Rodlike Polymers Containing Nickel and Cobalt Metal Bis(dicarbollide

Article pubs.acs.org/Organometallics

Rodlike Polymers Containing Nickel and Cobalt Metal Bis(dicarbollide) Anions: Synthesis and Characterization Kothanda Rama Pichaandi,† Alexander V. Safronov, Yulia V. Sevryugina, Thomas A. Everett, Satish S. Jalisatgi, and M. Frederick Hawthorne* International Institute of Nano and Molecular Medicine (I2NM2), University of Missouri, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: Two approaches for the synthesis of rodlike polymers having metal bis(dicarbollide) anions connected by organic linkers, such as acetylene and 1,4-dibutyl-2,5diethynylbenzene, in the backbone are reported. Route 1 involved connecting nido-bis(carborane) compounds, containing the linkers, via π coordination with Co or Ni metals. This reaction yielded small oligomers having one to six repeating units. Route 2 utilized a palladium-catalyzed cross-coupling copolymerization of diiodo-substituted cobaltocarboranes and a zinc-derivatized aromatic linker. This approach gave a mixture of oligomers and polymers. The polymers were separated from the smaller oligomers by dialysis and were characterized by 1H NMR, 11B NMR, 11B{1H} NMR, IR, UV−vis, TEM, and AFM techniques. End-group analysis and dynamic-light scattering experiments (DLS) determined that the average number of repeating units in the polymers ranged between 35 and 44. GPC analyses failed to aid the molecular weight determination due to the adsorption of the polymers to the stationary phase of the column. Dilute-solution viscometry experiments supported the semirigid nature of these polymers.

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INTRODUCTION The development of conducting π-conjugated polymers containing icosahedral boron clusters is an emerging area of research with applications in light-emitting diodes, solar cells, electrochromic devices, nonlinear optics, chemical sensors, and biosensors.1−8 The incorporation of boron clusters into the polymer backbone or side-chain branches provides low nucleophilicity, high-temperature stability, hardness, chemical inertness, and electron-withdrawing characteristics.8 In contrast to conventional organic polymers, the ability of boron clusters to prevent bundling and π stacking makes these materials better candidates for optoelectronic devices.9 The commonly used icosahedral clusters in these polymers have been closo[B12H12]2−, closo-[CB11H12]−, nido-[C2B9H12]−, and ortho, meta, and para isomers of closo-C2B10H12 carboranes. The organic linkers used to connect these boron clusters have traditionally been p-phenyleneethynylene, polyfluorene, silane, and siloxane bridging units.8 Some of the previously reported linear rigid-rod molecules called “carbo(ra)rods” contained repeating carborane fragments linked via either ethynylene or butadiyne linkers.10,11 The highlight of this work was the synthesis of an enormous carbo(ra)rod containing, on average, 16-carborane modules each consisting of a 12-vertex pcarborane.11 Here, we present continuation of that work with the known bis(dicarbollide)metal complexes, [3,3′-M(1,2C2B9H11)2], where M is either Ni or Co, respectively. The bis(dicarbollyl)nickel system, a well-studied complex in this family, is known for its capability as a molecular motor with redox-controlled motion.12−16 Metallacarborane-decorated dendrimers6,17 and conducting conjugated-polypyrrole poly© XXXX American Chemical Society

mers either doped with a bis(dicarbollyl)cobalt complex or having the same side chain were shown to have enhanced thermal stabilities and overoxidation thresholds.8,18−21 Therefore, polymers with a bis(dicarbollyl)metal moiety in their backbone linked by rigid unsaturated linkers are expected to be conductive and serve as molecular wires.8,21−23 However, introducing transition-metal bis(dicarbollides) into the polymer backbone has remained a challenge due to the synthetic difficulties associated with substitution at the para position of metal bis(dicarbollides). Recently, we reported a series of precursors: (i) the multigram synthesis of 8-iodo-1,2-dicarbacloso-dodecaborane and the corresponding diiodo-substituted metal bis(dicarbollides) (1)24 and (ii) nido-bis(carboranes) with an acetylene linker (2),25 which paved the way for the introduction of metal bis(dicarbollides) into the polymer backbone. We envisaged the two possible routes 1 and 2 to introduce them in the polymer backbone (Scheme 1).25 Route 1 involves the connection of nido-bis(carborane) compounds containing the desired linkers, via π coordination, with metals. Route 2 is a Pd-catalyzed cross-coupling copolymerization of 1 with metal derivatives of the desired linker molecules. In this report, we advanced that chemistry to realize the envisaged carborods.

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RESULTS AND DISCUSSION Route 1. The three nido-bis(carborane) precursors 2 and 3 (C−H units in the carborane cage in 2 are replaced with C− Received: July 29, 2017

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DOI: 10.1021/acs.organomet.7b00578 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Two Routes to Metallacarborane-Based Molecular Rodsa,25

Reaction conditions are as follows. Route 1: (i) nBuLi, THF, −78 °C → 25 °C, 3 h; (ii) M(acac)2 (M = Ni, Co), THF, 65 °C, 24 h. Route 2: (i) Pd(PPh3)4, THF, 65 °C, 48 h. a

was made first from the reaction of nido-bis(carboranes) with nbutyllithium and then metalated with nickel(II) acetylacetonate (Ni(acac)2) or cobalt(II) acetylacetonate (Co(acac)2) (Scheme 1, route 1). Precursors 2−4 resulted in oligomers 5−7 (a, Ni; b, Co) respectively. Ni-based oligomers were paramagnetic and were characterized by electron spray ionization (ESI) mass spectroscopy. Co-based oligomers were diamagnetic and were characterized by ESI mass spectroscopy as well as NMR. The end groups of the oligomers obtained were identified as nidocarboranes. In the 11B{1H} NMR spectrum, the signals at −35 ppm corresponding to the terminal open cage boron atoms (with bridged hydrogen and accounting for four in each oligomer) are distinct from the rest of the boron atoms. These were used to determine the average number of repeating units (n) and were calculated by the following equation with the count of boron atoms on different segments of the oligomer:

CH3) and 4 (nido-bis(carborane) linked with 1,4-dibutyl-2,5diethynylbenzene) (Figure 1) were utilized. The syntheses of

Figure 1. Reported (1 and 2) as well as new precursors (3 and 4) for metallacarborane-based molecular rods.24,25

n=

total − terminal open cages (18) cobalt bis(dicarbollide) (18)

(1)

Oligomers 5a,b had limited solubility. ESI mass spectroscopy identified just one metal bis(dicarbollide) unit, and attempts to characterize other oligomers were not successful due to the insolubility of these species in common solvents. The oligomers 6a (n = 1−6) were identified by ESI mass spectroscopy (Figure

precursors 3 and 4 were achieved by a route analogous to that for 2, mentioned in our earlier report25 (see the Supporting Information for schemes and structural characterization). For route 1 the general methodology of synthesis and characterization is described here. The bis(dicarbollide) dianion

Figure 2. ESI mass spectroscopy of oligomers 6a with n = 1−6 obtained from Ni(acac)2 and 3. B


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Organometallics

cellulose) using acetone as a solvent. The lower molecular weight (MW) fractions came out with the mother solvent, and the polymers were retained in the bag. ESI-MS analysis of lower molecular weight (MW) fractions identified up to seven repeating units (Figures S7−S12 in the Supporting Information represent the oligomers for n = 1−7). A representative mass spectrum of an oligomer with n = 4 is given in Figure 4.

2). As the number of repeating units increased from 1 to 6, the overall charge increased with a decrease in the difference in the mass peaks. Characterizations of the oligomers 6b and 7b are described here. Due to the sparingly soluble nature of 6b in solvents such as methylene chloride and acetone, the ESI technique identified oligomers with repeating units for n = 1−4 (Figures S1−S4 in the Supporting Information). Figure 3 represents the 11B{1H}

Figure 3. 11B{1H} NMR spectrum of 6b used for the determination of the average number of repeating units. Figure 4. ESI mass spectroscopy of oligomer 9 with n = 4.

NMR spectrum of 6b, and the average number of repeating units determined was 4.1 using eq 1. Oligomers 7b had a maximum of three repeating units, as identified by ESI mass spectroscopy (Figure S5 in the Supporting Information),, and 11 1 B{ H} NMR determined the average number of repeating units as 1.2, using eq 1 (see the Figure S6 in the Supporting Information). Synthesis of oligomers from 7a was not attempted. Overall, precursor 3 gave oligomers with a higher number of repeating units (up to 6) in comparison to 2, which can be attributed to the increased THF solubility afforded by the presence of methyl groups in the boron cage. However, precipitation of these oligomers from the reaction mixture prevented further reaction and the formation of polymer. For precursor 4, solubility was not a limitation, as no precipitation was observed during the reaction. Therefore, the absence of higher oligomers/polymer formation can be attributed to the failure of the nido centers to π-coordinate with the metal in the elongated chains. Since the use of 1,4-dibutyl-2,5-diethynylbenzene resulted in better solubility, route 2 with Pd-catalyzed cross-coupling copolymerization of the diiodo derivative 1 with zinc derivatives of the same linker molecule (8) was attempted (Scheme 1, route 2) and is discussed below. Route 2. The diiodo-substituted cobaltocarboranes (1)24 and the zinc derivative of the 1,4-dibutyl-2,5-diethynylbenzene linker (8)26 were prepared according to the literature. Reaction between complex 1 and 1.2 equiv of zinc derivative 8 with a 5% Pd(0) catalyst was carried out for 24 h. The reaction was monitored by 11B NMR by the decrease in the resonance at δ −12.7 ppm corresponding to the iodinated boron vertex in 1. A second supply of 0.5 equiv of 8 and the 5% Pd(0) catalyst was added, and the reaction was continued for another 24 h. The addition of an excess of 8 was made to ensure that all of 1 was consumed and that the linker was positioned at the end of the polymer chain. Filtration of the reaction mixture and passing it through a short silica plug removed the remnants of the Pd catalyst and phosphines. Then, the reaction mixture was dialyzed using 12−14k MWCO dialysis bags (regenerated

The polymer fraction retained in the bag after dialysis was 30−40% in yield between batches and was characterized by 1H NMR, 11B NMR, 11B{1H} NMR, IR, and UV−vis techniques as well as by the imaging tools AFM and TEM. Figure 5 represents the 1H NMR (CD3COCD3) spectrum of polymer 9 (with SiMe3 as terminal groups). In the 1H NMR spectrum, the aromatic region (Figure 5A) was shifted downfield to 8.1 ppm in comparison with 1,4-dibutyl-2,5-diethynylbenzene (7.2 ppm, Figure 5C). This confirmed the presence of the aromatic linker, and the downfield chemical shift was consistent with polymer formation.27,28 The presence of the C−H protons corresponding to the boron cage at approximately 4.5 ppm in the 1H NMR spectrum (Figure 5B), the 11B NMR spectrum analogous to that of oligomer 7b with the absence of signals for the nido end (Figures S13 and S14 in the Supporting Information), and the IR signal corresponding to a typical B−H stretch at 2569 cm−1 (Figure S15 in the Supporting Information) confirmed the presence of cobalt bis(dicarbollide) in the polymer chain. In comparison to monomeric complex 1, the UV−vis spectrum of the polymer was much broader and red-shifted (Figure 6). This is consistent with the NMR observation of cobalt bis(dicarbollide) in the polymer backbone and the emphasized π conjugation. Further slow evaporation of the solvent from the polymer solution resulted in a thin film on the surface of the glass, which is a typical characteristic of a polymer. Determination of molecular weight of these rigid-rod polymers posed challenges similar to those of conjugated polyelectrolytes. 29−31 The traditional GPC method to determine the MW of polymers was ineffective, as these molecules adsorbed onto the GPC column due to the ionic natures of the polymers.27 MALDI is another common technique used in the determination of the molecular weight of polymers but was also not helpful due to the polymers possessing multiple charges, resulting in difficult identification of m/z values below 500.27 Hence we used the following alternative strategies. C


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Figure 5. (A, B) Aromatic and aliphatic regions of the 1H NMR (CD3COCD3) spectrum of polymer 9 (with SiMe3 as terminal groups) after dialysis with a 12−14k MWCO bag (p, CH3Si; q, CH of the cage; s, NMR solvent; g, grease). (C) 1H NMR (CD3COCD3) spectrum of the aromatic region of 1,4-dibutyl-2,5-diethynylbenzene (i, CHCl3).

Figure 6. Comparison of UV−vis spectra of polymer 9 with complex 1: (- - -) 1; (−) 9.

(i) Dialysis of the polymers (obtained from the 12k-14k MWCO bags) with 25k bags resulted in zero retention of the polymers. This indicated that the average MW of the polymers was between 12000 and 25000. (ii) Next, we used end-group analysis to determine the number of repeating units by quenching the reaction mixture at the end of the polymerization reaction with Me3SiCl. Since an excess of the zinc derivative of linker 9 in comparison with 2 was used in the reaction, the polymers are expected to have Me3Si groups at their ends. The number of repeating units was determined by the integral ratio of the methyl group of Me3Si to the C−H protons of the boron cage in the 1H NMR spectrum (Figure 5). The repeating units varied in the range of 35−44 between batches, corresponding to an estimated MW of 20k−25k (anionic part). (iii) Dynamic-light scattering analysis of the polymers showed that the average size was ∼42 nm (Figure S16 in the Supporting Information). The length of the monomeric unit was estimated to be ∼1.7 nm,24,25 and the estimated average size of 60−75 nm (from end group analysis) reasonably agrees with the DLS observed. (iv) Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of 9 are in good agreement with the results obtained by DLS (Figure 7). The images show that the polymer takes on micelle-like structures when it is cast

Figure 7. AFM and TEM images of 9: (A, B) tapping mode AFM height images of polymer 9 cast from acetone onto freshly cleaved mica showing a micelle-like morphology (height bar ranges from 0 to 30 nm); (C, D) TEM images showing spherical particles of various sizes (scale bar is 50 nm in (C) and 10 nm in (D)).

onto a supporting substrate. The size of the particles ranges from ∼120 nm down to tens of nanometers. The wide size range obtained from the images can be explained by the aggregation of oligomers in solution upon drying. Overall, the nonretention of the polymer by dialysis with the 25k MWCO bags in combination with DLS and imaging techniques supports the findings of the end-group analysis that the polymers possess 35−44 repeating units. The viscometric behavior of a polymer solution can give insight into the macroscopic properties, especially the rigidity of the polymer.9,27 In a dilute solution, a polyion will form a rodlike shape due to electrostatic self-repulsion. At a high concentration, the chains become flexible due to the dominance of the small interchain separation over the rod length. Therefore, an increase in the polymer concentration will increase the specific viscosity. Figure 8 represents a plot of specific viscosity vs polymer concentration, showing a linear D


Organometallics

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EXPERIMENTAL SECTION

General Considerations. Unless otherwise stated, all reactions were carried out under an argon atmosphere using standard Schlenkline techniques. Decaborane-14 was purchased from Katchem Ltd. (Czech Republic). Iodine chloride (ICl), cobalt and nickel acetylacetonates, n-butyllithium, ethyl propiolate, diethyl sulfide (Aldrich), and potassium tert-butoxide (Fluka) were used as purchased. Tetrabutylammonium fluoride was purchased from TCI America as a 1 M solution in THF and used as received. Anhydrous tetrahydrofuran (THF), toluene, dichloromethane (DCM), acetonitrile (ACN), and hexane were obtained using a Vacuum Atmospheres solvent purification system. Thin-layer chromatography was performed using Merck precoated glass plates (Silica 60 F254). Column chromatography was performed using Merck silica gel (63−200 mesh). 8-Iodo-1,2-dicarba-closo-dodecaborane and the tetrabutylammonium (TBA) salt of diiodocobalt bis(dicarbollide) (1) were prepared according to the published procedure.24 1,4-Dibutyl-2,5diethynylbenzene was prepared by following the reported procedures.26,32−35 Caution! All operations with decaboranes must be carried out in a well-ventilated hood using skin protection! Physical Measurements. 1H, 11B, 11B{1H}, and 13C{1H}, DEPT NMR spectra were measured on Bruker Avance-400 and Avance-500 NMR spectrometers; boron NMR spectra were referenced to 15% BF3·Et2O in CDCl3. 1H NMR and 13C{1H} NMR spectra were referenced to the residual solvent peak impurity. Chemical shifts were reported in ppm and coupling constants in hertz. Mass spectra were obtained on an ABI QSTAR and Mariner Biospectrometry Workstation by PerSeptive Biosystems. The purity of all intermediates was confirmed by HRMS, 1H NMR, 13 C{1H} NMR, 11B NMR, and 11B{1H} NMR. HRMS measurements given were compared between the highest intensity signals obtained and the theoretical value. GPC analyses were attempted using columns from Agilent filled with cross-linked polystyrene beads using THF as a solvent at 25 °C. TEM samples were prepared by drop-casting a dilute solution of the polymer from acetone onto a 200 mesh carbon-coated TEM grid. This was allowed to dry and subsequently imaged. No electron diffraction was observed and cryo-sections of the polymer were also imaged but did not yield any useful data. For AFM the samples were prepared in dilute acetone and ∼5 μL was drop-cast onto the mica and allowed to dry. Imaging was completed in an air environment. Synthesis of 1,2-Dimethyl-8-iodo-o-carborane. A Schlenk flask was loaded with 8-iodo-1,2-dicarba-closo-dodecaborane (1.0 g, 3.7 mmol) and 10 mL of THF and cooled to −78 °C with stirring. nButyllithium (3.1 mL of 2.5 M hexane solution) was added using a syringe over a period of 15 min. The reaction mixture was brought to room temperature and stirred for 3 h. Methyl iodide (0.60 mL, 9.6 mmol) was added to the reaction mixture over a period of 10 min and stirring continued overnight at room temperature. Then the THF solvent was removed by rotary evaporation and 1,2-dimethyl-8iodocarborane was extracted three times with ether (100 mL each time). The ethereal fraction on subsequent rotary evaporation and crystallization (heptane) gave 1,2-dimethyl-8-iodocarborane as colorless crystals. Yield: 0.93 g (85%) 1H NMR (500.13 MHz, CDCl3): δ 3.39−1.53 (br s, 9H, B-H), 2.04 ppm (s, 6H, C-CH3). 13C{1H} NMR (125.75 MHz, CDCl3): δ 74.52 (carboranyl C), 23.16 ppm (C-CH3). 11 1 B{ H} NMR (160.46 MHz, CDCl3): δ −3.35 (2B), −6.04 to −12.62 (t, br s, 7B), −23.22 (1B, B-I). HRMS (EI; m/z): C4H15B10I calculated, 298.1508; found, 298.1274 (M−). Mp: 93−102 °C. IR (cm−1): 2591 (B−H), 2942, 2990. Synthesis of 1,2-Bis(1′,2′-dimethyl-8′-carboranyl)acetylene. A Schlenk flask fitted with a condenser was loaded with 1,2-dimethyl8-iodo-o-carborane (1.0 g, 3.3 mmol), 1,1,1,2,2,2-hexabutyldistannane (0.80 mL, 1.5 mmol), tetrakis(triphenylphosphine)palladium(0) (0.19 g, 0.17 mmol), lithium chloride (1.4 g, 33 mmol), a few crystals of BHT, and 10 mL of acetonitrile and the mixture refluxed overnight under an argon atmosphere. After the reaction mixture was cooled to room temperature, another lot of 1,1,1,2,2,2-hexabutyldistannane (0.25

Figure 8. Plot of the specific viscosity vs the polymer concentration for 9.

trend and consistency with the rigid-rod nature in dilute solution and flexibility at higher concentrations. In contrast to the behavior of ordinary polymer solutions, charged polymers exhibit an increase in reduced viscosity (ηsp/ c) with a decrease in the polymer segment concentration. Polymer 9, reported here, followed the same trend. At a higher concentration the viscosity became independent of the polymer concentration (Figure 9). Overall, the viscometric study

Figure 9. Plot of the reduced viscosity vs the polymer concentration for 9.

showed that the carborods behaved similarly to a semiflexible polyion, where at low chain concentrations the chains were stiffer than at high segment concentrations.

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Article

CONCLUSION

We describe two routes for synthesis of carborods from metal bis(dicarbollides). The route involving the connection of nidobis(carborane) compounds containing the desired linkers via π coordination with metals gave oligomers with up to six repeating units of metal bis(dicarbollides), as determined by ESI mass spectrometry analysis. The insolubility of the oligomers and the difficulty of π coordination with metals in the long chains prevented the formation of polymers using this route. On the other hand, a palladium-catalyzed cross-coupling copolymerization methodology gave oligomers and polymers, which were separated by dialysis with 12−14k MWCO dialysis bags. NMR, IR, and UV−vis spectroscopy of the polymers confirmed the presence of both organic linkers as well as cobalt bis(dicarbollide) moieties. The end-group analysis determined that the average number of repeating units was 35−44, which was supported by DLS experiments, imaging experiments, and the nonretention of the polymer when it was dialyzed with 25k MWCO bags. Dilute-solution viscometry experiments predicted the polymers to have a rigid structure in dilute solutions. E


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Organometallics mL, 0.47 mmol), tetrakis(triphenylphosphine)palladium(0) (0.45 g, 0.05 mmol), and a few crystals of BHT were added and heated to reflux for 24 h. The mixture was cooled to room temperature and filtered through Celite. The filtrate was rotary evaporated to remove the solvent and chromatographed using silica gel (hexane/ethyl acetate 1/0.3). 1,2-Bis(1′,2′-dimethyl-8′-carboranyl)acetylene was obtained as colorless crystals. Yield: 0.20 g (33%). 1H NMR (500.13 MHz, CDCl3): δ 3.15−1.43 (br s, 9H, B-H), 2.00 ppm (s, 12H, C-CH3). 13 C{1H} NMR (125.75 MHz, CDCl3): δ 71.8 (carboranyl C), 23.3 ppm (C-CH3). 11B{1H} NMR (160.46 MHz, CDCl3): δ −4.32 (4B), −6.2 to −15.3 (t, br s, 14B). HRMS (m/z): C10H30B20 calculated, 366.4927; found, 366.5008 (M−). Mp: 242−245 °C. IR (KBr): 2591 (B−H), 2866, 3299 (CC). Synthesis of nido-Biscarborane 3. A Schlenk flask fitted with a condenser was loaded with 1,2-bis(1′,2′-dimethyl-8′-carboranyl)acetylene (0.16 g, 0.44 mmol), tetrabutylammonium fluoride (4.50 mL of 1 M solution in THF, 7.08 mmol), and 5 mL of THF and heated to reflux for 12 h. The reaction mixture was cooled to room temperature, and 70 mL of water was added to precipitate nidobis(carborane) (3) as its TBA salt, which was filtered and recrystallized from methanol. Yield: 0.36 g (quantitative). 1H NMR (400.13 MHz, CD3COCD3): δ 3.44 (t, J = 9.1 Hz, 4H, NCH2CH2CH2CH3), 2.83 ppm (s, 4H), 1.83 (m, 16H), 1.44 (m, 16H), 1.3 (s, 12H), 0.97 (t, 24H). 13C{1H} NMR (125.75 MHz, CD3COCD3): δ 71.8 (carboranyl C), 23.3 ppm (C-CH3). 11B{1H} NMR (160.46 MHz, CD3COCD3): δ −4.32 (4B), −6.2 to −15.3 (t, br s, 14B). HRMS (m/z): z = 2, C10H30B18 calculated, 172.7335; found, 172.7559. Mp: 162−169 °C. IR (KBr, cm−1): 1467, 2518, 2869, 2959. Synthesis of 1,4-Bis(carboranyl)-2,5-dibutylbenzene. In a 10 mL Schlenk flask were placed 1,4-dibutyl-2,5-diethynylbenzene (0.11 g, 0.46 mmol) and 2 mL of THF, and the mixture was cooled to −78 °C. Lithium hexamethyldisilazide (0.94 mL of 1 M soln in THF) was added, brought to 0 °C, and stirred for 30 min. The reaction mixture was further cooled to −78 °C, ZnBr2 (0.23 g, 1 mmol) in 3 mL of THF was added, and this mixture was warmed to room temperature and stirred for 3 h. 8-Iodo-1,2-dicarba-closo-dodecaborane (0.24 g, 0.88 mmol) and Pd(PPh3)4 (0.05 g, o.04 mmol) were added to the reaction mixture and heated to reflux for another 24 h. The reaction was monitored by the presence of the B−I signal at −22.5 ppm corresponding to 8-iodocarborane by 11B NMR. The next day another lot of bisorganozinc halide (from 0.09 g of 1,4-dibutyl-2,5diethynylbenzene) was freshly prepared and added to the reaction mixture and heated to reflux overnight. Complete consumption of 8iodocarborane was confirmed by 11B NMR with the absence of a signal at −22.5 ppm corresponding to 8-iodo-1,2-dicarba-closo-dodecaborane. The mixture was cooled to room temperature and filtered through Celite. The filtrate was rotary-evaporated to remove the solvent and chromatographed using silica gel (hexane/ethyl acetate 1/0.1).Yield: 0.21 g (50%). 1H NMR (CD3COCD3): δ 1.04 (t, 6H), 1.49 (m, 4H), 1.71 (m, 4H), 2.82 (m, 4H), 4.70 (s, 4H), 7.31 (s, 2H). 13C{1H} NMR (CD3COCD3): δ 13.35, 22.44, 32.74, 33.60, 54.70 (carboranyl C), 122.99, 131.96, 142.44. 11B{1H} NMR (CD3COCD3): δ −16.9, −14.3, −13.2, −10.5, −8.1, −2.2. HRMS (m/z) could not be determined due to difficulty in ionizing with the APCI method. Mp: 165−172 °C. IR (cm−1): 2591, 2866, 2955, 3299. Synthesis of nido-Biscarborane 4. 1,4-Bis(carboranyl)-2,5dibutylbenzene (0.12 g, 0.23 mmol) was placed in a 10 mL Schlenk flask, and tetrabutylammonium fluoride (2.5 mL, 1 M solution in THF, 3.9 mmol) was added to it. The reaction mixture was heated to reflux for 12 h. The reaction mixture was rotary evaporated to remove the solvent. The solid obtained was washed with water, filtered, and dried under vacuum to give the desired nido compound 4. Yield: 0.23 g (quantitative), 1H NMR (CD3COCD3): δ 2.85 (t, 4H), 3.52 (t, 16H), 7.18 (s, 2H). 13C{1H} NMR (CD3COCD3): δ 12.8, 13.5, 19.4, 22.4, 23.5, 32.8, 33.9, 42.8 (br, carboranyl C), 58.5, 124.2, 131.3, 141.7. 11 1 B{ H} NMR (CD3COCD3): δ −34.5, −33,1, −20.8, −16.6, −14.8, −10.5. HRMS (m/z): z = 2, C22H42B18 calculated, 250.7834; observed, 250.6472. Mp: 112−119 °C. IR (KBr, cm−1): 2523, 2864, 2915. Route 1: Connecting the nido-Biscarborane Compounds Containing the Linkers via π-Coordination with Metals. nido-

Bis(carboranes) 2−4 were taken up in 3 mL of THF and cooled to −78 °C. n-Butyllithium (4.1 equiv with respect to nido-bis(carborane)) was added, and the reaction mixture was brought to room temperature and stirred for 3 h. Co(acac)2 or Ni(acac)2 (3 equiv with respect to nido-bis(carborane)) dissolved in 2 mL of THF was added and the mixture stirred for 1 h. It was further heated to 65 °C and maintained at that temperature for 24 h. The reaction mixture was filtered and passed through a silica plug and the filtrate was rotary-evaporated to remove the solvent. Characterization is given in the Results and Discussion. Route 2: Pd-Catalyzed Cross-Coupling Copolymerization of Diidodo Derivative 1 with Zinc Derivatives of Linker Molecule 8. 1,4-Dibutyl-2,5-diethynylbenzene (0.04 g, 0.18 mmol) was dissolved in tetrahydrofuran (2 mL) and cooled to −78 °C. Lithium hexamethyldisilazide (0.4 mL, 1 M solution in THF) was added over 10 min. The reaction mixture was brought to 0 °C and stirred for 30 min. The reaction mixture was further cooled to −78 °C, and ZnBr2 (0.09 g, 0.38 mmol) dissolved in 2 mL of THF was added to it and was brought to room temperature and stirred for 3 h to make bisorganozinc reagent 8. Diiodocobalt bis(dicarbollide) (1) as its tetrabutylammonium salt (0.12 g, 0.15 mmol) and Pd(PPh3)4 (0.01 g, 0.01 mmol) were added to the bisorganozinc reagent and refluxed for 24 h. Another lot of freshly prepared 8 (starting with 0.09 mmol of 1,4-dibutyl-2,5-diethynylbenzene) with Pd(PPh3)4 (0.01 mmol) was added and refluxing was continued for another 24 h period. Then the reaction mixture was filtered and washed with acetone. The filtrate was passed through a silica plug and was rotary-evaporated. The solid obtained was further dialyzed with 12−14k dialysis bags. The yield of polymer retained in the bag varies from 36 to 48 mg between batches. For the end-group analysis, the reaction mixture at the end of polymerization was quenched with excess SiMe3Cl. The workup of the polymer remained the same.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00578. NMR data for intermediates and NMR and DLS characterization of oligomers as well as polymers (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for M.F.H.: [email protected]. ORCID

Thomas A. Everett: 0000-0003-1322-759X Satish S. Jalisatgi: 0000-0001-7477-6344 Present Address †

K.R.P.: Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for this research was provided by the University of Missouri-Columbia. REFERENCES

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