Lokale Lithiumfluktuationen mit niedriger Aktivierungsenergie in zwei Festkörperphasen des Lithiumsilylamids [(Me2SiNtBuLi)2O]2

Abstract

AbstractLocal Lithium Fluctuations with low Activation Energies in two solid Phases of the Lithiumsilylamide [(Me2SiNtBuLi)2O]2The dilithium silylamide [(Me2SiNtBuLi)2O]2 shows in the temperature range between ambient temperature and 150 K three solid state phases with transition points at 253 K and 163 K which were characterised by means of X‐ray diffraction andsolid state NMR spectroscopy. In the high temperature phase the dimeric molecules of [(Me2SiNtBuLi)2O]2 crystallize in the tetragonal space group P42/nmc. It can be deducted from X‐ray diffraction studies that, at room temperature, the four lithium atoms in [(Me2SiNtBuLi)2O]2 are statistically distributed on two split positions at a distance of 0.89(1) Å. By cooling below 253 K the crystal transforms into an intermediate phase with space group P42/n. The symmetry of the molecule ist reduced from point symmetry 2m (D2d) to (S4). The split positions of the lithium atoms are found in different planes and their distance is slightly reduced from0.89 Å to 0.81 Å. Solid state NMR spectroscopic measurements and powder diffraction analysis reveal the existence of a monoclinic low temperature phase below 163 K. From 7Li MAS and 13C CP/MAS NMR spectroscopic measurements follows furthermore that the disorder of the lithium atoms in the two higher temperature phases (above 163 K) is due to a local dynamic exchange of the four lithium atoms over eight possible positions in the molecule. 7Li NMR measurements at variable MAS rates reveal a very small activation energy of 11 KJ/mol for the dynamic process with exchange rates of the lithium atoms in the order of 5 to 15 KHz. With respect to the lithium motions, the dynamics are intramolecular with an intermolecular coupling process due to van‐der‐Waals forces.