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-tetramethylsapphyrin
(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-tetramethylsapphyrin
(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 1H and 31P 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 104
M-1 in
methanol and 102 M-1 in 10 mM
aqueous bis-Tris, pH 6.1.