Chemoselective, Practical Synthesis of Cobaltocenium Carboxylic

Cobaltocenium carboxylic acid (carboxycobaltocenium) hexafluorophosphate, a key compound for other monofunctionalized cobaltocenium salts, has been ...
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Chemoselective, Practical Synthesis of Cobaltocenium Carboxylic Acid Hexafluorophosphate Stefan Vanicek,† Holger Kopacka,† Klaus Wurst,† Thomas Müller,‡ Herwig Schottenberger,† and Benno Bildstein*,† †

Institute of General, Inorganic and Theoretical Chemistry, Center for Chemistry and Biomedicine, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria ‡ Institute of Organic Chemistry, Center for Chemistry and Biomedicine, University of Innsbruck, Innrain 80-82, A-6020 Innsbruck, Austria S Supporting Information *

ABSTRACT: Cobaltocenium carboxylic acid (carboxycobaltocenium) hexafluorophosphate, a key compound for other monofunctionalized cobaltocenium salts, has been synthesized in >70% overall yield starting from cobaltocenium hexafluorophosphate by a synthetic sequence involving (i) nucleophilic addition of lithium (trimethylsilyl)ethynide, (ii) hydride removal by tritylium hexafluorophosphate, and (iii) oxidative cleavage of the alkynyl substituent by potassium permanganate.

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gates.8 The synthetic procedure in this contribution is based in part on our earlier work9 and is motivated by our general interest in developing advanced cobaltocenium-containing materials.

n metallocene chemistry, ferrocene and its derivatives represent by far the most important class of compounds, due to their advantageous chemical and physical properties such as air stability, reversible redox chemistry, well-developed synthetic chemistry, planar chirality of appropriate derivatives, useful material properties in homogeneous catalysis, electrochemical sensing, bioorganometallic chemistry, supramolecular chemistry, etc.1 In comparison, much less is known of the isoelectronic cobaltocenium salts, although they have some analogous advantageous properties: e.g., high chemical stability and reversible redox chemistry. In contrast to neutral ferrocenes with their 18 valence electrons, the isoelectronic cobaltocenium salts with 18 valence electrons are positively charged and therefore water-soluble, a potentially useful feature for applications in green chemistry or for functional hybrid materials. On the other hand, the cationic cobaltocenium core makes functionalized cobaltocenium salts much more difficult to synthesize and they are very few in number in comparison to ferrocene derivatives. Historically, the parent cobaltocene2 was prepared shortly after the discovery of ferrocene and functionalized cobaltocenium salts have been known since 1970 from the seminal work of Sheats and Rausch;3 a comprehensive review from 1979 by Sheats, the pioneer in this area of research, covers most of the known cobaltocene/cobaltocenium chemistry.4 In this contribution, we report reliable syntheses of ethynylcobaltocenium hexafluorophosphate, a useful synthon for triazolyl cobaltocenium materials via “click” chemistry,5 and of cobaltocenium carboxylic acid hexafluorophosphate (carboxycobaltocenium hexafluorophosphate, CcCO2H+PF6−), the key starting material for further monofunctionalized cobaltocenium derivatives: e.g. CcCO2Cl+PF6− and CcNH2+PF6−.3 Current applications of these latter building blocks comprise organometallic electrodes,6 block copolymers,7 and peptide bioconju© 2014 American Chemical Society

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RESULTS AND DISCUSSION According to the protocol of Sheats and Rausch,3 carboxycobaltocenium hexafluorophosphate is obtained in low overall yield (6.61%) by a statistical reaction from a mixture of cyclopentadiene, methylcyclopentadiene, and cobaltous chloride in the presence of a base, followed by metal-centered oxidation by air (Co(II) to Co(III)) and methyl side chain oxidation by potassium permanganate. The product mixture of this reaction consists of hexafluorophosphates of cobaltocenium (CcH+), cobaltocenium carboxylic acid (CcCO2H+), and cobaltocenium dicarboxylic acid (Cc′(CO2H)2+), separable by their different solubilities in acetone. Recent attempts by others7a or by us10 to improve the yield of the desired CcCO2H+PF6− by optimizing the conditions, changing the stoichiometry, or using other starting materials, e.g. Cp-Li/ MeCp-Li instead of CpH/MeCpH/pyrrolidine, gave only marginall better yields of 7.5%6a or 11.0%.10 Clearly there is a need for a more practical preparation of CcCO2H+PF6−. To this end, various potential synthetic approaches were tried out, resulting finally in the three-step protocol given in Scheme 1. For the starting material we chose air-stable cobaltocenium hexafluorophosphate (1), a commercially available but rather expensive chemical. If desired, 1 may be prepared from sodium cyclopentadienide and CoCl2, followed by air oxidation11 according to the initial preparation by Wilkinson and Cotton.2d However, this standard procedure Received: November 19, 2013 Published: February 24, 2014 1152

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Scheme 1. Synthetic Protocol for the Synthesis of 5 and 6a

cyclopentadiene)(η5-cyclopentadienyl)cobalt(I) is possible in 80% yield; however, no experimental details were reported and various attempts by us to repeat this work failed. In our opinion, the (trimethylsilyl)ethynyl moiety is therefore the best group for the attachment of a carbon moiety on CcH+PF6−, because it combines π conjugation with minimal steric hindrance, thereby allowing the sequence nucleophilic addition/hydride abstraction to be performed in a satisfying 87% yield. The final step in the synthesis of the target compound CcCO2H+PF6− (5) comprises an oxidative cleavage of the alkyne substituent by potassium permanganate, in analogy to the preparation of organic carboxylic acids from alkynes.15 It has been shown that water is a necessary component in this reaction;15a therefore, the high solubility of cobaltocenium derivative 4 in an acetonitrile/water solvent mixture is beneficial for this transformation. Furthermore, in general only terminal alkynes are cleaved by KMnO4;15 therefore, the trimethylsilyl protecting group was first removed in situ by addition of sodium fluoride to the reaction mixture (see the Experimental Section). In this manner, CcCO2H+PF6− (5) was obtained from 4 in 94% yield in a purity of 98% according to NMR analysis. Interestingly, the only (unexpected) byproduct in 2% yield was unsubstituted CcH+PF6− (1), although the starting material 4 contained no traces of 1 according to NMR analysis. Attempts to avoid this impurity by changing the reaction conditions (stoichiometry, temperature, etc.) have met so far with failure. However, for most further chemistry of the carboxyl functionality of 5 the presence of small amounts of unsubstituted CcH+PF6− is tolerable. Nevertheless, if desired or necessary, 100% pure 5 can easily be obtained from this material in 91% isolated yield by short column chromatography on neutral alumina, although with some loss of material. Overall, the three-step synthetic sequence from CcH+PF6− (1) to CcCO2H+PF6− (5) afforded the final product in 80.6% yield in 98% purity or in 73.3% yield in 100% purity, respectively. In an alternative approach, we investigated also the oxidative cleavage of nonprotected ethynylcobaltocenium hexafluorophosphate (6) by potassium permanganate, to see if thereby better yields and/or better initial purity of 5 with no concomitant formation of unsubstituted 1 could be obtained. To this end (Scheme 1), ethynylcobaltocenium hexafluorophosphate (6) was prepared in 43% yield according to the recently reported method by Tang5d via desilylation of 2 to (exo-ethynyl-η4-cyclopentadiene)(η5-cyclopentadienyl)cobalt(I) (3) followed by hydride removal by tritylium hexafluorophosphate. A significantly better yield of 76% of 6 was obtained by the inverse sequence (see the Experimental Section): first oxidation of 2 by tritylium hexafluorophosphate and second desilylation by potassium fluoride, indicating the advantageous properties of the apolar trimethylsilyl protecting group in the workup procedure in the oxidation step. Unsubstituted ethynylcobaltocenium hexafluorophosphate (6) is an important synthon for click chemistry5 and an interesting redox-active ligand precursor with up to now unexplored coordination chemistry; therefore, this chemistry might be useful in the future. However, for our purpose of synthesizing cobaltocenium carboxylic acid hexafluorophosphate (5) via oxidative cleavage of the alkynyl substituent, unsubstituted 6 proved slightly inferior to silylated 4 in terms of yield of product (77%) but slighlty superior in terms of its purity (no cobaltocenium hexafluorophosphate (1) as impurity). Overall, the four-step synthetic pathway 1−2−4−6−5 affords the target

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Reaction conditions: (i) (H3C)3SiCCLi/THF; (ii) K2CO3/ CH3OH;5d (iii) (H5C6)3C+PF6−/CH2Cl2/n-hexane; (iv) NaF/ CH3CN, KMnO4/CH3CN, H2O/HPF6; (v) NaF/CH3CN, H2O.

involves in the first step a rather tedious sublimation of cobaltocene; in our hands a one-pot reaction proved much more convenient, affording 1 in 66% yield (see the Experimental Section). Nucleophilic addition of organolithium reagents to CcH+PF6− is a general reaction in cobaltocene chemistry,12 resulting in η5/η4 Co(I) compounds containing the nucleophile in the exo position and one of the former cyclopentadienide hydrogens in an endo configuration.4 Earlier work by us has shown that lithium acetylide in tetramethylethylenediamine (TMEDA) as solvent added to 1 in 68% isolated yield.9 We modified the reagents, simplified the reaction conditions ((trimethylsilyl)acetylide instead of acetylide, THF instead of TMEDA), and optimized the workup procedure, thereby making 2 accessible in an almost quantitative yield of 98%. Interestingly, Tang and co-workers5d reported very recently and independently from us exactly the same result. The improved yield obtained by using (trimethylsilyl)acetylide instead of acetylide/TMEDA is due to the superior nucleophilicity of (trimethylsilyl)alkynide in comparison to unsubstituted acetylide. In the next step, Co(I) species 2 was oxidized to substituted Co(III) derivative 4 by a chemoselective endo-hydride abstraction. The best reagents for this process are triphenylcarbenium (tritylium) salts containing noncoordinating anions, either tritylium tetrafluoroborate ((C6H5)3C+BF4−) or tritylium hexafluorophosphate ((C6H5)3C+PF6−).13 Other oxidation agents for similar hydride abstractions, e.g. simple Fe3+ or Ce4+ salts, have been reported14 but proved ineffective in our case. Under optimized conditions, hydride removal from 2 by (C6H5)3C+PF6− gave the alkynyl-substituted cobaltocenium derivative 4 in 87% yield. In our experience and in accord with literature reports9,11 only π-conjugated carbanions (e.g., phenyl, cyclopentadienyl, ethynyl) as exo substituents allow clean endohydride removal, indicating selective hydride abstraction due to conjugation in the product. On the way to the desired target compound CcCO2H+PF6− (5), one would naturally choose methyllithium as the smallest carbon nucleophile and as the most atom-economical synthon; however, hydride abstraction of its cobaltocenium adduct by (C6H5)3C+PF6− is nonselective, giving methylcobaltocenium hexafluorophosphate in only 24% yield together with unsubstituted CcH+PF6−, accompanied by formation of (C6H5)3CH and (C6H5)3CCH3.10 We are aware of a short note in the literature12c where it has been claimed that endo-hydride abstraction from (exo-methyl-η 4 1153

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of the endo-hydride abstraction; it is shown that π conjugation of the exo substituent is essential. In addition, an improved synthesis of ethynylcobaltocenium hexafluorophosphate is reported (75% yield starting from cobaltocenium hexafluorophosphate). With these preparative syntheses of cobaltocenium carboxylic acid hexafluorophosphate and ethynylcobaltocenium hexafluorophosphate, we wish to contribute to advancements in the chemistry of monofunctionalized cobaltocenium compounds and materials.

compound 5 in 58% yield. Hence, we can summarize that the shorter three-step synthetic sequence 1−2−4−5 (overall yield 73%) discussed above is the most convenient and practical method to synthesize cobaltocenium carboxylic acid hexafluorophosphate (5) on a preparative scale. Cobaltocenium carboxylic acid hexafluorophosphate (5) was fully characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and IR spectroscopy (see the Experimental Section and Supporting Information), and spectroscopic data are in agreement with published results.3,7a Cobaltocenium carboxylic acid hexafluorophosphate (5) is an air-stable, highmelting solid (mp 206 °C dec) soluble in acetone, acetonitrile, and water. Due to the cationic charge of the cobaltocenium moiety, it is a strong acid with an estimated pKa ≤ −1.74, indicated by pKa determination by acid−base titration in an acetonitrile/water solvent mixture according to the Henderson−Hasselbalch equation.16 An early single-crystal structure analysis was published17 in 1978, and a more precise redetermination by us (Figure 1,

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EXPERIMENTAL SECTION

General Considerations. Standard methods and procedures of organometallic synthesis, spectroscopic characterization, and singlecrystal structure analysis were performed as described recently.18 Starting materials were obtained commercially and used as received. Cobaltocenium Hexafluorophosphate (1). A Schlenk flask was charged with 100 mL of a 2.0 M NaCp solution in THF (0.2 mol, 2 equiv) and 400 mL of dry THF at −40 °C under protection from air by an argon atmosphere. After the mixture was stirred for 10 min at −40 °C, 12.984 g of anhydrous CoCl2 (0.1 mol, 1 equiv) was added in one portion, the cooling bath was removed, and stirring was continued for 45 min. The dark purple reaction mixture was sonicated for an additional 15 min and further stirred at room temperature overnight. The solvent was removed in vacuo, affording a dark brown residue. A 300 mL portion of hot water was added, and pressurized air was injected through a glass frit for 4 h. The remaining brown residue was filtered off, and a further Soxhlet extraction over 8 h with water gave a yellow aqueous solution of the product. The combined cold aqueous solutions were washed two times with 100 mL of diethyl ether, decolorized with activated charcoal, and concentrated on a rotary evaporator until the total volume was approximately 80 mL. A 250 mL portion of an aqueous KPF6 solution (18.406 g, 0.1 mol, 1 equiv) was added, and crystallization was facilitated by cooling in an ice bath. The yellow powdery product was filtered off, thoroughly washed once with ice−water and twice with diethyl ether, and dried in vacuo, giving pure 1 in 66% yield (22.013 g, 65.9 mmol). The compound is soluble in acetonitrile, acetone, and water: yellow solid, mp >300 °C. 1H NMR (300 MHz, CD3CN): δ 5.67 (s, 10H, Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 85.9 (s, Cp) ppm. IR (ATR): 3126 (νC−H), 1416 (νCC), 1012, 809 (νP−F), 553, 503, 451 cm−1. These data concur with literature values.4 For spectra, see the Supporting Information. (η5-Cyclopentadienyl)[η4-(exo-5-trimethylsilylethynyl)-1,3cyclopentadiene]cobalt(I) (2) and ((Trimethylsilyl)ethynyl)cobaltocenium Hexafluorophosphate (4). A Schlenk flask was charged under an atmosphere of argon with 400 mL of dry THF and 4.7 mL of (trimethylsilyl)acetylene (34.1 mmol, 1.14 equiv). The mixture was cooled to −15 °C, and 19.7 mL of a 1.6 M nBuLi solution (31.5 mmol, 1.05 equiv) was added. Stirring was continued for 45 min, and 10.000 g of 1 (29.9 mmol, 1 equiv) was added. The reaction mixture was slowly warmed to room temperature for 30 min with vigorous stirring, whereby the nucleophilic addition was indicated by a color change from yellow to orange to red. The reaction mixture was sonicated for an additional 15 min at room temperature, resulting in a dark red homogeneous solution. The solvent was removed in vacuo, and the intermediate 2 was isolated by solid-phase extraction over several cycles with 600 mL of dry hexane under argon. In the case of larger synthetic approaches a Soxhlet extraction of the intermediate 2 (which shows air stability over several days) is recommended. The hexane was removed in vacuo and stored for the succeeding workup of 4. After the volatiles were completely removed, a total amount of 8.402 g (98%, 29.3 mmol) of 2 was obtained as an intense red solid. Spectral data have been published earlier.9 This material, soluble in hexane or dichloromethane, can be used without further purification for the synthesis of 4: a 1000 mL Schlenk flask equipped with a reflux condenser was charged at room temperature under an argon atmosphere and under exclusion of light with 300 mL of dry dichloromethane, 8.402 g of 2 (29.3 mmol), and 15.118 g of triphenylcarbenium hexafluorophosphate (38.9 mmol, 1.3 equiv). Note: triphenylcarbenium salts are light-sensitive. After 10 min of

Figure 1. Molecular structure of 5 showing thermal ellipsoids at 20% probability. Selected distances (Å): C(11)−O(1) = 1.263(3), C(11)− O(2) = 1.263(3), average value Co(1)−C(unsubstituted Cp) = 2.020(3), average value Co(1)−C(substituted Cp) = 2.038(2), average value C−C of unsubstituted Cp 1.385(6), average value C−C of substituted Cp 1.423(4), O(1)−O(2A) = 2.632(2).

Supporting Information) is included here. Overall, the structure of 5 consists of carboxylate O−H−O hydrogen-bonded dimers with hexafluorophosphate counterions, as expected. The final R1 value (I > 2σ(I)) obtained was 0.036, indicating a precise structure solution. However, there are three types of disorder that are more fully discussed in the Supporting Information: disorder of (i) hexafluorophosphate, (ii) carbon atoms of the unsubstituted Cp ring, and (iii) oxygen atoms of the carboxylate moiety.

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CONCLUSION In summary, a reliable synthesis of cobaltocenium carboxylic acid hexafluorophosphate was developed. Starting from unsubstituted cobaltocenium hexafluorophosphate, the synthetic protocol involves nucleophilic addition of lithium (trimethylsilyl)ethynide, followed by selective endo-hydride removal, and finally oxidative cleavage of the alkynyl substituent, affording the target compound in 73% overall yield. The key issue in this preparation is the chemoselectivity 1154

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stirring at room temperature, the previously collected 600 mL of hexane in the synthesis of 2 was added to the yellow-brown reaction mixture and the product was allowed to precipitate during 20 min of further stirring. The tan precipitate was filtered off over a Büchner funnel and thoroughly washed with three portions of diethyl ether, two portions of cold water, and finally three portions of diethyl ether. Further drying in vacuo afforded 4 in 90% yield over both steps (11.585 g, 26.9 mmol) as an amber powder: mp 175.8 °C dec. The compound is soluble in acetonitrile and dichloromethane. 1H NMR (300 MHz, CD3CN): δ 0.27 (s, 9H, trimethylsilyl, 5.67 (s, 5H, Cp), 5.68 (pseudo-t, 2H, J = 2.1 Hz, substituted Cp), 5.85 (pseudo-t, 2H, J = 2.1 Hz, substituted Cp) ppm. 13C NMR (75 MHz, CD3CN): δ −0.6 (trimethylsilyl), 85.9 (substituted Cp), 87.1 (substituted Cp), 87.4 (Cp), 95.5 (ethynyl), 103.1 (ethynyl) ppm. MS (FAB pos): m/z 285.13 (M+ − PF6−). IR (ATR): 3130, 2962, 1458, 1420, 1247, 827 (νP−F), 557, 447 cm−1. Spectra are given in the Supporting Information. Cobaltocenium Carboxylic Acid Hexafluorophosphate (5). A 500 mL round-bottom flask was charged with a magnetic stirring bar, 10.000 g of 4 (23.2 mmol, 1 equiv), 180 mL of acetonitrile, and 0.995 g of sodium fluoride (23.7 mmol, 1.02 equiv). To the amber solution was added with stirring an aqueous potassium permanganate solution (9.916 g, 62.8 mmol, 2.7 equiv., 350 mL of water), and the reaction mixture was refluxed for 2 h with vigorous stirring. The heterogeneous brown reaction mixture was filtered through a paper filter to give a yellow filtrate and solid manganese dioxide. The brown precipitate was thoroughly washed once with hot water and once with hot acetonitrile using ultrasound. The yellow aqueous solutions were combined and carefully concentrated on a rotary evaporator until the total volume was approximately 80 mL. Hexafluorophosphoric acid (4.1 mL of an aqueous solution, 65% (w/w), 30.2 mmol, 1.3 equiv) was added, and the product was allowed to crystallize in a refrigerator overnight at 4 °C. The yellow powdery product was filtered off over a Büchner funnel, thoroughly washed two times with ice water and two times with diethyl ether, and dried in vacuo to give 5 in 95% yield (8.340 g, 22.1 mmol) as a yellow solid in a purity of 97.7% according to NMR analysis. The compound is soluble in water or acetonitrile; the best solubility is observed in a 1/1 (v/v) mixture of these two solvents. The only impurity (2.3%) that occurs is unsubstituted cobaltocenium hexafluorophosphate (1), which is formed during the oxidation process by a decarboxylative pathway. For most further synthetic applications, a purity of 97.7% is sufficient. If necessary, byproduct 1 can easily be removed by flash chromatography on a short neutral alumina column, first with acetonitrile/diethyl ether (3/1 v/v) to remove the cobaltocenium hexafluorophosphate (1) and then with water to elute the product. The aqueous fractions were combined and concentrated on a rotary evaporator, and an equimolar amount of hexafluorophosphoric acid was added to initiate the crystallization process, which was further facilitated by cooling in an ice bath. The yellow powder was filtered off, washed once with ice water and twice with diethyl ether, and dried in vacuo, affording 5 in 87% yield (4.260 g, 1.1 mol from 5.000 g (1.3 mmol) of starting material with an initial purity of 97.7%) as a yellow solid: mp 206.0 °C dec. 1H NMR (300 MHz, CD3CN): δ 5.75 (s, 5H, Cp), 5.80 (pseudo-t, 2H, J = 2.1 Hz, C2/C5 of substituted Cp), 6.09 (pseudo-t, 2H, J = 2.1 Hz, C3/C4 of substituted Cp) ppm; signal of CO2H not observed due to rapid exchange. 13C NMR (75 MHz, CD3CN): δ 86.7 (C2/C5 of substituted Cp), 87.4 (Cp), 87.9 (C3/C4 of substituted Cp), 90.1 (quat carbon of substituted Cp), 165.0 (CO2H) ppm. MS (FAB pos): m/z 233.03 (M+ − PF6−). IR (ATR): 3125 (νO−H), 1705 (νCO), 1492, 1417, 1300, 814 (νP−F), 555, 473, 446 cm−1. Single crystals of 5 were obtained at room temperature from an aqueous acetonitrile solution (1/1 v/v). Spectra and crystallographic details are given in the Supporting Information. Ethynylcobaltocenium Hexafluorophosphate (6). A 1.000 g portion of 4 (2.32 mmol, 1 equiv) was dissolved in 25 mL of an acetonitrile−water mixture (3/2 v/v), and 0.106 g of sodium fluoride (2.53 mmol, 1.09 equiv) was added. Note: attempted deprotection using potassium carbonate in methanol gave inferior results. The amber solution was refluxed for 2 h, and stirring was continued at

room temperature overnight. Traces of dark solid byproduct were filtered off and thoroughly washed with acetonitrile. The washing fractions and the mother liquor were combined and concentrated on a rotary evaporator at a bath temperature of 50 °C until the volatile acetonitrile amount was completely removed, whereby the product started to precipitate. Evaporation was continued until the total volume of the aqueous solution was approximately 7 mL. To this warm heterogeneous mixture was added hot acetonitrile dropwise until the solid product was completely redissolved, and then 345 μL of an aqueous hexafluorophosphoric acid solution (65% w/w, 2.53 mmol, 1.09 equiv) was added, whereupon the solution turned brighter and the acetonitrile fraction was completely evaporated again. Due to the meager solubility of 6 in water, which drastically increases in the presence of traces of acetonitrile, the product readily crystallized after all the acetonitrile was removed. Crystallization was facilitated in an ice bath for 1 h, and the yellow powdery product was filtered off over a Büchner funnel, thoroughly washed two times with a small amount of ice−water and three times with diethyl ether, and dried in vacuo. In addition, the yellow aqueous mother liquor was extracted with three portions of dichloromethane, anhydrous magnesium sulfate was added to the combined organic fractions, the desiccant was filtered off, the solvent was removed on a rotary evaporator, and the solid product was dried in vacuo and combined with the first product fraction from above. In this manner, 6 was obtained in 87% yield (0.724 g, 2.02 mmol): mp 186.5 °C dec. The compound is most soluble in acetonitrile and acetone but only slightly soluble in dichloromethane or water. 1H NMR (300 MHz, CD3CN): δ 3.72 (s, 1H, ethynyl), 5.69 (pseudo-t, 2H, J = 4.2, C2/C5 of substituted Cp), 5.70 (s, 1H, Cp), 5.91 (pseudo-t, 2H, J = 4.2, C3/C4 of substituted Cp) ppm. 13C NMR (75 MHz, CD3CN): δ 75.0 (ethynyl), 85.0 (quat carbon of substituted Cp), 85.9 (ethynyl), 86.0 (C2/C5 of substituted Cp), 87.5 (Cp), 87.6 (C3/C4 of substituted Cp) ppm. MS (FAB pos): m/z 213.04 (M+ − PF6−). IR (ATR): 3274 (νC−H), 3208 (νC−H), 3124 (νC−H), 1453, 1419, 817 (νP−F), 702, 677, 554, 512, 461, 435 cm−1. Spectra are given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and text giving spectra (1H/13C NMR, IR, MS) and characterization data for 4−6 and tables and a CIF file giving crystallographic data for 5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for B.B.: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Thematic/special issue “Ferrocene−Beauty and Function”: Organometallics 2013, 32 (20), 5623−6146. (2) (a) Wilkinson, G. J. Am. Chem. Soc. 1952, 74, 6148−6149. (b) Fischer, E. O.; Jira, R. Z. Naturforsch., B 1953, 8b, 1−2. (c) Fischer, E. O.; Jira, R. Z. Naturforsch., B 1953, 8b, 327−328. (d) Wilkinson, G.; Cotton, F. A.; Birmingham, J. M. J. Inorg. Nucl. Chem. 1956, 2, 95− 113. (3) Sheats, J. E.; Rausch, M. D. J. Org. Chem. 1970, 35, 3245−3249. (4) Sheats, J. E. Organomet. Chem. Rev. 1979, 7, 461−521. (5) (a) Rapakousiou, A.; Wang, Y.; Ruiz, J.; Astruc, D. J. Inorg. Organomet. Polym. 2014, 24, 107−113. (b) Rapakousiou, A.; Wang, Y.; Belin, C.; Pinaud, N.; Ruiz, J.; Astruc, D. Inorg. Chem. 2013, 52, 6685− 6693. (c) Wang, Y.; Rapakousiou, A.; Latouche, C.; Daran, J.-C.; Singh, A.; Ledoux-Rak, I.; Ruiz, J.; Saillard, J.-Y.; Astruc, D. Chem. Commun. 2013, 49, 5862−5864. (d) Yan, Y.; Zhang, J.; Qiao, Y.; Tang, C. Macromol. Rapid Commun. 2014, 35, 254−259.

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dx.doi.org/10.1021/om401120h | Organometallics 2014, 33, 1152−1156