Porphyrin Derivatives

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Journal Name Cite this: DOI: 10.1039/c0xx00000x

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Long-Range Electronic Connection in Picket-Fence like FerrocenePorphyrin Derivatives† Charles H. Devillers,a Anne Milet,b Jean-Claude Moutet,b Jacques Pécaut,c Guy Royal,b Eric Saint-Amanb and Christophe Bucherb* 5

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Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X DOI: 10.1039/b000000x The effects of a direct connection between ferrocene and porphyrin units have been thoroughly investigated by electrochemical and spectroscopic methods. These data not only reveal that substitution of the porphyrin macrocycle by one, two, three or four ferrocenyl groups strongly affects the electronic properties of the porphyrin and ferrocenyl moieties, they also clearly demonstrate that the metallocene centres are “connected” through the porphyrin-based electronic network. The dynamic properties of selected ferrocene-porphyrin conjugates have been investigated by VT NMR and metadynamic calculations. 1,3-dithiolanyl protecting groups have been introduced on the upper rings of the ferrocene fragments to allow a straightforward and easy access to redox active picket-fence porphyrins. X-ray diffraction analyses of the zinc(II) 5-[1’-[2-(1,3-dithiolanyl)]ferrocenyl]-10,15,20-tri(p-tolyl)porphyrin and 5,15-bis[1’-[2-(1,3-dithiolanyl)]ferrocenyl]-10,20-bis(p-tolyl)porphyrin complex reveal the existence of S-Zn bonds involved in supramolecular arrays. The solid state analysis of the trans-5,15-di-(1’(formyl)ferrocenyl)-10,20-di-(p-tolyl)-porphyrinatozinc(II) complex, obtained by deprotection of the dithiolane substituted analog, is conversely found in the crystal lattice as a monomer exibiting an hexacoordinated zinc metal centre.

Introduction

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Ferrocene and porphyrin have already been associated in a wide range of molecular architectures to reach quite different objectives. Our group has recently published a comprehensive review on this topic.1 Their donor-acceptor properties have for instance been exploited to investigate photoinduced electron transfer processes and to mimic photosynthesis active sites. Such molecular architectures containing multiple redox active centres are also of fundamental importance for the development of molecular devices for uses in analysis or in electronics.2,3 Their ability to reversibly accept and/or release electrons at distinct potentials is particularly promising in the context of molecular electronics as each redox states can be considered as an elemental data storage.3,4 Recently, much efforts have been devoted to conjugated systems featuring several metallocenes directly connected to, or fused with, a -conjugated porphyrin, notably to enable an optimized “communication“ between metallocenes or between the metallocene and the macrocycle.5-7,8-14 As a general statement, the intramolecular “communication” between mutiple redox centres within molecules might occur through bonds in conjugated structures, or through space as a result of the electrostatic repulsion between electrogenerated charges. The magnitude of these phenomena mainly depends on a combination of structural factors (distances, geometry) as well as on the dielectric constant of the medium used for investigation.15,16 In mixed-valence chemistry, the interaction is usually characterized

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by the Vab parameter16 related to the coupling between metalcentred orbitals and estimated from the characteristics of the intervalence transition observed in the near IR spectrum. The interaction between two chemically equivalent redox centres exhibiting discrete Nernstian electron transfers can also be revealed by simple electrochemical measurements, for instance through the observation of two successive CV waves with disctinct half-wave potential values (E1/2). As a matter of fact, the Vab and E1/2 values depend on the same parameters and usually exhibit parallel variations,16,17 although electrochemical measurements simultaneously involve homovalent and mixedvalence species produced transitorily at the electrode interface. We now wish to report the synthesis and characterization of such derivatives showing up to four ferrocene subunits introduced at the meso positions of an aromatic porphyrin skeleton (Scheme 1). Dithiolanyl protecting groups have been introduced on the upper cyclopentadiene rings to enable further functionnalizations of the metallocene-based picket fences surrounding the porphyrin core. This article also reports on the determination of the wave splitting (E1/2) observed in the electrochemical signature of polyferrocenyl-porphyrin conjugates. We also report numerous experimental evidences supporting the existence of efficient electronic “communications” occuring between multiple chemically-equivalent redox centres.

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Experimental Reagents and Instrumentation

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Dichloromethane and dimethylformamide (Rathburn, HPLC grade) have been distilled over calcium hydride under argon and under reduced pressure over 3Å molecular sieves, respectively. Electrochemical experiments were conducted in a conventional three-electrode cell under an argon atmosphere at 20 °C using a CHI 660B electrochemical workstation. The working electrode was a vitreous carbon disc (3 mm in diameter) polished with 1 µm diamond paste before each record. The non aqueous Ag/Ag+ reference electrode was purchased from CH instrument, Inc. (10 mM AgNO3 in CH3CN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP)). Under these experimental conditions, the potential of the decamethylferrocene/ decamethylferrocenium (DMFc/DMFc+) redox couple, used as internal reference in dichloromethane and dimethylformamide, was observed at E1/2 = –345 mV and –410 mV, respectively.add footnote : In these conditions (DCM/TBAP), we found that E1/2[Fc/Fc+] = E1/2[DMFc/DMFc+] + 0.545 V Rotating disc electrode (RDE) voltammetry was carried out at a rotation rate of 600 rpm. Cyclic voltammetry (CV) curves were recorded at a scan rate of 0.1 V s– 1 . Electrolyses were performed at controlled potential using a Pt plate (2 cm2). Electrochemical simulations and best fitting of experimental data were performed by the Digisim software (vs. 3). High resolution mass spectra (HRMS) were recorded on a MicrOTOF Q Bruker instrument in ESI (positive mode) or on a Bruker Daltonics Ultraflex II spectrometer in the MALDI/TOF reflectron mode with dithranol as matrix and polyethylene glycol ion series as internal calibrant, at the Plateforme d’Analyse Chimique et de Synthèse Moléculaire de l’Université de Bourgogne (PACSMUB). NMR spectra were recorded on a Bruker AC-2000 250 MHz. 1H chemical shifts (ppm) were referenced to residual solvent peaks. UV-vis spectra were recorded on a Varian Cary 100 spectrophotometer using quartz cells. Synthesis 1-[2-(1,3-dithiolanyl)]-1’-formylferrocene (1). 1,2-dithioethane (5.40 mL, 64.2 mmol) was added to a cold (0°C) CH2Cl2 solution (450 mL) of 1,1’ diformylferrocene18 (15.9 g, 64.2 mmol). Trifluoroboride etherate (15.92 mL, 129.4 mmol) dissolved in 160 mL of CH2Cl2 was then added dropwise (30 min.) at 0 °C. After stirring the resulting solution at 0 °C for 5 h, an aqueous NaHCO3 solution (50 mL, 10 %) was added. The organic layer was then washed with 100 mL of an aqueous solution saturated with sodium bicarbonate, with 2×100 mL of water and finally with 100 mL of brine. The organic layer was then dried over anhydrous sodium sulphate, filtered and the solvent was evaporated under reduced pressure. The crude compound was purified by column chromatography on silica gel using n-hexane, with increasing amount of ethyl acetate (0 to 2 %), as the eluent to afford 14.3 g (yield: 70 %) of pure 1-[2-(1,3dithiolanyl)-1’-formylferrocene isolated as a red solid. NMR 1H (250 MHz, CDCl3, 298 K)  (ppm): 3.30 (m, 4H, thioethane); 4.27 (s, 2H, -Fc); 4.40 (s, 2H, -Fc); 4.61 (s, 2H, -Fc); 4.78 (s, 2H, -Fc); 5.42 (s, 1H, -HC(S-)2); 9.66 (s, 1H, -CHO). These data are consistent with those reported in reference [19].

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5-[1’-[2-(1,3-dithiolanyl)]ferrocenyl]-10,15,20-tri(p-tolyl)porphyrin (2H2); 5,10-bis[1’-[2-(1,3-dithiolanyl)]ferrocenyl]15,20-di(p-tolyl)porphyrin (3H2); 5,15-bis[1’-[2-(1,3-dithio lanyl)]ferrocenyl]-10,20-bi(p-tolyl)porphyrin (4H2); 5,10,15tris[1’-[2-(1,3-dithiolanyl)]ferrocenyl]-20-(p-tolyl)porphyrin (5H2). 5-tolyldipyrromethane20 (1.84 g, 7.8 mmol) and 1 (2.50 g, 7.8 mmol) were dissolved in 400 mL of anhydrous CH2Cl2 and Argon was bubbled through the solution for about 15 minutes. After protecting the mixture from light, trifluororacetic acid (0.60 mL, 7.8 mmol) was added dropwise. The solution was stirred at room temperature for an additional period of 30 minutes and neutralized with 2,4,6-trimethylpyridine (1.04 mL, 7.8 mmol). pchloranil (1.92 g, 7.8 mmol) was then added and the mixture was kept under stirring at room temperature for 3 h. The solvent was evaporated under reduced pressure. The resulting crude oil was suspended in 500 mL of a NaOH (2M) aqueous solution and the mixture was stirred for 1 h at room temperature. The dark precipitate was filtered off, washed with water and dried under vacuum. The crude compound was purified by chromatography on silica gel using CH2Cl2/n-hexane (75/25 v/v) as the eluent. Four successive fractions were collected when increasing amounts of ethyl acetate (0 to 2%) were added in the eluent to give 2H2 (340 mg, 10 %), 4H2 (541 mg, 13%), 3H2 (240 mg, 6%) and 5H2 (45 mg,  1%) isolated as dark green solids. 2H2: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): -2.31 (s, 2H, NH); 2.69 (s, 3H, -Me); 2.71 (s, 6H, -Me); 3.05 - 3.40 (m, 4H, S(CH2)2S-); 4.09 (m, 2H, -Fc); 4.36 (m, 2H, -Fc); 4.88 (m, 2H, Fc); 5.51 (s, 1H, -HC(S-)2); 5.57 (m, 2H, -Fc); 7.55 (m, 6H, Tol); 8.09 (m, 6H, -Tol); 8.80 (m, 6H, -pyrr); 9.94 (d, 3J = 5.00 Hz, 2H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 422 (331000); 510 (10800); 588 (9200); 671 (8300). HRMS (ESI/TOF) m/z calcd for C54H45N4FeS2: 869.2431; found: 869.2450 [M+H]+. 3H2: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): -1.83 (s, 2H, NH); 2.70 (s, 6H, -Me); 3.08 – 3.36 (m, 8H, -S(CH2)2S-); 3.99 (m, 4H, -Fc); 4.31 (m, 4H, -Fc); 4.87 (m, 4H, -Fc); 5.48 (s, 2H, HC(S-)2); 5.52 (m, 4H, -Fc); 7.54 (d, 3J = 7.25 Hz, 4H, -Tol); 8.05 (d, 3J = 8.25 Hz, 4H, -Tol); 8.68 (s, 2H, -pyrr); 8.74 (d, 3J = 4.75 Hz, 2H, -pyrr); 9.80 (s, 2H, -pyrr); 9.85 (d, 3J = 5.00 Hz, 2H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 427 (237000); 616 (12500); 692 (10400). HRMS (ESI/TOF) m/z calcd for C60H51N4Fe2S4: 1067.1693; found: 1067.1696 [M+H]+. 4H2: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): -1.70 (s, 2H, NH); 2.71 (s, 6H, -Me); 3.06 – 3.36 (m, 8H, -S(CH2)2S-); 4.01 (m, 4H, -Fc); 4.31 (m, 4H, -Fc); 4.86 (m, 4H, -Fc); 5.48 (s, 2H, HC(S-)2); 5.52 (m, 4H, -Fc); 7.55 (d, 3J = 8.25 Hz, 4H, -Tol); 8.06 (d, 3J = 7.50 Hz, 4H, -Tol); 8.69 (d, 3J = 4.75 Hz, 4H, pyrr); 9.78 (d, 3J = 5.00 Hz, 4H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 425 (247000); 615 (13700); 695 (14200). HRMS (ESI/TOF) m/z calcd for C60H51N4Fe2S4: 1067.1693; found: 1067.1736 [M+H]+. 5H2: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): -1.14 (s, 2H, NH); 2.69 (s, 4H, -Me); 3.06 – 3.36 (m, 12H, -S(CH2)2S-); 3.89 (m, 6H, -Fc); 4.25 (m, 6H, -Fc); 4.84 (m, 6H, -Fc); 5.42 (s, 3H, HC(S-)2); 5.44 (m, 6H, -Fc); 7.54 (d, 3J = 8.50 Hz, 2H, -Tol); 8.01 (d, 3J = 8.75 Hz, 2H, -Tol); 8.61 (d, 3J = 5.00 Hz, 2H, pyrr); 9.61 (d, 3J = 3.75 Hz, 4H, -pyrr); 9.74 (s, 4H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 430 (187000); 635 This journal is © The Royal Society of Chemistry [year]

(14700); 711 (12900). HRMS (ESI/TOF) m/z calcd for C66H57N4Fe3S6: 1265.0957; found: 1265.0990 [M+H]+.

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5,10,15,20-tetra[1’-[2-(1,3-dithiolanyl)]ferrocenyl]porphyrin (6H2). 1-[2-(1,3-dithiolanyl)-1’-formylferrocene (1) (700 mg, 2.2 mmol) and pyrrole (150 L, 2.2 mmol) were dissolved in 200 mL of anhydrous CH2Cl2 and argon was bubbled through the solution for about 15 minutes. After protecting the mixture from light, trifluororacetic acid (250 L, 7.8 mmol) was added dropwise. The solution was kept under stirring at room temperature for 2 h. p-Chloranil (810 mg, 3.3 mmol) and triethylamine (460 L, 3.3 mmol) were then added. After stirring the resulting solution at room temperature for 4 h, the solvent was evaporated under reduced pressure. The crude compound was purified by column chromatography on silica gel using CH2Cl2 as the eluent to give 240 mg (29 %) of 6H2 isolated as a violet solid. 6H2: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): -0.53 (s, 2H, NH); 3.02 – 3.38 (m, 16H, -S(CH2)2S-); 3.80 (m, 8H, -Fc); 4.19 (m, 8H, -Fc); 4.81 (m, 8H, -Fc); 5.34 (m, 8H, -Fc); 5.40 (s, 4H, HC(S-)2); 9.57 (s, 8H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 435 (144000); 663 (15000); 726 (12800). HRMS (ESI/TOF) m/z calcd for C72H63N4Fe4S8: 1463.0221; found: 1463.0224 [M+H]+. 5-(1’-(formyl)ferrocenyl)-10,15,20-tri(p-tolyl)porphyrin (7H2). A solution of 2H2 (50 mg, 0.057 mmol) in THF (100 mL) was added to a mixture of N-chlorosuccinimide (45.7 mg, 0.34 mmol) and AgNO3 (58.1 mg, 0.34 mmol) dissolved in CH3CN/H2O (35 mL, 85/15, v/v). After stirring the resulting solution at room temperature for 10 min., 1 mL of an aqueous solution saturated with Na2SO3, 1 mL of an aqueous solution saturated with Na2CO3 and 1 mL of an aqueous solution saturated with NaCl were successively added to the mixture. 50 mL of CH2Cl2 were then added and the mixture was filtered. The filtrate was washed with CH2Cl2. The organic phases were collected, washed with water and dried over anhydrous Na2SO4. The black crude product obtained upon evaporation of the solvent under reduced pressure was purified by column chromatography on silica gel using CH2Cl2 as the eluent to afford 32 mg (70 %) of 7H2 isolated as a violet solid. 1H NMR and MS analyses of 7H2 are consistent with those reported in reference [6]. 5,15-di(1’-(formyl)ferrocenyl)-10,20-di(p-tolyl)porphyrin (8H2). A solution of 4H2 (100 mg, 0.094 mmol) in THF (100 mL) was added to a mixture of N-chlorosuccinimide (150 mg, 1.1 mmol) and AgNO3 (191 mg, 1.1 mmol) dissolved in CH3CN/H2O (70 mL, 85/15 v/v). After stirring the resulting solution at room temperature for 10 min., 2 mL of an aqueous solution saturated with NaSO3, 2 mL of an aqueous solution saturated with Na2CO3 and 2 mL of an aqueous solution saturated with NaCl were successively added to the mixture. 100 mL of CH2Cl2 were then added and the mixture was filtered. The filtrate was washed with CH2Cl2. The organic phases were collected, washed with water and dried over anhydrous Na2SO4. The black crude product obtained upon evaporation of the solvent under reduced pressure was purified by column chromatography on silica gel using CH2Cl2 as the eluent to yield 43 mg (50%) of 8H2 isolated as a violet solid. This journal is © The Royal Society of Chemistry [year]

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8H2: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): -1.76 (s, 2H, NH); 2.72 (s, 6H, -Me); 4.38 (m, 4H, -Fc); 4.80 (m, 4H, -Fc); 4.92 (m, 4H, -Fc); 5.57 (m, 4H, -Fc); 7.57 (d, 3J = 7.50 Hz, 4H, Tol); 8.04 (d, 3J = 7.00 Hz, 4H, -Tol); 8.73 (d, 3J = 4,75 Hz, 4H, -pyrr); 9.69 (d, 3J = 4.50 Hz, 4H, -pyrr); 9.89 (s, 2H, -CHO). Mass spectroscopy (FAB+–MS), m/z: [M+1]+ = 915. HRMS (ESI/TOF) m/z calcd for C56H42O2N4Fe2Na: 937.1902; found: 937.1921 [M+Na]+. 5-ferrocenyl-10,15,20-tri(p-tolyl)porphyrin (9H2). Argon was bubbled for 15 minutes through a solution of 5tolyldipyrromethane (1 g, 4.23 mmol) and 1-carboxaldehydeferrocene (905 mg, 4.23 mmol) in anhydrous CH2Cl2 (200 mL). After protecting the mixture from light, trifluororacetic acid (0.47 mL, 6.3 mmol) was slowly added. The solution was kept under stirring at room temperature for 30 min. and neutralized with triethylamine (0.88 mL, 6.3 mmol). p-Chloranil (1.549 g, 6.3 mmol) was then added. After stirring for 3 h, the solvent was removed under reduced pressure. The resulting crude oil was then suspended in 500 mL of aqueous NaOH (2M) and the mixture was stirred for 1 h at room temperature. The black precipitate was filtered off, washed with water and dried. After evaporation of the solvent, the resulting solid was purified by column chromatography on silica gel using CH2Cl2/n-hexane (75/25 v/v) as the eluent. The first fraction collected was the targeted 5ferrocenyl-10,15,20-tri(p-tolyl)porphyrin 9H2 (90 mg, 5.5 %). 9H2: 1H NMR (400 MHz, CDCl3, 298 K)  (ppm): -2.29 (s, 2H, NH); 2.69 (s, 3H, -Me); 2.71 (s, 6H, -Me); 4.18 (m, 5H, -Fc); 4.82 (m, 2H, -Fc); 5.55 (m, 2H, -Fc); 7.51-7.60 (m, 6H, -Tol); 8.05-8.13 (m, 6H, -Tol); 8.82-8.74 (m, 6H, -pyrr); 9.98 (d, 3J = 4.80 Hz, 2H, -pyrr). HRMS (MALDI-TOF) m/z calcd for C51H41FeN4: 765.2682; found: 765.2714[M+H] +. 5,10,15,20-tetra(ferrocenyl)porphyrin (10H2). Argon was bubbled for 15 minutes through a solution of ferrocenecarboxaldehyde (470 mg, 2.2 mmol) and pyrrole (150 L, 2.2 mmol) in 200 mL of anhydrous CH2Cl2. After protecting the mixture from light, trifluororacetic acid (250 L, 3.3 mmol) was slowly added. The solution was kept under stirring at room temperature for 2 h and then neutralized with triethylamine (456 L, 3.3 mmol). p-Chloranil (440 mg, 3.3 mmol) was then added and after stirring the resulting solution at room temperature for 3 h, the solvent was removed under reduced pressure. The crude compound was purified by column chromatography on silica gel using CH2Cl2 as the eluent to give 136 mg (24 %) of 10H2. Spectroscopic data found for 10H2 are consistent with those reported in literature.21 Metallation of the free bases In a typical experiment, 1 mL of a saturated solution of Zn(OAc)2 in CH3OH was added to a stirred CH2Cl2 solution of the free base (50 mol/20 mL). The mixture was kept under stirring for 10 h at room temperature and then washed with water. The organic layer was dried over anhydrous Na2SO4, filtered and then removed under reduced pressure. The crude product was purified by column chromatography on silica gel using CH2Cl2 as the eluent to afford the targeted Zn(II) complexes in high yields (> 90 %). 2Zn: 1H NMR (500 MHz, CDCl3, 298 K)  (ppm): 2.21 (m, 2H, S(CH2)2S-); 2.65 (s, 3H, -Me); 2.67 (m, 8H, -Me and -S(CH2)2S); 3.97 (m, 2H, -Fc); 3.99 (m, 2H, -Fc); 4.45 (s, 1H, -HC(S-)2); Journal Name, [year], [vol], 00–00 | 3

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4.58 (m, 2H, -Fc); 5.37 (m, 2H, -Fc); 7.52 (m, 6H, -Tol); 8.03 (m, 6H, -Tol); 8.83 (m, 6H, -pyrr); 10.03 (d, 3J = 3,50 Hz, 2H, pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 425 (369000); 568 (12600); 619 (13800). HRMS (ESI/TOF) m/z calcd for C54H42N4FeS2Zn: 930.1488; found: 930.1508 [M]+. 3Zn: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): 2.40 – 2.60 (m, 4H, -S(CH2)2S-); 2.70 (s, 6H, -Me); 2.80 – 3.00 (m, 4H, S(CH2)2S-); 4.00 (m, 4H, -Fc); 4.11 (m, 4H, -Fc); 4.68 (m, 4H, Fc); 4.79 (s, 2H, -HC(S-)2); 5.44 (m, 4H, -Fc); 7.55 (d, 3J = 8.00 Hz, 4H, -Tol); 8.07 (d, 3J = 8.00 Hz, 4H, -Tol); 8.80 (s, 2H, pyrr); 8.85 (d, 3J = 4.50 Hz, 2H, -pyrr); 9.96 (s, 2H, -pyrr); 10.03 (d, 3J = 4.75 Hz, 2H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 429 (248000); 586 (10200); 640 (19500). HRMS (ESI/TOF) m/z calcd for C60H48N4Fe2S4Zn: 1128.0751; found: 1128.0775 [M]+. 4Zn: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): 2.71 (s, 6H, Me); 2.90 - 3.08 (m, 4H, -S(CH2)2S-); 3.09 - 3,24 (m, 4H, S(CH2)2S-); 4.12 (m, 4H, -Fc); 4.31 (m, 4H, -Fc); 4.82 (m, 4H, Fc); 5.31 (s, 2H, -HC(S-)2); 5.50 (m, 4H, -Fc); 7.56 (d, 3J = 7.50 Hz, 4H, -Tol); 8.07 (d, 3J = 6.75 Hz, 4H, -Tol); 8.81 (d, 3J = 5.00 Hz, 4H, -pyrr); 10.01 (d, 3J = 4.00 Hz, 4H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 428 (261000); 583 (9600); 648 (21000). HRMS (ESI/TOF) m/z calcd for C60H48N4Fe2S4Zn: 1128.0751; found: 1128.0783 [M]+. 5Zn: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): 2.24 - 2,50 (m, 6H, -S(CH2)2S-); 2,62 - 2,90 (m, 9H, -Me and -S(CH2)2S-); 3.76 – 4.08 (m, 12H, -Fc); 4.44 – 4.72 (m, 9H, -HC(S-)2 and Fc); 5.30 (m, 6H, -Fc); 7.54 (d, 3J = 7.25 Hz, 2H, -Tol); 8.06 (d, 3J = 7.50 Hz, 2H, -Tol); 8.75 (d, 3J = 5.25 Hz, 2H, -pyrr); 9.79 (d, 3J = 5.25 Hz, 2H, -pyrr); 9.89 (m, 4H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 432 (207000); 605 (10000); 661(29000). HRMS (ESI/TOF) m/z calcd for C66H54N4Fe3S6Zn: 1326.0015; found: 1326.0044 [M]+. 6Zn: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): 2.66 - 2,82 (m, 8H, -S(CH2)2S-); 2.92 – 3.10 (m, 8H, -S(CH2)2S-); 3.89 (m, 8H, -Fc); 4.11 (m, 8H, -Fc); 4.70 (m, 8H, -Fc); 5.00 (s, 4H, HC(S-)2); 5.33 (m, 8H, -Fc); 9.79 (s, 8H, -pyrr). UV-vis. (CH2Cl2) λmax, nm (ε, L mol–1 cm–1): 437 (138000); 627 (8700); 680 (29400). HRMS (ESI/TOF) m/z calcd for C72H61N4Fe4S8Zn: 1524.9357; found: 1524.9380 [M+H]+. 7Zn: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): 2.70 (s, 3H, Me); 2.72 (s, 6H, -Me); 4.32 (m, 2H, -Fc); 4.44 (m, 2H, -Fc); 4.56 (m, 2H, -Fc); 5.35 (m, 2H, -Fc); 7.44-7,64 (m, 6H, -Tol); 8.00-8.21 (m, 6H, -Tol); 8.63 (s, 1H, -CHO); 8.80-9.03 (m, 6H, -pyrr); 9.86 (d, 3J = 4.75 Hz, 2H, -pyrr). HRMS (ESI/TOF) m/z calcd for C52H38ON4FeZn: 854.1683; found: 854.1709 [M] +. 8Zn: 1H NMR (250 MHz, CDCl3, 298 K)  (ppm): 2.72 (s, 6H, Me); 4.54 (m, 4H, -Fc); 4.74 (m, 4H, -Fc); 4.79 (m, 4H, -Fc); 5.57 (m, 4H, -Fc); 7.55 (d, 3J = 7.25 Hz, 4H, -Tol); 8.06 (d, 3J = 7.75 Hz, 4H, -Tol); 8.79 (d, 3J = 5.25 Hz, 4H, -pyrr); 9.01 (s, 2H, -CHO); 9,86 (d, 3J = 4.75 Hz, 4H, -pyrr. HRMS (MALDITOF) m/z calcd for C56H40Fe2N4O2Zn: 976,1142; found: 976,1105 [M]+. 9Zn: 1H NMR (500 MHz, CDCl3, 298 K)  (ppm): 2.70 (s, 3H, Me); 2.71 (s, 6H, -Me); 4.24 (s, 5H, -Fc); 4.82 (m, 2H, -Fc); 5.55 (m, 2H, -Fc); 7.48-7.63 (m, 6H, -Tol); 8.00-8.16 (m, 6H, -Tol); 8.81-8.99 (m, 6H, -pyrr); 10.20 (d, 3J = 4.75 Hz, 2H, -pyrr). HRMS (ESI/TOF) m/z calcd for C51H38N4FeZn: 826.1734; found: 4 | Journal Name, [year], [vol], 00–00

826.1756 [M]+. 60

65

70

75

80

Crystallography Crystals of 2Zn, 4Zn, 8Zn and 10Zn were used for data collection on a SMART CCD diffractometer using Mo-K graphite-monochromatic radiation (= 0.71073 Å). Intensity data were corrected for Lorentz, polarization effects and absorption. Structure solution and refinement were performed with the SHELXTL (v. 5.10; Bruker Analytical X-ray Instruments: Madison, WI, 1997.package). Data collection and reduction were conducted with SMART (v. 5.054) and SAINT (v. 6.36A), respectively, from Bruker Analytical X-ray Instruments. A summary of the crystallographic data and structure refinement is given in the supplementary material. All non-hydrogen atoms were refined with anisotropic thermal parameters except for disordered solvent molecules of THF in 4. Hydrogen atoms were generated in idealized positions for compound 3 and 4; riding on the carrier atoms and were found and refined in complex 2, with isotropic thermal parameters for all. Crystal structures have been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition numbers CCDC 893172 – 893175. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK [fax: (inter.) + 44-1223/336-033; email: [email protected]]

Results and discussion 85

90

95

100

105

110

Synthesis of 2H2-6H2 and their zinc complexes 2Zn-6Zn In previous articles,5,7 we, and the other research groups, have reported the synthesis of a range of meso-ferrocenyl-porphyrins from commercially available ferrocenecarboxaldehyde and pyrrole using a standard Lindsey’s synthetic strategy.22 Purification of the crude products unfortunately proved quite difficult and could only be achieved efficiently on small scales. These significant drawbacks have hitherto considerably restricted the application scope of such molecules, like for instance as intermediates in the synthesis of ferrocene-based “picket-fence” or “basket handled” porphyrins. Our strategy to overcome these limitations and promote an easy and straightforward postfunctionnalization involves use of dithiolanyl-protected ferrocene-based starting materials. Synthesis of the targeted ferrocene-porphyrin conjugates is summarized in Scheme 1. It starts with the mono-protection of diformyl ferrocene to afford 1 in 70% yield19 (Scheme 1). The dithiolanyl group has been selected i) for its ability to resist to acidic conditions ii) to enhance the solubility and the polarity of the porphyrin products and iii) to allow an easy post-functionalization of the ferrocenyl fragments. The acid-catalyzed Mac Donald-type condensation of 1 with one equivalent of 5-tolyldipyrromethane in dichloromethane, followed by oxidation with p-chloranil, led to a crude mixture from which 2H2-5H2 (path A, Scheme 1) could be isolated in 10, 6, 13 and 1% yield, respectively. The tetrasubstituted derivative 6H2 was obtained in 29 % yield using the same procedure starting from pyrrole (path B, Scheme 1). Metallation of these free-base porphyrins with Zn2+, to yield 2Zn6Zn, was then achieved quantitatively using zinc acetate in a This journal is © The Royal Society of Chemistry [year]

MeOH–CH2Cl2 solvent mixture.

Scheme 1 Syntheses of the meso-(dithiolanyl)ferrocenyltolylporphyrins 2H2, 3H2, 4H2, 5H2 and 6H2; (i) TFA (1.5 eq.), CH2Cl2, Ar, 298 K, 30 min. ; (ii) Et3N (1.5 eq.), p-chloranil/THF, 3 h.22

2.2 to 2.8 Å. 5

10

15

20

25

Crystallography Single crystals of 2Zn were grown by slow evaporation of a deuterated dichloromethane solution (Fig. 1). 2Zn crystallizes in the C2/c space group of the monoclinic system, with 8 crystallographic independent molecular entities self-assembled in four distinct columnar structures. The Zn(II) atom is found to lie ~0.2 Å above the mean plane formed by the four nitrogen atoms. The ferrocene unit is slightly twisted with an interplanar angle between both cyclopentadienyl (Cp) rings of ~5.6°. The zinc ion is pentacoordinated in a square pyramidal geometry with four nitrogen atoms in equatorial positions and one sulfur atom, from the dithiolanyl of a neighbouring porphyrin, in apical position (Zn-S(1)) = 2.658(2) Å (Fig. 2). Iteration of this intermolecular coordination mode leads to an infinite columnar self-assembled network wherein each monomer interacts with two neighbours through Zn-S bonds. The interplanar angle between the covalently linked Cp and the porphyrin plane is of ~51°. In the coordination polymer, the interplanar angle between the porphyrin plane and the closest Cp ring, bearing the coordinated dithiolane, is of 88.6°. This intermolecualr arrangement allows the observation of rather short HFc-porph distnaces ranging from This journal is © The Royal Society of Chemistry [year]

Journal Name, [year], [vol], 00–00 | 5

S(1)

Fe

N(2)

N(3)

S(2)

Zn

Fe S(2)

S(2)

Zn N(4)

N(2)

N(1)

S(1)

Fe

N(1) N(2)

N(1)

S(1)

10

Fig. 3 Ortep23 view of 4Zn. Solvent molecules and hydrogen atoms have been omitted for reasons of clarity. Thermal ellipsoids are scaled to a 50% probability level.

Fig. 1 Ortep23 view of 2Zn, solvent molecules and hydrogen atoms have been omitted for clarity reasons. Thermal ellipsoids are scaled to a 50% probability level.

15

20

25

5

Fig. 2 Ortep23 Side view of the crystal packing in 2Zn. Solvent molecules, hydrogen atoms and tolyl rings have been omitted for clarity reasons. Thermal ellipsoids are scaled to a 50% probability level.

30

35

40


Fig. 4 Partial Ortep23 side view of the crystal packing in 4Zn. Solvent molecules, hydrogen atoms and tolyl rings have been omitted for clarity reasons. Thermal ellipsoids are scaled to a 50% probability level.

Single crystals of 4Zn have been obtained in THF. The resulting solid state structure is depicted in Fig. 3. The interplanar angle between the mean porphyrin plane and the covalently linked Cp ring is of 61.9° and both ferrocenes are found in a slightly more open conformation (Cp^Cp ~7.58°) than in 2Zn (~5.63°). This compound is isolated as a single isomer with both ferrocenyl groups in a syn configuration (,-atropoisomer), both ferrocenes pointing towards opposite directions. It should be emphasized that prior to this work, every solid-state structures of 5,15diferrocenylporphyrins reported in litterature were found in the presumably more stable ,-conformation.21 The unexpected ,-conformation adopted by 4Zn is thus most probably imposed by the formation of a self-assembled coordination oligomers/polymers involving the dithiolane substituents and the zinc(II) cations. The Zn(II) atom lies in the plane defined by the four nitrogen atoms. The interatomic Fe-Fe distance reaches 13.327(4) Å. The Zn(II) atom is hexacoordinated by four nitrogen atoms in equatorial positions and by two sulphur atoms from the dithiolane groups of two adjacent molecules ((Zn-S(2)) = 3.160(2) Å and S(2)-Zn-S(2) = 180°, Fig. 4). In the resulting coordination polymer, each monomer is linked to four neighbours through Zn-S coordination bonds. Single crystals of 10Zn have been obtained in THF (Fig. 5). It crystallizes in the C2/c space group of the monoclinic system,


5

10

Tol

b,c,d

with eight crystallographic independent molecular entities including the title compound, THF and water. The Zn(II) atom is found to lie ~ 0.27 Å above the mean plane of the porphyrin skeleton. It is pentacoordinated by four nitrogen atoms in equatorial positions and by one oxygen atom from a THF molecule in axial position ((Zn-O(1)) = 2.23(2) Å). As previously reported for the free base,11,21 the Zn(II) complex is isolated as a single isomer adopting an ,,, conformation.Add footnote : “It should be mentioned that previous works carried out on a trans-dichlorotin(IV)-tetraferrocenylporphyrin suggest that the configuration observed at the solid state might be imposed by solvent effects (see J. Porphyrins Phthalocyanines, 2011, 15, 612-621)”.

Tol

j

k i

l

m

a

-S-(CH2)2-S-

NH

2H2 2

6

6

6

21

d

2 2

4

9

2

c

a

b

2

i NH

3H2

22

22

a

4

4

4

4

b

42 i

4

4 4

8

4,2 i

4

4 4

8

6

2

4H2 4

4

a,b

NH 6

2

d

NH

c O(1)

5H2

Fe(3)

Fe(1)

42

2

2

2

9

6

6 6

12

3

2

i

Zn

a

6H2 Fe(4) Fe(2)

8

10

9

10 10 8

NH

48

997

868

577

466

8

355

8 8

244

16

133

2

002

-11 -1

-20 -2

 / ppm 35

15

Fig. 5 Side Ortep23 view of 10Zn. Thermal ellipsoids are scaled to a 50% probability level. The carbon atoms of the THF molecule (O(1)) have been omitted for clarity reasons. 40

20

25

Both ferrocene moieties located on the side of the coordinated THF molecule are pointing towards opposite directions (backward and frontward on the drawing depicted in Fig. 5), as opposed to the two other ones pointing in the same direction. The ferrocenes are tilted of about 41-52° relative to the porphyrin plane and the Fe-Fe distance ranges from 8.573(3) to 11.732(3)Å measured between two adjacent (Fe(2)-Fe(3)) or nonadjacent (Fe(1)-Fe(3) ferrocenes, respectively.

45

50

NMR spectroscopy

30

NMR spectroscopy allowed us to investigate the steric and electronic effects of the ferrocene fragments on the porphyrin ring. For the free base compounds (2H2-6H2), the inner NH’s resonate at high field, below 0 ppm, as the result of the ring current associated with the aromatic porphyrin (Fig. 6).8

55

60

65


Fig. 6 1H NMR spectra of 2H2-6H2 (250 MHz, CDCl3, 295 K). Attribution of each signal is detailed in Scheme 1. The numbers represent the relative integration values calculated for each signal.

The corresponding broad singlets are observed at –2.31, –1.83, – 1.70, –1.14 and –0.53 ppm on the NMR spectra of 2H2, 3H2, 4H2, 5H2 and 6H2, respectively. As reported by Nemykin et al,21 the NH resonance undergoes a downfield shift as the number of ferrocenyl substituents increases. These changes are clearly related to a progressive decrease in the porphyrin ring current which might result from a significant sharing of electron density between the ferrocene and porphyrin units and/or from the distortion of the porphyrin skeleton following its substitution by an increasing number of bulky ferrocenyl substituents.9,24 These steric effects are for instance revealed at the solid state for 10Zn through the saddle shape of the porphyrin ring (Fig. 5). As expected, metallation of 2H2-6H2 with zinc(II) was found to enhance the rigidity of the porphyrin skeleton leading to a larger ring current revealed on the 1H-NMR spectra by the shifts of the -pyrrolic signals towards lower fields (0.03 <  < 0.23 ppm). The 1H NMR signals attributed to the hydrogen atoms in the dithiolane ring are conversely observed at higher fields in the metallated species than in the free bases. For instance, the resonance of the thioacetal proton Hi (see Scheme 1 for attribution) resonates at 5.51 and 4.45 in the spectrum of 2H2 and 2Zn, respectively. This metal-induced shift is attributed to the formation of self-assembled coordination oligomers/polymers in solution, wherein the dithiolane protons dive into the shielding cone of the macrocycle, as observed on the X-ray structures of 2Zn and 4Zn. The long-range “communication” observed between metallocene fragments covalently linked through a porphyrin ring has often been attributed to steric effects prohibiting atropoisomerization.10,11 These dynamic issues have been adressed in the present study through detailed VT-NMR Journal Name, [year], [vol], 00–00 | 7

-1

-2

10

15

Ha, Ha’

35

40

45

the system from revisiting points in the configurational space of the collective variables. In the present study, we considered the dihedral angle CFc-CFc-Cporph-Cporph (see Fig. 8) as a collective variable. A total free energy activation of 10-10.5 kcal.mol-1 was obtained from the resulting energy profile (Fig. 8). This value is in very good agreement with the VT NMR experimental data detailed above and it is also found close to values previously reported for the meso-tetraferrocenyl free base12 and zinc25 porphyrins (10.4 kcal mol-1 and 11.7 kcal mol-1, respectively). The low activation energy found for the ferrocenyl-substituted porphyrins 11Zn is thus in agreement with a free rotation of the ferrocenyl subunit at room temperature

0 11Zn

2 Free energy / Kcal mol1

5

investigations conducted with the monoferrocenyl-porphyrin derivatives 2Zn and 9Zn. For both species, decreasing the temperature from 300 K to 185 K led to a splitting of the initial doublet attributed to the -pyrrolic protons Ha and Ha’ (Fig. 7). At 185 K, rotation does not occur and the ferrocene fragment adopts the tilted conformation observed at the solid state. As a result, Ha and Ha’ become chemically and magnetically unequivalent with a chemical shift between both signals reaching almost 2 ppm. The activation energies corresponding to the rotational motion of ferrocene in 2Zn† and 9Zn were estimated from coalescence temperatures (Tc)25,26 at 10.4 kcal.mol-1 (Tc = 230 ± 5 K) and 9.2 kcal.mol-1 (Tc= 215 ± 5 K), respectively. Here again, the higher value found for 2Zn most probably results from the existence of intermolecular S-Zn coordination bonds occuring in deuterated chloroform. This assumption is further confirmed by the important upfield shift of the -S-CH2-CH2-S- proton signals observed upon cooling (from 2.28 ppm at 298 K to -0.24 ppm at 233 K†).

9Zn

302.9 K 292.9 K

4 6 8 10 Energy barrier

12

10.2 Kcal mol1

288.2 K 283 K

14

Final minimum

First minimum

278 K

16 250

273 K 268 K

200

150

263.1 K

100 50 Angle / Degree

0

50

100

150

258 K

Fig. 8 Free energy profile corresponding to the rotation motion of one of the two ferrocenes of 11Zn around the Cmeso-CFc bond.

253.1 K 249.1 K

243 K 238 K

50

UV-visible absorption spectroscopy

233 K 228 K 223 K 217.7 K 213 K 207.8 K 202.9 K

55

197.6 K 192.7 K 187.6 K 185.3 K

11.00 20

ppm (t1)

Ha’

Ha 10.00

Table 1 Maximum absorption wavelengths (max) and molar extinction coefficients (measured for 2-6 and 2Zn-6Zn in DCM.

(ppm)

Fig. 7 Partial H NMR spectra of 9Zn in the 185.3 – 302.9 K temperature range (9.10-3 M, CD2Cl2, 500 MHz, 9.2 ≤  ≤ 11.2 ppm). 1

2H2 3H2 4H2 5H2 6H2 2Zn 3Zn 4Zn 5Zn 6Zn

Ab initio dynamics calculations

25

30

These results have been further confirmed by Born-Oppenheimer dynamics calculations at the DFT level conducted on the most symetric species 11Zn (Fig. 8). The ab initio dynamics calculations were performed using the CP2K-QuickStep program at the DFT level with the BLYP functional.28 The basis set was a double zeta polarized set of gaussian orbitals for the second and third row atoms and double zeta for the iron and zinc atoms with the GTH pseudo potentials.29 To work around the issue of rare events, the metadynamics30 or “hills methods” has been used. A series of small repulsive Gaussian potentials (hills) centred on the current values of the collective variables were added during the dynamics to prevent 8 | Journal Name, [year], [vol], 00–00

UV-vis. absorption spectrophotometry experiments carried out with 2-6 also suggest a progressive decrease of the porphyrinbased aromaticity with the increasing number of covalently linked ferrocenyl substituents†. As shown in Fig. 9, this effect is clearly revealed by the increasing bathochromic and hypochromic shifts of the Soret band occuring from 2H2 to 6H2 or from 2Zn up to 6Zn (Table 1).

max / nm 422

427

425

430

435

425

429

428

432

437

10 × 331 / M-1 cm-1

237

247

187

144

369

248

261

207

138

-3

60


10-3 x / L.cm-1.mol-1

350 300

25

250

20 15

200

5

100

0 500

600

650

700

750

Epa = 0.800 V

800

5H2

45

450

500

550 600 650 700 750 800  / nm Fig. 9 UV-vis. absorption spectra of 2Zn-6Zn recorded in DCM at 295 K.

5

10

15

20

25

30

35

40

It should also be noted that the larger  values found for the trans isomers (4H2 and 4Zn) than for the cis isomers (3H2 and 3Zn) suggest a more planar structure for the trans-disubstituted derivatives (see Table 1). For all the investigated species, metallation with Zn2+ resulted in the bathochromic shifts of the main absorption bands along with a significant increase of the associated molar extinction coefficients.

6H2


5 µA 0.1 V

Epc = 0.565 V

0.1 V

Epc = 0.520V

Fig. 10 Cyclic voltammograms of 2H2-6H2 limited to the potential range corresponding to the oxidation of the ferrocene units (5.10-4 M, 100 mV s1 , DCM 0.1 M TBAP, glassy carbon working electrode Ø = 3 mm, reference: DMFc/DMFc+). 50

55

Electrochemistry The electrochemical behavior of 2H2-6H2 and of their Zn2+ complexes has been investigated by cyclic voltammetry (CV) and by voltammetry at rotating disk electrodes (RDE) in dichloromethane (DCM) or in N,N-dimethylformamide (DMF) solutions. All potentials have been referenced towards the halfwave potential of decamethyl-ferrocene (DMFc/DMFc+) used as an internal reference. Electrochemical data determined from CV experiments are collected in Table 2. The electrochemical signature of all the investigated compounds arises from electron transfers centred on the ferrocene (Fc) and on the porphyrin (P) moieties. In DCM, the oxidation of the dithiolane substituents is also observed as two irreversible waves at Epa = 1.20 and 1.46 V†. As a general statement, the signature of the ferrocene and porphyrin units are strongly affected by the extended  conjugation within the molecules.9-11,24 The ferrocene groups are reversibly oxidized at potential values ranging form +500 to +750 mV (Fig. 10). The electron withdrawing effect of the porphyrin ring on the ferrocene centre is revealed on the CV of 2H2 through the anodic shift of the ferrocene-centred one-electron oxidation wave (E1/2 = + 0.64 V) as compared to that of ferrocene used as a reference (E1/2 = +0.54 V). For the trans disubstituted ferrocenyl compound 4H2, the CV curve displays two distinct waves whereas a single signal flanked by two shoulders is seen on the CV curve of the cis isomer 3H2. According to RDE voltammetry experiments, the overall number of exchanged electron is in agreement with the number of ferrocene units, ie two electrons/molecule. The electrochemical signatures of the tri- and tetra-substituted ferrocenyl compounds feature two consecutive waves observed with 1/2 and 1/3 intensity ratios, respectively. The shape of these signals indicates that the chemically equivalent Fc subunits in the di-, tri- and tetra-substituted porphyrins are electrochemically non equivalent, the oxidation of one Fc centre shifting the oxidation of the remaining ones towards higher potential values.

Epc = 580 V

Epa = 0.790 V

5 A

400

0.1V

Epc = 0.580 V

50 0 350

4H2

5A

0.1 V

Epc = 0.595 V

5Zn 6Zn

3Zn 2Zn 550

5 A

5A

0.1V

Epa = 0.790 V

3H2

2H2

4Zn

10

150

Epa = 0.730 V

Epa = 0.690 V

4Zn 2Zn 5Zn 3Zn 6Zn 4Zn 7Zn 5Zn 8Zn 6Zn

30

60

65

70

75

80

85

90

Simulation and best fitting of the experimental data were used to determine the formal potentials of each ferrocenyl subunits in 3H2, 4H2 and 5H2†. These data are collected in Table 2. Unfortunately, adsorption of the oxidized complex 6H2n+ onto the electrode surface, as revealed by the observation of a desorption peak on the reverse scan of the CV curve, precluded the determination of the corresponding formal potentials. Theoretical developments carried out with molecules submitted to n successive one-electron transfers centred on fully independent and chemically equivalent redox sites have established that the shift between each successive formal potentials should equal 35.6, 28.5 and 23.7 mV when n = 2, 3 and 4, respectively, the resulting CV curve however still displaying the same peak to peak separation of 60 mV.14 The latter feature is not observed on the ferrocene centred oxidation signals depicted in Fig. 10 showing multiple shoulders and splitting attributed to the interactions occuring between each redox site in the oxidized and/or reduced states.10,17,24 The extent of the “communication” between ferrocene centres furthermore evolves with the relative position of each metallocenes on the porphyrin skeleton. The differences in the shape of the CV waves recorded with the bisferrocenyl isomers 3H2 and 4H2 for instance suggest that the coupling is stronger in the trans isomer 4H2 than in the cis isomer 3H2. These findings contrast with previous works, carried out by Swarts13 or Nemikyn21 on similar molecules. Swarts and coworkers found no significant differences between the first and second ferrocenecentered oxydations of cis and trans porphyrin isomers (111 and 115 mV for “2HPtrans” and “2HPcis”, respectively and 102 and 103 mV for “ZnPtrans” and “ZnPcis”, respectively), whereas Nemykin reported a much higher E value for a cis isomer (208 mV) than for a trans isomer(150 mV).”In our experimental conditions, the potential shifts between both ferrocene oxidation processes in 3H2 and 4H2 were found to reach 85 mV and 115 mV, respectively. These data confirm the fact that the electronic “coupling” between both ferrocenyl substituents is stronger in 4H2, although the Fe···Fe distance happens to be much higher in the trans isomer (11.5 < d < 13.5 Å) than for in the cis one 3 (6.2 < d < 11.1 Å). The extent of the bended or ruffled deformations of the porphyrin skeleton induced by the presence of two bulky ferrocenyl substituents in the 5,10 or 5,15-positions of the macrocycle is another key factor which will undoubtly affect the mixing of molecular orbitals involved in the through-bond Journal Name, [year], [vol], 00–00 | 9

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“communication” between ferrocene centres. Unfortunately, failure to obtain X-ray data for the cis isomer did not allow to draw clear cut conclusions on these structural aspects. In that regard, it should be mentionned that 1H NMR data do not support the assumption of a weaker ring current in 3H2 than in 4H2. CV curves of porphyrins (P) usually display two successive oneelectron oxidation waves involving formation of P•+ and P2+, as well as two successive one-electron reduction waves leading to the formation of the radical anion (P •-) and di-anion (P2-).31 In all studied compounds, the potential separations between the first porphyrin oxidation and first porphyrin reduction potentials are in

15

20

accordance with the standard HOMO-LUMO gap of ca. 2.2 V commonly measured with porphyrins.32 The electrodonating effect of the ferrocenyl group(s) on the porphyrin ring is enlightened by comparison between the reduction potentials of 2H2-6H2 or 2Zn-6Zn and those of the reference compounds TPPH2 and TPPZn. The first reductions of 2Zn and TPPZn are For instance observed at -1.305 V and 1.205 V, respectively. In addition, the nature of the solvent used for analyses has been shown to affect quite significantly the number and the reversibility of the observed redox processes.

Table 2 Electrochemical data for 2H2, 3H2, 4H2, 5H2, 6H2 and their Zn complexes (5.10-4 M in DMF or CH2Cl2 0.25 M TBAP,  = 0.1 V s-1, E vs. DMFc/DMFc+). Solvent

E1/2

DMF

(P-•/P2-)

–1.530 –1.505

–1.510a –1.495

–1.530

DMF

(P/P-•)

–1.090 –1.100 +

2H2

3H2

4H2

5H2

6H2

7H2

8H2

2Zn – 1.720 – 1.350

3Zn –1.675 –1.345

4Zn

5Zn

6Zn

– 1.700a – 1.325

– 1.690a – 1.340

– 1.680a – 1.335

–1.080

–1.110

–1.120

b

b

b

0.535

b

b

b

b

b

b

1.020

0.855

0.910

0.920

1.000

b

DMF

(Fc/Fc )

0.590

b

DMF

(P/P+•)

1.080

1.210

DCM

(P-•/P2-)

–1.520

–1.590a

DCM

(P/P-•)

–1.125

–1.160

DCM

(Fc/Fc+)

0.640

0.615c 0.700c

0.605c 0.720c

0.585c 0.695c 0.725c

b

0.600

0.555c 0.645c

0.545c 0.655c

0.510c 0.665c 0.685c

b

DCM

(P/P+•)

1.135d

1.210d

1.180d

1.135d

1.070d

0.895

~1.070

0.975

~1.11

~1.08 5

– 1.745a – 1.305

6Zn

6Zn

– 1.740a – 1.310

a 25

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35

Epc, irreversible; bExtensive overlaps occurring between successive redox systems (ferrocene, porphyrin or electrolyte-centered) prohibits an accurate experimental determination of this E1/2 value; cfrom best fitting; dEpa, irreversible.

Two successive one-electron reduction waves are systematically observed in DMF medium, whereas formation of the di-cation P2+ species are only observed in CH2Cl2. The potential values as well as the reversibility of the porphyrin-centred electron transfers also turn out to undergo significant variations with the substitution pattern and with the insertion of Zn2+. As observed with most porphyrins,10,31 the redox processes of the free bases are shifted towards more negative potentials (ca. −200 mV) upon metallation with Zn2+. In dichloromethane, the irreversibility of the first oxidation of the free base porphyrins suggests the existence of a follow-up chemical reaction. On the contrary, the first oxidation of the zinc porphyrins appears reversible at the cyclic voltammetry time scale. Spectroelectrochemistry

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Absorption spectra were recorded periodically throughout bulk electrolyses experiments carried out in CH2Cl2. In all cases, oxidation of the ferrocene units leads to a decrease in the intensity of the Soret band along with a slight hypsochromic shift of its maximum wavelength. These changes are in agreement with the electron withdrawing character of the electrogenerated ferrocenium and with the delocalization of the positive charge over the whole aromatic macrocycle.11,17 The exhaustive oxidation of the ferrocene unit in 2H2 (Eapp = 0.81 V) was for instance accompanied with a progressive color change from brown/green to pale green. After the uptake of one electron, the initial Soret band at 422 nm ( = 331x103 M-1 cm-1) had lost 10 | Journal Name, [year], [vol], 00–00

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2/3 of its initial intensity ( = 118300 M-1 cm-1) at the expense of a new band developping at 448 nm ( = 76200 M-1 cm-1). The reversibility of these transformations was checked through a bulk back reduction (Eapp = 0.51 V) of the resulting ferricinium appended porphyrin allowing to restore the initial spectrum. Further oxidation carried out at 1.26 V and uptake of another one electron/molecule (1 < Qtotal < 2.1 electron/molecule, Fig. 11), led to the development of the signal at 448 nm ( = 2 x105 M-1 cm-1) and to the disappearance of the initial Soret band at 422 nm. The irreversiblity of the oxidation was revealed by the fact that the signature of the starting material could not be restored by back reduction of the doubly oxidized species. This irreversibility was further confirmed by CV analysis of the electrolyzed solution (obtained after uptake of two electrons from 2H2 at Eapp = 1.26 V) showing a reversible oxidation wave at 0.81 V and a reversible reduction at –0.36 V. This signature was not only found different from that of the starting material, as expected for a species produced irreversibly from the electrogenerated dicationic species 2H22+, it was most importantly found to hold great similarities with that of the doubly-protonated H4TPP2+ species.33 To confirm our hypothesis that protonation of the porphyrin ring might be involved, we carried out a detailled characterization of 2H42+ produced in CH2Cl2 from 2H2 upon addition of increasing amounts of trifluoroacetic acid (TFA). Addition of up to 2 molar equivalents of TFA led to the progressive disappearance of the Soret band at 422 nm along with the development of a new band at 448 nm attributed to 2H42+. Additionaly, the CV curve of the This journal is © The Royal Society of Chemistry [year]

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2.5

422

Abs. / a.u.

1.5

1.0

55

0.05

800

900 65

0.5

680 0.0 300 25

400

500

600  / nm

700

800

900

Fig. 11 Electrolysis of a 10-4 M solution of 2H2 followed by UV-Vis. spectroscopy (l = 1 mm, 0.25 M TBAP in CH2Cl2, Eapp = 1.26 V, -2.1 e).

30

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45

Oxidation of 2Zn+• carried out at 1.07 V, corresponding to the first porphyrin-centred electron transfer, is accompanied by a further decrease in the intensity of the Soret and Q bands. These changes turned out to be however fully irreversible at the electrolysis time scale since the signature of the starting solution could not be restored by back reduction. In summary, these results not only demonstrate that the doubly oxidized species 2Zn2+ is poorly stable at the electrolysis time scale, it also supports our assumption that the bulk oxydation of 2H2 yields quantitatively 2H42+ The UV-Vis absorption spectra recorded periodically as 4H2 was subjected to electrochemical oxidations were found to be similar to those monitored with 2H2. The successive oxidations carried out at 0.71 V (1 electron/molecule) and then at 0.86 V (1 additionnal electron/molecule) were accompanied by the progressive decrease in the intensity of the initial Soret band at 426 nm at the expense of a new band developping at 453 nm ( = 132500 M-1cm-1†). The overall process was found to be fully This journal is © The Royal Society of Chemistry [year]

0.08

822

0.06 0.04 0.02 500

600

700

800

900

/ nm

400

500

600

700

800

900

/ nm

Fig. 12 Electrolysis of a 10-4 M solution of 2Zn followed by UV-Vis. spectroscopy (l = 1 mm, 0.25 M TBAP in CH2Cl2, Eapp = 1.07 V, -2.1 e).

60

700

1.0

0.0 300

0.10

 / nm

1.5

0.5

680

600

0.12 0.10

0.15

0.00 500

425

2.0

0.20

448

2.0

Abs./ a.u.

0.25

2.5

Abs. / a.u.

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50

irreversible at the time scale of electrolysis and a CV analysis of the resulting oxidized solution revealed two reversible oxidation waves at 0.78 V and 0.90 V attributed to the oxidation of the Fc centres and one reduction process at -0.37 V attributed to the protonated porphyrin ring electrochemical response. As seen with the mono-ferrocenyl derivative, the electrochemical and spectrophotometric signatures of the electrolyzed solution match those of the diprotonated species 4H42+ which could be produced in situ by addition of TFA to a solution of the free base 4H2†.

Abs. / a.u.

5

diprotonated species 2H42+ and that of the species produced by bulk oxidation of 2H2 turned out to display similar key features including the reversible waves at 0.80 V and −0.36 V. These results led us to conclude that the electrogenerated dication 2H22+ is not stable in anhydrous CH2Cl2 and readily evolves towards 2H42+. It should be mentioned that similar proton exchanges involving oxidized tetrapyrrolic macrocycles or nitrogen containing aromatic compounds34,35 have been explained as resulting from the presence of oxidizable and protic substrate in the electrolyte (solvent, traces of water …). The proposed course of events was further confirmed with spectroelectrochemical investigations carried out with the metallated species 2Zn. The first, ferrocene-centred, electron transfer remains reversible at the time scale of electrolysis but the UV-vis. signature of the electrogenerated species 2Zn+• is completely different from that observed with 2H2. As seen on the spectra shown in Fig. 12, the intensity of the Soret band decreases down to = 151000 M-1 cm-1 while a new broad band appears at 822 nm. The magnitude of the modifications in the porphyrinbased signature, proceeding through well defined isosbetic points at 415, 558, 577, 601 and 622 nm, have been attributed to delocalization of the electron hole (radical cation character) over the whole molecule from the ferrocene centre to the porphyrin ring.

70

The oxidized forms of this bisferrocenylporphyrin derivative were found to be greatly stabilized by insertion of a zinc atom within the porphyrin ring and the reversibility of both ferrocenecentred oxidation processes was cheked from back-reduction experiments allowing to recover the UV-Vis absorption spectrum of the starting material 4Zn. The electrochemical oxidation of 4Zn into 4Zn2+ was revealed by the hypochromic shift of the Soret band from = 261000 M-1 cm-1 down to = 126000 M-1 cm-1 and by the growth of a new band at 850 nm (= 9700 M1 .cm-1). As discussed for the monoferrocenyl-porphyrin analog 2Zn, the magnitude of these changes, involving porphyrin-based -* transitions, support the assumption of an extended delocalization of the electrogenerated positive charges over the entire molecule, i.e. from the ferrocene centres to the porphyrin macrocycle. Removal of the 1,3-dithiolanyl protecting group

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The 1,3-dithiolanyl protecting groups have been introduced on the upper ring of the ferrocene fragments to allow a straightforward post-functionnalizations and an easy access to a range of redox active picket-fence porphyrins. Numerous chemical or electrochemical strategies have been considered to remove the dithiolane protecting groups in 2H2 and 4H2 and we found that the most efficient and mildest procedure for synthesizing the targeted formylated compounds 7H2 and 8H2 in good yields (Scheme 2) involves Ag+ in the presence of Nchlorosuccinimide.5 The 1H-NMR spectra of 2H2 and 7H2 indicates that the inner NH resonances are shifted upfield upon hydrolysis of the dithiolane substituents (-2.31 and -2.36 ppm, respectively). This shift is also observed for 4H2 and 8H2 (-1.70 and -1.76 ppm, respectively). Removal of the dithiolane group is further confirmed by the disappearance of the multiplets near 3.05-3.40 ppm for 2H2 and 4H2 along with the appearance of the Journal Name, [year], [vol], 00–00 | 11

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characteristic singlet of the aldehyde function(s) at 9.89 and 9.90 ppm for 8H2 and 7H2, respectively. The corresponding Zn2+ complexes 7Zn and 8Zn could be obtained using Zn(OAc)2 as a metal source following classical metalation procedures. Single crystals of 8Zn were obtained by slow diffusion of methanol into a solution of 8Zn in CHCl3. It is formulated as C58H48N4Fe2O4Zn and crystallizes in the P-1 space group of the triclinic system, revealing one crystallographic independent molecular entity consisting of one 8Zn entity and two methanol molecules. 8Zn is isolated as a single isomer with both ferrocenyl groups adopting a syn conformation (,-atropoisomer), this latter being favored due to hydrogen bonds between the two coordinated MeOH molecules and the aldehyde functions (Fig. 13). The interplanar angle between the covalently linked Cp and porphyrin planes is of ~34.7° whereas the ferrocene is slightly twisted with an interplanar angle between both Cp rings of ~6.3°. The Zn(II) atom lies inside the porphyrin plane and is coordinated by four nitrogen atoms of the macrocycle, with bond distances ranging from 2.043(2) to 2.059(2) Å and by two oxygen atoms from two methanol molecules bound in axial positions (Zn-O(2)) = 2.494(2) Å). The Fe-Fe distance is 12.775(2) Å and both hydroxyl groups are H-bonded to the oxygen atoms of the aldehyde units (O(1)···H-O(2) = 2.882(4) Å).

O(1)

Fe

O(2)

N(2) N(1) Zn

N(1)

N(2)

Fe

Fig. 13 Ortep23 views of 8Zn. Thermal ellipsoids are scaled to a 50% probability level. 30

35

40

Further evidences of the high electronic coupling between the ferrocene and porphyrin units came from electrochemical investigations carried out with the protected and deprotected derivatives 4H2 and 8H2, respectively. While the conversion of the dithiolane rings into formyl groups proved to have no significant effect on the shape of the observed CV waves, it led to large shifts of the associated half-wave oxidation and reduction potential towards more positive values. These results thus reveal that (i) the magnitude of the electronic coupling between both iron centres is similar in both species, and that (ii) the electronwithdrawing character of the formyl groups is efficiently transmitted to the ferrocene sub unit and to a lesser extent to the porphyrin, their electrochemical signatures being shifted positively of + 200 mV and + 70 mV, respectively.

5 µA

-1.25

-0.85

-0.45

-0.05

0.35

0.75

Potential / V vs. DMFc/DMFc+ 45

Fig. 14 Cyclic voltammograms of 4H2 (dotted line) and 8H2 (solid line) (5.10-4 M, 100 mV s-1, CH2Cl2 0.1 M TBAP, WE: glassy carbon Ø = 3 mm, CE: Pt wire, Ref: DMFc/DMFc+).

Conclusions 50

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Scheme 2 (i) NCS (6 eq.), AgNO3 (6 eq.) in THF/CH3CN/H2O, 70%; (ii) NCS (12 eq.), AgNO3 (12 eq.) in THF/CH3CN/H2O, 50%.35 55

60


A -conjugated porphyrin ring has been used as an electronconducting linker enabling effective electronic interactions between two, three or four ferrocene centers. Dithiolanylprotected ferrocenes have been introduced at the meso positions of the porphyrin ring using a classical Lindsey-like synthetic strategy involving use of pyrrole or dippyrromethane as starting materials. Dithiolanyl substituants have been selected to allow a straightforward post-functionnalization of the targeted redoxresponsive picket-fence porphyrins. Atropoisomerization issues have been adressed by VT NMR and by metadynamic calculations. The low activation energy of around 10 Kcal/mol found for the bisferrocenylporphyrin 11Zn is in agreement with a free rotation at room temperature of the ferrocenyl subunit around the CFc-CPorph bond. X-ray diffraction analyses of the mono- and bis-ferrocenyl This journal is © The Royal Society of Chemistry [year]

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porphyrin derivatives 2Zn and 4Zn revealed the existence of SZn bonds involved in supramolecular arrays. The solid state structure of the zinc complex 8Zn obtained after removing the dithiolanyl protecting group, is conversely found as a monomer exibiting an unusual hexacoordinated zinc metal centre. The electronic connection between ferrocene and porphyrin and between ferrocene centers has been investigated by spectrocopic and electrochemical methods. Interestingly, the interaction appears stronger for the trans di-substituted ferrocenyl macrocycle than for the cis isomer. The effective electronic “communication” through the whole molecule was further confirmed by bulk electrolyses experiments. Oxydation of the ferrocene centers in 2H2 or in 4H2 was found to produce the protonated species 2H42+ or 4H42+, respectively. The exact succession of electrochemical and chemical steps leading to these species is still unknown but their formation support the notion that the electrogenerated positive charge is significantly delocalized over the macrocycle. It is noteworthy to mention that deprotection of the dithiolanyl-substituted ferrocenes could be readily achieved without loss of the ferrocene and porphyrinbased electrochemical activity. Future works will be devoted to the synthesis of functionnalized ferrocene-based picket fence porphyrins which could find applications in supramolecular chemistry, in electrochemical recognition or in molecular electronics.

4. 5. 60

6. 7. 65

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Aknowledgement

30

The authors would like to thank the “centre national de la recherche scientifique”, the “université Joseph Fourier”, the “conseil régional de Bourgogne” and the “université de Bourgogne” for their financial support. C. H. D. would like to thank Dr. Fanny Chaux for carrying out the ESI-MS analyses. A. M. whishes to thank the “région Rhône-Alpes” as well as the CECIC calculation facility.

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10.

Notes and references 35

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Institut de Chimie Moléculaire de l’Université de Bourgogne, UMR CNRS 6302, Université de Bourgogne, BP 47870, 21078 Dijon Cedex, France. b Département de Chimie Moléculaire, UMR CNRS 5630, Université Joseph Fourier, BP 53, 38041, Grenoble Cedex 9, France. E-mail: [email protected]; Fax: (33)476514267; Tel: (33)47651 4682. c CEA/DRMFC/SCIB, Laboratoire de coordination et nanochimie, 17 rue des Martyrs, 38054, Grenoble Cedex 9, France. † Electronic Supplementary Information (ESI) available: Experimental details and characterization data. See DOI: 10.1039/b000000x/

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