Diboryldiborene: π-konjugierte B 4 -Ketten isoelektronisch zum Butadien-Dikation

Electron-Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate-Borenium Zwitterion and a 1,3-Azaborinine

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species.

Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH₂ ] (Dur=2,3,5,6-Me₄ C₆ H) and [(Me₃ Si)₂ NBH₂ ] led to the formation of bridging borylene complexes of the form [(Cp*RuH)₂ BR] (Cp*=C₅ Me₅ ; 1 a: R=Dur; 1 b: R=N(SiMe₃ )₂ ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF₃ )₂ -C₆ H₃ BH₂ ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)₄ [(Cp*Ru)₂ B₄ H₅ (3,5-(CF₃ )₂ C₆ H₃ )₄ ] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)₂ (μ-H)B₄ H₉ ] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B₄ chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two.

Reactivity of a Tetra(o-tolyl)diborane(4) Dianion as a Diarylboryl Anion Equivalent

Lithium and magnesium salts of tetra(o-tolyl)diborane(4) dianion, having B=B double bond character, were synthesized. It was clarified that the lithium salt of the dianion has a high-lying HOMO and a narrow HOMO-LUMO gap, which were perturbed by dissociation of Li⁺ cation, as judged by UV/Vis spectroscopy and DFT calculations. The lithium salt of the dianion reacted as two equivalents of a diarylboryl anion with CH₂ Cl₂ or S₈ to give boryl-substituted products.