Metal-ion selectivity by macrocyclic ligands. Part 1. The interaction of NiII and CuII with pyridinyl-derived N3O2 macrocycles; the X-ray structures of a free macrocycle, its endomacrocyclic complexes of NiII and CuII and an exomacrocyclic nickel(II) complex

Abstract

The interaction of Cu^II and Ni^II with two macrocycles L¹ and L², each containing an N₃O₂ donor set, has been investigated. Spectrophotometric studies in dimethyl sulphoxide reveal the formation of complexes with 1 : 1 and 1 : 2 metal : ligand ratios and conductometric studies in the same solvent indicated that all of the complexes were 1 : 2 electrolytes. Conductometric titration of the 1 : 1 copper(II) complexes with chloride in each case indicated the formation of a 1 : 1 electrolyte, presumably through co-ordination of a chloride ion to the central copper of each complex. Similar titration of the nickel(II) complexes gave evidence for the formation of dinuclear species; each of these was postulated to contain a bridging chloride anion. The stability constants of the complexes together with their enthalpies of co-ordination have been determined in 95% methanol. Extraction and related transport experiments were carried out and under the conditions employed Cu^II was favoured over Ni^II. The X-ray crystal structures of the free macrocycle L¹, the 1 : 1 complexes [CuL¹(H₂O)][ClO₄]₂ and [NiL¹(I)]I·MeOH, and the 1 : 2 complex [NiL¹₂(NO₃)]NO₃·2MeOH have been determined. The metal in [CuL¹(H₂O)][ClO₄]₂ is six-co-ordinate and lies within the folded macrocyclic cavity. The donor set comprises the five macrocyclic donor atoms and a water molecule. Overall, the co-ordination sphere corresponds to a restricted tetragonal rhombic arrangement. The complex [NiL¹(I)]I·MeOH also has all donors of the macrocycle co-ordinated with an iodide anion occupying the sixth site to yield a distorted-octahedral geometry. The structure of [NiL¹₂(NO₃)]NO₃·2MeOH shows that the nickel is again six-co-ordinated, to the aliphatic nitrogen atoms from two macrocycles (each showing exo co-ordination) and a bidentate nitrate anion.