Covalently Bonded Fluorine as a σ-Donor for Groups I and II Metal Ions in Partially Fluorinated Macrocycles

Novel fluoro cryptands and fluoro crown ethers were synthesized in good yields in the reactions of 1,3-bis(bromomethyl)-2-fluorobenzene with diaza-18-crown-6 [the resulting two macrocycles are abbreviated as FN 2 O 4 (1 + 1 product) and (FN 2 O 4 ) 2 (2 + 2 product)], diaza-15-crown-5 [product abbreviated as FN 2 O 3 ], aza-(3 n )-crown-( n ) ( n = 4, 5, 6) [ F(NO 3 ) 2, F(NO 4 ) 2, F(NO 5 ) 2 ], and the potassium salts of tetraethyleneglycol [ FO 5 ] or pentaethylene glycol [ FO 6 ] and in the reaction of 1-(bromomethyl)-3-methyl-2-fluorobenzene with aza-15-crown-5 [ FNO 4 ]. The analogous reactions of 1,3-bis(bromomethyl)benzene with diaza-18-crown-6 [ HN 2 O 4 ], diaza-15-crown-5 [ HN 2 O 3 ], and the potassium salt of tetraethylene glycol [ HO 5 ] led to the isolation of closely related fluorine-free cryptands and crown ethers. The 19 F NMR resonances of the fluoro macrocycles shift upon complexation of metal ions by between Δδ = −18.2 ppm and Δδ = +8.5 ppm relative to those of the respective free ligands. To explain the unexpected high-field shifts Δδ of most metal complexes, a qualitative model is proposed which considers conformative rearrangements of the oxyethylene chain to be mainly responsible for the Δδ values, with the deshielding effect of the positive charge of the metal ions being smaller in most cases. It was demonstrated that the 1 J CF coupling constants can be used to gauge the strength of metal−fluorine interactions since the values for 1 J CF range from 253 Hz ( FN 2 O 4 ) to 238.5 Hz (Li + · FN 2 O 4 ), 242 Hz (Na + · FN 2 O 4 ), 246.5 Hz (K + · FN 2 O 4 ), and 249 Hz (Rb + · FN 2 O 4 ). The larger the metal ion the less it is able to migrate into the cavity and contact the C−F unit. The stability constants of the metal complexes of the fluoro macrocycles and their fluorine free relatives were determined by picrate extraction experiments and NMR competition experiments. It is evident from both investigations that the fluorine-containing macrocycles form more stable metal complexes than the analogous fluorine free systems. This is taken as a proof for C−F to metal ion σ-donor bonds. It is significant that this stabilizing effect is only observed when the radii of the metal ion and macrocyclic cavity are complementary, since only then can the metal ion contact the C−F unit. The crystal structures of FN 2 O 3 and the metal complexes Li + · FN 2 O 3, Li + · HN 2 O 3, Na + · FN 2 O 3, Na + · FO 5, and Na + · FNO 4 were determined. In the solid state structures of all metal complexes with the exception of Na + · FNO 4, short metal−fluorine distances are observed, which are approximately as long as the respective metal−oxygen distances and much shorter than the metal−N distances [Li + · FN 2 O 3: Li + −F, 203.5(5) pm; Li + −O, 193.6(5), 201.3(5), 202.1(5) pm; Li + −N, 239.4(5), 255.5(5) pm], [Na + · FO 5: Na + −F, 237.4(5) pm], [Na + · FN 2 O 3: Na + −F, 257.5(2) pm]. A comparison of the structures of Li + · FN 2 O 3 and Li + · HN 2 O 3 illustrates the importance of fluorine coordination since this interaction is required to position the metal ion within the center of the cavity, whereas in Li + · HN 2 O 3 the cation is located on the outer edge of the cavity, completing its coordination sphere with one additional molecule of water.