COVALENTLY BONDED FLUORINE AS A SIGMA-DONOR FOR GROUP-I AND GROUP-II METAL-IONS IN PARTIALLY FLUORINATED MACROCYCLES

Novel fluoro cryptands and fluoro crown ethers were synthesized in goo d yields in the reactions of 1,3-bis(bromomethyl)-2-fluorobenzene with diaza-18-crown-6 [the resulting two macrocycles are abbreviated as FN 2O4 (1 + 1 product) and (FN2O4)(2) (2 + 2 product)], diaza-15-crown-5 [product abbreviated as FN2O3], aza-(3n)-crown-(n) (n = 4, 5, 6) [F(NO 3)(2), F(NO4)(2), F(NO5)(2)], and the potassium salts of tetraethylene glycol [FO5] or pentaethylene glycol [FO6] and in the reaction of 1-(b romomethyl)-3-methyl-2-fluorobenzene with aza-15-crown-5 [FNO4]. The a nalogous reactions of 1,3-bis(bromomethyl)benzene with diaza-18-crown- 6 [HN2O4], diaza-15-crown-5 [HN2O3], and the potassium salt of tetraet hylene glycol [HO5] led to the isolation of closely related fluorine-f ree cryptands and crown ethers. The F-19 NMR resonances of the fluoro macrocycles shift upon complexation of metal ions by between Delta del ta = -18.2 ppm and Delta delta = +8.5 ppm relative to those of the res pective free ligands. To explain the unexpected high-field shifts Delt a delta of most metal complexes, a qualitative model is proposed which considers conformative rearrangements of the oxyethylene chain to be mainly responsible for the Delta delta values, with the deshielding ef fect of the positive charge of the metal ions being smaller in most ca ses. It was demonstrated that the (I)J(CF) coupling constants can be u sed to gauge the strength of metal-fluorine interactions since the val ues for (I)J(CF) range from 253 Hz (FN2O4) to 238.5 Hz (Li+. FN2O4), 2 42 Hz (Na+. FN2O4), 246.5 Hz (K+. FN2O4), and 249 Hz (Rb+. FN2O4). The larger the metal ion the less it is able to migrate into the cavity a nd contact the C-F unit. The stability constants of the metal complexe s of the fluoro macrocycles and their fluorine free relatives were det ermined by picrate extraction experiments and NMR competition experime nts. It is evident from both investigations that the fluorine-containi ng macrocycles form more stable metal complexes than the analogous flu orine free systems. This is taken as a proof for C-F to metal ion sigm a-donor bonds. It is significant that this stabilizing effect is only observed when the radii of the metal ion and macrocyclic cavity are co mplementary, since only then can the metal ion contact the C-F unit. T he crystal structures of FN2O3 and the metal complexes Li+. FN2O3, Li. HN2O3, Na+. FN2O3, Na+. FO5, and Na+. FNO4 were determined. In the s olid state structures of all metal complexes with the exception of Na. FNO4, short metalfluorine distances are observed, which are approxim ately as long as the respective metal-oxygen distances and much shorte r than the metal-N distances [Li+. FN2O3: Li+-F, 203.5(5) pm; Li+-O, 1 93.6(5), 201.3(5), 202.1(5) pm; Li+-N, 239.4(5), 255.5(5) pm], [Na+. F OS: Na+-F, 237.4(5) pm], [Na+ FN2O3: Na+-F, 257.5(2) pm]. A comparison of the structures of Li+. FN2O3 and Li+. HN2O3 illustrates the import ance of fluorine coordination since this interaction is required to po sition the metal ion within the center of the cavity, whereas in Li+. HN2O3 the cation is located on the outer edge of the cavity, completin g its coordination sphere with one additional molecule of water.