LIGAND DESIGN FOR COMPLEXATION IN AQUEOUS-SOLUTION .2. CHELATE RING SIZE AS A BASIS FOR CONTROL OF SIZE-BASED SELECTIVITY FOR METAL-IONS

The role of chelate ring size in complex stability, and selectivity based on metal ion size, is examined. Formation constants of pairs of ligands which differ in that one member of the pair forms a five-membered chelate ring where the other forms a six-membered chelate ring are reported with metal ions ranging in ionic radius from small (e.g. Be(II), Cu(II), Al(III)) to medium sized (e.g. Zn(II), In(III)) and large (e.g. La(III), Pb(II)). Pairs of ligands studied, and whose formation constants are combined with literature values already available, are as follows: 9-aneN2O (l-oxa-4,7-diazacyclononane) and 10-aneN2O (l-oxa-4,8-diazacyclodecane); 15-aneN4O (l-oxa-4,7,10,13-tetraazacyclopentadecane) and 16-aneN4O (l-oxa-4,7,11,14-tetraazacyclohexa-decane); TIRON (4,5-dihydroxy-1,3-benzenedisulfonate) and CTA (chromotropic acid, 4,5-dihydroxynaphthalene-2,7-disulfonate); TM-cyclen (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). Except for the pair of rigid small macrocycles 9-aneN2O and 10-aneN2O, chelate ring size dominates macrocyclic ring size in controlling metal ion size-based selectivity so that complexes of larger metal ions are destabilized relative to those of smaller metal ions by increase in chelate ring size from five to six membered. For the TIRON/CTA pair of ligands, rigidity introduced into the single chelate ring by aromatic rings gave sharper size selectivity than for pairs of flexible ligands such as oxalate/malonate. With high levels of steric crowding, as in TM-cyclen and TMC, size selectivity was still controlled by chelate ring size. The strain energy (U) of five- and six-membered chelate rings of the ethylenediamine (EN) and 1,3-diaminopropane (TN) type was calculated by molecular mechanics (MM) as a function of M-N bond length and N-M-N angle. The MM showed that minimum U occurred for the EN chelate ring when the M-N bond length was 2.5 Å and the N-M-N angle was 70°, while for the TN chelate ring U was minimum for M-N bond length 1.6 Å and N-M-N angle 109.5°. MM studies on alkali-metal ion complexes with crown ethers with different chelate ring sizes present showed that even with highly ionic M-L bonding the effect of chelate ring size on metal ion selectivity should be present. © 1990, American Chemical Society. All rights reserved.