Ion-pair extraction of metalloporphyrins into a mixture of acetonitrile with water

Abstract

Extraction mechanism and solvent structure were studied for the solvent extraction of cationic porphyrin (5,10,15,20-tetrakis(1-methyl-pyridinium-4-yl) porphyrin; H2(tmpyp)4+) and its metalloporphyrins (MP(n+2)+) into an acetonitrile phase separated from a 1:1 (v/v) mixture of acetonitrile with water by addition of sodium chloride. Mn+ denotes Cu2+, Co3+, Fe3+, Li+, Mn3+, and Zn2+ and H2P4+ is the free base form of H2tmpyp4+. The separated acetonitrile phase contains a lot of water (more than 4 mol dm-3) and sodium chloride (1 × 10-2 mol dm-3) that are necessary to extract the highly charged chemical species of the above porphyrin or metalloporphyrins which can not be extracted into normal organic solvents such as chloroform. The extracted chemical species dissociate in the acetonitrile phase and exist in ionized forms. X-ray diffraction analysis indicates the formation of clusters of acetonitrile in the mixed-aqueous organic solvent. An acetonitrile molecule interacts with 2 neighbors at 3.45Å in antiparallel and 4.10 Å in parallel through dipole-dipole interaction. Water molecules that form hydrogen bonding between water molecules surround the acetonitrile clusters. The porphyrin or metalloporphyrins are preferentially solvated by acetonitrile molecules and are surrounded by acetonitrile clusters.