Anion Binding to a Ferric Porphyrin Complexed with Per-O-methylated β-Cyclodextrin in Aqueous Solution

5,10,15,20-Tetrakis(4-sulfonatophenyl)porphinato iron(III) (Fe(III)TPPS) forms a very stable 1:2 complex with heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TMe-β-CD), whose iron(III) center is located at a hydrophobic cleft formed by two face-to-face TMe-β-CD molecules. Various inorganic anions (X-) such as F-, Cl-, Br-, I-, N3-, and SCN- coordinate to Fe(III)TPPS(TMe-β-CD)2 to form five-coordinate high-spin Fe(III)TPPS(X)(TMe-β-CD)2, while no coordination occurs with ClO4-, H2PO4-, NO3-, and HSO4-. Except for F-, none of the anions investigated coordinate to Fe(III)TPPS in the absence of TMe-β-CD due to extensive hydration to the anions as well as to Fe(III)TPPS. The present system shows a high selectivity toward the N3- anion. The thermodynamics suggests that Lewis basicity, hydrophilicity, and shape of an X- anion are the main factors to determine the stability of the Fe(III)TPPS(X)(TMe-β-CD)2 complex.