Cp*-Substituted Boron Cations: The Effect of NHC ... - ACS Publications

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Cp*-Substituted Boron Cations: The Effect of NHC, NHO, and CAAC Ligands Jih-Sheng Huang,† Wan-Hua Lee,† Chao-Tang Shen,† Ya-Fan Lin,‡ Yi-Hung Liu,† Shie-Ming Peng,† and Ching-Wen Chiu*,† †

Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 10617, Taiwan Department of Medical and Applied Chemistry, Kaohsiung Medical University, No. 100, Shih-Chuan First Road, Kaohsiung 80708, Taiwan

‡

S Supporting Information *

ABSTRACT: The effect of a ligand on the electron deficiency and Lewis acidity of the Cp*-substituted boron dication has been investigated experimentally and theoretically. In addition to the reported IMes- and N-heterocyclic olefin (NHO)-stabilized boron dications, the related cyclic alkylamino carbene (CAAC)-coordinated boron mono- and dications have also been synthesized and structurally characterized. An electrochemical study of dications [3a−3c]2+ confirms the higher electron deficiency of the dicationic system than the related boron monocations. Moreover, the presence of a π-acidic CAAC ligand is critical for realizing stable radical species generated from the chemical reduction of boron cations. The nature of the axial ligand also significantly affects the selectivity of the hydride addition reaction of boron dications. While bulky superhydride reacts with [3a−3c]2+ in the same manner to give the cyclic boreniums, [BH4]− attacks three different electrophilic sites of boron dications: the sp2 carbon of Cp* of the IMes-coordinated system ([3a]2+), the central boron atom of the NHO-stabilized analogue ([3b]2+), and the ylidene carbon of the CAAC-containing boron dication ([3c]2+).

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INTRODUCTION Owing to the relative strong bonding between boron and carbon, neutral carbon ligands have played an imperative role in the recent advances of boron chemistry. The coordination of Arduengo’s carbene facilitates the isolation of diborene1 and diboryne,2 boron-centered neutral radicals, 3 and stable borenium cations that are capable of catalyzing hydrogenation of unsaturated substrates.4 In recent years, the cyclic alkylamino carbene (CAAC) developed by Bertrand’s group has also attracted much attention.5 The replacement of one of the nitrogen atoms of N-heterocyclic carbene (NHC) with an sp3 carbon markedly increases the σ-donating and π-accepting abilities of the ylidene center. The enhanced π acidity of CAAC has been proven to be effective in stabilizing several electronrich boron derivatives, including boryl anion,6 diboryne,7 and borylene and boron radicals.8 In addition to carbene ligands, the coordination of N-heterocyclic olefin (NHO) to a boron center has also been investigated.9 NHO forms stable complexes with neutral boranes and boron cations through coordination at the exocyclic polarized CC double bond. Ligand substitution reactions on the NHO−borane adduct © XXXX American Chemical Society

reveal that the NHO−boron interaction is weaker than the NHC−boron bond, suggesting the lower σ-donating ability of NHO.10 Besides, the lack of a π-accepting ability of NHO also hampers its ability in stabilizing electron-rich boron species.11 Recently, our group has reported the synthesis and reactivity of Cp*-substituted boron dications that feature neutral carbon ligands such as IMes ([3a]2+) and NHO ([3b]2+) at the boron.12 Our preliminary investigations on the IMes- and NHO-stabilized dications show that [3b]2+ is noticeably more reactive and difficult to handle compared to its NHC analogue. In order to elucidate the effect of a ligand on the hypercoordinated boron cations, we decided to investigate the electron reduction and hydride addition reactions of Cp*substituted boron dications that feature IMes, NHO, or CAAC at the boron center. The molecular structures of these ligands are depicted in Figure 1. The π-acidic nature of CAAC is expected to enhance the stability of radical species obtained from the chemical reduction of boron cations. Received: September 28, 2016

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DOI: 10.1021/acs.inorgchem.6b02336 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

around the boron center of 359.3°. Unfortunately, the presence of the disordered aluminate anion precluded a detailed comparison on the bond distances and angles of the molecule. Treating the borane−CAAC adduct with 2 equiv of AlCl3 in chlorobenzene afforded the corresponding boron dications [3c][AlCl4]2 in moderate yield. NMR characterizations of [3c][AlCl4]2 were carried out in CD3NO2. [3c]2+ exhibits a characteristic 11B NMR signal at −48.8 ppm and a sharp singlet at 2.3 ppm corresponding to the methyl groups of Cp* in the 1 H NMR spectrum. These observations are consistent with the η5-coordination of Cp* to the boron center. Similar to its NHO analogue ([3b][AlCl4]2), which decomposes rapidly in nitromethane, [3c][AlCl4]2 also slowly decays in CD3NO2. Crystals suitable for X-ray diffraction analysis were obtained by the slow evaporation of a chlorobenzene solution of [3c][AlCl4]2 in a glovebox. As shown in Figure 3, the ylidene−

Figure 1. Neutral carbon ligands utilized in the Cp*-substituted boron cation chemistry.

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RESULTS AND DISCUSSION Syntheses of [2c][Al(OC(CF3)3)4] and [3c][AlCl4]2. The general syntheses of the boron cations are summarized in Scheme 1. The synthesis and characterization of the IMes- and Scheme 1. General Syntheses of Cp*-Substituted Chloroboreniums and Boron Dications

Figure 3. Crystal structure of [3c][AlCl4]2. Counteranions and hydrogen atoms are omitted for clarity.

NHO-stabilized mono- and dications have been reported in our preliminary studies.12 While monochloride abstraction of 1a and 1b can be realized with trimethylsilyl trifluoromethanesulfonate, silver salts, or AlCl3 to yield the corresponding chloroboreniums ([2a]+ and [2b]+), [2c]+ can only be isolated from the reaction of 1c and Ag[Al(OC(CF3)3)4]. [2c]+ features a 11B NMR signal at 59.7 ppm, which is consistent with the formation of a tricoordinate borenium ion in the solution.12 The Cp* substituent in [2c]+ appears as a singlet at 1.53 ppm, suggesting the rapid sigmatropic rearrangement of the boron atom around the Cp* ring. Orange crystals suitable for X-ray diffraction analysis were grown by the slow evaporation of a dichloromethane (DCM) solution of [2c][Al(OC(CF3)3)4]. As shown in Figure 2, the boron center is tricoordinated with the sum of the bond angles

B bond distance of [3c]2+ [1.560(7) Å] is comparable to the C(ligand)−B bond distance in [3a]2+ [1.558(4) Å] and [3b]2+ (avg. 1.561 Å). The boron atom is sitting above the centroid of the Cp* ring with an average B−C bond length of 1.779 Å. The observed ylidene−B−Cp*centroid angle of 172.2° is between that of [3a]2+ (179.1°) and that of [3b]2+ (avg. 163.3°). Electrochemistry. To verify the ligand effect on the redox behavior of a Cp*-substituted boron dication, the reduction potentials of [3a−3c]2+ were examined with cyclic voltammetry. The reduction potentials of [3a]2+, [3b]2+, and [3c]2+ are respectively observed at −1.59, −0.82, and −0.85 V (vs Fc/ Fc+) in DCM (Figure 4). While irreversible reduction waves are detected for [3a]2+ and [3b]2+, the one-electron-reduction process of [3c]2+ is fully reversible. The reduction potentials of

Figure 2. Crystal structure of [2c][Al(OC(CF3)3)4]. The [Al(OC(CF3)3)4]− and hydrogen atoms are omitted for clarity.

Figure 4. Cyclic voltammograms of boron dications ([3a−3c]2+) in CH2Cl2. B


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Inorganic Chemistry

reduction of [2b]• does not result in the formation of a borabenzene derivative but a dissociation of the NHO−boron bond. On the other hand, one-electron reduction of [2c]+ is achieved with Cp2Co in benzene (Scheme 3). The electron paramagnetic resonance (EPR) signal of [2c]• at g = 2.0047 can be simulated with the hyperfine coupling constants shown in Figure 5. The observed a(14N) value of 6.50 G is comparable to

[3a−3c]2+ are markedly more anodic than those of boreniums, which normally display an irreversible reduction at −2 V (vs Fc/Fc+).3a,13 This result confirms that the accumulation of positive charges certainly enhances the electron deficiency of the molecule. In addition, this result also shows that the reduction behavior of the boron dication is highly dependent on the nature of the axial ligand. Unlike the irreversible reduction behavior of [3a]2+ and [3b]2+, the fully reversible reduction of [3c]2+ implies the existence of stable radical species with preserved η5-Cp*-B structure. In fact, the reduction potential of [3c]2+ (−0.85 V) resembles those of the CAAC-derived iminiums,14 suggesting that the reduction of [3c]2+ may occur at the ylidene carbon of CAAC instead of the nido-carborane cluster. Therefore, the CAAC-coordinated boron dication [3c]2+ (Form A, Scheme 2) can be alternatively described as a cationic boryl-substituted iminium (Form B), which is more relevant to the reduction at −0.85 V. Scheme 2. Resonance Structure of [3c]2+

Figure 5. EPR spectra of [2c]• (top) and [3c]•+ (bottom).

Chemical Reduction. In order to have a better understanding of the ligand effect on the redox chemistry of Cp*substituted boron cations, we have also carried out the chemical reduction of chloroboreniums [2a−2c]+ and boron dications [3a−3c]2+, and the results are summarized in Scheme 3. In all cases, the presence of CAAC−boron ligation is essential for realizing the persistent radical species.

those of CAAC-supported neutral carbon radicals (∼5.5 G)14,15 and boron radicals (∼6.6 G).8 Although delocalization of the spin density to the boron center is supported by the detection of hyperfine coupling to both boron [a(11B) = 1.60 G and a(10B) = 0.54 G] and chlorine [a(35Cl) = 1.50 G and a(37Cl) = 1.25G] atoms, the relative small values suggest that [2c]• should be ascribed as a carbon-centered radical. While chemical reductions of [3a][AlCl4]2 and [3b][AlCl4]2 in DCM result in the generation of the corresponding chloroboreniums, one-electron reduction of [3c][AlCl4]2 is accomplished via the addition of tetrakis(dimethylamino)ethylene. The EPR signal of [3c]•+ detected at g = 2.0032 can be simulated with hyperfine coupling constants of 9.00, 3.00, and 6.20 G to 11B, 10B, and 14N nuclei, respectively. The observed strong coupling to the boron and nitrogen centers implies that the unpaired electron at the ylidene carbon of CAAC is captodatively stabilized by the adjacent nitrogen atom and the cationic [C5B]+ moiety. Whereas the observed a(14N) is comparable to that of [2c]•, the 11B hyperfine coupling constant is significantly larger than those found in the CAACcoordinated boron radicals. The obtained 11B hyperfine coupling of 9.00 G in [3c]•+ is even greater than that measured for the radical cation of parent borylene [a(11B) = 6.432 G].8a Actually, the observed value is comparable to that determined for radical anions of triarylboranes,16 emphasizing the electronwithdrawing ability of the boron atom in the cationic nido cluster. Hydride Addition. The mechanistic study on the reaction of [3a]2+ and [Et3BH]− reported by our group showed that hydride addition took place at Cp* as a consequence of excessive kinetic protection at the boron center.17 Treating [3a]2+ with superhydride at 195 K yields a metastable 5borabicyclo[2.1.1]hex-2-ene borenium cation ([4a-CH]+),

Scheme 3. Chemical Reduction of Chloroboreniums and Boron Dications

Reduction of [2a][OTf] with 1 equiv of Cp*2Co in benzene resulted in the formation of a mixture that contains 1a and the IMes-coordinated borabenzene (Figure S21). We propose that the intuitive neutral radical [2a]• undergoes a further reduction in the presence of Cp*2Co to yield borabenzene and releases a chloride ion, which then attacks the remaining [2a]+ to give 1a. Treatment of [2b][OTf] with Cp2Co in benzene gave the borane adduct (1b) and protonated NHO (Figure S22). This result suggests that the neutral radical [2b]• undergoes a further reduction to release a chloride anion. However, the C


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Inorganic Chemistry which undergoes structural rearrangement to a boracyclohexa2,5-diene borenium cation ([5a-para]+) at elevated temperature. In an attempt to attain [4a-BH]+ from the hydride addition of [3a]2+, we have also carried out the same reaction with the least sterically demanding boron hydride, [Bu4N][BH4] (Scheme 4). Unfortunately, the reaction still yields [4a-

the addition of 1 equiv of Li[Et3BH] into the [3c][AlCl4]2 suspension in DCM at 195 K, all insoluble solids disappeared and the solution turned from colorless to light yellow. After warming to room temperature, all volatiles were removed to give a complicated mixture, from which [5c-para]+ was

Scheme 4. Hydride Addition to [3a]2+ and [3b]2+

Scheme 5. Hydride Addition to [3c]2+

obtained in moderate yield (Scheme 5). [5c-para]+ features a 11 B NMR resonance at 52.4 ppm comparable to that of [5apara]+ (50.1 ppm), suggesting the formation of a cyclic borenium cation.18 In the 1H NMR spectrum of [5c-para]+, the five methyl groups of Cp* split into three peaks with a 6:6:3 ratio. The p-methyl group detected as a doublet at 0.68 ppm is coupled to the p-hydrogen atom (2.96 ppm) with a 3JH−H coupling constant of 7.2 Hz. The structural connectivity of [5cpara]+ is further corroborated with single-crystal X-ray diffraction analysis. As shown in Figure 6, the formation of a

CH]+ exclusively. Thus, we are not able to distinguish the two electrophilic sites of [3a]2+, the boron atom and the sp2 carbon of Cp*, with different boron hydride reagents. In contrast, the hydride addition products of [3b]2+ and [3c]2+ are dependent on the hydride source. Upon the addition of [Et3BH]− to a DCM suspension of [3b][AlCl4]2 at 213 K, the characteristic 11B NMR signal of [3b]2+ shifted from −45.6 to −29 ppm. The observed highly shielded 11B resonance is consistent with the formation of a 5borabicyclo[2.1.1]hex-2-ene borenium cation, [4b-CH]+; Scheme 4). The five methyl groups of Cp* split into three signals at 1.14, 0.74, and 0.51 ppm with integration equal to six, three, and six hydrogen atoms, respectively. At 253 K, [4bCH]+ transforms into [5b-meta]+, which decomposes at temperatures higher than 263 K. [5b-meta]+ features a 11B chemical shift at 28 ppm and five distinct 1H NMR signals for the methyl groups of Cp*. Attempts in the deprotonation or isolation of [5b-meta]+ at low temperature failed because of facile cleavage of the boron−NHO bond. Interestingly, treating [3b]2+ with [BH4]− results in the formation of a hydridoborenium [4b-BH]+. The observed 11B resonance for [4b-BH]+ at 37 ppm is slightly downfield-shifted compared to that determined for [4a-BH]+ (21 ppm).17 Like [4a-BH]+, the five methyl groups of Cp* of [4b-BH]+ are detected as a sharp singlet at 1.52 ppm as a result of rapid sigmatropic rearrangement. The reaction with [BH4]− indicates that the introduction of a NHO ligand leads to a more exposed and reactive boron center. On the other hand, the reaction between [3c]2+ and [Et3BH]− is not as clean as that of [3a]2+ and [3b]2+. Upon

Figure 6. Crystal structure of [5c-para][AlCl4]. Counteranions and hydrogen atoms (except p-CH) are omitted for clarity.

six-membered cyclic borenium cation featuring an sp3 carbon para to the boron atom is confirmed. The relatively short B−C bond distances within the ring [1.537(4) and 1.538(4) Å] compared with the exocyclic B−CCAAC bond [1.614(4) Å] indicate the existence of delocalization of π electrons. Furthermore, the isolation of [5c-para]+ suggests that [3c]2+ undergoes hydride addition at Cp* to yield the 5borabicyclo[2.1.1]hex-2-ene borenium cation intermediate D


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Inorganic Chemistry ([4c-CH]+), which cannot be unambiguously identified in the reaction mixture. Unexpectedly, when [3c][AlCl4]2 is treated with [Bu4N][BH4], a completely different product is observed. The 11B NMR spectroscopy of [4c-C′H]+ at −44.4 ppm is referred to as a well-shielded boron atom, and the five methyl groups of Cp* are detected as a sharp singlet at 2.02 ppm. These two spectral features imply the preservation of the η5-Cp*-B moiety in [4cC′H]+. While the Cp* group remains symmetrical in solution, the CAAC fragment becomes highly unsymmetrical due to the addition of hydride at the ylidene carbon. All 1H signals belonging to CAAC are split into two sets, and the ylidene CH observed at 3.39 ppm as a singlet is identified from a 2D HSQC experiment. Unfortunately, [4c-C′H]+ decomposes in the reaction solution within 24 h, and attempts to isolate [4cC′H][AlCl4] were unsuccessful. Nevertheless, these spectral characterizations infer that hydride attacks the CAAC ligand of [3c]2+, and the addition product, [4c-C′H]+, can be described as a hypercoordinatively stabilized dialkylborinium cation. Theoretical Calculations. As shown in the previous sections, the electrochemical properties and hydride addition reaction of Cp*-substituted boron dications are highly dependent on the nature of the carbon ligand. To shed light on the effect of these carbon ligands on the boron center, we have carried out DFT calculations on [3b]2+ and compared the data with that reported for [3a−3c]2+.12b As indicated by the natural population analysis (NPA) calculations of [3a−3c]2+ (Figure 7), the boron center of [3b]2+ is the most electron

Table 1. Calculated CIA and FIA Lewis acids (LA)

CIA (kcal/mol)a

FIA (kcal/mol)b

2+

23.93 9.61 14.4 24.72 36.22 0

−48.25 −62.11 −58.09 −50.85 −26.79

[3a] [3b]2+ [3c]2+ [LutBCl2]+ B(C6F5)3 AlCl3 (ref) BEt3 (ref)

0

ΔH of the LA + [AlCl4]− → [LA-Cl]− + AlCl3 reaction. bΔH of the LA + [Et3BF]− → [LA-F]− + BEt3 reaction. a

to a CIA/FIA value comparable to that of [LutBCl2]+. A comparison between [3a]2+ and [3c]2+ confirms that the incorporation of π-acidic ligand to the system indeed enhances the Lewis acidity of the boron center. Interestingly, the most acidic boron dication in the series is [3b]2+. Once again, the high Lewis acidity of [3b]2+ can be explained by the deformed nido-[C5B] structure resulted from steric repulsion between NHO and Cp*. In other words, the Lewis acidity of the Cp*substituted boron dication is predominantly controlled by the interaction between the filled π-bonding orbitals of the Cp* anion and the empty p orbitals of the boron dication. To elucidate the selectivity of the hydride addition reaction, the addition products of these three boron dications are also examined computationally. As shown in Table S4, the addition products ([4a-CH]+, [4b-CH]+, and [4c-CH]+) are 8.5, 19.6, and 4.2 kcal/mol higher in energy compared to the related hydridoborenium ([4a-BH]+ and [4b-BH]+) and dialkylborinium ([4c-C′H]+) isomers, respectively. Thus, the addition of hydride at the sp2 carbon of Cp* of [3a−3c]2+ is merely a kinetic result. When the less sterically demanding [BH4]− is introduced, the more thermodynamically stable [4b-BH]+ and [4c-C′H]+ are achieved.

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CONCLUSION The effects of the carbon ligand on Cp*-substituted boron cations are investigated experimentally and computationally. Electrochemical studies of [3a−3c]2+ confirm the higher electron deficiency of boron dications than boreniums and boranes. Besides, the presence of CAAC is essential for persistent free-radical species. A neutral radical ([2c]•) and radical cation ([3c]•+) generated from chemical reduction of the corresponding chloroborenium and dication have been characterized using EPR spectroscopy. The reactions between [3a−3c]2+ and boron hydrides are found to be dependent on both the size of the hydride molecule and the nature of the axial ligand. When treated with [Et3BH]−, all three boron dications ([3a−3c]2+) transform into planar cyclic boreniums via the addition of hydride at Cp*. While [3a]2+ cannot distinguish [Et3BH]− and [BH4]− because of the excessive kinetic protection of the boron center, the angular geometry of [3b]2+ creates a more exposed boron center that selectively reacts with [BH4]− to yield the corresponding hydridoborenium cation. In the presence of πacidic CAAC, the reaction of [BH4]− and [3c]2+ affords a η5Cp*-coordinated dialkylborinium cation via the addition of hydride at the ylidene center of CAAC. NPA, CIA, and FIA calculations of [3a−3c]2+ reveal that [3a]2+ is the least acidic compound in the series. Although the introduction of a π-acidic CAAC ligand indeed increases the acidity of the boron center, disruption of the π-bonding

Figure 7. NPA charges of [3a−3c]2+ obtained from natural bond orbital analysis.

positive one in the series. This can be explained by the steric repulsion between Cp* and NHO that disrupts the π-bonding interactions between the Cp* anion and boron cation, leading to a more electrophilic boron center in [3b]2+. On the other hand, the higher positive charge at the boron center of [3c]2+ compared to [3a]2+ is attributed to the higher π-acidity of CAAC. This is in accordance with the lower Q(ligand) value of [3c]2+ than that of [3a]2+. In order to gain more information about the Lewis acidity of [3a−3c]2+, we have also calculated the chloride ion affinity (CIA) and fluoride ion affinity (FIA) of [3a−3c]2+, 2,6lutidine-stabilized dichloroborenium cation ([LutBCl2]+) and (C6F5)3B (Table 1) in DCM.19 As expected, both the CIA and FIA values of cationic species, [3a−3c]2+ and [LutBCl2]+, are higher than that of neutral borane, reflecting the additive columbic attraction on the Lewis acid−base interaction. However, the hypercoordinated environment in [3a] 2+ significantly restrained the acidity of the boron center, leading E


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Inorganic Chemistry

Hz, 6H, CHMe2). 13C NMR (100.6 MHz, CD3NO2, 298 K): δ 142.2 (o-C, Dipp), 129.7 (ipso-C, Dipp), 129.3 (p-C, Dipp), 123.6 (m-C, Dipp), 113.7 (C5, Cp*), 85.1 (CMe2), 58.8 (CCy), 42.3 (CH2), 29.1 (H2CCy), 25.5 (CHMe2), 24.8 (CMe2), 22.8 (CHMe2), 19.3 (CHMe2), 19.2 (H2CCy), 16.4 (H2CCy), 6.0 (Me, Cp*). 11B NMR (128.4 MHz, CD3NO2, 298 K): δ −48.8. Anal. Calcd for C33H50Al2BCl8N: C, 48.98; H, 6.23; N, 1.73. Found: C, 48.97; H, 6.27; N, 1.72. Generation of [4b-CH]+ and [5b-meta]+. A solution of [3b][AlCl4]2 (25 mg, 0.028 mmol) in 0.5 mL of CD2Cl2 in Young’s NMR tube was cooled to 213 K before the addition of Li[Et3BH] (1.0 M in THF, 30 μL, 0.030 mmol). The tube was then quickly transferred to an NMR spectrometer. Because [4b-CH]+ and [5b-meta]+ could not be isolated, spectroscopic characterizations of these two cations were performed on the reaction mixture at 233 and 253 K, respectively. [4bCH]+. 1H NMR (500.1 MHz, CD2Cl2, 233 K): δ 7.62 (t, 2H, p-Dipp), 7.52 (s, 2H, NCH), 7.41 (d, 4H, m-Dipp), 2.33 (m, 4H, CHMe2), 1.30 (d, 12H, CHMe2), 1.15 (d, 12H, CHMe2), 1.14 (s, 6H, Me, Cp*), 0.74 (d, 3H, Me, Cp*), 0.51 (s, 6H, Me, Cp*). 11B NMR (160.5 MHz, CD2Cl2, 233 K): δ −29. [5b-meta]+. 1H NMR (500.1 MHz, CD2Cl2, 253 K): δ 7.63 (t, 2H, p-Dipp), 7.51 (s, 2H, NCH), 7.39 (d, 4H, mDipp), 2.77 (d, 1H, IPrCH2), 2.45 (d, 1H, IPrCH2), 2.33 (m, 2H, CHMe2), 2.22 (m, 2H, CHMe2), 1.90 (s, 3H, Me, Cp*), 1.79 (s, 3H, Me, Cp*), 1.78 (s, 3H, Me, Cp*), 1.36 (d, 6H, CHMe2), 1.24 (s, 3H, Me, Cp*), 1.17 (d, 6H, CHMe2), 0.93 (d, 6H, CHMe2), 0.91 (d, 6H, CHMe2), 0.40 (d, 3H, Me, Cp*). 11B NMR (160.5 MHz, CD2Cl2, 253 K): δ 28. Generation of [4b-BH]+. A solution of [3b][AlCl4]2 (20 mg, 0.022 mmol) in 0.5 mL of CD2Cl2 in Young’s NMR tube was cooled to 213 K before the addition of [Bu4N][BH4] (0.50 M in CH2Cl2, 40 μL, 0.020 mmol). The tube was allowed to warm to room temperature slowly, and the spectroscopic characterizations were performed on the reaction mixture at room temperature. 1H NMR (400.2 MHz, CD2Cl2, 298 K): δ 7.71 (t, 2H, p-Dipp), 7.48 (d, 4H, m-Dipp), 7.41 (s, 2H, NCH), 2.32 (m, 4H, CHMe2), 1.82 (s, 2H, IPrCH2), 1.52 (s, 15H, Cp*), 1.28 (d, 12H, CHMe2), 1.24 (d, 12H, CHMe2). 11B NMR (128.4 MHz, CD2Cl2, 298 K): δ 37. Synthesis of [5c-para][AlCl4]. Li[Et3BH] (1.0 M in THF, 125 μL, 125 μmol) was added to a DCM suspension (5 mL) of [3c][AlCl4]2 (101 mg, 125 μmol) at 195 K. The pale-yellow solution was allowed to stir at room temperature for 4 h. Afterward, all volatiles were removed under vacuum, and the residue was washed with Et2O to yield a paleyellow solid mixture containing [5c-para][AlCl4] (38 mg). Crystals of [5c-para][AlCl4] suitable for X-ray diffraction analysis was grown through the slow evaporation of a benzene solution. 1H NMR (400.2 MHz, CDCl3, 298 K): δ 7.42 (t, J = 7.8 Hz, 1H, p-Dipp), 7.25 (d, J = 7.8 Hz, m-Dipp), 2.96 (q, J = 7.2 Hz, 1H, C5BMe5H), 2.71 (sept, J = 6.4 Hz, 2H, CHMe2), 2.59 (s, 2H, CH2), 1.98 (s, 6H, C5BMe5H), 1.95 (s, 6H, C5BMe5H), 1.54 (s, 6H, CMe2), 1.36 (d, J = 6.4 Hz, 6H, CHMe2), 1.05 (d, J = 6.4 Hz, 6H, CHMe2), 0.68 (d, J = 7.2 Hz, 3H, C5BMe5H). 11B NMR (128.4 MHz, CDCl3, 298 K): δ 52.4. Generation of [4c-C′H]+. A suspension of [3c][AlCl4]2 (50.0 mg, 61.8 μmol) in 0.8 mL of CD2Cl2 in Young’s NMR tube was cooled to 195 K before the addition of [Bu4N][BH4] (0.50 M in CH2Cl2, 124 μL, 62 μmol). The resulting mixture was allowed to thermoequilibrate inside an NMR spectrometer. Because the ammonium salt could not be separated from [4c-C′H]+, spectroscopic characterization of [4cC′H]+ was performed on the crude reaction mixture at 243 or 273 K. 1 H NMR (500.1 MHz, CD2Cl2, 243 K): δ 7.19−7.08 (m, 3H, Dipp), 3.30 (s, 1H, NC′HB), 3.29 (sept, J = 6.6 Hz, 1H, CHMe2), 2.72 (sept, J = 6.6 Hz, 1H, CHMe2), 2.31 (d, J = 13.4 Hz, 1H, CH2), 1.96 (s, 15H, Me, Cp*), 1.86−0.68 (m, 10H, H2CCy), 1.73 (d, J = 13.4 Hz, 1H, CH2), 1.41 (d, J = 6.6 Hz, 3H, CHMe2), 1.32 (s, 3H, CMe2), 1.28 (d, J = 6.6 Hz, 3H, CHMe2), 1.12 (d, J = 6.6 Hz, 3H, CHMe2), 1.07 (d, J = 6.6 Hz, 3H, CHMe2), 0.86 (s, 3H, CMe2). 13C NMR (125.8 MHz, CD2Cl2, 243 K): δ 149.2 (o-C, Dipp), 147.5 (o-C, Dipp), 144.8 (ipsoC, Dipp), 127.0 (m-C, Dipp), 126.7 (p-C, Dipp), 124.8 (m-C, Dipp), 113.6 (C5, Cp*), 65.3 (CMe2), 63.0 (br, NC′HB), 51.8 (CH2), 46.3 (CCy), 39.5 (H2CCy), 37.6 (H2CCy), 32.6 (CMe2), 29.1 (CHMe2), 28.5 (CMe2), 28.3 (CHMe2), 27.2 (CHMe2), 26.0 (CHMe2), 25.9 (CHMe2), 25.6 (H2CCy), 24.5 (CHMe2), 24.4 (H2CCy), 22.8

interaction between Cp* and boron atoms is indispensable to realizing a highly acidic boron dication. Thus, [3b]2+ featuring the most distorted [C5B] fragment possesses the strongest Lewis acidity in the series.

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EXPERIMENTAL SECTION

General Information. All manipulations were operated under an inert atmosphere using a standard Schlenk technique or inside a glovebox. Compounds [2a−2b][OTf] and [3a−3b][AlCl4]2 were prepared according to the reported procedures.12 Dichloromethane (DCM), toluene, and n-hexane were dried with a molecular-sieveloaded solvent purification system. Diethyl ether (Et2O) and tetrahydrofuran (THF) were dried over a sodium/potassium alloy and distilled under nitrogen. Chlorobenzene and CD2Cl2 were dried by P2O5 and distilled under reduced pressure. CD3NO2 and CDCl3 were dried over CaH2 and distilled under nitrogen. AlCl3 was purified via sublimation and stored in a glovebox. NMR spectra were recorded on Bruker AVIII-400 (1H, 400.2 MHz; 11B, 128.4 Hz; 13C, 100.6 MHz), Bruker DMX-500 (1H, 500.1 MHz; 11B, 160.5 Hz; 13C, 125.8 MHz), and Bruker AVIII-800 (13C, 201 MHz) FT-NMR spectrometers. Chemical shifts (δ) are given in ppm and are referenced to the signals of the residual solvent (1H and 13C) or external BF3·OEt2 (11B). EPR measurement was carried out on a Bruker EMXnano spectrometer. EPR spectra were simulated using Bruker WinEPR SimFonia software. Elemental analyses were performed on a Heraeus varioIII-NCH elemental analyzer. Synthesis of 1c. CAAC (3.04 mmol) in situ generated from the corresponding iminium salt was added to a hexane solution of CpBCl2 (3.0 mmol) at 195 K. After warming to room temperature, all volatiles were removed under vacuum and the solid residue was extracted with benzene to give 1c as a light-yellow solid. Yield: 844 mg (51%). 1H NMR (400.2 MHz, C6D6, 298 K): δ 7.10−7.00 (m, 3H, Dipp), 2.98 (sept, J = 6.5 Hz, 2H, CHMe2), 2.34 (s, 6H, Me, Cp*), 2.37−2.27 (m, 2H, H2CCy), 1.90 (s, 6H, Me, Cp*), 1.74 (s, 3H, Me, Cp*), 1.56 (s, 2H, CH2), 1.56−1.03 (m, 8H, H2CCy), 1.51 (d, J = 6.5 Hz, 6H, CHMe2), 1.14 (d, J = 6.5 Hz, 6H, CHMe2), 0.85 (s, 6H, CMe2). 13C NMR (201 MHz, C6D6, 298 K): δ 219.2 (Ccarbene), 147.6 (C5, Cp*), 144.4 (o-C, Dipp), 137.4 (ipso-C, Dipp), 133.3 (C5, Cp*), 128.8 (p-C, Dipp), 125.3 (m-C, Dipp), 80.7 (CMe2), 59.7 (CCy), 56.9 (C5, Cp*), 45.8 (CH2), 33.8 (H2CCy), 29.5 (CHMe2), 29.3 (CMe2), 28.0 (CHMe2), 26.9 (Me, Cp*), 24.9 (CHMe2), 23.8 (H2CCy), 22.6 (H2CCy), 15.1 (Me, Cp*), 11.8 (Me, Cp*). 11B NMR (128.4 MHz, C6D6, 298 K): δ 5.6. Synthesis of [2c][Al(OC(CF3)3)4]. The addition of AgAl(OC(CF3)3)4 (128 mg, 0.119 mmol) to a benzene solution of 1c (61.0 mg, 0.112 mmol) resulted in the precipitation of AgCl and the anticipated borenium cation, which was then extracted with DCM. Evaporation of the filtrate afforded [2c][Al(OC(CF3)3)4] as an orange solid. Yield: 120 mg (73%). 1H NMR (500.1 MHz, CDCl3, 298 K): δ 7.57 (t, J = 7.8 Hz, 1H, p-Dipp), 7.41 (d, J = 7.8 Hz, 2H, m-Dipp), 2.67 (br, 2H, CHMe2), 2.24 (s, 2H, CH2), 1.92−1.13 (m, 10H, H2CCy), 1.55 (s, 6H, CMe2), 1.53 (s, 15H, Me, Cp*), 1.28 (dd, J = 6.8 Hz, 12H, CHMe2). 13C NMR (125.8 MHz, CDCl3, 298 K): δ 209.4 (Ccarbene), 145.1 (o-C, Dipp), 132.4 (p-C, Dipp), 131.7 (ipso-C, Dipp), 127.9 (C5, Cp*), 127.4 (m-C, Dipp), 121.3 (q, J = 293 Hz, CF3), 86.1 (CMe2), 79.0 (br, C(CF3)3), 60.1 (CCy), 43.6 (CH2), 35.8 (H2CCy), 29.6 (CHMe2), 25.5 (CHMe2), 24.9 (CHMe2), 24.5 (H2CCy), 21.7 (H2CCy), 13.4 (Me, Cp*). 11B NMR (160.5 MHz, CDCl3, 298 K): δ 59.7. Anal. Calcd for C49H50AlBClF36NO4: C, 39.92; H, 3.42; N, 0.95. Found: C, 39.64; H, 3.31; N, 0.96. Synthesis of [3c][AlCl4]2. A solution of 1c (215 mg, 396 μmol) in chlorobenzene (10 mL) was allowed to react with AlCl3 (109 mg, 817 μmol) at room temperature for 30 min. After the reaction, Et2O (20 mL) was added and the mixture was stirred for 5 min. The precipitate was collected and washed with Et2O to yield [3c][AlCl4]2 as a white solid. Yield: 205 mg (64%). 1H NMR (400.2 MHz, CD3NO2, 298 K): δ 7.88−7.71 (m, 3H, Dipp), 2.74 (sept, J = 6.5 Hz, 2H, CHMe2), 2.73 (s, 2H, CH2), 2.30 (s, 15H, Me, Cp*), 2.02−1.53 (m, 10H, H2CCy), 1.82 (s, 6H, CMe2), 1.52 (d, J = 6.5 Hz, 6H, CHMe2), 1.33 (d, J = 6.5 F


Article

Inorganic Chemistry (H2CCy), 10.1 (Me, Cp*). 11B NMR (160.5 MHz, CD2Cl2, 243 K): δ −44.4. Structural Determination. Crystallographic data of [2c][Al(OC(CF3)3)4], [3c][AlCl4]2, and [5c-para][AlCl4] were collected with an Oxford Gemini Duo system diffractometer using graphitemonochromated Cu Kα radiation (200 K, λ = 1.54178 Å) or Mo Kα radiation (150 K, λ = 0.71073 Å). The structures were solved by direct methods and refined by least-squares cycles. The non-hydrogen atoms were refined anisotropically. All calculations were performed using the SHELXTL-97 package. Crystallographic data of [2c][Al(OC(CF3)3)4], [3c][AlCl4]2, and [5c-para][AlCl4] have been deposited at the Cambridge Crystallographic Data Center as CCDC 1505513, 1505515, and 1505514, respectively.

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4252−4253. (b) Ueng, S.-H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.; Lacôte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D. P. N-Heterocyclic Carbene Boryl Radicals: A New Class of Boron-Centered Radical. J. Am. Chem. Soc. 2009, 131, 11256−11262. (c) Walton, J. C.; Brahmi, M. M.; Monot, J.; Fensterbank, L.; Malacria, M.; Curran, D. P.; Lacôte, E. Electron Paramagnetic Resonance and Computational Studies of Radicals Derived from Boron-Substituted NHeterocyclic Carbene Boranes. J. Am. Chem. Soc. 2011, 133, 10312− 10321. (4) (a) Farrell, J. M.; Hatnean, J. A.; Stephan, D. W. Activation of Hydrogen and Hydrogenation Catalysis by a Borenium Cation. J. Am. Chem. Soc. 2012, 134, 15728−15731. (b) Farrell, J. M.; Posaratnanathan, R. T.; Stephan, D. W. A family of N-heterocyclic carbene-stabilized borenium ions for metal-free imine hydrogenation catalysis. Chem. Sci. 2015, 6, 2010−2015. (c) Eisenberger, P.; Bestvater, B. P.; Keske, E. C.; Crudden, C. M. Hydrogenations at Room Temperature and Atmospheric Pressure with Mesoionic Carbene-Stabilized Borenium Catalysts. Angew. Chem., Int. Ed. 2015, 54, 2467−2471. (5) (a) Lavallo, V.; Canac, Y.; Präsang, C.; Donnadieu, B.; Bertrand, G. Stable Cyclic (Alkyl) (Amino)Carbenes as Rigid or Flexible, Bulky, Electron-Rich Ligands for Transition-Metal Catalysts: A Quaternary Carbon Atom Makes the Difference. Angew. Chem., Int. Ed. 2005, 44, 5705−5709. (b) Soleilhavoup, M.; Bertrand, G. Cyclic (Alkyl) (Amino)Carbenes (CAACs): Stable Carbenes on the Rise. Acc. Chem. Res. 2015, 48, 256−266. (6) Ruiz, D. A.; Ung, G.; Melaimi, M.; Bertrand, G. Deprotonation of a Borohydride: Synthesis of a Carbene-Stabilized Boryl Anion. Angew. Chem., Int. Ed. 2013, 52, 7590−7592. (7) Böhnke, J.; Braunschweig, H.; Ewing, W. C.; Hörl, C.; Kramer, T.; Krummenacher, I.; Mies, J.; Vargas, A. Diborabutatriene: An Electron-Deficient Cumulene. Angew. Chem., Int. Ed. 2014, 53, 9082− 9085. (8) (a) Kinjo, R.; Donnadieu, B.; Celik, M. A.; Frenking, G.; Bertrand, G. Synthesis and Characterization of a Neutral Tricoordinate Organoboron Isoelectronic with Amines. Science 2011, 333, 610. (b) Ruiz, D. A.; Melaimi, M.; Bertrand, G. An efficient synthetic route to stable bis(carbene)borylenes [(L1)(L2)BH]. Chem. Commun. 2014, 50, 7837−7839. (c) Dahcheh, F.; Martin, D.; Stephan, D. W.; Bertrand, G. Synthesis and Reactivity of a CAAC−Aminoborylene Adduct: A Hetero-Allene or an Organoboron Isoelectronic with Singlet Carbenes. Angew. Chem., Int. Ed. 2014, 53, 13159−13163. (d) Bissinger, P.; Braunschweig, H.; Damme, A.; Krummenacher, I.; Phukan, A. K.; Radacki, K.; Sugawara, S. Isolation of a Neutral BoronContaining Radical Stabilized by a Cyclic (Alkyl) (Amino)Carbene. Angew. Chem., Int. Ed. 2014, 53, 7360−7363. (9) (a) Malcolm, A. C.; Sabourin, K. J.; McDonald, R.; Ferguson, M. J.; Rivard, E. Donor−Acceptor Complexation and Dehydrogenation Chemistry of Aminoboranes. Inorg. Chem. 2012, 51, 12905−12916. (b) Wang, Y.; Abraham, M. Y.; Gilliard, R. J.; Sexton, D. R.; Wei, P.; Robinson, G. H. N-Heterocyclic Olefin Stabilized Borenium Cations. Organometallics 2013, 32, 6639−6642. (c) Ghadwal, R. S.; Schürmann, C. J.; Engelhardt, F.; Steinmetzger, C. Unprecedented Borylene Insertion into a C−N Bond. Eur. J. Inorg. Chem. 2014, 2014, 4921− 4926. (10) (a) Berger, C. J.; He, G.; Merten, C.; McDonald, R.; Ferguson, M. J.; Rivard, E. Synthesis and Luminescent Properties of Lewis BaseAppended Borafluorenes. Inorg. Chem. 2014, 53, 1475−1486. (b) Momeni, M. R.; Shulman, L.; Rivard, E.; Brown, A. Interplay of donor-acceptor interactions in stabilizing boron nitride compounds: insights from theory. Phys. Chem. Chem. Phys. 2015, 17, 16525−16535. (11) Powers, K.; Hering-Junghans, C.; McDonald, R.; Ferguson, M. J.; Rivard, E. Improved synthesis of N-heterocyclic olefins and evaluation of their donor strengths. Polyhedron 2016, 108, 8−14. (12) (a) Shen, C.-T.; Liu, Y.-H.; Peng, S.-M.; Chiu, C.-W. A DiSubstituted Boron Dication and Its Hydride-Induced Transformation to an NHC-Stabilized Borabenzene. Angew. Chem., Int. Ed. 2013, 52, 13293−13297. (b) Lee, W.-H.; Lin, Y.-F.; Lee, G.-H.; Peng, S.-M.;

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02336. NMR spectrum of 1c, [2c][Al(OC(CF3)3)4], [3c][AlCl4]2, [4b-CH]+, [4b-BH]+, [5b-meta]+, [4c-C′H]+, and [5c-para]+, electrochemistry data, chemical reduction of [2a][OTf] and [2b][OTf], crystallographic data of [2c][Al(OC(CF3)3)4], [3c][AlCl4]2, and [5c-para][AlCl4], and DFT calculations (PDF) Cartesian coordinates of the optimized structures (XYZ) Crystallographic data (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology of Taiwan (Grants MOST 103-2113-M-002-005 and MOST 104-2113-M-002-018-MY3) and National Taiwan University (Grant NTU-CESRP-104R7619). Dr. Y.-F. Lin extends gratitude to MOST for a postdoctoral fellowship (Grant MOST 104-2811-M-002-130). We also thank ShouLing Huang for NMR experiments.

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REFERENCES

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