EQUILIBRIUM STUDIES OF SELENITE AND SELENATE IONS WITH THE RECEPTOR COMPLEX FORMED BY CU2-HEXAAZA-10,22-DIOXACYCLOTETRACOSANE (OBISDIEN)( AND 1,4,7,13,16,19)

The complexation of selenite (SeO32-) and selenate (SeO32-) ions in th e cavity of the macrocyclic ligand OBISDIEN was studied in the absence and presence of Cu2+ ions and in the presence of bromide ions. Potent iometric equilibrium determinations allowed us to determine the specie s formed in the absence and presence of Cu(II) ions for the systems st udied in the p[H] range 2-12. In the absence of metal ions the strengt h of binding of SeO42- increases from the monoprotonated to hexaproton ated species of OBISDIEN. The reverse effect occurs for the HSeO3- ion , with the strength of binding decreasing from the monoprotonated to t he most protonated species. In the OBISDIEN:Cu2+:Br- system, the bromi de ions contribute to the stabilisation of binuclear complexes. A simi lar role is played by selenite anions in the OBISDIEN:Cu2+:Br- :SeO32- systems. All SeO42- ions are complexed in the OBISDIEN:Cu2+:SeO32-:Se O42-:Br(-)1:2:1:1:6 system; in the p[H] range 2-12, selenite and bromi de ions are mostly free. At p[H] 8.1 which is the p[H] of sea water, i n the absence of other complexing agents, all the selenite and bromide ions are free while the selenate ions are complexed, (mu-hydroxo) (mu -selenate) dicopper(II)-OBISDIEN being the major species present.