Formation of Strong Boron Lewis Acid Sites on Silica.

Bis(1-methyl-ortho-carboranyl)borane (HBMeoCb2) is a very strong Lewis acid that reacts with the isolated silanols present on silica partially dehydroxylated at 700 °C (SiO2-700) to form the well-defined Lewis site MeoCb2B(OSi≡) (1) and H2. 11B{1H} magic-angle spinning (MAS) nuclear magnetic resonance (NMR) data of 1 are consistent with that of a three-coordinate boron site. Contacting 1 with O═PEt3 (triethylphosphine oxide TEPO) and measuring 31P{1H} MAS NMR spectra show that 1 preserves the strong Lewis acidity of HBMeoCb2. Hydride ion affinity and fluoride ion affinity calculations using small molecules analogs of 1 also support the strong Lewis acidity of the boron sites in this material. Reactions of 1 with Cp2Hf(13CH3)2 show that the Lewis sites are capable of abstracting methide groups from Hf to form [Cp2Hf-13CH3][H313C-B(MeoCb2)OSi≡], but with a low overall efficiency.

The Effect of Carborane Substituents on the Lewis Acidity of Boranes.

The Lewis acidity of primary, secondary, and tertiary boranes with phenyl, pentafluorophenyl, and all three isomers of the C-substituted icosahedral carboranes (ortho, meta, and para) was investigated by computing their fluoride, hydride, and ammonia affinities as well as their global electrophilicity indices and LUMO energies. From these calculations, it was determined that the substituent effects on the Lewis acidity of these boranes follow the trend of ortho-carborane > meta-carborane > para-carborane > C6F5 > C6H5.

Triple-Decker Iron and Cobalt Complexes Featuring a Bridging 1,2-Diboratabenzene Ligand.

Neutral triple-decker iron and cobalt complexes with a bridging 1,2-diboratabenzene ligand were accessed by reactions of a dilithium 1,2-diboratabenzene reagent with [Cp*FeCl]2 and [Cp*CoCl]2, respectively. While 1,2-diboratabenzene metal complexes are known, these represent the first examples of the ligand bridging two metals.

9-Borafluorenes: Synthesis, Properties, and Reactivity.

This review covers all aspects of 9-borafluorene chemistry, from the first attempted synthesis in 1960 to the present. This class of molecules consists of a tricyclic system featuring a central antiaromatic BC4 ring with two fused arene groups. The synthetic routes to all 9-borafluorenes and their adducts are presented. The Lewis acidity and photophysical properties outlined demonstrate potential utility as sensors and in electronic materials. The reactivity of borafluorenes reveals their prospects as reagents for the synthesis of other boron-containing molecules. The appealing traits of 9-borafluorenes have stimulated investigations into analogues that bear different aromatic groups fused to the central BC4 ring. Finally, we offer our views on the challenges and future of borafluorene chemistry.

Bis(1-Methyl-ortho-Carboranyl)Borane.

The Lewis superacid, bis(1-methyl-ortho-carboranyl)borane, is rapidly accessed in two steps. It is a very effective hydroboration reagent capable of B-H addition to alkenes, alkynes, and cyclopropanes. To date, this is the first identified Lewis superacidic secondary borane and most reactive neutral hydroboration reagent.

A High-Performance Single-Molecule Magnet Utilizing Dianionic Aminoborolide Ligands.

The first example of a homoleptic f-block borolide sandwich complex is presented and shown to be a high-performance single-molecule magnet (SMM). The bis(borolide) complex [K(2.2.2)][[1-(piperidino)-2,3,4,5-tetraphenylborolyl]2Dy] (1) features an unusual example of an anionic Ln3+ metallocene that supports short metal-ligand bonds and a high degree of linearity around the central Dy3+ ion, resulting in comparatively large barriers to magnetization reversal (Ueff = 1600 cm-1 for the most linear orientation) and, importantly, a high blocking temperature (TB, defined as T(τ100s)) of 66 K. These metrics put complex 1 among the very best performing SMMs reported to date and highlight the potential of dianionic borolide ligands to increase ligand field axiality, compared to monoanionic cyclic ligands, to ultimately maximize magnetic anisotropy in f-block-based SMMs.

Accessing Boron-Doped Pentaphene Analogues from 12-Boradibenzofluorene.

Borole-doped polycyclic aromatic hydrocarbons (PAHs) have garnered attention in recent years due to their attractive photophysical properties and potential utility in electronic devices. In this work, a borole-doped PAH, 12-boradibenzofluorene, is synthesized and formal intermolecular nitrene and oxygen atom insertion reactions were employed to access 1,2-azaborine- and 1,2-oxaborine-containing analogues of the carbonaceous PAH pentaphene. Iodosobenzene is established as a versatile reagent for oxygen atom insertion reactions into a variety of borole species to access 1,2-oxaborine systems.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid.

The synthesis of tris(ortho-carboranyl)borane (BoCb3 ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb3 is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C6 F5 )3 ].

Nickel-Borolide Complexes and Their Complex Electronic Structure.

Borolides (BC42-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr2NBC4Ph2)2 (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni2+ species bound to two monoanionic radical (BC4•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

Ligation of Boratabenzene and 9-Borataphenanthrene to Coinage Metals.

The reactions of boratabenzene and borataphenanthrene anions with group 11 Ph3PMCl reagents furnished η2 coordination complexes, with the exception of the copper boratabenzene species that adopted an η6 mode. The binding of arene ligands to copper in an η6 manner is rare, and altering the ancillary ligand on copper to an N-heterocyclic carbene switched the binding of the boratabenzene to η2, indicating that such ligands are capable of vacating coordination sites. The η2 coordination complexes bind side-on, akin to olefins, via a borataalkene unit, although with the carbon atom much more proximal to the metal center than boron.

The 9-Borataphenanthrene Anion.

The 9-borataphenanthrene anion is easily accessed by deprotonation of a 9,10-dihydro-9-boraphenanthrene and its diverse reactivity is investigated. Alkylation occurs at the carbon atom adjacent to boron, and room temperature hydroboration occurs across the B=C bond. The π-manifold of the central BC5 ring coordinates to chromium in an η6 fashion while only the B=C unit binds η2 to gold, indicating versatility of the 9-borataphenanthrene anion as a ligand. Supporting calculations rationalize the reactivity and aromaticity is corroborated by nucleus-independent chemical shift (NICS) indices.

Diverse Reactivity of Dienes with Pentaphenylborole and 1-Phenyl-2,3,4,5-Tetramethylborole Dimer.

The reactions of a monomeric borole and a dimeric borole with 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene were investigated. The monomeric borole reacted at ambient temperature whereas heat was required to crack the dimer to form the monomer and induce reactivity. 2,3-Dimethyl-1,3-butadiene reacts to give diverse products resulting from a cycloaddition process with the B-C moiety of the boroles acting as a dienophile, followed by rearrangements to furnish bicyclic species. For 1,3-cyclohexadiene, a [4+2] process is observed in which 1,3-cyclohexadiene serves as the dienophile and the boroles as the diene partner. The experimental results are corroborated with mechanistic theoretical calculations that indicate boroles can serve as either a diene or dienophile in cycloaddition reactions with dienes.

Evaluation of the σ-Donating and π-Accepting Properties of N-Heterocyclic Boryl Anions.

The relative σ-donating and π-accepting capacities of a range of synthetically relevant boryl anions have been evaluated by examining the geometric, thermochemical, and electronic properties of their adducts to the Li+ cation and Se atom, as compared to the properties of the analogous neutral N-heterocyclic carbenes (NHCs), by theoretical methods. The results indicate that boryl anions have a weaker π-accepting capability compared to NHCs, but it is still a non-negligible factor in the bonding contributions between boryl and the Se atom. The tunability of the π-accepting capacity of boryl anions is similar to that of NHCs, indicating a potential for the modification of the electronic properties of metal complexes incorporating either boryl or NHC ligands. In all cases, the boryl ligands were found to be superior σ-donors to NHCs.

Aromaticity of unsaturated BEC4 heterocycles (E = N, P, As, Sb, O, S, Se, Te).

A compendium of pnictogen and chalcogen substituted boron heterocycles were assessed for their aromatic character by first principles density functional theory. Group-15 and Group-16 elements were placed at the ortho-, meta-, and para-positions of six-membered rings relative to boron to assess their impact on the aromaticity of the unsaturated heterocycles. Aromaticity was analyzed by a multidimensional approach using nuclear independent chemical shifts, gauge-including magnetically induced current, as well as natural bond orbital and natural resonance theory analyses. Based on these methods, we observe a general decline of aromaticity in heavier pnictaborines while the chalcogen analogues maintain relatively strong aromatic character. These general trends result from complementary π-π* natural bond order interactions that sustain resonance within the ring of each heterocycle establishing a pattern of cyclic delocalization. Consequently, natural resonance theory displays strong resonance, which is corroborated with the signed modulus of ring current, toroidal vortices of current maps, and elevated average induced current throughout the ring. The 1,3-configurations for pnictaborines and chalcogenaborines are generally more aromatic compared to the 1,2- and 1,4-isomers, which contain π-holes that limit diatropism within the heterocycles. However, an energetic trend favors the 1,2-heterocycles in both groups, with a few exceptions driven in large-part by π-donation of the lone pair on the heteroatom to the pz orbital on the adjacent boron resulting in stabilization. The importance of planarity for high aromaticity is demonstrated, especially in the pnictaborine isomers where pyramidalization at the pnictogen is favored, while bond regularity seems a less important criterion.

Isomer Dependence on the Reactivity of Diazenes with Pentaphenylborole.

Reactions of the anti-aromatic pentaphenylborole with diazenes indicate that both the substitution and the isomer influence the reaction outcome. With respect to trans isomers, azobenzene underwent coordination and C-H addition across the diene of the borole, and 2',6'-dimethylazobenzene furnished a fused tricyclic system. Under photolytic conditions, both of the aforementioned diazenes generate the first 1,3,2-diazaborepin heterocycles, rationalized through reactivity with the cis isomers. This notion is corroborated by the reaction of pentaphenylborole with benzo[c]cinnoline, the tethered variant of azobenzene, that only exists as the cis conformer as the corresponding 1,3,2-diazaborepin was produced regardless if the reaction is conducted in the dark or light. The more aromatic pyridazine proved to be less reactive, forming a resilient adduct.

Exploiting Pincer Ligands to Perturb the Geometry at Boron.

Boranes are ubiquitous in synthesis and materials but advancements in their development have been primarily restricted to the geometric energetic minima, trigonal planar complexes. This report discloses a class of boranes with expanded bond angles achieved by taking advantage of the structural rigidity of tridentate pincer ligands. The bonding of these novel boranes is investigated by X-ray crystallography and computationally.

Expedient Synthesis of 1,2-Thiaborines by Means of Sulfur Insertion into Boroles.

The propensity of boroles to undergo ring expansion reactions has been exploited as a route to generate 1,2-thiaborines, molecules that can be viewed as hybrid inorganic/organic analogues of benzene. Computational studies as well as structural data indicate that the species reported here have a high degree of aromatic character.

Peculiar Reactivity of Isothiocyanates with Pentaphenylborole.

The reactions of isothiocyanates with the antiaromatic pentaphenylborole were investigated, revealing significantly different outcomes than the analogous reactions with isocyanates. The 1:1 stoichiometric reaction products isolated include a seven-membered BNC5 heterocycle and a fused bicyclic 4/5-ring system. Studies suggest that the seven-membered ring undergoes an intramolecular [2 + 2] electrocyclic ring closure to produce the bicyclic system. The only derivative for which stoichiometry influenced the reaction outcome was 4-methoxyphenylisothiocyanate. The reaction of borole with an excess of 4-methoxyphenylisothiocyanate resulted in the formation of a fused tetracyclic species with two equivalents of isothiocyanate incorporated into the product. Rational pathways for these unusual transformations are presented.

Ring expansion reactions of pentaphenylborole with dipolar molecules as a route to seven-membered boron heterocycles.

Reactions of pentaphenylborole with isocyanates, benzophenone, and benzaldehyde produced new seven-membered heterocycles in high yields. For 1-adamantyl isocyanate, a BNC5 heterocycle was obtained from the insertion of the C-N moiety into the five-membered borole, whereas for 4-methoxyphenyl isocyanate, a BOC5 heterocycle was generated from the insertion of the C-O unit. These reactions are believed to occur via a mechanism wherein coordination of the nucleophile to the borole (1-adamantyl, N-coordination or O-coordination for 4-methoxyphenyl) is followed by ring expansion to afford the observed seven-membered heterocycles. The selectivity to form B-O- or B-N-containing heterocycles is based on the polarization of the isocyanate implying tunable reactivity for the system. Having observed that isocyanates react as 1,2-dipoles with pentaphenylborole, we examined benzophenone and benzaldehyde, which both reacted to insert C-O units into the ring. This represents a new efficient method for preparing rare seven-membered boracycles.

Reactions of imines, nitriles, and isocyanides with pentaphenylborole: coordination, ring expansion, C-H bond activation, and hydrogen migration reactions.

The reactions of pentaphenylborole with imines, isocyanides, and acetonitrile were investigated experimentally and theoretically. On the basis of literature precedent, we envisioned that the dipolar substrates would undergo facile ring expansion reactions to yield new BNC5 heterocycles. For acetonitrile and one particular imine, this ring expansion process was observed. However, in many cases, unexpected reactivity occurred. This included hydride migration of an imine ring expanded product and the ortho C-H bond activation of an aryl group of an imine if two phenyl groups were present on the α-carbon. A bulky group on the nitrogen atom of an imine prevented coordination to the boron center, and no reaction was observed, indicating that coordination to the borole is a critical step for any type of reaction to occur. Isocyanides made coordination complexes, but heating to induce further reactivity resulted in mixtures. The mechanisms were elucidated via DFT calculations, which complement the experimental findings.