Access to adducts of parent iminoborane isomers, HBNH and NBH₂, using frustrated Lewis pair chelation

Abstract

A general frustrated Lewis pair chelation approach was used to access the parent iminoborane isomers HBNH and NBH2.

Frustrated Lewis Pair Chelation and Reactivity of Complexed Parent Iminoborane and Aminoborane

“…In a recent communication, we reported the synthesis of the aminoborane and iminoborane complexes [PB­{H 2 BNH 2 }] ( 1 ) and [PB­{HBNH}] ( 2 ). [PB­{H 2 BNH 2 }] ( 1 ) was obtained by combining H 3 B·NH 3 with i Pr 2 P­(C 6 H 4 )­BCy 2 ( PB ) at room temperature for 12 h in tetrahydrofuran (THF) (Scheme ), and the formation of 1 parallels the known frustrated Lewis pair (FLP)-instigated dehydrogenation of ammonia borane by the groups of Aldridge and Slootweg to yield FLP­{H 2 BNH 2 } complexes and dihydrogen. , As a start to this full study, we examined the energetics associated with the formation of [PB­{H 2 BNH 2 }] ( 1 ) from PB and H 3 B·NH 3 via computations at the M06-2X/cc-pVTZ level with a THF-polarizable continuum model (PCM).…”

“…Computations provide a partial reason for the failed dehydrogenation of 1 since the dehydrogenation of [PB­{H 2 BNH 2 }] ( 1 ) to give H 2 and [PB­{HBNH}] ( 2 ) is computed to be unfavorable with a THF-polarized continuum model [Δ H = 16.1 kcal mol –1 ; Δ G = 7.9 kcal mol –1 ; M06-2X/cc-pVTZ] . Of note, the synthesis of the HBNH adduct 2 required the initial formation of [PB­{HB­(I)–NH 2 }] ( D ) by treatment of 1 with Me 2 S·BI 3 followed by the addition of K­[N­(SiMe 3 ) 2 ] to D to promote HI loss and form [PB­{HBNH}] ( 2 ). As with 1 , [PB­{HBNH}] ( 2 ) did not show any signs of reactivity when exposed to the dehydrogenation pre-/catalysts mentioned above.…”

“…Overall, the metrical parameters of [PB­{H 2 BN­(Me)­H}] ( 4 ) are similar to those observed previously for [PB­{H 2 BNH 2 }] ( 1 ) with an internal B–N distance [B2–N1] of 1.581(2) Å, and a dative B–N distance involving the −BCy 2 unit [B1–N1] of 1.662(2) Å; notably, the latter B–N bond length in 4 is ca . 0.03 Å longer than in the corresponding adduct [PB­{H 2 BNH 2 }] ( 1 ) [1.628(3) Å, average of two independent molecules in the unit cell], reflecting the steric impact of the methyl group at N in 4 . Since it was observed that a longer reaction time was needed to afford [PB­{H 2 BN­(Me)­H}] ( 4 ) in similar yields as 1 (24 h vs 11 h), the formation of 4 from PB and H 3 B·NH 2 Me was explored computationally at an M06-2X/cc-pVTZ level of theory with a THF-polarized continuum model (Scheme S1).…”

“…0.03 Å longer than in the corresponding adduct [PB­{H 2 BNH 2 }] ( 1 ) [1.628(3) Å, average of two independent molecules in the unit cell], reflecting the steric impact of the methyl group at N in 4 . Since it was observed that a longer reaction time was needed to afford [PB­{H 2 BN­(Me)­H}] ( 4 ) in similar yields as 1 (24 h vs 11 h), the formation of 4 from PB and H 3 B·NH 2 Me was explored computationally at an M06-2X/cc-pVTZ level of theory with a THF-polarized continuum model (Scheme S1). As with the formation of 1 from PB and H 3 B·NH 3 (Scheme ), the initial dehydrogenation event to yield the mono-substituted aminoborane H 2 BNHMe and the hydrogenated FLP [ i Pr 2 P­{H}­(C 6 H 4 )­B­{H}­Cy 2 ] [PB­{H 2 }] was computed to be endergonic [Δ G = 8.5 kcal mol –1 ] (Scheme S1).…”

Synthesis of Iminoboryl o-Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

“…The parent iminoborane (HBNH) features a BN unit that is isoelectronic with a CC triple bond and can thus be regarded as “inorganic acetylene” . The spectroscopic identification and theoretical perspective dates back to early 1970s .…”