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

Abstract

The electron-rich, double-base stabilized dihydrido-diborane(4) [HB(hpp)] (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) combines two different, easily accessible reactive sites: the B-B and B-H bond. Herein, we report two basically similar reactions of [HB(hpp)] with catecholborane and B-chlorocatecholborane unsuspectedly resulting in two different reactivities. While reaction of the diborane with two equivalents of HB(cat) proceeded with formation of a cationic triborane with a (3c2e) bond, [{HB(μ-hpp)} (μ-BH )] , reaction with ClB(cat) led to hydride-chloride substitution yielding [ClB(hpp)] . The cationic triborane was also obtained by the addition of BH to the in situ generated [HB(hpp) B] . Proceeding from the new compound [ClB(hpp)] , we investigated its reactivity towards chloride abstraction with AlCl and GaCl . Irrespectively of the salt chosen, the reaction led to formation of a radical tricationic tetraborane [{B(hpp)} ] with a (4c,5e) bond. Moreover, the dichloro-diborane proves as a versatile reagent for further substitution cleanly reacting with the strong nucleophile nBuLi to a new dibutyl-diborane [nBuB(hpp)] .

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

“…[47] Thef ormal addition of three hydrides and two neutral two-electron donor ligands Lto(B 3 H 3 ) 4+ leads to amonocation (B 3 H 6 L 2 ) + that could alternatively be described as ad ouble base-stabilized (B 3 H 6 ) + analogue.D erivatives of this compound are indeed known. [52] Thes alts of this monocation are remarkably stable,s howing no sign of decomposition when heated up to 80 8 8C. [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.…”

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

“…Quantum-chemical calculations (B3LYP/def2-TZVP) found no minimum structure for this tetracationic tetraborane (4) molecule.The four skeletal electrons do not provide sufficient bond energy to compensate the massive Coulombic repulsion between the four HB + units ("Coulomb explosion"). [11][12][13] The dihydridodiborane [HB(hpp)] 2 (1,h pp = 1,3, 4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate,S cheme 2a)i sa ne specially electron-rich example. [10] Our group developed electron-rich diboranes suitable for the synthesis of cationic oligoboranes,i nw hich the boron atoms are bridged by guanidinate substituents.…”

“…[11] Hydride abstraction with B(C 6 F 5 ) 3 gave the dicationic tetraborane (6) [H 2 B 4 (hpp) 4 ] 2+ [14] with aplanar rhomboid B 4 core and (4c,4e) bonding between the four boron atoms.T he two remaining hydrides could not be abstracted with B(C 6 F 5 ) 3 ,e ven if al arge excess of the Lewis acid is used. Nevertheless,reaction of 2 with AlCl 3 or GaCl 3 in CH 2 Cl 2 or toluene again leads to the radical trication [B 4 (hpp) 4 ]C 3+ (Scheme 2a), [13] highlighting the great electron affinity of the hypothetical tetracation intermediate,being calculated at 13.5 eV (BP86/def2-SV(P)). Thea dditional electrons are provided by excess 1.T he dichlorodiborane 2 is less electron-rich than 1.T he calculated (adiabatic) ionization energy (B3LYP + D3/def2-TZVPP calculations) of 1 (536 kJ mol À1 ) is significantly lower than that of 2 (574 kJ mol À1 ).…”

Vier Boratome, vier positive Ladungen und vier Gerüstelektronen: ein fluoreszierendes σ‐aromatisches Tetraboran(4)

“…[9] Das weitere Entfernen von zwei Elektronen würde letztlich das Te trakation [B 4 H 4 ] 4+ mit nur 4GE ergeben. [11][12][13] Das Dihydrido-Diboran [HB(hpp)] 2 (1;h pp = 1,3,4,6,7,pyrimidinat;Schema 2a)ist ein besonders elektronenreiches Beispiel. Die vier Gerüstelektronen bieten keine ausreichende Bindungsenergie,u m die massive Coulomb-Abstoßung zwischen den vier HB + -Einheiten zu kompensieren ("Coulomb-Explosion").…”

“…Dagegen isolierten Timms et al das neutrale Tetraboran (8) [B 4 F 4 (BF 2 ) 4 ]mit einem gefalteten B 4 -Ring und 4GE(verteilt auf zwei (3c,2e)-Bindungen). [12] Die HOMO-Energien betragen À4.78 eV für 1 und À5.12 eV für 2.Nichtsdestotrotz liefert die Reaktion für 2 mit AlCl 3 oder GaCl 3 in CH 2 Cl 2 oder Toluol erneut das Radikaltrikation [B 4 (hpp) 4 ]C 3+ (Schema 2a) [13] aufgrund der hohen Elektronenaffinitätd es hypothetischen tetrakationischen Intermediats,d eren Wert zu 13.5 eV berechnet wurde (BP86/def2-SV(P)). [11][12][13] Das Dihydrido-Diboran [HB(hpp)] 2 (1;h pp = 1,3,4,6,7,pyrimidinat;Schema 2a)ist ein besonders elektronenreiches Beispiel.…”