A Solid-State ¹¹B NMR and Computational Study of Boron Electric Field Gradient and Chemical Shift Tensors in Boronic Acids and Boronic Esters

Abstract

The results of a solid-state 11B NMR study of a series of 10 boronic acids and boronic esters with aromatic substituents are reported. Boron-11 electric field gradient (EFG) and chemical shift (CS) tensors obtained from analyses of spectra acquired in magnetic fields of 9.4 and 21.1 T are demonstrated to be useful for gaining insight into the molecular and electronic structure about the boron nucleus. Data collected at 21.1 T clearly show the effects of chemical shift anisotropy (CSA), with tensor spans (Ω) on the order of 10−40 ppm. Signal enhancements of up to 2.95 were achieved with a DFS-modified QCPMG pulse sequence. To understand the relationship between the measured tensors and the local structure better, calculations of the 11B EFG and magnetic shielding tensors for these compounds were conducted. The best agreement was found between experimental results and those obtained from GGA revPBE DFT calculations. A positive correlation was found between Ω and the dihedral angle (ϕCCBO), which describes the orientation of the boronic acid/ester functional group relative to an aromatic system bound to boron. The small boron CSA is discussed in terms of paramagnetic shielding contributions as well as diamagnetic shielding contributions. Although there is a region of overlap, both Ω and the 11B quadrupolar coupling constants tend to be larger for boronic acids than for the esters. We conclude that the span is generally the most characteristic boron NMR parameter of the molecular and electronic environment for boronic acids and esters, and show that the values result from a delicate interplay of several competing factors, including hydrogen bonding, the value of ϕCCBO, and the electron-donating or withdrawing substituents bound to the aromatic ring.

Facile Monolayer Formation on SiO2 Surfaces via Organoboron Functionalities

“…Overall, the 11 B NMR spectra demonstrated primarily trigonal boron species, with some amount of tetrahedral boron species formed at the higher temperatures (the proportion of tetrahedral species may be underestimated herein owing to the selective excitation inherent to the spin-echo pulse sequence employed). For SiO 2 NPs treated with B1 and B2, the 11 B chemical-shift values ruled out the presence of halogenated (BÀCl) species, the chemical shifts of which are expected at 50-80 ppm (Figure 2), and suggested the existence of B À OH species, typically observed at 25-35 ppm, [10] in line with the FTIR spectroscopic results and XPS data (see below). Furthermore, R 2 B(OH) species were expected to show higher chemical shifts than those observed in this study, which suggests for asprepared samples the form SiÀOÀB(ÀR,ÀOH), in which R = phenyl and C 6 H 11 for B1 and B2, respectively.…”

Correlations between 11B NMR parameters and structural characters in borate and borosilicate minerals investigated by high-resolution MAS NMR and ab initio calculations

“…Many hydrous borate minerals, on the other hand, fall significantly away from the trend defined by anhydrous borates (Fig. 7), pointing to possible effects of hydrogen bonding (see also Weiss and Bryce 2010). Unfortunately, a quantitative evaluation of the effects of hydrogen bonding on the 11 B chemical shifts is presently not possible, owing to the fact that hydrogen positions are generally poorly determined in X-ray structural studies.…”

Solid‐state 11B and 13C NMR, IR, and X‐ray crystallographic characterization of selected arylboronic acids and their catechol cyclic esters

“…As highlighted previously, boronic acids and boronate esters are particularly important classes of compounds that have a wide range of uses in organic synthesis and catalysis, as well as in biochemistry, pharmaceutical chemistry, industry, crystal engineering, and various other applications . Boronic acids are used extensively in organic chemistry as chemical building blocks and intermediates, and the most widely used example is the Suzuki–Miyaura coupling, which is a useful synthetic route to biaryl compounds.…”

“…Ten boronic acids and ester derivatives were investigated in our previous study . In the present work, we report the experimental characterization and interpretation of 11 B chemical shift (CS) and electric field gradient (EFG) tensors in a novel series of solid arylboronic acids and their catechol cyclic esters (Scheme ; Table ), with an added focus on understanding the origins of spurious peaks in some of the spectra as being caused by impurities or the conditions used for spectral acquisition.…”