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-tetraazapentacyclo14.6.3.3^5.12.0^6.11.0^17.22]octacosane (L⁶) and 2,2,4,10,12,18,20-octamethyl-1,5,9,13-tetraazatricyclo11.3.2. 2^5.9]eicosane (L⁷) have been prepared in high yield by direct reaction of 1,3-dibromopropane with 3,14-dimethyl-2,6,13,17-tetraazatricyclo16.4.0.0^7.12]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.