Kontakt- und solvensgetrennte Ionenpaare lithiumorganischer Verbindungen im Festkörper : eine Festkörper-NMR-spektroskopische Studie

An aspect of general interest in organolithium chemistry is the structural alternative between contact and solvent-separated ion pairs (CIP vs SIP) because metal cations which are surrounded by an individual solvent cage are expected to show reactivities towards basic centers that differ from those of ions which are closely attached to carbanions. At the same time, the anionic center in a SIP is less shielded than in a CIP and thus expected to be more reactive. Although many organolithium compounds conserve their solution structure in the solid state, and numerous X-ray investigations have unravelled the solid state structure of contact and solvent-separated ion pairs, it is not possible to obtain suitable crystals for some systems. It was therefore of interest to extend 6,7Li NMR investigations of organolithium compounds to the study of ion pairs in order to evaluate the relation between solid-state NMR parameters and the structure more closely. In particular, the quadrupolar coupling constant χ, has been recognized as an important parameter related to bonding and structure. 7Li and 13C solid-state magic angle spinning NMR spectra were measured for different benzyllithium systems, stabilized by different ligands (THF, PMDTA, TMEDA, [12]crown-4). They were analyzed by the 7Li quadrupole parameters χ(6,7Li) and η(7Li), and further characterized by the chemical shifts δ (6,7Li) and δ (13C), respectively. On the basis of X-ray and solution NMR data from the literature, it is shown that solvent-separated ion pairs (SIP) can be distinguished from contact ion pairs (CIP) by the magnitude of χ(7Li) and typical shifts for δ (6,7Li) and δ (13C). All systems with two mol equivalents crown ether ligand exist exclusively as SIP structures and show χ(7Li) values of 15-30 kHz, whereas for CIP structures χ(7Li) values > 80 kHz were measured.