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

Abstract

Herein reported are the reactions of an electron‐rich, Lewis acidic diborane with N‐heterocyclic aromatics to give first members of an unprecedented family of highly charged cationic cyclophanes with diboranyl units. Tetracationic cyclophanes with 4,4‘‐bipyridine/ 1,2‐bis(4‐pyridyl)ethylene and diboranyl units were synthesized and their redox chemistry was studied. Cyclisation of two diboranyl and two pyrazine units is accompanied by electron transfer from the diboranyl unit to the pyrazine. Our results pave the way for the integration of redox‐active diboranyl units into cyclophanes and supramolecular structures.

Chemistry of Dicationic Diboranes

“…This step is in accordance with experimental findings which prove an easy substitution of the OTf substituents by organic Lewis bases (see chapters 5 and 6 for more information). [58][59][60] The two remaining steps to [B 4 (hpp) 4 ] 4+ proceed either in accordance with the pathway A or B (Scheme 12) and show a fascinating variety of reactivities conceivable for cationic borane compounds.…”

“…Indeed, reacting [(TfO)B(hpp)] 2 with 4,4′-bpy or dpe in CH 2 Cl 2 resulted in the formation of an unprecedented tetracationic tetraboron cyclophane 23 or 24 as triflate salts (Figure 9a, left). [60] Both 23(OTf ) 4 and 24(OTf ) 4 are water soluble and stable compounds that can be stored in air atmosphere.…”

“…Thermal ellipsoids are drawn at the 50 % probability level, anions and hydrogen atoms are omitted for clarity (see ref. [60] for more information).…”

“…(a) Synthesis of the first bis-viologen boracyclophane with two diborane linkers (23 and 24) and the tetraborane 26, and (b) Lewis representation of the cocrystallized octacationic side product in the reaction with 4,4′-bpy and a photo of its yellow crystals under the microscope. Reproduced from ref [60]. …”

Electrochromic Polycationic Organoboronium Macrocycles with Multiple Redox States

“…Although diverse polycationic macrocycles have been reported, [8] those based on cationic boronium moieties remain very limited [9] . Very recently, coordination‐driven self‐assembly to build up polycationic boron macrocycles was reported by Himmel and co‐workers [9c] . These macrocycles are composed of cationic diborane units that are linked together by pyrazine, 4,4′‐bipyridine, or 1,2‐di(4‐pyridyl)ethene.…”