Four Boron Atoms, Four Positive Charges, and Four Skeletal Electrons: A Fluorescent σ‐Aromatic Tetraborane(4)

Abstract

Reaction of aditriflatodiborane compound with the Lewis acids AlCl 3 or GaCl 3 leads to abstraction of the two triflate substituents and dimerization of the resulting dicationic diborane to give a s-aromatic tetracationic tetraborane with ap lanar,r homboid B 4 core.T he compound exhibits four skeletal s-electrons involved in two (3c,2e) bonds and represents the first stable fourfold base-stabilized [B 4 H 4 ] 4+ analogue.The product is isolated from the reaction mixture in the form of bright orange crystals that displayf luorescence. Further analysis shows that the new tetraborane(4) is stabilized in the solid state by the lattice energy.I te xhibits an extremely high electron affinity and is only stable in solution after oneelectron reduction to the radical cation. Angewandte ChemieCommunications 5897

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

“…Substitution 4 ] 4+ were calculated to be + 917 and + 1062 kJ mol À1 ,respectively (Scheme 18 a), [6] thus rendering its synthesis highly unlikely.T he Coulombic repulsion that opposes the formation of this compound could be diminished by the use of electron-donating substituents, which lead to delocalization of the positive charge into the substituents,asdoes the hpp group (see mesomeric structures of hpp in Schemes 10 and 18 a). Hence,[B 4 (hpp) 4 ] 4+ is astable derivative.E ach hpp group formally substitutes ap air (H,NH 3 ).…”

“…[80] for details). [6] Thesynthesis of [B 4 (hpp) 4 ] 4+ was achieved by abstraction of the two triflato groups of [B(OTf)(hpp)] 2 [87] with AlCl 3 or GaCl 3 (Scheme 18 b). [7] for details).…”

“…Scheme 17. a) Calculated (B3LYP/6-31G*) energy difference between D 2h -symmetric (planar B 4 unit) and puckered C 2v -symmetric structures of B 4 F 4 H 4 (see Ref. [6] Theexcess of chlorine in the counterions presumably results from chloride abstraction from the solvent (CH 2 Cl 2 )inthe presence of the strong Lewis acid. b) The first derivative of B 4 H 8 , B 4 (BF 2 ) 4 F 4 ,isolated and structurally characterizedbyT imms and coworkers by the low-temperature decomposition of B 3 F 5 (see Ref.…”

“…[6] Several dicationic diboranes with bridging hpp substituents and related bridging bicyclicg uanidinate substituents have so far been synthesized, starting either from [TfOB-(hpp)] 2 (example in Scheme 19 a) or from B 2 Cl 2 -(NMe 2 ) 2 (example in Scheme 19 b). bipyridine,S cheme 19 a).…”

“…On the other hand, very recently it was possible to synthesize the compound [B 4 (hpp) 4 ] 4+ (where hpp denotes the bicyclicg uanidinate 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2-a]pyrimidinate;S cheme 2), [6] af ourfold base-stabilized derivative of (B 4 H 4 ) 4+ ,b yd iminishing the Coulombic repulsion forces within the B 4 cycle through charge delocalization into the hpp groups. On the other hand, very recently it was possible to synthesize the compound [B 4 (hpp) 4 ] 4+ (where hpp denotes the bicyclicg uanidinate 1,3,4,6,7,8-hexahydro-2H-pyrimido-[1,2-a]pyrimidinate;S cheme 2), [6] af ourfold base-stabilized derivative of (B 4 H 4 ) 4+ ,b yd iminishing the Coulombic repulsion forces within the B 4 cycle through charge delocalization into the hpp groups.…”