Boron NMR Spectroscopy

For my first blog I have decided to talk a little bit about 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.

(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.