METAL INTERCHANGE OF CROWN-ETHER ALKALI-METAL CATION COMPLEXES IN SOLUTION - LI-7 NUCLEAR-MAGNETIC-RESONANCE STUDY OF THE EXCHANGE KINETICS OF LITHIUM 15-CROWN-5 AND LITHIUM MONOBENZO-15-CROWN-5 IN NITROMETHANE

The exchange kinetics of (Li:15-crown-5)+ and (Li:benzo-15-crown-5)+ in nitromethane were studied by Li-7 NMR. The lithium cation exchanges between two sites in solution: solvated and 1:1 complexed with the crown ether. The chemical exchange between these two lithium sites was determined to proceed via a metal interchange mechanism, I(M), in both cases, characterized by the following activation parameters: DELTA-H double-ended dagger = 21 +/- 1 and 32 +/- 1 kJ mol-1, DELTA-S double-ended dagger = -54 +/- 3 and -36 +/- 1 J mol K-1 for (Li:15C5)+ and (Li:B15C5)+, respectively. This interchange mechanism was predominant in the temperature range 263.0-300.0 K. The main factors controlling the exchange rate are conformational rearrangements of the ligand during the concerted partial decomplexation of a lithium cation and partial complexation of a second one.