Protonated sapphyrins. Highly effective phosphate receptors

The phosphate anion chelation properties of several sapphyrin derivatives, namely 3,12,13,22-tetraethyl-8,17-bis[bis(hydroxyethyl)amino)carbonylethyl]-2,7,18,23-tetramethysapphyrin (1), 3,12,13,22-tetraethyl-2,7,18,23-tetramethyl-8,17-bis(hydroxypropyl)sapphyrin (2), and 3,8,12,13,17,22-hexaethyl-2,7,18,23-tetramethysapphyrin (3) are reported. X-ray diffraction studies of the dihydroxylated and decaalkyl derivatives 2 and 3 reveal that the diprotonated forms of sapphyrin are capable of stabilizing 1:2 inner-sphere complexes with phosphate-derived monoanions, such as diphenyl phosphate and monobasic phenyl phosphate. Similar analyses reveal that the diprotonated form of dihydroxysapphyrin 2 is capable of forming a 1:1 chelate complex in the solid state with either mono- or dibasic phosphoric acid. Solution-phase studies, involving H-1 and P-31 NMR spectroscopy, confirm that these same sapphyrins are capable of binding phosphate anions in organic solution, a conclusion that is supported by qualitative fast atom bombardment mass spectrometric (FAB MS) and extractive partition studies. In the case of phenylphosphonic acid and sapphyrin 2, extraction studies were consistent with 2:1 and 1:1 phosphate-to-sapphyrin binding stochiometries at pH 1.68 and 5.6, respectively. Similar studies using NMR and visible spectroscopy carried out with the water-soluble tetrahydroxy sapphyrin derivative, 1, and 2 indicate that these species bind phosphate anion in both methanolic and aqueous solution. Calculated association constants are on the order of 10(4) M(-1) in methanol and 10(2) M(-1) in 10 mM aqueous bis-Tris, pH 6.1.