Enhancement of London Dispersion in Frustrated Lewis Pairs: Towards a Crystalline Encounter Complex

Abstract

The encounter complex, i.e., the pre-organized assembly consisting of a Lewis acid and a Lewis base, is a fundamental concept in frustrated Lewis pair (FLP) chemistry. However, this donor–acceptor complex is challenging to study due to its transient nature. Here, we present a combined theoretical and experimental investigation on the potential isolation of an encounter complex enabled by enhancement of London dispersion forces between a sterically encumbered Lewis acid and base pair. Guided by computational analyses, the FLP originating from the bulky triarylamine N(3,5-tBu₂C₆H₃)₃ and the novel triarylborane B(3,5-tBu₂C₆H₃)₃ was investigated, leading to the isolation of a 1:1 co-crystal of both FLP components.

Key words

frustrated Lewis pairs - encounter complex - London dispersion forces - Lewis acid - Lewis base

Publication History

Received: 15 July 2022

Accepted after revision: 19 August 2022

Accepted Manuscript online:
22 August 2022

Article published online:
11 October 2022

• References and Notes

• 1 Welch GC, Juan RR. S, Masuda JD, Stephan DW. Science 2006; 314: 1124
• 2a Stephan DW, Erker G. Angew. Chem. Int. Ed. 2010; 49: 46
• 2b Stephan DW. Acc. Chem. Res. 2015; 48: 306
• 2c Stephan DW. J. Am. Chem. Soc. 2015; 137: 10018
• 2d Stephan DW. Science 2016; 354: aaf7229
• 2e Paradies J. Coord. Chem. Rev. 2019; 380: 170
• 2f Jupp AR, Stephan DW. Trends Chem. 2019; 1: 35
• 3a Paradies J. Eur. J. Inorg. Chem. 2019; 283
• 3b Jupp AR. Dalton Trans. 2022; 51: 10681
• 4a Rokob TA, Hamza A, Stirling A, Soós T, Pápai I. Angew. Chem. Int. Ed. 2008; 47: 2435
• 4b Bistoni G, Auer AA, Neese F. Chem. Eur. J. 2017; 23: 865
• 4c Grimme S, Kruse H, Goerigk L, Erker G. Angew. Chem. Int. Ed. 2010; 49: 1402
• 4d Skara G, Pinter B, Top J, Geerlings P, De Proft F, De Vleeschouwer F. Chem. Eur. J. 2015; 21: 5510
• 4e Zeonjuk LL, Vankova N, Mavrandonakis A, Heine T, Röschenthaler GV, Eicher J. Chem. Eur. J. 2013; 19: 17413
• 5 Rocchigiani L, Ciancaleoni G, Zuccaccia C, Macchioni A. J. Am. Chem. Soc. 2014; 136: 112
• 6 Holtrop F, Jupp AR, van Leest NP, Paradiz Dominguez M, Williams RM, Brouwer AM, de Bruin B, Ehlers AW, Slootweg JC. Chem. Eur. J. 2020; 26: 9005
• 7 A π–π-stacked co-crystal of 1-Br-4-(NMe₂)C₆H₄ and B(C₆F₅)₃ was reported: Aramaki Y, Imaizumi N, Hatta M, Kumagai J, Ooi T. Chem. Sci. 2020; 11: 4305
• 8a Rösel S, Balestrieri C, Schreiner PR. Chem. Sci. 2017; 8: 405
• 8b Rösel S, Becker J, Allen WD, Schreiner PR. J. Am. Chem. Soc. 2018; 140: 14421
• 9 Holtrop F, Jupp AR, Kooij BJ, van Leest NP, de Bruin B, Slootweg JC. Angew. Chem. Int. Ed. 2020; 59: 22210
• 10 Geier SJ, Stephan DW. J. Am. Chem. Soc. 2009; 131: 3476
• 11a Pollice R, Bot M, Kobylianskii IJ, Shenderovich I, Chen P. J. Am. Chem. Soc. 2017; 139: 13126
• 11b Schümann JM, Wagner JP, Eckhardt AK, Quanz H, Schreiner PR. J. Am. Chem. Soc. 2021; 143: 41
• 12 The syntheses of all-meta-Ph-substituted triarylamine and triarylborane were attempted. However, only in common organic solvents insoluble products were obtained precluding purification, characterization, and further investigations.
• 13 Bedford RB, Betham M. Tetrahedron Lett. 2007; 48: 8947
• 14 Kahr B, Van Engen D, Gopalan P. Chem. Mater. 1993; 5: 729
• 15 Kolychev EL, Bannenberg T, Freytag M, Daniliuc CG, Jones PG, Tamm M. Chem. Eur. J. 2012; 18: 16938
• 16 Borger JE, Ehlers AW, Lutz M, Slootweg JC, Lammertsma K. Angew. Chem. Int. Ed. 2015; 55: 613
• 17 Osi A, Mahaut D, Tumanov N, Fusaro L, Wouters J, Champagne B, Chardon A, Berionni G. Angew. Chem. Int. Ed. 2022; 61: e202112342
• 18 Synthesis of B(3,5-tBu₂C₆H₃)₃ [Me₃NH]Cl (50 mg, 0.55 mmol) was added to a solution of Na[B(3,5-tBu₂C₆H₃)₄] (396 mg, 0.5 mmol) in MeCN (10 mL) at ambient temperature. The white suspension was stirred for 3 h and subsequently filtered over Celite. Volatiles were removed from the filtrate in vacuo. The resulting white solid was dissolved in THF (40 mL), and 2,2,6,6-tetramethylpiperidine (78 mg, 0.55 mmol, 0.093 mL) was added at ambient temperature. The mixture was stirred for 1 h, after which volatiles were removed in vacuo. The residue was extracted into diethyl ether (30 mL), and the filtrate was concentrated until incipient crystallization and stored at –30 °C to yield colorless analytically pure crystals of B(3,5-tBu₂C₆H₃)₃. Yield: 260 mg (0.45 mmol, 90%); mp 255 °C. HRMS (LIFDI): m/z calcd for C₄₂H₆₃B: 578.5030; found: 578.5028. IR (neat): ν = 2952, 2902, 2866, 1584, 1475, 1460, 1412, 1390, 1361, 1309, 1247, 1220, 993, 895, 885, 822, 725, 671, 539 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 7.60 (s, 3 H, p-CH), 7.51 (s, 6 H, o-CH), 1.35 (s, 54 H, tBu). ¹³C{¹H} NMR (125.6 MHz, CDCl₃): δ = 148.9 (m-C ₆H₃ tBu₂), 133.7 (o-C ₆H₃ tBu₂), 125.0 (p-C ₆H₃ tBu₂), 35.0 (C(CH₃)₃), 31.8 (C(CH₃)₃) (ipso-C ₆H₃ tBu₂ not observed). ¹¹B NMR (160.5 MHz, CD₃CN): δ = 68.4 (very br).
• 19 Sreenath K, Suneesh CV, Gopidas KR, Flowers RA. II. J. Phys. Chem. A 2009; 113: 6477
• 20 Zheng X, Wang X, Qiu Y, Li Y, Zhou C, Sui Y, Li Y, Ma J, Wang X. J. Am. Chem. Soc. 2013; 135: 14912
• 21 Lawrence EJ, Oganesyan VS, Wildgoose GG, Aschley AE. Dalton Trans. 2013; 42: 782
• 22 Welch GC, Stephan DW. J. Am. Chem. Soc. 2007; 129: 1880
• 23 Synthesis of N(3,5-tBu₂C₆H₃)₃/B(3,5-tBu₂C₆H₃)₃ N(3,5-tBu₂C₆H₃)₃ (10 mg, 0.017 mmol) and B(3,5-tBu₂C₆H₃)₃ (10 mg, 0.017 mmol) were dissolved in toluene-d ₈ (0.5 mL). Storage of the solution at –30 °C overnight afforded colorless crystals of N(3,5-tBu₂C₆H₃)₃/B(3,5-tBu₂C₆H₃)₃. Yield: 16 mg (0.014 mmol, 80%); mp 224 °C. IR (neat): ν = 2951, 2902, 2866, 1582, 1495, 1476, 1461, 1447, 1426, 1413, 1391, 1361, 1307, 1246, 1221, 1204, 1130, 1038, 1008, 993, 924, 896, 886, 865, 822, 727, 720, 711, 694, 671, 539, 463 cm^–1. ¹H NMR (500 MHz, C₆D₆): δ 8.03 (s, 3 H, p-CH), 7.86 (s, 6 H, o-CH), 7.46 (s, 3 H, p-CH), 7.27 (s, 6 H, o-CH), 1.32 (s, 54 H, tBu), 1.23 (s, 54 H, tBu).