GROUP 12 COMPLEXES OF IS(2-AMINOETHYL)-1,4,8,11-TETRAAZACYCLOTETRADECANE (L) - CRYSTAL-STRUCTURES OF [ZN2(OH)L][CLO4]3 AND [CD2CL(L)][CF3SO3]3 TOGETHER WITH FORMATION CONSTANT DETERMINATIONS AND PKA MEASUREMENTS FOR THE AQUATED BIMETALLIC SPECIES

The complex-forming properties of the ligand is(2-aminoethyl)-1,4,8,11 -tetraazacyclotetradecane (L) with zinc(II), cadmium(II) and mercury(I I) have been investigated by synthesis, X-ray crystallography and pote ntiometric titration. With zinc(II) in aqueous or aqueous methanol sol ution the only complexes which could be isolated were salts of the mu- hydroxo-bridged bimetallic cation [Zn2(OH)(L)]3+, although potentiomet ric titration data suggest the presence of small amounts of [Zn(H3L)]5 + at low pH. Crystallographic data for [Zn2(OH)(L)]3+ indicate that ea ch zinc(II) ion is five-co-ordinate, being bound by a pair of tertiary amines, a pair of primary amines and the bridging hydroxide. The mu-h ydroxo group could not be replaced, in attempted substitution reaction s, by other potentially bridging anions. The pK(a) for the aqua specie s which leads to the mu-hydroxo species is < 5.3. Complexation with ca dmium(II) and mercury(II) leads to both mono- and bi-metallic species. The monometallic species are the first isolated and can be recognised spectroscopically by the relatively high-field C-13 NMR chemical shif t for the carbon atoms beta to nitrogen. In the bimetallic complexes t he tendency for mu-hydroxo formation is lower than for zinc(II) and it is possible to isolate [Cd2L]4+ to which Cl- can be added forming [Cd 2Cl(L)]3+. X-Ray crystallography indicates a structure for [Cd2Cl(L)]3 + which is qualitatively similar to that of [Zn2(OH)(L)]3+. In aqueous solution the species [Cd2(OH)(L)]3+ can be detected from potentiometr ic titration data and forms in response to a pK(a) value of < 6.6 for the aquated precursor. The pK(a) values for the corresponding cobalt(I I), nickel(II) and copper(II) aquated species were measured, for compa rison, and found to be 7.3, 9.0 and 7.7, respectively.