Synthesis and Characterization of a Doubly Base‐Stabilized B₃H₆⁺ Analogue

Abstract

There is long-standing interest in the electronic structure of small-ring compounds containing boron.[1] However, the simple boron hydride B 3 H 6 + , being isoelectronic to the smallest aromatic organic compound, namely the 2p system C 3 H 3 + , has not yet been prepared. The possible structure of B 3 H 6 + was assessed in a number of quantum-chemical calculations using various levels of theory. [2,3] These calculations predicted a C 3v -symmetric, nonplanar global minimum structure with three bridging hydrogen atoms (see Scheme 1). The D 3h -symmetric planar form does not represent an energy minimum, is approximately 175 kJ mol À1 higher in energy than the C 3v -symmetric global energy minimum. B 3 H 6 + was therefore declared to be the "first three-membered nonplanar 2p aromatic" species.[2a]Korkin, Schleyer, and McKee also considered other isomers in their calculations.[3] They found in total four energy minimum structures, which are displayed in Scheme 1. According to their ab initio (MP2) calculations, a C 2v -symmetric isomer featuring a B 3 triangle with one short (150.1 pm) and two longer B À B bonds (184.9 pm) as well as two bridging hydrogen atoms (see Scheme 1) lies 49 kJ mol À1 higher in energy than the C 3v -symmetric global energy minimum. In two further C S -and C 2v -symmetric isomers a B-B-B chain is established. The energies of these two isomers are already 60 and 77 kJ mol À1 higher than the C 3v global energy minimum structure. Up to date, none of these isomers or even a derivative of B 3 H 6 + have been structurally characterized. Herein we report the synthesis and complete characterization of a doubly base-stabilized B 3 H 6 + analogue. In our studies we used the doubly base-stabilized diborane(4) [HB(m-hpp)] 2 (1), which was synthesized as described previously by catalytic dehydrogenation of the adduct H 3 B·hppH (Scheme 2).[4] We recently reported on different aspects of the reactivity of 1. For example, sulfuration was shown to give [HB(m-hpp)] 2 (m-S), [5] and protonation leads to the cation and doubly base-stabilized B 2 H 5[6] Moreover, the complexes [{HB(m-hpp)} 2 ZnX 2 ] and [{HB(m-hpp)} 2 ZnMe 2 ] were obtained as the products of reaction with zinc halides ZnX 2 (X = Cl, Br) or dimethylzinc, respectively.[7] These reactions demonstrated the high nucleophilicity of 1. In the light of this chemical behavior, we tested the reaction of 1 with BR 2 + (R = H or alkyl) cationic species. In the course of our research, we treated 1 with Nborabicyclo[3.3.1]nonan-9-yl (9-BBN) bis-(trifluoromethanesulfonyl)imide (2 a) and trifluoromethanesulfonate (2 b) (see Scheme 3), which are known as easily accessible sources of a highly electrophilic boronium ion.[8] We indeed observed the formation of a single reaction product.Both salts 3 a and 3 b could be crystallized from toluene or toluene/petroleum ether (40/60) solutions. However, only for the crystals of 3 b was a good-quality structure derived on the basis of the X-ray diffraction data. The unit cell contains two slightly different cationic unit...

On the Dual Reactivity of a Nucleophilic Dihydrido‐Diborane: Reaction at the B−B Bond and/or the B−H Bond

“…Reaction of the dihydrido‐diborane 1 with two equivalents of catecholborane, HB(cat), furnished a new cationic triborane 2 [{HB(hpp)} 2 (μ‐BH 2 )] + with a three‐center, two‐electron bond in 58 % isolated yield (see Scheme ). The compound turned out to be thermally stable up to at least 80 °C, in line with other compounds featuring a closed B‐B‐B three‐center, two‐electron bond . Exposed to a polar protic solvent like methanol or water the B 3 unit decomposed immediately under evolution of gas.…”

“…The 11 B NMR peak of the BH 2 unit could not be unambiguously detected neither by high‐ nor by low‐temperature NMR experiments (see Supporting Information, Figures S1 and S2). This is eventually reasoned by the high dynamics of the BH 2 moiety in solution, leading to an extreme signal broadening already observed for another cationic triborane synthesized in our group, [{HB(hpp)} 2 (μ‐BC 8 H 15 )] + . The experimentally derived structure of the cation in the solid state is displayed in Figure .…”

Electron‐Deficient Triborane and Tetraborane Ring Compounds: Synthesis, Structure, and Bonding

“…[45,46] At heoretical analysis found evidence for ac losed (3c,2e) s(B 3 )b ond, thus motivating its description as a s-aromatic compound. [51] Each hpp unit replaces ap air (H,L) in (B 3 H 6 L 2 ) + ,a si llustrated by the first two resonance structures of hpp in Scheme 10 b. Thes tructure of this compound has some similarities with (B 3 H 8 ) À ,b ut the electronic structure as well as the chemical properties are quite different. Thef irst example was prepared by reaction of the especially nucleophilic sp 3 -sp 3hybridized dihydrodiborane [HB(hpp)] 2 [48][49][50] with the strongly electrophilic boranes N-borabicyclo[3.3.1]nonan-9yl (9-BBN) trifluoromethanesulfonateo rb is(trifluoromethanesulfonyl)imide (Scheme 10 a).…”

“…The triangles denote bond critical points;see Ref [51]. The triangles denote bond critical points;see Ref [51].…”

Isolierbare, neutrale Analoga des [B2H5]−‐Ions als vielseitige und stark bindende η2‐Donorliganden

“…In 11 B-entkoppelten 1 H-NMR-Spektren (CD 2 Cl 2 )v erschieben sich durch die Koordination zum Kupfer(I)-Zentrum die Signale des Bor-gebundenen Wasserstoffatoms um Dd = 0.4-0.9 ppm zu hçherem Feld. [16,24] Im Infrarotspektrum von 2 werden weder in Lçsung noch im Festkçrper Banden terminaler B-H-Schwingungen erhalten. [16,24] Im Infrarotspektrum von 2 werden weder in Lçsung noch im Festkçrper Banden terminaler B-H-Schwingungen erhalten.…”