13C NMR longitudinal relaxation times (T1s) have been determined for 15-crown-5, 18-crown-6, and several C-and N-pivot lariat ethers in the absence and presence of Na+, K+, and Ca2+ cations. The C-pivot compounds are 2-substituted 15-crown-5 derivatives as follows, in which Me = CH3 and E = CH2CH2: 1, CH2O-n-Pr; 2, CH2OEOMe; 3, CH2OC6H4-2-OMe; 4, CH2OC6H4-4-OMe. The N-substituted monoaza-15-crown-5 derivatives are as follows: 5, Me; 6, n-Bu; 7, EOMe; 8, EOEOMe. The N-substituted monoaza-18-crown-6 derivatives are as follows: 9, EOMe; 10, EOEOMe. The relaxation time data are used to assess structural changes which accompany cation complexation, the degree of macroring vs. sidearm participation, and the effect of cation charge on the complexation process. In general, the C-pivot lariat ethers (1-4) appear to rely more strongly on the macroring for complexation of cations than do the N-pivot compounds (7-10) except when donor groups are absent from the sidearm (5, 6). For N-pivot lariat ethers 8 and 9, the result of Ca2+ complexation is a rigid, cryptate-like structure in solution, suggesting that Ca2+ plays a very strong organizing role on the ligand structure.
ASJC Scopus subject areas
- Colloid and Surface Chemistry