11B NMR

Boron because it’s a fascinating element that imparts remarkable properties to its compounds. And believe me, there are hundred of different compounds with very different properties containing boron centers. We find boron in compounds like boronic acids (A), boronic esters (B), and MIDA boronates (C), all of which are very popular in Suzuki coupling reactions to make new C–C bonds.1,2 Another popular family of compounds containing boron are the BODIPY dyes (D). These derivatives, composed of a dipyrromethene ligand supporting a BF2 unit, are used as stable functional dyes in several fields, but their application as biomolecular labels is the most investigated by far.3-5

FIGURE 1

Arguably the most important characteristic of a three coordinate boron center is the empty p-orbital, which allows effective conjugation of organic π systems with and through boron, and coordination of Lewis bases.6,7 Have you heard about Frustrated Lewis Pairs (FLPs)? In those types of compounds the Lewis acidity of the tri–coordinated boron center is exploited to active several molecules such as hydrogen and carbon dioxide. On the other hand, in conjugated π systems, such as boraanthracene (F), the boron imparts interesting photophysical properties because it allows to access new electronic levels compared to the all-carbon analogs.8

In terms of NMR properties, there are two stable isotopes of boron, 10B and 11B, and Table 1 summarizes their properties.9,10 Both nuclei are quadrupolar, thus the boron NMR signal is usually broad (>10Hz). 11B is commonly regarded as more suitable for NMR because of its higher sensitivity and better resolution at a given external magnetic field. Boron is well suited to low–field because the 11B chemical shifts range between +100 and –120 ppm.

Possible interference in 11B NMR region comes from NMR tubes made of borosilicate glass and most of the time the probe itself also contains borosilicate. As a result, there is a broad signal in the spectrum arising from the tube (background signal) between 30 and –30 ppm, commonly referred as a boron hump (Figure 2). It’s a common practice to use quartz NMR tubes that do not contain boron in order to avoid this hump, although these are much more expensive and fragile than regular tubes. In our instruments we do not see this background signal, therefore you don’t need to use quartz NMR tubes! Furthermore, our instrument’s probe is boron free!

FIGURE 2

Unlike other nuclei, the boron resonance (intensity and linewidth) highly depends on the environment, coordination and symmetry around the boron center. If you take a look at Figure 3 you will see the significant difference in linewidth and intensity between 2-thiopheneboronic acid MIDA ester (left) and sodium tetraphenylborate (right) acquired using the same parameters.

FIGURE 3

Nanalysis has the only benchtop NMR instrument capable of running 11B spectra, so feel free to contact us if you have any questions about our instruments or if you want to see whether our instrument will be suitable for your chemistry. In the meantime, if you want to see more representative 11B NMR spectra of boron containing compounds just click here......http://www.nanalysis.com/11b-spectra.

(1) Knapp, D. M.; Gillis, E. P.; Burke, M. D. J. Am. Chem. Soc. 2009, 131, 6961.
(2) Kohei, T.; Miyaura, N. In Cross-Coupling Reactions; Miyaura, N., Ed.; Springer Berlin / Heidelberg: 2002; Vol. 219, p 1.
(3) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891.
(4) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem. Int. Ed. 2008, 47, 1184.
(5) Wood, T. E.; Thompson, A. Chem. Rev. 2007, 107, 1831.
(6) Entwistle, C. D.; Marder, T. B. Chem. Mater. 2004, 16, 4574.
(7) Yamaguchi, S.; Wakamiya, A. Pure and Applied Chemistry 2006, 78, 1413.
(8) Wood, T. K.; Piers, W. E.; Keay, B. A.; Parvez, M. Angew. Chem. Int. Ed. 2009, 48, 4009.
(9) Eaton, G. R. J. Chem. Educ. 1969, 46, 547.
(10) Smith, W. L. J. Chem. Educ. 1997, 54, 469.

EXAMPLES
NUMBER 1

11B NMR (128 MHz, CDCl3) δ (ppm) = 22.5.
READ........http://organicsynthesisinternational.blogspot.in/2018/03/catalyst-free-and-solvent-free.html




Number 2
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2907063/

EXAMPLE 3

http://u-of-o-nmr-facility.blogspot.in/2017/08/boron-isotope-effects-in-fluorine-nmr.html

EXAMPLE 4
http://orgsyn.org/demo.aspx?prep=v90p0153

Potassium 1-naphthyltrifluoroborate has the following physical and spectroscopic properties: mp: > 300 ºC. IR (cm^-1): 2980, 2884, 2360, 1382, 940, 668, 651; ¹H NMR pdf (400 MHz, acetone-d⁶) δ: 7.30-7.36 (m, 3 H), 7.64 (d, J = 8.2 Hz, 1 H), 7.74-7.77 (m, 2 H), 8.57-8.59 (m, 1 H); ¹³C NMR pdf (100 MHz, acetone-d⁶) δ: 124.5, 124.8, 125.9, 126.7, 128.4, 130.0 (q, J_C-F = 3.2 Hz), 131.4, 134.5, 138.1, 147 (very broad); ¹¹B NMR (128 MHz, acetone-d⁶) δ: 4.06; ¹⁹F NMR pdf (377 MHz, acetone-d⁶) δ: -138.6; HRMS (ESI) m/z calcd. for C₁₀H₇BF₃ (M-K) 195.0593, found 195.0593. Anal calcd for C₁₀H₇BF₃K: C, 51.31; H, 3.01; Found: C, 51.50; H, 2.81.

1-naphthaleneboronic acid 1
Analytical data1 : mp: 194-196 °C. IR (cm^-1): 3261, 1575, 1508, 1348, 1318, 807, 778; ¹H NMR (500 MHz, DMSO-d⁶) d: 7.62-7.49 (m, 3 H), 7.78-7.76 (m, 1 H), 8.02-7.91 (m, 2 H), 8.35 (m, 3 H); ¹³C NMR (125 MHz, acetone-d⁶) d: 125.6, 125.9, 126.1, 128.7, 129.3, 129.7, 132.6, 133.4, 136.1; ¹¹B NMR (128 MHz, acetone-d⁶) d: 30.6.



EXAMPLE 5

http://orgsyn.org/demo.aspx?prep=v94p0234

 4,4,5,5-Tetramethyl-2-phenethyl-1,3,2-dioxaborolane (1) has the following physical and spectroscopic properties: R_f = 0.47 (3:97, ethyl acetate:pentane), the checkers report the following values for 1: R_f = 0.09 (3:97 ethyl acetate:pentane); R_f = 0.52 (10% EtOAc in hexanes); Merck silica gel 60 F254 plate; mp 38-39 °C; ¹H NMR pdf(CDCl₃, 400 MHz) δ: 1.18 (t, J = 8.4 Hz, 2H), 1.26 (s, 12H), 2.79 (t, J = 8.0 Hz, 2H), 7.16-7.22 (m, 1H), 7.23-7.32 (m, 4H); ¹³C NMR pdf(CDCl₃, 151 MHz) d: 25.0, 30.1, 83.2, 125.6, 128.1, 128.3, 144.6 [N.B. the carbon attached to boron was not observed due to quadrupolar relaxation]; HRMS (ESI⁺) calculated for C₁₄H₂₂BO₂⁺ = 233.1707, mass found = 233.1710; IR (film): 3026, 2978, 2929, 1372, 1318, 1139, 848, 755, 703 cm^-1; Anal. calcd for C₁₄H₂₁BO₂: C, 72.44; H, 9.12. Found: C, 72.18; H, 9.28.
MORE EXAMPLES.............

Boron NMR

Boron has two naturally occurring NMR active nuclei. Both nuclei have spins of greater than ½ and are quadrupolar. 11B has a spin of 3/2 and 10B is spin 3. 11B is the better nucleus in all respects, having the lower quadrupole moment and being more sensitive. Regular NMR tubes are made of borosilicate glass and therefore contain boron. As a result there is a broad signal in the spectrum arising from the tube. It is therefore preferable to use quartz tubes that do not contain boron although these are much more expensive and fragile than regular tubes. Both nuclei have the same wide chemical shift range. Each type of signal has a characteristic chemical shift range (fig. 1) which is the same for both nuclei.

Fig. 1. Chemical shift ranges for boron NMR

10Boron

10B-NMR (fig. 2) has a lower sensitivity and yields broader signals than 11B. It is therefore preferable to use 11B unless the sample is enriched in 10B as may be the case for samples intended for neutron capture applications.

Fig. 2. 10B-NMR spectrum of BF3.OEt2 in CDCl3

Coupling to proton is not usually observed except in the smallest and most symmetric molecules such as +BH4 (fig. 3).

Fig. 3. 10B-NMR spectrum of +BH4 in D2O

Properties of 10B

Property	Value
Spin	3
Natural abundance	19.9%
Chemical shift range	210 ppm, from -120 to 90
Frequency ratio (Ξ)	10.743658%
Reference compound	15% BF3.OEt2 in CDCl3
Linewidth of reference	9 Hz
T1 of reference	0.5 s
Receptivity rel. to 1H at natural abundance	3.96 × 10-3
Receptivity rel. to 1H when enriched	0.0199
Receptivity rel. to 13C at natural abundance	23.2
Receptivity rel. to 13C when enriched	117
Linewidth parameter	14 fm4

11Boron

11B-NMR (fig. 4) is more sensitive and yields sharper signals than 10B and is therefore usually the boron nuclide of choice. The coupling between boron and fluorine contain positive and negative components that cancel each other out so that a singlet is observed for BF3 (Fig. 4). A broad signal in the spectrum arising from the boron in the NMR tube is apparent in the spectrum below.

Fig. 4. 11B-NMR spectrum of BF3.OEt2 in CDCl3

Coupling to proton is not usually observed except in the smallest and most symmetric molecules such as +BH4 (fig. 5).

Fig. 5. 11B-NMR spectrum of +BH4 in D2O

The proton spectrum of +BH4 displays couplings to 11B (quartet) and 10B (septet) (fig. 6).

Fig. 6. 1H-NMR spectrum of +BH4 in D2O showing coupling to 10B and 11B

• Summary of B-11 Chemical Shifts

  

  General Literature References

  1.          R. Schaeffer, "NMR of Boron Hydrides and Related Compounds," Prog. Boron Chem., 1, 417 (1964).
  2.          G. R. Eaton, "NMR of Boron Compounds," J. Chem. Ed., 46, 547 (1969).
  3.          G. R. Eaton and W. N. Lipscomb, "NMR Studies of Boron Hydrides and Related Compounds," Benjamin, NY, 1969.
  4.          W. G. Henderson and E. F. Mooney, "Boron-11 NMR Spectroscopy," Ann. Rev. NMR Spectrosc., 2, 219 (1969).
  5.          H. Beall and C. H. Bushweller, "Dynamic Processes in Boranes, Borane Complexes, Carboranes, and Related Compounds," Chem. Rev., 73, 465 (1973).
  6.          W. L. Smith, "Boron-11 NMR," J. Chem. Ed., 54, 469 (1977).
  7.          H. Nöth and B. Wrackmeyer, "NMR Spectroscopy of Boron Compounds," NMR Basic Prin. Prog., 14, 1 (1978).
  8.          L. J. Todd and A. R. Siedle, "NMR Studies of Boranes, Carboranes and Heteroatom Boranes," Prog. Nucl. Magn. Reson. Spectrosc., 13, 87 (1979).
  9.          B. Wrackmeyer, "Carbon-13 NMR Spectroscopy of Boron Compounds," Prog. Nucl. Magn. Reson. Spectrosc.,12, 227 (1979).
  10.        A. R. Siedle, "Boron-11 NMR Spectroscopy,"Annu. Rep. NMR Spectrosc., 12, 177 (1982).
  11.        R. G. Kidd, "Boron-11," in "NMR of Newly Accessible Nuclei," Vol. 2, P. Laszlo, Ed., Academic Press, NY, 1983, Ch. 3.
  12.        J. D. Kennedy, "Boron," in "Multinuclear NMR," J. Mason, Ed., Plenum Press, NY, 1987, Ch. 8.
  13.        A. R. Siedle, "Boron-11 NMR Spectroscopy," Annu. Rep. NMR Spectrosc., 20, 205 (1988).
  14.        B. Wrackmeyer, "NMR Spectroscopy of Boron Compounds Containing Two-, Three- and Four-Coordinate Boron," Annu. Rep. NMR Spectrosc., 20, 61 (1988).
  15.        S. Hermanek, "11B NMR Spectra of Boranes, Main-Group Heteroboranes, and Substituted Derivatives. Factors Influencing Chemical Shifts of Skeletal Atoms," Chem. Rev., 92, 325 (1992).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2907063/

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