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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Regioselective Functionalization of the [closo-1-CB9H10]− Anion through Iodonium Zwitterions Remigiusz Ż urawinś ki,†,‡ Rafał Jakubowski,†,‡ Sławomir Domagała,§ Piotr Kaszynś ki,*,†,‡,∥ and Krzysztof Woźniak§ †

Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Łódź, Poland Department of Chemistry, Middle Tennessee State University, Murfreesboro, Tennessee 37130, United States § Department of Chemistry, University of Warsaw, 02-089 Warsaw, Poland ∥ Faculty of Chemistry, University of Łódź, 91-403 Łódź, Poland

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‡

S Supporting Information *

ABSTRACT: Reactions of [closo-1-CB9H9-1-R]− (2, R = H, COOH, C5H11) with PhI(OAc)2 lead to mixtures of regioisomers [closo-1-CB9H8-1-R-6-IPh] (5[6]) and [closo-1-CB9H8-1-R-10-IPh] (5[10]) in ratios of ∼3:1 to 1:1, of which the former isomer undergoes selective reactions with nucleophiles (MeCN, pyridine, MeC(NH)NH2, CN−). The products and the unreacted 10-isomers 5[10] are separated achieving kinetic resolution of the isomeric iodonium zwitterions. Pure 5[10] is reacted with nucleophiles (pyridine, 4C7H15OPyridine, Me2NCHS, PhCO2−, CN−, N3−, I−, MeC(NH)NH2, and MeCN), giving substitution products. The mechanism of the substitution is investigated with density functional theory (DFT) methods. Some of the nucleophilic substitution products are transformed further, expanding the scope of available functional groups for the [closo-1-CB9H10]− anion. Four derivatives are characterized with single-crystal XRD methods: [closo-1-CB9H9-10-N2] (4[10]a), [closo-1-CB9H9-6-NC5H5] (9[6]a), [closo-1-CB9H9-10-NC5H5] (9[10]a), and [closo-1-CB9H9-10-NHC(NH2)Me] (10[10]a). Spectroscopic data for selected derivatives are interpreted in terms of transmission of electronic effects through the {closo-1-CB9} cluster (NMR) and interaction with substituents (IR, UV). The latter results are compared to those of TD-DFT computational methods.

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used in materials design,18 partially because of its better availability for several decades, when compared to that of the smaller 10-vertex anion 2a. This situation changed in 2000 with the discovery of a new and convenient route to [closo-1CB9H10]− (2a) via the Brellochs reaction,19 which, after optimization by Kennedy,20,21 Sivaev,22 and Janoušek23 made 2a more readily available than its analogue 1a.24,25 Anions 1a and 2a exhibit significant differences in regioselectivity of electrophilic substitution1 which, in large part, are at the root of the lesser interest in the [closo-1CB9H10]− (2a) anion. While [closo-1-CB11H12]− (1a) predominantly yields the B(12) isomer (∼75%−85%), the 10vertex analogue 2a gives essentially only the B(6) substitution products. Even with I+, which is a mild electrophile, only 2% of the antipodal B(10) isomer is formed.26 The antipodal substitution in these clusters is important for certain applications such as liquid crystals,17 which require elongated molecular shapes. Generally, the lack of simple access to a variety of 1,10-disubstituted derivatives hampers the broader use of anion 2a in material and biological sciences.

INTRODUCTION closo-Carbaborates1 [closo-1-CB11H12]− (1a, see Figure 1) and [closo-1-CB9H10]− (2a) are of increasing interest as key components of ionic liquids,2,3 solid-state electrolytes,4−6 and polar7−12 and ionic13−15 liquid crystals.16,17 The chemistry of the former anion (1a) is better developed and more frequently

Figure 1. Structures of [closo-1-CB11H12]− (1a) and [closo-1CB9H10]− (2a) anions and the main regioisomers formed upon electrophilic substitution. Each vertex represents a BH group and the sphere is a carbon atom. © XXXX American Chemical Society

Received: June 20, 2018

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DOI: 10.1021/acs.inorgchem.8b01701 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry To remedy this problem, we developed the first practical method for the preparation of isomerically pure 1,10disubstituted derivatives of anion 2a (Figure 2) about a

isomers [closo-1-CB9H8-1-COOH-6-IPh] (5[6]b) and [closo1-CB9H8-1-COOH-10-IPh] (5[10]b) with warm MeCN, in which the former selectively reacted, while the pure antipodal analogue 5[10]b was isolated in an overall yield of 25%, based on the starting acid [closo-1-CB9H9-1-COOH]− (2b).30 The subsequent reaction of 5[10]b with 4-heptyloxypyridine gave [closo-1-CB9H8-1-COOH-10-(NC5H4OC7H15-4)] (6[10]b) in six steps and an overall yield of ∼9%, based on B10H14, which is a shorter and more-efficient (by a factor of >4) sequence than that reported previously.29,31 This result suggested that the iodonium zwitterion method is promising for regioselective functionalization of the [closo-1-CB9H10]− (2a) and significant expansion of the types of available substituents. Herein, we describe full details of regioselective substitution of the parent anion [closo-1-CB9H10]− (2a) and, to a lesser extent, its two C(1) derivatives, known [closo-1-CB9H9-1COOH]− (2b) and new [closo-1-CB9H9-1-C5H11]− (2c), via phenyliodonium zwitterions with a range of nucleophiles (pyridine, 4-C7H15OPyridine, Me2NCHS, PhCO2−, CN−, N3−, I−, MeC(NH)NH2, and MeCN), and some functional group interconversion in the resulting products to NH2, N2, NHCONHPh, NHCSNHAr, (CH2)5S, NHCOCH(Me)NHBoc. We compare the reactivity and efficiency of two leaving groups, N2 and PhI, in nucleophilic substitution of the parent anion 2a. The synthetic work is supplemented with single-crystal XRD analysis, analysis of electronic effects, UV spectroscopy, and mechanistic considerations for selected derivatives augmented with DFT computational results.

Figure 2. Previous preparation of isomerically pure 1,10-disubstituted derivatives of the [closo-1-CB9H10]− anion through nucleophilic substitution and functional group transformations (FGT) (see refs 27−29). In the numbering scheme, the first number corresponds to the boron substituent, the number in parentheses specifies the position of the B-substitution, and the letter corresponds to the C(1) substituent.

decade ago.27 The procedure was a multistep process and relied on the separation of a mixture of isomers formed during the rearrangement of derivatives of [closo-2-CB9H10]− that was first observed by Kennedy.20 Although the method was moderately efficient, the initially obtained acid 3[10]b provided a convenient starting point to a variety of functional groups through transformations of the COOH and I substituents (see Figure 2).13,16,28 The iodine was also converted to the dinitrogen group in 4[10]b, which offered another convenient synthetic handle through nucleophilic displacement reactions.28 While this work was a step in the right direction and permitted the development of new materials,17 it still lacked the efficiency29 and scope of available substituents already developed for the 12-vertex analogue 1a. Recently, we described a simple and efficient pathway to zwitterionic aryliodonium closo-borates, including anions 1a and 2a, and demonstrated their facile transformation to a range of derivatives through a nucleophilic displacement of the iodoarene (Figure 3).30 We also found that ArI(OAc)2 exhibits

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RESULTS Kinetic Separation of Iodonium Isomers: Isolation of [closo-1-CB9H8-1-R-10-IPh] (5[10]). Reactions30 of [closo-1CB9H9-1-R]− (2, R = H, COOH, C5H11) with PhI(OAc)2 in aqueous CF 3 COOH gave mixtures of two iodonium regioisomers, 5[6] and 5[10], typically in isolated yields of 95% (see Scheme 1). The ratio of the products vary from ∼3:1 for 2b to 1:1 for 2c, in response to electronic effects of the C(1) substituent. Their differential thermal stability was used to selectively transform the 6-iodonium isomer 5[6]b in warm MeCN (55 °C) and isolate the pure 10-iodonium derivative 5[10]b in an overall yield of 25%, based on anion 2b (see Scheme 1. Generation and Selective Transformations of Isomeric Iodonium Zwitterions 5[10] and 5[6]a

Figure 3. Functionalization of closo-borates. X = B (n = 2), X = C (n = 1) (see ref 30).

the highest regioselectivity in electrophilic substitution of the anions and, in reactions with anion 2a, gives rise to an ∼1:2 ratio of the 10- vs 6-substituted products, [closo-1-CB9H9-10IPh] (5[10]a) vs [closo-1-CB9H9-6-IPh] (5[6]a). This suggested an opportunity to access a broad range of 10substituted derivatives through a direct electrophilic substitution of the cluster, followed by reactions with nucleophiles. Preliminary experiments indicated that, unlike isomers of other clusters (including 1a), isomers 5[10]a and 5[6]a could not be separated by chromatography, but they exhibited a substantial difference in thermal stability, allowing for selective reactions.30 Indeed, this was demonstrated for a mixture of

Reagents and conditions: (i) PhI(OAc)2, 70% TFA/H2O, 0 °C, 1 h; (ii) Nu, MeCN, temperature = 0−50 °C, 2−12 h; and (iii) separation.

a

B


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Inorganic Chemistry Table 1. Yields for Isolated Products of Reaction of Isomeric Mixture of 5[10] and 5[6] with a Nucleophilea

a

Yields based on the starting anion 2 after chromatographic separation. See Scheme 1. bNot isolated. cData taken from ref 30. dIsolated after hydrolysis as [closo-1-CB9H8-1-COOH-6-NH3] (8[6]b).

Scheme 2. Preparation of [closo-1-CB9H9-1-C5H11]− [Et4N]+ (2c-[Et4N])a

Table 1). During the reaction, the 5[6]b isomer was converted to the 6-acetonitrilium zwitterion [closo-1-CB9H8-1-COOH-6NCMe] (7[6]b), which, upon acidic hydrolysis, gave amino acid [closo-1-CB9H8-1-COOH-6-NH3] (8[6]b) in 54% overall yield.30 This method was used to isolate the isomerically pure 5[10]a in an overall unoptimized yield of 22%, based on the parent anion 2a (Table 1) or ∼9%, based on B10H14.32 The regioisomeric iodonium zwitterions also demonstrate significant differences in reactivity toward strong nucleophiles in weakly nucleophilic solvents, which can be used for selective transformations of one isomer into the desired derivative in the presence of another. Thus, the reaction of a mixture of 5[6]a and 5[10]a with pyridine (1.8 equiv) and acetamidine (1.2 equiv) in MeCN at ambient temperature cleanly gave substitution products 9[6]a and 10[6]a, respectively, in yields of 54%. The unreacted 10-iodonium zwitterion 5[10]a was separated by chromatography in ∼30% overall yield, based on anion 2a (see Table 1). A similar reaction of a mixture 5[6]c and 5[10]c with [Et4N]+CN− (0.45 equiv) in MeCN at 0 °C gave nitrile 11[6]c and unreacted 10-iodonium zwitterion 5[10]c in 42% and 48% yield, respectively, based on the anion 2c. It is noteworthy that the rates of the reactions of 5[6] can be ranked in the following order: CN− (1 h at 0 °C) > acetamidine (1.5 h, room temperature (rt)) > pyridine (20 h, rt), which reflects the relative nucleophilicity of the reagents. The requisite pentyl derivative 2c was obtained in 71% yield by alkylation of the parent anion salt 2a[Cs] under typical conditions (see Scheme 2). The product was isolated as the

Reagents and conditions: (i) 1. n-BuLi, THF, −10 °C → rt. 2. C5H11I, rt. (ii) [Et4N]+Br−, 71% yield. a

[Et4N]+ salt by selective precipitation and separation from small amounts of unreacted anion 2a by chromatography, followed by recrystallization. Transformations of [closo-1-CB 9 H 8 -1-R-10-IPh] (5[10]). As demonstrated above, 10-phenyliodonium zwitterions 5[10] are significantly less reactive than the 6-analogues (5[6]) and require higher temperatures for effective conversion with nucleophiles. Similar to that observed with the isomeric mixtures of iodonium zwitterions, reactions of 5[10] can be conducted either with neat nucleophiles or with strong nucleophiles in MeCN solutions. The former is particularly effective for weakly nucleophilic reagents, such as MeCN, thian,30 and Me2NCHS. For instance, reactions of 5[10]a with pyridine and Me2NCHS at 45 and 80 °C, respectively, gave the expected derivatives 9[10]a and 12[10]a, respectively, as the only products isolated in yields of ∼90% (see Scheme 3). C


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Inorganic Chemistry

Scheme 5. Synthesis of 10-Pyridinium Derivatives 6[10]a

Scheme 3. Reactions of [closo-1-CB9H9-10-IPh] (5[10]a) with Nucleophiles

a

Reagents and conditions: (i) 4-C7H15OC5H4N, 80 °C, 6 h.

Scheme 6. Synthesis of 10-Iodo Derivative 3[10]c-[Et4N]a

a Reagents and conditions: (i) 1. n-BuLi, THF, − 5 °C, 1 h, 55%; 2. [Et4N]+Br−; (ii) [Et4N]+I−, MeCN, 60 °C, 97%.

Reactions of iodonium zwitterion 5[10]a with strong nucleophiles were conducted in MeCN solutions using small excess of reagents. Thus, reactions were conducted typically with 1.2−1.7 equiv of CN−, PhCOO−, N3−, or acetamidine (4 equiv), either at ambient temperature (CN−, N3−) or at 40−60 °C (acetamidine, PhCOO−), to give the expected products 11[10]a, 14[10]a, 10[10]a, and 13[10]a, respectively, in isolated yields of ∼90% (see Scheme 3). On the other hand, similar reactions with L-leucine benzyl ester, glycine benzyl ester, and D-1-phenylethylamine (MeCN, 40 °C, 16−24 h) did not lead to the formation of the desired products; while unreacted 5[10]a was recovered in the first two cases, the amine caused partial (∼10%) decomposition of the iodonium zwitterion. The effectiveness of the phenyliodonium zwitterion 5[10]a in nucleophilic substitution was compared to that of the dinitrogen analogue 4[10]a under identical conditions (solvent, concentration, temperature). Thus, reactions of 4[10]a with [Et4N]+CN− (ambient temperature, 48 h) and PhCO2−[Me4N]+ (40 °C, 5 h) resulted in the formation of complex mixtures of products (see Scheme 4). In the reaction

Surprisingly, the resulting iodo derivative 3[10]c was contaminated with 7% of [closo-1-CB9H8-1-C5H11-10-Ph]− anion, on the basis of 1H NMR and MS analyses. The pure salt 3[10]c-[Et4N] was isolated in 55% yield by extraction of the byproduct from the mixture of the [Et4N]+ salts with Et2O, followed by recrystallization. The second method turned out to be much more effective. Reaction of iodonium zwitterion 5[10]c with [Et4N]+I− (1.5 equiv) in MeCN gave the 10-iodo derivative 3[10]c-[Et4N] as the only product isolated in 97% yield. Transformations of [closo-1-CB9H9-6-Nu] and [closo1-CB9H9-10-Nu]. To expand the scope of substituents available for anion 2a, some of the functional groups obtained directly from the iodonium zwitterions were transformed to other substituents. Thus, the acetamidinium derivatives 10[10]a and 10[6]a were hydrolyzed under basic conditions to the corresponding amines 8[10]a and 8[6]a, respectively (see Schemes 7 and 8), demonstrating a convenient way to Scheme 7. Transformations of Amidinium 10[10]aa

a

Scheme 4. Reaction of 10-Dinitrogen Derivative 4[10]a

Reagents and conditions: (i) [Et 4 N] + CN − , MeCN, rt or [Me4N]+PhCO2−, MeCN, 40 °C.

a

Reagents and conditions: (i) KOH, MeOH, 60 °C, 16 h, 79% yield; (ii) NO+PF6−, pyridine, 65% yield.

a

PhCO2−

anion, no desired product 13[10]a was with the detected, whereas, in the reaction with the CN− anion, the expected product 11[10]a was a minor component, according to 11B NMR analysis. The B(10) substitution method through iodonium zwitterions developed for the parent anion 2a was extended to the C(1)-substituted derivatives used as precursors to liquid crystals. Thus, the reaction of C(1) carboxylic acid 5[10]b and C(1)-pentyl derivative 5[10]c with neat 4-heptyloxypyridine gave the desired products 6[10]b30 and 6[10]c, respectively, in high isolated yields (see Scheme 5). Finally, the formation of a 10-iodo derivative of 2a was demonstrated with two methods (see Scheme 6). Thus, following a procedure recently developed33 for the preparation of [closo-B10H8-1,10-2I]2−, 10-iodonium zwitterion 5[10]c was treated with n-BuLi in THF at −5 °C (see Scheme 6).

introduce this important functionality to the {closo-1-CB9} cluster. Interestingly, hydrolysis using 25% HCl (100 °C, 8 h) was ineffective and acetamidinium 10[10]a was fully recovered. Alternative methods for generation of 10-amino derivative 8[10]a via the reduction of [closo-1-CB9H9-10-N3]− (14[10]a), using either H2/Pd(C) in THF (25 °C) or Ph3P in MeCN (65 °C), were unsuccessful and the azide was fully recovered.32 Treatment of amine 8[10]a with NO+PF6− in pyridine, conditions previously used for generation of 4[10]b,28 gave the parent 10-dinitrogen derivative 4[10]a in 65% yield (see Scheme 7). Generation of the B(6) analogue of 4[10]a, 6dinitrogen [closo-1-CB9H9-6-N2] (4[6]a) was not attempted, D


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Inorganic Chemistry Scheme 8. Transformations of Amidinium 10[6]aa

Among the four investigated derivatives, three (9[6]a, 9[10]a, and 10[10]a) crystallize with one unique molecule in the asymmetric unit of the crystal lattice. The only exception is the 10-dinitrogen derivative 4[10]a, with the asymmetric part of the unit cell containing two halves of unique molecules.32 In this case, the first molecule of 4[10]a is located on a 2-fold rotation axis, while the second one is situated on a mirror plane. Thermal ellipsoid representations of molecules of investigated derivatives are shown in Figure 4. Derivatives 4[10]a and 9[6]a crystallize in the orthorhombic Pbcm and Pbca space groups, respectively, while compounds 9[10]a and 10[10]a crystallize in the monoclinic P21/n space group. Analysis of crystals packing indicates some specific features for each investigated derivative. Thus, neighboring molecules of 10-pyridinium 9[10]a are arranged in the crystal network in the characteristic zigzag manner with their molecular axes inclined by ∼80° (Figure 5). In contrast, the adjacent molecules of 6-pyridinium 9[6]a are antiparallel or inclined (considering the angle between mean-quadratic planes of non-hydrogen atom of the adjacent molecules) by ∼43° (Figure 5). In the crystal structure of the 10-dinitrogen 4[10]a, the molecular axes of the adjacent unique molecules are oriented to each other by an angle of 71.2°, which results in an overall criss-cross molecular packing.32 Finally, the molecules of acetamidinium 10[10]a are antiparallel or inclined to themselves by a relatively small angle of 38°. The intracage dimensions for all four derivatives, such as C− B and B−B bond distances and angles, are typical for {closo-1CB9} derivatives. Analysis of the data in Table 2 shows that cage dimensions are sensitive to the nature of the substituent at the B(10) position: upon increasing the electron-withdrawing character of the substituent, the distance between equatorial boron atoms in both belts is expanding and, to a smaller extent, also between the belts. At the same time, the height of the square pyramids decreases especially for the B(10)···B(6−9): from 1.090 Å in 2a to 1.022 Å in 4[10]a. Consequently, this results in the overall contraction of the cage, as measured by the C(1)···B(10) separation (3.529 Å in 2a to 3.448 Å in 4[10]a). The trend observed for the parent anion 2a parallels that for the 10-substituted derivatives of carboxylic acid [closo1-CB9H9-1-COOH]− (2b),21,28,32 and is similar to trends observed for derivatives of other clusters, such as 1a and [closoB10H10]2−.33 Interestingly, the substitution of the antipodal position with a −COOH complex in 4[10]b and 9[10]b has essentially no effect on the exocage interatomic distances.32 Further analysis of the molecular structures demonstrates that the pyridine ring in both derivatives 9[10]a and 9[6]a adopts an almost-eclipsed conformation, with respect to the cage (17° and 11°, respectively) and the B−N distance is shorter in the former (1.526(1) Å, Table 2) than in the 6pyridinium analogue (1.549(1) Å). These structural features are similar to those observed for the analogous 1-carboxylic acid derivatives 9[10]b and 9[6]b.28,34 Analysis of the molecular structure of the acetamidinium derivative 10[10]a revealed that the CH3 group exhibits rotational disorder in two positions with an occupancy ratio of 0.56(4)/0.44(4). The acetamidinyl group is planar, within the experimental error (see Figure 4), and adopts a pseudostaggered orientation, with respect to the cage (33°). The additional proton is located on the N(2) atom allowing for a full delocalization of the positive charge of the zwitterion (see Scheme 7 for the structure). This is evident from an almostequal length of both C−N bonds of ∼1.30 Å.

Reagents and conditions: (i) KOH, MeOH, 60 °C, 16 h, 76% yield; (ii) Et(i-Pr)2N, PhNCO, THF, rt, 4 h, 92% yield; (iii) NaOH, [Et4N]+Br−; (iv) Boc-Ala-OH, DCC, THF, rt, 18 h.

a

because of its expected low stability, as was previously demonstrated for 4[6]b.34 The amino group in 8[6]a was investigated as a synthetic handle to introduce the {closo-1-CB9} cluster into organic molecules. Thus, reaction of 8[6]a with phenyl isocyanate in the presence of EtN(i-Pr)2 (Hunig’s base) gave the urea derivative 15[6]a in 92% yield (see Scheme 8). A similar reaction of deprotonated 8[6]a with 3,5-(CF3)2C6H3NCS, an aryl isothiocyanate, gave the thiourea derivative 16[6]a in 69% isolated yield after 15 h at 40 °C. Attempted derivatization of 8[6]a with amino acids under standard conditions was much less successful. Thus, a reaction of Boc-protected alanine with 8[6]a-[Et4N] in the presence of DCC gave product 17[10]a, which could not be completely purified. An attempt at reacting a methyl ester of protected alanine (Boc-Ala-OMe) with 8[6]a (MeCN, 40 °C, 7 d) gave no reaction. Finally, the thioformamide derivative 12[10]a was cycloalkylated with Br(CH2)5Br under previously described7,10 general hydrolytic conditions and the cyclic sulfonium zwitterion 18[10]a was isolated in 71% yield (see Scheme 9). Scheme 9. Transformation of [closo-1-CB9H9-10SCHNMe2] (12[10]a)a

a Reagents and conditions: (i) Br(CH2)5Br, [Me4N]+OH−·5H2O, MeCN, 70 °C, 12 h, 71% yield.

Molecular and Crystal Structures. Colorless crystals of pyridinium 9[6]a and 9[10]a were obtained by slow evaporation of MeCN/hexane solutions, while crystals of 10dinitrogen 4[10]a and acetamidinium 10[10]a were grown from CH2Cl2/hexane solutions and their solid-state structures were determined by low-temperature single-crystal X-ray diffraction (XRD) analysis.32 Results are shown in Table 2 and in Figures 4 and 5, and details of data collection and analysis are provided in the Supporting Information. E


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Inorganic Chemistry Table 2. Selected Interatomic Distances and Angles for Selected Derivativesb bond distances (Å) C(1)−B(2) avg C(1)···B(2−5)d B(2)−B(3) avg B(2)−B(6) avg B(6)−B(7) avg B(6)−B(10) avg B(10)···B(6−9)d B(10)−N C(1)···B(10) bond angles (deg) B−C(1)−H B−B(10)−X

2a[Q]a,c

10[10]a

9[10]a

4[10]ac

1.598(4) 0.932(1) 1.835(3) 1.805(6) 1.839(7) 1.697(5) 1.090(3) 3.529

1.603(1) 0.932 1.845(6) 1.806(4) 1.846(3) 1.698(4) 1.086 1.5031(9) 3.523

1.604(2) 0.929 1.849(4) 1.809(2) 1.855(5) 1.6869(5) 1.059 1.526(1) 3.494

1.602(2) 0.923(1) 1.853(5) 1.810(2) 1.877(3) 1.675(1) 1.022(1) 1.494 3.448

125.7(33) 130.0(2)e

125.5(3) 129.7(22)f

125.4(6) 128.9(8)f

125.2(7) 127.6(12)f

a Q = [DME]3Mg2+ (see ref 6). bExcept for unique in each molecule distances B(10)−N and the cage size C(1)···B(10), all parameters are average values and the esd refers to the distribution of the measured values. cTwo molecules. dHeight of the square pyramid. eX = H. fX = N.

Figure 4. Atomic displacement ellipsoid representation of 4[10]a (molecule A), 9[6]a, 9[10]a, and 10[10]a. Pertinent geometrical dimensions for 9[6]a: B(6)−N(1), 1.549(1) Å; B(6)−B(10), 1.696(2) Å; B(7)−B(10), 1.705(2) Å; B(8)−B(10), 1.692(2) Å; B(9)−B(10), 1.704(2) Å; C(1)− B(2)avg, 1.605 Å; C(1)···B(10), 5.546(2) Å; B(6)−N(1)−C, 121.02(8)°; B(10)−B(6)−N(1)−C, − 10.8(1)°. For selected geometrical parameters of other derivatives, see Table 2. The corresponding ellipsoids are at the 50% probability level, and the numbering system is based on the chemical structure.

Figure 5. Partial packing diagram for 9[10]a (left) and for 9[6]a (right). Thermal ellipsoid diagram drawn at 50% probability.

Mechanistic Considerations. Two mechanisms for nucleophilic replacement of the PhI group in 5[6]a and 5[10]a with CN− and pyridine were investigated at the M06-

2x/6-31+G(2d,p)//M06-2x/6-31G(2d,p) level of theory in MeCN dielectric medium, and the results were compared to those of intramolecular rearrangements of the two iodonium F


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Inorganic Chemistry

Figure 6. Possible pathways for transformation of iodonium zwitterions 5[6]a and 5[10]a with a nucleophile and thermodynamic parameters (kcal mol−1) for selected steps obtained with the M06-2x/6-31+G(2d,p)//M06-2x/6-31G(2d,p) model in MeCN dielectric medium.

rearrangement of the 10-I-3 intermediate 5[10]a-Nu. In contrast, similar calculations for the 6-isomer show that the ylide pathway may compete with the 10-I-3 mechanism: while the process is still significantly endothermic (ΔH = +29.0 kcal mol−1), the change in free energy, ΔG298, is only +17.6 kcal mol−1, which is less by 1 kcal mol−1 than ΔG⧧298 for 5[6]aPyrTS but is 2.3 kcal mol−1 more than ΔG⧧298 for 5[6]aCN-TS (see Figure 6). Thus, for strong nucleophiles, substitution of 5[6]a likely proceeds via the 10-I-3 mechanism, whereas for less-nucleophilic reagents, such as pyridine and MeCN, the ylide pathway is possible. Calculations also show that the intramolecular rearrangement of iodonium zwitterions 5[n]a through 5[n]a-TS (Figure 6) and the formation of B-arylated products requires significantly higher activation energies, ΔH⧧ = 23.7 kcal mol−1 (ΔG⧧298 = 23.7 kcal mol−1) for 5[6]a-TS and ΔH⧧ = 26.9 kcal mol−1 (ΔG⧧298 = 28.2 kcal mol−1) for 5[10]a-TS, and it does not compete with substitution through the 10-I-3 intermediate. NMR Spectroscopy. The availability of a relatively broad range of substituents at the B(10) position allows for another glimpse into the transmission of electronic effects through the {closo-1-CB9} cage.36 A correlation of 1H NMR chemical shifts (δ) of the hydrogen atoms antipodal to the substituent in [closo-1-CB9H9-10-X]− (in CD3CN) and in Ph−X (in DMSOd6) revealed a linear correlation for six pairs of derivatives (r2 = 0.974), while data points for derivatives containing the sulfonium and phenyliodonium groups significantly deviate from this correlation (Figure 7). A plot of δC(4)H vs Hammett parameters37 σp for a series of Ph−X derivatives demonstrated that the C(4)−H chemical shift for PhMe2S+ fits the correlation, and the anticipated σp value for the PhI+ substituent is 0.77.32 A similar analysis of C(1)−H chemical shifts for [closo-1-CB9H9-10-X]− yields an σp value of 1.2 ± 0.1 for the sulfonium and 1.5 ± 0.1 for PhI+.32 Thus, results in Figure 7 suggest a significantly greater electron-withdrawing

zwitterions (see Figure 6). According to the rearrangement mechanism35 of the 10-I-3 intermediate (10-electron 3coordinated iodine atom), the nucleophile is initially coordinated to the zwitterion 5[n]a forming adduct 5[n]a− Nu (a λ3 iodane). The process is slightly more exothermic for the CN− adduct than for the pyridine complex, and for the 5[10]a isomer than for the 5[6]a isomer (see Figure 6). Because of a decrease in entropy, the formation of 5[n]a−Nu is moderately endergonic (approximately +7 kcal mol−1). The observed higher exotherm for the formation of the adduct with CN− than with pyridine is presumably due to the charge of the nucleophile and is reflected in the Nu···I distance in the complex: 2.43 Å in 5[n]a−CN vs ∼2.85 Å in 5[n]a−Pyr. The 10-I-3 adduct, 5[n]a-Nu, subsequently rearranges to the substitution product through the transition structure 5[n]aNuTS and expulsion of PhI as the leaving group (Figure 6). The calculated activation energy is lower for the 6-substituted derivatives, by 3.0 kcal mol−1 for the pyridine adduct (5[6]aPyr) and 4.3 kcal mol−1 for the CN− adduct (5[6]a-CN), than for the corresponding 10-isomers. In accord with experimental observations (vide supra), activation energies are lower for the CN− adduct than for formation of the pyridine adduct by at least 2 kcal mol−1. The entire nucleophilic substitution process in 5[n]a is highly exothermic: by ∼45 kcal mol−1 for the reaction of 5[n]a with pyridine, and ∼52 and 63 kcal mol−1 for reactions of 5[6]a and 5[10]a, respectively, with the CN− anion (see Figure 6). A pathway alternative to substitution through the 10-I-3 intermediate involves dissociation of the zwitterion 5[n]a, formation of a boronium ylide 19[n]a, and its subsequent trapping with a nucleophile (see Figure 6). Density functional theory (DFT) calculations in the MeCN medium demonstrate that the formation of ylide 19[10]a is significantly endothermic (ΔH = +36.9 kcal mol−1 and ΔG298 = +26.0 kcal mol−1) and therefore much less favorable than the G


Article

Inorganic Chemistry

(9[6]a) and also 10-dinitrogen (4[10]a) derivatives provides an opportunity to investigate their electronic structures and compare them to the previously described 1-isomers 9[1]a and 4[1]a.43 All three pyridinium derivatives 9[n]a display a single absorption band located at ∼270 nm and each exhibits a vibronic splitting of ∼800 cm−1 (Figure 8a). The excitation

Figure 7. Correlation of 1H NMR chemical shifts (δ) for the C(1)−H hydrogen in selected derivatives [closo-1-CB9H9-10-X]− (in CD3CN) and for the C(4)−H hydrogen in Ph−X (in DMSO-d6). The best fit line for six dots: δC(4)H = 4.6(2) + 0.59(5) × δC(1)H; r2 = 0.974. The outlying square data points are circled in the figure.

effect of the R2S+ and PhI+ substituents in the {closo-1-CB9} derivatives than in the benzene analogues for reasons presently unclear. The slope of 0.59(5) for the correlation in Figure 7 (excluding data for X = R2S+ and PhI+) indicates that the electronic effects are transmitted through the {closo-1-CB9} cage at ∼60% of that in benzene derivatives. This result is fully consistent with our previous findings for dissociation constant in a series of B(10) substituted acids [closo-1-CB9H8-1COOH-10-X]−, which indicated 56% efficiency of transmission of electronic effect through the {closo-1-CB9} cage, relative to benzene.36 IR Spectroscopy. Electronic interactions between the {closo-1-CB9} anion and substituents were probed using IR spectroscopy in three derivatives: 4[10]a, 11[10]a, and 13[10]a, which contained groups at the B(10) position that were distinctively IR-active. Data in Table 3 indicates that the

Figure 8. Electronic absorption spectra: (a) for pyridinium derivatives 9[1]a (red), 9[6]a (blue), and 9[10]a (black); and (b) for dinitrogen derivatives 4[1]a and 4[10]a in MeCN.

Table 3. Characteristic IR Stretching Vibrations of Selected Functional Groups in [closo-1-CB9H9-10-R]−, Ph−R, and tBu-R

energies are similar for 9[10]a and 9[6]a, and they are slightly lower for 9[1]a (Table 4), which also has the largest calculated ground-state dipole moment (μGS = 19.5 D in MeCN). The

IR Stretching Vibration, ν (cm−1) substituent R +

N2 CN OCOPh

{1-CB9}−Ra

Ph−R

t-Bu-R

2283 2201 1695

2297b 2228c 1731e

2235d 1714f

Table 4. Selected Experimental and Calculated Electronic Transition Energies and Oscillator Strength Values Experimentala

a

Obtained for neat samples. bKBr: data taken from ref 38. cFilm; data taken from ref 39. dFilm, data taken from ref 41. eKBr, data taken from ref 40. fFilm; data taken from ref 42.

NN, CN, and CO bonds in the {closo-1-CB9} derivatives are weaker than those for the analogous benzene derivatives and t-Bu derivatives, which suggests π-interactions between the cluster and the substituent and negative charge delocalization. This is consistent with trends in 4-substituted benzenediazonium (νNN),38 benzonitrile (νCN),39 and phenyl benzoate (νCO)40 derivatives, in which electrondonating substituents reduce the bond order through the resonance effect and shift the position of the stretching vibration band to lower wavenumbers. Electronic Absorption Spectra. The availability of the parent isomers 10-pyridinium (9[10]a) and 6-pyridinium

Theoreticalb

compound

λmax (nm)

log ε

π →π*

f

4[1]a 4[10]a 9[1]a 9[6]a 9[10]a 20[1]a

250.0c 215.0 273.5c,d 264.0d 265.5d [255]e [224]e [250]e [224]e

3.82 4.30 3.81 3.87 4.01

232.7 204.5 246.3 237.2 243.4 233.8 210.6 230.3 211.1

0.310 0.778 0.243 0.255 0.315 0.068 0.153 0.067 0.236

20[12]a

a

Recorded in MeCN. bObtained with the TD CAM-B3LYP/6-31+ +G(2d,p)//M062x/6-31G(2d,p) method in MeCN dielectric medium. cData taken from ref 43. dThe middle peak of the vibronic band. ePredicted based on trends in the {closo-1-CB9} derivatives; see the Supporting Information. H


Article

Inorganic Chemistry

Similar calculations for pyridinium derivatives of 1a, zwitterions [closo-1-CB11H11-1-NC5H5]− (20[1]) and [closo1-CB11H11-12-NC5H5]− (20[12]) revealed that the excitation energies are higher than those for the smaller {closo-1-CB9} analogues (see Table 4) by up to 1 eV, which is in agreement with trends for other pairs of such derivatives.9,44 The difference is mainly due to the lower energies of the highest occupied orbitals.

two dinitrogen derivatives 4[1]a and 4[10]a also exhibit a single absorption band above 200 nm, but in contrast to the pyridinium zwitterions, their excitation energies are significantly different (see Figure 8b, Table 4). The more-polar isomer 4[1]a (μGS = 9.5 D in MeCN) has a maximum absorption at 250 nm (4.96 eV), while the 4[10]a analogue (μGS = 4.1 D in MeCN) exhibits a more intense absorption band at λmax = 215 nm (5.77 eV). Similar to the pyridinium derivatives, however, the 10-isomer 4[10]a has a higher excitation energy and higher intensity of absorption than the C(1) analogue 4[1]a. Time-dependent density functional theory (TD-DFT) computational analysis of all five compounds in MeCN dielectric medium reproduced the experimental spectra and trends in excitation energies and also the relative intensities (Table 4). In most cases, the observed absorption band is due to a π−π* excitation from the HOMO, localized mainly on the {closo-1-CB9} cluster, to the LUMO, localized mainly on the substituent (Figure 9). Only in the case of 4[10]a the observed

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DISCUSSION Experiments demonstrated that the Brellochs reaction19,24 is efficient with aldehydes of moderate and enhanced electrophilicity, such as formaldehyde,23 glyoxalic acid,21 and aryl aldehydes, which have been used in the preparation of the parent anion 2a,23,25 carboxylic acid 2b,21 and C(1)−aryl derivatives,21,22,45,46 respectively. The preparation of the C(1)−alkyl derivatives can be accomplished efficiently by alkylation of the parent anion 2a. This was demonstrated before only for C(1) methylation,23 and now we described C(1) pentylation and the formation of 2c. Regioselective substitution of the readily available parent anion [closo-1CB9H10]− and its simple derivatives using the phenyliodonium zwitterion chemistry now opens access to a broad range of functional groups in positions B(6) and B(10). The formation of iodonium zwitterions of {closo-1-CB9} is exceptional: it occurs with the highest regioselectivity observed to date for the B(10) position in electrophilic substitution of this anion,20,26 even in the presence of the C(1)−COOH group,30 which is not the case for the [closo-1-CB11H11-1COOH]− (1b) acid.47 This offers a more facile and efficient synthetic route to isomerically pure 1,10-disubstituted derivatives of 2a than the previous method involving substitution of the {closo-2-CB9} derivatives followed by rearrangement and separation of isomers.20,27 Iodonium zwitterions 5[n] allow for the introduction of a variety of nucleophiles to the {closo-1-CB9} anion under mild conditions and highly controlled manner. Computational analysis supported with experimental observations demonstrates that substitution of the 5[10] zwitterionic isomers through rearrangement of the 10-I-3 adduct is clearly preferred kinetically and requires relatively low activation energies, dependent on the charge and nucleophilicity of the reagent. Another process, heterolysis of the B−I bond, requires higher energy and does not compete with the 10-I-3 rearrangement mechanism. Therefore, nucleophilic substitution in 5[10] is a clean and high yield reaction. In contrast, activation energies for the two substitution processes in the 6-isomers 5[6], the 10-I-3 rearrangement and B−I dissociation, are comparable and the latter probably occurs for weak nucleophiles. The observed differential reactivity of the two isomers, 5[10] and 5[6], is presumably due to the difference in electron density in the B−I bond. An intramolecular rearrangement through the 5[n]-TS structure requires sufficiently high activation energies such that they do not compete with the substitution process in both of the isomers. In this respect, the [closo-1-CB9H10]− anion and the [closo-B10H10]2−,30,33 are unique among closoboranes. In contrast, iodonium zwitterions of the [closoB12H12]2− dianion preferentially undergo intramolecular rearrangement rather than substitution.48 The nucleophilic substitution in the {closo-1-CB9} anion using iodonium zwitterions is much more efficient and has a broader scope than that observed using dinitrogen derivatives 4[n]. Previous experiments showed that the substitution of the

Figure 9. Contours and energies of molecular orbitals relevant to lowenergy excitations in 4[10]a, 9[6]a, and 9[10]a obtained at the CAM-B3LYP/6-31++G(2d,p)//M062x/6-31G(2d,p) level of theory. For 4[10]a, both degenerated MOs are shown.

band at 215 nm is due to the excitation from the HOMO−1 (Figure 9). Some predicted low-energy transitions have a calculated oscillator strength of f = 0, because of the high molecular symmetry (C4v), and are not observed experimentally. Analysis of the computational data shows that the origin of the different excitation energies in the pyridinium derivatives 9[n]a is related to the level of the LUMO (localized largely on the pyridine), which changes from −0.99 eV for 9[6]a to −1.47 eV for 9[1]a, while the HOMO remains almost the same at approximately −8.65 eV. In the dinitrogen derivatives 4[n]a, however, both active MOs have different energies: the HOMO is lower by 0.36 eV, while the LUMO higher by 0.44 eV in 4[10]a, relative to 4[1]a, which corresponds to a difference of 0.29 eV in the experimentally observed excitation energies. I


Article

Inorganic Chemistry −N2 group in 4[10] occurs presumably through heterolysis of the B−N bond and formation of the reactive boronium ylide 19[10] (see Scheme 10). This process is moderately effective

C(1)−N2 derivatives.13,43 The latter served as a precursor to the protected C(1)−SH group7,12,43 and as a reagent to an efficient diazocoupling reaction with phenol.13,43 The parent [closo-1-CB9H10]− anion (2a) was C(1)-alkylated,23 C(1)carboxylated51 and C(1)-halogenated.23 Until now, functionalities at the B(10) positions were derived from the [closo-1-CB9H8-1-COOH-10-I]− acid (3[10]b), in which the B(10)−I group was transformed to an alkyl12,13 and NH228,29 through the Pd-catalyzed coupling reactions. The latter was converted to the B(10)−N2,28 which served as a direct precursor to pyridinium,28,31 sulfonium,7 alkoxy,15 alkylsulfanyl,15 and protected mercaptan11,28,29 derivatives. The chemistry described here greatly simplifies and improves direct access to these B(10) derivatives and expands the scope of the functionalities to OCOR, CN, and N3. Also, access to functional groups in the B(6) position is expanded and now includes NH2, CN, and pyridine, available through nucleophilic displacement of the PhI, while electrophilic substitution provides OH20 and halogen26 at the B(6) position. It can be easily envisioned that sequential introduction of substituents using methods described above may lead to regiocontrolled polysubstitution and the formation of multifunctional derivatives of anion 2a. Finally, analysis of spectroscopic data for several derivatives of the {closo-1-CB9} anion revealed reasonably strong π−π interactions between the cage and substituents, which is consistent with our previous results for derivatives of 10-vertex closo-boranes.52,53 Such interactions facilitate transmission of electronic effects through the cage, estimated at ∼60% of that for benzene,36 and affect reactivity of the antipodal position. The latter has important consequences for regioselectivity of electrophilic substitution of the B(6) vs B(10) position of the {closo-1-CB9} cage. Thus, the presence of a weakly electrondonating pentyl group (σp = −0.15)37 at the C(1) position in 2c significantly increases the relative rate of formation of the B(10) phenyliodonium zwitterion 5[10]c by ∼2-fold, relative to the parent 2a, and pure 1,10-disubstituted derivative 5[10]c is isolated in a good yield of ∼48%. In contrast, substitution of the C(1) position in 2a with an electron-accepting COOH group (σp = +0.45)37 only moderately deactivates the B(10) position and product 5[10]b is formed in yields of ∼25%, relative to 29%−30% of the parent 5[10]a (see Table 1). This disproportionate electronic effect of an alkyl and carboxyl groups on the regioselectivity of substitution and also unexpectedly large effect of the sulfonium and phenyliodonium groups on NMR chemical shifts suggest that the {closo-1-CB9} is a different and nonconventional conduit of electronic effects, compared to classical π-systems, such as benzene. This warrants more-detailed investigation, which now becomes easier due to a better access to a variety of functional derivatives of the [closo-1-CB9H10]− anion.

Scheme 10. Precursors to Functional Derivatives of the [closo-1-CB9H10]− and Substitution Models

for neat reactions with small nucleophiles, such as solutions in pyridine or Me2NCSH, where the boronium ylide is trapped by the solvent.28 In contrast, reactions of the 4[1] isomer follow a different pathway: because of the strong electronaccepting nature of the N2+ group attached to the electronpoor C(1) atom, the C(1)−N2 group is an effective electron acceptor and undergoes a single electron transfer with nucleophiles, such as pyridine.43 Consequently, 4[1] reacts through a radical anion intermediate 21a (Scheme 10) and a mixture of products may be formed (e.g., with pyridine).43 It should be added that 4[1] undergoes efficient diazocoupling with phenols.13,43 Finally, the 6-dinitrogen isomers 4[6] are generally unavailable, because of their low thermodynamic stability,34 which makes the 6-iodonium zwitterions 5[6] particularly valuable intermediates in functionalization of this position in the [closo-1-CB9H10]− anion. The presented method for selective B−H bond activation toward nucleophilic substitution via iodonium zwitterions, in combination with the Brellochs method for construction of the {closo-1-CB9} skeleton, C(1)−H activation toward electrophilic substitution (via deprotonation), and classical electrophilic substitution selective for the B(6) position, constitutes a rich and powerful toolkit for the synthesis of a variety of functional derivatives of the [closo-1-CB9H10]− anion (2a). The scope and selectivity of these methods may quickly prove that anion 2a offers a greater flexibility in accessing specifically designed multifunctional materials than is currently available for the [closo-1-CB11H12]− anion (1a). To date, several key functional groups have been introduced to the C(1) position of the [closo-1-CB9H10]− anion (2a) by cyclization of C-substituted intermediates (nido and arachno prepared using Brellochs’ or similar reactions) and they include the COOH,21,27,29 NH2,49 and substituted aryl groups (remote functionality).21,22,46,50 The C(1)−COOH group has been transformed to CH2OH,21 CHO,21 amino,27,43 and numerous ester groups,16 while the amino functionality was alkylated12,49 and used to construct a pyridine ring9 and to generate the

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SUMMARY AND CONCLUSIONS The facile formation of the isomeric phenyliodonium zwitterions 5[6] and 5[10] with the highest selectivity to date for substitution of the B(10) position, combined with their significant difference in reactivity has provided a practical, relatively high yield method for regioselective preparation of a variety of 10- and 6-substituted derivatives of the [closo-1CB9H10]− anion. Substituents such as CN, RCOO, N3, I, MeC(NH)NH, SCHNMe2, and pyridine are introduced directly by displacement of iodobenzene in iodonium J


Article

Inorganic Chemistry

(2a-[Cs], 508 mg, 2.00 mmol, P2O5, 100 °C, 0.25 Tr, 12 h) was dissolved in freshly distilled THF (10 mL) and the solution was cooled to −10 °C. A solution of n-BuLi (2.5 M, 1.04 mL, 2.60 mmol) was added slowly over 10 min under Ar atmosphere. The solution was stirred for 1 h at −10 °C, warmed to room temperature, and stirred for an additional 30 min, while a milky precipitate was formed signifying the formation of the reactive carboranyllithium intermediate. The suspension was cooled to 0 °C and 1-iodopentane (515 mg, 0.34 mL, 2.6 mmol) was added dropwise. The reaction mixture was warmed to room temperature and stirred overnight. MeOH (5 mL) was added, and the solvents were evaporated to dryness. 6 M aqueous HCl (15 mL) and Et2O (15 mL) were added to the residue and the suspension was stirred for 1 h. The organic layer was separated and the aqueous layer was extracted with Et2O (4 × 15 mL). The organic extracts were combined and the solvent was removed in vacuo. The 11B NMR spectrum of the crude mixture showed ∼80% conversion. Water (10 mL) was added to the residue, followed by a slow dropwise addition of a concentrated aqueous solution of [Et4N]+Br− (0.85 equiv, 358 mg, 1.7 mmol). The resulting precipitate containing ∼90% of 2c (based on the 11B NMR spectrum) was collected, washed with H2O, and dried. Traces of the unreacted iodopentane were removed under vacuum (Kugel−Rohr, 80 °C, 0.25 Tr) and the crude product was separated from small amounts of the unreacted 2a by column chromatography (hexane/AcOEt 2:3) to give 454 mg (71% yield) of [closo-1-CB9H9-1-C5H11]−[Et4N]+ (2c[Et4N]) as a yellowish solid. The analytical sample was obtained by recrystallization (EtOH/H2O): mp 102−103 °C.59 Reaction of 2a with PhI(OAc)2.30 Following a literature procedure, 30 to a cold (0−5 °C) solution of [closo-1CB9H10]−[Et4N]+ (2a-[Et4N], 5.0 mmol)20,45 in 75% aqueous CF3COOH (45 mL), PhI(OAc)2 (3.38 g, 5.51 mmol) was added in five portions at 5 min intervals. The reaction mixture was stirred for 1 h at the same temperature and the resulting solid material was collected by filtration, washed with water and dried in vacuo, giving a white solid mixture of isomers 5[6]a and 5[10]a in a ratio of 2.7:1 (27% of 5[10]a by 1H NMR) and a typical yield of 95%.59 The mixture was used immediately for the next step. Reaction of 2c with PhI(OAc)2. Reaction with 2c-[Et4N] (300 mg, 0.94 mol)20,45 was conducted as described for 2a-[Et4N] with modified isolation of the iodonium zwitterions. Thus, the reaction mixture was diluted with H2O and stirred for an additional 5 min, giving a semicrystalline product sticking to the reaction flask walls. The supernatant liquid was decanted, the residue was washed with cold H2O, and dissolved in CH2Cl2. The resulting solution was washed with 5% aqueous NaHCO3 and passed through a thin layer of silica gel using a mixture of hexanes and CH2Cl2 (1:9) as the eluent. The solvents were evaporated under reduced pressure (cold bath!) giving an oily mixture of isomers 5[6]c and 5[10]c in an approximate ratio of 4:5 (55% of 5[10]c by 1H NMR) and yield of 95%.59 The mixture was used immediately for the next step. Isomer Separation. With MeCN: Preparation of [closo-1-CB9H910-IPh] (5[10]a).30 The mixture of regioisomers 5[6]a and 5[10]a (2.893 g, 8.97 mmol) was dissolved in MeCN (30 mL) and the solution was stirred at 60 °C for 16 h. The solvent was evaporated under reduced pressure and the residue was separated by column chromatography (SiO2, CH2Cl2/hexane 1:4) to give 0.670 g (22% yield based on 2a) of 5[10]a as a colorless solid: mp 129−131 °C, dec.59 With Pyridine in MeCN: Preparation of [closo-1-CB9H9-6-NC5H5] (9[6]a) and [closo-1-CB9H9-10-IPh] (5[10]a). To a stirred solution of a mixture of regioisomers 5[6]a and 5[10]a (450 mg, 1.39 mmol) in MeCN (4 mL), pyridine (200 mg, 2.53 mmol) was added. Stirring was continued for 20 h at room temperature. The solvent and the excess pyridine were evaporated under reduced pressure, and the residue was purified by column chromatography (CH2Cl2/hexane 1:3 gradient to 1:1) to give 149 mg (33% yield) of [closo-1-CB9H9-10IPh] (5[10]a) as the first fraction and 149 mg (54% yield) of 6pyridinium 9[6]a as a colorless solid: mp 118 °C.59 With Acetamidine in MeCN: Preparation of [closo-1-CB9H9-6NHC(NH2)Me] (10[6]a) and [closo-1-CB9H9-10-IPh] (5[10]a). To a

zwitterions 5[6] and 5[10] via the 10-I-3 intermediate, and the resulting functional derivatives are efficiently transformed further using standard organic synthesis methods. The expanded range of available substituents has provided a means for correlation of spectroscopic properties and for probing electronic interactions between the {closo-1-CB9} cluster and the substituents. Overall, the presented method provides the shortest and highest yield route to isomerically pure derivatives of 2a. It also complements other synthetic methods, which, in combination, constitute a powerful synthetic toolbox for the construction of specifically designed functional materials derived from the [closo-1-CB9H10]− anion.

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COMPUTATIONAL DETAILS Quantum-mechanical calculations were carried out using the Gaussian 09 suite of programs.54 Geometry optimizations of selected compounds were undertaken using the M06-2x/631G(2d,p) level of theory using tight convergence limits and appropriate symmetry constraints. Calculations involving iodine used the LANL2DZdp effective core potential basis set (available via the Internet at http://www.emsl.pnl.gov/ forms/basisform.html) and 6-31G(2d,p) for the remaining elements implemented with the GEN keyword. Transitionstate structures were located using the QST3 method, and input structures were obtained from relaxed scans of the PES. The nature of stationary points was confirmed with vibrational frequency calculations, which were also used to derive thermodynamic corrections. Zero-point energy (ZPE) corrections were scaled by 0.9806.55 Electronic excitation energies in MeCN dielectric medium were obtained at the CAM-B3LYP/6-31++G(2d,p)// M06-2x/6-31G(2d,p) level, using the TD-DFT method56 supplied in the Gaussian package. Energy change in reactions were obtained using single point calculations at the M06-2x/631+G(2d,p)//M06-2x/6-31G(2d,p) level of theory in MeCN dielectric medium implemented with the PCM model,57 using SCRF(solvent = CH3CN) as the keyword.

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EXPERIMENTAL SECTION

General. Reactions and subsequent manipulations were conducted in air, unless otherwise noted. Commercial-grade reagents and solvents were used without further purification, except as indicated below. THF and diethyl ether were distilled over Na/benzophenone prior to use. Thin-layer chromatography (TLC) was conducted on Silica Gel 60 F254 TLC (purchased from Merck). Column chromatography was performed using Merck silica gel (70−230 mesh). NMR spectra were obtained at 500 MHz (1H), 125 MHz (13C), and 160 MHz (11B) in CD3CN and acetone-d6. Chemical shifts were referenced to the solvent (1H and 13C: 1.94 and 1.39 ppm for CD3CN and 2.05 and 29.84 ppm for acetone-d6),58 or to an external sample of neat BF3·Et2O in CD3CN and acetone-d6 (11B, δ = 0.0 ppm). 11B chemical shifts are reported for 1H decoupled spectra (11B{1H} NMR). All chemical shifts (δ) are given in ppm, and the coupling constants (J) are given in Hz. Mass spectra were recorded in a negative-ion mode on a Waters Synapt HDMS instrument fitted with an atmospheric pressure ionization electrospray source; details are provided in the Supporting Information. Melting and boiling points are uncorrected. All analytical data are provided in the Supporting Information. The solid-state structures are deposited at the Cambridge Crystallograic Data Center (CCDC) (Nos. CCDC-1585235− CCDC-1585238). Preparation of Starting Materials. Preparation of [closo-1CB9H9-1-C5H11]−[Et4N]+ (2c-[Et4N]). Well-dried [closo-1-CB9H10]−Cs+ K


Article

Inorganic Chemistry

under reduced pressure and the crude product was purified by column chromatography (CH2Cl2/hexane 1:2, Rf = 0.17) to give 86 mg (93% yield) of 10-pyridinium derivative 9[10]a as a colorless solid: mp 134−135 °C.59 Preparation of [closo-1-CB9H9-10-NHC(NH2)Me] (10[10]a). Acetamidine hydrochloride (437 mg, 4.62 mmol) was added in one portion to sodium ethoxide solution, which was prepared from sodium (101 mg, 4.4 mmol) and ethanol (2.7 mL).60 The mixture was stirred for 10 min at room temperature and ethanol was evaporated at reduced pressure. MeCN (6 mL) was added to the residue, and the resulting suspension was filtered through Celite and concentrated to ca. 4 mL. [closo-1-CB9H9-10-IPh] (5[10]a, 354 mg, 1.10 mmol) was added and the resulting solution was stirred for 4 h at 40 °C. The solvent was evaporated in vacuo, and the crude product was purified via column chromatography (SiO2, CH2Cl2) to afford 171 mg (89% yield) of 10-acetamidinium derivative 10[10]a as a colorless solid: mp 135 °C.59 Preparation of [closo-1-CB9H9-10-CN]−[Et4N]+ (11[10]a-[Et4N]). A solution of [closo-1-CB9H9-10-IPh] (5[10]a, 45.0 mg, 0.140 mmol) and [Et4N]+[CN]− (26 mg, 0.168 mmol) in MeCN (1 mL) was stirred at room temperature for 7 h. After evaporation of the solvent, the crude product was purified by column chromatography (CH2Cl2 gradient to CH2Cl2/MeCN 10:1, Rf = 0.19 at CH2Cl2/MeCN 10:1), to give 37.0 mg (96% yield) of [closo-1-CB9H9-10-CN]−[Et4N]+ (11[10]a-[Et4N]) as a colorless solid: mp 237−239 °C, dec.59 Preparation of [closo-1-CB9H9-10-SCHNMe2] (12[10]a). A solution of [closo-1-CB9H9-10-IPh] (5[10]a, 70 mg, 0.217 mmol) in N,N-dimethylthioformamide (0.30 g, 3.36 mmol) was stirred at 80 °C for 1.5 h. Excess N,N-dimethylthioformamide was removed under reduced pressure, and the resulting crude product was purified by column chromatography (CH2Cl2/hexane 1:1, Rf = 0.28) to afford 39.0 mg (87% yield) of zwitterion 12[10]a as a white solid: mp 249− 250 °C.59 Preparation of [closo-1-CB9H9-10−OCOPh]−[Me4N]+ (13[10]a[Me4N]). To a stirred solution of [closo-1-CB9H9-10-IPh] (5[10]a, 35.0 mg, 0.109 mmol) in MeCN (0.4 mL) [Me4N]+PhCOO− (36 mg, 0.185 mmol) was added. The mixture was heated at 40 °C for 9 h. The solvent was evaporated under reduced pressure and the resulting crude product was purified by column chromatography (CH2Cl2 gradient to CH2Cl2/MeCN 5:1, Rf = 0.17 at CH2Cl2/MeCN 5:1) to give 31.0 mg (91% yield) of benzoyloxy derivative 13[10]a[Me4N] as a colorless solid: mp 236−238 °C.59 Preparation of [closo-1-CB9H9-10-N3]−[Bu4N]+ (14[10]a-[Bu4N]). To a stirred solution of [closo-1-CB9H9-10-IPh] (5[10]a, 28.0 mg, 0.087 mmol) in THF (1.2 mL), [Bu4N]+N3− (29 mg, 0.102 mmol) was added. After stirring for 2 h at room temperature, the solvent was evaporated under reduced pressure and the crude product was purified by column chromatography (CH2Cl2, Rf = 0.45) to yield 34.0 mg (95% yield) of azide 14[10]a-[Bu4N] as a colorless oil.59 Functional Group Transformations. Preparation of [closo-1CB9H9-10-N2] (4[10]a). To a cooled (−15 °C) stirred solution of amine 8[10]a (66 mg, 0.49 mmol) and pyridine (193 mg, 2.44 mmol) in MeCN (2 mL), [NO]+[BF4]− (171 mg, 1.46 mmol) was added in four portions at 5 min intervals. The reaction mixture was stirred for 2 h at the same temperature and then evaporated to dryness under reduced pressure. The crude product was purified by column chromatography (CH2Cl2/petroleum ether 1:2, Rf = 0.62) to give 46 mg (65% yield) of 4[10]a as a colorless solid: mp 125−139 °C, dec.59 Preparation of [closo-1-CB9H9-6-NH3] (8[6]a). Acetamidinium 10[6]a (100 mg, 0.567 mmol) was added to a solution of KOH (318 mg, 5.67 mmol) in MeOH (3.8 mL) and the reaction mixture was stirred at 60 °C for 16 h. After evaporation of MeOH under reduced pressure, the residue was acidified with ca. 18% HCl and extracted with Et2O (3 × 10 mL). The organic layers were combined and dried (Na2SO4). After evaporation of the solvent in vacuo, the crude material was purified by column chromatography (SiO2, CH2Cl2/ MeCN 10:1, Rf = 0.26) to afford 41 mg (76% yield) of [closo-1CB9H9-6-NH3] (8[6]a) as a colorless solid: mp >300 °C, dec.59

stirred solution of a mixture of regioisomers 5[6]a and 5[10]a (2.49 g, 7.72 mmol) in MeCN (23 mL), acetamidine60 (0.54 g, 9.30 mmol) was added. After stirring for 1.5 h at room temperature, acetic acid (180 mg, 3.00 mmol) was added. The solvent was evaporated under reduced pressure and the residue was subjected to column chromatography (CH2Cl2/hexane 1:2 gradient to CH2Cl2) to afford 753 mg (30% yield) of [closo-1-CB9H9-10-IPh] (5[10]a) as the first fraction and 760 mg (56% yield) of [closo-1-CB9H9-6-NHC(NH2)Me] (10[6]a) as a colorless solid: mp 150−151 °C.59 With [Et4N]+[CN]− in MeCN: Preparation of [closo-1-CB9H8-1C5H11-6-CN]−[Et4N]+ (11[6]c-[Et4N]) and [closo-1-CB9H8-1-C5H1110-IPh] (5[10]c). To a stirred solution of a mixture of regioisomers 5[6]c and 5[10]c (280.0 mg, 0.71 mmol) in MeCN (5 mL), [Et4N]+[CN]− (49.0 mg, 0.31 mmol) was added. The progress of the reaction was monitored using 11B NMR spectroscopy. After stirring for 1 h at 0 °C, the reaction was complete. After evaporation of the solvent (cold bath), the products were separated by column chromatography (SiO2), starting with hexanes (elution of iodobenzene), followed by hexanes/CH2Cl2 4:1 (elution of 5[10]c), and finally CH2Cl2/MeCN 10:1 (elution of 11[6]c-[Et4N]), giving 150 mg of 5[10]c (containing ∼3% 5[6]c) and 121 mg (49% yield) of ∼92% pure [closo-1-CB 9H 8-1-C 5 H11-6-CN] −[Et4 N] + (11[6]a[Et4N]) as a yellow oil. Pure [closo-1-CB9H8-1-C5H11-10-IPh] (5[10]c) was obtained by heating the product in MeCN at 60 °C. The progress of the reaction was monitored using 11B NMR spectroscopy, and, after 2 h, it showed that the entire [closo-1CB9H8-1-C5H11-6-IPh] (5[6]c) was consumed. The solvent was evaporated, and the residue was passed through a silica gel pad, using hexanes/CH2Cl2 (4:1) as an eluent, giving 140 mg (50% yield) of pure 5[10]c as a white solid, which was recrystallized from n-heptane: mp 65−66 °C.59 Crude [closo-1-CB9H8-1-C5H11-6-CN]-[Et4N]+ (11[6]a-[Et 4 N]) was purified by column chromatography (CH2Cl2/MeCN 30:1) to give 97 mg of a colorless oil.59 Conversions of [closo-CB9H8-1-R-10-IPh] (5[10]). Preparation of [closo-1-CB9H8-1-C5H11-10-I]−[Et4N]+ (3[10]c-[Et4N]): Method A. To a stirred solution of [closo-1-CB9H8-1-C5H11-10-IPh] (5[10]c, 50.0 mg, 0.127 mmol) in dry MeCN (1 mL) [Et4N]+I− (49 mg, 0.191 mmol) was added. The mixture was heated overnight at 60 °C. The solvent was evaporated under reduced pressure, and the resulting crude product was purified by column chromatography (CH2Cl2/ hexane 2:1 gradient to CH2Cl2), giving 55 mg (97% yield) of pure 3[10]c-[Et4N]. Method B. To a solution of [closo-1-CB9H8-1-C5H11-10-IPh] (5[10]c, 71 mg, 0.181 mmol) in dry THF (3 mL), 2.5 M n-BuLi in hexanes (0.18 mL, 0.45 mmol) was added dropwise at −10 °C under argon. The reaction mixture was stirred at −5 °C, and the progress of the reaction was monitored by TLC. After 1 h, H2O (3 mL) was added, THF was removed under reduced pressure, and the clear solution was extracted with hexanes (three times). The aqueous layer was treated with concentrated solution of [Et4N]+Br− (46 mg, 0.22 mmol). The resulting milky suspension was extracted with CH2Cl2 (three times) and dried (MgSO4). The solvent was evaporated to give 70 mg of crude product, which contained 7% of the [closo-1-CB9H8-1C5H11-10-Ph]− derivative. Pure 3[10]c-[Et4N] was isolated as a white solid by extraction of the 10-phenyl derivative from the crude solid with Et2O, followed by recrystallization from MeOH: mp 102−103 °C.59 Preparation of [closo-1-CB9H8-1-C5H11-10-(NC5H4OC7H15)] (6[10]c). A solution of [closo-1-CB9H9-1-C5H11-10-IPh] (5[10]c, 35.0 mg, 0.089 mmol) in 4-heptyloxypyridine (0.4 mL) was stirred at 85 °C for 6 h. After completion of the reaction (confirmed by TLC), all volatiles were removed via Kugelrohr distillation (80 °C, 0.25 Tr). The crude product was purified by passing through a silica gel pad using CH2Cl2 as an eluent, giving 33.0 mg (95% yield) of [closo-1CB9H8-1-C5H11-10-(NC5H4OC7H15)] (6[10]c) as white crystals. The analytical sample was obtained by crystallization from n-heptane: mp 94−95 °C.59 Preparation of [closo-1-CB9H9-10-NC5H5] (9[10]a). A solution of [closo-1-CB9H9-10-IPh] (5[10]a, 150 mg, 0.465 mmol) in pyridine (0.50 g, 6.3 mmol) was stirred at 45 °C for 6 h. Pyridine was removed L


Article

Inorganic Chemistry Preparation of [closo-1-CB9H9-10-NH3] (8[10]a). A solution of potassium hydroxide (0.44 g, 7.84 mmol) and acetamidinium 10[10]a (137 mg, 0.78 mmol) in methanol (7 mL) was stirred at 60 °C for 9 h. Solvents were removed under reduced pressure, and the residue was acidified with ca. 18% hydrochloric acid and extracted with diethyl ether (3 × 10 mL). The organic layers were combined and dried (Na2SO4). After evaporation of the solvent in vacuo, the crude material was purified by column chromatography (SiO2, CH2Cl2/MeCN, 10:1) to afford 83 mg (78% yield) of amine 8[10]a as a colorless crystalline solid: mp >250 °C.59 Preparation of [closo-1-CB9H9-6-(NHCONHPh)]−[Et4N]+ (15[6]a[Et4N]). To a stirred solution of [closo-1-CB9H9-6-NH3] (8[6]a, 25 mg, 0.185 mmol) and N,N-diisopropylethylamine (48 mg, 0.369 mmol) in THF (0.4 mL), PhNCO (31.0 mg, 0.259 mmol) was added. Stirring was continued for 4 h at ambient temperature and the mixture was concentrated at the reduced pressure. The residue was acidified with 5% aqueous solution of HCl and extracted with Et2O (3 × 15 mL). The organic layers were combined and concentrated under reduced pressure. A solution of [Et4N]+ OH− (1 mL, 35 wt % in water) was added and the mixture was extracted with CH2Cl2 (3 × 15 mL). The organic layers were combined and dried (Na2SO4). Evaporation of the solvent in vacuo, followed by column chromatography of the resulting crude product (CH2Cl2/MeCN 10:3, Rf = 0.15), gave 65 mg (92% yield) of urea 15[6]a-[Et4N] as a white solid: mp 68−69 °C.59 Preparation of [closo-1-CB 9 H 9 -6-(NHCSNHC 6 H 3 (CF 3 ) 2 3,5)]−[Et4N]+ (16[6]a-[Et4N]). A mixture of [closo-1-CB9H9-6-NH3] (8[6]a, 17 mg, 0.126 mmol), N,N-diisopropylethylamine (32 mg, 0.251 mmol) and 3,5-bis(trifluoromethyl)phenyl isothiocyanate (102 mg, 0.378 mmol) in THF (0.35 mL) was stirred at 40 °C for 15 h. The solvent was evaporated under reduced pressure, and the residue was acidified with 5% aqueous solution of HCl and extracted with Et2O (3 × 15 mL). The organic layers were combined and concentrated under reduced pressure. A solution of [Et4N]+ OH− (1 mL, 35 wt % in water) was added and a mixture was extracted with CH2Cl2 (3 × 15 mL). The organic layers were combined and dried (Na2SO4). After evaporation of the solvent the crude product was purified by a column chromatography (SiO2, CH2Cl2/MeCN 10:1, Rf = 0.22) to yield 46.0 mg (69% yield) of thiourea derivative 16[6]a[Et4N] as a colorless solid: mp 123−124 °C.59 Preparation of [closo-1-CB9H9-6-(NHAlaBoc)]− [Et4N]+ (17[10]a[Et4N]). To a solution of L-Boc-Ala-OH (19 mg, 0.1 mmol) in THF (0.6 mL) at 0 °C DCC (23 mg, 0.11 mmol) was added. After 25 min at 0 °C [closo-1-CB9H9-6-NH2]− [Et4N]+ (8[6]a-[Et4N], 26 mg, 0.1 mmol) was added and stirring was continued at ambient temperature for 18 h. The solvent was evaporated and the residue was purified by column chromatography (CH2Cl2/MeCN, 10:6) to give 18.0 mg of ∼90% pure amide 17[10]a-[Et4N].59 Preparation of [closo-1-CB9H9-10-(SC5H9)] (18[10]a). A mixture of [closo-1-CB9H9-10-SCHNMe2] (12[10]a, 33 mg, 0.159 mmol), 1,5-dibromopentane (40 mg, 0.175 mmol), [Me4N]+OH−·5H2O (86 mg, 0.477 mmol) in MeCN (2.5 mL) was stirred at 70 °C overnight. The solvent was removed in vacuo, and the residue was purified by column chromatography (CH2Cl2/hexane 1:3, Rf = 0.45) to give 25.0 mg (71% yield) of sulfonium 18[10]a as a white solid: mp 150−151 °C.59 Comparative Studies. Reaction of [closo-1-CB9H9-10-N2] (4[10]a) with [Me4N]+[PhCO2]−. A solution of [closo-1-CB9H9-10-N2] (4[10]a, 7 mg, 0.048 mmol) and [Me4N]+PhCO2− (16 mg, 0.081 mmol) in CD3CN (0.4 mL) was heated at 40 °C for 5 h. Analysis of the reaction mixture by 11B NMR spectroscopy revealed complete consumption of the starting material and the appearance of two major new sets of signals: −25.2 (4B), −21.2 (4B), 34.4 (1B), and −25.9 (4B), −21.9 (4B), 41.0 (1B) in an approximate ratio of 3:1. The expected product 13[10]a was not detected. Reaction of [closo-1-CB9H9-10-N2] (4[10]a) with [Et4N]+CN−. A solution of [closo-1-CB9H9-10-N2] (4[10]a, 7 mg, 0.048 mmol) and [Et4N]+CN − (9 mg, 0.057 mmol) in CD3CN (0.4 mL) was stirred at room temperature for 7 h. Analysis of the reaction mixture by 11B NMR spectroscopy revealed ∼ 5% of the starting material and 1% of

the 11[10]a product formation. After an additional 17 h (a total of 24 h), the conversion was ∼15% and 5% of the product 11[10]a formation. An additional portion of [Et4N]+CN− (15 mg, 0.096 mmol) was added, and, after 24 h, 11B NMR revealed ∼60% conversion and the formation of ∼20% of the expected product 11[10]a. Major two sets of signals not belonging to the starting material or the product were observed: −28.7, −24.4, 48.2 and −26.2, −22.4, 44.0 in a ratio of 3:1. The identity of these side products was not investigated.

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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.inorgchem.8b01701. Full synthetic details with complete analytical data, comparison of geometrical parameters for derivatives of acid 2b, comparison of NMR chemical shifts and Hammett constants, copies of 1H, 13C and 11B spectra, and archives of DFT calculations (PDF) Accession Codes

CCDC 1585235−1585238 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Piotr Kaszyński: 0000-0002-2325-8560 Krzysztof Woźniak: 0000-0002-0277-294X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF (No. DMR-1611250), NCN (No. OPUS 2015/17/B/ST5/02801), and FNP (No. TEAM/2016-3/24) grants. We thank Mr. John C. Lasseter (Middle Tennessee State University) for help with the preparation of [closo-1-CB9H10]− and Dr. Damian Trzybiński (University of Warsaw) for assistance with XRD reports.

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REFERENCES

(1) Grimes, R. N. Carboranes, 2nd Edition; Academic Press: Boston, 2011. (2) Zhu, Y.; Hosmane, N. S. Ionic Liquids: Recent Advances and Applications in Boron Chemistry. Eur. J. Inorg. Chem. 2017, 2017, 4369−4377. (3) Sivaev, I. B. Nitrogen heterocyclic salts of polyhedral borane anions: from ionic liquids to energetic materials. Chem. Heterocycl. Compd. 2017, 53, 638−658. (4) Tang, W. S.; Matsuo, M.; Wu, H.; Stavila, V.; Zhou, W.; Talin, A. A.; Soloninin, A. V.; Skoryunov, R. V.; Babanova, O. A.; Skripov, A. V.; Unemoto, A.; Orimo, S.; Udovic, T. J. Liquid-like ionic conduction in solid lithium and sodium monocarba-closo-decaborates near or at room temperature. Adv. Energy Mater. 2016, 6, 1502237. (5) Tang, W. S.; Yoshida, K.; Soloninin, A. V.; Skoryunov, R. V.; Babanova, O. A.; Skripov, A. V.; Dimitrievska, M.; Stavila, V.; Orimo, S.; Udovic, T. J. Stabilizing superionic-conducting structures via mixed-anion solid solutions of monocarba-closo-borate salts. ACS Energy Lett. 2016, 1, 659−664.

M


Article

Inorganic Chemistry (6) McArthur, S. G.; Jay, R.; Geng, L.; Guo, J.; Lavallo, V. Below the 12-vertex: 10-vertex carborane anions as non-corrosive, halide free, electrolytes for rechargeable Mg batteries. Chem. Commun. 2017, 53, 4453−4456. (7) Pecyna, J.; Kaszyński, P.; Ringstrand, B.; Bremer, M. Investigation of high Δε derivatives of the [closo-1-CB9H10]- anion for liquid crystal display applications. J. Mater. Chem. C 2014, 2, 2956−2964. (8) Pecyna, J.; Kaszyński, P.; Ringstrand, B.; Pociecha, D.; Pakhomov, S.; Douglass, A. G.; Young, V. G., Jr. Synthesis and characterization of quinuclidinium derivatives of the [closo-1CB11H12]− anion as potential polar components of liquid crystal materials. Inorg. Chem. 2016, 55, 4016−4025. (9) Pecyna, J.; Pociecha, D.; Kaszyński, P. Zwitterionic pyridinium derivatives of [closo-1-CB9H10]− and [closo-1-CB11H12]− as high Δε additives to a nematic host. J. Mater. Chem. C 2014, 2, 1585−1591. (10) Pecyna, J.; Ż urawiński, R.; Kaszyński, P.; Pociecha, D.; Zagórski, P.; Pakhomov, S. Polar liquid crystals derived from sulfonium zwitterions of the [closo-1-CB11H12]− anion. Eur. J. Inorg. Chem. 2016, 2016, 2923−2931. (11) Ringstrand, B.; Kaszynski, P. High Δε nematic liquid crystals: fluxional zwitterions of the [closo-1-CB9H10]− cluster. J. Mater. Chem. 2011, 21, 90−95. (12) Ringstrand, B.; Kaszynski, P.; Januszko, A.; Young, V. G., Jr. Polar derivatives of the [closo-1-CB9H10]− cluster as positive Δε additives to nematic hosts. J. Mater. Chem. 2009, 19, 9204−9212. (13) Ringstrand, B.; Monobe, H.; Kaszynski, P. Anion-driven mesogenicity: ionic liquid crystals based on the [closo-1-CB9H10]− cluster. J. Mater. Chem. 2009, 19, 4805−4812. (14) Ringstrand, B.; Jankowiak, A.; Johnson, L. E.; Pociecha, D.; Kaszynski, P.; Górecka, E. Anion-Driven Mesogenicity: A Comparative Study of Ionic Liquid Crystals Based on the [closo-1CB9H10]− and [closo-1-CB11H12]− Clusters. J. Mater. Chem. 2012, 22, 4874−4880. (15) Pecyna, J.; Sivaramamoorthy, A.; Jankowiak, A.; Kaszynski, P. Anion driven ionic liquid crystals: The effect of the connecting group in [closo-1-CB9H10]−derivatives on mesogenic properties. Liq. Cryst. 2012, 39, 965−971. (16) Ringstrand, B.; Kaszynski, P. Functionalization of the [closo-1CB9H10]− Anion for the Construction of New Classes of Liquid Crystals. Acc. Chem. Res. 2013, 46, 214−225. (17) Kaszynski, P. closo-Boranes as Structural Elements for Liquid Crystals. In Boron Science: New Technologies and Applications; Hosmane, N. S., Ed.; CRC Press: Boca Raton, FL, 2012; pp 305−338. (18) Körbe, S.; Schreiber, P. J.; Michl, J. Chemistry of the Carbacloso-dodecaborate(−) Anion, CB11H12. Chem. Rev. 2006, 106, 5208−5249. (19) Brellochs, B. New routes to carboranes. In Contemporary Boron Chemistry; Davidson, M. G., Hughes, A. K., Marder, T. B., Wade, K., Eds.; Royal Society of Chemistry: Cambridge, England, 2000; pp 212−214. (20) Franken, A.; Kilner, C. A.; Thornton-Pett, M.; Kennedy, J. D. Polyhedral monocarbaborane chemistry. A review of recent developments among C-aryl monocarbaborane systems. Collect. Czech. Chem. Commun. 2002, 67, 869−912. (21) Franken, A.; Carr, M. J.; Clegg, W.; Kilner, C. A.; Kennedy, J. D. Monocarbaborane anion chemistry. {COOH}, {CH2OH} and {CHO} units as functional groups on ten-vertex monocarbaborane anionic compounds. Dalton Trans. 2004, 3552−3561. (22) Sivaev, I. B.; Starikova, Z. A.; Petrovskii, P. V.; Bregadze, V. I.; Sjöberg, S. The Synthesis of functional derivatives of the [1-CB9H10]− anion by Brellochs reaction. J. Organomet. Chem. 2005, 690, 2790− 2795. (23) Ringstrand, B.; Bateman, D.; Shoemaker, R. K.; Janousek, Z. Improved Synthesis of [closo-1-CB9H10]− Anion and New CSubstituted Derivatives. Collect. Czech. Chem. Commun. 2009, 74, 419−431. (24) Brellochs, B.; Bačkovský, J.; Š tíbr, B.; Jelínek, T.; Holub, J.; Bakardjiev, M.; Hnyk, D.; Hofmann, M.; Císarová, I.; Wrackmeyer, B.

New ways to a series of parent representatives of the eight-, nine-, and ten-vertex monocarbaborane family. Eur. J. Inorg. Chem. 2004, 3605− 3611. (25) Franken, A.; Kennedy, J. D.; Clapper, J.; Sneddon, L. G. Monocarbaborane anions with 10 or 12 vertices. Inorg. Synth. 2014, 36, 174−185. (26) Ivanov, S. V.; Rockwell, J. J.; Miller, S. M.; Anderson, O. P.; Solntsev, K. A.; Strauss, S. H. Reactions of CB9H10− with electrophiles, including the regioselective mono- and dihalogenation of the lower belt. Inorg. Chem. 1996, 35, 7882−7891. (27) Ringstrand, B.; Balinski, A.; Franken, A.; Kaszynski, P. A practical synthesis of isomerically pure 1,10-difunctionalized derivatives of the [closo-1-CB9H10]− anion. Inorg. Chem. 2005, 44, 9561− 9566. (28) Ringstrand, B.; Kaszynski, P.; Young, V. G., Jr.; Janoušek, Z. Anionic Amino Acid [closo-1-CB9H8-1-COO-10-NH3]− and Dinitrogen Acid [closo-1-CB9H8-1-COOH-10-N2] as Key Precursors to Advanced Materials: Synthesis and Reactivity. Inorg. Chem. 2010, 49, 1166−1179. (29) Pecyna, J.; Denicola, R. P.; Ringstrand, B.; Jankowiak, A.; Kaszynski, P. The preparation of [closo-1-CB9H8-1-COOH-10-(4C3H7C5H9S)] as intermediate to polar liquid crystals. Polyhedron 2011, 30, 2505−2513. (30) Kaszynski, P.; Ringstrand, B. Functionalization of closo-borates via iodonium zwitterions. Angew. Chem., Int. Ed. 2015, 54, 6576− 6581. (31) Ringstrand, B.; Kaszynski, P. How much can an electric dipole stabilize a nematic phase? Polar and non-polar isosteric derivatives of [closo-1-CB9H10]− and [closo-1,10-C2B8H10]. J. Mater. Chem. 2010, 20, 9613−9615. (32) For details, see the Supporting Information. (33) Rzeszotarska, E.; Novozhilova, I.; Kaszyński, P. Convenient synthesis of [closo-B10H9-1-I]2− and [closo-B10H8-1,10−2I]2− anions. Inorg. Chem. 2017, 56, 14351−14356. (34) Ringstrand, B.; Kaszynski, P.; Young, V. G., Jr. Diazotization of the Amino Acid [closo-1-CB9H8-1-COOH-6-NH3] and Reactivity of the [closo-1-CB9H8-1-COO-6-N2]− Anion. Inorg. Chem. 2011, 50, 2654−2660. (35) Zhdankin, V. V. Hypervalent Iodine Chemistry; Wiley: Petaling Jaya, Selangor, Malaysia, 2014. (36) Pecyna, J. G.; Ringstrand, B.; Kaszynski, P. Transmission of Electronic Effects through the {closo-1-CB9} and {closo-1-CB11} Cages: Apparent Dissociation Constants for Series of [closo-1-CB9H81-COOH-10-X] and [closo-1-CB11H10-1-COOH-12-X] Acids. Inorg. Chem. 2012, 51, 5353−5359. (37) Hansch, C.; Leo, A.; Taft, R. W. A survey of Hammett substituent constants and resonance and field parameters. Chem. Rev. 1991, 91, 165−195. (38) Hubbard, A.; Okazaki, T.; Laali, K. K. Halo- and azidodediazoniation of arenediazonium tetrafluoroborates with trimethylsilyl halides and trimethylsilyl azide and sandmeyer-type bromodediazoniation with Cu(I)Br in [BMIM][PF6] ionic liquid. J. Org. Chem. 2008, 73, 316−319. (39) Zheng, S.; Yu, C.; Shen, Z. Ethyl cyanoacetate: a new cyanating agent for the palladium - catalyzed cyanation of aryl halides. Org. Lett. 2012, 14, 3644−3647. (40) Arisawa, M.; Igarashi, Y.; Kobayashi, H.; Yamada, T.; Bando, K.; Ichikawa, T.; Yamaguchi, M. Equilibrium shift in the rhodiumcatalyzed acyl transfer reactions. Tetrahedron 2011, 67, 7846−7859. (41) Westrum, E. F., Jr.; Ribner, A. Trimethylacetonitrile. Lowtemperature heat capacity, vapor pressure, and chemical thermodynamics of the crystalline, liquid, and gaseous phases. J. Phys. Chem. 1967, 71, 1216−1224. (42) Degennaro, L.; Maggiulli, D.; Carlucci, C.; Fanelli, F.; Romanazzi, G.; Luisi, R. A direct and sustainable synthesis of tertiary butyl esters enabled by flow microreactors. Chem. Commun. 2016, 52, 9554−9557. (43) Ringstrand, B.; Kaszynski, P.; Franken, A. Synthesis and Reactivity of [closo-1-CB9H9-1-N2]: Functional Group InterconverN


Article

Inorganic Chemistry sion at the Carbon Vertex of the {closo-1-CB9} Cluster. Inorg. Chem. 2009, 48, 7313−7329. (44) Pecyna, J.; Ringstrand, B.; Domagała, S.; Kaszyński, P.; Woźniak, K. Synthesis and characterization of 12-pyridinium derivatives of the [closo-1-CB11H12]− anion. Inorg. Chem. 2014, 53, 12617−12626. (45) Franken, A.; Kilner, C. A.; Thornton-Pett, M.; Kennedy, J. D. Polyhedral monocarbaborane chemistry: positional isomerism within the 10-vertex monocarbaborane cluster: synthesis and characterisation of the [2-Ph-closo-2-CB9H9] anion. Inorg. Chem. Commun. 2002, 5, 581−584. (46) Franken, A.; Kilner, C. A.; Kennedy, J. D. Monocarbaborane anion chemistry: use of an anilinyl residue as an anchor group for the construction of rod-like monocarbaboranyl diazo and imino compounds. Inorg. Chem. Commun. 2003, 6, 1104−1108. (47) No stable iodonium zwitterion could be isolated from reactions of 1b with PhI(AcO)2. Pecyna, J.; Ringstrand, B.; Kaszyński, P. Personal communication. (48) Tokarz, P.; Kaszyński, P.; Domagała, S.; Woźniak, K. The [closo-B12H11-1-IAr]− zwitterion as a precursor to monosubstituted derivatives of [closo-B12H12]2−. J. Organomet. Chem. 2015, 798, 70− 79. (49) Jelínek, T.; Stibr, B.; Plešek, J.; Thornton-Pett, M.; Kennedy, J. D. Ten-vertex monocarbaborane chemistry. Synthesis of carbonsubstituted ligand derivatives 1-L-closo-1-CB9H9 and crystal and molecular structure of 1-(Me3N)-closo-1-CB9H9. J. Chem. Soc., Dalton Trans. 1997, 4231−4236. (50) Bullen, N. J.; Franken, A.; Kilner, C. A.; Kennedy, J. D. Plyhedral monocarbaborane chemistry. Routes to neutral, monoanionic and dianionic carbo-carbaborane rods. Chem. Commun. 2003, 1684−1685. (51) Knoth, W. H. B10H12CNH3, B9H9CH−, B11H11CH−, and metallomonocarboranes. Inorg. Chem. 1971, 10, 598−605. (52) Pakhomov, S.; Kaszynski, P.; Young, V. G., Jr. 10-Vertex closoboranes as potential π linkers for electronic materials. Inorg. Chem. 2000, 39, 2243−2245. (53) Kaszynski, P.; Pakhomov, S.; Young, V. G., Jr. Investigations of Electronic Interactions Between closo-Boranes and Triple-Bonded Substituents. Collect. Czech. Chem. Commun. 2002, 67, 1061−1083. (54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (55) Scott, A. P.; Radom, L. Harmonic Vibrational Frequencies: An Evaluation of Hartree−Fock, Møller−Plesset, Quadratic Configuration Interaction, Density Functional Theory, and Semiempirical Scale Factors. J. Phys. Chem. 1996, 100, 16502−16513. (56) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. An efficient implementation of time-dependent density-functional theory for the calculation of excitation energies of large molecules. J. Chem. Phys. 1998, 109, 8218−8224. (57) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New developments in the polarizable continuum model for quantum mechanical and classical calculations on molecules in solution. J. Chem. Phys. 2002, 117, 43−54 and references therein. (58) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR

Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist. Organometallics 2010, 29, 2176−2179. (59) For full analytical data, see the Supporting Information. (60) Crossland, I.; Grevil, F. S.; Schaumburg, K.; Samuelsson, B.; Ö rn, U. A Convenient Preparation of Acetamidine. Acta Chem. Scand. 1981, 35B, 605−605.

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