Structural Chemistry of Borazines

Abstract

AbstractThe 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)