Synthesis and characterization of new adjacent-bridged tetraaza macrocyclic compounds with C-alkyl groups: crystal structure and spectral properties of a copper(II) complex

New structurally constrained tetraaza macrocycles 2,3-dimethyl-1,5,12,16-tetraazapentacyclo[14.6.3.35.12.06.11.017.22]octacosane (L6) and 2,2,4,10,12,18,20-octamethyl-1,5,9,13-tetraazatricyclo[11.3.2. 25.9]eicosane (L7) have been prepared in high yield by direct reaction of 1,3-dibromopropane with 3,14-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.07.12]docosane (L3) or 2,5,5,7,9,12,12,14-octamethyl-1,4,8,11-tetraazacyclodecane (L5). The macrocycle L6 readily reacts with anhydrous copper(II) ion to yield [CuL6]2+ in dry methanol but does not react with nickel(II) ion, showing a high copper(II) ion selectivity. On the other hand, neither of the metal ions is inserted into the cavity of L7 in similar conditions. The copper(II) complex [CuL6](ClO4)2 has a severely distorted square-planar coordination polyhedron with a rarely observed trans-IV type N-conformation. The visible absorption spectrum of [CuL6]2+ shows the d–d transition band at 490 nm (ε=690 M−1 cm−1); the wavelength is quite similar to that of [CuL3]2+, but the molar absorption coefficient is extraordinarily larger than those of [CuL3]2+ and other related tetraaza macrocyclic copper(II) complexes.