Solution chemistry in non-co-ordinating solvents (CH2Cl2, CHCl3) of nickel(II) and nickel(III) complexes of the lipophilic macrocycle 1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane. The emphasized role of the anion co-ordination

Abstract

Nickel(II) complexes of the lipophilic tetramine macrocycle N-cetylcyclam (1-hexadecyl-1,4,8,11-tetra-azacyclotetradecane) L¹, dissolve in non-co-ordinating solvents such as CH₂Cl₂ and CHCl₃ as intact Ni(L¹)X₂ species, whose spin state and colour depend on the co-ordinating tendencies of the apically bound X^– anions: Cl^–, and SCN^–, blue, high-spin complexes; ClO₄^–, BF₄^–, and I^–, yellow, low-spin complexes. The complex Ni(L¹)Br₂ exists in solution as an equilibrium mixture of the blue and yellow species and the blue-to-yellow conversion is exothermic; moreover, the equilibrium is displaced to the right on further addition of the background electrolyte, tetra-alkylammonium or tetraethylphosphonium bromide. A general model is proposed to explain the thermodynamic aspects of the high-spin/low-spin interconversion in both co-ordinating (e.g. water) and non-co-ordinating media. Finally, Ni^II(L¹)X₂ complexes in CH₂Cl₂ solution (0.1 mol dm^–3 Buⁿ₄NX), undergo reversible one-electron oxidation processes to give a [Ni^III(L¹)X₂]X species. The Ni^III–Ni^II redox potential decreases dramatically with increasing co-ordinating tendency of the X^– ions. This permits us to obtain a spectrochemical series for electrochemically inert inorganic anions. On this basis, it appears that ClO₄^– is a more strongly co-ordinating ligand than BF₄^–.