From Carbene-Dithiolene Zwitterion Mediated B-H Bond Activation to BH3·SMe2-Assisted Boron-Boron Bond Formation

The 1:1 reaction of the carbene-stabilized dithiolene zwitterion 1 with BH3·SMe2 gave the dithiolene-based hydroborane 2 and the doubly hydrogen-capped CAAC species 3 via hydride-coupled reverse electron transfer processes. The mechanism of this transformation was probed computationally using density functional theory. The subsequent 2:1 reaction of 2 with 1 resulted in 4 and 3, suggesting that 1 can mediate the B-H bond activation not only for BH3 but also for monohydroboranes. In the presence of BH3·SMe2, 2 was unexpectedly converted to the corresponding diborane(4) complex 5 through a dehydrocoupling reaction at an elevated temperature.

A Crystalline B4N2 Dewar Benzene as a Building Block for Conjugated B,N-Chains

Dewar benzene, one of the isolable valence isomers of C₆H₆, has been extensively studied since its first synthesis in 1962. By contrast, the chemistry of inorganic congeners of Dewar benzene, which can be formally gained by replacing the skeletal carbon atoms with heteroatoms, has been less developed despite their peculiar structural and electronic features. Among them, the extant B,N-Dewar benzenes are limited to the B₃N₃ system. Herein, we report the development of the first example of an isolable B₄N₂ Dewar benzene, 3. As predicted by DFT calculations, a judicious selection of the substituents allows synthesizing 3. Single-crystal X-ray analysis, NMR, and computational studies confirmed that 3 possesses a high-lying B(sp³)-B(sp³) σ-bond at the bridgehead position. Reactions with ethylene and phenylacetylene proceeded smoothly under mild conditions, affording the fused B₄C₄N₂ ring systems (4 and 5). Structural characterization as well as DFT calculations revealed that compounds 4 and 5 involve a rigid and conjugated (BN)₄ tetraene scaffold. Formation of 4 and 5 demonstrates that 3 may serve as a building block for the construction of conjugated B,N-chains.

trans-Selective Insertional Dihydroboration of a cis-Diborene: Synthesis of Linear sp3 -sp2 -sp3 -Triboranes and Subsequent Cationization

The reaction of aryl- and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3-trans-dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis-1,2-μ-H-3-hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base-stabilized B₃ H₄ ⁺ analogues.

Steric Effects Dictate the Formation of Terminal Arylborylene Complexes of Ruthenium from Dihydroboranes

The steric and electronic properties of aryl substituents in monoaryl borohydrides (Li[ArBH₃ ]) and dihydroboranes were systematically varied and their reactions with [Ru(PCy₃ )₂ HCl(H₂ )] (Cy: cyclohexyl) were studied, resulting in bis(σ)-borane or terminal borylene complexes of ruthenium. These variations allowed for the investigation of the factors involved in the activation of dihydroboranes in the synthesis of terminal borylene complexes. The complexes were studied by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and density functional theory (DFT) calculations. The experimental and computational results suggest that the ortho-substitution of the aryl groups is necessary for the formation of terminal borylene complexes.

Single and Double Hydroboration of B-B Triple Bonds and Convergent Routes to a Cationic Tetraborane

A compound with a boron-boron triple bond is shown to undergo stepwise hydroboration reactions with catecholborane to yield an unsymmetrical hydro(boryl)diborene and a 2,3-dihydrotetraborane. Abstraction of H^- from the latter compound produces an unusual cationic, planar tetraborane with a hydrogen atom bridging the central B₂ moiety. Spectroscopic and crystallographic data and DFT calculations support a "protonated diborene" structure for this compound, which can also be accessed via direct protonation of the corresponding diborene.

Boron-Boron Dehydrocoupling of Boranes Initiated by Reaction with Iodine

A new boron-boron dehydrocoupling strategy was established, providing convenient access to some diborane(4) compounds starting from simple borane adducts under mild conditions. In contrast to the traditional pathway using a reducing reagent, the reduction from B^III to B^II was paradoxically initiated by the addition of the oxidation-reagent iodine. A reaction pathway for this unusual reaction was proposed based on quantum-chemical calculations.

Reactivity of Tetrahalo- and Difluorodiboranes(4) toward Lewis Basic Platinum(0): Bis(boryl), Borylborato, and Doubly Boryl-Bridged Platinum Complexes

The reaction of the tetrahalodiboranes(4) B₂F₄, B₂Cl₄, and B₂Br₄ with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy₃P)₂Pt(BF₂)₂] (1) and the novel borylborato complexes trans-[(Cy₃P)₂Pt{B(X)-BX₃}] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F₂B-BMes₂ and the new derivative F₂B-BAn₂ (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R₃P)₂Pt(BF₂)(BMes₂)] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me₃P)(Cy₃P)Pt(BF₂)(BMes₂)] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{( iPr₃P)Pt}₂(μ-BF₂)(μ-BAr₂)] (8, Ar = Mes; 9, Ar = An), which feature two different μ₂-bridging boryl ligands.

Synthesis and Applications of Perfunctionalized Boron Clusters

This Viewpoint describes major advances pertaining to perfunctionalized boron clusters in synthesis and their respective applications. The first portion of this work highlights key synthetic methods, allowing one to access a wide range of polyhedral boranes (B4 and B6-B12 cluster cores) that contain exhaustively functionalized vertices. The second portion of this Viewpoint showcases the historical developments in using these molecules for applications ranging from materials science to medicine. Last, we suggest potential new directions for these clusters as they apply to both synthetic methods and applications.

Magnesium Boryl Reactivity with 9-BBN and Ph3 B: Rational B-B' Bond Formation and Diborane Isomerization

Reactions of a magnesium‐based pinacolatoboryl nucleophile with the electrophilic organoboranes, 9‐BBN and Ph₃B, provide facile B−B′ single bond formation. Although the Ph₃B derivative is thermally stable, when heated, the unsymmetrical diborane(5) anion derived from 9‐BBN is found to isomerize to two regioisomeric species via a proposed mechanism involving dehydroboration of the borabicyclo[3.3.1]nonane and syn‐diboration of the resultant alkenyl carbocycle.

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.

Simple solution-phase syntheses of tetrahalodiboranes(4) and their labile dimethylsulfide adducts

Convenient solution-phase syntheses of tetrahalodiboranes(4) and the dimethylsulfide adducts B₂Cl₄(SMe₂)₂ and B₂Br₄(SMe₂)₂ are presented.

Uncatalyzed Hydrogenation of First-Row Main Group Multiple Bonds

Room temperature hydrogenation of an SIDep-stabilized diboryne (SIDep=1,3-bis(diethylphenyl)-4,5-dihydroimidazol-2-ylidene) and a cAAC-supported diboracumulene (cAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) provided the first selective route to the corresponding 1,2-dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol^-1 ) two-step asynchronous H₂ addition mechanism proceeding via a bridging hydride.

New outcomes of Lewis base addition to diboranes(4): electronic effects override strong steric disincentives

Two surprising new outcomes of the reaction of Lewis bases with dihalodiboranes(4) are presented, including sp(2)-sp(3) diboranes in which the Lewis base unit is bound to a highly sterically congested boron atom, and a rearranged double base adduct. The results provide a fuller understanding of the reactivity of diboranes, a poorly-understood class of molecule of critical importance to synthetic organic chemistry.

A Radical Tricationic Rhomboid Tetraborane(4) with Four-Center, Five-Electron Bonding

The red-colored tetraborane(4) [B4 (hpp)4 ](3+.) (3; hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) with a rhomboid B4 skeleton stabilized by four N donors, was synthesized by the reaction of the strong hydride abstraction reagent [(acridine)BCl2 ][AlCl4 ] with the electron-rich diborane(4) [HB(hpp)]2 (1). The salt 3[AlCl4 ]3 was structurally characterized and the presence of unpaired electrons proven by EPR measurements. The unprecedented radical tricationic 3 is distinguished by a high positive charge and boron atoms in a low oxidation state (less than two).

Thermal Dehydrogenation of Base-Stabilized B2H5(+) Complexes and Its Role in C-H Borylation

Thermally induced dehydrogenation of the H-bridged cation L2B2H5(+) (L=Lewis base) is proposed to be the key step in the intramolecular C-H borylation of tertiary amine boranes activated with catalytic amounts of strong "hydridophiles". Loss of H2 from L2B2H5(+) generates the highly reactive cation L2B2H3(+), which in its sp(2)-sp(3) diborane(4) form then undergoes either an intramolecular C-H insertion with B-B bond cleavage, or captures BH3 to produce L2B3H6(+). The effect of the counterion stability on the outcome of the reaction is illustrated by formation of LBH2C6F5 complexes through disproportionation of L2B2H5(+) HB(C6F5)3(-) .

Aluminum chain in Li2Al3H8−as suggested by photoelectron spectroscopy and ab initio calculations

Group 13 elements are very rarely observed to catenate into linear chains and experimental observation of such species is challenging.