Unraveling the reactivity of a cationic iminoborane: avenues to unusual boron cations

A cationic terminal iminoborane [Mes*N 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 B ← IPr2Me2][AlBr4] (3+[AlBr4]−) (Mes* = 2,4,6-tri-tert-butylphenyl and IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) has been synthesized and characterized. The employment of an aryl group and N-heterocyclic carbene (NHC) ligand enables 3+[AlBr4]− to exhibit both B-centered Lewis acidity and BN multiple bond reactivities, thus allowing for the construction of tri-coordinate boron cations 5+–12+. More importantly, initial reactions involving coordination, addition, and [2 + 3] cycloadditions have been observed for the cationic iminoborane, demonstrating the potential to build numerous organoboron species via several synthetic routes.


Introduction
Exploration of boron cations has attracted extensive attention not only due to their unique bonding environments and electronic congurations but also due to their potential applications in bond activation and catalysis. 1,2 Generally, boron cations can be classied into borinium [R 2 B] + , borenium [R 2 BL] + , and boronium [R 2 BL 2 ] + cations based on the coordination number of boron (Fig. 1a). By installing p-donating ligands at the boron atom to compensate for the inherent electron deciency, a few borinium ions have been isolated since the rst report of diamido-substituted boron cations I and II by the Nöth group (Fig. 1b). 3 Subsequently, Stephan and coworkers obtained the persistent borinium ion III supported by phosphoraniminato groups. 4 In 2014, a seminal report on the synthesis of dimesityl boron cation IV was published by Shoji et al. 5 IV exhibits high Lewis acidity and electrophilic reactivity towards CO 2 , CS 2 , H 2 and Et 3 SiH. 6 In addition, twofold 1,2carboration of diphenylacetylene with IV afforded divinylborinium ion V, in which the positive charge is delocalized over the entire p-conjugated system. 7 More recently, the Inoue group demonstrated the asymmetric borinium ion VI ligated by imidazolin-2-imino and silyl substituents. 8 Cationic boron species of type [R n E^B] + possess different bonding characters from classical boron cations and should be versatile reagents for organic synthesis through the functionalization of multiple bonds. 9,10 Nevertheless, such species are still hitherto unknown. Even with Lewis base stabilization, only two corresponding [R n E^BL] + and [R n E^BL 2 ] + derivatives, exemplied by iminoboryl-CAAC adducts VII and cationic thioxoborane VIII, have been described by the Bertrand, Stephan and Inoue groups, respectively (Fig. 1c). 11,12 VII-I with an iodide counter anion is in equilibrium with the neutral idodoiminoborane in solution, which alternatively served as a dipole to cyclize with CO 2 . Comparatively, VII-BPh 4 and VII-B(C 6 F 5 ) 4 retaining the cationic structure both in the solid-state and in solutions do not exhibit dipole reactivity and are inert toward CO 2 . Neutral iminoboranes are extensively employed as synthons for constructing organoboron compounds through addition and cycloaddition reactions. 13 Therefore, development of persistent yet highly reactive cationic counterparts and unravelling their reactivity would provide new avenues to cationic boron species. Herein, we report the access to the cationic iminoborane IX supported by an NHC, as well as its utility for constructing unusual B-containing frameworks (Fig. 1d).

Synthesis and characterization of 3 + [AlBr 4 ]
Recently, we demonstrated a facile approach for the synthesis of iminoborane-NHC complexes by employing NHCs as Brønsted bases and ancillary ligands. 14 In an analogous fashion, addition of two equiv. of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IPr 2 Me 2 ) into Mes*NHBBr 2 1 in toluene afforded compound 2 as colorless crystals in 56% yield (Scheme 1). The multi-nuclear NMR spectroscopic studies coupled with single crystal X-ray diffraction conrmed the formulation of 2 to be Mes*N]B(Br) IPr 2 Me 2 ( 11 B NMR: 9.4 ppm). In the solid state, the boron center of 2 adopts a planar geometry with the sum of angles at 359.9 . The Mes* and IPr 2 Me 2 groups are located on the opposite sides of the B1]N1 double bond (Fig. 2a).
Halide abstraction of 2 was next examined. We reacted 2 with an equal molar amount of AlBr 3 in toluene at ambient temperature for 5 minutes to obtain colorless solids of 3 + [AlBr 4 ] À quantitatively. The 11 B NMR spectrum of 3 + shows a broad signal at 6.9 ppm, which is close to the chemical shis seen for the iminoboranes ArB^NR (2.8-4.3 ppm), 13,15 borinium III (11.1 ppm) 4 and cationic iminoboranes VII 11 (7.7 and 7.4 ppm), but strongly upeld-shied with respect to those of two-coordinate boron cations I, II, IV and V (36.7, 38.7, 93.3 and 74.0 ppm, respectively). 3,5,7 A sharp singlet peak at 80.8 ppm assigned to [AlBr 4 ] À was observed in the 27 Al NMR spectrum, demonstrating the smooth bromide abstraction of 2 by AlBr 3 . Species 3 + [AlBr 4 ] À exhibits high thermal stability under an inert atmosphere and does not decompose even upon heating at 80 C for 24 h in C 6 D 6 ( Fig. S71 †).
To further understand the electronic structure of 3 + , DFT calculations at the B3LYP/6-311G* level of theory were performed. The calculated geometrical parameters agree well with those observed for the crystal structure of 3 + . The 11 B NMR chemical shi (5.8 ppm) obtained from GIAO calculations is consistent with the experimental value (6.9 ppm, Fig. S82 †). The Wiberg Bond Index (WBI) for the B-N bond in 3 + is 1.98 (Table  S2 †), which is smaller than that of VII (2.10, Table S3 †), while those for the C1-N1 and B1-C19 bonds are 1.05 and 0.94, respectively. The natural population analysis (NPA) shows that the boron atom has a positive charge of +0.75, whereas the N1 atom is negatively charged (À0.70 a.u.) ( Fig. 3a and Table S4 †), suggesting the pronounced polar nature of the B-N triple bond. Indeed, natural bond orbital (NBO) analysis reveals one sand two p-bonding features of the BN unit with signicant contribution from the N atom (76%, 76% and 74%, respectively, Table S5 †). The Mes* and IPr 2 Me 2 groups bear +0.28 and +0.67 charges, respectively. Furthermore, second-order perturbation Scheme 1 Synthesis of 2 and 3 + [AlBr 4 ] À (Mes* ¼ 2,4,6-tri-tertbutylphenyl). analysis conrmed the electronic conjugation between the B^N triple bond and Mes* as well as imidazole rings with stabilization energies of 13.9 and 6.9 kcal mol À1 , respectively (Fig. S85 †).
The HOMO and HOMOÀ2 of 3 + represent out-of-plane and in-plane B-N p-bonding orbitals, with contributions from p orbitals of Mes* or imidazole moieties, whereas the LUMO and LUMO+1 predominately involve their p*-components (Fig. 3b). The UV/Vis and IR spectroscopic analyses of 3 + [AlBr 4 ] À were also carried out. The former showed no detectable absorption between 240 and 400 nm (Fig. S15 †), whereas the signal at a shorter wavelength (<240 nm) was too noisy or saturated for a reliable assignment. The latter measured in the solid state exhibited a characteristic peak atṽ ¼ 2004 cm À1 for the B^N stretching vibration (Fig. S16 †), which is comparable to that for t BuB^N t Bu (2018 cm À1 ) 17 and is in good agreement with the calculated result (2008 cm À1 , Fig. S84 †).

1,2-Addition of 3 + [AlBr 4 ] À
The addition of unactivated C-H bonds across iminoboranes to form aminoboranes is extremely challenging. It was found that in the presence of sodium cyanate (NaOCN), 3 + [AlBr 4 ] À converted into the product 7 + [AlBr 4 ] À ( 11 B NMR: 40.8 ppm) at 50 C (Scheme 3). Both NMR spectroscopic and X-ray single-crystal diffraction studies conrmed 7 + [AlBr 4 ] À to be a cyclic aminoborenium formed via an intramolecular C-H bond addition across the B^N triple bond (Fig. 5a). NaOCN likely acts as a Lewis-base for coordination activation of the B^N bond to enhance the basicity of N for capturing a proton from the methyl group. Note that a neutral three-coordinate borane enabled intramolecular C-H activation affording borocycles has been described by the Knochel research group. 19 In addition, intermolecular addition proceeded smoothly between 3 + [AlBr 4 ] À and more acidic acetonitrile at ambient temperature to give the acyclic aminoborenium 8 + [AlBr 4 ] À ( 11 B NMR: 37.2 ppm; Fig. 5b). It is proposed that this transformation may occur in successive steps involving acetonitrile complexation, proton migration and rearrangement of an aza-allene intermediate (Fig. S81 †). 20 Both reactions formed anti-addition products.
Inspired by a few successful examples of catalyst-free transfer hydrogenation (TH) of unsaturated compounds, 21 the reactions of 3 + [AlBr 4 ] À with H 3 N$BH 3 or Me 2 HN$BH 3 were also conducted. The conversions were complete within half an hour at ambient temperature, generating the corresponding aminoborenium salt 9 + [AlBr 4 ] À ( 11 B NMR: 34.1 ppm) with concomitant formation of the respective borazine and cyclodiborazane as the by-products (Fig. S72 and S73 †). The solid-state structure of 9 + [AlBr 4 ] À revealed its trans stereochemistry (Fig. 5c), which is consistent with Braunschweig's results on TH of neutral iminoboranes with H 3 N$BH 3 . 22 Deuterium labelling experiments using H 3 N$BD 3 , D 3 N$BH 3 and D 3 N$BD 3 resulted in deuterium incorporation at boron, nitrogen and both boron and nitrogen atoms, respectively, as evidenced by 2 H NMR with characteristic peaks at 6.64 ppm (N-D) and 4.78 ppm (B-D) (Fig. S74 †). This conrmed the polarity match mechanism of TH to 3 + [AlBr 4 ] À . This is different from the aminoborane iPr 2 N]BH 2 , which readily undergoes reverse TH with Me 2 -HN$BH 3 , 23 and no further hydrogenation of species 9 + [AlBr 4 ] À to amineboronium could be observed in the presence of excess H 3 N$BH 3 or Me 2 HN$BH 3 under heating conditions (Fig. S75 †).
When 3 + [AlBr 4 ] À was treated with trimethylsilyl azide (Me 3 SiN 3 ) in toluene, a white precipitate of 11 was formed immediately. The 11 B NMR of 11 showed a signal at 26.0 ppm, whereas 1 H NMR revealed that no Me 3 Si-group was in the product. Single crystal X-ray diffraction revealed 11 to be a zwitterionic tetraazaborole species with cationic boron and  anionic aluminium centers (Fig. 6b), resulting from a [2 + 3] cycloaddition and subsequent Al/Si exchange as well as Me 3 SiBr elimination. With respect to those of 1,4-diphenyl-5-terphenyltetraazaborole reported by the Braunschweig group, 25 11 features comparable B-N bond lengths (avg. 1.432Å) and N-N double bond length (1.276(6)Å) as well as an essentially planar BN 4 ve-membered ring (the mean deviation from the plane is 0.0186Å).
Moreover, 3 + [AlBr 4 ] À also cyclized with diazomethyl(trimethyl)silane to give product 12 + [AlBr 4 ] À ( 11 B NMR: 22.6 ppm). Its solid structure conrmed the migration of the trimethylsilyl group from carbon to the nitrogen atom aer cycloaddition to form an unprecedented cationic triazaborole skeleton (Fig. 6c). 26 The B1-C30 bond length is 1.462(5)Å and its WBI value is 1.2978, thus conrming the typical double bond character. 27 All ve atoms of the BCN 3 ring are perfectly coplanar with the sum of internal pentagon angles of 540 , and the calculated nucleus-independent chemical-shi values of NICS(0) and NICS(1) are À11.0 and À28.2, respectively, supporting its aromatic character. 28 The computational studies reveal that the generation of aromatic 12 + from isomerization of the non-migrated intermediate is strongly exergonic (À37.1 kcal mol À1 ) and thus is thermodynamically favorable (Fig. S91 †).

Conclusions
Collectively, this work unravels the elementary reactions of a cationic iminoborane, laying the foundation for efficient alternatives to build organoboron species. Employment of aryl and NHC ligands afforded isolable cationic iminoborane 3 + . Both the boron atom and B^N triple bond in 3 + could behave as reactive sites for derivatization enabling coordination, addition, and cycloaddition leading to a facile access to three-coordinate boron cations. This clearly conrms that substituent modication could boost the reactivity of cationic iminoboranes since analogous VII-BPh 4 lacks the dipole reactivity toward CO 2 , 11 CH 3 CN, H 3 N$BH 3 and Me 3 SiN 3 (Figs. S76-S78 †). These systematic results indicate that unsaturated boron cations, which integrate both Lewis-acidity and multiple bond reactivities, are attractive synthons in organic synthesis. Investigations on the isolation and applications of more functional boron cations are continuously underway in our laboratory.

Data availability
Experimental and computational data has been provided as ESI. †

Author contributions
L. K. designed the study and supervised the project. R. G. synthesized the complexes and conducted most of the measurements. X. Z., L. L. L. and L. K. performed the theoretical calculations. T. L. collected the crystal structures. Q. L. conducted the HRMS measurement. D. A. R., L. L. L., C.-H. T. and L. K. wrote the manuscript. All authors discussed the experimental results and commented on the manuscript.

Conflicts of interest
The authors declare no competing nancial interests.