Dynamic 7Li NMR investigation of dianionic dibenzotetraaza[14]annulene derivatives in solution

Abstract

Deprotonation of the two dibenzotetraaza[14]annulenes ( 1, H 2L, 6, 8, 15, 17-tetramethyldibenzotetraaza[14]annulene and 6, 15-dimethyl-8, 17-diphenyldibenzotetraaza[14]annulene) by LiCH 3 in ether yielded the corresponding dilithium dibenzotetraaza[14]annulene complexes 2 (Li 2L × n ether) as solvates. These extremely moisture-sensitive compounds have been isolated as deep red solids and characterized by 1H, 13C, variable-temperature 7Li NMR and UV spectroscopies. Based on solution NMR investigations both complexes contain two different Li + environments: one Li + ion is coordinated to the nitrogen centres of the dianionic annulene ([LiL] −), while the second Li + ion is coordinated by ether oxygen atoms and weakly to nitrogen centres of the dianionic annulene. Variable-temperature 7Li NMR spectroscopy between 213 and 300 K indicates a rapid exchange process ( E A ca 47±3 kJ mol −1) between the two types of Li + environments. In the presence of TMEDA (N,N′-tetramethylethylenediamine), the exchange process is slowed ( E A ca 59±5 kJ mol −1) due to stronger coordination of the TMEDA-solvated Li +. These results are discussed with respect to the known coordination behaviour of the Li + ion towards macrocyclic nitrogen donors.