Group 12 complexes of N,N′,N″,N‴-tetrakis(2-aminoethyl)-1,4,8,11-tetraazacyclotetradecane (L). Crystal structures of [Zn2(OH)L][CLO4]3 and [Cd2CI(L)][CF3SO3]3 together with formation constant determinations and pKa measurements for the aquated bimetallic species

Abstract

The complex-forming properties of the ligand N,N′,N″,N‴-tetrakis(2-aminoethyl)-1,4,8,11-tetraaza-cyclotetradecane (L) with zinc(II), cadmium(II) and mercury(II) have been investigated by synthesis, X-ray crystallography and potentiometric titration. With zinc(II) in aqueous or aqueous methanol solution the only complexes which could be isolated were salts of the µ-hydroxo-bridged bimetallic cation [Zn₂(OH)(L)]³⁺, although potentiometric titration data suggest the presence of small amounts of [Zn(H₃L)]⁵⁺ at low pH. Crystallographic data for [Zn₂(OH)(L)]³⁺ indicate that each 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 µ-hydroxo group could not be replaced, in attempted substitution reactions, by other potentially bridging anions. The pK_a for the aqua species which leads to the µ-hydroxo species is <5.3. Complexation with cadmium(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 ¹³C NMR chemical shift for the carbon atoms β to nitrogen. In the bimetallic complexes the tendency for µ-hydroxo formation is lower than for zinc(II) and it is possible to isolate [Cd₂L]⁴⁺ to which Cl^– can be added forming [Cd₂Cl(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 potentiometric 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(II), nickel(II) and copper(II) aquated species were measured, for comparison, and found to be 7.3, 9.0 and 7.7, respectively.