1-Cyano-1′-ethynyl-ferrocene: Synthesis and reaction chemistry

Journal of Organometallic Chemistry 786 (2015) 1e9

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1-Cyano-10 -ethynyl-ferrocene: Synthesis and reaction chemistry Frank Strehler, Marcus Korb, Elisabeth A. Poppitz, Heinrich Lang* €t Chemnitz, Faculty of Natural Sciences, Institute of Chemistry, Inorganic Chemistry, D-09107 Chemnitz, Germany Technische Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2014 Received in revised form 17 February 2015 Accepted 23 February 2015 Available online 20 March 2015

Several consecutive synthetic methodologies for the preparation of Fe(h5-C5H4C^N)(h5-C5H4C^CH) (3) are described. Ferrocene Fe(h5-C5H4C^N)(h5-C5H4C(O)Me) (1) reacts under typical Vilsmeier conditions to give as the main product Fe(h5-C5H4C^N)(h5-C5H4CCl]CH2) (2) and in minor yield Fe(h5C5H4C^N)(h5-C5H4CCl]CHC(O)H) (4). Compound 2 could be directly converted to 3 by addition of KOtBu. The title compound is also accessible by the gradual reaction of 4 with NaOH to give Fe(h5C5H4C(O)NH2)(h5-C5H4C^CH) (5), which upon treatment with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and P(O)Cl(OEt)2 produced 3. Organometallic 3 could be homo-coupled to [Fe(h5-C5H4C^N)(h5C5H4C^C)]2 (7) in an Eglinton coupling upon addition of [Cu(OAc)2]. With [CuI] and NEt2H in dichloromethane, compound Fe(h5-C5H4C^N)(h5-C5H4C^CCH2NEt2) (6) was produced by coppercatalyzed three-component coupling. The structures of 3e5 in the solid state were determined by single crystal X-ray structure analysis. While the X-ray structures of 3 and 4 show no peculiarities, the structure of 5 possesses a network structure due to hydrogen bridge bond formation. The electrochemical behavior of 3, 6 and 7 was studied by cyclic voltammetry. It could be shown that 3 possesses a reversible redox event at 550 mV (DE ¼ 65 mV), while in homo-coupled 7 two consecutive redox processes at 524 and 680 mV were found indicating that the ferrocenyl units in 7 can be oxidized in a stepwise manner to 7þ and 72þ, respectively. The redox separation DE with 156 mV implies a possible electron transfer in the mixed-valent species 7þ, which was confirmed by spectroelectrochemical studies. From these studies, 7 could be classified as a weakly coupled class II system according to Robin and Day. © 2015 Elsevier B.V. All rights reserved.

Keywords: Ferrocene Alkyne Nitrile Electrochemistry Solid state structure

Introduction There is considerable interest in the synthesis of transition metal compounds in which the metals are bridged by p-conjugated organic units, because such species allow to studying electron transfer processes in their mixed-valent state [1e8]. These molecules can be considered as model compounds for molecular wires with possible application in, for example, the down-scaling of electronic devices [9e17]. In this respect, ethynyl linkers were recently be introduced as connectivities between redox-active terminal organometallic groups, including ferrocenes and halfsandwich iron, ruthenium and osmium moieties [1e4,17e27]. It was found that compounds incorporating diverse organic units between the ethynyl building blocks such as heterocycles and transition metal-containing fragments show only weak electronic interactions between the redox-active terminal groups [7,28e39].

* Corresponding author. Tel.: þ49 (0) 371 531 21210; fax: þ49 (0) 371 531 21219. E-mail address: [email protected] (H. Lang). http://dx.doi.org/10.1016/j.jorganchem.2015.02.049 0022-328X/© 2015 Elsevier B.V. All rights reserved.

Alkynides are isoelectronic to nitriles, which are, as compared to alkynyl ligands, good two-electron s donors and also have been recently used as bridging units in homo- and hetero-bimetallic transition metal chemistry to study electron transfer in the appropriate mixed-valent species [40e42]. In 2011, Astruc and coworkers reported about the synthesis and electronic communication between iron atoms bridged by phenylene bis- and trisnitrile units [43]. It was found that the respective nitrile entities efficiently quench the electronic interaction between the metal centers. In contrast, Taube, Henry and Lewis found that in heteronuclear mixed ion-containing ruthenium and ferrocenyl metalorganic and organometallic complexes inter-valence charge transfer processes took place between the appropriate metal centers [44,45]. Within these molecules, the ferrocenecarbonitrile fragment was directly bonded to the ruthenium ion. In addition, Vahrenkamp et al. reported on the weak electron delocalization between cyanide bridged transition metal units [46]. This prompted us to introduce both, the ethynyl and the nitrile unit in a redox-active species, which opens the possibility to directly compare the influence of these groups on the electronic

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interaction in mixed-valent transition metal complexes. We herein report on the synthesis of Fe(h5-C5H4C^N)(h5-C5H4C^CH) and its reaction chemistry. The electrochemical and spectroelectrochemical properties of [Fe(h5-C5H4C^N)(h5-C5H4C^C)]2 and Fe(h5-C5H4C^N)(h5-C5H4C^CCH2NEt2) are discussed as well. Materials and methods General procedure All reactions were carried out under an argon atmosphere using standard Schlenk techniques. Solvents were purified as follows: N,N-dimethylformamide by distillation from P4O10; tetrahydrofuran by distillation from sodium/benzophenone ketyl; pyridine by distillation from CaH2. Dichloromethane and hexane were dried using a MBraun MB SPS-800 system (double column solvent filtration, working pressure 0.5 bar). Reagents 1,8-Diazabicyclo[5.4.0]undec-7-ene, diethyl chlorophosphate, copper(I) iodide, copper(II) acetate and nbutyllithium were purchased from commercial suppliers and were used as received. 10 Acetyl-1-cyano ferrocene (1) was synthesized according to a published procedure [47]. Instruments Infrared spectra were recorded with a Thermo Nicolet 200 FT-IR spectrometer using the KBr press technique for sample preparation. NMR spectra were recorded using a Bruker Avance III 500 FT NMR spectrometer (1H NMR at 500.303 MHz, 13C{1H} NMR at 125.813 MHz) at ambient temperature, unless otherwise noted. Chemical shifts (d) are reported in parts per million (ppm) relative to tetramethylsilane using the solvent as internal reference (CDCl3: 1 H NMR d 7.26 ppm; 13C{1H} NMR d 77.16 ppm) [48]. Coupling constants (J) are reported in Hertz (Hz) and integrations are reported in numbers of protons. The following abbreviations were used to describe peak patterns: s ¼ singlet, d ¼ doublet, pt ¼ pseudo-triplet, pq pseudo-quintet. The melting points (sealed off in argon-flushed capillaries) were determined using a Gallenkamp MFB 595 010 M melting point apparatus. Microanalyses were performed with a Thermo FLASHEA 1112 Series instrument. Highresolution mass spectra were recorded with a Bruker micrOTOF QII with an Apollo II ESI source. Electrochemistry Electrochemical measurements were conducted in 1.0 mmol L1 solutions in dichloromethane containing 0.1 mol L1 of [nBu4N] [B(C6F5)4] as supporting electrolyte under an argon atmosphere at 25  C utilizing a Voltalab PGZ 100 radiometer electrochemical workstation interfaced with a personal computer. A three electrode cell, which utilized a Pt auxiliary electrode, a glassy carbon working electrode, and an Ag/AgCl (0.01 mol L1 [AgNO3]) reference electrode, mounted on a Luggin capillary was used. The working electrode was pretreated by polishing on a Buehler microcloth first with a 1 mm and then with a 0.25 mm diamond paste. The reference electrode was constructed from a silver wire inserted into a solution of 0.01 mol L1 of [AgNO3] and 0.1 mol L1 of [nBu4N][B(C6F5)4] in acetonitrile, in a Luggin capillary with a porous Vycor tip. This Luggin capillary was inserted into a second Luggin capillary with porous Vycor tip filled with a 0.1 mol L1 of a [nBu4N][B(C6F5)4] solution in dichloromethane. Successive experiments under the same experimental conditions showed that all formal reduction

and oxidation potentials were reproducible within 5 mV. Experimental potentials were referenced against an Ag/AgCl reference electrode but results are presented referenced against ferrocene or decamethylferrocene (Fc*) as an internal standard, as required by IUPAC [49]. To achieve this, since the ferrocene couple FcH/FcHþ (Fc ¼ Fe(h5-C5H4)(h5-C5H5)) interferes with the ferrocenyl potentials, each experiment was first performed in the absence of any internal standard and then repeated in the presence of 1 mmol L1 of ferrocene. Data were then manipulated on a Microsoft Excel worksheet to set the formal reduction potentials of the FcH/FcHþ couple to 0.0 V. Under our conditions the Fc*/Fc*þ couple was at 614 mV vs FcH/FcHþ, DEp ¼ 60 mV, while the FcH/FcHþ couple itself was at 220 mV vs Ag/AgCl, DEp ¼ 61 mV [50]. Single crystal X-ray diffraction analysis Diffraction data for 3e5 were collected with an Oxford Gemini S diffractometer, with graphite-monochromated Mo Ka radiation (l ¼ 0.71073 Å) using oil-coated shock-cooled crystals. The structures were solved by direct methods and refined by full-matrix least-squares procedures on F2 [51,52]. Graphics of the molecular structures have been created by using SHELXTL [52] and ORTEP [53]. Crystal data of 3: C13H9FeN, M ¼ 235.06 g mol1, orange prism, 0.4  0.25  0.15 mm, space group P21/c, monoclinic, a ¼ 8.0003(4) Å, b ¼ 9.6596(5) Å, c ¼ 13.5278(6) Å, b ¼ 105.859(4) , V ¼ 1005.63(9) Å3, Z ¼ 4, rcalcd ¼ 1.553 g cm3, T ¼ 100 K, q range 3.38e25.25 , 3807 reflections collected, 1803 independent reflections, Rint ¼ 0.0732, R1 ¼ 0.0572, wR2 ¼ 0.1361 (I > 2s(I)). Crystal data of 4: C14H10ClFeNO, M ¼ 299.53 g mol1, orange prism, 0.4  0.2  0.2 mm, space group P421c, tetragonal, a ¼ b ¼ 16.8903(3) Å, c ¼ 8.4110(3) Å, V ¼ 2399.51(10) Å3, Z ¼ 8, rcalcd ¼ 1.658 g cm3, T ¼ 110.00(10) K, q range 3.41e25.99 , 5486 reflections collected, 2236 independent reflections, Rint ¼ 0.0293, R1 ¼ 0.0258, wR2 ¼ 0.0518 (I > 2s(I)), absolute structure parameter [54] 0.172(19). Crystal data of 5: C13H11FeNO, M ¼ 253.08 g mol1, orange prism, 0.4  0.3  0.2 mm, space group Cc, monoclinic, a ¼ 22.2883(6) Å, b ¼ 11.0335(3) Å, c ¼ 13.1325(4) Å, b ¼ 99.751(3) , V ¼ 3182.86(16) Å3, Z ¼ 12, rcalcd ¼ 1.584 g cm3, T ¼ 100 K, q range 3.34e26.00 , 11,669 reflections collected, 4701 independent reflections, Rint ¼ 0.0386, R1 ¼ 0.0309, wR2 ¼ 0.0668 (I > 2s(I)), absolute structure parameter [54] 0.007(14). Synthesis Synthesis of 2 and 4 1-Cyano-10 -acetyl-ferrocene (1) (5.24 g, 20.7 mmol) was dissolved in 10 mL of DMF under an argon atmosphere in a threenecked flask, equipped with a dropping funnel and a gas in- and outlet. The mixture was cooled to 0  C. In a second Schlenk flask 6 mL (10.1 g, 65.7 mmol) of POCl3 were added carefully to 6 mL (5.7 g, 80.0 mmol) of DMF under ice bath cooling. The resulting mixture was transferred to the dropping funnel and added dropwise to the solution of 1 and DMF within 30 min. The reaction mixture was stirred for 2 h at 0  C. The dropping funnel was replaced by a reflux condenser and 15 mL of diethyl ether were carefully added and stirred for 10 min. With continuous ice cooling, 23 g (0.17 mol) sodium acetate trihydrate were added for neutralization followed by 2 mL of water. The solution was stirred for additional 4 h at ambient temperature, whereby further 2 mL of diethyl ether were added after the first hour. The mixture was transferred to a separation funnel together with 50 mL of water and diethyl ether each. The aqueous phase was extracted several times with 20 mL portions of diethyl ether. The combined organic layers

F. Strehler et al. / Journal of Organometallic Chemistry 786 (2015) 1e9

were cautiously washed several times with saturated sodium bicarbonate solution until the gas evolution has stopped. Afterward, the diethyl ether phase was washed twice with water and dried over magnesium sulfate, filtered and all volatiles were removed under reduced pressure. The crude product was purified by flash chromatography on SiO2 using a mixture of hexane/diethyl ether of ratio 2:1 (v/v) as eluant giving two main fractions, whereby fraction 1 contained compound 2 as brown oil and fraction 2 contained compound 4 (dark red solid). 2: Yield 3.54 g (13.0 mmol, 63% based on 1). 1H NMR (CDCl3, 297 K, d): 4.43 (pt, 3JHH ¼ 1.8 Hz, 2H, C5H4), 4.45 (pt, 3JHH ¼ 1.8 Hz, 2H, C5H4), 4.65 (pq, 3JHH ¼ 1.7 Hz, 4H, C5H4), 5.38 (d, 2JHH ¼ 1.5 Hz, H, CH2), 5.54 (d, 2JHH ¼ 1.5 Hz, 2H, CH2). 13C{1H} NMR (CDCl3, 297 K, d): 69.6 (C5H4), 72.1 (C5H4), 72.8 (C5H4), 73.5 (C5H4), 111.0 (CH2), quaternary carbon atoms were not observed. MS (ESI-Qq-TOF, m/z): 270.9848 (Mþ, 100%). Anal. Calcd (%) for C13H10ClFeN: C 57.50, H 3.71, N 5.16. Found: C 57.76, H 3.60, N 5.06. 4: Mp.: 89  C. Yield 1.25 g (4.2 mmol, 20% based on 1): 1H NMR (CDCl3, 297 K, d): 4.47 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.69 (pt, 3 JHH ¼ 1.9 Hz, 2H, C5H4), 4.70 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.87 (pt, 3 JHH ¼ 1.9 Hz, 2H, C5H4), 6.42 (d, 3JHH ¼ 7.0 Hz, 1H, CH), 10.12 (d, 2 JHH ¼ 7.0 Hz, 2H, CHO). 13C{1H} NMR (CDCl3, 297 K, d): 55.0 (iC, C5H4), 70.6 (C5H4), 73.3 (C5H4), 73.9 (C5H4), 74.32 (C5H4), 82.9 (iC, C5H4), 122.4 (CN), 151.7 (CCl), 190.7 (CHO). IR (KBr, cm1): ~n ¼ 2226 (CN), 1670 (CHO), 1603 (C]C). MS (ESI-Qq-TOF, m/z): 321.9698 (Mþ þ Na, 100%), 299.9885 (Mþ, 57%). Anal. Calcd (%) for C14H10ClFeNO: C 56.14, H 3.37, N 4.68. Found: C 56.46, H 3.38, N 4.73. Synthesis of 3 from 2 1.65 g (6.0 mmol) of 2 and 673 mg (6.0 mmol) of KOtBu were dissolved in 15 mL of tetrahydrofuran under an argon atmosphere and stirred for 2 h at ambient temperature. Then 15 mL of water were added and the aqueous phase was extracted twice with 10 mL of diethyl ether. The combined organic phases were dried over MgSO4. All volatiles were removed under reduced pressure. The crude product was purified by flash chromatography using SiO2 and diethyl ether/hexane of ratio 1:1 (v/v) yielding 850 mg (3.62 mmol, 60% based on 2) of 3. Synthesis of 3 and 5 from 4 500 mg (1.65 mmol) of 4 were dissolved in 100 mL of 1,4dioxane and the solution was heated to 100  C. Then 10 mL of a hot 1 N sodium hydroxide solution were added in a single portion and the reaction mixture was stirred for 20 min at 100  C. After cooling to ambient temperature, the reaction mixture was poured onto ice and was neutralized with 1 N hydrochloric acid. The solution was extracted three times with 25 mL portions of dichloromethane and the combined organic phases were dried over MgSO4. The drying agent was filtered off and all volatiles were removed under reduced pressure. The crude product was separated and purified by flash chromatography using SiO2 and dichloromethane as eluant. The first fraction contained 68 mg (0.29 mmol, 18% based on 4) of 3 and the second one contained 311 mg (1.23 mmol, 74% based on 4) of 5. Synthesis of 3 from 5 Compound 5 (50 mg, 0.20 mmol) was dissolved in 10 mL of dichloromethane and 90 mg (0.60 mmol, 0.09 mL) of DBU and 68.2 mg (0.40 mmol, 0.06 mL) of diethyl chlorophosphate were added in a single portion. The reaction mixture was stirred for 2 h at ambient temperature. Afterward, 20 g of crushed ice were added and the solution was extracted twice with 10 mL of dichlormethane and the combined organic phases were dried over MgSO4. The drying agent was filtered off and all volatiles were removed under

3

reduced pressure. The crude product was purified by flash chromatography using SiO2 and diethyl ether/hexane of ratio 1:1 (v/v) yielding 30 mg (0.13 mmol, 64% based on 5) of 3. 3: Mp.: 104  C. 1H NMR (CDCl3, 297 K, d): 2.84 (s, 1H, CH), 4.38 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.44 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.59 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.67 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4). 13C {1H} NMR (CDCl3, 297 K, d): 54.0 (iC, C5H4), 71.4 (C5H4), 73.0 (C5H4), 73.4 (eC^), 73.5 (C5H4), 73.9 (C5H4), 75.7 (^CH), 80.1 (iC, C5H4), 119.1 (CN). IR (KBr, cm1): ~n ¼ 3247 (HCC), 2227 (CN), 2111 (CC). Anal. Calcd (%) for C13H9FeN: C 66.42, H 3.86, N 5.96. Found: C 66.05, H 3.75, N 5.99. 5: Mp.: 133  C. 1H NMR (CDCl3, 297 K, d): ¼ 2.81 (s, 1H, CH), 4.28 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.44 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.46 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.64 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 5.68 (s (broad), 2H, NH2). 13C{1H} NMR (CDCl3, 297 K, d): 65.4 (iC, C5H4), 70.8 (C5H4), 71.2 (C5H4), 72.8 (C5H4), 73.8 (C5H4), 74.9 (^CH), 76.3 (eC^), 81.5 (iC, C5H4), 171.9 (CONH2). IR (KBr, cm1): ~n ¼ 3452 (NeH), 3331 (HCC), 2098 (C^C), 1654 (C]O), 1599 (NeH), MS (ESIQq-TOF, m/z): 529.0235 (2M þ Na, 5%) 276.0106 (M þ Na, 100%), 253.0185 (Mþ, 16%). Anal. Calcd (%) for C13H11FeNO: C 61.70, H 4.38, N 5.53. Found: C 61.47, H 4.41, N 5.49. Synthesis of 6 Compound 3 (50 mg, 0.21 mmol) and 20.5 mg (0.11 mmol) of [CuI] were suspended in 10 mL of dichloromethane. Afterward, 0.5 mL (350 mg, 4.79 mmol) of dry diethylamine were added in a single portion. The reaction mixture was stirred over-night at room temperature. Then all volatiles were removed under reduced pressure. The residue was purified by flash chromatography on Al2O3 using a mixture of hexane/ethyl acetate of ratio 1:1 (v/v) giving pure 6. Yield 26 mg (0.08 mmol, 39% based on 3). 1H NMR (CDCl3, 297 K, d): 1.12 (t, 3JHH ¼ 7.2 Hz, 6H, CH3), 2.62 (t, 3 JHH ¼ 7.2 Hz, 4H, CH2), 3.56 (s, 2H, CH2), 4.34 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.41 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.52 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.64 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4). 13C{1H} NMR (CDCl3, 297 K, d): 12.8 (CH3), 41.8 (CH2), 47.5 (CH2), 53.7 (iC, C5H4), 68.8 (C^C), 71.1 (C5H4), 72.8 (C5H4), 73.4 (C5H4), 73.6 (C5H4), 80.7 (iC, C5H4), 83.0 (C^C), 119.5 (C^N). IR (KBr, cm1): ~n ¼ 2223 (C^N), 2153 (C^C). MS (ESI-Qq-TOF, m/z): 321.1184 (M þ H, 100%). Anal. Calcd (%) for C18H20FeN2: C 67.52, H 6.30, N 8.75. Found: C 67.32, H 6.23, N 8.47. Synthesis of 7 Compound 3 (104 mg, 0.45 mmol) was dissolved in 15 mL of pyridine. 90 mg (0.45 mmol) of Cu(OAc)2$H2O were added in a single portion. The reaction mixture was stirred for 2 h at ambient temperature. All volatiles were removed under reduced pressure. The residue was purified by flash chromatography on Al2O3 using a mixture of hexane/ethyl acetate of ratio 5:1 (v/v) to remove impurities. Using solely dichloromethane as second eluant gave pure 7. Yield: 89 mg (0.19 mmol, 85% based on 3). M.p.: 220  C (decomp.). 1 H NMR (CDCl3, 297 K, d): 4.44 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.51 (pt, 3 JHH ¼ 1.9 Hz, 2H, C5H4), 4.65 (pt, 3JHH ¼ 1.9 Hz, 2H, C5H4), 4.73 (pt, 3 JHH ¼ 1.9 Hz, 2H, C5H4). 13C{1H} NMR (CDCl3, 297 K, d): 54.0, 66.5, 72.1 (C5H4), 72.5, 73.2 (C5H4), 73.6 (C5H4), 74.3 (C5H4), 77.7. Not all quaternary carbon atoms were found due to low solubility. IR (KBr, cm1): ~n ¼ 2224 (C^N); 2152 (C^C). MS (ESI-Qq-TOF, m/z): 490.9983 (M þ Na, 100%), 469.0071 (M þ H, 2%). Anal. Calcd (%) for C26H16Fe2N2: C 66.71H 3.45, N 5.98. Found: C 67.02, H 3.81, N 5.71. Results and discussion Synthesis For the synthesis of the title compound Fe(h5-C5H4C^N)(h5C5H4C^CH) (3) three consecutive synthetic methodologies starting

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from 1-acetyl-10 -cyano ferrocene (1) were developed as depicted in Scheme 1. Ferrocene 1, however, with its nitrile group limits the potential synthetic pathways to 3, since nucleophiles, in general, must be strictly avoided, due to their nucleophilic attack at the C,N triple bond [55e59]. This restricted us to use the straightforward transformation of acetyl groups into ethynyl units by applying n BuLi/diisopropylamine as nucleophile [60]. This disposed us to apply the two-step reaction strategy typically used for acetyl ferrocene (Scheme 1, reaction path (ia)/(iiia)) [61]. Within these reactions the corresponding ferrocene Fe(h5-C5H4C^N)(h5C5H4C^CH) (3) was only produced in minor yield, due to the formation of expected Fe(h5-C5H4C^N)(h5-C5H4CCl]CHC(O)H) (4) along with unexpected Fe(h5-C5H4C^N)(h5-C5H4CCl]CH2) (2) and Fe(h5-C5H4C(O)NH2)(h5-C5H4C^CH) (5) of which the latter two compounds have been produced as major components (Experimental Part). Ferrocene 5 was formed by the formal addition of H2O to the C,N triple bond in good yields (Scheme 1, reaction (iiib)). Upon reaction of 5 with DBU (¼1,8-diazabicyclo[5.4.0]undec-7ene) and P(O)(OEt)2Cl in the molar ratio of 2:3 resulted in the release of water and hence ferrocene 3 was obtained in 60% yield (Experimental Part; Scheme 1, reaction (iv)). The main product of the Vilsmeier reaction is Fe(h5-C5H4C^N)(h5-C5H4 CCl]CH2) (2) (Scheme 1, reaction (ib)) which could be successfully converted to 3 upon addition of KOtBu (reaction (ii)) in tetrahydrofuran at ambient temperature in excellent yield (Experimental Part). In a first attempt to capitalize on the various functionalities of ferrocene 3, we tried to prepare either [Pt(PnBu3)2(C^C-fc-C^N)2] (8), [PtCl2(N^C-fc-C^CH)2] (9) and [Pt(C^C-fc-C^N)2]n (10) (fc ¼ Fe(h5-C5H4)2). However, it appeared that neither the copper(I)-assisted platinumecarbon coupling to afford 8 or 10 (reaction of 3 with [Pt(PnBu3)2Cl2] or [PtCl2]) nor the dative-binding of the nitrile building block to platinum(II) to give 9 (reaction of 3 with [PtCl2] or [PtCl2(MeC≡N)2]) was successful (Scheme 2). Always, the respective educts could be recovered, even applying different synthesis conditions. Changing to equimolar amounts of 3 and [CuI] and diethylamine as base gave Fe(h5-C5H4C^N)(h5-

C5H4C^CCH2NEt2) (6) by the copper(I)-catalyzed three-component coupling (Scheme 2, path (i)). In contrast, the Eglinton homocoupling of 3 with an equimolar amount of [Cu(OAc)2] in pyridine afforded, as expected, the butadiyne [Fe(h5-C5H4C^N)(h5C5H4C^C)]2 (7) (Scheme 2, path (ii)). Characterization Organometallic compounds 2e7 were characterized by elemental analysis, IR and NMR (1H, 13C{1H}) spectroscopy and high resolution ESI mass-spectrometry (Experimental Part). In addition, electrochemical (3, 6, 7) and spectroelectrochemical (3,7) studies were carried out. The molecular structures of 3e5 in the solid state are reported. The IR spectra of all newly synthesized ferrocenes are characterized by distinct absorptions typical for the nitrile and ethynyl functionalities with vibrations at 2220e2230 cm1 (~nC^N), 3200e3250 cm1 (~n^CeH), and 2090e2150 cm1 (~nC^C) (Experimental Part) [62]. As expected, the amide unit in 5 shows two typical absorptions (~nNH ¼ 3452 cm1, ~nCO ¼ 1654 cm1), while for 4 a band at 1666 cm1 is observed for the carbonyl stretching frequency. The progress of the reaction of 1 to give 3 can be monitored by IR spectroscopy, since the typical vibration of the acetyl group disappears (1685 cm1) in the first reaction step (Scheme 1, path (i)), while new bands at 3247 cm1 (~n^CeH) and 2111 cm1 (~nC^C) appear for the ethynyl unit in 3 (Scheme 1, paths (ii)e(iv)). The successful formation of 6 and 7 from 3 is evidenced by the disappearing of the ~n^CeH vibration (vide supra). 1 H and 13C{1H} NMR spectroscopy allows to identify all organic groups present (Experimental Part). Characteristic signals in the 13C {1H} NMR spectra are the carbon resonance signals for the nitrile and the ethynyl groups, which are found at ca. 120 ppm (C^N) and in the range of 68e83 ppm (C^C) (Experimental Part). Further characteristic signals are the carbonyl carbon atoms for ferrocenes 4 (C(O)H) (190.7 ppm) and 5 (C(O)NH2) (171.9 ppm). Solid state structures

Scheme 1. Synthetic strategies for the preparation of 3 ((ia/b) Vilsmeier reagents (P(O) Cl3/N,N-dimethylformamide (DMF), 0  C; (ii) KOtBu, tetrahydrofuran; (iiia/b) NaOH, 1,4dioxane/water (1:1 v/v), 100  C; (iv) DBU, P(O)Cl(OEt)2).

The molecular structures of 3e5 in the solid state have been determined by single crystal X-ray diffraction analysis (Figs. 1e3). Suitable crystals for 3, 4, and 5 were obtained by slow diffusion of hexane into a chloroform solution containing either 3, 4 or 5 (Experimental Part) at ambient temperature. Selected bond distances (Å), bond angles ( ) and torsion angles ( ) are summarized in the captions of Figs. 1e3, while for the crystal and structure refinement data see Experimental Part. The ferrocenes 3e5 crystallize in the monoclinic space groups P21/c (3), Cc (5) or in tetragonal P421c (4) with one (3, 4) or three (5) molecules in the asymmetric unit. The bond distances and angles of the ferrocenyl backbones are in the range typical for ferrocene derivatives [63e66] with characteristic C^C bond distances (1.170(5)e1.187(6) Å) (for comparison, 1.151(7)e1.194(10) Å [66]). This also holds for the C^N moiety (3, 1.145(5); 4, 1.137(3); for comparison, 1.133(6)e1.150(2) Å [64]). The ferrocenes protrude from an ecliptic conformation (3, 3.6(6); 4, 9.43(17); 5, 2.1(2)e 7.2(3)  ) with all substituents arranged synperiplanar (Figs. 1e3). All substituents slightly exceed the cyclopentadienyl plane, where the C^X (X ¼ CH, N) groups expose a maximum deviation of 0.061(8) (3, X ¼ N) and 0.275(11) Å (5, X ¼ CH), respectively, similar to the amide with 0.443(8) Å (5, O1) and the CClCHCHO functionality with 0.301(6) Å (4, Cl1), which is attributed to the conjugation of the p-systems. Furthermore, compound 5 exhibits a hydrogen bridge bond pattern, which involves one hydrogen atom of the amide function and the ethynylic hydrogen atom. Therein, one hydrogen atom of

F. Strehler et al. / Journal of Organometallic Chemistry 786 (2015) 1e9

5

Scheme 2. Reaction behavior of 3, synthesis of 6 and 7 ((i) [CuI], HNEt2, CH2Cl2; (ii) [Cu(OAc)2], pyridine).

every NH2 moiety interacts with a carbonyl oxygen atom (Fig. 4) of the next adjacent molecule. The O/HeC hydrogen bridge bond only exists between the O1 and C26 atoms. Due to these bindings an intermolecular network of a columnar one dimensional structure is formed along the crystallographic c-axis (Fig. 4 and Fig. SI1). Electrochemistry

Fig. 1. ORTEP diagram (50% probability level) of the molecular structure of 3 with the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles ( ) and torsion angles ( ): (D1 denotes the cyclopentadienyl centroid of C5H4C^N, D2 denotes the cyclopentadienyl centroid of C5H4C^CH) C11eC12 1.183(6), C13eN1 1.145(5), C1eC11eC12 178.3(4), C6eC13eN1 178.1 (4), D1eFe1eD2 177.39(3), Fe1eD1 1.6603(5), Fe1eD2 1.6543(5), C6eC13 1.431(6), C1eC11 1.425(6).

Fig. 2. ORTEP diagram (50% probability level) of the molecular structure of 4 with the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles ( ) and torsion angles ( ): (D1 denotes the cyclopentadienyl centroid of C5H4C^N, D2 denotes the cyclopentadienyl centroid of C5H4CCl]CHCHO) C11eC12 1.331(4), C12eC13 1.451(4), C13eO1 1.207(3), C11eCl1 1.748(3), C14eN1 1.137(3), Fe1eD1 1.6519(3), Fe1eD2 1.6473(3), C12eC13eO1 124.4(3), C11eC12eC13 126.1(2), C1eC11eC12 126.2(2), Cl1eC11eC12 119.9(2), C10eC14eN1 178.5(3), D1eFe1eD2 177.13(3).

The redox properties of 3, 6 and 7 have been determined by cyclic voltammetry (¼ CV) and square-wave voltammetry (¼ SWV) (Fig. 5). Dichloromethane solutions containing the analyte (1.0 mmol L1) and [nBu4N][B(C6F5)4] (0.1 mol L1) as supporting electrolyte were used [67e69]. The data of the CV measurements have been recorded at a scan rate of 100 mV s1 and are summarized in Table 1. All redox potentials are referenced to the FcH/FcHþ redox couple (E0 ¼ 0.00 mV, FcH ¼ Fe(h5-C5H5)2) as recommend by IUPAC [50]. Fig. 5 shows the cyclic and square wave voltammograms of 6 and 7. From Fig. 5 and Table 1 it can be seen that 3 and 6 show one reversible (3, DEp ¼ 65 mV) or quasi-reversible (6, DEp ¼ 100 mV) redox event corresponding to the ferrocene moiety. Further characteristic for 6 is the appearance of an irreversible oxidation process at 460 mV, which can be assigned to the amine NEt2 oxidation and is typical for alkyl-substituted amines [70e72]. The irreversibility of this process can be explained by the strong tendency of oxidized amines to undergo follow-up reactions, like iminium ion formation by deprotonation [73] or dealkynation [74]. Also an attachment of the oxidized species towards the carbon and platinum surfaces is possible [75]. Therefore the small redox peak at about 920 mV and the broad reduction process at 750 mV may be explained by follow-up reactions or surface attachment of the oxidized species. For ferrocene 7 two consecutive reversible one-electron processes for the oxidation of both ferrocenyl termini, which take place at 525 (DEp ¼ 66 mV) and 680 mV (DEp ¼ 67 mV) in the cyclic voltammogram (Fig. 5, Table 1) are observed. The redox separation DE0 was determined to 155 mV. A similar behavior has been reported for other diferrocenyl- or di(biferrocenyl)-butadiynes [76e79]. This redox-splitting points to an electronic communication between the respective iron centers Fe(II)/Fe(III) in the mixedvalent species.

6

F. Strehler et al. / Journal of Organometallic Chemistry 786 (2015) 1e9

Fig. 3. ORTEP diagram (50% probability level) of the molecular structure of 5 with the atom numbering scheme showing three independent molecules in the asymmetric unit. Hydrogen atoms have been omitted for clarity. Selected bond distances (Å), angles ( ) and torsion angles ( ): (D1, D3, D5 denotes the cyclopentadienyl centroids of C5H4CONH2, D2, D4, D6 denotes the cyclopentadienyl centroids of C5H4C^CH) C1eC2 1.482(5), C1eO1 1.244(4), C1eN1 1.325(5), C12eC13 1.187(6), C14eO2 1.231(4), C14eN2 1.345(5), C14eC15 1.490(5), C25eC26 1.176(5), C27eO3 1.255 (4), C27eN3 1.326(5), C27eC28 1.465(5), C38eC39 1.170(5), Fe1eD1 1.661(12), Fe1eD2 1.660(12), Fe2eD3 1.655(13), Fe2eD4 1.650(13), Fe3eD5 1.663(12), Fe3eD6 1.660(12), O1eC1eN1 122.6(4), O1eC1eC2 118.9(3), N1eC1eC2 118.4(3), C11eC12eC13 174.6(4), O2eC14eN2 122.1(3), O2eC14eC15 120.7(3), N2eC14eC15 117.1(3), C24eC25eC26 178.5(4), O3eC27eN3 120.6(3), O3eC27eC28 120.4(3), N3eC27eC28 118.9(3), C37eC38eC39 177.5(4), average D1eFe1eD2 178.0.

Fig. 4. Ball-and-Stick model of the hydrogen bridge bond pattern including the involved hydrogen atoms along the crystallographic a- (left) and the c-axis (right) of 5 with selected atom numbering. (Red: oxygen; Blue: nitrogen; Gray: hydrogen). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Using only electrochemical measurements, however, it cannot be stated if electron transfer takes place in mixed-valent 7þ, or if the appropriate redox splitting is caused solely by electrostatic repulsion between the ferrocenium and the ferrocenyl units. To

get a deeper insight into the electronic properties of the individual oxidation states of the latter compound, in situ spectroelectrochemical UVeVis/NIR measurements have been carried out.

Fig. 5. Cyclic voltammograms of 6 (left) and 7 (right) at 25  C; supporting electrolyte: 0.1 mol L1 [NnBu4][B(C6F5)4] in dichloromethane, glassy carbon working electrode (surface area 0.031 cm2), scan rate 100 mV s1. All potentials are measured vs FcH/FcHþ. Inset (right): square wave voltammogram at 25  C; supporting electrolyte: 0.1 mol L1 [nBu4N] [B(C6F5)4] in dichloromethane; duration: 5 s, amplitude: 5 mV, pulse: 25 mV.

F. Strehler et al. / Journal of Organometallic Chemistry 786 (2015) 1e9 Table 1 Cyclic voltammetry data (potentials vs FcH/FcHþ), scan rate 100 mV s1 at a glassycarbon electrode of 1.0 mmol L1 solutions of the analytes in dichloromethane containing 0.1 mol L1 of [nBu4N][B(C6F5)4] as supporting electrolyte at 25  C. Compd.

E1 0 (mV)a (DEp (mV))b

E2 0 (mV)a (DEp (mV))b

DE 0 (mV)c

3 6 7

550 (65) 460d 525 (66)

e 760 (100) 680 (67)

e e 155

a b c d

E 0 ¼ formal potential. DEp ¼ difference between the oxidation and the reduction potential. DE 0 ¼ potential difference between the two ferrocenyl-related redox processes. Oxidation of the tertiary amine.

Spectroelectrochemistry Spectroelectrochemical studies were performed by a stepwise increase of the potential (step heights: 50 or 100 mV) from 500 to 1200 mV vs Ag/AgCl in an OTTLE (¼ Optically Transparent Thin-Layer Electrochemistry) cell [80] using a dichloromethane solution of 7 (0.0025 mol L1) containing [nBu4N][B(C6F5)4] (0.1 mol L1) as electrolyte at 25  C. During this procedure 7 was oxidized to mixed-valent 7þ and finally to dicationic 72þ (Fig. 6). Upon formation of 7þ during successive oxidation increasing absorptions at the edge of the UVeVis (850 nm) and NIR (1750 nm) region were observed, which can be assigned to LMCT (¼ Ligand-to-Metal Charge Transfer) and IVCT (¼ Inter Valence Charge Transfer) bands for mixed-valent 7þ (Fig. 6). The IVCT transition can be described by Gaussian shaped function using the method of deconvolution (Fig. 4 (right); εmax ¼ 700 L mol1 cm1, ~nmax ¼ 5725 cme1, D~n1/2 ¼ 4120 cm1). Deconvolution also results in a very small band which can be assigned to a LF (¼ Ligand Field) transition (εmax ¼ 190 L mol1 cm1, ~nmax ¼ 3820 cme1, D~n1/2 ¼ 890 cm1). The forbidden metal-centered ligand field (LF) electronic transition occurs commonly at approx. 4000 cm1 with a very low intensity (100 L mol1 cm1) (Fig. 6) [81,82]. The predictions of intramolecular interactions in 7 discussed within the electrochemical part (vide supra) are validated with these results. With these considerations [Fe(h5-C5H4C^N)(h5-C5H4C^C)]2 (7) could be classified as a weakly coupled class II system according to Robin and Day [83]. Comparing the electrochemical and UVeVis/NIR properties of 7 with FcC^CeC^CFc (Fc ¼ Fe(h5-C5H4)(h5-C5H5)) in which two

7

ferrocenyl moieties are bridged by an all-carbon C4 unit, the influence of the electron-withdrawing nitrile substituents are noticeable. As a result thereof, the nitrile-functionalized ferrocenyls in 7 are oxidized at higher potential (7, E10 ¼ 525 and E20 ¼ 680 mV; FcC^CeC^CFc, E10 ¼ 135 and E20 ¼ 290 mV [76]), but they show the same redox separation (7, DE0 ¼ 155 mV; FcC^CeC^CFc, DE0 ¼ 155 mV [76]). From NIR spectroscopy it was found that 7 possesses a higher extinction coefficient (7, εmax ¼ 700 L mol1 cm1, ~nmax ¼ 5725 cme1; FcC^CeC^CFc, εmax ¼ 515 L mol1 cm1, ~nmax ¼ 7866 cme1 [79]) indicating that in 7 the electron transfer is more pronounced than in FcC^CeC^CFc. However, these results must be considered with care, since different electrolyte solutions were applied. The stretching vibrations of the eC^CeC^Ce bridging unit provide a suitable probe for IR spectroscopy during in situ oxidation of 7 (Fig. 7). The measurements were conducted within an OTTLE cell by stepwise increase of the potential similar to the previous described UVeVis/NIR measurements (vide infra, concentration of analyte: 0.010 mol L1). The IR spectra from 3 and in situ generated 3þ were measured for comparison (Fig. 7). Compound 3 possesses the typical C^C and C^N vibrations at 2115 and 2229 cm1, respectively. Upon oxidation to 3þ both modes were shifted to higher wavenumbers (2122 and 2248 cm1). The C^N vibration of 7 (2224 cm1) shows a similar behavior after in situ generation of 7þ (2249 cm1), while the nC≡C band is also shifted from 2154 to 2166 cm1. In addition, a strong vibration mode at 2195 cm1 was found. This is in good agreement with the unsubstituted ferrocenylanalog FceC^CeC^CeFc and biferrocenyl-analog bfceC^CeC^Cebfc (bfc ¼ 1:100 -biferrocenyl), which possess frequencies at 2205 or 2191 cm1 for the mono-oxidized or trioxidized species [78]. The observation of two modes in 7þ for the C^C vibration is an indication that 7þ shows a weak and slow charge delocalization over the eC^CeC^Ce backbone, which is in agreement with the results of the UVeVis/NIR measurements. Upon further oxidation to 72þ the nC≡N bands at 2249 and 2195 cm1 disappear, while the third mode is slightly shifted to 2163 cm1. Conclusions In this study the synthesis of the 1-cyano-10 -ethynyl ferrocene Fe(h5-C5H4C^N)(h5-C5H4C^CH) from Fe(h5-C5H4C^N)(h5C5H4C(O)Me) is described. Several consecutive synthetic

Fig. 6. Left: UVeVis/NIR spectra of 7 at rising potentials vs Ag/AgCl: 500e900 mV (bottom), 950e1200 mV (top). Right: deconvolution of the NIR absorptions at 900 mV of in situ generated 7þ using three Gaussian-shaped bands. Measurement conditions: 25  C, dichloromethane, 0.1 mol L1 [nBu4N] [B(C6F5)4] as supporting electrolyte.

F. Strehler et al. / Journal of Organometallic Chemistry 786 (2015) 1e9

100

100

80

80

Transmission [a.u.]

Transmission [a.u.]

8

60

40

20

0

60

40

20

0

2200

2100

2000

2200

Wavenumber [cm-1]

2100

2000

Wavenumber [cm-1]

Fig. 7. Left: IR spectra of 3 during electrochemical oxidation to 3þ (500e1200 mV). Right: IR spectra of 7 during electrochemical oxidation (500e1100 mV (7 / 7þ)) at 20  C in dichloromethane (10.0 mmol L1), supporting electrolyte [nBu4N][B(C6F5)4].

methodologies were established including the formation of Fe(h5C5H4C^N)(h5-C5H4CCl]CH2), Fe(h5-C5H4C^N)(h5-C5H4CCl] 5 CHC(O)H) and Fe(h -C5H4C(O)NH2)(h5-C5H4C^CH). Eglinton homo-coupling of Fe(h5-C5H4C^N)(h5-C5H4C^CH) gave the respective butadiyne [Fe(h5-C5H4C^N)(h5-C5H4C^C)]2, while addition of [CuI], NEt2H in dichloromethane produced Fe(h5C5H4C^N)(h5-C5H4C^CCH2NEt2). The structure of Fe(h5C5H4C^N)(h5-C5H4C^CH), Fe(h5-C5H4C^N)(h5-C5H4CCl]CHC(O) H) and Fe(h5-C5H4C(O)NH2)(h5-C5H4C^CH) in the solid state were determined by single crystal X-ray structure analysis exhibiting an ecliptic conformation with all substituents arranged synperiplanar. The bond distances and angles of the ferrocenyl backbones are in the range typical for ferrocene derivatives. Fe(h5-C5H4C(O)NH2)(h5C5H4C^CH) exhibits an hydrogen bridge bond pattern including both, the hydrogen atoms of the amide and the ethynyl functionality, forming an intermolecular network of a columnar one dimensional structure. The electrochemical behavior of Fe(h5C5H4C^N)(h5-C5H4C^CH) possesses a reversible redox event at 550 mV with DE ¼ 65 mV in the cyclic voltammogram, while for Fe(h5-C5H4C^N)(h5-C5H4C^CCH2NEt2) a quasi-reversible process (E20 ¼ 761 mV with DE ¼ 100 mV) was found. In addition, the amino moiety will be irreversibly oxidized at E10 ¼ 460 mV. In homo-coupled [Fe(h5-C5H4C^N)(h5-C5H4C^C)]2 two well separated redox events were observed at 525 and 680 mV, respectively. The redox separation DE with 155 mV implies a possible electron transfer in the mixed-valent species [Fe(h5-C5H4C^N)(h5C5H4C^C)]þ 2 which could be confirmed by UVeVis/NIR and IR spectroelectrochemical studies allowing to classify this species as a weakly coupled class II system according to Robin and Day [83]. Compared with unsubstituted [Fe(h5-C5H5)(h5-C5H4C^C)]2 the electron transfer is more pronounced in the nitrile functionalized compound.

be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Appendix B. Supplementary material Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2015.02.049. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Acknowledgment We are grateful to the Fonds der Chemischen Industrie. M.K. thanks the FCI for a Ph.D. fellowship and E.P. thanks the Federal Cluster of Excellence EXC 1075 “MERGE Technologies for Multifunctional Lightweight Structures” for generous financial support. Appendix A. Supplementary material CCDC 1032801 (3), 1032799 (4) and 1032800 (5) contain the supplementary crystallographic data for this paper. These data can

[24] [25] [26]

[27]

[28] [29]

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