Formation of macrocyclic ring systems by carbonylation of trifunctional P/B/B frustrated Lewis pairs

The trifunctional P/B/B frustrated Lewis pairs 11a–c featuring bulky aryl groups at phosphorus [Dmesp (a), Tipp (b), Mes* (c)] were synthesized. Compounds 11a,b react with carbon monoxide and form the macrocyclic dimers 17a,b, while the carbonylation reaction of the Mes*P/B/B FLP 11c gives the macrocyclic trimer 18c.

In situ generation of compound 11a S8 Synthesis of compound 14a S11 Characterization of compound 15py S15 Synthesis of compound 14b S18 In situ reaction of compound 14b (in situ generated) with H2 S22 Synthesis of compound 14c S24 Preparation of compound 17a S28 Preparation of 13 C labeled compound 17a S34 Preparation of compound 17b S37 Preparation of 13 C labeled compound 17b S43 In situ generation of compound 18c S46 In situ generation of 13 C labeled compound 18c S52 Preparation of compound 18c S54 Stability of compound 18c in dichloromethane-d2 solution S57 Preparation of compound 3 Scheme S1.
A solution of styrene 1 (145.8 mg, 1.4 mmol, 1.4 eq.) in pentane (6 mL) was added to a suspension of HB(C6F5)2 2 (345.9 mg, 1.0 mmol, 1 eq.) in pentane (2 mL) at room temperature. While stirring the reaction suspension became a clear solution and then a suspension. The precipitate was collected by filtration, then it was washed with cold pentane (2 mL×2, -30 o C) to give compound 3 as a white solid (320.0 mg, 0.71 mmol, yield 71 %). 11

Reaction of compound 3 with CO gas Scheme S2.
A solution of compound 3 (22.5 mg, 0.05 mmol) in C6D6 (1 mL) was filled in a J-Young tube, which was cooled to -78 o C (dry ice / isopropanol bath) and then evacuated carefully. Then the reaction mixture was exposed to CO gas (1.5 bar) at room temperature. After storing for 24 h at room temperature, the mixture was characterized by NMR experiments. In the glove box, compound 8a (19.9 mg, 0.05 mmol, 1.0 eq.) and compound 2 (34.6 mg, 0.10 mmol, 2.0 eq.) were mixed with dichloromethane-d2 (1 mL) to give a yellow solution. The yellow solution was characterized by NMR experiments.  In a Schlenk flask, a solution of compound 8a (119.6 mg, 0.30 mmol, 1.0 eq.) and compound 2 (207.6 mg, 0.6 mmol, 2.0 eq.) in dichloromethane (6 mL) was stirred for 15 min at room temperature to give a yellow solution. The solution was cooled to -78 °C (dry ice / isopropanol bath) and the flask was evacuated carefully. Then the dry ice bath was removed and the reaction mixture was exposed to H2 gas (1.5 bar) at -78 o C. Subsequently the obtained suspension was stirred at room temperature for 30 min, then the H2 was released under argon atmosphere followed by the addition of pentene (42.1 mg, 0.6 mmol, 2 eq.). The reaction mixture was stirred at room temperature for 2 days until the precipitate had dissolved again. Then all volatiles were removed in vacuo to give a white solid, which was washed with pentane (2 mL × 3) and dried in vacuo to give compound 14a as a white solid (174.7 mg, 0.234 mmol, yield 78 %). The combined washing solutions were collected and pyridine (23.7 mg, 0.3 mmol, 1.0 eq.) was added. The mixture was stored at -36 o C to give compound 15py as a white solid (108.4 mg, 0.219 mmol, yield 73 %). Crystals suitable for the X-ray crystal structure analysis were obtained by slow diffusion of pentane to a solution of compound 14a in dichloromethane at -36 o C. X-ray crystal structure analysis of compound 14a: A colorless plate-like specimen of C40H34BF10P, approximate dimensions 0.044 mm x 0.206 mm x 0.388 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured. A total of 342 frames were collected. The total exposure time was 6.65 hours. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a monoclinic unit cell yielded a total of 46282 reflections to a maximum θ angle of 25.03° (0.84 Å resolution), of which 7340 were independent (average redundancy 6.305, completeness = 99.9%, Rint = 15.27%, Rsig = 8.51%) and 4607 (62.77%) were greater than 2σ(F 2 ). The final cell constants of a = 13.2212(8) Å, b = 16.1944(12) Å, c = 19.5088(13) Å, β = 94.223(2)°, volume = 4165.7(5) Å 3 , are based upon the refinement of the XYZ-centroids of 5505 reflections above 20 σ(I) with 4.605° < 2θ < 50.76°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.915. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.9490 and 0.9940. The final anisotropic full-matrix least-squares refinement on F 2 with 479 variables converged at R1 = 6.07%, for the observed data and wR2 = 14.36% for all data. The goodness-of-fit was 1.020. The largest peak in the final difference electron density synthesis was 0.306 e -/Å 3 and the largest hole was -0.285 e -/Å 3 with an RMS deviation of 0.065 e -/Å 3 . On the basis of the final model, the calculated density was 1.190 g/cm 3 and F(000), 1536 e -. The position of the hydrogen atom at P1 was refined freely; others hydrogen atoms were calculated and refined as riding atoms.  In a Schlenk flask, a solution of compound 8b (86.5 mg, 0.30 mmol, 1.0 eq.) and compound 2 (217.9 mg, 0.63 mmol, 2.1 eq.) in pentane (10 mL) and stirred at room temperature for 24 h. Then the mixture was filtered via cannula. The obtained solution was cooled to -78 °C (dry ice / isopropanol bath) and the flask was evacuated carefully. Then the dry ice bath was removed and the reaction mixture was exposed to H2 gas (1.5 bar). Subsequently the formed suspension was stirred at room temperature for 2 h. Then all volatiles were removed in vacuo and dichloromethane (6 mL) was added to give a suspension. After addition of pentene (42.1 mg, 0.6 mmol, 2 eq.), the reaction mixture was stirred at room temperature for 1 day until the solid was dissolved. Then the volatiles were removed in vacuo to give a white solid, which was washed with pentane (2 mL × 3) and dried in vacuo to give compound 14b as a white solid (133.6 mg, 0.210 mmol, 70 % yield). The combined washing solutions were collected and pyridine (23.7 mg, 0.3 mmol, 1.0 eq.) was added. The mixture was stored at -36 o C to give compound 15py as a white solid (81.7 mg, 0.165 mmol, 55 % yield). The obtained NMR data of compound 15py were consistant with those listed above (page 49  Crystals suitable for the X-ray crystal structure analysis were obtained from slow diffusion of pentane to a solution of compound 14b in dichloromethane at -36 o C. X-ray crystal structure analysis of compound 14b: A colorless plate-like specimen of C31H32BF10P, approximate dimensions 0.030 mm x 0.180 mm x 0.300 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured. A total of 1809 frames were collected. The total exposure time was 26.63 hours. The frames were integrated with the Bruker SAINT software package using a wide-frame algorithm. The integration of the data using a monoclinic unit cell yielded a total of 43312 reflections to a maximum θ angle of 67.37° (0.84 Å resolution), of which 5066 were independent (average redundancy 8.550, completeness = 96.5%, Rint = 9.19%, Rsig = 4.59%) and 3915 (77.28%) were greater than 2σ(F 2 ). The final cell constants of a = 17.6792(14) Å, b = 8.5320(7) Å, c = 19.5972(15) Å, β = 99.052(4)°, volume = 2919.2(4) Å 3 , are based upon the refinement of the XYZ-centroids of 9978 reflections above 20 σ(I) with 6.261° < 2θ < 133.0°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.770. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.6450 and 0.9530. The final anisotropic fullmatrix least-squares refinement on F 2 with 398 variables converged at R1 = 4.50%, for the observed data and wR2 = 10.02% for all data. The goodness-of-fit was 1.035. The largest peak in the final difference electron density synthesis was 0.287 e -/Å 3 and the largest hole was -0.336 e -/Å 3 with an RMS deviation of 0.054 e -/Å 3 . On the basis of the final model, the calculated density was 1.448 g/cm 3 and F(000), 1312 e -. The position of the hydrogen atom at P1 was refined freely; others hydrogen atoms were calculated and refined as riding atoms. In a Schlenk flask, a solution of compound 8b (11.5 mg, 0.04 mmol, 1.0 eq.) and compound 2 (29.0 mg, 0.084 mmol, 2.1 eq.) in pentane (2 mL) was stirred at room temperature for 12 h. Then the mixture was filtered. After the solution was filled in a J-Young tube which was cooled to -78 °C (dry ice / isopropanol bath), it was evacuated carefully. Then the dry ice bath was removed and the reaction mixture was exposed to H2 gas (1.5 bar). The formed suspension was kept at room temperature for 16 h. Then the supernatant was removed by decantation and the collected solid was dissolved in dichloromethane-d2 (1 mL). In a Schlenk flask, a solution of compound 8c (99.1 mg, 0.30 mmol, 1.0 eq.) and compound 2 (207.6 mg, 0.6 mmol, 2.0 eq.) in dichloromethane (6 mL) was stirred at room temperature for 15 min to give a yellow solution. After the solution was cooled to -78 °C (dry ice / isopropanol bath) and the flask was evacuated carefully, the dry ice bath was removed and the reaction mixture was exposed to H2 gas (1.5 bar). Subsequently the formed suspension was stirred at room temperature for 30 min, then the H2 was released under argon atmosphere followed by the addition of pentene (42.1 mg, 0.6 mmol, 2 eq.). The reaction mixture was stirred at room temperature for 2 days until the precipitate had dissolved again. Then all volatiles were removed in vacuo to give a white solid which was washed with pentane (2 mL × 3) and dried in vacuo to give compound 14c as a white solid (140.4 mg, 0.207 mmol, 69 % yield). The combined washing solutions were collected and pyridine (23.7 mg, 0.3 mmol, 1.0 eq.) was added. The mixture was stored at -36 o C to give compound 15py as a white solid (89.1 mg, 0.180 mmol, 60 % yield). The obtained NMR data of compound 15py were consistant with those listed above (page 49   In a Schlenk flask, compound 8a (79.7 mg, 0.20 mmol, 1.0 eq.) and compound 2 (138.4 mg, 0.40 mmol, 2.0 eq.) were mixed with dichloromethane (4 mL) and stirred at room temperature for 15 min to give a yellow solution. The solvent was removed in vacuo and the residue was dissolved in pentane (4 mL). Then the solution was cooled to -78 °C (dry ice / isopropanol bath) and subsequently the flask was evacuated carefully. Then, after the dry ice bath was removed and the reaction mixture was exposed to CO gas (1.5 bar) at room temperature, the obtained reaction mixture was stirred at room temperature for 30 min to give a white suspension. Filtration by using a filter canula gave a white solid. The white solid was washed with pentane (1 mL × 2) and dried in vacuo. Compound 17a was obtained as a white solid (135.0 mg, 0.061 mmol, 61 %).      Crystals suitable for the X-ray crystal structure analysis of compound 17a were obtained from slow diffusion of pentane to a solution of the white solid in dichloromethane at -36 o C. X-ray crystal structure analysis of compound 17a: A colorless needle-like specimen of C106H66B4F40O2P2, approximate dimensions 0.046 mm x 0.064 mm x 0.173 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured. A total of 1640 frames were collected. The total exposure time was 39.38 hours. The frames were integrated with the Bruker SAINT software package using a wide-frame algorithm. The integration of the data using an orthorhombic unit cell yielded a total of 130938 reflections to a maximum θ angle of 60.00° (0.89 Å resolution), of which 12974 were independent (average redundancy 10.092, completeness = 95.5%, Rint = 11.46%, Rsig = 10.86%) and 10263 (79.10%) were greater than 2σ(F 2 ). The final cell constants of a = 23.2043(10) Å, b = 30.9754(12) Å, c = 14.8030(6) Å, volume = 10639.8(8) Å 3 , are based upon the refinement of the XYZ-centroids of 287 reflections above 20 σ(I) with 11.88° < 2θ < 73.31°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.881. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7900 and 0.9370. The final anisotropic full-matrix least-squares refinement on F 2 with 1399 variables converged at R1 = 10.81%, for the observed data and wR2 = 20.58% for all data. The goodness-of-fit was 1.161. The largest peak in the final difference electron density synthesis was 0.578 e -/Å 3 and the largest hole was -0.465 e -/Å 3 with an RMS deviation of 0.103 e -/Å 3 . On the basis of the final model, the calculated density was 1.396 g/cm 3 and F(000), 4512 e -. Preparation of 13 C labeled compound 17a Scheme S10.
In the glove box, compound 8a (19.9 mg, 0.05 mmol, 1.0 eq.) and compound 2 (34.6 mg, 0.10 mmol, 2.0 eq.) were mixed with pentane (1 mL) and stirred at room temperature for 1 hour to give a yellow solution. After the obtained solution was filled in a J-Young tube, it was carefully evacuated at -78 o C (dry ice / isopropanol bath) and then the solution was exposed to 13 CO (1.5 bar) for 10 min at room temperature to give a white suspension. Decantation of the suspension gave a white solid, which was washed with pentane (0.5 mL) and dried in vacuo to give a white solid. The white solid product was characterized by NMR and IR spectroscopy.  In a Schlenk flask, compound 8b (57.7 mg, 0.20 mmol, 1.0 eq.) and compound 2 (145.3 mg, 0.42 mmol, 2.1 eq.) were mixed with pentane (6 mL) and stirred at room temperature for 24 h. After the reaction mixture was filtered via cannula, the obtained solution was cooled to -78 °C (dry ice / isopropanol bath) and then the flask was evacuated carefully. Then the dry ice bath was removed and the solution was exposed to CO gas (1.5 bar) and stirred at room temperature for 30 min to give a white suspension. Filtration by using a filter cannula gave a white solid. The white solid was washed with pentane (1 mL × 2) and dried in vacuo to finally give compound 17b as a white solid (107.6 mg, 0.054 mmol, 54 %).   Crystals suitable for the X-ray crystal structure analysis of compound 17b were obtained from slow diffusion of pentane to a solution of the white solid in dichloromethane at -36 o C. X-ray crystal structure analysis of compound 17b: A prism-like specimen of C88H62B4F40O2P2, approximate dimensions 0.093 mm x 0.161 mm x 0.223 mm, was used for the X-ray crystallographic analysis. The X-ray intensity data were measured. A total of 1327 frames were collected. The total exposure time was 14.74 hours. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The integration of the data using a triclinic unit cell yielded a total of 80163 reflections to a maximum θ angle of 26.37° (0.80 Å resolution), of which 17300 were independent (average redundancy 4.634, completeness = 99.8%, Rint = 4.21%, Rsig = 3.78%) and 13572 (78.45%) were greater than 2σ(F 2 ). The final cell constants of a = 11.1630(5) Å, b = 16.2067(8) Å, c = 24.8378(11) Å, α = 76.4320(10)°, β = 87.505(2)°, γ = 76.034(2)°, volume = 4238.6(3) Å 3 , are based upon the refinement of the XYZ-centroids of 9782 reflections above 20 σ(I) with 4.756° < 2θ < 54.95°. Data were corrected for absorption effects using the multi-scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.958. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.9590 and 0.9830. The structure was solved and refined using the Bruker SHELXTL Software Package, using the space group P-1, with Z = 2 for the formula unit, C88H62B4F40O2P2. The final anisotropic full-matrix least-squares refinement on F 2 with 1237 variables converged at R1 = 3.90%, for the observed data and wR2 = 8.60% for all data. The goodness-of-fit was 1.024. The largest peak in the final difference electron density synthesis was 0.452 e -/Å 3 and the largest hole was -0.361 e -/Å 3 with an RMS deviation of 0.056 e -/Å 3 . On the basis of the final model, the calculated density was 1.580 g/cm 3 and F(000), 2032 e -. Preparation of 13 C labeled compound 17b Scheme S12.
In the glove box, a solution of compound 8b (14.4 mg, 0.05 mmol, 1.0 eq.) and compound 2 (36.3 mg, 0.105 mmol, 2.1 eq.) in pentane (1 mL) was stirred for 24 h at room temperature and then filtered. Subsequently the yellow solution was filled in a J-Young tube which was carefully evacuated at -78 o C (dry ice / isopropanol bath) and then the solution was exposed to 13 CO (1.5 bar) at room temperature for 10 min to give a white suspension. After decanting, the collected solid was washed with pentane (0.5 mL) and dried under vacuum to give compound f17b as a white solid. The white solid was characterized by NMR and IR spectroscopy.  In a Schlenk flask, compound 8c (16.5 mg, 0.05 mmol, 1.0 eq.) and compound 2 (34.6 mg, 0.10 mmol, 2.0 eq.) were mixed with dichloromethane-d2 (2 mL) and stirred at room temperature for 15 min to give a yellow solution. After the solution (1 mL) was transferred to a J-Young NMR tube, it was cooled to -78 °C (dry ice / isopropanol bath). Then the NMR tube was evacuated carefully and subsequntly the reaction mixture was exposed to CO gas (1.5 bar) to give a colorless solution at -78 o C. The in situ generated solution was characterized by NMR experiments.  In the glove box, compound 8a (8.3 mg, 0.025 mmol, 1.0 eq.) and compound 2 (17.3 mg, 0.05 mmol, 2.0 eq.) were mixed with dichloromethane-d2 (1 mL) at room temperature to give a yellow solution. The solution was filled in a J-Young tube, which was carefully evacuated at -78 o C (dry ice / isopropanol bath). Then the solution was exposed to 13 CO (1.5 bar) at -78 o C and characterized by NMR experiments.  In a Schlenk flask, compound 8c (66.1 mg, 0.20 mmol, 1.0 eq.) and compound 2 (138.4 mg, 0.40 mmol, 2.0 eq.) were mixed with dichloromethane (4 mL) at room temperature. Then all volatiles were removed in vacuo and the obtained residue was dissolved in pentane (4 mL). After the solution was cooled to -78 °C (dry ice / isopropanol bath), the flask was evacuated carefully. Then the dry ice bath was removed and the reaction mixture was exposed to CO gas (1.5 bar). The reaction mixture was stirred at room temperature for 30 min to give a white suspension. After filtration by using a filter cannula, the collected white solid was washed with pentane (1 mL × 2) and dried in vacuo to give compound 18c as a white solid (170.  Compound 18c (white solid (see page 35), 15.8 mg, 0.005 mmol) was dissolved in dichloromethane-d2 (1 mL) at room temperature and the obtained solution was characterized by NMR experiments, immediately (after ca. about 10 minutes).
Control experiment: In a Schlenk flask, a solution of compound 8c (16.5 mg, 0.05 mmol, 1.0 eq.) and compound 2 (34.6 mg, 0.10 mmol, 2.0 eq.) in dichloromethane-d2 (1 mL) was stirred for 15 min at room temperature to give a yellow solution of compound 11c.  Compound 17b (white solid, 15 mg) was mixed with benzene-d6 (1 mL) in a J-Young tube at room temperature (suspension). The suspension in J-Young tube was heated at 80 o C for 6h to give a colorless solution which were characterized by NMR experiments. Control experiment: (1) Compound 17b (white solid, 15 mg) in dichloromethane-d2 (the solubility of compound 17b in benzene-d6 is too low for the measurements). (2) In a Schlenk flask, a solution of compound 8b (14.4 mg, 0.05 mmol, 1.0 eq.) and compound 2 (35.5 mg, 0.1026 mmol, 2.05 eq.) in benzene-d6 (1 mL) were stirred for 12 h at room temperature. After filtration, a colorless solution of compound 11b was obtained and analyzed by NMR experiments.    To understand the importance of the 'second' borane, we computed Gibbs free energy for the CO insertion step with and without the ancillary borane, respectively (Fig S96). The results show that the addition of the second borane changes the thermodynamics of the CO insertion step. For example, a typical FLP could capture the CO, but CO can not be inserted into the C-B bond because the computed ΔG is 11.4 kcal mol -1 . After introducing the second borane, the CO inserting becomes possible with a computed ΔG of -4.3 kcal -1 . To evaluate whether the observed cyclodimers/trimers might be kinetic or thermodynamic products, we computed the Gibbs free energies for alternative dimer or trimer formation in the cases of Mes* and Tipp (see Fig S97). In the Mes*-case the trimer is energetically more stable than the dimer (-16.0 v.s -6.2 kcal mol -1 per one monomer molecule). In the case of Tipp, the dimer is energetically more stable than the trimer (-12.9 v.s -7.6 kcal mol -1 per one monomer molecule).   11.74 T) spectrometer using a rotational frequency of 12.5 kHz in 4 mm NMR double and triple resonance probes. For quantitative analysis the 11 B-MAS NMR spectra were measured using short pulses in the range of 0.5-0.7 µs to achieve uniform excitation and a relaxation delay between 10 and 20 s. During acquisition proton decoupling was performed (SWFTPPM15 57 kHz RF-Power).
For direct excitation of phosphorous in 31 P MAS NMR experiments a 5.2 µs pulse (48 kHz RF-Power) was applied followed by a relaxation delay of 60 s (18c) and 800 s (17b) respectively. During acquisition proton decoupling was performed (SWFTPPM15 60 kHz RF-Power). 11 B{ 31 P} J-resolved NMR spectra were acquired using a 11 B excitation pulse of 5 µs and reconversion pulses of 10 µs for boron and 10.4 µs for phosphorous. After evolution a z-filter was utilized. During evolution and acquisition 1 H decoupling was performed (SWFTPPM15 42 kHz RF-Power). A relaxation delay of 4 s was sufficient. Spectral deconvolution was done with DMFIT software (version 2011) 25 DFT calculation of NMR parameters were conducted using crystal structures obtained by x-ray diffraction. Chemical shifts calculation were performed with TURBOMOLE (version 6.5) 26-27 using B3LYP [28][29] as functional in combination with a def2-SVP basis set for 18c and a def2-TZVP basis set 20 for 17c (less computational effort). Quadrupolar coupling parameters were calculated on a GGA DFT level using GAUSSIAN (version GAUSSIAN09) 30 and the B97-D functional 31 . Here for both molecules the def2-TZVP basis set obtained from EMSL database 32-33 could be used with additional functions for boron from the cc-pCVTZ basis set [34][35] to enhance the accuracy near the nucleus.  11 B isotropic shift (±0.2 ppm), quadrupolar coupling constant CQ (±0.1 MHz) and electric field gradient asymmetry parameter ηQ (±0..05) obtained via lineshape deconvolution of 11 B{ 1 H} MAS NMR spectra of 17b depicted in figure S98 and 11 B spectra extracted from 11 B{ 31 P} CP-INEPT NMR experiments (figure S103) [a] Calculated shifts assigned to the species in such a way that the trend form high to low shift values is matched. Quadrupolar parameters of 11 B were assigned to the respective species.
[b] 31 P species listed in table 2 assigned to boron species using 11 B{ 31 P} CP-INEPT NMR data.  Figure S98. 11 B{ 1 H} MAS NMR spectrum measured of 17b at a magnetic field strength of 7.05 T and a spinning frequency of 12.5 kHz. 1 H decoupling was applied during acquisition using the SWFTPPM15 scheme. Figure S99. 31 P{ 1 H} CPMAS NMR spectrum of 17b measured at a magnetic field strength of 7.05 T and a spinning frequency of 12.5 kHz. 1 H decoupling was applied during acquisition using the SWFTPPM15 scheme. Figure S100. 31 P{ 1 H} CPMAS NMR spectrum of 17b measured at a magnetic field strength of 7.05 T and a spinning frequency of 12.5 kHz. 11 B and 1 H decoupling was applied during acquisition using the SWFTPPM15 scheme. Figure S101. 31 P{ 1 H} MAS NMR spectrum of 17b measured at a magnetic field strength of 7.05 T and a spinning frequency of 12.5 kHz. 11 B and 1 H decoupling was applied during acquisition using the SWFTPPM15 scheme. Figure S102. 11 B{ 31 P}J-resolved NMR spectrum of 17b measured at 7.05 T with a spinning frequency of 12.5 kHz. SWFTPPM15 1 H decoupling scheme was used during evolution and acquisition.   11 B isotropic chemical shift (±0.2 ppm), quadrupolar coupling constant CQ(± 0.1 MHz) and electric field gradient asymmetry parameter ηQ (± 0.05) obtained via lineshape deconvolution of 11 B{ 1 H} MAS NMR spectra of 18c-batch 1 depicted in figure S104, S105 and S112 and 11 B spectra extracted from 11 B{ 31 P} CP-INEPT NMR experiments.

S99
[a] Calculated shifts assigned to the species in such a way that the trend form high to low shift values is matched. Quadrupolar parameters of 11 B were assigned to the respective species.
[b] 31 P species listed in table 2 assigned to boron species using 11 B{ 31 P} CP-INEPT NMR data. ------------4 S101 Figure S104. 11 B{ 1 H} MAS NMR spectrum of 18c-batch 1 measured at a field strength of 7.05 T and a spinning speed of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. Figure S105. 11 B{ 1 H} MAS NMR spectrum of 18c-batch 1 measured at a field strength of 11.74 T and a spinning speed of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. S102 Figure S106. 31 P{ 1 H} CPMAS NMR spectrum of 18c-batch 1 measured at 7.05 T using a spinning frequency of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. The symbol + denotes a spectral component attributed to an impurity. Figure S107. 31 P{ 1 H} CPMAS NMR spectrum of 18c-batch 1 measured at 7.05 T using a spinning frequency of 12.5 kHz. SWFTPPM15 1 H and 11 B decoupling was applied during acquisition. The symbol + denotes a spectral component attributed to an impurity. S103 Figure S108. 31 P{ 1 H} MAS NMR spectrum of 18c-batch 1 measured at 7.05 T using a spinning frequency of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. The symbol + denotes a spectral component attributed to an impurity. Figure S109. 31 P{ 1 H} CPMAS NMR spectrum of 18c-batch 1 measured at 11.74 T using a spinning frequency of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. The symbol + denotes a spectral component attributed to an impurity. Figure S110. 11 B{ 31 P} REDOR NMR spectra of 18c-batch 1 measured at 7.05 T with a spinning frequency of 12.5 kHz. Depicted are 1D spectra after an evolution time of 2.9 ms. SWFTPPM15 1 H decoupling was applied during acquisition.    Figure S113. 31 P{ 1 H} CPMAS NMR spectrum of 18c-batch 2 measured at 7.05 T using a spinning frequency of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. The symbol + denotes a spectral component attributed to an impurity. Figure S114. 31 P{ 1 H} CPMAS NMR spectrum of 18c-batch 1 (a) and batch 2 (b) measured at 7.05 T using a spinning frequency of 12.5 kHz. SWFTPPM15 1 H decoupling was applied during acquisition. The symbol + denotes a spectral component attributed to an impurity, based on the fact that its contribution to the spectra varies with sample batch.