Neutral (bis-β-diketonato) iron(iii), cobalt(ii), nickel(ii), copper(ii) and zinc(ii) metallocycles: structural, electrochemical and solvent extraction studies

Abstract

Neutral dimeric metallocyclic complexes of type [M₂(L¹)₂B_n] (where M = cobalt(II), nickel(II) and zinc(II), L¹ is the doubly deprotonated form of a 1,3-aryl linked bis-β-diketone ligand of type 1,3-bis(RC(O)CH₂C(O))C₆H₄ (R = Me, n-Pr, t-Bu) and B is pyridine (Py) or 4-ethylpyridine (EtPy)) have been synthesised, adding to similar complexes already reported for copper(II). New lipophilic ligand derivatives with R = octyl or nonyl were also prepared for use in solvent extraction experiments. Structural, electrochemical and solvent extraction investigations of selected metal complex systems from the above series are reported, with the X-ray structures of [Co₂(L¹)₂(Py)₄]·2.25CHCl₃·0.5H₂O (R = Pr), [Co₂(L¹)₂(EtPy)₄] (R = t-Bu), [Ni₂(L¹)₂(EtPy)₄] (R = t-Bu), [Zn₂(L¹)₂(EtPy)₂] (R = Me) and [Zn₂(L¹)₂(EtPy)₄] (R = t-Bu) being presented. The electrochemistry of H₂L¹ (R = t-Bu) and of [Fe₂(L¹)₃], [Co₂(L¹)₂(Py)₄], [Ni₂(L¹)₂(Py)₄], [Cu₂(L¹)₂] and [Zn₂(L¹)₂(Py)₂] has been examined. Oxidative processes for the complexes are dominantly irreversible, but several examples of quasireversible behaviour were observed and support the assignment of an anodic process, seen between +1.0 and +1.6 V, as a metal-centred oxidation. The reduction processes for the respective metal complexes are not simple, and irreversible in most cases. Solvent extraction studies (water/chloroform) involving variable concentrations of metal, bis-β-diketone and heterocyclic base have been performed for cobalt(II) and zinc(II) using a radiotracer technique to probe the stoichiometries of the extracted species in each case. Synergism was observed when 4-ethylpyridine was added to the bis-β-diketone ligand in the chloroform phase. Competitive extraction studies show a clear uptake preference for copper(II) over cobalt(II), nickel(II), zinc(II) and cadmium(II).