Metal-ion selectivity by macrocyclic ligands. Part 2. The interaction of ZnII, CdII and HgII with pyridinyl-derived N3O2 macrocycles; X-Ray structures of one zinc and two cadmium endomacrocyclic complexes and of one zinc and one mercury exomacrocyclic complex

Abstract

The interaction of Zn^II, Cd^II and Hg^II with the macrocycles L¹ and L², each containing an N₃O₂-donor set, has been investigated. Conductometric titration of the 1:1 complexes of Zn^II and Cd^II with chloride in each case indicated the sequential formation of a 1:1 electrolyte of the type [ML(Cl)]⁺ and a 2:1 electrolyte of the type [MLCl₂] in each case. Proton NMR titration of the ligands with M(O₂CMe)₂ in CD₃OD gave evidence for the formation of 1:1 and 1:2 (metal : ligand) species. The stability constants of the complexes have been determined in 95% methanol. The X-ray crystal structures of the 1:1 complexes [ZnL¹I₂]·H₂O, [ZnL¹(NO₃)]NO₃, [CdL¹(NO₃)(MeOH)]NO₃, [CdL²-(ClO₄)(MeCN)]ClO₄ and [HgL¹I₂] have been determined. The metal atoms in [ZnL¹I₂]·H₂O and [HgL¹I₂] are four-co-ordinated in a tetrahedral geometry and lie outside the macrocyclic cavity. The donor set in each molecule comprises the two secondary amine nitrogen atoms from the macrocycle and the two iodide anions. The complex [ZnL¹(NO₃)]NO₃ has all donors of the macrocycle co-ordinated with one oxygen from a nitrate anion occupying the sixth site to yield an approximately trigonal-prismatic geometry. The structure of [CdL¹(NO₃)(MeOH)]NO₃ shows that the metal is seven-co-ordinated in an approximately pentagonal-bipyramidal environment consisting of the full donor set of the macrocycle together with one oxygen each from a nitrate anion and a methanol of solvation. The structure of [CdL²(ClO₄)(MeCN)]ClO₄ shows that the metal has a similar co-ordination environment consisting of the full donor set of the macrocycle together with an oxygen from a perchlorate anion and a nitrogen from acetonitrile of solvation.