Characterization of Tricoordinate Boron Chemical Shift Tensors: Definitive High-Field Solid-State NMR Evidence for Anisotropic Boron Shielding

Polar covalent apex-base bonding in borapyramidanes probed by solid-state NMR and DFT calculations

Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF3 -Et2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C-11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C6 F5 )2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp2 -carbon-sp2 -boron σ-bond with 1 J(13 C-11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations.

Systematic Electronic and Structural Studies of 9-Carbene-9-Borafluorene Monoanions and Transformations into Luminescent Boron Spirocycles

The impact of the exact spatial arrangement of the alkali metal on the electronic properties of 9-carbene-9-borafluorene monoanions is assessed, and a series of [K][9-CAAC-9-borafluorene] complexes (1-4) have been isolated (CAAC = cyclic(alkyl)(amino) carbene, (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene). Compound 1, which contains [B]-K(THF)₃ interactions, is compared to charge-separated 2-4, which were prepared by capturing the potassium cations with 18-crown-6, 2.2.2-cryptand, or 1,10-phenanthroline. Notably, the ¹¹B NMR spectra of charge-separated borafluorene monoanions 2-4 show distinct low-field signatures compared to 1. Theoretical calculations indicate that charge separation may be exploited to influence the nucleophilic and electron transfer properties of 9-carbene-9-borafluorene monoanions. When [K(2.2.2-cryptand)][9-CAAC-9-borafluorene] (3) is reacted with 9,10-phenanthrenequinone and 1,10-phenanthroline-5,6-dione, the carbene ligand is displaced, and new air-stable R₂BO₂ spirocycles are formed (5 and 6, respectively). Remarkably, compounds 5 and 6 display fluorescence under UV light in both the solid and solution phases with quantum yields of up to 20%. In addition, a drastic red-shift in the emission color is observed in 6 because of the presence of the nitrogen atoms on the phenanthroline moiety. Mechanistic insights into the formation of these spirocycles are also described based on density functional theory calculations.

Solid-state 11B NMR studies of coinage metal complexes containing a phosphine substituted diboraanthracene ligand

Transition metal interactions with Lewis acids (M → Z linkages) are fundamentally interesting and practically important. The most common Z-type ligands contain boron, which contains an NMR active ¹¹B nucleus. We measured solid-state ¹¹B{¹H} NMR spectra of copper, silver, and gold complexes containing a phosphine substituted 9,10-diboraanthracene ligand (B₂P₂) that contain planar boron centers and weak M → BR₃ linkages ([(B₂P₂)M][BAr^F₄] (M = Cu (1), Ag (2), Au (3)) characterized by large quadrupolar coupling (C_Q) values (4.4-4.7 MHz) and large span (Ω) values (93-139 ppm). However, the solid-state ¹¹B{¹H} NMR spectrum of K[Au(B₂P₂)]^- (4), which contains tetrahedral borons, is narrow and characterized by small C_Q and Ω values. DFT analysis of 1-4 shows that C_Q and Ω are expected to be large for planar boron environments and small for tetrahedral boron, and that the presence of a M → BR₃ linkage relates to the reduction in C_Q and ¹¹B NMR shielding properties. Thus solid-state ¹¹B NMR spectroscopy contains valuable information about M → BR₃ linkages in complexes containing the B₂P₂ ligand.

4,7-Bis[3-(dimesitylboryl)thien-2-yl]benzothiadiazole: Solvato-, Thermo-, and Mechanochromism Based on the Reversible Formation of an Intramolecular B-N Bond

4,7-Bis-[3-(dimesitylboryl)thien-2-yl]benzothiadiazole (1) and monoborylated derivative 2 were synthesized and their chromic behavior was investigated. Photophysical measurements, single-crystal XRD analysis, and theoretical calculations revealed that an intramolecular B-N coordination bond formed reversibly. The equilibrium of this reversible bond formation depends on the solid-state structure, solvent, temperature, and mechanical forces, and leads to significant changes in the electronic structure and chromic behavior of these molecules. The responsiveness toward external stimuli, resulting in the reversible formation of open and closed forms of this system, is achieved through weak intramolecular B-N coordination bonds induced by the steric bulk of the mesityl groups on the boron centers.

Geminal bis-borane formation by borane Lewis acid induced cyclopropyl rearrangement and its frustrated Lewis pair reaction with carbon dioxide

The borylated tetrahydroborole obtained by the reaction of cyclopropylacetylene with Piers' borane adds carbon dioxide under frustrated Lewis pair conditions.

Cyclopropylacetylene reacts with two molar equivalents of Piers' borane [HB(C₆F₅)₂] under mild conditions by an addition/rearrangement sequence with cyclopropyl ring opening to give a mixture of two α-B(C₆F₅)₂ substituted tetrahydroboroles. This compound forms an active frustrated Lewis pair with P^tBu₃ that heterolytically splits dihydrogen and adds carbon dioxide as a geminal chelate bis-boryl component. The respective reactions of the two-fold HB(C₆F₅)₂ addition to Ph-CH₂CH₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH were studied as a geminal Lewis acid reference. Most of the products were characterized by X-ray diffraction.

Spin-Spin Coupling between Quadrupolar Nuclei in Solids: (11)B-(75)As Spin Pairs in Lewis Acid-Base Adducts

Solid-state (11)B NMR measurements of Lewis acid-base adducts of the form R3AsBR'3 (R = Me, Et, Ph; R' = H, Ph, C6F5) were carried out at several magnetic field strengths (e.g., B0 = 21.14, 11.75, and 7.05 T). The (11)B NMR spectra of these adducts exhibit residual dipolar coupling under MAS conditions, allowing for the determination of effective dipolar coupling constants, Reff((75)As,(11)B), as well as the sign of the (75)As nuclear quadrupolar coupling constants. Values of Reff((75)As,(11)B) range from 500 to 700 Hz. Small isotropic J-couplings are resolved in some cases, and the sign of (1)J((75)As,(11)B) is determined. Values of CQ((75)As) measured at B0 = 21.14 T for these triarylborane Lewis acid-base adducts range from -82 ± 2 MHz for Et3AsB(C6F5)3 to -146 ± 1 MHz for Ph3AsBPh3. For Ph3AsBH3, two crystallographically nonequivalent sites are identified with CQ((75)As) values of -153 and -151 ± 1 MHz. For the uncoordinated Lewis base, Ph3As, four (75)As sites with CQ((75)As) values ranging from 193.5 to 194.4 ± 2 MHz are identified. At these applied magnetic field strengths, the (75)As quadrupolar interaction does not satisfy high-field approximation criteria, and thus, an exact treatment was used to describe this interaction in (11)B and (75)As NMR spectral simulations. NMR parameters calculated using the ADF and CASTEP program packages support the experimentally derived parameters in both magnitude and sign. These experiments add to the limited body of literature on solid-state (75)As NMR spectroscopy and serve as examples of spin-spin-coupled quadrupolar spin pairs, which are also rarely treated in the literature.

Background-free solution boron NMR spectroscopy

The appearance of background signals arising from the NMR probe and tube is a well-known problem of boron NMR spectroscopy. Background suppression may be achieved by using DEPTH, which increases the signal-to-background (S/B) ratio. Although, the quality of such spectra is often adequate, but in the case of rapid relaxation broadened resonances (T(1)  < 1 ms), the residual background signals may still hamper the interpretation of the spectra. It was observed that the background signals are practically invisible in solution (10) B NMR. The unusual isotopic effect on the (S/B) ratio was interpreted as an inherent consequence of the integer versus half-integer spin of (10) B and (11) B, respectively. The practicability of (10/11) B NMR was compared for a selected set of boron compounds covering the typical range of (S/B) ratio. The application of (11) B is more favourable than (10) B as long as it is possible to achieve the desired spectral quality by using DEPTH. Otherwise, the 'background-free' appearance of (10) B NMR spectra makes (10) B a reasonable alternative of (11) B DEPTH. This was found typical for compounds having relaxation broadened resonances. The variable temperature (VT) NMR study of an adduct formation process was also presented here as an example of the advantage of (10) B over (11) B.

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

Nine arylboronic acids, seven arylboronic catechol cyclic esters, and two trimeric arylboronic anhydrides (boroxines) are investigated using (11)B solid-state NMR spectroscopy at three different magnetic field strengths (9.4, 11.7, and 21.1 T). Through the analysis of spectra of static and magic-angle spinning samples, the (11)B electric field gradient and chemical shift tensors are determined. The effects of relaxation anisotropy and nutation field strength on the (11)B NMR line shapes are investigated. Infrared spectroscopy was also used to help identify peaks in the NMR spectra as being due to the anhydride form in some of the arylboronic acid samples. Seven new X-ray crystallographic structures are reported. Calculations of the (11)B NMR parameters are performed using cluster model and periodic gauge-including projector-augmented wave (GIPAW) density functional theory (DFT) approaches, and the results are compared with the experimental values. Carbon-13 solid-state NMR experiments and spectral simulations are applied to determine the chemical shifts of the ipso carbons of the samples. One bond indirect (13)C-(11)B spin-spin (J) coupling constants are also measured experimentally and compared with calculated values. The (11)B/(10)B isotope effect on the (13)C chemical shift of the ipso carbons of arylboronic acids and their catechol esters, as well as residual dipolar coupling, is discussed. Overall, this combined X-ray, NMR, IR, and computational study provides valuable new insights into the relationship between NMR parameters and the structure of boronic acids and esters.

9-Borafluorenes – NMR spectroscopy and DFT calculations. Molecular structure of 1,2-(2,2′-diphenylylene)-1,2-diethyldiborane

9-Borafluorene derivatives 1 (9-R = Et (a), Ph (b), Cl (c), NEt2 (d)), the pyridine adduct 1py+ and 1,2-(2,2′-biphenylylene)-1,2-diethyldiborane(6) (3), were studied by 11B and 13C NMR spectroscopy to obtain a fairly complete data set for the first time. The molecular structure of the doubly hydrogen-bridged 1,2-diphenylenediborane 3 was determined by X-ray diffraction. The gas-phase structures of the compounds 1, related derivatives, and of some doubly hydrogen-bridged 1,2-diphenylenediboranes were optimized by quantum chemical calculations (B3LYP/6-311+G(d,p) level of theory) and NMR parameters, such as chemical shifts, 11B chemical shift tensors and indirect nuclear 13C–11B spin–spin coupling constants were calculated at the same level of theory and compared with experimental data.

A solid-state (11)b NMR and computational study of boron electric field gradient and chemical shift tensors in boronic acids and boronic esters

The results of a solid-state (11)B 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 (Omega) 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 (11)B 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 Omega and the dihedral angle (phi(CCBO)), 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 Omega and the (11)B 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 phi(CCBO), and the electron-donating or withdrawing substituents bound to the aromatic ring.

Theoretical investigation on multinuclear NMR chemical shifts of some tris(trifluoromethyl)boron complexes

Tris(trifluoromethyl)boron complexes have unusual properties and may find applications in many fields of chemistry, biology, and physics. To gain insight into their NMR properties, the isotropic 11B, 13C, and 19F NMR chemical shifts of a series of tris(trifluoromethyl)boron complexes were systematically studied using the gauge-included atomic orbitals (GIAO) method at the levels of B3LYP/6-31 + G(d,p)//B3LYP/6-31G* and B3LYP/6-311 + G(d,p)//B3LYP/6-311 + G(d,p). Solvent effects were taken into account by polarizable continuum models (PCM). The calculated results were compared with the experimental values. The reason that the structurally inequivalent fluorine atoms in a specific species give a same chemical shift in experimental measurements is attributed to the fast rotation of CF3 group around the B-C(F3) bond because of the low energy barrier. The calculated 11B, 13C(F3), and 19F chemical shifts are in good agreement with the experimental measurements, while the deviations of calculated 13C(X, X = O, N) chemical shifts are slightly large. For the latter, the average absolute deviations of the results from B3LYP/6-311 + G(d,p)//B3LYP/6-311 + G(d,p) are smaller than those from B3LYP/6-31 + G(d,p)//B3LYP/6-31G*, and the inclusion of PCM reduces the deviation values. The calculated 19F and 11B chemical shieldings of (CF3)3BCO are greatly dependent on the optimized structures, while the influence of structural parameters on the calculated 13C chemical shieldings is minor.

The NMR response of boroxol rings: a density functional theory study

Cluster models of boron oxide glasses are studied computationally using density functional theory. It is shown that the isotropic chemical shielding of boron in boroxol rings is about 5 ppm less than for boron in non-ring BO3/2 units, and that the quadrupole coupling in ring sites is about 0.1 MHz larger than in non-ring sites, confirming assignments made in glasses and crystalline model compounds. The chemical shielding anisotropy of these sites is computed and shown to be in agreement with recent experimental measurements. Furthermore, it is shown that the reason for the different responses is not the co-planarity of BO3/2 groups bound in rings, but rather the contraction in the B-O-B bond angle from about 134 degrees in relaxed structures to 120 degrees as found in rings.