly, we found that in CVs of 3 and 4

Results and Discussion Synthesis and Structure. Mixtures of 1,4-, 3,4-, and 1,6-asdihydroindacene (3.5: 2.5:1 ratio) and of 1,5- and 1,7-sdihydroindacenes (1:1) were synthesized according to the literature procedures.9,10 By reaction with NBS and water in DMSO at 25 °C the as-/s-dihydroindacenes were con(16) 1,4-Dihydro-, 3,4-dihydro-, and 1,6-dihydro-2,5-dibromo-as-indacenes; 1,5-dihydro- and 1,7-dihydro-2,6-dibromo-s-indacenes.

(17) (a) Negishi, E.; Liu, F. In Metal Catalyzed Cross Coupling Reactions; Diederich, F., Stang, O. J., Eds.; Wiley VCH: New York, 1997. (b) Anderson, J. C.; White, C.; Stenson, K. P. Synlett 2002, 9, 1511. (18) 1 is a mixture of 1,4-, 3,6- (1a þ 1b, 10%), 3,4- (1c, 70%), and 1,6(1d, 20%) dihydro-2-bromo-5-ferrocenyl-as-indacene; 2 is a mixture of 1,5- (2a, 45%) and 1,7- (2b, 55%) dihydro-2-bromo-6-ferrocenyl-sindacenes; 3 is a mixture of 1,4- (3a, 70%), 3,4- (3b, 20%), and 1,6(3c, 10%) dihydro-2,5-diferrocenyl-as-indacene; 4 is a mixture of 1,5(4a, 55%) and 1,7- (4b, 45%) dihydro-2,6-diferrocenyl-s-indacenes. (19) Santi, S.; Ceccon, A.; Crociani, L.; Gambaro, A.; Ganis, P.; Tiso, M.; Venzo, A.; Bacchi, A. Organometallics 2002, 21, 565.

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Donoli et al. Table 1. Near-IR Data in CH2Cl2

-1

1þd 2þd 3þe,f 32þe 32þd 4þe,f 42þe

ν~max (cm )

type

εmax (M-1cm-1)

(Δν~1/2)obsd (cm-1)

8756 8532 5570 9280 9230 5576 8900

LMCT LMCT IVCT LMCT LMCT IVCT LMCT

1508 1958 1286f 2930 2630 2036g 3170

2710 2510 2690 2600 2730 2740 2740

(Δν~1/2)Husha (cm-1)

Γb

3590

0.25

3590

0.24

f (cm-1)c 0.019 0.023 0.016 0.035 0.033 0.026 0.040

a (Δν~1/2)Hush(cm-1) = (16RTln2 ν~max)1/2 calculated for the deconvoluted low-energy Gaussian component; T = 20 °C. b Γ = 1 - (Δν~1/2)obsd/ (Δν~1/2)Hush (ref 4g). c f = (4.6  10-9)εmaxΔν~1/2 is the oscillator strength (ref 4a). d nBu4NPF6 as supporting electrolyte. e nBu4NB(C6F5) as supporting electrolyte. f For data in nBu4NPF6, see ref 21. g The concentrations of 3þ and 4þ have been determined taking into account the Kc values and the dication concentration.

in perfluorinated electrolyte (Figure 2b,e) the wave splits into two waves due to subsequent 3/3þ, 3þ/32þ and 4/4þ, 4þ/42þ oxidations, as confirmed by differential pulse voltammetry (DPV) (Figure 2c,f). One may argue that the splitting can be merely due to different electrochemical properties of the isomers present in solution.18 However, in the case of 3, the isomeric ratio 7:2:1 disagrees with the relative 1:1 intensity of the peaks in the DPV voltammogram. Concerning 4, the selective generation of 4þ by oxidation, as demostrated by its spectroscopic characterization (see below), clearly indicates that the DPV peaks are due to subsequent redox processes. The 1:1 ratio of the two peak intensities indicates that the isomers of 3 and 4 have identical electrochemical behavior; otherwise more than two peaks or two peaks of different intensity should be observed. The wave separation appreciably increases for 4 (Ep1 = 0.30, Ep2 = 0.41 V, ΔE = 110 mV) with respect to 3 (Ep1 = 0.35, Ep2 = 0.41 V, ΔE = 60 mV). The determination of the ΔE values allows finding the ΔGc = -nFΔE and the related comproportionation constant Kc = exp(FΔE/RT) for the equilibrium given in eq 1 (Kc = 11 and 78 for 3 and 4, respectively). The magnitude of Kc is diagnostic of low thermodynamic stability of 3þ and 4þ, indicating that significant disproportionation occurs upon oxidation.

½Fc- spacer- Fc þ ½Fcþ - spacer- Fcþ  ¼ 2½Fc- spacer- Fcþ

ð1Þ

Different factors determine the magnitude of ΔGc (and ΔE), among which ΔGr, the resonance factor that accounts for the metal-metal electronic coupling, and ΔGe, the electrostatic factor that reflects the repulsion of charged and linked redox centers.5e Since electrostatic repulsion is expected to be more effective in 3 than in 4, as suggested by the nonlinear structure of the as-indacene bridge, which is nearer the ferrocenyl groups, metal-metal electronic coupling is expected to be higher in 4 than in 3 due to a better conjugation between the Fc groups caused by the linear structure of s-indacene. Optical Spectroscopy. Once the mono- and dioxidized species were obtained, the charge transfer processes were probed by the analysis of the charge transfer absorption bands of [1-4]nþ (n = 1, 2) in the near-IR region. Oxidation of yellow CH2Cl2 solutions of 1 and 2 by incremental addition of 1 equiv of acetylferricinium afforded deep red 1þ and 2þ cations and single absorption bands in the near-IR spectral range at 8756 and 8532 cm-1, respectively (Table 1 and Supporting Information). As previously reported for the

Figure 3. Near-IR spectra of 3þ/2þ and 4þ/2þ in CH2Cl2/0.1 M nBu4NTFAB solution at 20 °C. Spectral changes upon the oxidation of (a) 3.1  10-3 M 3 to 3þ and 32þ and (b) 2.9  10-3 M 4 to 4þ and 42þ by incremental addition of acetylferricinium. Gaussian deconvolution (open circles) of the red line curves obtained by oxidation of (c) 3 and (d) 4.

structurally correlated cationic intermediates of (ferrocenyl)indenes,20 the bands of 1þ and 2þ can be confidently attributed to LMCT transitions. Similarly, oxidant addition up to 2 equiv gave exclusively deep red 32þ and 42þ cations and single absorption bands at wavenumber values very close to 1þ and 2þ (Table 1, Figure 3). The oscillator strengths f = (4.6  10-9)εmaxΔν~1/24a of the dications (Table 1) 32þ and 42þ are approximately twice those of the monocations 1þ and 2þ due to the presence of two Fcþ acceptor groups per molecule in the dications, indicating that the absorptions of 32þ and 42þ are due to LMCT transitions.7 Differently, oxidant addition to a solution of 3 containing 0.1 M nBu4NTFAB initially induced the concomitant appearance of two separated absorptions (Figure 3a) corresponding to the formation of the dication 32þ (9280 cm-1) and a small amount of monocation 3þ (5570 cm-1), according to the very low value of the comproportionation constant (20) Santi, S.; Orian, L.; Donoli, A.; Durante, C.; Bisello, A.; Ganis, P.; Ceccon, A.; Crociani, L.; Benetollo, F. Organometallics 2007, 26, 5867. (21) When a deep red solution of 32þ or 42þ in CH2Cl2/0.1 M nBu4NPF6 was mixed with 2 equiv of the neutral 3 or 4, small bands of 3þ and 4þ around 5600-5900 cm-1 were observed as shoulders, values close to those found in nBu4NTFAB. Further addition of the neutral compounds gave degradation of the solution. For the method see: Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 693.

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Organometallics, Vol. 30, No. 5, 2011

Kc. Further oxidant addition up to 2 equiv caused the quantitative formation of 32þ. Notably, addition to 4 of 1 equiv of acetylferricinium afforded a persistent green solution and the appearance of an intense and low-energy band at 5576 cm-1 corresponding to the selective formation of cation 4þ (Figure 3b).21 Gaussian deconvolution of the spectra in Figure 3c,d allows for the determination of the spectral parameters of the low-energy bands of the cations 3þ and 4þ (Table 1) and for their analysis by using the classical two-state electron transfer model (Hush theory). The narrowness of these bands is revealed by the comparison of the experimental and calculated half-bandwidths, (Δν~1/2)Hush (cm-1) = [16RT ln2 ν~max]1/2.4a The magnitude of Γ = 1 - (Δν~1/2)obsd/ (Δν~1/2)Hush4g (0.24-0.25, Table 1), a criterion proposed to classify the mixed valence species, is consistent with a moderately coupled class II. The relevant narrowness of the low-energy IT band for 3þ and 4þ prevents a correct evaluation of the electronic coupling in the framework of the Hush model, Hab = [0.0205(εmaxν~maxΔν~1/2)1/2]/d, where d is the unknown adiabatic electron transfer distance and (Δν~1/2)calcd ≈ (Δν~1/2)obsd.4 Nevertheless, the higher molar absorption coefficient (εmax, Table 1) of 4þ with respect to 3þ is in favor of higher electronic interaction,4h in agreement with the ΔE separation.

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platform bonded to two terminal redox groups displays a redox chemistry that can be switched from a single twoelectron transfer to two successive one-electron transfers by changing the supporting electrolyte. Moreover, in the stepwise oxidation, ΔE separation is sensitive to the geometry of the organic bridge. Despite the small ΔE values, generation by chemical oxidation of the mixed valence diiron cations and their characterization by optical spectroscopy were accomplished. Hush analysis of the IVCT bands in the near-IR region clearly indicates that 3þ and 4þ are class II moderate coupled mixed valence systems. In particular, as a result of higher ΔE separation, 4þ can be selectively produced in solution. The molar absorption coefficient of its IVCT band is higher than that of 3þ, suggesting that the linear s-indacene is a more efficient bridging ligand than the nonlinear asindacene in transmitting charge from the ferrocene to the ferrocenium terminal groups. Indacenes are structurally related to the phenylene-transvinylene monomer, whose oligomeric chains have exhibited the fastest electron transfer rates of all conjugated πsystems.22

Acknowledgment. We thank Progetto di Ateneo 2008 of University of Padova (CPDA089018/08) for financial support of this work. We wish to gratefully acknowledge Dr. Barbara Biondi for ESI-MS analysis.

Conclusions We have synthesized a family s- and as-indacenes with one and two terminal ferrocenyl groups. Cyclic voltammetric studies show that the two-electron wave observed for 3 and 4 with the standard electrolyte nBu4NPF6 can be split into two one-electron waves by simple variation of the counterion of the supporting electrolyte PF6- with the weak nucleophilic and ion-pairing B(C6F5)4-. The rigid and planar indacene

Supporting Information Available: Characterization data for new compounds; CIF file giving crystallographic data set. This material is available free of charge via the Internet at http:// pubs.acs.org. (22) (a) Amatore, C.; Gazard, S.; Maisonhaute, E.; Pebay, C.; Sch€ ollhorn, B.; Syssa-Magale, J.-L.; Wadhawan, J. ChemPhysChem 2007, 8, 1321. (b) Sikes, H. D.; Smalley, J. F.; Dudek, S. P.; Cook, A. R.; Newton, M. D.; Chidsey, C. E. D.; Feldberg, S. W. Science 2001, 291, 1529.