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

The square-planar nickel(II) complex of the octaaza macrohexacyclic li gand ahexacyclo[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 nic kel(II) complex of 7-bis(2-aminoethyl)-1,3,5,7-tetraazabicyclo[3.3.1] nonane with formaldehyde and trans-cyclohexane-1,2-diamine. The comple x exhibits an unusually strong ligand-field strength compared with oth er square-planar nickel(II) complexes of 14-membered azamacrocycles. C yclic voltammetry indicated a one-electron reduction potential at -1.3 2 V vs. Ag-AgCl and no oxidation peak up to +1.59 V. The complex cryst allizes 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 solv ed by the heavy-atom method and refined by full-matrix least squares t o a final R = 0.0447 for 2092 observed [I > 2sigma(I)] reflections. Th e ligand contains bicyclononane and tricyclotetradecane cage ring moie ties 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) a nd 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 c avity size of the ligand is similar to that of 1,4,7,10-tetraazacyclod odecane. Reduction of [NiL4][PF6]2 with Na/Hg gave [NiL4]PF6, which sh ows an anisotropic axial EPR powder spectrum with g values of g(parall el-to) > g(perpendicular).