Reactivity Enhancement of a Zerovalent Diboron Compound by Desymmetrization

Transcription

1 Reactivity Enhancement of a Zerovalent Diboron Compound by Desymmetrization Julian Böhnke, a,b Merle Arrowsmith, a,b Holger Braunschweig a,b, * a. Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, Würzburg (Germany). b. Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians- Universität Würzburg, Am Hubland, Würzburg (Germany). Table of contents Methods and materials...2 Synthetic procedures...3 NMR Spectra...8 UV-vis spectroscopy IR spectroscopy X-ray crystallographic details Computational details References S1

2 Methods and materials All manipulations were performed either under an atmosphere of dry argon or in vacuo using standard Schlenk line or glovebox techniques. Deuterated solvents were dried over molecular sieves and degassed by three freeze-pump-thaw cycles prior to use. All other solvents were distilled and degassed from appropriate drying agents. Solvents (both deuterated and nondeuterated) were stored under argon over activated 4 Å molecular sieves. NMR spectra were acquired on a Bruker Avance 500 NMR spectrometer at 298 K unless otherwise stated ( 1 H and 1 H{ 11 B}: MHz, 11 B{ 1 H}: MHz, 13 C{ 1 H} and 13 C{ 1 H, 11 B}: MHz). Chemical shifts (δ) are given in ppm and internally referenced to the carbon nuclei ( 13 C{ 1 H}) or residual protons ( 1 H) of the solvent. 11 B{ 1 H} NMR spectra were referenced to [BF 3 OEt 2 ] as an external standard. UV/Vis spectra were acquired on a JASCO-V660 UV/Vis spectrometer under inert conditions inside a glovebox. Solid state IR spectra were acquired on a Bruker Alpha spectrometer using a setup with a Bruker diamond crystal single reflection ATR system. High-resolution mass spectrometry data was obtained from a Thermo Scientific Exactive Plus spectrometer in ASAP or LIFDI mode. Microanalyses (C, H, N) could not performed due to the extremely air-, moisture-, or temperature-sensitivity of the compounds. Solvents and reagents were were purchased from Sigma Aldrich or Alfa Aesar. Deuterated solvents were degassed with three freeze-pump-thaw cycles and stored over molecular sieves in Young ampoules or in a glovebox. (caac) 2 B 2 (1) 1, (IMe Me ) 2 and [(caac)bh] 3 2 were synthesized following literature procedures. S2

3 Synthetic procedures (caac) 2 B 2 (IMe Me ), 2 Compound 1 (124 mg, 209 µmol) und IMe Me (26.0 mg, 209 µmol) were combined in 10 ml of benzene. The suspension was stirred for 5 h at room temperature prior to removal of volatiles in vacuo. The green solid residue was washed with 3 x 5 ml pentane and recrystallized twice from a benzene solution. Green crystals of 2 were obtained by slow evaporation of a benzene solution at room temperature (91.7 mg, 128 µmol, 61%). 1 H NMR (400.1 MHz, C 6 D 6 ): δ = (overlapping dd and t, 2H, m/p-ar-h), 7.20 (dd, 1H, 3 J = 6.7 Hz, 2 J = 2.7 Hz, m-ar-h), 7.04 (dd, 1H, 3 J = 7.6 Hz, 2 J = 1.9 Hz, m-ar-h), 7.00 (t, 1H, 3 J = 7.4 Hz, p-ar-h), 6.84 (dd, 1H, 3 J = 7.2 Hz, 2 J = 1.9 Hz, m-ar-h), 4.25 (sept, 1H, 3 J = 6.6 Hz, CH ipr ), 3.69 (sept, 1H, 3 J = 6.9 Hz, CH ipr ), 3.58 (sept, 1H, 3 J = 6.9 Hz, CH ipr ), 3.48 (sept, 1H, 3 J = 6.8 Hz, CH ipr ), 3.15 (s, 3H, NCH 3 ), 2.35 (s, 3H, NCH 3 ), 2.08 (d, 1H, 2 J = 11.6 Hz, CH 2 ), 2.05 (d, 3H, 3 J = 6.8 Hz, CH 3-iPr ), 1.84, 1.81 (two d, 1H each, 2 J = 11.6, 11.9 Hz, CH 2 ), 1.65 (s, 3H, C(CH 3 ) 2 ), 1.64 (d, 1H, 2 J = 11.9 Hz, CH 2 ), 1.54 (s, 3H, C(CH 3 ) 2 ), 1.52, 1.51 (two d, 3H each, 3 J = 6.6, 6.8 Hz, CH 3-iPr ), 1.47, 1.45 (two s, 3H each, CH 3-NHC ), 1.46 (overlapping d, 3H, CH 3-iPr ), 1.42 (s, 3H, C(CH 3 ) 2 ), 1.38, 1.37 (two d, 3H each, 3 J = 6.8, 6.9 Hz, CH 3-iPr ), 1.22 (d, 3H, 3 J = 6.9 Hz, CH 3-iPr ), 1.14, 1.00, 0.96, 0.87, 0.79 (five s, 3H each, C(CH 3 ) 2 ), 0.30 (d, 3H, 3 J = 6.9 Hz, CH 3-iPr ). 13 C{ 1 H} NMR (C 6 D 6, MHz): (C carbene-nhc, detected by HMBC), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C carbene-caac, detected by HMBC), (C ar ), (C carbene-caac, detected by HMBC), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (CCH 3-NHC ), (CCH 3-NHC ), 64.7 (NC(CH 3 ) 2 ), 64.4 (NC(CH 3 ) 2 ), 59.9 (CH 2 ), 58.8 (CH 2 ), 46.2 (NCH 3 ), 43.1 (NCH 3 ), 35.9 (C(CH 3 ) 2 ), 35.2 (C(CH 3 ) 2 ), 33.6 (C(CH 3 ) 2 ), 33.3 (C(CH 3 ) 2 ), 33.0 (C(CH 3 ) 2 ), 32.0 (C(CH 3 ) 2 ), 31.6 (C(CH 3 ) 2 ), 31.1 (C(CH 3 ) 2 ), 29.6 (CH -ipr ), 29.4 (CH 3-iPr ), 27.7(CH -ipr ), (C(CH 3 ) 2, 27.5 (CH -ipr ), 27.4 (CH -ipr ), 27.0 (C(CH 3 ) 2, CH 3-iPr ), 26.2 (CH 3-iPr ), 25.4 (CH 3-iPr ), 25.0 (CH 3-iPr ), (CH 3-iPr ), 24.2 (CH 3-iPr ), 24.0 (CH 3-iPr ), 8.4 (CH 3-NHC ), 8.2 (CH 3-NHC ). Note: the 13 C NMR C carbene resonances could not be detected. 11 B NMR (128.4 MHz, C 6 D 6 ): δ = (sp-b), 6.2 (B(IMe Me )). LIFDI-MS [C 47 H 74 B 2 N 4 ]: m/z (calculated) = ; m/z (found): (caac) 2 B 2 (IMe Me )(CO), mg (34.9 µmol) of 2 were dissolved in 0.7 ml of benzene and left standing under a CO atmosphere at room temperature. A color change from yellow-green to red was observed and after one hour all volatiles were removed in vacuo and the deep red residue was washed with S3

4 3 x 1 ml pentane. Red crystals of 3 were obtained by slow evaporation of a benzene solution at room temperature (20.3 mg, 27.3 µmol, 78%). 1 H NMR (400.1 MHz, C 6 D 6 ): δ = 7.33 (dd, 1H, 3 J = 7.5 Hz, 2 J = 2.0 Hz, m-ar-h), 7.27 (dd, 1H, 3 J = 7.5 Hz, 2 J = 2.0 Hz, m-ar-h), 7.22, 7.21 (two overlapping t, 1H each, 3 J = 7.5, 7.2 Hz, p-ar-h), (m overlapping with residual NMR solvent, m-ar-h), 7.03 (dd, 1H, 3 J = 7.0 Hz, 2 J = 2.3 Hz, m-ar-h), 4.27 (sept, 1H, 3 J = 6.8 Hz, CH ipr ), 4.17 (sept, 1H, 3 J = 6.6 Hz, CH ipr ), 3.49 (s, 3H, NCH 3 ), 3.46 (br, 1H, CH ipr ), 3.00 (br s, 3H, NCH 3 ), 2.70 (br sept, 1H, 3 J = 6.8 Hz, CH ipr ), 1.90 (d, 1H, 2 J = 11.2 Hz, CH 2 ), 1.76 (d, 1H, 2 J = 11.2 Hz, CH 2 ), 1.73 (s, 3H, C(CH 3 ) 2 ), (br, 3H, CH 3-iPr, identified by COSY), (br, 2H, CH 2 ), 1.57 (d, 3H, 3 J = 6.6 Hz, CH 3-iPr ), 1.53, 1.51 (two d, 3H each, 3 J = 6.8 Hz, CH 3-iPr ), 1.44 (d, 3H, 5 J = 0.5 Hz, CH 3-NHC ), 1.42 (br s, 3H, C(CH 3 ) 2 ), 1.39 (d, 3H, 5 J = 0.5 Hz, CH 3-NHC ), 1.26 (s, 3H, C(CH 3 ) 2 ), 1.25 (br s, 3H, C(CH 3 ) 2 ), 1.22 (d, 3H, 3 J = 6.6 Hz, CH 3-iPr ), 1.18 (s, 3H, C(CH 3 ) 2 ), (br, 3H, CH 3-iPr, identified by COSY), 1.06 (d, 3H, 3 J = 6.8 Hz, CH 3-iPr ), 0.90 (br s, 3H, C(CH 3 ) 2 ), 0.37 (br d, 3H, CH 3- ipr). Note: due to line broadening caused by fluxional processes, two C(CH 3 ) 2 resonances could not be detected. 13 C{ 1 H} NMR (C 6 D 6, MHz): (BCO, detected by 11 B- decoupling), (B(CO)C Carbene-cAAC, detected by selective 11 B-decoupling), (C Carbene- NHC, detected by HMBC), (B(IMe Me )C Carbene-cAAC, detected by selective decoupling), (C ar, detected by HMBC), (C ar ), (C ar ), (C ar, detected by HMBC), (C ar ), (C ar ), (C ar, detected by HSQC), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (CCH 3-NHC ), (CCH 3-NHC ), 66.9 (NC(CH 3 ) 2 ), 61.8 (NC(CH 3 ) 2 ), 60.5 (CH 2 ), 55.7 (CH 2 ), 47.6 (C(CH 3 ) 2 ), 45.0 (C(CH 3 ) 2 ), 35.8 (NCH 3-NHC ), 34.3 (NCH 3-NHC ), 33.8 (C(CH 3 ) 2 ), 33.5 (C(CH 3 ) 2 ), 32.8 (C(CH 3 ) 2 ), 32.2 (C(CH 3 ) 2 ), 28.4 (CH ipr ), 28.2 (CH 3-iPr ), 28.0 (CH ipr ), 27.9 (CH ipr ), 26.9 (CH ipr ), 27.0 (C(CH 3 ) 2 ), 26.7 (CH 3-iPr ), 26.6 (C(CH 3 ) 2, 26.3 (CH 3-iPr ), 26.2 (CH 3-iPr ), 24.8 (CH 3-iPr ), 23.4 (CH 3-iPr ), 8.5 (NCCH 3-NHC ), 8.3 (NCCH 3-NHC ). Note: due to line broadening caused by fluxional processes, two ipr-ch 3 and C(CH 3 ) 2 resonances could not be detected. 11 B NMR (128.4 MHz, C 6 D 6 ): δ = 9.3 (B(IMe Me )), 16.6 (BCO) ppm. LIFDI-MS [C 48 H 74 B 2 N 4 O]: m/z (calculated) = ; m/z (found): B- [(caac) 2 B 2 (IMe Me ) 2 ], 4 Compound 1 (150 mg, 253 µmol) und IMe Me (62.9 mg, 506 µmol) were combined in 10 ml benzene. The suspension was stirred for 5 h at room temperature prior to removal of volatiles in vacuo. The solid red residue was extracted with pentane. Red crystals of 4 suitable for X- ray crystallographic analysis were obtained by slow evaporation of a hexane solution at room S4

5 temperature. However, crystallization was systematically accompanied by decomposition of 4 to free IMe Me and 2, the latter slowly undergoing intramolecular C-H activation to 5. Due to co-crystallization of these decomposition products, isolated samples of 4 analyzed by NMR spectroscopy always contained varying amounts of 2 and/or 5 and/or IMe Me. Furthermore, compound 4 proved highly fluxional in solution due to strongly hindered rotation, which caused all NMR resonances to broaden, thus precluding the interpretation of 1 H NMR spectra even of contaminated samples. As a result, the compound could only be characterized by X- ray crystallography, HRMS and its 11 B NMR shift. 11 B NMR (128.4 MHz, C 6 D 6 ): δ = 14.6 ppm. LIFDI-MS [C 54 H 86 B 2 N 6 ]: m/z (calculated) = ; m/z (found): (caach)b(ch 2 C(Me)CH 2 C(Me 2 )N(Dip)C)B(IMe Me ), 5 A) 10.0 mg (14.0 µmol) of 2 were dissolved in in 0.7 ml C 6 D 6 and heated to 80 C for 24 h. After removal of volatiles in vacuo the yellow residue was washed with 3 x 2 ml pentane. Yellow crystals of 5 were obtained by slow evaporation of a benzene solution at room temperature (8.91 mg, 12.4 µmol, 89%). B) 25.0 mg (34.9 µmol) of 2 und 1.0 mg GaCl 3 (5.7 µmol) were dissolved in 0.7 ml C 6 D 6. The reaction mixture turned yellow immediately. After removal of volatiles in vacuo the yellow residue was washed with 3 x 2 ml pentane. Yellow crystals of 5 were obtained by slow evaporation of a benzene solution at room temperature (22.8 mg, 31.8 µmol, 91%). 1 H NMR (500.1 MHz, C 6 D 6 ): δ = 7.32 (dd, 1H, 3 J = 7.6 Hz, 4 J = 2.0 Hz, m-ar-h), 7.27 (t, 1H, 3 J = 7.6 Hz, p-ar-h), 7.21 (dd, 1H, 3 J = 7.6 Hz, 4 J = 2.0 Hz, m-ar-h), (m, 2H, m/p- Ar-H), (m, 1H, m-ar-h), 4.50 (sept, 1H, 3 J = 6.8 Hz, CH ipr ), 4.29 (s, 1H, BC caac H), 4.02 (sept, 1H, 3 J = 6.8 Hz, CH ipr ), 3.60 (sept, 1H, 3 J = 6.8 Hz, CH ipr ), 3.22 (s, 3H, NCH 3 ), 3.11 (sept, 1H, 3 J = 6.5 Hz, CH ipr ), 2.60 (s, 3H, NCH 3 ), 2.10 (d, 1H, 2 J = 11.9 Hz, CH 2 ), 2.01 (s, 3H, BCH 2 CCH 3 ), 2.00 (d, 1H, 2 J = 16.3 Hz, BCH 2 ), 1.96, 1.95, 1.93 (three overlapping d, 1H each, 2 J = 12.3, 11.9, 12.3 Hz, CH 2 ), 1.70 (d, 3H, 3 J = 6.7 Hz, CH 3-iPr ), 1.58 (s, 3H, C(CH 3 ) 2 ), 1.53 (s, 3H, C(CH 3 ) 2 ), 1.45 (d, 3H, 5 J = 0.8 Hz, CCH 3-NHC ), 1.43, 1.41, 1.39 (three d, 3H each, 3 J = 6.8 Hz, CH 3-iPr ), 1.37 (s, 3H, C(CH 3 ) 2 ), 1.29 (s, 3H, C(CH 3 ) 2 ), 1.25 (d, 3H, 5 J = 0.8 Hz, CCH 3-NHC ), 1.23 (d, 3H, 3 J = 6.8 Hz, CH 3-iPr ), 1.22 (s, 3H, C(CH 3 ) 2 ), 1.18 (d, 1H, 2 J = 16.3 Hz, BCH 2 ), 1.18 (d, 3H, 3 J = 6.8 Hz, CH 3-iPr ), 1.11 (d, 3H, 3 J = 6.5 Hz, CH 3-iPr ), 0.82 (s, 3H, C(CH 3 ) 2 ), 0.36 (d, 3H, 3 J = 6.5 Hz, CH 3-iPr ). 13 C{ 1 H} NMR (C 6 D 6, MHz): (C Carbene-cAAC, detected by HMBC), (C Carbene-NHC, detected by HMBC), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (CCH 3-NHC ), (CCH 3-NHC ), 74.7 S5

6 (BCH), 71.1 (NC(CH 3 ) 2 ), 62.2 (NC(CH 3 ) 2 ), 60.3 (CH 2 ), 58.5 (BCH 2 C), 54.8 (CH 2 ), 47.1 (BCH 2 ), 42.6 (B(CH)C), 38.4 (BCH 2 CCH 3 ), 33.6 (NCH 3 ), 33.5 (NCH 3 ), 32.8 (C(CH 3 ) 2 ), 31.2 (C(CH 3 ) 2 ), 30.0 (C(CH 3 ) 2 ), 29.9 (C(CH 3 ) 2 ), 29.1 (C(CH 3 ) 2 ), 28.9 (CH ipr ), 28.1 (CH ipr ), 27.8 (CH ipr ), 27.7 (CH ipr ), 27.6 (CH 3-iPr ), 26.5 (C(CH 3 ) 2 ), 26.4 (CH 3-iPr ), (CH 3-iPr ), (CH 3-iPr ), (CH 3-iPr ), (CH 3-iPr ), 25.1 (CH 3-iPr ), 24.1 (CH 3-iPr ), 8.3 (CCH 3-NHC ), 8.1 (CCH 3-NHC ). 11 B NMR (160.5 MHz, C 6 D 6 ): δ = 91.4 (BH), 9.4 (B(IMe Me )). LIFDI-MS [C 47 H 74 B 2 N 4 ]: m/z (calculated) = ; m/z (found): (caac)(ime Me )BB(H)(cAACH), 6 The diborene [(caac)bh] 2 (48 mg, 81 µmol) and IMe Me (15 mg, 0.12 mmol) were dissolved in 0.5 ml C 6 D 6 in a Young s NMR tube. The reaction mixture was heated at 60 C for 8 h. Removal of volatiles in vacuo yielded an orange solid, which was extracted with 0.5 ml of a 1:1 benzene:hexanes mixture. Slow evaporation of the solvent at room temperature yielded a crop of orange crystals of 6 (36 mg, 50 µmol, 62%). At room temperature, the compound was highly fluxional and 1 H NMR spectra only showed broad signals. At 60 C, the resonances of the neutral caac ligand were sharpened but those associated with IMe Me and the protonated caac ligands remained very broad (stated as br in the data analysis). However, it also started to slowly decompose at that temperature, which precluded the acquisition of clean 1 H and 13 C NMR spectra. NMR shifts are provided from spectra of ca. 80% pure 6 recorded at 60 C. 1 H NMR (500.1 MHz, C 6 D 6, 333 K): δ = (m, 3H, Ar-H), 6.90 (t, 1H, 3 J = 7.1 Hz, p-ar-h), 6.81 (d, 2H, 3 J = 7.1 Hz, m-ar-h), (br, 2H, CH ipr ), 3.32 (s, 1H, BCH), 3.19 (br, 2H, CH ipr ), 2.53 (br, 6H, NCH 3 ), 2.30 (s, 6H, C(CH 3 ) 2 ), 2.06 (s, 2H, CH 2 ), 2.01 (br, 2H, CH 2 ), (br, 6H, C(CH 3 ) 2 ), 1.41 (s, 6H, C(CH 3 ) 2 ), (br m, 9H, C(CH 3 ) 2 + CH 3-iPr ), 1.25 (d, 3 J = 6.5 Hz, CH 3-iPr ), 1.24 (s, 6H, NCCH 3-NHC ), (br m, 6H, CH 3-iPr ), 1.12 (d, 3 J = 6.5 Hz, CH 3-iPr ), 1.00 (br, 3H, C(CH 3 ) 2 ) ppm. 13 C{ 1 H} NMR (C 6 D 6, MHz, 333 K): δ = (br, C carbene-nhc ), (br, C carbene-caac ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (C ar ), (br, NCCH 3-NHC ), (C ar ), 123.8, (C ar ), 76.1 (br, BCH), 64.5 (NC(CH 3 ) 2 ), 64.1 (NC(CH 3 ) 2 ), 62.2 (CH 2 ), 61.4 (CH 2 ), 48.6 (C(CH 3 ) 2 ), 43.1 (C(CH 3 ) 2 ), 38.8 (br, CH -ipr ), 33.3 (CH -ipr ), 29.7 (br, CH -ipr ), 28.2 (CH 3-iPr ), 28.1 (CH 3-iPr ), 27.4 (br, CH 3-iPr ), 26.5 (br, CH 3-iPr ), 25.7 (br, CH 3-iPr ), 24.8 (br, CH 3-iPr ), 8.9 (CCH 3-NHC ) ppm. 11 B NMR (128.4 MHz, C 6 D 6, 333 K): δ = 81.4 (br s, fwmh 670 Hz, BH), 15.7 (B(IMe Me )). LIFDI-MS [C 47 H 76 B 2 N 4 ]: m/z (calculated) = ; m/z (found): S6

7 (caac)(ime Me )BH, 7 A) 72.0 mg (100 µmol) of 2 were dissolved in 5 ml of benzene and stirred for 1 h under a H 2 atmosphere at room temperature. After removal of volatiles in vacuo the deep orange residue was washed with 3 x 2 ml pentane and dried in vacuo. The mixture was found to contain both 6 and (caac)bh 3. By addition of one equiv. caac per (caac)bh 3, (caac)(caach)bh 2 was formed which was removed by fractional crystallization from hexanes at 30 C. From the residual filtrate yellow crystals of 7 suitable for X-ray diffraction analysis were obtained by slow solvent evaporation. This method, however, did not provide sufficient quantities of analytically pure 7 for complete characterization. B) (caac)bbr 2 H (100 mg, 0.22 mmol), IMe Me (135 mg, 1.09 mmol, 5 equiv.) and KC 8 (135 mg, 1.00 mmol) were combined in toluene and the mixture stirred for 5 h at rt. After removal of volatiles the mixture was extracted with 3 ml pentane and the solvent removed. The resulting orange solid was analyzed by NMR spectroscopy and revealed a 1:2 mixture of 7 and IMe Me. The latter was removed by sublimation at 100 C, 10 3 mbar. Analytically pure 7 was obtained by recrystallization from a saturated hexanes solution at 30 C (35 mg, 83 µmol, 38%). 1 H NMR (500.1 MHz, C 6 D 6 ): δ = (m, 3H, Ar-H), 3.97 (sept, 1H, 3 J = 6.6 Hz, CH ipr ), 3.24 (s, 6H, NCH 3 ), 2.51 (s 1H, BH), 2.14 (s, 2H, CH 2 ), 1.73 (d, 6H, 3 J = 6.6 Hz, CH 3-iPr ), 1.57 (d, 6H, 3 J = 6.6 Hz, CH 3-iPr ), 1.47 (s, 6H, C(CH 3 ) 2 ), 1.38 (s, 6H, C(CH 3 ) 2, 1.26 (s, 6H, NCCH 3 ). 13 C{ 1 H} NMR (C 6 D 6, MHz): δ = (C carbene-nhc, identified by HMBC), (C carbene-caac, identified by HMBC), (o-ar-c), (i-ar-c), (p- Ar-C), (m-ar-c), (NCCH 3-NHC ), 62.9 (NC(CH 3 ) 2 ), 59.7 (CH 2 ), 43.2 (C(CH 3 ) 2 ), 34.2 (NCH 3-NHC ), 32.3 (C(CH 3 ) 2 ), 30.6 (C(CH 3 ) 2 ), 29.0 (CH -ipr ), 28.3 (CH 3-iPr ), 25.0 (CH 3-iPr ), 8.52 (CCH 3-NHC ). 11 B NMR (128.4 MHz, C 6 D 6 ): δ = 2.9 ppm (d, 1 J = Hz). LIFDI-MS [C 27 H 44 BN 3 +H] + : m/z (calculated) = ; m/z (found): S7

8 NMR spectra Figure S1. 1 H NMR spectrum of 2 in C 6 D 6. S8

9 Figure S2. 11 B NMR spectrum of 2 in C 6 D 6. S9

10 Figure S3. 13 C{ 1 H} NMR spectrum of 2 in C 6 D 6. S10

11 Figure S4. 1 H NMR spectrum of 3 in C 6 D 6. S11

12 Figure S5. 11 B NMR spectrum of 3 in C 6 D 6. S12

13 Figure S6. 13 C{ 1 H} NMR spectrum of 3 in C 6 D 6. S13

14 Figure S7. 13 C{ 1 H, 11 B} NMR spectrum of 3 in C 6 D 6 : region of boron-bound caac, IMe Me and CO ligands. S14

15 Figure S8. 11 B NMR spectrum of 4 in C 6 D 6. S15

16 Figure S9. 1 H NMR spectrum of 5 in C 6 D 6. S16

17 Figure S B NMR spectrum of 5 in C 6 D 6. S17

18 Figure S C{ 1 H} NMR spectrum of 5 in C 6 D 6. S18

19 Figure S B NMR spectrum of the reaction mixture of 2 + H 2 in C 6 D 6 after 24 h at rt, showing both products, borylene 7 and (caac Me )BH 3, formed in a 1:1 ratio. S19

20 Figure S B NMR spectrum of 6 at 60 C in C 6 D 6 S20

21 Figure S14. 1 H NMR spectrum of 7 in C 6 D 6. S21

22 Figure S B NMR spectrum of 7 in C 6 D 6. S22

23 Figure S C{ 1 H} NMR spectrum of 7 in C 6 D 6. S23

24 UV-vis spectroscopy Figure S17. UV-vis spectrum of 2 in THF. c = 2.79 x 10 5 M. λ max = 434 nm. S24

25 Figure S18. UV-vis spectrum of 3 in THF. c = 5.37 x 10 5 M. λ max = 390 nm. S25

26 IR spectroscopy Figure S19. Solid state IR spectrum of 3. S26

27 X-ray crystallographic details Crystal data of all compounds were collected on a Bruker X8-APEX II diffractometer with a CCD area detector (3, 5 and 7), or a Bruker D8 Quest diffractometer with a CMOS area detector (2, 4 and 6), both equipped with m-layer mirror monochromated Mo Kα radiation. The structures were solved using intrinsic phasing methods, 4 refined with the ShelXL software package 5 and expanded using Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen aroms were refined isotropically and assigned to idealized positions, except for boron-bound hydrogen atoms, which were located in the difference Fourier map and freely refined. Cif files of crystallographic structures have been deposited with the Cambridge Crystallographic Data Centre: CCDC (2), (3), (4), (5), (6) and (7). Crystal data for 2: C 47 H 74 B 2 N 4, M r = , yellow plate, mm 3, triclinic space group P 1, a = 9.953(2) Å, b = (18) Å, c = (7) Å, α = (14), β = (18), γ = (17), V = (10) Å 3, Z = 2, ρ calcd = g cm 3, µ = mm 1, F(000) = 788, T = 100(2) K, R 1 = , wr 2 = , 8907 independent reflections [2θ ] and 498 parameters. Crystal data for 3: C 48 H 74 B 2 N 4 O, M r = , red block, mm 3, triclinic space group P 1, a = (6) Å, b = (6) Å, c = (13) Å, α = (18), β = (17), γ = 71.33(2), V = 2247(2) Å 3, Z = 2, ρ calcd = g cm 3, µ = mm 1, F(000) = 816, T = 103(2) K, R 1 = , wr 2 = , 9190 independent reflections [2θ ] and 516 parameters. Crystal data for 4: C 60 H 92 B 2 N 6, M r = , red block, mm 3, monoclinic space group P2 1 /n, a = (6) Å, b = (15) Å, c = (5) Å, β = 92.18(3), V = 5495(4) Å 3, Z = 4, ρ calcd = g cm 3, µ = mm 1, F(000) = 2016, T = 100(2) K, R 1 = , wr 2 = , independent reflections [2θ ] and 637 parameters. Crystal data for 5: C 47 H 74 B 2 N 4, M r = , yellow block, mm 3, monoclinic space group P2 1 /c, a = 9.589(5) Å, b = (14) Å, c = (18) Å, β = 95.42(5), V = 4369(6) Å 3, Z = 4, ρ calcd = g cm 3, µ = mm 1, F(000) = 1576, S27

28 T = 103(2) K, R 1 = , wr 2 = , 8891 independent reflections [2θ ] and 497 parameters. Crystal data for 6: C 47 H 76 B 2 N 4, M r = , yellow block, mm 3, triclinic space group P 1, a = (15) Å, b = (18) Å, c = (2) Å, α = (4), β = (5), γ = (5), V = (5) Å 3, Z = 2, ρ calcd = g cm 3, µ = mm 1, F(000) = 792, T = 100(2) K, R 1 = , wr 2 = , 9728 independent reflections [2θ ] and 502 parameters. Crystal data for 7: C 27 H 44 BN 3, M r = , orange block, mm 3, triclinic space group P 1, a = 9.284(3) Å, b = (3) Å, c = (4) Å, α = (9), β = 89.48(2), γ = (9), V = (6) Å 3, Z = 2, ρ calcd = g cm 3, µ = mm 1, F(000) = 464, T = 103(2) K, R 1 = , wr 2 = , 5222 independent reflections [2θ ] and 296 parameters. S28

29 Figure S20. a) Simplified view of the solid-state structure of 4 displaying the shortest IMe Me - to-ime Me (red) and IMe Me -to-dip (blue) interactions as dotted lines with distances in Å. Thermal ellipsoids drawn at the 30% probability level. Methyl and isopropyl substituents and hydrogen atoms omitted for clarity; b) View along the B-B axis, showing the staggered arrangement of the aromatic IMe Me rings (red) and the relative position of the flanking Dip rings (blue); c) Plot of the HOMO-16 of 4 at the B3LYP/6-311G(d) level showing π overlap between the IMe Me rings. Hydrogens omitted for clarity. Isovalue: Energies (ev) given in brackets. S29

30 Computational details Compounds 2, 3, 4 and 7 were analyzed with the Gaussian 09 software package 6 in the gas phase at the B3LYP/6-311G(d) 7-9 level of theory. All optimized structures were subjected to frequency analysis to confirm their identity as true ground state minima with no imaginary frequencies. (caac) 2 B 2 (IMe Me ), 2 LUMO ( 0.60) HOMO ( 3.29) HOMO 1 ( 3.99) Figure S21. Top: optimized structure of 2 with relevant bond lengths (Å). Plots of frontier MOs at B3LYP/6-311G(d) level. Hydrogens omitted for clarity. Energies (ev) in brackets. S30

31 (caac) 2 B 2 (IMe Me )(CO), 3 LUMO ( 0.70) HOMO ( 3.43) HOMO 1 ( 4.17) Figure S22. Top: optimized structure of 3 with relevant bond lengths (Å). Plots of frontier MOs at B3LYP/6-311G(d) level. Hydrogens omitted for clarity. Energies (ev) in brackets. S31

32 (caac) 2 B 2 (IMe Me ) 2, 4 LUMO ( 0.10) HOMO ( 3.03) HOMO 1 ( 3.56) HOMO 2 Figure S23. Top: optimized structure of 4 with relevant bond lengths (Å). Plots of frontier MOs at B3LYP/6-311G(d) level. Hydrogens omitted for clarity. Energies (ev) in brackets. S32

33 [(caac)(ime Me )BH], 7 LUMO ( 0.03) HOMO ( 3.25) Figure S24. Top: optimized structure of 7 with relevant bond lengths (Å). Plots of frontier MOs at B3LYP/6-311G(d) level. Hydrogens omitted for clarity. Energies (ev) in brackets. S33

34 Table S1. Cartesian coordinates (Å) and energy (a.u.) for compound 2 optimized at the B3LYP/6-311G(d) level. E = N C B N C B C H H H N C H H H N C H H C C H H H C H H H C C C H C H H H C H C H H H C C H S34

35 C H C H C H H H C H H H C C C H H H C H H H C H H C C H H H C H H H C C C H C H H H C H H H C H C H C S35

36 H C C H C H H H C H H H C C H H H C C H H H C C H H H C H H H S36

37 Table S2. Cartesian coordinates (Å) and energy (a.u.) for compound 3 optimized at the B3LYP/6-311G(d) level. E = O N B C N B C C H H H N C H H H N C H H C C H H H C H H H C C C H C H H H C H C H H H C S37

38 H C H C C H C H H H C H H H C C C H H H C H H H C H H C C H H H C H H H C C C H C H H H C H H H C H C S38

39 H C H C C H C H H H C H H H C C H H H C C H H H C C H H H C H H H C S39

40 Table S3. Cartesian coordinates (Å) and energy (a.u.) for compound 4 optimized at the B3LYP/6-311G(d) level. E = N N N B N N N C C C C C H H H C H H H C C C C H H H C C C H H H C C C C C H H H C H C H C S40

41 H H H C C H H H C H C H H C C C H H H C H C H H H C H C H C C C H H H C H C H C H C H H H C H H C H H S41

42 H C H H H C H H H C H H H C H H H C H H H C H H H C H H H C H C H H H C H H H C H H H C H H H C H H H S42

43 B S43

44 Table S4. Cartesian coordinates (Å) and energy (a.u.) for compound 7 optimized at the B3LYP/6-311G(d) level. E = N H N N C C C C C C C C C H C H H H C H H H C H H H C H C H C H H C C H C H C H H H C H H H S44

45 C H H H C H H H C H H H C H H H C H H H C H H H C H H H B S45

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