Complexation of anions including nucleotide anions by open-chain host compounds with amide, urea, and aryl functions

A systematical evaluation of association constants between halide, phosphat e, and carboxylate anions with N-methylformamide (1) and the related bident ate receptors 2-6 (derived from, e.g., phthalic acid or ethylenediamine) in CDCl3 as solvent yielded increments of complexation free-energy Delta Delt a G for each single H-bond, which varied like, e.g., 5.1 kJ/mol (for Cl-), 4.0 kJ/mol (for Br-), 4.0 kJ/mol (for I-) (with values taken from Tables 1 and 2), in line with expected H-bond strength. The observed complexation in duced NH-NMR shift (CIS) values also showed a regular change, in the case o f 1 e.g., from 5.0 to 2.8 to 2.1 ppm (Table 1), with about half of these va lues with the bidentate ligands (Tables 2 and 3). Tridentate hosts led to a substantial binding increase, if strain-free convergence of all NH donor f unctions towards the anion was possible. The tris[urea] ligand 10 yielded, even in the polar solvent DMSO, with Cl- a Delta G of - 21.5 kJ/mol and wit h Br- of - 10.5 kJ/mol, whereas with I-, no association was detectable. The results demonstrated that small, inexpensive, and conformationally mobile host compounds can exhibit high affinities as well as descrimination with a nions, as much as more preorganized receptors do which require multistep sy nthesis. The corresponding adamantyl derivative 13 allowed measurements als o in CDCl3, with K = 4.3 . 10(4) M-1 for chloride (Table 7). Complexes with nucleotide anions were again particularly strong with the tridentate urea- based ligands, the latter being optimal ligands for chloride complexation. For the association of 10 with AMP(2-) and GMP(2-)in (D-6)DMSO, the associa tion constants were 3 . 10(4) M-1 (Table 8) and almost the: same as with Cl -. In the case of the urea derivatives 17 18, and 21, containing only one p henyl or pyrenyl substituent, however, the Delta G values decreased in the order A > C > T > G (e.g. - 13.6, - 11.6, - 7.6, - 10.5 kJ/mol in the case of 17 resp.; Table 8). In H2O, the pyrenyl-substituted urea derivatives all ow measurements with fluorescence, and, unexpectedly show only smaller nucl eobase discrimination, with constants around 3 . 10(3) M-1.