1,3,2-Diazaborole-derived carbene complexes of boron

Hunter P. Hickox , Yuzhong Wang , Kaitlin M. Luedecke , Yaoming Xie , Pingrong Wei , Deidrah Carrillo , Nathaniel L. Dominique , Dongtao Cui , Henry F. Schaefer III and Gregory H. Robinson *
Department of Chemistry and the Center for Computational Chemistry, The University of Georgia, Athens, Georgia 30602-2556, USA. E-mail: robinson@uga.edu

Received 29th October 2017 , Accepted 16th November 2017

First published on 16th November 2017


Reaction of 2-bromo-1,3,2-diazaborole (1) with excess BBr3 induces 1,2-hydrogen migration, giving 1,3,2-diazaborole-derived carbene complexes of boron bromide (2). Compound 2 exists in a dynamic solution equilibrium with 1. The 1H NMR study shows that the equilibrium lies to the right side of the dissociation reaction of 2. Parallel reaction of 1 with excess BI3 gives the corresponding 1,3,2-diazaborole-derived carbene boron iodide complex (3). Notably, in contrast to 2, the dissociation reaction of 3 largely lies to the left side, favouring the formation of 3. The dynamic solution equilibrium behaviours of 2 and 3 are probed by both experimental and theoretical methods.


The formula of the elusive parent carbene, H2C:, perhaps exemplifies why chemists long considered this entire class of molecules to be too reactive to ever be isolated.1 As is frequently the case in science, the wisdom of such presumptions appears perfectly reasonable until the moment that factual proof to the contrary is presented—in this case, the ground-breaking syntheses of stable carbenes:2 Bertrand's (phosphine)(silyl)carbene3,4 and Arduengo's imidazol-2-ylidene.5 It is interesting to note that N-heterocyclic carbenes (NHCs, I, in Fig. 1) are currently playing pivotal roles in research fields spanning from organic and transition-metal catalysis6 to low-oxidation-state main group chemistry.7–15 Subsequent to the discovery of normal N-heterocyclic carbenes (I), less stable “abnormal” N-heterocyclic carbenes (aNHCs) (II, in Fig. 1), bearing a C4 carbene centre, were prepared and shown to exhibit stronger electron-donating capabilities than normal NHCs. Consequently, aNHCs have found applications in catalysis.16–21 Notably, the first metal-free aNHC was only obtained recently.22 This laboratory, by lithiation of NHC (I), synthesized an anionic N-heterocyclic dicarbene (NHDCs, III, in Fig. 1), containing both C2 and C4 carbene centres.23,24 Multi-anionic NHDCs were subsequently reported.25,26 Indeed, the utilization of III in main group and transition metal chemistry is burgeoning.27
image file: c7dt04079b-f1.tif
Fig. 1 Imidazole-based carbenes (I–III), CAAC (IV), 1,2-azaborole-derived CAAC (V), and 1,3,2-diazaborole-derived carbene (VI).

The increasing utilization of cyclic (alkyl)(amino)carbenes (CAACs, IV, in Fig. 1),28 developed by Bertrand, is noteworthy.13,29,30 Since CAACs have both a higher energy HOMO and a lower energy LUMO than NHCs, CAACs have been reported to be stronger σ-donors and π-acceptors than NHCs (I). In addition, the singlet–triplet gap for CAACs is significantly smaller than that for NHCs.13,31 Our computations at the B3LYP/6-311G** level (Fig. 2) are consistent with these findings.


image file: c7dt04079b-f2.tif
Fig. 2 Energy (eV) of the frontier orbitals and ΔEST (singlet–triplet gap, kcal mol−1) in parentheses of carbenes (I, IV–VI) calculated at the B3LYP/6-311G** level of theory.

Kinjo recently reported the synthesis of 1,2-azaborole-derived CAAC (V, in Fig. 1)–borane adducts via borane-induced 1,2-hydrogen migration. This finding may provide unique access to asymmetrical diborenes and allenic diborenes.32,33 Prior to this discovery, only transient carbene formation from alkenes through 1,2-hydrogen migration had been documented.34–38 2-Bromo-1,3,2-diazaborole (1, in Fig. 3) has provided effective access to nucleophilic 1,3,2-diazaborolyllithium39 and its main group and transition-metal derivatives.40,41 However, the utility of this diazaborole in carbene chemistry has not been explored. Could 1,3,2-diazaborole-derived carbene (VI)-based compounds be prepared? Herein, we report the synthesis,42 structures,42 and computations42 of 1,3,2-diazaborole-derived carbene–boron halides (2 and 3) via 1,2-hydrogen migration.


image file: c7dt04079b-f3.tif
Fig. 3 Synthesis of 2 and 3 and their dynamic solution equilibrium with 1.

Reaction of 2-bromo-1,3,2-diazaborole (1) with excess BBr3 in hexane gives 2-bromo-1,3,2-diazaborole-derived carbene–boron bromide complex (2) in quantitative yield (based on 1H NMR data) (Fig. 3, R = 2,6-diisopropylphenyl).42 In contrast to 1,2-azaborole-derived CAAC–boron halides that do not dissociate in organic solvents (i.e., pentane and toluene),322 exists in a dynamic equilibrium with 1 (eqn (1) in Fig. 3). This is supported by the presence of three sets of 11B NMR resonances for the C6D6 solution of 2 (Fig. 4a) [i.e., +30.28 and −14.42 ppm (2), +20.10 ppm (1),43 and +37.14 ppm (BBr3)]. The borole 1H NMR resonances in C6D6 (5.87 ppm for 2 and 6.13 ppm for 1) indicate an approximate 1[thin space (1/6-em)]:[thin space (1/6-em)]2 molar ratio of 2 to 1 in the equilibrium mixture. Thus, the equilibrium lies to the right in the dissociation reaction of 2 (eqn (1) in Fig. 3). Addition of BBr3 to the equilibrium mixture shifts the equilibrium to the left. Furthermore, our computations suggest that 1,2-azaborole-derived CAAC (V in Fig. 2), like bicyclic (alkyl)(amino)carbenes (BICAACs),44 is an even stronger σ-donor and π-acceptor than CAAC (IV in Fig. 2) (considering the relatively higher energy HOMO and lower energy LUMO of V than IV). In addition, the singlet–triplet gap of V (35.3 kcal mol−1) (Fig. 2) is also smaller than that for CAAC (IV) (49.5 kcal mol−1).42 However, 1,3,2-diazaborole derived carbene (VI, in Fig. 2) exhibits an obviously lower energy HOMO, and thus a weaker electron-donating capability than V. These energetics may explain why 2 readily undergoes partial dissociation. In addition, compound 2 may be hydrolysed by the presence of trace amounts of moisture, giving the protonated by-product 4 as colourless crystals (Fig. 3). While 4 was only characterized by X-ray single crystal diffraction, reaction of 2 with HCl does not give the protonated analogue of 4, but 2-chloro-1,3,2-diazaborole. While combination of 1 with BF3 or BCl3 does not give the analogues of 2, the parallel reaction of 1 with excess BI3 indeed gives 3 (in a quantitative yield) (Fig. 3).42 Notably, the borole 1H NMR resonances for the C6D6 solution of 3 (6.58 ppm for 3 and 6.13 ppm for 1) indicate an approximate 17[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio of 3 to 1 in the equilibrium mixture. Thus, the dissociation reaction of 3 (eqn (2) in Fig. 3) largely lies to the left side, favouring the formation of 3. This may be ascribed to an increase of Lewis acidity of boron trihalides down the group. The 11B NMR spectrum of 3 in C6D6 at room temperature (Fig. 4b) shows not only the resonances for 3 (+30.38 ppm and −69.52 ppm), 1 (+20.21 ppm),43 and BI3 (−8.07 ppm) due to the equilibrium in eqn (2) (Fig. 3) but also a pair of resonances (+25.26 ppm and −45.60 ppm) for an uncharacterized species. Further investigation of the properties of this unknown species is being conducted in this laboratory.


image file: c7dt04079b-f4.tif
Fig. 4 The 11B NMR spectra of 2 (a) and 3 (b) in C6D6 at room temperature.

X-ray structural analysis42 shows that compound 2 crystallizes in the orthorhombic space group Pbca. The asymmetric unit cell contains two independent molecules of 2 (Fig. 5, for clarity, only one molecule of 2 is shown). Two hydrogen atoms of the borole-derived carbene ring of 2 were located from difference Fourier map. The carbon–carbon bond in the carbene ring of 2 [1.488(11) Å, av.], similar to that of 2-Me model42 (1.498 Å), is obviously longer than the C[double bond, length as m-dash]C double bond for 1 [1.330(9) Å],43 indicating its single bond character [WBI(C–C bond)borole ring of 2-Me = 1.04].42 The C–B single bond in 2 [1.590(12) Å, av.], comparable to that [1.623(7) Å] of L:BBr3 [L: = :C{N(2,6-Pri2C6H3)CH}2],45 is significantly polarized toward the carbene carbon (71.1% toward carbon and 28.9% toward boron for 2-Me model). The Bborole–Br bond distance [1.891(9) Å, av.] in 2 is almost the same as that [1.898(7) Å] for 1, but marginally longer than those of the BBr3 unit in 2 [1.963(10)–2.063(10) Å]. Compound 3, crystalizing in the orthorhombic space group P212121, is isostructural to 2 (Fig. 5). Both 2 and 3 exhibit similar C–B, C–C, N–B, C–N, and B–Br bond distances. The B–I bond distances [2.200(8)–2.265(8) Å] of 3 are consistent with the theoretical values (2.281–2.295 Å) of 3-Me model.42 Compound [4]+[BBr4] crystallizes in the monoclinic space group P21/c. All three hydrogen atoms residing at C(1) and C(2) in [4]+ were located from difference Fourier map (Fig. 5). The C–C bond in the BN2C2 ring of [4]+ [1.470(5) Å] is only slightly shorter than those in 2 [1.488(11) Å, av.] and 3 [1.491(8) Å].


image file: c7dt04079b-f5.tif
Fig. 5 Molecular structures of 2, 3, and [4]+. Thermal ellipsoids represent 30% probability: hydrogen atoms except for those residing at C(1) and C(2) are omitted for clarity. Selected bond distances (Å) and angles (°) are as follows. For 2, B(1)–Br(1) 1.888(9), B(2)–C(1) 1.579(11), C(1)–C(2) 1.493(10), B(2)–Br(2) 2.006(10), B(2)–Br(3) 2.063(10), B(2)–Br(4) 1.963(10); N(1)–B(1)–Br(1) 124.1(6), N(1)–C(1)–B(2) 131.7(7), C(2)–C(1)–B(2) 121.1(7). For 3, B(1)–Br(1) 1.899(6), B(2)–C(1) 1.615(9), C(1)–C(2) 1.491(8), B(2)–I(1) 2.200(8), B(2)–I(2) 2.229(8), B(2)–I(3) 2.265(8); N(1)–B(1)–Br(1) 123.6(4), N(1)–C(1)–B(2) 131.7(5), C(2)–C(1)–B(2) 119.9(5). For [4]+, C(1)–C(2) 1.470(5), B(1)–Br(1) 1.882(4); N(1)–B(1)–N(2) 106.0(3), N(1)–B(1)–Br(1) 128.0(3).

Conclusions

Boron bromide and boron iodide were employed to induce 1,2-hydrogen migration of 2-bromo-1,3,2-diazaborole (1), giving 1,3,2-diazaborole-derived carbene complexes of boron tribromide (2) and boron triiodide (3), respectively. The dynamic solution equilibrium behaviour of 2 and 3 is consistent with weak electron-donating capability of 2-bromo-1,3,2-diazaborole-derived carbene, which is supported by our theoretical study.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Science Foundation for support: CHE-1565676 (G. H. R., Y. W.) and CHE-1361178 (H. F. S.). We submit this paper in honour of Professor Philip P. Power and his transformative contributions to inorganic chemistry.

Notes and references

  1. G. Bertrand, Carbene Chemistry: From Fleeting Intermediates to Powerful Reagents, Marcel Dekker, New York, 2002 Search PubMed .
  2. D. Bourissou, O. Guerret, F. P. Gabbaie and G. Bertrand, Chem. Rev., 2000, 100, 39–91 CrossRef CAS PubMed .
  3. A. Igau, H. Grutzmacher, A. Baceiredo and G. Bertrand, J. Am. Chem. Soc., 1988, 110, 6463–6466 CrossRef CAS .
  4. A. Igau, A. Baceiredo, G. Trinquier and G. Bertrand, Angew. Chem., Int. Ed. Engl., 1989, 28, 621–622 CrossRef .
  5. A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem. Soc., 1991, 113, 361–363 CrossRef .
  6. S. P. Nolan, N-Heterocyclic Carbenes in Synthesis, Wiley-VCH, Weinheim, 2006 Search PubMed .
  7. Y. Wang and G. H. Robinson, Inorg. Chem., 2014, 53, 11815–11832 CrossRef CAS PubMed .
  8. Y. Wang and G. H. Robinson, Dalton Trans., 2012, 41, 337–345 RSC .
  9. Y. Wang and G. H. Robinson, Inorg. Chem., 2011, 50, 12326–12337 CrossRef CAS PubMed .
  10. D. Martin, M. Melaimi, M. Soleilhavoup and G. Bertrand, Organometallics, 2011, 30, 5304–5313 CrossRef CAS PubMed .
  11. C. D. Martin, M. Soleilhavoup and G. Bertrand, Chem. Sci., 2013, 4, 3020–3030 RSC .
  12. D. Martin, M. Soleilhavoup and G. Bertrand, Chem. Sci., 2011, 2, 389–399 RSC .
  13. M. Soleilhavoup and G. Bertrand, Acc. Chem. Res., 2015, 48, 256–266 CrossRef CAS PubMed .
  14. H. Braunschweig and R. D. Dewhurst, Angew. Chem., Int. Ed., 2013, 52, 3574–3583 CrossRef CAS PubMed .
  15. R. S. Ghadwal, R. Azhakar and H. W. Roesky, Acc. Chem. Res., 2013, 46, 444–456 CrossRef CAS PubMed .
  16. S. Grundemann, A. Kovacevic, M. Albrecht, J. W. Faller and R. H. Crabtree, Chem. Commun., 2001, 2274–2275 RSC .
  17. S. Gruendemann, A. Kovacevic, M. Albrecht, J. W. Faller and R. H. Crabtree, J. Am. Chem. Soc., 2002, 124, 10473–10481 CrossRef CAS .
  18. A. R. Chianese, A. Kovacevic, B. M. Zeglis, J. W. Faller and R. H. Crabtree, Organometallics, 2004, 23, 2461–2468 CrossRef CAS .
  19. O. Schuster, L. Yang, H. G. Raubenheimer and M. Albrecht, Chem. Rev., 2009, 109, 3445–3478 CrossRef CAS PubMed .
  20. P. L. Arnold and S. Pearson, Coord. Chem. Rev., 2007, 251, 596–609 CrossRef CAS .
  21. R. H. Crabtree, Coord. Chem. Rev., 2013, 257, 755–766 CrossRef CAS .
  22. E. Aldeco-Perez, A. J. Rosenthal, B. Donnadieu, P. Parameswaran, G. Frenking and G. Bertrand, Science, 2009, 326, 556–559 CrossRef CAS PubMed .
  23. Y. Wang, Y. Xie, M. Y. Abraham, P. Wei, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2010, 132, 14370–14372 CrossRef CAS PubMed .
  24. Y. Wang, Y. Xie, M. Y. Abraham, P. Wei, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, Organometallics, 2011, 30, 1303–1306 CrossRef CAS .
  25. A. El-Hellani and V. Lavallo, Angew. Chem., Int. Ed., 2014, 53, 4489–4493 CrossRef CAS PubMed .
  26. M. J. Asay, S. P. Fisher, S. E. Lee, F. S. Tham, D. Borchardt and V. Lavallo, Chem. Commun., 2015, 51, 5359–5362 RSC .
  27. J. B. Waters and J. M. Goicoechea, Coord. Chem. Rev., 2015, 293, 80–94 CrossRef .
  28. V. Lavallo, Y. Canac, C. Prasang, B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2005, 44, 5705–5709 CrossRef CAS PubMed .
  29. M. Melaimi, M. Soleilhavoup and G. Bertrand, Angew. Chem., Int. Ed., 2010, 49, 8810–8849 CrossRef CAS PubMed .
  30. S. Roy, K. C. Mondal and H. W. Roesky, Acc. Chem. Res., 2016, 49, 357–369 CrossRef CAS PubMed .
  31. B. Rao, H. R. Tang, X. M. Zeng, L. Liu, M. Melaimi and G. Bertrand, Angew. Chem., Int. Ed., 2015, 54, 14915–14919 CrossRef CAS PubMed .
  32. W. Lu, Y. Li, R. Ganguly and R. Kinjo, J. Am. Chem. Soc., 2017, 139, 5047–5050 CrossRef CAS PubMed .
  33. W. Lu, Y. Li, R. Ganguly and R. Kinjo, Angew. Chem., Int. Ed., 2017, 56, 9829–9832 CrossRef CAS PubMed .
  34. G. V. Shustov and M. T. H. Liu, Can. J. Chem., 1998, 76, 851–861 CrossRef CAS .
  35. T. H. Chan and D. Massuda, J. Am. Chem. Soc., 1977, 99, 936–937 CrossRef CAS .
  36. P. E. Eaton and K. L. Hoffmann, J. Am. Chem. Soc., 1987, 109, 5285–5286 CrossRef CAS .
  37. P. E. Eaton and R. B. Appell, J. Am. Chem. Soc., 1990, 112, 4055–4057 CrossRef CAS .
  38. N. Chen, M. Jones, W. R. White and M. S. Platz, J. Am. Chem. Soc., 1991, 113, 4981–4992 CrossRef CAS .
  39. Y. Segawa, M. Yamashita and K. Nozaki, Science, 2006, 314, 113–115 CrossRef CAS PubMed .
  40. L. Weber, Eur. J. Inorg. Chem., 2012, 5595–5609 CrossRef CAS .
  41. L. Weber, Eur. J. Inorg. Chem., 2017, 3461–3488 CrossRef CAS .
  42. See the ESI for synthetic, computational, and crystallographic details.
  43. Y. Segawa, Y. Suzuki, M. Yamashita and K. Nozaki, J. Am. Chem. Soc., 2008, 130, 16069–16079 CrossRef CAS PubMed .
  44. E. Tomas-Mendivil, M. M. Hansmann, C. M. Weinstein, R. Jazzar, M. Melaimi and G. Bertrand, J. Am. Chem. Soc., 2017, 139, 7753–7756 CrossRef CAS PubMed .
  45. Y. Wang, B. Quillian, P. Wei, C. S. Wannere, Y. Xie, R. B. King, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2007, 129, 12412–12413 CrossRef CAS PubMed .

Footnote

Electronic supplementary information (ESI) available: Synthetic and computational details and structural and spectral characterization. CCDC 1581024–1581026. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt04079b

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