NEUTRAL LIGANDS FOR SELECTIVE CHLORIDE ANION COMPLEXATION - -5,10,15,20-TETRAKIS(2-(ARYLUREA)PHENYL)PORPHYRINS

A series of neutral, urea-appended, free-base porphyrins and their Zn( II) complexes have been synthesized and characterized. The -5,10,15,20 -tetrakis(2-(arylurea)phenyl)porphyrins bind strongly (K (M-1) > 10(3) -10(5)) to chloride anion in DMSO-d(6) and also in the more competitiv e solvent system DMSO-d(6)/D2O (88:12, v/v) and bromide anion in DMSO- d(6), as revealed by H-1 NMR titration studies. The porphyrin derivati ves exhibited significant binding selectivity since they complexed wit h the spherical Cl- and Br- to a much greater extent than with the tet rahedral H2PO4- and HSO4- and the trigonal NO3- anions in DMSO-d(6). I ndeed, the selectivity trend Cl- > Br- much greater than H2PO4- > HSO4 - > NO3- is novel for any neutral urea-anion binding system. On the ot her hand, the corresponding metalloporphyrins exhibited a decrease in binding strength and selectivity in DMSO-d(6). The stoichiometry of bi nding for the anions and porphyrins was determined to be 1:1. The enth alpy of complexation was determined to be highly favorable and the ent ropy of complexation determined to be unfavorable from a variable-temp erature H-1 NMR experiment with a 1:1 tetrabutylammonium bromide/porph yrin complex. X-ray crystallography revealed 20-tetrakis(2-(4-chloroph enylurea)phenyl)porphyrin to be the first coordination complex of an a nion (chloride and bromide) bound by a neutral free-base porphyrin. Th e binding motif consisted of the halide, buried deep within the porphy rin pocket, bound by two adjacent urea functional groups via four hydr ogen bonds, with the two remaining urea functional groups involved in hydrogen bonding to solvent molecules. The crystal structure of the te trabutylammonium halide-porphyrin complex showed additional Coulombic interaction between the electron-poor sulfur of a pocket-bound, hydrog en-bonded DMSO and halide anion.