Structural Chemistry of Borazines

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 perpendic...

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Bibliographic Details
Published in:European Journal of Inorganic Chemistry 2008-07, Vol.2008 (20), p.3186-3199
Main Authors: Anand, Benadir, Nöth, Heinrich, Schwenk-Kircher, Holger, Troll, Alexander
Format: Article
Language:English
Subjects:
Borazines
Ring distortion
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Summary: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) The BN bond lengths and the planar borazine ring are distorted in the solid state by substituents in three different ways: a) by retaining the planarity of the borazine ring but with substituents bent up and down thering, b) by substituents that induce different BN bond lengths and bond ring bond angles and c) by distortion of the borazine ring into a chair shaped or twisted ring.
ISSN:1434-1948
1099-0682
DOI:10.1002/ejic.200800169