Ligand design and metal-ion recognition. Comparison of the interaction of cobalt(II) and nickel(II) with 16- to 19-membered mixed-donor macrocycles

Abstract

The interaction of cobalt(II) and nickel(II) with a range of 16- to 19-membered ring macrocycles incorporating nitrogen, oxygen and/or sulfur heteroatoms is reported. These ligands constitute an extensive array of related macrocyclic structures in which the positions of the donor atoms, their spacing, and the macrocyclic ring size all vary in a systematic manner. Emphasis has been given to the examination of structure–function relationships in the complexation behaviour. Physical measurements confirm the 1 : 1 metal to macrocyclic ligand stoichiometry of the respective complexes. Stability constants for the metal complexes have been determined potentiometrically in 95% methanol (I= 0.1 mol dm^–3, NEt₄ClO₄). An X-ray crystallographic study of [NiL¹⁸(H₂O)][NO₃]₂(L¹⁸= 5,6,7,8,10,11,12,13,19,20-decahydrodibenzo[f,I][1,8,11,4,15]oxadithiadiazacycloheptadecine) confirms that the nickel ion is six-co-ordinate with the complex cation exhibiting a distorted-octahedral geometry defined by all five donor atoms of the ON₂S₂ macrocycle and a water molecule; the macrocyclic backbone incorporating the N–O–N donor fragment is arranged meridionally. Molecular mechanics modelling of selected nickel(II) complexes has also been undertaken. As well as their considerable intrinsic interest, the results provide a potentially useful background upon which the design of new reagents for metal-ion discrimination may be based.