Neutral ligands for selective chloride anion complexation:: (α,α,α,α)-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 (alpha,alpha,alpha,alpha)-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 competitive 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 derivatives exhibited significant binding selectivity since they complexed with the spherical Cl- and Br- to a much greater extent than with the tetrahedral H2PO4- and HSO4- and the trigonal NO3- anions in DMSO-d(6). Indeed, the selectivity trend Cl- > Br- much greater than H2PO4- > HSO4- > NO3- is novel for any neutral urea-anion binding system. On the other hand, the corresponding metalloporphyrins exhibited a decrease in binding strength and selectivity in DMSO-d(6). The stoichiometry of binding for the anions and porphyrins was determined to be 1:1. The enthalpy of complexation was determined to be highly favorable and the entropy of complexation determined to be unfavorable from a variable-temperature H-1 NMR experiment with a 1:1 tetrabutylammonium bromide/porphyrin complex. X-ray crystallography revealed (alpha,alpha,alpha,alpha)-5,10,15,20-tetrakis(2-(4-chlorophenylurea)phenyl)porphyrin to be the first coordination complex of an anion (chloride and bromide) bound by a neutral free-base porphyrin. The binding motif consisted of the halide, buried deep within the porphyrin pocket, bound by two adjacent urea functional groups via four hydrogen bonds, with the two remaining urea functional groups involved in hydrogen bonding to solvent molecules. The crystal structure of the tetrabutylammonium halide-porphyrin complex showed additional Coulombic interaction between the electron-poor sulfur of a pocket-bound, hydrogen-bonded DMSO and halide anion.