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Synthesis of closo- and nido-biscarboranes with rigid unsaturated linkers as precursors to linear metallacarborane-based molecular rods† Alexander V. Safronov, Yulia V. Sevryugina, Kothanda Rama Pichaandi,‡ Satish S. Jalisatgi and M. Frederick Hawthorne* Several biscarborane-type derivatives of 8-iodo-1,2-dicarba-closo-dodecaborane (1), suitable as the precursors of linear metallacarborane-based molecular rods, were prepared. The synthesized closocompounds contained two carborane moieties connected through a rigid linear unsaturated linker. The linkers were based on ethynylene and para-phenylene fragments and their combinations. The deborona-
Received 19th September 2013, Accepted 16th October 2013 DOI: 10.1039/c3dt52596a www.rsc.org/dalton
tion of all reported closo-compounds was selective and afforded nido-products with open pentagonal faces on opposite sides of the molecule, parallel to each other and perpendicular to the main molecular axis. All of the closo and nido products were characterized by a variety of physical methods (NMR, HRMS, IR). The structures of closo-carboranes 3, 6, 9, and 14 and nido-carboranes 15 and 17 were established by X-ray diffraction.
Introduction Rigid-rod molecules—organic, organometallic, or inorganic— have received attention during the past two decades. Numerous reviews and articles are dedicated to the design, synthesis, and application of these molecules, which can be discreet or oligomeric in nature.1 The rigid molecules of fixed length can be used as building blocks for supramolecular assemblies;2 i.e., as “molecular tinkertoys”.3 The discovery of porous coordination polymers, also known as metal–organic frameworks (MOFs), was the direct result of the development of molecularrod chemistry. Transition-metal-containing MOFs show catalytic activity in a variety of organic reactions.4 Although they are best known for their application as gas storage and separation materials,5 MOFs also form thin films that can be used as sensors.6 The application of molecular rods in molecular electronics is even more promising. Such molecules can potentially be used as unimolecular diodes,7 rectifiers,8 molecular wires that provide electron transfer through a molecule, components of computer memory devices, and in several additional areas.9
International Institute of Nano and Molecular Medicine (I2NM2), University of Missouri, Columbia, MO 65211, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. CCDC 961327–961332. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52596a ‡ Present address: Department of Chemistry and Energy Center, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA.
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Progress in molecular electronic applications of molecular rods has occurred parallel to advances in physics, where theoretical and practical approaches to single-molecule conductivity measurements and other types of characterization have been developed.10 Several reports have been previously published on the design and synthesis of rigid-rod molecules based on carborane fragments called “carbo(ra)rods”.11 Dimers and tetramers of para-carborane containing direct Ccarborane–Ccarborane bonds were prepared and characterized,11c as well as several derivatives of para- and meta-carboranes in which the polyhedral moieties were connected through ethynylene and butadiyne1,4-diyl linkers attached both to cage carbon and to boron atoms.11a,b This line of investigation was continued in the current study with the molecular rods based on metallacarborane fragments. The synthesis of such molecules seemed attractive due to the numerous known bis(dicarbollide)metal complexes, many of which exist in more than one oxidation state.12 Rigid-rod molecules with metallacarboranes connected through rigid unsaturated linkers might be conductive and possibly serve as molecular wires. There are two possible ways to prepare these compounds: (1) Pd-catalyzed cross-coupling copolymerization of diiodosubstituted metallacarboranes with metal derivatives of the desired linker molecules, and (2) connection of nido-biscarborane compounds containing the desired linkers via π-coordination with metals (Fig. 1). To maintain structural rigidity, the metallacarborane units in rod molecules must be substituted at the apical positions,
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Fig. 1
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Two possible routes to metallacarborane-based molecular rods.
Fig. 2 Selective deboronation of 1 and the formation of metallacarboranes with apical functionalization.
B(10) and B(10′), as shown in Fig. 1. In this case, conformational interconversions of metallacarborane units have no effect on the rod geometry. However, until recently, no reliable procedure has existed for the preparation of a carborane precursor that would allow for synthesis of axially functionalized metallacarborane complexes. In 2012 publication,13 a convenient synthetic route to a less-common iodinated ortho-carborane molecule, 8-iodo-1,2dicarba-closo-dodecaborane, was described (1, Fig. 2). The compound revealed an unusual and selective deboronation pattern: in the resulting 1-iodo-7,8-dicarba-nido-undecaborate anion, the B–I bond was perpendicular to the plane of the open pentagonal C2B3 face. The compound formed the corresponding bis(dicarbollide)cobalt and nickel complexes in high yields. In these complexes, both the B–I bonds and the metal atom were collinear, as required for the synthesis of molecular rods. Cross-coupling polymerization of these complexes is currently under investigation in our laboratory. Reported here is the synthesis and characterization of closo and nido precursors to metallacarborane-based molecular rods via a single-step cross-coupling of 8-iodo-1,2-dicarbacloso-dodecaborane with ethynylene and phenylene-based nucleophiles.
Since then, several important discoveries have been made concerning the differences in reactivity of arylhalides and halocarboranes. First, in contrast to organic chemistry, only compounds with B–I bonds participate in cross-coupling reactions. Other derivatives (B–Cl, B–Br) appeared to be completely nonreactive towards the oxidative addition to palladium. Second, unlike organic cross-couplings, which can be catalyzed by complexes of different transition metals, in borane and carborane chemistry, these reactions are exclusively palladiumcatalyzed. Third, in addition to the nature of the nucleophile, the nature of the additive plays an important role the yield of the product. For example, cross-coupling according to the Suzuki protocol15 in the presence of CsF (a transmetallationassisting agent) proceeds with high yields only in the case of para-carborane because meta- and ortho-carboranes react faster with fluoride anion to form of nido-carboranes. Fourth, in some cases, cross-coupling reactions are accompanied by undesirable side reactions, most often by dehalogenation of the starting material.16 In the current study, four compounds of interest were modeled; all of them contained two 1,2-dicarba-closo-dodecaboran-8-yl moieties connected through an unsaturated linker: ethynylene, 1,4-phenylene, phenylene-1,4-diethynyl, or butadiyn-1,4-diyl. For simplicity, the synthesis of most of the target molecules was performed via a one-step cross-coupling reaction, i.e. through the coupling of two iodocarborane molecules with one linker molecule containing two nucleophilic centers. This approach has never been used in carborane cross-coupling chemistry where reactions of a single carborane molecule with one or several nucleophiles are the most common case. This study began with the synthesis of a bis(carboranyl)acetylene compound. The commercially available bis(trimethylstannyl)acetylene (2, Scheme 1) was chosen as an acetylene-containing nucleophile. The reaction of 8-iodo-1,2dicarba-closo-dodecaborane (1) with 2 in dioxane in the presence of Pd(PPh3)4 gave target compound 3 with 47% isolated yield. The use of less-volatile bis(tributylstannyl)acetylene in this reaction did not increase the reaction yield. In addition to the coupling product 3, 20–30% of deiodinated product (orthocarborane) was regularly observed in this reaction. Analysis of the literature data showed that this reaction represents the first example of Stille cross-coupling in the carborane series. Synthesis of the next target compound was achieved through the generation of an organozinc derivative from 1,4-dibromobenzene (4, Scheme 2). The recently discovered cobalt-assisted synthesis of organozincs17 appeared to be the most convenient way to produce labile compound 5. The reaction of 5 with 1 in acetonitrile in the presence of a Pd(0) catalyst gave 6 in moderate yield. Most likely, side reactions of 5,
Results and discussion Synthesis of closo-biscarboranes Palladium-catalyzed cross-coupling reactions using carborane and borane substrates were discovered in the early 1980s.14
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Scheme 1
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Scheme 2
of acetylenes—is usually conducted in the presence of copper compounds in air in an amine solution at elevated temperatures. In the case of ortho-carboranes, these reaction conditions are not favorable because amines cause cage deboronation at elevated temperatures.19 Instead, oxidative coupling of 13 was performed in DMSO in air in the presence of Pd/C and CuI catalysts20 to produce the target compound 14 in 91% isolated yield. The overall yield of compound 14 was 84% based on the initial amount of the starting carborane 1. 2. Synthesis of nido-biscarboranes
Scheme 3
Scheme 4
Several reagents can be used for deboronation of closo-carboranes. Alkali-metal alkoxides and cesium and tetrabutylammonium (TBAF) fluorides are most commonly used because they are inexpensive and produce target nido-carboranes in very high yields.21 In the current study, TBAF was used for two reasons. First, the products after reaction with TBAF are easy to isolate, and product loss during the isolation process is minimal. Second, TBA salts of nido-carboranes are soluble in most organic solvents. In this study, solubility was important for proper characterization of the target compounds bearing double negative charge. With an excess of 1 M TBAF in THF at reflux, the reactions of compounds 3, 6, 9, and 14 were complete within 5 h and produced single products according to NMR analyses (see below). The corresponding nido-biscarboranes 15–18 (Scheme 5) were isolated with excellent yields after water treatment of the evaporated reaction mixtures (see Experimental section). 3. Characterization of closo- and nido-biscarboranes
such as polymerization and decomposition, can explain the reduced yield of 6. Compared with the syntheses of 3 and 6, the synthesis of compound 9 was relatively simple because the acetylenes in the linker were separated by a 1,4-phenylene fragment. The organozinc compound 8 was produced from the commercially available 1,4-diethynylbenzene (7, Scheme 3) by an exchange reaction of the initially generated bis(ethynyllithium) with anhydrous zinc bromide. The subsequent cross-coupling reaction of 8 with closo-carborane 1 in the presence of Pd(0) gave product 9 with a 75% isolated yield. Initially, synthesis the last target compound (14) containing the butadiyn-1,4-diyl linker was planned through the double cross-coupling of 1,4-trimethylstannylbutadiyne with compound 1. However, a more efficient step-by-step synthetic route was chosen because of the cost of the corresponding organotin derivative and the possible low yield of the product. TMS-protected acetylenes18 can be coupled with iodocarboranes in a variety of ways with excellent yields. The reaction of compound 1 with the organozinc derivative of ethynyltrimethylsilane (11, Scheme 4) in the presence of Pd(PPh3)4 afforded compound 12 in 93% isolated yield. Deprotection of 12 was achieved quantitatively by a standard procedure using K2CO3 in methanol. The next step—oxidative coupling
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All of the new closo- and nido-biscarboranes were characterized by multinuclear NMR. Because of the symmetry of the compounds and the unsaturated nature of the linkers, both 1H and 13 C{1H} spectra provided limited structural information. For example, in the 1H NMR spectra of compounds 3 and 14, which contain ethynylene and butadiyne linkers, respectively, only one sharp signal corresponding to the carborane C–H and a broad signal corresponding to the B–H bonds were observed. In the 1H NMR of compounds 6 and 9, additional signals were observed from the phenyl rings of the linkers, which appeared as singlets at δ 7.39 and 7.34 ppm, respectively. The same trend was observed in the 1H NMR spectra of nido-biscarboranes 15–18 (see Experimental section).
Scheme 5
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In the 13C{1H} NMR spectra of all the compounds, signals from the cage carbon atoms were observed in the normal range for closo-compounds (see Experimental section). However, spectra of both the closo and nido compounds acquired under standard conditions did not contain signals corresponding to the carbon atoms connected directly to boron. This absence of signal could result from a number of causes. First, all of the carbons connected to carborane cages were quaternary, and the resonances of such atoms are sometimes difficult to acquire, even in relatively simple organic systems. Second, quaternary carbons were connected to boron atoms that contain two quadrupolar isotopes, 10B and 11B. The splitting on these nuclei significantly decreases the intensity of the carbon signal, which makes it difficult to distinguish them from the background. Signals from the rest of the linkers’ carbon atoms were acquired without any difficulties and corresponded to the expected structures. In the case of the intermediate compound 13, a 2J coupling of the terminal carbon atom with the 11B isotope of the B(8) atom of the carborane cage was observed. The corresponding signal appeared as a quartet with a 2J value of 21 Hz. Such coupling had been observed previously for carboranes with ethynyl substituents in different positions on the cage.18 The 1H and 13C{1H} NMR spectra of nido-biscarboranes 15–18 also provided limited structural information. In the 1H NMR spectra of all of the compounds, signals of TBA counter ions partially overlapped with resonances of carborane C–H and B–H vertices. In the case of compounds 6 and 9, signals from the phenyl rings of the linkers were also observed. All spectra contained a broad characteristic peak in the range of δ −2–(−3) ppm, corresponding to the bridging or “extra” hydrogen. Similarly, in the 13C{1H} NMR spectra of 15–18, signals from all of the carbon atoms except for the ones directly connected to the boron atoms were observed. The majority of the structural information for the prepared compounds was provided by their 11B, 11B{1H}, and [11B–11B] COSY NMR spectra. The spectra of closo compounds 3, 9, and 14 were very similar and had the expected signal pattern of 2 : 1 : 1 : 2 : 3 : 1 in the typical range for closo-carboranes between δ 0 and −20 ppm. A singlet corresponding to the B–C bond appeared in the 11B NMR spectra of 3, 9, and 14 at δ −8.1, −8.0, and −9.2 ppm, respectively. The assignment of the signals was based on the [11B–11B] COSY NMR spectrum of 3. This assignment confirmed that all of the reported compounds contained a closo-carborane fragment substituted at the B(8) position of the cage. Because of the nature of the linker in 6, its 11B spectra were slightly different. Similar to the previously reported 11B NMR of 9-aryl-substituted carboranes,22 the signal corresponding to the B–C bond was observed in the positive part of the spectrum at δ +2.56 ppm. The symmetry of resonances corresponding to the remainder of the molecule stayed unchanged. Similar to the spectra of closo-biscarboranes, the 11B NMR of the nido products provided more structural information. The spectra of all of the nido-biscarboranes contained the same 2 : 2 : 1 : 2 : 1 : 1 pattern in the range of 0 to −40 ppm,
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which is characteristic of nido-carboranes. Most importantly, the 11B NMR spectra of all compounds contained just one set of signals which implied formation of a single nido-isomer for both carborane cages. Assignment of the signals via the [11B–11B] COSY NMR spectrum of 3 confirmed that the resulting compounds contain nido-carborane fragments substituted at position B(1) and that the regioselectivity characteristic of deboronation of the parent 8-iodo-1,2-dicarba-closo-dodecaborane persisted in its coupling products. High-resolution mass spectral data provided additional support to the NMR characterization of the reported closo and nido compounds. For all of the closo-biscarboranes, molecular ion peak envelopes with the expected isotope distribution patterns were observed. The spectra of the nido-biscarboranes contained two peak envelopes corresponding to the molecular ion bearing a double negative charge (Δm/z = 0.5) and to its agglomerate with the TBA cation bearing an overall single negative charge (Δm/z = 1) (see Experimental section). X-ray quality crystals of the closo-biscarborane compounds, 3, 6, 9, and 14 and the two nido-biscarborane compounds, 15 and 17 were obtained. A graphic representation of the refined structures is shown in Fig. 3–8. In all of the structures, two carborane units were connected to a linker fragment by a B–C bond. The boron cluster cages showed no distortion, with the B–B and B–C distances within the normal range. Similarly, the para-substituted phenyl rings in compounds 6, 9, and 17 were undistorted, displaying averaged C–C bond distances of 1.395, 1.389, and 1.380 Å, respectively. The B–C distances between the carborane cages and the linker carbon atoms differed slightly for all of the compounds (Table S2†). The shortest B–C distance was observed for closo-
Fig. 3 The ORTEP representation of compound 3, drawn at the 40% probability level.
Fig. 4 The ORTEP representation of compound 6, drawn at the 40% probability level.
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Fig. 5 The ORTEP representation of compound 9, drawn at the 40% probability level. Only one of the two molecules in the asymmetric unit is displayed. Chloroform molecules are omitted for clarity.
Fig. 6 The ORTEP representation of compound 14, drawn at the 40% probability level.
Fig. 7 The ORTEP representation of compound 15, drawn at the 40% probability level. The B(21)–B(31) nido-carborane cage exhibits a disorder due to averaging of two rotamers [in gauche- and trans-orientations with respect to the B(1)–B(11) cage], modeled with the 0.5 : 0.5 ratio. For clarity purposes, only trans-rotamer is displayed, and two NBu4+ cations are omitted.
compound 3 (1.527(5) Å) with an ethynylene linker, whereas the longest was observed for closo-compound 6 (1.584(3) Å) containing a para-phenylene linker. In closo-biscarborane compounds 9 and 14, the B–C distances were nearly identical: 1.544 and 1.545 Å, respectively. In the nido-biscarboranes 15 and 17, the B–C distances were 1.553 and 1.550 Å, respectively.
Fig. 8
In 3, 9, 14, 15, and 17, the CuC bond length decreased in the order 3 > 15 > 14 > 9 > 17. For an analogous carborane/ nido-carborane pair, such as 3/15 or 9/17, the CuC distance was slightly longer in the neutral closo-biscarborane compounds (3 and 9), and an insertion of the para-phenylene linker shortened the CuC distance by approximately 0.02 Å, such as in 9 vs. 3 and in 17 vs. 15. These effects can also be observed in the overall lengths of the prepared molecules. The distances, measured between the most remote B-atoms in 3, 6, and 9, or between the centroids of the open pentagonal C2B3 faces of nido-carboranes in 15 and 17, decreased in the order 9 (17.967 Å) > 17 (16.024 Å) > 14 (13.654 Å) > 6 (12.855 Å) > 3 (10.999 Å) > 15 (9.241 Å). The degree of linearity of the linkers connecting the carborane cages was also examined. The angles between the lines connecting substituted B-atoms and the centers of mass of the molecules were compared. In all compounds containing the benzene ring—6, 9, and 17—the linkers formed straight lines with almost perfect 180.0° angles between the substituted B-atoms and centroids of the para-phenylene ring. Similarly, the linker in compound 14 displayed an ideal 180.0° angle, whereas that in compound 15 demonstrated a slight bend (177.6°). The bend in the crystal structure of 3 containing the ethynylene linker was even more pronounced with the corresponding angle equal to 171.7°. In their solid states, the compounds also differed by the rotational conformations of their carborane cages. To describe the relative orientation of the carborane cage carbon atoms with respect to each other around the axis of the linker, the use of the well-established for alkane chemistry cis-, trans-, and gauche-nomenclature is convenient. The carborane carbons adopted the trans-conformation in 6, 9, 14, and 17 and the gauche-conformation in compound 3. In the crystal structure of 15, the trans- and gauche-conformers were averaged to
The ORTEP representation of compound 17, drawn at the 40% probability level. Two NBu4+ cations are omitted for clarity.
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cause a positional disorder, which was modeled in the form of two equally present rotamers.
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Conclusions Several closo-biscarboranes were prepared starting from 8-iodo1,2-dicarba-closo-dodecaborane. In all of the reported compounds both carborane cages were deboronated selectively in the para-positions with respect to the substituent. Structural analysis of the nido-compounds showed that the open pentagonal faces of the nido-cages were parallel to each other. This fact makes the synthesized compounds attractive starting materials for the preparation of linear molecular rods with metallacarborane fragments in the main chain.
Experimental section Materials Unless otherwise stated, all reactions were carried out under an argon atmosphere using standard Schlenk-line techniques. 8-Iodo-1,2-dicarba-closo-dodecaborane (1) was prepared according to the published procedure.13 Bis(trimethylstannyl)acetylene (2) was purchased from Acros Organics. 1,4-Dibromobenzene (4), 1,4-diethynylbenzene (7), and trimethylsilylacetylene (10) were purchased from Aldrich and were used without further purification. Tetrabutylammonium fluoride (TBAF) was purchased from TCI America as a 1 M solution in THF and was used as received. Tetrahydrofuran (THF) was distilled in an argon atmosphere from sodium benzophenone ketyl before use. Acetonitrile (ACN) was distilled under an argon atmosphere from calcium hydride before use. Anhydrous methanol was purchased from Taylor Scientific (St. Louis, MO) and was used without further purification. Chromatography separations were performed in air using SorbTech silica (60 Å, 63–200 μm). Thin-layer chromatography was performed using Merck precoated glass plates (Silica 60 F254) using a palladium stain solution for spot developing. Physical measurements The 1H, 11B, 11B{1H}, and 13C{1H} NMR spectra were obtained using a Bruker Avance-300, DRX 500 and Avance-500 NMR spectrometers; 2D spectra were recorded on a Bruker Avance500 NMR spectrometer. Boron NMR spectra were referenced to 15% BF3·Et2O in CDCl3 taken as 0 ppm. 1H and 13C{1H} NMR spectra were referenced to the residual peak of the nondeuterated solvent. Chemical shifts were reported in ppm, and the coupling constants were reported in Hz. Mass spectra were obtained on an ABI QSTAR and Mariner Biospectrometry Workstation by PerSeptive Biosystems. Melting points were measured in sealed capillaries with an SRS OptiMelt apparatus. IR spectra were recorded on a Nicolet Nexus 470 FT-IR spectrometer.
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Synthesis of closo-biscarboranes 1,2-Bis(1′,2′-dicarba-closo-dodecaboran-8′-yl)ethyne (3). In a glove-box, 8-iodo-1,2-dicarba-closo-dodecaborane (1, 0.500 g, 1.85 mmol), bis(trimethylstannyl)acetylene (2, 0.325 g, 0.925 mmol), Pd(PPh3)4 (0.107 g, 0.0925 mmol), anhydrous LiCl (0.235 g, 5.55 mmol), and a few crystals of BHT were combined in a flame-dried reaction tube and sealed with a septum. Anhydrous ACN (5 mL) was added, and the reaction mixture was stirred at 80 °C for 48 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 10 mL of silica and treated by column chromatography (40 mL of silica) using EtOAc gradient (0 → 50%) in hexane. After evaporation and drying, compound 3 was isolated as a beige solid (0.134 g, 47%). NMR δH (500 MHz; CDCl3) 3.47 (4 H, br s, Ccb– H), 2.9–1.5 (18 H, m, B–H); δB (160 MHz; CDCl3; BF3·Et2O) −1.4 (2 B, d, JB–H 151), −8.1 (1 B, s, B–C), −10.2 (1 B, d, JB–H 152), −13.2 (2 B, d, JB–H 160), −14.5 (3 B, d, JB–H 163), −17.6 (1 B, br d, JB–H 159); δC (125 MHz; CDCl3) 52.7 (Ccb–H); m/z (–APCI) 311.3741 [M + H]− (calcd for C6H22B20 310.3726); mp 338–340 °C; νmax/cm−1 (nujol) 3072, 3062 (CH), 2648, 2635, 2604, 2570 (BH). 1,4-Bis(1′,2′-dicarba-closo-dodecaboran-8′-yl)benzene (6). In a glove-box, activated Zn powder (0.453 g, 6.93 mmol), anhydrous ZnBr2 (52.0 mg, 0.231 mmol), and anhydrous CoCl2 (30.0 mg, 0.231 mmol) were combined in a flame-dried onenecked round-bottom flask equipped with a stirring bar and a septum, and anhydrous ACN (2 mL) was added. A drop of TFA was added to the suspension followed by the addition of 4-bromotoluene (42.0 μL, 0.345 mmol), and the reaction mixture was stirred at RT for 30 min. A solution of 1,4-dibromobenzene (4, 0.546 g, 2.31 mmol) in ACN (4 mL) was added dropwise, and the resulting reaction suspension was stirred at RT for 3 h. The solution of 5 was added to a suspension of 1 (1.00 g, 3.70 mmol) and Pd(PPh3)4 (0.427 g, 0.370 mmol) in ACN (5 mL), and the resulting reaction mixture was stirred at 80 °C for 48 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 20 mL of silica and treated by column chromatography (80 mL of silica) using EtOAc gradient (0 → 50%) in hexane. After evaporation and drying, compound 6 was isolated as a white solid (0.154 g, 23%). NMR δH (500 MHz; acetone-d6) 7.39 (4 H, s, C6H4), 4.59 (4 H, br s, Ccb– H), 2.9–1.6 (18 H, m, B–H); δB (160 MHz; acetone-d6; BF3·Et2O) 2.5 (1 B, s, B–C), −1.5 (2 B, d, JB–H 146), −9.4 (1 B, d, JB–H 149), −12.4 (2 B, d, JB–H 125), −13.1 (3 B, d, JB–H 208), −16.7 (1 B, br d, JB–H 175); δC (125 MHz; acetone-d6) 133.1 (C6H4), 55.8 (Ccb–H); m/z (–APCI) 363.4056 [M + H]− (calcd for C10H26B20 362.4041); mp >350 °C (decomp); νmax/cm−1 (nujol) 2515br (BH). 1,4-Bis((1′,2′-dicarba-closo-dodecaboran-8′-yl)ethynyl)benzene (9). A 2.5 M solution of BuLi in hexane (450 μL, 1.12 mmol) was added dropwise to a solution of 1,4-diethynylbenzene (7, 74.0 mg, 0.586 mmol) in THF (3 mL) at 0 °C over a period of 5 min. The reaction mixture was allowed to warm to RT, was stirred for 1 h, and was subsequently cooled to −70 °C; a solution of anhydrous ZnBr2 (0.291 g, 1.29 mmol) in THF (4 mL)
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was then added over a period of 5 min. The reaction mixture was allowed to warm to RT and was then mixed with a solution of 1 (0.316 g, 1.11 mmol) and Pd(PPh3)4 (65.0 mg, 0.056 mmol) in THF (3 mL). The reaction mixture was stirred at reflux for 24 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 5 mL of silica and treated by column chromatography (20 mL of silica) using EtOAc gradient (0 → 50%) in hexane. After evaporation and drying, compound 9 was isolated as a beige solid (0.173 g, 75%). NMR δH (500 MHz; CDCl3) 7.37 (4 H, s, C6H4), 3.56 (4 H, br s, Ccb–H), 3.3–1.6 (18 H, m, B–H); δB (160 MHz; CDCl3; BF3·Et2O) −1.5 (2 B, d, JB–H 151), −8.0 (1 B, s, B–C), −10.1 (1 B, d, JB–H 152), −13.3 (2 B, d, JB–H 160), −14.5 (3 B, d, JB–H 151), −17.0 (br d, 1B); δC (125 MHz; CDCl3) 131.7, 123.5 (C6H4), 52.9 (Ccb–H); m/z (–APCI) 410.4075 [M]− (calcd for C14H26B20 410.4044); mp 222–224 °C (decomp); νmax/cm−1 (nujol) 3069 (CH), 2598, 2571 (BH), 2191 (CuC). 1,4-Bis(1′,2′-dicarba-closo-dodecaboran-8′-yl)butadiyne (14). (a) A 2.5 M solution of BuLi in hexane (1.56 mL, 3.89 mmol) was added dropwise to a solution of trimethylsilylacetylene (10, 549 μL, 3.89 mmol) in THF (10 mL) at 0 °C over a period of 5 min. The reaction mixture was allowed to warm to RT, was stirred for 1 h, and was cooled to −70 °C. A solution of anhydrous ZnBr2 (1.01 g, 4.47 mmol) in THF (10 mL) was subsequently added over a period of 5 min. After warming to RT, the prepared solution of trimethylsilylethynylzinc bromide 11 was mixed with a solution of 1 (0.700 g, 2.59 mmol) and Pd(PPh3)4 (0.150 g, 0.129 mmol) in THF (10 mL), and the reaction mixture was stirred at reflux for 24 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 10 mL of silica and treated by column chromatography (40 mL of silica) using DCM gradient (0 → 30%) in hexane. After evaporation and drying, TMS-protected compound 12 was isolated as a white solid (0.580 g, 93%). NMR δH (500 MHz; CDCl3) 3.51 (2 H, br s, Ccb–H), 3.3–1.5 (9 H, m, B–H), 0.15 (9 H, s, TMS); δB (160 MHz; CDCl3; BF3·Et2O): δ −1.5 (2 B, d, JB–H 151), −8.7 (1 B, s, B–C), −10.1 (1 B, d, JB–H 175), −13.3 (2 B, d, JB–H 154), −14.4 (3 B, d, JB–H 148), −17.3 (1 B, br d, JB–H 165); δC (125 MHz; CDCl3) 104.8 (TMC–Cu), 52.9 (Ccb–H), 0.24 (TMS); m/z (–TIS) 240.2409 [M − H]− (calcd for C7H20B10Si 240.2337); mp 110–112 °C; νmax/cm−1 (nujol) 3070, 3065 (νC–H), 2663, 2652, 2638, 2608, 2586, 2567 (BH), 2073 (CuC). (b) Anhydrous methanol (5 mL) was added to a solid mixture of 12 (0.200 g, 0.833 mmol) and anhydrous K2CO3 (0.173 g, 1.25 mmol), and the resulting suspension was stirred at RT for 2 h (TLC control). The reaction mixture was dissolved in water (10 mL) and extracted with ether (3 × 5 mL). The combined organic extracts were passed through a plug containing 0.5 mL of silica and 4 mL of anhydrous Na2SO4 and were evaporated to dryness to give 13 (0.134 g, 96%) as a white solid. NMR δH (500 MHz; CDCl3) 3.56 (2 H, br s, Ccb–H), 2.9–1.6 (9 H, m, B–H), 2.27 (1 H, s, uC–H); δB (160 MHz; CDCl3; BF3·Et2O) −1.4 (2 B, d, JB–H 151), −8.9 (1 B, s, B–C), −9.8 (1 B, d, JB–H 154), −13.1 (2 B, d, JB–H 163), −14.2 (2 B, d, JB–H 166), −14.7 (1 B, d), −17.0 (1 B, d, JB–H 180); δC (125 MHz; CDCl3) 85.6 (q*, 2 J 21, uC–H), 53.2 (Ccarb–H); m/z (–TIS) 168.2075 [M]− (calcd
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for C4H12B10 168.1939); mp 148–150 °C; νmax/cm−1 (nujol) 3064br (CH), 2609, 2582 (B–H), 2129 (CuC). (c) To a solution of 13 (0.200 g, 1.19 mmol) in 4 mL of DMSO were added 10% Pd/C (64.0 mg, 60.0 μmol) and CuI (12.0 mg, 60.0 μmol), and the resulting suspension was stirred in air for 24 h (TLC control). The reaction mixture was filtered, poured into 20 mL of water, and extracted with ethyl acetate (3 × 10 mL). The combined extracts were washed with water (2 × 20 mL) and brine (5 mL), dried over Na2SO4, and co-evaporated with 4 mL of silica. The compound was isolated by column chromatography (16 mL of silica) using EtOAc (0 → 50%) in hexane to give, after evaporation and drying, 14 (0.180 g, 91%) as a beige solid. NMR δH (500 MHz; acetone-d6) 4.59 (4 H, br s, Ccb–H), 3.1–1.5 (18 H, m, B–H), 2.27 (1 H, s, uC–H); δB (160 MHz; CDCl3, BF3·Et2O) −1.0 (2 B, d, JB–H 148), −8.4 (1 B, s, B–C), −9.5 (1 B, d, JB–H 159), −13.1 (2 B, d), −13.9 (3 B, d, JB–H 158), −17.0 (1 B, d); δC (125 MHz; CDCl3) 82.4 (br, uC–Cu), 55.9 (Ccb–H); m/z (–TIS) 334.3712 [M]− (calcd for C8H22B20 334.3727); mp >350 °C (decomp); νmax/cm−1 (nujol) 3064 (CH), 2641, 2628, 2610, 2581 (BH), 2108 (CuC). Synthesis of nido-biscarboranes General procedure for the synthesis of TBA salts of nido-biscarboranes 15–18. To a solution of the corresponding closocarborane (1 mmol) in 3 mL of THF was added 10 mL of 1 M TBAF in THF (10 mmol). The reaction mixture was heated to reflux until the TLC analysis showed the absence of the starting material (2–5 h). The reaction mixture was evaporated to dryness, and 5 mL of water was added to the residue in one portion. The formed precipitate was filtered on a glass frit, washed with water (3 × 3 mL) and ether (5 mL), and dried under vacuum. Tetrabutylammonium 1,2-bis(7′,8′-dicarba-nido-undecaborate1′-yl)ethyne (15). White solid (94%). NMR δH (500 MHz; acetone-d6) 3.44 (16 H, m, TBA–CH2), 2.34–(−0.21) (16 H, br m, B–H), 1.82 (16 H, m, TBA–CH2), 1.73 (4 H, br s, Ccarb–H), 1.43 (16 H, m, TBA–CH2), 0.98 (24 H, m, TBA–CH3), −2.40 (2 H, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O): −12.8 [2 B, d, JB–H 132, B(9,11)], −16.5 [2 B, d, JB–H 137, B(5,6)], −18.2 (1 B, d, JB–H 159, B(3)], −22.6 (2 B, d, JB–H 147, B(2,4)], −34.9 [1 B, dd, JB–H term 123, JB–H bridge 38, B(10)], −35.5 [1 B, s, B(1)]; δC (125 MHz; acetone-d6) 59.4 (TBA), 43.1 (br, Ccarb–H), 24.4, 20.4, 13.8 (TBA); m/z (–APCI) 144.7116 [M]2−, 531.7629 [M + TBA]− (calcd for C14H26B18 289.3504, M2−144.6749, M + TBA 531.6363); νmax/cm−1 (nujol): 2534, 2507 (BH). Tetrabutylammonium 1,4-bis(7′,8′-dicarba-nido-undecaborate-1′-yl)benzene (16). White solid (92%). NMR δH (500 MHz; acetone-d6) 7.53 (4 H, s, C6H4), 3.30 (16 H, m, TBA–CH2), 2.46– (−0.08) (16 H, m, B–H), 1.88 (Ccarb–H), 1.74 (16 H, m, TBA– CH2), 1.40 (16 H, m, TBA–CH2), 0.96 (24 H, m, TBA–CH3), −2.23 (2 H, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O) −11.5 (2 B, d, JB–H 134), −15.6 (2 B, d, JB–H 134), −17.5 (1 B, d, JB–H 154), −21.8 (2 B, d, JB–H 145), −25.9 [1 B, s, B(1)], −33.6 (1 B, dd, JB–H term 127, JB–H bridge 43); δC (125 MHz; acetone-d6) 134.7 (C6H4), 59.2 (TBA), 43.9 (br, Ccarb–H), 24.4, 20.3, 13.8 (TBA); m/z (–TIS) 170.7397 [M]2−, 583.8452 [M + TBA]− (calcd
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for C8H22B18 341.3820, M2− 170.6907, M + TBA 583.6679); νmax/cm−1 (thin film) 2962, 2934, 2875 (CH), 2507br (BH). Tetrabutylammonium 1,4-bis((7′,8′-dicarba-nido-undecaborate-1′-yl)ethynyl)benzene (17). Beige solid (96%). NMR δH (500 MHz; acetone-d6) 7.24 (4 H, s, C6H4), 3.41 (16 H, m, TBA– CH2), 2.43–(−0.15) (16 H, br m, B–H), 1.80 (20 H, m, Ccarb–H, TBA–CH2), 1.42 (16 H, m, TBA–CH2), 0.93 (24 H, m, TBA–CH3), −2.34 (2 H, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O) −10.3 (2 B, d, JB–H 135), −14.7 (2 B, d, JB–H 136), −16.4 (1 B, d, JB–H 163), −20.7 (2 B, d, JB–H 145), −32.9 (1 B, dd, JB–H term 127, JB–H bridge 42), −34.5 [1 B, s, B(1)]; δC (125 MHz; acetone-d6) 131.9, 125.7 (C6H4), 59.4 (TBA), 43.4 (br, Ccarb–H), 24.4, 20.4, 13.8 (TBA); m/z (–APCI) 194.7012 [M]2−, 631.6817 [M + TBA]− (calcd for C14H26B18 389.3818, M2− 194.6909, M + TBA 631.6666); νmax/cm−1 (nujol) 2576, 2552, 2515 (BH), 2183 (CuC). Tetrabutylammonium 1,4-bis(7′,8′-dicarba-nido-undecaborate1′-yl)butadiyne (18). Beige solid (95%). NMR δH (500 MHz; acetone-d6) 3.44 (16 H, m, TBA–CH2), 2.39–(−0.19) (16 H, br m, B–H), 1.82 (20 H, m, Ccarb–H, TBA–CH2), 1.44 (16 H, m, TBA– CH2), 0.98 (24 H, m, TBA–CH3), −2.39 (2B, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O) −11.4 (2 B, d, JB–H 134), −15.8 (2 B, d, JB–H 135), −17.5 (1 B, d, JB–H 165), −21.7 (2 B, d, JB–H 146), −33.9 (1 B, dd, JB–H term 127, JB–H bridge 43), −35.8 [1 B, s, B(1)]; δC (125 MHz; acetone-d6) 59.4 (TBA), 43.4 (br, Ccarb–H), 24.5, 20.4, 13.9 (TBA); m/z (–TIS) 156.7275 [M]2−, 555.8129 [M + TBA]− (calcd for C8H22B18 313.3506, M2− 156.6750, M + TBA 555.6364); νmax/cm−1(thin film) 2962, 2934 (CH), 2518br (BH). X-ray diffraction studies The X-ray quality crystals of 3, 6, 9, 14, 15, and 17 were obtained by slow evaporation of chloroform (3, 9, 14) or acetone (6, 15, 17) solutions. Data collection for the crystals was performed at −100 °C on a Bruker SMART 1000 CCD area detector system using the ω scan technique with Mo Kα radiation (λ = 0.71073 Å) from a graphite monochromator. Data reduction and integration were performed with the software package SAINT.23 Data were corrected for absorption using SADABS.24 The crystal structures were solved with the direct methods program SHELXS-97 and were refined by full matrix least squares techniques with the SHELXTL25 suite of programs. All non-hydrogen atoms were refined with anisotropic thermal parameters, except for the disordered chlorine atoms in the crystal structure of 9. The hydrogen atoms in 3, 6, and 14 were located in the difference Fourier maps and were refined individually. In 9, all hydrogen atoms, except those of the disordered chloroform, were located in the difference Fourier maps and were refined individually, whereas the chloroform hydrogens were included at geometrically idealized positions; therefore, the refinement of H-atoms was mixed. In 15, only the hydrogens of the open pentagonal C2B3 faces of the nido-carboranes were located in the difference Fourier maps and were refined individually, whereas the remaining BH hydrogens, as well as those of the tetrabutylammonium (TBA) cations, were included at geometrically idealized
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positions. Therefore, the refinement of the H-atoms was mixed. In 17, only the hydrogens of the nido-carborane groups were located in the difference Fourier maps and refined individually, whereas the hydrogens of the aromatic group and the TBA cations were included at geometrically idealized positions; therefore, the refinement of the H-atoms was mixed. The crystal structure of 15 revealed two types of disorder. One type of disorder was caused by the three fluxional arms of the TBA cation. This disorder was modeled to provide the 0.6 : 0.4 ratio of the superimposed forms. In the second type of disorder, one of the nido-carborane cages exhibited rotational disorder, which made distinguishing between the boron and carbon atoms at two atom sites difficult. This disorder was modeled using partial occupancies for the boron and carbon atoms, which provided a 0.5 : 0.5 ratio for the superimposed rotamers. The TBA cation in the crystal structure of 17 exhibited disorder of its the two fluxional arms. This disorder was modeled to provide 0.65 : 0.35 and 0.52 : 0.48 ratios for the two superimposed forms of each arm. The crystal structure of 9 contained two disordered molecules of chloroform in an asymmetric unit. The disorders were modeled in the form of four rotamers in a 0.60 : 0.14 : 0.11 : 0.15 ratio for one of the chloroform molecules and in the form of two rotamers in a 0.5 : 0.5 ratio for the other molecule. The crystal structure of 3 was solved in space group Pnn2; however, because the compound is a weak anomalous scatterer (i.e., in contains only light atoms B, C, and H) and because data were collected using Mo Kα radiation, its absolute configuration could not be reliably determined. Therefore, Friedel pairs were merged, and any references to the Flack parameter were removed. The crystallographic data and the details of the data collection and structure refinements of 3, 6, 9, 14, 15, and 17 are provided in Table S1.† CCDC 961327 (3), CCDC 961328 (6), CCDC 961329 (9), CCDC 961330 (14), CCDC 961331 (15) and CCDC 961332 (17) contain the supplementary crystallographic data for this paper.
Acknowledgements The authors gratefully acknowledge the National Science Foundation (CHE-0702774) for financial support of this work, Dr Mark Lee and Mr Brett Meyers for measuring the mass spectra of the prepared compounds. The authors also thank Dr Ilia Guzei (UW-Madison) for the in-house crystallographic program “ModiCIFer”.
References 1 See, for example: (a) P. F. H. Schwab, M. D. Levin and J. Michl, Chem. Rev., 1999, 99, 1863; (b) P. F. H. Schwab, J. R. Smith and J. Michl, Chem. Rev., 2005, 105, 1197. 2 P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997, 30, 502. 3 J. Michl and T. F. Magnera, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4788.
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4 J.-Y. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S.-B. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450. 5 See, for example: (a) J.-R. Li, Y. Ma, M. C. McCarthy, J. Sculley, J. Yu, H.-K. Jeong, P. B. Balbuena and H.-C. Zhou, Coord. Chem. Rev., 2011, 255, 1791; (b) L. J. Murray, M. Dinca and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294; (c) X.-J. Wang, P.-Z. Li, Y. Chen, Q. Zhang, H. Zhang, X. X. Chan, R. Ganguly, Y. Li, J. Jiang and Y. Zhao, Sci. Rep., 2013, 3, 1149, DOI: 10.1038/srep01149. 6 A. Bétard and R. A. Fischer, Chem. Rev., 2012, 112, 1055. 7 See, for example: (a) M. Elbing, R. Ochs, M. Koentopp, M. Fischer, C. von Hänisch, F. Weigend, F. Evers, H. Weber and M. Mayor, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 8815; (b) M. J. Kumar, Recent Pat. Nanotechnol., 2007, 1, 51. 8 R. M. Metzger, Chem. Rev., 2003, 103, 3803. 9 R. L. Carroll and C. B. Gorman, Angew. Chem., Int. Ed., 2001, 41, 4378. 10 (a) D. K. James and J. M. Tour, Chem. Mater., 2004, 16, 4423; (b) B. A. Mantooth and P. S. Weiss, Proc. IEEE, 2003, 91, 1785. 11 (a) A. Herzog, S. S. Jalisatgi, C. B. Knobler, T. J. Wedge and M. F. Hawthorne, Chem.–Eur. J., 2005, 11, 7155; (b) W. Jiang, D. E. Harwell, M. D. Mortimer, C. B. Knobler and M. F. Hawthorne, Inorg. Chem., 1996, 35, 4355; (c) X. Yang, W. Jiang, C. B. Knobler and M. F. Hawthorne, J. Am. Chem. Soc., 1992, 114, 9719. 12 M. F. Hawthorne, Acc. Chem. Res., 1968, 1, 281. 13 A. V. Safronov, Y. S. Sevryugina, S. S. Jalisatgi, R. D. Kennedy, C. L. Barnes and M. F. Hawthorne, Inorg. Chem., 2012, 51, 2629. 14 See, for example: (a) L. I. Zakharkin, A. I. Kovredov, V. A. Ol’shevskaya and Zh. S. Shaugumbekova, Izv. Akad.
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17
18 19 20 21
22
23 24 25
Nauk. SSSR, Ser. Khim., 1980, 1691; (b) L. I. Zakharkin, A. I. Kovredov, V. A. Ol’shevskaya and Zh. S. Shaugumbekova, J. Organomet. Chem., 1982, 226, 217; (c) L. I. Zakharkin, A. I. Kovredov and V. A. Ol’shevskaya, Izv. Akad. Nauk. SSSR, Ser. Khim., 1985, 888. L. Eriksson, I. P. Beletskaya, V. I. Bregadze, I. B. Sivaev and S. Sjöberg, J. Organomet. Chem., 2002, 657, 267. (a) W. J. Marshall, R. J. Young and V. V. Grushin, Organometallics, 2001, 20, 523; (b) L. I. Zakharkin, E. V. Balagurova and V. N. Lebedev, Russ. J. Gen. Chem., 1998, 68, 972. (a) H. Fillon, C. Gosmini and J. Périchon, J. Am. Chem. Soc., 2003, 125, 3867; (b) I. Kazmierski, C. Gosmini, J.-M. Paris and J. Périchon, Tetrahedron Lett., 2003, 44, 6417. H. Himmelspach and M. Finze, Eur. J. Inorg. Chem., 2010, 2012 and references therein. L. I. Zakharkin and V. N. Kalinin, Tetrahedron Lett., 1965, 407. T. Kurita, M. Abe, T. Maegawa, Y. Monguchi and H. Sajiki, Synlett, 2007, 2521. (a) M. F. Hawthorne, D. C. Young, P. M. Garrett, D. A. Owen, S. G. Schwerin, F. N. Tebbe and P. A. Wegner, J. Am. Chem. Soc., 1968, 90, 862; (b) M. A. Fox, W. R. Gill, P. L. Herbertson, J. A. H. MacBride, K. Wade and H. M. Colquhoun, Polyhedron, 1996, 15, 565; (c) J. Yoo, J.-W. Hwang and Y. Do, Inorg. Chem., 2001, 40, 568. See, for example, ESI† to A. M. Spokoyny, T. C. Li, O. K. Farha, C. W. Machan, C. She, C. L. Stern, T. J. Marks, J. T. Hupp and C. A. Mirkin, Angew. Chem., Int. Ed., 2010, 49, 5339. Bruker, SAINT v 7.68A, Bruker AXS Inc., Madison, WI, 2009. G. M. Sheldrick, SADABS 2008/1, Gottingen, 2008. G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112.
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Synthesis of closo- and nido-biscarboranes with rigid unsaturated linkers as precursors to linear metallacarborane-based molecular rods† Alexander V. Safronov, Yulia V. Sevryugina, Kothanda Rama Pichaandi,‡ Satish S. Jalisatgi and M. Frederick Hawthorne* Several biscarborane-type derivatives of 8-iodo-1,2-dicarba-closo-dodecaborane (1), suitable as the precursors of linear metallacarborane-based molecular rods, were prepared. The synthesized closocompounds contained two carborane moieties connected through a rigid linear unsaturated linker. The linkers were based on ethynylene and para-phenylene fragments and their combinations. The deborona-
Received 19th September 2013, Accepted 16th October 2013 DOI: 10.1039/c3dt52596a www.rsc.org/dalton
tion of all reported closo-compounds was selective and afforded nido-products with open pentagonal faces on opposite sides of the molecule, parallel to each other and perpendicular to the main molecular axis. All of the closo and nido products were characterized by a variety of physical methods (NMR, HRMS, IR). The structures of closo-carboranes 3, 6, 9, and 14 and nido-carboranes 15 and 17 were established by X-ray diffraction.
Introduction Rigid-rod molecules—organic, organometallic, or inorganic— have received attention during the past two decades. Numerous reviews and articles are dedicated to the design, synthesis, and application of these molecules, which can be discreet or oligomeric in nature.1 The rigid molecules of fixed length can be used as building blocks for supramolecular assemblies;2 i.e., as “molecular tinkertoys”.3 The discovery of porous coordination polymers, also known as metal–organic frameworks (MOFs), was the direct result of the development of molecularrod chemistry. Transition-metal-containing MOFs show catalytic activity in a variety of organic reactions.4 Although they are best known for their application as gas storage and separation materials,5 MOFs also form thin films that can be used as sensors.6 The application of molecular rods in molecular electronics is even more promising. Such molecules can potentially be used as unimolecular diodes,7 rectifiers,8 molecular wires that provide electron transfer through a molecule, components of computer memory devices, and in several additional areas.9
International Institute of Nano and Molecular Medicine (I2NM2), University of Missouri, Columbia, MO 65211, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available. CCDC 961327–961332. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52596a ‡ Present address: Department of Chemistry and Energy Center, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA.
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Progress in molecular electronic applications of molecular rods has occurred parallel to advances in physics, where theoretical and practical approaches to single-molecule conductivity measurements and other types of characterization have been developed.10 Several reports have been previously published on the design and synthesis of rigid-rod molecules based on carborane fragments called “carbo(ra)rods”.11 Dimers and tetramers of para-carborane containing direct Ccarborane–Ccarborane bonds were prepared and characterized,11c as well as several derivatives of para- and meta-carboranes in which the polyhedral moieties were connected through ethynylene and butadiyne1,4-diyl linkers attached both to cage carbon and to boron atoms.11a,b This line of investigation was continued in the current study with the molecular rods based on metallacarborane fragments. The synthesis of such molecules seemed attractive due to the numerous known bis(dicarbollide)metal complexes, many of which exist in more than one oxidation state.12 Rigid-rod molecules with metallacarboranes connected through rigid unsaturated linkers might be conductive and possibly serve as molecular wires. There are two possible ways to prepare these compounds: (1) Pd-catalyzed cross-coupling copolymerization of diiodosubstituted metallacarboranes with metal derivatives of the desired linker molecules, and (2) connection of nido-biscarborane compounds containing the desired linkers via π-coordination with metals (Fig. 1). To maintain structural rigidity, the metallacarborane units in rod molecules must be substituted at the apical positions,
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Fig. 1
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Two possible routes to metallacarborane-based molecular rods.
Fig. 2 Selective deboronation of 1 and the formation of metallacarboranes with apical functionalization.
B(10) and B(10′), as shown in Fig. 1. In this case, conformational interconversions of metallacarborane units have no effect on the rod geometry. However, until recently, no reliable procedure has existed for the preparation of a carborane precursor that would allow for synthesis of axially functionalized metallacarborane complexes. In 2012 publication,13 a convenient synthetic route to a less-common iodinated ortho-carborane molecule, 8-iodo-1,2dicarba-closo-dodecaborane, was described (1, Fig. 2). The compound revealed an unusual and selective deboronation pattern: in the resulting 1-iodo-7,8-dicarba-nido-undecaborate anion, the B–I bond was perpendicular to the plane of the open pentagonal C2B3 face. The compound formed the corresponding bis(dicarbollide)cobalt and nickel complexes in high yields. In these complexes, both the B–I bonds and the metal atom were collinear, as required for the synthesis of molecular rods. Cross-coupling polymerization of these complexes is currently under investigation in our laboratory. Reported here is the synthesis and characterization of closo and nido precursors to metallacarborane-based molecular rods via a single-step cross-coupling of 8-iodo-1,2-dicarbacloso-dodecaborane with ethynylene and phenylene-based nucleophiles.
Since then, several important discoveries have been made concerning the differences in reactivity of arylhalides and halocarboranes. First, in contrast to organic chemistry, only compounds with B–I bonds participate in cross-coupling reactions. Other derivatives (B–Cl, B–Br) appeared to be completely nonreactive towards the oxidative addition to palladium. Second, unlike organic cross-couplings, which can be catalyzed by complexes of different transition metals, in borane and carborane chemistry, these reactions are exclusively palladiumcatalyzed. Third, in addition to the nature of the nucleophile, the nature of the additive plays an important role the yield of the product. For example, cross-coupling according to the Suzuki protocol15 in the presence of CsF (a transmetallationassisting agent) proceeds with high yields only in the case of para-carborane because meta- and ortho-carboranes react faster with fluoride anion to form of nido-carboranes. Fourth, in some cases, cross-coupling reactions are accompanied by undesirable side reactions, most often by dehalogenation of the starting material.16 In the current study, four compounds of interest were modeled; all of them contained two 1,2-dicarba-closo-dodecaboran-8-yl moieties connected through an unsaturated linker: ethynylene, 1,4-phenylene, phenylene-1,4-diethynyl, or butadiyn-1,4-diyl. For simplicity, the synthesis of most of the target molecules was performed via a one-step cross-coupling reaction, i.e. through the coupling of two iodocarborane molecules with one linker molecule containing two nucleophilic centers. This approach has never been used in carborane cross-coupling chemistry where reactions of a single carborane molecule with one or several nucleophiles are the most common case. This study began with the synthesis of a bis(carboranyl)acetylene compound. The commercially available bis(trimethylstannyl)acetylene (2, Scheme 1) was chosen as an acetylene-containing nucleophile. The reaction of 8-iodo-1,2dicarba-closo-dodecaborane (1) with 2 in dioxane in the presence of Pd(PPh3)4 gave target compound 3 with 47% isolated yield. The use of less-volatile bis(tributylstannyl)acetylene in this reaction did not increase the reaction yield. In addition to the coupling product 3, 20–30% of deiodinated product (orthocarborane) was regularly observed in this reaction. Analysis of the literature data showed that this reaction represents the first example of Stille cross-coupling in the carborane series. Synthesis of the next target compound was achieved through the generation of an organozinc derivative from 1,4-dibromobenzene (4, Scheme 2). The recently discovered cobalt-assisted synthesis of organozincs17 appeared to be the most convenient way to produce labile compound 5. The reaction of 5 with 1 in acetonitrile in the presence of a Pd(0) catalyst gave 6 in moderate yield. Most likely, side reactions of 5,
Results and discussion Synthesis of closo-biscarboranes Palladium-catalyzed cross-coupling reactions using carborane and borane substrates were discovered in the early 1980s.14
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Scheme 1
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Scheme 2
of acetylenes—is usually conducted in the presence of copper compounds in air in an amine solution at elevated temperatures. In the case of ortho-carboranes, these reaction conditions are not favorable because amines cause cage deboronation at elevated temperatures.19 Instead, oxidative coupling of 13 was performed in DMSO in air in the presence of Pd/C and CuI catalysts20 to produce the target compound 14 in 91% isolated yield. The overall yield of compound 14 was 84% based on the initial amount of the starting carborane 1. 2. Synthesis of nido-biscarboranes
Scheme 3
Scheme 4
Several reagents can be used for deboronation of closo-carboranes. Alkali-metal alkoxides and cesium and tetrabutylammonium (TBAF) fluorides are most commonly used because they are inexpensive and produce target nido-carboranes in very high yields.21 In the current study, TBAF was used for two reasons. First, the products after reaction with TBAF are easy to isolate, and product loss during the isolation process is minimal. Second, TBA salts of nido-carboranes are soluble in most organic solvents. In this study, solubility was important for proper characterization of the target compounds bearing double negative charge. With an excess of 1 M TBAF in THF at reflux, the reactions of compounds 3, 6, 9, and 14 were complete within 5 h and produced single products according to NMR analyses (see below). The corresponding nido-biscarboranes 15–18 (Scheme 5) were isolated with excellent yields after water treatment of the evaporated reaction mixtures (see Experimental section). 3. Characterization of closo- and nido-biscarboranes
such as polymerization and decomposition, can explain the reduced yield of 6. Compared with the syntheses of 3 and 6, the synthesis of compound 9 was relatively simple because the acetylenes in the linker were separated by a 1,4-phenylene fragment. The organozinc compound 8 was produced from the commercially available 1,4-diethynylbenzene (7, Scheme 3) by an exchange reaction of the initially generated bis(ethynyllithium) with anhydrous zinc bromide. The subsequent cross-coupling reaction of 8 with closo-carborane 1 in the presence of Pd(0) gave product 9 with a 75% isolated yield. Initially, synthesis the last target compound (14) containing the butadiyn-1,4-diyl linker was planned through the double cross-coupling of 1,4-trimethylstannylbutadiyne with compound 1. However, a more efficient step-by-step synthetic route was chosen because of the cost of the corresponding organotin derivative and the possible low yield of the product. TMS-protected acetylenes18 can be coupled with iodocarboranes in a variety of ways with excellent yields. The reaction of compound 1 with the organozinc derivative of ethynyltrimethylsilane (11, Scheme 4) in the presence of Pd(PPh3)4 afforded compound 12 in 93% isolated yield. Deprotection of 12 was achieved quantitatively by a standard procedure using K2CO3 in methanol. The next step—oxidative coupling
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All of the new closo- and nido-biscarboranes were characterized by multinuclear NMR. Because of the symmetry of the compounds and the unsaturated nature of the linkers, both 1H and 13 C{1H} spectra provided limited structural information. For example, in the 1H NMR spectra of compounds 3 and 14, which contain ethynylene and butadiyne linkers, respectively, only one sharp signal corresponding to the carborane C–H and a broad signal corresponding to the B–H bonds were observed. In the 1H NMR of compounds 6 and 9, additional signals were observed from the phenyl rings of the linkers, which appeared as singlets at δ 7.39 and 7.34 ppm, respectively. The same trend was observed in the 1H NMR spectra of nido-biscarboranes 15–18 (see Experimental section).
Scheme 5
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In the 13C{1H} NMR spectra of all the compounds, signals from the cage carbon atoms were observed in the normal range for closo-compounds (see Experimental section). However, spectra of both the closo and nido compounds acquired under standard conditions did not contain signals corresponding to the carbon atoms connected directly to boron. This absence of signal could result from a number of causes. First, all of the carbons connected to carborane cages were quaternary, and the resonances of such atoms are sometimes difficult to acquire, even in relatively simple organic systems. Second, quaternary carbons were connected to boron atoms that contain two quadrupolar isotopes, 10B and 11B. The splitting on these nuclei significantly decreases the intensity of the carbon signal, which makes it difficult to distinguish them from the background. Signals from the rest of the linkers’ carbon atoms were acquired without any difficulties and corresponded to the expected structures. In the case of the intermediate compound 13, a 2J coupling of the terminal carbon atom with the 11B isotope of the B(8) atom of the carborane cage was observed. The corresponding signal appeared as a quartet with a 2J value of 21 Hz. Such coupling had been observed previously for carboranes with ethynyl substituents in different positions on the cage.18 The 1H and 13C{1H} NMR spectra of nido-biscarboranes 15–18 also provided limited structural information. In the 1H NMR spectra of all of the compounds, signals of TBA counter ions partially overlapped with resonances of carborane C–H and B–H vertices. In the case of compounds 6 and 9, signals from the phenyl rings of the linkers were also observed. All spectra contained a broad characteristic peak in the range of δ −2–(−3) ppm, corresponding to the bridging or “extra” hydrogen. Similarly, in the 13C{1H} NMR spectra of 15–18, signals from all of the carbon atoms except for the ones directly connected to the boron atoms were observed. The majority of the structural information for the prepared compounds was provided by their 11B, 11B{1H}, and [11B–11B] COSY NMR spectra. The spectra of closo compounds 3, 9, and 14 were very similar and had the expected signal pattern of 2 : 1 : 1 : 2 : 3 : 1 in the typical range for closo-carboranes between δ 0 and −20 ppm. A singlet corresponding to the B–C bond appeared in the 11B NMR spectra of 3, 9, and 14 at δ −8.1, −8.0, and −9.2 ppm, respectively. The assignment of the signals was based on the [11B–11B] COSY NMR spectrum of 3. This assignment confirmed that all of the reported compounds contained a closo-carborane fragment substituted at the B(8) position of the cage. Because of the nature of the linker in 6, its 11B spectra were slightly different. Similar to the previously reported 11B NMR of 9-aryl-substituted carboranes,22 the signal corresponding to the B–C bond was observed in the positive part of the spectrum at δ +2.56 ppm. The symmetry of resonances corresponding to the remainder of the molecule stayed unchanged. Similar to the spectra of closo-biscarboranes, the 11B NMR of the nido products provided more structural information. The spectra of all of the nido-biscarboranes contained the same 2 : 2 : 1 : 2 : 1 : 1 pattern in the range of 0 to −40 ppm,
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which is characteristic of nido-carboranes. Most importantly, the 11B NMR spectra of all compounds contained just one set of signals which implied formation of a single nido-isomer for both carborane cages. Assignment of the signals via the [11B–11B] COSY NMR spectrum of 3 confirmed that the resulting compounds contain nido-carborane fragments substituted at position B(1) and that the regioselectivity characteristic of deboronation of the parent 8-iodo-1,2-dicarba-closo-dodecaborane persisted in its coupling products. High-resolution mass spectral data provided additional support to the NMR characterization of the reported closo and nido compounds. For all of the closo-biscarboranes, molecular ion peak envelopes with the expected isotope distribution patterns were observed. The spectra of the nido-biscarboranes contained two peak envelopes corresponding to the molecular ion bearing a double negative charge (Δm/z = 0.5) and to its agglomerate with the TBA cation bearing an overall single negative charge (Δm/z = 1) (see Experimental section). X-ray quality crystals of the closo-biscarborane compounds, 3, 6, 9, and 14 and the two nido-biscarborane compounds, 15 and 17 were obtained. A graphic representation of the refined structures is shown in Fig. 3–8. In all of the structures, two carborane units were connected to a linker fragment by a B–C bond. The boron cluster cages showed no distortion, with the B–B and B–C distances within the normal range. Similarly, the para-substituted phenyl rings in compounds 6, 9, and 17 were undistorted, displaying averaged C–C bond distances of 1.395, 1.389, and 1.380 Å, respectively. The B–C distances between the carborane cages and the linker carbon atoms differed slightly for all of the compounds (Table S2†). The shortest B–C distance was observed for closo-
Fig. 3 The ORTEP representation of compound 3, drawn at the 40% probability level.
Fig. 4 The ORTEP representation of compound 6, drawn at the 40% probability level.
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Fig. 5 The ORTEP representation of compound 9, drawn at the 40% probability level. Only one of the two molecules in the asymmetric unit is displayed. Chloroform molecules are omitted for clarity.
Fig. 6 The ORTEP representation of compound 14, drawn at the 40% probability level.
Fig. 7 The ORTEP representation of compound 15, drawn at the 40% probability level. The B(21)–B(31) nido-carborane cage exhibits a disorder due to averaging of two rotamers [in gauche- and trans-orientations with respect to the B(1)–B(11) cage], modeled with the 0.5 : 0.5 ratio. For clarity purposes, only trans-rotamer is displayed, and two NBu4+ cations are omitted.
compound 3 (1.527(5) Å) with an ethynylene linker, whereas the longest was observed for closo-compound 6 (1.584(3) Å) containing a para-phenylene linker. In closo-biscarborane compounds 9 and 14, the B–C distances were nearly identical: 1.544 and 1.545 Å, respectively. In the nido-biscarboranes 15 and 17, the B–C distances were 1.553 and 1.550 Å, respectively.
Fig. 8
In 3, 9, 14, 15, and 17, the CuC bond length decreased in the order 3 > 15 > 14 > 9 > 17. For an analogous carborane/ nido-carborane pair, such as 3/15 or 9/17, the CuC distance was slightly longer in the neutral closo-biscarborane compounds (3 and 9), and an insertion of the para-phenylene linker shortened the CuC distance by approximately 0.02 Å, such as in 9 vs. 3 and in 17 vs. 15. These effects can also be observed in the overall lengths of the prepared molecules. The distances, measured between the most remote B-atoms in 3, 6, and 9, or between the centroids of the open pentagonal C2B3 faces of nido-carboranes in 15 and 17, decreased in the order 9 (17.967 Å) > 17 (16.024 Å) > 14 (13.654 Å) > 6 (12.855 Å) > 3 (10.999 Å) > 15 (9.241 Å). The degree of linearity of the linkers connecting the carborane cages was also examined. The angles between the lines connecting substituted B-atoms and the centers of mass of the molecules were compared. In all compounds containing the benzene ring—6, 9, and 17—the linkers formed straight lines with almost perfect 180.0° angles between the substituted B-atoms and centroids of the para-phenylene ring. Similarly, the linker in compound 14 displayed an ideal 180.0° angle, whereas that in compound 15 demonstrated a slight bend (177.6°). The bend in the crystal structure of 3 containing the ethynylene linker was even more pronounced with the corresponding angle equal to 171.7°. In their solid states, the compounds also differed by the rotational conformations of their carborane cages. To describe the relative orientation of the carborane cage carbon atoms with respect to each other around the axis of the linker, the use of the well-established for alkane chemistry cis-, trans-, and gauche-nomenclature is convenient. The carborane carbons adopted the trans-conformation in 6, 9, 14, and 17 and the gauche-conformation in compound 3. In the crystal structure of 15, the trans- and gauche-conformers were averaged to
The ORTEP representation of compound 17, drawn at the 40% probability level. Two NBu4+ cations are omitted for clarity.
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cause a positional disorder, which was modeled in the form of two equally present rotamers.
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Conclusions Several closo-biscarboranes were prepared starting from 8-iodo1,2-dicarba-closo-dodecaborane. In all of the reported compounds both carborane cages were deboronated selectively in the para-positions with respect to the substituent. Structural analysis of the nido-compounds showed that the open pentagonal faces of the nido-cages were parallel to each other. This fact makes the synthesized compounds attractive starting materials for the preparation of linear molecular rods with metallacarborane fragments in the main chain.
Experimental section Materials Unless otherwise stated, all reactions were carried out under an argon atmosphere using standard Schlenk-line techniques. 8-Iodo-1,2-dicarba-closo-dodecaborane (1) was prepared according to the published procedure.13 Bis(trimethylstannyl)acetylene (2) was purchased from Acros Organics. 1,4-Dibromobenzene (4), 1,4-diethynylbenzene (7), and trimethylsilylacetylene (10) were purchased from Aldrich and were used without further purification. Tetrabutylammonium fluoride (TBAF) was purchased from TCI America as a 1 M solution in THF and was used as received. Tetrahydrofuran (THF) was distilled in an argon atmosphere from sodium benzophenone ketyl before use. Acetonitrile (ACN) was distilled under an argon atmosphere from calcium hydride before use. Anhydrous methanol was purchased from Taylor Scientific (St. Louis, MO) and was used without further purification. Chromatography separations were performed in air using SorbTech silica (60 Å, 63–200 μm). Thin-layer chromatography was performed using Merck precoated glass plates (Silica 60 F254) using a palladium stain solution for spot developing. Physical measurements The 1H, 11B, 11B{1H}, and 13C{1H} NMR spectra were obtained using a Bruker Avance-300, DRX 500 and Avance-500 NMR spectrometers; 2D spectra were recorded on a Bruker Avance500 NMR spectrometer. Boron NMR spectra were referenced to 15% BF3·Et2O in CDCl3 taken as 0 ppm. 1H and 13C{1H} NMR spectra were referenced to the residual peak of the nondeuterated solvent. Chemical shifts were reported in ppm, and the coupling constants were reported in Hz. Mass spectra were obtained on an ABI QSTAR and Mariner Biospectrometry Workstation by PerSeptive Biosystems. Melting points were measured in sealed capillaries with an SRS OptiMelt apparatus. IR spectra were recorded on a Nicolet Nexus 470 FT-IR spectrometer.
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Synthesis of closo-biscarboranes 1,2-Bis(1′,2′-dicarba-closo-dodecaboran-8′-yl)ethyne (3). In a glove-box, 8-iodo-1,2-dicarba-closo-dodecaborane (1, 0.500 g, 1.85 mmol), bis(trimethylstannyl)acetylene (2, 0.325 g, 0.925 mmol), Pd(PPh3)4 (0.107 g, 0.0925 mmol), anhydrous LiCl (0.235 g, 5.55 mmol), and a few crystals of BHT were combined in a flame-dried reaction tube and sealed with a septum. Anhydrous ACN (5 mL) was added, and the reaction mixture was stirred at 80 °C for 48 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 10 mL of silica and treated by column chromatography (40 mL of silica) using EtOAc gradient (0 → 50%) in hexane. After evaporation and drying, compound 3 was isolated as a beige solid (0.134 g, 47%). NMR δH (500 MHz; CDCl3) 3.47 (4 H, br s, Ccb– H), 2.9–1.5 (18 H, m, B–H); δB (160 MHz; CDCl3; BF3·Et2O) −1.4 (2 B, d, JB–H 151), −8.1 (1 B, s, B–C), −10.2 (1 B, d, JB–H 152), −13.2 (2 B, d, JB–H 160), −14.5 (3 B, d, JB–H 163), −17.6 (1 B, br d, JB–H 159); δC (125 MHz; CDCl3) 52.7 (Ccb–H); m/z (–APCI) 311.3741 [M + H]− (calcd for C6H22B20 310.3726); mp 338–340 °C; νmax/cm−1 (nujol) 3072, 3062 (CH), 2648, 2635, 2604, 2570 (BH). 1,4-Bis(1′,2′-dicarba-closo-dodecaboran-8′-yl)benzene (6). In a glove-box, activated Zn powder (0.453 g, 6.93 mmol), anhydrous ZnBr2 (52.0 mg, 0.231 mmol), and anhydrous CoCl2 (30.0 mg, 0.231 mmol) were combined in a flame-dried onenecked round-bottom flask equipped with a stirring bar and a septum, and anhydrous ACN (2 mL) was added. A drop of TFA was added to the suspension followed by the addition of 4-bromotoluene (42.0 μL, 0.345 mmol), and the reaction mixture was stirred at RT for 30 min. A solution of 1,4-dibromobenzene (4, 0.546 g, 2.31 mmol) in ACN (4 mL) was added dropwise, and the resulting reaction suspension was stirred at RT for 3 h. The solution of 5 was added to a suspension of 1 (1.00 g, 3.70 mmol) and Pd(PPh3)4 (0.427 g, 0.370 mmol) in ACN (5 mL), and the resulting reaction mixture was stirred at 80 °C for 48 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 20 mL of silica and treated by column chromatography (80 mL of silica) using EtOAc gradient (0 → 50%) in hexane. After evaporation and drying, compound 6 was isolated as a white solid (0.154 g, 23%). NMR δH (500 MHz; acetone-d6) 7.39 (4 H, s, C6H4), 4.59 (4 H, br s, Ccb– H), 2.9–1.6 (18 H, m, B–H); δB (160 MHz; acetone-d6; BF3·Et2O) 2.5 (1 B, s, B–C), −1.5 (2 B, d, JB–H 146), −9.4 (1 B, d, JB–H 149), −12.4 (2 B, d, JB–H 125), −13.1 (3 B, d, JB–H 208), −16.7 (1 B, br d, JB–H 175); δC (125 MHz; acetone-d6) 133.1 (C6H4), 55.8 (Ccb–H); m/z (–APCI) 363.4056 [M + H]− (calcd for C10H26B20 362.4041); mp >350 °C (decomp); νmax/cm−1 (nujol) 2515br (BH). 1,4-Bis((1′,2′-dicarba-closo-dodecaboran-8′-yl)ethynyl)benzene (9). A 2.5 M solution of BuLi in hexane (450 μL, 1.12 mmol) was added dropwise to a solution of 1,4-diethynylbenzene (7, 74.0 mg, 0.586 mmol) in THF (3 mL) at 0 °C over a period of 5 min. The reaction mixture was allowed to warm to RT, was stirred for 1 h, and was subsequently cooled to −70 °C; a solution of anhydrous ZnBr2 (0.291 g, 1.29 mmol) in THF (4 mL)
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was then added over a period of 5 min. The reaction mixture was allowed to warm to RT and was then mixed with a solution of 1 (0.316 g, 1.11 mmol) and Pd(PPh3)4 (65.0 mg, 0.056 mmol) in THF (3 mL). The reaction mixture was stirred at reflux for 24 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 5 mL of silica and treated by column chromatography (20 mL of silica) using EtOAc gradient (0 → 50%) in hexane. After evaporation and drying, compound 9 was isolated as a beige solid (0.173 g, 75%). NMR δH (500 MHz; CDCl3) 7.37 (4 H, s, C6H4), 3.56 (4 H, br s, Ccb–H), 3.3–1.6 (18 H, m, B–H); δB (160 MHz; CDCl3; BF3·Et2O) −1.5 (2 B, d, JB–H 151), −8.0 (1 B, s, B–C), −10.1 (1 B, d, JB–H 152), −13.3 (2 B, d, JB–H 160), −14.5 (3 B, d, JB–H 151), −17.0 (br d, 1B); δC (125 MHz; CDCl3) 131.7, 123.5 (C6H4), 52.9 (Ccb–H); m/z (–APCI) 410.4075 [M]− (calcd for C14H26B20 410.4044); mp 222–224 °C (decomp); νmax/cm−1 (nujol) 3069 (CH), 2598, 2571 (BH), 2191 (CuC). 1,4-Bis(1′,2′-dicarba-closo-dodecaboran-8′-yl)butadiyne (14). (a) A 2.5 M solution of BuLi in hexane (1.56 mL, 3.89 mmol) was added dropwise to a solution of trimethylsilylacetylene (10, 549 μL, 3.89 mmol) in THF (10 mL) at 0 °C over a period of 5 min. The reaction mixture was allowed to warm to RT, was stirred for 1 h, and was cooled to −70 °C. A solution of anhydrous ZnBr2 (1.01 g, 4.47 mmol) in THF (10 mL) was subsequently added over a period of 5 min. After warming to RT, the prepared solution of trimethylsilylethynylzinc bromide 11 was mixed with a solution of 1 (0.700 g, 2.59 mmol) and Pd(PPh3)4 (0.150 g, 0.129 mmol) in THF (10 mL), and the reaction mixture was stirred at reflux for 24 h (TLC and 11B NMR control). The reaction mixture was co-evaporated with 10 mL of silica and treated by column chromatography (40 mL of silica) using DCM gradient (0 → 30%) in hexane. After evaporation and drying, TMS-protected compound 12 was isolated as a white solid (0.580 g, 93%). NMR δH (500 MHz; CDCl3) 3.51 (2 H, br s, Ccb–H), 3.3–1.5 (9 H, m, B–H), 0.15 (9 H, s, TMS); δB (160 MHz; CDCl3; BF3·Et2O): δ −1.5 (2 B, d, JB–H 151), −8.7 (1 B, s, B–C), −10.1 (1 B, d, JB–H 175), −13.3 (2 B, d, JB–H 154), −14.4 (3 B, d, JB–H 148), −17.3 (1 B, br d, JB–H 165); δC (125 MHz; CDCl3) 104.8 (TMC–Cu), 52.9 (Ccb–H), 0.24 (TMS); m/z (–TIS) 240.2409 [M − H]− (calcd for C7H20B10Si 240.2337); mp 110–112 °C; νmax/cm−1 (nujol) 3070, 3065 (νC–H), 2663, 2652, 2638, 2608, 2586, 2567 (BH), 2073 (CuC). (b) Anhydrous methanol (5 mL) was added to a solid mixture of 12 (0.200 g, 0.833 mmol) and anhydrous K2CO3 (0.173 g, 1.25 mmol), and the resulting suspension was stirred at RT for 2 h (TLC control). The reaction mixture was dissolved in water (10 mL) and extracted with ether (3 × 5 mL). The combined organic extracts were passed through a plug containing 0.5 mL of silica and 4 mL of anhydrous Na2SO4 and were evaporated to dryness to give 13 (0.134 g, 96%) as a white solid. NMR δH (500 MHz; CDCl3) 3.56 (2 H, br s, Ccb–H), 2.9–1.6 (9 H, m, B–H), 2.27 (1 H, s, uC–H); δB (160 MHz; CDCl3; BF3·Et2O) −1.4 (2 B, d, JB–H 151), −8.9 (1 B, s, B–C), −9.8 (1 B, d, JB–H 154), −13.1 (2 B, d, JB–H 163), −14.2 (2 B, d, JB–H 166), −14.7 (1 B, d), −17.0 (1 B, d, JB–H 180); δC (125 MHz; CDCl3) 85.6 (q*, 2 J 21, uC–H), 53.2 (Ccarb–H); m/z (–TIS) 168.2075 [M]− (calcd
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for C4H12B10 168.1939); mp 148–150 °C; νmax/cm−1 (nujol) 3064br (CH), 2609, 2582 (B–H), 2129 (CuC). (c) To a solution of 13 (0.200 g, 1.19 mmol) in 4 mL of DMSO were added 10% Pd/C (64.0 mg, 60.0 μmol) and CuI (12.0 mg, 60.0 μmol), and the resulting suspension was stirred in air for 24 h (TLC control). The reaction mixture was filtered, poured into 20 mL of water, and extracted with ethyl acetate (3 × 10 mL). The combined extracts were washed with water (2 × 20 mL) and brine (5 mL), dried over Na2SO4, and co-evaporated with 4 mL of silica. The compound was isolated by column chromatography (16 mL of silica) using EtOAc (0 → 50%) in hexane to give, after evaporation and drying, 14 (0.180 g, 91%) as a beige solid. NMR δH (500 MHz; acetone-d6) 4.59 (4 H, br s, Ccb–H), 3.1–1.5 (18 H, m, B–H), 2.27 (1 H, s, uC–H); δB (160 MHz; CDCl3, BF3·Et2O) −1.0 (2 B, d, JB–H 148), −8.4 (1 B, s, B–C), −9.5 (1 B, d, JB–H 159), −13.1 (2 B, d), −13.9 (3 B, d, JB–H 158), −17.0 (1 B, d); δC (125 MHz; CDCl3) 82.4 (br, uC–Cu), 55.9 (Ccb–H); m/z (–TIS) 334.3712 [M]− (calcd for C8H22B20 334.3727); mp >350 °C (decomp); νmax/cm−1 (nujol) 3064 (CH), 2641, 2628, 2610, 2581 (BH), 2108 (CuC). Synthesis of nido-biscarboranes General procedure for the synthesis of TBA salts of nido-biscarboranes 15–18. To a solution of the corresponding closocarborane (1 mmol) in 3 mL of THF was added 10 mL of 1 M TBAF in THF (10 mmol). The reaction mixture was heated to reflux until the TLC analysis showed the absence of the starting material (2–5 h). The reaction mixture was evaporated to dryness, and 5 mL of water was added to the residue in one portion. The formed precipitate was filtered on a glass frit, washed with water (3 × 3 mL) and ether (5 mL), and dried under vacuum. Tetrabutylammonium 1,2-bis(7′,8′-dicarba-nido-undecaborate1′-yl)ethyne (15). White solid (94%). NMR δH (500 MHz; acetone-d6) 3.44 (16 H, m, TBA–CH2), 2.34–(−0.21) (16 H, br m, B–H), 1.82 (16 H, m, TBA–CH2), 1.73 (4 H, br s, Ccarb–H), 1.43 (16 H, m, TBA–CH2), 0.98 (24 H, m, TBA–CH3), −2.40 (2 H, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O): −12.8 [2 B, d, JB–H 132, B(9,11)], −16.5 [2 B, d, JB–H 137, B(5,6)], −18.2 (1 B, d, JB–H 159, B(3)], −22.6 (2 B, d, JB–H 147, B(2,4)], −34.9 [1 B, dd, JB–H term 123, JB–H bridge 38, B(10)], −35.5 [1 B, s, B(1)]; δC (125 MHz; acetone-d6) 59.4 (TBA), 43.1 (br, Ccarb–H), 24.4, 20.4, 13.8 (TBA); m/z (–APCI) 144.7116 [M]2−, 531.7629 [M + TBA]− (calcd for C14H26B18 289.3504, M2−144.6749, M + TBA 531.6363); νmax/cm−1 (nujol): 2534, 2507 (BH). Tetrabutylammonium 1,4-bis(7′,8′-dicarba-nido-undecaborate-1′-yl)benzene (16). White solid (92%). NMR δH (500 MHz; acetone-d6) 7.53 (4 H, s, C6H4), 3.30 (16 H, m, TBA–CH2), 2.46– (−0.08) (16 H, m, B–H), 1.88 (Ccarb–H), 1.74 (16 H, m, TBA– CH2), 1.40 (16 H, m, TBA–CH2), 0.96 (24 H, m, TBA–CH3), −2.23 (2 H, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O) −11.5 (2 B, d, JB–H 134), −15.6 (2 B, d, JB–H 134), −17.5 (1 B, d, JB–H 154), −21.8 (2 B, d, JB–H 145), −25.9 [1 B, s, B(1)], −33.6 (1 B, dd, JB–H term 127, JB–H bridge 43); δC (125 MHz; acetone-d6) 134.7 (C6H4), 59.2 (TBA), 43.9 (br, Ccarb–H), 24.4, 20.3, 13.8 (TBA); m/z (–TIS) 170.7397 [M]2−, 583.8452 [M + TBA]− (calcd
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for C8H22B18 341.3820, M2− 170.6907, M + TBA 583.6679); νmax/cm−1 (thin film) 2962, 2934, 2875 (CH), 2507br (BH). Tetrabutylammonium 1,4-bis((7′,8′-dicarba-nido-undecaborate-1′-yl)ethynyl)benzene (17). Beige solid (96%). NMR δH (500 MHz; acetone-d6) 7.24 (4 H, s, C6H4), 3.41 (16 H, m, TBA– CH2), 2.43–(−0.15) (16 H, br m, B–H), 1.80 (20 H, m, Ccarb–H, TBA–CH2), 1.42 (16 H, m, TBA–CH2), 0.93 (24 H, m, TBA–CH3), −2.34 (2 H, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O) −10.3 (2 B, d, JB–H 135), −14.7 (2 B, d, JB–H 136), −16.4 (1 B, d, JB–H 163), −20.7 (2 B, d, JB–H 145), −32.9 (1 B, dd, JB–H term 127, JB–H bridge 42), −34.5 [1 B, s, B(1)]; δC (125 MHz; acetone-d6) 131.9, 125.7 (C6H4), 59.4 (TBA), 43.4 (br, Ccarb–H), 24.4, 20.4, 13.8 (TBA); m/z (–APCI) 194.7012 [M]2−, 631.6817 [M + TBA]− (calcd for C14H26B18 389.3818, M2− 194.6909, M + TBA 631.6666); νmax/cm−1 (nujol) 2576, 2552, 2515 (BH), 2183 (CuC). Tetrabutylammonium 1,4-bis(7′,8′-dicarba-nido-undecaborate1′-yl)butadiyne (18). Beige solid (95%). NMR δH (500 MHz; acetone-d6) 3.44 (16 H, m, TBA–CH2), 2.39–(−0.19) (16 H, br m, B–H), 1.82 (20 H, m, Ccarb–H, TBA–CH2), 1.44 (16 H, m, TBA– CH2), 0.98 (24 H, m, TBA–CH3), −2.39 (2B, br m, B–H–B); δB (160 MHz; acetone-d6; BF3·Et2O) −11.4 (2 B, d, JB–H 134), −15.8 (2 B, d, JB–H 135), −17.5 (1 B, d, JB–H 165), −21.7 (2 B, d, JB–H 146), −33.9 (1 B, dd, JB–H term 127, JB–H bridge 43), −35.8 [1 B, s, B(1)]; δC (125 MHz; acetone-d6) 59.4 (TBA), 43.4 (br, Ccarb–H), 24.5, 20.4, 13.9 (TBA); m/z (–TIS) 156.7275 [M]2−, 555.8129 [M + TBA]− (calcd for C8H22B18 313.3506, M2− 156.6750, M + TBA 555.6364); νmax/cm−1(thin film) 2962, 2934 (CH), 2518br (BH). X-ray diffraction studies The X-ray quality crystals of 3, 6, 9, 14, 15, and 17 were obtained by slow evaporation of chloroform (3, 9, 14) or acetone (6, 15, 17) solutions. Data collection for the crystals was performed at −100 °C on a Bruker SMART 1000 CCD area detector system using the ω scan technique with Mo Kα radiation (λ = 0.71073 Å) from a graphite monochromator. Data reduction and integration were performed with the software package SAINT.23 Data were corrected for absorption using SADABS.24 The crystal structures were solved with the direct methods program SHELXS-97 and were refined by full matrix least squares techniques with the SHELXTL25 suite of programs. All non-hydrogen atoms were refined with anisotropic thermal parameters, except for the disordered chlorine atoms in the crystal structure of 9. The hydrogen atoms in 3, 6, and 14 were located in the difference Fourier maps and were refined individually. In 9, all hydrogen atoms, except those of the disordered chloroform, were located in the difference Fourier maps and were refined individually, whereas the chloroform hydrogens were included at geometrically idealized positions; therefore, the refinement of H-atoms was mixed. In 15, only the hydrogens of the open pentagonal C2B3 faces of the nido-carboranes were located in the difference Fourier maps and were refined individually, whereas the remaining BH hydrogens, as well as those of the tetrabutylammonium (TBA) cations, were included at geometrically idealized
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positions. Therefore, the refinement of the H-atoms was mixed. In 17, only the hydrogens of the nido-carborane groups were located in the difference Fourier maps and refined individually, whereas the hydrogens of the aromatic group and the TBA cations were included at geometrically idealized positions; therefore, the refinement of the H-atoms was mixed. The crystal structure of 15 revealed two types of disorder. One type of disorder was caused by the three fluxional arms of the TBA cation. This disorder was modeled to provide the 0.6 : 0.4 ratio of the superimposed forms. In the second type of disorder, one of the nido-carborane cages exhibited rotational disorder, which made distinguishing between the boron and carbon atoms at two atom sites difficult. This disorder was modeled using partial occupancies for the boron and carbon atoms, which provided a 0.5 : 0.5 ratio for the superimposed rotamers. The TBA cation in the crystal structure of 17 exhibited disorder of its the two fluxional arms. This disorder was modeled to provide 0.65 : 0.35 and 0.52 : 0.48 ratios for the two superimposed forms of each arm. The crystal structure of 9 contained two disordered molecules of chloroform in an asymmetric unit. The disorders were modeled in the form of four rotamers in a 0.60 : 0.14 : 0.11 : 0.15 ratio for one of the chloroform molecules and in the form of two rotamers in a 0.5 : 0.5 ratio for the other molecule. The crystal structure of 3 was solved in space group Pnn2; however, because the compound is a weak anomalous scatterer (i.e., in contains only light atoms B, C, and H) and because data were collected using Mo Kα radiation, its absolute configuration could not be reliably determined. Therefore, Friedel pairs were merged, and any references to the Flack parameter were removed. The crystallographic data and the details of the data collection and structure refinements of 3, 6, 9, 14, 15, and 17 are provided in Table S1.† CCDC 961327 (3), CCDC 961328 (6), CCDC 961329 (9), CCDC 961330 (14), CCDC 961331 (15) and CCDC 961332 (17) contain the supplementary crystallographic data for this paper.
Acknowledgements The authors gratefully acknowledge the National Science Foundation (CHE-0702774) for financial support of this work, Dr Mark Lee and Mr Brett Meyers for measuring the mass spectra of the prepared compounds. The authors also thank Dr Ilia Guzei (UW-Madison) for the in-house crystallographic program “ModiCIFer”.
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