Design of base metal extractants. Part 1. Inter-ligand hydrogen bonding in the assembly of pseudo-macrocyclic bis(aminosulfonamidato)M(ii) complexes

Abstract

Monosulfonyl derivatives of simple 1,2- and 1,3-diamines (R²HN–R–NHSO₂R¹ = L) have been shown to be easily deprotonated to give neutral 2 : 1 complexes, [M(L − H)₂], with Co(II), Ni(II), Cu(II) or Zn(II). The Ni(II) and Cu(II) complexes with deprotonated N-tosyl-1,2-diaminoethane have a planar N₄²⁻ donor set and a 14-membered pseudo-macrocyclic structure based on head-to-tail SO⋯H–N_(amine) bonding between the two bidentate ligands. In the related tetrahedral Zn(II) complex the ends of the mutually perpendicular bidentate N₂⁻ units are too far apart to form a cyclic H-bonded system. X-Ray structure determinations on five free ligands provide evidence for extensive inter-molecular H-bonding, which in the case of N-tosyl-1,3-diaminopropane and its N′-tert-butyl derivative involves formation of dimeric 16-membered pseudo-macrocycles. Despite favourable inter-ligand H-bonding in the neutral 2 : 1 complexes, these ligands are relatively weak extractants, showing >50% loading of Cu(II) in “pH-swing” equilibria, 2 L_(org) + M²⁺ = [M(L − H)₂]_(org) + 2 H⁺, only when the pH of the aqueous phase is raised above 4.