Diprotonated Sapphyrin: A Fluoride Selective Halide Anion Receptor

Prior solid-state structural evidence (Sessler, J. L.; Cyr, M. J.; Lynch, V.; McGhee, E.; Ibers, J. A. J. Am. Chem. Soc. 1990, 112, 2810-2813) served to indicate that the diprotonated form of 3,8,12,13,17,22-hexaethyl-2,7,18,23-tetramethylsapphyrin ([2·2H]²⁺), as its mixed fluoride-hexafluorophosphate salt, [1•2H•F]•PF₆, is capable of encapsulating fluoride anion within its ca. 5.5 Å diameter pentaaza macrocyclic core. The present report describes solution-phase studies related to these earlier solid-state ones. Optical titration studies, carried out in MeOH, reveal that fluoride anion is bound to [1•2H]²⁺ with an affinity constant, K_s, of ca. 1 × 10⁵ M^-1 whereas neither chloride nor bromide anion is complexed (K_s, ≤ 10² M^-1 for both). In dichloromethane, a large but incomplete (i.e. biphasic) decrease in the fluorescence lifetimes was observed for the dihydrochloride (τ_f = 0.88 (93%) and 4.8 (7%) ns) and dihydrobromide (τ_f = 0.31 (78%) and 4.3 (22%) ns) salts of [1•2H]²⁺ under conditions where the fluorescence lifetime of the dihydrofluoride salt (rf = 5.0 ns) remained high. From the fractional amplitudes involved, association constants of (1.8 ± 0.4) × 10⁷ and (1.5 ± 0.3) × 10⁶ M^-1 were derived respectively for the binding of Cl^- and Br^- to [1•2H]²⁺ in CH₂Cl₂; a lower bound for fluoride anion complexation, K_s, > 10⁸ M^-1, was also determined. In this same solvent, the fluoride salt of [1•2H]²⁺ also displayed both oxidation and reduction waves that were shifted to more negative values (by ca. 100 and 150 mV, respectively) as compared to those of the chloride and bromide salts. Also, in CD₂Cl₂, substantial upfield shifts and a significant concentration dependence on the observed position of the central pyrrolic NH signals are found for both the difluoride and mixed fluoride-hexafluorophosphate salts of [1•2H]²⁺ (e.g., for 1•2HF: δ(NH) ≈ -4.6, -5.8, -6.0 at 1 mM and -7.6, -8.6, -8.8 ppm at 30 mM) that are not seen in either the dihydrochloride or dihydrobromide salts (δ(NH) ≈ -4.2, -4.5, -4.8, and -4.6, -4.8, -5.1 ppm, respectively). Taken together, these data are consistent with a solution-state model in which a single fluoride anion may be complexed within the plane of the diprotonated sapphyrin core whereas both chloride and bromide anions would necessarily need be bound in an out-of-plane ion-pair-like fashion. Support for an extension of this model to the solid state was obtained from an X-ray structure of the dihydrochloride salt, 1• 2HCl, which revealed that the two chloride counteranions are complexed above and below the diprotonated sapphyrin core within hydrogen bonding distance of the nearest pyrrolic nitrogen centers.