Synthesis, structure and oxidation of [NiL1]2+[L1= 5-(4-amino-2-azabutyl)-5-methyl-3,7-diazanonane-1,9-diamine]: evidence for rearrangement within the complexed nickel(III) species

Abstract

The ligand 5-(4-amino-2-azabutyl)-5-methyl-3,7-diazanonane-1,9-diamine, L¹, has been synthesised and a nickel(II) complex, [NiL¹]Br₂, characterized by X-ray crystallography (monoclinic, space group P2₁/n, a= 14.143, b= 9.618, c= 13.552, β= 106.14°, Z= 4, R= 0.0429, R′= 0.0418). The Cation adopts a distorted octahedral geometry typical of similar nickel(II) complexes. The complex [NiL¹]²⁺ can be oxidized, under unutral conditions, either electrochemically or by use of S₂O₈^2– to a moderately stable nickel(III) complex. By monitoring the reaction with S₂O₈^2– using ESR spectro-scopy, evidence has been obtained for three species, with characteristic ESR signals, g₁, g₂ and g₃. These represent the initial nickel(III) complexed ion and two structural modifications formed through substitution by sulfate. The final product is a distorted tetragonal complex with axial sulfate ligation. These reactions represent a rare example of ligand substitution at an NiN₆³⁺ chromophore. A second-order rate constant of 0.26 ± 0.05 dm³ mol^–1 s^–1 for the reaction of [NiL¹]²⁺ with S₂O₈^2– has been obtained.