Diborane Concatenation Leads to New Planar Boron Chemistry

Abstract

Diborane has long been realized to be analogous to ethylene in terms of its bonding MOs, both as to symmetries and splitting patterns. This naturally suggests an investigation to see whether other similar conjugated hydrocarbons manifest a similar boron‐substituted and H2 supplemented borane. That is, for a conjugated hydrocarbon structure with a neighbor‐paired resonance pattern, we propose to look at boranes where each carbon atom is replaced by a boron atom, and an H‐atom pair is added to each double bond of the resonance structure, with one H above the molecular plane and one below. This construction of concatenated diboranes is uniformly different than that for the previously known stable boranes of 4 or more B atoms. We find from quantum‐chemical computations that our so constructed polyboranes are stable. All this suggests a possible novel new chapter in borane chemistry – a chapter with some promise of understandings related to that for (alternant) conjugated hydrocarbons.

Hybrid Boron-Carbon Chemistry

“…However, this stability is increased by along the series for k = {2, 3}, and three {C=C ↔ B(H 2 )B} substitutions leads to a structure—D 3h cyclic hexaborane(12) B 6 H 12 —with a singlet-triplet energy gap which is only 8 kJ·mol −1 lower as compared to benzene. This result is striking and remains at the very origin of the recent proposal we have put forward [ 24 ]: To every (poly)cyclic planar conjugated hydrocarbon C n H m there corresponds a boron equivalent structure B n H m + n which is also an energy minimum.…”

“…The recent experimental isolation of borophane layers [ 18 ], a 2D (BH) 1 system structurally equivalent to graphene and the characterisation of chemical structures where one C=C double bond is substituted by one B(H 2 )B moiety [ 19 , 20 , 21 , 22 , 23 ] calls for the possibility of creating a new field of research within boron chemistry, namely, the synthesis of finite planar neutral borane molecules. We have recently proved that there is a one-to-one structural correspondence between any planar conjugated hydrocarbon C n H m and the planar borane B n H ( m + n ) , indeed with the same number of electrons and n more hydrogen atoms in the latter [ 24 ]. This transformation can be carried out by substituting all C=C double bonds by a perpendicular planar B(H 2 )B moiety, which is the central rhombus of diborane(6).…”

On the 3D → 2D Isomerization of Hexaborane(12)

“…In styx notation, s stands for number of bridge hydrogens, t for the number of two-electron three-center boron bonds, y for the number of two-center two-electron boron bonds, and x for the number of BH 2 groups. Reactant (C 2 ) and Product (D 3h ) [7,8] in the 3D → 2D hexaborane(12) isomerization correspond to isomers 4212 and 6030, respectively, and structure 5121 to an intermediate of the 3D → 2D isomerization, as shown below. Isomer 3303 lies 13 kJ•mol −1 above R (4212).…”

“…However, recent experiments [6] called our attention to the possibility of isolating planar finite borane molecules. Moreover, a one-to-one correspondence between any conjugated hydrocarbon C n H m and the structurally equivalent borane B n H m+n can be easily drawn [7]. For instance, benzene C 6 H 6 can be transformed into planar hexaborane(12) B 6 H 12 by substituting carbon atoms with boron atoms and every π two-electron bond with one perpendicular H 2 moiety at the mid-point of the former C=C bond; in other words, with three {C=C} → {BH 2 B} substitutions [7,8].…”

“…Moreover, a one-to-one correspondence between any conjugated hydrocarbon C n H m and the structurally equivalent borane B n H m+n can be easily drawn [7]. For instance, benzene C 6 H 6 can be transformed into planar hexaborane(12) B 6 H 12 by substituting carbon atoms with boron atoms and every π two-electron bond with one perpendicular H 2 moiety at the mid-point of the former C=C bond; in other words, with three {C=C} → {BH 2 B} substitutions [7,8]. Therefore, potential synthesis of planar borane molecules encompasses a new field of research within boron chemistry.…”