Halide-anion binding by singly and doubly N-confused porphyrins

The halide-binding properties of N-confused porphyrin (NCP, 1) and doubly N-confused porphyrins (trans-N₂CP (2), cis-N₂CP (3)) were examined in CH₂Cl₂. In the free-base forms, cis-N ₂CP (3) showed the highest affinity to each anion (Cl^-, Br^-, I^-) with association constants K_a = 7.8 × 10³, 1.9 × 10³, and 5.8 × 10 ² M^-1, respectively. As metal complexes, on the other hand, trans-N²CP 2-Cu exhibited the highest affinity to Cl ^-, Br^-, and I^- with K_a = 9.0 × 10⁴, 2.7 × 10⁴, and 1.9 × 10³ M^-1, respectively. The corresponding K_a values for cis-N₂CP 3-Cu and NCP 1-Cu were about 1/10 and 1/2, respectively, of those of 2-Cu. With the help of density functional theory (DFT) calculations and complementary affinity measurements of a series of trisubstituted N-confused porphyrins, the efficient anion binding of NCPs was attributed to strong hydrogen bonding at the highly polarized NH moieties owing to the electron-deficient C₆F₅ groups at meso positions as well as the ideally oriented dipole moments and large molecular polarizability. The orientation and magnitude of the dipole moments in NCPs were suggested to be important factors in the differentiation of the affinity for anions.