Stickstoffverbindungen von Elementen der dritten Hauptgruppe mit intra- und intermolekularen Donor-Akzeptor-Bindungen. - III : Dynamische Phänomene und Lewis-Säure-Stärke

Abstract

Compounds of the general formula tBuN(H)-SiMe2-N(tBu)ElX2 (El=B, X=Me (1m), F (1f), Cl (1c); El=Al, X=Me (2m), Cl (2c), Br (2b); El=Ga, X=Me (3m), Cl (3c), Br (3b); El=In, X=Me (4m), Cl (4c), Br (4b); El=Tl, X=Me (5m)) and tBuN(D)-SiMe2-N(tBu)ElMe2 (El=Al (2d), Ga (3d), In (4d)) have been characterized by IR and temperature dependent 1H NMR techniques with respect to their structural changes in solution. A reversible, dynamic, intromolecular equilibrium between two enantiomeric four-membered rings has been establisehd to be responsible for the changes in the spectra. The cyclic form B1, which arises from Lewis acid base interaction of the tert-butyl-amino with the ElX2 group, transforms to the mirror-image B2 via an acyclic intermediate, in which reorientation of the groups by rotation or inversion can take place. In the boron derivatives (1m, 1c, and 1f) no comparable N-B interaction can be detected. The Lewis acidity of the ElX2 group deduced from NH wave numbers increases within the following series: TlMe2<GaMe2?InMe2<AlMe2<GaBr2 (approximately-equal) InBr2<GaCl2 (approximately-equal) AlBr2 (approximately-equal) InCl2<AlCl2. Intramolecular hydrogen transfer from one nitrogen atom to the other can be excluded by comparison of the rate constant kH vs. kD. The free enthalpy of activation delta G (unequal) depends on the acidity of ElX2, the steric requirements and the relative masses of X. The synthesis and characterisation of 1f, 2b, 3b, 4b, 2d, 3d, 4d are described.