Spying on the boron-boron triple bond using spin-spin coupling measured from 11B solid-state NMR spectroscopy

The Structure of Boron Monoxide

Boron monoxide (BO), prepared by the thermal condensation of tetrahydroxydiboron, was first reported in 1955; however, its structure could not be determined. With the recent attention on boron-based two-dimensional materials, such as borophene and hexagonal boron nitride, there is renewed interest in BO. A large number of stable BO structures have been computationally identified, but none are supported by experiments. The consensus is that the material likely forms a boroxine-based two-dimensional material. Herein, we apply advanced 11B NMR experiments to determine the relative orientations of B(B)O2 centers in BO. We find that the material is composed of D2h-symmetric O2B-BO2 units that organize to form larger B4O2 rings. Further, powder diffraction experiments additionally reveal that these units organize to form two-dimensional layers with a random stacking pattern. This observation is in agreement with earlier density functional theory (DFT) studies that showed B4O2-based structures to be the most stable.

Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity

The use of 1,3,4,5-tetramethylimidazol-2-ylidene (IMe) to coordinate with diatomic B₂ species afforded a tetrakis(N-heterocyclic carbene)-diboron(0) [(IMe)₂B-B(IMe)₂] (2). The singly bonded B₂ moiety therein possesses a valence electronic configuration 1σ_g²1π_u²1π_g*² with four vacant molecular orbitals (1σ_u*, 2σ_g, 1π_u', 1π_g'*) coordinated with IMe. Its unprecedented electronic structure is analogous to the energetically unfavorable planar hydrazine with a D_2h symmetry. The two highly reactive π_g* antibonding electrons enable double single-electron-transfer (SET) reactivity in small-molecule activation. Compound 2 underwent a double SET reduction with CO₂ to form two carbon dioxide radical anions CO₂^•-, which then reduced pyridine to yield a carboxylated pyridine reductive coupling dianion [O₂CNC₅(H)₅-C₅(H)₅NCO₂]^2- and converted compound 2 to the tetrakis(N-heterocyclic carbene)-diborene dication [(IMe)₂B═B(IMe)₂]²⁺ (3²⁺). This is a remarkable transition-metal-free SET reduction of CO₂ without ultraviolet/visible (UV/vis) light conditions.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B₂, C, C₂, Si, Si₂, Ge, Ge₂, Sn, P₂, As₂, Sb₂) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

In Search of the Perfect Triple BB Bond: Mechanical Tuning of the Host Molecular Trap for the Triple Bond B≡B Fragment

The coordination of the B₂ fragment by two σ-donor ligands L: could lead to a diboryne compound with a formal triple bond L:→B≡B←:L. σ-Type coordination L:→B leads to an excess of electrons around the B₂ central fragment, whereas π-back-donation from the B≡B moiety to ligand L has a compensation effect. Coordination of the σ-donor and π-acceptor ligand is accompanied by the lowering of the BB bond order. Here, we propose a new approach to obtain the perfect triple BB bond through the incorporation of the BB unit into a rigid molecular capsule. The idea is the replacement of π-back-donation, as the principal stabilization factor in the linear NBBN structure, with the mechanical stabilization of the BB fragment in the inert molecular capsule, thus preserving the perfect B≡B triple bond. Quantum-chemical calculations show that the rigid molecular capsule provided a linear NBBN structure and an unusually short BB bond of 1.36 Å. Quantum-chemical calculations of the proposed diboryne adducts show a perfect triple bond B≡B without π-back-donation from the B₂ unit to the host molecule. Two mechanisms were tested for the molecular design of a diboryne adduct with a perfect B≡B triple bond: the elimination of π-back-donation and the construction of a suitable molecular trap for the encapsulation of the B₂ unit. The second factor that could lead to the strengthening or stretching of a selected chemical bond is molecular strain produced by the rigid molecular host capsule, as was shown for B≡B and for C≡C triple bonds. Different derivatives of icosane host molecules exhibited variation in BB bond length and the corresponding frequency of the BB stretch. On the other hand, this group of molecules shows a perfect triple BB bond character and they all possess a similar level of HOMO.

Revisiting the π-Back-Donation in the NHC-B≡B-NHC Molecule

As the first thermal stable molecule with a B≡B bond, the diboryne complex protected by N-heterocyclic carbene ligands (NHC-B≡B-NHC) has attracted much interest. Researchers point out that π-back-donation highly stabilizes the B≡B bond besides σ-donation, both of which are induced by NHC ligands. In this work, details of the π-back-donation are revisited by using DFT calculations. There are two delocalized π* orbitals in NHC, and the symmetry of one π* orbital is highly adaptive to the π orbitals in B≡B bond, whereas the other cannot be involved in the π-back-donation. In staggered configuration, two orthogonal π orbitals of B≡B interact with this π* orbital in each NHC ligand, respectively, to form π-back-donations in both sides. This interaction has proven to be more intensive than π-conjunction, resulting in the lower energy of the staggered isomer compared with the eclipsed one containing greater π-conjunction. Moreover, intensity of the π-back-donation can be enhanced by reducing the energy levels of the matched π* orbitals in ligands, which gives references for the design of stable diborynes.

EE triple bonds (E = Group 13) promoted by charge transfer from alkali metals

Chemical bonding analysis shows that strong charge transfer arises from M4 (M = Li and Na) motifs to E2 (E = Group 13), further making an EE triple bond composed of two π bonds and one delocalized σ bond.

π-Complexes of Diborynes with Main Group Atoms

We present herein an in-depth study of complexes in which a molecule containing a boron-boron triple bond is bound to tellurate cations. The analysis allows the description of these salts as true π complexes between the B-B triple bond and the tellurium center. These complexes thus extend the well-known Dewar-Chatt-Duncanson model of bonding to compounds made up solely of p block elements. Structural, spectroscopic and computational evidence is offered to argue that a set of recently reported heterocycles consisting of phenyltellurium cations complexed to diborynes bear all the hallmarks of π-complexes in the π-complex/metallacycle continuum envisioned by Joseph Chatt. Described as such, these compounds are unique in representing the extreme of a metal-free continuum with conventional unsaturated three-membered rings (cyclopropenes, azirenes, borirenes) occupying the opposite end.

A Karplus Equation for the Conformational Analysis of Organic Molecular Crystals

Vicinal scalar couplings (³ J) are extensively used for the conformational analysis of organic compounds in the liquid state through empirical Karplus equations. In contrast, there are no examples of such use for the structural investigation of solids. With the support of first principles calculations, we demonstrate here that ¹³ C-¹³ C ³ J coupling constants (³ J_CC ) measured on a series of isotopically enriched solid amino acids and sugars can be related to dihedral angles by a simple Karplus-like relationship, and we provide a parameterized Karplus function for the conformational analysis of organic molecular crystals. Under the experimental conditions discussed, torsional angles can be estimated from the experimental ³ J_CC values with an accuracy of 10° using this function. These results open new perspectives towards the use of ³ J_CC as a new analytical tool that could considerably simplify structure determination of functional organic solids.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Synthesis of a hydrogen-bridged tetraborane(6): a substituent effect of a diaminoboryl group toward the B–B multiple bond character

A hydrogen-bridging tetraborane(6) was synthesized from boryllithium, a boron nucleophile, in three steps.

Striking transformations of the hydroborylene ligand in a HB:→NiII complex with isocyanides and CO

The first reactivity of the hydroborylene ligand is described, giving access to a remarkable range of unprecedented boron-centered species.

For the first time, the reactivity of the metal- and N-heterocyclic carbene-supported monovalent hydroborylene is reported. Isocyanides react with the hydroborylene Ni^II complex [{cat(^TMSL)Si}(Cl)Ni←:BH(NHC)₂] 1 (cat = ortho-C₆H₄O₂; ^TMSL = N(SiMe₃)(Dipp); Dipp = 2,6-Prⁱ₂C₆H₃; NHC = :C[(Prⁱ)NC(Me)]₂) to form the hydride-bridged hydroborylene-Ni^II complexes 2. The reaction of 1 with isoelectronic CO, however, is reversible and furnishes the related unprecedented hydride- and CO-bridged hydroborylene Ni^II complex 2-CO, which undergoes isomerisation through silyl/NHC exchange at ambient temperature to afford the corresponding hydro(silyl)boryl Ni^II complex 3. Markedly, 2 readily and quantitatively react with one further molar equiv. of isocyanide to give, under borylene liberation and H/Cl ligand exchange, boraketiminium species, which represent cationic B^I complexes. These latter compounds are highly reactive in solution, and can undergo quantitative transformation into previously unknown cyanoborenium cations.

Boron–boron, carbon–carbon and nitrogen–nitrogen bonding in N-heterocyclic carbenes and their diazaboryl and triazole analogues: Wanzlick equilibrium revisited

Diazaboroles and triazoles are predicted to be unstable as dimers, in contrast to carbenes with small alkyl substituents or flexible side-groups.

Multinuclear solid-state magnetic resonance study of oxo-bridged diniobium and quadruply-bonded dimolybdenum carboxylate clusters

Carboxylate paddlewheels and their oxo-bridged analogues constitute ideal building blocks for the assembly of two- and three-dimensional framework materials. Here, we present a multinuclear (¹H, ¹³C, ⁹³Nb, ⁹⁵Mo) magnetic resonance study of solid samples of Nb₂OCl₆(O₂Ph)₂ (1), Mo₂(O₂CMe)₄ (2), and Mo₂(O₂CCHF₂)₄ (3). High-resolution proton and ¹³C CP/MAS NMR spectra provide valuable information on structure and crystal symmetry and on cocrystallized solvent. ⁹³Nb solid-state NMR spectra of 1 provide quadrupolar coupling constants and chemical shift tensors which are characteristic of the axially asymmetric Nb-O-Nb bridging environment. ⁹⁵Mo solid-state NMR spectra of 2 and 3 provide quadrupolar coupling constants and chemical shift tensors which are directly characteristic of the molybdenum-molybdenum quadruple bonds in these compounds. The quadruple bonds are characterized by particularly large ⁹⁵Mo chemical shift tensor spans on the order of 5500ppm. Density functional theoretical computations provide good agreement with the ⁹³Nb and ⁹⁵Mo experimental data, with some exceptions noted. This work demonstrates possible NMR approaches to characterize more complex framework materials and provides key insight into the Mo-Mo quadruple bond.

Dynamic Disorder and Electronic Structures of Electron-Precise Dianionic Diboranes: Insights from Solid-State Multinuclear Magnetic Resonance Spectroscopy

The J(¹¹B,¹¹B) coupling constants of various salts of the electron-precise hexacyanodiborane(6) dianion, [B₂(CN)₆]^2-, were obtained using ¹¹B double-quantum-filtered (DQF) J-resolved solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Our results show that the magnitude of the DQF J splitting is influenced by both the crystallographic symmetry of the system and the presence of dynamics. The splittings are amplified by a factor of 3 as compared to the corresponding theoretical J coupling constants for cases where (1) there is an absence of dynamics but the boron pairs are crystallographically equivalent or (2) the boron pairs are crystallographically inequivalent but are rendered magnetically equivalent on the time scale of the experiment due to dynamic disorder, which was identified by ¹¹B and ¹³C SSNMR experiments. Consequently, molecular motions need to be taken into consideration when interpreting the results of DQF J-resolved experiments, and conversely, these experiments may be used to identify dynamic disorder. Variable-temperature NMR data support the notion of three different motional processes with correlation times ranging from 10² to 10⁶ s^-1 over the temperature range of 248-306 K. When molecular motion and crystallographic symmetry are both accounted for, the J(¹¹B,¹¹B) coupling constants for various [B₂(CN)₆]^2- salts were measured to range from 29.4 to 35.8 Hz, and their electronic origins were determined using natural localized molecular orbital and natural bond orbital analyses. The coupling constants were found to strongly correlate to the hybridization states of the boron orbitals that form the B-B bonds and to the strength of the B-B bonds. This study provides a novel tool to study dynamics in ordered and disordered solids and provides new perspectives on electron-precise dianionic diboranes featuring two-center-two-electron bonds in the context of related compounds featuring multiply and singly bonded boron spin pairs.

NMR spin-spin coupling constants: bond angle dependence of the sign and magnitude of the vicinal (3)JHF coupling

The dependence of the magnitude and sign of (3)JHFF on the bond angle in fluoro-cycloalkene compounds is evaluated by electronic structure calculations using different levels of theory, viz. DFT, SOPPA(CCSD) and SOPPA(CC2). Localized molecular orbital contributions to (3)JHFF are analyzed to assess which orbitals are responsible for (3)JHFF and which are the most important coupling transmission mechanisms for each compound. Fluoro-ethylene is used as a model system to evaluate the dependence of the (3)JHFF coupling constant on the angle between the σCα-F and σCα'-HF vectors. Through-space and hyperconjugative transmission pathways and ring strain are identified as responsible for the opposite trend between (3)JHFF and bond angle, and for the negative signs obtained for the two molecules, respectively. One of the fluorine lone pairs, σCα'-HF, σCα-F, σCα'-Cβ' bonding orbitals and the σ*Cα-F antibonding orbital are involved in the J-coupling pathways, according to analyses of pairwise-steric and hyperconjugative energies.

Quantitative structure parameters from the NMR spectroscopy of quadrupolar nuclei

Nuclear magnetic resonance (NMR) spectroscopy is one of the most important characterization tools in chemistry, however, 3/4 of the NMR active nuclei are underutilized due to their quadrupolar nature. This short review centers on the development of methods that use solid-state NMR of quadrupolar nuclei for obtaining quantitative structural information. Namely, techniques using dipolar recoupling as well as the resolution afforded by double-rotation are presented for the measurement of spin–spin coupling between quadrupoles, enabling the measurement of internuclear distances and connectivities. Two-dimensional J-resolved-type experiments are then presented for the measurement of dipolar and J coupling, between spin-1/2 and quadrupolar nuclei as well as in pairs of quadrupolar nuclei. Select examples utilizing these techniques for the extraction of structural information are given. Techniques are then described that enable the fine refinement of crystalline structures using solely the electric field gradient tensor, measured using NMR, as a constraint. These approaches enable the solution of crystal structures, from polycrystalline compounds, that are of comparable quality to those solved using single-crystal diffraction.

Donor–acceptor bonding in novel low-coordinated compounds of boron and group-14 atoms C–Sn

Donor–acceptor complexes of one, two or three atoms E = B, Si–Sn which are stabilized by σ-donor ligands L are discussed.

25 years of N-heterocyclic carbenes: activation of both main-group element-element bonds and NHCs themselves

N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C-N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.

Dative Bonding between Group 13 Elements Using a Boron-Centered Lewis Base

An electron-rich monovalent boron compound is used as a Lewis base to prepare adducts with Group 13 Lewis acids using both its boron and nitrogen sites. The hard Lewis acid AlCl3 binds through a nitrogen atom of the Lewis base, while softer Lewis acids GaX3 (Cl, Br, I) bind at the boron atom. The latter are the first noncluster Lewis adducts between a boron-centered Lewis base and a main-group Lewis acid.

The boron-boron triple bond in NHC→B 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 B←NHC

Thorough examination of the electronic structure of the compound B₂(NHC^Me)₂ provides convincing evidence for a B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B triple bond.

Quantum chemical calculations of the compound B₂(NHC^Me)₂ and a thorough examination of the electronic structure with an energy decomposition analysis provide strong evidence for the appearance of boron–boron triple bond character. This holds for the model compound and for the isolated diboryne B₂(NHC^R)₂ of Braunschweig which has an even slightly shorter B–B bond. The bonding situation in the molecule is best described in terms of NHC^Me→B₂←NHC^Me donor–acceptor interactions and concomitant π-backdonation NHC^Me←B₂→NHC^Me which weakens the B–B bond, but the essential features of a triple bond are preserved. An appropriate formula which depicts both interactions is the sketch NHC^Me⇄B Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> B⇄NHC^Me. Calculations of the stretching force constants F_BB which take molecules that have genuine single, double and triple bonds as references suggest that the effective bond order of B₂(NHC^Me)₂ has the value of 2.34. The suggestion by Köppe and Schnöckel that the strength of the boron–boron bond in B₂(NHC^H)₂ is only between a single and a double bond is repudiated. It misleadingly takes the force constant F_BB of OBBO as the reference value for a B–B single bond which ignores π bonding contributions. The alleged similarity between the B–O bonds in OBBO and the B–C bonds in B₂(NHC^Me)₂ is a mistaken application of the principle of isolable relationship.

Generation of 1,2-azaboretidines via reduction of ADC borane adducts

ADC borane adducts RBX₂·ADC (R = Mes, Dur; X = Cl, Br; ADC = :C(NiPr₂)₂) have been prepared and reduced by KC₈ to afford air stable 1,2-azaboretidines with high selectivity.

Reaction of the acyclic (diamino)carbene (ADC) :C(NiPr₂)₂ (1) with different dihaloboranes of the type RBX₂ (R = Mes, Dur; X = Cl, Br) smoothly afforded a novel class of ADC-stabilized borane adducts. For MesBBr₂ however, the reaction did not stop at the adduct level, but an uncommon rearrangement process occurred, which eventually resulted in the formation of a 5-membered boracycle after elimination of mesitylene. Chemical reduction of the ADC borane adducts by KC₈ selectively yielded air stable 1,2-azaboretidines. Detailed DFT studies suggest a reduction mechanism involving a highly reactive borylene intermediate, which is converted into the boracycles via a rearrangement/C–H activation sequence.

Synthesis of cyclic diborenes with unprecedented cis-configuration

The first cyclic and cis-configured diborenes are prepared in a convenient one-pot synthesis.