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

Abstract

Electron‐deficient small boron rings are unique in their formation of σ‐ and π‐delocalized electron systems as well as the avoidance of “classical” structures with two‐center‐two‐electron (2c,2e) bonds. These rings are tolerant of several skeletal electron numbers, which makes their redox chemistry highly interesting. In the past few decades, a range of stable compounds have been synthesized with various electron numbers in their B3 and B4 cores. The electronic structures were evaluated by quantum‐chemical calculations. On the other hand, the chemistry of these rings is still very much underdeveloped, being generally limited to the protonation and redox reactions of individual systems. The linkage of several B3 and/or B4 ring systems should give compounds with attractive electronic properties, thus leading the way to novel boron‐based materials. By summarizing important experimental and theoretical results, this Review intends to provide the basis for the exploration of the chemistry of these rings and, in particular, their integration into larger molecular architectures.

Electron‐Rich, Lewis Acidic Diborane Meets N‐Heterocyclic Aromatics: Formation and Electron Transfer in Cyclophane Boranes

“…In our previous investigations we focused on the elimination of the triflate substituents in the diborane 1 and the synthesis of cationic boron species. The abstraction of the triflates in 1 was achieved both with the Lewis acids GaCl 3 and AlCl 3 leading to a tetracationic tetraborane and with neutral Lewis bases under formation of diborane dications (Scheme ) …”

“…[24,25] In our previousi nvestigations we focusedo nt he elimination of the triflate substituents in the diborane 1 and the synthesis of cationic boron species. The abstraction of the triflates in 1 was achieved both with the Lewis acids GaCl 3 and AlCl 3 leading to at etracationic tetraborane [26,27] and with neutral Lewis bases under formation of diboraned ications (Scheme 1). [28,29] In previous studies, these abstraction reactions werec arried out with Lewis bases that offer two equivalent Lewis basic centers for coordination to the two boron atoms.…”

Chemistry of Dicationic Diboranes

“…Small electron-deficient compounds with rhomboid B 4 core fascinate by the interplay between σand π-electron delocalization. [48] These unprecedented oligoboranes, which can be formally obtained by the dimerization of mono-and/or dicationic diboranes, have been subject of both theoretical and experimental studies. Recently, the introduction of bridging bicyclic guanidinate substituents by Himmel et al allowed the synthesis of new highly charged cationic tetraboranes being stabilized by the delocalization of the positive charge into the guanidinate ligands.…”

B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

“…The fast and quantitative formation of M[ 3 ] in THF demonstrates that M[ 2 H] is sufficiently nucleophilic to outcompete even excess THF as a ligand of 1 . B 3 clusters comparable to [ 3 ] − have been obtained by Himmel and co‐workers from the reaction of C (Figure ) with sources of borinium cations [BR 2 ] + (R=H, alkyl) …”