Structures and reactivity of binuclear nickel(II) complexes of 5,5-bis(4′-amino-2′-azabutyl)-3,7-diazanonane-1,9-diamine

Abstract

Three different forms of binuclear nickel(II) complexes of the acyclic polyamine ligand, 5,5-bis(4′-amino-2′-azabutyl)-3,7-diazanonane-1,9-diamine (tabm) have been prepared and characterised structurally by X-ray crystallography. The complex [Ni₂(tabm)][ClO₄]₄·H₂O (space group P2₁/c, monoclinic, a= 21.884(4), b= 8.349(3), c= 16.852(3)Å, β= 96.88(2)°, R= 0.0733, R′= 0.0875) has both nickel(II) centres in a square-planar geomety. In [(NiCl₂)₂(tabm)]·2H₂O [space group P(no. 2), triclinic, a= 8.637(2), b= 22.106(7), c= 6.812(2)Å, α= 94.95(7)°, β= 106.25(3)°, γ= 93.78(4)°, R=R′= 0.0496) both the nickel(II) centres are octahedral. The third structure is of the novel complex [Ni₂Cl(OH₂)(tabm)][ClO₄]₃·H₂O [space group P2₁2₁2₁(no. 19), orthorhombic, a= 13.734(3), b= 16.645(4), c= 13.021(3)Å, R= 0.0648, R′= 0.070] in which there is present in the same cation both a square-planar and an octahedral nickel(II) centre. The bond lengths and angles for the square-planar NiN₄ moieties provide evidence for an ‘ideal’ Ni–N bond length which is based on an analysis of the present and other similar data. The ligand tabm in these complexes has two tetraaza units emanating from a single carbon which are paired in a rigid, nearly orthogonal orientation. As a result, the complexes show extraordinary kinetic stability in acidic media relative to 3,7-diazanonane-1,9-diaminenickel(II). All three complexes may be oxidised to corresponding nickel(III) species which according to time-dependent ESR spectra undergo a gradual change in geometry to a single final solution configuration. The integrity of these structures in solution depends upon the nature of the solvent, as demonstrated by ultraviolet–visible and ESR spectroscopy. Cyclic voltammetry of [Ni₂(tabm)]⁴⁺ in acetonitrile (0.1 mol dm^–3 NEt₄BF₄) shows two waves, and from differential pulse polarography a comproportionation constant (K_c= 72.5) for the mixed-valence (Ni^IINi^III) species was evaluated.