Synthesis and characterization of water-operative cationic and anionic metal-ion activated molecular receptors for aromatic anions.
Two new, octadentate, water-soluble, macrocyclic ligands, 1,4,7,10-tetrakis((2S)-(-)-2-hydroxy-3-[3'-(N,N,N-trimethylammonium)-phenoxy]-propyl)-1,4,7,10-tetraazacyclododecane tetratriflate, ((S)-tmappc12 triflate, L1 triflate) and 1,4,7,10-tetrakis((2S)-(-)-2-hydroxy-3-[2'-sulfo-4'-methylphenoxy]-propyl)-1,4,7,10-tetraazacyclododecane, ((S)-sthmppc12, L2H4) have been prepared with a view to using them to study anion sequestration in aqueous solution. Their pKa and metal-ion binding constant values with a range of alkaline earth, transition, and post-transition metals are reported. The eight-coordinate, water-soluble Cd(II) complexes of (L1)4+ and (L2)4-, [CdL1](CF3SO3)6 and (NH4)2-[CdL2], the former cationic and the latter anionic, have both been shown to be capable of acting as anion receptors in aqueous solution. The binding constant values (log(K/M-1) given in parentheses) for binding by the cationic receptor to a range of aromatic anions in water are p-nitrophenolate (1.7), p-formylphenolate (2.1), p-nitrobenzoate (3.0), p-aminobenzoate (4.5), p-dimethylaminobenzoate (>4.5), D- and L-tryptophanate (1.6, 2.2), phenoxyacetate (2.1), and acetate (2.3). With the anionic receptor, nonzero binding constants were only measurable for p-nitrobenzoate (approximately 0.4), p-aminobenzoate (2.0), and p-dimethylaminobenzoate (1.8). By reference to the X-ray determined structures of related, but water-insoluble inclusion complexes, anion retention is thought to occur within a hydrophobic cavity, with four convergent hydroxy groups at its base, which develops in (L1)4+ and (L2)4- through the juxtapositioning of aromatic rings that occurs as a consequence of octadentate coordination.