Structural Chemistry of Borazines

Abstract

The solid-state structure of (HN=BF)₃ has been redetermined. It is characterised by a stacking of the molecules, similar to hexagonal boron nitride. In contrast, (F₃CH₂N=BF)₃ shows no intermolecular interactions between the planar borazine rings. In (Cl₂BN=BCl)₃, the Cl₂B groups are almost perpendicularly oriented to the planar borazine ring and its B–Cl bonds are shorter than the Cl–B bonds to the ring boron atoms. Reactions of (Cl₂BN=BCl)₃ with Me₃SiNMe₂ allows a successive Cl/Me₂N exchange. Depending on the molar ratio, the compounds [Cl(Me₂N)BN=BCl]₃, [(Me₂N)₂BN=BCl]₃ and [(Me₂N)₂BN=BNMe₂]₃ are obtained. The latter two react with CH₂Cl₂ by B–N bond cleavage of one ring boron bonded Me₂N group with formation of an N–H bond. These molecules not only possess a strongly distorted BN hexagon with short B–NH bonds but also a nonplanar borazine ring. The nonplanarity of the borazine ring is even more pronounced in [Me₃SiN=BCl]₃. A planar borazine ring is observed for (MeN=BBr)₃. Its molecules are ordered into stacks by translation with borazine planes 4.2 Å apart from each other. Characteristic of the structure are close contacts between neighbouring bromine atoms. Due to the steric demand of pentafluorophenyl groups, the borazine ring of (F₅C₆N=BBr)₃ forms no stacks because one of the three F₅C₆ rings stands almost perpendicular to the borazine plane while the other two are twisted by 70°. Aminolysis of (F₅C₆N=BBr)₃ yields (F₅C₆N=BNH₂)₃ with NH₂ groups that are coplanar with the borazine ring. The phenyl groups of (HN=BPh)₃ are twisted by only 30–40° relative to the borazine ring. A threefold crystallographic axis stands perpendicular to the ring plane. The sixfold crystallographic axis of the unit cell generates a channel structure with an internal diameter of 4.5 Å. The two isomeric borazines, (iPrN=BMe)₃ and (MeN=BiPr)₃ are structurally rather similar although the first of these has longer B–N bonds due to the shorter N–C bonds. Moreover, the B–N–B and N–B–N bond angles are slightly different.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)