SYNTHESES OF SQUARE-PLANAR NICKEL-(II) AND NICKEL-(I) COMPLEXES OF AN OCTAAZA MACROHEXACYCLIC LIGAND AND CRYSTAL-STRUCTURE OF THE NICKEL(II) COMPLEX

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).I(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 Pna2(1), with a = 17.934(2), b = 16.056(3), c = 9.612(l ) angstrom 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 > 2sigma(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) angstrom. The N-N bite distances are 2.706(6) and 2.698(6) angstrom for the five-membered chelate rings and 2.703(6) and 2.665(6) angstrom 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(parallel-to) > g(perpendicular).