Synthetic, structural, electrochemical and solvent extraction studies of neutral trinuclear Co(ii), Ni(ii), Cu(ii) and Zn(ii) metallocycles and tetrahedral tetranuclear Fe(iii) species incorporating 1,4-aryl-linked bis-β-diketonato ligands

Abstract

Uncharged complexes, formulated as trimeric metallocycles of type [M₃(L¹)₃(Py)₆] (where M = cobalt(II), nickel(II) and zinc(II) and L¹ is the doubly deprotonated form of a 1,4-phenylene linked bis-β-diketone ligand of type 1,4-bis(RC(O)CH₂C(O))C₆H₄ (R = t-Bu)) have been synthesised, adding to related, previously reported complexes of these metals with L¹ (R = Ph) and copper(II) with L¹ (R = Me, Et, Pr, t-Bu, Ph). New lipophilic ligand derivatives with R = hexyl, octyl or nonyl were also prepared for use in solvent extraction experiments. The X-ray structures of H₂L¹ (R = t-Bu) and of its trinuclear (triangular) nickel(II) complex [Ni₃(L¹)₃(Py)₆]·3.5Py (R = t-Bu) are also presented. Electrochemical studies of H₂L¹, [Co₃(L¹)₃(Py)₆], [Ni₃(L¹)₃(Py)₆], [Cu₃(L¹)₃], [Zn₃(L¹)₃(Py)₆] and [Fe₄(L¹)₆] (all with R = t-Bu) show that oxidative processes for the complexes are predominantly irreversible, but several examples of quasireversible behaviour also occur and support the assignment of an anodic process, seen between +1.0 and +1.6 V, as involving metal-centred oxidations. The reduction behaviour for the respective metal complexes is not simple, being irreversible in most cases. Solvent extraction studies (water/chloroform) involving the systematic variation of the metal, bis-β-diketone and heterocyclic base concentrations have been performed for cobalt(II) and zinc(II) using a radiotracer technique in order to probe the stoichiometries of the respective extracted species. Significant extraction synergism was observed when 4-ethylpyridine was also present with the bis-β-diketone ligand in the chloroform phase. Competitive extraction studies demonstrated a clear uptake preference for copper(II) over cobalt(II), nickel(II), zinc(II) and cadmium(II).