Activation of H2 and Et3SiH by the Borinium Cation [Mes2B]+:

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Activation of H2 and Et3SiH by the Borinium Cation [Mes2B]+: Avenues to Cations [MesB(µ-H)2(µ-Mes)BMes]+ and [H2B(µ-H)(µ-Mes)B(µ-Mes)(µ-H)BH2]+ Karlee L. Bamford, Zheng-Wang Qu, and Douglas W. Stephan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02510 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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Journal of the American Chemical Society

Activation of H2 and Et3SiH by the Borinium Cation [Mes2B]+: Avenues to Cations [MesB(-H)2(-Mes)BMes]+ and [H2B(-H)(Mes)B(-Mes)(-H)BH2]+ Karlee L. Bamford,a Zheng-Wang Qu,*b and Douglas W. Stephan*a aDepartment

of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada, email: [email protected] bMulliken Center for Theoretical Chemistry, University of Bonn, Beringstr. 4, D-53115 Bonn, Germany, e-mail: [email protected] Supporting Information Placeholder ABSTRACT: Reaction of dihydrogen and the borinium cation [Mes2B][B(C6F5)4] gave an arene-stabilized mesityl-borenium cation, which isotopically scrambles HD. Similarly reaction with triethylsilane gave the diboranium cation, [MesB(H)2(-Mes)BMes]+ featuring three bridge-bonds between two boron centres. Reaction of [Mes2B][B(C6F5)4] with Brønsted acid also afforded the diboranium whereas the corresponding reaction with (MesBH2)2 yielded the triboron cation [H2B(H)(-Mes)B(-Mes)(-H)BH2]+.

In seminal work, Power described the reaction of germene and germyne with dihydrogen.1 While this was the first such report for a main group species, it was the uncovering2 of the notion of frustrated Lewis pair (FLP) chemistry that emerged from the reactivity of the combination of Lewis acidic boranes and basic phosphines with H2 that led to the widespread focus on main group reactivity and applications in synthesis and catalysis. Since those initial findings, catalytic hydrogenation of a variety of organic substrates as well as strategies to highly selective metal-free asymmetric reductions have emerged. In addition, FLP chemistry has expanded conceptually3-4 to include a diverse range of Lewis acids and bases,5 allowing the capture and activation of a broad array of small molecule substrates. Select FLP systems have been shown to react via transient radical pairs,6 while seemingly stable classical Lewis acid-base adducts have been shown to exhibit FLP reactivity.7 Moreover, the concept has been creatively applied in the development of new materials and polymers, and in describing transition metal and enzymatic chemistry as well as heterogeneous catalysts.8 Despite these developments, FLP activation of dihydrogen remains of interest. Computational studies revealed a concerted reaction involving an encounter complex as the Lewis acid and Lewis base approach with the subsequent activation of H2 in an almost barrierless process.9-11 This mechanism appears operative for intermolecular combinations of Lewis acids and bases as well as intramolecular FLPs in which the Lewis acid and base sites are tethered together.

Interestingly, systems in which the acidic and basic sites are directly linked have been shown to exhibit similar reactivity with H2 (Figure 1). For example, the species (C6F5)2BPR2 reacts with H2 despite the presence of a covalent B-P bond.12 In this case, the reactivity was attributed to the electronic mismatch in energies of the vacant p-orbital on boron and the lone pair on phosphorus. While B2H6 is known to effect H-D exchange in the presence of D2,13 Piers and coworkers reported the activation of H2 with a highly electrophilic perfluoroaryl-substituted borole in 2010.14-15 Bourissou subsequently described the reaction of a cationic phosphinestabilized borenium with H2.16-17 Li, Wang and Nikonov18-19 showed that electrophilic hydridoboranes could mediate isotopic scrambling of HD, demonstrating activation of H2 by an electrophilic center without the presence of a Lewis base. Similarly, both apolar symmetric diboranes and polar, neutral and charged diboron species20-21 react with H2, further suggesting that electrophilic centers interact with the σorbital of H2, leading to polarization and reactivity of the HH bond.15, 22 Dipp Dipp

Ge

F

Ge

Dipp

Dipp

C6F5

C6F5 C6F5 B C6F5

C6F5

F

Piers, 2010

R 2P

R

C6F5 C6F5

R = tBu or Cy R = tBu or Mes Mes = C6H3-2,4,6-Me3

Stephan, 2006

Power, 2005

P B

B(C6F5)3

B(C6F5)2 F

Dipp = C6H3-2,6-iPr2

R

PR3

F

Mes2P

B

Stephan, 2007 Mes NTf2

R = Ph or iPr Bourissou, 2015/2016

ArF

H B

Stephan, 2011

ArF

ArF = C6F5 or C6H2(CF3)3 Li & Wang, 2011 Nikonov, 2012

Figure 1. Examples of main group reagents that react with directly with H2. Our interest in FLP reactivity prompted the study of boron cations. Three coordinate borenium cations proved to be highly effective catalysts for imine and enamine hydrogenations and hydrosilylations.23-26 The application of

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borenium cations towards catalysis has recently been reviewed.27 Despite the fact that two-coordinate borinium cations were first prepared in the 1980s, the reactivity of such species have garnered little attention.28-31 However, in a relatively recent and seminal finding, Shoji and coworkers described the synthesis of the borinium cation [Mes2B][B(C6F5)4] 1. This species has been shown to be highly reactive towards CO2, CS2 and alkyne.30-32 In this work, we explore the reactivity of Shoji’s borinium cation with dihydrogen and silane. We show that this highly electrophilic species affords access to a diboranium ([B2X5]+) cation. The mechanism of these reactions are probed computationally and based on the nature of this product, other synthetic avenues to this species and a related [B3X8]+ species are developed. The borinium salt 1 in 1,2-dichlorobenzene (ODCB) was pressurized with 4 atm of HD (Scheme 1) and monitored by 1H and 2H{1H} NMR spectroscopy over a 24 hour period at 25 °C. Slow isotopic scrambling generating H2 D2 was apparent while the 11B resonance for 1 (δ 93.5 ppm) gradually disappeared and a new signal at δ 25.2 ppm appeared. 1H and 2H{1H} NMR spectra were consistent with and the formation of mesitylene/mesitylene-d1. These observations infer HD cleavage generates a B-H/D bond and induces protonation of a mesityl substituent, suggesting the generation of the borenium-mesitylene adduct [MesB(E)(C6Me3H2E)]+ (E = H, D) (A+, Scheme 1).

B

4 atm. HD ODCB

1+

E

HD

B E A+

E E B

-H2 or D2 E = H, D

TSAH2+

E

E

Scheme 1. Reaction of 1 with HD (anions are omitted). The mechanism of this isotopic scrambling was considered. The possibility that Friedel-Crafts chemistry of 1 with solvent generates Brønsted acid which mediates isotopic scrambling was dismissed as a solution of [H(MesH)][B(C6F5)4]33-34 alone under an HD atmosphere showed no reaction over several days. Similarly, a transient hydridoborane effecting HD scrambling was also dismissed as such reactions are only seen at room temperature with highly electrophilic hydridoboranes.18-19, 35-36

Figure 2. DFT computed free energy paths (in kcal/mol, at 298 K, ref. conc.: 1M) for the reaction of borinium 1+ with H2 in ODCB solution. The crucial B, C and H atoms are highlighted as grey, pink and white balls, respectively, with selected bond lengths shown in Å.

DFT computations at the PW6B95-D3 + COSMO-RS // TPSSD3 + COSMO level of theory37-51 in ODCB solution showed that the borinium cation 1+ slowly reacts with H2 to add across a BC bond. This reaction is 3.3 kcal/mol exergonic over a barrier of 31.6 kcal/mol (via transition structure TS1+H2) and generates the cation A+ with a long B···C bonding to the coordinating mesitylene (Figure 2). Another H2 molecule is then activated by the BH bond of A+ over a barrier of 18.8 kcal/mol (via TSA+H2), leading to an efficient H-isotope scrambling. Dimerization of A+ is almost energetically neutral and gives (A+)2 while loss of mesitylene from A+ is 18.5 kcal/mol endergonic affording B+. Neither (A+)2 nor B+ participate in the observed isotope scrambling. The instability of the boron product above prompted the corresponding reaction with triethylsilane (HSiEt3) at 25 °C. An equimolar mixture of 1 and HSiEt3 produced a major product 2 that exhibited a 11B NMR signal at δ 4.5 ppm and a characteristically broad B-H resonance at δ 4.76 ppm in the 1H NMR spectrum. Comparison of these spectra with those for authentic samples of dimesitylborane ((Mes2BH)2 3), and mesitylborane ((MesBH2)2, 4) confirmed 2 was a new species (Scheme 2). Layering these reaction mixtures with hexanes and cooling to -17 °C yielded golden-colored crystals of 2 amidst an amorphous mixture of 2 and unidentified byproducts. X-ray crystallographic and spectroscopic (NMR, IR, Raman) analysis unambiguously affirmed the formulation of 2 as the diboranium cation [MesB(-H)2(Mes)BMes][B(C6F5)4] (Figure 3). Compound 2 is highly reactive towards typical haloalkane and arene solvents affording complex mixtures of uncharacterized products however it is stable in ODCB and ODFB solvent. The IR spectrum displayed absorptions at 1372 and 1606 cm-1 diagnostic of B-H-B and bridging aryl C-H vibrations, respectively, as confirmed by our DFT calculations. The Raman spectrum featured bands at 1373, 1381 and 1606 cm-1. Efforts to use 1H-29Si HMBC NMR experiments to characterize the Si-containing by-product(s) were unsuccessful. However, DART-MS of the crude mixture confirmed the presence of MesSiEt3 (m/z 235.2). In addition, the formation of [Et3Si]+ as a reaction by-product was indirectly confirmed by X-ray crystallography following the isolation of crystals of [Et3Si-HSiEt3][B(C6F5)4]52-53 in a control reaction of one equivalent of 1 and two equivalents of HSiEt3. DFT calculations (see ESI) show that the reaction is initiated by rapid HSi addition across the BC bond of 1+ and is 1.5 kcal/mol exergonic over a barrier of only 11.6 kcal/mol affording the reactive cation C+, analogous to A+. Reaction of a further equivalent of silane prompted barrierless loss of [Et3Si-H-SiEt3]+ which is 0.8 kcal/mol exergonic and generates transient HBMes2. Coupling of the latter species with C+ yields the major product 2+ with liberation of SiEt3Mes. Alternatively, dimerization of HBMes2 affords the minor product borane 3. The molecular structure of 2+ (Figure 3) revealed bridging mesityl and two bridging hydrides between two boron centres in distorted tetrahedral environments (∠C(1)-B(1)-C(2) = ∠C(3)-B(2)-C(2) = 132.2(3)°). The terminal B-C bond length (1.541(6) Å) is longer than that reported for 1 (B-Cipso(avg.) = 1.4585 Å),30 and slightly shorter than that seen in the dimer 3 (B-Cipso(avg.) = 1.5966 Å).54 The bridging mesityl substituent features an elongated B-C(2) bond length of 1.679(6) Å that is greater than the sum of covalent radii (ΣCOV(B-C) = 1.59 Å).55 The bridging mesityl substituent rep resents a rare bonding motif for boron compounds, yet one that is relevant to


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Journal of the American Chemical Society substituent scrambling observed for hydrido(aryl)boranes, as suggested in the work of Köster.56 A related aryl-bridged 2,7di-tert-butyl-9H-9-borafluorene species was reported by Wagner and coworkers.57 The Cortho-Cbridge-Cortho’ angle in this species was 118.0(2)° similar to that see in 2 (∠Cortho-C(2)Cortho = 120.1(4)°), reflecting sp2 hybridization.

Figure 3. POV-ray depiction of the cation of compound 2. Hydrogens (except BH) and the [B(C6F5)4]- anion are omitted for clarity. H: light grey; B: yellow-green; C: black. Compound 2 represents the first isolable [B2X5]+ cation featuring three bridge bonds between the two boron centers. The parent diboranium cation [B2H5]+ was first observed in ion cyclotron resonance and pyrolysis mass spectrometry studies of B2H6.58-60 Olah61 subsequently generated diboranium transiently via treatment of B2H6 with magic acid. Computational studies have predicted varying geometries,61-66 although a trihydride-bridged D3h structure is supported by photoionization mass spectrometry studies.67 HSiEt3 - MesSiEt3 - [Et3Si]+ [B(C6F5)4] or

B 1

0.5 (MesBH2)2 4 [Et3Si(tol)]+ [B(C6F5)4] - MesSiEt3, tol

B

H H

B B H H 2

or [H(MesH)]+ [B(C6F5)4] - 2 MesH

B

[B(C6F5)4]

the central BC, BH, B'H, and B'B pairs are 0.70, 0.43, 0.34 and 0.80, consistent with two BHB and one BCB 3c-2e bonds in 2+ rather than a BB single bond. AIM calculations are consistent with direct BB bonding within the cation of 2. Efforts to prepare bulk 2 from 1 and HSiEt3 were unsuccessful as 11B NMR signals at δ 90.8 and 28.3 ppm consistent with the presence of 1 and 3 were observed, while 1H NMR spectra showed residual SiEt3 fragments in the product. Thus, several other synthetic strategies to 2 were probed. Reactions of 3 with [Et3Si(tol)][B(C6F5)4] as well as reaction of 1 and 4 in 1 : 0.5 stoichiometry generated 2 as the major product, however in both cases the minor by-products of 1 and 3 persisted in solution. In contrast, combination of the dimer 3 with one equivalent of [H(MesH)][B(C6F5)4] led to clean conversion to 2 in 84 % isolated yield, most likely via exergonic proton transfer to one ipso-mesityl site of 3 followed by MesH elimination (see ESI). Treatment of the dimeric borane 4 with [H(MesH)][B(C6F5)4], yielded a new species 5 as evidenced by 1H and 11B NMR spectroscopy. Using a 1.5 : 1 stoichiometry furnished compound 5 in 91% isolated yield (Scheme 2). The 11B NMR spectrum of isolated 5 features a triplet and a broad resonance at δ -7.5 and -10.5 ppm, respectively, while the vibrational spectra feature bands associated with the respective terminal and bridging B-H-B stretching bands (IR: 2513, 1644 cm-1; Raman: 2528, 1643 cm-1). Single crystals of 5 were obtained from the reaction mixture following layering with hexanes and cooling to 17 °C. X-ray crystallography revealed the formulation of 5 as [H2B(-H)(-Mes)B(Mes)(-H)BH2][B(C6F5)4] (Figure 4). Compound 5 reacts similarly to 2 with most organic solvents but is stable in ODCB and ODFB. The central boron atom of the cation of 5 is bridged by mesityl and hydride substituents to the two terminal BH2 units, providing a B(1)-B(2)-B(3) angle of 146.0(2)°. The BC bond lengths to the central and terminal boron centers were found to average 1.705(4) Å and 1.633(4) Å, respectively. Compound 5 featuring four 3c-2e bonds is the only isolable analogue of the cation [B3H8]+ predicted by Korkin,70 while related [B3H6]+ analogues have been prepared by Himmel and coworkers.71-72 AIM calculations show no direct BB bond path is found for 5, consistent with the longer BB distances (1.823(5), 1.830(5) Å).

3

1.5

H

B

H H

B

H

[H(MesH)]+ [B(C6F5)4]

H

H H H H H B B B

- 2 MesH

4

5 [B(C6F5)4]

Scheme 2. Synthetic pathways to 2 and 5. The question of B-B bonding in polyboron compounds featuring 3c-2e bonds is of interest.68-69 In 2+, the B···B distance of 1.604(6) Å falls within the sum of covalent radii (ΣCOV(B-B) = 1.68 Å)55 which is shorter than that reported by Himmel et al. (2.229 Å) for the cationic diboron species [B2H3(hpp)2][I] (hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a-pyrimidinate).69 Nonetheless, it is also longer than that predicted for [B2H5]+ (1.518 Å) at the QCISD(T)/6-311G** level.65 The DFT-computed fractional Wiberg bond indices for

Figure 4. POV-ray depiction of the cation of 5. Hydrogen (except BH) and the [B(C6F5)4]- anion are omitted for clarity. H: light grey; B: yellow-green; C: black. In summary, the borinium cation 1 is shown to react with H2 and HSiEt3, affecting hydride delivery to boron and protonation or silylation of a mesityl group. In the latter case, the diboranium salt 2 is obtained. These reactions are reminiscent of the activation of H2 by our (C6F5)2BPR2,



Bourissou’s borenium cation [R2PC10H6BMes][NTf2]15 and Piers’ electrophilic borole (C6F5C)4B(C6F5),13 among other notable examples. Compound 2 represents the first isolable example of a diboron cation exhibiting three 3c-2e bonds. Moreover, a synthetic route to pure 2 has been demonstrated via protonation of the borane 3. This latter finding also enabled the preparation of a unique triboron cation 5 featuring four 3c-2e bonds supported by two mesityl and two hydride substituents.

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ASSOCIATED CONTENT Supporting Information Synthetic and spectroscopic characterization are deposited. In addition, the CIF files for the crystallographic data are deposited in the CCDC # 1901342-1901343. The Supporting Information is available free of charge on the ACS Publications website.

13.

AUTHOR INFORMATION

15.

14.

Corresponding Author Email: [email protected]

16.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors thanks NSERC of Canada for financial support. D.W.S. is grateful for the award of a Canada Research Chair and an Einstein Visiting Professorship at TU Berlin. K.L.B is grateful for the award of an Alexander Graham Bell Canada Graduate Scholarship. Z.-W.Q. is grateful to the German Science Foundation (DFG) for financial support (Gottfried Wilhelm Leibnitz prize to Prof. Stefan Grimme). The authors thank Prof. Robert Morris and Brian Tsui (University of Toronto) for assistance with IR spectroscopy, and Prof. Saurabh Chitnis and Katherine Marczenko (Dalhousie University) for assistance with Raman spectroscopy.

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Activation of H2 and Et3SiH by the Borinium Cation [Mes2B]+:

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