Syntheses of square-planar nickel-(II) and -(I) complexes of an octaaza macrohexacyclic ligand and crystal structure of the nickel(II) complex

Abstract

The square-planar nickel(II) complex of the octaaza macrohexacyclic ligand 1,3,6,8,10,12,15,17-octaazahexacyclo[15.6. 1.1^3,17.1^6,0.1^8,12.0^18,23]heptacosane, [NiL⁴][PF₆]₂, has been synthesized by template condensation of the nickel(II) complex of 3,7-bis(2-aminoethyl)-1,3,5,7-tetraazabicyclo-[3.3.1]nonane with formaldehyde and trans-cyclohexane-1,2-diamine. The complex exhibits an unusually strong ligand-field strength compared with other square-planar nickel(II) complexes of 14-membered azamacrocycles. Cyclic voltammetry indicated a one-electron reduction potential at –1.32 V vs. Ag–AgCl and no oxidation peak up to +1.59 V. The complex crystallizes in the orthorhombic space group Pna2₁, with a= 17.934(2), b= 16.056(3), c= 9.612(1)Å and Z= 4. The structure was solved by the heavy-atom method and refined by full-matrix least squares to a final R= 0.0447 for 2092 observed [I > 2σ(I)] reflections. The ligand contains bicyclononane and tricyclotetradecane cage ring moieties fused at both ends of two ethylenediamine groups. The average Ni–N distance is 1.904(7)Å. The N–N bite distances are 2.706(6) and 2,698(6)Å for the five-membered chelate rings and 2.703(6) and 2.665(6)Å for the six-membered chelate rings so that the cavity size of the ligand is similar to that of 1,4,7,10-tetraazacyclododecane. Reduction of [NiL⁴][PF₆]₂ with Na/Hg gave [NiL⁴]PF₆, which shows an anisotropic axial EPR powder spectrum with g values of g_∥ > g_⊥.